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Network Working Group                                           K. HoltmanRequest for Comments: 2506                                             TUEBCP: 31                                                            A. MutzCategory: Best Current Practice                            Hewlett-Packard                                                                 T. Hardie                                                                   Equinix                                                                March 1999Media Feature Tag Registration ProcedureStatus of this Memo   This document specifies an Internet Best Current Practices for the   Internet Community, and requests discussion and suggestions for   improvements.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1999).  All Rights Reserved.ABSTRACT   Recent Internet applications, such as the World Wide Web, tie   together a great diversity in data formats, client and server   platforms, and communities.  This has created a need for media   feature descriptions and negotiation mechanisms in order to identify   and reconcile the form of information to the capabilities and   preferences of the parties involved.   Extensible media feature identification and negotiation mechanisms   require a common vocabulary in order to positively identify media   features.  A registration process and authority for media features is   defined with the intent of sharing this vocabulary between   communicating parties. In addition, a URI tree is defined to enable   sharing of media feature definitions without registration.   This document defines a registration procedure which uses the   Internet Assigned Numbers Authority (IANA) as a central registry for   the media feature vocabulary.   Please send comments to the CONNEG working group at <ietf-   medfree@imc.org>.  Discussions of the working group are archived at   <URL:http://www.imc.org/ietf-medfree/>.Holtman, et. al.         Best Current Practice                  [Page 1]

RFC 2506        Media Feature Tag Registration Procedure      March 1999TABLE OF CONTENTS1 Introduction .................................................22 Media feature tag definitions ................................32.1 Media feature tag purpose .................................32.2 Media feature tag syntax ..................................42.3 Media feature tag values ..................................42.4  ASN.1 identifiers for media feature tags .................53 Media feature tag registration ...............................53.1 Registration trees ........................................63.1.1 IETF tree ...............................................63.1.2 Global tree .............................................63.1.3 URL tree ................................................63.1.4 Additional registration trees ...........................73.2 Location of registered media feature tag list .............73.3 IANA procedures for registering media feature tags ........73.4 Registration template .....................................74 Security Considerations ......................................105 Acknowledgments ..............................................106 References ...................................................107 Authors' Addresses ...........................................118 Full Copyright Statement .....................................121 Introduction   Recent Internet applications, such as the World Wide Web, tie   together a great diversity in data formats, client and server   platforms, and communities.  This has created a need for media   feature descriptions and negotiation mechanisms in order to identify   and reconcile the form of information to the capabilities and   preferences of the parties involved.   Extensible media feature identification and negotiation mechanisms   require a common vocabulary in order to positively identify media   features.  A registration process and authority for media features is   defined with the intent of sharing this vocabulary between   communicating parties. In addition, a URI tree is defined to enable   sharing of media feature definitions without registration.   This document defines a registration procedure which uses the   Internet Assigned Numbers Authority (IANA) as a central registry for   the media feature vocabulary.   This document uses the terms MUST, MUST NOT, SHOULD, SHOULD NOT and   MAY according to usage described in [8].Holtman, et. al.         Best Current Practice                  [Page 2]

RFC 2506        Media Feature Tag Registration Procedure      March 19992 Media feature tag definitions2.1 Media feature tag purpose   Media feature tags represent individual and simple characteristics   related to media capabilities or properties associated with the   resource to which they are applied.  Examples of such features are:   * the color depth of the screen on which something is to be displayed   * the type of paper available in a printer   * the support of the `floating 5 dimensional tables' feature   * the fonts which are available to the recipient   * the capability to display graphical content   Each media feature tag identifies a single characteristic. Values   associated with a specific tag must use the data type defined for   that tag.  The list of allowed data types is presented below, insection 2.3.   Examples of media feature tags with values are:   * the width of a display in pixels per centimeter represented as an   integer value.   * a font available to a recipient, selected from an enumerated list.   * the version of a protocol composed of integers "i.j.k", defined as   either a value in an enumerated list or with a defined mapping to   make the value isomorphic to a subset of integers (e.g. i*100 + j*10   +k, assuming j<=9 and k<=9).   Further examples of media feature tags are defined in detail   elsewhere [4].   Feature collections may be composed using a number of individual   feature tags [2]. Composition of feature collections is described   elsewhere [2].  Examples of feature collections requiring multiple   media feature tags are:   * the set of all fonts used by a document   * the width and height of a display   * the combination of color depth and resolution a display can support   This registry presumes the availability of the MIME media type   registry, and MIME media types MUST NOT be re-registered as media   feature tags.  Media feature tags which are currently in use by   individual protocols or applications MAY be registered with this   registry if they might be applied outside of their current domain.Holtman, et. al.         Best Current Practice                  [Page 3]

RFC 2506        Media Feature Tag Registration Procedure      March 1999   The media feature tag namespace is not bound to a particular   transport protocol or capability exchange mechanism.  The registry is   limited, however, to feature tags which express a capability or   preference related to how content is presented.  Feature tags related   to other axes of negotiation are not appropriate for this registry.   Capability exchange mechanisms may, of course, be used to express a   variety of capabilities or preferences.2.2 Media feature tag syntax   A media feature tag is a string consisting of one or more of the   following US-ASCII characters: uppercase letters, lowercase letters,   digits, colon (":"), slash ("/"), dot (".") percent ("%"), and dash   ("-"). Feature tags are case-insensitive.  Dots are understood to   potentially imply hierarchy; a feature can be subtyped by describing   it as tree.feature.subfeature and by indicating this in the   registration.  Tags should begin with an alphabetic character.   In ABNF [6], this may be represented as:   Feature-tag = ALPHA *( ALPHA / DIGIT / ":" / "/" / "." / "-" /"%" )   Registrants should take care to avoid creating tags which might   conflict with the creation of new registration trees; in general this   means avoiding tags which begin with an alphabetic character followed   by a dot.  The current registration trees are described insection 3   below.2.3 Media feature tag values   The registry will initially support the use of the following data   types as tag values:      - signed integers      - rational numbers      - tokens, with equality relationship      - tokens, with defined ordering relationship      - strings, with standard (octet-by-octet) equality relationship      - strings, with defined equality and/or comparison relationship   "Token" here means the token data type as defined by [7], which may   be summarized as:Holtman, et. al.         Best Current Practice                  [Page 4]

RFC 2506        Media Feature Tag Registration Procedure      March 1999      token          = 1*<any CHAR except CTLs or tspecials>      tspecials      = "(" / ")" / "<" / ">" / "@"                     / "," / ";" / ":" / "\" / <">                     / "/" / "[" / "]" / "?" / "="                     / "{" / "}" / SP / HT   At the time of registration, each tag must be associated with a   single data type.  If that data type implies a defined comparison or   an ordering, the registrant must define the ordering or comparison.   For ordered tokens, this may be by full enumeration of the tokens and   their order or by reference to an ordering mechanism.  For defined   comparisons, a full description of the rules for comparison must be   provided or included by reference.   Media feature tags related to spatial or temporal characteristics   must be registered with a single canonical unit.  It is strongly   preferred that units be in the SI system; where current practice has   defined units in other systems (such as pixels per inch), a   conversion method to SI units must be provided.  Conversion methods   should include a defined rounding practice.2.4  ASN.1 identifiers for media feature tags   Certain protocols use ASN.1 identifiers rather than human-readable   representations for capability exchange.  In order to allow both   systems to interoperate, registrants may provide an ASN.1 identifier   or ask that IANA assign an ASN.1 identifier during registration.   These identifiers are not required for registration, but may provide   assistance to those building gateways or other cross-protocol   systems.  Note that ASN.1 identifiers assigned by IANA will be   treated as tokens, not as elements from which sub-delegated   identifiers may be created or derived.3 Media feature tag registration   Media feature tags can be registered in several different   registration trees, with different requirements as discussed below.   The vocabulary for these requirements is taken from [5]. In general,   a feature tag registration proposal is circulated and reviewed in a   fashion appropriate to the tree involved.  The feature tag is then   registered if the proposal is accepted.   Review of a feature tag in the URI tree is not required.Holtman, et. al.         Best Current Practice                  [Page 5]

RFC 2506        Media Feature Tag Registration Procedure      March 19993.1 Registration trees   The following subsections define registration "trees", distinguished   by the use of faceted names (e.g., names of the form "tree.feature-   name").3.1.1 IETF tree   The IETF tree is intended for media feature tags of general interest   to the Internet Community, and proposals for these tags must meet the   "IETF Consensus" policies described in [5].   Registration in the IETF tree requires approval by the IESG and   publication of the feature tag specification as an RFC.  Submissions   for feature tag registration in the IETF tree can originate in any WG   of the IETF or as an individual submission to the IESG.   Feature tags in the IETF tree normally have names that are not   explicitly faceted, i.e., do not contain period (".", full stop)   characters.3.1.2 Global tree   Tags in the global tree will be distinguished by the leading facet   "g.".  An organization may propose either a designation indicative of   the feature, (e.g., "g.blinktags") or a faceted designation including   the organization name (e.g., "g.organization.blinktags").   Organizations which have registered media types under the MIME vendor   tree should use the same organizational name for media feature tags   if they propose a faceted designation. The acceptance of the proposed   designation is at the discretion of the IANA. If the IANA believes   that a designation needs clarification it may request a new proposal   from the proposing organization or otherwise coordinate the   development of an appropriate designation.   Registrations of feature tags in the global tree must meet the   "Expert Review" policies described in [5].  In this case, a   designated area expert will review the proposed tag, consulting with   the members of a related mailing list.  A registration may be   proposed for the global tree by anyone who has the need to allow for   communication on a particular capability or preference.3.1.3 URI tree   A feature tag may be defined as a URI using the restricted character   set defined above. Feature tags in the URI tree are identified by the   leading facet "u.". The leading facet u. is followed by a URI [9]   which conforms to the character limitations specified in thisHoltman, et. al.         Best Current Practice                  [Page 6]

RFC 2506        Media Feature Tag Registration Procedure      March 1999   document.  The author of the URI is assumed to be registration   authority regarding features defined and described by the content of   the URI.  These tags are considered unregistered for the purpose of   this document.3.1.4 Additional registration trees   From time to time and as required by the community, the IANA may,   with the advice and consent of the IESG, create new top-level   registration trees. These trees may be created for external   registration and management by (for example) well-known permanent   bodies, such as scientific societies for media feature types specific   to the sciences they cover.  Establishment of these new trees will be   announced through RFC publication approved by the IESG.3.2 Location of registered feature tag list   Feature tag registrations will be posted in the anonymous FTP   directory:  "ftp://ftp.isi.edu/in-notes/iana/assignments/media-feature-tags/" and all registered feature tags will be listed in the   periodically issued "Assigned Numbers" RFC [currently STD 2,RFC-1700].  The feature tag description and other supporting material may   also be published as an Informational RFC by sending it to "rfc-   editor@rfc-editor.org".3.3 IANA procedures for registering feature tags   The IANA will only register feature tags in the IETF tree in response   to a communication from the IESG stating that a given registration   has been approved.   Global tags will be registered by the IANA after review by a   designated expert.  That review will serve to ensure that the tag   meets the technical requirements of this specification.3.4 Registration template   To: media-feature-tags@apps.ietf.org (Media feature tags mailing list)   Subject: Registration of media feature tag XXXX    | Instructions are preceded by `|'.  Some fields are optional.   Media feature tag name:   ASN.1 identifier associated with feature tag:       [optional]    | To have IANA assign an ASN.1 identifier,    | use the value "New assignment by IANA" here.Holtman, et. al.         Best Current Practice                  [Page 7]

RFC 2506        Media Feature Tag Registration Procedure      March 1999   Summary of the media feature indicated by this feature tag:    | Include a short (no longer than 4 lines) description or summary    | Examples:    |   `Use of the xyzzy feature is indicated by ...'    |   `Support of color display is indicated by ...'    |   `Number of colors in a palette which can be defined ...'   Values appropriate for use with this feature tag:     [ ] 1. The feature tag is Boolean and may have values of          TRUE or FALSE.   A value of TRUE indicates an available          capability.  A value of FALSE indicates the capability          is not available.    | If you wish to indicate two mutually exclusive possibilities    | that cannot be expressed as the availability or lack of a    | capability, use a two-token list, rather than a Boolean value.     [ ] 2. The feature has an associated numeric or enumerated value.   For case 2: Indicate the data type of the value:      [ ] 2a. Signed Integer      [ ] 2b. Rational number      [ ] 2c. Token (equality relationship)      [ ] 2d. Token (ordered)      [ ] 2e. String (equality relationship)      [ ] 2f. String (defined comparison)    |IMPORTANT: You may only chose one of the above data types.   (Only for case 2) Detailed description of the feature value meaning,   and of the format and meaning of the feature tag values for the   alternative results.    | If you have selected 2d you must provide the ordering mechanism    | or a full and ordered enumeration of possible values.  If you    | have selected 2f, you must provide a definition of the comparison.    | Definitions by included reference must be to stable and readily    | available specifications:    |    | If the number of alternative results is small, you may    | enumerate the identifiers of the different results and describe    | their meaning.    |    | If there is a limited useful numeric range of result (2b, 2c),Holtman, et. al.         Best Current Practice                  [Page 8]

RFC 2506        Media Feature Tag Registration Procedure      March 1999    | indicate the range.    |    | The identifiers of the alternative results could also be    | described by referring to another IANA registry, for example    | the paper sizes enumerated by the Printer MIB.   The feature tag is intended primarily for use in the following   applications, protocols, services, or negotiation mechanisms:                                                   [optional]    | For applications, also specify the number of the first version    | which will use the tag, if applicable.   Examples of typical use:                               [optional]   Related standards or documents:                        [optional]   Considerations particular to use in individual applications,   protocols, services, or negotiation mechanisms:        [optional]   Interoperability considerations:                       [optional]   Security considerations:     Privacy concerns, related to exposure of personal information:     Denial of service concerns related to consequences of specifying     incorrect values:     Other:   Additional information:                                [optional]     Keywords:                                            [optional]     Related feature tags:                                [optional]     Related media types or data formats:                 [optional]     Related markup tags:                                 [optional]   Name(s) & email address(es) of person(s) to contact for   further information:   Intended usage:    | one of COMMON, LIMITED USE or OBSOLETEHoltman, et. al.         Best Current Practice                  [Page 9]

RFC 2506        Media Feature Tag Registration Procedure      March 1999   Author/Change controller:   Requested IANA publication delay:                      [optional]    | A delay may only be requested for final placement in the global    | or IETF trees, with a maximum of two months.  Organizations    | requesting a registration with a publication delay should note    | that this delays only the official publication of the tag    | and does not prevent information on it from being disseminated    | by the members of the relevant mailing list.   Other information:                                     [optional]    |  Any other information that the author deems interesting may be    |  added here.4 Security Considerations   Negotiation mechanisms reveal information about one party to other   parties.  This may raise privacy concerns, and may allow a malicious   party to make better guesses about the presence of specific security   holes.5 Acknowledgments   The details of the registration procedure in this document were   directly adapted from [1].  Much of the text insection 3 was   directly copied from this source.   The idea of creating a vocabulary of areas of media features,   maintained in a central open registry, is due to discussions on   extensible negotiation mechanisms [3] in the IETF HTTP working group.   The authors wish to thank Larry Masinter, Graham  Klyne, Al Gilman,   Dan Wing, Jacob Palme, and Martin Duerst for their contributions to   discussions about media feature tag registration.6 References   [1] Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet Mail       Extensions (MIME) Part Four: Registration Procedures",BCP 13,RFC 2048, November 1996.   [2] Klyne, G.,"An algebra for describing media feature sets", Work       in Progress.   [3] Holtman, K. and  A. Mutz, "Transparent Content Negotiation in       HTTP.RFC 2295, March 1998.Holtman, et. al.         Best Current Practice                 [Page 10]

RFC 2506        Media Feature Tag Registration Procedure      March 1999   [4] Masinter, L., Holtman, K., Mutz, A. and D. Wing, "Media Features       for Display, Print, and Fax",RFC 2534, March 1999.   [5] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA       Considerations Section in RFCs",BCP 26,RFC 2434, October 1998.   [6] Crocker, D., Ed., "Augmented BNF for Syntax Specifications:       ABNF",RFC 2234, November 1997.   [7] Fielding, R., Gettys, J., Mogul, J. Frystyk, H. and T. Berners-       Lee, "Hypertext Transfer Protocol -- HTTP/1.1",RFC 2068, January       1997.   [8] Bradner, S., "Key words for use in RFCs to Indicate Requirement       Levels",BCP 14,RFC 2119, March 1997.   [9] Berners-Lee, T., "Universal Resource Identifiers in WWW,"RFC1630, June 1994.7 Authors' Addresses   Koen Holtman   Technische Universiteit Eindhoven   Postbus 513   Kamer HG 6.57   5600 MB Eindhoven   The Netherlands   EMail: koen@win.tue.nl   Andrew H. Mutz   Hewlett-Packard Company   11000 Wolfe Rd. 42UO   Cupertino CA 95014 USA   Fax +1 408 447 4439   EMail: andy_mutz@hp.com   Ted Hardie   Equinix   901 Marshall Street   Redwood City, CA 94063 USA   EMail: hardie@equinix.comHoltman, et. al.         Best Current Practice                 [Page 11]

RFC 2506        Media Feature Tag Registration Procedure      March 19998 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.Holtman, et. al.         Best Current Practice                 [Page 12]