Internet Engineering Task Force SIP WG
Internet Draft J. Rosenberg
dynamicsoft
H. Schulzrinne
Columbia U.
draft-ietf-sip-callerprefs-07.txt
November 4, 2002
Expires: May 2003
Session Initiation Protocol (SIP) Caller Preferences and Callee Capabilities
STATUS OF THIS MEMO
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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 URIs a request gets routed to, and to specify certain
request handling directives in proxies and redirect servers. It does
so by defining four new request headers, Accept-Contact, Reject-
Contact, Require-Contact and Request-Disposition, which specify the
caller's preferences. The extension also defines new parameters for
the Contact header that describe the capabilities and characteristics
of a UA.
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Table of Contents
1 Introduction ........................................ 3
2 Terminology ......................................... 4
3 Definitions ......................................... 4
4 Overview of Operation ............................... 6
5 Usage of the Content Negotiation Framework .......... 7
6 UA Behavior ......................................... 8
6.1 Expressing Capabilities in a Registration ........... 8
6.2 Expressing Preferences in a Request ................. 10
6.2.1 Request Handling Preferences ........................ 11
6.2.2 Feature Set Preferences ............................. 11
6.3 Indicating Feature Sets in Remote Target URIs ....... 12
6.4 Request Handling and Feature Set Preferences ........ 13
7 Proxy Behavior ...................................... 14
7.1 Request-Disposition Processing ...................... 14
7.2 Preference and Capability Matching .................. 14
7.2.1 Extracting Explicit Preferences ..................... 14
7.2.2 Extracting Implicit Preferences ..................... 15
7.2.2.1 Priority ............................................ 15
7.2.2.2 Methods ............................................. 16
7.2.2.3 Event Packages ...................................... 16
7.2.2.4 Media Types ......................................... 17
7.2.2.5 Languages ........................................... 17
7.3 Constructing Contact Predicates ..................... 18
7.4 Matching ............................................ 19
8 Header Field Definitions ............................ 21
8.1 Request Disposition ................................. 21
8.2 Accept-Contact, Reject-Contact, and Require-
Contact Header Fields .......................................... 23
8.3 Contact Header Field ................................ 23
9 Media Feature Tag Definitions ....................... 24
9.1 Attendant ........................................... 24
9.2 Audio ............................................... 25
9.3 Automata ............................................ 25
9.4 Class ............................................... 26
9.5 Duplex .............................................. 26
9.6 Image ............................................... 27
9.7 Message ............................................. 28
9.8 Mobility ............................................ 28
9.9 Description ......................................... 29
9.10 Event Packages ...................................... 29
9.11 Priority ............................................ 30
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9.12 Methods ............................................. 31
9.13 Schemes ............................................. 32
9.14 Text ................................................ 33
9.15 Video ............................................... 33
9.16 Voicemail ........................................... 34
10 Augmented BNF ....................................... 34
11 Mapping Feature Parameters and Feature Set
Predicates ..................................................... 35
12 Security Considerations ............................. 38
13 IANA Considerations ................................. 39
13.1 Media Feature Tags .................................. 39
13.2 SIP Header Fields ................................... 39
13.3 SIP Option Tags ..................................... 40
14 Acknowledgements .................................... 40
15 Author's Addresses .................................. 40
16 Normative References ................................ 41
17 Informative References .............................. 42
A Overview of RFC 2533 ................................ 43
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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) [16]. 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 requestor 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 provides
specific request handling directives for a server. The other, called
feature preferences, are present in the Accept-Contact, Reject-
Contact, and Require-Contact header fields. They allow the caller to
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provide a feature set [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 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
[17].
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 [3] 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 [18], 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 [18], a feature tag is a
name that identifies a feature.
Media Feature: As defined in RFC 2703, [18], a media feature is
information that indicates facilities assumed to be
available for the message content to be properly rendered
or otherwise presented. Media features are not intended to
include information that affects message transmission.
In the context of this specification, a media feature is
information that indicates facilities for handling SIP
requests, rather than specifically for content. In that
sense, it is used synonymously with feature.
Feature Collection: As defined in RFC 2533 [2], a feature
collection is a collection of different media features and
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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 [18], a feature set is
Information about a sender, recipient, data file 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.
These preferences are carried in the Accept-Contact,
Reject-Contact and Require-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, Reject-Contact
and Require-Contact header fields. The feature parameters
represent an encoding of a feature set. There is a one-one
mapping between a set of feature parameters and a feature
set predicate, as both represent alternative encodings of a
feature set.
Capability: As defined in RFC 2703 [18], a capability is an
attribute of a sender or receiver (often the receiver)
which indicates an ability to generate or process a
particular type of message content.
Filter: A single expression in a feature 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.
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
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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 a
feature tag, as defined in RFC 2703 [18], 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 each 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
feature set to its peer. For example, by including the "voicemail"
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, Reject-Contact, and Require-Contact
header fields. These header fields also contain feature sets,
represented by the same feature parameters that are used in the
Contact header. 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-
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Contact header field contains feature sets which, if matched by a UA,
imply that the request should not be routed to that UA. The Require-
Contact header field contains feature sets which, if not matched by a
UA, imply that the request should not be routed to that UA. Require-
Contact and Accept-Contact are similar, but Require-Contact is more
forceful. Contacts which don't match are outright rejected, whereas
with Accept-Contact, they are tried as fallbacks.
Proxies use the information in the Accept-Contact, Reject-Contact and
Require-Contact header fields to select amongst registered contacts.
Proxies also compute 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
the content negotiation work carried out within the IETF, and
documented in several RFCs. The ones relevant to this specification
are RFC 2506 [4] 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 [5], which provides a
minor update to RFC 2533, and RFC 2703 [18] 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
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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 messaging. 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
at hand.
6 UA Behavior
UA behavior covers four separate cases. The first is registration,
where a UA can declare its capabilities. The second is expression of
preferences, 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.
6.1 Expressing Capabilities in a Registration
When a UA registers, it MAY construct a feature predicate for each
Contact URI it registers. In the text that follows, this process is
described in terms of RFC 2533 [2] (and its minor update, [5]) syntax
and constructs, followed by a conversion to the syntax used in this
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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.
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
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 [4]; this document registers several that are
appropriate for SIP. It is also permissible to use the URI tree [4]
for expressing vendor-specific feature tags. Feature tags in any
other trees created through IANA MAY also be used.
A UA MAY include the "schemes" feature tag in its feature parameters.
However, this tag MUST include a value that matches the scheme of the
URI being registered. For example, if a SIP URI is being registered,
the schemes parameter can include a SIP and TEL URI [6]. If this
feature tag is omitted, the proxy will assume an implicit value for
it, equal to the scheme of the registered URI.
It is RECOMMENDED that a UA provide complete information in its
feature parameters. 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. This includes
features that are supported, and those that are not. For example, if
a UA does not support video, it SHOULD include a 'video="FALSE"'
parameter in its registered Contact. 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 ommission of a value from a list means that
the value is not supported.
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.
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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)
(voicemail=TRUE)
(mobility=fixed)
(| (methods=INVITE) (methods=BYE) (methods=OPTIONS) (methods=ACK)
(methods=CANCEL)))
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"
;voicemail="TRUE";mobility="fixed"
;methods="INVITE,BYE,OPTIONS,ACK,CANCEL"
Content-Length: 0
6.2 Expressing Preferences in a Request
A caller wishing to express preferences for a request includes
Accept-Contact, Reject-Contact, Require-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, Require-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.
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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 or capability is executed. As
such, inclusion 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, Reject-
Contact, and Require-Contact header field values. Each header field
value is either a URI or the wildcard "*", along with 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. That is,
when represented by a feature set predicate, each predicate MUST be a
conjunction of terms. 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 or range.
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.
Note that the UAC can express explicit preferences for the methods,
event packages and priorities supported by a UA. As described in
Section 7.2.2, a proxy will compute implicit preferences from the
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request if explicit ones are not provided.
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 URI
or a "*" and is parameterized by a set of feature parameters. Any UA
whose capabilities match the feature set described by the feature
parameters, and whose URI matches the URI in the value (if
specified), matches the value. A value of "*" indicates a wildcard
operation on the URI, so that any URI matches. 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.
The Require-Contact header field allows the UAC to specify that a UA
should not be contacted if it doesn't match all of the values of the
header field. Each value of the Require-Contact header field contains
a URI or a "*" and is parameterized by set of feature parameters. Any
UA whose capabilities match the feature set described by the feature
parameters, and whose URI matches the URI in the value (if
specified), matches the value. A value of "*" indicates a wildcard
operation on the URI, so that any URI matches. 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.
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. If a UA matches none of the values, it should be
contacted as a last resort. Each value of the Accept-Contact header
field contains a URI or a "*" and is parameterized by a set of
feature parameters. Any UA whose capabilities match the feature set
described by the feature parameters, and whose URI matches the URI in
the value (if specified), matches the value. A value of "*" indicates
a wildcard operation on the URI, so that any URI matches. The q-value
provides a weighting operation, allowing the UAC to request
preferential routing to UAs that match that value above other values.
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
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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 request, 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
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 Request Handling and Feature Set Preferences
When a UAS compliant to this specification receives a request whose
request-URI correspods 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.
It also adds the uri-user and uri-domain terms to the conjunction as
described in Section 7.2.1.
Having a UAS perform the matching operations as if it were
a proxy has many benefits. First, it allows caller
preferences to be honored even if the proxy doesn't support
the extension. Secondly, and perhaps more importantly,
feature set processing of preferences for the URI will only
occur at a UA, not at a proxy. Thats because the UA is the
only one that adds the uri-user and uri-domain terms to the
feature set describing a request target.
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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,
Reject-Contact, and Require-Contact header fields.
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 wth 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.
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
The first step in proxy processing is to extract explicit
preferences. To do that, it looks for the Accept-Contact, Reject-
Contact and Require-Contact header fields.
For each value of those header fields, it SHOULD convert all
parameters except for the q-value to the syntax of RFC 2533, based on
the rules in Section 11. If a value of the header field was not a
"*", it SHOULD take the URI in that value, and add two terms to the
top level conjunction:
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(uri-user=<user part of URI>)
and
(uri-domain=<host portion of URI>)
If the user part of the SIP URI is absent, the uri-user term is not
added, only the uri-domain one. No URI parameters are used. Note that
these are not "real" feature tags; they are not registered with IANA
and cannot appear anywhere in actual form. They are merely added in
order to perform the matching operation.
The result will be a set of feature set predicates in conjunctive
normal form, each of which is associated with one of the three
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.
7.2.2 Extracting Implicit Preferences
The proxy then applies any "implicit" preferences. These preferences
are ones not explicitly stated in the three header fields, but
implied by the presence of other header fields in the request.
7.2.2.1 Priority
The Priority header field is an indication of a caller preference - a
desire to be routed to a UA that can handle requests of the desired
priority. If the request contained a Priority header field, the proxy
looks for feature tags with the value "priority" in all feature set
predicates. If that feature tag is not used in any of the predicates,
the proxy creates a new feature set predicate, and associates it with
the Accept-Contact header field (note that there is no modification
of the message implied - only an association for the purposes of
processing). The new predicate looks like:
(& (priority>=[numeric value of the Priority header field]))
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The numeric value of the Priority header field is obtained through
the procedures described in Section 9.11. For example, if the request
had a Priority header field with a value of urgent, the proxy would
create the following predicate:
(& (priority >= 3))
7.2.2.2 Methods
Another 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 looks for feature tags with the value
"methods" in all feature set predicates. If that feature tag is not
used in any of the predicates, the proxy examines the predicates
associated with the Require-Contact header field. If there are no
predicates associated with that header field, the proxy creates a new
empty feature set predicate, and associates it with the Require-
Contact header field (note that there is no modification of the
message implied - only an association for the purposes of
processing). In this case, an empty predicate is one with a
conjunction, but no terms in that conjunction yet.
For all predicates associated with the Require-Contact header field
(including the one which may have just been created), the proxy
SHOULD add a term to the conjunction of the following form:
(methods=[method of request])
7.2.2.3 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 implement that implicit preference, the
proxy looks for feature tags with the value "events" in all feature
set predicates. If that feature tag is not used in any of the
predicates, the proxy examines the predicates associated with the
Require-Contact header field. If there are no predicates associated
with that header field, the proxy creates a new empty feature set
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predicate, and associates it with the Require-Contact header field
(note that there is no modification of the message implied - only an
association for the purposes of processing). In this case, an empty
predicate is one with a conjunction, but no terms yet.
For all predicates associated with the Require-Contact header field
(including the one which may have just been created), the proxy
SHOULD add a term of the following form:
(events=[value of the Event header field])
7.2.2.4 Media Types
Another implicit preference is for the sessions that are to be
established. If a UA generates an INVITE request with a session
description that includes video, this is an implicit preference to be
connected to a UA that supports video. To implement this implicit
preference, the proxy looks for feature tags with the values "audio",
"video", "application", "message", "text" or "image" in all feature
set predicates. If none of those feature tags are used in any of the
predicates, the proxy MAY create a new feature set predicate, and
associate it with the Accept-Contact header field (note that there is
no modification of the message implied - only an association for the
purposes of processing). This predicate has a term for each top-level
media type listed in the session description, with a value of TRUE.
For example, if the request is an INVITE request, with a Session
Description Protocol (SDP) [19] body, where the SDP contains an audio
and a video media description, the proxy would construct the
following predicate:
(& (audio=TRUE)
(video=TRUE))
This implicit preference is added with MAY strength, and not SHOULD,
since it requires the proxy to examine the body of the request. This
can have performance implications, and won't always be possible. For
example, if the body is encrypted, the proxy cannot examine it.
7.2.2.5 Languages
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The languages understood by the caller is another form of implicit
preference. The Accept-Language header field contains a list of the
languages that content should be returned in. It is reasonable to
imply that the caller would like the call to be routed to a user that
speaks those languages as well. To implement that implicit
preference, the proxy looks for feature tags with the value
"language" in all feature set predicates. If that feature tag is not
used in any of the predicates, the proxy creates a new feature set
predicate for each value in the Accept-Language header field, and
associates it with the Accept-Contact header field (note that there
is no modification of the message implied - only an association for
the purposes of processing). Each predicate is of the following form:
(& (language=[value of the Accept-Language header field]))
Furthermore, if an Accept-Language header field value had a q-value
associated with it, that q-value is associated with the corresponding
feature set predicate.
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 target URI was obtained through a registration, the
proxy computes the contact predicate by taking all Contact URI
parameters except for the q and expires parameters, and converting
them to RFC 2533 syntax using the rules of Section 8.1.
If the contact predicate doesn't already contain a "schemes" feature
tag, the proxy SHOULD add a term containing one, whose value is equal
to the scheme of the URI.
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, if a REGISTER request had the following Contact URI:
Contact: sip:1.2.3.4;mobility="fixed";q=0.8
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The proxy would compute the following contact predicate, associating
it with a q-value of 0.8:
(& (mobility=fixed)
(schemes=sip))
7.4 Matching
It is important to note that the proxy does not have to know anything
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 to know whether the feature tag is a
numeric, token, quoted string, etc.
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 (and of course, its contact predicate is discarded as well).
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 Require-
Contact header field.
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For each contact predicate that remains, each Require-Contact
predicate is examined. The Require-Contact predicate is matched to
the contact predicate using the matching operation of RFC 2533 [2].
If the result is not a match, the URI corresponding to that contact
predicate is discarded from the target set, as is the contact
predicate itself.
For each contact predicate that remains, each Accept-Contact
predicate is examined. The Accept-Contact predicate is matched to the
contact predicate using the matching operation of RFC 2533 [2]. If
the result is a match, the URI associated with the contact predicate
is considered a candidate URI. The set of Accept-Contact predicates
which matched the contact predicate is called its matching set.
The q-value of URIs from the target set are then modified for this
transaction only, in order to incorporate the caller's preferences.
If the URI in the target set is not a candidate URI, its q-value is
set to zero. If the URI is a candidate URI, its q-value is combined
with those from the matching set. This document does not prescribe a
specific algorithm for combining q-values. Among many possibilities,
a server MAY set the q-value to the average of the original value
specified in the registration, and the average q-value amongst the
predicates in the matching set. This gives equal weight to caller and
callee preferences. The only requirement for the combining process is
that if a target URI has a q-value of q1, and the q values amongst
the predicates in the matching set are q2,q3,..qn, the combined q
value, qm, must satisfy:
MIN(q1,q2,q3,..qn) <= qm <= MAX(q1,q2,q3,..,qn)
Note that this preference computation only determines the
ordering of request attempts, so that the properties of the
preference computation are of secondary importance. The q-
value ordering provides only limited flexibility to
indicate, for example, that a particular parameter is more
important than another one or that combinations of two
parameters should be weighed heavily.
If the server proxies, the target set is then sorted according to the
updated q-value. Processing from this point depends on the
configuration and policy of the server. If the server elects to do a
sequential proxy, it SHOULD try the highest q-value contact entry
first, trying addresses with decreasing q-values as each attempt
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fails. If the server elects to do a parallel proxy, it SHOULD group
contact entries with "close" q-values together, and try the group
with the highest q-value first, then the group with the next lowest
q-values, and so on. The precise method of the grouping is left to
the implementor. A reasonable choice is to round each q-value to the
nearest tenth, and group those with the same rounded value.
If a proxy server is recursing, it SHOULD apply the caller
preferences to the Contact header fields returned in the redirect
responses. Any target URI remaining after the application of caller
preferences SHOULD be added to the list of untried addresses. This
list is then resorted based on q values. The server uses this list
for subsequent proxy operations.
If the server is redirecting, it SHOULD return all entries in the
target set, including a q-value for each as obtained through the
combining process. This SHOULD include any URI with a zero q-value.
If the server is executing any other type of policy, as a general
guideline, it SHOULD prefer target URI with higher q values than
those with lower q values.
8 Header Field Definitions
This specification defines four new header fields - Accept-Contact,
Reject-Contact, Require-Contact and Request-Disposition.
Table 1 is an extension of Tables 2 and 3 in [1] for the Accept-
Contact, Reject-Contact, Require-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], and "NOT" is for the NOTIFY method
[7].
Header field where proxy ACK BYE CAN INV OPT REG PRA UPD SUB NOT INF
_____________________________________________________________________________
Accept-Contact R r o o o o o - o o o o o
Reject-Contact R r o o o o o - o o o o o
Require-Contact R r o o o o o - o o o o o
Request-Disposition R r o o o o o o o o o o o
Table 1: Accept-Contact, Reject-Contact, Require-Contact and
Request-Disposition header fields
8.1 Request Disposition
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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. There can only be one value of a directive per
header field (i.e., you can't have both "proxy" and "redirect" in the
same Request-Disposition header field).
When the caller specifies a directive, the server SHOULD treat it as
a hint, not as a requirement and MAY ignore the directive.
The directives have the following semantics:
proxy-directive: This directive indicates whether the caller
would like each server to proxy or redirect. If the server
is incapable of performing the requested directive, it
SHOULD ignore it.
cancel-directive: This directive indicates whether the caller
would like each proxy server to send a CANCEL request
downstream 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. 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 directive indicates whether a proxy should
fork a request, or proxy to only a single address. 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 directive indicates whether a proxy
server receiving a 3xx response should send requests to the
addresses listed in the response (i.e., recurse), or
forward the list of addresses upstream towards the caller.
The directive is ignored if "redirect" has been requested.
parallel-directive: For a forking proxy server, this directive
indicates whether the caller would like the proxy server to
proxy the request to all known addresses at once, 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. The directive is ignored if
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"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 rather than
rejected immediately. 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 "tunnelled" through this header field.
8.2 Accept-Contact, Reject-Contact, and Require-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 feature-tag is 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 parameter to include tag-set,
whose BNF is described in Section 10. Tag-set is a set of feature
parameters that describes the feature set of the UA associated with
the URI in the Contact header field.
It is important to note that there is no way to differentiate, by
syntax, Contact parameters that are part of tag-set or just other
extensions. It turns out that this does not matter. If a proxy should
mistakenly take a contact parameter used by another extension, and
assume it is a feature parameter when its not, it will be ignored by
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the matching algorithm unless the same parameter appears in the
Accept-Contact or Reject-Contact header fields. However, it won't
ever appear in these header fields, since those header fields only
ever contain feature parameters, and the parameter is not actually a
feature parameter.
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
[4]. 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, message, text and image feature tags
(each of which indicate a top level media type 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:1.2.3.4;type="video/H263";video="TRUE"
9.1 Attendant
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.
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The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Routing a call to a phone that 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 MIME
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 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
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indicates that the UA represents an automata.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Choosing to communicate with a message
recording device instead of a user.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.4 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.]]
9.5 Duplex
Media feature tag name: duplex
ASN.1 Identifier: New assignment by IANA.
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Summary of the media feature indicated by this tag: The duplex
media feature tag lists whether a communications device can
simultaneously send and receive media ("full"), alternate
between sending and receiving ("half"), can only receive
("receive-only") or only send ("send-only").
Values appropriate for use with this feature tag: Token with an
equality relationship. Typical values include:
full: The device can simultaneously send and receive media.
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.]]
9.6 Image
Media feature tag name: image
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: This feature
tag indicates that the device supports image as a MIME
media type.
Values appropriate for use with this feature tag: Boolean.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
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Examples of typical use: Routing a call to a phone that can
support image transfer.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.7 Message
Media feature tag name: message
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: This feature
tag indicates that the device supports message as a MIME
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 messaging.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.8 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 wired.
mobile: The device is wireless.
The feature tag is intended primarily for use in the following
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applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Choosing to communicate with a wireless
phone instead of a desktop phone.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.9 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.]]
9.10 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
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equality relationship. Typical values include:
presence: SIP event package for for user presence [20].
winfo: SIP event package for watcher information [21].
refer: The SIP REFER event package [22].
dialog: The SIP dialog event package [23].
conference: The SIP conference event package [24].
reg: The SIP registration event package [25].
message-summary: The SIP message summary event package
[26].
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 [26].
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.11 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.
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 1. The device supports non-
urgent calls.
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normal: Integral value of 2. The device supports normal
calls.
urgent: Integral value of 3. The device supports urgent
calls.
emergency: Integral value of 4. The device supports
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 a the
emergency cell phone of a user, instead of their regular
phone.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.12 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. Typical 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].
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REGISTER: The SIP REGISTER method [1].
INFO: The SIP INFO method [8].
UPDATE: The SIP UPDATE method [10].
SUBSCRIBE: The SIP SUBSCRIBE method [7].
NOTIFY: The SIP NOTIFY method [7].
PRACK: The SIP PRACK method [9].
MESSAGE: The SIP MESSAGE method [17].
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.]]
9.13 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 [13] that are supported by a UA.
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 [6].
http: The HTTP URI scheme [14].
https: The HTTPS URI scheme [27].
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cid: The CID URI scheme [15].
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.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.14 Text
Media feature tag name: text
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: This feature
tag indicates that the device supports text as a MIME media
type.
Values appropriate for use with this feature tag: Boolean.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Routing a call to a phone that can
support text.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.15 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 MIME
media type.
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Values appropriate for use with this feature tag: Boolean.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Routing a call to a phone that can
support video.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
9.16 Voicemail
Media feature tag name: voicemail
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 voicemail system which
will record messages for a user.
Values appropriate for use with this feature tag: Boolean.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation
mechanisms: This feature tag is most useful in a
communications application, for describing the capabilities
of a device, such as a phone or PDA.
Examples of typical use: Requesting that a call not be routed to
voicemail.
Related standards or documents: RFC XXXX [[Note to IANA: Please
replace XXXX with the RFC number of this specification.]]
10 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"
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cancel-directive = "cancel" / "no-cancel"
fork-directive = "fork" / "no-fork"
recurse-directive = "recurse" / "no-recurse"
parallel-directive = "parallel" / "sequential"
queue-directive = "queue" / "no-queue"
Accept-Contact = ("Accept-Contact" / "a") HCOLON feature-set
*(COMMA feature-set)
Reject-Contact = ("Reject-Contact" / "j") HCOLON feature-set-noq
*(COMMA feature-set-noq)
Require-Contact = "Require-Contact"
HCOLON feature-set-noq *(COMMA feature-set-noq)
feature-set = ( name-addr / addr-spec / "*")
*(SEMI tag-set) [q-param]
feature-set-noq = ( name-addr / addr-spec / "*")
*(SEMI tag-set)
tag-set = feature-tag EQUAL LDQUOT (tag-value-list
/ string-value / boolean / numeric) RDQUOT
feature-tag = ftag ; From RFC 2533
tag-value-list = tag-value *("," tag-value)
tag-value = ["!"] token-nobang
token-nobang = 1*(alphanum / "-" / "." / "%" / "*"
/ "_" / "+" / "`" / "'" / "~" )
boolean = "TRUE" / "FALSE"
numeric = "#" (lessthan / greaterthan / equality /
range)
lessthan = ">=" number
greaterthan = "<=" number
equality = "=" number
range = "R" number ".." number
number = integer / rational
integer = [ "+" / "-" ] 1*DIGIT
rational = [ "+" / "-" ] 1*DIGIT "/" 1*DIGIT
string-value = LDQUOT "<" qdtext ">" RDQUOT
q-param = "q" EQUAL qvalue
contact-params = c-p-q / c-p-expires / tag-set
= / contact-extension
11 Mapping Feature Parameters and Feature Set Predicates
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Mapping between feature parameters and feature set predicates,
formatted according to the syntax of RFC 2533 [2] is trivial.
Starting from a set of feature parameters, the procedure is as
follows. Construct a conjunction. Each term in the conjunction
derives from one feature parameter. If the feature parameter value is
a comma separated list, the element of the conjunction is a
disjunction. There is one term in the disjunction for each value in
the comma separated list. Call each value a "phrase". If the feature
parameter value was not a comma separate list, the term in the
conjunction is obtained from the value. That value is also a
"phrase".
Consider now the construction of a filter from the phrase. If the
phrase starts with a bang (!), the filter is of the form:
(! (name=remainder))
where name is the name of the feature parameter, and remainder is the
remainder of the text in the phrase after the bang.
If the phrase starts with an octothorpe (#), the filter is a numeric
comparison. The comparator is either =, >= or <= based on the next
characters in the phrase. In this case, the filter is of the form:
(name comparator remainder)
where name is the name of the feature parameter, comparator is either
=, >= or <=, and remainder is the remainder of the text in the phrase
after the equal.
If the value after the octothorpe is R, the filter is a range. The
format of the filter is:
(name=[remainder])
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where name is the name of the feature parameter, and remainder is the
remainder of the text in the phrase after the R. According to the
BNF, this will be of the form "value..value", which specifies the
range.
If the phrase begins with a left angle bracket ("<") and ends with a
right angle bracket (">"), this implies that the value is a string,
rather than a token. This is converted to a filter of the form:
(name="bracketed")
where name is the name of the feature parameter, and bracketed is the
text from the phrase between the left and right angle brackets. Note
the explicit usage of quotes, 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 latter, and sensitive for the
former. 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.
Otherwise, the filter is of the following form:
(name=phrase)
where name is the name of the feature parameter, and phrase is the
phrase.
As an example, the Contact header:
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Contact:*;mobility="fixed";events="!presence,winfo";language="en,de"
;description="<PC>"
would be converted to the following feature predicate:
(& (mobility=fixed)
(| (! (events=presence)) (events=winfo))
(| (language=en) (language=de))
(description="PC"))
As another example, the following Accept-Contact header field:
Accept-Contact: *;methods="SUBSCRIBE";resolution="#R5..100"
would be converted to the following feature set predicate:
(& (methods=SUBSCRIBE)
(resolution=[5..100]))
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.
12 Security Considerations
The presence of caller preferences in a request has a significant
effect on the ways in which the request is handled at a server. As a
result, is is especially important that requests with caller
preferences be authenticated and integrity-protected. The same holds
true for registrations with feature parameters in the Contact header
field.
Processing of caller preferences requires set operations and searches
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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 S/MIME or 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
This specification registers a number of new Media feature tags
according to the procedures of RFC 2506 [4]. 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 four 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
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The following is the registration for the Require-Contact header
field:
RFC Number: RFC XXXX [Note to IANA: Fill in with the RFC number
of this specification.]
Header Field Name: Require-Contact
Compact Form: none defined
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
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 Acknowledgements
The initial set of media feature tags used by this specification were
influenced by Scott Petrack's CMA design. Jonathan Lennox, and John
Hearty provided helpful comments. Graham Klyne provided assistance on
the usage of RFC 2533. Paul Kyzivat contributed significantly to this
work, assisting in the generation of use cases, and poking holes in
past versions of the document.
15 Author's Addresses
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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
16 Normative References
[1] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
initiation protocol," RFC 3261, Internet Engineering Task Force, June
2002.
[2] G. Klyne, "A syntax for describing media feature sets," RFC 2533,
Internet Engineering Task Force, Mar. 1999.
[3] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
[4] K. Holtman, A. Mutz, and T. Hardie, "Media feature tag
registration procedure," RFC 2506, Internet Engineering Task Force,
Mar. 1999.
[5] G. Klyne, "Corrections to "A syntax for describing media feature
sets"," RFC 2738, Internet Engineering Task Force, Dec. 1999.
[6] A. Vaha-Sipila, "URLs for telephone calls," RFC 2806, Internet
Engineering Task Force, Apr. 2000.
[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.
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[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] T. Berners-Lee, R. Fielding, and L. Masinter, "Uniform resource
identifiers (URI): generic syntax," RFC 2396, Internet Engineering
Task Force, Aug. 1998.
[14] R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P.
Leach, and T. Berners-Lee, "Hypertext transfer protocol -- HTTP/1.1,"
RFC 2616, Internet Engineering Task Force, June 1999.
[15] E. Levinson, "Content-id and message-id uniform resource
locators," RFC 2392, Internet Engineering Task Force, Aug. 1998.
17 Informative References
[16] J. Lennox and H. Schulzrinne, "Call processing language
framework and requirements," RFC 2824, Internet Engineering Task
Force, May 2000.
[17] B. Campbell and J. Rosenberg, "Session initiation protocol
extension for instant messaging," Internet Draft, Internet
Engineering Task Force, Sept. 2002. Work in progress.
[18] G. Klyne, "Protocol-independent content negotiation framework,"
RFC 2703, Internet Engineering Task Force, Sept. 1999.
[19] M. Handley and V. Jacobson, "SDP: session description protocol,"
RFC 2327, Internet Engineering Task Force, Apr. 1998.
[20] J. Rosenberg, "Session initiation protocol (SIP) extensions for
presence," Internet Draft, Internet Engineering Task Force, May 2002.
Work in progress.
[21] J. Rosenberg, "A session initiation protocol (SIP)event
template-package for watcher information," Internet Draft, Internet
Engineering Task Force, May 2002. Work in progress.
[22] R. Sparks, "The SIP refer method," Internet Draft, Internet
Engineering Task Force, July 2002. Work in progress.
[23] J. Rosenberg and H. Schulzrinne, "A session initiation protocol
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(SIP) event package for dialog state," Internet Draft, Internet
Engineering Task Force, June 2002. Work in progress.
[24] 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.
[25] J. Rosenberg, "A sip event package for registration state,"
Internet Draft, Internet Engineering Task Force, Oct. 2002. Work in
progress.
[26] R. Mahy, "A message summary and message waiting indication event
package for the session initiation protocol (SIP)," Internet Draft,
Internet Engineering Task Force, June 2002. Work in progress.
[27] E. Rescorla, "HTTP over TLS," RFC 2818, Internet Engineering
Task Force, May 2000.
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 union of
points - feature collections - that make up the space. If a
particular feature collection is in the space, it means that the
rendering described by that feature collection is supported by the
device with that feature set.
How does one represent a feature set? There are many ways to describe
an N-dimensional space. One way is to identify mathematical functions
which identify its contours. Clearly, that is too complex to be
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useful. The solution taken in RFC 2533 is to define the space with a
feature set predicate. A feature set predicate is a boolean function
over an N-dimensional space. The input to the function is a point in
that space - a feature collection. If the result of the boolean
function is TRUE, the feature collection is a member of the space. If
the result of the boolean function is FALSE, the feature collection
is not in the space.
RFC 2533 describes a syntax for writing down these N-dimensional
boolean functions. 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
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
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specification of the feature collections that the recipient supports.
There is a match when the spaces defined by both feature sets
overlap. When there is overlap, there exists at least one feature
collection that exists in both feature sets, and therefore a modality
or rendering desired by the sender which is supported by the
recipient.
This leads directly to the definition of a match. Two feature sets
match if there exists at least one feature collection present in both
feature sets.
Computing a match for two general feature set predicates is not easy.
Section 5 of RFC 2533 presents an algorithm for doing it by expanding
an arbitrary expression into disjunctive normal form. However, the
feature set predicates used by this specification are constrained.
They are always in conjunctive normal form, with each term in the
conjunction describing values for different media features. This
makes computation of a match easy. It is computed independently for
each media feature, and then the feature sets overlap if media
features specified in both sets overlap. Computing the overlap of a
single media feature is very straightforward, and is a simple matter
of computing whether two finite sets overlap.
Full Copyright Statement
Copyright (c) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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