IETF conneg working group Graham Klyne Internet draft 5GM/Content Technologies Category: Work-in-progress 12 February 1999 Expires: August 1999 Identifying composite media features Status of this memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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." To view the list Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society 1999. All Rights Reserved. Abstract In "A syntax for describing media feature sets" [1], an expression format is presented for describing media feature capabilities as a combination of simple media feature tags [2]. This document proposes an abbreviated format for a composite media feature set, based upon a hash of the feature expression describing that composite. Klyne Work-in-progress [Page 1] Internet Draft Identifying composite meadia features 12 February 1999 Table of contents 1. Introduction ............................................2 1.1 Organization of this document 2 1.2 Terminology and document conventions 3 2. Motivation and goals ....................................3 3. Composite feature representation ........................4 3.1 Feature set reference format 4 3.2 Hash value calculation 5 3.3 Dereferencing feature set expressions 6 3.3.1 Inline feature set details 6 3.3.2 URI reference 6 3.4 The birthday problem 7 4. Feature set resolution and matching .....................8 5. Examples ................................................8 6. Internationalization considerations .....................8 7. Security considerations .................................9 8. Full copyright statement ................................9 9. Acknowledgements ........................................10 10. References .............................................10 11. Author's address .......................................11 Appendix A: Revision history ...............................11 1. Introduction In "A syntax for describing media feature sets" [1], an expression format is presented for describing media feature capabilities as a combination of simple media feature tags [2]. This document proposes an abbreviated format for a composite media feature set, based upon a hash of the feature expression describing that composite. 1.1 Organization of this document Section 2 sets out somne of the background and goals for feature set references. Section 3 preents a syntax for feature set references, and describes how they are related to feature set expressions. Section 4 discusses how feature set references are used in conction with feature set matching. Klyne Work-in-progress [Page 2] Internet Draft Identifying composite meadia features 12 February 1999 1.2 Terminology and document conventions This section defines a number of terms and other document conventions, which are used with specific meaning in this memo. The terms are listed in alphabetical order. dereference the act of replacing a feature set reference with its corresponding feature set expression. feature set some set of media features described by a media feature assertion, as described in "A syntax for describing media feature sets" [1]. (See that memo for a more formal definition of this term.) feature set expression a string that describes some feature set, formulated according to the rules in "A syntax for describing media feature sets" [1] (and possibly extended by other specifications). feature set reference a brief construct that references some feature set. (See also: "dereference".) This specification uses syntax notation and conventions described in RFC2234 "Augmented BNF for Syntax Specifications: ABNF" [3]. NOTE: Comments like this provide additional nonessential information about the rationale behind this document. Such information is not needed for building a conformant implementation, but may help those who wish to understand the design in greater depth. 2. Motivation and goals The range of media feature capabilities of a message handling system can be quite extensive, and the corresponding feature set expression [1] can reach a significant size. A requirement has been identified to allow recurring feature sets to be identified by a single reference value, which can be combined with other elements in a feature set expression. It is anticipated that mechanisms will be provided that allow the recipient of such a feature set reference to discover the corresponding feature set expression. Klyne Work-in-progress [Page 3] Internet Draft Identifying composite meadia features 12 February 1999 Thus, the goals for this proposal are: o to provide an abbreviated form for referencing an arbitrary feature set expression. o the meaning of (i.e. the corresponding feature set expression) a feature set reference should be independent of any particular mechanism that may be used to dereference it. o to be able to verify whether a given feature set expression corresponds to some feature set reference without having to perform an explicit dereferencing operation (i.e. without incurring additional network traffic). o for protocol processors that conform to [1] to be able to sensibly handle a feature set reference without explicit knowledge of its meaning (i.e. the introduction of feature set references should not break existing feature expression processors). o to allow, but not require, some indication of how to dereference a feature set reference to be included in a feature set expression. This proposal does not attempt to address the "override" or "default" problem. (Also called "delegation", where a feature set may be referenced and selectively overridden.) 3. Composite feature representation This specification hinges on two central ideas: o the use of auxiliary predicates (introduced in [1]) to form the basis of a feature set reference, and o the use of a token based on a hash function computed over the referenced feature set expression. 3.1 Feature set reference format This specification introduces a special form of auxililiary predicate name with the following syntax: fname = "h." 1*HEXDIG The sequence of hexadecimal digits is the value of a hash function calculated over the corresponding feature set expression (see next section), represented as a hexadecimal number. Klyne Work-in-progress [Page 4] Internet Draft Identifying composite meadia features 12 February 1999 Thus, within a feature set expression, a feature set reference would have the following form: (h.123456789abcdef0123456789abcdef0) NOTE: Base64 representation (per MIME [4]) would be more compact (21 rather than 32 characters for the hash value), but an auxiliary predicate name is defined (by [1]) to have the same syntax as a feature tag, and the feature tag matching rules (per [2]) state that feature tag matching is case in sensitive. 3.2 Hash value calculation The hash value is calculated using the MD5 algorithm [6] over the text of the referenced feature set expression subjected to certain normalizations. The feature expression must conform to the syntax given in "A syntax for describing media feature sets" [1] for 'filter': filter = "(" filtercomp ")" *( ";" parameter ) The steps for calculating a hash value are: 1. Whitespace normalization: all spaces, CR, LF, TAB and any other layout control characters that may be embedded in the feature expression string are removed (or ignored for the purpose of hash value computation). 2. Case normalization: all lower case letters in the feature expression, other than those contained within quoted strings, are converted to upper case. That is, unquoted characters with values 97 to 122 (decimal) are changed to corresponding characters in the range 65 to 90. 3. Hash computation: the MD5 algorithm [6] is applied to the normalized feature expression string. The result obtained in step 3 is a 128-bit number that is converted to a hexadecimal representation to form the feature set reference. NOTE: under some circumstances, removal of ALL whitespace may result in an invalid feature expression string. This should not be a problem as significantly different feature expressions are expected to differ in ways other than their whitespace. NOTE: case normalization is deemed appropriate since feature tag and token matching is case insensitive. Klyne Work-in-progress [Page 5] Internet Draft Identifying composite meadia features 12 February 1999 3.3 Dereferencing feature set expressions This memo does not mandate any particular mechanism for defeferencing a feature set reference. It is expected that specific dereferencing mechanisms will be specified for any application that uses them. The following sections describe two specific ways that feature set dereferencing information may be incorporated into a feature set expression. Both of these mechanisms are based on auxiliary predicate definitions within a "where" clause [1]. NOTE: both of the forms described below may be used with feature set references that are not constructed as "h." values described above. The consequence of not using hash-based reference values is that feature set differences, changes or other errors may be undetectable. 3.3.1 Inline feature set details The feature set expression associated with a reference value may be specified directly in a "where" clause, using the auxiliary predicate definition syntax [1]; e.g. (& (dpi=100) (h.1234567890) ) where (h.1234567890) :- (& (pix-x<=200) (pix-y<=150) ) This form might be used on request (where the request mechanism is defined by the invoking application protocol), or when the originator believes the recipient may not understand the reference. 3.3.2 URI reference This and associates a URI with a feature set reference. NOTE: How a calling application interprets the URI is not specified here. For URIs that are URLs, one reasonable approach would be to use the URL scheme protocol to access the corresponding feature set expression. But other mechanisms are possible. [[[e.g. RESCAP?]]] An auxiliary predicate name is defined to be a feature tag [1], and one allowable form for a feature tag is 'u.' [2]. Thus a standard form of auxiliary predicate definition can be used to associate a URI with a feature set reference: (h.1234567890) :- (u.http://www.acme.com/widget-feature/modelT) Klyne Work-in-progress [Page 6] Internet Draft Identifying composite meadia features 12 February 1999 [[[The range of URI forms allowed by [2] is restricted, and that restriction would apply to the above proposal. Another approach would be to introduce some new syntax... A new form of auxiliary predicate definition is introduced, extending the feature expression syntax [1]: named-pred =/ "(" fname ")" ":-" "<" URI ">" URI = An example predicate definition using this form is: (h.1234567890) :- ...]]] 3.4 The birthday problem NOTE: this entire section is commentary, and does not affect the feature set reference specification in any way. The use of a hash value to represent an arbitrary feature set is based on a presumption that no two distinct feature sets will yield the same hash value. There is clearly a small but distinct possibility that two different feature sets will indeed yield the same hash value. We assume that the hash function distributes hash values for feature sets with even very small differences randomly and evenly through the range of 2^128 (approximately 10^38) possible values. This is a fundamental property of a good digest algorithm like MD5. Thus, the chance that any two distinct feature set expressions yield the same hash is roughly 1 in 10^38. This is negligible when compared with, say, the probability that a receiving system will fail having received data conforming to a negotiated feature set. But when the number of distinct feature sets in circulation increases, the probability of clashing hash values increases surprisingly. This is illustrated by the "birthday paradox": given a random collection of just 23 people, there is a greater than even chance that there exists some pair with the same birthay. This topic is discussed further in sections 7.4 and 7.5 of Bruce Scheier's "Applied Cryptography" [7]. [[[TODO: Include some numbers to illustrate actual probabilities of clash with 10^3, 10^6, 10^9, 10^12, 10^15, 10^18 feature sets in circulation.]]] Klyne Work-in-progress [Page 7] Internet Draft Identifying composite meadia features 12 February 1999 If original feature set expressions are generated manually, or only in response to some manually constrained process, the total number of feature sets in circulation is likely to remain very small in relation to the total number of possible hash values. The outcome of all this is: assuming that the feature sets are manually generated, even taking account of the birthday paradox effect, the probability of incorrectly identifying a feature set using a hash value is still negligibly small when compared with other possible failure modes. 4. Feature set resolution and matching This section discusses the use of feature references in conjunction with feature set matching [1]. The definitive position on matching feature sets containing feature set references is given by dereferencing all of the references; i.e. every feature set reference is replaced by the corresponding expression. Sometimes, it may be desirable to process feature sets without performing dereferencing. The rules below may facilitate this while achieving results that are consistent with the definitive position. (& ... (h.) (h.) ... ) --> (& ... (h.) ... ) (| ... (h.) (h.) ... ) --> (& ... (h.) ... ) (& ... (h.) (! (h.) ) ... ) --> FALSE (| ... (h.) (! (h.) ) ... ) --> TRUE If some referenced feature set is known to be TRUE or FALSE, then the corresponding references may be replaced by the corresponding TRUE or FALSE value. [[[Can more be said?]]] 5. Examples [[[TODO]]] 6. Internationalization considerations Feature set expressions are currently defined to consist of only characters from the US-ASCII repertoire; under these circumstances this specification is not impacted by internationalization considerations. Klyne Work-in-progress [Page 8] Internet Draft Identifying composite meadia features 12 February 1999 But, if future revisions of the feature set syntax permit non-US- ASCII characters (e.g. within quoted strings), then some canonical representation must be defined for the purposes of calculating hash values. One choice might be to use a UTF-8 equivalent representation as the basis for calculating the feature set hash. Another choice might be to leave this as an application protocol issue (but this could lead to non-interoperable feature sets between different protocols). Another conceivable issue is that of up-casing the feature expression in preparation for computing a hash value. This does not apply to the content of strings so is not likely to be an issue. But if changes are made that do permit non-US-ASCII characters in feature tags or token strings, consideration must be given to properly defining how case conversion is to be performed. 7. Security considerations <<>> 8. Full copyright statement Copyright (C) The Internet Society 1999. All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS 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. Klyne Work-in-progress [Page 9] Internet Draft Identifying composite meadia features 12 February 1999 9. Acknowledgements This proposal is developed from a suggestion by Larry Masinter. Some of the ideas have been honed in early discussions with Martin Duerst, Al Gilman, Ted Hardie and Bill Newman. 10. References [1] "A syntax for describing media feature sets" Graham Klyne, 5GM/Content Technologies Internet draft: " Work in progress, September 1998. [2] "Media Feature Tag Registration Procedure" Koen Holtman, TUE Andrew Mutz, Hewlett-Packard Ted Hardie, NASA Internet draft: Work in progress, July 1998. [3] RFC 2234, "Augmented BNF for Syntax Specifications: ABNF" D. Crocker (editor), Internet Mail Consortium P. Overell, Demon Internet Ltd. November 1997. [4] RFC 2045, "Multipurpose Internet Mail Extensions (MIME) Part 1: Format of Internet message bodies" N. Freed, Innosoft N. Borenstein, First Virtual November 1996. [5] RFC 2396, "Uniform Resource Identifiers (URI): Generic Syntax", Tim Berners-Lee, World Wide Web Consortium/MIT Roy T. Fielding, University of California, Irvine Larry Masinter, Xerox PARC August 1998. [6] RFC 1321, "The MD5 Message-Digest Algorithm", R. Rivest, MIT Laboratory for Computer Science and RSA Data Security, Inc., April 1992. [7] "Applied Cryptography" Bruce Schneier John Wiley and Sons, 1996 (second edition) ISBN 0-471-12845-7 (cloth) ISBN 0-471-11709-9 (paper) Klyne Work-in-progress [Page 10] Internet Draft Identifying composite meadia features 12 February 1999 11. Author's address Graham Klyne 5th Generation Messaging Ltd. Content Technologies Ltd. 5 Watlington Street Forum 1, Station Road Nettlebed Theale Henley-on-Thames, RG9 5AB Reading, RG7 4RA United Kingdom United Kingdom. Telephone: +44 1491 641 641 +44 118 930 1300 Facsimile: +44 1491 641 611 +44 118 930 1301 E-mail: GK@ACM.ORG Appendix A: Revision history 00a 10-Feb-1999 Initial draft. Klyne Work-in-progress [Page 11]