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Versions: 00 01 02 03 04 05 RFC 3892

Network Working Group                                          R. Sparks
Internet-Draft                                               dynamicsoft
Expires: September 20, 2004                               March 22, 2004


                     The SIP Referred-By Mechanism
                      draft-ietf-sip-referredby-05

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
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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 September 20, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

   The SIP REFER method provides a mechanism where one party (the
   referrer) gives a second party (the referee) an arbitrary URI to
   reference. If that URI is a SIP URI, the referee 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 REFER request to the refer target using the
   referee 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
   1.1 Requirements notation  . . . . . . . . . . . . . . . . . . . .  3
   2.  The Referred-By Mechanism  . . . . . . . . . . . . . . . . . .  4
   2.1 Referrer behavior  . . . . . . . . . . . . . . . . . . . . . .  4
   2.2 Referee 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 . . . . . . . .  8
   5.  The 429 Provide Referrer Identity error response . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.1 Identifying the referee in the Referred-by Token . . . . . . . 10
   7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.1 Basic REFER  . . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.2 Insecure REFER . . . . . . . . . . . . . . . . . . . . . . . . 14
   7.3 Requiring Referrer Identity  . . . . . . . . . . . . . . . . . 14
   7.4 Nested REFER . . . . . . . . . . . . . . . . . . . . . . . . . 18
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   9.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 22
       Normative References . . . . . . . . . . . . . . . . . . . . . 23
       Informative References . . . . . . . . . . . . . . . . . . . . 23
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 23
       Intellectual Property and Copyright Statements . . . . . . . . 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 referee) with an arbitrary URI
   to reference. If that URI is a SIP URI, the referee 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 referee might have sent to the refer target.

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

   There are applications of REFER, such as call transfer [8], 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 referee 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 referee, 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 referee.

1.1 Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119 [1].






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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 Referee copies this header and the
   token, if provided, into the triggered request (shown here as an
   INVITE).

    Referrer                       Referee                   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>         |
       |                              |---------------------------->|


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 message/sipfrag [4] body
   indicating a final response of 429 "Provide Referrer Identity" to the
   referenced request if the refer target requires a valid Referred-By
   token to accept the request. This 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



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   present in order to accept the REFER.

   If a referrer wishes to reattempt to refer a referee after receiving
   a 429 response or a NOTIFY containing a 429, it MAY submit a new
   REFER request containing a Referred-By token.

2.2 Referee behavior

   A UA accepting a REFER request (a referee) 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 referee MAY reject a REFER request that does not contain a
   Referred-By token with a 429 "Provide Referrer Identity" response. A
   referee SHOULD NOT reject a request that contains a Referred-By token
   encrypted to a key it does not possess simply because it cannot
   decrypt the token. (One scenario where such rejection would be
   appropriate is when the referee is attempting to remain anonymous
   (see Section 6.1)). Note that per [3] the referee should still be
   able to verify the signature of such an encrypted token.

   A referee SHOULD present the same identity to the referrer and the
   refer target.

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 referee
   might have sent. Thus, the UA would apply exactly the admissions
   policies and processing described in [5] 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



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   Identity" error response defined in Section 5. The 428 error response
   from [7] 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 [5]. 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 sip-clean-msg-id

      sip-clean-msg-id = LDQUOT dot-atom "@" (dot-atom / host) RDQUOT

      dot-atom = atom *( "." atom )

      atom     = 1*( alphanum / "-" / "!" / "%" / "*" /
                          "_" / "+" / "'" / "`" / "~"   )


   Since the Content-ID appears as a SIP header parameter value which
   must conform to the expansion of gen-value defined in [5], this
   grammar produces values in the intersection of the expansions of
   gen-value and msg-id from [9]. The double-quotes surrounding the
   sip-clean-msg-id MUST be replaced with left and right angle brackets
   to derive the Content-ID used in the message's MIME body. For
   example,

       Referred-By: sip:r@ref.example;cid="2UWQFN309shb3@ref.example"
          indicates the token is in the body part containing



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       Content-ID: <2UWQFN309shb3@ref.example>

   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.

       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
   [3]. This body part MUST be identified with a MIME [6] Content-ID:
   field.

   The sipfrag inside a Referred-By token MUST contain copies of the
   Refer-To, Referred-By, and Date header fields from the REFER request.

   The token SHOULD NOT contain the Call-ID header field from the REFER
   request as that information is not useful to the refer target and may
   even be an information leak. The token SHOULD NOT contain the From
   header field from the REFER request since the identity being claimed
   is represented in the Referred-By header field.

   The token MAY contain the To header field from the REFER request, but
   it SHOULD NOT be included unless the referrer has cryptographically
   identified the referee. Some ways this authentication can be achieved
   include inspecting the certificates used in a TLS association between
   the referrer and the referee or encrypting the Refer-To header in the
   REFER request using the S/MIME encryption techniques detailed in [5].

   When inspecting the certificates used to establish TLS associations,
   the identity asserted in the token's To header field URI is compared
   to the subjectAltNames from the referee's certificate. The sip and
   sips URI schemes MUST be treated as equivalent for this comparison.
   If the URI is an exact match, confidence in the authentication is
   high and the To header field MAY be added to the token. If the
   certificate subjects contain only a hostname matching the hostname
   portion of the URI, an application level warning SHOULD be issued to
   the referrer agent's user seeking that user's consent before
   including the To header field in the token.




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   Including the To header field in the token significantly strengthens
   the claim being asserted by the token, but may have privacy
   implications as discussed in Section 6.1.

   Additional header fields and body parts MAY be included in the token.

   As described in [3], a Referred-By token MAY be encrypted as well as
   signed. The subjectAltName of the certificate used for these
   operations SHOULD exactly match the identity claimed in the
   referrer-uri in the Referred-By header field in the token.

4.1 Refer target inspection of a Referred-By token

   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. This
   verification SHOULD include at least the request method and any
   indicated end-to-end header field values. Note that the URI in the
   Refer-To header field may not match the request URI in the received
   request due to request retargetting between the referee and the refer
   target.

   The target SHOULD verify that the identity in the Referred-By header
   field in the token exactly matches the SubjectAltName from the
   signing certificate, reporting discrepancies to its user as described
   in [3].

   If the token contains a To header field, the target SHOULD verify
   that the identity it expresses matches the referrer. One way of
   verifying this is to exactly match the identity in the token's To
   header field with the subjectAltName of the certificate used by the
   referee to sign the aib protecting the request itself. The 428
   response defined in [7] can be used to request such an aib if one is
   not already present.

5. The 429 Provide Referrer Identity error response

   The 429 client error response code is used by a refer target to
   indicate that the referee must provide a valid Referred-By token. As
   discussed in the behavior section, the referee 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".





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6. Security Considerations

   This mechanism defined in this specification relies on an
   intermediary (the referee) to forward information from the referrer
   to the refer target. This necessarily establishes the referee 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 [5]. In contrast to a
   proxy,  the referee'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 referee 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
   target to detect manipulation of the Referred-By information by the
   referee. 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 referee 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 referee's ability to inject or modify
      information without knowing the key used for that signature).
   o  A referee 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 referee 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 referee from
   deleting ALL referred-by information from the referenced request. A
   refer target can not detect such deletion. This introduces no new



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   problems since removing all referred-by information from a referenced
   request transforms it into an ordinary SIP request as described in
   [5]. Thus the referee 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 referee 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 requests between three endpoints, the
   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 [5].

   Proxies between the participants may collect referred-by information
   and reinsert it in future requests or make it available to hostile
   endpoints. The end-to-end confidentiality capabilities discussed in
   [5] 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 referee and the refer target are identical to abuse by
   the referee and the considerations discussed for malicious referee
   applies. The abuse possibilities in subsequent requests by proxies
   (or endpoints that they may leak information to) between the referrer
   and the referee are similar to those discussed for the presentation
   of Authenticated Identity Bodies in [7].

6.1 Identifying the referee in the Referred-by Token

   To a refer target, a Referred-By token minimally asserts "The
   identity expressed by this Referred-By header field asked at the time
   indicated in this Date header field that the request indicated by
   this Refer-To header field be sent". This assertion makes no claims
   at all about who is being asked to send the request. This is
   sufficient to enable policies such as "Accept any requests referred
   by Alice", but not "Only accept requests from Bob if he can prove
   that Alice referred him to us". Thus, there is an opportunity for a
   cut-and-paste attack. If Mallory sees Alice refer Carol to us using a
   minimal token, he can copy that token into his own request (as long
   as it matches what is indicated in the embedded Refer-To header), and



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   it will appear to us that Alice referred Mallory to us. This risk is
   best mitigated by protecting the REFER Alice sends to Carol from
   eavesdropping, using TLS or the S/MIME mechanisms detailed in [5].

   Including the To header field from the REFER request in the
   Referred-by token enables the "Only accept requests from Bob if he
   can prove that Alice referred him to us". Alice is constrained to add
   this header to the token only if she is sure she is sending the REFER
   request to Bob. We, in turn, ensure it was Bob that sent the
   referrenced request to us in addition to validating Alice's signature
   of the token. Mallory's earlier attack is not effective with this
   token.

   Including the To header field in the Referred-By token has privacy
   implications, however. Carol, above, might wish to contact us
   anonymously. That wish would be defeated if Carol's identity appeared
   in the token Alice created. If Alice encrypted the token to us, Carol
   will not even be aware of the information leak. To protect herself
   when she wishes anonymity, Carol will have to reject any REFER
   requests containing a Referred-By token she can not inspect.

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



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    F1 REFER sip:referee@referee.example SIP/2.0
       Via: SIP/2.0/UDP referrer.example;branch=z9hG4bK392039842
       To: sip:referee@referee.example
       From: sip:referrer@referrer.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:referrer@referrer.example>
          ;cid="20398823.2UWQFN309shb3@referrer.example"
       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@referrer.example>
       Content-Length: (appropriate value)

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

       Date: Thu, 21 Feb 2002 13:02:03 GMT
       Refer-To: <sip:refertarget@target.example>
       Referred-By: <sip:referrer@referrer.example>
          ;cid="20398823.2UWQFN309shb3@referrer.example"

       --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 referee.example;branch=z9hG4bKffe209934aac
       To: <sip:refertarget@target.example>
       From: <sip:referee@referee.example>;tag=2909034023
       Call-ID: fe9023940-a3465@referee.example
       CSeq: 889823409 INVITE
       Max-Forwards: 70



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       Contact: <sip:referee@referee.example>
       Referred-By: <sip:referrer@referrer.example>
          ;cid="20398823.2UWQFN309shb3@referrer.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=referee 2890844526 2890844526 IN IP4 referee.example
       s=Session SDP
       c=IN IP4 referee.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@referrer.example>
       Content-Length: (appropriate value)

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

       Date: Thu, 21 Feb 2002 13:02:03 GMT
       Refer-To: <sip:refertarget@target.example>
       Referred-By: <sip:referrer@referrer.example>
          ;cid="20398823.2UWQFN309shb3@referrer.example"

       --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:referee@referee.example SIP/2.0
       Via: SIP/2.0/UDP referrer.example;branch=z9hG4bK392039842
       To: <sip:referee@referee.example>
       From: <sip:referrer@referrer.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:referrer@referrer.example>
       Content-Length: 0

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

       v=0
       o=referee 2890844526 2890844526 IN IP4 referee.example
       s=Session SDP
       c=IN IP4 referee.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
   chooses 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                    Referee                   Refer Target
        | F1 REFER                  |                            |
        |-------------------------->|                            |
        | 202 Accepted              |                            |
        |<--------------------------|                            |
        | NOTIFY                    |                            |
        |<--------------------------| F2 INVITE                  |
        | 200 OK                    |--------------------------->|
        |-------------------------->| F3 429 Provide Referrer Identity
        |                           |<---------------------------|
        |                           | ACK                        |
        | F4 NOTIFY                 |--------------------------->|
        |<--------------------------|                            |
        | 200 OK                    |                            |
        |-------------------------->|                            |
        | F5 REFER                  |                            |
        |-------------------------->|                            |
        | 202 Accepted              |                            |
        |<--------------------------|                            |
        | NOTIFY                    |                            |
        |<--------------------------| F6 INVITE                  |
        | 200 OK                    |--------------------------->|
        |-------------------------->| 200 OK                     |
        |                           |<---------------------------|
        |                           | ACK                        |
        | NOTIFY                    |--------------------------->|
        |<--------------------------|                            |
        | 200 OK                    |                            |
        |-------------------------->|                            |
        |                           |                            |

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

    F4 NOTIFY sip:referrer@referrer.example SIP/2.0



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       Via: SIP/2.0/UDP referee.example;branch=z9hG4bK2934209da390
       To: <sip:referrer@referrer.example>;tag=39092342
       From: <sip:referee@referee.example>;tag=199949923
       Call-ID: 2203900ef0299349d9209f023a
       CSeq: 3920390 NOTIFY
       Event: refer;id=1239930
       Subscription-State: terminated
       Content-Type: message/sipfrag
       Content-Length: (appropriate value)

       SIP/2.0 429 Provide Referrer Identity

    F5 REFER sip:referee@referee.example SIP/2.0
       Via: SIP/2.0/UDP referrer.example;branch=z9hG4bK98823423
       To: <sip:referee@referee.example>
       From: <sip:referrer@referrer.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:referrer@referrer.example>
          ;cid="20342EFXEI.390sdefn2@referrer.example"
       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@referrer.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: aib; handling=optional             *
       *                                                         *
       * Date: Thu, 21 Feb 2002 13:02:03 GMT                     *
       * Refer-To: <sip:refertarget@target.example>              *
       * Referred-By: <sip:referrer@referrer.example>            *



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       *    ;cid="20342EFXEI.390sdefn2@referrer.example"         *
       ***********************************************************

       --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 referee.example;branch=z9hG4bK3920390423
       To: <sip:refertarget@target.example>
       From: <sip:referee@referee.example>;tag=1342093482342
       Call-ID: 23499234-9239842993@referee.example
       CSeq: 19309423 INVITE
       Max-Forwards: 70
       Referred-By: <sip:referrer@referrer.example>
          ;cid="20342EFXEI.390sdefn2@referrer.example"
       Contact: <sip:referee@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=referee 2890844526 2890844526 IN IP4 referee.example
       s=Session SDP
       c=IN IP4 referee.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@referrer.example>
       Content-Length: (appropriate value)

       --boundary42
       Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
         name=smime.p7m



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

       ***********************************************************
       * Content-Type: message/sipfrag                           *
       * Content-Disposition: aib; handling=optional             *
       *                                                         *
       * Date: Thu, 21 Feb 2002 13:02:03 GMT                     *
       * Refer-To: <sip:refertarget@target.example>              *
       * Referred-By: <sip:referrer@referrer.example>            *
       *    ;cid="20342EFXEI.390sdefn2@referrer.example"         *
       ***********************************************************

       --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.example;method=REFER?Refer-To="<sip:D.example>">

    This reference would result in the following flow:


     A                   B                   C                   D
     | F1 REFER          |                   |                   |
     |------------------>|                   |                   |
     | 202 Accepted      |                   |                   |
     |<------------------|                   |                   |



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

    F1 REFER sip:B SIP/2.0
       Via: SIP/2.0/UDP A.example;branch=z9hG4bK3802394232
       To: <sip:B.example>
       From: <sip:A.example>;tag=23490234
       Call-ID: 2304098023@A.example
       CSeq: 2342093 REFER
       Max-Forwards: 70
       Contact: <sip:A.example>
       Refer-To: <sip:C.example;method=REFER?Refer-To="<sip:D>.example">
       Referred-By: <sip:A.example>;
           cid="23094202342.10123091233@A.example"
       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.example>
       Content-Length: (appropriate value)

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

       Date: Thu, 21 Feb 2002 13:02:03 GMT
       Refer-To: <sip:C.example;method=REFER?Refer-To="<sip:D.example>">
       Referred-By: <sip:A.example>;
           cid="23094202342.10123091233@A.example"

       --dragons39
       Content-Type: application/pkcs7-signature; name=smime.p7s
       Content-Transfer-Encoding: base64



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

       (appropriate signature goes here)

       --dragons39--
       --unique-boundary-1--

    F2 REFER sip:C.example SIP/2.0
       Via: SIP/2.0/UDP B.example;branch=z9hG4bK00239842
       To: <sip:C.example>
       From: <sip:B.example>;tag=2934u23
       Call-ID: 203942834@B.example
       CSeq: 8321039 REFER
       Max-Forwards: 70
       Contact: <sip:B.example>
       Refer-To: <sip:D.example>
       Referred-By: <sip:A.example>;
           cid="23094202342.10123091233@A.example"
       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.example>
       Content-Length: (appropriate value)

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

       Date: Thu, 21 Feb 2002 13:02:03 GMT
       Refer-To: <sip:C.example;method=REFER?Refer-To="<sip:D.example>">
       Referred-By: <sip:A.example>;cid="23094202342.1012309123@A.example"

       --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--




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

       SIP/2.0 202 Accepted

    F4 INVITE sip:D.example SIP/2.0
       Via: SIP/2.0/UDP C.example;branch=z9hG4bK29348234
       To: <sip:D.example>
       From: <sip:C.example>;tag=023942334
       Call-ID: 23489020352@C.example
       CSeq: 1230934 INVITE
       Max-Forwards: 70
       Contact: <sip:C.example>
       Referred-By: <sip:A.example>;
           cid="23094202342.10123091233@A.example"
       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.example
       s=Session SDP
       c=IN IP4 C.example
       t=0 0
       m=audio 49172 RTP/AVP 0
       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.example>
       Content-Length: (appropriate value)

       --dragons39



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       Content-Type: message/sipfrag
       Content-Disposition: aib; handling=optional

       Date: Thu, 21 Feb 2002 13:02:03 GMT
       Refer-To: <sip:C.example;method=REFER?Refer-To="<sip:D.example>">
       Referred-By: <sip:A.example>;
           cid="23094202342.1012309123@A.example"

       --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).

      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. Contributors

   Rohan Mahy distilled RFC2822's msg-id into this document's definition
   of sip-clean-msg-id.



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Normative References

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

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

   [3]  Peterson, J., "SIP Authenticated Identity Body (AIB) Format",
        draft-ietf-sip-authid-body-02 (work in progress), July 2003.

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

   [5]  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.

   [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]  Peterson, J., "Enhancements for Authenticated Identity
        Management in the Session  Initiation Protocol (SIP)",
        draft-ietf-sip-identity-01 (work in progress), March 2003.

   [8]  Sparks, R. and A. Johnston, "Session Initiation Protocol Call
        Control - Transfer", draft-ietf-sipping-cc-transfer-01 (work in
        progress), February 2003.

   [9]  Resnick, P., "Internet Message Format", RFC 2822, April 2001.


Author's Address

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

   EMail: rsparks@dynamicsoft.com







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Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
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Full Copyright Statement

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   This document and translations of it may be copied and furnished to
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   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION



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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Acknowledgment

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











































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