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Versions: 00 01 02 03 RFC 3840

SIP                                                         J. Rosenberg
Internet-Draft                                               dynamicsoft
Expires: December 22, 2003                                H. Schulzrinne
                                                     Columbia University
                                                              P. Kyzivat
                                                           Cisco Systems
                                                           June 23, 2003


 Indicating User Agent Capabilities in the Session Initiation Protocol
                                 (SIP)
                     draft-ietf-sip-callee-caps-00

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."

   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 December 22, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

   This specification defines mechanisms by which a Session Initiation
   Protocol (SIP) user agent can convey its capabilities and
   characteristics to other user agents. These capabilities are conveyed
   as parameters of the Contact header field.







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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.    Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.    Definitions  . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.    Usage of the Content Negotiation Framework . . . . . . . . .  7
   5.    Computing Capabilities . . . . . . . . . . . . . . . . . . .  8
   6.    Expressing Capabilities in a Registration  . . . . . . . . . 11
   7.    Indicating Feature Sets in Remote Target URIs  . . . . . . . 13
   8.    OPTIONS Processing . . . . . . . . . . . . . . . . . . . . . 14
   9.    Contact Header Field . . . . . . . . . . . . . . . . . . . . 15
   10.   Media Feature Tag Definitions  . . . . . . . . . . . . . . . 17
   10.1  Attendant  . . . . . . . . . . . . . . . . . . . . . . . . . 17
   10.2  Audio  . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   10.3  Application  . . . . . . . . . . . . . . . . . . . . . . . . 18
   10.4  Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   10.5  Control  . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   10.6  Automata . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   10.7  Class  . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   10.8  Duplex . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   10.9  Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   10.10 Description  . . . . . . . . . . . . . . . . . . . . . . . . 23
   10.11 Event Packages . . . . . . . . . . . . . . . . . . . . . . . 23
   10.12 Priority . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   10.13 Methods  . . . . . . . . . . . . . . . . . . . . . . . . . . 25
   10.14 SIP Extensions . . . . . . . . . . . . . . . . . . . . . . . 25
   10.15 Schemes  . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   10.16 Video  . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   10.17 Message Server . . . . . . . . . . . . . . . . . . . . . . . 27
   10.18 Is Focus . . . . . . . . . . . . . . . . . . . . . . . . . . 27
   10.19 URI User . . . . . . . . . . . . . . . . . . . . . . . . . . 28
   10.20 URI Domain . . . . . . . . . . . . . . . . . . . . . . . . . 28
   11.   Security Considerations  . . . . . . . . . . . . . . . . . . 30
   12.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 31
   12.1  Media Feature Tags . . . . . . . . . . . . . . . . . . . . . 31
   12.2  SIP Option Tag . . . . . . . . . . . . . . . . . . . . . . . 31
   13.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 32
         Normative References . . . . . . . . . . . . . . . . . . . . 33
         Informative References . . . . . . . . . . . . . . . . . . . 34
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 34
   A.    Overview of RFC 2533 . . . . . . . . . . . . . . . . . . . . 36
         Intellectual Property and Copyright Statements . . . . . . . 39









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1. Introduction

   Session Initiation Protocol (SIP) [1] user agents vary widely in
   their capabilities and in the types of devices they represent.
   Frequently, it is important for another SIP element to learn the
   capabilities and characteristics of a SIP UA. Some of the
   applications of this information include:

   o  One user agent, a PC-based application, is communicating with
      another that is embedded in a limited-function device. The PC
      would like to be able to "grey out" those components of the user
      interface that represent features or capabilities not supported by
      its peer. To do that, there needs to be a way to exchange
      capability information within a dialog.

   o  A user has two devices at their disposal. One is a videophone, and
      the other, a voice-only wireless phone. A caller wants to interact
      with the user using video. As such, they would like their call
      preferentially routed to the device which supports video. To do
      this, the INVITE request can contain parameters that express a
      preference for routing to a device with the specified capabilities
      [11].

   o  A network application would like to asynchronously send
      information to a user agent in a MESSAGE [15] request. However,
      before sending it, they would like to know if the UA has the
      capabilites necessary to receive the message. To do that, they
      would ideally query a user database managed by the domain which
      holds such information. Population of such a database would
      require that a UA convey its capabilities as part of its
      registration. Thus, there is a need for conveying capabilities in
      REGISTER requests.

   SIP has some support for expression of capabilities. The Allow,
   Accept, Accept-Language and Supported header fields convey some
   information about the capabilities of a user agent. However, these
   header fields convey only a small part of the information that is
   needed. They do not provide a general framework for expression of
   capabilities. Furthermore, they only specify capabilities indirectly;
   the header fields really indicate the capabilities of the UA as they
   apply to this request.

   As a result, this specification provides a more general framework for
   indication of capabilities in SIP. Capability information about a UA
   is carried as parameters of the Contact header field. These
   capabilities can be used within REGISTER requests and responses,
   OPTIONS responses, and requests and responses that create dialogs
   (such as INVITE).



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2. Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [2] and
   indicate requirement levels for compliant implementations.













































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3. Definitions

   Feature: As defined in RFC 2703 [16], a piece of information about
      the media handling properties of a message passing system
      component or of a data resource. For example, the SIP methods
      supported by a UA represent a feature.

   Feature Tag: As defined in RFC 2703 [16], a feature tag is a name
      that identifies a feature. An example is ``methods''.

   Media Feature: As defined in RFC 2703, [16], a media feature is
      information that indicates facilities assumed to be available for
      the message content to be properly rendered or otherwise
      presented.  Media features are not intended to include information
      that affects message transmission.

         In the context of this specification, a media feature is
         information that indicates facilities for handling SIP
         requests, rather than specifically for content. In that sense,
         it is used synonymously with feature.

   Feature Collection: As defined in RFC 2533 [4], a feature collection
      is a collection of different media features and associated values.
      This might be viewed as describing a specific rendering of a
      specific instance of a document or resource by a specific
      recipient.

   Feature Set: As defined in RFC 2703 [16], a feature set is
      Information about a sender, recipient or other participant in a
      message transfer which describes the set of features that it can
      handle. Where a 'feature' describes a single identified attribute
      of a resource, a 'feature set' describes a full set of possible
      attributes.

   Feature Parameters: A set of SIP header field parameters that can
      appear in the Contact header field. The feature parameters
      represent an encoding of a feature set. Each set of feature
      parameters maps to a feature set predicate.

   Capability: As defined in RFC 2703 [16], a capability is an attribute
      of a sender or receiver (often the receiver) which indicates an
      ability to generate or process a particular type of message
      content.

   Filter: A single expression in a feature set predicate.






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   Simple Filter: An expression in a feature predicate which is a
      comparison (equality or inequality) of a feature tag against a
      feature value.

   Disjunction: A boolean OR operation across some number of terms.

   Conjunction: A boolean AND operation across some number of terms.

   Predicate: A boolean expression.

   Feature Set Predicate: From RFC 2533 [4], a feature set predicate is
      a function of an arbitrary feature collection value which returns
      a Boolean result.  A TRUE result is taken to mean that the
      corresponding feature collection belongs to some set of media
      feature handling capabilities defined by this predicate.

   Contact Predicate: The feature set predicate associated with a URI
      registered in the Contact header field of a REGISTER request. The
      contact predicate is derived from the feature parameters in the
      Contact header field.































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4. Usage of the Content Negotiation Framework

   This specification makes heavy use of the terminology and concepts in
   the content negotiation work carried out within the IETF, and
   documented in several RFCs. The ones relevant to this specification
   are RFC 2506 [3] which provides a template for registering media
   feature tags, RFC 2533 [4] which presents a syntax and matching
   algorithm for media feature sets, RFC 2738 [5], which provides a
   minor update to RFC 2533, and RFC 2703 [16] which provides a general
   framework for content negotiation.

   In case the reader does not have the time to read those
   specifications, Appendix A provides a brief overview of the concepts
   and terminology in those documents that is critical for understanding
   this specification.

   Since the content negotiation work was primarily meant to apply to
   documents or other resources with a set of possible renderings, it is
   not immediately apparent how it is used to model SIP user agents. A
   feature set is composed of a set of feature collections, each of
   which represents a specific rendering supported by the entity
   described by the feature set. In the context of a SIP user agent, a
   feature collection represents an instantaneous modality. That is, if
   you look at the run time processing of a SIP UA, and take a snapshot
   in time, the feature collection describes what it is doing at that
   very instant.

   This model is important, since it provides guidance on how to
   determine whether something is a value for a particular feature tag,
   or a feature tag by itself. If two properties can be exhibited by a
   UA simultaneously, so that both are present in an instantaneous
   modality, they need to be represented by separate media feature tags.
   For example, a UA may be able to support some number of media types -
   audio, video, and control. Should each of these be different values
   for a single "media-types" feature tag, or should each of them be a
   separate boolean feature tag? The model provides the answer. Since,
   at any instance of time, a UA could be handling both audio and video,
   they need to be separate media feature tags. However, the SIP methods
   supported by a UA can each be represented as different values for the
   same media feature tag (the "methods" tag), because fundamentally, a
   UA processes a single request at a time. It may be multi-threading,
   so that it appears that this is not so, but at a purely functional
   level, it is true.

   Clearly, there are weaknesses in this model, but it serves as a
   useful guideline for applying the concepts of RFC 2533 to the problem
   at hand.




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5. Computing Capabilities

   To construct a set of Contact header field parameters which indicate
   capabilities, a UA constructs a feature predicate for that contact.
   This process is described in terms of RFC 2533 [4] (and its minor
   update, RFC 2738 [5]) syntax and constructs, followed by a conversion
   to the syntax used in this specification. However, this represents a
   logical flow of processing. There is no requirement that an
   implementation actually use RFC 2533 syntax as an intermediate step.

   A UA MAY use any feature tags that are registered through IANA in the
   IETF or global trees [3]; this document registers several that are
   appropriate for SIP. The feature tags discussed in this specification
   are referred to as base tags. While other tags can be used, in order
   to identify them as feature parameters (as opposed to parameters for
   another SIP extension) they are encoded with a leading "+" sign in
   the Contact header field. It is also permissible to use the URI tree
   [3] for expressing vendor-specific feature tags. Feature tags in any
   other trees created through IANA MAY also be used.

   When using the "methods" feature tag, a UA MUST NOT include values
   that correspond to methods not standardized in IETF standards track
   RFCs. When using the "events" feature tag, a UA MUST NOT include
   values that correspond to event packages not standardized in IETF
   standards track RFCs. When using the "schemes" feature tag, a UA MUST
   NOT include values that correspond to schemes not standardized in
   IETF standards track RFCs. When using the "sip-extensions" feature
   tag, a UA MUST NOT include values that correspond to option tags not
   standardized in IETF standards track RFCs.

   A UA SHOULD include the "uri-user" and "uri-domain" feature tag in
   its contact predicate. The value of those tags SHOULD be equal to the
   user and domain part of the registered URI, respectively. Setting
   them differently is likely to result in odd behavior, and should only
   be done if some unforseen service neccesitates it. Note that the
   "uri-user" feature tag is a quoted string (implying case sensitive
   matching), and the "uri-domain" feature tag is a token, implying case
   insensitive matching.

   Note that the "schemes" feature tag is not a peer of the "uri-user"
   and "uri-domain" feature tags. That is, it does not indicate the
   scheme of the registered URI. Rather, it indicates schemes that a UA
   is capable of sending requests to, should such a URI be received in a
   web page or Contact header field of a redirect response.

   It is RECOMMENDED that a UA provide complete information in its
   contact predicate. That is, it SHOULD provide information on as many
   feature tags as possible. The mechanisms in this specification work



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   best when user agents register complete feature sets. Furthermore,
   when a UA registers values for a particular feature tag, it MUST list
   all values that it supports. For example, when including the
   "methods" feature tag, a UA MUST list all methods it supports.

   The contact predicate constructed by a UA MUST be an AND of terms
   (called a conjunction). Each term is either an OR (called a
   disjunction) of simple filters or negations of simple filters , or a
   single simple filter or negation of a single filter. In the case of a
   disjunction, each filter in the disjunction MUST indicate feature
   values for the same feature tag (i.e., the disjunction represents a
   set of values for a particular feature tag), and each element of the
   conjunction MUST be for a different feature tag. Each simple filter
   can be an equality, or in the case of numeric feature tags, an
   inequality or range. This contact predicate is then converted to a
   list of feature parameters, following the procedure outlined below.

   The contact predicate is a  conjunction of terms. Each term indicates
   constraints on a single feature tag, and each term is represented by
   a separate feature parameter. The name of this parameter depends on
   the feature tag. Any forward slashes in the feature tag are converted
   to a single quote, and any colons are converted to an exclamation
   point. If the feature tag name is not amongst the base tags specified
   in Section 9, a plus sign is added to the front of the feature
   parameter name. The plus sign MUST NOT be added if the feature tag
   name is amongst the base tags. The result is the feature parameter
   name.

   The value of the feature parameter depends on the the term of the
   conjunction. If the term is a boolean expression with value of true,
   i.e., (audio=TRUE), the contact parameter has no value. If the term
   of the conjunction is a disjunction, the value of the contact
   parameter is a quoted string. The quoted string is a comma separated
   list of strings, each one derived from one of the terms in the
   disjunction. If the term of the conjunction is a negation, the value
   of the contact parameter is a quoted string. The quoted string begins
   with an exclamation point (!), and the remainder is constructed from
   the expression being negated.

   The remaining operation is to compute a string from a primitive
   filter (i.e., no and, or, or nots). If the filter is a simple filter
   that is performing a numeric comparison, the string starts with an
   octothorpe (#), followed by the comparator in the filter (=, >, or
   <), followed by the value from the filter. If the value from the
   filter is expressed in rational form (X / Y), then X and Y are
   divided, yielding a decimal number, and this decimal number is output
   to the string.




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      RFC 2533 uses a fractional notation to describe rational numbers.
      This specification use a decimal form. The above text merely
      converts between the two representations. Practically speaking,
      this conversion is not needed since the numbers are the same in
      either case. However, it is described in case implementations wish
      to directly plug the predicates generated by the rules in this
      section into an RFC 2533 implementation.

   If the filter is a range (foo=X..Y), the string is equal to X:Y,
   where X and Y have been converted from fractional numbers (A / B) to
   their decimal equivalent.

   If the filter is an equality over a token or boolean, then that token
   or boolean value ("TRUE" or "FALSE") is output to the string.

   If the filter is an equality over a quoted string, the output is a
   less than (<) followed by the quoted string, followed by a greater
   than (>).

   As an example, feature predicate:

   (& (mobility=fixed)
      (| (! (events=presence)) (events=winfo))
      (| (language=en) (language=de))
      (description="PC")
      (newparam=TRUE)
      (rangeparam=-4..5125/1000))

   would be converted into the following feature parameters:

   mobility="fixed";events="!presence,winfo";language="en,de"
    ;description="<PC>";+newparam;+rangeparam="#-4:+5.125"



















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6. Expressing Capabilities in a Registration

   When a UA registers, it can choose to indicate a feature set
   associated with a registered contact. Whether or not a UA does so
   depends on what the registered URI represents. If the registered URI
   represents a UA instance (the common case in registrations), a UA
   compliant to this specification SHOULD indicate a feature set using
   the mechanisms described here. If, however, the registered URI
   represents an address-of-record, or some other resource that is not
   representable by a single feature set, it SHOULD NOT include a
   feature set. As an example, if a user wishes to forward calls from
   sip:user1@example.com to sip:user2@example.org, it could generate a
   registration that looks like, in part:

   REGISTER sip:example.com SIP/2.0
   To: sip:user1@example.com
   Contact: sip:user2@example.org

   In this case, the registered contact is not identifying a UA, but
   rather, another address-of-record. In such a case, the registered
   contact would not indicate a feature set.

   However, in some cases a UA may wish to express feature parameters
   for an address-of-record. One example is an AOR which represents a
   mutliplicity of devices in a home network, and routes to a proxy
   server in the user's home. Since all devices in the home are for
   personal use, the AOR itself can be described with the
   "class=personal" feature parameter. A registration that forwards
   calls to this home AOR could make use of that feature parameter.
   Generally speaking, a feature parameter can only be associated with
   an address-of-record if all devices bound to that address-of-record
   share the exact same set of values for that feature parameter.

   The remainder of this section assumes that a UA would like to
   associate a feature set with a contact that it is registering. This
   feature set is constructed and converted to a series of Contact
   header field parameters, as described in Section 5, and those feature
   parameters are added to the the Contact header field value containing
   the URI that the parameters apply to.

   The REGISTER request MAY contain a Require header field with the
   value  "pref" if the client wants to be sure that the registrar
   understands the extensions defined in this specification. This means
   that the registrar will store the feature parameters, and make them
   available to elements accessing the location service within the
   domain. In absence of the Require header field, a registrar that does
   not understand this extension will simply ignore the Contact header
   field parameters.



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   If a UA registers against multiple separate addresses-of-record, and
   the contacts registered for each have different capabilities, a UA
   MUST use different URIs in each registration. This is so that the UA
   can uniquely determine the feature set that is associated with the
   request URI of an incoming request.

   As an example, a UA that supports audio and video media types, is a
   voicemail server, and is not mobile would construct a feature
   predicate like this:

   (& (audio=TRUE)
      (video=TRUE)
      (msgserver=TRUE)
      (automata=TRUE)
      (attendant=TRUE)
      (mobility=fixed)
      (| (methods=INVITE) (methods=BYE) (methods=OPTIONS) (methods=ACK)
         (methods=CANCEL))
      (uri-user="user")
      (uri-domain=host.example.com)

   These would be converted into feature parameters and included in the
   REGISTER request:


   REGISTER sip:example.com SIP/2.0
   From: sip:user@example.com;tag=asd98
   To: sip:user@example.com
   Call-ID: hh89as0d-asd88jkk@host.example.com
   CSeq: 9987 REGISTER
   Max-Forwards: 70
   Via: SIP/2.0/UDP host.example.com;branch=z9hG4bKnashds8
   Contact: <sip:user@host.example.com>;audio="TRUE";video="TRUE"
     ;msgserver="TRUE";automata;attendant;mobility="fixed"
     ;methods="INVITE,BYE,OPTIONS,ACK,CANCEL"
     ;uri-user="<user>"
     ;uri-domain="host.example.com"
   Content-Length: 0

   Note that a voicemail server is usually an automata and an attendant,
   as defined below.










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7. Indicating Feature Sets in Remote Target URIs

   Target refresh requests and responses are used to establish and
   modify the remote target URI in a dialog. The remote target URI is
   conveyed in the Contact header field. A UAC or UAS MAY add feature
   parameters to the Contact header field value in target refresh
   requests and responses, for the purpose of indicating the
   capabilities of the UA. To do that, it constructs a set of feature
   parameters according to the Section 5. These are then added as
   Contact header field parameters in the request or response.

   The feature parameters can be included in both initial requests and
   mid-dialog requests, and MAY change mid-dialog to signal a change in
   UA capabilities.

   There is overlap in the caller preferences mechanism with the Allow,
   Accept, Accept-Language, and Allow-Events [9] header fields, which
   can also be used in target refresh requests. Specifically, the Allow
   header field and "methods" feature tag indicate the same information.
   The Accept header field and the "type" feature tag indicate the same
   information. The Accept-Language header field and the "language"
   feature tag indicate the same information. The Allow-Events header
   field and the "events" feature tag indicate the same information. It
   is possible that other header fields and feature tags defined in the
   future may also overlap. When there exists a feature tag that
   describes a capability that can also be represented with a SIP header
   field, a UA MUST use the header field to describe the capability. A
   UA receiving a message that contains both the header field and the
   feature tag MUST use the header field, and not the feature tag.






















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8. OPTIONS Processing

   When a UAS compliant to this specification receives an OPTIONS
   request, it MAY add feature parameters to the Contact header field in
   the OPTIONS response for the purpose of indicating the capabilities
   of the UA. To do that, it constructs a set of feature parameters
   according to Section 5. These are then added as Contact header field
   parameters in OPTIONS response. Indeed, if feature parameters were
   included in the registration generated by that UA, those same
   parameters SHOULD be used in the OPTIONS response.









































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9. Contact Header Field

   This specification extends the Contact header field. In particular,
   it allows for the Contact header field parameters to include
   feature-param. Feature-param is a feature parameter that describes a
   feature of the UA associated with the URI in the Contact header
   field. Feature parameters are identifiable because they either belong
   to the well known set of base feature tags, or they begin with a plus
   sign.


   feature-param    =  enc-feature-tag [EQUAL LDQUOT (tag-value-list
                       / string-value ) RDQUOT]
   enc-feature-tag  =  base-tags / other-tags
   base-tags        =  "attendant" / "audio" / "automata" /
                       "class" / "duplex" / "data" /
                       "control" / "mobility" / "description" /
                       "events" / "priority" / "methods" /
                       "schemes" / "application" / "video" /
                       "msgserver" / "language" / "type" /
                       "isfocus" / "uri-user" / "uri-domain"
   other-tags      =  "+" ftag-name
   ftag-name       =  ALPHA *( ALPHA / DIGIT / "!" / "'" /
                      "." / "-" / "%" )
   tag-value-list  =  tag-value *("," tag-value)
   tag-value       =  ["!"] (token-nobang / boolean / numeric)
   token-nobang    =  1*(alphanum / "-" / "." / "%" / "*"
                      / "_" / "+" / "`" / "'" / "~" )
   boolean         =  "TRUE" / "FALSE"
   numeric         =  "#" numeric-relation number
   numeric-relation  =  ">=" / "<=" / "=" / (number ":")
   number          =  [ "+" / "-" ] 1*DIGIT ["." 0*DIGIT]
   string-value    =  "<" qdtext ">"

   Note that the tag-value-list uses an actual comma instead of the
   COMMA construction. Thats because it appears within a quoted string,
   where line folding cannot take place.

   The production for qdtext can be found in RFC 3261 [1].

   There are additional constraints on usage of feature-param that
   cannot be represented in a BNF. There MUST only be one instance of
   any feature tag in feature-param. Any numbers present in a feature
   parameter MUST be representable using an ANSI C double.

   The following production updates the one in RFC 3261 [1] for
   contact-params:




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   contact-params    =  c-p-q / c-p-expires / feature-param
                        / contact-extension

















































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10. Media Feature Tag Definitions

   This specification defines an initial set of media feature tags for
   use with this specification. New media feature tags SHOULD be
   registered with IANA, based on the process defined for feature tag
   registrations [3]. This section also serves as the IANA registration
   for these feature tags.

   Any registered feature tags MAY be used with this specification.
   However, several existing ones appear to be particularly applicable.
   These include the language feature tag [6], which can be used to
   specify the language of the human or automata represented by the UA,
   and the type feature tag [7], which can be used to specify the MIME
   types of the media formats supported by the UA. However, the usage of
   the audio, video, application, data and control feature tags (each of
   which indicate a media type, as defined in RFC 2327 [8]) supported by
   the UA are preferred to indicating support for specific media
   formats. When the type feature tag is present, there SHOULD also be a
   feature tag present for the its top-level MIME type with a value of
   TRUE. In other words, if a UA indicates in a registration that it
   supports the video/H263 MIME type, it should also indicate that it
   supports video generally:

      Contact: sip:192.0.2.1;type="video/H263";video="TRUE"

   If a new SDP media type were to be defined, such as "message", a new
   feature tag registration SHOULD be created for it. The name of the
   feature tag MUST equal that of the media type, unless there is an
   unlikely naming collision between the new media type and an existing
   feature tag registration. As a result of this, implementations can
   safely construct caller preferences and callee capabilities for the
   new media type before it is registered, as long as there is no naming
   conflict.

   If a new media feature tag is registered with the intent of using
   that tag with this specification, the registration is done for the
   unencoded form of the tag (see Section Section 5). In other words, if
   a new feature tag "foo" is registered, the IANA registration would be
   for the tag "foo" and not "+foo".

10.1 Attendant

   Media feature tag name: attendant

   ASN.1 Identifier: New assignment by IANA.






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   Summary of the media feature indicated by this tag: This feature tag
      indicates that the device is an automated or human attendant that
      will answer if the actual user of the device is not available.

   Values appropriate for use with this feature tag: Boolean.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Routing a call to a phone that has an
      auto-attendant feature.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.2 Audio

   Media feature tag name: audio

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: This feature tag
      indicates that the device supports audio as a media type.

   Values appropriate for use with this feature tag: Boolean.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Routing a call to a phone that can support
      audio.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.3 Application

   Media feature tag name: application

   ASN.1 Identifier: New assignment by IANA.





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   Summary of the media feature indicated by this tag: This feature tag
      indicates that the device supports application as a media type.
      This feature tag exists primarily for completeness. Since so many
      MIME types are underneath application, indicating the ability to
      support applications provides little useful information. In most
      cases, the concrete MIME type is a better parameter to use in a
      predicate representing a preference.

   Values appropriate for use with this feature tag: Boolean.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Routing a call to a phone that can supports
      gaming application.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.4 Data

   Media feature tag name: data

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: This feature tag
      indicates that the device supports data as a media type.

   Values appropriate for use with this feature tag: Boolean.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Routing a call to a phone that can supports
      a data streaming application.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.5 Control





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   Media feature tag name: control

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: This feature tag
      indicates that the device supports control as a media type.

   Values appropriate for use with this feature tag: Boolean.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Routing a call to a phone that can supports
      a floor control application.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.6 Automata

   Media feature tag name: automata

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: The automata
      feature tag is a boolean value that indicates whether the UA
      represents an automata (such as a voicemail server, conference
      server, IVR, or recording device) or a human.

   Values appropriate for use with this feature tag: Boolean. TRUE
      indicates that the UA represents an automata.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Choosing to communicate with a message
      recording device instead of a user.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]






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10.7 Class

   Media feature tag name: class

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: This feature tag
      indicates the setting, business or personal, in which a
      communications device is used.

   Values appropriate for use with this feature tag: Token with an
      equality relationship. Typical values include:

      business: The device is used for business communications.

      personal: The device is used for personal communications.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Choosing between a business phone and a home
      phone.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.8 Duplex

   Media feature tag name: duplex

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: The duplex media
      feature tag lists whether a communications device can
      simultaneously send and receive media ("full"), alternate between
      sending and receiving ("half"), can only receive ("receive-only")
      or only send ("send-only").

   Values appropriate for use with this feature tag: Token with an
      equality relationship. Typical values include:

      full: The device can simultaneously send and receive media.






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      half: The device can alternate between sending and receiving
         media.

      receive-only: The device can only receive media.

      send-only: The device can only send media.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Choosing to communicate with a broadcast
      server, as opposed to a regular phone, when making a call to hear
      an announcement.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.9 Mobility

   Media feature tag name: mobility

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: The mobility
      feature tag indicates whether the device is fixed (meaning that it
      is associated with a fixed point of contact with the network), or
      mobile (meaning that it is not associated with a fixed point of
      contact). Note that cordless phones are fixed, not mobile, based
      on this definition.

   Values appropriate for use with this feature tag: Token with an
      equality relationship. Typical values include:

      fixed: The device is stationary.

      mobile: The device can move around with the user.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Choosing to communicate with a wireless
      phone instead of a desktop phone.




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   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.10 Description

   Media feature tag name: description

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: The description
      feature tag provides a textual description of the device.

   Values appropriate for use with this feature tag: String with an
      equality relationship.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Indicating that a device is of a certain
      make and model.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.11 Event Packages

   Media feature tag name: events

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: The event
      packages [9] supported by a SIP UA. The values for this tag equal
      the event package names that are registered by each event package.

   Values appropriate for use with this feature tag: Token with an
      equality relationship. Values are taken from the IANA SIP Event
      types namespace registry.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.





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   Examples of typical use: Choosing to communicate with a server that
      supports the message waiting event package, such as a voicemail
      server [12].

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.12 Priority

   Media feature tag name: priority

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: The priority
      feature tag indicates the call priorities the device is willing to
      handle. A value of X means that the device is willing to take
      requests with priority X and higher.

   Values appropriate for use with this feature tag: An integer. Each
      integral value corresponds to one of the possible values of the
      Priority header field as specified in SIP [1]. The mapping is
      defined as:

      non-urgent: Integral value of 10. The device supports non-urgent
         calls.

      normal: Integral value of 20. The device supports normal calls.

      urgent: Integral value of 30. The device supports urgent calls.

      emergency: Integral value of 40. The device supports calls in the
         case of an emergency situation.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Choosing to communicate with the emergency
      cell phone of a user.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]







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10.13 Methods

   Media feature tag name: methods

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: The methods (note
      the plurality) feature tag indicates the SIP methods supported by
      this UA. In this case, "supported" means that the UA can receive
      requests with this method. In that sense, it has the same
      connotation as the Allow header field.

   Values appropriate for use with this feature tag: Token with an
      equality relationship. Values are taken from the Methods table
      defined in the IANA SIP parameters registry.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Choosing to communicate with a presence
      application on a PC, instead of a PC phone application.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.14 SIP Extensions

   Media feature tag name: sip-extensions

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: The
      sip-extensions feature tag is a list of SIP extensions (each of
      which is defined by an option-tag registered with IANA) that are
      understood by the UA. Understood, in this context, means that the
      option tag would be included in a Supported header field in a
      request.

   Values appropriate for use with this feature tag: Token with an
      equality relationship. Values are taken from the option tags table
      in the IANA SIP parameters registry.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for



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      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Choosing to communicate with a phone that
      supports quality of service preconditions instead of one that does
      not.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.15 Schemes

   Media feature tag name: schemes

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: The set of URI
      schemes [10] that are supported by a UA. Supported implies, for
      example, that the UA would know how to handle a URI of that scheme
      in the Contact header field of a redirect response.

   Values appropriate for use with this feature tag: Token with an
      equality relationship. Values are taken from the IANA URI scheme
      registry.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Choosing to get redirected to a phone number
      when a called party is busy, rather than a web page.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.16 Video

   Media feature tag name: video

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: This feature tag
      indicates that the device supports video as a media type.






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   Values appropriate for use with this feature tag: Boolean.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Routing a call to a phone that can support
      video.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.17 Message Server

   Media feature tag name: msgserver

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: This feature tag
      indicates that the device is a messaging server which will record
      messages for a user. An example of such a device is a voicemail
      server.

   Values appropriate for use with this feature tag: Boolean.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Requesting that a call not be routed to
      voicemail.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.18 Is Focus

   Media feature tag name: isfocus

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: This feature tag
      indicates that the UA is a conference server, also known as a
      focus, and will mix together the media for all calls to the same



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      URI [13].

   Values appropriate for use with this feature tag: Boolean.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Indicating to a UA that the server it has
      connected to is a conference server.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.19 URI User

   Media feature tag name: uri-user

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: The uri-user
      feature tag provides the user part of the SIP URI that represents
      the device.

   Values appropriate for use with this feature tag: String with an
      equality relationship.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Requesting to route a call to a specific
      device, identified by a URI.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]


10.20 URI Domain

   Media feature tag name: uri-domain

   ASN.1 Identifier: New assignment by IANA.





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   Summary of the media feature indicated by this tag: The uri-domain
      feature tag indicates the hostname of a device.

   Values appropriate for use with this feature tag: Token with a
      case-insensitive equality relationship.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Requesting to route a call to a specific
      device, identified by a URI.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]



































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11. Security Considerations

   Feature sets contained in REGISTER requests can reveal sensitive
   information about a user or UA (for example, the languages spoken).
   If this information is sensitive, confidentiality SHOULD be provided
   by using the SIPS URI scheme, as described in RFC 3261 [1].













































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12. IANA Considerations

   There are a number of IANA considerations associated with this
   specification.

12.1 Media Feature Tags

   This specification registers a number of new Media feature tags
   according to the procedures of RFC 2506 [3]. Those registrations are
   contained in Section Section 10, and are meant to be placed into the
   IETF tree for media feature tags.

12.2 SIP Option Tag

   This specification registers a single SIP option tag, pref.  The
   required information for this registration, as specified in RFC 3261
   [1], is:

   Name: pref

   Description: This option tag is used in a Require header field of a
      registration to ensure that the registrar supports the caller
      preferences extensions.




























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13. Acknowledgments

   The initial set of media feature tags used by this specification were
   influenced by Scott Petrack's CMA design. Jonathan Lennox, Bob
   Penfield, Ben Campbell, Mary Barnes, Rohan Mahy and John Hearty
   provided helpful comments. Graham Klyne provided assistance on the
   usage of RFC 2533.












































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Normative References

   [1]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [2]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [3]   Holtman, K., Mutz, A. and T. Hardie, "Media Feature Tag
         Registration Procedure", BCP 31, RFC 2506, March 1999.

   [4]   Klyne, G., "A Syntax for Describing Media Feature Sets", RFC
         2533, March 1999.

   [5]   Klyne, G., "Corrections to "A Syntax for Describing Media
         Feature Sets"", RFC 2738, December 1999.

   [6]   Hoffman, P., "Registration of Charset and Languages Media
         Features Tags", RFC 2987, November 2000.

   [7]   Klyne, G., "MIME Content Types in Media Feature Expressions",
         RFC 2913, September 2000.

   [8]   Handley, M. and V. Jacobson, "SDP: Session Description
         Protocol", RFC 2327, April 1998.

   [9]   Roach, A., "Session Initiation Protocol (SIP)-Specific Event
         Notification", RFC 3265, June 2002.

   [10]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
         Resource Identifiers (URI): Generic Syntax", RFC 2396, August
         1998.


















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Informative References

   [11]  Rosenberg, J., Schulzrinne, H. and P. Kyzivat, "Caller
         Preferences and Callee Capabilities for the Session Initiation
         Protocol (SIP)", draft-ietf-sip-callerprefs-08 (work in
         progress), March 2003.

   [12]  Mahy, R., "A Message Summary and Message Waiting Indication
         Event Package for the  Session Initiation Protocol (SIP)",
         draft-ietf-sipping-mwi-02 (work in progress), March 2003.

   [13]  Rosenberg, J., "A Framework for Conferencing with the Session
         Initiation Protocol",
         draft-ietf-sipping-conferencing-framework-00 (work in
         progress), May 2003.

   [14]  Howes, T. and M. Smith, "LDAP: String Representation of Search
         Filters", draft-ietf-ldapbis-filter-04 (work in progress),
         March 2003.

   [15]  Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and
         D. Gurle, "Session Initiation Protocol (SIP) Extension for
         Instant Messaging", RFC 3428, December 2002.

   [16]  Klyne, G., "Protocol-independent Content Negotiation
         Framework", RFC 2703, September 1999.


Authors' Addresses

   Jonathan Rosenberg
   dynamicsoft
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

   Phone: +1 973 952-5000
   EMail: jdrosen@dynamicsoft.com
   URI:   http://www.jdrosen.net












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   Henning Schulzrinne
   Columbia University
   M/S 0401
   1214 Amsterdam Ave.
   New York, NY  10027
   US

   EMail: schulzrinne@cs.columbia.edu
   URI:   http://www.cs.columbia.edu/~hgs


   Paul Kyzivat
   Cisco Systems
   Mail Stop LWL3/12/2
   900 Chelmsford St.
   Lowell, MA  01851
   US

   EMail: pkzivat@cisco.com
































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Appendix A. Overview of RFC 2533

   This section provides a brief overview of RFC 2533 and related
   specifications that form the content negotiation framework. This
   section does not represent normative behavior. In the event of any
   conflict between the tutorial material here and the normative text in
   RFC 2533, RFC 2533 takes precedence.

   A critical concept in the framework is that of a feature set. A
   feature set is information about an entity (in our case, a UA), which
   describes a set of features it can handle. A feature set can be
   thought of as a region in N-dimensional space. Each dimension in this
   space is a different media feature, identified by a media feature
   tag. For example, one dimension (or axis) might represent languages,
   another might represent methods, and another, MIME types. A feature
   collection represents a single point in this space. It represents a
   particular rendering or instance of an entity (in our case, a UA).
   For example, a ``rendering'' of a UA would define an instantaneous
   mode of operation that it can support. One such rendering would be
   processing the INVITE method, which carried the application/sdp MIME
   type, sent to a UA for a user that is speaking English.

   A feature set can therefore be defined as a set of feature
   collections. In other words, a feature set is a region of
   N-dimensional feature-space, that region being defined by the set of
   points - feature collections - that make up the space. If a
   particular feature collection is in the space, it means that the
   rendering described by that feature collection is supported by the
   device with that feature set.

   How does one represent a feature set? There are many ways to describe
   an N-dimensional space. One way is to identify mathematical functions
   which identify its contours. Clearly, that is too complex to be
   useful. The solution taken in RFC 2533 is to define the space with a
   feature set predicate.  A feature predicate defines a relation over
   an N-dimensional space; its input is any point in that space (i.e. a
   feature collection), and is true for all points that are in the
   region thus defined.

   RFC 2533 describes a syntax for writing down these N-dimensional
   boolean functions, borrowed from LDAP [14]. It uses a prolog-style
   syntax which is fairly self-explanatory. This representation is
   called a feature set predicate. The base unit of the predicate is a
   filter, which is a boolean expression encased in round brackets. A
   filter can be complex, where it contains conjunctions and
   disjunctions of other filters, or it can be simple. A simple filter
   is one that expresses a comparison operation on a single media
   feature tag.



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   For example, consider the feature set predicate:

   (& (foo=A)
      (bar=B)
      (| (baz=C) (& (baz=D) (bif=E))))

   This defines a function over four media features - foo, bar, baz and
   bif. Any point in feature space with foo equal to A, bar equal to B,
   and either baz equal to C, or baz equal to D and bif equal to E, is
   in the feature set defined by this feature set predicate.

   Note that the predicate doesn't say anything about the number of
   dimensions in feature space. The predicate operates on a feature
   space of any number of dimensions, but only those dimensions labeled
   foo, bar, baz and bif matter. The result is that values of other
   media features don't matter. The feature collection
   {foo=A,bar=B,baz=C,bop=F} is in the feature set described by the
   predicate, even though the media feature tag ``bop'' isn't mentioned.
   Feature set predicates are therefore inclusive by default. A feature
   collection is present unless the boolean predicate rules it out. This
   was a conscious design choice in RFC 2533.

   RFC 2533 also talks about matching a preference with a capability
   set. This is accomplished by representing both with a feature set. A
   preference is a feature set - its a specification of a number of
   feature collections, any one of which would satisfy the requirements
   of the sender. A capability is also a feature set - its a
   specification of the feature collections that the recipient supports.
   There is a match when the spaces defined by both feature sets
   overlap. When there is overlap, there exists at least one feature
   collection that exists in both feature sets, and therefore a modality
   or rendering desired by the sender which is supported by the
   recipient.

   This leads directly to the definition of a match. Two feature sets
   match if there exists at least one feature collection present in both
   feature sets.

   Computing a match for two general feature set predicates is not easy.
   Section 5 of RFC 2533 presents an algorithm for doing it by expanding
   an arbitrary expression into disjunctive normal form. However, the
   feature set predicates used by this specification are constrained.
   They are always in conjunctive normal form, with each term in the
   conjunction describing values for different media features. This
   makes computation of a match easy. It is computed independently for
   each media feature, and then the feature sets overlap if media
   features specified in both sets overlap. Computing the overlap of a
   single media feature is very straightforward, and is a simple matter



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   of computing whether two finite sets overlap.


















































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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement

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