IETF conneg working group Graham Klyne, editor Internet draft 5GM/Content Technologies Category: Work-in-progress Larry Masinter Xerox Corporation 16 June 1999 Expires: December 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 RFC 2026. 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. Copyright Notice Copyright (C) The Internet Society 1999. All Rights Reserved. Abstract In RFC 2533 [1], an expression format is presented for describing media feature capabilities as a combination of simple media feature tags [2]. This document describes an abbreviated format for a composite media feature set, based upon a hash of the feature expression describing that composite, or the URI of a resource containing the feature expression. Klyne & Masinter Work-in-progress [Page 1] Internet Draft Identifying composite media features 16 June 1999 Table of contents 1. Introduction ............................................2 1.1 Organization of this document 2 1.2 Terminology and document conventions 3 1.3 Discussion of this document 3 2. Motivation and goals ....................................4 3. Composite feature representation ........................5 3.1 Feature set hashed reference format 5 3.1.1 Hash value calculation 6 3.1.2 Base-32 value representation 7 3.2 Feature set URI reference format 8 3.3 Resolving feature set references 9 3.3.1 URI reference 9 3.3.2 Inline feature set details 10 4. Examples ................................................11 5. Internationalization considerations .....................11 6. Security considerations .................................12 7. Full copyright statement ................................12 8. Acknowledgements ........................................13 9. References ..............................................13 10. Authors' addresses .....................................14 Appendix A The birthday problem ............................14 Appendix B: Revision history ...............................16 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 some of the background and goals for feature set references. Section 3 presents a syntax for feature set references, and describes how they are related to feature set expressions. Klyne & Masinter Work-in-progress [Page 2] Internet Draft Identifying composite media features 16 June 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. Also called "resolution". 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".) feature set tag a name that conforms to the syntax of a feature tag [1] that is used to denote a feature set rather than a single feature. resolution (See "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. 1.3 Discussion of this document Discussion of this document should take place on the content negotiation and media feature registration mailing list hosted by the Internet Mail Consortium (IMC). Klyne & Masinter Work-in-progress [Page 3] Internet Draft Identifying composite media features 16 June 1999 Please send comments regarding this document to: ietf-medfree@imc.org To subscribe to this list, send a message with the body 'subscribe' to "ietf-medfree-request@imc.org". To see what has gone on before you subscribed, please see the mailing list archive at: http://www.imc.org/ietf-medfree/ 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. 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. Klyne & Masinter Work-in-progress [Page 4] Internet Draft Identifying composite media features 16 June 1999 NOTE: This proposal does not attempt to address the "override" or "default" problem. (Also called "delegation", where a feature set may be referenced and selectively modified.) 3. Composite feature representation This specification hinges on three 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. o the use of an expression containing a URI to indicate an identifier and possibly a service for resolution of a composite feature set. A key reason to use a hash function to generate an identifier is to define a global name space without requiring a central naming authority. New feature set tags can be introduced by any party following the appropriate rules of formulation, without reference to any centralized authority. Local resolution services may be needed to map feature set tags to their corresponding feature set expressions, but these are not able to vary the meaning of any given tag. Failure of a resolution service to return the correct expression is detectable by a calling application, which should reject any incorrect value supplied. This memo also suggests that an expression containing a URI in the format '' may be used to indicate a mechanism and location of a service to perform feature set resolution. 3.1 Feature set hashed reference format This specification introduces a special form of auxiliary predicate name with the following syntax: fname = "h." 1*BASE32DIGIT BASE32DIGIT = DIGIT / "A" / "B" / "C" / "D" / "E" / "F" / "G" / "H" / "I" / "J" / "K" / "L" / "M" / "N" / "O" / "P" / "Q" / "R" / "S" / "T" / "U" / "V" The sequence of base-32 digits represents the value of a hash function calculated over the corresponding feature set expression Klyne & Masinter Work-in-progress [Page 5] Internet Draft Identifying composite media features 16 June 1999 (see following sections). Note that the above syntax allows upper- or lower- case letters for base-32 digits (per RFC 2234 [3]). Thus, within a feature set expression, a hashed feature set reference would have the following form: (h.123456789abcdefghijklmnopq) 3.1.1 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 for 'filter' in RFC 2533 [1]: 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, described in RFC 1321 [6], is applied to the normalized feature expression string. The result obtained in step 3 is a 128-bit (16 octet) value that is converted to a base-32 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 this is done only for the purpose of calculating a hash value, and 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 & Masinter Work-in-progress [Page 6] Internet Draft Identifying composite media features 16 June 1999 3.1.2 Base-32 value representation RFC 1321 [6] describes how to calculate an MD5 hash value that is a sequence of 16 octets. This is then required to be coded as a base-32 value, which is a sequence of base-32 digit characters. Each successive character in a base-32 value represents 5 successive bits of the underlying octet sequence. Thus, each group of 8 characters represents a sequence of 5 octets (40 bits): 1 2 3 01234567 89012345 67890123 45678901 23456789 +--------+--------+--------+--------+--------+ |< 1 >< 2| >< 3 ><|.4 >< 5.|>< 6 ><.|7 >< 8 >| +--------+--------+--------+--------+--------+ <===> 8th character <====> 7th character <===> 6th character <====> 5th character <====> 4th character <===> 3rd character <====> 2nd character <===> 1st character The value (i.e. sequence of bits) represented by each base-32 digit character is indicated by the following table: "0" 0 "A" 10 "K" 20 "U" 30 "1" 1 "B" 11 "L" 21 "V" 31 "2" 2 "C" 12 "M" 22 "3" 3 "D" 13 "N" 23 "4" 4 "E" 14 "O" 24 "5" 5 "F" 15 "P" 25 "6" 6 "G" 16 "Q" 26 "7" 7 "H" 17 "R" 27 "8" 8 "I" 18 "S" 28 "9" 9 "J" 19 "T" 29 When encoding a base-32 value, each full group of 5 octets is represented by a sequence of 8 characters indicated above. If a group of less than 5 octets remain after this, they are encoded using as many additional characters as may be needed: 1, 2, 3 or 4 octets are encoded by 2, 4, 5 or 7 characters respectively. Any spare bits represented by the base-32 digit characters are selected to be zero. When decoding a base-32 value, the reverse mapping is applied: each full group of 8 characters codes a sequence of 5 octets. A final group of 2, 4, 5 or 7 characters codes a sequence of 1, 2, 3 or 4 octets respectively. Any spare bits represented by the final group of characters are discarded. Klyne & Masinter Work-in-progress [Page 7] Internet Draft Identifying composite media features 16 June 1999 Thus, for a 128-bit (16 octet) MD5 hash value, the first 15 octets are coded as 24 base 32 digit characters, and the final octet is coded by two characters. NOTE: Base64 representation (per MIME [4]) would be more compact (21 rather than 26 characters for the MD5 128-bit 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 insensitive. Base36 representation was considered (i.e. using all letters "A"-"Z") but was not used because this would require extended precision multiplication and division operations to encode and decode the hash values. 3.2 Feature set URI reference format This section introduces a new form of feature set predicate by extending the feature set syntax [1] as follows: filter =/ "<" URI ">" *( ";" parameter ) where 'URI' is described by "Uniform Resource Identifiers (URI): Generic Syntax" [5]. The meaning of this construct is defined to be the meaning of the expression in which '' is replaced by a copy of the resource indicated by 'URI'. The indicated resource is required to be a text value containing a valid feature set expression, NOT itself containing a '' reference. If a '' reference is used within a feature expression that defines a hash reference, then the hash value is calculated over the expression obtained after the resource has been subsituted. Thus, the following are examples of feature set expressions using URI references: (& (dpi=100) ) This specification does not indicate: o any specific URI schemes to be supported, o any meaning if the resource cannot be accessed, of if the value obtained does not correspond to some recognized format. Klyne & Masinter Work-in-progress [Page 8] Internet Draft Identifying composite media features 16 June 1999 These details must be indicated by the specification of any application or protocol that relies upon this form of a feature predicate. If the URI uses characters other than a designated subset of US- ASCII then those additional characters should be represented by a sequence of US-ASCII characters allowed by RFC 2396 [5]. NOTE: the syntax above allows a '' reference to appear almost anywhere in a feature set expression. In some contexts, the appearance of these references may be restricted to specific locations (e.g. the right hand side of a auxiliary feature predicate, as indicated in section 3.3.1). The specification of any application or protocol should fully describe any such restriction that it may impose. 3.3 Resolving feature set references 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 or protocol that uses them. The following sections describe some 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]. When a hashed feature set reference is used, conformance to the hashing rules takes precedence over any other determination of the feature expression. Any expression, however obtained, may not be substituted for the hash-based reference unless it yields the correct hash value. 3.3.1 URI reference The two formats for feature set references described above may be combined by defining the meaning of a hashed reference to be a URI- based reference. For example: (& (dpi=100) (h.1234567890) ) where (h.1234567890) :- end This indicates that the meaning of the hash-based form is contained in the resource whose URI is given. In this case, an HTTP resource retrieval is suggested. Klyne & Masinter Work-in-progress [Page 9] Internet Draft Identifying composite media features 16 June 1999 The hash value used is calculated over the feature set expression obtained by defererencing the URI form expression. IMPORTANT: How a calling application processes a 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 might be used; e.g. protocols developed by the IETF resource capability (RESCAP) working group. In any case, any mechanism used must be specified by an application or protocol that uses URI references in this way. When a hashed feature set reference is resolved using a URI value, the retrieving program should use the feature expression thus obtained only if it hashes to the correct value. 3.3.2 Inline feature set details In this case, a reference is resolved by including its definition inline in an expression. 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) ) end 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. It is an error if the inline feature expression does not yield the hash value contained in auxiliary predicate name. NOTE: viewed in isolation, this format does not have any obvious value, in that the (h.xxx) form of auxiliary predicate could be replaced by any arbitrary name. Klyne & Masinter Work-in-progress [Page 10] Internet Draft Identifying composite media features 16 June 1999 It is anticipated that this form might be used as a follow-up response in a sequence along the lines of: A> Capabilities are: (& (dpi=100) (h.1234567890) ) B> Do not understand: (h.1234567890) A> Capabilities are: (& (dpi=100) (h.1234567890) ) where (h.1234567890) :- (& (pix-x<=200) (pix-y<=150) ) end 4. Examples The following are some examples of feature set expressions containing feature set references: (& (dpi=100) (h.1234567890abcdefghijklmnop) ) (& (dpi=100) ) (& (dpi=100) (h.1234567890abcdefghijklmnop) ) where (h.1234567890abcdefghijklmnop) :- end 5. Internationalization considerations Feature set expressions and URI strings are currently defined to consist of only characters from the US-ASCII repertoire [1,5]; under these circumstances this specification is not impacted by internationalization considerations (other than any already applicable to URIs [5]). 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). Klyne & Masinter Work-in-progress [Page 11] Internet Draft Identifying composite media features 16 June 1999 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. 6. Security considerations For the most part, security considerations are the same as those that apply for capability identification in general [1,2,9]. A possible added consideration is that use of a specific feature set tag may reveal more information about a system than is necessary for a transaction at hand. 7. 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 & Masinter Work-in-progress [Page 12] Internet Draft Identifying composite media features 16 June 1999 8. Acknowledgements Much of the initial work for URI references to feature sets was provided by Bill Newman. Some of the ideas here have been improved by early discussions with Martin Duerst, Al Gilman and Ted Hardie. 9. References [1] RFC 2533, "A syntax for describing media feature sets" Graham Klyne, 5GM/Content Technologies March 1999. [2] RFC 2506, "Media Feature Tag Registration Procedure" Koen Holtman, TUE Andrew Mutz, Hewlett-Packard Ted Hardie, Equinix March 1999. [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 & Masinter Work-in-progress [Page 13] Internet Draft Identifying composite media features 16 June 1999 [8] "Protocol-independent content negotiation framework" Graham Klyne, 5GM/Content Technologies Internet draft: Work in progress, March 1999. [9] "Numerical Recipes" William H Press, Brian P Flannery, Saul A Teukolski and William T Vetterling Cambridge University Press (1986) ISBN 0 521 30811 9 (The Gamma function approximation is presented in chapter 6 on "Special Functions". There have been several later editions of this book published, so the chapter reference may change.) 10. Authors' addresses 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 Larry Masinter Xerox Corporation 3333 Coyote Hill Road Palo Alto, CA 94304 Facsimile: +1 650 812 4333 EMail: masinter@parc.xerox.com http://www.parc.xerox.com/masinter Appendix A 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 a small but distinct possibility that two different feature sets will indeed yield the same hash value. Klyne & Masinter Work-in-progress [Page 14] Internet Draft Identifying composite media features 16 June 1999 We assume that the 128-bit hash function distributes hash values for feature sets, even those with very small differences, randomly and evenly through the range of 2^128 (approximately 3*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 less than 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 repeating a hash value 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 Schneier's "Applied Cryptography" [7]. The table below shows the "birthday paradox" probabilities that at least one pair of feature sets has the same hash value for different numbers of feature sets in use. Number of feature Probability of two sets in use sets with the same hash value 1 0 2 3E-39 10 1E-37 1E3 1E-33 1E6 1E-27 1E9 1E-21 1E12 1E-15 1E15 1E-9 1E18 1E-3 The above probability computations are approximate, being performed using logarithms of a Gamma function approximation by Lanczos [9]. The probability formula is 'P=1-(m!/((m-n)! m^n))', where 'm' is the total number of possible hash values (2^128) and 'n' is the number of feature sets in use. 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. Klyne & Masinter Work-in-progress [Page 15] Internet Draft Identifying composite media features 16 June 1999 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. Appendix B: Revision history [[[RFC editor: please remove this section on publication]]] 00a 10-Feb-1999 Initial draft. 01a 16-Feb-1999 Added pointers to mailing list for discussion. 01b 25-Mar-1999 Name all authors. Add some terms to the glossary. Expand on meaning of URI tag used as auxiliary predicate name. Update references. Rework section 3 to deal more evenly with both hash and URI based feature set references. State absolute requirement for hash-based references to be resolved to expressions that yield the correct hash value. 01c 06-Apr-1999 Define form of URI reference using new '<...>' syntax, and adjust other text accordingly. 01d 06-Apr-1999 Editorial revisions. Include values in table of probabilities for hash value clashes. Remove discussion of algebraic simplification of hash references. Correct syntax of some examples. 02a 16-Jun-1999 Move birthday problem to an appendix. Remove RESCAP citation. Use base-32 to represent feature hashes; describe base-32 encoding. 02b 16-Jun-1999 Add note that the form of feature reference may not be allowed at arbitrary locations in all contexts. Klyne & Masinter Work-in-progress [Page 16]