





Network Working Group                                       J. Rosenberg
Request for Comments: 3840                                   dynamicsoft
Category: Standards Track                                 H. Schulzrinne
                                                     Columbia University
                                                              P. Kyzivat
                                                           Cisco Systems
                                                             August 2004


                 Indicating User Agent Capabilities in
                 the Session Initiation Protocol (SIP)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

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




















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Usage of the Content Negotiation Framework . . . . . . . . . .  6
   5.  Computing Capabilities . . . . . . . . . . . . . . . . . . . .  7
   6.  Expressing Capabilities in a Registration  . . . . . . . . . . 10
   7.  Indicating Feature Sets in Remote Target URIs  . . . . . . . . 12
   8.  OPTIONS Processing . . . . . . . . . . . . . . . . . . . . . . 13
   9.  Contact Header Field . . . . . . . . . . . . . . . . . . . . . 13
   10. Media Feature Tag Definitions  . . . . . . . . . . . . . . . . 14
       10.1.  Audio . . . . . . . . . . . . . . . . . . . . . . . . . 15
       10.2.  Application . . . . . . . . . . . . . . . . . . . . . . 16
       10.3.  Data. . . . . . . . . . . . . . . . . . . . . . . . . . 16
       10.4.  Control . . . . . . . . . . . . . . . . . . . . . . . . 17
       10.5.  Video . . . . . . . . . . . . . . . . . . . . . . . . . 17
       10.6.  Text. . . . . . . . . . . . . . . . . . . . . . . . . . 18
       10.7.  Automata. . . . . . . . . . . . . . . . . . . . . . . . 18
       10.8.  Class . . . . . . . . . . . . . . . . . . . . . . . . . 19
       10.9.  Duplex. . . . . . . . . . . . . . . . . . . . . . . . . 20
       10.10. Mobility. . . . . . . . . . . . . . . . . . . . . . . . 20
       10.11. Description . . . . . . . . . . . . . . . . . . . . . . 21
       10.12. Event Packages. . . . . . . . . . . . . . . . . . . . . 22
       10.13. Priority. . . . . . . . . . . . . . . . . . . . . . . . 22
       10.14. Methods . . . . . . . . . . . . . . . . . . . . . . . . 23
       10.15. Extensions. . . . . . . . . . . . . . . . . . . . . . . 24
       10.16. Schemes . . . . . . . . . . . . . . . . . . . . . . . . 24
       10.17. Actor . . . . . . . . . . . . . . . . . . . . . . . . . 25
       10.18. Is Focus. . . . . . . . . . . . . . . . . . . . . . . . 26
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 26
       11.1.  Considerations for Media Feature Tags . . . . . . . . . 26
       11.2.  Considerations for Registrations. . . . . . . . . . . . 27
       11.3.  Considerations for OPTIONS Responses. . . . . . . . . . 28
       11.4.  Considerations for Dialog Initiating Messages . . . . . 28
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 28
       12.1.  SIP Media Feature Tag Registration Tree . . . . . . . . 28
       12.2.  Media Feature Tags. . . . . . . . . . . . . . . . . . . 29
       12.3.  SIP Option Tag. . . . . . . . . . . . . . . . . . . . . 30
   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 30
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
       14.1.  Normative References. . . . . . . . . . . . . . . . . . 31
       14.2.  Informative References. . . . . . . . . . . . . . . . . 31
   Appendix.  Overview of RFC 2533. . . . . . . . . . . . . . . . . . 33
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 36





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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 [16] request.  However,
      before sending it, they would like to know if the UA has the
      capabilities 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.  SIP also has no ability to convey
   characteristics, that is, information that describes a UA.

   As a result, this specification provides a more general framework for
   an indication of capabilities and characteristics in SIP.  Capability
   and characteristic information about a UA is carried as parameters of





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   the Contact header field.  These parameters can be used within
   REGISTER requests and responses, OPTIONS responses, and requests and
   responses that create dialogs (such as INVITE).

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 BCP 14, RFC 2119
   [2] and indicate requirement levels for compliant implementations.

3.  Definitions

   Feature: As defined in RFC 2703 [17], 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 [17], a feature tag is a name
      that identifies a feature.  An example is "sip.methods".

   Media Feature: As defined in RFC 2703, [17], 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 [17], 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.







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   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 [17], 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.  A capability is distinct from a characteristic in that a
      capability may or may not be utilized in any particular call,
      whereas a characteristic is a non-negotiable property of a UA.
      SIP itself will often negotiate whether or not capabilities are
      used in a call.

   Characteristic: A characteristic is like a capability, but describes
      an aspect of a UA which is not negotiable.  As an example, whether
      or not a UA is a mobile phone is a characteristic, not a
      capability.  The semantics of this specification do not
      differentiate between capability and characteristic, but the
      distinction is useful for illustrative purposes.  Indeed, in the
      text below, when we say "capability", it refers to both
      capabilities and characteristics, unless the text explicitly says
      otherwise.

   Filter: A single expression in a feature set predicate.

   Simple Filter: An expression in a feature set 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 [17], 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 in 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 "sip.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 that 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
   SIP tree (Established in Section 12.1), IETF, or global trees [3];
   this document registers several into the SIP tree.  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 "sip.methods" feature tag, a UA MUST NOT include
   values that correspond to methods not standardized in IETF standards
   track RFCs.  When using the "sip.events" feature tag, a UA MUST NOT
   include values that correspond to event packages not standardized in
   IETF standards track RFCs.  When using the "sip.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.

   Note that the "sip.schemes" feature tag 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
   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
   "sip.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



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   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), while 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.   If a string (as defined in RFC 2533 [4]) is
   used as the value of a simple filter, that value MUST NOT include the
   "<" or ">" characters, the simple filter MUST NOT be negated, and it
   MUST be the only simple filter for that particular feature tag.  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 that will be present in the Contact
   header field.  The syntax of this parameter depends on the feature
   tag.  Each forward slash in the feature tag is converted to a single
   quote, and each colon are converted to an exclamation point.  For the
   base tags - that is, those feature tags documented in this
   specification (sip.audio, sip.automata, sip.class, sip.duplex,
   sip.data, sip.control, sip.mobility, sip.description, sip.events,
   sip.priority, sip.methods, sip.extensions, sip.schemes,
   sip.application, sip.video, language, type, sip.isfocus, sip.actor
   and sip.text), the leading "sip.", if present, is stripped.  For
   feature tags not in this list, the leading "sip." MUST NOT be
   stripped if present, and indeed, a plus sign ("+") MUST be added as
   the first character of the Contact header field parameter.  The
   result is the feature parameter name.  As a result of these rules,
   the base tags appear "naked" in the Contact header field - they have
   neither a "+" nor a "sip." prefix.  All other tags will always have a
   leading "+" when present in the Contact header field, and will
   additionally have a "sip." if the tag is in the SIP tree.

   The value of the feature parameter depends on the term of the
   conjunction.  If the term is a boolean expression with a value of
   true, i.e., (sip.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. If the filter is a simple filter that is performing a numeric
   comparison, the string starts with an octothorpe (#), followed by the



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   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.

      RFC 2533 uses a fractional notation to describe rational numbers.
      This specification uses 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, this feature predicate:

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

   would be converted into the following feature parameters:

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

   These feature tags would then appear as part of the Contact header
   field:

   Contact: <sip:user@pc.example.com>
             ;mobility="fixed";events="!presence,message-summary"
             ;language="en,de";description="<PC>"
             ;+sip.newparam;+rangeparam="#-4:+5.125"

   Notice how the leading "sip." was stripped from the sip.mobility,
   sip.events and sip.description feature tags before encoding them in



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   the Contact header field.  This is because these feature tags are
   amongst the base tags listed above.  It is for this reason that these
   feature tags were not encoded with a leading "+" either.  However,
   the sip.newparam feature tag was encoded with both the "+" and its
   leading "sip.", and the rangeparam was also encoded with a leading
   "+".  This is because neither of these feature tags are defined in
   this specification.  As such, the leading "sip." is not stripped off,
   and a "+" is added.

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
   multiplicity 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.

   Similarly, in some cases, a UA can exhibit one characteristic or
   another, but the characteristic is not known in advance.  For
   example, a UA could represent a device that is a phone with an
   embedded answering machine.  The ideal way to treat such devices is
   to model them as if they were actually a proxy fronting two devices -
   a phone (which is never an answering machine), and an answering



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   machine (which is never a phone).  The registration from this device
   would be constructed as if it were an AOR, as per the procedures
   above.  Generally, this means that, unless the characteristic is
   identical between the logical devices, that characteristic will not
   be present in any registration generated by the actual device.

   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 Contact header field value containing the
   URI to which the parameters apply.  The Allow, Accept, Accept-
   Language and Allow-Events [9] header fields are allowed in REGISTER
   requests, and also indicate capabilities.  However, their semantic in
   REGISTER is different, indicating capabilities, used by the
   registrar, for generation of the response.  As such, they are not a
   substitute or an alternate for the Contact feature parameters, which
   indicate the capabilities of the UA generally speaking.

   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 the absence of the Require header field, a registrar that
   does not understand this extension will simply ignore the Contact
   header field parameters.

   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 allows the UA to
   uniquely determine the feature set that is associated with the
   request URI of an incoming request.

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

   (& (sip.audio=TRUE)
      (sip.video=TRUE)
      (sip.actor=msg-taker)
      (sip.automata=TRUE)
      (sip.mobility=fixed)
      (| (sip.methods=INVITE) (sip.methods=BYE) (sip.methods=OPTIONS)
         (sip.methods=ACK) (sip.methods=CANCEL)))






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   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;video
     ;actor="msg-taker";automata;mobility="fixed"
     ;methods="INVITE,BYE,OPTIONS,ACK,CANCEL"
   Content-Length: 0

   Note that a voicemail server is usually an automata and a message
   taker.

   When a UAC refreshes its registration, it MUST include its feature
   parameters in that refresh if it wishes for them to remain active.
   Furthermore, when a registrar returns a 200 OK response to a REGISTER
   request, each Contact header field value MUST include all of the
   feature parameters associated with that URI.

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 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 callee capabilities 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 "sip.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 "sip.events" feature tag indicate
   the same information.  It is possible that other header fields and



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   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.

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.

   The guidelines in Section 7 regarding the overlap of the various
   callee capabilities feature tags with SIP header fields applies to
   the generation of OPTIONS responses as well.  In particular, they
   apply when a Contact header field is describing the UA which
   generated the OPTIONS response.  When a Contact header field in the
   OPTIONS response is identifying a different UA, there is no overlap.

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        =  "audio" / "automata" /
                       "class" / "duplex" / "data" /
                       "control" / "mobility" / "description" /
                       "events" / "priority" / "methods" /
                       "schemes" / "application" / "video" /
                       "language" / "type" / "isfocus" /
                       "actor" / "text" / "extensions"
   other-tags      =  "+" ftag-name
   ftag-name       =  ALPHA *( ALPHA / DIGIT / "!" / "'" /
                      "." / "-" / "%" )



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   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-no-abkt / quoted-pair ) ">"
   qdtext-no-abkt  =  LWS / %x21 / %x23-3B / %x3D
                           / %x3F-5B / %x5D-7E / UTF8-NONASCII

   Note that the tag-value-list uses an actual comma instead of the
   COMMA construction 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 the 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:

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

10.  Media Feature Tag Definitions

   This specification defines an initial set of media feature tags for
   use with this specification.  This section serves as the IANA
   registration for these feature tags, which are made into the SIP
   media feature tag tree.  New media feature tags are registered in the
   IETF or global trees based on the process defined for feature tag
   registrations [3], or in the SIP tree based on the process defined in
   Section 12.1.

   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 that a SIP UA can receive in a SIP message.  The audio, video,
   application, data, and control feature tags in the SIP tree (each of
   which indicate a media type, as defined in RFC 2327 [8]) are
   different.  They do not indicate top level MIME types which can be



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   received in SIP requests.  Rather, they indicate media types that can
   be used in media streams, and as a result, match up with the types
   defined in RFC 2327 [8].

   If a new SDP media type were to be defined, such as "message", a new
   feature tag registration SHOULD be created for it in the SIP tree.
   The name of the feature tag MUST equal "sip." concatenated with the
   name 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, 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 5).  In other words, if a new
   feature tag "foo" is registered in the IETF tree, the IANA
   registration would be for the tag "foo" and not "+foo".  Similarly,
   if a new feature tag "sip.gruu" is registered in the SIP tree, the
   IANA registration would be for the tag "sip.gruu" and not "+sip.gruu"
   or "gruu".  As such, all registrations into the SIP tree will have
   the "sip." prefix.

   The feature tags in this section are all registered in the SIP media
   feature tag tree created by Section 12.1.

10.1.  Audio

   Media feature tag name: sip.audio

   ASN.1 Identifier: 1.3.6.1.8.4.1

   Summary of the media feature indicated by this tag: This feature tag
      indicates that the device supports audio as a streaming 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 3840




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   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

10.2.  Application

   Media feature tag name: sip.application

   ASN.1 Identifier: 1.3.6.1.8.4.2

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

   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 a
      media control application.

   Related standards or documents: RFC 3840

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

10.3.  Data

   Media feature tag name: sip.data

   ASN.1 Identifier: 1.3.6.1.8.4.3

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





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

   Related standards or documents: RFC 3840

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

10.4.  Control

   Media feature tag name: sip.control

   ASN.1 Identifier: 1.3.6.1.8.4.4

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

   Related standards or documents: RFC 3840

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

10.5.  Video

   Media feature tag name: sip.video

   ASN.1 Identifier: 1.3.6.1.8.4.5

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

   Values appropriate for use with this feature tag: Boolean.







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   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 3840

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

10.6.  Text

   Media feature tag name: sip.text

   ASN.1 Identifier: 1.3.6.1.8.4.6

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

   Related standards or documents: RFC 3840

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

10.7.  Automata

   Media feature tag name: sip.automata

   ASN.1 Identifier: 1.3.6.1.8.4.7

   Summary of the media feature indicated by this tag: The sip.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.




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   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: Refusing to communicate with an automata
      when it is known that automated services are unacceptable.

   Related standards or documents: RFC 3840

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

10.8.  Class

   Media feature tag name: sip.class

   ASN.1 Identifier: 1.3.6.1.8.4.8

   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 3840

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.






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10.9.  Duplex

   Media feature tag name: sip.duplex

   ASN.1 Identifier: 1.3.6.1.8.4.9

   Summary of the media feature indicated by this tag: The sip.duplex
      media feature tag indicates 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.

      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 3840

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

10.10.  Mobility

   Media feature tag name: sip.mobility

   ASN.1 Identifier: 1.3.6.1.8.4.10

   Summary of the media feature indicated by this tag: The sip.mobility
      feature tag indicates whether the device is fixed (meaning that it
      is associated with a fixed point of contact with the network), or



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      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.

   Related standards or documents: RFC 3840

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

10.11.  Description

   Media feature tag name: sip.description

   ASN.1 Identifier: 1.3.6.1.8.4.11

   Summary of the media feature indicated by this tag: The
      sip.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 3840




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   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

10.12.  Event Packages

   Media feature tag name: sip.events

   ASN.1 Identifier: 1.3.6.1.8.4.12

   Summary of the media feature indicated by this tag: Each value of the
      sip.events (note the plurality) feature tag indicates a SIP event
      package [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.

   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 3840

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

10.13.  Priority

   Media feature tag name: sip.priority

   ASN.1 Identifier: 1.3.6.1.8.4.13

   Summary of the media feature indicated by this tag: The sip.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.  This does not imply that a
      phone has to reject calls of lower priority.  As always, the
      decision on handling of such calls is a matter of local policy.







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   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 3840

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

10.14.  Methods

   Media feature tag name: sip.methods

   ASN.1 Identifier: 1.3.6.1.8.4.14

   Summary of the media feature indicated by this tag: Each value of the
      sip.methods (note the plurality) feature tag indicates a SIP
      method 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.



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   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 3840

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

10.15.  Extensions

   Media feature tag name: sip.extensions

   ASN.1 Identifier: 1.3.6.1.8.4.15

   Summary of the media feature indicated by this tag: Each value of the
      sip.extensions feature tag (note the plurality) is a SIP extension
      (each of which is defined by an option-tag registered with IANA)
      that is 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
      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 3840

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

10.16.  Schemes

   Media feature tag name: sip.schemes

   ASN.1 Identifier: 1.3.6.1.8.4.16

   Summary of the media feature indicated by this tag: Each value of the
      sip.schemes (note the plurality) media feature tag indicates a URI
      scheme [10] that is supported by a UA.  Supported implies, for



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      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 3840

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

10.17.  Actor

   Media feature tag name: sip.actor

   ASN.1 Identifier: 1.3.6.1.8.4.17

   Summary of the media feature indicated by this tag: This feature tag
      indicates the type of entity that is available at this URI.

   Values appropriate for use with this feature tag: Token with an
      equality relationship.  The following values are defined:

      principal: The device provides communication with the principal
         that is associated with the device.  Often this will be a
         specific human being, but it can be an automata (for example,
         when calling a voice portal).

      attendant: The device provides communication with an automaton or
         person that will act as an intermediary in contacting the
         principal associated with the device, or a substitute.

      msg-taker: The device provides communication with an automaton or
         person that will take messages and deliver them to the
         principal.

      information: The device provides communication with an automaton
         or person that will provide information about the principal.




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   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 3840

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

10.18.  Is Focus

   Media feature tag name: sip.isfocus

   ASN.1 Identifier: 1.3.6.1.8.4.18

   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
      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 to which
      it has connected is a conference server.

   Related standards or documents: RFC 3840

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC 3840.

11.  Security Considerations

11.1.  Considerations for Media Feature Tags

   This section discusses security considerations for the media feature
   tags, including, but not limited to, this specification.






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   The media feature tags defined in Section 10 reveal sensitive
   information about a user or the user agent they are describing.  Some
   of the feature tags convey capability information about the agent -
   for example, the media types it can support, the SIP methods it can
   support, and the SIP extensions it can support.  This capability
   information might be used for industrial espionage, for example, and
   so its protection may be important.  Other attributes, such as the
   mobility, priority, and isfocus attributes, reveal characteristics of
   the user agent.  These attributes are more sensitive than the
   capability information.  They describe the way in which a user agent
   is utilized by a user, and thus reveal information about user
   preferences and the ways in which they want calls handled.  Some
   feature tags, such as languages, reveal information about the user
   themself.  As a result, applications which utilize these media
   feature tags SHOULD provide a means for ensuring their
   confidentiality.

   The media feature tags can be used in ways which affect application
   behaviors.  For example, the SIP caller preferences extension [11]
   allows for call routing decisions to be based on the values of these
   parameters.  Therefore, if an attacker can modify the values of these
   feature tags, they may be able to affect the behavior of
   applications.  As a result of this, applications which utilize these
   media feature tags SHOULD provide a means for ensuring their
   integrity.  Similarly, media feature tags should only be trusted as
   valid when they come from the user or user agent described by those
   feature tags.  As a result, mechanisms for conveying feature tags
   SHOULD provide a mechanism for guaranteeing authenticity.

11.2.  Considerations for Registrations

   As per the general requirements in Section 11.1, when media feature
   tags are carried in a registration, authenticity, confidentiality,
   and integrity need to be provided.  To accomplish this, registrations
   containing capability information SHOULD be made by addressing the
   registration to a SIPS URI (in other words, the Request URI of the
   request would be sips:example.com when creating a registration in the
   example.com domain).  Furthermore, the registrar SHOULD challenge the
   UA using digest over TLS, to verify its authenticity.  The
   combination of TLS and digest provide integrity, confidentiality, and
   authenticity, as required.

   It is not necessary for the Contact in the registration to itself
   contain a sips URI, since the feature tags are not carried in
   incoming requests sent to the UA.






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11.3.  Considerations for OPTIONS Responses

   When including information on capabilities in a response to an
   OPTIONS request, a UA SHOULD verify with the user (either through a
   user interface or though prior configuration) whether or not
   capability information should be divulged to the requester.  If the
   identity of the requester cannot be cryptographically verified (using
   digest or the SIP identity enhancements [15]), the user SHOULD also
   be alerted to this fact, and be allowed to choose whether such
   information should be divulged.

   If the user does wish to reveal capability information to the
   requester, and wishes to guarantee its confidentiality, but the
   request did not arrive using SIPS, the UAS SHOULD redirect the
   request to a sips URI.  This will cause the UAC to send the OPTIONS
   request using SIPS instead, and therefore provide confidentiality of
   any responses sent over the secure connections.

   Furthermore, S/MIME MAY be used in the OPTIONS response.  In that
   case, the capability information would be contained only in the
   secured S/MIME body, and not in the header fields of the OPTIONS
   response.

11.4.  Considerations for Dialog Initiating Messages

   When a UAS generates a response that will initiate a dialog, and they
   wish to include capability information in the Contact header field,
   the same considerations as described in Section 11.3 apply.

   When a UAC generates a request that will initiate a dialog, it SHOULD
   obtain permission from the user (either through a user interface or
   apriori configuration) before including capability information in the
   Contact header field of the request.  Confidentiality and integrity
   of the information SHOULD be provided using SIPS.  S/MIME MAY be
   used.

12.  IANA Considerations

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

12.1.  SIP Media Feature Tag Registration Tree

   This specification serves to create a new media feature tag
   registration tree, per the guidelines of Section 3.1.4 of RFC 2506
   [3].  The name of this tree is the "SIP Media Feature Tag
   Registration Tree", and its prefix is "sip.".  It is used for the
   registration of media feature tags that are applicable to the Session



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   Initiation Protocol, and whose meaning is only defined within that
   usage.

   The addition of entries into this registry occurs through IETF
   consensus, as defined in RFC 2434 [18].  This requires the
   publication of an RFC that contains the registration.  The
   information required in the registration is identical to the IETF
   tree.  As such, specifications adding entries to the registry should
   use the template provided in Section 3.4 of RFC 2506.  Note that all
   media feature tags registered in the SIP tree will have names with a
   prefix of "sip.".  No leading "+" is used in the registrations in any
   of the media feature tag trees.

12.2.  Media Feature Tags

   This specification registers a number of new Media feature tags
   according to the procedures of RFC 2506 [3].  These registrations are
   all made in the newly created SIP tree for media feature tags.  These
   registrations are:

   sip.audio: The information for registering the sip.audio media
      feature tag is contained in Section 10.1.

   sip.application: The information for registering the sip.application
      media feature tag is contained in Section 10.2.

   sip.data: The information for registering the sip.data media feature
      tag is contained in Section 10.3.

   sip.control: The information for registering the sip.control media
      feature tag is contained in Section 10.4.

   sip.video: The information for registering the sip.video media
      feature tag is contained in Section 10.5.

   sip.text: The information for registering the sip.text media feature
      tag is contained in Section 10.6.

   sip.automata: The information for registering the sip.automata media
      feature tag is contained in Section 10.7.

   sip.class: The information for registering the sip.class media
      feature tag is contained in Section 10.8.

   sip.duplex: The information for registering the sip.duplex media
      feature tag is contained in Section 10.9.





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   sip.mobility: The information for registering the sip.mobility media
      feature tag is contained in Section 10.10.

   sip.description: The information for registering the sip.description
      media feature tag is contained in Section 10.11.

   sip.events: The information for registering the sip.events media
      feature tag is contained in Section 10.12.

   sip.priority: The information for registering the sip.priority media
      feature tag is contained in Section 10.13.

   sip.methods: The information for registering the sip.methods media
      feature tag is contained in Section 10.14.

   sip.extensions: The information for registering the sip.extensions
      media feature tag is contained in Section 10.15.

   sip.schemes: The information for registering the sip.schemes media
      feature tag is contained in Section 10.16.

   sip.actor: The information for registering the sip.actor media
      feature tag is contained in Section 10.17.

   sip.isfocus: The information for registering the sip.isfocus media
      feature tag is contained in Section 10.18.

12.3.  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.

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.  Thanks to Allison Mankin for her comments and
   support, and to Ted Hardie for his guidance on usage of the media
   feature tags.



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14.  References

14.1.  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.B., "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.

14.2.  Informative References

   [11]  Rosenberg, J., Schulzrinne, H. and P. Kyzivat, "Caller
         Preferences for the Session Initiation Protocol (SIP)", RFC
         3841, August 2004.

   [12]  Mahy, R., "A Message Summary and Message Waiting Indication
         Event Package for the Session Initiation Protocol (SIP)", RFC
         3842, August 2004.






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   [13]  Rosenberg, J., "A Framework for Conferencing with the Session
         Initiation Protocol", Work in Progress, May 2003.

   [14]  Howes, T. and M. Smith, "LDAP: String Representation of Search
         Filters", Work in Progress, March 2003.

   [15]  Peterson, J., "Enhancements for Authenticated Identity
         Management in the Session  Initiation Protocol (SIP)", Work in
         Progress, March 2003.

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

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

   [18]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434, October
         1998.































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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





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   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.

   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 baz equal to either C or 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



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   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
   of computing whether two finite sets overlap.

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

   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
   1414 Massachusetts Avenue
   BXB500 C2-2
   Boxboro, MA  01719
   US

   EMail: pkyzivat@cisco.com















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Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.









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