Internet Engineering Task Force J. Elwell Internet Draft Siemens R. Jesske Deutsche Telekom J. McMillen Avaya Inc. draft-elwell-sipping-redirection-reason-01.txt 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. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress. " The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This examines the need for signalling additional information concerning the reason for redirection in SIP and proposes an extension to the Reason header. Elwell Expires - April 2005 [Page 1] Indicating redirection reasons in SIP October 2004 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 Elwell Expires - April 2005 [Page 2] Indicating redirection reasons in SIP October 2004 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 cases: 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 Elwell Expires - April 2005 [Page 3] Indicating redirection reasons in SIP October 2004 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 reachable. 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]: Elwell Expires - April 2005 [Page 4] Indicating redirection reasons in SIP October 2004 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 | | | | Elwell Expires - April 2005 [Page 5] Indicating redirection reasons in SIP October 2004 | 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: ;tag=2 To: Call-ID: 12345600@example.com CSeq: 1 INVITE History-Info: ;index=1 ... The call is then redirected to a contact URI in a 302 response. The response would be as follows: F3 (302 Moved Temporarily) Bob -> Proxy SIP/2.0 302 Moved temporarily From: ;tag=2 To: ;tag=3 Call-ID: 12345600@example.com CSeq: 1 INVITE Contact: Reason: Redirection;cause=5;text="Forward immediate"  Elwell Expires - April 2005 [Page 6] Indicating redirection reasons in SIP October 2004 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) ¡ Proxy -> Carol INVITE sip:Carol@example.com SIP/2.0 From: ;tag=2 To: Call-ID: 12345600@example.com CSeq: 1 INVITE History-Info: ;index=1, ;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 | | |------------->| Elwell Expires - April 2005 [Page 7] Indicating redirection reasons in SIP October 2004 | | 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 client.atlanta.example.com:5061;branch=z9hG4bK74bf9 ;received=192.0.2.103 From: Alice ;tag=1234567 To: Bob 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 client.atlanta.example.com:5061;branch=z9hG4bK74bf9 ;received=192.0.2.103 Record-Route: Max-Forwards: 69 From: Alice ;tag=1234567 To: Bob Call-ID: 12345600@atlanta.example.com CSeq: 1 INVITE Contact: History-Info: ;index=1, ;index=2 Content-Type: application/sdp Content-Length: ... v=0 o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com s=Session SDP c=IN IP4 client.atlanta.example.com Elwell Expires - April 2005 [Page 8] Indicating redirection reasons in SIP October 2004 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 Elwell Expires - April 2005 [Page 9] Indicating redirection reasons in SIP October 2004 Technology Drive Beeston 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). Intellectual Property Statement 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. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE 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. 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. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Elwell Expires - April 2005 [Page 11]