





Network Working Group                                          R. Sparks
Request for Comments: 3892                                          Xten
Category: Standards Track                                 September 2004


      The Session Initiation Protocol (SIP) Referred-By Mechanism

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   The Session Initiation Protocol (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 . . . . . . . . . . . . . . . . . . . . . . . . . . .  2
       1.1.  Requirements Notation  . . . . . . . . . . . . . . . . .  3
   2.  The Referred-By Mechanism  . . . . . . . . . . . . . . . . . .  3
       2.1.  Referrer Behavior  . . . . . . . . . . . . . . . . . . .  4
       2.2.  Referee Behavior . . . . . . . . . . . . . . . . . . . .  4
       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  . . . . . . . . . . . . . . . . . . .  8
       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  . . . . . . . . . . . . . . . . . . . . . 23
   9.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 23
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
       10.1. Normative References . . . . . . . . . . . . . . . . . . 23
       10.2. Informative References . . . . . . . . . . . . . . . . . 24
   11. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 24
   12. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 25

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      |
         |                  |------------------->|









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   There are applications of REFER, such as call transfer [8], where it
   is desirable to provide the refer target with particular 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 document defines one set of mechanisms to provide that
   information.

   All of the mechanisms in this document 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 document 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 document 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].

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











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   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
   present in order to accept the REFER.

   If a referrer wishes to re-attempt 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.



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   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 cannot
   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
   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.



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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 the 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

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

   If the referrer-uri contains a comma, question mark, or semicolon,
   (for example, if it contains URI parameters) the URI MUST be enclosed
   in angle brackets (< and >). Any URI parameters are contained within
   these brackets. If the URI is not enclosed in angle brackets, any
   semicolon-delimited parameters are header-parameters, not URI
   parameters.

   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.





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   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.

   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.





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   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 re-targeting 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".

6.  Security Considerations

   The 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.



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   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
   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.





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

   Applications using the mechanisms in this document 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 re-insert it in future requests or make it available to hostile
   endpoints.  The end-to-end confidentiality capabilities discussed in
   [5] can help reduce the 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 the abuse
   by the referee, and the considerations discussed for a 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
   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].




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   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
   referenced 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--







Sparks                      Standards Track                    [Page 13]

RFC 3892             The SIP Referred-By Mechanism        September 2004


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
   to the messages detailed in Section 7.2.






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RFC 3892             The SIP Referred-By Mechanism        September 2004


   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













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RFC 3892             The SIP Referred-By Mechanism        September 2004


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










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RFC 3892             The SIP Referred-By Mechanism        September 2004


      ***********************************************************
      * 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--

   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







Sparks                      Standards Track                    [Page 17]

RFC 3892             The SIP Referred-By Mechanism        September 2004


      --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
      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>">



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RFC 3892             The SIP Referred-By Mechanism        September 2004


   This reference would result in the following flow:

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





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RFC 3892             The SIP Referred-By Mechanism        September 2004


     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
     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"







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RFC 3892             The SIP Referred-By Mechanism        September 2004


     --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--

  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)









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RFC 3892             The SIP Referred-By Mechanism        September 2004


     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
     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--

















Sparks                      Standards Track                    [Page 22]

RFC 3892             The SIP Referred-By Mechanism        September 2004


8.  IANA Considerations

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

   The following changes are reflected at:

      http:///www.iana.org/assignments/sip-parameters

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

      Header Name             Compact Form               Reference
      Referred-By                  b                     [RFC3892]

   The following row has been added to the response code section under
   the Request Failure 4xx heading.

      429 Provide Referrer Identity                     [RFC3892]

9.  Contributors

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

10.  References

10.1.  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., "Session Initiation Protocol (SIP) Authenticated
        Identity Body (AIB) Format", RFC 3893, September 2004.

   [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.






Sparks                      Standards Track                    [Page 23]

RFC 3892             The SIP Referred-By Mechanism        September 2004


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

10.2.  Informative References

   [7]  Peterson, J., "Enhancements for Authenticated Identity
        Management in the Session  Initiation Protocol (SIP)", Work in
        Progress, March 2003.

   [8]  Sparks, R. and A. Johnston, "Session Initiation Protocol Call
        Control - Transfer", Work in Progress, February 2003.

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

11.  Author's Address

   Robert J. Sparks
   Xten
   5100 Tennyson Parkway
   Suite 1000
   Plano, TX  75024

   EMail: RjS@xten.com



























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RFC 3892             The SIP Referred-By Mechanism        September 2004


12.  Full Copyright Statement

   Copyright (C) The Internet Society (2004).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/S HE
   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
   INTERNET ENGINEERING TASK FORCE DISCLAIM 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.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the IETF's procedures with respect to rights in IETF Documents can
   be found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.

Acknowledgement

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







Sparks                      Standards Track                    [Page 25]

