HTTP Working Group R. Fielding, UC Irvine
INTERNET-DRAFT H. Frystyk, MIT/LCS
<draft-ietf-http-v11-spec-02.html> T. Berners-Lee, MIT/LCS
J. Gettys, DEC
Jeffrey C. Mogul, DEC
Expires September 23, 1996 April 23, 1996
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NOTE: This specification is for discussion purposes only. It is not claimed to represent the consensus of the HTTP working group, and contains a number of proposals that either have not been discussed or are controversial. The working group is discussing significant changes in many areas, including - support for caching, persistent connections, range retrieval, content negotiation, MIME compatibility, authentication, timing of the PUT operation.
The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. It is a generic, stateless, object-oriented protocol which can be used for many tasks, such as name servers and distributed object management systems, through extension of its request methods (commands). A feature of HTTP is the typing and negotiation of data representation, allowing systems to be built independently of the data being transferred.
HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification defines the protocol referred to as "HTTP/1.1".
This document is still organized to minimize changes from the previous draft, to ease reviewers work in finding new material (and because the editor has not had time to reorganize it).. However, the current organization is now quite poor for new readers of this document. We recommend that new readers of this document not read it in the current order of presentation, but may want to skip ahead after reading sections 1-9 and read sections 11, 12 13 and 14 before reading section 10 which defines the header field definitions. Section 10 itself is now also not in alphabetical order, again, to avoid renumbering sections to be able to easily compare between drafts.
If you are reading the version of this document showing revision markup, note that we've tried to preserve significant changes from the previous version, though a few changes may have slipped through unmarked. We make no guarantees that all changes have revision marks, though we've tried to preserve them as an aid to those who wish to check a specific change has been reflected in this draft.
Note that some sections are still marked as SLUSHY and a few are marked FLUID; these are still undergoing drafting.
Note that text in bold in the text are as yet
incompletely resolved issues. Opinions are solicited…
HtmlDirectHypertext Transfer Protocol -- HTTP/1.1 1Status of this Memo 1Abstract 1Note to Readers of This Document 1Table of Contents 31. Introduction 91.1 Purpose 91.2 Requirements 91.3 Terminology 101.4 Overall Operation 111.4 HTTP and MIME 132. Notational Conventions and Generic Grammar 132.1 Augmented BNF 132.2 Basic Rules 143. Protocol Parameters 163.1 HTTP Version 163.2 Uniform Resource Identifiers 163.2.1 General Syntax 173.2.2 http URL 183.3 Date/Time Formats 183.3.1 Full Date 183.3.2 Delta Seconds 193.4 Character Sets 193.5 Content Codings 203.6 Transfer Codings 213.7 Media Types 223.7.1 Canonicalization and Text Defaults 223.7.2 Multipart Types 233.8 Product Tokens 233.9 Quality Values 243.10 Language Tags 243.12 Full Date Values 253.13 Opaque Validators 253.14 Variant IDs 253.15 Validator Sets 253.16 Variant Sets 263.17 HTTP Protocol Parameters Related to Ranges 263.17.1SLUSHY Range Units 263.17.2 SLUSHY Byte Ranges 263.17.3 SLUSHY: Content Ranges 274. HTTP Message 284.1 Message Types 284.2 Message Headers 284.3 General Header Fields 295. Request 295.1 Request-Line 305.1.1 Method 305.1.2 Request-URI 305.2 Request Header Fields 326. Response 326.1 Status-Line 336.1.1 Status Code and Reason Phrase 336.2 Response Header Fields 357. Entity 357.1 Entity Header Fields 357.2 Entity Body 367.2.1 Type 367.2.2 Length 368. Method Definitions 378.1 OPTIONS 378.2 GET 378.3 HEAD 388.4 POST 388.4.1 SLUSHY: Entity Transmission Requirements 398.5 PUT 408.9 DELETE 408.12 TRACE 419. Status Code Definitions 419.1 Informational 1xx 419.2 Successful 2xx 419.3 Redirection 3xx 439.4 Client Error 4xx 459.5 Server Error 5xx 4710. Header Field Definitions 4810.1 Accept 4810.2 Accept-Charset 5010.3 Accept-Encoding 5010.4 Accept-Language 5110.5 Allow 5210.6 Authorization 5210.7 Cache-Control 53Check: is this true? 5410.7.1 SLUSHY: Restrictions on What is Cachable 5410.7.2 Restrictions On What May be Stored by a Cache 5510.7.3 Modifications of the Basic Expiration Mechanism 5510.7.4 SLUSHY: Controls over cache revalidation and reload 5610.7.5 FLUID: Restrictions on use count and demographic reporting 5710.7.6 Miscellaneous restrictions 5810.8 Connection 5810.8.1 Persist 5810.9 Content-Base 5810.10 Content-Encoding 5910.11 Content-Language 5910.12 Content-Length 6010.13 Content-MD5 6010.14 SLUSHY Content-Range 6110.14.1 MIME multipart/byteranges content-type 6110.14.2 Additional rules for Content-Range 6210.15 Content-Type 6210.16 Content-Location 6310.17 Date 6310.19 SLUSHY Expires 6410.20 Via 6410.21 From 6510.22 Host 6610.23 If-Modified-Since 6610.25 Last-Modified 6710.27 Location 6810.29 Pragma 6810.30 Proxy-Authenticate 6910.31 Proxy-Authorization 6910.32 Public 6910.33 Range 7010.34 Referer 7010.36 Retry-After 7010.37 Server 7110.38 Title 7110.39 Transfer Encoding 7110.41 Upgrade 7210.43 User-Agent 7210.44 WWW-Authenticate 7310.45 Max-Forwards 7310.46 Age 7310.47 CVal 7410.48 If-Invalid 7410.49 If-Valid 7510.50 If-Unmodified-Since 7610.51 Warning 7610.52 Vary 7710.53 Alternates 7910.54 SLUSHY: Accept-Ranges 7910.55 SLUSHY: Range-If 8011. Access Authentication 8011.1 Basic Authentication Scheme 8211.2 Digest Authentication Scheme 8212. Content Negotiation 8212.1 Negotiation facilities defined in this specification 8313 Caching in HTTP 8313.1 Semantic Transparency 8313.2 Expiration Model 8413.2.1 Server-Specified Expiration 8513.2.2 Limitations on the Effect of Expiration Times 8513.2.3 Heuristic Expiration 8513.2.4 Client-controlled Behavior 8513.2.5 Exceptions to the Rules and Warnings 8613.2.6 Age Calculations 8613.2.7 Expiration Calculations 8713.2.8 UT Mandatory 8813.3 Validation Model 8813.3.1 Last-modified Dates 8913.3.2 Opaque Validators 8913.3.3 Weak and Strong Validators 8913.3.4 Rules for When to Use Opaque Validators and Last-modified Dates 9113.3.5 SLUSHY: Non-validating conditionals 9213.3.6 FLUID: Other Issues 9213.4 Cache-control Mechanisms 9213.5 Warnings 9213.6 Explicit Indications Regarding User-specified Overrides 9313.7
The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. HTTP has been in use by the World-Wide Web global information initiative since 1990. The first version of HTTP, referred to as HTTP/0.9, was a simple protocol for raw data transfer across the Internet. HTTP/1.0, as defined by RFC xxxx [6], improved the protocol by allowing messages to be in the format of MIME-like entities, containing metainformation about the data transferred and modifiers on the request/response semantics. However, HTTP/1.0 does not sufficiently take into consideration the effect of hierarchical proxies and caching, the desire for persistent connections and virtual hosts, and a number of other details that slipped through the cracks of existing implementations. In addition, the proliferation of incompletely-implemented applications calling themselves "HTTP/1.0" has necessitated a protocol version change in order for two communicating applications to determine each other's true capabilities.
This specification defines the protocol referred to as "HTTP/1.1". This protocol is backwards-compatible with HTTP/1.0, but includes more stringent requirements in order to ensure reliable implementation of its features.
Practical information systems require more functionality than simple retrieval, including search, front-end update, and annotation. HTTP allows an open-ended set of methods that indicate the purpose of a request. It builds on the discipline of reference provided by the Uniform Resource Identifier (URI) [3], as a location (URL) [4] or name (URN) [20], for indicating the resource on which a method is to be applied. Messages are passed in a format similar to that used by Internet Mail [9] and the Multipurpose Internet Mail Extensions (MIME) [7].
HTTP is also used as a generic protocol for communication between user agents and proxies/gateways to other Internet protocols, such as SMTP [16], NNTP [13], FTP [18], Gopher [2], and WAIS [10], allowing basic hypermedia access to resources available from diverse applications and simplifying the implementation of user agents.
This specification uses the same words as RFC 1123 [8] for defining the significance of each particular requirement. These words are:
An implementation is not compliant if it fails to satisfy one or more of the MUST requirements for the protocols it implements. An implementation that satisfies all the MUST and all the SHOULD requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the MUST requirements but not all the SHOULD requirements for its protocols is said to be "conditionally compliant".
This specification uses a number of terms to refer to the roles played by participants in, and objects of, the HTTP communication.
Any given program MAY be capable of being both a client and a server; our use of these terms refers only to the role being performed by the program for a particular connection, rather than to the program's capabilities in general. Likewise, any server MAY act as an origin server, proxy, gateway, or tunnel, switching behavior based on the nature of each request.
The HTTP protocol is based on a request/response paradigm. A client sends a request to the server in the form of a request method, URI, and protocol version, followed by a MIME-like message containing request modifiers, client information, and possible body content over a connection with a server. The server responds with a status line, including the message's protocol version and a success or error code, followed by a MIME-like message containing server information, entity metainformation, and possible body content.
Most HTTP communication is initiated by a user agent and consists of a request to be applied to a resource on some origin server. In the simplest case, this may be accomplished via a single connection (v) between the user agent (UA) and the origin server (O).
request chain ------------------------>
UA -------------------v------------------- O
<----------------------- response chain
A more complicated situation occurs when one or more intermediaries are present in the request/response chain. There are three common forms of intermediary: proxy, gateway, and tunnel. A proxy is a forwarding agent, receiving requests for a URI in its absolute form, rewriting all or parts of the message, and forwarding the reformatted request toward the server identified by the URI. A gateway is a receiving agent, acting as a layer above some other server(s) and, if necessary, translating the requests to the underlying server's protocol. A tunnel acts as a relay point between two connections without changing the messages; tunnels are used when the communication needs to pass through an intermediary (such as a firewall) even when the intermediary cannot understand the contents of the messages.
request chain -------------------------------------->
UA -----v----- A -----v----- B -----v----- C -----v----- O
<------------------------------------- response chain
The figure above shows three intermediaries (A, B, and C) between the user agent and origin server. A request or response message that travels the whole chain MUST pass through four separate connections. This distinction is important because some HTTP communication options may apply only to the connection with the nearest, non-tunnel neighbor, only to the end-points of the chain, or to all connections along the chain. Although the diagram is linear, each participant may be engaged in multiple, simultaneous communications. For example, B may be receiving requests from many clients other than A, and/or forwarding requests to servers other than C, at the same time that it is handling A's request.
Any party to the communication which is not acting as a tunnel may employ an internal cache for handling requests. The effect of a cache is that the request/response chain is shortened if one of the participants along the chain has a cached response applicable to that request. The following illustrates the resulting chain if B has a cached copy of an earlier response from O (via C) for a request which has not been cached by UA or A.
request chain ---------->
UA -----v----- A -----v----- B - - - - - - C - - - - - - O
<--------- response chain
Not all responses are cachable, and some requests may contain modifiers which place special requirements on cache behavior. HTTP requirements for cache behavior and cachable responses are defined in Section 13.
On the Internet, HTTP communication generally takes place over TCP/IP connections. The default port is TCP 80 [19], but other ports can be used. This does not preclude HTTP from being implemented on top of any other protocol on the Internet, or on other networks. HTTP only presumes a reliable transport; any protocol that provides such guarantees can be used; the mapping of the HTTP/1.1 request and response structures onto the transport data units of the protocol in question is outside the scope of this specification.
However, HTTP/1.1 implementations SHOULD implement persistent connections (See section 14). Both clients and servers MUST be capable of handling cases where either party closes the connection prematurely, due to user action, automated time-out, or program failure. In any case, the closing of the connection by either or both parties always terminates the current request, regardless of its status.
HTTP/1.1 uses many of the constructs defined for MIME, as defined in RFC 1521 [7]. Appendix C describes the ways in which the context of HTTP allows for different use of Internet Media Types than is typically found in Internet mail, and gives the rationale for those differences.
All of the mechanisms specified in this document are described in both prose and an augmented Backus-Naur Form (BNF) similar to that used by RFC 822 [9]. Implementers will need to be familiar with the notation in order to understand this specification. The augmented BNF includes the following constructs:
name = definition
"<"
and ">") and is
separated from its definition by the equal character "=".
Whitespace is only significant in that indentation of continuation
lines is used to indicate a rule definition that spans more than
one line. Certain basic rules are in uppercase, such as SP,
LWS, HT,
CRLF, DIGIT,
ALPHA, etc. Angle brackets are
used within definitions whenever their presence will facilitate
discerning the use of rule names."literal"
rule1 | rule2
"I")
are alternatives, e.g., "yes | no"
will accept yes or no.
(rule1 rule2)
(elem (foo | bar)
elem)" allows the token sequences "elem
foo elem" and "elem
bar elem".*rule
*"
preceding an element indicates repetition. The full form is "<n>*<m>element"
indicating at least <n>
and at most <m> occurrences
of element. Default values are
0 and infinity so that "*(element)"
allows any number, including zero; "1*element"
requires at least one; and "1*2element"
allows one or two.[rule]
[foo
bar]" is equivalent to "*1(foo
bar)".rule
<n>(element)"
is equivalent to "<n>*<n>(element)";
that is, exactly <n> occurrences
of (element). Thus 2DIGIT
is a 2-digit number, and 3ALPHA
is a string of three alphabetic characters.#rule
"#"
is defined, similar to "*",
for defining lists of elements. The full form is "<n>#<m>element"
indicating at least <n>
and at most <m> elements,
each separated by one or more commas (",")
and optional linear whitespace (LWS). This makes the usual form
of lists very easy; a rule such as "( *LWS element
*( *LWS "," *LWS element ))"
can be shown as "1#element".
Wherever this construct is used, null elements are allowed, but
do not contribute to the count of elements present. That is, "(element),
, (element)" is permitted, but counts
as only two elements. Therefore, where at least one element is
required, at least one non-null element MUST be present. Default
values are 0 and infinity so
that "#(element)" allows
any number, including zero; "1#element"
requires at least one; and "1#2element"
allows one or two.; comment
implied *LWS
LWS)
can be included between any two adjacent words (token
or quoted-string), and between
adjacent tokens and delimiters (tspecials),
without changing the interpretation of a field. At least one delimiter
(tspecials) MUST exist between
any two tokens, since they would otherwise be interpreted as a
single token. However, applications SHOULD attempt to follow "common
form" when generating HTTP constructs, since there exist
some implementations that fail to accept anything beyond the common
forms.
The following rules are used throughout this specification to describe basic parsing constructs. The US-ASCII coded character set is defined by [21].
OCTET = <any 8-bit sequence of data>
CHAR = <any US-ASCII character (octets 0 - 127)>
UPALPHA = <any US-ASCII uppercase letter "A".."Z">
LOALPHA = <any US-ASCII lowercase letter "a".."z">
ALPHA = UPALPHA | LOALPHA
DIGIT = <any US-ASCII digit "0".."9">
CTL = <any US-ASCII control character
(octets 0 - 31) and DEL (127)>
CR = <US-ASCII CR, carriage return (13)>
LF = <US-ASCII LF, linefeed (10)>
SP = <US-ASCII SP, space (32)>
HT = <US-ASCII HT, horizontal-tab (9)>
<"> = <US-ASCII double-quote mark (34)>
HTTP/1.1 defines the octet sequence CR
LF as the end-of-line marker for all protocol
elements except the Entity-Body
(see Appendix B for tolerant applications).
The end-of-line marker within an Entity-Body
is defined by its associated media type, as described in Section 3.7.
CRLF = CR LF
HTTP/1.1 headers can be folded onto multiple lines
if the continuation line begins with a space or horizontal tab.
All linear whitespace, including folding, has the same semantics
as SP.
LWS = [CRLF] 1*( SP | HT )
The TEXT rule is
only used for descriptive field contents and values that are not
intended to be interpreted by the message parser. Words of *TEXT
MAY contain octets from character sets other than US-ASCII only
when encoded according to the rules of RFC 1522 [14].
TEXT = <any OCTET except CTLs,
but including LWS>
Recipients of header field TEXT
containing octets outside the US-ASCII character set range MAY
assume that they represent ISO-8859-1 characters if there is no
other encoding indicated by an RFC 1522 mechanism.
Hexadecimal numeric characters are used in several protocol elements.
HEX = "A" | "B" | "C" | "D" | "E" | "F"
| "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
Many HTTP/1.1 header field values consist of words
separated by LWS or special characters.
These special characters MUST be in a quoted string to be used
within a parameter value.
word = token | quoted-string
token = 1*<any CHAR except CTLs or tspecials>
tspecials = "(" | ")" | "<" | ">" | "@"
| "," | ";" | ":" | "\" | <">
| "/" | "[" | "]" | "?" | "="
| "{" | "}" | SP | HT
Comments can be included in some HTTP header fields
by surrounding the comment text with parentheses. Comments are
only allowed in fields containing "comment"
as part of their field value definition. In all other fields,
parentheses are considered part of the field value.
comment = "(" *( ctext | comment ) ")"
ctext = <any TEXT excluding "(" and ")">
A string of text is parsed as a single word if it is quoted using double-quote marks.
quoted-string = ( <"> *(qdtext) <"> )
qdtext = <any CHAR except <"> and CTLs,
but including LWS>
The backslash character ("\") may be used as a single-character quoting mechanism only within quoted-string and comment constructs.
quoted-pair = "\" CHAR
HTTP uses a "<major>.<minor>" numbering scheme to indicate versions of the protocol. The protocol versioning policy is intended to allow the sender to indicate the format of a message and its capacity for understanding further HTTP communication, rather than the features obtained via that communication. No change is made to the version number for the addition of message components which do not affect communication behavior or which only add to extensible field values. The <minor> number is incremented when the changes made to the protocol add features which do not change the general message parsing algorithm, but which may add to the message semantics and imply additional capabilities of the sender. The <major> number is incremented when the format of a message within the protocol is changed.
The version of an HTTP message is indicated by an
HTTP-Version field in the first
line of the message. If the protocol version is not specified,
the recipient MUST assume that the message is in the simple HTTP/0.9
format [6].
HTTP-Version = "HTTP" "/" 1*DIGIT "." 1*DIGIT
Note that the major and minor numbers SHOULD be treated as separate integers and that each MAY be incremented higher than a single digit. Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is lower than HTTP/12.3. Leading zeros SHOULD be ignored by recipients and never generated by senders.
Applications sending Full-Request
or Full-Response messages, as
defined by this specification, MUST include an HTTP-Version
of "HTTP/1.1". Use
of this version number indicates that the sending application
is at least conditionally compliant with this specification.
Proxy and gateway applications MUST be careful in forwarding requests that are received in a format different than that of the application's native HTTP version. Since the protocol version indicates the protocol capability of the sender, a proxy/gateway MUST never send a message with a version indicator which is greater than its native version; if a higher version request is received, the proxy/gateway MUST either downgrade the request version, respond with an error, or switch to tunnel behavior. Requests with a version lower than that of the application's native format MAY be upgraded before being forwarded; the proxy/gateway's response to that request MUST follow the server requirements listed above.
Note: Converting between versions of HTTP may involve addition or deletion of headers required or forbidden by the version involved. It is likely more involved than just changing the version indicator.
URIs have been known by many names: WWW addresses, Universal Document Identifiers, Universal Resource Identifiers [3], and finally the combination of Uniform Resource Locators (URL) [4] and Names (URN) [20]. As far as HTTP is concerned, Uniform Resource Identifiers are simply formatted strings which identify--via name, location, or any other characteristic--a network resource.
URIs in HTTP can be represented in absolute form or relative to some known base URI [11], depending upon the context of their use. The two forms are differentiated by the fact that absolute URIs always begin with a scheme name followed by a colon.
URI = ( absoluteURI | relativeURI ) [ "#" fragment ]
absoluteURI = scheme ":" *( uchar | reserved )
relativeURI = net_path | abs_path | rel_path
net_path = "//" net_loc [ abs_path ]
abs_path = "/" rel_path
rel_path = [ path ] [ ";" params ] [ "?" query ]
path = fsegment *( "/" segment )
fsegment = 1*pchar
segment = *pchar
params = param *( ";" param )
param = *( pchar | "/" )
scheme = 1*( ALPHA | DIGIT | "+" | "-" | "." )
net_loc = *( pchar | ";" | "?" )
query = *( uchar | reserved )
fragment = *( uchar | reserved )
pchar = uchar | ":" | "@" | "&" | "=" | "+"
uchar = unreserved | escape
unreserved = ALPHA | DIGIT | safe | extra | national
escape = "%" HEX HEX
reserved = ";" | "/" | "?" | ":" | "@" | "&" | "="
extra = "!" | "*" | "'" | "(" | ")" | ","
safe = "$" | "-" | "_" | "." | "+"
unsafe = CTL | SP | <"> | "#" | "%" | "<" | ">"
national = <any OCTET excluding ALPHA, DIGIT,
reserved, extra, safe, and unsafe>
For definitive information on URL syntax and semantics, see RFC
1738 [4] and RFC 1808 [11].
The BNF above includes national characters not allowed
in valid URLs as specified by RFC 1738, since HTTP servers are
not restricted in the set of unreserved characters
allowed to represent the rel_path part of addresses,
and HTTP proxies may receive requests for URIs not defined by
RFC 1738.
The HTTP protocol does not place any a-priori limit on the length of a URI. Servers MUST be able to handle the URI of any resource they serve, and SHOULD be able to handle URIs of unbounded length if they provide GET-based forms that could generate such URIs. A server SHOULD return a status code of
414 Request-URI Too Large
if a URI is longer than the server can handle. See section 9.4.
Note: Servers SHOULD be cautious about depending on URI lengths above 255 bytes, because some older client or proxy implementations may not properly support these.
All client and proxy implementations MUST be able to handle a URI of any finite length.
The "http" scheme is used to locate network resources via the HTTP protocol. This section defines the scheme-specific syntax and semantics for http URLs.
http_URL = "http:" "//" host [ ":" port ] [ abs_path ]
host = <A legal Internet host domain name
or IP address (in dotted-decimal form),
as defined by Section 2.1 of RFC 1123>
port = *DIGIT
If the port is empty or not given, port 80 is assumed.
The semantics are that the identified resource is located at the
server listening for TCP connections on that port
of that host, and the Request-URI for
the resource is abs_path. The use of IP addresses
in URL's SHOULD be avoided whenever possible. See RFC 1900[24].
If the abs_path is not present in the URL, it MUST
be given as "/" when used as a Request-URI
for a resource (Section 5.1.2).
Note: Although the HTTP protocol is independent of the transport layer protocol, the http URL only identifies resources by their TCP location, and thus non-TCP resources MUST be identified by some other URI scheme.
The canonical form for "http" URLs is obtained by converting
any UPALPHA characters in host to their
LOALPHA equivalent (hostnames are case-insensitive),
eliding the [ ":" port ] if the port is
80, and replacing an empty abs_path with "/".
HTTP applications have historically allowed three different formats for the representation of date/time stamps:
Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123
Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, made obsolete by RFC 1036
Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
The first format is preferred as an Internet standard and represents a fixed-length subset of that defined by RFC 1123 [8] (an update to RFC 822 [9]). The second format is in common use, but is based on the obsolete RFC 850 [12] date format and lacks a four-digit year. HTTP/1.1 clients and servers that parse the date value MUST accept all three formats, though they MUST only generate the RFC 1123 format for representing date/time stamps in HTTP message fields.
Note: Recipients of date values are encouraged to be robust in accepting date values that may have been generated by non-HTTP applications, as is sometimes the case when retrieving or posting messages via proxies/gateways to SMTP or NNTP.
All HTTP date/time stamps MUST be represented in Universal Time (UT), also known as Greenwich Mean Time (GMT), without exception. This is indicated in the first two formats by the inclusion of "GMT" as the three-letter abbreviation for time zone, and SHOULD be assumed when reading the asctime format.
HTTP-date = rfc1123-date | rfc850-date | asctime-date
rfc1123-date = wkday "," SP date1 SP time SP "GMT"
rfc850-date = weekday "," SP date2 SP time SP "GMT"
asctime-date = wkday SP date3 SP time SP 4DIGIT
date1 = 2DIGIT SP month SP 4DIGIT
; day month year (e.g., 02 Jun 1982)
date2 = 2DIGIT "-" month "-" 2DIGIT
; day-month-year (e.g., 02-Jun-82)
date3 = month SP ( 2DIGIT | ( SP 1DIGIT ))
; month day (e.g., Jun 2)
time = 2DIGIT ":" 2DIGIT ":" 2DIGIT
; 00:00:00 - 23:59:59
wkday = "Mon" | "Tue" | "Wed"
| "Thu" | "Fri" | "Sat" | "Sun"
weekday = "Monday" | "Tuesday" | "Wednesday"
| "Thursday" | "Friday" | "Saturday" | "Sunday"
month = "Jan" | "Feb" | "Mar" | "Apr"
| "May" | "Jun" | "Jul" | "Aug"
| "Sep" | "Oct" | "Nov" | "Dec"
Note: HTTP requirements for the date/time stamp format apply only to their usage within the protocol stream. Clients and servers are not required to use these formats for user presentation, request logging, etc.
Additional rules for requirements on parsing and representation of dates and other potential problems with date representations include:
Expires date as earlier than the proper value, but
MUST NOT internally represent a parsed Expires date
as later than the proper value.
Some HTTP header fields allow a time value to be specified as an integer number of seconds, represented in decimal, after the time that the message was received. This format SHOULD only be used to represent short time periods or periods that cannot start until receipt of the message.
delta-seconds = 1*DIGIT
HTTP uses the same definition of the term "character set" as that described for MIME:
The term "character set" is used in this document to refer to a method used with one or more tables to convert a sequence of octets into a sequence of characters. Note that unconditional conversion in the other direction is not required, in that not all characters may be available in a given character set and a character set may provide more than one sequence of octets to represent a particular character. This definition is intended to allow various kinds of character encodings, from simple single-table mappings such as US-ASCII to complex table switching methods such as those that use ISO 2022's techniques. However, the definition associated with a MIME character set name MUST fully specify the mapping to be performed from octets to characters. In particular, use of external profiling information to determine the exact mapping is not permitted.
Note: This use of the term "character set" is more commonly referred to as a "character encoding." However, since HTTP and MIME share the same registry, it is important that the terminology also be shared.
HTTP character sets are identified by case-insensitive tokens. The complete set of tokens is defined by the IANA Character Set registry [19]. However, because that registry does not define a single, consistent token for each character set, we define here the preferred names for those character sets most likely to be used with HTTP entities. These character sets include those registered by RFC 1521 [7] -- the US-ASCII [21] and ISO-8859 [22] character sets -- and other names specifically recommended for use within MIME charset parameters.
charset = "US-ASCII"
| "ISO-8859-1" | "ISO-8859-2" | "ISO-8859-3"
| "ISO-8859-4" | "ISO-8859-5" | "ISO-8859-6"
| "ISO-8859-7" | "ISO-8859-8" | "ISO-8859-9"
| "ISO-2022-JP" | "ISO-2022-JP-2" | "ISO-2022-KR"
| "UNICODE-1-1" | "UNICODE-1-1-UTF-7" | "UNICODE-1-1-UTF-8"
| token
Although HTTP allows an arbitrary token to be used as a charset value, any token that has a predefined value within the IANA Character Set registry [19] MUST represent the character set defined by that registry. Applications SHOULD limit their use of character sets to those defined by the IANA registry.
Note: This use of the term "character set" is more commonly referred to as a "character encoding." However, since HTTP and MIME share the same registry, it is important that the terminology also be shared.
The character set of an entity body SHOULD be labeled as the lowest common denominator of the character codes used within that body, with the exception that no label is preferred over the labels US-ASCII or ISO-8859-1.
Content coding values indicate an encoding transformation that has been or can be applied to a resource. Content codings are primarily used to allow a document to be compressed or encrypted without losing the identity of its underlying media type. Typically, the resource is stored in this encoding and only decoded before rendering or analogous usage.
content-coding = "gzip" | "x-gzip" | "compress" | "x-compress" | token
Note: For historical reasons, HTTP applications SHOULD consider "x-gzip" and
"x-compress" to be equivalent to "gzip" and "compress", respectively.
All content-coding
values are case-insensitive. HTTP/1.1 uses content-coding
values in the Accept-Encoding
(Section 10.3) and Content-Encoding
(Section 10.10) header fields.
Although the value describes the content-coding, what is more
important is that it indicates what decoding mechanism will be
required to remove the encoding. Note that a single program MAY
be capable of decoding multiple content-coding formats. Two values
are defined by this specification:
Note: Use of program names for the identification of encoding formats is not desirable and should be discouraged for future encodings. Their use here is representative of historical practice, not good design.
HTTP defines a registration process which uses the
Internet Assigned Numbers Authority (IANA) as a central registry
for content-coding value tokens. Additional content-coding value
tokens beyond the four defined in this document (gzip
x-gzip compress x-compress) SHOULD be registered
with the IANA. To allow interoperability between clients and servers,
specifications of the content coding algorithms used to implement
a new value SHOULD be publicly available and adequate for independent
implementation, and MUST conform to the purpose of content coding
defined in this section.
Transfer coding values are used to indicate an encoding
transformation that has been, can be, or may need to be applied
to an Entity-Body in order to
ensure safe transport through the network. This differs from a
content coding in that the transfer coding is a property of the
message, not of the original resource.
transfer-coding = "chunked" | transfer-extension
transfer-extension = token
All transfer-coding values are case-insensitive.
HTTP/1.1 uses transfer coding values in the Transfer-Encoding
header field (Section 10.39).
Transfer codings are analogous to the Content-Transfer-Encoding values of MIME [7], which were designed to enable safe transport of binary data over a 7-bit transport service. However, "safe transport" has a different focus for an 8bit-clean transfer protocol. In HTTP, the only unsafe characteristic of message bodies is the difficulty in determining the exact body length (Section 7.2.2), or the desire to encrypt data over a shared transport.
All HTTP/1.1 applications MUST be able to receive and decode the "chunked" transfer coding , and MUST ignore chunked extensions they do not understand. The chunked encoding modifies the body of a message in order to transfer it as a series of chunks, each with its own size indicator, followed by an optional footer containing entity-header fields. This allows dynamically-produced content to be transferred along with the information necessary for the recipient to verify that it has received the full message.
Chunked-Body = *chunk
"0" CRLF
footer
CRLF
chunk = chunk-size [ chunk-ext ] CRLF
chunk-data CRLF
chunk-size = hex-no-zero *HEX
chunk-ext = *( ";" chunk-ext-name [ "=" chunk-ext-value ] )
chunk-ext-name = token
chunk-ext-val = token | quoted-string
chunk-data = chunk-size(OCTET)
footer = *< Content-MD5 and future headers that specify
they are allowed in footer>>
hex-no-zero = <HEX excluding "0">
Note that the chunks are ended by a zero-sized chunk,
followed by the footer and terminated by an empty line. An example
process for decoding a Chunked-Body
is presented in Appendix C.5.
HTTP uses Internet Media Types [17]
in the Content-Type (Section 10.15)
and Accept (Section 10.1)
header fields in order to provide open and extensible data typing
and type negotiation.
media-type = type "/" subtype *( ";" parameter )
type = token
subtype = token
Parameters may follow the type/subtype in the form of attribute/value pairs.
parameter = attribute "=" value
attribute = token
value = token | quoted-string
The type, subtype, and parameter attribute names
are case-insensitive. Parameter values may or may not be case-sensitive,
depending on the semantics of the parameter name.
LWS MUST NOT be generated between the type
and subtype, nor between an attribute and its value. Upon receipt
of a media type with an unrecognized parameter, a user agent SHOULD
treat the media type as if the unrecognized parameter and its
value were not present.
Some older HTTP applications do not recognize media type parameters. HTTP/1.1 applications SHOULD only use media type parameters when they are necessary to define the content of a message.
Media-type values are registered with the Internet Assigned Number Authority (IANA [19]). The media type registration process is outlined in RFC 1590 [17]. Use of non-registered media types is discouraged.
Internet media types are registered with a canonical form. In
general, an Entity-Body transferred via HTTP MUST
be represented in the appropriate canonical form prior to its
transmission. If the body has been encoded with a Content-Encoding,
the underlying data SHOULD be in canonical form prior to being
encoded.
Media subtypes of the "text" type use CRLF
as the text line break when in canonical form. However, HTTP allows
the transport of text media with plain CR or LF
alone representing a line break when used consistently within
the Entity-Body. HTTP applications MUST accept CRLF,
bare CR, and bare LF as being representative
of a line break in text media received via HTTP.
In addition, if the text media is represented in a character set
that does not use octets 13 and 10 for CR and LF
respectively, as is the case for some multi-byte character sets,
HTTP allows the use of whatever octet sequences are defined by
that character set to represent the equivalent of CR
and LF for line breaks. This flexibility regarding
line breaks applies only to text media in the Entity-Body;
a bare CR or LF SHOULD NOT be substituted
for CRLF within any of the HTTP control structures
(such as header fields and multipart boundaries).
The "charset" parameter is used with some media types to define the character set (Section 3.4) of the data. When no explicit charset parameter is provided by the sender, media subtypes of the "text" type are defined to have a default charset value of "ISO-8859-1" when received via HTTP. Data in character sets other than "ISO-8859-1" or its subsets MUST be labeled with an appropriate charset value in order to be consistently interpreted by the recipient.
Note: Many current HTTP servers provide data using charsets other than "ISO-8859-1" without proper labeling. This situation reduces interoperability and is not recommended. To compensate for this, some HTTP user agents provide a configuration option to allow the user to change the default interpretation of the media type character set when no charset parameter is given.
MIME provides for a number of "multipart" types -- encapsulations
of one or more entities within a single message's Entity-Body.
All multipart types share a common syntax, as defined in Section
7.2.1 of RFC 1521 [7], and MUST include
a boundary parameter as part of the media type value. The message
body is itself a protocol element and MUST therefore use only
CRLF to represent line breaks between body-parts.
Unlike in RFC 1521, the epilogue of any multipart message MUST
be empty; HTTP applications MUST NOT transmit the epilogue even
if the original resource contains an epilogue.
In HTTP, multipart body-parts MAY contain header fields which are significant to the meaning of that part.
In general, an HTTP user agent SHOULD follow the same or similar behavior as a MIME user agent would upon receipt of a multipart type. If an application receives an unrecognized multipart subtype, the application MUST treat it as being equivalent to "multipart/mixed".
Note: The "multipart/form-data" type has been specifically defined for carrying form data suitable for processing via the POST request method, as described in RFC 1867 [15].
Product tokens are used to allow communicating applications to identify themselves via a simple product token, with an optional slash and version designator. Most fields using product tokens also allow sub-products which form a significant part of the application to be listed, separated by whitespace. By convention, the products are listed in order of their significance for identifying the application.
product = token ["/" product-version]
product-version = token
Examples:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
Server: Apache/0.8.4
Product tokens SHOULD be short and to the point --
use of them for advertising or other non-essential information
is explicitly forbidden. Although any token character may appear
in a product-version, this token
SHOULD only be used for a version identifier (i.e., successive
versions of the same product SHOULD only differ in the product-version
portion of the product value).
HTTP content negotiation (Section 12) uses short "floating point" numbers to indicate the relative importance ("weight") of various negotiable parameters. The weights are normalized to a real number in the range 0 through 1, where 0 is the minimum and 1 the maximum value. In order to discourage misuse of this feature, HTTP/1.1 applications MUST not generate more than three digits after the decimal point. User configuration of these values SHOULD also be limited in this fashion.
qvalue = ( "0" [ "." 0*3DIGIT ] )
| ( "." 0*3DIGIT )
| ( "1" [ "." 0*3("0") ] )
"Quality values" is a slight misnomer, since these values actually measure relative degradation in perceived quality. Thus, a value of "0.8" represents a 20% degradation from the optimum rather than a statement of 80% quality.
A language tag identifies a natural language spoken,
written, or otherwise conveyed by human beings for communication
of information to other human beings. Computer languages are explicitly
excluded. HTTP uses language tags within the Accept-Language,
and Content-Language fields.
The syntax and registry of HTTP language tags is the same as that defined by RFC 1766 [1]. In summary, a language tag is composed of 1 or more parts: A primary language tag and a possibly empty series of subtags:
language-tag = primary-tag *( "-" subtag )
primary-tag = 1*8ALPHA
subtag = 1*8ALPHA
Whitespace is not allowed within the tag and all tags are case-insensitive. The namespace of language tags is administered by the IANA. Example tags include:
en, en-US, en-cockney, i-cherokee, x-pig-latin
where any two-letter primary-tag is an ISO 639 language
abbreviation and any two-letter initial subtag is an ISO 3166
country code. The last three tags above are not registered tags,
but examples of tags which could be registered in future.
Contents moved to section 3.3.
Opaque validators are quoted strings whose internal structure is not visible to clients or caches.
opaque-validator = strong-opaque-validator | weak-opaque-validator
| null-validator
strong-opaque-validator = quoted-string
weak-opaque-validator = quoted-string "/W"
null-validator = <"> <">
Note that the "/W" tag is considered part of a weak opaque validator; it MUST NOT be removed by any cache or client.
There are two comparison functions on opaque validators:
The weak comparison function MAY be used for simple (non-subrange) GET requests. The strong comparison function MUST be used in all other cases.
The null validator is a special value, defined as never matching the current validator of an existing resource, and always matching the "current" validator of a resource that does not exist.
Variant-IDs are used to identify specific entities (variants) of a varying resource; see section 13.8.3 for how they are used.
variant-id = quoted-string
Variant-IDs are compared using string octet-equality; case is significant.
Validator sets are used for doing conditional retrievals on varying resources; see section 13.8.4.
validator-set = 1#validator-set-item
validator-set-item = opaque-validator
Validator sets are used for doing conditional retrievals on varying resources; see section 13.8.3.
variant-set = 1#variant-set-item
variant-set-item = opaque-validator ";" variant-id
This section defines certain HTTP protocol parameters used in range requests and related responses.
A resource may be broken down into subranges according to various structural units.
range-unit = bytes-unit
other-range-unit
bytes-unit = "bytes"
The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1 implementations may ignore ranges specified using other units. other-range-unit = token
Since all HTTP entities are represented in HTTP messages as sequences of bytes, the concept of a byte range is meaningful for any HTTP entity. (However, not all clients and servers need to support byte-range operations.)
Byte range specifications in HTTP apply to the sequence of bytes that would be transferred by the protocol if no transfer-encoding were being applied.
This means that if Content-encoding is applied to the data, the byte range specification applies to the resulting content-encoded byte stream, not to the unencoded byte stream. It also means that if the entity-body's media-type is a composite type (e.g., multipart/* and message/rfc822), then the composite's body-parts may have their own content-encoding and content-transfer-encoding, and the byte range applies to the result of the those encodings.
A byte range operation may specify a single range of bytes, or a set of ranges within a single entity.
ranges-specifier = byte-ranges-specifier
byte-ranges-specifier = bytes-unit "=" byte-range-set
byte-range-set = 1#( byte-range-spec | suffix-byte-range-spec )
byte-range-spec = first-byte-pos "-" [last-byte-pos]
first-byte-pos = 1*DIGIT
last-byte-pos = 1*DIGIT
The first-byte-pos value in a byte-range-spec gives the byte-offset of the first byte in a range. The last-byte-pos value gives the byte-offset of the last byte in the range; that is, the byte positions specified are inclusive. Byte offsets start at zero.
If the last-byte-pos value is present, it must be greater than or equal to the first-byte-pos in that byte-range-spec, or the byte-range-spec is invalid. The recipient of an invalid byte-range-spec must ignore it.
If the last-byte-pos value is absent, it is assumed to be equal to the current length of the entity in bytes.
If the last-byte-pos value is larger than the current length of the entity, it is assumed to be equal to the current length of the entity.
suffix-byte-range-spec = "-" suffix-length
suffix-length = 1*DIGIT
A suffix-byte-range-spec is used to specify the suffix of the entity, of a length given by the suffix-length value. (That is, this form specifies the last N bytes of an entity.) If the entity is shorter than the specified suffix-length, the entire entity is used.
Examples of byte-ranges-specifier values (assuming an entity of length 10000):
bytes=0-499
bytes=500-999
bytes=-500
bytes=9500-
bytes=0-0,-1
bytes=500-600,601-999
bytes=500-700,601-999
When a server returns a partial response to a client, it must describe both the extent of the range covered by the response, and the length of the entire entity.
content-range-spec = byte-content-range-spec
byte-content-range-spec = bytes-unit SP first-byte-pos "-"
last-byte-pos "/" entity-length
entity-length = 1*DIGIT
Unlike byte-ranges-specifier values, a byte-content-range-spec may only specify one range, and must contain absolute byte positions for both the first and last byte of the range.
A byte-content-range-spec whose last-byte-pos value, is less than its first-byte-pos value, or whose entity-length value is less than its last-byte-pos value, is invalid. The recipient of an invalid byte-content-range-spec must ignore it and any content transferred along with it.
Examples of byte-content-range-spec values, assuming that the entity contains a total of 1234 bytes:
bytes 0-499/1234
bytes 500-999/1234
bytes 500-1233/1234
bytes 734-1233/1234
HTTP messages consist of requests from client to server and responses from server to client.
HTTP-message = Full-Request ; HTTP/1.1 messages
| Full-Response
| NULL-Request
A NULL-Request (an empty line where a request would normally be expected) MUST be ignored. Clients SHOULD NOT send a NULL-Request, but there are some error and testing circumstances in which a NULL-Request might be sent by mistake and MUST NOT cause failure on the server.
NULL-Request = CRLF
Full-Request and Full-Response
use the generic message format of RFC 822 [9]
for transferring entities. Both messages may include optional
header fields (also known as "headers") and an entity
body. The entity body is separated from the headers by a null
line (i.e., a line with nothing preceding the CRLF).
Full-Request = Request-Line ; Section 5.1
*( General-Header ; Section 4.3
| Request-Header ; Section 5.2
| Entity-Header ) ; Section 7.1
CRLF
[ Entity-Body ] ; Section 7.2
Full-Response = Status-Line ; Section 6.1
*( General-Header ; Section 4.3
| Response-Header ; Section 6.2
| Entity-Header ) ; Section 7.1
CRLF
[ Entity-Body ] ; Section 7.2
HTTP header fields, which include General-Header
(Section 4.3), Request-Header
(Section 5.2), Response-Header
(Section 6.2), and Entity-Header
(Section 7.1) fields, follow the
same generic format as that given in Section 3.1 of RFC 822 [9].
Each header field consists of a name followed by a colon (":")
and the field value. Field names are case-insensitive. The field
value may be preceded by any amount of LWS,
though a single SP is preferred.
Header fields can be extended over multiple lines by preceding
each extra line with at least one SP
or HT.
HTTP-header = field-name ":" [ field-value ] CRLF
field-name = token
field-value = *( field-content | LWS )
field-content = <the OCTETs making up the field-value
and consisting of either *TEXT or combinations
of token, tspecials, and quoted-string>
The order in which header fields with differing field
names are received is not significant. However, it is "good
practice" to send General-Header
fields first, followed by Request-Header
or Response-Header fields, and
ending with the Entity-Header fields.
Multiple HTTP-header
fields with the same field-name
may be present in a message if and only if the entire field-value
for that header field is defined as a comma-separated list [i.e.,
#(values)]. It MUST be possible
to combine the multiple header fields into one "field-name:
field-value" pair, without changing the semantics of the
message, by appending each subsequent field-value to the first,
each separated by a comma. Thus, the order in which multiple
header fields with the same field-name are received may be significant
to the interpretation of the combined field-value.
There are a few header fields which have general applicability for both request and response messages, but which do not apply to the entity being transferred. These headers apply only to the message being transmitted.
General-Header = Cache-Control ; Section 10.8
| Connection ; Section 10.9
| Date ; Section 10.17
| Via ; Section 10.20
| Keep-Alive ; Section 10.24
| Pragma ; Section 10.29
| Upgrade ; Section 10.41
General header field names can be extended reliably
only in combination with a change in the protocol version. However,
new or experimental header fields may be given the semantics of
general header fields if all parties in the communication recognize
them to be general header fields. Unrecognized header fields are
treated as Entity-Header fields.
A request message from a client to a server includes, within the first line of that message, the method to be applied to the resource, the identifier of the resource, and the protocol version in use. For backwards compatibility with the more limited HTTP/0.9 protocol, there are two valid formats for an HTTP request:
Request = Full-Request | NULL-Request
Full-Request = Request-Line ; Section 5.1
*( General-Header ; Section 4.3
| Request-Header ; Section 5.2
| Entity-Header ) ; Section 7.1
CRLF
[ Entity-Body ] ; Section 7.2
NULL-Request = CRLF
A NULL-Request MUST be ignored.
The Request-Line
begins with a method token, followed by the Request-URI
and the protocol version, and ending with CRLF.
The elements are separated by SP
characters. No CR or LF
are allowed except in the final CRLF
sequence.
Request-Line = Method SP Request-URI SP HTTP-Version CRLF
The Method token indicates the method to be performed
on the resource identified by the Request-URI. The
method is case-sensitive.
Method = "OPTIONS" ; Section 8.1
| "GET" ; Section 8.2
| "HEAD" ; Section 8.3
| "POST" ; Section 8.4
| "PUT" ; Section 8.5
| "DELETE" ; | "TRACE" ; Section 8.12
| extension-method
extension-method = token
The list of methods acceptable by a specific resource can be specified
in an Allow header field (Section 10.5).
However, the client is always notified through the return code
of the response whether a method is currently allowed on a specific
resource, as this can change dynamically. Servers SHOULD return
the status code 405 (method not allowed) if the method is known
by the server but not allowed for the requested resource, and
501 (not implemented) if the method is unrecognized or not implemented
by the server. The list of methods known by a server can be listed
in a Public response header field (Section 10.32).
The methods GET and HEAD MUST be supported by all general-purpose
servers. Servers which provide Last-Modified dates
for resources MUST also support the conditional GET method. All
other methods are optional; however, if the above methods are
implemented, they MUST be implemented with the same semantics
as those specified in Section 8.
The Request-URI is a Uniform Resource Identifier
(Section 3.2) and identifies the resource upon
which to apply the request.
Request-URI = "*" | absoluteURI | abs_path
To allow for transition to absoluteURIs in all requests
in future versions of HTTP, HTTP/1.1 servers MUST accept the absoluteURI
form in requests, even though HTTP/1.1 clients will not normally
generate them. Versions of HTTP after HTTP/1.1 may require absoluteURIs
everywhere, after HTTP/1.1 or later have become the dominant implementations.
The three options for Request-URI are dependent on
the nature of the request. The asterisk "*" means that
the request does not apply to a particular resource, but to the
server itself, and is only allowed when the Method used does not
necessarily apply to a resource. One example would be
OPTIONS * HTTP/1.1
The absoluteURI form is only allowed to an origin
server if the client knows the server supports HTTP/1.1 or later.
If the absoluteURI form is used, any Host
request-header included with the request MUST be ignored. The
absoluteURI form is required when the request is
being made to a proxy. The proxy is requested to forward the request
and return the response. If the request is GET or
HEAD and a prior response is cached, the proxy may
use the cached message if it passes any restrictions in the Cache-Control
and Expires header fields. Note that the proxy MAY
forward the request on to another proxy or directly to the server
specified by the absoluteURI. In order to avoid request
loops, a proxy MUST be able to recognize all of its server names,
including any aliases, local variations, and the numeric IP address.
An example Request-Line would be:
GET /TheProject.html HTTP/1.1
The most common form of Request-URI is that used
to identify a resource on an origin server or gateway. In this
case, only the absolute path of the URI is transmitted (see Section 3.2.1,
abs_path). For example, a client wishing to retrieve
the resource above directly from the origin server would create
a TCP connection to port 80 of the host "www.w3.org"
and send the lines:
GET /pub/WWW/TheProject.html HTTP/1.1
Host:www.w3.org
followed by the remainder of the Full-Request. Note
that the absolute path cannot be empty; if none is present in
the original URI, it MUST be given as "/" (the server
root).
If a proxy receives a request without any path in the Request-URI
and the method used is capable of supporting the asterisk form
of request, then the last proxy on the request chain MUST forward
the request with "*" as the final Request-URI.
For example, the request
OPTIONS http://www.ics.uci.edu:8001 HTTP/1.1
would be forwarded by the proxy as
OPTIONS * HTTP/1.1
after connecting to port 8001 of host "www.ics.uci.edu".
The Request-URI is transmitted as an encoded string,
where some characters may be escaped using the "% HEX HEX"
encoding defined by RFC 1738 [4]. The origin
server MUST decode the Request-URI in order to properly
interpret the request. In requests that they forward, proxies
MUST NOT rewrite the "abs_path" part of
a Request-URI in any way except as noted above to
replace a null abs_path with "*". Illegal
Request-URIs SHOULD be responded to with an appropriate
status code. (Proxies MAY transform the Request-URI
for internal processing purposes, but SHOULD NOT send such a transformed
Request-URI in forwarded requests. Transformations
for use in cache updates and lookups are subject to additional
requirements; see section 13 on caching. The main reason for
this rule is to make sure that the form of Request-URIs
is well specified, to enable future extensions without fear that
they will break in the face of some rewritings. Another is that
one consequence of rewriting the Request-URI is that
integrity or authentication checks by the server may fail; since
rewriting MUST be avoided in this case, it may as well be proscribed
in general.
Note: servers writers SHOULD be aware that some existing proxies do some rewriting.
The request header fields allow the client to pass additional information about the request, and about the client itself, to the server. These fields act as request modifiers, with semantics equivalent to the parameters on a programming language method (procedure) invocation.
Request-Header = Accept ; Section 10.1
| Accept-Charset ; Section 10.2
| Accept-Encoding ; Section 10.3
| Accept-Language ; Section 10.4
| Authorization ; Section 10.6
| From ; Section 10.21
| Host ; Section 10.22
| If-Modified-Since ; Section 10.23
| Proxy-Authorization ; Section 10.31
| Range ; Section 10.33
| Referer ; Section 10.34
| User-Agent ; Section 10.43
| Max-Forwards ; Section 10.45
Request-Header field names can be extended
reliably only in combination with a change in the protocol version.
However, new or experimental header fields MAY be given the semantics
of request header fields if all parties in the communication recognize
them to be request header fields. Unrecognized header fields are
treated as Entity-Header fields.
After receiving and interpreting a request message, a server responds in the form of an HTTP response message.
Response = Full-Response
Full-Response = Status-Line ; Section 6.1
*( General-Header ; Section 4.3
| Response-Header ; Section 6.2
| Entity-Header ) ; Section 7.1
CRLF
[ Entity-Body ] ; Section 7.2
The first line of a Full-Response
message is the Status-Line, consisting
of the protocol version followed by a numeric status code and
its associated textual phrase, with each element separated by
SP characters. No CR
or LF is allowed except in the
final CRLF sequence.
Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
The Status-Code element is a 3-digit integer result
code of the attempt to understand and satisfy the request. The
Reason-Phrase is intended to give a short textual
description of the Status-Code. The Status-Code
is intended for use by automata and the Reason-Phrase
is intended for the human user. The client is not required to
examine or display the Reason-Phrase.
The first digit of the Status-Code defines the class
of response. The last two digits do not have any categorization
role. There are 5 values for the first digit:
The individual values of the numeric status codes defined for
HTTP/1.1, and an example set of corresponding Reason-Phrase's,
are presented below. The reason phrases listed here are only recommended
-- they may be replaced by local equivalents without affecting
the protocol. These codes are fully defined in Section 9.
Status-Code = "100" ; Continue
| "101" ; Switching Protocols
| "200" ; OK
| "201" ; Created
| "202" ; Accepted
| "203" ; Non-Authoritative Information
| "204" ; No Content
| "205" ; Reset Content
| "206" ; Partial Content
| "300" ; Multiple Choices
| "301" ; Moved Permanently
| "302" ; Moved Temporarily
| "303" ; See Other
| "304" ; Not Modified
| "305" ; Use Proxy
| "400" ; Bad Request
| "401" ; Unauthorized
| "402" ; Payment Required
| "403" ; Forbidden
| "404" ; Not Found
| "405" ; Method Not Allowed
| "406" ; Not Acceptable
| "407" ; Proxy Authentication Required
| "408" ; Request Time-out
| "409" ; Conflict
| "410" ; Gone
| "411" ; Length Required
| "412" ; Precondition Failed
| "413" ; Request Entity Too Large
| "414" ; Request URI Too Large
| "415" ; Unsupported Media Type
| "416" ; None Acceptable
| "500" ; Internal Server Error
| "501" ; Not Implemented
| "502" ; Bad Gateway
| "503" ; Service Unavailable
| "504" ; Gateway Time-out
| "505" ; HTTP Version not supported
| extension-code
extension-code = 3DIGIT
Reason-Phrase = *<TEXT, excluding CR, LF>
HTTP status codes are extensible. HTTP applications are not required to understand the meaning of all registered status codes, though such understanding is obviously desirable. However, applications MUST understand the class of any status code, as indicated by the first digit, and treat any unrecognized response as being equivalent to the x00 status code of that class, with the exception that an unrecognized response MUST not be cached. For example, if an unrecognized status code of 431 is received by the client, it can safely assume that there was something wrong with its request and treat the response as if it had received a 400 status code. In such cases, user agents SHOULD present to the user the entity returned with the response, since that entity is likely to include human-readable information which will explain the unusual status.
The response header fields allow the server to pass
additional information about the response which cannot be placed
in the Status-Line. These header
fields give information about the server and about further access
to the resource identified by the Request-URI.
Response-Header = Location ; Section 10.27
| Proxy-Authenticate ; Section 10.30
| Public ; Section 10.32
| Retry-After ; Section 10.36
| Server ; Section 10.37
| WWW-Authenticate ; Section 10.44
Response-Header field names can be extended
reliably only in combination with a change in the protocol version.
However, new or experimental header fields MAY be given the semantics
of response header fields if all parties in the communication
recognize them to be response header fields. Unrecognized header
fields are treated as Entity-Header
fields.
Full-Request and Full-Response
messages MAY transfer an entity within some requests and responses.
An entity consists of Entity-Header
fields and (usually) an Entity-Body.
In this section, both sender and
recipient refer to either the client
or the server, depending on who sends and who receives the entity.
Entity-Header fields define optional
metainformation about the Entity-Body
or, if no body is present, about the resource identified by the
request.
Entity-Header = Allow ; Section 10.5
| Content-Base ; Section 10.9
| Content-Encoding ; Section 10.10
| Content-Language ; Section 10.11
| Content-Length ; Section 10.12
| Content-Location ; Section 10.16
| Content-MD5 ; Section 10.13
| Content-Range ; Section 10.14
| Content-Type ; Section 10.15
| Expires ; Section 10.19
| Last-Modified ; Section 10.25
| Title ; Section 10.38
| Transfer-Encoding ; Section 10.39
| extension-header
extension-header = HTTP-header
The extension-header
mechanism allows additional Entity-Header
fields to be defined without changing the protocol, but these
fields cannot be assumed to be recognizable by the recipient.
Unrecognized header fields SHOULD be ignored by the recipient
and forwarded by proxies.
The entity body (if any) sent with an HTTP request
or response is in a format and encoding defined by the Entity-Header
fields.
Entity-Body = *OCTET
An entity body is included with a request message
only when the request method calls for one. The presence of an
entity body in a request is signaled by the inclusion of a Content-Length
and/or Content-Type header field
in the request message headers.
For response messages, whether or not an entity body
is included with a message is dependent on both the request method
and the response code. All responses to the HEAD request method
MUST not include a body, even though the presence of entity header
fields may lead one to believe they do. All 1xx (informational),
204 (no content), and 304 (not modified) responses MUST not include
a body. All other responses MUST include an entity body or a Content-Length
header field defined with a value of zero (0).
When an entity body is included with a message, the data type
of that body is determined via the header fields Content-Type,
Content-Encoding, and Transfer-Encoding.
These define a three-layer, ordered encoding model:
entity-body :=
Transfer-Encoding( Content-Encoding( Content-Type( data ) ) )
The default for both encodings is none (i.e., the identity function).
Content-Type specifies the media type of the underlying
data. Content-Encoding may be used to indicate any
additional content codings applied to the type, usually for the
purpose of data compression, that are a property of the resource
requested. Transfer-Encoding may be used to indicate
any additional transfer codings applied by an application to ensure
safe and proper transfer of the message. Note that Transfer-Encoding
is a property of the message, not of the resource.
Any HTTP/1.1 message containing an entity body SHOULD include
a Content-Type header field defining the media type
of that body. If and only if the media type is
not given by a Content-Type header, the recipient
may attempt to guess the media type via inspection of its content
and/or the name extension(s) of the URL used to identify the resource.
If the media type remains unknown, the recipient SHOULD treat
it as type "application/octet-stream".
When an entity body is included with a message, the length of
that body may be determined in one of several ways. If a Content-Length
header field is present, its value in bytes represents the length
of the entity body. Otherwise, the body length is determined by
the Transfer-Encoding (if the "chunked"
transfer coding has been applied) or by the server closing the
connection.
Note: Any response message which MUST NOT include an entity body (such as the 1xx, 204, and 304 responses and any response to a HEAD request) is always terminated by the first empty line after the header fields, regardless of the entity header fields present in the message.
Closing the connection cannot be used to indicate the end of a
request body, since it leaves no possibility for the server to
send back a response. For compatibility with HTTP/1.0 applications,
HTTP/1.1 requests containing an entity body MUST include a valid
Content-Length header field unless the server is
known to be HTTP/1.1 compliant. HTTP/1.1 servers MUST accept the
"chunked" transfer coding (Section 3.6),
thus allowing this mechanism to be used for a request when Content-Length
is unknown.
If a request contains an entity body and Content-Length
is not specified, the server SHOULD respond with 400 (bad request)
if it cannot determine the length of the request message's content,
or with 411 (length required) if it wishes to insist on receiving
a valid Content-Length.
Messages MUST NOT include both a Content-Length header
field and the "chunked" transfer coding. If both are
received, the Content-Length MUST be ignored.
When a Content-Length is given in a message where
an entity body is allowed, its field value MUST exactly match
the number of OCTETs in the entity body. HTTP/1.1
user agents MUST notify the user when an invalid length is received
and detected.
The set of common methods for HTTP/1.1 is defined below. Although this set can be expanded, additional methods cannot be assumed to share the same semantics for separately extended clients and servers.
The Host request-header
field (Section 10.22) MUST accompany all
HTTP/1.1 requests.
The OPTIONS method
represents a request for information about the communication options
available on the request/response chain identified by the Request-URI.
This method allows the client to determine the options and/or
requirements associated with a resource, or the capabilities of
a server, without implying a resource action or initiating a resource
retrieval.
Unless the server's response is an error, the response
MUST NOT include entity information other than what can be considered
as communication options (e.g., Allow
is appropriate, but Content-Type
is not) and MUST include a Content-Length
with a value of zero (0). Responses to this method are not cachable.
If the Request-URI
is an asterisk ("*"), the OPTIONS
request is intended to apply to the server as a whole. A 200 response
SHOULD include any header fields which indicate optional features
implemented by the server (e.g., Public),
including any extensions not defined by this specification, in
addition to any applicable general or response header fields.
As described in Section 5.1.2, an "OPTIONS
*" request can be applied through a proxy
by specifying the destination server in the Request-URI
without any path information.
If the Request-URI
is not an asterisk, the OPTIONS
request applies only to the options that are available when communicating
with that resource. A 200 response SHOULD include any header fields
which indicate optional features implemented by the server and
applicable to that resource (e.g., Allow),
including any extensions not defined by this specification, in
addition to any applicable general or response header fields.
If the OPTIONS request passes
through a proxy, the proxy MUST edit the response to exclude those
options known to be unavailable through that proxy.
The GET method means
retrieve whatever information (in the form of an entity) is identified
by the Request-URI. If the Request-URI
refers to a data-producing process, it is the produced data which
shall be returned as the entity in the response and not the source
text of the process, unless that text happens to be the output
of the process.
The semantics of the GET
method change to a "conditional GET"
if the request message includes an If-Modified-Since
header field. A conditional GET
method requests that the identified resource be transferred only
if it has been modified since the date given by the If-Modified-Since
header, as described in Section 10.23.
The conditional GET method is
intended to reduce unnecessary network usage by allowing cached
entities to be refreshed without requiring multiple requests or
transferring data already held by the client.
The semantics of the GET
method change to a "partial GET"
if the request message includes a Range
header field. A partial GET requests
that only part of the identified resource be transferred, as described
in Section 10.33. The partial GET
method is intended to reduce unnecessary network usage by allowing
partially-retrieved entities to be completed without transferring
data already held by the client.
The response to a GET
request may be cachable if and only if it meets the requirements
for HTTP caching described in Section 13.
The HEAD method
is identical to GET except that
the server MUST not return any Entity-Body
in the response. The metainformation contained in the HTTP headers
in response to a HEAD request
SHOULD be identical to the information sent in response to a GET
request. This method can be used for obtaining metainformation
about the resource identified by the Request-URI
without transferring the Entity-Body
itself. This method is often used for testing hypertext links
for validity, accessibility, and recent modification.
The response to a HEAD
request may be cachable in the sense that the information contained
in the response may be used to update a previously cached entity
from that resource. If the new field values indicate that the
cached entity differs from the current resource (as would be indicated
by a change in Content-Length,
Content-MD5, or Content-Version),
then the cache MUST discard the cached entity.
There is no "conditional HEAD"
or "partial HEAD" request
analogous to those associated with the GET
method. If an If-Modified-Since
and/or Range header field is
included with a HEAD request,
they SHOULD be ignored.
The POST method
is used to request that the destination server accept the entity
enclosed in the request as a new subordinate of the resource identified
by the Request-URI in the Request-Line.
POST is designed to allow a uniform method to cover the following
functions:
The actual function performed by the POST method
is determined by the server and is usually dependent on the Request-URI.
The posted entity is subordinate to that URI in the same way that
a file is subordinate to a directory containing it, a news article
is subordinate to a newsgroup to which it is posted, or a record
is subordinate to a database.
For compatibility with HTTP/1.0 applications, all
POST requests MUST include a
valid Content-Length header field
unless the server is known to be HTTP/1.1 compliant. When sending
a POST request to an HTTP/1.1
server, a client MUST use a valid Content-Length
or the "chunked" Transfer-Encoding.
The server SHOULD respond with a 400 (bad request) message if
it cannot determine the length of the request message's content,
or with 411 (length required) if it wishes to insist on receiving
a valid Content-Length.
A successful POST
does not require that the entity be created as a resource on the
origin server or made accessible for future reference. That is,
the action performed by the POST
method might not result in a resource that can be identified by
a URI. In this case, either 200 (ok) or 204 (no content) is the
appropriate response status, depending on whether or not the response
includes an entity that describes the result.
If a resource has been created on the origin server, the response SHOULD be 201 (created) and contain an entity (preferably of type "text/html") which describes the status of the request and refers to the new resource.
Responses to this method are not cachable. However, the 303 (see other) response can be used to direct the user agent to retrieve a cachable resource.
POST requests must obey the entity transmission requirements set out in section 8.4.1.
The following rules apply to any method that is subject to the two-phase mechanism.
Upon receiving such a method from an HTTP/1.1 (or later) client, an HTTP/1.1 (or later) server immediately either respond with "100 Continue" and continue to read from the input stream, or respond with an error status. If it responds with an error status, it MAY close the transport (TCP) connection or it MAY continue to read and discard the rest of the request. It MUST not perform the requested action if returns an error status.
HTTP/1.1 servers are encouraged to maintain persistent connections and use TCP's flow control mechanisms to resolve temporary overloads, rather than terminating connections with the expectation that clients will retry. The latter technique can exacerbate network congestion.
An HTTP/1.1 (or later) client doing a PUT-like method SHOULD monitor
the network connection for an error status while it is transmitting
the body of the request including any encoding mechanism used
to transmit the body. If the client sees an error status, it
SHOULD immediately cease transmitting the body. If the body
was proceeded by a Content-length header, the client
MUST either close the connection or if the body is being sent
using a Chunked encoding, use a 0 length chunk, to mark the end
of the message.
An HTTP/1.1 (or later) client MUST be prepared to accept a 100 Continue status followed by a regular response.
An HTTP/1.1 (or later) client that sees the connection close before receiving any status from the server SHOULD retry the request, but if it does so, it MUST use the two-phase mechanism. In the two-phase mechanism, the client first sends the request headers, then waits for the server to respond with either a 100 Continue, in which case the client SHOULD continue, or an error status, in which case the client MUST NOT continue and MUST close the connection if it has not already completed sending the full request body including any encoding mechanism used to transmit the body.
If the client knows that the server is an HTTP/1.1 (or later) server, because of the server protocol version returned with a previous request on the same persistent connection [alternatively: within the past <N> hours], it MUST wait for a response. If the client believes that the server is a 1.0 or earlier server, it SHOULD continue transmitting its request after waiting at least [5] seconds for a status response.
An HTTP/1.1 (or later) client that sees the connection close after receiving a "100 Continue" but before receiving any other status SHOULD retry the request, and need not use the two-phase method (but MAY do so if this simplifies the implementation).
An HTTP/1.1 (or later) server that receives a request from a 1.0 (or earlier) client MUST NOT transmit the "100 Continue" response; it SHOULD either wait for the request to be completed normally (thus avoiding an interrupted request) or close the connection prematurely.
The PUT method requests that the enclosed entity
be stored under the supplied Request-URI.
If the Request-URI refers to
an already existing resource, the enclosed entity SHOULD be considered
as a modified version of the one residing on the origin server.
If the Request-URI does not point
to an existing resource, and that URI is capable of being defined
as a new resource by the requesting user agent, the origin server
can create the resource with that URI. If a new resource is created,
the origin server MUST inform the user agent via the 201 (created)
response. If an existing resource is modified, either the 200
(ok) or 204 (no content) response codes SHOULD be sent to indicate
successful completion of the request. If the resource could not
be created or modified with the Request-URI,
an appropriate error response SHOULD be given that reflects the
nature of the problem.
If the request passes through a cache and the Request-URI
identifies a currently cached entity, that entity MUST be removed
from the cache. Responses to this method are not cachable.
The fundamental difference between the POST
and PUT requests is reflected
in the different meaning of the Request-URI.
The URI in a POST request identifies
the resource that will handle the enclosed entity as an appendage.
That resource may be a data-accepting process, a gateway to some
other protocol, or a separate entity that accepts annotations.
In contrast, the URI in a PUT
request identifies the entity enclosed with the request -- the
user agent knows what URI is intended and the server MUST NOT
attempt to apply the request to some other resource. If the server
desires that the request be applied to a different URI, it MUST
send a 301 (moved permanently) response; the user agent MAY then
make its own decision regarding whether or not to redirect the
request.
A single resource MAY be identified by many different
URIs. For example, an article may have a URI for identifying "the
current version" which is separate from the URI identifying
each particular version. In this case, a PUT
request on a general URI may result in several other URIs being
defined by the origin server.
For compatibility with HTTP/1.0 applications, all
PUT requests MUST include a valid
Content-Length header field unless
the server is known to be HTTP/1.1 compliant. When sending a PUT
request to an HTTP/1.1 server, a client MUST use a valid Content-Length
or the "chunked" Transfer-Encoding.
The server SHOULD respond with a 400 (bad request) message if
it cannot determine the length of the request message's content,
or with 411 (length required) if it wishes to insist on receiving
a valid Content-Length.
The actual method for determining how the resource
is placed, and what happens to its predecessor, is defined entirely
by the origin server. If the entity being PUT
was derived from an existing resource which included a Content-Version
header field, the new entity MUST include a Derived-From
header field corresponding to the value of the original Content-Version
header field. Multiple Derived-From
values may be included if the entity was derived from multiple
resources with Content-Version
information. Applications are encouraged to use these fields for
constructing versioning relationships and resolving version conflicts.
PUT requests must obey the entity transmission requirements set out in section 8.4.1.
The DELETE method requests that the origin server
delete the resource identified by the Request-URI.
This method MAY be overridden by human intervention (or other
means) on the origin server. The client cannot be guaranteed that
the operation has been carried out, even if the status code returned
from the origin server indicates that the action has been completed
successfully. However, the server SHOULD not indicate success
unless, at the time the response is given, it intends to delete
the resource or move it to an inaccessible location.
A successful response SHOULD be 200 (OK) if the response includes an entity describing the status, 202 (accepted) if the action has not yet been enacted, or 204 (no content) if the response is OK but does not include an entity.
If the request passes through a cache and the Request-URI
identifies a currently cached entity, that entity MUST be removed
from the cache. Responses to this method are not cachable.
The TRACE method is used to invoke a remote, application-layer
loop back of the request message. The final recipient of the
request SHOULD reflect the message received back to the client
as the entity body of a 200 (OK) response. The final recipient
is either the origin server or the first proxy or gateway to receive
a Max-Forwards value of zero
(0) in the request (see Section 10.45).
A TRACE request MUST NOT include an entity body and MUST include
a Content-Length header field
with a value of zero (0).
TRACE allows the client to see what is being received
at the other end of the request chain and use that data for testing
or diagnostic information. The value of the Via
header field (Section 10.20) is of particular
interest, since it acts as a trace of the request chain. Use
of the Max-Forwards header field
allows the client to limit the length of the request chain, which
is useful for testing a chain of proxies forwarding messages in
an infinite loop.
If successful, the response SHOULD contain the entire
request message in the entity body, with a Content-Type
of "message/http", "application/http", or
"text/plain". Responses to this method MUST NOT be
cached.
Each Status-Code
is described below, including a description of which method(s)
it can follow and any metainformation required in the response.
This class of status code indicates a provisional
response, consisting only of the Status-Line
and optional headers, and is terminated by an empty line. Since
HTTP/1.0 did not define any 1xx status codes, servers MUST NOT
send a 1xx response to an HTTP/1.0 client except under experimental
conditions.
The client may continue with its request. This interim response is used to inform the client that the initial part of the request has been received and has not yet been rejected by the server. The client SHOULD continue by sending the remainder of the request or, if the request has already been completed, ignore this response. The server MUST send a final response after the request has been completed.
The server understands and is willing to comply with the client's
request, via the Upgrade message header field (Section 10.41),
for a change in the application protocol being used on this connection.
The server will switch protocols to those defined by the response's
Upgrade header field immediately after the empty
line which terminates the 101 response.
The protocol should only be switched when it is advantageous to do so. For example, switching to a newer version of HTTP is advantageous over older versions, and switching to a real-time, synchronous protocol may be advantageous when delivering resources that use such features.
This class of status code indicates that the client's request was successfully received, understood, and accepted.
The request has succeeded. The information returned with the response is dependent on the method used in the request, as follows:
GET
HEAD
Entity-Body;
POST
TRACE
If the entity corresponds to a resource, the response MAY include
a Content-Location header field giving the actual
location of that specific resource for later reference.
The request has been fulfilled and resulted in a new resource
being created. The newly created resource can be referenced by
the URI(s) returned in the entity of the response, with the most
specific URL for the resource given by a Location
header field. The origin server SHOULD create the resource before
using this Status-Code. If the action cannot be carried
out immediately, the server MUST include in the response body
a description of when the resource will be available; otherwise,
the server SHOULD respond with 202 (accepted).
The request has been accepted for processing, but the processing has not been completed. The request MAY or MAY NOT eventually be acted upon, as it MAY be disallowed when processing actually takes place. There is no facility for re-sending a status code from an asynchronous operation such as this.
The 202 response is intentionally non-committal. Its purpose is to allow a server to accept a request for some other process (perhaps a batch-oriented process that is only run once per day) without requiring that the user agent's connection to the server persist until the process is completed. The entity returned with this response SHOULD include an indication of the request's current status and either a pointer to a status monitor or some estimate of when the user can expect the request to be fulfilled.
The returned metainformation in the Entity-Header is not the definitive set as available from the origin server, but is gathered from a local or a third-party copy. The set presented MAY be a subset or superset of the original version. For example, including local annotation information about the resource MAY result in a superset of the metainformation known by the origin server. Use of this response code is not required and is only appropriate when the response would otherwise be 200 (OK).
The server has fulfilled the request but there is no new information to send back. If the client is a user agent, it SHOULD not change its document view from that which caused the request to be generated. This response is primarily intended to allow input for actions to take place without causing a change to the user agent's active document view. The response MAY include new metainformation in the form of entity headers, which SHOULD apply to the document currently in the user agent's active view.
The 204 response MUST not include an entity body, and thus is always terminated by the first empty line after the header fields.
The server has fulfilled the request and the user agent SHOULD
reset the document view which caused the request to be generated.
This response is primarily intended to allow input for actions
to take place via user input, followed by a clearing of the form
in which the input is given so that the user can easily initiate
another input action. The response MUST include a Content-Length
with a value of zero (0) and no entity body.
The server has fulfilled the partial GET request
for the resource. The request MUST have included a Range
header field (Section 10.33) indicating the
desired range. The response MUST include a Content-Range
header field (Section 10.14) indicating
the range included with this response. All entity header fields
in the response MUST describe the partial entity transmitted rather
than what would have been transmitted in a full response. In particular,
the Content-Length header field in the response MUST
match the actual number of OCTETs transmitted in
the entity body. It is assumed that the client already has the
complete entity's header field data.
The server has determined that the requested range(s) are not present in the requested resource, and so there is no content to return. This status code should be handled by the client the same as 204 No Content.
This could be a compatibility problem if there is an installed base. If treating this status code as the generic 2xx code by such implementations would lead to an error, it will have to be replace by 204.
This class of status code indicates that further
action needs to be taken by the user agent in order to fulfill
the request. The action required MAY be carried out by the user
agent without interaction with the user if and only if the method
used in the second request is GET
or HEAD. A user agent SHOULD
NOT automatically redirect a request more than 5 times, since
such redirections usually indicate an infinite loop.
This status code is reserved for future use by a planned content
negotiation mechanism. HTTP/1.1 user agents receiving a 300 response
which includes a Location header field can treat
this response as they would treat a 303 (See Other) response.
If no Location header field is included, the appropriate
action is to display the entity enclosed in the response to the
user.
The requested resource has been assigned a new permanent URI and
any future references to this resource SHOULD be done using one
of the returned URIs. Clients with link editing capabilities SHOULD
automatically re-link references to the Request-URI
to one or more of the new references returned by the server, where
possible. This response is cachable unless indicated otherwise.
If the new URI is a location, its URL MUST be given by the Location
field in the response. Unless it was a HEAD request,
the Entity-Body of the response SHOULD contain a
short hypertext note with a hyperlink to the new URI(s).
If the 301 status code is received in response to a request other
than GET or HEAD, the user agent MUST
NOT automatically redirect the request unless it can be confirmed
by the user, since this might change the conditions under which
the request was issued.
Note: When automatically redirecting a POST request after receiving a 301 status code, some existing HTTP/1.0 user agents will erroneously change it into a GET request.
The requested resource resides temporarily under a different URI.
Since the redirection MAY be altered on occasion, the client SHOULD
continue to use the Request-URI for future requests.
This response is only cachable if indicated by a Cache-Control
or Expires header field.
If the new URI is a location, its URL MUST be given by the Location
field in the response. Unless it was a HEAD request,
the Entity-Body of the response SHOULD contain a
short hypertext note with a hyperlink to the new URI(s).
If the 302 status code is received in response to a request other
than GET or HEAD, the user agent MUST
NOT automatically redirect the request unless it can be confirmed
by the user, since this might change the conditions under which
the request was issued.
The response to the request can be found under a different URI
and SHOULD be retrieved using a GET method on that resource. This
method exists primarily to allow the output of a POST-activated
script to redirect the user agent to a selected resource. The
new resource is not a update reference for the original Request-URI.
The 303 response is not cachable, but the response to the second
request MAY be cachable.
If the new URI is a location, its URL MUST be given by the Location
field in the response. Unless it was a HEAD request, the Entity-Body
of the response SHOULD contain a short hypertext note with a hyperlink
to the new URI(s).
Note: When automatically redirecting a POST request after receiving a 302 status code, some existing HTTP/1.0 user agents will erroneously change it into a GET request.
If the client has performed a conditional GET request and access
is allowed, but the document has not been modified since the date
and time specified in the If-Modified-Since field,
the server MUST respond with this status code and not send an
Entity-Body to the client. Header fields contained
in the response SHOULD only include information which is relevant
to cache managers or which MAY have changed independently of the
entity's Last-Modified date. Examples of relevant
header fields include: Date, Server,
Content-Length, Content-MD5, Content-Version,
Cache-Control and Expires.
A cache SHOULD update its cached entity to reflect any new field
values given in the 304 response. If the new field values indicate
that the cached entity differs from the current resource (as would
be indicated by a change in Content-Length, Content-MD5,
or Content-Version), then the cache MUST disregard
the 304 response and repeat the request without an If-Modified-Since
field.
The 304 response MUST NOT include an entity body, and thus is always terminated by the first empty line after the header fields.
The requested resource MUST be accessed through the proxy given
by the Location field in the response. In other words,
this is a proxy redirect.
The 4xx class of status code is intended for cases in which the client seems to have erred. If the client has not completed the request when a 4xx code is received, it SHOULD immediately cease sending data to the server. Except when responding to a HEAD request, the server SHOULD include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method.
Note: If the client is sending data, server implementations on TCP SHOULD be careful to ensure that the client acknowledges receipt of the packet(s) containing the response prior to closing the input connection. If the client continues sending data to the server after the close, the server's controller will send a reset packet to the client, which may erase the client's unacknowledged input buffers before they can be read and interpreted by the HTTP application.
The request could not be understood by the server due to malformed syntax. The client SHOULD not repeat the request without modifications.
The request requires user authentication. The response MUST include
a WWW-Authenticate header field (Section 10.44)
containing a challenge applicable to the requested
resource. The client MAY repeat the request with a suitable Authorization
header field (Section 10.6). If the
request already included Authorization credentials, then the 401
response indicates that authorization has been refused for those
credentials. If the 401 response contains the same challenge as
the prior response, and the user agent has already attempted authentication
at least once, then the user SHOULD be presented the entity that
was given in the response, since that entity MAY include relevant
diagnostic information. HTTP access authentication is explained
in Section 11.
This code is reserved for future use.
The server understood the request, but is refusing to fulfill
it. Authorization will not help and the request SHOULD not be
repeated. If the request method was not HEAD and
the server wishes to make public why the request has not been
fulfilled, it SHOULD describe the reason for the refusal in the
entity body. This status code is commonly used when the server
does not wish to reveal exactly why the request has been refused,
or when no other response is applicable.
The server has not found anything matching the Request-URI.
No indication is given of whether the condition is temporary or
permanent. If the server does not wish to make this information
available to the client, the status code 403 (forbidden) can be
used instead. The 410 (gone) status code SHOULD be used if the
server knows, through some internally configurable mechanism,
that an old resource is permanently unavailable and has no forwarding
address.
The method specified in the Request-Line is not allowed
for the resource identified by the Request-URI. The
response MUST include an Allow header containing a list of valid
methods for the requested resource.
The resource identified by the request is only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request.
HTTP/1.1 servers are allowed to return responses which are not acceptable according to the accept headers sent in the request. In some cases, this may even be preferable over sending a 406 response. User agents are encouraged to inspect the headers of an incoming response to determine if it is acceptable. If the response is not acceptable, user agents SHOULD interrupt the receipt of the response if doing so would save network resources. If it is unknown whether an incoming response would be acceptable, a user agent SHOULD temporarily stop receipt of more data and query the user for a decision on further
actions.
This code is similar to 401 (unauthorized), but indicates that
the client MUST first authenticate itself with the proxy. The
proxy MUST return a Proxy-Authenticate header field
(Section 10.30) containing a
challenge applicable to the proxy for the requested resource.
The client MAY repeat the request with a suitable Proxy-Authorization
header field (Section 10.31).
HTTP access authentication is explained in Section 11.
The client did not produce a request within the time that the server was prepared to wait. The client MAY repeat the request without modifications at any later time.
The request could not be completed due to a conflict with the current state of the resource. This code is only allowed in situations where it is expected that the user MAY be able to resolve the conflict and resubmit the request. The response body SHOULD include enough information for the user to recognize the source of the conflict. Ideally, the response entity would include enough information for the user or user-agent to fix the problem; however, that MAY not be possible and is not required.
Conflicts are most likely to occur in response to a PUT
request. If versioning is being used and the entity being PUT
includes changes to a resource which conflict with those made
by an earlier (third-party) request, the server MAY use the 409
response to indicate that it can't complete the request. In this
case, the response entity SHOULD contain a list of the differences
between the two versions in a format defined by the response Content-Type.
The requested resource is no longer available at the server and
no forwarding address is known. This condition SHOULD be considered
permanent. Clients with link editing capabilities SHOULD delete
references to the Request-URI after user approval.
If the server does not know, or has no facility to determine,
whether or not the condition is permanent, the status code 404
(not found) SHOULD be used instead. This response is cachable
unless indicated otherwise.
The 410 response is primarily intended to assist the task of web maintenance by notifying the recipient that the resource is intentionally unavailable and that the server owners desire that remote links to that resource be removed. Such an event is common for limited-time, promotional services and for resources belonging to individuals no longer working at the server's site. It is not necessary to mark all permanently unavailable resources as "gone" or to keep the mark for any length of time -- that is left to the discretion of the server owner.
The server refuses to accept the request without a defined Content-Length.
The client MAY repeat the request if it adds a valid Content-Length
header field containing the length of the entity body in the request
message.
The precondition given in one or more of the request header fields evaluated to false when it was tested on the server. This response code allows the client to place preconditions on the current resource metainformation (header field data) and thus prevent the requested method from being applied to a resource other than the one intended.
The server is refusing to process a request because it considers the request entity to be larger than it is willing or able to process. The server SHOULD close the connection if that is necessary to prevent the client from continuing the request.
If the client manages to read the 413 response, it MUST honor it and SHOULD reflect it to the user.
If this restriction is considered temporary, the server MAY include
a Retry-After header field to indicate that it is
temporary and after what time the client MAY try again.
The server is refusing to service the request because the Request-URI
is longer than the server is willing to interpret. This rare condition
is only likely to occur when a client has improperly converted
a POST request to a GET request with long query information, when
the client has descended into a URL "black hole" of
redirection (e.g., a redirected URL prefix that points to a suffix
of itself), or when the server is under attack by a client attempting
to exploit security holes present in some servers using fixed-length
buffers for reading or manipulating the Request-URI.
The server is refusing to service the request because the entity body of the request is in a format not supported by the requested resource for the requested method.
This status code is reserved for future use by a planned content
negotiation mechanism. HTTP/1.1 user agents receiving a 416 response
which includes a Location header can treat this response
as they would treat a 303 (See Other) response. If no Location
header is included, the appropriate action is to display the entity
enclosed in the response to the user.
Response status codes beginning with the digit "5" indicate cases in which the server is aware that it has erred or is incapable of performing the request. If the client has not completed the request when a 5xx code is received, it SHOULD immediately cease sending data to the server. Except when responding to a HEAD request, the server SHOULD include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. These response codes are applicable to any request method and there are no required header fields.
The server encountered an unexpected condition which prevented it from fulfilling the request.
The server does not support the functionality required to fulfill the request. This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource.
The server, while acting as a gateway or proxy, received an invalid response from the upstream server it accessed in attempting to fulfill the request.
The server is currently unable to handle the request due to a
temporary overloading or maintenance of the server. The implication
is that this is a temporary condition which will be alleviated
after some delay. If known, the length of the delay MAY be indicated
in a Retry-After header. If no Retry-After
is given, the client SHOULD handle the response as it would for
a 500 response.
Note: The existence of the 503 status code does not imply that a server must use it when becoming overloaded. Some servers MAY wish to simply refuse the connection.
The server, while acting as a gateway or proxy, did not receive a timely response from the upstream server it accessed in attempting to complete the request.
The server does not support, or refuses to support, the HTTP protocol version that was used in the request message. The server is indicating that it is unable or unwilling to complete the request using the same major version as the client, as described in Section 3.1, other than with this error message. The response SHOULD contain an entity describing why that version is not supported and what other protocols are supported by that server.
This section defines the syntax and semantics of
all standard HTTP/1.1 header fields. For Entity-Header
fields, both sender and
recipient refer to either the client
or the server, depending on who sends and who receives the entity.
The Accept request-header
field can be used to specify certain media types which are acceptable
for the response. Accept headers
can be used to indicate that the request is specifically limited
to a small set of desired types, as in the case of a request for
an in-line image.
The field MAY be folded onto several lines and more than one occurrence of the field is allowed, with the semantics being the same as if all the entries had been in one field value.
Accept = "Accept" ":" #(
media-range
[ ( ":" | ";" )
range-parameter
*( ";" range-parameter ) ]
| extension-token )
media-range = ( "*/*"
| ( type "/" "*" )
| ( type "/" subtype )
) *( ";" parameter )
range-parameter = ( "q" "=" qvalue )
| extension-range-parameter
extension-range-parameter = ( token "=" token )
extension-token = token
The asterisk "*" character is used to group
media types into ranges, with "*/*" indicating all media
types and "type/*" indicating all subtypes of that type.
The range-parameter q is used to indicate the media type quality
factor for the range, which represents the user's preference for
that range of media types. The default value is q=1. In Accept
headers generated by HTTP/1.1 clients, the character separating
media-ranges from range-parameters SHOULD be a ":".
HTTP/1.1 servers SHOULD be tolerant of use of the ";"
separator by HTTP/1.0 clients.
The example
Accept: audio/*: q=0.2, audio/basic
SHOULD be interpreted as "I prefer audio/basic, but send me any audio type if it is the best available after an 80% mark-down in quality."
If no Accept header
is present, then it is assumed that the client accepts all media
types. If Accept headers are
present, and if the server cannot send a response
which is acceptable according to the Accept
headers, then the server SHOULD send an error response with the
406 (not acceptable) status code, though the sending of an unacceptable
response is also allowed.
A more elaborate example is
Accept: text/plain: q=0.5, text/html,
text/x-dvi: q=0.8, text/x-c
Verbally, this would be interpreted as "text/html and text/x-c are the preferred media types, but if they do not exist, then send the text/x-dvi entity, and if that does not exist, send the text/plain entity."
Media ranges can be overridden by more specific media ranges or specific media types. If more than one media range applies to a given type, the most specific reference has precedence. For example,
Accept: text/*, text/html, text/html;level=1, */*
have the following precedence:
1) text/html;level=1
2) text/html
3) text/*
4) */*
The media type quality factor associated with a given type is determined by finding the media range with the highest precedence which matches that type. For example,
Accept: text/*:q=0.3, text/html:q=0.7, text/html;level=1,
*/*:q=0.5
would cause the following values to be associated:
text/html;level=1 = 1
text/html = 0.7
text/plain = 0.3
image/jpeg = 0.5
text/html;level=3 = 0.7
Note: A user agent MAY be provided with a default set of quality values for certain media ranges. However, unless the user agent is a closed system which cannot interact with other rendering agents, this default set SHOULD be configurable by the user.
The Accept-Charset
request-header field can be used to indicate what character sets
are acceptable for the response. This field allows clients capable
of understanding more comprehensive or special-purpose character
sets to signal that capability to a server which is capable of
representing documents in those character sets. The ISO-8859-1
character set can be assumed to be acceptable to all user agents.
Accept-Charset = "Accept-Charset" ":"
1#( charset [ ";" "q" "=" qvalue ] )
Character set values are described in Section 3.4. Each charset may be given an associated quality value which represents the user's preference for that charset. The default value is q=1. An example is
Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
If no Accept-Charset
header is present, the default is that any character set is acceptable.
If an Accept-Charset header is
present, and if the server cannot send a response which is acceptable
according to the Accept-Charset
header, then the server SHOULD send an error response with the
406 (not acceptable) status code, though the sending of an unacceptable
response is also allowed.
The Accept-Encoding
request-header field is similar to Accept,
but restricts the content-coding values (Section 3.5)
which are acceptable in the response.
Accept-Encoding = "Accept-Encoding" ":"
#( content-coding )
An example of its use is
Accept-Encoding: compress, gzip
If no Accept-Encoding
header is present in a request, the server MAY assume that the
client will accept any content coding. If an Accept-Encoding
header is present, and if the server cannot send a response which
is acceptable according to the Accept-Encoding
header, then the server SHOULD send an error response with the
406 (not acceptable) status code.
The Accept-Language
request-header field is similar to Accept,
but restricts the set of natural languages that are preferred
as a response to the request.
Accept-Language = "Accept-Language" ":" 1#( language-range [ ";" "q" "=" qvalue ] ) language-range = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) )| "*" )
Each language-range MAY be given an associated quality value which represents an estimate of the user's comprehension of the languages specified by that range. The quality value defaults to "q=1" (100% comprehension).For example,
Accept-Language: da, en-gb;q=0.8, en;q=0.7
would mean: "I prefer Danish, but will accept
British English (with 80% comprehension) and other types of English(with
70% comprehension)." A language-range matches a language-tag
if it exactly equals the tag, or if it exactly equals a prefix
(a sub-sequence starting at the first character) of the tag such
that the first tag character following the prefix is "-".
The special range "*", if present in the Accept-Language
field, matches every tag not matched by any other ranges present
in the Accept-Language field.
Note: This use of a prefix matching rule does not imply that language tags are assigned to languages in such a way that it is always true that if a user understands a language with a certain tag, then this user will also understand all languages with tags for which this tag is a prefix. The prefix rule simply allows the use of prefix tags if this is the case.
The language quality factor assigned to a language-tag
by the Accept-Language field
is the quality value of the longest language-range
in the field that matches the language-range.
If no language-range in the
field matches the tag, the language quality factor assigned is
0. If no Accept-Language header
is present in the request, the server SHOULD assume that all languages
are equally acceptable. If an Accept-Language
header is present, then all languages which are assigned a quality
factor greater than 0 are acceptable. If the server cannot generate
a response for an audience capable of understanding at least one
acceptable language, it can send a response that uses one or more
un-accepted languages.
It may be contrary to the privacy expectations of
the user to send an Accept-Language
header with the complete linguistic preferences of the user in
every request. For a discussion of this issue, see Section 14.7.
Note: As intelligibility is highly dependent on the individual user, it is recommended that client applications make the choice of linguistic preference available to the user. If the choice is not made available, then theAccept-Languageheader field MUST not be given in the request.
The Allow entity-header
field lists the set of methods supported by the resource identified
by the Request-URI. The purpose
of this field is strictly to inform the recipient of valid methods
associated with the resource. An Allow
header field MUST be present in a 405 (method not allowed) response.
The Allow header field is not
permitted in a request using the POST method, and thus SHOULD
be ignored if it is received as part of a POST entity.
Allow = "Allow" ":" 1#method
Example of use:
Allow: GET, HEAD, PUT
This field cannot prevent a client from trying other
methods. However, the indications given by the Allow
header field value SHOULD be followed. The actual set of allowed
methods is defined by the origin server at the time of each request.
The Allow header
field MAY be provided with a PUT request to recommend the methods
to be supported by the new or modified resource. The server is
not required to support these methods and SHOULD include an Allow
header in the response giving the actual supported methods.
A proxy MUST not modify the Allow
header field even if it does not understand all the methods specified,
since the user agent MAY have other means of communicating with
the origin server.
The Allow header
field does not indicate what methods are implemented at the server
level. Servers MAY use the Public response header field (Section 10.32)
to describe what methods are implemented on the server as a whole.
A user agent that wishes to authenticate itself with
a server--usually, but not necessarily, after receiving a 401
response--MAY do so by including an Authorization
request-header field with the request. The Authorization
field value consists of credentials
containing the authentication information of the user agent for
the realm of the resource being requested.
Authorization = "Authorization" ":" credentials
HTTP access authentication is described in Section 11.
If a request is authenticated and a realm
specified, the same credentials
SHOULD be valid for all other requests within this realm.
When a shared cache (see section 13.10) receives
a request containing an Authorization
field, it MUST NOT return the corresponding response as a reply
to any other request, unless one of the following specific exceptions
holds:
proxy-revalidate"
Cache-Control directive, the
cache MAY use that response in replying to a subsequent request,
but a proxy cache MUST first revalidate it with the origin server,
using the request headers from the new request to allow the origin
server to authenticate the new request.
must-revalidate"
Cache-Control directive, the
cache MAY use that response in replying to a subsequent request,
but all caches MUST first revalidate it with the origin server,
using the request headers from the new request to allow the origin
server to authenticate the new request.
public"
Cache-Control directive, it may
be returned in reply to any subsequent request.
The Cache-Control
general-header field is used to specify directives that MUST be
obeyed by all caching mechanisms along the request/response chain.
The directives specify behavior intended to prevent caches from
adversely interfering with the request or response. . These directives
typically override the default caching algorithms. Cache directives
are unidirectional in that the presence of a directive in a request
does not imply that the same directive should be given in the
response.
Cache directives must be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives may be applicable to all recipients along the request/response chain. It is not possible to specify a cache-directive for a specific cache.
Cache-Control = "Cache-Control" ":" 1#cache-directive
cache-directive = "public"
| "private" [ "=" <"> 1#field-name <"> ]
| "no-cache" [ "=" <"> 1#field-name <"> ]
| "no-store"
| "no-transform"
| "must-revalidate"
| "proxy-revalidate"
| "only-if-cached"
| "max-age" "=" delta-seconds
| "max-stale" "=" delta-seconds
| "min-fresh" "=" delta-seconds
| "min-vers" "=" HTTP-Version
and perhaps
| "max-uses" "=" 1*DIGIT
| "use-count" "=" 1*DIGIT
The cache-control directives can be broken down into these general categories:
Caches never add or remove Cache-Control
directives to requests or responses.
Unless specifically constrained by a Cache-Control
directive, a caching system may always store a successful response
as a cache entry, may return it without validation if it is fresh,
and may return it after successful validation. If there is neither
a cache validator nor an explicit expiration time associated with
a response, we do not expect it to be cached, but certain caches
may violate this expectation (for example, when little or no network
connectivity is available) as long as they explicit mark their
responses using the Warning mechanism describe in
section 10.51.
Note that some HTTP/1.0 caches are known to violate this expectation
without providing any Warning.
However, in some cases it may be inappropriate for a cache to
retain a resource value, or to return it in response to a subsequent
request. This may be because absolute semantic transparency is
deemed necessary by the service author, or because of security
or privacy considerations. Certain Cache-Control
directives are therefore provided so that the server can indicate
that certain resources, or portions thereof, may not be cached
regardless of other considerations.
Note that section 10.6 normally prevents a shared cache from saving
and returning a response to a previous request if that request
included an Authorization header.
The following Cache-Control response directives add
or remove restrictions on what is cachable:
Authorization
header. However, any other constraints on caching still apply.
Note: This usage of the word "private"
only controls where the response may cached, and cannot ensure
the privacy of the message content. Note in particular that HTTP/1.0
caches will not recognize or obey this directive.
Note: HTTP/1.0 caches will not recognize or obey this directive.
TBS: precedence relations between public, private, and no-cache.
The "no-store" directive applies to the
entire message, and may be sent either in a response or in a request.
If sent in a request, a cache MUST NOT store any part of either
this request or any response to it. If sent in a response, a cache
MUST NOT store any part of either this response or the request
that elicited it. This directive applies to both non-shared and
shared caches.
Even when this directive is associated with a response, users may explicitly store such a response outside of the caching system (e.g., with a "Save As" dialog). History buffers may store such responses as part of their normal operation.
The purpose of this directive is to meet the stated requirements of certain users and service authors who are concerned about accidental releases of information via unanticipated accesses to cache data structures. While the use of this directive may improve privacy in some cases, we caution that it is NOT in any way a reliable or sufficient mechanism for ensuring privacy. In particular, HTTP/1.0 caches will not recognize or obey this directive, malicious or compromised caches may not recognize or obey this directive, and all communications networks may be vulnerable to eavesdropping.
The "min-vers" directive applies to the
entire message, and may be sent either in a response or in a request.
If sent in a request, a cache whose HTTP version number is less
than the specified version MUST NOT store any part of either this
request or any response to it. If sent in a response, a cache
whose HTTP version number is less than the specified version MUST
NOT store any part of either this response or the request that
elicited it, nor may any cache transmit a stored (non-firsthand)
copy of the response to any client with a lower HTTP version number.
This directive applies to both non-shared and shared caches, and
is made mandatory to allow for future protocol extensions that
may affect caching.
Note that the lowest version that can be sensibly included in a "min-vers" directive is HTTP/1.1, since HTTP/1.0 caches do not obey it.
The expiration time of a resource may be specified by the origin
server using the Expires header (see section TBS).
Alternatively, it may be specified using the "max-age"
directive in a response.
If a response includes both an Expires header and
a max-age: directive, the max-age: directive overrides the Expires
header, even if the Expires header is more restrictive.
This rule allows an origin server to provide, for a given response,
a longer expiration time to an HTTP/1.1 (or later) cache than
to an HTTP/1.0 cache. This may be useful if certain HTTP/1.0 caches
improperly calculate ages or expiration times, perhaps due to
badly unsynchronized clocks.
Other directives allow an end-user client to modify the basic expiration mechanism, making it either stricter or looser. These directives may be specified on a request:
max-age Indicates that the client is willing to accept a response
whose age is no greater than the specified time in seconds. Unless
"max-stale" is also included, the client
is not willing to accept a stale response. This directive overrides
any policy of the cache.
min-fresh Indicates that the client is willing to accept a response whose freshness lifetime is no less than its current age plus the specified time in seconds. That is, the client wants a that response will still be fresh for at least the specified number of seconds.
max-stale Indicates that the client is willing to accept a response
that has exceeded its expiration time by no more than the specified
number of seconds. If a cache returns a stale response in response
to such a request, it MUST mark it as stale using the Warning
header.
Note that HTTP/1.0 caches will ignore these directives.
If a cache returns a stale response, either because of a max-stale
directive on a request, or because the cache is configured to
override the expiration time of a response, the cache MUST attach
a Warning header to the stale response, using Warning
10 (Response is stale).
Sometimes an end-user client may want or need to insist that a cache revalidate its cache entry with the origin server (and not just with the next cache along the path to the origin server), or to reload its cache entry from the origin server. End-to-end revalidation may be necessary if either the cache or the origin server has overestimated the expiration time of the cached response. End-to-end reload may be necessary if the response value has become corrupted for some reason, and the fact that its validator is up-to-date is irrelevant.
End-to-end revalidation may be requested either when the client does not have its own local cached copy, in which case we call it "unspecified end-to-end revalidation", or when the client does have a local cached copy, in which case we call it "specific end-to-end revalidation."
The client can specify these three kinds of action using Cache-Control
request directives:
End-to-end reload The request includes "Cache-Control:
no-cache" or, for compatibility with HTTP/1.0 clients, "Pragma:
no-cache". No field names may be included with the "no-cache"
directive in a request. The server MUST NOT use a cached copy
when responding to such a request.
Specific end-to-end revalidation The request includes "Cache-Control:
max-age=0", which forces each cache along the path to the
origin server to revalidate its own entry, if any, with the next
cache or server. The initial request includes a cache-validating
conditional with the client's current validator.
Unspecified end-to-end revalidation The request includes "Cache-Control:
max-age=0", which forces each cache along the path to the
origin server to revalidate its own entry, if any, with the next
cache or server. The initial request does not include a cache-validating
conditional; the first cache along the path (if any) that holds
a cache entry for this resource includes a cache-validating conditional
with its current validator.
Note that HTTP/1.0 caches will ignore these directives, except
perhaps for "Pragma: no-cache".
When an intermediate cache is forced, by means of a "max-age=0"
directive, to revalidate its own cache entry, and the client has
supplied its own validator in the request, the supplied validator
may differ from the validator currently stored with the cache
entry. In this case, the cache may use either validator in making
its own request without affecting semantic transparency.
However, the choice of validator may affect performance. The best approach is for the intermediate cache to use its own validator when making its request. If the server replies with 304 (Not Modified), then the cache should return its now validated copy to the client with a 200 (OK) response. If the server replies with a new Entity-body and cache validator, however, the intermediate cache should compare the returned validator with the one provided in the client's request, using the strong comparison function. If the client's validator is equal to the origin server's, then the intermediate cache simply returns 304 (Not Modified). Otherwise, it returns the new Entity-body with a 200 (OK) response.
If a request includes the "no-cache" directive,
it should not include "fresh-min", "max-stale",
or "max-age".
In some cases, such as times of extremely poor network connectivity,
a client may want a cache to return only those responses that
it currently has stored, and not to reload or revalidate with
the origin server. To do this, the client may include the "only-if-cached"
directive in a request. If it receives this directive, a cache
SHOULD either respond using a cached value that is consistent
with the other constraints of the request, or respond with a 504
(Gateway Timeout) status. However, if a group of caches is being
operated as a unified system with good internal connectivity,
such a request MAY be forwarded within that group of caches.
Because a cache may be configured to ignore a server's specified
expiration time, and because a client request may include a max-stale
directive, which has a similar effect, the protocol also includes
a mechanism for the origin server to require revalidation of a
cache entry on any subsequent use. When the "must-revalidate"
directive is present in a response received by a cache, that cache
MUST NOT use the value after it becomes stale to respond to a
subsequent request without first revalidating it with the origin
server. (I.e., the cache must do an end-to-end revalidation every
time.)
The "must-revalidate" directive is necessary
to support reliable operation for cookies and certain other protocol
features. In all circumstances an HTTP/1.1 cache MUST obey the
"must-revalidate" directive; in particular,
if the cache cannot reach the origin server for any reason, it
MUST generate a 504 (Gateway Timeout) response. Note that HTTP/1.0
caches will ignore this directive.
Servers should send the "must-revalidate"
directive if and only if failure to revalidate a request on the
entity could result in significantly incorrect operation, such
as a silently unexecuted financial transaction. Recipients MUST
NOT take any automated action that violates this directive, and
MUST NOT automatically provide an unvalidated copy of the entity
if revalidation fails.
Although this is not recommended, user agents operating under severe connectivity constraints may violate this directive but if so, MUST explicitly warn the user that an unvalidated response has been provided. The warning MUST be provided on each unvalidated access, and SHOULD require explicit user confirmation.
The "proxy-revalidate" directive has the
same meaning as the "must-revalidate" directive,
except that it does not apply to user-agent caches. This directive
is meant to support digest authentication.
This section is highly debatable and is likely to be removed to a separate I.D.
The "max-uses" response directive allows
a cache to use a response at most a certain limited number of
times. For example, "max-uses=10" means
that the response should be returned in reply to the current request,
and may be returned in reply to no more than nine subsequent requests
(subject to other caching constraints), unless revalidated.
A cache may subdivide its remaining use-count among several of its own clients. For example, if the incoming response includes "max-uses=10", the recipient may forward this as two responses, each with "max-uses=5". The idea is that the total number of uses allowed in a cache hierarchy should not exceed the specified limit. (The heuristics a cache uses to sub-allocate its max-uses value are beyond the scope of the HTTP spec.)
The "use-count" request directive allows a cache to tell a server how many times it has actually used the cache entry specified in the associated request. If a cache receives a use-count value from one of its clients, and it has a corresponding cache entry, it should add the incoming use-count to its local count.
When a cache removes an entry, it MAY first send a HEAD request on the associated URI, including its use-count value, to inform the server of the actual use-count. If the server has sent a max-uses limit, the cache SHOULD perform this notification.
A cache that is willing to perform such notifications and that is willing to obey the max-uses limit SHOULD send a ``use-count=0'' directive on its first (non-conditional) request on a resource. This informs the server that the cache intends to use these two directives in the manner described here.
In certain circumstances, an intermediate cache (proxy) may find it useful to convert the encoding of an entity body. For example, a proxy might use a compressed content-coding to transfer the body to a client on a slow link.
Because end-to-end authentication of entity bodies and/or entity headers relies on the specific encoding of these values, such transformations may cause authentication failures. Therefore, an intermediate cache MUST NOT change the encoding of an entity body if the response includes the "no-transform" directive.
HTTP version 1.1 provides a new request and response
header field called "Connection".
This header field allows the client and server to specify options
which should only exist over that particular connection and MUST
NOT be communicated by proxies over further connections. The connection
header field MAY have multiple tokens separated by commas (referred
to as connection-tokens).
HTTP version 1.1 proxies MUST parse the Connection
header field and for every connection-token in this field, remove
a corresponding header field from the request before the request
is forwarded. The use of a connection option is specified by the
presence of a connection token in the Connection
header field, not by the corresponding additional header field
(which may not be present).
When a client wishes to establish a persistent connection
it MUST send a "Persist"
connection-token:
Connection: persist
The Connection header has the following grammar:
Connection-header = "Connection" ":"
connection-token 0#( "," connection-token )
When the Persist connection-token has been transmitted with a request or a response a Persist header field MAY also be included. The Persist header field takes the following form:
Persist-header = "Persist" ":" 0#pers-param
pers-param = param-name "=" value
The Persist header itself is optional, and is used only if a parameter is being sent. HTTP/1.1 does not define any parameters.
If the Persist header is sent, the corresponding connection token MUST be transmitted. The Persist header MUST be ignored if received without the connection token.
The Content-Base
entity-header field may be used to specify the base URI for resolving
relative URLs within the entity. This header field is described
as "Base" in RFC 1808 [11],
which is expected to be revised soon.
Content-Base = "Content-Base" ":" absoluteURI
If no Content-Base
field is present, the base URI of an entity is defined either
by its Content-Location or the
URI used to initiate the request, in that order of precedence.
Note, however, that the base URI of the contents within the entity
body may be redefined within that entity body.
The Content-Encoding
entity-header field is used as a modifier to the media-type.
When present, its value indicates what additional content codings
have been applied to the resource, and thus what decoding mechanisms
MUST be applied in order to obtain the media-type
referenced by the Content-Type
header field. Content-Encoding
is primarily used to allow a document to be compressed without
losing the identity of its underlying media type.
Content-Encoding = "Content-Encoding" ":" 1#content-coding
Content codings are defined in Section 3.5. An example of its use is
Content-Encoding: gzip
The Content-Encoding
is a characteristic of the resource identified by the Request-URI.
Typically, the resource is stored with this encoding and is only
decoded before rendering or analogous usage.
If multiple encodings have been applied to a resource, the content codings MUST be listed in the order in which they were applied. Additional information about the encoding parameters MAY be provided by other Entity-Header fields not defined by this specification.
The Content-Language
entity-header field describes the natural language(s) of the intended
audience for the enclosed entity. Note that this may not be equivalent
to all the languages used within the entity.
Content-Language = "Content-Language" ":" 1#language-tag
Language tags are defined in Section 3.10.
The primary purpose of Content-Language
is to allow a selective consumer to identify and differentiate
resources according to the consumer's own preferred language.
Thus, if the body content is intended only for a Danish-literate
audience, the appropriate field is
Content-Language: dk
If no Content-Language
is specified, the default is that the content is intended for
all language audiences. This may mean that the sender does not
consider it to be specific to any natural language, or that the
sender does not know for which language it is intended.
Multiple languages MAY be listed for content that is intended for multiple audiences. For example, a rendition of the "Treaty of Waitangi," presented simultaneously in the original Maori and English versions, would call for
Content-Language: mi, en
However, just because multiple languages are present
within an entity does not mean that it is intended for multiple
linguistic audiences. An example would be a beginner's language
primer, such as "A First Lesson in Latin," which is
clearly intended to be used by an English-literate audience. In
this case, the Content-Language
should only include "en".
Content-Language MAY be applied to any
media type -- it SHOULD not be limited to textual documents.
The Content-Length
entity-header field indicates the size of the Entity-Body,
in decimal number of octets, sent to the recipient or, in the
case of the HEAD method, the
size of the Entity-Body that
would have been sent had the request been a GET.
Content-Length = "Content-Length" ":" 1*DIGIT
An example is
Content-Length: 3495
Applications SHOULD use this field to indicate the
size of the Entity-Body to be
transferred, regardless of the media type of the entity. A valid
Content-Length field value is
required on all HTTP/1.1 request messages containing an entity
body.
Any Content-Length
greater than or equal to zero is a valid value. Section 7.2.2
describes how to determine the length of an Entity-Body
if a Content-Length is not given.
Note: The meaning of this field is significantly different from the corresponding definition in MIME, where it is an optional field used within the "message/external-body" content-type. In HTTP, it SHOULD be used whenever the entity's length can be determined prior to being transferred.
The Content-MD5
entity-header field is an MD5 digest of the entity-body, as defined
in RFC 1864 [23], for the purpose of providing
an end-to-end message integrity check (MIC) of the entity-body.
(Note: an MIC is good for detecting accidental modification of
the entity-body in transit, but is not proof against malicious
attacks.)
ContentMD5 = "Content-MD5" ":" md5-digest
md5-digest = <base64 of 128 bit MD5 digest as per RFC 1864>
The Content-MD5
header may be generated by an origin server to function as an
integrity check of the entity-body. Only origin-servers may generate
the Content-MD5 header field;
proxies and gateways MUST NOT generate it, as this would defeat
its value as an end-to-end integrity check. Any recipient of the
entity-body, including gateways and proxies, MAY check that the
digest value in this header field matches that of the entity-body
as received.
The MD5 digest is computed based on the content of
the entity body, including any Content-Encoding
that has been applied, but not including any Transfer-Encoding.
If the entity is received with a Transfer-Encoding,
that encoding must be removed prior to checking the Content-MD5
value against the received entity.
This has the result that the digest is computed on
the octets of the entity body exactly as, and in the order that,
they would be sent if no Transfer-Encoding
were being applied.
HTTP extends RFC 1864 to permit the digest to be computed for MIME composite media-types (e.g., multipart/* and message/rfc822), but this does not change how the digest is computed as defined in the preceding paragraph.
Note: There are several consequences of this. The entity-body for composite types many contain many body-parts, each with its own MIME and HTTP headers (includingContent-MD5,Content-Transfer-Encoding, andContent-Encodingheaders). If a body-part has aContent-Transfer-EncodingorContent-Encodingheader, it is assumed that the content of the body-part has had the encoding applied, and the body-part is included in theContent-MD5digest as is -- i.e., after the application. Also, the HTTPTransfer-Encodingheader makes no sense within body-parts; if it is present, it is ignored -- i.e. treated as ordinary text.
Note: while the definition ofContent-MD5is exactly the same for HTTP as in RFC 1864 for MIME entity-bodies, there are several ways in which the application ofContent-MD5to HTTP entity-bodies differs from its application to MIME entity-bodies. One is that HTTP, unlike MIME, does not useContent-Transfer-Encoding, and does useTransfer-EncodingandContent-Encoding. Another is that HTTP more frequently uses binary content types than MIME, so it is worth noting that in such cases, the byte order used to compute the digest is the transmission byte order defined for the type. Lastly, HTTP allows transmission of text types with any of several line break conventions and not just the canonical form using CR-LF. Conversion of all line breaks to CR-LF should not be done before computing or checking the digest: the line break convention used in the text actually transmitted should be left unaltered when computing the digest.
The Content-Range header is sent with a partial entity body to specify where in the full entity body the partial body should be inserted. It also indicates the total size of the entity.
Content-Range = "Content-Range" ":" content-range-spec
When an HTTP message includes the content of a single range (for example, a response to a request for a single range, or to request for a set of ranges that overlap without any holes), this content is transmitted with a Content-Range header, and a Content-length header showing the number of bytes actually transferred.
For example,
HTTP/1.0 206 Partial content
Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-range: 21010-47021/47022
Content-length: 26012
Content-type: image/gif
When an HTTP message includes the content of multiple ranges (for example, a response to a request for multiple non-overlapping ranges), these are transmitted as a multipart MIME message. The multipart MIME content-type used for this purpose is defined in this specification to be "multipart/byteranges".
The MIME multipart/byteranges content-type includes two or more
parts, each with its own Content-type and Content-Range fields.
The parts are separated using a MIME boundary parameter.
For example:
HTTP/1.0 206 Partial content
Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-type: multipart/byteranges; boundary=THIS_STRING_SEPARATES
--THIS_STRING_SEPARATES
Content-type: application/pdf
Content-range: bytes 500-999/8000
...the first range...
--THIS_STRING_SEPARATES
Content-type: application/pdf
Content-range: bytes 7000-7999/8000
...the second range...
--THIS_STRING_SEPARATES-
A client that cannot decode a MIME multipart/byteranges message should not ask for multiple byte-ranges in a single request.
When a client requests multiple byte-ranges in one request, the server SHOULD return them in the order that they appeared in the request.
If the server ignores a byte-range-spec because it is invalid, or because it specifies a range that starts beyond the end of the entity, it may omit the corresponding Content-Range field and partial entity body.
If none of the byte-range-spec values in a request specify part of the current entity (i.e., start before the last byte of the entity), then the server should return a status of 207 (Range Out Of Bounds).
The Content-Type
entity-header field indicates the media type of the Entity-Body
sent to the recipient or, in the case of the HEAD
method, the media type that would have been sent had the request
been a GET.
Content-Type = "Content-Type" ":" media-type
Media types are defined in Section 3.7. An example of the field is
Content-Type: text/html; charset=ISO-8859-4
Further discussion of methods for identifying the media type of an entity is provided in Section 7.2.1.
The Content-Location
entity-header field is used to define the location of the specific
resource associated with the entity enclosed in the message. A
server SHOULD provide a Content-Location
if, when including an entity in response to a GET request on a
negotiated resource, the entity corresponds to a specific, non-negotiated
location which can be accessed via the Content-Location
URI. A server SHOULD provide a Content-Location
with any 200 (OK) response which was internally (not visible to
the client) redirected to a resource other than the one identified
by the request and for which correct interpretation of that resource
MAY require knowledge of its actual location. The recipient MAY
make future requests on this location instead of on the Request-URI.
Content-Location = "Content-Location" ":" absoluteURI
If no Content-Base
header field is present, the value of Content-Location
also defines the base URL for the entity (see Section 10.9).
Note: Since the Content-Location
field re-interprets the source of an entity, recipients must take
care in not allowing a "spoofed" location to deny access
to the real resource. This is described in Section 15.9.
The Date general-header
field represents the date and time at which the message was originated,
having the same semantics as orig-date
in RFC 822. The field value is an HTTP-date,
as described in Section 3.3.
Date = "Date" ":" HTTP-date
An example is
Date: Tue, 15 Nov 1994 08:12:31 GMT
If a message is received via direct connection with
the user agent (in the case of requests) or the origin server
(in the case of responses), then the date can be assumed to be
the current date at the receiving end. However, since the date--as
it is believed by the origin--is important for evaluating cached
responses, origin servers SHOULD always include a Date
header. Clients SHOULD only send a Date
header field in messages that include an entity body, as in the
case of the PUT and POST requests, and even then it is optional.
A received message which does not have a Date
header field SHOULD be assigned one by the recipient if the message
will be cached by that recipient or gatewayed via a protocol which
requires a Date.
In theory, the date SHOULD represent the moment just before the entity is generated. In practice, the date can be generated at any time during the message origination without affecting its semantic value.
Note: An earlier version of this document incorrectly
specified that this field SHOULD contain the creation date of
the enclosed Entity-Body. This
has been changed to reflect actual (and proper) usage.
Origin servers MUST send a Date
field in every response. However, if a cache receives a response
without a Date field, it SHOULD
attach one with the cache's best estimate of the time at which
the response was originally generated.
The format of the Date
is an absolute date and time as defined by HTTP-date in Section
3.3; it MUST be in RFC1123-date format.
The Expires entity-header
field gives the date/time after which the entity should be considered
stale. A stale cache entry may not normally be returned by a cache
(either a proxy cache or an end-user cache) unless it is first
validated with the origin server (or with an intermediate cache
that has a fresh copy of the resource). See section 13.2 for further
discussion of the expiration model.
The presence of an Expires
field does not imply that the original resource will change or
cease to exist at, before, or after that time.
The format is an absolute date and time as defined by HTTP-date in Section 3.3; it MUST be in rfc1123-date format:
Expires = "Expires" ":" HTTP-date
An example of its use is
Expires: Thu, 01 Dec 1994 16:00:00 GMT
Note: if a response includes aCache-Controlfield with the max-age directive, that directive overrides theExpiresfield.
HTTP/1.1 clients and caches MUST treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired").
To mark a response as "already expired,"
an origin server should use an Expires
date that is equal to the Date
header value. (See the rules for expiration calculations in section
13.2.7.)
To mark a response as "never expires,"
an origin server should use Expires
date approximately one year from the time the response is generated.
HTTP/1.1 servers should not send Expires
dates more than one year in the future.
The Via general-header
field MUST be used by gateways and proxies to indicate the intermediate
protocols and recipients between the user agent and the server
on requests, and between the origin server and the client on responses.
It is analogous to the "Received" field of RFC 822 [9]and
is intended to be used for tracking message forwards, avoiding
request loops, and identifying the protocol capabilities of all
senders along the request/response chain.
Via = "Via" ":" 1#( received-protocol received-by [ comment ] )
received-protocol = [ protocol-name "/" ] protocol-version
protocol-name = token
protocol-version = token
received-by = ( host [ ":" port ] ) | pseudonym
pseudonym = token
The received-protocol indicates the protocol version
of the message received by the server or client along each segment
of the request/response chain. The received-protocol version
is appended to the Via field
value when the message is forwarded so that information about
the protocol capabilities of upstream applications remains visible
to all recipients.
The protocol-name is optional if and only if it would be "HTTP". The received-by field is normally the host and optional port number of a recipient server or client that subsequently forwarded the message. However, if the real host is considered to be sensitive information, it MAY be replaced by a pseudonym. If the port is not given, it MAY be assumed to be the default port of the received-protocol.
Multiple Via field
values represent each proxy or gateway that has forwarded the
message. Each recipient MUST append their information such that
the end result is ordered according to the sequence of forwarding
applications.
Comments MAY be used in the Via
header field to identify the software of the recipient proxy or
gateway, analogous to the User-Agent and Server header fields.
However, all comments in the Via
field are optional and MAY be removed by any recipient prior to
forwarding the message.
For example, a request message could be sent from
an HTTP/1.0 user agent to an internal proxy code-named "fred",
which uses HTTP/1.1 to forward the request to a public proxy at
nowhere.com, which completes the request by forwarding it to the
origin server at www.ics.uci.edu. The request received by www.ics.uci.edu
would then have the following Via
header field:
Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)
Proxies and gateways used as a portal through a network firewall SHOULD NOT, by default, forward the names and ports of hosts within the firewall region. This information SHOULD only be propagated if explicitly enabled. If not enabled, the received-by host of any host behind the firewall SHOULD be replaced by an appropriate pseudonym for that host.
For organizations that have strong privacy requirements
for hiding internal structures, a proxy MAY combine an ordered
subsequence of Via header field
entries with identical received-protocol values into a single
such entry. For example,
Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
could be collapsed to
Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
Applications SHOULD NOT combine multiple entries unless they are all under the same organizational control and the hosts have already been replaced by pseudonyms. Applications MUST NOT combine entries which have different received-protocol values.
Note: TheViaheader field replaces theForwardedheader field which was present in earlier drafts of this specification.
The From request-header
field, if given, SHOULD contain an Internet e-mail address for
the human user who controls the requesting user agent. The address
SHOULD be machine-usable, as defined by mailbox
in RFC 822 [9] (as updated by RFC 1123
[8]):
From = "From" ":" mailbox
An example is:
From: webmaster@w3.org
This header field MAY be used for logging purposes
and as a means for identifying the source of invalid or unwanted
requests. It SHOULD NOT be used as an insecure form of access
protection. The interpretation of this field is that the request
is being performed on behalf of the person given, who accepts
responsibility for the method
performed. In particular, robot agents SHOULD include this header
so that the person responsible for running the robot can be contacted
if problems occur on the receiving end.
The Internet e-mail address in this field MAY be separate from the Internet host which issued the request. For example, when a request is passed through a proxy the original issuer's address SHOULD be used.
Note: The client SHOULD not send the From
header field without the user's approval, as it may conflict with
the user's privacy interests or their site's security policy.
It is strongly recommended that the user be able to disable, enable,
and modify the value of this field at any time prior to a request.
The Host request-header
field specifies the Internet host and port number of the resource
being requested, as obtained from the original URL given by
the user or referring resource (generally an HTTP URL, as described
in Section 3.2.2). The Host
field value MUST represent the network location of the origin
server or gateway given by the original URL. This allows the
origin server or gateway to differentiate between internally-ambiguous
URLs, such as the root "/" URL of a server for multiple
host names on a single IP address.
Host = "Host" ":" host [ ":" port ] ; see Section 3.2.2
A "host" without any trailing port information
implies the default port for the service requested (e.g., "80"
for an HTTP URL). For example, a request on the origin server
for </>
MUST include:
GET /pub/WWW/ HTTP/1.1
Host: www.w3.org
The Host header
field MUST be included in all HTTP/1.1 request messages on the
Internet (i.e., on any message corresponding to a request for
a URL which includes an Internet host address for the service
being requested). If the Host
field is not already present, an HTTP/1.1 proxy MUST add a Host
field to the request message prior to forwarding it on the Internet.
All Internet-based HTTP/1.1 servers MUST respond with a 400 status
code to any HTTP/1.1 request message which lacks a Host
header field.
The If-Modified-Since
request-header field is used with the GET
method to make it conditional: if the requested resource has not
been modified since the time specified in this field, a copy of
the resource will not be returned from the server; instead, a
304 (not modified) response will be returned without any Entity-Body.
If-Modified-Since = "If-Modified-Since" ":" HTTP-date
An example of the field is:
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A GET method with an If-Modified-Since
header and no Range header requests
that the identified resource be transferred only if it has been
modified since the date given by the If-Modified-Since
header. The algorithm for determining this includes the following
cases:
If-Modified-Since
date is invalid, the response is exactly the same as for a normal
GET. A date which is later than
the server's current time is invalid.
If-Modified-Since
date, the response is exactly the same as for a normal GET.
If-Modified-Since date,
the server MUST return a 304 (not modified) response.
The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead.
Note that theRangerequest-header field modifies the meaning ofIf-Modified-Since; see section 13.9 for full details.
Note that If-Modified-Since
is ignored for varying resources.
Note that If-Modified-Since
times are interpreted by the server, whose clock may not be synchronized
with the client.
Note that if a client uses an arbitrary date in theIf-Modified-Sinceheader instead of a date taken from theLast-Modifiedheader for the same request, the client should be aware of the fact that this date is interpreted in the server's understanding of time. The client should consider unsynchronized clocks and rounding problems due to the different representations of time between the client and server. This includes the possibility of race conditions if the document has changed between the time it was first request and theIf-Modified-Sincedate of a subsequent request, and the possibility of clock-skew-related problems if theIf-Modified-Datedate is derived from the client's clock without correction to the server's clock. Corrections for different time bases between client and server are at best approximate due to network latency.
The Last-Modified
entity-header field indicates the date and time at which the sender
believes the resource was last modified. The exact semantics of
this field are defined in terms of how the recipient SHOULD interpret
it: if the recipient has a copy of this resource which is older
than the date given by the Last-Modified
field, that copy SHOULD be considered stale.
Last-Modified = "Last-Modified" ":" HTTP-date
An example of its use is
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
The exact meaning of this header field depends on the implementation of the sender and the nature of the original resource. For files, it may be just the file system last-modified time. For entities with dynamically included parts, it may be the most recent of the set of last-modify times for its component parts. For database gateways, it may be the last-update time stamp of the record. For virtual objects, it may be the last time the internal state changed.
An origin server MUST not send a Last-Modified
date which is later than the server's time of message origination.
In such cases, where the resource's last modification would indicate
some time in the future, the server MUST replace that date with
the message origination date.
An origin server should obtain the Last-Modified
value of the entity as close as possible to the time that it generates
the Date value of its response.
This allows a recipient to make an accurate assessment of the
entity's modification time, especially if the entity changes near
the time that the response is generated.
The Location response-header
field is used to redirect the recipient to a location other than
the Request-URI for completion
of the request or identification of a new resource. For 201 responses,
the Location is that of the new
resource which was created by the request. For 3xx responses,
the location SHOULD indicate the server's preferred URL for automatic
redirection to the resource. The field value consists of a single
absolute URL.
Location = "Location" ":" absoluteURI
An example is
Location: /People.html
Note: TheContent-Locationheader field (Section 10.16) differs fromLocationin that the former identifies the original location of the entity enclosed in the request. It is therefore possible for a response to contain header fields for bothLocationandContent-Location.
The Pragma general-header field is used to include implementation-specific directives that may apply to any recipient along the request/response chain. All pragma directives specify optional behavior from the viewpoint of the protocol; however, some systems MAY require that behavior be consistent with the directives.
Pragma = "Pragma" ":" 1#pragma-directive
pragma-directive = "no-cache" | extension-pragma
extension-pragma = token [ "=" word ]
When the "no-cache"
directive is present in a request message, an application SHOULD
forward the request toward the origin server even if it has a
cached copy of what is being requested. This pragma directive
has the same semantics as the "no-cache" cache-directive
(see Section 10.8) and is defined
here for backwards compatibility with HTTP/1.0. Clients SHOULD
include both header fields when a "no-cache" request
is sent to a server not known to be HTTP/1.1 compliant.
Pragma directives MUST be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives may be applicable to all recipients along the request/response chain. It is not possible to specify a pragma for a specific recipient; however, any pragma directive not relevant to a recipient SHOULD be ignored by that recipient.
HTTP/1.1 clients SHOULD NOT send the Pragma
request header. HTTP/1.1 caches SHOULD treat "Pragma:
no-cache" as if the client had sent "Cache-control:
no-cache". No new Pragma
directives will be defined in HTTP.
The Proxy-Authenticate
response-header field MUST be included as part of a 407 (proxy
authentication required) response. The field value consists of
a challenge that indicates the authentication scheme and parameters
applicable to the proxy for this Request-URI.
Proxy-Authentication = "Proxy-Authentication" ":" challenge
The HTTP access authentication process is described
in Section 11. Unlike WWW-Authenticate, the
Proxy-Authenticate header field
applies only to the current connection and MUST not be passed
on to downstream clients.
The Proxy-Authorization
request-header field allows the client to identify itself (or
its user) to a proxy which requires authentication. The Proxy-Authorization
field value consists of credentials containing the authentication
information of the user agent for the proxy and/or realm of the
resource being requested.
Proxy-Authorization = "Proxy-Authorization" ":" credentials
The HTTP access authentication process is described
in Section 11. Unlike Authorization, the Proxy-Authorization
applies only to the current connection and MUST not be passed
on to upstream servers. If a request is authenticated and a realm
specified, the same credentials SHOULD be valid for all other
requests within this realm.
The Public response-header
field lists the set of non-standard methods supported by the server.
The purpose of this field is strictly to inform the recipient
of the capabilities of the server regarding unusual methods. The
methods listed may or may not be applicable to the Request-URI;
the Allow header field (Section 10.5) SHOULD
be used to indicate methods allowed for a particular URI. This
does not prevent a client from trying other methods. The field
value SHOULD not include the methods predefined for HTTP/1.1 in
Section 5.1.1.
Public = "Public" ":" 1#method
Example of use:
Public: OPTIONS, MGET, MHEAD
This header field applies only to the server directly
connected to the client (i.e., the nearest neighbor in a chain
of connections). If the response passes through a proxy, the proxy
MUST either remove the Public
header field or replace it with one applicable to its own capabilities.
HTTP retrieval requests using conditional or unconditional GET methods may request one or more sub-ranges of the entity, instead of the entire entity. This is done using the Range request header:
Range = "Range" ":" ranges-specifier
A server MAY ignore the Range header. However, HTTP/1.1 origin servers and intermediate caches SHOULD support byte ranges whenever possible, since this supports efficient recovery from partially failed transfers, and it supports efficient partial retrieval of large entities.
I the server supports the Range
header and the specified range or ranges are appropriate for the
entity:
Range
header in an unconditional GET modifies what is returned if the
GET is otherwise successful. In other words, the response carries
a status code of 206 (Partial content) instead of 200 (OK).
Range
header in a conditional GET (a request using one or both of If-Modified-Since
and If-Invalid, or one or both
of If-Unmodified-Since and If-Valid)
modifies what is returned if the GET is otherwise successful and
the condition is true. It does not affect the 304 (Not Modified)
response returned if the conditional is false.
In some cases, it may be more appropriate to use
the Range-If header (see section
10.104) instead of the Range
header.
The Referer(sic)
request-header field allows the client to specify, for the server's
benefit, the address (URI) of the resource from which the Request-URI
was obtained. This allows a server to generate lists of back-links
to resources for interest, logging, optimized caching, etc. It
also allows obsolete or mistyped links to be traced for maintenance.
The Referer field MUST not be
sent if the Request-URI was obtained
from a source that does not have its own URI, such as input from
the user keyboard.
Referer = "Referer" ":" ( absoluteURI | relativeURI )
Example:
Referer: http://www.w3.org/hypertext/DataSources/Overview.html
If a partial URI is given, it SHOULD be interpreted
relative to the Request-URI.
The URI MUST not include a fragment.
Note: Because the source of a link may be private information or may reveal an otherwise private information source, it is strongly recommended that the user be able to select whether or not theRefererfield is sent. For example, a browser client could have a toggle switch for browsing openly/anonymously, which would respectively enable/disable the sending ofRefererandFrominformation.
The Retry-After
response-header field can be used with a 503 (service unavailable)
response to indicate how long the service is expected to be unavailable
to the requesting client. The value of this field can be either
an HTTP-date or an integer number of seconds (in decimal) after
the time of the response.
Retry-After = "Retry-After" ":" ( HTTP-date | delta-seconds )
Two examples of its use are
Retry-After: Wed, 14 Dec 1994 18:22:54 GMT
Retry-After: 120
In the latter example, the delay is 2 minutes.
The Server response-header
field contains information about the software used by the origin
server to handle the request. The field can contain multiple product
tokens (Section 3.8) and comments identifying
the server and any significant subproducts. By convention, the
product tokens are listed in order of their significance for identifying
the application.
Server = "Server" ":" 1*( product | comment )
Example:
Server: CERN/3.0 libwww/2.17
If the response is being forwarded through a proxy, the proxy application MUST not add its data to the product list. Instead, it SHOULD include a Via field (as described in Section 10.20).
Note: Revealing the specific software version of the server may allow the server machine to become more vulnerable to attacks against software that is known to contain security holes. Server implementers are encouraged to make this field a configurable option.
The Title entity-header
field indicates the title of the entity
Title = "Title" ":" *TEXT
An example of the field is
Title: Hypertext Transfer Protocol -- HTTP/1.1
This field is isomorphic with the <TITLE> element in HTML [5].
The Transfer-Encoding
general-header field indicates what (if any) type of transformation
has been applied to the message body in order to safely transfer
it between the sender and the recipient. This differs from the
Content-Encoding in that the
transfer coding is a property of the message, not of the original
resource.
Transfer-Encoding = "Transfer-Encoding" ":" 1#transfer-coding
Transfer codings are defined in Section 3.6. An example is:
Transfer-Encoding: chunked
Many older HTTP/1.0 applications do not understand
the Transfer-Encoding header.
The Upgrade general-header
allows the client to specify what additional communication protocols
it supports and would like to use if the server finds it appropriate
to switch protocols. The server MUST use the Upgrade
header field within a 101 (switching protocols) response to indicate
which protocol(s) are being switched.
Upgrade = "Upgrade" ":" 1#product
For example,
Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
The Upgrade header
field is intended to provide a simple mechanism for transition
from HTTP/1.1 to some other, incompatible protocol. It does so
by allowing the client to advertise its desire to use another
protocol, such as a later version of HTTP with a higher major
version number, even though the current request has been made
using HTTP/1.1. This eases the difficult transition between
incompatible protocols by allowing the client to initiate a request
in the more commonly supported protocol while indicating to the
server that it would like to use a "better" protocol
if available (where "better" is determined by the server,
possibly according to the nature of the method and/or resource
being requested).
The Upgrade header
field only applies to switching application-layer protocols upon
the existing transport-layer connection. Upgrade
cannot be used to insist on a protocol change; its acceptance
and use by the server is optional. The capabilities and nature
of the application-layer communication after the protocol change
is entirely dependent upon the new protocol chosen, although the
first action after changing the protocol MUST be a response to
the initial HTTP request containing the Upgrade
header field.
The Upgrade header
field only applies to the immediate connection. Therefore, the
"upgrade" keyword MUST be supplied within a Connection
header field (Section 10.8) whenever Upgrade
is present in an HTTP/1.1 message.
The Upgrade header
field cannot be used to indicate a switch to a protocol on a different
connection. For that purpose, it is more appropriate to use a
301, 302, 303, or 305 redirection response.
This specification only defines the protocol name "HTTP" for use by the family of Hypertext Transfer Protocols, as defined by the HTTP version rules of Section 3.1 and future updates to this specification. Any token can be used as a protocol name; however, it will only be useful if both the client and server associate the name with the same protocol.
The User-Agent request-header
field contains information about the user agent originating the
request. This is for statistical purposes, the tracing of protocol
violations, and automated recognition of user agents for the sake
of tailoring responses to avoid particular user agent limitations.
Although it is not required, user agents SHOULD include this field
with requests. The field can contain multiple product tokens (Section 3.8)
and comments identifying the agent and any subproducts which form
a significant part of the user agent. By convention, the product
tokens are listed in order of their significance for identifying
the application.
User-Agent = "User-Agent" ":" 1*( product | comment )
Example:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
The WWW-Authenticate
response-header field MUST be included in 401 (unauthorized) response
messages. The field value consists of at least one challenge
that indicates the authentication scheme(s) and parameters applicable
to the Request-URI.
WWW-Authenticate = "WWW-Authenticate" ":" 1#challenge
The HTTP access authentication process is described
in Section 11. User agents MUST take special
care in parsing the WWW-Authenticate
field value if it contains more than one challenge, or if more
than one WWW-Authenticate header
field is provided, since the contents of a challenge may itself
contain a comma-separated list of authentication parameters.
The Max-Forwards
general-header field may be used with the TRACE method (Section 8.12)
to limit the number of times that a proxy or gateway can forward
the request to the next inbound server. This can be useful when
the client is attempting to trace a request chain which appears
to be failing or looping in mid-chain.
Max-Forwards = "Max-Forwards" ":" 1*DIGIT
The Max-Forwards
value is a decimal integer indicating the remaining number of
times this request message may be forwarded.
Each proxy or gateway recipient of a TRACE request
containing a Max-Forwards header
field SHOULD check and update its value prior to forwarding the
request. If the received value is zero (0), the recipient SHOULD
NOT forward the request; instead, it SHOULD respond as the final
recipient with a 200 response containing the received request
message as the response entity body (as described in Section 8.12).
If the received Max-Forwards
value is greater than zero, then the forwarded message SHOULD
contain an updated Max-Forwards
field with a value decremented by one (1).
The Max-Forwards
header field SHOULD be ignored for all other methods defined by
this specification and for any extension methods for which it
is not explicitly referred to as part of that method definition.
Caches transmit age values using:
Age = "Age" ":" age-value
age-value = delta-seconds
Age values are non-negative decimal integers, representing time in seconds.
If a cache receives a value larger than the largest
positive integer it can represent, or if any of its age calculations
overflows, it MUST NOT transmit an Age
header. Otherwise, HTTP/1.1 caches MUST send an Age
header in every response. Caches SHOULD use a representation
with at least 31 bits of range.
The CVal
header is used to transmit opaque cache validators in HTTP/1.1
responses.
CVal = "CVal" ":" cval-info
cval-info = opaque-validator [ ";" variant-id ]
Examples:
CVal: "xyzzy"
CVal: "xyzzy"/W
CVal: "xyzzy";3
CVal: "xyzzy"/W;3
CVal: ""
Note that the variant-id is not part of the opaque
validator. The CVal field is
used to transmit a variant-id simply as a matter of compact representation
of responses.
TBS: does the protocol allow the combination of a null validator and a variant-ID?
The If-Invalid request-header
field is used with a method to make it conditional. A client that
has a cache entry for the relevant entity supplies the associated
validator using the If-Invalid
header; if this validator matches the server's current validator
for the entity, the server SHOULD return a 304 (Not modified)
response without any Entity-Body.
If the validators do not match, the server should
treat the request as if the If-Invalid
header was not present.
See section 13.3.3 for rules on how to determine if two validators match.
If the If-Invalid
header is used to make a conditional request on varying resource,
it may be used to pass a set of validators. This is done using
the variant-set mechanism if the client has variant IDs for the
corresponding cache entries (see sections 13.8.3 and 3.16), or
the validator-set mechanism if the client has no variant IDs (see
sections 13.8.4 and 3.15).
If-Invalid = "If-Invalid" ":" if-invalid-rhs
if-invalid-rhs = variant-set | validator-set
Examples of single-entity form:
If-Invalid: "xyzzy"
If-Invalid: "xyzzy"/W
Examples of multiple-entity form:
If-Invalid: "xyzzy";4
If-Invalid: "xyzzy";3, "r2d2xxxx";5, "c3piozzzz";7
If-Invalid: "xyzzy"/W;3, "r2d2xxxx"/W;5, "c3piozzzz"/W;7
If-Invalid: "xyzzy", "r2d2xxxx", "c3piozzzz"
If the request would, without the If-Invalid
header, result in anything other than a 2xx status, then the If-Invalid
header is ignored.
The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead.
The If-Valid request-header
field is used with a method to make it conditional. A client that
has a cache entry for the relevant entity supplies the associated
validator using the If-Valid
header; if this validator matches the server's current validator
for the entity, the server SHOULD perform the requested operation
as if the If-Valid header were
not present.
If the validators do not match, the server MUST NOT
perform the requested operation, and MUST return a 412 (Precondition
failed) response with no Entity-Body.
This behavior is most useful when the client wants to prevent
an updating method, such as PUT or POST, from modifying a resource
whose value has changed since the client last checked it.
When the If-Valid
header is used, the server should use the strong comparison function
(see section 3.13) to compare validators.
If the If-Valid
header is used to make a conditional request on varying resource,
it may be used to pass a set of validators. This is done using
the variant-set mechanism if the client has variant IDs for the
corresponding cache entries (see sections 13.8.3 and 3.16), or
the validator-set mechanism if the client has no variant IDs (see
sections 13.8.4 and 3.15).
If-Valid = "If-Valid" ":" if-valid-rhs
if-valid-rhs = validator-set | variant-set
An updating request (e.g., a PUT or POST) on a multi-entity resource should include only one variant-set-item, the one associated with the particular variant whose value is being conditionally updated.
Examples of single-entity form:
- If-Valid: "xyzzy"
- If-Valid: "xyzzy"/W
Examples of multiple-entity form:
- If-Valid: "xyzzy";4
- If-Valid: "xyzzy";3, "r2d2xxxx";5, "c3piozzzz";7
- If-Valid: "xyzzy", "r2d2xxxx", "c3piozzzz"
- If-Valid: "xyzzy"/W;3, "r2d2xxxx"/W;5, "c3piozzzz"/W;7
If the request would, without the If-Valid
header, result in anything other than a 2xx status, then the If-Valid
header is ignored.
The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. It is also used, on updating requests, to prevent inadvertent modification of the wrong instance of a resource.
The If-Unmodified-Since
request-header field is used with a method to make it conditional.
If the requested resource has not been modified since the time
specified in this field, the server should perform the requested
operation as if the If-Unmodified-Since
header were not present.
If the requested resource has been modified since
the specified time, the server MUST NOT perform the requested
operation, and MUST return a 412 (Precondition failed) response
with no Entity-Body.
If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
An example of the field is:
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
If the request normally (i.e., without the If-Unmodified-Since
header) would result in anything other than a 2xx status, the
If-Unmodified-Since header should
be ignored.
If the specified date is invalid, the header is ignored.
Warning headers are sent with responses using:
Warning = "Warning" ":" warn-code SP warn-agent SP warn-text
[SP language-tag [SP charset]]
warn-code = 2DIGIT
warn-agent = ( host [ ":" port ] ) | pseudonym
; the name or pseudonym of the server adding
; the Warning header, for use in debugging
warn-text = quoted-string
A response may carry more than one Warning
header.
The warn-text should be in a natural language and
character set that is most likely to be intelligible to the human
user receiving the response. This decision may be based on any
available knowledge, such as the location of the cache or user,
the Accept-Language field in
a request, the Content-Language
field in a response, etc. The default language is English and
the default character set is ISO-8599-1.
Any server or cache may add Warning
headers to a response. New Warning
headers should be added after any existing Warning
headers. A cache MUST NOT delete any Warning
header that it received with a response. However, if a cache successfully
validates a cache entry, it SHOULD remove any Warning
headers previously attached to that entry. It MUST then add any
Warning headers received in the
validating response. In other words, Warning
headers are those that would be attached to the most recent relevant
response.
This needs clarification. Someplace else in the specification, we need to make a clear distinction between headers that are stored with a cache entry and those that aren't, and we have to define carefully what headers are simply deleted when a cache entry is updated. Section 13.7.3 already talks about combining headers, but doesn't provide a way to remove, say, a "Response is stale" Warning after a fresh response is received.
When multiple Warning
headers are attached to a response, the user agent SHOULD display
as many of them as possible, in the order that they appear in
the response. If it is not possible to display all of the warnings,
the user agent should follow these heuristics:
This is a list of the currently-defined warn-codes, each with a recommended warn-text in English, and a description of its meaning.
TBS XXX anything else?
The Vary response-header
field is used by an origin server to signal that the resource
identified by the current request is a varying resource. A varying
resource has multiple entities associated with it, all of which
are representations of the content of the resource. If a GET
or HEAD request on a varying resource is received, the origin
server will select one of the associated entities as the entity
best matching the request. Selection of this entity is based
on the contents of particular header fields in the request message,
or on other information pertaining to the request, like the network
address of the sending client.
If a resource is varying, this has an important effect
on cache management, particularly for caching proxies which service
a diverse set of user agents. All 200 (OK) responses from varying
resources MUST contain at least one Vary
header or Alternates header (Section
10.53) to signal variance.
If no Vary headers
and no Alternates headers are
present in a 200 (OK) response, then caches may assume, as long
as the response is fresh, that the resource in question is not
varying, and has only one associated entity. Note however that
this entity can still change through time, as possibly indicated
by a Cache-Control response header
(section 10.cc).
After selection of the entity best matching the current
request, the origin server will usually generate a 200 (OK) response,
but it can also generate other responses like 206 (Partial Content)
or 304 (Not modified) if headers which modify the semantics of
the request, like Range (Section
10.ran) or If-Valid (Section
10.ifva), are present. An origin server need not be capable of
selecting an entity for every possible incoming request on a varying
resource; it can choose to generate a 3xx (redirection) or 4xx
(client error) type response for some requests.
In a request message on a varying resource, the selecting
request headers are those request headers whose contents were
used by the origin server to select the entity best matching the
request. The Vary header field
specifies the selecting request headers and any other selection
parameters that were used by the origin server.
Vary = "Vary" ":" 1#selection-parameter
selection-parameter = request-header-name
| "{accept-headers}"
| "{other}"
| "{" extension-parameter "}"
request-header-name = field-name
extension-parameter = token
The presence of a request-header-name signals that the request-header field with this name is selecting. Note that the name need not belong to a request-header field defined in this specification, and that header names are case-insensitive. The presence of the "{accept-headers}" parameter signals that all request headers whose names start with "accept" are selecting.
The inclusion of the "{other}" parameter
in a Vary field signals that
parameters other than the contents of request headers, for example
the network address of the sending party, play a role in the selection
of the response.
Note: This specification allows the origin server to express that other parameters were used, but does not allow the origin server to specify the exact nature of these parameters. This is left to future extensions.
If an extension-parameter unknown to the cache is
present in a Vary header, the
cache MUST treat it as the "{other}" parameter. If multiple
Vary and Alternates
header fields are present in a response, these MUST be combined
to give all selecting parameters.
The field name "Host"
MUST never be included into a Vary
header; clients MUST ignore it if it is present. The names of
fields which change the semantics of a GET request, like "Range"
and "If-Valid" MUST
also never be included, and MUST be ignored when present.
Servers which use access authentication are not obliged
to send "Vary: Authorization"
headers in responses. It MUST be assumed that requests on authenticated
resources can always produce different responses for different
users. Note that servers can signal the absence of authentication
by including a "Cache-Control: public"
header in the response.
A cache MAY store and refresh 200 (OK) responses from a varying resource according to the rules in Section 13.7.2. The partial entities in 206 (Partial Content) responses from varying resources MAY also be used by the cache.
When getting a request on a varying resource, a cache can only return a cached 200 (OK) response to one of its clients in two particular cases.
First, if a cache gets a request on a varying resource
for which it has cached one or more responses with Vary
or Alternates headers, it can
relay that request towards the origin server, adding an If-Invalid
header listing the cval-info
values in the CVal headers (Section
10.47) of the cached responses. If it then gets back a 304 (Not
Modified) response with the cval-info
of a cached 200 (OK) response in its CVal
header, it can return this cached 200 (OK) response to its client,
after merging in any of the 304 response headers as specified
in Section 13.7.2.
Second, if a cache gets a request on a varying resource,
it can return to its client a cached, fresh 200 (OK) response
which has Vary or Alternates
headers, provided that
Vary and
Alternates headers of this fresh
response specify that only request header fields are selecting
parameters,
cval-info
value in its CVal header as the
cached, fresh 200 (OK) response.
Two sequences of selecting request header fields match if and only if the first sequence can be transformed into the second sequence by only adding or removing whitespace at places in fields where this is allowed according to the syntax rules in this specification.
If a cached 200 (OK) response MAY be returned to
a request on a varying resource which includes a Range
request header, then a cache MAY also use this 200 (OK) response
to construct and return a 206 (Partial Content) response with
the requested range.
Note: Implementation of support for the second case above is mainly interesting in user agent caches, as a user agent cache will generally have an easy way of determining whether the sequence of request header fields of the current request equals the sequence sent in an earlier request on the same resource. Proxy caches supporting the second case would have to record diverse sequences of request header fields previously relayed; the implementation effort associated with this may not be balanced by a sufficient payoff in traffic savings. A planned specification of a content negotiation mechanism will define additional cases in which proxy caches can return a cached 200 (OK) response without contacting the origin server. The implementation effort associated with support for these additional cases is expected to have a much better cost/benefit ratio.
The Alternates response-header
field is used by origin servers to signal that the resource identified
by the current request has the capability to send different responses
depending on the accept headers in the request message. This
has an important effect on cache management, particularly for
caching proxies which service a diverse set of user agents. This
effect is covered in Section 10.v.
Alternates = "Alternates" ":" opaque-field
opaque-field = field-value
The Alternates header
is included into HTTP/1.1 to make HTTP/1.1 caches compatible with
a planned content negotiation mechanism. HTTP/1.1 allows a future
content negotiation standard to define the format of the Alternates
header field-value, as long as the defined format satisfies the
general rules in Section 4.2.
To ensure compatibility with future experimental
or standardized software, caching HTTP/1.1 clients MUST treat
all Alternates headers in a response
as synonymous to the following Vary
header:
Vary: {accept-headers}
and follow the caching rules associated with the
presence of this Vary header,
as covered in Section 10.v. HTTP/1.1 allows origin servers to
send Alternates headers under
experimental conditions.
In some cases, a client may want to know if the server accepts range requests using a certain range unit. The server may indicate its acceptance of range requests for a resource by providing this header in a response for that resource:
Accept-Ranges = "Accept-Ranges" ":" acceptable-ranges
acceptable-ranges = 1#range-unit | "none"
Origin servers that accept byte-range requests MAY send
Accept-Ranges: bytes
but are not required to do so. Clients MAY generate byte-range requests without having received this header for the specific resource involved, but the server MAY ignore such requests.
Should this say that the server SHOULD send "Accept-Ranges: bytes", or is MAY good enough
Origin servers that do not accept any kind of range request for a specific resource MAY send
Accept-Ranges: none
to advise the client not to attempt a range request.
We're still not quite sure why this header is in the protocol. We gather that Netscape uses it for something, but nobody from Netscape has even tried to explain to me whether it is necessary for anything. The only thing we can think of is that a client would have to know in advance if a server accepted partial-content PUTs (i.e., PUT+Content-Range), but we don't see any indication that this is what Netscape wants.
If a client has a partial copy of an entity in its
cache, and wished to have an up-to-date copy of the entire entity
in its cache, it could use Range
request header with a conditional GET (using either of both of
If-Unmodified-Since and If-Valid.)
However, if the condition fails because the entity has been modified,
the client would then have to make a second request to obtain
the entire current entity body.
The Range-If header
allows a client to ``short-circuit'' the second request. Informally,
its meaning is ``if the entity is unchanged, send me the part(s)
that I am missing; otherwise, send me the entire new entity.''
Range-If: if-valid-rhs
The Range-If header
should only be used together with a Range
header,and must be ignored if the request does not include a Range
header, or if the server does not support the sub-range operation.
If the validator given in the Range-If
header matches the current validator for the entity, then the
server should provide the specified sub-range of the entity using
a 206 (Partial content) response. If the validator does not match,
then the server should return the entire entity using a 200 (OK)
response.
This description may need slight modification to deal with(1) the use of a last-modified date as a validator (but this |can perhaps be hidden in the definition of if-valid-rhs), and|(2) its application to multi-entity resources.
HTTP provides a simple challenge-response authentication mechanism which MAY be used by a server to challenge a client request and by a client to provide authentication information. It uses an extensible, case-insensitive token to identify the authentication scheme, followed by a comma-separated list of attribute-value pairs which carry the parameters necessary for achieving authentication via that scheme.
auth-scheme = token
auth-param = token "=" quoted-string
The 401 (unauthorized) response message is used by
an origin server to challenge the authorization of a user agent.
This response MUST include a WWW-Authenticate
header field containing at least one challenge
applicable to the requested resource.
challenge = auth-scheme 1*SP realm *( "," auth-param )
realm = "realm" "=" realm-value
realm-value = quoted-string
The realm attribute (case-insensitive) is required for all authentication schemes which issue a challenge. The realm value (case-sensitive), in combination with the canonical root URL of the server being accessed, defines the protection space. These realms allow the protected resources on a server to be partitioned into a set of protection spaces, each with its own authentication scheme and/or authorization database. The realm value is a string, generally assigned by the origin server, which may have additional semantics specific to the authentication scheme.
A user agent that wishes to authenticate itself with
a server--usually, but not necessarily, after receiving a 401
or 411 response--MAY do so by including an Authorization
header field with the request. The Authorization
field value consists of credentials
containing the authentication information of the user agent for
the realm of the resource being requested.
credentials = basic-credentials
| auth-scheme *("," auth-param )
The domain over which credentials can be automatically applied by a user agent is determined by the protection space. If a prior request has been authorized, the same credentials MAY be reused for all other requests within that protection space for a period of time determined by the authentication scheme, parameters, and/or user preference. Unless otherwise defined by the authentication scheme, a single protection space cannot extend outside the scope of its server.
If the server does not wish to accept the credentials
sent with a request, it SHOULD return a 401 (unauthorized) response.
The response MUST include a WWW-Authenticate
header field containing the (possibly new) challenge
applicable to the requested resource and an entity explaining
the refusal.
The HTTP protocol does not restrict applications to this simple challenge-response mechanism for access authentication. Additional mechanisms MAY be used, such as encryption at the transport level or via message encapsulation, and with additional header fields specifying authentication information. However, these additional mechanisms are not defined by this specification.
Proxies MUST be completely transparent regarding
user agent authentication. That is, they MUST forward the WWW-Authenticate
and Authorization headers untouched,
and MUST not cache the response to a request containing Authorization.
HTTP/1.1 allows a client to pass authentication information
to and from a proxy via the Proxy-Authenticate
and Proxy-Authorization headers.
The "basic" authentication scheme is based
on the model that the user agent must authenticate itself with
a user-ID and a password for each realm. The realm value should
be considered an opaque string which can only be compared for
equality with other realms on that server. The server will service
the request only if it can validate the user-ID and password for
the protection space of the Request-URI.
There are no optional authentication parameters.
Upon receipt of an unauthorized request for a URI within the protection space, the server SHOULD respond with a challenge like the following:
WWW-Authenticate: Basic realm="WallyWorld"
where "WallyWorld" is the string assigned
by the server to identify the protection space of the Request-URI.
To receive authorization, the client sends the user-ID
and password, separated by a single colon (":") character,
within a base64 [7] encoded string in
the credentials.
basic-credentials = "Basic" SP basic-cookie
basic-cookie = <base64 [7] encoding of userid-password,
except not limited to 76 char/line>
userid-password = [ token ] ":" *TEXT
If the user agent wishes to send the user-ID "Aladdin" and password "open sesame", it would use the following header field:
Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
The basic authentication scheme is a non-secure method of filtering unauthorized access to resources on an HTTP server. It is based on the assumption that the connection between the client and the server can be regarded as a trusted carrier. As this is not generally true on an open network, the basic authentication scheme should be used accordingly. In spite of this, clients SHOULD implement the scheme in order to communicate with servers that use it.
The "digest" authentication scheme is [currently described in an expired Internet-Draft, and this description will have to be improved to reference a new draft or include the old one].
A varying resource has multiple entities associated with it, all of which are representations of the content of the resource. Content negotiation is the process of selecting the best representation when a GET or HEAD request is made on the varying resource. HTTP/1.1 has provisions for two kinds of content negotiation: opaque negotiation and transparent negotiation.
With opaque negotiation, the selection of the best
representation is done by an algorithm located at the origin server,
and unknown to the proxies and user agents involved. Selection
is based on the contents of particular header fields in the request
message, or on other information pertaining to the request, like
the network address of the sending client. A typical example
of opaque negotiation would be the selection of a text/html response
in a particular language based on the contents of the Accept-Language
request header field. A disadvantage of opaque negotiation is
that the request headers may not always contain enough information
to allow for selection. If the Accept
header
Accept: text/*: q=0.3, text/html, */*: q=0.5
is sent in a request on a varying resource which has a video/mpeg and a video/quicktime representation, the selection algorithm in the origin server will either have to make a default choice, or return an error response which allows the user to decide on further actions.
With transparent negotiation, the selection of the best representation is done by a distributed algorithm which can perform computation steps in the origin server, in proxies, or in the user agent. Transparent negotiation guarantees that, if the user agent supports the transparent negotiation algorithm and is correctly configured, the request will always correctly yield either the video/mpeg representation, the video/quicktime representation, or an error message indicating that the resource cannot be displayed by the user agent.
This specification defines all protocol facilities
for opaque negotiation, but does not define the distributed algorithm
for transparent negotiation. This specification only defines
the basic facilities (Vary, Alternates,
Accept) in the core protocol
allowing requests on transparently negotiated resources to be
correctly handled by HTTP/1.1 caches. All other information about
transparent content negotiation is found in a separate document[29].
If a varying resource is opaquely negotiated, successful
responses to requests on the resource will always include a Vary
header. If a varying resource is transparently negotiated, successful
responses to requests on the resource will always include an Alternates
header. If a successful response contains an Alternates
header, it will also always contain a Content-Location
header. A future specification may allow a combination of opaque
and transparent negotiation that would lead to the inclusion of
both a Vary header and an Alternates
header in a response.
.
The World Wide Web is a distributed system, and so its performance can be improved by the use of caches. These caches are typically placed at proxies and in the clients themselves. The HTTP/1.1 protocol includes a number of elements intended to make caching work as well as possible. Because these elements are inextricable from other aspects of the protocol, and because they interact with each other, it is useful to describe the basic caching design of HTTP separately from the detailed descriptions of methods, headers, response codes, etc.
Ideally, an HTTP/1.1 cache would be "semantically transparent." That is, use of the cache would not affect either the clients or the servers in any way except to improve performance. When a client makes a request via a semantically transparent cache, it receives exactly the same entity headers and entity body it would have received if it had made the same request to the origin server, at the same time.
In the real world, requirements for performance, availability, and disconnected operation require us to relax the goal of semantic transparency in many cases. The HTTP/1.1 protocol allows origin servers, caches, and clients to explicitly reduce transparency when necessary. However, because non-transparent operation may confuse non-expert users, and may be incompatible with certain server applications (such as those for ordering merchandise), the protocol requires that transparency may not be relaxed
Therefore, the HTTP/1.1 protocol provides these important elements:
A basic principle is that it must be possible for the clients to detect any potential breakdown of semantic transparency.
Caching would be useless if it did not significantly improve performance in many cases. The goal of caching in HTTP/1.1 is to eliminate the need to send requests in many cases, and to eliminate the need to send full responses in many other cases. The former reduces the number of network round-trips required for many operations; we use an "expiration" mechanism for this purpose (see section 13.2). The latter reduces network bandwidth requirements; we use a "validation" mechanism for this purpose (see section 13.3).
The server, cache, or client implementer may be faced with design decisions not explicitly discussed in this specification. If decision may affect semantic transparency, the implementer ought to err on the side of maintaining transparency unless a careful and complete analysis shows significant benefits in breaking transparency.
A note on terminology: we say that a resource is "cachable" if a cache is allowed to store a copy of this resource, when it arrives in a response message, and then later use that copy to respond to a subsequent request. Even if a resource is cachable, there may be additional constraints on when and whether a cache can use a cached copy of it.
In order to describe the associated mechanisms, we introduce several terms for describing responses returned by a cache in response to a client's request:
HTTP caching works best when caches can entirely avoid making requests to the origin server. The primary mechanism for avoiding requests is for an origin server to provide an explicit expiration time in the future, indicating that a response may be used to satisfy subsequent requests. In other words, a cache can return a fresh response without first contacting the server.
Our expectation is that servers will assign future explicit expiration times to responses in the belief that the entity is not likely to change, in a semantically significant way, before the expiration time is reached. This normally preserves semantic transparency, as long as the server's expiration times are carefully chosen.
If an origin server wishes to force a semantically transparent cache to validate every request, it may assign an explicit expiration time in the past. This means that the response is always stale, and so the cache SHOULD validate it before using it for subsequent requests. (Note that a firsthand response MUST always be returned to the requesting client, independent of its expiration time, unless the connection to the client is lost.)
If an origin server wishes to force any HTTP/1.1 cache, no matter
how it is configured, to validate every request, it should use
the "must-revalidate" Cache-Control
directive (see section 10.8).
Servers specify explicit expiration times using either the Expires
header, or the max-age directive of the Cache-Control
header.
An expiration time cannot be used to force a user agent to refresh its display or reload a resource; its semantics apply only to caching mechanisms, and such mechanisms need only check a resource's expiration status when a new request for that resource is initiated.
User agents often have history mechanisms, such as "Back" buttons and history lists, which can be used to redisplay an entity retrieved earlier in a session. By default, an expiration time does not apply to history mechanisms. If the entity is still in storage, a history mechanism should display it even if the entity has expired, unless the user has specifically configured the agent to refresh expired history documents.
Since origin servers do not always provide explicit expiration
times, HTTP caches typically assign heuristic expiration times,
employing algorithms that use other header values (such as the
Last-Modified time) to estimate a plausible expiration
time. The HTTP/1.1 specification does not provide specific algorithms,
but does impose worst-case constraints on their results. Since
heuristic expiration times may compromise semantic transparency,
they should be used cautiously, and we encourage origin servers
to provide explicit expiration times as much as possible.
While the origin server (and to a lesser extent, intermediate
caches) are the primary source of expiration information, in some
cases the client may need to control a cache's decision about
whether to return a cached response without validating it. Clients
do this using several directives of the Cache-Control
header.
A client's request may specify the maximum age it is willing to accept for an unvalidated response; specifying a value of zero forces the cache(s) to revalidate all responses. A client may also specify the minimum time remaining before a response expires. Both of these options increase constraints on the behavior of caches, and so cannot decrease semantic transparency.
A client may also specify that it will accept stale responses, up to some maximum amount of staleness. This loosens the constraints on the caches, and so may violate semantic transparency, but may be necessary to support disconnected operation, or high availability in the face of poor connectivity.
In some cases, the operator of a cache may choose to configure
it to return stale responses even when not requested by clients.
This decision not be made lightly, but may be necessary for reasons
of availability or performance, especially when the cache is poorly
connected to the origin server. Whenever a cache returns a stale
response, it MUST mark it as such (using a Warning
header). This allows the client software to alert the user that
there may be a potential problem.
It also allows the user to take steps to obtain a firsthand or fresh response, if the user so desires. For this reason, a cache MUST NOT return a stale response if the client explicitly requests a first-hand or fresh one, unless it is impossible to comply.
In order to know if a cached entry is fresh, a cache needs to know if its age exceeds its freshness lifetime. We discuss how to calculate the latter in section 13.2.7; this section describes how to calculate the age of a response or cache entry.
In this discussion, we use the term "now" to mean "the current value of the clock at the host performing the calculation." All HTTP implementations, but especially origin servers and caches, should use NTP [RFC1305] or some similar protocol to synchronize their clocks to a globally accurate time standard.
Also note that HTTP/1.1 requires origin servers to send a Date
header with every response, giving the time at which the response
was generated. We use the term "date_value" to denote
a representation of the value of the Date header,
in a form appropriate for arithmetic operations.
HTTP/1.1 uses the "Age" response header
to help convey age information between caches. The Age
header value is the sender's estimate of the amount of time since
the response was generated at the origin server. In the case of
a cached response that has been revalidated with the origin server,
the Age value is based on the time of revalidation,
not of the original response.
In essence, the Age value is the sum of the time
that the response has been resident in each of the caches along
the path from the origin server, plus the amount of time it has
been in transit along network paths.
We use the term "age_value" to denote a representation
of the value of the Age header, in a form appropriate
for arithmetic operations.
An response's age can be calculated in two entirely independent ways:
Given that we have two independent ways to compute the age of a response when it is received, we can combine these as
corrected_received_age = max(now - date_value, age_value)
and as long as we have either nearly synchronized clocks or all-HTTP/1.1 paths, one gets a reliable (conservative) result.
Note that this correction is applied at each HTTP/1.1 cache along the path, so that if there is an HTTP/1.0 cache in the path, the correct received age is computed as long as the receiving cache's clock is nearly in sync. We don't need end-to-end clock synchronization (although it is good to have), and there is no explicit clock synchronization step.
Because of network-imposed delays, some significant interval may pass from the time that a server generates a response, and the time it is received at the next outbound cache or client. If uncorrected, this delay could result in improperly low ages.
Because the request that resulted in the returned Age
value must have been initiated prior to that Age
value's generation, we can correct for delays imposed by the network
by recording the time at which the request was initiated. Then,
when an Age value is received, it MUST be interpreted
relative to the time the request was initiated, not the time that
the response was received. This algorithm results in conservative
behavior no matter how much delay is experienced. So, we compute:
corrected_initial_age = corrected_received_age + (now - request_time)
where "request_time" is the time (according to the local clock) when the request that elicited this response was sent.
Summary of age calculation algorithm, when a cache receives a response:
/*
* age_value
* is the value of Age: header received by the cache with
* this response.
* date_value
* is the value of the origin server's Date: header
* request_time
* is the (local) time when the cache made the request
* that resulted in this cached response
* response_time
* is the (local) time when the cache received the
* response
* now
* is the current (local) time
*/
apparent_age = max(0, now - date_value);
corrected_received_age = max(apparent_age, age_value);
response_delay = now - request_time;
corrected_initial_age = corrected_received_age + response_delay;
resident_time = now - response_time;
current_age = corrected_initial_age + resident_time;
When a cache sends a response, it must add to the corrected_initial_age
the amount of time that the response was resident locally. It
must then transmit this total age, using the Age
header, to the next recipient cache.
In order to decide whether a response is fresh or stale, we need to compare its freshness lifetime to its age. The age is calculated as described in section 13.2.6; this section describes how to calculate the freshness lifetime, and to determine if a response has expired.
We use the term "expires_value" to denote a representation
of the value of the Expires header, in a form appropriate
for arithmetic operations. We use the term "max_age_value"
to denote an appropriate representation of the number of seconds
carried by the max-age directive of the Cache-Control
header in a response (see section 10.8).
The max-age directive takes priority over Expires,
so if max-age is present in a response, the calculation is simply:
freshness_lifetime = max_age_value
Otherwise, if Expires is present in the response,
the calculation is:
freshness_lifetime = expires_value - date_value
Note that neither of these calculations is vulnerable to clock skew, since all of the information comes from the origin server.
If neither Expires nor Cache-Control
max-age appears in the response, and the response does not include
other restrictions on caching, the cache MAY compute a freshness
lifetime using a heuristic. This heuristic is subject to certain
limitations; the minimum value may be zero, and the maximum value
MUST be no more than 24 hours.
Also, if the response does have a Last-Modified time,
the heuristic expiration value SHOULD be no more than some fraction
of the interval since that time. A typical setting of this fraction
might be 10%.
The calculation to determine if a response has expired is quite simple:
response_is_fresh = (freshness_lifetime > current_age)
All expiration-related calculations must be done in Universal Time (GMT). The local time zone MUST not influence the calculation or comparison of an age or expiration time.
If an HTTP header incorrectly carries a date value with a time zone other than GMT, it must be converted into GMT using the most conservative possible conversion.
When a cache has a stale value that it would like to use as a response to a client's request, it first has to check with the origin server (or possibly an intermediate cache with a fresh response) to see if its cached value is still usable. We call this "validating" the cache entry. Since we do not want to have to pay the overhead of retransmitting the full response if the cached value is good, and we do not want to pay the overhead of an extra round trip if the cached value is invalid, the HTTP/1.1 protocol supports the use of conditional methods.
The key protocol features for supporting conditional methods are those concerned with "cache validators." When an origin server generates a full response, it attaches some sort of validator to it, which is kept with the cache entry. When a client (end-user or cache) makes a conditional request for a resource for which it has a cache entry, it includes the associated validator in the request.
The server then checks that validator against the current validator for the resource, and if they match, it responds with a special status code (usually, "304 Not Modified") and no entity body. Otherwise, it returns a full response (including entity body). Thus, we avoid transmitting the full response if the validator matches, and we avoid an extra round trip if it does not match.
Note: the comparison functions used to decide if validators match are defined in section 13.3.3.
In HTTP/1.1, a conditional request looks exactly the same as a normal request for the same resource, except that it carries a special header (which includes the validator) that implicitly turns the method (usually, GET) into a conditional.
The protocol includes both positive and negative senses of cache-validating conditions. That is, it is possible to request either that a method be performed if and only if the validators match, or if and only if the validators do not match.
Note: a response that lacks a cache validator may still be cached, and served from cache until it expires, unless this is explicitly prohibited by aCache-Controldirective. However, a cache cannot do a conditional retrieval if it does not have a cache validator for the entity, which means it will not be refreshable after it expires.
In HTTP/1.0, the only cache validator is the Last-Modified
time carried by a response. Clients validate entities using the
If-Modified-Since header. In simple terms, a cache
entry is considered to be valid if the actual resource has not
been modified since the original response was generated.
HTTP/1.1 introduces the possibility of using an "opaque"
validator, for situations where the Last-Modified
date is not appropriate. This may include server implementations
where it is not convenient to store modification dates, or where
the one-second resolution of HTTP date values is insufficient,
or where the origin server wishes to avoid certain paradoxes that
may arise from the use of modification dates.
An opaque validator is simply a string of octets whose internal structure is not known to clients or caches. Caches store opaque validators and return them when making conditional requests. Also, when a cache receives a conditional request for a resource for which it has a fresh cache entry, it may compare opaque validators using strict octet-equality. Otherwise, opaque validators have no semantic value to clients or caches.
To preserve compatibility with HTTP/1.0 clients and caches, and
because the Last-Modified date may be useful for
purposes other than cache validation, HTTP/1.1 servers SHOULD
send Last-Modified whenever feasible.
The headers used to convey opaque validators are described in sections 10.47, 10.48, 10.49, and 10.55.
Since both origin servers and caches will compare two validator values to decide if they represent the same or different values for the entire resource, one normally would expect that if the resource value (the entity body or any entity headers) changes in any way, then the associated validator would change as well. If this is true, then we call this validator a "strong validator."
However, there may be cases when a server prefers to change the validator only on semantically significant changes, and not when insignificant aspects of the resource change. A validator that does not always change when the resource changes is a "weak validator."
One can think of a strong validator as one that changes whenever the bits of an entity changes, while a weak value changes whenever the meaning of an entity changes. Alternatively, one can think of a strong validator as part of an identifier for a specific instance of an entity, while a weak validator is part of an identifier for a set of semantically equivalent instances of an entity.
Note: One example of a strong validator is an integer that is incremented in stable storage every time an entity is changed.
An entity's modification time, if represented with one-second resolution, could be a weak validator, since it is possible that the resource may be modified twice during a single second.
Opaque validators are normally "strong," but the protocol provides a mechanism to tag an opaque validator as "weak."
A "use" of a validator is either when a client generates a request and includes the validator in a validating header field, or when a server compares two validators.
Strong validators are usable in any context. Weak validators are only usable in contexts that do not depend on exact equality of an entity. For example, either kind is usable for a conditional GET of a full entity. However, only a strong validator is usable for a sub-range retrieval, since otherwise the client may end up with an internally inconsistent entity body.
The only function that the HTTP/1.1 protocol defines on validators is comparison. There are two validator comparison functions, depending on whether the comparison context allows the use of weak validators or not:
The weak comparison function should be used for simple (non-subrange) GET requests. The strong comparison function must be used in all other cases.
An opaque validator is strong unless it is explicitly tagged as weak. Section 3.13 gives the syntax for opaque validators.
A Last-Modified time, when used as a validator in
a request, is implicitly weak unless it is possible to deduce
that it is strong, using the following rules:
or
If-Modified-Since
or If-Unmodified-Since header, because the client
has a cache entry for the associated entity, and
Date value, which
gives the time when the origin server generated the original response,
and
Last-Modified time is at least
60 seconds before the Date value.
or
Date value, which
gives the time when the origin server generated the original response,
and
Last-Modified time is at least
60 seconds before the Date value.
This method relies on the fact that if two different responses
were generated by the origin server during the same second, but
both had the same Last-Modified time, then at least
one of those responses would have a Date value equal
to its Last-Modified time. The arbitrary 60-second
limit guards against the possibility that the Date
and Last-Modified values are generated from different
clocks, or at somewhat different times during the preparation
of the response. An implementation may use a value larger than
60 seconds, if it is believed that 60 seconds is too short.
If a client wishes to perform a sub-range retrieval on a value
for which it has only a Last-Modified time and no
opaque validator, it may do this only if the Last-Modified
time is strong in the sense described here.
A cache or origin server receiving a cache-conditional request, other than a full-body GET request, must use the strong comparison function to evaluate the condition.
This allows HTTP/1.1 caches and clients to safely perform sub-range retrievals on values that have been obtained from HTTP/1.0 servers.
We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types should be used, and for what purposes.
HTTP/1.1 origin servers:
Last-Modified value if it is feasible
to send one, unless the risk of a breakdown in semantic transparency
that could result from using this date in an If-Modified-Since
header would lead to serious problems.
In other words, the preferred behavior for an HTTP/1.1 origin
server is to send both a strong opaque validator and a Last-Modified
value.
In order to be legal, a strong opaque validator MUST change whenever the associated entity value changes in any way. A weak opaque validator SHOULD change whenever the associated entity value changes in a semantically significant way.
Note: in order to provide semantically transparent caching, an origin server should avoid reusing a specific strong opaque validator value for two different instances of an entity, or reusing a specific weak opaque validator value for two semantically different instances of an entity. Caches entries may persist for arbitrarily long periods, regardless of expiration times, so it may be inappropriate to expect that a cache will never again attempt to validate an entry using a validator that it obtained at some point in the past.
HTTP/1.1 clients:
If-Valid or If-Invalid).
Last-Modified value has been provided
by the origin server, SHOULD use that value in non-subrange cache-conditional
requests (using If-Modified-Since).
Last-Modified value has been provided
by an HTTP/1.0 origin server, MAY use that value in subrange cache-conditional
requests (using If-Unmodified-Since:). The user agent
should provide a way to disable this, in case of difficulty.
Last-Modified
value have been provided by the origin server, SHOULD use both
validators in cache-conditional requests. This allows both HTTP/1.0
and HTTP/1.1 caches to respond appropriately.
An HTTP/1.1 cache, upon receiving a request, MUST use the most
restrictive validator when deciding whether the client's cache
entry matches the cache's own cache entry. This is only an issue
when the request contains both an opaque validator and a last-modified-date
validator (If-Modified-Since or If-Unmodified-Since:).
A note on rationale: The general principle behind these rules is that HTTP/1.1 servers and clients should transmit as much non-redundant information as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative assumptions about the validators they receive.
HTTP/1.0 clients and caches will ignore opaque validators. Generally, last-modified values received or used by these systems will support transparent and efficient caching, and so HTTP/1.1 origin servers should provideLast-Modifiedvalues. In those rare cases where the use of aLast-Modifiedvalue as a validator by an HTTP/1.0 system could result in a serious problem, then HTTP/1.1 origin servers should not provide one.
TBS
The principle behind opaque validators is that only the service
author knows the semantics of a resource well enough to select
an appropriate cache validation mechanism, and the specification
of any validator comparison function more complex than byte-equality
would open up a can of worms. Thus, comparisons of any other headers
(except Last-Modified, for compatibility with HTTP/1.0)
are never used for purposes of validating a cache entry.
TBS: what if no validator present in response?
The basic cache mechanisms in HTTP/1.1 (server-specified
expiration times and validators) are implicit directives to caches.
In some cases, a server or client may need to provide explicit
directives to the HTTP caches. We use the Cache-Control
header for this purpose.
The Cache-Control
header allows a client or server to transmit a variety of directives
in either requests or responses. These directives typically override
the default caching algorithms. As a general rule, if there is
any apparent conflict between header values, the most restrictive
interpretation should be applied (that is, the one that is most
likely to preserve semantic transparency). However, in some cases,
Cache-Control directives are
explicitly specified as weakening semantic transparency (for example,
"max-stale" or "public").
The Cache-Control
directives are described in detail in section 10.7.
Whenever a cache returns a response that is not semantically
transparent, it must attach a warning to that effect, using a
Warning response header. This
warning allows clients and user agents to take appropriate action.
Warnings may be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error status code, distinguish these responses from true failures.
Warnings are always cachable, because they never weaken the transparency of a response. This means that warnings can be passed to HTTP/1.0 caches without danger; such caches will simply pass the warning along as a entity header in the response.
Warnings are assigned numbers between 0 and 99. This specification defines the code numbers and meanings of each warning, allowing a client or cache to take automated action in some (but not all) cases.
Warnings also carry a warning message text in any
appropriate natural language (perhaps based on the client's Accept
headers), and an optional indication of what language and character
set are used.
Multiple warning messages may be attached to a response (either by the origin server or by a cache), including multiple warnings with the same code number. For example, a server may provide the same warning with texts in both English and Basque.
When multiple warnings are attached to a response, it may not be practical or reasonable to display all of them to the user. This version of HTTP does not specify strict priority rules for deciding which warnings to display and in what order, but does suggest some heuristics.
The Warning header
and the currently defined warnings are described in section 10.106.
Many user agents make it possible for users to override
the basic caching mechanisms. For example, the user agent may
allow the user to specify that cached entities (even explicitly
stale ones) are never validated. Or the user agent might habitually
add "Cache-Control: max-stale=3600"
or "Cache-Control: reload"
to every request. We recognize that there may be situations which
require such overrides, although user agents SHOULD NOT default
to any behavior contrary to the HTTP/1.1 specification. That is,
the user should have to explicitly request either non-transparent
behavior, or behavior that results in abnormally ineffective caching.
If the user has overridden the basic caching mechanisms, the user agent should explicitly indicate to the user whenever this results in the display of information that might not meet the server's transparency requirements (in particular, if the displayed resource is known to be stale). Since the protocol normally allows the user agent to determine if responses are stale or not, this indication need only be displayed when this actually happens. The indication need not be a dialog box; it could be an icon (for example, a picture of a rotting fish) or some other visual indicator.
If the user has overridden the caching mechanisms in a way that would abnormally reduce the effectiveness of caches, the user agent should continually display an indication (for example, a picture of currency in flames) so that the user does not inadvertently consume excess resources or suffer from excessive latency.
The purpose of an HTTP cache is to store information received in response to requests, for use in responding to future requests. In many cases, a cache simply returns the appropriate parts of a response to the requester. However, if the cache holds a cache entry based on a previous response, it may have to combine parts of a new response with what is held in the cache entry.
For the purpose of defining the behavior of caches and non-caching proxies, we divide HTTP headers into two categories:
The following HTTP/1.1 headers are hop-by-hop headers:
Connection
Keep-Alive
Upgrade
Public
Proxy-Authenticate
Transfer-Encoding
All other headers defined by HTTP/1.1 are end-to-end headers.
Hop-by-hop headers introduced in future versions of HTTP MUST
be listed in a Connection header, as described in
section 10.9.
Some features of the HTTP/1.1 protocol, such as Digest Authentication
(see TBS), depend on the value of certain end-to-end headers.
A cache or non-caching proxy SHOULD NOT modify an
end-to-end header unless the definition of that header requires
or specifically allows that.
A cache or non-caching proxy MUST NOT modify any of the following fields in a request or response, nor may it add any of these fields if not already present:
Content-Type
Content-Encoding
Content-Length
Expires
Last-Modified
Content-Range
Content-Location
Warning: unnecessary modification of end-to-end headers may cause authentication failures if stronger authentication mechanisms are introduced in later versions of HTTP. Such authentication mechanisms may rely on the values of header fields not listed here.
When a cache makes a validating request to a server, and the server provides a 304 Not Modified response, the cache must construct a response to send to the requesting client. The cache uses the entity-body stored in the cache entry as the entity-body of this outgoing response. It uses the end-to-end headers from the incoming response, not from the cache entry. Unless it decides to remove the cache entry, it must also replace the end-to-end headers stored with the cache entry with those received in the incoming response.
In other words, the complete set of end-to-end headers received
in the incoming response overrides all end-to-end headers stored
with the cache entry. The cache may add Warning headers
(see section 10.106) to this set.
A cache MUST preserve the order of all headers as received in an incoming response.
These rule allows an origin server to completely control the response seen by the client of a cache when the cache revalidates an entry, and may be necessary for preserving semantic transparency or for certain kinds of security mechanisms or future extensions.
A response may transfer only a subrange of the bytes of an entity,
either because the request included one or more Range
specifications, or because a connection was broken prematurely.
After several such transfers, a cache may have received several
ranges of the same entity.
If a cache has a stored non-empty set of subranges for an entity, and an incoming response transfers another subrange, the cache MAY combine the new subrange with the existing set if both the following conditions are met:
If either requirement is not meant, the cache must use only the
most recent partial response (based on the Date values
transmitted with every response, and using the incoming response
if these values are equal or missing), and must discard the other
partial information.
HTTP/1.1's expiration model is that as soon as any variant of a URI becomes stale, all variants becomes stale as well. Thus, "freshness" applies to all the variants of URI, rather than any particular variant. Dates and expires etc. apply to any cached variant that a proxy might have with a URI and not just the one particular entity.
The HTTP content negotiation mechanism interacts with caching in several ways:
Origin servers may respond to requests for varying resources use
the Vary header (see section 10.vary for a full description)
to inform the cache which header fields of the request were used
to select the variant returned in the response. A cache can use
that response to reply to a subsequent request only if the two
requests not only specify the same URI, but also have the same
value for all headers specified in the Vary response-header.
The Vary header may also inform the cache that the
variant was selected using criteria not limited to the request
headers; in this case, the response MUST NOT be used in a reply
to a subsequent request except if the cache relays the new request
to the origin server in a conditional request, and the origin
server responds with 304 (Not Modified) and includes the same
variant-ID (see 13.8.3).
Origin servers may respond to requests for varying resources with
a status of 300 (Multiple choice), using the Alternates
header (see section 10.alternates) to inform the requesting client
that describes the set of possible choices, including specific
URIs for each variant.
Roy says this response also includes a Content-Location
header.
In this case, the client may choose one of the available
variants and make a subsequent request using the specific URI
for that variant. Since such an URI is bound to just one entity,
the origin server's response to this request includes neither
a Vary header nor an Alternates
header, and a cache may treat it as it would any non-varying resource.
If a cache receives an Alternates
header in a response from the origin server, it should act as
if the response carried a "Vary:{accept-headers}"
header. This means that the response may be returned in reply
to a subsequent request with Accept-*
headers identical to those in the current request.
Note that section 13.14.1 prevents caching of 300 (Multiple choices) responses unless this is explicitly allowed by anExpiresorCache-controlheader.
A cache stores copies of specific entity instances, not copies of varying resources per se. Therefore, the URI of a varying resource is not sufficient for use as a cache key. In certain interactions between a cache and an origin server, it is convenient to encode the cache key using a more compact representation than the full set of selecting request headers. Or, if the selection criteria are not known to the cache, it may be impossible to express the actual cache key to the cache. For these reasons, the HTTP protocol provides two different optional mechanisms to encode a cache key:
Variant-IDs are the preferred mechanism, since they generally allow more efficient management of caches.
If an origin server chooses to use the variant-ID mechanism, it
assigns a variant-ID (see section 3.14) to each distinct variant.
This assignment can only be done by the origin server. It then
returns the appropriate variant-ID with each response that applies
to a specific variant, using the CVal header (see
10.47).
If an origin server provides a variant-ID for any variant of a resource, it SHOULD provide a variant-ID for all variants of that resource.
When a cache receives a successful response with a variant-ID, it SHOULD use this information to replace any existing cache entries for the same variant of the corresponding URI. That is, it forms a cache key using the URI of the request and the variant-ID of the response. If this key matches the key of an existing cache entry, it SHOULD replace the existing entry with the new response (subject to all of the other rules on caching). See section 13.12 for more details on update.
When a cache performs a conditional request on a varying resource, and it has one or more cache entries for the resource that include variant-IDs, the cache MUST transmit the (cache-validator, variant-ID) tuples in the conditional request, using the variant-set mechanism (see section 3.16). This tells the server which variants are currently in the requester's cache.
The client MAY choose to transmit only a subset of the (cache-validator, variant-ID) tuples corresponding to its cache entries for this resource.
When a server receives a conditional request that includes a variant-set, and the server is able to reply with an appropriate variant (either because it is the origin server, or because it is an intermediate cache that can properly implement the variant selection algorithm), once it has selected the variant it should examine the elements of the supplied variant-set. If one of these matches the variant-ID of the selected variant, and if the cache validators match, the server SHOULD reply with a 304 (Not Modified) response, including the variant-ID of the selected variant. Otherwise, the server should reply as if the request were unconditional.
The server may optionally use the variant-set information in its selection algorithm. For example, if the selection algorithm yields several variants with equal preference, and one of these is already in the requester's cache, the server could select that variant and avoid an extra data transfer. This is a performance optimization; otherwise, the variant-selection mechanism is orthogonal to the variant-ID mechanism.
If the origin server prefers not to provide variant-IDs, it MAY at its option use the "selecting opaque validator" mechanism. A selecting opaque validator is an opaque validator whose value is unique across all variants of a resource.
If the origin server cannot generate opaque validators that are guaranteed to be unique across all variants of a varying resource, it MUST NOT send any opaque validators for that resource.
When a cache receives a successful response with an opaque validator and no variant-ID, it MAY either replace any cache entries for the resource with the new response, or it may keep multiple such entries. See section 13.12 for more details on update.
When a cache performs a conditional request on a varying resource, and it has one or more cache entries for the resource that include opaque validators, the cache SHOULD transmit the set of opaque validators in the conditional request, using the validator-set mechanism (see section 3.15). This tells the server which variants are currently in the requester's cache.
The client MAY chose to transmit only a subset of the opaque validators from its cache entries for this resource.
When a server receives a conditional request that includes a validator-set, and the server is able to reply with an appropriate variant (either because it is the origin server, or because it is an intermediate cache that can properly implement the variant selection algorithm), once it has selected the variant it should examine the elements of the supplied validator-set. If one of these matches the cache validator of the selected variant, the server SHOULD reply with a 304 (Not Modified) response, including that cache validator. Otherwise, the server should reply as if the request were unconditional.
For reasons of security and privacy, it is necessary to make a distinction between "shared" and "non-shared" caches. A non-shared cache is one that is accessible only to a single user. Accessibility in this case SHOULD be enforced by appropriate security mechanisms. All other caches are considered to be "shared." Other sections of this specification place certain constraints on the operation of shared caches in order to prevent loss of privacy or failure of access controls.
This section is somewhat miscellaneous, and its contents might be shifted to other locations in the document.
Note that a client can usually tell if a response is firsthand
by comparing the Date to its local request-time,
and hoping that the clocks are not badly skewed.
Because expiration values are assigned optimistically, it is possible that two caches may contain fresh values for the same resource that are different.
If a client performing a retrieval receives a non-firsthand response
for a resource that was already fresh in its own cache, and the
Date header in its existing cache entry is newer
than the Date on the new response, then the client
MAY ignore the response. If so, it MAY retry the request with
a "Cache-Control: max-age=0" directive
(see section 10.8), to force a check with the origin server.
If a cache that is pooling cached responses from other caches
sees two fresh responses for the same resource with different
validators, it SHOULD use the one with the newer Date
header.
Because a client may be receiving responses via multiple paths, so that some responses flow through one set of caches and other responses flow through a different set of caches, a client may receive responses in an order different from that in which the origin server generated them. We would like the client to use the most recently generated response, even if older responses are still apparently fresh.
Neither the opaque validator nor the expiration value can impose
an ordering on responses, since it is possible that a later response
intentionally carries an earlier expiration time. However, the
HTTP/1.1 specification requires the transmission of Date
headers on every response, and the Date values are
ordered to a granularity of one second.
If a client performs a request for a resource that it already
has in its cache, and the response it receives contains a Date
header that appears to be older than the one it already has in
its cache, then the client SHOULD repeat the request unconditionally,
and include
Cache-Control: max-age=0
to force any intermediate caches to validate their copies directly with the origin server, or
Cache-Control: no-cache
to force any intermediate caches to obtain a new copy from the origin server. This prevents certain paradoxes arising from the use of multiple caches.
If the Date values are equal, then the client may
use either response (or may, if it is being extremely prudent,
request a new response). Servers MUST NOT depend on clients being
able to choose deterministically between responses generated during
the same second, if their expiration times overlap.
A "cache key" is a value used to identify a cache entry. HTTP caches three different kinds of cache keys, for use in different contexts:
Request-URI,
some request-header fields, and some response-header fields.
Request-URI,
constitute the "update key" of a response.
Request-URI, constitute
the "lookup key" of a request.
When a cache receives a request, it builds a lookup key from that request, then tries to find (lookup) a cache entry with a matching entry key according to the key matching procedure in section 13.12.3. If such a match exists, then the cache can decide (based on the other caching rules) whether to return that entry in reply to the request.
When a cache receives a response, it builds a update key from that response, and from the request that elicited it. It uses this key to find any previously stored entry with a matching entry key. If such an entry exists, the cache replaces the old entry with the new one.
The term "update" means to remove the old entry from the cache, and then to insert the new entry. It does not imply a modification of an existing entry.
This section describes specifically how the three kinds of keys are constructed, and how a cache determines if keys match.
When a response is received for a non-varying resource (that is,
the response includes no Vary, Alternates,
or Content-Location headers), the update key for
the response is simply the Request-URI of the request
that elicited it: (Request-URI, null). The entry
key for the response is (Request-URI, null, null).
If a response includes a Vary header, then we use
the notation "sel-hdr-values" to denote
the canonical form of the headers in the corresponding request
whose field-names are given in the Vary header. If
the response does not include a Vary header, then
sel-hdr-values is assigned the null value. Section
10.52 on Vary defines the canonical form for selecting
headers.
The canonical form of the headers is defined to be a set whose elements are sequences of request headers with identical field-names. For a given field-name, the corresponding element is the concatenation of the request headers with that field-name, in exactly the order that these fields appear in the request
If the response contains "Vary: {other}",
then sel-hdr-values is assigned a non-null value
that is defined as never matching a set of request headers.
When a response is received that includes a variant-ID in a CVal
header (see section 10.102), but no Content-Location
header, then the update key is (Request-URI, variant-ID),
and the entry key for the response is (Request-URI,
variant-ID, sel-hdr-values).
When a response is received that includes a Vary
header and an opaque validator, but no variant-ID or Content-Location
header, then the update key is (Request-URI, opaque-validator),
and the entry key for the response is (Request-URI,
opaque-validator, sel-hdr-values).
This rule supports the "selecting opaque validators" mechanism described in section 13.8.4. The cache should distinguish between actual variant-IDs and opaque-validators in the variant-ID element of the entry key; a non-null opaque-validator in an entry key DOES match a null variant-ID in a lookup key.
When a response is received that includes both a variant-ID in
a CVal header, and a Content-Location
header, then the update key is (content-location-URI,
variant-ID), and the entry key for the response is
(content-location-URI, variant-ID, sel-hdr-values).
When a response is received that includes a Content-Location
header but no variant-ID, then the update key is (content-location-URI,
null), and the entry key for the response is (content-location-URI,
null, sel-hdr-values).
We express entry keys as the tuple (URI, variant-ID,
sel-hdr-values), in which the variant-ID may be null,
and the sel-hdr-values may either be null, or may
be a set of request headers.
We express update keys as a tuple (URI, variant-ID),
in which the variant-ID may be null. A update key
matches an entry key if both their URI elements match and their
variant-ID elements match. (A null variant-ID
does not match a non-null variant-ID.)
We express lookup keys as a tuple (URI, variant-ID,
all-request-headers), in which the variant-ID
may be null. The all-request-headers element of the tuple is
not always used, but is included here as a notational convenience.
A lookup key matches an entry key if both their URI elements
match and their variant-ID elements match, and either
sel-hdr-values element of the entry key is
null
or
sel-hdr-values element of the entry key matches
the appropriate headers in the all-request-headers element of
the lookup key, according to the matching rules in section on
Vary, section 10.52.
This description matching algorithm is clearly not the most efficient
implementation of an equivalent algorithm. A cache may use any
algorithm that yields equivalent results. For example, it may
use a hierarchical approach where cache entries are grouped into
sets by the URI and variant-ID, and only if a set includes non-null
sel-hdr-values elements does the cache need to consider
the other request headers.
If on a cache lookup there are two or more fresh entries that
appear to match the request, then the one with the most recent
Date value MUST be used.
A cache, when comparing two URIs to decide if they match or not, a cache MUST use a case-sensitive octet-by-octet comparison of the entire URIs, with these exceptions:
Following the rules from section 3.2.2:
Characters except those in the reserved set and the unsafe set
(see section 3.2) are equivalent to their ""%"
HEX HEX" encodings.
For example, the following three URIs are equivalent:
http://abc.com:80/~smith/home.html
http://ABC.com/%7Esmith/home.html
http://ABC.com:/%7esmith/home.html
TBS
This section will list a few problems that are NOT addressed in HTTP/1.1, with the intention of encouraging implementers not to adopt proprietary solutions inconsistent with possible future protocol revisions..
A cache that receives an incomplete response (for example, with fewer bytes of data than specified in a Content-length: header) may store the response. However, the cache MUST treat this as a partial response. Partial responses may be combined as described in section 13.7.4; the result might be a full response or might still be partial. A cache MUST NOT return a partial response to a client without explicitly marking it as such, using the 206 (Partial Content) status code. A cache MUST NOT return a partial response using a status code of 200 (OK).
A cache that receives a response with a zero-length
Entity-body and no explicit indication that the correct length
is zero (such as "Content-Length: 0")
MUST NOT not store the response. The same rule applies to a response
of any length received without an explicit length indication if
the transport connection was terminated in any unusual way.
If a cache receives a response carrying Retry-After
header (see section 10.36), it may either forward this response
to the requesting client, or act as if the server failed to respond.
In the latter case, it MAY return a previously received response,
although it MUST follow all of the rules applying to stale responses.
In particular, it MUST NOT override the "must-revalidate"
Cache-Control directive (see
section 10.7).
A response received with a status code of 200 or 206 may be stored by a cache and used in reply to a subsequent request, subject to the expiration mechanism, unless a Cache-control directive prohibits caching.
A response received with any other status code MUST not be returned in a reply to a subsequent request unless it carries at least one of the following:
Expires header
Cache-control directive
Cache-control directive
Cache-control directive
If a cache receives a response carrying a Retry-After header (see
section 10.36), it may either forward this response to the requesting
client, or act as if the server failed to respond. In the latter
case, it MAY return a previously received response, although it
MUST follow all of the rules applying to stale responses. In
particular, it MUST not override the "must-revalidate"
Cache-control directive (see section 10.7).
TBS
If anything should be here, it should be a collection of warnings about what HTTP/1.1 systems should not assume about HTTP/1.0 systems.
Unless the origin server explicitly prohibits the caching of their responses, the application of GET and HEAD methods to any resources SHOULD NOT have side effects that would lead to erroneous behavior if these responses are taken from a cache. They may still have side effects, but a cache is not required to consider such side effects in its caching decisions. Caches are always expected to observe an origin server's explicit restrictions on caching.
We note one exception to this rule: since some applications have traditionally used GETs and HEADs with query URLs (those containing a "?" in the rel_path part) to perform operations with significant side effects, caches MUST NOT treat responses to such URLs as fresh unless the server provides an explicit expiration time.
This specifically means that responses from HTTP/1.0 servers for such URIs should not be taken from a cache.
See section 15.2 for related information.
The effect of certain methods at the origin server may cause one or more existing cache entries to become non-transparently invalid. That is, although they may continue to be "fresh," they do not accurately reflect what the origin server would return for a new request.
There is no way for the HTTP protocol to guarantee that all such cache entries are marked invalid. For example, the request that caused the change at the origin server may not have gone through the proxy where a cache entry is stored. However, several rules help reduce the likelihood of erroneous behavior.
In this section, the phrase "invalidate an entity" means that the cache should either remove all instances of that entity from its storage, or should mark these as "invalid" and in need of a mandatory revalidation before they can be returned in response to a subsequent request.
Some HTTP methods invalidate a single entity. This
is either the entity referred to by the Request-URI,
or by the Location or Content-Location
response headers (if present). These methods are:
In order to prevent denial of service attacks, an
invalidation based on the URI in a Location
or Content-Location
header MUST only be performed if the host part is the same as
in the Request-URI.
All methods that may be expected to cause modifications to the origin server's resources MUST be written through to the origin server. This currently includes all methods except for GET and HEAD. A cache MUST NOT reply to such a request from a client before having transmitted the request to the inbound server, and having received a corresponding response from the inbound server.
The alternative (known as "write-back" or "copy-back" caching) is not allowed in HTTP/1.1, due to the difficulty of providing consistent updates and the problems arising from server, cache, or network failure prior to write-back.
If the correct handling of responses from a varying
resource (Section 10.xxx) by HTTP/1.0 proxy caches in the response
chain is important, HTTP/1.1 origin servers can include the following
Expires (Section 10.exp) response
header in all responses from the varying resource:
Expires: Thu, 01 Jan 1980 00:00:00 GMT
If this Expires
header is included, the server should usually also include a Cache-Control
header for the benefit of HTTP/1.1 caches, for example
Cache-Control: max-age=604800
which overrides the freshness lifetime of zero seconds
specified by the included Expires
header.
If a new 200 (OK) response is received from a non-varying resource while an old 200 (OK) response is cached, caches can delete this old response from cache memory and insert the new response. For 200 (OK) responses from varying resources (Section 13.12.3), cache replacement is more complex.
HTTP/1.1 allows the authors of varying resources to guide cache update by the inclusion of elements of so-called update keys in the responses of these resources. The update key of a varying response consists of two elements, both of which may be empty strings, separated by a semicolon:
update-key = variant-id ";" absoluteURI
The variant-id element of the update key is the variant-id
value in the CVal header of the
response, if a CVal header which
such a value is present, and an empty string otherwise. The absoluteURI
element of the update key is the absolute URI given in, or derived
from, the Content-Location header
of the response if present, and an empty string if no Content-Location
header is present.
If a cache has stored in memory a 200 (OK) response with a certain update key, and receives, from the same resource, a new 200 (OK) response which has the same update key, this should be interpreted as a signal from the resource author that the old response can be deleted from cache memory and replaced by the new response.
The update key mechanism cannot cause deletion from cache memory of old responses with update keys that will no longer be used. It is expected that the normal "least recently used" update heuristics employed by caches will eventually cause such old responses to be deleted.
All 200 (OK) responses from varying resources should
include update key elements. Resource authors may not assume
that caches will be able to cache responses not including update
key elements. If a Vary header is used to signal variance, the
response should include a variant-id
value as the update key element. The Content-Location
header should only be used to supply a update key element if an
Alternates header is present
in the response.
TBS
There may be enough said elsewhere already, but we haven't checked.
TBS
History lists as implemented in many user agents and caches are different. In particular history lists SHOULD NOT try to show a semantically transparent view of the current state of a resource. Rather, a history list is meant to show exactly what the user saw at the time when the resource was retrieved .
This should not be construed to prohibit the history mechanism from telling the user that a view may be stale.
HTTP's greatest strength and its greatest weakness has been its simplicity. Prior to persistent connections, a separate TCP connection was established to fetch each URL, increasing the load on HTTP servers, and causing congestion on the Internet. The use of inline images and other associated data often requires a client to make multiple requests of the same server in a short amount of time. An excellent analysis of these performance problems is available [2]; analysis and results from a prototype implementation are in [32, 33].
Persistent HTTP connections have a number of advantages, including:
HTTP implementations SHOULD implement persistent connections.
Persistent connections provides a mechanism by which
a client and a server can negotiate the use of a TCP connection
for an extended conversation.. This negotiation takes place using
the Connection and Persist
header fields. Once this option has been negotiated the client
can make multiple HTTP requests over a single transport connection.
To request the use of persistent connections, a client sends a
Connection header with a connection-token "Persist".
If the server wishes to accept persistent connections it will
respond with the same connection-token. Both the client and server
MUST send this connection-token with every request and response
for the duration of the persistent connection. If either the client
or the server omits the Persist token from the Connection
header, that request becomes the last one for the connection.
A server MUST NOT establish a persistent connection with an HTTP/1.0
client that uses the above form of the Persist header
due to problems with the interactions between 1.1 clients and
1.0 proxy servers (See section E.2.5 for more information on backwards
compatibility with HTTP 1.0 clients).
Clients and servers which support persistent connections MAY "pipe-line" their requests and responses. When pipe-lining, a client will send multiple requests without waiting for the responses. The server MUST then send all of the responses in the same order that the requests were made.
A client MAY pipeline multiple requests immediately if it has
previous knowledge that the server it is connecting to supports
persistent connections. A client MAY assume that a server supports
persistent connections if the same server has accepted persistent
connections within the past 24 hours. Clients which assume persistent
connections and pipeline immediately SHOULD be prepared to retry
their connection if the first pipe-lined attempt fails. If a client
does such a retry, it MUST NOT pipeline without first receiving
an explicit Persist token from the server. Clients
MUST also be prepared to resend their requests if the server closes
the connection before sending all of the corresponding responses.
When using persistent connections both the client and the server MUST mark the exact endings of transmitted entity-bodies using one of the following three techniques:
Content-length field in the header with
the exact number of bytes in the entity-body.
Sending the Content-length is the preferred technique.
Chunked encoding SHOULD be used when the size of the entity-body
is not known before beginning to transmit the entity-body. Finally,
the connection MAY be closed and fall back to non-persistent connections,
if neither 1 or 2 are possible.
Clients and servers that support persistent connections MUST correctly support receiving via all three techniques.
It is especially important that proxies correctly
implement the properties of the Connection
header field as specified in 14.2.1.
The proxy server MUST negotiate persistent connections separately with its clients and the origin servers (or other proxy servers) that it connects to. Each persistent connection applies to only one transport link.
A proxy server MUST NOT establish a persistent connection with an HTTP 1.0 client.
It is expected that the Session extension will operate with both SHTTP [31] and SSL [32]. When used in conjunction with SHTTP, the SHTTP request is prepared normally and the persist connection-token is placed in the outermost request block (the one containing the "Secure" method). When used in conjunction with SSL, a SSL session is started as normal and the first HTTP request made using SSL contains the persistent connection header.
Servers will usually have some time-out value beyond which they will no longer maintain an inactive connection. Proxy servers might make this a higher value since it is likely that the client will be making more connections through the same server. The use of persistent connections places no requirements on the length of this time-out for either the client or the server.
When a client or server wishes to time-out it SHOULD issue a graceful close on the transport connection. Clients and servers SHOULD both constantly watch for the other side of the transport close, and respond to it as appropriate. If a client or server does not detect the other sides close promptly it could cause unnecessary resource drain on the network.
A client, server, or proxy MAY close the transport connection at any time. For example, a client MAY have started to send a new request at the same time that the server has decided to close the "idle" connection. From the server's point of view, the connection is being closed while it was idle, but from the client's point of view, a request is in progress.
This means that clients, servers, and proxies MUST be able to recover from asynchronous close events. Client software SHOULD reopen the transport connection and retransmit the aborted request without user interaction. However, this automatic retry SHOULD NOT be repeated if the second request fails.
Servers SHOULD always respond to at least one request per connection, if at all possible. Servers SHOULD NOT close a connection in the middle of transmitting a response, unless a network or client failure is suspected.
It is suggested that clients which use persistent connections SHOULD limit the number of simultaneous connections that they maintain to a given server. A single-user client SHOULD maintain AT MOST 2 connections with any server of proxy. A proxy SHOULD use up to 2*N connections to another server or proxy, where N is the number of simultaneously active users. These guidelines are intended to improve HTTP response times and avoid congestion of the Internet or other networks.
This section is meant to inform application developers, information providers, and users of the security limitations in HTTP/1.1 as described by this document. The discussion does not include definitive solutions to the problems revealed, though it does make some suggestions for reducing security risks.
As mentioned in Section 11.1,
the Basic authentication scheme is not a secure method of user
authentication, nor does it in any way protect the Entity-Body,
which is transmitted in clear text across the physical network
used as the carrier. HTTP does not prevent additional authentication
schemes and encryption mechanisms from being employed to increase
security or the addition of enhancements (such as schemes to use
one-time passwords) to Basic authentication.
The most serious flaw in Basic authentication is that it results in the essentially clear text transmission of the user's password over the physical network. It is this problem which Digest Authentication attempts to address.
Because Basic authentication involves the clear text transmission of passwords it SHOULD never be used (without enhancements) to protect sensitive or valuable information.
A common use of Basic authentication is for identification purposes -- requiring the user to provide a user name and password as a means of identification, for example, for purposes of gathering accurate usage statistics on a server. When used in this way it is tempting to think that there is no danger in its use if illicit access to the protected documents is not a major concern. This is only correct if the server issues both user name and password to the users and in particular does not allow the user to choose his or her own password. The danger arises because naive users frequently reuse a single password to avoid the task of maintaining multiple passwords.
If a server permits users to select their own passwords, then the threat is not only illicit access to documents on the server but also illicit access to the accounts of all users who have chosen to use their account password. If users are allowed to choose their own password that also means the server must maintain files containing the (presumably encrypted) passwords. Many of these may be the account passwords of users perhaps at distant sites. The owner or administrator of such a system could conceivably incur liability if this information is not maintained in a secure fashion.
Basic Authentication is also vulnerable to spoofing
by counterfeit servers. If a user can be led to believe that
he is connecting to a host containing information protected by
basic authentication when in fact he is connecting to a hostile
server or gateway then the attacker can request a password, store
it for later use, and feign an error. This type of attack is
not possible with Digest Authentication[26]. Server implementers
SHOULD guard against the possibility of this sort of counterfeiting
by gateways or CGI scripts. In particular it is very dangerous
for a server to simply turn over a connection to a gateway since
that gateway can then use the persistent connection mechanism
to engage in multiple transactions with the client while impersonating
the original server in a way that is not detectable by the client.
The writers of client software should be aware that the software represents the user in their interactions over the Internet, and should be careful to allow the user to be aware of any actions they may take which may have an unexpected significance to themselves or others.
In particular, the convention has been established
that the GET and HEAD
methods should never have the significance of taking an action
other than retrieval. These methods should be considered "safe.
" This allows user agents to represent other methods, such
as POST, PUT
and DELETE, in a special way,
so that the user is made aware of the fact that a possibly unsafe
action is being requested.
Naturally, it is not possible to ensure that the
server does not generate side-effects as a result of performing
a GET request; in fact, some
dynamic resources consider that a feature. The important distinction
here is that the user did not request the side-effects, so therefore
cannot be held accountable for them.
A server is in the position to save personal data about a user's requests which may identify their reading patterns or subjects of interest. This information is clearly confidential in nature and its handling may be constrained by law in certain countries. People using the HTTP protocol to provide data are responsible for ensuring that such material is not distributed without the permission of any individuals that are identifiable by the published results.
Like any generic data transfer protocol, HTTP cannot
regulate the content of the data that is transferred, nor is there
any a priori method of determining the sensitivity of any particular
piece of information within the context of any given request.
Therefore, applications SHOULD supply as much control over this
information as possible to the provider of that information. Four
header fields are worth special mention in this context: Server,
Via, Referer
and From.
Revealing the specific software version of the server
may allow the server machine to become more vulnerable to attacks
against software that is known to contain security holes. Implementers
SHOULD make the Server header
field a configurable option.
Proxies which serve as a portal through a network
firewall SHOULD take special precautions regarding the transfer
of header information that identifies the hosts behind the firewall.
In particular, they SHOULD remove, or replace with sanitized versions,
any Via fields generated behind
the firewall.
The Referer field
allows reading patterns to be studied and reverse links drawn.
Although it can be very useful, its power can be abused if user
details are not separated from the information contained in the
Referer. Even when the personal
information has been removed, the Referer
field may indicate a private document's URI whose publication
would be inappropriate.
The information sent in the From
field might conflict with the user's privacy interests or their
site's security policy, and hence it SHOULD not be transmitted
without the user being able to disable, enable, and modify the
contents of the field. The user MUST be able to set the contents
of this field within a user preference or application defaults
configuration.
We suggest, though do not require, that a convenient
toggle interface be provided for the user to enable or disable
the sending of From and Referer
information.
Implementations of HTTP origin servers SHOULD be
careful to restrict the documents returned by HTTP requests to
be only those that were intended by the server administrators.
If an HTTP server translates HTTP URIs directly into file system
calls, the server MUST take special care not to serve files that
were not intended to be delivered to HTTP clients. For example,
UNIX, Microsoft Windows, and other operating systems use ".."
as a path component to indicate a directory level above the current
one. On such a system, an HTTP server MUST disallow any such construct
in the Request-URI if it would
otherwise allow access to a resource outside those intended to
be accessible via the HTTP server. Similarly, files intended for
reference only internally to the server (such as access control
files, configuration files, and script code) MUST be protected
from inappropriate retrieval, since they might contain sensitive
information. Experience has shown that minor bugs in such HTTP
server implementations have turned into security risks.
HTTP clients are often privy to large amounts of personal information (e.g. the user's name, location, mail address, passwords, encryption keys, etc.), and SHOULD be very careful to prevent unintentional leakage of this information via the HTTP protocol to other sources. We very strongly recommend that a convenient interface be provided for the user to control dissemination of such information, and that designers and implementers be particularly careful in this area. History shows that errors in this area are often both serious security and/or privacy problems, and often generate very adverse publicity for the implementer's company.
Accept request headers can reveal information
about the user to all servers which are accessed. The Accept-Language
header in particular can reveal information the user would consider
to be of a private nature, because the understanding of particular
languages is often strongly correlated to the membership of a
particular ethnic group. User agents which offer the option to
configure the contents of an Accept-Language
header to be sent in every request are strongly encouraged to
let the configuration process include a message which makes the
user aware of the loss of privacy involved.
An approach that limits the loss of privacy would
be for a user agent to omit the sending of Accept-Language
headers by default, and to ask the user whether it should start
sending Accept-Language headers
to a server if it detects, by looking for any Vary
or Alternates response headers
generated by the server, that such sending could improve the quality
of service.
Elaborate user-customized accept header fields sent in every request, in particular if these include quality values, can be used by servers as relatively reliable and long-lived user identifiers. Such user identifiers would allow content providers to do click-trail tracking, and would allow collaborating content providers to match cross-server click-trails or form submissions of individual users. Note that for many users not behind a proxy, the network address of the host running the user agent will also serve as a long-lived user identifier. In environments where proxies are used to enhance privacy, user agents should be conservative in offering accept header configuration options to end users. As an extreme privacy measure, proxies could filter the accept headers in relayed requests. General purpose user agents which provide a high degree of header configurability should warn users about the loss of privacy which can be involved.
Clients using HTTP rely heavily on the Domain Name Service, and are thus generally prone to security attacks based on the deliberate miss-association of IP addresses and DNS names. The deployment of DNSSEC[27] should help this situation. In advance of this deployment, however, clients need to be cautious in assuming the continuing validity of an IP number/DNS name association.
In particular, HTTP clients SHOULD rely on their name resolver for confirmation of an IP number/DNS name association, rather than caching the result of previous host name lookups. Many platforms already can cache host name lookups locally when appropriate, and they SHOULD be configured to do so. These lookups should be cached, however, only when the TTL (Time To Live) information reported by the name server makes it likely that the cached information will remain useful.
If HTTP clients cache the results of a host name lookups in order to achieve a performance improvement, they MUST observe the TTL information reported by DNS.
If HTTP clients do not observe this rule, they could be spoofed when a previously-accessed server's IP address changes. As renumbering is expected to become increasingly common[24], the possibility of this form of attack will grow. Observing this requirement thus reduces this potential security vulnerability.
This requirement also improves the load-balancing behavior of clients for replicated servers using the same DNS name and reduces the likelihood of a user's experiencing failure in accessing sites which use that strategy.
If a single server supports multiple organizations
that do not trust one another, then it must check the values of
Location and Content-Location
headers in responses that are generated under control of said
organizations to make sure that they do not attempt to invalidate
resources over which they have no authority.
This specification makes heavy use of the augmented BNF and generic constructs defined by David H. Crocker for RFC 822 [9]. Similarly, it reuses many of the definitions provided by Nathaniel Borenstein and Ned Freed for MIME [7]. We hope that their inclusion in this specification will help reduce past confusion over the relationship between HTTP and Internet mail message formats.
The HTTP protocol has evolved considerably over the past four years. It has benefited from a large and active developer community--the many people who have participated on the www-talk mailing list--and it is that community which has been most responsible for the success of HTTP and of the World-Wide Web in general. Marc Andreessen, Robert Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois Groff, Phillip M. Hallam-Baker, Håkon W. Lie, Ari Luotonen, Rob McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc VanHeyningen deserve special recognition for their efforts in defining early aspects of the protocol.
This document has benefited greatly from the comments of all those participating in the HTTP-WG. In addition to those already mentioned, the following individuals have contributed to this specification:
Gary Adams Harald Tveit Alvestrand
Keith Ball Brian Behlendorf
Paul Burchard Maurizio Codogno
Mike Cowlishaw Roman Czyborra
Michael A. Dolan Jim Gettys
Marc Hedlund Koen Holtman
Alex Hopmann Bob Jernigan
Shel Kaphan Rohit Khare
Martijn Koster Alexei Kosut
David M. Kristol Daniel LaLiberte
Paul J. Leach Albert Lunde
John C. Mallery Jean-Philippe Martin-Flatin
Larry Masinter Mitra
Jeffrey Mogul Gavin Nicol
Bill Perry Jeffrey Perry
Owen Rees Luigi Rizzo
David Robinson Marc Salomon
Rich Salz Jim Seidman
Chuck Shotton Eric W. Sink
Simon E. Spero Richard N. Taylor
Robert S. Thau François Yergeau
Mary Ellen Zurko David Morris
Greg Herlihy Scott Powers
Allan M. Schiffman Alan Freier
Bill (BearHeart) Weinman
Much of the content and presentation of the caching design is due to suggestions and comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris, Larry Masinter, and Roy Fielding.
Most of the specification of ranges is based on work originally done by Ari Luotonen and John Franks, with additional input from Steve Zilles and Roy Fielding.
XXX need acks for subgroup work.
Roy T. Fielding
Department of Information and Computer Science
University of California
Irvine, CA 92717-3425, USA
Fax: +1 (714) 824-4056
Email: fielding@ics.uci.edu
Henrik Frystyk Nielsen
W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
Email: frystyk@w3.org
Tim Berners-Lee
Director, W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
Email: timbl@w3.org
Jim Gettys
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
Email: jg@w3.org
Jeffrey C. Mogul
Western Research Laboratory
Digital Equipment Corporation
250 University Avenue
Palo Alto, California, 94305, U.S.A.
Email: mogul@wrl.dec.com
These appendices are provided for informational reasons only -- they do not form a part of the HTTP/1.1 specification.
In addition to defining the HTTP/1.1 protocol, this document serves as the specification for the Internet media type "message/http". The following is to be registered with IANA [17].
Media Type name: message
Media subtype name: http
Required parameters: none
Optional parameters: version, msgtype
version: The HTTP-Version number of the enclosed message
(e.g., "1.1"). If not present, the version can be
determined from the first line of the body.
msgtype: The message type -- "request" or "response". If not
present, the type can be determined from the first
line of the body.
Encoding considerations: only "7bit", "8bit", or "binary" are
permitted
Security considerations: none
Although this document specifies the requirements for the generation of HTTP/1.1 messages, not all applications will be correct in their implementation. We therefore recommend that operational applications be tolerant of deviations whenever those deviations can be interpreted unambiguously.
Clients SHOULD be tolerant in parsing the Status-Line
and servers tolerant when parsing the Request-Line.
In particular, they SHOULD accept any amount of SP
or HT characters between fields,
even though only a single SP
is required.
The line terminator for HTTP-header
fields is the sequence CRLF.
However, we recommend that applications, when parsing such headers,
recognize a single LF as a line
terminator and ignore the leading CR.
HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC 822 [9]) and the Multipurpose Internet Mail Extensions (MIME [7]) to allow entities to be transmitted in an open variety of representations and with extensible mechanisms. However, RFC 1521 discusses mail, and HTTP has a few features that are different than those described in RFC 1521. These differences were carefully chosen to optimize performance over binary connections, to allow greater freedom in the use of new media types, to make date comparisons easier, and to acknowledge the practice of some early HTTP servers and clients.
At the time of this writing, it is expected that RFC 1521 will be revised. The revisions may include some of the practices found in HTTP/1.1 but not in RFC 1521.
This appendix describes specific areas where HTTP differs from RFC 1521. Proxies and gateways to strict MIME environments SHOULD be aware of these differences and provide the appropriate conversions where necessary. Proxies and gateways from MIME environments to HTTP also need to be aware of the differences because some conversions may be required.
RFC 1521 requires that an Internet mail entity be converted to canonical form prior to being transferred, as described in Appendix G of RFC 1521 [7]. Section 3.6.1 of this document describes the forms allowed for subtypes of the "text" media type when transmitted over HTTP. RFC 1521 requires that content with a typeof "text" represent line breaks as CRLF and forbids the use of CR or LF outside of line break sequences. HTTP allows CRLF, bare CR, and bare LF to indicate a line break within text content when a message is transmitted over HTTP.
Where it is possible, a proxy or gateway from HTTP
to a strict RFC 1521 environment SHOULD translate all line breaks
within the text media types described in Section 3.6.1
of this document to the RFC 1521 canonical form of CRLF.
Note, however, that this may be complicated by the presence of
a Content-Encoding and by the
fact that HTTP allows the use of some character sets which do
not use octets 13 and 10 to represent CR
and LF, as is the case for some
multi-byte character sets.
HTTP/1.1 uses a restricted set of date formats (Section 3.3)
to simplify the process of date comparison. Proxies and gateways
from other protocols SHOULD ensure that any Date
header field present in a message conforms to one of the HTTP/1.1
formats and rewrite the date if necessary.
RFC 1521 does not include any concept equivalent
to HTTP/1.1's Content-Encoding
header field. Since this acts as a modifier on the media type,
proxies and gateways from HTTP to MIME-compliant protocols MUST
either change the value of the Content-Type
header field or decode the Entity-Body
before forwarding the message. (Some experimental applications
of Content-Type for Internet
mail have used a media-type parameter of ";conversions=<content-coding>"
to perform an equivalent function as Content-Encoding.
However, this parameter is not part of RFC 1521.)
HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC 1521. Proxies and gateways from MIME-compliant protocols to HTTP MUST remove any non-identity CTE ("quoted-printable" or "base64") encoding prior to delivering the response message to an HTTP client.
Proxies and gateways from HTTP to MIME-compliant protocols are responsible for ensuring that the message is in the correct format and encoding for safe transport on that protocol, where "safe transport" is defined by the limitations of the protocol being used. Such a proxy or gateway SHOULD label the data with an appropriate Content-Transfer-Encoding if doing so will improve the likelihood of safe transport over the destination protocol.
In RFC 1521, most header fields in multipart body-parts
are generally ignored unless the field name begins with "Content-".
In HTTP/1.1, multipart body-parts may contain any HTTP header
fields which are significant to the meaning of that part.
HTTP/1.1 introduces the Transfer-Encoding
header field (Section 10.39).
Proxies/gateways MUST remove any transfer coding prior to forwarding
a message via a MIME-compliant protocol. The process for decoding
the "chunked" transfer coding (Section 3.6)
can be represented in pseudo-code as:
length := 0
read chunk-size and CRLF
while (chunk-size > 0) {
read chunk-data and CRLF
append chunk-data to Entity-Body
length := length + chunk-size
read chunk-size and CRLF
}
read entity-header
while (entity-header not empty) {
append entity-header to existing header fields
read entity-header
}
Content-Length := length
Remove "chunked" from Transfer-Encoding
HTTP is not a MIME-compliant protocol (see Appendix C).
However, HTTP/1.1 messages may include a single MIME-Version
general-header field to indicate what version of the MIME protocol
was used to construct the message. Use of the MIME-Version
header field indicates that the message is in full compliance
with the MIME protocol (as defined in [7]).
Proxies/gateways are responsible for ensuring full compliance
(where possible) when exporting HTTP messages to strict MIME environments.
MIME-Version = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
MIME version "1.0"
is the default for use in HTTP/1.1. However, HTTP/1.1 message
parsing and semantics are defined by this document and not the
MIME specification.
This section will summarize major differences between versions 1.0 and 1.1 of the Hypertext Transfer Protocol.
The requirements that clients and servers support
the Host request-header, report
an error if the Host request-header
is missing from an HTTP/1.1 request (Section 10.22), and accept
absolute URIs (Section 5.1.2) are among the most important changes
from HTTP/1.0.
In HTTP/1.0 there is a one-to-one relationship of IP addresses and servers. There is no other way to distinguish the intended server of a request than the IP address to which that request is directed. The HTTP/1.1 change will allow the Internet, once HTTP/1.0 clients and servers are no longer common, to support multiple Web sites from a single IP address, greatly simplifying large operational Web servers, where allocation of many IP addresses to a single host has created serious problems. The Internet will also be able to recover the IP addresses that have been used for the sole purpose of allowing root-level domain names to be used in HTTP URLs. Given the rate of growth of the Web, and the number of servers already deployed, it is extremely important that implementations of HTTP/1.1 correctly implement these new requirements:
Host
request-header
Host request-headers are required
in HTTP/1.1 requests.
Host request-header
This appendix documents protocol elements used by some existing HTTP implementations, but not consistently and correctly across most HTTP/1.1 applications. Implementers should be aware of these features, but cannot rely upon their presence in, or interoperability with, other HTTP/1.1 applications.
The PATCH method is similar to PUT except
that the entity contains a list of differences between the original
version of the resource identified by the Request-URI
and the desired content of the resource after the PATCH
action has been applied. The list of differences is in a format
defined by the media type of the entity (e.g., "application/diff")
and MUST include sufficient information to allow the server to
recreate the changes necessary to convert the original version
of the resource to the desired version.
If the request passes through a cache and the Request-URI
identifies a currently cached entity, that entity MUST be removed
from the cache. Responses to this method are not cachable.
For compatibility with HTTP/1.0 applications, all PATCH
requests MUST include a valid Content-Length header
field unless the server is known to be HTTP/1.1 compliant. When
sending a PATCH request to an HTTP/1.1 server, a
client MUST use a valid Content-Length or the "chunked"
Transfer-Encoding. The server SHOULD respond with
a 400 (bad request) message if it cannot determine the length
of the request message's content, or with 411 (length required)
if it wishes to insist on receiving a valid Content-Length.
The actual method for determining how the patched resource is
placed, and what happens to its predecessor, is defined entirely
by the origin server. If the original version of the resource
being patched included a Content-Version header field,
the request entity MUST include a Derived-From header
field corresponding to the value of the original Content-Version
header field. Applications are encouraged to use these fields
for constructing versioning relationships and resolving version
conflicts.
PATCH requests must obey the entity transmission requirements set out in section 8.4.1.
Caches that implement PATCH should invalidate
cached responses as defined in section 13.17 for PUT.
The LINK method establishes one or more Link relationships between
the existing resource identified by the Request-URI
and other existing resources. The difference between LINK and
other methods allowing links to be established between resources
is that the LINK method does not allow any Entity-Body
to be sent in the request and does not directly result in the
creation of new resources.
If the request passes through a cache and the Request-URI
identifies a currently cached entity, that entity MUST be removed
from the cache. Responses to this method are not cachable.
Caches that implement LINK should invalidate cached responses as defined in section 13.17 for PUT.
The UNLINK method removes one or more Link relationships from
the existing resource identified by the Request-URI.
These relationships may have been established using the LINK method
or by any other method supporting the Link header. The removal
of a link to a resource does not imply that the resource ceases
to exist or becomes inaccessible for future references.
If the request passes through a cache and the Request-URI
identifies a currently cached entity, that entity MUST be removed
from the cache. Responses to this method are not cachable.
Caches that implement UNLINK should invalidate cached responses as defined in section 13.17 for PUT.
The Content-Version entity-header field defines the
version tag associated with a rendition of an evolving entity.
Together with the Derived-From field described in
Section 10.18, it allows a group of
people to work simultaneously on the creation of a work as an
iterative process. The field SHOULD be used to allow evolution
of a particular work along a single path. It SHOULD NOT be used
to indicate derived works or renditions in different representations.
It MAY also me used as an opaque value for comparing a cached
entity's version with that of the current resource.
Content-Version = "Content-Version" ":" quoted-string
Examples of the Content-Version field include:
Content-Version: "2.1.2"
Content-Version: "Fred 19950116-12:26:48"
Content-Version: "2.5a4-omega7"
The value of the Content-Version field SHOULD be
considered opaque to all parties but the origin server. A user
agent MAY suggest a value for the version of an entity transferred
via a PUT request; however, only the origin server can reliably
assign that value.
The Derived-From entity-header field can be used
to indicate the version tag of the resource from which the enclosed
entity was derived before modifications were made by the sender.
This field is used to help manage the process of merging successive
changes to a resource, particularly when such changes are being
made in parallel and from multiple sources.
Derived-From = "Derived-From" ":" quoted-string
An example use of the field is:
Derived-From: "2.1.1"
The Derived-From field is required for PUT
and PATCH requests if the entity being sent was previously
retrieved from the same URI and a Content-Version
header was included with the entity when it was last retrieved.
The Link entity-header field provides a means for
describing a relationship between two resources, generally between
the requested resource and some other resource. An entity MAY
include multiple Link values. Links at the metainformation level
typically indicate relationships like hierarchical structure and
navigation paths. The Link field is semantically
equivalent to the <LINK> element in HTML [5].
Link = "Link" ":" #("<" URI ">" *( ";" link-param )
link-param = ( ( "rel" "=" relationship )
| ( "rev" "=" relationship )
| ( "title" "=" quoted-string )
| ( "anchor" "=" <"> URI <"> )
| ( link-extension ) )
link-extension = token [ "=" ( token | quoted-string ) ]
relationship = sgml-name
| ( <"> sgml-name *( SP sgml-name) <"> )
sgml-name = ALPHA *( ALPHA | DIGIT | "." | "-" )
Relationship values are case-insensitive and MAY be extended within the constraints of the sgml-name syntax. The title parameter MAY be used to label the destination of a link such that it can be used as identification within a human-readable menu. The anchor parameter MAY be used to indicate a source anchor other than the entire current resource, such as a fragment of this resource or a third resource.
Examples of usage include:
Link: <http://www.cern.ch/TheBook/chapter2>; rel="Previous"
Link: <mailto:timbl@w3.org>; rev="Made"; title="Tim Berners-Lee"
The first example indicates that chapter2 is previous to this resource in a logical navigation path. The second indicates that the person responsible for making the resource available is identified by the given e-mail address.
The URI header field has, in past versions of this specification,
been used as a combination of the existing Location,
Content-Location, and Alternates header fields. Its
primary purpose has been to include a list of additional URIs
for the resource, including names and mirror locations. However,
it has become clear that the combination of many different functions
within this single field has been a barrier to consistently and
correctly implementing any of those functions. Furthermore, we
believe that the identification of names and mirror locations
would be better performed via the Link header field. The URI header
field is therefore deprecated in favor of those other fields.
URI-header = "URI" ":" 1#( "<" URI ">" )
Some clients and servers may wish to be compatible with some previous implementations of persistent connections in HTTP version 1.0 clients and servers.
When connecting to an origin server an HTTP client MAY send the
Keep-Alive connection-token in addition to the Persist
connection-token:
Connection: Keep-Alive,Persist
An HTTP/1.0 server would then respond with the Keep-Alive
connection token and the client may proceed with an HTTP/1.0 (or
Keep-Alive) persistent connection.
An HTTP/1.1 server may also establish persistent connections with
HTTP/1.0 clients upon receipt of a Keep-Alive connection
token.
A persistent connection based on the Keep-Alive connection
token MUST only use the "Content-Length"
technique for marking the ending boundaries of entity-bodies.
It MAY use pipe-lining.
A client MUST NOT send the Keep-Alive connection
token to a proxy server as HTTP/1.0 proxy servers do not obey
the rules of HTTP/1.1 for parsing the Connection header field.
When the Keep-Alive connection-token has been transmitted
with a request or a response a Keep-Alive header
field MAY also be included. The Keep-Alive header
field takes the following form:
Keep-Alive-header = "Keep-Alive" ":"
0# keepalive-param
keepalive-param = param-name "=" value
The Keep-Alive header itself is optional, and is
used only if a parameter is being sent. HTTP/1.1 does not define
any parameters.
If the Keep-Alive header is sent, the corresponding
connection token MUST be transmitted. The Keep-Alive
header MUST be ignored if received without the connection token.
It is beyond the scope of a protocol specification to mandate compliance with previous versions. HTTP /1.1 was deliberately designed, however, to make supporting previous versions easy. While we are contemplating a separate document containing advice to implementers, we feel it worth noting that at the time of composing this specification, we would expect commercial HTTP/1.1 servers to::
And we would expect HTTP/1.1 clients to:
For most implementations of HTTP/1.0, each connection is established by the client prior to the request and closed by the server after sending the response. A few implementations implement the Keep-Alive version of persistent connections described in Section E.2.5.1.
The material in appendix G should go into a separate implementation guide as an informational RFC, rather than in this specification. (since it mostly describes 3 possible cache implementation strategies possible within the protocol, rather than just the two protocol facilities (transparent and opaque)). For the purposes of this (02) draft, we will leave it in as an appendix as it clarifies some points of how caching might work in the context of the HTTP/1.1 protocol.
If a resource is varying, this has an important effect on cache management, particularly for caching proxies which service a diverse set of user agents. Such proxy caches must correctly handle requests on varying resources in order not to disturb the negotiation process.
This specification distinguishes six levels of correct support for content negotiation by proxy caches. The text below describes these levels, but does not exhaustively list all mechanisms associated with support on these levels. In particular, mechanisms for handling partial requests on varying resources are not discussed.
Vary
header or Alternates header).
When receiving a request on a varying resource, the proxy will
thus always forward the request towards an upstream server. A
level 1 proxy cache never makes selection decisions itself.
If-Invalid
request header field listing the CVal
header values of the associated cached 200 responses, as described
in Section 10.52. If a 304 (Not Modified) response is received
from the upstream server, the proxy updates, with the 304 response
headers, the stored 200 response which has the same CVal
header field as the 304 response. It then passes either the updated
200 response or the 304 response on to its client, the choice
depending on the presence and contents of an If-Invalid
header in the original request. If a 200 response is received
from the upstream server, the proxy will update the set of responses
it has for the varying resource by using the cache update algorithm
described in Section 13.20, and pass on the 200 response to its
client.
CVal
header identical to the CVal
header of that cached 200 response. When receiving a request
on the varying resource, the proxy will iterate over all cached,
fresh 200 responses associated with the resource. For each fresh
200 response, it will search the associated list of selecting
request header sequences to see if a match to the headers of the
current request can be found. If a match is found, the proxy
will return the fresh 200 response in question. If no match is
found, the proxy will switch to level 1 behavior and pass on the
request to an upstream server. The response received from the
upstream server may refresh a stale 200 response that was cached
for the varying resource a side effect. XXX previous sentence
doesn't make sense…
Note: Implementation of support levels 4 to 6 is only possible when the planned content negotiation specification [29] is completed. The level numbers above were assigned to reflect expected caching efficiency in an environment where the proxy cache is serving a diverse set of clients. It is expected that level 4 proxies will be easier to implement than level 3 proxies.
Level 3 and level 6 proxy caches not only cache the
responses from an opaquely varying resource, they also cache the
mappings from request headers to particular entities computed
by the opaque selection algorithm located at the origin server.
If this selection algorithm is changed by the resource author,
for example because a Spanish text entity is added to a resource
which previously only had English and French entities available,
it is important to make the level 3 and 6 caches refresh their
cached mappings. This can be done by changing the CVal
header fields sent along with the original English and French
responses. This change will eventually cause the proxies to replace
the old English and French responses in cache memory, along with
their associated lists of selecting request header sequences,
by `new' English and French responses with fresh lists of selecting
request header sequences. In order to guarantee an upper time
bound for this update process, the resource author can include
an appropriate Cache-control: max-age=...
directive in the responses from the varying resource.
This should probably be in the cookie ID, and not in this document at all.
HTTP implementations often support facilities for
state management, often called "cookies"[35]. Cookies
can not be cached by public (shared) caches, but since public
documents may make up part of a "stateful dialog," and
in particular the first document in a stateful dialog may be (for
example) a public and cachable home page, servers that wish to
receive the client's cookie on each request, or to issue a new
cookie on requests for a document, must set the document up to
require validation on each request (Cache-Control:
must-revalidate)
In general, the cache control headers for responses control what a proxy has to do. If a document is fresh in a cache, a request containing a cookie does not have to be forwarded to the origin server, since (by definition) if the document can be served from a cache the origin server must have said there are no important side effects at the origin relating to requests for that document, and so, no changes to the cookie.
One important state issue bearing on caching is that for conditional requests that go through to the origin server, for which the origin server responds with 304 and also with a set-cookie header, caches must splice the set-cookie sent by the origin server into their own response. For example, this allows a home page to be cached, but stale, so that the only traffic to the origin server is to validate the home page, receiving a 304 and potentially a new cookie.
TBS
Should go into an implementers Informational RFC.
This should also go into an implementers Informational RFC, and become grist for HTML's mill.
Many HTTP caching configurations involve hierarchies of caches, often designed to reduce bandwidth requirements rather than improving latency. However, if a cache at a low level in the hierarchy is sure that the cache(s) above it do not contain a cache entry to match a given request, that low-level cache can transmit the request directly to the origin server. This improves retrieval latency without increasing total bandwidth requirements (it even eliminates some packet transmissions) and is entirely appropriate for resources whose values are explicitly not cached.
We call this technique "request bypassing." Note that although the bypassing decision might be done by the ultimate client, in many cases the use of firewalls or unsophisticated clients means that the decision must be made by an intermediate-level cache.
In order for request bypassing to work in the most efficient possible way, the caches must be able to determine from the request whether the response is likely to be cachable. (It is important to err on the side of assuming cachability, since the assuming converse could seriously reduce the effectiveness of the higher-level caches.)
The current HTTP/1.1 draft specification does not include a foolproof mechanism to mark requests in this way. While we generally do not allow caching of responses to GET requests for URLs with a "?" in the rel_path part (see section 13.16), we also allow the origin server to mark responses to such queries as cachable. Therefore, any bypassing done using this heuristic runs the risk of giving up perfectly good opportunities to cache some resources.
XXX we have discussed various approaches for marking requests, all of which apparently require some kind of change to HTML to allow the origin server to pass the marks to the ultimate client. Some people suggest using special methods that are explicitly always cachable ("POST_WITH_NO_SIDE_EFFECTS", or more concisely "POSTC") or never cachable ("GET_QUERY", or more concisely "GETQ"). Others have suggested adding tags to HTML that would cause subsequent requests to carry some special sort of header. Neither solution has resulted in a consensus.
An origin server would be able to use POSTC only
withHTTP/1.1 clients and proxies, and so would have to return
different HTML forms depending on the protocol version in the
request header. This would also imply using the proposed Vary:
header with some token that indicates "varies based on request
HTTP version," since we don't want a cache returning one
of these HTML responses to an HTTP/1.0 client