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Versions: 00 01 02 03 04 05 RFC 3892

Network Working Group                                          R. Sparks
Internet-Draft                                               dynamicsoft
Expires: August 13, 2003                               February 12, 2003


                     The SIP Referred-By Mechanism
                      draft-ietf-sip-referredby-01

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
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at http://
   www.ietf.org/ietf/1id-abstracts.txt.

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

   This Internet-Draft will expire on August 13, 2003.

Copyright Notice

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

Abstract

   The SIP REFER method [2] provides a mechanism where one party (the
   referrer) gives a second party (the referree) an arbitrary URI to
   reference.  If that URI is a SIP URI, the referree will send a SIP
   request, often an INVITE, to that URI (the refer target).  This
   document extends the REFER method allowing the referrer to provide
   information about the reference to the refer target using the
   referree as an intermediary.  This information includes the identity
   of the referrer and the URI to which the referrer referred.  The
   mechanism utilizes S/MIME to help protect this information from a
   malicious intermediary.  This protection is optional, but a recipient
   may refuse to accept a request unless it is present.




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Table of Contents

   1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  The Referred-By Mechanism  . . . . . . . . . . . . . . . . . .  4
   2.1 Referrer behavior  . . . . . . . . . . . . . . . . . . . . . .  4
   2.2 Referree behavior  . . . . . . . . . . . . . . . . . . . . . .  5
   2.3 Refer Target behavior  . . . . . . . . . . . . . . . . . . . .  5
   3.  The Referred-By Header Field . . . . . . . . . . . . . . . . .  6
   4.  The Referred-By Token  . . . . . . . . . . . . . . . . . . . .  7
   4.1 Refer target inspection of a Referred-By token . . . . . . . .  7
   5.  The 429 Provide Referrer Identity error response . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   7.1 Basic REFER  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   7.2 Insecure REFER . . . . . . . . . . . . . . . . . . . . . . . . 13
   7.3 Requiring Referrer Identity  . . . . . . . . . . . . . . . . . 13
   7.4 Nested REFER . . . . . . . . . . . . . . . . . . . . . . . . . 17
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   9.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 22
   10. Changes from -00 . . . . . . . . . . . . . . . . . . . . . . . 22
       Normative References . . . . . . . . . . . . . . . . . . . . . 23
       Informative References . . . . . . . . . . . . . . . . . . . . 23
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 23
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 24



























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1. Overview

   The SIP REFER method [2] provides a mechanism where one party (the
   referrer) provides a second party (the referree) with an arbitrary
   URI to reference.  If that URI is a SIP URI, the referree will send a
   SIP request, often an INVITE, to that URI (the refer target).
   Nothing provided in [2] distinguishes this referenced request from
   any other request the referree might have sent to the refer target.

   ---------------------------------------------------------------------


      Referrer           Referree            Refer Target
         |                  |                    |
         | REFER            |                    |
         | Refer-To: target |                    |
         |----------------->| INVITE target      |
         |                  |------------------->|

                             Classic REFER

   ---------------------------------------------------------------------

   There are applications of REFER, such as call transfer [7], where it
   is desirable to provide the refer target with certain information
   about the referrer and the REFER request itself.  This information
   may include, but is not limited to, the referrer's identity, the
   referred to URI, and the time of the referral.  The refer target can
   use this information when deciding whether to admit the referenced
   request.  This draft defines one set of mechanisms to provide that
   information.

   All of the mechanisms in this draft involve placing information in
   the REFER request that the referee copies into the referenced
   request.  This necessarily establishes the referee as an eavesdropper
   and places the referree in a position to launch man-in-the-middle
   attacks on that information.

   At the simplest level, this draft defines a mechanism for carrying
   the referrer's identity, expressed as a SIP URI in a new header:
   Referred-By.  The refer target can use that information, even if it
   has not been protected from the referree, at the perils and with the
   limitations documented here.  The draft proceeds to define an S/MIME
   based mechanism for expressing the identity of the referrer and
   capturing other information about the REFER request, allowing the
   refer target to detect tampering (and other undesirable behaviors) by
   the referree.




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2. The Referred-By Mechanism

   The following figure summarizes how Referred-By information is
   carried to the Refer Target.  The Referrer provides a Referred-By
   header with its SIP address-of-record, optionally associating an S/
   MIME protected token reflecting the identity of the referrer and
   details of the REFER request.  The Referree copies this header and
   the token, if provided, into the triggered request (shown here as an
   INVITE).

   ---------------------------------------------------------------------


    Referrer                       Referree                   Refer Target
       |                              |                             |
       | REFER                        |                             |
       | Refer-To: target             |                             |
       | Referred-By: referrer;cid=X  |                             |
       |                              |                             |
       | (one of the body parts is)   |                             |
       | Content-ID: X                |                             |
       | <Referred-By Token>          |                             |
       |----------------------------->|                             |
       |                              | INVITE target               |
       |                              | Referred-By: referrer;cid=X |
       |                              |                             |
       |                              | (one of the body parts is)  |
       |                              | Content-ID: X               |
       |                              | <Referred-By token>         |
       |                              |---------------------------->|

                         REFER with Referred-By

   ---------------------------------------------------------------------


2.1 Referrer behavior

   A UA sending a REFER request (a referrer) MAY provide a Referred-By
   header field value in the request.  A REFER request MUST NOT contain
   more than one Referred-By header field value.

   A referrer MAY include a Referred-By token in a REFER request.  A
   REFER request containing a Referred-By token MUST contain a Referred-
   By header field value with a cid parameter value equal to the
   Content-ID of the body part containing the token.

   The referrer will receive a NOTIFY with a sipfrag indicating a final



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   response of 429 "Provide Referrer Identity" to the referenced request
   if the refer target requires a valid Referred-By token to accept the
   request.  The can occur when either no token is provided or a
   provided token is invalid.

   The referrer will receive a 429 "Provide Referrer Identity" response
   to the REFER if the referee requires a Referred-By token to be
   present in order to accept the REFER.

2.2 Referree behavior

   A UA receiving a REFER request (a referree) to a SIP URI (using
   either the sip: or sips: scheme) MUST copy any Referred-By header
   field value and token into the referenced request without
   modification.

   A referree MAY reject a REFER request that does not contain a
   Referred-By token with a 429 "Provide Referrer Identity" response.  A
   referree SHOULD NOT reject a request that contains a Referred-By
   token encrypted to a key it does not possess.  Note that per [5] the
   referee should still be able to verify the signature of such an
   encrypted token.

2.3 Refer Target behavior

   A UA receiving a non-REFER SIP request MAY inspect the request for a
   Referred-By header field and token.

   If a Referred-By header field value is not present, this UA can not
   distinguish this request from any other the UA acting as the referree
   might have sent.  Thus, the UA would apply exactly the admissions
   policies and processing described in [1] to the request.

   If a Referred-By header field value is present, the receiving UA can
   consider itself a refer target and MAY apply additional admission
   policies based on the contents of the Referred-By header field and
   token.

   The referee is in a position to modify the contents of the Referred-
   By header field value, or falsely provide one even if no REFER
   actually exists.  If such behavior could affect admission policy
   (including influencing the agent's user by rendering misleading
   content), the refer target SHOULD require that a valid Referred-By
   token be present.

   The refer target MAY reject a request if no Referred-By token is
   present or if the token is stale using the 429 "Provide Referrer
   Identity" error response defined in Section 5.  The 428 error



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   response from [4] is not appropriate for this purpose - it is needed
   for the refer target to request an authentication token from the
   referee.

   If no Referred-By token is present, the refer target MAY proceed with
   processing the request.  If the agent provides any information from
   the Referred-By header to its user as part of processing the request,
   it  MUST notify the user that the information is suspect.

   The refer target MUST reject an otherwise well-formed request with an
   invalid Referred-By token (see Section 4) with a 429 error response.

3. The Referred-By Header Field

   Referred-By is a request header field as defined by [1].  It can
   appear in any request.  It carries a SIP URI representing the
   identity of the referrer and, optionally, the Content-ID of a body
   part (the Referred-By token) that provides a more secure statement of
   that identity.


   ---------------------------------------------------------------------



        Referred-By  =  ("Referred-By" / "b") HCOLON referrer-uri
                       *( SEMI (referredby-id-param / generic-param) )

        referrer-uri = ( name-addr / addr-spec )

        referredby-id-param = "cid" EQUAL msg-id

        msg-id = TO BE INCORPORATED from rfc2822 (at great pain)


                           Referred-By Syntax

   ---------------------------------------------------------------------

   The Referred-By header field  MAY appear in any SIP request, but is
   meaningless for ACK and CANCEL.  Proxies do not need to be able to
   read Referred-By header field values and MUST NOT remove or modify
   them.

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





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       Header field              where       proxy ACK BYE CAN INV OPT REG
       ___________________________________________________________________
       Referred-By                 R                -   o   -   o   o   o


4. The Referred-By Token

   The Referred-By token is an Authenticated Identity Body as defined by
   [5].  This body part MUST be identified with a MIME [6] Content-ID:
   field.

   In addition to the From, Date, and Call-ID header fields required by
   [5], the sipfrag inside a Referred-By token MUST contain copies of
   the Refer-To and Referred-By header fields from the REFER request.
   As in [5] additional header fields and body parts MAY be included.

      OPEN ISSUE: The Call-ID header from the referrer will not be
      useful to the refer target.  It can even be argued that including
      it leaks information to the refer target that it should not get to
      see.  Should we require that this field be populated with a
      minimal, meaningless constant value?

   As described in [5], a Referred-By token MAY be encrypted as well as
   signed.

4.1 Refer target inspection of a Referred-By token

      (Editor's note: This section is new, replacing and modifying text
      that was removed from sip-identity.  Please review it carefully.)

   A refer target MUST treat a Referred-By token with an invalid
   signature as an invalid token.  A target SHOULD treat a token with an
   aged Date header field value as invalid.

   A target SHOULD verify that the request it receives matches the
   reference in the Refer-To header field in the token.  Note that the
   URI in that header field may not match the request URI in the
   received request due to request retargetting between the referree and
   the refer target.

   The target SHOULD verify that the identity in the From header field
   in the token exactly matches the SubjectAltName from the signing
   certificate.

      OPEN ISSUE: [5] suggests this check with a non-normative "should".
      Can we expect a referrer to always have a certificate that matches
      whatever From header field value it happened to be using in the
      middle of a call? Is From even the right field to be looking at?



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      The referrer may need to provide a different identity to the refer
      target than it provides to the referee.  Should we be basing this
      on the Referred-By header field instead? If no, then we should
      remove Referred-By from the token.  If yes, is there any value in
      including real information in the From field, or should we
      recommend using a minimal, meaningless constant value.

      OPEN ISSUE: Can the target make any meaningful use of the To
      header field in the token? This value could quite reasonably have
      no relation to the identity that the referree presents to the
      refer target.  Is it appropriate to restrict the referree to reuse
      the To value from the original dialog (helpdesk@example.com
      perhaps) as the From in the referenced request? Or do we need a
      way for the referrer to tell the referree "Use this identity for
      me in the token you build"? As noted in Section 9, a similar
      problem may exist in sip-identity and the solution may belong
      there.


5. The 429 Provide Referrer Identity error response

   The 429 client error response code is used by a refer target to
   indicate that the referree must provide a valid Referred-By token.
   As discussed in the behavior section, the referree will forward this
   error response to the referrer in a NOTIFY as the result of the
   REFER.  The suggested text phrase for the 429 error response is
   "Provide Referrer Identity".

6. Security Considerations

   This mechanism defined in this specification relies on an
   intermediary (the referree) to forward information from the referrer
   to the refer target.  This necessarily establishes the referree as an
   eavesdropper of that information and positions him perfectly to
   launch man-in-the-middle attacks using the mechanism.

   A SIP proxy is similarly positioned.  Protecting SIP messaging from
   malicious proxy implementations is discussed in [1].  In contrast to
   a proxy,  the referree's agent is an endpoint.  Proxies will
   typically be managed and monitored by service providers.  Malicious
   behavior by a proxy is more likely to be noticed and result in
   negative repercussions for the provider than malicious behavior by an
   endpoint would be.  The behavior of an endpoint can be entirely under
   the control of a single user.  Thus, it is more feasible for an
   endpoint acting as referree to behave maliciously than it is for a
   proxy being operated by a service provider.

   This specification uses an S/MIME based mechanism to enable the refer



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   target to detect manipulation of the Referred-By information by the
   referree.  Use of this protection is optional! The community has
   asserted that there are systems where trust in the validity of this
   information is either not important or can be established through
   other means.  Any implementation choosing not to use this optional
   mechanism needs to provide its own defense to the following risks:

   o  The Referred-By information is highly likely to influence request
      admission policy.  For instance, it may be displayed to the user
      of the agent with a "This call was transferred to you by X.
      Accept?" prompt.  A malicious referree can unduly influence that
      policy decision by providing falsified referred-by information.
      This includes falsely claiming to have been referred in the first
      place.  (The S/MIME mechanism protects the information with a
      signature, hampering the referree's ability to inject or modify
      information without knowing the key used for that signature).

   o  A referree is by definition an eavesdropper of the referred-by
      information.  Parts of that information may be sensitive.  (The S/
      MIME mechanism allows encryption).

   o  The referree may store any referred-by information it sees and
      paste it into future unrelated requests.  (The S/MIME mechanism
      allows detection of stale assertions by covering a timestamp with
      the signature and allows detection of use in unrelated requests by
      covering the Refer-To header field with the signature).

   The mechanisms in this specification do NOT prevent the referree from
   deleting ALL referred-by information from the referenced request.  A
   refer target can not detect such deletion.  This introduces no new
   problems since removing all referred-by information from a referenced
   request transforms it into an ordinary SIP request as described in
   [1].  Thus the referree gains no new influence over processing logic
   at the refer target by removing the referred-by information.

   Refer targets can protect themselves from the possibility that a
   malicious referree removed a token (leaving an unsecured identity in
   the Referred-By header field) by using the 429 error response.

   Applications using the mechanisms in this draft may be able to take
   advantage of pre-existing relationships between the participants to
   mitigate the risks of its use.  In some transfer scenarios, A has the
   choice of referring B to C or referring C to B.  If A and B have a
   pre-existing trust relationship leading A to have greater confidence
   that B will not behave maliciously (B is A's administrative assistant
   for example), referring B to C may make more sense.

   This mechanism involves two SIP messages between three endpoints, the



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   REFER and the referenced request.  The content of those messages
   (including the referred-by information) is subject to the security
   considerations and protection mechanisms documented in [1].

   Proxies between the participants may collect referred-by information
   and reinsert it in future request or make them available to hostile
   endpoints.  The end-to-end confidentiality capabilities discussed in
   [1] can help reduce risk of exposing sensitive referred-by
   information to these proxies.  The abuse possibilities in subsequent
   requests by proxies (or endpoints that they may leak information to)
   between the referree and the refer target are identical to abuse by
   the referree and the considerations discussed for malicious referree
   applies.  The abuse possibilities in subsequent requests by proxies
   (or endpoints that they may leak information to) between the referrer
   and the referree are identical to those discussed for the
   presentation of Authenticated Identity Bodies in [4].

7. Examples

7.1 Basic REFER

   This example shows the secured Referred-By mechanism applied to a
   REFER to an SIP INVITE URI.

   Details are shown only for those messages involved in exercising the
   mechanism defined in this document.


     Referrer                    Referree                   Refer Target
        | F1 REFER                  |                            |
        |-------------------------->|                            |
        | 202 Accepted              |                            |
        |<--------------------------|                            |
        |                           | F2 INVITE                  |
        |                           |--------------------------->|
        |                           | 200 OK                     |
        |                           |<---------------------------|
        |                           | ACK                        |
        | NOTIFY                    |--------------------------->|
        |<--------------------------|                            |
        | 200 OK                    |                            |
        |-------------------------->|                            |
        |                           |                            |

    F1 REFER sip:referree@referree.example SIP/2.0
       Via: SIP/2.0/UDP referrer.example;branch=z9hG4bK392039842
       To: sip:referree@referree.example
       From: sip:referror@referror.example;tag=39092342



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       Call-ID: 2203900ef0299349d9209f023a
       CSeq: 1239930 REFER
       Max-Forwards: 70
       Contact: <sip:referrer.example>
       Refer-To: sip:refertarget@target.example
       Referred-By: sip:referror@referror.example
          ;cid=%3C20398823.2UWQFN309shb3@referror.example%3E
       Content-Type: multipart/mixed; boundary=unique-boundary-1
       Content-Length: (appropriate value)

       --unique-boundary-1

       Content-Type: multipart/signed;
         protocol="application/pkcs7-signature";
         micalg=sha1; boundary=dragons39
       Content-ID: <20398823.2UWQFN309shb3@referror.example>
       Content-Length: (appropriate value)

       --dragons39
       Content-Type: message/sipfrag
       Content-Disposition: auth-id; handling=optional

       From: sip:referror@referror.example
       Date: Thu, 21 Feb 2002 13:02:03 GMT
       Call-ID: 2203900ef0299349d9209f023a
       Refer-To: sip:refertarget@target.example
       Referred-By: sip:referror@referror.example
          ;cid=%3C20398823.2UWQFN309shb3@referror.example%3E

       --dragons39
       Content-Type: application/pkcs7-signature; name=smime.p7s
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7s;
          handling=required

       (appropriate signature goes here)

       --dragons39--

       --unique-boundary-1--

    F2 INVITE sip:refertarget@target.example SIP/2.0
       Via: SIP/2.0/UDP referree.example;branch=z9hG4bKffe209934aac
       To: sip:refertarget@target.example
       From: sip:referree@referree.example;tag=2909034023
       Call-ID: fe9023940-a3465@referree.example
       CSeq: 889823409 INVITE
       Max-Forwards: 70



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       Contact: <sip:referree@referee.example>
       Referred-By: sip:referror@referror.example
          ;cid=%3C20398823.2UWQFN309shb3@referror.example%3E
       Content-Type: multipart/mixed; boundary=my-boundary-9
       Content-Length: (appropriate value)

       --my-boundary-9

       Content-Type: application/sdp
       Content-Length: (appropriate value)

       v=0
       o=referree 2890844526 2890844526 IN IP4 referree.example
       s=Session SDP
       c=IN IP4 referree.example
       t=0 0
       m=audio 49172 RTP/AVP 0
       a=rtpmap:0 PCMU/8000

       --my-boundary-9

       Content-Type: multipart/signed;
         protocol="application/pkcs7-signature";
         micalg=sha1; boundary=dragons39
       Content-ID: <20398823.2UWQFN309shb3@referror.example>
       Content-Length: (appropriate value)

       --dragons39
       Content-Type: message/sipfrag
       Content-Disposition: auth-id; handling=optional

       From: sip:referror@referror.example
       Date: Thu, 21 Feb 2002 13:02:03 GMT
       Call-ID: 2203900ef0299349d9209f023a
       Refer-To: sip:refertarget@target.example
       Referred-By: sip:referror@referror.example
          ;cid=%3C20398823.2UWQFN309shb3@referror.example%3E

       --dragons39
       Content-Type: application/pkcs7-signature; name=smime.p7s
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7s;
          handling=required

       (appropriate signature goes here)

       --dragons39--
       --my-boundary-9--



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7.2 Insecure REFER

   The flow for this example is the same as that of Section 7.1.  Here,
   the referrer has opted to not include a Referred-By token, and the
   refer target is willing to accept the referenced request without one.



    F1 REFER sip:referree@referree.example SIP/2.0
       Via: SIP/2.0/UDP referrer.example;branch=z9hG4bK392039842
       To: sip:referree@referree.example
       From: sip:referror@referror.example;tag=39092342
       Call-ID: 2203900ef0299349d9209f023a
       CSeq: 1239930 REFER
       Max-Forwards: 70
       Contact: <sip:referrer.example>
       Refer-To: sip:refertarget@target.example
       Referred-By: sip:referror@referror.example
       Content-Length: 0

    F2 INVITE sip:refertarget@target.example SIP/2.0
       Via: SIP/2.0/UDP referree.example;branch=z9hG4bKffe209934aac
       To: sip:refertarget@target.example
       From: sip:referree@referree.example;tag=2909034023
       Call-ID: fe9023940-a3465@referree.example
       CSeq: 889823409 INVITE
       Max-Forwards: 70
       Contact: <sip:referree@referee.example>
       Referred-By: sip:referror@referror.example
       Content-Type: application/sdp
       Content-Length: (appropriate value)

       v=0
       o=referree 2890844526 2890844526 IN IP4 referree.example
       s=Session SDP
       c=IN IP4 referree.example
       t=0 0
       m=audio 49172 RTP/AVP 0
       a=rtpmap:0 PCMU/8000



7.3 Requiring Referrer Identity

   In contrast to the example in Section 7.2, the refer target requires
   a Referred-By token to accept the referenced request.  The referrer
   choses to provide an encrypted token (note that the block surrounded
   by asterisks represents encrypted content).  F1 and F2 are identical



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   to the messages detailed in Section 7.2.


     Referrer                    Referree                   Refer Target
        | F1 REFER                  |                            |
        |-------------------------->|                            |
        | 202 Accepted              |                            |
        |<--------------------------|                            |
        |                           | F2 INVITE                  |
        |                           |--------------------------->|
        |                           | F3 429 Provide Referrer Identity
        |                           |<---------------------------|
        |                           | ACK                        |
        | F4 NOTIFY                 |--------------------------->|
        |<--------------------------|                            |
        | 200 OK                    |                            |
        |-------------------------->|                            |
        | F5 REFER                  |                            |
        |-------------------------->|                            |
        | 202 Accepted              |                            |
        |<--------------------------|                            |
        |                           | F6 INVITE                  |
        |                           |--------------------------->|
        |                           | 200 OK                     |
        |                           |<---------------------------|
        |                           | ACK                        |
        | NOTIFY                    |--------------------------->|
        |<--------------------------|                            |
        | 200 OK                    |                            |
        |-------------------------->|                            |
        |                           |                            |

    F3 SIP/2.0 429 Provide Referrer Identity
       Via: SIP/2.0/UDP referree.example;branch=z9hG4bKffe209934aac
       To: sip:refertarget@target.example;tag=392093422302334
       From: sip:referree@referree.example;tag=2909034023
       Call-ID: fe9023940-a3465@referree.example
       CSeq: 889823409 INVITE
       Content-Length: 0

    F4 NOTIFY sip:referror@referror.example SIP/2.0
       Via: SIP/2.0/UDP referree.example;branch=z9hG4bK2934209da390
       To: sip:referror@referror.example;tag=39092342
       From: sip:referree@referree.example;tag=199949923
       Call-ID: 2203900ef0299349d9209f023a
       CSeq: 3920390 NOTIFY
       Event: refer;id=1239930
       Subscription-State: terminated



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       Content-Type: message/sipfrag
       Content-Length: (appropriate value)

       SIP/2.0 429 Provide Referrer Identity

    F5 REFER sip:referree@referree.example SIP/2.0
       Via: SIP/2.0/UDP referrer.example;branch=z9hG4bK98823423
       To: sip:referree@referree.example
       From: sip:referror@referror.example;tag=39092342
       Call-ID: 2203900ef0299349d9209f023a
       CSeq: 1239931 REFER
       Max-Forwards: 70
       Contact: <sip:referrer.example>
       Refer-To: sip:refertarget@target.example
       Referred-By: sip:referror@referror.example
          ;cid=%3C20342EFXEI.390sdefn2@referror.example%3E
       Content-Type: multipart/mixed; boundary=unique-boundary-1
       Content-Length: (appropriate value)

       --unique-boundary-1

       Content-Type: multipart/signed;
         protocol="application/pkcs7-signature";
         micalg=sha1; boundary=boundary42
       Content-ID: <20342EFXEI.390sdefn2@referror.example>
       Content-Length: (appropriate value)

       --boundary42

       Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
         name=smime.p7m
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7m
         handling=required
       Content-Length: (appropriate value)

       ***********************************************************
       * Content-Type: message/sipfrag                           *
       * Content-Disposition: auth-id; handling=optional         *
       *                                                         *
       * From: sip:referror@referror.example                     *
       * Call-ID: 2203900ef0299349d9209f023a                     *
       * Date: Thu, 21 Feb 2002 13:02:03 GMT                     *
       * Refer-To: sip:refertarget@target.example                *
       * Referred-By: sip:referror@referror.example              *
       *    ;cid=%3C20342EFXEI.390sdefn2@referror.example%3E     *
       ***********************************************************




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       --boundary42

       Content-Type: application/pkcs7-signature; name=smime.p7s
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7s;
          handling=required

       (appropriate signature)

       --boundary42--

    F6 INVITE sip:refertarget@target.example SIP/2.0
       Via: SIP/2.0/UDP referree.example;branch=z9hG4bK3920390423
       To: sip:refertarget@target.example
       From: sip:referree@referree.example;tag=1342093482342
       Call-ID: 23499234-9239842993@referree.example
       CSeq: 19309423 INVITE
       Max-Forwards: 70
       Referred-By: sip:referror@referror.example
          ;cid=%3C20342EFXEI.390sdefn2@referror.example%3E
       Contact: <sip:referree@referee.example>
       Content-Type: multipart/mixed; boundary=my-boundary-9
       Content-Length: (appropriate value)

       --my-boundary-9

       Content-Type: application/sdp
       Content-Length: (appropriate value)

       v=0
       o=referree 2890844526 2890844526 IN IP4 referree.example
       s=Session SDP
       c=IN IP4 referree.example
       t=0 0
       m=audio 49172 RTP/AVP 0
       a=rtpmap:0 PCMU/8000

       --my-boundary-9

       Content-Type: multipart/signed;
         protocol="application/pkcs7-signature";
         micalg=sha1; boundary=boundary42
       Content-ID: <20342EFXEI.390sdefn2@referror.example>
       Content-Length: (appropriate value)

       --boundary42

       Content-Type: application/pkcs7-mime; smime-type=enveloped-data;



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         name=smime.p7m
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7m
         handling=required
       Content-Length: (appropriate value)

       ***********************************************************
       * Content-Type: message/sipfrag                           *
       * Content-Disposition: auth-id; handling=optional         *
       *                                                         *
       * From: sip:referror@referror.example                     *
       * Call-ID: 2203900ef0299349d9209f023a                     *
       * Date: Thu, 21 Feb 2002 13:02:03 GMT                     *
       * Refer-To: sip:refertarget@target.example                *
       * Referred-By: sip:referror@referror.example              *
       *    ;cid=%3C20342EFXEI.390sdefn2@referror.example%3E     *
       ***********************************************************

       --boundary42

       Content-Type: application/pkcs7-signature; name=smime.p7s
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7s;
          handling=required

       (appropriate signature)

       --boundary42--
       --my-boundary-9--


7.4 Nested REFER

   The Refer-To URI may be a SIP URI indicating the REFER method.
   Consider The following URI which A uses to refer B to send a REFER
   request to C which refers C to send an INVITE to D.

   Note that A provides a Referred-By token which gets passed through B
   and C to D.  In particular, B does not provide its own Referred-By
   token to C.  Also note that A is notified of the outcome of the
   request it triggered at B (the REFER), not at C (the INVITE).

    Refer-To: <sip:C;method=REFER?Refer-To=%3Csip:D%3E>

    This reference would result in the following flow:


     A                   B                   C                   D



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     | F1 REFER          |                   |                   |
     |------------------>|                   |                   |
     | 202 Accepted      |                   |                   |
     |<------------------|                   |                   |
     |                   | F2 REFER          |                   |
     |                   |------------------>|                   |
     |                   | 202 Accepted      |                   |
     |                   |<------------------|                   |
     | F3 NOTIFY         |                   | F4 INVITE         |
     |<------------------|                   |------------------>|
     | 200 OK            |                   | 200 OK            |
     |------------------>|                   |<------------------|
     |                   |                   | ACK               |
     |                   |                   |------------------>|
     |                   | NOTIFY            |                   |
     |                   |<------------------|                   |
     |                   | 200 OK            |                   |
     |                   |------------------>|                   |
     |                   |                   |                   |

    F1 REFER sip:B SIP/2.0
       Via: SIP/2.0/UDP A;branch=z9hG4bK3802394232
       To: sip:B
       From: sip:A;tag=23490234
       Call-ID: 2304098023@A
       CSeq: 2342093 REFER
       Max-Forwards: 70
       Contact: <sip:A>
       Refer-To: <sip:C;method=REFER?Refer-To=%3Csip:D%3E>
       Referred-By: <sip:A>;cid=%3C23094202342.10123091233@A%3E
       Content-Type: multipart/mixed; boundary=unique-boundary-1
       Content-Length: (appropriate value)

       --unique-boundary-1

       Content-Type: multipart/signed;
         protocol="application/pkcs7-signature";
         micalg=sha1; boundary=dragons39
       Content-ID: <23094202342.10123091233@A>
       Content-Length: (appropriate value)

       --dragons39
       Content-Type: message/sipfrag
       Content-Disposition: auth-id; handling=optional

       From: sip:A
       Call-ID: 2304098023@A
       Date: Thu, 21 Feb 2002 13:02:03 GMT



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       Refer-To: <sip:C;method=REFER?Refer-To=%3Csip:D%3E>
       Referred-By: <sip:A>;cid=%3C23094202342.101230912342A%3E

       --dragons39
       Content-Type: application/pkcs7-signature; name=smime.p7s
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7s;
          handling=required

       (appropriate signature goes here)

       --dragons39--

       --unique-boundary-1--

    F2 REFER sip:C SIP/2.0
       Via: SIP/2.0/UDP B;branch=z9hG4bK00239842
       To: sip:C
       From: sip:B;tag=2934u23
       Call-ID: 203942834@B
       CSeq: 8321039 REFER
       Max-Forwards: 70
       Contact: <sip:B>
       Refer-To: <sip:D>
       Referred-By: <sip:A>;cid=%3C23094202342.10123091233@A%3E
       Content-Type: multipart/mixed; boundary=unique-boundary-1
       Content-Length: (appropriate value)

       --unique-boundary-1

       Content-Type: multipart/signed;
         protocol="application/pkcs7-signature";
         micalg=sha1; boundary=dragons39
       Content-ID: <23094202342.10123091233@A>
       Content-Length: (appropriate value)

       --dragons39
       Content-Type: message/sipfrag
       Content-Disposition: auth-id; handling=optional

       From: sip:A
       Call-ID: 2304098023@A
       Date: Thu, 21 Feb 2002 13:02:03 GMT
       Refer-To: <sip:C;method=REFER?Refer-To=%3Csip:D%3E>
       Referred-By: <sip:A>;cid=%3C23094202342.101230912342A%3E

       --dragons39
       Content-Type: application/pkcs7-signature; name=smime.p7s



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       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7s;
          handling=required

       (appropriate signature goes here)

       --dragons39--

       --unique-boundary-1--

    F3 NOTIFY sip:A SIP/2.0
       Via: SIP/2.0/UDP A;branch=z9hG4bK3802394232
       To: sip:A;tag=23490234
       From: sip:B;tag=5923020
       Call-ID: 2304098023@A
       CSeq: 29420342 NOTIFY
       Event: refer;id=2342093
       Subscription-State: terminated
       Max-Forwards: 70
       Contact: <sip:B>
       Content-Type: message/sipfrag
       Content-Length: (appropriate value)

       SIP/2.0 202 Accepted

    F4 INVITE sip:D SIP/2.0
       Via: SIP/2.0/UDP C;branch=z9hG4bK29348234
       To: sip:D
       From: sip:C;tag=023942334
       Call-ID: 23489020352@C
       CSeq: 1230934 INVITE
       Max-Forwards: 70
       Contact: <sip:C>
       Referred-By: <sip:A>;cid=%3C23094202342.10123091233@A%3E
       Content-Type: multipart/mixed; boundary=unique-boundary-1
       Content-Length: (appropriate value)

       --unique-boundary-1

       Content-Type: application/sdp
       Content-Length: (appropriate value)

       v=0
       o=C 2890844526 2890844526 IN IP4 C
       s=Session SDP
       c=IN IP4 C
       t=0 0
       m=audio 49172 RTP/AVP 0



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       a=rtpmap:0 PCMU/8000

       --unique-boundary-1

       Content-Type: multipart/signed;
         protocol="application/pkcs7-signature";
         micalg=sha1; boundary=dragons39
       Content-ID: <23094202342.10123091233@A>
       Content-Length: (appropriate value)

       --dragons39
       Content-Type: message/sipfrag
       Content-Disposition: auth-id; handling=optional

       From: sip:A
       Call-ID: 2304098023@A
       Date: Thu, 21 Feb 2002 13:02:03 GMT
       Refer-To: <sip:C;method=REFER?Refer-To=%3Csip:D%3E>
       Referred-By: <sip:A>;cid=%3C23094202342.101230912342A%3E

       --dragons39
       Content-Type: application/pkcs7-signature; name=smime.p7s
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7s;
          handling=required

       (appropriate signature goes here)

       --dragons39--

       --unique-boundary-1--



8. IANA Considerations

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

   This document defines a new SIP header field name with a compact form
   (Referred-By and b respectively).  It also defines an new SIP client
   error response code (429).

   The following changes should be made to http:///www.iana.org/
   assignments/sip-parameters

   The following row should be added to the header field section
   (replacing any existing row for Referred-By).



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      Header Name             Compact Form               Reference
      Referred-By                  b                     [RFCXXXX]

   The following row should be added to the response code section under
   the Request Failure 4xx heading

      429 Provide Referrer Identity                     [RFCXXXX]


9. Open Issues

   1.  This mechanism proves to the target that the referrer sent a
       REFER with this particular Refer-To and Referred-By header field
       values.  It DOES NOT prove to the target that the referrer sent
       that REFER to this particular referree (which may enable an
       intercept/cut-paste attack).  Including the REFER start line (the
       Request-URI in particular) is not sufficient to tighten this up -
       location services may arbitrarily retarget the REFER and the
       target will generally have no way to reconcile the REFER Request-
       URI with the actual identity of the referree.  Do we need to
       tighten this? If so, I believe the solution needs to lie in the
       identity service mechanism.  The same attack applies to that
       mechanism in general, resulting in theft of identity.

   2.  Is Call-ID in a token a security leak? Is it even useful? See
       Section 4.

   3.  Should the identity expressed by the token reflect From or
       Referred-By? See Section 4.

   4.  Is the To field in the token useful to the target? See Section 4


10. Changes from -00

   o  Resolved open issue: A referree is not allowed to change the
      content-id for the body part containing a token while copying the
      header and body part into the referenced request.  A copy of the
      Referred-By header is in the token.  Allowing the referree to
      change the copy outside the token adds complexity to the
      acceptance logic at a refer target.

   o  Updated to reflect the current identity drafts

   o  Identified open issues with using the token on reciept

   o  Added SIP to the draft title




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   o  Updated References

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]  Sparks, R., "The SIP Refer Method", draft-ietf-sip-refer-07
        (work in progress), December 2002.

   [3]  Sparks, R., "Internet Media Type message/sipfrag", RFC 3420,
        November 2002.

   [4]  Peterson, J., "Enhancements for Authenticated Identity
        Management in the Session  Initiation Protocol (SIP)", draft-
        ietf-sip-identity-00 (work in progress), October 2002.

   [5]  Peterson, J., "SIP Authenticated Identity Body (AIB) Format",
        draft-ietf-sip-authid-body-00 (work in progress), October 2002.

   [6]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part One: Format of Internet Message Bodies",
        RFC 2045, November 1996.

Informative References

   [7]  Sparks, R. and A. Johnston, "Session Initiation Protocol Call
        Control - Transfer", draft-ietf-sipping-cc-transfer-00 (work in
        progress), October 2002.


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 (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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



















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