Internet Engineering Task Force                           Steve Donovan
Internet Draft                                             MCI Worldcom
October, 1999                                       Expires April, 2000
                                    draft-ietf-sip-session-timer-00.txt


                           SIP Session Timer

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
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Abstract

   This document proposes an extension to the SIP specification [1].
   This extension adds a new message header, Session-Expires, which is
   used to specify the duration of a requested session.

   A session timer can be used to control the duration of a session if,
   for instance, one of the participants in the session wants to limit
   the cost of the session. Call Stateful SIP Proxy Servers can also use
   a session timer to track the status of sessions for which session
   state exists on the servers. Currently a call stateful SIP Proxy
   Server that is not handling the media stream(s) for the session has
   no mechanism to definitively determine the state of all sessions for
   which it has state.  While the SIP Specification does provide the BYE
   method for terminating the session, there is no mechanism for a SIP
   Proxy Server to detect the end of a session when the BYE message is
   not sent or is lost due to network problems.








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1.0 Introduction

   The need for the addition of a session timer was initially motivated
   by the realization that call stateful proxy servers currently have no
   method of determining the end of a session in certain anomalous situ-
   ations.  For instance, when a user agent fails to send a BYE message
   at the end of a session or the BYE message gets lost due to network
   problems.  In this situation, the call stateful proxy will retain
   state for the session and has no deterministic method of determining
   when the state no longer applies.

   With the addition of a session timer to the SIP protocol, the call
   stateful proxy server will have the option of tearing down the ses-
   sion upon the expiration of the session timer.

   A session timer can also be used for other purposes.  For instance,
   it could be used in the implementation of a prepaid service.  In this
   case all session related signaling would be routed through a SIP Pre-
   paid Server that would control the session time based on a sub-
   scriber's prepaid account. The SIP Prepaid Server could use the ses-
   sion timer to force the tear down of the session within a specific
   time.  The session timer could also be used by the SIP Prepaid Server
   as a trigger to indicate to the subscriber that the prepaid account
   requires more funds to extend the call.

   This document proposes the addition of the Session-Expires header to
   various SIP messages.  In addition, a new Request Failure message is
   proposed.  This message would be used to indicate session timer prob-
   lems, including the need for a session-expires header in an INVITE
   message and the need for a smaller session-expires time.

2.0 Session-Expires Header Field Definition

   User agents or proxy servers needing to indicate the maximum duration
   of a session shall use the Session-Expires header.

   The format of the Session-Expires header shall be equivalent to the
   format of the Expires header (see section 6.20 in [1]).  As such, the
   end of the session can be specified as either an absolute or delta
   time.

   The Session-Expires header shall be valid in INVITE and ACK requests.
   In addition it shall be valid in certain response messages.  This
   includes the 200 OK response and the new Request Failure responses
   defined in this document.

   The called UA or any SPS in the path of the session signaling shall
   have the option of rejecting an INVITE that does not contain the Ses-
   sion-Expires header if the service context in which the session
   request is made requires the use of the session timer.



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   The called UA has the option of adding the Session-Expires header to
   the 200 OK response in the case where there was not a Session-Expires
   header in the INVITE message.

   An SPS shall also have the option of adding the Session-Expires
   header to an INVITE message.

   In addition, the called UA and SPS shall have the option of modifying
   the value in the Session-Expires header.  This modification shall
   only be to decrease the requested duration of the session.

   The start time of the session shall be based on the sending of the
   ACK by the calling UA and receipt of the ACK message by the SPS and
   called UA.

   If the called UA or the SPS requires a session timer for a requested
   session and the calling UA does not include the Session-Expires
   header in the INVITE message, then the called UA or the SPS may
   reject the request with a 487 request failure message.

   If the use of a session timer is desirable but optional for the ses-
   sion and the calling UA does not include the Session-Expires header
   in the INVITE then the called UA or SPS may add a Session-Expires
   header to the next session setup message. In this case the SPS shall
   add the Session-Expires header to the INVITE message and the called
   UA shall add the Session-Expires header to the 200 OK response mes-
   sage.

   The calling UA shall have the ability to request an extension to the
   duration of the session by sending a re-INVITE message for the ses-
   sion with a new Session-Expires header.  See section 1.4.6, Changing
   an Existing Session, in [1].

   The called UA or any SPS in the path of the session signaling shall
   have the ability to reject the change in the session duration.

   Upon expiration of the session timer, the calling UA and called UA
   shall have the option of terminating the session using the normal BYE
   method.

3.0 Behavior of SIP User Agents

3.1 Behavior of User Agent Clients

   A User Agent Client (UAC) shall have the ability to include the Ses-
   sion-Expires header in an INVITE message.

   The UAC shall have the ability to request an extension of the session
   timer by sending an INVITE message for an existing session with a
   Session-Expires header.  This should be done enough in advance of the
   session timeout to prevent the called UA or the SPS from timing out
   the session prior to receipt of the new INVITE.

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   When requesting an extension of the session, the UAC shall use a
   value in the Session-Expires header that is less-than or equal to the
   value of the Session-Expires header received in the 200 OK from the
   original setup of the session.

   The UAC shall have the ability to receive a Session-Expires header in
   a 200 OK message.  This is independent of whether or not a Session-
   Expires was included in the original INVITE message.

   The UAC shall have the ability to accept the session time indicated
   in the 200 OK Session-Expires header by including unchanged Session-
   Expires header in the ACK generated as a result of the 200 OK.

   The UAC shall have the ability to reject the session time indicated
   in the 200 OK message by not including a Session-Expires header in
   the ACK generated as a result of the 200 OK message.

   In general, if the UAC is able to support session timers then it
   should accept the session time included in the 200 OK, even if it
   changed between the INVITE and the 200 OK.

3.2 Behavior of User Agent Servers

   The User Agent Server shall have the ability to reject an INVITE mes-
   sage that does not contain a Session-Expires header if the INVITE is
   received in a service context that requires a session timer.  The UAS
   shall reject the INVITE using the following response:

        487 Session-Expires Header Problem
        Session-Expires: n


   The UAS shall have the ability to provisionally reject an INVITE
   request based on the contents of the Session-Expires header received
   in the INVITE message.  For instance, the UAS may choose to reject
   the INVITE if the requested session time is longer than the UAS
   desires to participate in the session.  The UAS shall use the follow-
   ing message to reject the request:

        487 Session-Expires Header Problem
        Session-Expires: n


   In both cases, the UAS shall include in the 487 response an accept-
   able delta time for the session.

   If the Session-Expires header contains a delta time then the UAS
   shall calculate the end of the session based on receipt of the ACK
   message that completes a successful session initiation:

      End of session time = Receipt of ACK time + Session-Expires time


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   The UAS shall have the ability to reject a request to extend the
   length of the session.  The UAS shall do so by sending the following
   response:

        487 Session Time Request Problem
        Session-Expires: n


   A value of zero (0) in the Session-Expires header shall indicate that
   the extension request is rejected.  Any other value should be less
   than that received in the re-INVITE message and should indicate an
   acceptable extension to the session timer.

   If the Session-Expires header in the re-INVITE contains a delta time
   then the UAS shall calculate the new end of the session from the pre-
   vious end of session time:

    New end of session time = Previous end of session time + delta time

   If a request to extend the session time is rejected by the UAS then
   the end of session time that existed prior to the extension request
   shall be remain in effect.

   The UAS shall have the ability to add a Session-Expires header to a
   200 OK response for an INVITE request that did not contain a session-
   Expires header.

   If the UAS adds a Session-Expires header to the 200 OK response and
   the resulting ACK does not contain a Session-Expires header then the
   UAS has the following options:

   - The UAS can choose to participate in the session without a session
   timer.

   - The UAS can choose to not participate in the session.  In this
   case, the UAS shall send a BYE message to tear down the session.

   The UAS shall have the ability to decrease the requested session time
   in the INVITE Session-Expires header.

   If the UAS is sending a 200 OK to an INVITE that contained a Session-
   Expires header then the UAS shall include a Session-Expires header in
   the 200 OK.  The content of the Session-Expires header shall be
   either the content of the Session-Expires header contained in the
   INVITE request, if the UAS accepts the requested duration, or a
   decreased value of the session timer desired by the UAS.

4.0 Behavior of SIP Proxy Servers

   A SIP Proxy Server has the option of tracking the duration of ses-
   sions for which it is a proxy.


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   If the SPS receives an INVITE without a Session-Expires header, it
   has the option of sending the following request failure response
   indicating that the Session-Expires header is required:

        487 Session-Expires Header Problem
        Session-Expires: n


   A SPS has the option of rejecting an INVITE with a Session-Expires
   header based on the time specified in the header.  For instance, if
   the time specified is too long. The proxy server shall do so by send-
   ing the following request failure response:

        487 Session-Expires Header Problem
        Session-Expires: n


   In both cases, the SPS shall include in the 487 response an accept-
   able delta time for the session.

   If the Session-Expires header contains a delta time then the SPS
   shall use the method of calculating the session time specified in
   section 3.2.

   A SPS has the option of rejecting a request for an extension of the
   session timer for an existing session by sending the following
   request failure response:

        487 Session-Expires Header Problem
        Session-Expires: n


   A value of zero (0) in the Session-Expires header shall indicate that
   the extension request is rejected. Any other value should be less
   than that received in the re-INVITE message and should indicate an
   acceptable extension to the session timer.

   If the SPS accepts the request to extend the session timer then it
   shall use the method of calculating the new end of session time spec-
   ified in section 3.2.

   The SPS shall have the ability to add a Session-Expires header to an
   INVITE request that does not contain a Session-Expires header.

   The SPS shall have the ability to decrease the requested session time
   in the INVITE Session-Expires header.

   The content of the Session-Expires header included in the INVITE sent
   by the SPS shall be either the content of the Session-Expires header
   contained in the INVITE request, if the UAS accepts the requested
   duration, or a decreased value of the session timer desired by the
   SPS.

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   If the SPS adds a Session-Expires header to the 200 OK response and
   the resulting ACK does not contain a Session-Expires header then the
   SPS has the following options:

   - The SPS can choose to allow the session without a session timer.

   - The SPS can choose to not allow the session.


5.0 Header Field Definitions

   The following is an extension of tables 4 and 5 in [1] for the Ses-
   sion-Expires header:

                     where  enc.  e-e  ACK  BYE  CAN  INV  OPT  REG
    Session-Expires    R           e    o    -    -    o    -    -
    Session-Expires   200          e    -    -    -    o    -    -
    Session-Expires   487          e    -    -    -    o    -    -


6.0 Request Failure Messages

6.1 487 Session-Expires Header Problem

   The server received an INVITE request with one of the following prob-
   lems:

   - The server requires a Session-Expires header.  In this case the
   response should contain a Session-Expires header with an acceptable
   session time.

   - The server received an INVITE with a Session-Expires header with an
   expiration time longer than the server is willing to accept.  In this
   case the response should contain a Session-Expires header with an
   acceptable session time.

   - The server received an INVITE with a Session-Expires header for an
   existing session and the server does not wish to extend the session.
   In this case the response should contain a Session-Expires header
   with a value of zero (0).

   - The server received an INVITE with a Session-Expires header for an
   existing session with an expiration time longer than the server is
   willing to accept. In this case the response should contain a Ses-
   sion-Expires header with an acceptable session time.

7.0 Security Considerations

   There are no new security considerations introduced by the Session-
   Expires header.



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8.0 Examples

   The following examples are meant to illustrate the functionality
   associated with the Session-Expires header.  In the interest of
   brevity, other headers are intentionally left out of the SIP mes-
   sages.

8.1 Basic session-timer setup with UAS detected timeout

     Calling UA -> Called UA
            INVITE
            Session-Expires: 120

     Calling UA <- Called UA
            200 OK
            Session-Expires: 120

     Calling UA -> Called UA
            ACK                        Calling UA starts session timer
            Session-Expires: 120

     Called UA Receipt of ACK          Called UA starts session timer

     Calling UA determines need to extend session time

     Calling UA -> Called UA
            INVITE
            Session-Expires: 120

     Calling UA <- Called UA
            200 OK
            Session-Expires: 120

     Calling UA -> Called UA
            ACK                        Calling UA resets session end time
            Session-Expires: 120

     Called UA Receipt of ACK          Called UA resets session timer

     Session timeout detected by Called UA

     Calling UA <- Called UA
            BYE

     Calling UA -> Called UA
            200 OK







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8.2 Basic negotiation

     Calling UA -> SPS
             INVITE
             Session-Expires: 240

     SPS -> Called UA
             INVITE                    SPS wants a shorter timer
             Session-Expires: 180

     SPS <- Called UA
             200 OK                    Called UA wants a shorter timer
             Session-Expires: 120

     Calling UA <- SPS
             200 OK
             Session-Expires: 120

     Calling UA -> SPS
            ACK                        Calling UA starts session timer
            Session-Expires: 120

     SPS Receipt of ACK                SPS starts session timer

     SPS -> Called UA
            ACK                        Called UA starts session timer
            Session-Expires: 120

     Calling UA -> SPS
             BYE

     SPS -> Called UA
            BYE

     SPS <- Called UA
            200 OK

     Calling UA <- SPS
            200 OK














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8.3 No Session-Expires Header Rejection by SPS

     Calling UA -> SPS
            INVITE                     No Session-Expires header

     Calling UA <- SPS
            4xx Session-Expires Header Required
            Session-Expires: 120       Suggested session time

     Calling UA -> SPS
            INVITE
            Session-Expires: 120

     Session setup continues


8.4 Invalid Session-Expires Header Rejection by Called UA

     Calling UA -> Called UA
            INVITE                     No Session-Expires header
            Session-Expires: 1000

     Calling UA <- Called UA
            4xx Session Time Request Rejected
            Session-Expires: 120       Suggested session time

     Calling UA -> Called UA
            INVITE
            Session-Expires: 120

     Session setup continues






















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8.5 Session-Expires Header Added by SPS, Rejected by UAC

     Calling UA -> SPS
            INVITE

     SPS -> Called UA
            INVITE                 SPS adds S-E header
            Session-Expires: 180

     SPS <- Called UA
            200 OK                 Called UA wants shorter timer
            Session-Expires: 120

     Calling UA <- SPS
            200 OK
            Session-Expires: 120

     Calling UA -> SPS
            ACK                    Calling UA does not include S-E header


   The SPS has the choice of allowing or not allowing the call as a
   result of the Calling UA not supporting the Session-Expires function.

9.0 Changes in this version

   The changes in this version of the document were primarily editorial
   in nature.

   10.0 References

[] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
   Session Initiation Protocol", RFC 2543, March 1999.

11.0 Author's Address

   Steven R. Donovan
   MCI Worldcom
   1493/678
   901 International Parkway
   Richardson, Texas 75081
   Email: steven.r.donovan@wcom.com











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