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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



Donovan                     Standards Track                     [Page 1]

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.



Donovan                     Standards Track                     [Page 2]

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.






Donovan                     Standards Track                     [Page 4]

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.






Donovan                     Standards Track                     [Page 7]

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






























Donovan                     Standards Track                     [Page 8]

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.



















Donovan                     Standards Track                     [Page 9]