Internet Engineering Task Force                                MMUSIC WG
Internet Draft                                     Schulzrinne/Rosenberg
draft-ietf-mmusic-sip-caller-00.txt        Columbia U./Bell Laboratories
February 26, 1999
Expires: August 26, 1999

             SIP Caller Preferences and Callee Capabilities


   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
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   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as work in progress.

   The list of current Internet-Drafts can be accessed at

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   This document describes a set of extensions to SIP which allow a
   caller to express preferences about request handling in servers. It
   also extends the SIP Contact header to allow users to describe their
   communications capabilities and characteristics.

1 Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and
   indicate requirement levels for compliant SIP implementations.

2 Introduction

   When a SIP server receives a request, there are a number of decisions

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   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;

        o whether to return just the first 200-class response, or all
          2xx responses.

   The server can base these decisions on a large number of factors,
   such as time of day, caller or callee identity, call urgency, caller
   preferences, network status, and the content of external databases.

   There are three parties which have an interest in influencing the
   call processing logic at the server: (1) the administrator of the
   server, (2) the callee, and (3) the caller. The directives of the
   administrator are usually embedded in the configuration of the
   server, and can be expressed, for example, in the form of SIP CGI
   scripts [2]. The preferences of the callee can be expressed most
   easily through a script written in the call processing language (CPL)
   [3]. This document specifies SIP extensions which express caller
   preferences and describe callee capabilities by adding three new
   header fields (Request-Disposition, Accept-Contact, Reject-Contact)
   and extending the parameter list for the Contact header field.

        This draft is the result of partitioning the SIP call
        control extensions draft [4] into two components, this one
        and another on third-party call control, to be issued.

   If the client wants to be sure that the server understands the
   headers described in this specification, it MUST include a Require
   option of org.ietf.sip.caller-preferences.

   Since the Accept-Contact and Request-Disposition header are advisory
   only, a client can save an extra round-trip time if it only includes
   the Require option if the Reject-Contact header is present.

3 Design Alternatives

   There are a number of alternatives for expressing caller preferences.
   Caller preferences and callee preferences must "meet" at the proxy or

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   redirect server responsible for the callee, so that the appropriate
   forwarding decision can be made. This server will always be under the
   control of the callee or an entity trusted by the callee, so that
   installing a call processing language script expressing caller
   preferences is not appropriate (unless, of course, the callee would
   like to accomodate the known preferences of a certain caller).
   Uploading a caller preference script to every possible server is also
   clearly not feasible. As another alternative, one could embed a
   script in the request, to be executed by proxy or redirect servers
   when making forwarding decisions. This would be an application-layer
   version of active networks. However, the generality of a script does
   not seem to be needed. It also makes combining caller and callee
   preferences a rather difficult problem and raises possible
   performance and security issues. Unlike the callee script, which
   needs to handle unknown callers, with a wide range of call
   properties, at unknown times in the future, a caller knows all but
   the set of communications capabilities of the callee. The caller can
   present the servers with its preferences on a call-by-call basis.
   Callers can thus place their preferences for this particular call in
   the request message. We propose a simple ordered list of preferences
   to make it possible to reconcile caller and callee preferences

   In summary, there is a strong asymmetry in how preferences for
   callers and callees can be presented to the network. While a caller
   takes an active role by initiating the call, the callee takes a
   passive role in waiting for calls. This motivates the use of callee-
   supplied scripts and caller preferences included in the call request.

   This asymmetry is also reflected in the appropriate relationship
   between caller and callee preferences. A server for a callee SHOULD
   respect the wishes of the caller to avoid certain locations, while
   the preferences among locations has to be the callee's choice, as it
   determines where, for example, the phone rings and whether the callee
   incurs mobile telephone charges for incoming calls.

   The problem of feature negotation has also been approached in a more
   general way by [5]. However, that proposal is far more complicated
   than appears to be needed here, with syntax that does not fit into
   the current SIP syntax structure.

4 Overview

   The extension specified in this document consists of three new header
   fields:  Reject-Contact, Accept-Contact, and Request-Disposition. The
   first two express preferences about which URIs the client would like
   the request to reach. The Reject-Contact header field (Section 6.3)
   contains a list of URIs the user does not wish the request to be

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   proxied (or redirected) to. The Accept-Contact header field (Section
   6.2) is a list of addresses that the caller would like to be proxied
   (or redirected) to, with the strength of preferences expressed
   through a q parameter, ranging in value from zero to one, with one
   indicating the highest preference. The Request-Disposition header
   field (Section 6.4) contains a set of tags which request particular
   processing for this request, such as whether or not the caller
   prefers proxy or redirection.

5 Determining Request Forwarding

   The Accept-Contact header field indicates preferences for certain
   destinations among those chosen by a proxy server. The Reject-Contact
   header field describes those locations that the proxy server should
   not direct the request to.

   Listing only URIs is of limited use, since the caller often will not
   know where the callee is located. For this reason, this specification
   proposes a number of URI parameters which describe characteristics of
   the user. These parameters include whether the location is a home or
   work address, whether it is fixed or mobile, and what media types are
   available. A server may know about a number of URIs for a user, along
   with parameters describing each one, for example, through user
   registration (REGISTER request). The combination of a URI along with
   a set of parameters is called a contact entry; a set of contact
   entries is called the contact list. When a request arrives with
   either Reject-Contact or Accept-Contact header field, the server
   performs a matching operation, described below, to create an ordered
   list of contact addresses that reflect the joint caller's and
   callee's preferences.

        There is some overlap between the indication of receiver
        capabilities in the session description message body and
        the Accept-Contact and Reject-Contact header fields.
        However, current session description formats cannot express
        the preferences described here. Also, the capabilities
        described here are fundamental to call-routing and thus
        should not depend on the particulars of the various session
        description formats that might be used.

5.1 Matching Rules

   The server matches rules in the Accept-Contact and Reject-Contact
   request headers to contact entries in the contact list according to
   the following rules.

        o If rule does not contain URI, only parameters are compared. If

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          the request header rule contains a URI, both the URI and
          parameters must match.

        o A URI in a rule matches a SIP URI in a contact entry if the
          user and host parts are equal, where equality is based on
          string equivalence. Either part may be omitted from the rule,
          and in this case it matches any value. For example, the rule "
 " matches " " or " ",
          while the rule " joe@ " matches " " and "
 ". Matches are case-insensitive. Other URIs
          are matched according to the equality rules for that URI

        o The parameters in a rule match the parameters in a contact
          entry if all parameters in the rule either have a matching
          value in the contact entry, or the parameter is not present in
          the contact entry.

             Ignoring the parameter if it does not exist in the
             contact list avoids that a parameter that the server
             does not know about causes the match to fail.

   The pseudo-code below describes the matching procedure between the
   rule in the request header and the contact entry. For simplicity, we
   assume that the list of parameters in each rule is stored as an
   associative array, so that rule.para[duplex] yields the value of the
   attribute duplex, and is undefined if duplex is not specified.

   struct {
     uri_t URI;           /* URI */
     parameter_t para[];  /* list of parameters */
   } rule, entry;

   boolean MATCH(rule r, entry e) {
     boolean match;

     match = TRUE;
     if (r.URI != "") {
       if (r.URI.scheme == e.URI.scheme) {
         if (r.URI.scheme == "sip") {
           match = ( == "" || == &&
                   (r.URI.user == "" || r.URI.user == e.URI.user)
         } else {
           match = scheme-appropriate comparison;
       } else {
         return FALSE;

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     if(match == FALSE) return FALSE;

     /* compare parameters */
     foreach parameter p in r.para[] {
       if (isdefined(e.para[p])) {
         if (r.para[p][0] == "!")
           if (r.para[p] intersect e.para[p] != "") {
             return FALSE;
         } else {
           if (r.para[p] is not subset of e.para[p]) {
             return FALSE;
     return TRUE;

   Parameter names are matched by case-insensitive string comparison.
   Parameter values are compared by set-comparisons. Parameter values in
   quoted strings are interpreted as sets, with elements separated by
   commas. The names of elements are case-insensitive. Parameter values
   that are tokens are interpreted as sets with one element. There are
   two cases: if the quoted-string parameter value in a rule starts with
   an exclamation mark (!), the rule matches if the intersection of the
   set in the rule and in the contact entry is empty. Otherwise, the
   rule matches if the intersection is the rule set itself, i.e., if the
   rule set is a subset of the contact entry parameter.

        The syntax for empty intersection is ugly. Using operators
        instead of equal may be preferable, but breaks the basic
        SIP parser model.

   For example, the rule


   matches the contact entry containing


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   but not any of


   As another example, the rule


   matches the contact entry


   but not


   A server SHOULD NOT be aware of the particular semantics of any of
   the parameters. This allows for the definition of new parameters and
   values without explicitly programming them into the servers.

5.2 Contact List Processing

   It is assumed that the server has a contact list for the callee. In
   this case, the server MAY elect to follow the procedure below for
   merging caller preferences and callee preferences. If the server has
   a call processing language for the callee, or some other form of
   complex logic, the way in which the caller preferences are merged is
   not defined, but left to the implementor. In general, the server
   SHOULD NOT proxy or redirect to a contact entry which matches a rule
   in the Reject-Contact request header. If the server was operating as
   a proxy, it SHOULD behave as if it had proxied, and had received a
   404 "Not Found" in response. If the server was acting as a redirect
   server, if SHOULD NOT place the contact entry in the Contact header
   in the response. The rules in the Accept-Contact header SHOULD be
   treated as a hint, and the preferences honored when all else is

   In the case when the server has a simple, preference ordered contact
   list for the callee, the procedure is as follows.

   The server first removes any contact entry from the contact list that

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   matches a rule in the Reject-Contact header field.

   A contact entry may contain a priority parameter. This means that a
   request should not be proxied or redirected to that contact entry
   unless the request is of equal or higher priority. The priority value
   of the request is derived from the Priority header field. If the
   request does not contain a Priority header field, the request
   priority is set to "non-urgent". Priorities are ordered from "non-
   urgent" (lowest), "normal", "urgent" to "emergency" (highest).
   Priority values not known to the server are mapped to "non-urgent".
   The server then removes any contact entry whose priority value is
   higher than that of the request.

   Each rule in the Accept-Contact header field is then processed. If
   the rule matches a contact entry, the q value of that entry is
   updated, in order to incorporate the caller's preferences. If the
   rule does not match a contact entry, nothing is done. This document
   does not prescribe a certain algorithm for updating. Among many
   possibilities, a server MAY set the q value to the average of the
   original value specified by the callee, and the average q value of
   the caller's rules that match the contact entry. This gives equal
   weight to caller and callee preferences. If a rule or contact entry
   does not have a q value, it is taken to be one (this is in agreement
   with the HTTP defaults).

        Note that this preference computation only determines the
        ordering of call 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 contact list is then sorted according to
   the q value. The server first attempts to contact those with the
   highest q value in parallel. If these contact entries do not respond
   with a 2xx or 6xx response, the server tries the entries with the
   next-highest q value.

        Due to round-off errors and the computation of joint
        preferences, there may be an excessive bias here towards
        serialization rather than parallel attempts. Maybe a server
        should group all q values within, say, 0.1 into a single
        parallel-search group.

   If the server receives a redirection, and elects to recurse

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   (depending on its configuration and the preference specified in the
   Request-Disposition header field), the contact addresses are added to
   the contact list and the algorithm continues. Note that this may
   cause addresses that were added by redirection to be tried before
   contact entries in the original contact list.

5.3 IANA Registration

   New URI parameters and values can be defined at any time and
   registered with IANA. When registering new parameters and values, the
   following information MUST be provided:

   Contact: Name, organization, email address, and phone number of
        person registering the attributes.

   Attributes: A list of the new attributes being registered. For each,
        the meaning of the attribute must be described, in sufficient
        detail so that a user agent would be able to ascertain whether
        the parameter applies to it, and if so, which value to use. The
        attributes MUST also be associated with a finite set of values,
        each of which is a valid unicode string. For each value, a
        description of the value must be provided.

5.4 Use with REGISTER

   A user agent normally registers with one or more servers, providing
   each with a list of Contact addresses. The user agent MAY add the
   parameters cp-param described in Section 6.2 to the Contact header

   Furthermore, the REGISTER request MAY contain a Require header with
   the name of this extension if the client wants to be sure that the
   server honors callee preferences.

6 Header Field Definitions

   The table below specifies which requests can contain which headers.

   Since all three headers specify call routing logic, they can apply to
   any request which can normally be proxied.

6.1 Contact, Accept-Contact and Reject-Contact Parameters

   This specification adds the following extension parameters to the
   Contact header field and defines the same parameters for the Accept-
   Contact and Reject-Contact header fields.

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                                 type ACK BYE INV OPT REG CAN
            Accept-Contact        R    -   o   o   o   o   -
            Reject-Contact        R    -   o   o   o   o   -
            Request-Disposition   R    -   o   o   o   o   -

   Table 1: Summary of header fields. "o": optional "-": not applicable,
   "R':  request header, "r": response header, "g": general header, "*":
   needed if message body is not empty. A numeric value  in  the  "type"
   column indicates the status code the header field is used with.

   cp-params       =  class-param | duplex-param |
                      features-param | language-param | media-param |
                      mobility-param | priority-param | service-param
   class-param     =  "class" "=" <"> [<!>] class-tag <">
   duplex-param    =  "duplex" "=" <"> [<!>] duplex-tag <">
   feature-param   =  "feature" "=" <"> [<!>] 1#feature-tag <">
   language-param  =  "language" "=" <"> [<!>] 1#language-tag <">
   media-param     =  "media" "=" <"> [<!>] 1#media-tag <">
   mobility-param  =  "mobility" "=" <"> [<!>] mobility-tag <">
   service-param   =  "service" "=" <"> [<!>] service-tag <">
   language-tag    =  primary-tag *( "-" subtag )
   primary-tag     =  1*8ALPHA
   subtag          =  1*8ALPHA
   mobility-tag    =  "fixed" | "mobile"
   class-tag       =  "personal" | "business"
   duplex-tag      =  "full" | "half" | "receive-only" | "send-only"
   service-tag     =  "fax" | "IP" | "PSTN" | "ISDN" | "text"
   media-tag       =  ( "*/*" | (type "/" "*") |
                      (type "/" subtype) )
   feature-tag     =  "voice-mail" | "attendant"

   The exclamation mark in the parameter value MUST NOT be included if
   the cp-params are included in a Contact header.

   class: The class parameter indicates whether this terminal is found
        in a residential or business setting. (A caller may defer a
        personal call if only a business line is available, for

   duplex: The duplex parameter lists whether the terminal can
        simultaneously send and receive ("full"), alternate between
        sending and receiving ("half"), can only receive ("receive-
        only") or only send ("send-only"). Typically, a caller will
        prefer a full-duplex terminal over a half-duplex terminal and
        these over receive-only or send-only terminals.

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   features: The feature list enumerates additional features. It is
        assumed that these features are orthogonal, and could occur in
        any combination. "Voice-mail" includes the recording of any
        multimedia stream, as appropriate.

   language: The language parameter lists the languages spoken by the
        caller or callee. This feature may, for example, be used to have
        a caller automatically be directed to the appropriate attendant
        or customer service representative. Note that this parameter has
        a different functionality than the Accept-Language and Content-
        Language header fields, which describe the acceptable languages
        and languages used in the request and the media description, not
        the actual communications.

   media: The media tag lists the media types supported by the terminal.
        Media types can be the standard Internet media types ("audio",
        "video", "text", "application"), optionally followed by a
        subtype (e.g., "text/html"). In addition, the type
        "application/email" is defined.

   mobility: The mobility parameter indicates if the terminal is fixed
        or mobile. In some locales, this may affect audio quality or

   service: The service tag describes what service is being provided by
        the terminal. The "text" service refers to a device that can
        send or receive unformatted ASCII text, such as a pager or a
        TTY. The "IP" tag indicates a device, such as a personal
        computer, with Internet connectivity. The "fax" tag indicates a
        fax machine. The "PSTN" tag indicates a telephone connected to
        the public switched telephone network. The "ISDN" tag indicates
        a telephone connected to the Integrated Services Digital

        The service tags were chosen to maximize the orthogonality
        of the mobility and service parameters.

   In addition, the Contact header field may contain the description-
   param and priority-param parameters.  The description parameter
   further describes, as text, the terminal.  The user agent MAY present
   this text when it is contained in a Contact header field in a 3xx
   response. The description parameter MUST NOT be used in the matching
   operation described above. The priority parameter indicates the
   minimum priority level this terminal is to be used for. It can be
   used for automatically restricting the choice of terminals available
   to the user, as described in the procedure above.

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   priority-param     =  "priority" "=" <"> priority-tag <">
   description-param  =  "description" "=" quoted-string
   priority-tag       =  "emergency" | "urgent" | "normal" | "non-urgent"

   The first example below describes a SIP terminal whose owner speaks
   English, Spanish and German. The terminal is capable of sending and
   receiving audio and video and can participate in a chat session.
   However, the owner only wants callers to use the terminal if the call
   is of priority "urgent" or higher. This Contact header would normally
   be included in a REGISTER message.

   Contact: Carol <> ;language="en,es,de"

6.2 Accept-Contact

   The syntax for the Accept-Contact header is defined below:

   Accept-Contact   =  "Accept-Contact" ":" 1# rule
   rule             =  ( name-addr | addr-spec )
                       [ *( ";" (cp-params | extension-param | q-param) ) ]
   q-param          =  "q" "=" qvalue
   extension-param  =  extension-name "=" extension-value
   extension-name   =  token
   extension-value  =  token | quoted-string

   The header field specifies contact addresses which are acceptable to
   the caller.

   In the following example, the caller would prefer not to talk to She has a slight preference for fixed as opposed to
   mobile phones.

   Accept-Contact: ;q=0,
     ;media="!video" ;q=0.1,
     ;mobility="fixed"  ;q=0.6,
     ;mobility="!fixed" ;q=0.4

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6.3 Reject-Contact

   The Reject-Contact header field specifies a list of URIs that the
   caller does not wish to communicate with. The BNF for the header is:

   Reject-Contact  =  "Reject-Contact" ":"
                      1# ( ( name-addr | addr-spec )
                      [ *( ";" new-params ) ] )

6.4 Request-Disposition

   The Request-Disposition header field specifies caller preferences for
   how a proxy or user agent server should process a request. Its value
   is a list of tokens, each of which specifies a particular feature.

   When the caller specifies a feature, the server SHOULD treat it as a
   hint, not as a requirement and MAY ignore the feature request.

   The header field has the following syntax:

   Request-Disposition  =  "Request-Disposition" ":"
                           1# (proxy-feature | cancel-feature |
                           fork-feature | recurse-feature |
                           parallel-feature | queue-feature)
   proxy-feature        =  "proxy" | "redirect"
   cancel-feature       =  "cancel" | "no-cancel"
   fork-feature         =  "fork" | "no-fork"
   recurse-feature      =  "recurse" | "no-recurse"
   parallel-feature     =  "parallel" | "sequential"
   queue-feature        =  "queue" | "no-queue"

   proxy-feature: This feature indicates whether the caller would like
        each server to proxy or redirect.

   cancel-feature: This feature 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, or whether this
        function should be left to the caller.

   fork-feature: This feature 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. The feature is ignored if "redirect" has
        been requested.

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   recurse-feature: This feature indicates whether a proxy server
        receiving a 300-class 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 feature is
        ignored if "redirect" has been requested.

   parallel-feature: For a forking proxy server, this feature 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-200 or non-600 final response for the previous
        one. The feature is ignored if "redirect" has been requested.

   queue-feature: 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
        pending call be terminated by a SIP CANCEL or BYE request.

   The normal Proxy-Require/Require/Unsupported mechanism is used to
   indicate to the caller and/or downstream proxies that a particular
   service is required to complete the request. Otherwise, the service
   indication is to be taken as a hint.


     Request-Disposition: proxy, recurse, parallel

7 Acknowledgements

   Parameters of the terminal negotiation mechanism in Section 6.1 were
   influenced by Scott Petrack's CMA design.  Jonathan Lennox provided
   helpful comments.

8 Bibliography

   [1] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.

   [2] J. Lennox, J. Rosenberg, and H. Schulzrinne, "Common gateway
   interface for SIP," Internet Draft, Internet Engineering Task Force,
   Nov. 1998.  Work in progress.

   [3] H. Schulzrinne and J. Lennox, "Call processing language
   requirements," Internet Draft, Internet Engineering Task Force, Aug.
   1998.  Work in progress.

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   [4] H. Schulzrinne and J. Rosenberg, "SIP call control services,"
   Internet Draft, Internet Engineering Task Force, Feb. 1998.  Work in

   [5] G. Klyne, "A syntax for describing media feature sets," Internet
   Draft, Internet Engineering Task Force, Dec. 1998.  Work in progress.

   Full Copyright Statement

   Copyright (c) The Internet Society (1999). All Rights Reserved.

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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an

                           Table of Contents

   1          Terminology .........................................    1
   2          Introduction ........................................    1
   3          Design Alternatives .................................    2
   4          Overview ............................................    3
   5          Determining Request Forwarding ......................    4

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   5.1        Matching Rules ......................................    4
   5.2        Contact List Processing .............................    7
   5.3        IANA Registration ...................................    9
   5.4        Use with REGISTER ...................................    9
   6          Header Field Definitions ............................    9
   6.1        Contact, Accept-Contact and Reject-Contact
   Parameters .....................................................    9
   6.2        Accept-Contact ......................................   12
   6.3        Reject-Contact ......................................   13
   6.4        Request-Disposition .................................   13
   7          Acknowledgements ....................................   14
   8          Bibliography ........................................   14

Schulzrinne/Rosenberg                                        [Page 16]