Network Working Group J. Gregorio, Ed. Internet-Draft Google Intended status: Standards Track M. Hadley, Ed. Expires: September 27, 2008 Sun Microsystems M. Nottingham, Ed. D. Orchard BEA Systems, Inc. Mar 26, 2008 URI Template draft-gregorio-uritemplate-03 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 27, 2008. 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 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 Gregorio, et al. Expires September 27, 2008 [Page 1] Internet-Draft URI Template Mar 2008 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. Applicability . . . . . . . . . . . . . . . . . . . . . . 4 1.4. Notational Conventions . . . . . . . . . . . . . . . . . . 4 2. Characters . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. URI Template . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.1. Variables . . . . . . . . . . . . . . . . . . . . . . . . 5 4.2. Template Expansions . . . . . . . . . . . . . . . . . . . 6 4.3. Error Handling . . . . . . . . . . . . . . . . . . . . . . 6 4.4. URI Template Substitution . . . . . . . . . . . . . . . . 7 4.4.1. ('var') substitution . . . . . . . . . . . . . . . . . 8 4.4.2. The 'opt' operator . . . . . . . . . . . . . . . . . . 8 4.4.3. The 'neg' operator . . . . . . . . . . . . . . . . . . 8 4.4.4. The 'prefix' operator . . . . . . . . . . . . . . . . 9 4.4.5. The 'suffix' operator . . . . . . . . . . . . . . . . 9 4.4.6. The 'join' operator . . . . . . . . . . . . . . . . . 9 4.4.7. The 'list' operator . . . . . . . . . . . . . . . . . 10 4.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 11 5. Security Considerations . . . . . . . . . . . . . . . . . . . 12 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 7. Appendix A - Parsing URI Template Expansions . . . . . . . . . 13 8. Normative References . . . . . . . . . . . . . . . . . . . . . 13 Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 14 Appendix B. Revision History . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15 Intellectual Property and Copyright Statements . . . . . . . . . . 16 Gregorio, et al. Expires September 27, 2008 [Page 2] Internet-Draft URI Template Mar 2008 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 Here is an example that constructs a query from multiple variables: http://www.example.com/?{-join|&|query,number} And the following variables query := "mycelium" number := 100 The expansion of the URI Template is: http://www.example.com/?query=mycelium&number=100 Gregorio, et al. Expires September 27, 2008 [Page 3] Internet-Draft URI Template Mar 2008 The template expansion describes in a machine readable manner how the URI is to be constructed. http://www.example.com/?{-join|&|query,number} \____________________/ | | Join 'var=value' for each variable in ['query', 'number'] with '&'. 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. 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. Applicability While URI Templates use a notation that is similar to some URI path matching notations in web frameworks, URI Templates were not designed for that use case, nor are they appropriate for that purpose. URI Templates are not URIs, they do not identify an abstract or physical resource, they are not to be treated like URIs, nor should not be used in places where a URI would be expected. 1.4. Notational Conventions This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234], 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. Gregorio, et al. Expires September 27, 2008 [Page 4] Internet-Draft URI Template Mar 2008 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 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. Terminology o template processor - A program or library that converts a URI Template into a URI. o template expansion - The text between '{' and '}', including the enclosing brackets. 4. URI Template A URI Template is a sequence of characters that contains any number of embedded template expansions, see Section 4.2. Each expansion references one or more variables whose values are used when determining the substition value for an expansion. A URI Template becomes a URI when all the template expansions are substituted with their values (see Section 4.4). The generated URI will be a URI- reference, i.e. either an absolute URI or a relative reference. 4.1. Variables Every variable is either a Unicode string or a list of Unicode strings. Gregorio, et al. Expires September 27, 2008 [Page 5] Internet-Draft URI Template Mar 2008 A template expansion MAY reference variables that are unknown to the template processor. Those variables are 'undefined' and template expansion takes into consideration 'undefined' variables. Conversely, every variable that he template processor knows about is considered 'defined'. A variable that contains a string of length zero MUST NOT be considered 'undefined' by the template processor. A list variable that contains no members, that is of zero length, MUST NOT be considered 'undefined' by the template processor. Beyond the scope of this specification is the allowable programming constructs that can be used for a list variable. 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. Some variables may be supplied with default values. The default value must comde from ( unreserved / pct-encoded ). Note that there is no notation for supplying default values to list variables. 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. 4.2. 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 ) "}" 4.3. Error Handling During template substitution error conditions may arise. The exact circumstances for those errors are described in Section 4.4. When an error occurs the template processor MUST NOT return a URI. It is language specific and beyond the scope of this document how the template processor signals that an error has occured and that a URI will not be generated from the template. Gregorio, et al. Expires September 27, 2008 [Page 6] Internet-Draft URI Template Mar 2008 4.4. 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. A template processor take two inputs, a URI Template and a set of variables, and returns a URI-reference. Before substitution the template processor MUST convert every variable value into a sequence of characters in ( unreserved / pct- encoded ). The template processor does that using the following algorithm: The template processor normalizes the string using NFKC, converts it to UTF-8 [RFC3629], and then every octet of the UTF-8 string that falls outside of ( unreserved ) MUST be percent-encoded, as per [RFC3986], section 2.1. For variables that are lists, the above algorithm is applied to each value in the list. 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 KC (NFKC, Compatibility Decomposition, followed by Canonical Composition). Since different Normalized Forms unicode strings will have different UTF-8 represenations the only way to guarantee that template processors will produce the same URI is to require a common Normalized Form. Requiring that all characters outside of ( unreserved ) be percent encoded means that the only characters outside of ( unreserved ) that will appear in the generated URI-reference will come from outside the template expansions in the URI Template or from the argument of a template expansion. This means that the designer of the URI Template determines the placement of reserved characters in the resulting URI, and thus the structure of the resulting generated URI-reference. 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. The result of substitution MUST match the URI-reference rule and SHOULD also match any known rules for the scheme of the resulting URI. If a template processor encounters an operator that it does not understand then it MUST fail and MUST NOT produce a URI from the URI Template. The list of operators that a template processor knows is not constrained by this specification, that is, later specifications may add new operators. Gregorio, et al. Expires September 27, 2008 [Page 7] Internet-Draft URI Template Mar 2008 Every expansion consists of either a variable ('var') or an operator expression ('operator'), and the rules for how to expand each of these is given below. For every expansion a template MUST have at least one variable name in the template expansion. It is an error if no variables are supplied. All of the variables supplied to a template expansion MAY be undefined and the expansion rules below specify how to process the template expansion in that situation. 4.4.1. ('var') substitution In a variable ('var') expansion, if the variable is defined then substitute the value of the variable, otherwise substitute the default value. If no default value is given then substitute with the empty string. Example: foo := "fred" "{foo}" -> "fred" "{bar=wilma}" -> "wilma" "{baz}" -> "" 4.4.2. The 'opt' operator If each variable is undefined or an empty list then substitute the empty string, otherwise substitute the value of 'arg'. Example: foo := "fred" "{-opt|fred@example.org|foo}" -> "fred@example.org" "{-opt|fred@example.org|bar}" -> "" 4.4.3. The 'neg' operator If each variable is undefined or an empty list then substitute the value of arg, otherwise substitute the empty string. Example: foo := "fred" "{-neg|fred@example.org|foo}" -> "" "{-neg|fred@example.org|bar}" -> "fred@example.org" Gregorio, et al. Expires September 27, 2008 [Page 8] Internet-Draft URI Template Mar 2008 4.4.4. The 'prefix' operator The prefix operator MUST only have one variable in its expansion. More than one variable is an error condition. If the variable is undefined or an empty list then substitute the empty string. If the variable is a defined non-list then substitute the value of arg preceeded by the value of the variable. If the variable is a defined list then substitute the concatenation of every list value preceeded by the arg. Example: foo := "fred" bar := ["fee", "fi", "fo", "fum"] baz := [] "{-prefix|/|foo}" -> "/fred" "{-prefix|/|bar}" -> "/fee/fi/fo/fum" "{-prefix|/|baz}" -> "" "{-prefix|/|qux}" -> "" 4.4.5. The 'suffix' operator The prefix operator MUST only have one variable in its expansion. More than one variable is an error condition. If the variable is undefined or an empty list then substitute the empty string. If the variable is a defined non-list then substitute the value of arg followed by the value of the variable. If the variable is a defined list then substitute the concatenation of every list value followed by the arg. Example: foo := "fred" bar := ["fee", "fi", "fo", "fum"] baz := [] "{-suffix|/|foo}" -> "fred/" "{-suffix|/|bar}" -> "fee/fi/fo/fum/" "{-suffix|/|baz}" -> "" "{-suffix|/|qux}" -> "" 4.4.6. The 'join' operator Supplying a list variable to the join operator is an error. 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 Gregorio, et al. Expires September 27, 2008 [Page 9] Internet-Draft URI Template Mar 2008 values of arg to create the substitution value. The order of variables MUST be preserved during substitution. Example: foo := "fred" bar := "barney" baz := "" "{-join|&|foo,bar,baz,qux}" -> "foo=fred&bar=barney&baz=" "{-join|&|bar}" -> "bar=barney" "{-join|&|qux}" -> "" 4.4.7. The 'list' operator The listjoin operator MUST have only one variable in its expansion and that variable must be a list. More than one variable is an error. If the list is non-empty then substitute the concatenation of all the list members with intervening values of arg. If the list is empty or the variable is undefined them substitute the empty string. Example: foo := ["fred", "barney", "wilma"] bar := ["a", "", "c"] baz := ["betty"] qux := [] "{-list|/|foo}" -> "fred/barney/wilma" "{-list|/|bar}" -> "a//c" "{-list|/|baz}" -> "betty" "{-list|/|qux}" -> "" "{-list|/|corge}" -> "" Gregorio, et al. Expires September 27, 2008 [Page 10] Internet-Draft URI Template Mar 2008 4.5. Examples Given the following template variable names and values: +---------+----------------------------------+ | Name | Value | +---------+----------------------------------+ | foo | \u03d3 | | bar | fred | | baz | 10,20,30 | | qux | ["10","20","30"] | | corge | [] | | grault | | | garply | a/b/c | | waldo | ben & jerrys | | fred | ["fred", "", "wilma"] | | plugh | ["\u017F\u0307", "\u0073\u0307"] | | 1-a_b.c | 200 | +---------+----------------------------------+ Table 1 The variable 'foo' is the unicode character GREEK UPSILON WITH ACUTE AND HOOK SYMBOL. This character was chosen because it is one of only three characters that has a different normal form for each of the four normalization forms (NFC, NFD, NFKC, NFKD). The name 'xyzzy' has not been defined, the value of 'grault' is the empty string. The variables qux, corge, fred, and plugh are lists. The following URI Templates will be expanded as shown: ---- Gregorio, et al. Expires September 27, 2008 [Page 11] Internet-Draft URI Template Mar 2008 http://example.org/?q={bar} http://example.org/?q=fred /{xyzzy} / http://example.org/?{-join|&|foo,bar,xyzzy,baz} http://example.org/?foo=%CE%8E&bar=fred&baz=10%2C20%2C30 http://example.org/?d={-list|,|qux} http://example.org/?d=10,20,30 http://example.org/?d={-list|&d=|qux} http://example.org/?d=10&d=20&d=30 http://example.org/{bar}{bar}/{garply} http://example.org/fredfred/a%2Fb%2Fc http://example.org/{bar}{-prefix|/|fred} http://example.org/fred/fred//wilma {-neg|:|corge}{-suffix|:|plugh} :%E1%B9%A1:%E1%B9%A1: ../{waldo}/ ../ben%20%26%20jerrys/ telnet:192.0.2.16{-opt|:80|grault} telnet:192.0.2.16:80 :{1-a_b.c}: :200: ---- 5. 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. 6. 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. Gregorio, et al. Expires September 27, 2008 [Page 12] Internet-Draft URI Template Mar 2008 7. 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. >>> parse_expansion("-join|&|a,b,c=1") ('join', '&', {'a': None, 'c': '1', 'b': None}) >>> parse_expansion("c=1") (None, None, {'c': '1'}) 8. 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. Gregorio, et al. Expires September 27, 2008 [Page 13] Internet-Draft URI Template Mar 2008 [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. [RFC4395] Hansen, T., Hardie, T., and L. Masinter, "Guidelines and Registration Procedures for New URI Schemes", BCP 115, RFC 4395, February 2006. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [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] Appendix A. Contributors The following people made significant contributions to this specification: Michaeljohn Clement, DeWitt Clinton, John Cowan, James H. Manger, and James Snell. Appendix B. Revision History 03 - Added more examples. Introduced error conditions and defined their handling. Changed listjoin to list. Changed -append to -suffix, and allowed -prefix and -suffix to accept list variables. Clarified the handling of unicode. 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. Gregorio, et al. Expires September 27, 2008 [Page 14] Internet-Draft URI Template Mar 2008 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/ Gregorio, et al. Expires September 27, 2008 [Page 15] Internet-Draft URI Template Mar 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Gregorio, et al. Expires September 27, 2008 [Page 16]