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, 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.
H. Schulzrinne et. al. [Page 1]
Internet Draft SIP April 23, 2002
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 ........................................ 7
H. Schulzrinne et. al. [Page 2]
Internet Draft SIP April 23, 2002
1 Introduction
The same SIP [1] 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 an INVITE is rejected, not because the call is declined, but
because some aspect of the request was not acceptable, if the INVITE
was forked, the error response is not forwarded towards the UAC by
the forking proxy. 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 [2].
Initially, the Reason header field defined here appears to be most
useful for BYE and CANCEL requests, but it can appear in any 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 [3] and
indicate requirement levels for compliant SIP implementations.
2 The Reason Request Header Field
The Reason header field can appear in any request and in 155 (Update
Requested) responses. The syntax of the header field follows the
standard SIP parameter syntax.
H. Schulzrinne et. al. [Page 3]
Internet Draft SIP April 23, 2002
Reason = "Reason" HCOLON protocol *(SEMI reason-params)
Protocol = 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: 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
H. Schulzrinne et. al. [Page 4]
Internet Draft SIP April 23, 2002
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 |
| |----------------->|
H. Schulzrinne et. al. [Page 5]
Internet Draft SIP April 23, 2002
| 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
H. Schulzrinne et. al. [Page 6]
Internet Draft SIP April 23, 2002
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 and Rohan May 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: schulzrinne@cs.columbia.edu
David R. Oran
Cisco Systems, Inc.
Acton, MA
USA
electronic mail: oran@cisco.com
Gonzalo Camarillo
Ericsson
Advanced Signalling Research Lab.
FIN-02420 Jorvas
Finland
electronic mail: Gonzalo.Camarillo@ericsson.com
8 Bibliography
[1] J. Rosenberg, H. Schulzrinne, et al. , "SIP: Session initiation
protocol," Internet Draft, Internet Engineering Task Force, Feb.
2002. Work in progress.
[2] J. Rosenberg, "The SIP UPDATE method," Internet Draft, Internet
H. Schulzrinne et. al. [Page 7]
Internet Draft SIP April 23, 2002
Engineering Task Force, Mar. 2002. Work in progress.
[3] 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. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS 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.
H. Schulzrinne et. al. [Page 8]
