Internet Engineering Task Force                                   SIP WG
Internet Draft                                     Schulzrinne/Rosenberg
draft-ietf-sip-callerprefs-02.txt                Columbia U./dynamicsoft
July 13, 2000
Expires: January 2001


             SIP Caller Preferences and Callee Capabilities

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
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   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.

Abstract

   This document describes a set of extensions to SIP which allow a
   caller to express preferences about request handling in servers.
   These preferences include the ability to select which URIs a call
   gets proxied or redirected to, and to specify certain request
   handling directives in proxies and redirect servers. It does so by
   defining three new request headers, Accept-Contact, Reject-Contact
   and Request-Disposition, which specify the callers preferences. The
   extension also defines new parameters for the Contact header. These
   extra parameters are present in the Contact header in REGISTER
   requests, and are used to associated attributes with particular
   addresses.


1 Introduction

   When a SIP [1] server receives a request, there are a number of



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   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 programmtic
   execution or database access.

   However, the administrator of the server is the not the only entity
   with an interest in call processing. There are at least three parties
   which have an interest: (1) the administrator of the server, (2) the
   callee, and (3) the caller. The directives of the administrator are
   embedded in the policy of the server. The preferences of the callee
   can be expressed most easily through a script written in the call
   processing language (CPL) [2]. However, no mechanism exists to
   incorporate the preferences of the caller. This extension fills that
   gap by specifying mechanisms by which a caller can provide
   preferences on processing of a call. These preferences include the
   ability to select which URIs a call gets proxied or redirected to,
   and to specify certain request handling directives in proxies and
   redirect servers. It does so by defining three new request headers,
   Accept-Contact, Reject-Contact and Request-Disposition, which specify
   the callers preferences. The extension also defines new parameters
   for the Contact header. These extra parameters are present in the
   Contact header in REGISTER requests, and are used to associated
   attributes with particular addresses.

2 Overview of Operation

   This extension defines a set of additional parameters to the Contact
   header. These parameters specify attributes that define the
   characteristics of the UA at the address in the header. For example,
   there is a mobility parameter which indicates whether the UA is fixed
   or mobile. When a UA registers, it places these parameters in the
   Contact headers to characterize the URIs it is registering. This
   allows the proxy to have information about the contact addresses for
   a user.

   The INVITE message, and its response, also contain Contact headers



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   used to route subsequent messaging. This extension allows these
   headers to contain extension parameters to provide additional
   information about the type of user agent being used. For example, by
   including the feature parameter with value "voicemail" 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 an INVITE, it can optionally include new headers
   which request certain handling at a proxy. These preferences fall
   into two categories. The first category, carried in the Request-
   Disposition header, describe desired server behavior. This includes
   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 are carried in both the Accept-
   Contact and Reject-Contact headers. These preferences contain rules
   that describe the set of desired URIs that the caller would like the
   server to proxy or redirect to. These rules are matched against the
   Contact headers sent in a registration (or through some other
   configuration means). If a rule in a Reject-Contact header matches a
   Contact header, that address is not proxied or redirected to. If a
   rule in a Accept-Contact header matches a Contact header, the q
   values in the rule are combined with the q values in the Contact
   header, resulting in a "merged" q value. This merged q value is then
   used by the proxy to determine the ordering of addresses to proxy or
   redirect to.

   Note that this second category of preferences can only be applied at
   a proxy which accesses a registration database.

3 Design Alternatives

   There are a number of alternatives for expressing caller preferences.
   Ideally, caller preferences, callee preferences, and administrator
   prefernces "meet" at each server which makes processing decisions. In
   practicality, a callee cannot install logic at each server in the
   network. It can only do so (using the CPL, for example), at those
   servers with which it has some kind of established trust
   relationship. These servers are those whose main goal is to provide
   services for the callee.

   One might try to place caller logic at these "callee servers" in much
   the same way the callee places logic there - through the CPL or some
   other programmtic directives. However, this is also infeasible. A
   caller cannot apriori install logic in every server for every



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   individual he might call.

   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 algorithmically.

   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 [3]. 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 Terminology

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

5 Header Field Definitions

   Table 5 specifies an extension of Table 5 in RFC 2543 [1] for the
   three new headers defined here.




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                             where enc e-e ACK BYE CAN INV OPT REG
         _________________________________________________________
         Accept-Contact        R    n   h   -   o   o   o   o   -
         Reject-Contact        R    n   h   -   o   o   o   o   -
         Request-Disposition   R    n   h   -   o   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.


5.1 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. These parameters apply to a
   single URI. When used in a Contact header, they specify
   characteristics of that URI. When used in the Accept-Contact or
   Reject-Contact headers, they specify rules to apply for matching
   URIs.



   cp-params  =  class-param | duplex-param |
                 feature-param | language-param | media-param |
                 mobility-param | other-param
   class-param     =  "class" "=" <"> [<!>] 1#class-value <">
   duplex-param    =  "duplex" "=" <"> [<!>] 1#duplex-value <">
   feature-param   =  "feature" "=" <"> [<!>] 1#feature-value <">
   language-param  =  "language" "=" <"> [<!>] 1#language-tag <">
   media-param     =  "media" "=" <"> [<!>] 1#media-value <">
   mobility-param  =  "mobility" "=" <"> [<!>] 1#mobility-value <">

   other-param     =  other-name "=" <"> [<!>] 1#other-value <">
   mobility-value  =  "fixed" | "mobile" | other-value
   class-value     =  "personal" | "business" | other-value
   duplex-value    =  "full" | "half" | "receive-only" |
                      "send-only" | other-value
   media-value     =  ( "*/*" | (type "/" "*") |
                      (type "/" subtype) )
   feature-value   =  "voice-mail" | "attendant" | other-value
   other-name      =  UTF8-TOKEN
   other-value     =  UTF8-TOKEN
   UTF8-TOKEN      =  <any UTF-8 character encoding
                       except separator, CTL, and LWS>



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   The BNF and semantics of the language-tag are defined in Section 3.10
   of RFC 2616 [5]. Note, however, that in their usage here they are
   case sensitive, and MUST appear as all lowercase. Also note that
   there MUST NOT be any linear white space between the tokens and
   quoted strings of the media-value. This is to align with HTTP 1.1
   [5].

   The exclamation mark in the parameter value MUST NOT be included if
   the cp-params are included in a Contact header. Most importantly,
   there MUST NOT be more than one class-value, duplex-value, or
   mobility-value when cp-params is included in a Contact header. These
   parameters refer to attributes which are mutually exclusive. As a
   result, a URI can only have one as a characteristic, whereas a rule
   in the Accept-Contact or Reject-Contact can specify more than one.

   The parameters and their values have the following meanings:

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


        duplex: The duplex parameter lists whether the UA can
             simultaneously send and receive media ("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 UA over a half-duplex UA
             and these over receive-only or send-only UAs.


        features: The feature parameter enumerates additional features
             of the UA. It is assumed that these features are
             orthogonal, and could occur in any combination. "voice-
             mail" means that an automated system exists at this UA,
             which is capable of recording messages. "attendant" means
             that a human operator is available to take messages.


        language: The language parameter lists the languages spoken by
             user or system behind the UA. This parameter 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.



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        media: The media parameter lists the media types supported by
             the UA. In this context, supported means that the media
             type is acceptable as part of the media session established
             by SIP (and usually described by SDP [6]). It does not
             refer to the media types which can be supported within the
             bodies of SIP messages. Media types can be the standard
             Internet media types ("audio", "video", "text",
             "application"), optionally followed by a subtype (e.g.,
             "text/html").


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


   In addition, the Contact header field may contain the description-
   param, methods-param and priority-param parameters.

   The description parameter further describes, as text, the terminal.
   The description parameter MUST NOT be used in the matching operation
   described in Section 6.3.1.

   The priority parameter indicates the minimum priority level this UA
   is to be used for. It can be used for automatically restricting the
   choice of terminals available to the caller. The priority parameter
   is not used in the matching operation described in Section 6.3.1. Its
   application is described in the procedure in Section 6.3.2.

   The methods parameter indicates the methods this UA understands. It
   MUST NOT be used in any request excepting REGISTER. The methods
   parameter is not used in the matching operation described in Section
   6.3.1. Its application is described in the procedure in Section
   6.3.2.


   priority-param     =  "priority" "=" <"> priority-value <">
   description-param  =  "description" "=" quoted-string
   methods-param      =  "methods" "=" <"> 1#methods-value <">
   methods-value      =  ( "INVITE" | "OPTIONS" | "BYE" | "REGISTER"
                         | token)


   Note that priority-value is defined in section 6.25 of [1].


        There is some overlap between the indication of receiver
        capabilities in the session description message body and



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        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.

5.2 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 | q-param | scheme-param) ) ]
   q-param         =  "q" "=" qvalue
   scheme-param    =  "scheme" "=" <"> [<!>] 1#scheme <">


   The header field specifies contact addresses which are acceptable to
   the caller. If a "*" is specified instead of a name-addr or addr-
   spec, it means the UAC doesn't care about the URI of the user
   eventually reached. Only the parameters of the Contact header are
   important. If the name-addr or addr-spec is present, and the userinfo
   field of the SIP URL is not present, it means the UAC doesn't care
   about the username of the user eventually reached. If the host
   portion of the SIP URL is a hostname, and has the value "x", it means
   the UAC doesn't care about the host portion of the URI eventually
   reached. If the name-addr or addr-spec is present, and contains URI
   parameters, if means the UAC wishes to be connected to an address
   that has been registered with these parameters.


        We use "x" as the wildcard domain because of the URI
        formatting constraints. The domain must be present in a SIP
        URL, and cannot be the "*" character. The "x" character is
        allowed and looks kind of similar.

   The scheme parameter describes the set of URI schemes which the
   caller is willing to accept redirects to or communicate with. The BNF
   for scheme is given in RFC 2396 [7], and can be any valid URI scheme.

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






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   Accept-Contact: sip:sales@acme.com ;q=0,
     *;media="!video" ;q=0.1,
     *;mobility="fixed"  ;q=0.6,
     *;mobility="!fixed" ;q=0.4



   In the next example, the caller would prefer to speak to someone from
   sales.org that supports video:



   Accept-Contact: sip:sales.org;media="video"



5.3 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 | "*")
                      [ *( ";" cp-params | scheme-param ) ] )


   If name-addr or addr-spec is not present (the "*" is present), it
   means the UAC does not care about the particular user or domain the
   request is routed to. The cp-params are used to filter out contact
   addresses based on their parameters alone. This process is described
   in Section 6.3.1. If either name-addr or addr-spec is present, and
   the URI does not contain a userinfo field, it means the UAC does not
   have a preference regarding the user name and/or password of the UA
   eventually reached. If domain of the URI is equal to "x", it means
   the UAC does not have a preference regarding the domain of the UA
   eventually reached.

   The scheme parameter describes the set of URI schemes which the
   caller is not willing to accept redirects to or communicate with. The
   BNF for scheme is given in RFC 2396 [7], and can be any valid URI
   scheme.

5.4 Contact Header

   The cp-params parameter is allowed as an extension attribute to the
   Contact header, along with the priority-param, methods-param and
   description-param. This effectively means that the BNF for



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   extension-attribute, defined in Section 6.13 of RFC 2543 [1] can be
   redefined as:


   extension-attribute  =  (cp-params |
                           priority-param | methods-param |
                           description-param |
                           (extension-name [ "=" extension-value]))


   The 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 <sip:carol@example.com> ;language="en,es,de"
     ;media="audio,video,application/chat"
     ;duplex="full"
     ;priority="urgent"



   As another example, an INVITE message is sent with a Contact header
   that includes some of the parameters defined here:



   INVITE sip:user@example.com SIP/2.0
   Via: SIP/2.0/UDP host.example.com
   From: sip:caller@university.edu
   To: sip:user@example.com
   Call-ID: 9sdnasdbasd@1.2.3.4
   CSeq: 3 INVITE
   Contact: Joe Caller <sip:caller@university.edu>;mobility="mobile"



   In this case, Joe is indicating he is calling from a mobile host.

5.5 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 feature.



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   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 |
                           extension-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"
   extension-feature    =  token


        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 (generally the one with the
             highest q value). The feature is ignored if "redirect" has
             been requested.


        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



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             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.


   Example:


     Request-Disposition: proxy, recurse, parallel



   The Request-Disposition header also allows an extension-feature,
   which can be used to specify additional dispositions. Section 8
   specifies procedures for registration of new extension-features. Note
   that features requested in Request-Disposition MUST always be
   optional for the proxy to apply. In other words, if the client has a
   feature it insists on using, the Request-Disposition header MUST NOT
   be used for that purpose.

6 Protocol Semantics

6.1 UAS Behavior

   User agent servers MAY include cp-params, priority-param, methods-
   param or description-param parameters as part of each Contact
   addresses they register. These parameters can be set through
   configuration, user input, or any means the implementor seeks to use.
   They SHOULD reflect actual characteristics of the URLs being
   registered.

   Furthermore, the REGISTER request MAY contain a Require header with
   the option tag "pref" if the client wants to be sure that the
   registration server honors caller preferences.

   When a UAS receives a request with the Accept-Contact, Reject-Contact
   and Request-Disposition, it MAY ignore these headers so long as it
   does not redirect the request. If the request is redirected, the UAS
   SHOULD follow the rules described in Section 6.3 for a proxy/redirect



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   server.

6.2 UAC Behavior

   A UAC wishing to express preferences for a request includes the
   Accept-Contact, Reject-Contact, or Request-Disposition headers in the
   request, depending on its particular preferences. No additional
   behavior is required after the request is sent.

   If the client wants to be sure that servers understand the headers
   described in this specification, it MAY include a Proxy-Require and
   Require option tag of "pref". However, this is NOT RECOMMENDED, as it
   leads to interoperability problems. In any case, client preferences
   can only be considered as preferences - there is no guarantee that
   the requested service or capability is executed. As such, inclusion
   of Proxy-Require and Require does not mean the preferences will be
   executed.

6.3 Proxy Behavior

   The behavior described here assumes a server (proxy or redirect) has
   received a valid request with either the Accept-Contact or Reject-
   Contact headers, and that this proxy has a list of Contact headers
   obtained from looking up the Request-URI in the location service. The
   location service may have obtained this data through registrations,
   as described in Section 6.1, but other means may exist.

   The processing depends heavily on a rule matching operation. This
   operation takes a rule (defined as a single element from the comma
   separate list of elements in the Accept-Contact or Reject-Contact
   headers), and matches it against the contact list obtained from the
   location service.

6.3.1 Rule Matching Procedures

   The contact list is composed of a set of contact entries. Each
   contact entry consists of a URI along with a set of parameters. A
   rule, like a contact entry, consists of a URI (or the "*" character),
   and a set of parameters. If the rule does not contain a URI (just the
   "*" character), the rule matches the contact entry if and only if the
   parameters in the rule and the parameters in the contact entry match.
   If the rule contains a URI, both the URI and parameters must match
   for the rule to match the contact entry.

   The URI in the rule and the URI in the contact entry match depending
   on the scheme. For non-SIP URIs, matching is based on the URI
   equivalency rules for that scheme. For SIP URLs, the userinfo, host,
   and URI parameters must match, where matching is defined as follows.



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   Note that these matching rules are not the same as the general URI
   matching rules in SIP [1].

   If the rule contains a userinfo field, that userinfo field must match
   the userinfo field in the URI in the contact entry. Matching is based
   on case sensitive string comparison. If the rule contains a userinfo
   field, but the URI in the contact address does not, the userinfo in
   the rule does not match the userinfo in the contact entry. If the
   rule does not contain a userinfo field, the userinfo component
   matches.

   If the rule contains a host not equal to "x", the host in the URI of
   the rule must match the host of the URI in the contact entry.
   Matching is based on case insensitive string comparison. If the rule
   has a host equal to "x", it matches any value of the host in the URI
   in the contact entry.

   If the URI in the rule contains URI parameters (port is considered a
   URI parameter for purposes of this discussion), each parameter in the
   URI in the rule must match a parameter in the URI in the contact
   entry. Matching is based on case sensitive string comparison of both
   parameter names and values. Note, however, if the URI in the rule
   contains a parameter with a default value, this matches a contact
   entry with a URI that does not contain this parameter. If the URI in
   the rule contains a URI parameter that is not the default value, this
   does not match a contact entry whose URI does not contain this
   parameter. If there are no URI parameters in the rule, this is
   considered a match to any set of URI parameters in the contact entry.

   To determine if the parameters in the rule match the parameters in
   the contact entry, the following process is followed.

   The parameters match if and only if each parameter in the rule
   matches the contact entry. A single parameter in the rule matches the
   contact entry if that parameter is present in the contact entry, and
   their values match. If a parameter exists in the rule, and there is
   no parameter of the same name in the contact entry, whether or not
   this is a match is context dependent. If the rule is present in the
   Accept-Contact header, it is considered a match. If the rule is
   present in the Reject-Contact header, it is not considered a match.

   Parameter names are matched by case-sensitive comparison. Parameter
   values are matched by set-comparisons. Parameter values in quoted
   strings are interpreted as sets, with elements separated by commas.
   Two elements in the set match if they are equal based on a case
   sensitive string comparison. 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



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   the contact entry is empty. Otherwise, the rule matches if the
   intersection of the rule set with the contact set is non-empty. Note
   that this process does not apply to the priority-param, methods-
   param, description-param or scheme-param.


        Case sensitive comparisons are necessary because of
        internationalization. Case insensitive matching in UTF-8
        depends on regional rules, and overly complicates the
        procedure.

   If there is a scheme-param in the rule, and the quoted-string
   parameter value in the rule starts with an exclamation mark, the
   scheme of the URI in the contact entry must not match any of the
   schemes listed in the rule. If the quoted-string parameter value in
   the scheme-param doesn't start with an exclamation mark, the scheme
   of the URI in the contact entry must match one of the schemes listed
   in the rule. Matching of schemes is done by case insensitive string
   comparison [7].

   The C code below describes the matching procedure between a rule and
   a contact entry. The function intersect() takes two arrays of
   strings, and returns true if there are any values common to both
   arrays, false otherwise. The function getparameterbyname() takes a
   rule and a string defining a parameter name. It returns a parameter
   from the rule with that name. However, if the parameter name is
   "scheme", the function returns a "scheme" parameter. This parameter
   structure has the name set to "scheme", the exclamation set to false,
   and the values array with a single value, containing the name of the
   scheme in the URI of the rule.




   /* context values */
   #define ACCEPT_CONTACT 0
   #define REJECT_CONTACT 1

   typedef int boolean;

   typedef struct uri_parameters_s {
     char *name;
     char *value;
   } uri_parameters_t;

   typedef struct uri_s {
     char *scheme;
     char *userinfo;



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     char *host;
     uri_parameters_t **params;
   } uri_t;

   typedef struct parameter_s {
     char *name;           /* parameter name */
     boolean exclamation;   /* whether ! was present in value */
     char **values;        /* list of elements in the value */
   } parameter_t;

   typedef struct rule_s {
     uri_t *URI;            /* URI */
     parameter_t **para;   /* list of parameters */
   } rule_t;


   /* little helper function to look up a parameter by its
      name within an entry */

   parameter_t *getparameterbyname(rule_t *r, char *name) {

     int i;
     parameter_t *p;

     if(strcmp(name,"scheme") == 0) {

       p = calloc(1, sizeof(parameter_t));
       p->name = "scheme";
       p->values = calloc(2, sizeof(char *));
       p->values[0] = malloc(sizeof(char) * (strlen(r->URI->scheme) + 1));
       strcpy(p->values[0], r->URI->scheme);

       return(p);
     }

     for(i=0; r->para[i] != NULL; i++) {

       if(strcmp(r->para[i]->name, name) == 0)
          return(r->para[i]);
     }

     return(NULL);
   }

   /* check if two sets of strings share at least one
      common value */

   boolean intersect(char *a[], char *b[]) {



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     int i,j;

     for(i = 0; a[i] != NULL; i++) {
       for(j = 0; b[j] != NULL; j++) {

         if(strcmp(a[i], b[j]) == 0)
           return(TRUE);
       }
     }

     return(FALSE);
   }

   /* returns the default value of a URI parameter */
   char *defaultvalue(char *name) {

     if(strcmp(name, "transport") == 0)
       return("udp");

     return("some-value-which-matches-no parameter");
   }

   boolean matchuriparameters(uri_parameters_t **r, uri_parameters_t **e) {
     int i,j;
     boolean match;

     /* for each rule */
     for(i=0; r[i] != NULL; i++) {

       match = FALSE;
       for(j=0; e[j] != NULL; j++) {

         /* found the matching URI parameter in the entry */
         if(strcmp(r[i]->name, e[j]->name) == 0) {

           /* if they're not equal, return FALSE. Otherwise, set match
              to TRUE, indicating that we found a matching parameter*/
           if(strcmp(r[i]->value, e[j]->value) != 0)
             return(FALSE);
           else
             match = TRUE;
         }
       }

       /* parameter in rule not in entry */
       if(match == FALSE) {

         /* check if rule contains default value */



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         if(strcmp(defaultvalue(r[i]->name), r[i]->value) != 0)
           return(FALSE);

       }
     }

     return(TRUE);
   }



   boolean MATCH(rule_t *r, rule_t *e, int context) {
     boolean match;
     int i;
     parameter_t *p, *q;

     match = TRUE;

     /* We represent a rule with a * as the match for URIs, as
        a URI with a scheme of * */

     if (strcmp(r->URI->scheme, "*") != 0) {

       /* the schemes must match */
       if (strcasecmp(r->URI->scheme, e->URI->scheme) == 0) {

        /* for sip, perform our SIP rules */
        if (strcasecmp(r->URI->scheme, "sip") == 0) {

          /* check for match of user and host */
          match=(((strcasecmp(r->URI->host, "x") == 0) ||
                  (strcasecmp(r->URI->host, e->URI->host) == 0)) &&
                 ((r->URI->userinfo == NULL) ||
                  ((e->URI->userinfo != NULL) &&
                   (strcmp(r->URI->userinfo, e->URI->userinfo) == 0))));

          if(match == FALSE) return(FALSE);

          /* match URI parameters */
          match = matchuriparameters(r->URI->params, e->URI->params);
          if(match == FALSE) return(FALSE);

        } else {
          /* match = scheme-appropriate comparison; */
          if(match == FALSE) return(FALSE);
        }
      } else {
        /* schemes don't match */



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        return FALSE;
      }
     }


     /* compare parameters */
     for(i = 0; r->para[i] != NULL; i++) {

       p = r->para[i];

       /* is this parameter defined in the contact entry */
       if ((q =getparameterbyname(e, p->name)) != NULL) {

         /* is this an empty set match */
         if (p->exclamation == TRUE) {

           if (intersect(p->values, q->values) == TRUE) {
             return FALSE;
           }
         } else {

           /* not an empty set case */
           if (intersect(p->values, q->values) == FALSE) {
             return FALSE;
           }
         }
       } else {
         /* this parameter is not present in the entry.
            whether its a match or not is dependent on
            context */

         if(context == REJECT_CONTACT)
           return(FALSE);
       }
     }
     return TRUE;
   }



   For example, the rule:


   sip:example.com;language="!en,de"


   matches the contact entry:




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   sip:joe@example.com;language="es,nl"


   but not any of:


   sip:joe@example.com;language="en"
   sip:bob@example.com;language="de,en"
   sip:alice@example.com;language="en,es,fi"



   As another example, the rule


   *;duplex="full,half"


   matches the contact entry


   sip:user@host;duplex="full"


   but not


   sip:user@host;duplex="send-only"



   The rule


   *;scheme="http"


   matches the contact entry


   http://www.example.com



   A server need 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.




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6.3.2 Contact List Processing

   Given the matching rule above, the formal processing rules at the
   server proceed as follows. The server begins with a contact list for
   the callee, and a set of rules in the Accept-Contact and Reject-
   Contact headers.

   The server first removes any contact entry from the contact list that
   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 from the contact list whose
   priority value is higher than that of the request.

   A contact entry may contain a methods parameter. This means that a
   request should not be proxied or redirected to that contact entry
   unless the method of the request is listed among those in the contact
   entry. The server removes any contact entry from the contact list
   whose method list does not include the method of the SIP request.

   Each rule in the Accept-Contact header field is then processed. If
   the rule matches a contact entry (according to the matching rule in
   section 6.3.1, 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
   specific algorithm for updating the q value. 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). The only requirement for the updating
   process is that if a contact entry has a q value of q1, and the q
   values among the matching rules are q2,q3,..qn, the merged q value,
   qm, must satisfy:



   MIN(q1,q2,q3,..qn) <= qm <= MAX(q1,q2,q3,..,qn)





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   For those contact entries which did not match any rule in the
   Accept-Contact header, their final q value is set to zero.


        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. 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 fails. If the
   server elects to do a forking 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 headers returned in the redirect
   responses. Any contact entries 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
   contact list, including those with a zero q value.

   If the server is executing any other type of policy, as a general
   guideline, it SHOULD prefer contact entries with higher q values than
   those with lower q values.

6.3.3 Request-Disposition Processing

   If the request contains a Request-Disposition header, the server
   SHOULD execute the behaviors described by the tokens, unless it has
   local policy configured to direct it otherwise.

7 Interactions with CPL

   When the called party has a Call Processing Language (CPL) [8] script
   present, feature interactions are introduced. CPL addresses this by



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   allowing the CPL script to control whether caller preferences are
   applied to the location list or not. CPL also allows the called party
   to discard certain rules from the caller preferences before their
   application. For more information, see [8].

8 IANA Registration Procedures

8.1 cp-params

   New cp-params 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
             UTF8-TOKEN. For each value, a description of the value must
             be provided. The registration MUST indicate whether the
             parameter values are mutually exclusive or not; that is,
             whether only one, or more than one, can appear in the
             Contact header.

8.2 Request-Disposition

   New request disposition values can be defined at any time and
   registered with IANA. Request dispositions MUST always be optional
   for a proxy to grant. When registering new values, the following
   information MUST be provided:

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


        Behavior: The requested behavior when the attribute appears in
             the request disposition.


        Interactions with other Values: Any interactions with other
             values specified or here registered with IANA.




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9 Changes since -01

        o Specified that parameters can be included in Contact in INVITE
          and its 200 OK response as well.

        o Allow Request-Disposition in REGISTER.

        o Added IANA registration for request-disposition values.

        o Added methods to contact parameters.

        o Alterned bnf so that cp-params can be applied to Contact,
          Accept-Contact, and Reject-Contact.

        o Updated code, compiled and verified

        o Removed ring-feature

10 Open Issues

        o Is IANA registrations for request-disposition values a good
          idea?

11 Security Considerations

   The presence of caller preferences in a request has a significant way
   in which the request is handled at a server. As a result, is is
   especially important that requests with caller preferences be
   authenticated. The same holds true for registrations with contact
   parameters.

   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.

12 Acknowledgements

   Parameters of the terminal negotiation mechanism in Section 5.1 were
   influenced by Scott Petrack's CMA design.  Jonathan Lennox and John
   Hearty provided helpful comments.

13 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




14 Bibliography

   [1] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
   session initiation protocol," Request for Comments 2543, Internet
   Engineering Task Force, Mar. 1999.

   [2] J. Lennox and H. Schulzrinne, "Call processing language framework
   and requirements," Request for Comments 2824, Internet Engineering
   Task Force, May 2000.

   [3] G. Klyne, "A syntax for describing media feature sets," Request
   for Comments 2533, Internet Engineering Task Force, Mar. 1999.

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

   [5] R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P.
   Leach, and T. Berners-Lee, "Hypertext transfer protocol -- HTTP/1.1,"
   Request for Comments 2616, Internet Engineering Task Force, June
   1999.

   [6] M. Handley and V. Jacobson, "SDP: session description protocol,"
   Request for Comments 2327, Internet Engineering Task Force, Apr.
   1998.

   [7] T. Berners-Lee, R. Fielding, and L. Masinter, "Uniform resource
   identifiers (URI): generic syntax," Request for Comments 2396,
   Internet Engineering Task Force, Aug. 1998.

   [8] J. Lennox and H. Schulzrinne, "CPL: a language for user control



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   of internet telephony services," Internet Draft, Internet Engineering
   Task Force, Mar.  1999.  Work in progress.





                           Table of Contents



   1          Introduction ........................................    1
   2          Overview of Operation ...............................    2
   3          Design Alternatives .................................    3
   4          Terminology .........................................    4
   5          Header Field Definitions ............................    4
   5.1        Contact, Accept-Contact and Reject-Contact
   Parameters .....................................................    5
   5.2        Accept-Contact ......................................    8
   5.3        Reject-Contact ......................................    9
   5.4        Contact Header ......................................    9
   5.5        Request-Disposition .................................   10
   6          Protocol Semantics ..................................   12
   6.1        UAS Behavior ........................................   12
   6.2        UAC Behavior ........................................   13
   6.3        Proxy Behavior ......................................   13
   6.3.1      Rule Matching Procedures ............................   13
   6.3.2      Contact List Processing .............................   21
   6.3.3      Request-Disposition Processing ......................   22
   7          Interactions with CPL ...............................   22
   8          IANA Registration Procedures ........................   23
   8.1        cp-params ...........................................   23
   8.2        Request-Disposition .................................   23
   9          Changes since -01 ...................................   24
   10         Open Issues .........................................   24
   11         Security Considerations .............................   24
   12         Acknowledgements ....................................   24
   13         Author's Addresses ..................................   24
   14         Bibliography ........................................   25












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