Internet Engineering Task Force SIP WG Internet Draft H. Schulzrinne Columbia University D. Oran Cisco G. Camarillo Ericsson
draft-ietf-sip-reason-00.txt April 23,draft-ietf-sip-reason-01.txt May 14, 2002 Expires: August 2002 The Reason Header Field for the Session Initiation Protocol 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 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 To view the list Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. Abstract For creating services, it is often useful to know why a SIP request was issued. This document defines a header field, Reason, that provides this information. The Reason header field is also intended to be used to encapsulate a final status code in a provisional response. This functionality is needed to resolve the "Heterogeneous Error Response Forking Problem", or HERFP. Table of Contents 1 Introduction ........................................ 3 1.1 Terminology ......................................... 3 2 The Reason Request Header Field ..................... 3 3 Examples ............................................ 5 3.1 Call Completed Elsewhere ............................ 5 3.2 Refusing an Offer that Comes in a Response .......... 5 3.3 Third Party Call Control ............................ 5 3.4 ISUP interworking ................................... 6 4 IANA Considerations ................................. 6 5 Security Considerations ............................. 7 6 Acknowledgments ..................................... 7 7 Authors' Addresses .................................. 7 8 Bibliography ........................................Normative References ................................ 7 1 Introduction The same SIP  request can be issued for a variety of reasons. For example, a SIP CANCEL request can be issued if the call has completed on another branch or was abandoned before answer. While the protocol and system behavior is the same in both cases, namely, alerting will cease, the user interface may well differ. In the second case, the call may be logged as a missed call, while this would not be appropriate if the call was picked up elsewhere. Third party call controllers sometimes generate a SIP request upon reception of a SIP response from another dialog. Gateways generate SIP requests after receiving messages from a different protocol than SIP. In both cases the client may be interested in knowing what triggered the SIP request. SIP responses already have a means of informing the user of why a request failed. The simple mechanism in this draft accomplishes something roughly similar for requests. When anAn INVITE is rejected,can sometimes be rejected not because the call issession initiation was declined, but because some aspect of the request was not acceptable, ifacceptable. If the INVITE was forked,forked and resulted in a rejection, the error response is notmay never be forwarded towardsto the UAC byclient unless all the forking proxy.other branches also reject the request. This problem is known as the "Heterogeneous Error Response Forking Problem", or HERFP. The header field defined in this draft allows encapsulating the final error response in a 155 (Update Requested) provisional response . Initially, the Reason header field defined here appears to be most useful for BYE and CANCEL requests, but it can appear in any request within a dialog, in any CANCEL request and in 155 (Update Requested) responses. When used in requests, clients and servers are free to ignore this header field. It has no impact on protocol processing. 1.1 Terminology In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119  and indicate requirement levels for compliant SIP implementations. 2 The Reason Request Header Field The Reason header field can appear in any request within a dialog, in any CANCEL request and in 155 (Update Requested) responses. The syntax of the header field follows the standard SIP parameter syntax. Reason = "Reason" HCOLON reason-value *(COMMA reason-value) reason-value = protocol *(SEMI reason-params) Protocolprotocol = SIP / Q.850 / token reason-params = protocol-cause / reason-text / reason-extension protocol-cause = "cause" EQUAL cause cause = 1*DIGIT reason-text = "text" EQUAL quoted-string reason-extension = generic-param The following values for the protocol field have been defined: SIP/2.0:SIP: The cause parameter contains a SIP status code. Q.850: The cause parameter contains an ITU-T Q.850 cause value in decimal representation. Examples are: Reason: SIP ;cause=200 ;text="Call completed elsewhere" Reason: Q.850 ;cause=16 ;text="Terminated" Reason: SIP ;cause=600 ;text="Busy Everywhere" Reason: SIP ;cause=580 ;text="Precondition Failure" Proxies generating a CANCEL request upon reception of a CANCEL from the previous hop that contains a Reason header field SHOULD copy it into the new CANCEL request. In normal SIP operation, a SIP status codes in a response provides the client with information about the request that triggered the response, the session parameters, or the user. For example, a 405 (Method not allowed) response indicates that the request contained an unsupported method. A 488 (Not Acceptable Here) indicates that the session parameters are unacceptable and a 486 (Busy Here) provides information about the status of the user. Any SIP status code MAY appear in the Reason header field of a request. However, status codes that provide information about the user and about session parameters are typically useful for implementing services whereas status codes intended to report errors about a request are typically useful for debugging purposes. A SIP message MAY contain more than one Reason values (i.e., multiple Reason lines), but all of them MUST have different protocol values (e.g., one SIP and another Q.850). A implementation is free to ignore Reason values that it does not understand. 3 Examples This section contains a number of examples that illustrate the use of the Reason header field. 3.1 Call Completed Elsewhere A proxy forks an INVITE request and one of the branches returns a 200 (OK). The forking proxy includes this status code in the CANCEL that it sends to the rest of the branches. 3.2 Refusing an Offer that Comes in a Response A client sends an empty INVITE and receives an unacceptable offer in a 200 (OK) response. The client sends an ACK with a correctly formatted answer and immediately sends a BYE to terminate the session. The client includes a 488 (Not Acceptable Here) status code in a Reason header field. 3.3 Third Party Call Control The third party call controller of figure 1 tries to establish a session between A and B. However, user B is busy. The controller sends a BYE with the status code 486 (Busy Here) in a Reason header field. A Controller B | INV no SDP | | |<------------------| | | | | | 200 SDP A1 | | |-----------------> | | | | | | ACK SDP held | | |<------------------| | | | | | | INV no SDP | | |----------------->| | | | | | 486 Busy Here | | |<-----------------| | | | | | ACK | | |----------------->| | BYE (486) | | |<------------------| | | | | | 200 OK | | |-----------------> | | | | | Figure 1: Third Party Call Control 3.4 ISUP interworking The PSTN gateway of figure 2 generates an INVITE that has to be CANCELed when a REL (release) message is received from the ISUP side. The CANCEL request contains the Q.850 cause value (16 Normal Call Clearing) of the REL message. A Gateway B | IAM | | |-----------------> | | | | INVITE | | |----------------->| | | | | | 100 Trying | | |<-----------------| | REL (16) | | |-----------------> | | | | CANCEL (Q.850 16)| | |----------------->| | | 200 OK | | |<-----------------| Figure 2: ISUP Interworking 4 IANA Considerations IANA registers new protocol values for the Reason header field. IANA also registers new values for the triggered parameter. These values MUST refer to either an ITU-T Recommendation number, an IETF protocol name or the recognized protocol identifier from another standardization organization. 5 Security Considerations While spoofing or removing the Reason header field from a request has no impact on protocol operation, the user interface may change and end systems may provide services based on this header field. Spoofing or removing the Reason header field from a 155 (Update Requested) response can make impossible for a client to update properly its previous request, making therefore session establishment impossible. Thus, it is RECOMMENDED that this header field is protected by a suitable integrity mechanism. 6 Acknowledgments Jonathan Rosenberg andRosenberg, Rohan MayMahy and Vijay K. Gurbani provided helpful comments and suggestions. 7 Authors' Addresses Henning Schulzrinne Dept. of Computer Science Columbia University 1214 Amsterdam Avenue New York, NY 10027 USA electronic mail: email@example.com David R. Oran Cisco Systems, Inc. Acton, MA USA electronic mail: firstname.lastname@example.org Gonzalo Camarillo Ericsson Advanced Signalling Research Lab. FIN-02420 Jorvas Finland electronic mail: Gonzalo.Camarillo@ericsson.com 8 BibliographyNormative References  J. Rosenberg, H. Schulzrinne, et al. , "SIP: Session initiation protocol," Internet Draft, Internet Engineering Task Force, Feb. 2002. Work in progress.  J. Rosenberg, "The SIP UPDATE method," Internet Draft, Internet Engineering Task Force, Mar. 2002. Work in progress.  S. Bradner, "Key words for use in RFCs to indicate requirement levels," RFC 2119, Internet Engineering Task Force, Mar. 1997. Full Copyright Statement Copyright (c) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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