Copyright  © 2009 W3C ® ( MIT , ERCIM , Keio ), All Rights Reserved. W3C liability , trademark and document use rules apply.
This document is an editors' copy that has no official standing.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/ .
This document reflects group resolutions on comments received on the previous Last Call Working Draft .
Publication as a Group Working Draft of a proposed normative Recommendation does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This document has been produced by the Mobile Web Best Practices Working Group as part of the Mobile Web Initiative . Please send comments on this document to the Working Group's public email list public-bpwg-ct@w3.org , a publicly archived mailing list .
This document was produced under the 5 February 2004 W3C Patent Policy . W3C maintains a public list of patent disclosures made in connection with this document; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) with respect to this specification must disclose the information in accordance with section 6 of the W3C Patent Policy .
1
Introduction
(Non-Normative)
    1.1
Purpose
    1.2
Audience
    1.3
Scope
    1.4
Principles
        1.4.1
IAB
Considerations
        1.4.2
Priority
of
Intention
2
Terminology
(Normative)
    2.1
Types
of
Proxy
    2.2
Types
of
Transformation
3
Conformance
(Normative)
    3.1
Classes
of
Product
    3.2
Normative
and
Informative
Parts
    3.3
Normative
Language
for
Conformance
Requirements
    3.4
Transformation
Deployment
Conformance
4
Behavior
of
Components
(Normative)
    4.1
Proxy
Forwarding
of
Request
        4.1.1
Applicable
HTTP
Methods
        4.1.2
no-transform
directive
in
Request
        4.1.3
Treatment
of
Requesters
that
are
not
Web
browsers
        4.1.4
Serving
Cached
Responses
        4.1.5
Alteration
of
HTTP
Header
Field
Values
            4.1.5.1
Content
Tasting
            4.1.5.2
Avoiding
"Request
Unacceptable"
Responses
            4.1.5.3
User
Selection
of
Restructured
Experience
            4.1.5.4
Sequence
of
Requests
            4.1.5.5
Original
Header
Fields
        4.1.6
Additional
HTTP
Header
Fields
            4.1.6.1
Proxy
Treatment
of
Via
Header
Field
    4.2
Proxy
Forwarding
of
Response
to
User
Agent
        4.2.1
Applicable
Responses
        4.2.2
User
Preferences
        4.2.3
Receipt
of
Cache-Control:
no-transform
        4.2.4
Use
of
Cache-Control:
no-transform
        4.2.5
Server
Rejection
of
HTTP
Request
        4.2.6
Receipt
of
Vary
HTTP
Header
Field
        4.2.7
Link
to
"handheld"
Representation
        4.2.8
WML
Content
        4.2.9
Proxy
Decision
to
Transform
            4.2.9.1
Alteration
of
Response
            4.2.9.2
Link
Rewriting
            4.2.9.3
HTTPS
Link
Rewriting
5
Testing
(Normative)
A
References
B
Conformance
Statement
C
Internet
Content
Types
associated
with
Mobile
Content
D
DOCTYPEs
Associated
with
Mobile
Content
E
URI
Patterns
Associated
with
Mobile
Web
Sites
F
Example
Transformation
Interactions
(Non-Normative)
    F.1
Basic
Content
Tasting
by
Proxy
    F.2
Optimization
based
on
Previous
Server
Interaction
    F.3
Optimization
based
on
Previous
Server
Interaction,
Server
has
Changed
its
Operation
    F.4
Server
Response
Indicating
that
this
Representation
is
Intended
for
the
Target
Device
    F.5
Server
Response
Indicating
that
another
Representation
is
Intended
for
the
Target
Device
G
Informative
Guidance
for
Origin
Servers
(Non-Normative)
    G.1
Server
Response
to
Proxy
        G.1.1
Use
of
HTTP
406
Status
        G.1.2
Use
of
HTTP
403
Status
        G.1.3
Server
Origination
of
Cache-Control:
no-transform
        G.1.4
Varying
Representations
            G.1.4.1
Use
of
Vary
HTTP
Header
Field
            G.1.4.2
Indication
of
Intended
Presentation
Media
Type
of
Representation
H
Applicability
to
Transforming
Solutions
which
are
Out
of
Scope
(Non-Normative)
I
Scope
for
Future
Work
(Non-Normative)
    I.1
POWDER
    I.2
link
HTTP
Header
Field
    I.3
Sources
of
Device
Information
    I.4
Inter
Proxy
Communication
    I.5
Amendment
to
and
Refinement
of
HTTP
J
Acknowledgments
(Non-Normative)
The overall objective of this document is to provide a means, as far as is practical, for users to be provided with at least a "functional user experience" [Device Independence Glossary] of the Web, when mobile, taking into account the fact that an increasing number of content providers create experiences specially tailored to the mobile context which they do not wish to be altered by third parties. Equally it takes into account the fact that there remain a very large number of Web sites that do not provide a functional user experience when perceived on many mobile devices.
The W3C Mobile Web Best Practices Working Group (BPWG) is not chartered to create new technology - its role is to advise on best practice for use of existing technology. In satisfying Content Transformation requirements, existing HTTP header fields, directives and behaviors must be respected, and as far as is practical, no extensions to [RFC 2616 HTTP] are to be used.
The recommendations in this document refer to interactions of a proxy and do not refer to any presumed aspects of the internal operation of the proxy. For this reason, the document does not discuss use of "allow" and "disallow" lists (though it does discuss behavior that is induced by the implementation of such lists). In addition it does not discuss details of how transformation is carried out except if this is reflected in interoperability. For this reason, it does not discuss the insertion or insertion of headers and footers or any other specific behaviors (though it does discuss the need for essential user interaction of some form).
The BPWG made reference to Internet Architecture Board (IAB) work on "Open Pluggable Edge Services" [RFC 3238 OPES] for various principles that underlie behavior of proxies. In this work the IAB expressed its concerns about privacy, control, monitoring, and accountability of such services.
Alteration of HTTP requests and responses is not prohibited by HTTP other than in the circumstances referred to in [RFC 2616 HTTP] Section 13.5.2 and Section 14.9.5 .
HTTP defines two types of proxy: transparent proxies and non-transparent proxies. As discussed in [RFC 2616 HTTP] Section 1.3, Terminology :
"A transparent proxy is a proxy that does not modify the request or response beyond what is required for proxy authentication and identification. A non-transparent proxy is a proxy that modifies the request or response in order to provide some added service to the user agent, such as group annotation services, media type transformation, protocol reduction, or anonymity filtering. Except where either transparent or non-transparent behavior is explicitly stated, the HTTP proxy requirements apply to both types of proxies."
This document elaborates the behavior of non-transparent proxies, when used for Content Transformation in the context discussed in [CT Landscape] .
There are three classes of operation on responses:
Restructuring
content
is
a
process
whereby
the
original
layout
is
altered
so
that
content
is
added
or
removed
or
where
the
spatial
or
navigational
relationship
of
parts
of
content
is
altered,
e.g.
linearization
(i.e.
reordering
presentation
elements,
especially
tables,
so
that
they
fit
on
a
narrow
display
and
can
be
traversed
without
horizontal
scrolling)
or
pagination
(i.e.
splitting
a
document
too
large
to
be
stored
in
or
transmitted
to
the
terminal
in
one
piece,
so
that
it
can
be
nevertheless
accessed
by
browsing
through
a
succession
of
smaller
interlinked
documents).
It
also
includes
rewriting
URIs
so
that
subsequent
requests
are
routed
via
the
proxy
handling
the
response.
It
includes
also
rewriting
of
URIs
so
that
subsequent
requests
route
via
the
proxy
handling
this
response.
Recoding content
Optimizing content
The Content Transformation Guidelines specification has one class of products:
A Transformation Deployment is the provision of non-transparent components in the path of HTTP requests and responses. Provisions that are applicable to a Transformation Deployment are identified in this document by use of the term "transforming proxy" or "proxy" in the singular or plural.
The key words must , must not , required , shall , shall not , should , should not , recommended , not recommended , may , and optional in this Recommendation have the meaning defined in [RFC 2119] .
A Transformation Deployment conforms to these guidelines if it follows the statements in 4.1 Proxy Forwarding of Request , 4.2 Proxy Forwarding of Response to User Agent and 5 Testing (Normative) .
A Transformation Deployment that wishes to claim conformance must make available a conformance statement B Conformance Statement that specifies the reasons for non-compliance with any clauses containing the key words should and should not , recommended and not recommended .
Proxies should not intervene in methods other than GET, POST, HEAD.
If the HTTP method is altered from HEAD to GET, proxies should (providing such action is in accordance with normal HTTP caching rules) cache the response so that a second GET request for the same content is not required (see also 4.1.4 Serving Cached Responses ).
Other than to convert between HEAD and GET proxies must not alter request methods.
no-transform
directive
in
Request
If
the
request
contains
a
Cache-Control:
no-transform
directive,
proxies
must
not
alter
the
request
other
than
to
comply
with
transparent
HTTP
behavior
defined
in
[RFC
2616
HTTP]
sections
section
14.9.5
and
section
13.5.2
and
to
add
header
fields
as
described
in
4.1.6
Additional
HTTP
Header
Fields
below.
Note:
An
example
of
the
use
of
Cache-Control:
no-transform
is
the
issuing
of
asynchronous
HTTP
requests,
perhaps
by
means
of
XMLHTTPRequest
XMLHttpRequest
[XHR]
,
which
may
include
such
a
directive
in
order
to
prevent
transformation
of
both
the
request
and
the
response.
Aside from the usual caching procedures defined in [RFC 2616 HTTP] , in some circumstances, proxies may paginate responses and where this is the case a request may be for a subsequent page of a previously requested resource. In this case proxies may for the sake of consistency of representation serve stale data but when doing so should notify the user that this is the case and must provide a simple means of retrieving a fresh copy.
Aside
from
the
usual
procedures
defined
in
[RFC
2616
HTTP]
proxies
should
not
modify
the
values
of
header
fields
other
than
the
User-Agent
,
Accept
,
Accept-Charset
,
and
Accept-Encoding
,
and
Accept-Language
header
fields
and
must
not
delete
header
fields.
It
must
be
possible
for
the
server
to
reconstruct
the
original
User
Agent
originated
header
fields
by
copying
directly
from
the
corresponding
X-Device
header
field
values
(see
4.1.5.5
Original
Header
Fields
).
Other than to comply with transparent HTTP operation, proxies should not modify any request header fields unless:
the user would be prohibited from accessing content as a result of the server responding that the request is "unacceptable" (see 4.2.5 Server Rejection of HTTP Request );
the user has specifically requested a restructured desktop experience (see 4.1.5.3 User Selection of Restructured Experience );
the request is part of a sequence of requests to the same Web site and either it is technically infeasible not to adjust the request because of earlier interaction, or because doing so preserves consistency of user experience.
These circumstances are detailed in the following sections.
Note:
In this section, the concept of "Web site" is used (rather than "origin server") as some origin servers host many different Web sites. Since the concept of "Web site" is not strictly defined, proxies should use heuristics including comparisons of domain name to assess whether resources form part of the same "Web site".
Note:
The heuristics discussed in 4.2.9 Proxy Decision to Transform relating to URI patterns are not part of the decision to alter HTTP Header Field values.
A proxy may reissue a request with altered HTTP header field values if a previous request with unaltered values resulted in the origin server rejecting the request as "unacceptable" (see 4.2.5 Server Rejection of HTTP Request ). A proxy may apply heuristics of various kinds to assess, in advance of sending unaltered header field values, whether the request is likely to cause a "request unacceptable" response. If it determines that this is likely then it may alter header field values without sending unaltered values in advance, providing that it subsequently assesses the response as described under 4.2.6 Receipt of Vary HTTP Header Field below, and is prepared to reissue the request with unaltered header fields, and alter its subsequent behavior in respect of the Web site so that unaltered header fields are sent.
A proxy must not reissue a POST request with altered header fields when the response to the unaltered POST request has HTTP status code 200 (in other words, it may only send the altered request for a POST/PUT request when the unaltered one resulted in an HTTP 406 response, and not a "request unacceptable" response).
Proxies may offer users an option to choose to view a restructured experience even when a Web site offers a choice of user experience. If a user has made such a choice then proxies may alter header field values when requesting resources in order to reflect that choice, but must , on receipt of an indication from a Web site that it offers alternative representations (see G.1.4.2 Indication of Intended Presentation Media Type of Representation ), inform the user of that and allow them to select an alternative representation.
Proxies should assume that by default users will wish to receive a representation prepared by the Web site. Proxies must assess whether a user's expressed preference for a restructured representation is still valid if a Web site changes its choice of representations (see 4.2.6 Receipt of Vary HTTP Header Field ).
When
requesting
resources
that
are
included
resources
(e.g.
style
sheets,
images),
proxies
should
make
the
request
for
such
resources
with
the
same
User-Agent
header
field
as
the
request
for
the
resource
from
which
they
are
referenced.
For
the
purpose
of
consistency
of
representation,
proxies
may
request
linked
resources
(e.g.
those
referenced
using
the
a
element)
that
form
part
of
the
same
Web
site
as
a
previously
requested
resource
with
the
same
header
fields
as
the
resource
from
which
they
are
referenced.
When requesting linked resources that do not form part of the same Web site as the resource from which they are linked, proxies should not base their choice of header fields on a consistency of presentation premise.
Specifically the following mapping must be used:
| Original | Replacement | Ref |
|---|---|---|
User-Agent
|
X-Device-User-Agent
| RFC2616 Section 14.43 |
Accept
|
X-Device-Accept
| RFC2616 Section 14.1 |
Accept-Charset
|
X-Device-Accept-Charset
| RFC2616 Section 14.2 |
Accept-Encoding
|
X-Device-Accept-Encoding
| RFC2616 Section 14.3 |
Accept-Language
|
X-Device-Accept-Language
| RFC2616 Section 14.4 |
The
X-Device-
prefix
was
chosen
primarily
on
the
basis
that
this
is
a
already
existing
convention.
It
is
noted
that
the
values
encoded
in
such
header
fields
may
not
ultimately
derive
from
a
device,
they
are
merely
received
fields.
The
treatment
of
received
X-Device
header
fields,
which
may
happen
where
there
are
multiple
transforming
proxies,
is
undefined
(see
I
Scope
for
Future
Work
).
Irrespective
of
the
presence
of
a
no-transform
directive:
Via
Header
Field
When
forwarding
Via
header
fields,
proxies
should
not
alter
them
by
removing
comments
from
them.
According
to
[RFC
2616
HTTP]
Section
14.45
Via
header
field
comments
"
may
be
removed
by
any
recipient
prior
to
forwarding
the
message".
However,
the
justification
for
removing
such
comments
is
based
on
memory
limitations
of
early
proxies,
most
proxies.
Most
modern
proxies
do
not
suffer
such
limitations.
Proxies should not intervene in the response if the request method was not HEAD, GET or POST.
Proxies must provide a means for users to express preferences for inhibiting content transformation. Those preferences must be maintained on a user by user and Web site by Web site basis. Proxies must solicit re-expression of preferences in respect of a server if the server starts to indicate that it offers varying responses as discussed under 4.2.6 Receipt of Vary HTTP Header Field .
Cache-Control:
no-transform
If
the
response
includes
a
Cache-Control:
no-transform
directive
then
proxies
must
not
alter
it
other
than
to
comply
with
transparent
HTTP
behavior
as
described
in
[RFC
2616
HTTP]
Section
13.5.2
and
Section
14.9.5
and
other
than
as
follows.
If a proxy determines that a resource as currently represented is likely to cause serious misoperation of the user agent then it may advise the user that this is the case and must provide the option for the user to continue with unaltered content.
Cache-Control:
no-transform
Proxies
may
use
Cache-Control:
no-transform
to
inhibit
transformation
by
further
proxies.
Vary
HTTP
Header
Field
A
proxy
may
not
be
carrying
out
content
tasting
as
described
under
4.1.5.2
Avoiding
"Request
Unacceptable"
Responses
if
it
anticipates
receiving
a
"request
unacceptable"
response.
However,
if
it
makes
a
request
with
altered
header
fields
in
these
circumstances,
and
receives
a
response
containing
a
Vary
header
field
referring
to
one
of
the
altered
header
fields
then
it
should
request
the
resource
again
with
unaltered
header
fields.
It
should
also
update
whatever
heuristics
it
uses
so
that
unaltered
header
fields
are
presented
first
in
subsequent
requests
for
this
resource.
.
oops
a
reference
to
something
Note:
If the content is WML proxies should act in a transparent manner.
Note:
This does not affect the operation of proxies that are also WAP Gateways.
the
content
is
HTML
and
contains
<link
rel="alternate"
media="handheld"
href=""/>
the
DOCTYPE
of
the
content
(if
it
has
one)
indicates
XHTML-MP,
XHTML
Basic,
WML
or
iMode
as
listed
in
D
DOCTYPEs
Associated
with
Mobile
Content
.
the
Content-Type
has
a
value
listed
in
C
Internet
Content
Types
associated
with
Mobile
Content
.
the
URI
of
the
response
(following
redirection
or
as
indicated
by
the
Content-Location
HTTP
header
field)
matches
a
pattern
listed
in
E
URI
Patterns
Associated
with
Mobile
Web
Sites
.
a
claim
of
mobileOK
Basic
[mobileOK
Basic
Tests]
conformance
is
indicated;
Editorial
Note:
a
resolution
was
taken
to
make
this
mandatory
but
as
indicated
(see
[mobileOK
Scheme]
for
how
such
a
feature
at
risk,
not
sure
I
understand
that
claim
may
be
indicated);
Other factors that a proxy may take into account:
The Web site (see note ) has previously shown that it is contextually aware, even if the present response does not indicate this;
the user agent has features (such as linearization or zoom) that allow it to present the content unaltered;
the response contains client side scripts that may misoperate if the resource is restructured;
the
response
is
an
HTML
response
and
it
includes
<link>
elements
specifying
alternatives
according
to
presentation
media
type.
Other than as noted in this section the nature of restructuring that is carried out, any character encoding alterations and what is omitted and what is inserted is, as discussed in 1.3 Scope , out of scope of this document.
If a proxy alters the response then:
It
must
add
a
Warning
214
Transformation
Applied
HTTP
header
field;
The altered content should validate according to an appropriate published formal grammar and if XML must be well-formed ;
It should indicate to the user that the content has been transformed for mobile presentation and provide an option to view the original, unmodified content.
Proxies must not rewrite links when content transformation is prohibited.
Editorial Note: 1s: await outcome of Chaals's ACTION-969 here
The practice of intercepting HTTPS links is strongly NOT RECOMMENDED .
If
a
proxy
rewrites
HTTPS
links,
replacement
links
must
have
the
scheme
https
.
When
forwarding
requests
originating
from
HTTPS
links
proxies
must
include
a
Via
header
field
as
discussed
under
4.1.6.1
Proxy
Treatment
of
Via
Header
Field
.
When forwarding responses from servers proxies must notify the user of invalid server certificates.
See example conformance statement from Francois (link below) and his covering note
See http://www.w3.org/2005/MWI/BPWG/Group/TaskForces/CT/editors-drafts/Guidelines/ics-081107
Request resource with original header fields
If the response is a 406 response:
If
the
response
contains
Cache-Control:
no-transform
,
forward
it
Otherwise request again with altered header fields
If the response is a 200 response:
Otherwise assess whether the 200 response is a form of "Request Unacceptable"
Proxy receives a request for resource P that it has not encountered before
Response is a desktop oriented representation of the resource
Proxy transforms this response into content that the user agent can display well and forwards it
Proxy receives a further request for the resource P
Response is a desktop oriented representation of the resource
Proxy transforms this response into content that the user agent can display well and forwards it
Proxy receives a request for resource P, that it has previously encountered as in F.2 Optimization based on Previous Server Interaction
Proxy forwards request with altered header fields
Response
is
200
OK
containing
a
Vary:
User-Agent
header
field
Proxy notices that behavior has changed and reissues the request with original header fields
Response is 200 OK and proxy forwards it
Content providers may wish to follow these procedures in order to improve interoperability.
Vary
HTTP
Header
Field
If
a
server
varies
its
representation
according
to
examination
of
received
HTTP
header
fields
then
[RFC
2616
HTTP]
describes
how
to
use
the
Vary
header
field
to
indicate
this.
Servers
that
are
aware
of
the
presence
of
a
transforming
proxy,
as
identified
by
a
Via
HTTP
Header
field
might
alter
their
responses
according
to
their
knowledge
of
specific
proxy
behavior.
When
doing
so
it
is
good
practice
to
make
sure
that
the
Internet
content
type
for
a
response
is
correct
for
the
actual
content
(e.g.
a
server
should
not
choose
Content-Type:
application/vnd.wap.xhtml+xml
because
it
suspects
that
proxies
will
not
transform
content
of
this
type,
if
its
content
is
not
valid
XHTML-MP).
If
a
server
has
distinct
representations
that
vary
according
to
the
target
presentation
media
type,
it
can
inhibit
transformation
of
the
response
by
including
a
Cache-Control:
no-transform
directive
(see
G.1.3
Server
Origination
of
Cache-Control:
no-transform
).
In
addition,
in
HTML
content
it
can
indicate
the
medium
for
which
the
representation
is
intended
by
including
a
link
element
identifying
in
its
media
attribute
the
target
presentation
media
types
of
this
representation
and
setting
the
href
attribute
to
"Same-Document
Reference"
(see
[RFC
3986]
section
4.4
)
and
in
particular
an
empty
href
attribute
is
a
"Same
Document
Reference".
In
addition
it
is
good
practice
but
do
we
have
a
reference
for
this
to
include
link
elements
identifying
the
target
presentation
media
types
of
other
available
representations
in
a
similar
manner.
If
content
for
more
than
one
presentation
media
type
is
served
from
the
same
URI,
it
is
better
not
to
use
a
link
element
identifying
the
presentation
media
types
as
the
URI
will
appear
to
be
a
"same
document
reference",
indicating
to
a
client
that
this
representation
is
suitable
for
all
the
named
presentation
media
types.
Instead,
use
a
Vary
HTTP
header
field
indicating
that
the
response
varies
according
to
the
received
User-Agent
HTTP
header
field.
I'm
really
not
sure
this
is
right
actually.
Think
we
need
to
bang
on
the
TAG's
door
again.
Note:
Some
examples
of
the
use
of
the
link
element
are
included
above
in
F
Example
Transformation
Interactions
.
The BPWG believes that POWDER will represent a powerful mechanism by which a server may express transformation preferences. Future work in this area may recommend the use of POWDER to provide a mechanism for origin servers to indicate more precisely which alternatives they have and what transformation they are willing to allow on them, and in addition to provide for Content Transformation proxies to indicate which services they are able to perform.
At present HTTP does not provide a mechanism for communicating original header field values. The scheme based on X-Device prefixed fields described under 4.1.5 Alteration of HTTP Header Field Values records and clarifies an approach used to achieve this effect by some content transformation proxies. This scheme relies upon non-standard HTTP fields, which are identified by their prefix as experimental according to IETF standards (notably RFC 822 and RFC 2076), and are not included in the IANA registry of HTTP header fields. While the mechanism defined in that section, based on current practice, applies to conforming transformation proxy deployments, it is possible that in future, in collaboration with the IETF, this approach will be reconsidered. This implies that the specified X-Device prefixed fields may, at some time, become deprecated in favor of new equivalent fields, or that an entirely different approach will be taken to representing such values.
A
number
of
mechanisms
exist
in
HTTP
which
might
be
exploited
given
more
precise
definition
of
their
operation
-
for
example
the
OPTIONS
method
and
the
HTTP
300
(Multiple
Choices)
Status.
The editor acknowledges contributions of various kinds from members of the Mobile Web Best Practices Working Group Content Transformation Task Force .
The editor acknowledges significant written contributions from: