Internet Engineering Task Force SIP WG
Internet Draft J. Rosenberg
dynamicsoft
H. Schulzrinne
Columbia U.
P. Kyzivat
Cisco
draft-ietf-sip-callerprefs-08.txt
March 2, 2003
Expires: September 2003
Caller Preferences and Callee Capabilities for the Session
Initiation Protocol (SIP)
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document describes a set of extensions to the Session Initiation
Protocol (SIP) which allow a caller to express preferences about
request handling in servers. These preferences include the ability to
select which Uniform Resource Identifiers (URI) a request gets routed
to, and to specify certain request handling directives in proxies and
redirect servers. It does so by defining three new request header
fields, Accept-Contact, Reject-Contact, and Request-Disposition,
which specify the caller's preferences. The extension also defines
new parameters for the Contact header field that describe the
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capabilities and characteristics of a User Agent (UA).
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Table of Contents
1 Introduction ........................................ 5
2 Terminology ......................................... 6
3 Definitions ......................................... 6
4 Overview of Operation ............................... 8
5 Usage of the Content Negotiation Framework .......... 9
6 UA Behavior ......................................... 11
6.1 Expressing Capabilities in a Registration ........... 11
6.2 Expressing Preferences in a Request ................. 14
6.2.1 Request Handling Preferences ........................ 15
6.2.2 Feature Set Preferences ............................. 15
6.3 Indicating Feature Sets in Remote Target URIs ....... 16
6.4 Processing Request Handling and Feature Set
Preferences .................................................... 17
6.5 OPTIONS Processing .................................. 17
7 Proxy Behavior ...................................... 18
7.1 Request-Disposition Processing ...................... 18
7.2 Preference and Capability Matching .................. 18
7.2.1 Extracting Explicit Preferences ..................... 18
7.2.2 Extracting Implicit Preferences ..................... 19
7.2.2.1 Methods ............................................. 19
7.2.2.2 Event Packages ...................................... 20
7.3 Constructing Contact Predicates ..................... 20
7.4 Matching ............................................ 21
7.4.1 Example ............................................. 27
8 Header Field Definitions ............................ 29
8.1 Request Disposition ................................. 29
8.2 Accept-Contact and Reject-Contact Header Fields ..... 31
8.3 Contact Header Field ................................ 31
9 Media Feature Tag Definitions ....................... 32
9.1 Attendant ........................................... 32
9.2 Audio ............................................... 33
9.3 Application ......................................... 33
9.4 Data ................................................ 34
9.5 Control ............................................. 35
9.6 Automata ............................................ 35
9.7 Class ............................................... 36
9.8 Duplex .............................................. 36
9.9 Mobility ............................................ 37
9.10 Description ......................................... 38
9.11 Event Packages ...................................... 38
9.12 Priority ............................................ 39
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9.13 Methods ............................................. 40
9.14 SIP Extensions ...................................... 41
9.15 Schemes ............................................. 42
9.16 Video ............................................... 43
9.17 Message Server ...................................... 43
9.18 Is Focus ............................................ 44
9.19 URI User ............................................ 44
9.20 URI Domain .......................................... 45
10 Augmented BNF ....................................... 45
11 Mapping Feature Parameters and Feature Set
Predicates ..................................................... 47
12 Security Considerations ............................. 50
13 IANA Considerations ................................. 50
13.1 Media Feature Tags .................................. 50
13.2 SIP Header Fields ................................... 51
13.3 SIP Option Tags ..................................... 51
14 Acknowledgments ..................................... 52
15 Author's Addresses .................................. 52
16 Normative References ................................ 52
17 Informative References .............................. 54
A Overview of RFC 2533 ................................ 55
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1 Introduction
When a Session Initiation Protocol (SIP) [1] server receives a
request, there are a number of decisions it can make regarding
processing of the request. These include:
o whether to proxy or redirect the request
o which URIs to proxy or redirect to
o whether to fork or not
o whether to search recursively or not
o whether to search in parallel or sequentially
The server can base these decisions on any local policy. This policy
can be statically configured, or can be based on programmatic
execution or database access.
However, the administrator of the server is the not the only entity
with an interest in request processing. There are at least three
parties which have an interest: (1) the administrator of the server,
(2) the user that sent the request, and (3) the user to whom the
request is directed. The directives of the administrator are embedded
in the policy of the server. The preferences of the user to whom the
request is directed (referred to as the callee, even though the
request may not be INVITE) can be expressed most easily through a
script written in some type of scripting language, such as the Call
Processing Language (CPL) [22]. However, no mechanism exists to
incorporate the preferences of the user that sent the request (also
referred to as the caller, even though the request may not be
INVITE). For example, the caller might want to speak to a specific
user, but want to reach them only at work, because the call is a
business call. As another example, the caller might want to reach a
user, but not their voicemail, since it is important that the caller
talk to the called party. In both of these examples, the caller's
preference amounts to having a proxy make a particular routing choice
based on the preferences of the caller.
This extension allows the caller to have these preferences met. It
does so by specifying mechanisms by which a caller can provide
preferences on processing of a request. There are two types of
preferences. One of them, called request handling preferences, are
encapsulated in the Request-Disposition header field. They provide
specific request handling directives for a server. The other, called
feature preferences, are present in the Accept-Contact and Reject-
Contact header fields. They allow the caller to provide a feature set
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[2] that expresses its preferences on the characteristics of the UA
that is to be reached. These are matched with a feature set carried
in the Contact header field of a REGISTER request, which describes
the capabilities of the UA represented by the Contact URI. The
extension is very general purpose, and not tied to a particular
service. Rather, it is a tool that can be used in the development of
many services.
Indeed, the feature sets uploaded to the server in REGISTER requests
can be used for a variety of purposes, not just meeting caller
preferences. Applications can use this information to tailor
information sent to a user as part of an instant message, for example
[3].
One example of the a service enabled by caller preferences is a "one
number" service. A user can have a single identity (their SIP URI)
for all of their devices - their cell phone, PDA, work phone, home
phone, and so on. If the caller wants to reach the user at their
business phone, they simply select "business phone" from a pull-down
menu of options when calling that URI. Users would no longer need to
maintain and distribute separate identities for each device.
2 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [4] and
indicate requirement levels for compliant SIP implementations.
3 Definitions
Caller: Within the context of this specification, a caller
refers to the user on whose behalf a UAC is operating. It
is not limited to a user who's UAC sends the INVITE method.
Feature: As defined in RFC 2703 [23], 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 [23], a feature tag is a
name that identifies a feature. An example is "methods".
Media Feature: As defined in RFC 2703, [23], 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.
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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 [2], 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 [23], 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 full set of possible attributes.
Feature Preferences: Caller preferences that described desired
properties of a UA that the request is to be routed to.
Feature preferences can be made explicitly with the
Accept-Contact and Reject-Contact header fields.
Request Handling Preferences: Caller preferences that describe
desired request treatment at a server. These preferences
are carried in the Request-Disposition header field.
Feature Parameters: A set of SIP header field parameters that
can appear in the Contact, Accept-Contact and Reject-
Contact header fields. 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 [23], 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.
Target Set: A target set is a set of candidate URI that a proxy
or redirect server can send or redirect a request to.
Frequently, target sets are obtained from a registration,
but they need not be.
Explicit Preference: A caller preference indicated explicitly in
the Accept-Contact or Reject-Contact header fields.
Implicit Preference: A caller preference that is implied through
the presence of other aspects of a request. For example, if
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the request method is INVITE, it represents an implicit
caller preference to route the request to a UA that
supports the INVITE method.
Filter: A single expression in a feature set predicate.
Simple Filter: An expression in a feature predicate which is a
comparison (equality or inequality) of a feature tag
against a feature value.
Disjunction: A boolean OR operation across some number of terms.
Conjunction: A boolean AND operation across some number of
terms.
Predicate: A boolean expression.
Feature Set Predicate: From RFC 2533 [2], 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.
4 Overview of Operation
This extension defines a set of additional parameters to the Contact
header field, called feature parameters. Each parameter name is an
encoded feature tag, as defined in RFC 2703 [23], that defines a
capability for the UA associated with the Contact header field value.
For example, there is a parameter for the SIP methods supported by
the UA. Each feature parameter has a value; that value is the set of
feature values for that feature tag. Put together, all of the feature
parameters specify a feature set that is supported by the UA
associated with that Contact header field value.
When a UA registers, it places these parameters in the Contact header
field value to provide a feature set for a URI it is registering. The
feature parameters are also mirrored in the Contact header field in a
REGISTER response. The proxy can use this feature set to route
requests based on caller preferences. Furthermore, Contact header
fields in requests and responses that establish a dialog can contain
these parameters. That allows a UA in a dialog to indicate its
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feature set to its peer. For example, by including the "msgserver"
feature tag with value "TRUE" in the 200 OK to an INVITE, the UAS can
indicate to the UAC that it is a voicemail server. This information
is useful for user interfaces, as well as automated call handling.
When a caller sends a request, it can optionally include new header
fields which request certain handling at a server. These preferences
fall into two categories. The first category, called request handling
preferences, are carried in the Request-Disposition header field.
They describe specific behavior that is desired at a server. Request
handling preferences include whether the caller wishes the server to
proxy or redirect, and whether sequential or parallel search is
desired. These preferences can be applied at every proxy or redirect
server on the call signaling path.
The second category of preferences, called feature preferences, are
carried in the Accept-Contact and Reject-Contact header fields. These
header fields also contain feature sets, represented by the same
feature parameters that are used in the Contact header field. Here,
the feature parameters represent the caller's preferences. The
Accept-Contact header field contains feature sets that describe UAs
that the caller would like to reach. The Reject-Contact header field
contains feature sets which, if matched by a UA, imply that the
request should not be routed to that UA.
Proxies use the information in the Accept-Contact and Reject-Contact
header fields to select amongst contacts in their target set. When
neither of those header fields are present, the proxy computes
implicit preferences from the request. These are caller preferences
that are not explicitly placed into the request, but can be inferred
from the presence of other message components. As an example, if the
request method is INVITE, this is an implicit preference to route the
call to a UA that supports the INVITE method.
Both request handling and feature preferences can appear in any
request, not just INVITE. However, they are only useful in requests
where proxies need to determine a request target. If the domain in
the request URI is not owned by any proxies along the request path,
those proxies will never access a location service, and therefore,
never have the opportunity to apply the caller preferences. This
makes sense; typically, the request URI will identify a UAS for mid-
dialog requests. In those cases, the routing decisions were already
made on the initial request, and it makes no sense to redo them for
subsequent requests in the dialog.
5 Usage of the Content Negotiation Framework
This specification makes heavy use of the terminology and concepts in
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the content negotiation work carried out within the IETF, and
documented in several RFCs. The ones relevant to this specification
are RFC 2506 [5] which provides a template for registering media
feature tags, RFC 2533 [2] which presents a syntax and matching
algorithm for media feature sets, RFC 2738 [6], which provides a
minor update to RFC 2533, and RFC 2703 [23] 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 the SIP entities at
hand. The goal of this specification is to allow a UA to express its
feature set, and for a caller to express a feature set that describes
properties of a desirable (or undesirable) UA. Therefore, we are
using feature sets to describe 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 instant of time, a UA could be handling both audio and video,
they need to be separate media feature tags. However, the SIP methods
supported by a UA can each be represented as different values for the
same media feature tag (the "methods" tag), because fundamentally, a
UA processes a single request at a time. It may be multi-threading,
so that it appears that this is not so, but at a purely functional
level, it is true.
Clearly, there are weaknesses in this model, but it serves as a
useful guideline for applying the concepts of RFC 2533 to the problem
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at hand.
6 UA Behavior
UA behavior covers five separate cases. The first is registration,
where a UA can declare its capabilities. The second is expression of
preferences in a request, where a UA can tell a proxy how it wants
the request to be processed and routed. The third is expressing of
capabilities, through a feature set, in the Contact header field of a
target refresh request or response. The fourth is UAS processing of
the request handling and feature preferences. The fifth is UAS
processing of an OPTIONS request.
6.1 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.
If a UA does not include feature parameters for a contact, that
contact will be immune from the caller preference processing.
Therefore, if a registering client does not want caller preferences
applied to a contact, it omits all feature parameters. Addresses-of-
record in particular often need to be immune from caller preferences
processing. If they were not, such a URI might be eliminated from
consideration, even though a downstream UA satisfies the desired
constraints.
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However, in some cases a UA may wish to express feature parameters
for an address-of-record. One example is an AOR which represents a
mutliplicity of devices in a home network, and routes to a proxy
server in the user's home. Since all devices in the home are for
personal use, the AOR itself can be described with the
"class=personal" feature parameter. A registration that forwards
calls to this home AOR could make use of that feature parameter.
Generally speaking, a feature parameter can only be associated with
an address-of-record if all devices bound to that address-of-record
share the exact same set of values for that feature parameter.
The remainder of this section assumes that a UA would like to
associate a feature set with a contact that it is registering. To do
that, it constructs a feature predicate for that contact. In the
text that follows, this process is described in terms of RFC 2533 [2]
(and its minor update, [6]) 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.
The feature predicate constructed by a UA MUST be an AND of terms
(called a conjunction). Each term is either an OR of simple filters
(called a disjunction), or a single simple filter. In the case of an
OR of simple filters, each filter MUST indicate feature values for
the same feature tag (i.e., the disjunction represents a set of
values for a particular feature tag), and each element of the
conjunction MUST be for a different feature tag. Each filter can be
an equality, the negation of an equality, or in the case of numeric
feature tags, an inequality, range, or negation of an inequality or
range. This feature predicate is then converted to a list of feature
parameters using the procedure specified in Section 11. Those feature
parameters are added to the the Contact header field value containing
the URI that the parameters apply to.
A UA MAY use any feature tags that are registered through IANA in the
IETF or global trees [5]; this document registers several that are
appropriate for SIP. It is also permissible to use the URI tree [5]
for expressing vendor-specific feature tags. Feature tags in any
other trees created through IANA MAY also be used.
A UA SHOULD include the "uri-user" and "uri-domain" feature tag in
its feature parameters. The value of those tags SHOULD be equal to
the user and domain part of the registered URI, respectively. Setting
them differently is likely to result in odd behavior, and should only
be done if some unforseen service neccesitates it. Note that the
"uri-user" feature tag is a quoted string (implying case sensitive
matching), and the "uri-domain" feature tag is a token, implying case
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insensitive matching.
Note that the "schemes" feature tag is not a peer of the "uri-user"
and "uri-domain" feature tags. That is, it does not indicate the
scheme of the registered URI. Rather, it indicates schemes that a UA
is capable of sending requests to, should such a URI be received in a
web page or Contact header field of a redirect response.
It is RECOMMENDED that a UA provide complete information in its
feature 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
"methods" feature tag, a UA MUST list all methods it supports. The
matching algorithms in this specification assume that omission of a
value from a list means that the value is not supported.
When using the "methods" feature tag, a UA MUST NOT include values
that correspond to methods not standardized in IETF standards track
RFCs. When using the "events" feature tag, a UA MUST NOT include
values that correspond to event packages not standardized in IETF
standards track RFCs. When using the "schemes" feature tag, a UA MUST
NOT include values that correspond to schemes not standardized in
IETF standards track RFCs. When using the "sip-extensions" feature
tag, a UA MUST NOT include values that correspond to option tags not
standardized in IETF standards track RFCs.
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. In absence
of the Require header field, a server that does not understand this
extension will simply ignore the Contact header field parameters.
As an example, a UA that supports audio and video media types, is a
voicemail server, and is not mobile would construct a feature
predicate like this:
(& (audio=TRUE)
(video=TRUE)
(msgserver=TRUE)
(automata=TRUE)
(attendant=TRUE)
(mobility=fixed)
(| (methods=INVITE) (methods=BYE) (methods=OPTIONS) (methods=ACK)
(methods=CANCEL))
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(uri-user="user")
(uri-domain=host.example.com)
These would be converted into feature parameters and included in the
REGISTER request:
REGISTER sip:example.com SIP/2.0
From: sip:user@example.com;tag=asd98
To: sip:user@example.com
Call-ID: hh89as0d-asd88jkk@host.example.com
CSeq: 9987 REGISTER
Max-Forwards: 70
Via: SIP/2.0/UDP host.example.com;branch=z9hG4bKnashds8
Contact: <sip:user@host.example.com>;audio="TRUE";video="TRUE"
;msgserver="TRUE";automata;attendant;mobility="fixed"
;methods="INVITE,BYE,OPTIONS,ACK,CANCEL"
;uri-user="<user>"
;uri-domain="host.example.com"
Content-Length: 0
Note that a voicemail server is usually an automata and an attendant,
as defined below.
6.2 Expressing Preferences in a Request
A caller wishing to express preferences for a request includes
Accept-Contact, Reject-Contact or Request-Disposition header fields
in the request, depending on their particular preferences. No
additional behavior is required after the request is sent.
The Accept-Contact, Reject-Contact and Request-Disposition header
fields in an ACK for a non-2xx final response, or in a CANCEL
request, MUST be equal to the values in the original request being
acknowledged or cancelled. This is to ensure proper operation through
stateless proxies.
If the UAC wants to be sure that servers understand the header fields
described in this specification, it MAY include a Proxy-Require
header field with a value of "pref". However, this is NOT
RECOMMENDED, as it leads to interoperability problems. In any case,
caller preferences can only be considered preferences - there is no
guarantee that the requested service is executed. As such, inclusion
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of a Proxy-Require header field does not mean the preferences will be
executed, just that the caller preferences extension is understood by
the proxies.
6.2.1 Request Handling Preferences
The Request-Disposition header field specifies caller preferences for
how a server should process a request. Its value is a list of tokens,
each of which specifies a particular processing directive.
The syntax of the header field can be found in Section 10, and the
semantics of the directives are described in Section 8.1.
6.2.2 Feature Set Preferences
A UAC can indicate caller preferences for the capabilities of a UA
that should be reached or not reached as a result of sending a SIP
request. To do that, it adds one or more Accept-Contact and Reject-
Contact header field values. Each header field value contains a set
of feature parameters that define a feature set. In the case of
Accept-Contact, each value can also have a q-value parameter.
Each feature set MUST follow the constraints of Section 6.1. The
feature sets placed into these header fields MAY overlap; that is, a
UA MAY indicate preferences for feature sets that match according to
the matching algorithm of RFC 2533 [2]. The UA MAY use any feature
tag in an IANA registry or in a vendor defined URI tree.
A UAC can express explicit preferences for the methods and event
packages supported by a UA. It is RECOMMENDED that a UA include a
term in an Accept-Contact feature set with the "methods" feature tag,
whose value includes the method of the request. When a UA sends a
SUBSCRIBE request, it is RECOMMENDED that a UA include a term in an
Accept-Contact feature set with the "events" feature tag, whose value
includes the event package of the request. Whether these terms are
placed into a new feature set, or whether they are included in each
feature set, is at the discretion of the implementor. In most cases,
the right effect is achieved by including a term in each feature set.
The Reject-Contact header field allows the UAC to specify that a UA
should not be contacted if it matches any of the values of the header
field. Each value of the Reject-Contact header field contains a "*",
purely to align the syntax with guidelines for SIP extensions [24],
and is parameterized by a set of feature parameters. Any UA whose
capabilities match the feature set described by the feature
parameters matches the value. As with registrations, it is not
necessary for a UAC to construct the feature set in RFC 2533 syntax
as an intermediate step. The only requirement is that the feature
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parameters, if converted back to RFC 2533 format, meet the
requirements above.
The Accept-Contact header field allows the UAC to specify that a UA
should be contacted if it matches some or all of the values of the
header field. Each value of the Accept-Contact header field contains
a "*" and is parameterized by a set of feature parameters. Any UA
whose capabilities match the feature set described by the feature
parameters matches the value. The q-value parameter provides a
weighting operation. A q-value parameter with a particular value
means that the caller's preference for a UA described by the feature
parameters equals that value. The processing rules at a proxy will
also favor those UA that are a "better" match to a particular value.
Here, better means that more of its capabilities explicitly match the
feature preferences. The value may also contain an "explicit"
parameter, which indicates that only UA whose capabilities explicitly
match are considered a match. If one of the values contains the
"require" parameter, it means that the UA must match that value. As
with registrations, it is not necessary for a UAC to construct the
feature set in RFC 2533 syntax as an intermediate step. The only
requirement is that the feature parameters, if converted back to RFC
2533 format, meet the requirements above.
6.3 Indicating Feature Sets in Remote Target URIs
Target refresh requests and responses are used to establish and
modify the remote target URI. The remote target URI is contained 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 feature set predicate according to the
constraints of Section 6.1, and converts it to a set of feature
parameters using the rules in Section 11. These are then added as
Contact header field parameters in the request or response.
The feature parameters can be included in both initial requests and
mid-dialog requests, and MAY change mid-dialog to signal a change in
UA capabilities.
There is overlap in the caller preferences mechanism with the Allow,
Accept, Accept-Language, and Allow-Events [7] header fields, which
can also be used in target refresh requests. Specifically, the Allow
header field and "methods" feature tag indicate the same information.
The Accept header field and the "type" feature tag indicate the same
information. The Accept-Language header field and the "language"
feature tag indicate the same information. The Allow-Events header
field and the "events" feature tag indicate the same information. It
is possible that other header fields and feature tags defined in the
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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.
6.4 Processing Request Handling and Feature Set Preferences
When a UAS compliant to this specification receives a request whose
request-URI corresponds to one of its registered Contacts, it SHOULD
apply the behavior described in Section 7 as if it were a proxy for
the domain in the request-URI. The UAS acts as if its location
database contains a single request target for the request-URI. That
target is associated with a feature set. The feature set is the same
as the one placed in the registration of the URI in the request-URI.
This processing occurs after the client authenticates and authorizes
the request, but before the remainder of the general UAS processing
described in Section 8.2.1 of RFC 3261.
If a UA registers against two separate addresses-of-record, and the
contacts registered for each have different capabilities, a UA MUST
use different URIs in each registration. This is so that the UA can
uniquely determine the feature set that is associated with the
request URI of an incoming request.
If, after performing this processing, there are no URI left in the
target set, the UA SHOULD reject the request with a 480 response. If
there is a URI remaining (there was only one to begin with), the UA
proceeeds with request processing as per RFC 3261.
Having a UAS perform the matching operations as if it were
a proxy allows certain caller preferences to be honored
even if the proxy doesn't support the extension.
6.5 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 feature set predicate
according to the constraints of Section 6.1, and converts it to a set
of feature parameters using the rules in Section 11. These are then
added as Contact header field parameters in OPTIONS response. Indeed,
if feature parameters were included in the registration generated by
that UA, those same parameters SHOULD be used in the OPTIONS
response.
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7 Proxy Behavior
Proxy behavior consists of two orthogonal sets of rules - one for
processing the Request-Disposition header field, and one for
processing the URI and feature set preferences in the Accept-Contact
and Reject-Contact header fields.
In addition to processing these headers, a proxy MAY add one if not
present, or add a value to an existing header field, as if it were a
UAC. This is useful for a proxy to request processing in downstream
proxies in the implementation of a feature. However a proxy MUST NOT
modify or remove an existing header field or header field value. This
is particularly important when S/MIME is used. The message signature
could include the caller preferences header fields, allowing the UAS
to verify that, even though proxies may have added header fields, the
original caller preferences were still present.
7.1 Request-Disposition Processing
If the request contains a Request-Disposition header field, the
server SHOULD execute the directives as described in Section 8.1,
unless it has local policy configured to direct it otherwise.
7.2 Preference and Capability Matching
A proxy compliant to this specification MUST NOT apply the
preferences matching operation described here to a request unless it
is the owner of the domain in the request URI, and accessing a
location service that has capabilities associated with request
targets. However, if it is the owner of the domain, and accessing a
location service that has capabilities associated with request
targets, it SHOULD apply the processing described in this section.
Typically, this is a proxy that is using a registration database to
determine the request targets. However, if a proxy knows about
capabilities through some other means, it SHOULD apply the processing
defined here as well. If it does perform the processing, it MUST do
so as described below.
The processing is described through a conversion from the syntax
described in this specification to RFC 2533 syntax, followed by a
matching operation and a sorting of resulting contact values. The
usage of RFC 2533 syntax as an intermediate step is not required, it
only serves as a useful tool to describe the behavior required of the
proxy. A proxy can use any steps it likes so long as the results are
identical to the ones that would be achieved with the processing
described here.
7.2.1 Extracting Explicit Preferences
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The first step in proxy processing is to extract explicit
preferences. To do that, it looks for the Accept-Contact and Reject-
Contact header fields.
For each value of those header fields, it extracts the feature
parameters. These are the header field parameters whose name is one
of the base-tags (see Section 10), or whose name begins with a plus
(+). The proxy converts all of those parameters to the syntax of RFC
2533, based on the rules in Section 11.
The result will be a set of feature set predicates in conjunctive
normal form, each of which is associated with one of the two
preference header fields. If there was a q parameter associated with
a header field value in the Accept-Contact header field, the feature
set predicate derived from that header field value is assigned a
preference equal to that q value. If there was a req-parameter
associated with a header field value in the Accept-Contact header
field, the feature set predicate derived from that header field value
is said to have its require flag set. Similarly, if there was an
explicit-param associated with a header field value in the Accept-
Contact header field, the feature set predicate derived from that
header field value is said to have its explicit flag set.
7.2.2 Extracting Implicit Preferences
If, and only if, the proxy did not find any explicit preferences in
the request (because there was no Accept-Contact or Reject-Contact
header field), the proxy extracts implicit preferences. These
preferences are ones implied by the presence of other information in
the request.
First, the proxy creates a conjunction with no terms. This
conjunction represents a feature set that will be associated with the
Accept-Contact header field, as if it were included there. Note that
there is no modification of the message implied - only an association
for the purposes of processing. Furthermore, this feature set has its
require flag set, but not its explicit flag.
The proxy then adds terms to the conjunction for the two implicit
preference types below.
7.2.2.1 Methods
One implicit preference is the method. When a UAC sends a request
with a specific method, it is an implicit preference to have the
request routed only to UAs that support that method. To support this
implicit preference, the proxy adds a term to the conjunction of the
following form:
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(methods=[method of request])
7.2.2.2 Event Packages
For requests that establish a subscription [7], the Event header
field is another expression of an implicit preference. It expresses a
desire for the request to be routed only to a server than supports
the given event package. To support this implicit preference, the
proxy adds a term to the conjunction of the following form:
(events=[value of the Event header field])
7.3 Constructing Contact Predicates
The proxy then takes each URI in the target set (the set of URI it is
going to proxy or redirect to), and obtains its capabilities as an
RFC 2533 formatted feature set predicate. This is called a contact
predicate. If the target URI was obtained through a registration, the
proxy computes the contact predicate by extracting the feature
parameters from the Contact header field and the converting them to a
feature predicate. To extract the feature parameters, the proxy
follows these steps:
1. Create an initial, empty list of feature parameters.
2. If the Contact URI parameters included the "attendant",
"audio", "automata", "class", "duplex", "data", "control",
"mobility", "description", "events", "priority", "methods",
"schemes", "application", "video", "msgserver", "language",
"isfocus", "uri-user", "uri-domain" or "type" parameters,
those are copied into the list.
3. If any Contact URI parameter name begins with a "+", it is
copied into the list if the list does not already contain
that name with the plus removed. In other words, if the
"video" feature parameter is in the list, the "+video"
parameter would not be placed into the list. This conflict
should never arise if the client were compliant to this
specification, since it is illegal to use the + form for
encoding of a feature tag in the base set.
If the URI in the target set had no feature parameters, it is said to
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be immune to caller preference processing. This means that the URI is
removed from the target set temporarily, the caller preferences
processing described below is executed, and then the URI is added
back in.
Assuming the URI has feature parameters, they are converted to RFC
2533 syntax using the rules of Section 11.
The resulting predicate is associated with a q-value. If the contact
predicate was learned through a REGISTER request, the q-value is
equal to the q-value in the Contact header field parameter, else
"1.0" if not specified.
As an example, consider the following registered Contact header
field:
Contact: <sip:user@example.com>;audio;video;mobility="fixed";
+message="TRUE";other-param=66372;
methods="INVITE,OPTIONS,BYE,CANCEL,ACK";schemes="sip,http";
uri-user="<user>";uri-domain="example.com"
This would be converted into the following predicate:
(& (audio=TRUE)
(video=TRUE)
(mobility=fixed)
(message=TRUE)
(| (methods=INVITE) (methods=OPTIONS) (methods=BYE)
(methods=CANCEL) (methods=ACK))
(| (schemes=sip) (schemes=http))
(uri-user="user")
(uri-domain="example.com"))
Note that "other-param" was not considered a featuer parameter, since
it is neither a base tag nor did it begin with a leading +.
7.4 Matching
It is important to note that the proxy does not have to know anything
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about the meaning of the feature tags that it is comparing in order
to perform the matching operation. The rules for performing the
comparison depend on syntactic hints present in the values of each
feature tag. For example, a predicate such as:
(foo>=4)
implies that the feature tag "foo" is a numeric value. The matching
rules in RFC 2533 only require an implementation to know whether the
feature tag is a numeric, token, or quoted string (booleans can be
treated as tokens). Quoted strings are always matched using a case-
sensitive matching operation. Tokens are matched using case-
insensitive matching. Numerics are matched using normal mathematical
comparisons.
First, the proxy applies the predicates associated with the Reject-
Contact header field.
For each contact predicate, each Reject-Contact predicate (that is,
each predicate associated with the Reject-Contact header field) is
examined. If that Reject-Contact predicate contains a filter for a
feature tag, and that feature tag is not present anywhere in the
contact predicate, that Reject-Contact predicate is discarded for the
processing of that contact predicate. If the Reject-Contact predicate
is not discarded, it is matched to the contact predicate using the
matching operation of RFC 2533 [2]. If the result is a match, the URI
corresponding to that contact predicate is discarded from the target
set.
The result is that Reject-Contact will only discard URIs where the UA
has explicitly indicated support for the features that are not
wanted.
Next, the proxy applies the predicates associated with the Accept-
Contact header field. For each contact that remains in the target
set, the proxy constructs a matching set, Ms. Initially, this set
contains all of the Accept-Contact predicates. Each of those
predicates is examined. It is matched to the contact predicate using
the matching operation of RFC 2533 [2]. If the result is not a match,
and the Accept-Contact predicate had its require flag set, the URI
corresponding to that contact predicate is discarded from the contact
set. If the result is not a match, but the Accept-Contact predicate
did not have its require flag set, that contact URI is not discarded
from the contact set, however, the Accept-Contact predicate is
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removed from the matching set for that contact.
For each contact that remains in the target set, the proxy computes a
score for that contact against each predicate the contact's matching
set. Let the number of terms in the Accept-Contact predicate
conjunction be equal to N. Each term in that predicate contains a
single feature tag. If the contact predicate has a term containing
that same feature tag, the score is incremented by 1/N. If the
feature tag was not present in the contact predicate, the score
remains unchanged. Based on these rules, the score can range between
zero and one.
The require and explicit tags are then applied, resulting in
potential modification of the score and the target set. This process
is summarized in Figure 1. If the score for the contact predicate
against that Accept-Contact predicate was less than one, and the
Accept-Contact predicate had an explicit tag, if the predicate also
had a require tag, the Contact URI corresponding to that contact
predicate is dropped. If, however, the predicate did not have a
require tag, the score is set to zero. If there was no explicit tag,
the score is unchanged.
The next step is to combine the scores and the q-values associated
with the predicates in the matching set, to arrive at an overall
caller preference, Qa. For those URIs in the target set which remain,
there will be a score which indicates its match against each Accept-
Contact predicate in the matching set. If there are M Accept-Contact
predicates in the matching set, there will be M scores S1 through SM,
for each contact. There will also be a preference associated with
each Accept-Contact predicate (derived from the q-value parameter, as
discussed in Section 7.2.1), X1..XM. The caller preference, Qa, is
computed as shown in Figure 2.
Note that in the limit as all Si go to zero, Qa equals the arithmetic
average of Xi.
This algorithm was chosen carefully so as to exhibit certain
properties:
o If Si is 1 for i=j, and zero for all other i, Qa=Xi. In other
words, if a contact predicate matches one of the Accept-
Contact predicates with a score of one (referred to as an
explicit match), and all others match with a score of zero
(referred to as an implicit match), the caller's preference
equals the q-value of that predicate.
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o If Si is the same for all predicates in the matching set, Qa
is equal to the average of the q-values for the predicates.
o If the contact predicate matches only one Accept-Contact
predicate, Qa is equal to the q-value of that predicate,
independent of the score.
The final step is to combine the overall caller preference for the
contact (Qa) with the q-value provided for that contact by the callee
(which we denote as Qb). The proxy can use any averaging mechanism at
its disposal, prefentially treating the callers preference and the
callee's preference as policy dictates. In the absence of policy
indicating otherwise, the two values are arithmetically averaged.
This results in an overall q-value for that contact, Qo, equal to:
Qa + Qb
Qo = ---------
2
At this point, any URI that were removed from the target set because
they were immune from caller preferences are added back in, and Qo
for that URI is set to its original q-value, or 1.0 if there was no
q-value specified.
If there were no URIs in the target set after the application of the
processing in this section, and the caller preferences were based on
implicit preferences (Section 7.2.2), the processing in this section
is discarded, and the original target set, along with their original
q-values, is used.
This handles the case where implicit preferences for the
method or event packages resulted in the elimination of all
potential targets. By going back to the original target
set, those URIs will be tried, and result in the generation
of a 405 or 489. The UAC can then use this information to
try again, or report the error to the user. Without
reverting to the original target set, the UAC would see a
480 response, and have no knowledge of why their request
failed. Of course, the target set can also be empty after
the application of explicit preferences. This will result
in the generation of a 480 by the proxy. This behavior is
acceptable, and indeed, desirable in the case of explicit
preferences. When the caller makes an explicit preference,
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T
+----------> DROP Contact
|
|
/ \
/ \
T / \ F
+---->/require\------> Set score=0
| \ /
| \ /
/ \ \ /
/ \ \/
score<1 / \
+-------> /explicit----> Score unchanged
| \ / F
| \ /
/ \ \ /
/ \ \/
+--------+ / \
-->|Compute |--> /Score \ --------> Score unchanged
| Score | \ / score=1
+--------+ \ /
\ /
\/
Figure 1: Score Computation
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/
/
/
| 0, if M=0
|
|
|
| M
| ------
| \
| \
| / Si*Xi
| /
Qa = | ------
| i=1
| if M>0
| ------------------
|
| M
| ------
| \
| \
| / Si
| /
| ------
\ i=1
\
\
Figure 2: Computation of Qa
it is agreeing that its request might fail because of a
preference mismatch. One might try to return an error
indicating the capabilities of the callee, so that the
caller could perhaps try again. However, doing so results
in the leaking of potentially sensitive information to the
caller without authorization from the callee, and therefore
this specification does not provide a means for it.
Any proxy processing that takes the q-values as inputs (for example,
a forking operation as described in Section 16.6 of RFC 3261 [1])
would use Qo instead of the original q-value associated with the
contact, for this specific transaction only. To avoid preferring one
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contact to another because of a relatively small difference in their
overall q-value, it is RECOMMENDED that the values be rounded to the
nearest tenth before they are used by the proxy.
If a proxy server is recursing, it applies the caller preferences to
the Contact header fields returned in the redirect responses. Any URI
remaining after the application of caller preferences are added to
the proxy's target set if it is not already in the target set. This
list is then resorted based on q values. The server uses this list
for subsequent proxy operations.
If the server is redirecting, it returns all entries in the target
set, including a q-value of Qo for each Contact URI as obtained
through the process above. This includes any URI with a zero q-value.
However, it MUST NOT include the feature parameters for the entries
in the target set. If it did, the upstream proxy server would apply
the same caller preferences once more, resulting in a double
application of those preferences. If the redirect server does wish to
include the feature parameters in the Contact header field, it MUST
redirect using the original target set and original q-values, before
the application of caller preferences.
It is the usage of these modified q-values that allows the caller
preferences to be taken into account, while at the same time giving
the proxy flexibility in how it processes the request.
7.4.1 Example
Consider the following example, which is contrived but illustrative
of the various components of the matching process. There are five
registered Contacts for sip:user@example.com. They are:
Contact: sip:u1@h.example.com;audio;video;methods="INVITE,BYE";q=0.1
Contact: sip:u2@h.example.com;audio="FALSE";
methods="INVITE";msgserver;q=0.2
Contact: sip:u3@h.example.com;audio;msgserver;
methods="INVITE";video;q=0.3
Contact: sip:u4@h.example.com;audio;methods="INVITE,OPTIONS";q=0.4
Contact: sip:u5@h.example.com;q=0.5
an INVITE sent to sip:user@example.com contained the following caller
preferences header fields:
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Reject-Contact: *;msgserver;video
Accept-Contact: *;audio;require;q=0.5, *;video;explicit;q=0.4,
*;methods="BYE";class="business";q=1.0
There are no implicit preferences in this example, because explicit
preferences are provided.
The proxy first removes u5 from the target set, since it is immune
from caller preferences processing.
Next, the proxy processes the Reject-Contact header field. It is a
match for all four remaining contacts, but only an explicit match for
u3. Thats because u3 is the only one that explicitly indicated
support for video, and explicitly indicated it is a messaging server.
So, u3 gets discarded, and the others remain.
Next, each of the remaining three contacts is compared against each
of the three Accept-Contact predicates. u1 is a match to all three,
earning a score of 1.0 for the first two predicates, and 0.5 for the
third (the methods feature tag was present in the contact predicate,
but the class tag was not). u2 doesn't match the first predicate.
Because that predicate has a require tag, u2 is discarded. u4 matches
the first predicate, earning a score of 1.0. u4 does match the second
predicate, but since the match is not explicit (the score is 0.0, in
fact), the score is set to zero (it was already zero, so nothing
changes). u4 does not match the third predicate.
At this point, u1 and u4 remain. u1 matched all three Accept-Contact
predicates, so that its matching set contains all three, with scores
of 1, 1, and 0.5. u4 matches the first two predicates, with scores of
1.0 and 0.0.
Qa for u1 is then computed as:
1.0*0.5 + 1.0*0.4 + 0.5*1.0
--------------------------- = 0.56
1.0 + 1.0 + 0.5
Qa for u4 is then computed as:
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1.0*0.5 + 0.0*0.4
------------------ = 0.5
1.0 + 0.0
Qo for u1 is the average of 0.56 and the registered q-value of 0.1,
which equals 0.33. Qo for u4 is the average of 0.5 and the registered
q-value of 0.4, which equals 0.45. Rounding these to the nearest
tenth, Qo for u1 is 0.3 and Qo for u4 is 0.5.
Now, u5 is added back in. It retains its original q-value of 0.5.
Since its q-value matches that of u4, both u4 and u5 would be tried
in parallel. Should both fail, u1 would be tried.
8 Header Field Definitions
This specification defines three new header fields - Accept-Contact,
Reject-Contact, and Request-Disposition.
Tables 1 and 2 are an extension of Tables 2 and 3 in [1] for the
Accept-Contact, Reject-Contact and Request-Disposition header fields.
The column "INF" is for the INFO method [8], "PRA" is for the PRACK
method [9], "UPD" is for the UPDATE method [10], "SUB" is for the
SUBSCRIBE method [7], "NOT" is for the NOTIFY method [7], and "MSG"
is for the MESSAGE method [3].
Header field where proxy ACK BYE CAN INV OPT REG
_________________________________________________________
Accept-Contact R ar o o o o o -
Reject-Contact R ar o o o o o -
Request-Disposition R ar o o o o o o
Table 1: Accept-Contact, Reject-Contact and Request-Disposition
header fields
8.1 Request Disposition
The Request-Disposition header field specifies caller preferences for
how a server should process a request. Its value is a list of tokens,
each of which specifies a particular directive. Its syntax is
specified in Section 10. Note that a compact form, using the letter
d, has been defined. The directives are grouped into types. There can
only be one directive of each type per request (i.e., you can't have
both "proxy" and "redirect" in the same Request-Disposition header
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Header field where proxy PRA UPD SUB NOT INF MSG
_________________________________________________________
Accept-Contact R ar o o o o o o
Reject-Contact R ar o o o o o o
Request-Disposition R ar o o o o o o
Table 2: Accept-Contact, Reject-Contact, and Request-Disposition
header fields
field).
When the caller specifies a directive, the server SHOULD honor that
directive.
The following types of directives are defined:
proxy-directive: This type of directive indicates whether the
caller would like each server to proxy ("proxy") or
redirect ("redirect").
cancel-directive: This type of directive indicates whether the
caller would like each proxy server to send a CANCEL
request downstream ("cancel") in response to a 200 OK from
the downstream server (which is the normal mode of
operation, making it somewhat redundant), or whether this
function should be left to the caller ("no-cancel"). If a
proxy receives a request with this parameter set to "no-
cancel", it SHOULD NOT CANCEL any outstanding branches on
receipt of a 2xx. However, it would still send CANCEL on
any outstanding branches on receipt of a 6xx.
fork-directive: This type of directive indicates whether a proxy
should fork a request ("fork"), or proxy to only a single
address ("no-fork"). If the server is requested not to
fork, the server SHOULD proxy the request to the "best"
address (generally the one with the highest q-value). The
directive is ignored if "redirect" has been requested.
recurse-directive: This type of directive indicates whether a
proxy server receiving a 3xx response should send requests
to the addresses listed in the response ("recurse"), or
forward the list of addresses upstream towards the caller
("no-recurse"). The directive is ignored if "redirect" has
been requested.
parallel-directive: For a forking proxy server, this type of
directive indicates whether the caller would like the proxy
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server to proxy the request to all known addresses at once
("parallel"), or go through them sequentially, contacting
the next address only after it has received a non-2xx or
non-6xx final response for the previous one ("sequential").
The directive is ignored if "redirect" has been requested.
queue-directive: If the called party is temporarily unreachable,
e.g., because it is in another call, the caller can
indicate that it wants to have its call queued ("queue") or
rejected immediately ("no-queue"). If the call is queued,
the server returns "182 Queued". A queued call can be
terminated as described in [1].
Example:
Request-Disposition: proxy, recurse, parallel
The set of request disposition directives is purposefully not
extensible. This is to avoid a proliferation of new extensions to SIP
that are "tunneled" through this header field.
8.2 Accept-Contact and Reject-Contact Header Fields
The syntax for these header fields is described in Section 10. A
compact form, with the letter a, has been defined for the Accept-
Contact header field, and with the letter j for the Reject-Contact
header field.
The enc-feature-tag is an encoded version of any valid feature tag, a
number of which are applicable to SIP, and defined in Section 9. Note
that string-value uses the qdtext production from RFC 3261. This
production allows UTF-8 characters. This is in contrast to RFC 2533,
which only allows ASCII characters in quoted strings. Usage of UTF-8
here is permissible since these values are never compared except
using case sensitive matching rules.
8.3 Contact Header Field
This specification extends the Contact header field. In particular,
it allows for the Contact header field parameters to include
feature-param, whose BNF is described in Section 10. 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.
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9 Media Feature Tag Definitions
This specification defines an initial set of media feature tags for
use with this specification. New media feature tags MAY be registered
with IANA, based on the process defined for feature tag registrations
[5]. This section also serves as the IANA registration for these
feature tags.
Any registered feature tags MAY be used with this specification.
However, several existing ones appear to be particularly applicable.
These include the language feature tag [11], which can be used to
specify the language of the human or automata represented by the UA,
and the type feature tag [12], which can be used to specify the MIME
types of the media formats supported by the UA. However, the usage of
the audio, video, application, data and control feature tags (each of
which indicate a media type, as defined in RFC 2327 [13] supported by
the UA) are preferred to indicating support for specific media
formats. When the type feature tag is present, there SHOULD also be a
feature tag present for the its top-level MIME type with a value of
TRUE. In other words, if a UA indicates in a registration that it
supports the video/H263 MIME type, it should also indicate that it
supports video generally:
Contact: sip:192.0.2.1;type="video/H263";video="TRUE"
If a new SDP media type were to be defined, such as "message", a new
feature tag registration SHOULD be created for it. The name of the
feature tag MUST equal that of the media type, unless there is an
unlikely naming collision between the new media type and an existing
feature tag registration. As a result of this, implementations can
safely construct caller preferences and callee capabilities for the
new media type before it is registered, as long as there is no naming
conflict.
If a new media feature tag is registered with the intent of using
that tag with this specification, the registration is done for the
unencoded form of the tag (see Section 11). In other words, if a new
feature tag "foo" is registered, the IANA registration would be for
the tag "foo" and not "+foo". When that parameter is used within the
Contact, Accept-Contact and Reject-Contact header fields, it would be
encoded using its + form.
9.1 Attendant
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Media feature tag name: attendant
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: This feature
tag indicates that the device is an automated or human
attendant that will answer if the actual user of the device
is not available.
Values appropriate for use with this feature tag: Boolean.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Routing a call to a phone that has an
auto-attendant feature.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.2 Audio
Media feature tag name: audio
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: This feature
tag indicates that the device supports audio as a media
type.
Values appropriate for use with this feature tag: Boolean.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Routing a call to a phone that can
support audio.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.3 Application
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Media feature tag name: application
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: This feature
tag indicates that the device supports application as a
media type. This feature tag exists primarily for
completeness. Since so many MIME types are underneath
application, indicating the ability to support applications
provides little useful information. In most cases, the
concrete MIME type is a better parameter to use in a
predicate representing a preference.
Values appropriate for use with this feature tag: Boolean.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Routing a call to a phone that can
supports gaming application.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.4 Data
Media feature tag name: data
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: This feature
tag indicates that the device supports data as a media
type.
Values appropriate for use with this feature tag: Boolean.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Routing a call to a phone that can
supports a data streaming application.
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Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.5 Control
Media feature tag name: control
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: This feature
tag indicates that the device supports control as a media
type.
Values appropriate for use with this feature tag: Boolean.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Routing a call to a phone that can
supports a floor control application.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.6 Automata
Media feature tag name: automata
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: The automata
feature tag is a boolean value that indicates whether the
UA represents an automata (such as a voicemail server,
conference server, or recording device) or a human.
Values appropriate for use with this feature tag: Boolean. TRUE
indicates that the UA represents an automata.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Choosing to communicate with a message
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recording device instead of a user.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.7 Class
Media feature tag name: class
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: This feature
tag indicates the setting, business or personal, in which a
communications device is used.
Values appropriate for use with this feature tag: Token with an
equality relationship. Typical values include:
business: The device is used for business communications.
personal: The device is used for personal communications.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Choosing between a business phone and a
home phone.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.8 Duplex
Media feature tag name: duplex
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: The duplex
media feature tag lists whether a communications device can
simultaneously send and receive media ("full"), alternate
between sending and receiving ("half"), can only receive
("receive-only") or only send ("send-only").
Values appropriate for use with this feature tag: Token with an
equality relationship. Typical values include:
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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 XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.9 Mobility
Media feature tag name: mobility
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: The mobility
feature tag indicates whether the device is fixed,
wireless, or somewhere in-between.
Values appropriate for use with this feature tag: Token with an
equality relationship. Typical values include:
fixed: The device is stationary.
mobile: The device can move around with the user.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Choosing to communicate with a wireless
phone instead of a desktop phone.
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Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.10 Description
Media feature tag name: description
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: The
description feature tag provides a textual description of
the device.
Values appropriate for use with this feature tag: String with an
equality relationship.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Indicating that a device is of a
certain make and model.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.11 Event Packages
Media feature tag name: events
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: The event
packages [7] 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. Typical values include:
presence: SIP event package for for user presence [25].
winfo: SIP event package for watcher information [26].
refer: The SIP REFER event package [27].
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dialog: The SIP dialog event package [28].
conference: The SIP conference event package [29].
reg: The SIP registration event package [30].
message-summary: The SIP message summary event package
[31].
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 [31].
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.12 Priority
Media feature tag name: priority
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: The priority
feature tag indicates the call priorities the device is
willing to handle. A value of X means that the device is
willing to take requests with priority X and higher.
Values appropriate for use with this feature tag: An integer.
Each integral value corresponds to one of the possible
values of the Priority header field as specified in SIP
[1]. The mapping is defined as:
non-urgent: Integral value of 10. The device supports non-
urgent calls.
normal: Integral value of 20. The device supports normal
calls.
urgent: Integral value of 30. The device supports urgent
calls.
emergency: Integral value of 40. The device supports
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emergency calls.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Choosing to communicate with the
emergency cell phone of a user.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.13 Methods
Media feature tag name: methods
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: The methods
(note the plurality) feature tag indicates the SIP methods
supported by this UA. In this case, "supported" means that
the UA can receive requests with this method. In that
sense, it has the same connotation as the Allow header
field.
Values appropriate for use with this feature tag: Token with an
equality relationship. Values include:
INVITE: The SIP INVITE method [1].
ACK: The SIP ACK method [1].
BYE: The SIP BYE method [1].
CANCEL: The SIP CANCEL method [1].
OPTIONS: The SIP OPTIONS method [1].
REGISTER: The SIP REGISTER method [1].
INFO: The SIP INFO method [8].
UPDATE: The SIP UPDATE method [10].
SUBSCRIBE: The SIP SUBSCRIBE method [7].
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NOTIFY: The SIP NOTIFY method [7].
PRACK: The SIP PRACK method [9].
MESSAGE: The SIP MESSAGE method [3].
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Choosing to communicate with a presence
application on a PC, instead of a PC phone application.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.14 SIP Extensions
Media feature tag name: sip-extensions
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: The sip-
extensions feature tag is a list of SIP extensions (each of
which is defined by an option-tag registered with IANA)
that are understood by the UA. Understood, in this context,
means that the option tag would be included in a Supported
header field in a request.
Values appropriate for use with this feature tag: Token with an
equality relationship. Values include:
100rel: The UA supports reliability of provisional
responses [9].
path: The UA supports the SIP Path header field [14].
precondition: The UA supports the preconditions mechanism
described in RFC 3312 [15].
privacy: The UA supports the privacy extension described in
RFC 3323 [16].
sec-agree: The UA supports the security agreement extension
described in RFC 3329 [17].
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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: Choosing to communicate with a phone
that supports quality of service preconditions instead of
one that does not.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.15 Schemes
Media feature tag name: schemes
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: The set of
URI schemes [18] that are supported by a UA. Supported
implies, for example, that the UA would know how to handle
a URI of that scheme in the Contact header field of a
redirect response.
Values appropriate for use with this feature tag: Token with an
equality relationship. Typical values include:
sip: The SIP URI scheme [1].
sips: The SIPS URI scheme [1].
tel: The tel URI scheme [19].
http: The HTTP URI scheme [20].
https: The HTTPS URI scheme [32].
cid: The CID URI scheme [21].
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 get redirected to a phone
number when a called party is busy, rather than a web page.
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Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.16 Video
Media feature tag name: video
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: This feature
tag indicates that the device supports video as a media
type.
Values appropriate for use with this feature tag: Boolean.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Routing a call to a phone that can
support video.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.17 Message Server
Media feature tag name: msgserver
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: This feature
tag indicates that the device is a messaging server which
will record messages for a user. An example of such a
device is a voicemail server.
Values appropriate for use with this feature tag: Boolean.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Requesting that a call not be routed to
voicemail.
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Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.18 Is Focus
Media feature tag name: isfocus
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: This feature
tag indicates that the UA is a conference server, also
known as a focus, and will mix together the media for all
calls to the same URI [33].
Values appropriate for use with this feature tag: Boolean.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Indicating to a UA that the server it
has connected to is a conference server.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.19 URI User
Media feature tag name: uri-user
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: The uri-user
feature tag provides the user part of the SIP URI that
represents the device.
Values appropriate for use with this feature tag: String with an
equality relationship.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Requesting to route a call to a
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specific device, identified by a URI.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.20 URI Domain
Media feature tag name: uri-domain
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: The uri-
domain feature tag indicates the hostname of a device.
Values appropriate for use with this feature tag: Token with a
case-insensitive equality relationship.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Requesting to route a call to a
specific device, identified by a URI.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
10 Augmented BNF
Request-Disposition = ( "Request-Disposition" / "d" ) HCOLON
directive *(COMMA directive)
directive = proxy-directive / cancel-directive /
fork-directive / recurse-directive /
parallel-directive / queue-directive)
proxy-directive = "proxy" / "redirect"
cancel-directive = "cancel" / "no-cancel"
fork-directive = "fork" / "no-fork"
recurse-directive = "recurse" / "no-recurse"
parallel-directive = "parallel" / "sequential"
queue-directive = "queue" / "no-queue"
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Accept-Contact = ("Accept-Contact" / "a") HCOLON ac-value
*(COMMA ac-value)
Reject-Contact = ("Reject-Contact" / "j") HCOLON rc-value
*(COMMA rc-value)
ac-value = "*" *(SEMI ac-params)
rc-value = "*" *(SEMI rc-params)
ac-params = feature-param / c-p-q / req-param
/ explicit-param / generic-param
rc-params = feature-param / req-param
/ explicit-param / generic-param
feature-param = enc-feature-tag [EQUAL LDQUOT (tag-value-list
/ string-value ) RDQUOT]
enc-feature-tag = base-tags / other-tags
base-tags = "attendant" / "audio" / "automata" /
"class" / "duplex" / "data" /
"control" / "mobility" / "description" /
"events" / "priority" / "methods" /
"schemes" / "application" / "video" /
"msgserver" / "language" / "type" /
"isfocus" / "uri-user" / "uri-domain"
other-tags = "+" ftag-name
ftag-name = ALPHA *( ALPHA / DIGIT / "!" / ""' /
"." / "-" / "%" )
tag-value-list = tag-value *("," tag-value)
tag-value = ["!"] (token-nobang / boolean / numeric)
token-nobang = 1*(alphanum / "-" / "." / "%" / "*"
/ "_" / "+" / "`" / "'" / "~" )
boolean = "TRUE" / "FALSE"
numeric = "#" numeric-relation number
numeric-relation = ">=" / "<=" / "=" / (number ":")
number = [ "+" / "-" ] 1*DIGIT ["." 0*DIGIT]
string-value = "<" qdtext ">"
req-param = "require"
explicit-param = "explicit"
Note that the tag-value-list uses an actual comma instead of the
COMMA construction. Thats because it appears within a quoted string,
where line folding cannot take place.
The productions for c-p-q, name-addr, addr-spec, qdtext and generic-
param can be found in RFC 3261 [1].
Despite the BNF, there MUST NOT be more than one c-p-q, req-param or
explicit-param in an ac-params or rc-params. Furthermore, there can
only be one instance of any feature tag in feature-param.
Any numbers present in a feature parameter MUST be representable
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using an ANSI C double.
The following production updates the one in RFC 3261 for contact-
params:
contact-params = c-p-q / c-p-expires / feature-param
/ contact-extension
11 Mapping Feature Parameters and Feature Set Predicates
Mapping between feature parameters and a feature set predicate,
formatted according to the syntax of RFC 2533 [2] is trivial.
Starting from a set of feature-param, the procedure is as follows.
Construct a conjunction. Each term in the conjunction derives from
one feature-param. If the feature-param has no value, it is
equivalent, in terms of the processing which follows, as if it had a
value of "TRUE".
If the feature-param value is a tag-value-list, the element of the
conjunction is a disjunction. There is one term in the disjunction
for each tag-value in the tag-value-list.
Consider now the construction of a filter from a tag-value. If the
tag-value starts with a bang (!), the filter is of the form:
(! <filter from remainder>)
where "filter from remainder" refers to the filter that would be
constructed from the tag-value if the bang had not been present.
If the tag-value starts with an octothorpe (#), the filter is a
numeric comparison. The comparator is either =, >=, <= or a range
based on the next characters in the phrase. If the next characters
are =. >= or <=, the filter is of the form:
(name comparator compare-value)
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where name is the name of the feature parameter after it has been
decoded (see below), and comparator is either =, >= or <= depending
of the initial characters in the phrase. If the remainder of the text
in the tag-value after the equal contains a decimal point (implying a
rational number), the decimal point is shifted right N times until it
is an integer, I. Compare-value above is then set to "I / 10**N",
where 10**N is the result of computing the number 10 to the Nth
power.
RFC 2533 uses a fractional notation to describe rational
numbers. This specification use a decimal form. The above
text merely converts between the two representations.
Practically speaking, this conversion is not needed since
the numbers are the same in either case. However, it is
described in case implementations wish to directly plug the
predicates generated by the rules in this section into an
RFC 2533 implementation.
If the value after the octothorpe is a number, the filter is a range.
The format of the filter is:
(name=[remainder])
where name is the feature-tag after it has been decoded (see below),
and remainder is the remainder of the text in the tag-value after the
#, with any decimal numbers converted to a rational form, and the
colon replaced by a double dot (..).
If the tag-value does not begin with an octothorpe (it is a token-
nobang or boolean), the filter is of the form:
(name=tag-value)
where name is the feature-tag after it has been decoded (see below).
If the feature-param contains a string-value (based on the fact that
it begins with a left angle bracket ("<") and ends with a right angle
bracket (">")), the filter is of the form:
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(name="qdtext")
Note the explicit usage of quotes around the qdtext, which indicate
that the value is a string. In RFC 2533, strings are compared using
case sensitive rules, and tokens, case insensitive.
In RFC 2533, when an feature tag value is unquoted, its a
token, and when quoted, its a string. The comparison rules
are case insensitive for the former, and sensitive for the
latter. The presence of quotes, or lack thereof, is the
means by which an implementation can tell whether to apply
sensitive or insensitive comparison rules. In the syntax
described here, we cannot use quoted strings, since there
is already a quoted string around each contact parameter
value. So, we use an angle bracket to signify that the
value is to be interpreted as a case sensitive string. If
no brackets are present, the proxy would perform matching
operations in a case insensitive manner, and if they are
present, case sensitive.
Feature tags, as specified in RFC 2506, cannot be directly
represented as header field parameters in the Contact, Accept-Contact
and Reject-Contact header fields. This is due to an inconsistency in
the grammars, and in the need to differentiate feature parameters
from parameters used by other extensions. As such, feature tag values
are encoded from RFC2506 format to yield an enc-feature-tag, and then
are decoded into RFC 2506 format. The decoding process is simple. If
there is a leading plus (+) sign, it is removed. Any exclamation
point (!) is converted to a colon (:) and any single quote (') is
converted to a forward slash (/). The encoding process is similarly
performed. Any forward slashes in the feature tag are converted to a
single quote, and any colons are converted to an exclamation point.
If the feature tag name is not amongst the base tags specified in
Section 10, a plus sign is added to the front of the feature tag to
create the encoded feature tag. The plus sign MUST NOT be added if
the feature tag name is amongst the base tags.
As an example, the Accept-Contact header:
Accept-Contact:*;mobility="fixed";events="!presence,winfo";language="en,de"
;description="<PC>";+newparam;+rangeparam="#-4:+5.125"
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would be converted to the following feature predicate:
(& (mobility=fixed)
(| (! (events=presence)) (events=winfo))
(| (language=en) (language=de))
(description="PC")
(newparam=TRUE)
(rangeparam=-4..5125/1000))
The conversion of an RFC 2533 formatted feature set to a set of
feature parameters proceeds in the same way, but in reverse. The
conversion can only be done for feature sets constrained as described
in Section 6.1. The feature tag has to be encoded into a feature
parameter using the process described above.
12 Security Considerations
The presence of caller preferences in a request has an effect on the
ways in which the request is handled at a server. As a result, it is
especially important that requests with caller preferences be
integrity-protected. The same holds true for registrations with
feature parameters in the Contact header field. User agents who are
concerned with protecting the integrity of their requests SHOULD use
the SIPS URI scheme.
Processing of caller preferences requires set operations and searches
which can require some amount of computation. This enables a DOS
attack whereby a user can send requests with substantial numbers of
caller preferences, in the hopes of overloading the server. To
counter this, servers SHOULD reject requests with too many rules. A
reasonable number is around 20.
Feature sets contained in REGISTER requests can reveal sensitive
information about a user or UA (for example, the languages spoken).
If this information is sensitive, confidentiality SHOULD be provided
by using the SIPS URI scheme, as described in RFC 3261 [1].
13 IANA Considerations
There are a number of IANA considerations associated with this
specification.
13.1 Media Feature Tags
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This specification registers a number of new Media feature tags
according to the procedures of RFC 2506 [5]. Those registrations are
contained in Section 9, and are meant to be placed into the IETF tree
for media feature tags.
13.2 SIP Header Fields
This specification registers three new SIP header fields, according
to the process of RFC 3261 [1].
The following is the registration for the Accept-Contact header
field:
RFC Number: RFC XXXX [Note to IANA: Fill in with the RFC number
of this specification.]
Header Field Name: Accept-Contact
Compact Form: a
The following is the registration for the Reject-Contact header
field:
RFC Number: RFC XXXX [Note to IANA: Fill in with the RFC number
of this specification.]
Header Field Name: Reject-Contact
Compact Form: j
The following is the registration for the Request-Disposition header
field:
RFC Number: RFC XXXX [Note to IANA: Fill in with the RFC number
of this specification.]
Header Field Name: Request-Disposition
Compact Form: d
13.3 SIP Option Tags
This specification registers a single SIP option tag, pref. The
required information for this registration, as specified in RFC 3261,
is:
Name: pref
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Description: This option tag is used in a Proxy-Require header
field by a UAC to ensure that caller preferences are
honored at each proxy along the path. However, this usage
is discouraged. It can also be used in the Require header
field of a registration to ensure that the registrar
supports the caller preferences extensions.
14 Acknowledgments
The initial set of media feature tags used by this specification were
influenced by Scott Petrack's CMA design. Jonathan Lennox, Rohan
Mahy and John Hearty provided helpful comments. Graham Klyne provided
assistance on the usage of RFC 2533.
15 Author's Addresses
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
email: jdrosen@dynamicsoft.com
Henning Schulzrinne
Columbia University
M/S 0401
1214 Amsterdam Ave.
New York, NY 10027-7003
email: schulzrinne@cs.columbia.edu
Paul Kyzivat
Cisco Systems
Mail Stop LWL3/12/2
900 Chelmsford St.
Lowell, MA 01851
email: pkzivat@cisco.com
16 Normative References
[1] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. R. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
initiation protocol," RFC 3261, Internet Engineering Task Force, June
2002.
J. Rosenberg et. al. [Page 52]
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[2] G. Klyne, "A syntax for describing media feature sets," RFC 2533,
Internet Engineering Task Force, Mar. 1999.
[3] "Session initiation protocol (SIP) extension for instant
messaging," RFC 3428, Internet Engineering Task Force, Dec. 2002.
[4] S. Bradner, "Key words for use in rfcs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
[5] K. Holtman, A. Mutz, and T. Hardie, "Media feature tag
registration procedure," RFC 2506, Internet Engineering Task Force,
Mar. 1999.
[6] G. Klyne, "Corrections to "A syntax for describing media feature
sets"," RFC 2738, Internet Engineering Task Force, Dec. 1999.
[7] A. B. Roach, "Session initiation protocol (sip)-specific event
notification," RFC 3265, Internet Engineering Task Force, June 2002.
[8] S. Donovan, "The SIP INFO method," RFC 2976, Internet Engineering
Task Force, Oct. 2000.
[9] J. Rosenberg and H. Schulzrinne, "Reliability of provisional
responses in session initiation protocol (SIP)," RFC 3262, Internet
Engineering Task Force, June 2002.
[10] J. Rosenberg, "The session initiation protocol (SIP) UPDATE
method," RFC 3311, Internet Engineering Task Force, Oct. 2002.
[11] P. Hoffman, "Registration of charset and languages media
features tags," RFC 2987, Internet Engineering Task Force, Nov. 2000.
[12] G. Klyne, "MIME content types in media feature expressions," RFC
2913, Internet Engineering Task Force, Sept. 2000.
[13] M. Handley and V. Jacobson, "SDP: session description protocol,"
RFC 2327, Internet Engineering Task Force, Apr. 1998.
[14] D. Willis and B. Hoeneisen, "Session initiation protocol (SIP)
extension header field for registering non-adjacent contacts," RFC
3327, Internet Engineering Task Force, Dec. 2002.
[15] "Integration of resource management and session initiation
protocol (SIP)," RFC 3312, Internet Engineering Task Force, Oct.
2002.
[16] J. Peterson, "A privacy mechanism for the session initiation
protocol (SIP)," RFC 3323, Internet Engineering Task Force, Nov.
J. Rosenberg et. al. [Page 53]
Internet Draft SIP Caller Preferences March 2, 2003
2002.
[17] J. Arkko, V. Torvinen, G. Camarillo, A. Niemi, and T. Haukka,
"Security mechanism agreement for the session initiation protocol
(SIP)," RFC 3329, Internet Engineering Task Force, Jan. 2003.
[18] T. Berners-Lee, R. Fielding, and L. Masinter, "Uniform resource
identifiers (URI): generic syntax," RFC 2396, Internet Engineering
Task Force, Aug. 1998.
[19] A. Vaha-Sipila, "Urls for telephone calls," RFC 2806, Internet
Engineering Task Force, Apr. 2000.
[20] R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, L. Masinter, P.
J. Leach, and T. Berners-Lee, "Hypertext transfer protocol --
HTTP/1.1," RFC 2616, Internet Engineering Task Force, June 1999.
[21] E. Levinson, "Content-id and message-id uniform resource
locators," RFC 2392, Internet Engineering Task Force, Aug. 1998.
17 Informative References
[22] J. Lennox and H. Schulzrinne, "Call processing language
framework and requirements," RFC 2824, Internet Engineering Task
Force, May 2000.
[23] G. Klyne, "Protocol-independent content negotiation framework,"
RFC 2703, Internet Engineering Task Force, Sept. 1999.
[24] J. Rosenberg and H. Schulzrinne, "Guidelines for authors of
extensions to the session initiation protocol (SIP)," internet draft,
Internet Engineering Task Force, Nov. 2002. Work in progress.
[25] J. Rosenberg, "A presence event package for the session
initiation protocol (SIP)," internet draft, Internet Engineering Task
Force, Jan. 2003. Work in progress.
[26] J. Rosenberg, "A watcher information event template-package for
the session initiation protocol (SIP)," internet draft, Internet
Engineering Task Force, Jan. 2003. Work in progress.
[27] R. Sparks, "The SIP refer method," internet draft, Internet
Engineering Task Force, Dec. 2002. Work in progress.
[28] J. Rosenberg and H. Schulzrinne, "A session initiation protocol
(SIP) event package for dialog state," internet draft, Internet
Engineering Task Force, June 2002. Work in progress.
J. Rosenberg et. al. [Page 54]
Internet Draft SIP Caller Preferences March 2, 2003
[29] J. Rosenberg and H. Schulzrinne, "A session initiation protocol
(SIP) event package for conference state," internet draft, Internet
Engineering Task Force, June 2002. Work in progress.
[30] J. Rosenberg, "A session initiation protocol (SIP) event package
for registrations," internet draft, Internet Engineering Task Force,
Oct. 2002. Work in progress.
[31] R. Mahy, "A message summary and message waiting indication event
package for the session initiation protocol (SIP)," internet draft,
Internet Engineering Task Force, Nov. 2002. Work in progress.
[32] E. Rescorla, "HTTP over TLS," RFC 2818, Internet Engineering
Task Force, May 2000.
[33] J. Rosenberg, "A framework for conferencing with the session
initiation protocol," internet draft, Internet Engineering Task
Force, Feb. 2003. Work in progress.
[34] M. Smith and T. Howes, "LDAP: string representation of search
filters," internet draft, Internet Engineering Task Force, Aug. 2002.
Work in progress.
A Overview of RFC 2533
This section provides a brief overview of RFC 2533 and related
specifications that form the content negotiation framework.
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
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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 [34]. It uses a prolog-style
syntax which is fairly self-explanatory. This representation is
called a feature set predicate. The base unit of the predicate is a
filter, which is a boolean expression encased in round brackets. A
filter can be complex, where it contains conjunctions and
disjunctions of other filters, or it can be simple. A simple filter
is one that expresses a comparison operation on a single media
feature tag.
For example, consider the feature set predicate:
(& (foo=A)
(bar=B)
(| (baz=C) (& (baz=D) (bif=E))))
This defines a function over four media features - foo, bar, baz and
bif. Any point in feature space with foo equal to A, bar equal to B,
and either baz equal to C, or baz equal to D and bif equal to E, is
in the feature set defined by this feature set predicate.
Note that the predicate doesn't say anything about the number of
dimensions in feature space. The predicate operates on a feature
space of any number of dimensions, but only those dimensions labeled
foo, bar, baz and bif matter. The result is that values of other
media features don't matter. The feature collection
foo=A,bar=B,baz=C,bop=F is in the feature set described by the
predicate, even though the media feature tag "bop" isn't mentioned.
Feature set predicates are therefore inclusive by default. A feature
collection is present unless the boolean predicate rules it out. This
was a conscious design choice in RFC 2533.
RFC 2533 also talks about matching a preference with a capability
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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 the caller preferences specification
are constrained. They are always in conjunctive normal form, with
each term in the conjunction describing values for different media
features. This makes computation of a match easy. It is computed
independently for each media feature, and then the feature sets
overlap if media features specified in both sets overlap. Computing
the overlap of a single media feature is very straightforward, and is
a simple matter of computing whether two finite sets overlap.
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The IETF invites any interested party to bring to its attention any
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rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
J. Rosenberg et. al. [Page 57]
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Director.
Full Copyright Statement
Copyright (c) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
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J. Rosenberg et. al. [Page 58]
