Network Working Group N. Walsh
Internet-Draft Sun Microsystems
Intended status: Best Current R. Tobin
Practice University of Edinburgh
Expires: November 15, 2007 May 14, 2007
Human Readable Resource Identifiers
draft-walsh-tobin-hrri-01
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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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Human Readable Resource Identifiers . . . . . . . . . . . . 5
4. Relation to IRIs . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . 11
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1. Introduction
The syntactic constraints of IRIs (RFC 3987[3]) and URIs (RFC
3986[1]) 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].
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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], the href attribute in
XLink[5], and XML Base attributes[6]. 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.
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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:
o the control characters #x0 to #x1F and #x7F to #x9F
o space #x20
o the delimiters "<" #x3C, ">" #x3E, and """ #x22
o the unwise characters "{" #x7B, "}" #x7D, "|" #x7C, "\" #x5C,
"^" #x5E, and "`" #x60
These characters are percent encoded by applying steps 2.1 to 2.3 of
Section 3.1 of RFC 3987[3] 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] 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].
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.
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4. Relation to IRIs
Internationalized Resource Identifiers (IRIs)[3] 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.
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5. Security Considerations
A Human Readable Resource Identifier does not in itself pose a
security threat. However, 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.
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6. IANA Considerations
This document has no actions for IANA.
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7. References
7.1. Normative References
[1] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", RFC 3986,
January 2005.
[2] Bradner, S., "Key words for us in RFCs to Indicate Requirement
Levels", March 1997.
[3] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
7.2. Informative References
[4] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and F.
Yergeau, "Extensible Markup Language (XML) 1.0 (Forth Edition)",
September 2006.
[5] DeRose, S., Maler, E., and D. Orchard, "XML Linking Language
(XLink) Version 1.0", June 2001.
[6] Marsh, J., "XML Base", June 2001.
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Authors' Addresses
Norman Walsh
Sun Microsystems
1 Network Drive
Building #2 MS UBUR02-201
Burlington, MA 01803
US
Email: Norman.Walsh@Sun.COM
Richard Tobin
University of Edinburgh
HCRC, School of Informatics
2 Buccleuch Place
Edinburgh EH8 9LW
UK
Email: richard@inf.ed.ac.uk
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