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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 3841

SIP                                                         J. Rosenberg
Internet-Draft                                               dynamicsoft
Expires: April 21, 2004                                   H. Schulzrinne
                                                     Columbia University
                                                              P. Kyzivat
                                                           Cisco Systems
                                                        October 22, 2003


      Caller Preferences for the Session Initiation Protocol (SIP)
                     draft-ietf-sip-callerprefs-10

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at http://
   www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on April 21, 2004.

Copyright Notice

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

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.




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Table of Contents

   1.      Introduction . . . . . . . . . . . . . . . . . . . . . . .  3
   2.      Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   3.      Definitions  . . . . . . . . . . . . . . . . . . . . . . .  6
   4.      Overview of Operation  . . . . . . . . . . . . . . . . . .  7
   5.      UAC Behavior . . . . . . . . . . . . . . . . . . . . . . .  8
   5.1     Request Handling Preferences . . . . . . . . . . . . . . .  8
   5.2     Feature Set Preferences  . . . . . . . . . . . . . . . . .  8
   6.      UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . 11
   7.      Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 12
   7.1     Request-Disposition Processing . . . . . . . . . . . . . . 12
   7.2     Preference and Capability Matching . . . . . . . . . . . . 12
   7.2.1   Extracting Explicit Preferences  . . . . . . . . . . . . . 13
   7.2.2   Extracting Implicit Preferences  . . . . . . . . . . . . . 13
   7.2.2.1 Methods  . . . . . . . . . . . . . . . . . . . . . . . . . 13
   7.2.2.2 Event Packages . . . . . . . . . . . . . . . . . . . . . . 14
   7.2.3   Constructing Contact Predicates  . . . . . . . . . . . . . 14
   7.2.4   Matching . . . . . . . . . . . . . . . . . . . . . . . . . 15
   7.2.5   Example  . . . . . . . . . . . . . . . . . . . . . . . . . 19
   8.      Mapping Feature Parameters to a Predicate  . . . . . . . . 21
   9.      Header Field Definitions . . . . . . . . . . . . . . . . . 24
   9.1     Request Disposition  . . . . . . . . . . . . . . . . . . . 24
   9.2     Accept-Contact and Reject-Contact Header Fields  . . . . . 26
   10.     Augmented BNF  . . . . . . . . . . . . . . . . . . . . . . 27
   11.     Security Considerations  . . . . . . . . . . . . . . . . . 28
   12.     IANA Considerations  . . . . . . . . . . . . . . . . . . . 29
   13.     Acknowledgments  . . . . . . . . . . . . . . . . . . . . . 30
           Normative References . . . . . . . . . . . . . . . . . . . 31
           Informative References . . . . . . . . . . . . . . . . . . 32
           Authors' Addresses . . . . . . . . . . . . . . . . . . . . 32
           Intellectual Property and Copyright Statements . . . . . . 33



















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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) [11]. 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



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   feature set [2] that expresses its preferences on the characteristics
   of the UA that is to be reached. These are matched with a feature
   sets provided by a UA to its registrar [3].  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.

   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.






































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2. Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [4] and
   indicate requirement levels for compliant implementations.













































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3. Definitions

   Much of the terminology used in this specification is presented in
   [3]. This specification defines the following additional terms:

   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 Preferences: Caller preferences that describe 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.

   Target Set: A target set is a set of candidate URIs 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 the
      request method is INVITE, it represents an implicit caller
      preference to route the request to a UA that supports the INVITE
      method.




















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4. Overview of Operation

   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 contain feature sets, represented by the same feature
   parameters that are used to indicate capabilities [3]. 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.










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5. UAC Behavior

   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 determine whether servers along the path
   understand the header fields described in this specification, it
   includes a Proxy-Require header field with a value of "pref" [3] in
   its request. If the request should fail with a 420, the UAC knows
   that the extension is not supported. In that case, it SHOULD retry,
   and may decide whether or not to use caller preferences. A UA should
   only use Proxy-Require if knowledge about support is essential for
   handling of the request. Note that, in any case, caller preferences
   can only be considered preferences - there is no guarantee that the
   requested service 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.

5.1 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 9.1.

5.2 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. The syntax of
   the header field can be found in Section 10, and a discussion of
   their usage in Section 9.2.

   Each feature set is constructed as described in Section 5 of [3]. The
   feature sets placed into these header fields MAY overlap; that is, a
   UA MAY indicate preferences for feature sets that match according to



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   the matching algorithm of RFC 2533 [2].

   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 "sip.methods" feature
   tag (note, however, that even though the name of this feature tag is
   sip.methods, it would be encoded into the Accept-Contact header field
   as just "methods"), 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 "sip.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.

   As an example, the following Accept-Contact header field expresses a
   desire to route a call to a mobile device:



   Accept-Contact: *;mobility="mobile";methods="INVITE"

   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 [12],
   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 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 precise behavior depends heavily on
   whether the "require" and "explicit" feature parameters are present.
   When both of them are present, a proxy will only forward the request
   to contacts which have explicitly indicated that they support the
   desired feature set. Any others are discarded. As such, a UAC should
   only use "require" and "explicit" together when it wishes the call to
   fail unless a contact definitively matches. It's possible that a UA
   supports a desired feature, but did not indicate it in its
   registration. When a UAC uses both "explicit" and "require", such a
   contact would not be reached. As a result, this combination is often
   not the one a UAC will want.




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   When only "require" is present, it means that a contact will not be
   used if it doesn't match. If it does match, or if its not known
   whether its a complete match, the contact is still used. A UAC would
   use "require" alone when a non-matching contact is useless. This is
   common for services where the request simply can't be serviced
   without the neccesary features. An example is support for specific
   methods or event packages. When only "require" is present, the proxy
   will also preferentially route the request to the UA which represents
   the "best" match. Here, "best" means that the UA has explicitly
   indicated it supports more of the desired features than any other.
   Note, however, that this preferential routing will never override an
   ordering providing by the called party. The preferential routing will
   only choose amongst contacts of equal q-value.

   When only "explicit" is present, it means that all contacts provided
   by the callee will be used. However, if the contact isn't an explicit
   match, it is tried last amongst all other contacts with the same
   q-value. The principle difference, therefore, between this
   configuration and the usage of both "require" and "explicit" is the
   fallback behavior for contacts that don't match explicitly. Here,
   they are tried as a last resort. If "require" is also present, they
   are never tried.

   Finally, if neither "require" nor "explicit" are present, it means
   that all contacts provided by the callee will be used. However, if
   the contact doesn't match, it is tried last amongst all other
   contacts with the same q-value. If it does match, the request is
   routed preferentially to the "best" match. This is a common
   configuration for preferences that, if not honored, will still allow
   for a successful call, and the greater the match, the better.





















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6. UAS Behavior

   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.
   If a UA had registered against multiple separate adresses-of-record,
   and the contacts registered for each had different capabilities, it
   will have used a different URI in each registration, so it can
   determine which feature set to use.

   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, 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.

























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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 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 9.1,
   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 [2] 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.





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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 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, or whose name begins with a plus (+) [3]. The proxy
   converts all of those parameters to the syntax of RFC 2533, based on
   the rules in Section 8.

   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 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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   (sip.methods=[method of request])


7.2.2.2 Event Packages

   For requests that establish a subscription [5], 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:


   (sip.events=[value of the Event header field])


7.2.3 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 [3] 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 "audio", "automata",
       "class", "duplex", "data", "control", "mobility", "description",
       "events", "priority", "methods", "schemes", "application",
       "video", "actor", "language", "isfocus" 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
   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.




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   Assuming the URI has feature parameters, they are converted to RFC
   2533 syntax using the rules of Section 8.

   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"

   This would be converted into the following predicate:



   (& (audio=TRUE)
      (video=TRUE)
      (sip.mobility=fixed)
      (message=TRUE)
      (| (sip.methods=INVITE) (sip.methods=OPTIONS) (sip.methods=BYE)
         (sip.methods=CANCEL) (sip.methods=ACK))
      (| (sip.schemes=sip) (sip.schemes=http)))

   Note that "other-param" was not considered a feature parameter, since
   it is neither a base tag nor did it begin with a leading +.

7.2.4 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 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



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   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 target
   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 target set, however, the Accept-Contact predicate is 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 in 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.









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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 7: Applying the Score

   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 7. 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. The overall caller preference, Qa,
   is the arithmetic average of S1 through SM.

   At this point, any URIs that were removed from the target set because
   they were immune from caller preferences are added back in, and Qa



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   for that URI is set to 1.0.

   The purpose of the caller preference Qa is to provide an ordering for
   any contacts remaining in the target set, if the callee has not
   provided an ordering. To do this, the contacts remaining in the
   target set are sorted by the q-value provided by the callee. Once
   sorted, they are grouped into equivalence classes, such that all
   contacts with the same q-value are in the same equivalence class.
   Within each equivalence class, the contacts are then ordered based on
   their values of Qa. The result is an ordered list of contacts that is
   used by the proxy.

   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, ordered by 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, 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.

   If a proxy server is recursing, it adds the Contact header fields
   returned in the redirect responses to the target set, and re-applies
   the caller preferences algorithm.

   If the server is redirecting, it returns all entries in the target
   set. It assigns q-values to those entries arbitrarily, so that the
   ordering is identical to the ordering determined by the processing
   above. 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



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   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.

7.2.5 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=0.2
   Contact: sip:u2@h.example.com;audio="FALSE";
     methods="INVITE";actor="msg-taker";q=0.2
   Contact: sip:u3@h.example.com;audio;actor="msg-taker";
     methods="INVITE";video;q=0.3
   Contact: sip:u4@h.example.com;audio;methods="INVITE,OPTIONS";q=0.2
   Contact: sip:u5@h.example.com;q=0.5

   An INVITE sent to sip:user@example.com contained the following caller
   preferences header fields:


   Reject-Contact: *;actor="msg-taker";video
   Accept-Contact: *;audio;require
   Accept-Contact: *;video;explicit
   Accept-Contact: *;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 message taker.
   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



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   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 0.83 and Qa for u4 is 0.5. u5 is added back
   in with a Qa of 1.0.

   Next, the remaining contacts in the target set are sorted by q-value.
   u5 has a value of 0.5, u1 has a q-value of 0.2 and so does u4. There
   are two equivalnce classes. The first has a q-value of 0.5, and
   consists of just u5. Since there is only one member of the class,
   sorting within the class has no impact. The second equivalence class
   as a q-value of 0.2. Within that class, the two contacts, u1 and u4,
   are ordered based on their values of Qa. u1 has a Qa of 0.83, and u4,
   a Qa of 0.5. Thus, u1 comes first, followed by u4. The resulting
   overall ordered set of contacts in the target set is u5, u1 and then
   u4.
































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8. Mapping Feature Parameters to a Predicate

   Mapping between feature parameters and a feature set predicate,
   formatted according to the syntax of RFC 2533 [2] is trivial. It is
   just the opposite of the process described in Section 5 of [3].

   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)

   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.

   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



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   #, 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:

   (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.

   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 RFC 2506 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 (/). If there was no
   leading plus sign, and the remainder of the encoded name was
   "automata", "class", "duplex", "mobility", "description", "events",
   "priority", "methods", "schemes", "isfocus" or "actor", the prefix
   "sip." is added to remainder of the encoded name to compute the
   feature tag name.

   As an example, the Accept-Contact header:


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

   would be converted to the following feature predicate:








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













































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9. Header Field Definitions

   This specification defines three new header fields - Accept-Contact,
   Reject-Contact, and Request-Disposition.

   Figure 17 and Figure 18 are an extension of  Tables 2 and 3 in RFC
   3261 [1] for the Accept-Contact, Reject-Contact and
   Request-Disposition header fields. The column "INF" is for the INFO
   method [6], "PRA" is for the PRACK method [7], "UPD" is for the
   UPDATE method [8], "SUB" is for the SUBSCRIBE method [5], "NOT" is
   for the NOTIFY method [5], "MSG" is for the MESSAGE method [9], and
   "REF" is for the REFER method [10].


   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

   Figure 17: Accept-Contact, Reject-Contact and Request-Disposition
                             header fields



   Header field          where  proxy  PRA UPD SUB NOT INF MSG REF

   Accept-Contact          R      ar    o   o   o   o   o   o   o
   Reject-Contact          R      ar    o   o   o   o   o   o   o
   Request-Disposition     R      ar    o   o   o   o   o   o   o

   Figure 18: Accept-Contact, Reject-Contact and Request-Disposition
                             header fields


9.1 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
   field).

   When the caller specifies a directive, the server SHOULD honor that
   directive.



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   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 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




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   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.

9.2 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.









































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10. Augmented BNF

   The BNF for the Request-Disposition header field is:


   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"

   The BNF for the Accept-Contact and Reject-Contact header fields is:


   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 / req-param
                         / explicit-param / generic-param
                       ;;feature param from RFC XXXX
                       ;;generic-param from RFC 3261
   rc-params       =  feature-param / generic-param
   req-param       =  "require"
   explicit-param  =  "explicit"

   Despite the BNF, there MUST NOT be more than one req-param or
   explicit-param in an acrc-params. Furthermore, there can only be one
   instance of any feature tag in feature-param.














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11. 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.

   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.






































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12. IANA Considerations

   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

















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13. Acknowledgments

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












































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Normative References

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

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

   [3]   Rosenberg, J., "Indicating User Agent Capabilities in the
         Session Initiation Protocol  (SIP)",
         draft-ietf-sip-callee-caps-00 (work in progress), June 2003.

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

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

   [6]   Donovan, S., "The SIP INFO Method", RFC 2976, October 2000.

   [7]   jdrosen@dynamicsoft.com and schulzrinne@cs.columbia.edu,
         "Reliability of Provisional Responses in Session Initiation
         Protocol (SIP)", RFC 3262, June 2002.

   [8]   Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
         Method", RFC 3311, October 2002.

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

   [10]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
         Method", RFC 3515, April 2003.

















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Informative References

   [11]  Lennox, J. and H. Schulzrinne, "Call Processing Language
         Framework and Requirements", RFC 2824, May 2000.

   [12]  Rosenberg, J., "Guidelines for Authors of Extensions to the
         Session Initiation Protocol  (SIP)",
         draft-ietf-sip-guidelines-06 (work in progress), November 2002.


Authors' Addresses

   Jonathan Rosenberg
   dynamicsoft
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

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


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

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


   Paul Kyzivat
   Cisco Systems
   1414 Massachusetts Avenue
   BXB500 C2-2
   Boxboro, MA  01719
   US

   EMail: pkzivat@cisco.com









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