Human Readable Resource Identifiers
draft-walsh-tobin-hrri-01c

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Copyright Notice

Copyright © The IETF Trust (2007).

Abstract

This memo defines Human Readable Resource Identifiers, strings which are interpreted as IRIs, but which allow the use of characters which must be escaped in a legal IRI, such as delimiters and a few other ASCII characters.

Table of Contents

1.  Introduction
2.  Motivation
3.  Human Readable Resource Identifiers
4.  Relation to IRIs
5.  Security Considerations
6.  IANA Considerations
7.  References
7.1.  Normative References
7.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

The syntactic constraints of IRIs (RFC 3987[3] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.)) and URIs (RFC 3986[1] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.)) mandate that certain common punctuation characters (such as spaces, quotation marks, and various sorts of delimiters) must be percent encoded. However, it is often inconvenient for authors to encode these characters.

Historically, XML system identifiers and, more generally, the value of XML attributes that are intended to contain IRIs or URIs have allowed authors to provide values that use these characters literally.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119[2] (Bradner, S., “Key words for us in RFCs to Indicate Requirement Levels,” March 1997.).

2.  Motivation

Several XML-related specifications use strings which are interpreted as IRIs, but which allow the use of characters which must be escaped in a legal IRI, such as delimiters and a few other ASCII characters. Examples include XML System Identifiers[4] (Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and F. Yergeau, “Extensible Markup Language (XML) 1.0 (Forth Edition),” September 2006.), the href attribute in XLink[5] (DeRose, S., Maler, E., and D. Orchard, “XML Linking Language (XLink) Version 1.0,” June 2001.), and XML Base attributes[6] (Marsh, J., “XML Base,” June 2001.). These specifications all describe, with slightly different wording, the same algorithm for converting that string to a URI or IRI. The purpose of this RFC is to provide a single definition which can be referenced by these specifications, and to provide a name for strings of this type: Human Readable Resource Identifiers.

3.  Human Readable Resource Identifiers

A Human Readable Resource Identifier (HRRI) is a sequence of Unicode characters that can be converted into an IRI by the application of a few simple encoding rules.

To convert a Human Readable Resource Identifier to an IRI reference, the following characters MUST be percent encoded:

• the control characters #x0 to #x1F and #x7F to #x9F
• space #x20
• the delimiters “<” #x3C, “>” #x3E, and “"” #x22
• the unwise characters “{” #x7B, “}” #x7D, “|” #x7C, “\” #x5C, “^” #x5E, and “`” #x60
• characters in the Unicode private use area (#xE000-#xF8FF), except where they appear in the query part of the resulting IRI.

These characters are percent encoded by applying steps 2.1 to 2.3 of Section 3.1 of RFC 3987[3] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.) to them.

Some strings are not Human Readable Resource Identifiers; for example, the string “%%%” is not an HRRI. A string is a legal Human Readable Resource Identifier if and only if the string generated by applying the encoding rules above is a legal IRI.

Note that in XML, the control character #x0 can never appear. Also, authors of HRRIs are advised to percent encode space characters themselves, rather than rely on the processor to do so, because spaces are often used to separate HRRIs in a sequence.

Processing a relative identifier against a base is handled straightforwardly; the algorithms of RFC 3986[1] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) can be applied directly, treating the characters additionally allowed in HRRIs in the same way that unreserved characters are treated in URI references.

If required, the IRI reference resulting from percent encoding an HRRI can be converted to a URI reference by following the prescriptions of Section 3.1 of RFC 3987[3] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.).

Conversion from a Human Readable Resource Identifier to an IRI or a URI MUST be performed only when absolutely necessary and as late as possible in a processing chain. In particular, neither the process of converting a relative Human Readable Resource identifier to an absolute one nor the process of passing a Human Readable Resource Identifier to a process or software component responsible for dereferencing it SHOULD trigger percent encoding.

4.  Relation to IRIs

Internationalized Resource Identifiers (IRIs)[3] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.) extend URIs by allowing unescaped non-ascii characters. Human Readable Resource Identifiers go further by allowing various ASCII characters that are illegal in both URIs and IRIs. By escaping these characters Human Readable Resource Identifiers can be converted to IRIs, which can in turn be converted to URIs if required.

5.  Security Considerations

Human Readable Resource Identifiers have the same security considerations as IRIs, see Section 8 of [3] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.). In addition, the use of control characters may allow malicious users to manipulate the displayed version of an HRRI.

Human Readable Resource Identifers are often converted to IRIs or URIs and subsequently used to provide a compact set of instructions for access to network resources, care must be taken to properly interpret the data within a Human Readable Resource Identifier, to prevent that data from causing unintended access, and to avoid including data that should not be revealed in plain text.

6.  IANA Considerations

This document has no actions for IANA.

7.  References

7.1. Normative References

7.2. Informative References

Authors' Addresses

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