Keywords: [--------|p], ua, contact header field
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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RFC 3840 SIP Capabilities August 2004
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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RFC 3840 SIP Capabilities August 2004
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.
Rosenberg, et al. Standards Track [Page 31]
RFC 3840 SIP Capabilities August 2004
[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.
Rosenberg, et al. Standards Track [Page 32]
RFC 3840 SIP Capabilities August 2004
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
Rosenberg, et al. Standards Track [Page 35]
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Rosenberg, et al. Standards Track [Page 36]