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Versions: 00 01 02 03 04 05 06 07 08 RFC 6570
Network Working Group J. Gregorio, Ed.
Internet-Draft Google
Intended status: Standards Track M. Hadley, Ed.
Expires: May 29, 2008 Sun Microsystems
M. Nottingham, Ed.
D. Orchard
BEA Systems, Inc.
Nov 26, 2007
URI Template
draft-gregorio-uritemplate-02
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
A URI Template is a compact sequence of characters used for the
construction of URIs. This specification defines the URI Template
syntax and the process for expanding a URI Template into a URI, along
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with guidelines and security considerations for the use of URI
Templates on the Internet. The URI Template syntax allows for the
construction of strings that are a superset of URIs, allowing an
implementation to process any URI Template without knowing the
scheme-specific requirements of every possible resulting URI.
Editorial Note
To provide feedback on this Internet-Draft, join the W3C URI mailing
list (http://lists.w3.org/Archives/Public/uri/) [1].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Design Considerations . . . . . . . . . . . . . . . . . . 4
1.3. Notational Conventions . . . . . . . . . . . . . . . . . . 4
2. Characters . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. URI Template . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Variables . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Template Expansions . . . . . . . . . . . . . . . . . . . 6
3.3. URI Template Substitution . . . . . . . . . . . . . . . . 6
3.3.1. The 'opt' operator . . . . . . . . . . . . . . . . . . 7
3.3.2. The 'neg' operator . . . . . . . . . . . . . . . . . . 7
3.3.3. The 'prefix' operator . . . . . . . . . . . . . . . . 7
3.3.4. The 'append' operator . . . . . . . . . . . . . . . . 7
3.3.5. The 'join' operator . . . . . . . . . . . . . . . . . 7
3.3.6. The 'listjoin' operator . . . . . . . . . . . . . . . 7
3.4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Appendix A - Parsing URI Template Expansions . . . . . . . . . 10
7. Normative References . . . . . . . . . . . . . . . . . . . . . 11
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 11
Appendix B. Revision History . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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1. Introduction
A URI Template provides a simple and extensible format for URI
construction. A URI Template is a string that contains embedded
expansions, text marked off in matching braces ('{', '}'), that
denotes a part of the string that is to be substituted by a template
processor to produce a URI. A URI Template is transformed into a URI
by substituting the expansions with their calculated value.
Several specifications have defined URI Templates with varying levels
of formality, such as WSDL, WADL and OpenSearch. This specification
is derived from these concepts, giving a rigorous definition to such
templates.
This specification uses the terms "character" and "coded character
set" in accordance with the definitions provided in [RFC2978], and
"character encoding" in place of what [RFC2978] refers to as a
"charset".
1.1. Overview
A URI Template allows a structural description of URIs while allowing
a consumer of the template to construct a final URI by providing the
values of the expansion variables. For example, given the following
URI Template:
http://www.example.com/users/{userid}
And the following variable value
userid := fred
The expansion of the URI Template is:
http://www.example.com/users/fred
URI Templates can be used as a machine-readable forms language. By
allowing clients to form their own identifiers based on templates
given to them by the URI's authority it's possible to construct
dynamic systems that use more of the URI than traditional HTML forms.
For example:
http://www.example.org/products/{upc}/buyers?page={page_num}
URI Templates can also be used to compose URI-centric protocols
without impinging on authorities' control of their URI space. For
example, there are many emerging conventions for passing around login
information between sites using URIs. Forcing people to use a well-
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known query parameter isn't good practice, but using URI Templates
allows different sites to specify local ways of conveying the same
information:
http://auth.example.com/userauth;{return-uri}
http://login.example.org/login?back={return-uri}
1.2. Design Considerations
The URI Template syntax has been designed to carefully balance the
need for a powerful substitution mechanism with ease of
implementation and security. The syntax is designed to be easy to
parse while at the same time providing enough flexibility to express
many common templating scenarios. On the balance, the template
processing is not Turing complete, thus avoiding a number of security
issues, ala the billion-laughs attack of XML DTDs.
Another consideration was to keep the syntax and processing in-line
with the pre-existing templating schemes present in OpenSearch, WSDL
and WADL.
The final design consideration was control over the placement of
reserved characters in the URI generated from a URI Template. The
reserved characters in a URI Template can only appear in the non-
expansion text, or in the argument to an operator, both locations are
dictated by the URI Template author. Given the percent-encoding
rules for variable values this means that the source of all
structure, i.e reserved characters, in a URI generated from a URI
Template is decided by the URI Template author.
1.3. Notational Conventions
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC4234], including the following core ABNF syntax rules
defined by that specification: ALPHA (letters) and DIGIT (decimal
digits). See [RFC3986] for the definitions of the URI-reference,
percent-encoded, reserved, and unreserved rules.
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 [RFC2119].
2. Characters
A URI Template is a sequence of characters, and has the same issues
as URIs with regard to codepoints and character sets. That is, URI
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Template characters are frequently encoded as octets for transport or
presentation. This specification does not mandate any particular
character encoding for mapping between URI characters and the octets
used to store or transmit those characters. When a URI appears in a
protocol element, the character encoding is defined by that protocol;
without such a definition, a URI is assumed to be in the same
character encoding as the surrounding text.
The ABNF notation defines its terminal values to be non-negative
integers (codepoints) based on the US-ASCII coded character set
[ASCII]. Because a URI is a sequence of characters, we must invert
that relation in order to understand the URI syntax. Therefore, the
integer values used by the ABNF must be mapped back to their
corresponding characters via US-ASCII in order to complete the syntax
rules.
3. URI Template
A URI Template is a sequence of characters that contains one or more
embedded template expansions, see Section 3.2. Each expansion
references one or more variables whose values are used in when
determining the substition value for an expansion. A URI Template
becomes a URI when the template expansions are substituted with their
values (see Section 3.3). The generated URI will be a URI-reference,
i.e. either an absolute URI or a relative reference.
3.1. Variables
The value of every non-list variable, and the individual values in
list variables, must come from ( unreserved / pct-encoded ). For
variable values that are strings that have characters outside that
range, the entire string must be converted into UTF-8 [RFC3629], and
then every octet of the UTF-8 string that falls outside of (
unreserved / pct-encoded ) MUST be percent-encoded, as per [RFC3986],
section 2.1.
This does not imply that every variable value can be decoded into a
Unicode string. For example, a variable value may be a binary blob
that has been percent-encoded before being passed into the template
processor.
The Unicode Standard [UNIV4] defines various equivalences between
sequences of characters for various purposes. Unicode Standard Annex
#15 [UTR15] defines various Normalization Forms for these
equivalences, in particular Normalization Form C (NFC, Canonical
Decomposition, followed by Canonical Composition) and Normalization
Form KC (NFKC, Compatibility Decomposition, followed by Canonical
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Composition). Since different Normalized Forms unicode strings will
have different UTF-8 represenations it is RECOMMEDED that unicode
strings use Normalized Form NFC.
The meaning of 'defined' for a variable is progamming language and
library specific and beyond the scope of this specification. Also
beyond the scope of this specification is the allowable programming
constructs that can be used for a list variable used in the
'listjoin' operator. For example, a Python implementation might
allow only built-in list types, or it may allow any iterable to be
used as the source for a list variable.
A variable may appear in more than one expansion in a URI Template.
The value used for that variable must remain the same for every
template expansion when converting a URI Template into a URI.
3.2. Template Expansions
Template expansions are the parameterized components of a URI
Template. A template expansion MUST match the 'expansion' rule.
op = 1*ALPHA
arg = *(reserved / unreserved / pct-encoded)
var = varname [ '=' vardefault ]
vars = var [ *("," var) ]
varname = (ALPHA / DIGIT)*(ALPHA / DIGIT / "." / "_" / "-" )
vardefault = *(unreserved / pct-encoded)
operator = "-" op "|" arg "|" vars
expansion = "{" ( var / operator ) "}"
3.3. URI Template Substitution
Template substitution is the process of turning a URI Template into a
URI given definitions for the variables used in the template.
Substitution replaces each expansion with its calculated value.
Every expansion consists of either a variable ('var') or an operator
expression. In a variable ('var') expansion, if the variable is
defined and non-empty then substitute the value of the variable,
otherwise substitute the default value. If no default value is given
then substitute with the empty string.
If the expansion is an operator then the substitution value is
determined by the given operator. Each operator works only on the
variables that are defined within their expansion.
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3.3.1. The 'opt' operator
If the one or more of the variables are defined and non-empty then
substitute the value of 'arg', otherwise substitute the empty string.
3.3.2. The 'neg' operator
If all of the variables are un-defined or empty then substitute the
value of arg, otherwise substitute the empty string.
3.3.3. The 'prefix' operator
The prefix operator MUST only have one variable in its expansion. If
the variable is defined and non-empty then substitute the value of
arg followed by the value of the variable, otherwise substitute the
empty string.
3.3.4. The 'append' operator
The append operator MUST only have one variable in its expansion. If
the variable is defined and non-empty then substitute the value of
the variable followed by the value of arg, otherwise substitute the
empty string.
3.3.5. The 'join' operator
For each variable that is defined and non-empty create a keyvalue
string that is the concatenation of the variable name, "=", and the
variable value. Concatenate more than one keyvalue string with
intervening values of arg to create the substitution value.
3.3.6. The 'listjoin' operator
The listjoin operator MUST have only one variable in its expansion
and that variable must be a list. If the list is non-empty then
substitute the concatenation of all the list members with intevening
values of arg.
The result of substitution MUST match the URI-reference rule and
SHOULD also match any known rules for the scheme of the resulting
URI.
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3.4. Examples
Given the following template variable names and values:
+----------+--------------------+
| Name | Value |
+----------+--------------------+
| a | foo |
| b | bar |
| data | 10,20,30 |
| points | ["10","20", "30"] |
| list0 | [] |
| str0 | |
| reserved | :/?#[]@!$&'()*+,;= |
| u | \u2654\u2655 |
| a_b | baz |
+----------+--------------------+
Table 1
The name 'foo' has not been defined, the value of 'str0' is the empty
string, and both list0 and points are lists. The variable 'u' is a
string of two unicode characters, the WHITE CHESS KING (0x2654) and
the WHITE CHESS QUEEN (0x2655).
The following URI Templates will be expanded as shown:
----
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http://example.org/?q={a}
http://example.org/?q=foo
http://example.org/{foo}
http://example.org/
relative/{reserved}/
relative/%3A%2F%3F%23%5B%5D%40%21%24%26%27%28%29%2A%2B%2C%3B%3D/
http://example.org/{foo=fred}
http://example.org/fred
http://example.org/{foo=%25}/
http://example.org/%25/
/{-prefix|#|foo}
/
./{-prefix|#|str0}
./
/{-append|/|a}{-opt|data|points}{-neg|@|a}{-prefix|#|b}
/foo/data#bar
http://example.org/q={u}
http://example.org/q=%E2%99%94%E2%99%95
http://example.org/?{-join|&|a,data}
http://example.org/?a=foo&data=10%2C20%2C30
http://example.org/?d={-listjoin|,|points}&{-join|&|a,b}
http://example.org/?d=10,20,30&a=foo&b=bar
http://example.org/?d={-listjoin|,|list0}&{-join|&|foo}
http://example.org/?d=&
http://example.org/?d={-listjoin|&d=|points}
http://example.org/?d=10&d=20&d=30
http://example.org/{a}{b}/{a_b}
http://example.org/foobar/baz
http://example.org/{a}{-prefix|/-/|a}/
http://example.org/foo/-/foo/
----
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4. Security Considerations
A URI Template does not contain active or executable content. Other
security considerations are the same as those for URIs, see section 7
of RFC3986.
5. IANA Considerations
In common with RFC3986, URI scheme names form a registered namespace
that is managed by IANA according to the procedures defined in
[RFC4395]. No IANA actions are required by this document.
6. Appendix A - Parsing URI Template Expansions
Parsing a valid URI Template expansion does not require building a
parser from the given ABNF. Instead, the set of allowed characters
in each part of URI Template expansion has been chosen to avoid
complex parsing, and breaking an expansion into its component parts
can be achieved by a series of splits of the character string.
Here is example Python code that parses a URI Template expansion and
returns the operator, argument, and variables as a tuple. The
variables are returned as a dictionary of variable names mapped to
their default values. If no default is given then the name maps to
None.
def parse_expansion(expansion):
if "|" in expansion:
(op, arg, vars_) = expansion.split("|")
op = op[1:]
else:
(op, arg, vars_) = (None, None, expansion)
vars_ = vars_.split(",")
variables = {}
for var in vars_:
if "=" in var:
(varname, vardefault) = var.split("=")
else:
(varname, vardefault) = (var, None)
variables[varname] = vardefault
return (op, arg, variables)
And here is an example of the parse_expansion() function being used.
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>>> parse_expansion("-join|&|a,b,c=1")
('join', '&', {'a': None, 'c': '1', 'b': None})
>>> parse_expansion("c=1")
(None, None, {'c': '1'})
7. Normative References
[ASCII] American National Standards Institute, "Coded Character
Set - 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, October 2000.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[RFC4395] Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
Registration Procedures for New URI Schemes", BCP 115,
RFC 4395, February 2006.
[UNIV4] The Unicode Consortium, "The Unicode Standard, Version
4.0.1, defined by: The Unicode Standard, Version 4.0
(Reading, MA, Addison-Wesley, 2003. ISBN 0-321-18578-1),
as amended by Unicode 4.0.1
(http://www.unicode.org/versions/Unicode4.0.1/)",
March 2004.
[UTR15] Davis, M. and M. Duerst, "Unicode Normalization Forms",
Unicode Standard Annex # 15, April 2003.
[1] <http://lists.w3.org/Archives/Public/uri/>
Appendix A. Contributors
The following people made significant contributions to this
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specification: DeWitt Clinton and James Snell.
Appendix B. Revision History
02 - Added operators and came up with coherent percent-encoding and
reserved character story. Added large examples section which is
extracted and tested against the implementation.
01
00 - Initial Revision.
Authors' Addresses
Joe Gregorio (editor)
Google
Email: joe@bitworking.org
URI: http://bitworking.org/
Marc Hadley (editor)
Sun Microsystems
Email: Marc.Hadley@sun.com
URI: http://sun.com/
Mark Nottingham (editor)
Email: mnot@pobox.com
URI: http://mnot.net/
David Orchard
BEA Systems, Inc.
Email: dorchard@bea.com
URI: http://bea.com/
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