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Versions: 00 01 02 03 04 05 06 RFC 3515

Network Working Group                                          R. Sparks
Internet-Draft                                               dynamicsoft
Expires: January 15, 2003                                  July 17, 2002


                          The SIP Refer Method
                        draft-ietf-sip-refer-06

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 groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   The list of current Internet-Drafts can be accessed at http://
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on January 15, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document defines the REFER method.  This SIP extension requests
   that the recipient REFER to a resource provided in the request.  It
   provides a mechanism allowing the party sending the REFER to be
   notified of the outcome of the referenced request.  This can be used
   to enable many applications, including call transfer.

   In addition to the REFER method, this document defines the the refer
   event package and the Refer-To request header.







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Table of Contents

   1.    Overview . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.    The REFER Method . . . . . . . . . . . . . . . . . . . . . .  3
   2.1   The Refer-To Header Field  . . . . . . . . . . . . . . . . .  3
   2.2   Header Field Support for the REFER Method  . . . . . . . . .  4
   2.3   Message Body Inclusion . . . . . . . . . . . . . . . . . . .  6
   2.4   Behavior of SIP User Agents  . . . . . . . . . . . . . . . .  6
   2.4.1 Forming a REFER request  . . . . . . . . . . . . . . . . . .  6
   2.4.2 Processing a REFER request . . . . . . . . . . . . . . . . .  6
   2.4.3 Accessing the Referred-to Resource . . . . . . . . . . . . .  7
   2.4.4 Using SIP Events to Report the Results of the Reference  . .  7
   2.4.5 The Body of the NOTIFY . . . . . . . . . . . . . . . . . . .  8
   2.4.6 Multiple REFER Requests in a Dialog  . . . . . . . . . . . .  9
   2.4.7 Using the Subscription-State Header Field with Event Refer .  9
   2.5   Behavior of SIP Registrars/Redirect Servers  . . . . . . . . 10
   2.6   Behavior of SIP Proxies  . . . . . . . . . . . . . . . . . . 10
   3.    Package Details: Event refer . . . . . . . . . . . . . . . . 10
   3.1   Event Package Name . . . . . . . . . . . . . . . . . . . . . 10
   3.2   Event Package Parameters . . . . . . . . . . . . . . . . . . 10
   3.3   SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . . 10
   3.4   Subscription Duration  . . . . . . . . . . . . . . . . . . . 10
   3.5   NOTIFY Bodies  . . . . . . . . . . . . . . . . . . . . . . . 11
   3.6   Notifier processing of SUBSCRIBE requests  . . . . . . . . . 11
   3.7   Notifier Generation of NOTIFY Requests . . . . . . . . . . . 11
   3.8   Subscriber Processing of NOTIFY Requests . . . . . . . . . . 11
   3.9   Handling of Forked Requests  . . . . . . . . . . . . . . . . 11
   3.10  Rate of Notifications  . . . . . . . . . . . . . . . . . . . 11
   3.11  State Agents . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.    Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.1   Prototypical REFER callflow  . . . . . . . . . . . . . . . . 12
   4.2   Multiple REFERs in a dialog  . . . . . . . . . . . . . . . . 14
   5.    Security Considerations  . . . . . . . . . . . . . . . . . . 17
   5.1   Circumventing Privacy  . . . . . . . . . . . . . . . . . . . 17
   5.2   Circumventing Security . . . . . . . . . . . . . . . . . . . 17
   5.3   Limiting the Breach  . . . . . . . . . . . . . . . . . . . . 18
   6.    Historic Material  . . . . . . . . . . . . . . . . . . . . . 18
   7.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 18
   8.    Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 19
         Normative References . . . . . . . . . . . . . . . . . . . . 19
         Author's Address . . . . . . . . . . . . . . . . . . . . . . 20
         Full Copyright Statement . . . . . . . . . . . . . . . . . . 21









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1. Overview

   This document defines the REFER method.  This SIP [1] extension
   requests that the recipient REFER to a resource provided in the
   request.  This can be used to enable many applications, including
   Call Transfer.

2. The REFER Method

   REFER is a SIP method as defined by RFC3261 [1].  The REFER method
   indicates that the recipient (identified by the Request-URI) should
   contact a third party using the contact information provided in the
   request.

   Unless stated otherwise, the protocol for emitting and responding to
   a REFER request are identical to those for a BYE request in [1].  The
   behavior of SIP entities not implementing the REFER (or any other
   unknown) method is explicitly defined in [1].

   A REFER request implicitly establishes a subscription to the refer
   event.  Event subscriptions are defined in [2].

   A REFER request MAY be placed outside the scope of a dialog created
   with an INVITE.  REFER creates a dialog, and MAY be Record-Routed,
   hence MUST contain a single Contact header field value.  REFERs
   occurring inside an existing dialog MUST follow the Route/Record-
   Route logic of that dialog.

2.1 The Refer-To Header Field

   Refer-To is a request header field (request-header) as defined by
   [1].  It only appears in a REFER request.  It provides a URL to
   reference.

      Refer-To = ("Refer-To" / "r") HCOLON ( name-addr / addr-spec ) *
      (SEMI generic-param)

   The following should be interpreted as if it appeared in Table 3 of
   RFC 3261.

       Header field              where       proxy ACK BYE CAN INV OPT REG
       ___________________________________________________________________
       Refer-To                    R                -   -   -   -   -   -


   The Refer-To header field MAY be encrypted as part of end-to-end
   encryption.




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        The Contact header field is an important part of the Route/Record-Route
        mechanism and is not available to be used to indicate the target of the
        reference.

   Examples

         Refer-To: sip:alice@atlanta.com

         Refer-To: <sip:bob@biloxi.com?Accept-Contact=sip:bobsdesk.
                   biloxi.com&Call-ID%3D55432%40alicepc.atlanta.com>

         Refer-To: <sip:dave@denver.com?Replaces=12345%40192.168.118.3%3B
                   to-tag%3D12345%3Bfrom-tag%3D5FFE-3994>

         Refer-To: <sip:carol@cleveland.com;method=SUBSCRIBE>

         Refer-To: http://www.ietf.org

   Long headers field values are line-wrapped here for clarity only.

2.2 Header Field Support for the REFER Method

   This table adds a column to tables 2 and 3 in [1], describing header
   field presence in a REFER method.  See [1] for a key for the symbols
   used.  A row for the Refer-To request-header should be inferred,
   mandatory for REFER.  Refer-To is not applicable for any other
   methods.  The proxy column in [1] applies to the REFER method
   unmodified.

            Header                    Where   REFER
            Accept                      R       o
            Accept                     2xx      -
            Accept                     415      c
            Accept-Encoding             R       o
            Accept-Encoding            2xx      -
            Accept-Encoding            415      c
            Accept-Language             R       o
            Accept-Language            2xx      -
            Accept-Language            415      c
            Alert-Info                          -
            Allow                       Rr      o
            Allow                      405      m
            Authentication-Info        2xx      o
            Authorization               R       o
            Call-ID                     c       m
            Call-Info                           -
            Contact                     R       m
            Contact                    1xx      -



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            Contact                    2xx      m
            Contact                   3-6xx     o
            Content-Disposition                 o
            Content-Encoding                    o
            Content-Language                    o
            Content-Length                      o
            Content-Type                        *
            CSeq                        c       m
            Date                                o
            Error-Info                3-6xx     o
            Expires                     R       o
            From                        c       m
            In-Reply-To                         -
            Max-Forwards                R       m
            Min-Expires                         -
            MIME-Version                        o
            Organization                        o
            Priority                    R       -
            Proxy-Authenticate         401      o
            Proxy-Authenticate         407      m
            Proxy-Authorization         R       o
            Proxy-Require               R       o
            Record-Route                R       o
            Record-Route           2xx,18x      o
            Reply-To                            -
            Require                             c
            Retry-After        404,413,480,486  o
            Retry-After              500,503    o
            Retry-After              600,603    o
            Route                       R       c
            Server                      r       o
            Subject                     R       -
            Supported                 R,2xx     o
            Timestamp                           o
            To                         c(1)     m
            Unsupported                420      o
            User-Agent                          o
            Via                        c(2)     m
            Warning                     r       o
            WWW-Authenticate           401      m
            WWW-Authenticate           407      o

                 Table 1: Header Field Support








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2.3 Message Body Inclusion

   A REFER method MAY contain a body.  This specification assigns no
   meaning to such a body.  A receiving agent may choose to process the
   body according to its Content-Type.

2.4 Behavior of SIP User Agents

2.4.1 Forming a REFER request

   REFER is a SIP request and is constructed as defined in [1].  A REFER
   request MUST contain exactly one Refer-To header field value.

2.4.2 Processing a REFER request

   A UA accepting a well-formed REFER request SHOULD request approval
   from the user to proceed (this request could be satisfied with an
   interactive query or through accessing configured policy).  If
   approval is granted, the UA MUST contact the resource identified by
   the URI in the Refer-To header field value as discussed in Section
   2.4.3.

   If the approval sought above for a well-formed REFER request is
   immediately denied, the UA MAY decline the request.

   An agent responding to a REFER method MUST return a 400 (Bad Request)
   if the request contained zero or more than one Refer-To header field
   values.

   An agent (including proxies generating local responses) MAY return a
   100 (Trying) or any appropriate 4xx-6xx class response as prescribed
   by [1].

   Care should be taken when implementing the logic that determines
   whether or not to accept the REFER request.  A UA not capable of
   accessing non-SIP URIs SHOULD NOT accept REFER requests to them.

   If no final response has been generated according to the rules above,
   the UA MUST return a 202 Accepted response before the REFER
   transaction expires.

   If a REFER request is accepted (that is, a 2xx class response is
   returned), the recipient MUST create a subscription and send
   notifications of the status of the refer as described in Section
   2.4.4.






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2.4.3 Accessing the Referred-to Resource

   The resource identified by the Refer-To URI is contacted using the
   normal mechanisms for that URI type.  For example, if the URI is a
   SIP URI indicating INVITE (using a method=INVITE URI parameter for
   example), the UA would issue a new INVITE using all of the normal
   rules for sending an INVITE defined in [1].

2.4.4 Using SIP Events to Report the Results of the Reference

   The NOTIFY mechanism defined in [2] MUST be used to inform the agent
   sending the REFER of the status of the reference.  The dialog
   identifiers (To, From, and Call-ID) of each NOTIFY must match those
   of the REFER as they would if the REFER had been a SUBSCRIBE request.

   Each NOTIFY MUST contain an Event header field with a value of refer
   and possibly an id parameter (see Section 2.4.6).

   Each NOTIFY MUST contain a body of type "message/sipfrag" [3].

   The creation of a subscription as defined by [1] always results in an
   immediate NOTIFY.  Analogous to the case for SUBSCRIBE described in
   that document, the agent that issued the REFER MUST be prepared to
   receive a NOTIFY before the REFER transaction completes.

   The implicit subscription created by a REFER is the same as a
   subscription created with a SUBSCRIBE request.  The agent issuing the
   REFER can terminate this subscription prematurely by unsubscribing
   using the mechanisms described in [2].  Terminating a subscription,
   either by explicitly unsubscribing or rejecting NOTIFY, is not an
   indication that the referenced request should be withdrawn or
   abandoned.  In particular, an agent acting on a REFER request SHOULD
   NOT issue a CANCEL to any referenced SIP requests because the agent
   sending the REFER terminated its subscription to the refer event
   before the referenced request completes.

   The agent issuing the REFER may extend its subscription using the
   subscription refresh mechanisms described in [2].

   REFER is the only mechanism that can create a subscription to event
   refer.  If a SUBSCRIBE request for event refer is received for a
   subscription that does not already exist, it MUST be rejected with a
   403.

   Notice that unlike SUBSCRIBE, the REFER transaction does not contain
   a duration for the subscription in either the request or the
   response.  The lifetime of the state being subscribed to is
   determined by the progress of the referenced request.  The duration



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   of the subscription is chosen by the agent accepting the REFER and is
   communicated to the agent sending the REFER in the subscription's
   initial NOTIFY (using the Subscription-State expires header
   parameter).  Note that agents accepting REFER and not wishing to hold
   subscription state can terminate the subscription with this initial
   NOTIFY.

2.4.5 The Body of the NOTIFY

   Each NOTIFY MUST contain a body of type "message/sipfrag" [3].  The
   body of a NOTIFY MUST begin with a SIP Response Status-Line as
   defined in [1].  The response class in this status line indicates the
   status of the referred action.  The body MAY contain other SIP header
   fields to provide information about the outcome of the referenced
   action.  This body provides a complete statement of the status of the
   referred action.  The refer event package does not support state
   deltas.

   If a NOTIFY is generated when the subscription state is pending, its
   body should consist only of a status line containing a response code
   of 100.

   A minimal, but complete, implementation can respond with a single
   NOTIFY containing either the body:


      SIP/2.0 100 Trying


   if the subscription is pending, the body:


      SIP/2.0 200 OK


   if the reference was successful, the body:


      SIP/2.0 503 Service Unavailable


   if the reference failed, or the body:


      SIP/2.0 603 Declined


   if the REFER request was accepted before approval to follow the



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   reference could be obtained and that approval was subsequently denied
   (see Section 2.4.7).

   An implementation MAY include more of a SIP message in that body to
   convey more information.  Warning header field values received in
   responses to the referred action are good candidates.  In fact, if
   the reference was to a SIP URI, the entire response to the referenced
   action could be returned (perhaps to assist with debugging).
   However, doing so could have grave security repercussions (see
   Section 5).  Implementers must carefully consider what they choose to
   include.

   Note that if the reference was to a non-SIP URI, status in any
   NOTIFYs to the referrer must still be in the form of SIP Response
   Status-Lines.  The minimal implementation discussed above is
   sufficient to provide a basic indication of success or failure.  For
   example, if a client receives a REFER to a HTTP URL, and is
   successful in accessing the resource, its NOTIFY to the referrer can
   contain the message/sipfrag body of "SIP/2.0 200 OK".  If the
   notifier wishes to return additional non-SIP protocol specific
   information about the status of the request, it may place it in the
   body of the sipfrag message.

2.4.6 Multiple REFER Requests in a Dialog

   A REFER creates an implicit subscription sharing the dialog
   identifers in the REFER request.  If more than one REFER is issued in
   the same dialog (a second attempt at transferring a call for
   example), the dialog identifiers do not provide enough information to
   associate the resulting NOTIFYs with the proper REFER.

   Thus, for the second and subsequent REFER requests a UA receives in a
   given dialog, it MUST include an id parameter[2] in the Event header
   field of each NOTIFY containing the sequence number (the number from
   the CSeq header field value) of the REFER this NOTIFY is associated
   with.  This id parameter MAY be included in NOTIFYs to the first
   REFER a UA receives in a given dialog.  A SUBSCRIBE sent to refresh
   or terminate this subscription MUST contain this id parameter.

2.4.7 Using the Subscription-State Header Field with Event Refer

   Each NOTIFY must contain a Subscription-State header field as defined
   in [2].  The final NOTIFY sent in response to a REFER MUST indicate
   the subscription has been "terminated" with a reason of "noresource".
   (The resource being subscribed to is the state of the referenced
   request).

   If a NOTIFY indicates a reason that indicates a re-subscribe is



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   appropriate according to  [2], the agent sending the REFER is NOT
   obligated to re-subscribe.

   In the case where a REFER was accepted with a 202, but approval to
   follow the reference was subsequently denied, the reason and retry-
   after elements of the Subscription-State header field can be used to
   indicate if and when the REFER can be re-attempted (as described for
   SUBSCRIBE in [2]).

2.5 Behavior of SIP Registrars/Redirect Servers

   A registrar that is unaware of the definition of the REFER method
   will return a 501 response as defined in [1].  A registrar aware of
   the definition of REFER SHOULD return a 405 response.

   This specification places no requirements on redirect server behavior
   beyond those specified in [1].  Thus, it is possible for REFER
   requests to be redirected.

2.6 Behavior of SIP Proxies

   SIP proxies do not require modification to support the REFER method.
   Specifically, as required by [1], a proxy should process a REFER
   request the same way it processes an OPTIONS request.

3. Package Details: Event refer

   This document defines an event package as defined in [2].

3.1 Event Package Name

   The name of this event package is "refer".

3.2 Event Package Parameters

   This package uses the "id" parameter defined in [2].  Its use in
   package is described in Section 2.4.6.

3.3 SUBSCRIBE Bodies

   SUBSCRIBE bodies have no special meaning for this event package.

3.4 Subscription Duration

   The duration of an implicit subscription created by a REFER request
   is initially determined by the agent accepting the REFER and
   communicated to the subscribing agent in the Subscription-State
   header field's expire parameter in the first NOTIFY sent in the



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   subscription.  Reasonable choices for this initial duration depend on
   the type of request indicated in the Refer-To URI.  The duration
   SHOULD be chosen to be longer than the time the referenced request
   will be given to complete.  For example, if the Refer-To URI is a SIP
   INVITE URI, the subscription interval should be longer than the
   Expire value in the INVITE.  Additional time MAY be included to
   account for time needed to authorize the subscription.  The
   subscribing agent MAY extend the subscription by refreshing it, or
   terminate it by unsubscribing.  As described in Section 2.4.7, the
   agent accepting the REFER will terminate the subscription when it
   reports the final result of the reference, indicating that
   termination in the Subscription-State header field.

3.5 NOTIFY Bodies

   The bodies of NOTIFY requests for event refer are discussed in
   Section 2.4.5.

3.6 Notifier processing of SUBSCRIBE requests

   Notifier processing of SUBSCRIBE requests is discussed in Section
   2.4.4.

3.7 Notifier Generation of NOTIFY Requests

   Notifier generation of NOTIFY requests is discussed in Section 2.4.4.

3.8 Subscriber Processing of NOTIFY Requests

   Subscriber processing of NOTIFY requests is discussed in Section
   2.4.4.

3.9 Handling of Forked Requests

   A REFER sent within the scope of an existing dialog will not fork.  A
   REFER sent outside the context of a dialog MAY fork, and if it is
   accepted by multiple agents, MAY create multiple subscriptions.
   These subscriptions are created and managed as per "Handling of
   Forked Requests" in [2] as if the REFER had been a SUBSCRIBE.  The
   agent sending the REFER manages the state associated with each
   subscription separately.  It does NOT merge the state from the
   separate subscriptions.  The state is the status of the referenced
   request at each of the accepting agents.

3.10 Rate of Notifications

   An event refer NOTIFY might be generated each time new knowledge of
   the status of a referenced requests becomes available.  For instance,



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   if the REFER was to a SIP INVITE, NOTIFYs might be generated with
   each provisional response and the final response to the INVITE.
   Alternatively, the subscription might only result in two NOTIFY
   requests, the immediate NOTIFY and the NOTIFY carrying the final
   result of the reference.  NOTIFYs to event refer SHOULD NOT be sent
   more frequently than once per second.

3.11 State Agents

   Separate state agents are not defined for event refer.

4. Examples

4.1 Prototypical REFER callflow

          Agent A                  Agent B
             |                        |
             |   F1 REFER             |
             |----------------------->|
             |        F2 202 Accepted |
             |<-----------------------|
             |        F3 NOTIFY       |
             |<-----------------------|
             |  F4 200 OK             |
             |----------------------->|
             |                        |
             |                        |
             |                        |------->
             |                        |  (whatever)
             |                        |<------
             |                        |
             |         F5 NOTIFY      |
             |<-----------------------|
             |   F6 200 OK            |
             |----------------------->|
             |                        |
             |                        |

   Here are examples of what the four messages between Agent A and Agent
   B might look like if the reference to (whatever) that Agent B makes
   is successful.  The details of this flow indicate this particular
   REFER occurs outside a session (there is no To tag in the REFER
   request).  If the REFER occurs inside a session, there would be a
   non-empty To tag in the request.

      Message One (F1)





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       REFER sip:b@agentland SIP/2.0
       Via: SIP/2.0/UDP agenta.agentland;branch=z9hG4bK2293940223
       To: <sip:b@agentland>
       From: <sip:a@agentland>;tag=193402342
       Call-ID: 898234234@agenta.agentland
       CSeq: 93809823 REFER
       Max-Forwards: 70
       Refer-To: (whatever URI)
       Contact: sip:a@agentland
       Content-Length: 0

      Message Two (F2)

       SIP/2.0 202 Accepted
       Via: SIP/2.0/UDP agenta.agentland;branch=z9hG4bK2293940223
       To: <sip:b@agentland>;tag=4992881234
       From: <sip:a@agentland>;tag=193402342
       Call-ID: 898234234@agenta.agentland
       CSeq: 93809823 REFER
       Contact: sip:b@agentland
       Content-Length: 0

      Message Three (F3)

       NOTIFY sip:a@agentland SIP/2.0
       Via: SIP/2.0/UDP agentb.agentland;branch=z9hG4bK9922ef992-25
       To: <sip:a@agentland>;tag=193402342
       From: <sip:b@agentland>;tag=4992881234
       Call-ID: 898234234@agenta.agentland
       CSeq: 1993402 NOTIFY
       Max-Forwards: 70
       Event: refer
       Subscription-State: active;expires=(depends on Refer-To URI)
       Contact: sip:b@agentland
       Content-Type: message/sipfrag;version=2.0
       Content-Length: 20

       SIP/2.0 100 Trying


      Message Four (F4)










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       SIP/2.0 200 OK
       Via: SIP/2.0/UDP agentb.agentland;branch=z9hG4bK9922ef992-25
       To: <sip:a@agentland>;tag=193402342
       From: <sip:b@agentland>;tag=4992881234
       Call-ID: 898234234@agenta.agentland
       CSeq: 1993402 NOTIFY
       Contact: sip:a@agentland
       Content-Length: 0

      Message Five (F5)

       NOTIFY sip:a@agentland SIP/2.0
       Via: SIP/2.0/UDP agentb.agentland;branch=z9hG4bK9323394234
       To: <sip:a@agentland>;tag=193402342
       From: <sip:b@agentland>;tag=4992881234
       Call-ID: 898234234@agenta.agentland
       CSeq: 1993403 NOTIFY
       Max-Forwards: 70
       Event: refer
       Subscription-State: terminated;reason=noresource
       Contact: sip:b@agentland
       Content-Type: message/sipfrag;version=2.0
       Content-Length: 16

       SIP/2.0 200 OK


      Message Six (F6)

       SIP/2.0 200 OK
       Via: SIP/2.0/UDP agentb.agentland;branch=z9hG4bK9323394234
       To: <sip:a@agentland>;tag=193402342
       From: <sip:b@agentland>;tag=4992881234
       Call-ID: 898234234@agenta.agentland
       CSeq: 1993403 NOTIFY
       Contact: sip:a@agentland
       Content-Length: 0


4.2 Multiple REFERs in a dialog

   Message One above brings an implicit subscription dialog into
   existence.  Suppose Agent A issued a second REFER inside that dialog:








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          Agent A                  Agent B
             |                        |
             |   F7 REFER             |
             |----------------------->|
             |        F8 202 Accepted |
             |<-----------------------|
             |        F9 NOTIFY       |
             |<-----------------------|
             |  F10 200 OK            |
             |----------------------->|
             |                        |------->
             |                        |  (something different)
             |                        |<------
             |                        |
             |         F11 NOTIFY     |
             |<-----------------------|
             |   F12 200 OK           |
             |----------------------->|
             |                        |
             |                        |

      Message Seven (F7)

       REFER sip:b@agentland SIP/2.0
       Via: SIP/2.0/UDP agenta.agentland;branch=z9hG4bK9390399231
       To: <sip:b@agentland>;tag=4992881234
       From: <sip:a@agentland>;tag=193402342
       Call-ID: 898234234@agenta.agentland
       CSeq: 93809824 REFER
       Max-Forwards: 70
       Refer-To: (some different URI)
       Contact: sip:a@agentland
       Content-Length: 0

      Message Eight (F8)

       SIP/2.0 202 Accepted
       Via: SIP/2.0/UDP agenta.agentland;branch=z9hG4bK9390399231
       To: <sip:b@agentland>;tag=4992881234
       From: <sip:a@agentland>;tag=193402342
       Call-ID: 898234234@agenta.agentland
       CSeq: 93809824 REFER
       Contact: sip:b@agentland
       Content-Length: 0

      Message Nine (F9)





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       NOTIFY sip:a@agentland SIP/2.0
       Via: SIP/2.0/UDP agentb.agentland;branch=z9hG4bK9320394238995
       To: <sip:a@agentland>;tag=193402342
       From: <sip:b@agentland>;tag=4992881234
       Call-ID: 898234234@agenta.agentland
       CSeq: 1993404 NOTIFY
       Max-Forwards: 70
       Event: refer
       Subscription-State: active;expires=(depends on Refer-To URI)
       Contact: sip:b@agentland
       Content-Type: message/sipfrag;version=2.0
       Content-Length: 20

       SIP/2.0 100 Trying


      Message Ten (F10)

       SIP/2.0 200 OK
       Via: SIP/2.0/UDP agentb.agentland;branch=z9hG4bK9320394238995
       To: <sip:a@agentland>;tag=193402342
       From: <sip:b@agentland>;tag=4992881234
       Call-ID: 898234234@agenta.agentland
       CSeq: 1993404 NOTIFY
       Contact: sip:a@agentland
       Content-Length: 0

      Message Eleven (F11)

       NOTIFY sip:a@agentland SIP/2.0
       Via: SIP/2.0/UDP agentb.agentland;branch=z9hG4bK2994a93eb-fe
       To: <sip:a@agentland>;tag=193402342
       From: <sip:b@agentland>;tag=4992881234
       Call-ID: 898234234@agenta.agentland
       CSeq: 1993405 NOTIFY
       Max-Forwards: 70
       Event: refer;id=93809824
       Subscription-State: terminated;reason=noresource
       Contact: sip:b@agentland
       Content-Type: message/sipfrag;version=2.0
       Content-Length: 16

       SIP/2.0 200 OK


      Message Twelve (F12)





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       SIP/2.0 200 OK
       Via: SIP/2.0/UDP agentb.agentland;branch=z9hG4bK2994a93eb-fe
       To: <sip:a@agentland>;tag=193402342
       From: <sip:b@agentland>;tag=4992881234
       Call-ID: 898234234@agenta.agentland
       CSeq: 1993405 NOTIFY
       Contact: sip:a@agentland
       Content-Length: 0


5. Security Considerations

   The security considerations  of [1] apply to the REFER method.

   This mechanism relies on providing contact information for the
   referred-to resource to the party being referred.  Care should be
   taken to provide a suitably restricted URI if the referred to
   resource should be protected.

   Using message/sipfrag bodies to return the progress and results of a
   REFER request is extremely powerful.  Careless use of that capability
   will compromise security and privacy.  Here are a couple of simple,
   somewhat contrived, examples to demonstrate the potential for harm.

5.1 Circumventing Privacy

   Suppose Alice has a user agent that accepts REFER requests to SIP
   INVITE URIs, and NOTIFYs the referrer of the progress of the INVITE
   by copying each response to the INVITE into the body of a NOTIFY.

   Suppose further that Carol has a reason to avoid Mallory and has
   configured her system at her proxy to only accept calls from a
   certain set of people she trusts (including Alice), so that Mallory
   doesn't learn when she's around, or what user agent she's actually
   using.

   Mallory can send a REFER to Alice, with a Refer-To URI indicating
   Carol.  If Alice can reach Carol, the 200 OK Carol sends gets
   returned to Mallory in a NOTIFY, letting him know not only that Carol
   is around, but also the IP address of the agent she's using.

5.2 Circumventing Security

   Suppose Alice, with the same user agent as above, is working at a
   company that is working on the greatest SIP device ever invented -
   the SIP FOO.  The company has been working for months building the
   device and the marketing materials, carefully keeping the idea, even
   the name of the idea secret (since a FOO is one of those things that



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   anybody could do if they'd just had the idea first).  FOO is up and
   running, and anyone at the company can use it, but it's not available
   outside the company firewall.

   Mallory has heard rumor that Alice's company is onto something big,
   and has even managed to get his hands on a URI that he suspects might
   have something to do with it.  He sends a REFER to ALICE with the
   mysterious URI and as Alice connects to the FOO, Mallory gets NOTIFYs
   with bodies containing


      Server: FOO/v0.9.7


5.3 Limiting the Breach

   For each of these cases, and in general, returning a carefully
   selected subset of the information available about the progress of
   the reference through the NOTIFYs mitigates risk.  The minimal
   implementation described in Section 2.4.5 exposes the least
   information about what the agent operating on the REFER request has
   done, and is least likely to be a useful tool for malicious users.

6. Historic Material

   This method was initially motivated by the call-transfer application.
   Starting as TRANSFER, and later generalizing to REFER, this method
   improved on the BYE/Also concept of the expired draft-ietf-sip-cc-01
   by disassociating transfers from the processing of BYE.  These
   changes facilitate recovery of failed transfers and clarify state
   management in the participating entities.

   Early versions of this work required the agent responding to REFER to
   wait until the referred action completed before sending a final
   response to the REFER.  That final response reflected the success or
   failure of the referred action.  This was infeasible due to the
   transaction timeout rules defined for non-INVITE requests in [1].  A
   REFER must always receive an immediate (within the lifetime of a non-
   INVITE transaction) final response.

7. IANA Considerations

   (Note to RFC Editor: Please fill in all occurences of  XXXX in this
   section with the RFC number of this specification).

   This document defines a new SIP method name (REFER), a new SIP header
   field name with a compact form (Refer-To and r respectively), and an
   event package (refer).



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   The following row should be added to the method sub-registry under
   http://www.iana.org/assignments/sip-parameters.

      REFER              [RFCXXXX]

   The following information should be added to the header sub-registry
   under http://www.iana.org/assignmemts/sip-parameters.

      Header Name: Refer-To

      Compact Form: r

      Reference: RFC XXXX

   This specification registers an event package, based on the
   registration procedures defined in [2].  The following is the
   information required for such a registration:

      Package Name: refer

      Package or Package-Template: This is a package.

      Published Specification: RFC XXXX

      Person to Contact: Robert Sparks, rsparks@dynamicsoft.com


8. Acknowledgments

   This draft is a collaborative product of the SIP working group.

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

   [3]  Sparks, R., "Internet Media Type message/sipfrag", Internet-
        Draft <draft-sparks-sip-mimetypes-03, April 2002.









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Author's Address

   Robert J. Sparks
   dynamicsoft
   5100 Tennyson Parkway
   Suite 1200
   Plano, TX  75024

   EMail: rsparks@dynamicsoft.com










































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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
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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
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   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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