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Versions: 00 01 02

Internet Engineering Task Force                        Steven R. Donovan
INTERNET DRAFT                                              MCI Worldcom
February 25, 1999                                Expires August 25, 1999
                            <draft-ietf-mmusic-sip-session-timer-01.txt>

                         SIP Session Timer

Status of this document

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
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Abstract

This document proposes an extension to the SIP specification.  This
extension adds a new message header that is used to specify the
duration of a requested session.

The 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.  It can also be used by stateful SIP
Proxy Servers to track the status of sessions for which session state
exists on the servers. Currently a 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.









Donovan                                                         [Page 1]


Internet draft          The SIP Session Timer          February 25, 1999

1.0 Introduction

The need for the addition of a session timer was initially motivated by
the realization that stateful proxy servers currently have no method of
determining the end of a session in certain anomalous situations.  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 stateful proxy will retain state for the session
and has no deterministic method for determining when the state
no longer applies.

With the addition of a session timer to the SIP protocol, the stateful
proxy server will have the option of tearing down the session 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 server that
would control the session time based on a subscriber's prepaid account.
The SIP server could use the session timer to force tear down of the
session within a specific time.  The session timer could also be used
by the user agent 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, two new Request Failure messages
are proposed.  The first to allow a SIP Proxy Server (SPS) server or a
User Agent Server (UAS) to indicate the need for the Session-Expires
header in INVITE messages.  The second to allow a SPS or UAS to reject
a User Agent Client's (UAC) session timer request.  This can be the
UAC's initial INVITE or the UAC's request to extent an existing session.

2.0 Session-Expires Header Field Definition

The Session-Expires header shall be used by a user agents or proxy
servers to indicate the maximum duration of the session.  The format
of the contents 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
Session-Expires header if the service context in which the session
request is made requires the use of the session timer.



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Internet draft          The SIP Session Timer          February 25, 1999

The called UA and SPS shall also have the option of adding the
Session-Expires header to and INVITE message.

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

The begin time of the session shall start upon receipt of the ACK
message by the SPS and called UA indicating successful setup of the
session.

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 should reject the
request with a 487 message.  If the use of a session timer is
desirable but optional for the session then and the calling UA does
not include the Session-Expires header in the INVITE then the called
UA or SPS should add a Session-Expires header to the next session
setup message.

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 session
with a new Session-Expires header.  See section 1.4.6, Changing an
Existing Session, in [1].

The calling 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 called UA shall have the
option of terminating the session using the normal BYE method.

In addition, a SPS shall have the option of tearing down an expired
session by sending a BYE to both the called UA and the calling UA.  It
is recommended that the SPS delay tearing down the session long enough
for a retry of a lost re-INVITE to be received by the SPS.  This will
also reduce the possibility of both the UAS and SPS attempting to tear
down the call simultaneously.

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 Session-
Expires header in an INVITE message.

The UAC may need to keep a session timer in order to determine
the need for extending the session.





Donovan                                                         [Page 3]


Internet draft          The SIP Session Timer          February 25, 1999

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.

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 the header unchanged
in the ACK to 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
to 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, event 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
message 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 reject an INVITE request based on
the Session-Expires header.  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 following 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 acceptable
delta time for the session.





Donovan                                                         [Page 4]


Internet draft          The SIP Session Timer          February 25, 1999

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

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: 0

A value of zero (0) in the Session-Expires header shall indicate that
the extension request is rejected.

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
previous 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 used.

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:

1 - The UAS can choose to participate in the session without a session
    timer.
2 - 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 that INVITE 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 the decreased
value of the session timer desired by the UAS.

Upon expiration of the session timer, the UAS shall explicitly
terminate the session by sending a BYE message.



Donovan                                                         [Page 5]


Internet draft          The SIP Session Timer          February 25, 1999

4.0 Behavior of SIP Proxy Servers

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

If the SPS receives an INVITE without a Session-Expires header, it has
the option of sending the following message 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
sending the following response:

     487 Session-Expires Header Problem
     Session-Expires: n

In both cases, the SPS shall include in the 487 response an acceptable
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 message:

     487 Session-Expires Header Problem
     Session-Expires: 0

A value of zero (0) in the Session-Expires header shall indicate that
the extension request is rejected.

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
specified 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 the decreased value of the session timer desired by the
SPS.



Donovan                                                         [Page 6]


Internet draft          The SIP Session Timer          February 25, 1999

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:

1 - The SPS can choose to allow the session without a session timer.
2 - The SPS can choose to not allow the session.  In this case, SPS
    shall send a BYE message to the calling and called parties to
    tear down the session.

A SPS shall have the option of tearing down sessions that have expired.
Note that the SPS should wait for the called UA to terminate the call
using normal mechanisms.  If the called UA fails to send a BYE as a
result of session expiration then the SPS can choose to end the call.

The SPS shall terminate the call by sending BYE requests to the calling
UA and the called UA.

5.0 Header Field Definitions

The following is an extension of tables 4 and 5 in [1] for the Session-
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
problems:

- 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 except.  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).

7.0 Security Considerations

There are no security considerations unique to the Session-Expires
header.



Donovan                                                         [Page 7]


Internet draft          The SIP Session Timer          February 25, 1999

8.0 Examples

The following examples are meant to illustrate the functionality
associated with the Session-Expires header.  As such, other headers
are intentionally left out of the SIP messages.

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










Donovan                                                         [Page 8]


Internet draft          The SIP Session Timer          February 25, 1999

8.2 Basic negotiation with SPS Detected timeout

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

Session timeout detected by SPS

Calling UA <- SPS
        BYE

SPS -> Called UA
       BYE

Calling UA -> SPS
       200 OK

SPS <- Called UA
       200 OK








Donovan                                                         [Page 9]


Internet draft          The SIP Session Timer          February 25, 1999

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 SPS

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

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

Calling UA -> SPS
       INVITE
       Session-Expires: 120

     . . .                        Session setup continues






















Donovan                                                        [Page 10]


Internet draft          The SIP Session Timer          February 25, 1999

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.


7.0 Changes in this version

The following is the list of the major changes to this version of
the document:

- Added the session time negotiation capability.  This resulted in
  added support for the Session-Expires header in the 200 OK response
  and ACK request.
- Consolidated the two 4xx responses into one.
- Suggested the value of 487 for the new response message.
- Added the ability to include a suggested session time in the 487
  response.

8.0 Issues

The following are the currently understood issues with the document:

- It may be useful to allow the Called UA to initiate a request to
  extend the session.  This would be in the case that the Calling UA
  does not support the Session Timer capability.









Donovan                                                        [Page 11]


Internet draft          The SIP Session Timer          February 25, 1999

9.0 References

[1] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg,
    "SIP: Session Initiation Protocol", Internet Draft, Internet
    Engineering Task Force, January 15, 1999.  Work in progress.

10.0 Author's Address

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







































Donovan                                                        [Page 12]

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