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Versions: 00 01 02                                                      
Internet Engineering Task Force                              J. Elwell
Internet Draft                                                 Siemens
                                                             R. Jesske
                                                      Deutsche Telekom
                                                           J. McMillen
                                                            Avaya Inc.
Expires: April 2005                                       October 2004

      SIP Reason header extension for indicating redirection reasons

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   or will be disclosed, and any of which I become aware will be
   disclosed, in accordance with RFC 3668.

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Copyright Notice

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


   This examines the need for signalling additional information
   concerning the reason for redirection in SIP and proposes an
   extension to the Reason header.

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Table of Contents

   1 Introduction....................................................3
   2 Overview of proposed solution...................................4
   3 Extension to the Reason header..................................4
   4 Examples........................................................5
   4.1 Redirection with reason "Forward Immediate"...................5
   4.2 Call Forwarding Unconditional.................................7
   5 IANA considerations.............................................9
   6 Security Considerations.........................................9
   7 Author's Addresses..............................................9
   8 Normative References...........................................10

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1 Introduction

   Central to SIP [2] is the concept of redirecting or retargeting a
   request by a proxy, whereby the Request-URI in the original request
   is replaced before forwarding the request on the next hop. Sometimes
   this is due to normal rerouting behaviour of the proxy (e.g.,
   resolving an address-of-record URI to a contact URI). At other times
   it is due to more application-related reasons, e.g., where a user has
   made arrangements for calls to that user under certain conditions to
   be forwarded to a different destination. Also retargeting can be
   performed as a result of a 3xx response from a redirect server.
   Different 3xx response codes reflect different reasons for rejecting
   the request. A 3xx response can be accompanied by a Reason header [1]
   giving more information.

   The History-Info header [3] provides a means for conveying
   information about a retarget to the final destination UAS and also
   back to the UAC. In addition to providing the retargeted-from and
   retargeted-to URIs for each recorded retarget, this header also
   conveys a reason by means of the Reason header. The Reason header
   accompanies the retargeted-from URI and reflects the reason why
   attempts to reach that target failed, normally in the form of the SIP
   response code concerned.

   However, there is nothing in either a 3xx response or the History-
   Info header, including the Reason header, to indicate an explicit
   reason for the redirection request or the retarget respectively. At
   present the reason is implicit in the reason for failure of the
   request to the original target. Sometimes this might give an accurate
   picture of what is happening, but not always. Consider the following

     1. A device acts as a redirect server because it is busy. None of
     the 3xx response codes can reflect that the reason for retargeting
     to the URI given in the Contact header of the 3xx response is
     because the existing target is busy.

     2. A device acts as a redirect server because it alerts the user
     but fails to get a reply within a certain time. None of the 3xx
     response codes can reflect that the reason for retargeting to the
     URI given in the Contact header of the 3xx response is because the
     existing target failed to answer.

     3. A proxy is scripted to retarget requests without first
     attempting to forward them to the original target. Retargeting may
     be unconditional or based on certain conditions such as date, time,
     the source of the request or caller preferences. Because it does

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     this without forwarding the request to the original target, no SIP
     response code is applicable.

     4. A proxy is scripted to perform hunting or distribution of calls
     among a number of different targets. When forwarding a request to a
     target selected from a list of candidate targets, the reason for
     retargeting is because of hunting or distribution, rather than
     because of any failure of the existing target.

     5. In the hunting or distribution scenario above, forwarding a
     request to one target from the list of candidate targets may fail
     for a particular reason (e.g., busy), leading to selection of
     another target from the list. However, the reason for retargeting
     is because of hunting or distribution, not specifically because the
     previous target had a certain condition.

   With these considerations in mind, there is a need to provide a
   solution for conveying a more precise reason for redirection in a 3xx
   response or a History-Info header.

   In addition, in some circumstances a proxy can generate a 181 "Call
   Is Being Forwarded" response. In this case no History-Info header
   would be included, since the response does not come from a UAS. At
   present a 181 response does not convey a precise reason for
   forwarding. Provision of a reason for forwarding might be of use to
   the UAC, which otherwise would need to wait for a later response
   containing a History-Info header.

2 Overview of proposed solution

   The proposed solution is to enhance the Reason header with new
   values. This can be done by defining a new "protocol" value and then
   defining specific new reason values under the umbrella of the new
   "protocol" value.

   Candidate reasons include forward unavailable, forward busy, forward
   no reply, forward immediate, deflection, hunting, mobile not

   Note that selection of the new target may depend on several other
   conditions (e.g., relating to date, time, the source of the request
   or caller preferences), but the reasons suggested above should be
   sufficient to convey the main circumstance leading to the retarget.

3 Extension to the Reason header

   This document defines the following new protocol value for the
   protocol field of the Reason header field in [1]:

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     Redirection: The cause parameter contains a reason for redirection
     of a request.

   This document defines the following redirection cause codes:

     Value  Default Text           Description

     1      Normal redirection     The call has been retargeted for
                                   normal routing reasons

     2      Forward unavailable    The call has been retargeted
                                   because the called user is
                                   unavailable (no registered contact).

     3      Forward busy           The call has been retargeted
                                   because the called user is busy.

     4      Forward no reply       The call has been retargeted
                                   because the called user has been
                                   alerted but has failed to reply.

     5      Forward immediate      The call has been retargeted
                                   immediately without determining
                                   whether the called user is
                                   unavailable or busy and without
                                   alerting the user.

     6      Deflection             The call has been retargeted as a
                                   result of a request by the called
                                   user in response to alerting.

     7      Hunting                The call has been retargeted to an
                                   individual member of the hunt group
                                   at which it was previously targeted.

     8      Mobile not reachable   The call has been retargeted
                                   because the called mobile user is
                                   not reachable

   Example syntax is as follows:

     Reason: redirection;cause=3 ;text="Forward busy"

4 Examples

4.1 Redirection with reason "Forward Immediate"

           Alice            Proxy           Bob              Carol
             |                |              |                   |

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             |    INVITE F1   |              |                   |
             |--------------->|   INVITE F2  |                   |
             |                |------------->|                   |
             |(100 Trying) F3 |              |                   |
             |<---------------|  302 Moved Temporarily F4        |
             |                |<-------------|                   |
             |                |     ACK F5   |                   |
             |                |------------->|                   |
             |                |              |    INVITE F6      |
             |                |--------------------------------->|
             |                |              | 180 Ringing F7    |
             |                |<---------------------------------|
             | 180 Ringing F8 |              |                   |
             |<---------------|              |      200 OK F9    |
             |                |<---------------------------------|
             |    200 OK F10  |              |                   |
             |<---------------|              |                   |
             |     ACK F11    |              |                   |
             |--------------->|              |     ACK F12       |
             |                |--------------------------------->|

   Assuming the entity sending the INVITE supports the History-Info
   header, the INVITE would look like this:

   F1 (INVITE) Alice -> Proxy

     INVITE  sip:Bob@example.com; SIP/2.0
     From: <sip:Alice@example.com>;tag=2
     To: <sip:Bob@example.com >
     Call-ID: 12345600@example.com
     CSeq: 1 INVITE
     History-Info: <sip:Bob@example.com>;index=1

   The call is then redirected to a contact URI <sip:carol@example.com>
   in a 302 response. The response would be as follows:

   F3 (302 Moved Temporarily) Bob -> Proxy

     SIP/2.0 302 Moved temporarily
     From: <sip:Alice @example.com>;tag=2
     To: <sip:Bob@example.com>;tag=3
     Call-ID: 12345600@example.com
     CSeq: 1 INVITE
     Contact: <sip:Carol@example.com>
     Reason: Redirection;cause=5;text="Forward immediate"

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   The call would be retargeted to the contact URI. The first History-
   Info header would be augmented with the two reasons for retargeting
   (SIP 302 and redirection 5)). A second History-Info header would be
   added with the new retargeted-to Request-URI:

   F6 (INVITE) ยก Proxy -> Carol

     INVITE  sip:Carol@example.com SIP/2.0
     From: <sip:Alice@example.com>;tag=2
     To: <sip:Bob@example.com>
     Call-ID: 12345600@example.com
     CSeq: 1 INVITE
     History-Info: <sip:Bob@example.com?Reason: SIP;cause=302;
     text="Moved temporarily"?Reason: Redirection; cause=5;text="Forward
     immediate">;index=1, <sip:Carol@example.com>;index=2

   The "index 1" entry indicates that the call to Bob was retargeted
   because of SIP response code 302 and redirection reason CFI.

   The "index 2" entry indicates that the call to Carol has not yet been
   further retargeted.

4.2 Call Forwarding Unconditional

   This example is taken from [4], augmented to show the use of History-
   Info and Reason headers.

            Alice           Proxy          Gateway
             |                |              |
             |    INVITE F1   |              |
             |--------------->|              |
             |(100 Trying) F2 |              |
             |<---------------|              |
             | (181 Call Is Being Forwarded) F3
             |<---------------|   INVITE F4  |
             |                |------------->|
             |                |180 Ringing F5|
             | 180 Ringing F6 |<-------------|
             |<---------------|  200 OK F7   |
             |    200 OK F8   |<-------------|
             |<---------------|              |
             |     ACK F9     |              |
             |--------------->|    ACK F10   |
             |                |------------->|
             |    Both way RTP Established   |
             |     BYE F11    |              |
             |--------------->|   BYE F12    |
             |                |------------->|

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             |                |   200 OK F13 |
             |    200 OK F14  |<-------------|
             |<---------------|              |
             |                |              |

   Bob wants all calls forwarded to a destination in the PSTN (which is
   just another URI to the proxy server). Alice calls Bob. The proxy
   server rewrites the Request URI, and forwards the INVITE to a
   Gateway.  Details of messaging behind the Gateway are not shown.

   Message Details

     F3 (181 Call is Being Forwarded) Proxy -> Alice

     SIP/2.0 181 Call is Being Forwarded
     Via: SIP/2.0/TLS
     From: Alice <sips:alice@atlanta.example.com>;tag=1234567
     To: Bob <sips:bob@biloxi.example.com>
     Call-ID: 12345600@atlanta.example.com
     CSeq: 1 INVITE
     Content-Length: 0
     Reason: Redirection;cause=5;text="Forward immediate"

     F4 INVITE Proxy -> Gateway

     INVITE sips:+19727293660@gw1.example.com;user=phone SIP/2.0
     Via: SIP/2.0/TLS ss1.example.com:5061;branch=z9hG4bK83749.1
     Via: SIP/2.0/TLS
     Record-Route: <sips:ss1.example.com;lr>
     Max-Forwards: 69
     From: Alice <sips:alice@atlanta.example.com>;tag=1234567
     To: Bob <sips:bob@biloxi.example.com>
     Call-ID: 12345600@atlanta.example.com
     CSeq: 1 INVITE
     Contact: <sips:alice@client.atlanta.example.com>
     History-Info: <sips:bob@biloxi.example.com?Reason: Redirection;
     cause=5;text="Forward immediate">;index=1,
     Content-Type: application/sdp
     Content-Length: ...

     o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
     s=Session SDP
     c=IN IP4 client.atlanta.example.com

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     t=3034423619 0
     m=audio 49170 RTP/AVP 0
     a=rtpmap:0 PCMU/8000

5 IANA considerations

   This document defines one new value for the SIP Reason header [1]
   protocol namespace. The new value is "Redirection" and indicates the
   use of cause value defined in this document.

   This document also creates an IANA registry for cause values that
   populate the cause field of the Reason header when protocol value
   "Redirection" is used and corresponding default values that populate
   the text field. The current cause and text values in this new
   registry are as follows:

     Cause value  Default text value   Reference
     -----------  ------------------------------
     1            Normal redirection   This document
     2            Forward unavailable  This document
     3            Forward busy         This document
     4            Forward no reply     This document
     5            Forward immediate    This document
     6            Deflection           This document
     7            Hunting              This document
     8            Mobile not reachable This document

   New values for this registry can only be defined by means of a
   published standard.

6 Security Considerations

   The security considerations of [1] apply. When the Reason header
   field is embedded within a History-Info header field, the security
   considerations of [3] apply.

   Unauthorised insertion, deletion of modification of the Reason header
   field can provide misleading information to users and applications.
   Eavesdropping on this header field can reveal information about a
   user. Securing of SIP connections by TLS can combat this problem.

   A SIP entity that can provide a redirection reason in a Reason header
   field SHOULD be able to suppress this in accordance with privacy
   requirements of the user concerned.

7 Author's Addresses

   John Elwell
   Siemens Communications

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   Technology Drive
   Nottingham, UK, NG9 1LA
   email: john.elwell@siemens.com

   Roland Jesske
   Deutsch Telekom
   Am Kavalleriesand 3
   Germany-64295 Darmstadt
   email: r.jesske@t-com.net

   Joanne McMillen
   Avaya Inc.
   1300 W. 120th Ave.
   Westminster, CO 80234-2726
   email: joanne@avaya.com

8 Normative References

   [1] H. Schulzrinne, D. Oran, G. Camarillo, "The Reason Header for the
   Session Initiation Protocol (SIP)", RFC 3326.

   [2] J. Rosenberg, H. Schulzrinne, et al., "SIP: Session initiation
   protocol", RFC 3261.

   [3] M. Barnes, "An Extension to the Session Initiation Protocol for
   Request History Information", draft-ietf-sipping-history-info-03
   (work in progress).

   [4] A. Johnston et alia, "Session Initiation Protocol Service
   Examples", draft-ietf-sipping-service-examples-07 (work in progress).

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