The SIP INFO Method
RFC 2976

Network Working Group S. Donovan
Request for Comments: 2976 dynamicsoft
Category: Standards Track October 2000
 The SIP INFO Method
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 (2000). All Rights Reserved.
Abstract
 This document proposes an extension to the Session Initiation
 Protocol (SIP). This extension adds the INFO method to the SIP
 protocol. The intent of the INFO method is to allow for the carrying
 of session related control information that is generated during a
 session. One example of such session control information is ISUP and
 ISDN signaling messages used to control telephony call services.
 This and other example uses of the INFO method may be standardized in
 the future.
Table of Contents
 1 Introduction................................................2
 1.1 Example Uses................................................2
 2 INFO Method.................................................3
 2.1 Header Field Support for INFO Method........................3
 2.2 Responses to the INFO Request Method........................4
 2.3 Message Body Inclusion......................................5
 2.4 Behavior of SIP User Agents.................................6
 2.5 Behavior of SIP Proxy and Redirect Servers..................6
 2.5.1 Proxy Server................................................6
 2.5.2 Forking Proxy Server........................................6
 2.5.3 Redirection Server..........................................6
 3. INFO Message Bodies.........................................6
 4. Guidelines for extensions making use of INFO................7
 5. Security Considerations.....................................7
 6. References..................................................8
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RFC 2976 SIP INFO Method October 2000
 7. Acknowledgments.............................................8
 8. Author's Address............................................8
 Full Copyright Statement....................................9
1. Introduction
 The SIP protocol described in [1] defines session control messages
 used during the setup and tear down stages of a SIP controlled
 session.
 In addition, the SIP re-INVITE can be used during a session to change
 the characteristics of the session. This is generally to change the
 properties of media flows related to the session or to update the SIP
 session timer.
 However, there is no general-purpose mechanism to carry session
 control information along the SIP signaling path during the session.
 The purpose of the INFO message is to carry application level
 information along the SIP signaling path.
 The INFO method is not used to change the state of SIP calls, or the
 parameters of the sessions SIP initiates. It merely sends optional
 application layer information, generally related to the session.
 It is necessary that the mid-session signaling information traverse
 the post session setup SIP signaling path. This is the path taken by
 SIP re-INVITEs, BYEs and other SIP requests that are tied to an
 individual session. This allows SIP proxy servers to receive, and
 potentially act on, the mid-session signaling information.
 This document proposes an extension to SIP by defining the new INFO
 method. The INFO method would be used for the carrying of mid-call
 signaling information along the session signaling path.
 1.1 Example Uses
 The following are a few of the potential uses of the INFO message:
 - Carrying mid-call PSTN signaling messages between PSTN
 gateways.
 - Carrying DTMF digits generated during a SIP session.
 - Carrying wireless signal strength information in support of
 wireless mobility applications.
 - Carrying account balance information.
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RFC 2976 SIP INFO Method October 2000
 - Carrying images or other non streaming information between the
 participants of a session.
 These are just potential uses; this document does not specify such
 uses nor does it necessarily recommend them.
 It can also be envisioned that there will be other telephony and
 non-telephony uses of the INFO method.
2. INFO Method
 The INFO method is used for communicating mid-session signaling
 information along the signaling path for the call.
 The INFO method is not used to change the state of SIP calls, nor
 does it change the state of sessions initiated by SIP. Rather, it
 provides additional optional information which can further enhance
 the application using SIP.
 The signaling path for the INFO method is the signaling path
 established as a result of the call setup. This can be either direct
 signaling between the calling and called user agents or a signaling
 path involving SIP proxy servers that were involved in the call setup
 and added themselves to the Record-Route header on the initial INVITE
 message.
 The mid-session information can be communicated in either an INFO
 message header or as part of a message body. The definition of the
 message body and/or message headers used to carry the mid-session
 information is outside the scope of this document.
 There are no specific semantics associated with INFO. The semantics
 are derived from the body or new headers defined for usage in INFO.
 2.1 Header Field Support for INFO Method
 Tables 1 and 2 add a column to tables 4 and 5 in the [1]. Refer
 to Section 6 of [1] for a description of the content of the
 tables. Note that the rules defined in the enc. and e-e columns
 in tables 4 and 5 in [1] also apply to use of the headers in the
 INFO request and responses to the INFO request.
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RFC 2976 SIP INFO Method October 2000
 2.2 Responses to the INFO Request Method
 If a server receives an INFO request it MUST send a final
 response.
 A 200 OK response MUST be sent by a UAS for an INFO request with
 no message body if the INFO request was successfully received for
 an existing call. Beyond that, no additional operations are
 required.
 Header Where INFO
 ------ ----- ----
 Accept R o
 Accept-Encoding R o
 Accept-Language R o
 Allow 200 -
 Allow 405 o
 Authorization R o
 Call-ID gc m
 Contact R o
 Contact 1xx -
 Contact 2xx -
 Contact 3xx -
 Contact 485 -
 Content-Encoding e o
 Content-Length e o
 Content-Type e *
 CSeq gc m
 Date g o
 Encryption g o
 Expires g o
 From gc m
 Hide R o
 Max-Forwards R o
 Organization g o
 Table 1 Summary of header fields, A-0
 Handling of INFO messages that contain message bodies is outside
 the scope of this document. The documents defining the message
 bodies will also need to define the SIP protocol rules associated
 with those message bodies.
 A 481 Call Leg/Transaction Does Not Exist message MUST be sent by
 a UAS if the INFO request does not match any existing call leg.
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RFC 2976 SIP INFO Method October 2000
 If a server receives an INFO request with a body it understands,
 but it has no knowledge of INFO associated processing rules for
 the body, the body MAY be rendered and displayed to the user. The
 INFO is responded to with a 200 OK.
 If the INFO request contains a body that the server does not
 understand then, in the absence of INFO associated processing
 rules for the body, the server MUST respond with a 415 Unsupported
 Media Type message.
 Header Where INFO
 ------ ----- ----
 Priority R o
 Proxy-Authenticate 407 o
 Proxy-Authorization R o
 Proxy-Require R o
 Require R o
 Retry-After R -
 Retry-After 404,480,486 o
 Retry-After 503 o
 Retry-After 600,603 o
 Response-Key R o
 Record-Route R o
 Record-Route 2xx o
 Route R o
 Server r o
 Subject R o
 Timestamp g o
 To gc(1) m
 Unsupported 420 o
 User-Agent g o
 Via gc(2) m
 Warning r o
 WWW-Authenticate 401 o
 Table 2 Summary of header fields, P-Z
 Bodies which imply a change in the SIP call state or the sessions
 initiated by SIP MUST NOT be sent in an INFO message.
 Other request failure (4xx), Server Failure (5xx) and Global
 Failure (6xx) responses MAY be sent for the INFO Request.
 2.3 Message Body Inclusion
 The INFO request MAY contain a message body.
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RFC 2976 SIP INFO Method October 2000
 2.4 Behavior of SIP User Agents
 Unless stated otherwise, the protocol rules for the INFO request
 governing the usage of tags, Route and Record-Route,
 retransmission and reliability, CSeq incrementing and message
 formatting follow those in [1] as defined for the BYE request.
 An INFO request MAY be cancelled. A UAS receiving a CANCEL for an
 INFO request SHOULD respond to the INFO with a "487 Request
 Cancelled" response if a final response has not been sent to the
 INFO and then behave as if the request were never received.
 However, the INFO message MUST NOT change the state of the SIP
 call, or the sessions initiated by SIP.
 2.5 Behavior of SIP Proxy and Redirect Servers
 2.5.1 Proxy Server
 Unless stated otherwise, the protocol rules for the INFO
 request at a proxy are identical to those for a BYE request as
 specified in [1].
 2.5.2 Forking Proxy Server
 Unless stated otherwise, the protocol rules for the INFO
 request at a proxy are identical to those for a BYE request as
 specified in [1].
 2.5.3 Redirection Server
 Unless stated otherwise, the protocol rules for the INFO
 request at a proxy are identical to those for a BYE request as
 specified in [1].
3. INFO Message Bodies
 The purpose of the INFO message is to carry mid-session information
 between SIP user agents. This information will generally be carried
 in message bodies, although it can be carried in headers in the INFO
 message.
 The definition of the message bodies or any new headers created for
 the INFO method is outside the scope of this document. It is
 expected that separate documents will be created to address
 definition of these entities.
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RFC 2976 SIP INFO Method October 2000
 In addition, the INFO method does not define additional mechanisms
 for ensuring in-order delivery. While the CSeq header will be
 incremented upon the transmission of new INFO messages, this should
 not be used to determine the sequence of INFO information. This is
 due to the fact that there could be gaps in the INFO message CSeq
 count caused by a user agent sending re-INVITES or other SIP
 messages.
4. Guidelines for extensions making use of INFO
 The following are considerations that should be taken into account
 when defining SIP extensions that make use of the INFO method.
 - Consideration should be taken on the size of message bodies to be
 carried by INFO messages. The message bodies should be kept small
 due to the potential for the message to be carried over UDP and the
 potential for fragmentation of larger messages.
 - There is potential that INFO messages could be forked by a SIP
 Proxy Server. The implications of this forking of the information
 in the INFO message need to be taken into account.
 - The use of multi-part message bodies may be helpful when defining
 the message bodies to be carried by the INFO message.
 - The extensions that use the INFO message MUST NOT rely on the
 INFO message to do anything that effects the SIP call state or the
 state of related sessions.
 - The INFO extension defined in this document does not depend on
 the use of the Require or Proxy-Require headers. Extensions using
 the INFO message may need the use of these mechanisms. However,
 the use of Require and Proxy-Require should be avoided, if
 possible, in order to improve interoperability between SIP
 entities.
5. Security Considerations
 If the contents of the message body are private then end-to-end
 encryption of the message body can be used to prevent unauthorized
 access to the content.
 There are no other security issues specific to the INFO method.
 The security requirements specified in the SIP specification apply
 to the INFO method.
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RFC 2976 SIP INFO Method October 2000
6. References
 [1] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
 "SIP: Session Initiation Protocol", RFC 2543, March 1999.
7. Acknowledgements
 The author would like to thank Matthew Cannon for his contributions
 to this document. In addition, the author would like to thank the
 members of the MMUSIC and SIP working groups, especially Jonathan
 Rosenberg, for comments and suggestions on how to improve the
 document.
8. Author's Address
 Steve Donovan
 dynamicsoft
 5100 Tennyson Parkway, Suite 200
 Plano, Texas 75024
 Email: sdonovan@dynamicsoft.com
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RFC 2976 SIP INFO Method October 2000
9. Full Copyright Statement
 Copyright (C) The Internet Society (2000). 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.
Acknowledgement
 Funding for the RFC Editor function is currently provided by the
 Internet Society.
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