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Internet Engineering Task Force                                   SIP WG
Internet Draft                                     S.Donovan,J.Rosenberg
draft-ietf-sip-session-timer-02.txt                          dynamicsoft
July 14, 2000
Expires: January 2001


                         The 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 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

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.


Abstract

   This document proposes an extension to the Session Initiation
   Protocol (SIP). This extension allows for a periodic refresh of SIP
   sessions through a re-INVITE. The refresh allows both user agents and
   call stateful proxies to determine if the SIP session is still
   active. The extension defines a new general header, Session-Expires,
   which conveys the lifetime of the session.


1 Introduction

   The Session Initiation Protocol (SIP) [1], does not currently define
   a keepalive mechanism. The result is that call stateful proxies will
   not always be able to determine whether a call is still active or
   not. 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



S.Donovan,J.Rosenberg                                         [Page 1]


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   problems, a call stateful proxy will not know when the session has
   ended. In this situation, the call stateful proxy will retain state
   for the call and has no deterministic method of determining when the
   call state information no longer applies.

   To resolve this problem, this extension defines a keepalive mechanism
   for SIP sessions. UAs send periodic re-INVITEs to keep the session
   alive. The interval for the re-INVITEs is determined through a
   negotiation mechanism defined here. If a re-INVITE is not received
   before the interval passes, the session is considered terminated.
   Both UAs are supposed to send a BYE, and call stateful proxies can
   remove any state for the call.

   This refresh mechanism has additional applications. For the same
   reasons a call stateful proxy server would like to determine whether
   the session is still active, a user agent would like to make this
   determination. This determination can be made at a user agent without
   the use of SIP level mechanisms; for audio sessions, periodic RTCP
   packets serve as an indication of liveness [2]. However, it is
   desirable to separate SIP call liveness from the details of the
   particular session.

   Another important application of the session timer is in NAT and
   firewall control [3]. In order for SIP to flow through a NAT or
   firewall, holes and/or address bindings must be dynamically created
   to allow the media for the session to flow. These holes and/or
   address bindings represent state which must be eventually removed.
   Relying on a BYE to trigger the removal of state, besides being
   unreliable, introduces a potential denial of service attack.

   This document proposes an extension to SIP that defines a session
   expiration mechanism. Periodic refreshes, through re-INVITEs, are
   used to keep the session active. The extension is sufficiently
   backwards compatible with SIP that it works so long as one of the two
   participants in a call leg understand the extension. A new general
   header, the Session-Expires header, is defined. It conveys the
   expiration time of the session.

2 Protocol Overview

   UACs which support the session timer extension defined here include a
   Supported header in all requests, excepting ACK, containing the
   option tag "timer" [4]. When a UAC makes a call, it MAY include a
   Session-Expires header in the INVITE request. The presence of the
   Session-Expires header indicates that the UAC wishes to use the
   session timer for this call. Its value indicates the desired
   expiration time of the session.




S.Donovan,J.Rosenberg                                         [Page 2]


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   Proxies on the signaling path may have their own requirements for the
   refresh interval of the session. If the Supported header in the
   request lists the option tag "timer", a proxy can be certain the UAC
   understands the session timer. In this case, if no Session-Expires
   was present, the proxy can insert one if it so desires. If one was
   present, the proxy can lower, but not increase, the expiration time
   of the session. The proxy remembers the value of Session-Expires it
   placed into the request, and also remembers that the UAC supported
   session timer. The UAC will ultimately be responsible for sending the
   refreshes for this call leg.

   If the Supported header was absent from the request, or was present
   but didn't include the tag "timer", the proxy knows the UAC cannot
   generate refreshes, but the called party may be able to. If no
   Session-Expires was present, the proxy can insert one if it so
   desires. If one was present, the proxy can lower, but not increase,
   the expiration time of the session. The proxy remembers the value of
   Session-Expires it placed into the request, and also remembers that
   the UAC did not support the session timer. The UAS may be responsible
   for sending the refreshes for this call leg. If the proxy wishes to
   insist that the call is only established if the UAS supports session
   timer, it MAY insert a Require header into the request, with the
   value "timer".

   Eventually, the initial INVITE reaches the UAS. There are two cases -
   the UAS supports session timer, or it does not. If it does, the UAS
   MAY add a session timer or reduce the session timer from the request.
   If the UAS accepts the call, it places the final value of the session
   timer in the Session-Expires in the 200 OK response. If the request
   also contained the Supported header with the value "timer", the UAS
   knows the UAC can do refreshes. To make sure the UAC is aware it must
   actually do them, the UAS MUST add a Require to the 200 OK response,
   with the tag "timer". The UAS does not perform the refreshes in this
   case. If the request did not contain the Supported header with the
   value "timer", the UAS knows the UAC cannot perform refreshes. So, it
   assumes the responsibility. It MUST add the value of the Session-
   Timer to the response, but it MUST NOT add a Require header with
   value "timer". This is because the UAC does not support the
   extension; the UAS cannot insist on its usage at the UAC, which is
   what a Require header in the response would accomplish.

   If the UAS does not support session timer, it behaves as a normal
   UAS. It will, in this case, neither insert Session-Expires in the
   response nor a Require header with value "timer".

   This response travels backwards through the proxies. When it reaches
   a proxy which remembers that it asked for session timer, the proxy
   examines the response. If the response did not contain a Session-



S.Donovan,J.Rosenberg                                         [Page 3]


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   Expires header, but the proxy remembers that the UAC supported
   session timer, the proxy knows that the UAC supports session timer,
   but the UAS did not. So, it inserts the Session-Expires header into
   the response and also adds a Require header with a value of "timer".
   If the response the proxy received did have a Session-Expires header,
   but no Require header with value "timer", the proxy knows that the
   UAS understands session timer, but not the UAC. It simply forwards
   this request upstream. If the proxy gets a response without Session-
   Expires, and the proxy remembers that the UAC did not support session
   timer, the proxy knows that session timer cannot be used, since
   neither UAS nor UAC support it. Finally, if the response contains the
   Session-Expires and Require header with the value "timer", the proxy
   knows that both UAC and UAC support session timer, and that the UAC
   will be performing refreshes.

   The response then arrives at the UAC. If it contains a Require header
   with the value "timer", the UAC knows it is responsible for
   refreshes. The response will also contain a Session-Expires header,
   and the value of that header is used as the interval for refreshes.

   The UAC then ACKs the INVITE. The Session-Expires MUST NOT be
   included in the ACK.

   If the calling UA is supposed to perform refreshes and wishes to keep
   the session alive, it MUST send a re-INVITE before the expiration
   time. This re-INVITE MAY contain a Session-Expires header. The
   processing of this re-INVITE by proxies and UAS is identical to that
   of the initial INVITE.

   If the called UA is supposed to perform refreshes and wishes to keep
   the session alive, it MUST send a re-INVITE before the expiration
   time. This re-INVITE MAY contain a Session-Expires header. The
   processing of this re-INVITE by proxies and UAS is identical to that
   of the initial INVITE.

   If the calling UA or the called UA is not performing refreshes, and
   does not receive a re-INVITE refreshing the session before the
   session expires, they SHOULD send a BYE to terminate the call. If a
   refreshing UA does not receive a response to the re-INVITE used to
   refresh the session, it SHOULD send a BYE to terminate the call.
   Similarly, if a proxy doesn't receive a re-INVITE before expiration
   of the session, it MAY remove associated call state, and MAY free any
   resources associated with the call. Unlike the UA, it MUST NOT send a
   BYE.

3 Session-Expires Header Field Definition

   The Session-Expires header conveys the expiration time for a SIP



S.Donovan,J.Rosenberg                                         [Page 4]


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   session. It is placed in only in INVITE requests, and is allowed in
   any response to an INVITE. Like the SIP Expires header, it can
   contain either an absolute time or a delta-time. If it contains an
   absolute time, this time indicates the time at which a proxy or UA
   may safely destroy any state associated with the call. If it contains
   a delta time, the expiration time of the session is defined as that
   delta plus the time at which the header is observed in a response.
   For example, if a UAS generates a 200 OK response to a re-INVITE that
   contained a Session-Expires header with a value of 3600, the UAS
   computes the expiration time of the session as one hour after the
   time when the 200 OK was sent. For each proxy, the expiration time is
   one hour after the time when the 200 OK was received or sent
   (assuming these two are sufficiently close together). For the UAC,
   the expiration time is one hour after the receipt of the 200 OK.

   The syntax of the Session-Expires header is:



        Session-Expires  =  "Session-Expires" ":" ( SIP-date |
                            delta-seconds )


   Both SIP-Date and delta-seconds are defined in Section 6.20 of RFC
   2543 [1].

   Table 1 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     n     h    -    -    -    o    -    -
    Session-Expires    r     n     h    -    -    -    o    -    -



4 UAC Behavior

   A UAC which supports the session timer extension defined here MUST
   include a Supported header in each request (excepting ACK), listing
   the option tag "timer" [4]. It MUST do so even if the UAC is not
   requesting keepalives for the call.

   If the UAC wishes to request keepalives for this call, it MUST
   include a Session-Expires in the INVITE request used to initiate the
   call. The value of this header indicates the time when the UAC will
   consider the call expired if no refresh is sent. If the request is



S.Donovan,J.Rosenberg                                         [Page 5]


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   being authenticated, the Session-Expires header MUST appear before
   the Authorization or Proxy-Authorization headers.

   The UAC MAY include a Require in the request with the value "timer"
   to indicate that the UAS must support the session timer to
   participate in the session. In addition, the UAC MAY include a
   Proxy-Require header in the request with the value "timer" to
   indicate that proxies must support session timer in order to
   correctly process the request. However, usage of either Require or
   Proxy-Require by the UAC is NOT RECOMMENDED. They are not needed,
   since the extension works even when only the UAC supports the
   extension.

   When the response to the initial INVITE request arrives, it may or
   may not contain a Session-Expires header, and may or may not contain
   a Require header with the value "timer". UACs MUST be prepared to
   receive a Session-Expires header in a response even if none were
   present in the request.

   Table 4 describes the behavior of the UAC after receiving a 200 OK
   response to an initial INVITE, or any final response to a re-INVITE.


   Require  Session-Timer  Behavior
   N        N              Do nothing.
   N        Y              Do nothing.
   Y        N              Should never happen. Do nothing.
   Y        Y              UAC SHOULD perform refreshes.


   If the UAC must refresh, it follows the following procedure. The UAC
   computes the expiration time of the session. If the Session-Expires
   contains an absolute time, that is the time of expiration. If it
   contains a delta-time, the expiration time is the time of reception
   of the response plus that delta time. Let the difference in time
   between the reception of the response and the session expiration time
   be called the refresh interval. Note that this expiration applies
   only to the call leg associated with the response. It is explicitly
   allowed for there to be differing session timers (or none at all) on
   differing call legs.

   If UA wishes to continue with the session beyond the expiration, it
   MUST generate a refresh before the expiration time. It is RECOMMENDED
   that this refresh be sent once half the refresh interval has elapsed.
   This refresh is accomplished by sending a re-INVITE request on the
   given call leg. Sending of the refresh (in terms of this extension),
   and processing the response are exactly identical to the rules above.




S.Donovan,J.Rosenberg                                         [Page 6]


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   A UA MAY use the refreshing re-INVITE as a normal SIP re-INVITE; that
   is, this re-INVITE MAY contain an updated session description. In the
   case where the re-INVITE contains an updated session description, the
   session description MUST somehow indicate that it has changed. In the
   case of SDP [5], this is accomplished by using a different value for
   the origin field.

   If the refreshing re-INVITE is used solely for refreshing, it still
   contains SDP. However, this is exactly the same SDP as sent
   previously by the UA.

   If no response to a refreshing re-INVITE is received before the
   expiration of the session, the UA SHOULD send a BYE request to
   terminate the call. It SHOULD send this BYE slightly before
   expiration of the session. Ten seconds is RECOMMENDED.


        Firewalls and NATs may be very unforgiving about allowing
        SIP traffic to pass after the expiration time of the
        session. It is for this reason that the BYE should be sent
        before the expiration.

   Note that it is possible that the calling UA is generating refreshes,
   and then it receives a re-INVITE. After following the rules for UAS
   described below, the calling UA now determines it is not supposed to
   generate refreshses. The UA SHOULD cease generating refreshes in this
   case, and let the other UA perform them. This also implies that the
   responsibility for generating refreshes may change many times during
   a call.

   Also note that a non-200 response to an initial INVITE MAY indicate a
   session expiration. This happens when a UA crashes and reboots
   between refreshes. When the refresh arrives at the rebooted UA, it
   decides to reject the call. In order to alert the UAC that it
   believes the call is down (the UAC believes this request to be a re-
   INVITE, and so a non-200 OK final response will not cause it to
   destroy the call), it MAY include a Session-Expires and Require into
   the non-200 response (assuming session timer is supported by the
   UAC), with an immediate expiration time.

5 Proxy Behavior

   Session expirations are mostly of interest to call stateful proxy
   servers. However, a stateful proxy server may also follow the rules
   described here. Stateless proxies MUST NOT attempt to request session
   timers.





S.Donovan,J.Rosenberg                                         [Page 7]


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        The proxy processing rules require the proxy to remember
        information between the request and response, ruling out
        stateless proxies.

   Due to local policy, a proxy may have guidelines about the desired
   maximum lifetime for a call initiated through it. When an initial
   INVITE is received to establish a call, a proxy MAY insert a
   Session-Expires header in the request before forwarding it, if none
   was present in the request. This Session-Expires header may contain
   any desired expiration time the proxy would like. If the request
   already had a Session-Expires header, the proxy MAY reduce the value
   in the Session-Expires header before forwarding the request, but MUST
   NOT increase it.

   Assuming the proxy wishes to use session timer (and thus has possibly
   inserted the Session-Expires header or reduced it), the proxy MUST
   remember that it is using session timer, and also remember the value
   in the request it forwarded, until the final response to the request
   arrives, or the transaction times out. If the request also contained
   the Supported header with the value "timer", the proxy MUST remember
   this as well.

   If the request did not contain a Supported header with the value
   "timer", the proxy MAY insert a Require header into the request, with
   the value "timer". This allows the proxy to insist on session timer
   for the session. This header is not needed if a Supported header was
   in the request; in this case, the proxy can already be sure that the
   session timer can be used for the session.

   When the final response to the request arrives, it is examined by the
   proxy. If it does not contain a Session-Expires header, and the proxy
   remembers that the UAC supports session timer, the proxy MUST insert
   a Session-Timer header into the response with the value it remembered
   placing into the forwarded request. The proxy MUST also insert the
   Require header into the response, with the value "timer", before
   forwarding it upstream. The value of the Session-Timer in the
   forwarded response represents the expiration time of the session.

   If the final response to the request arrives without a Session-
   Expires header, and the proxy remembers that the proxy did not
   support session timer, then session timer is not available for this
   session.

   If the final response does contain a Session-Expires header, its
   value represents the expiration time of the session.

   In all cases, the proxy MUST NOT modify the value of the Session-
   Expires header received in the response before forwarding it



S.Donovan,J.Rosenberg                                         [Page 8]


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   upstream.

   The expiration of the call will occur at the time indicated in the
   Session-Expires header. If the Session-Expires header contains a
   delta-time, the expiration time is the time of receipt of the final
   response, plus that delta time.

   Re-INVITE requests may arrive from either UA, refreshing the session
   and extending the expiration time. Processing of these re-INVITEs by
   a proxy is identical to the procedure for processing the initial
   INVITE.

   If the session expires without having seen a response to a re-INVITE,
   the proxy MAY consider the call leg terminated. This means it MAY
   flush any state associated with that call leg.

6 UAS Behavior

   When a UAS receives an INVITE for a new call, and that INVITE
   contains a Session-Expires header, the UAS MUST place a Session-
   Expires header in a 200 OK response (assuming the UAS accepts the
   call). The UAS MAY reduce the expiration time when it places this
   header into the response, but it MUST NOT increase it.  If the inital
   INVITE did not contain a Session-Expires header, but it did contain a
   Supported header containing the option tag "timer", the UAS MAY
   insert a Session-Expires header into the response. This header MAY
   have any desired expiration time.

   If the UAS places a Session-Expires header into the response, and the
   request contained a Supported header with the value "timer", the UAS
   MUST place a Require header into the response with the value "timer".
   In this case, the UAC will generate the refreshes.

   If the UAS places a Session-Expires header into the response, and the
   request did not contain a Supported header with the value "timer",
   the UAS MUST NOT place a Require header into the response with the
   value "timer". In this case, the UAS will generate the refreshes.

   If the UAS is generating refreshes, it computes the expiration time
   of the session based on the value of the Session-Expires header in
   the response it sent. The processing from this point is as described
   in section 4 once the UAC determined it was performing refreshses.

   As described in Section 4, the refreshing UA will send periodic re-
   INVITEs to refresh the session. A UAS MUST be prepared to receive and
   process these re-INVITEs. Processing of the re-INVITE, as far as the
   session timer is concerned, is identical to the rules for the initial
   INVITE, described above. Note that if the 200 OK to the re-INVITE has



S.Donovan,J.Rosenberg                                         [Page 9]


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   no Session-Expires, no expiration time exists for the session.

7 Security Considerations

   The session timer introduces the capability of a proxy to effectively
   force clients to send refreshes at a rate of the proxies choosing. It
   can also force the clients to send a BYE by setting the expirations
   to times that are too short. This introduces the possibility of rogue
   proxies introducing denial-of-service attacks. Use of short refresh
   intervals allows the proxies to create network load. The session
   timer mechanism allows the proxy to be able to terminate established
   calls - a capability a normal proxy doesn't have in [1].

   As a result of these potential attacks, it is RECOMMENDED that IP or
   transport level security is used when communicating between proxies.

8 Examples

   The following examples are meant to illustrate the functionality
   associated with the session timer. In the interest of brevity, all
   headers excepting Supported, Session-Expires and Require are
   intentionally left out of the SIP messages.

8.1 Basic session-timer setup with UAS detected timeout



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

   Calling UA <- Called UA
          200 OK
          Require: timer
          Session-Expires: 120    Called UA starts session timer on send
                                  Calling UA starts session timer on receipt
   Calling UA -> Called UA
          ACK

   n60 seconds later:

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

   Calling UA <- Called UA



S.Donovan,J.Rosenberg                                        [Page 10]


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          200 OK
          Require: timer
          Session-Expires: 120    Called UA starts session timer on send
                                  Calling UA starts session timer on receipt
   Calling UA -> Called UA
          ACK

   110 seconds later the called UA did not receive a re-INVITE. It
   therefore considers the call terminated and sends a BYE:

   Calling UA <- Called UA
          BYE

   Calling UA -> Called UA
          200 OK




8.2 Basic negotiation of Session Time

   In this configuration, two UAs talk through a single stateful proxy
   server (SPS). Both the SPS and the UAS reduce the session timer.



   Calling UA -> SPS
           INVITE
           Supported: timer
           Session-Expires: 240

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

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

   Calling UA <- SPS
           200 OK
           Session-Expires: 120  Proxy starts timer on send
           Require: timer        Calling UA starts timer on receipt

   Calling UA -> SPS
          ACK



S.Donovan,J.Rosenberg                                        [Page 11]


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   SPS -> Called UA
          ACK

   For whatever reason, the calling UA decides not to refresh. So, after
   120 seconds, it sends a BYE.

   Calling UA -> SPS
           BYE

   SPS -> Called UA
          BYE

   SPS <- Called UA
          200 OK

   Calling UA <- SPS
          200 OK



8.3 No Session-Expires Header in INVITE

   In this scenario, the UA sends an INVITE without a Session-Expires
   header and with a Supported header containing the option tag "timer".
   Since the proxy requires session timer for the call, it adds the
   Session-Expires header.



   Calling UA -> SPS
          INVITE                 No Session-Expires
          Supported: timer

   SPS -> Called UA
          INVITE
          Supported: timer
          Session-Expires: 120

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

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




S.Donovan,J.Rosenberg                                        [Page 12]


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   Calling UA -> SPS
          ACK

   SPS -> Called UA
          ACK



8.4 Session timer without Calling UA Support

   In this scenario, the calling UA sends and INVITE without a Session-
   Expires header and without a Supported header containing the option
   tag "timer". Since the proxy requires session timer for the call it
   adds Session-Expires header before proxying the INVITE to the called
   UA.



   Calling UA -> SPS
           INVITE

   SPS -> Called UA
           INVITE                SPS adds session expires
           Session-Expires: 180

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

   Calling UA <- SPS
           200 OK
           Session-Expires: 120  Proxy starts timer on send

   Calling UA -> SPS
          ACK

   SPS -> Called UA
          ACK

   Sixty seconds later:

   SPS <- Called UA
          INVITE
          Supported: timer
          Session-Expires: 120

   Calling UA <- SPS
          INVITE



S.Donovan,J.Rosenberg                                        [Page 13]


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          Supported: timer
          Session-Expires:120

   Calling UA -> SPS
          200 OK

   SPS -> Called UA
          200 OK

   SPS <- Called UA
          ACK

   Calling UA <- SPS
          ACK

   The Calling UA terminates the session for non timer
   related reasons:

   Calling UA -> SPS
          BYE

   SPS -> Called UA
          BYE

   SPS <- Called UA
          200 OK

   Calling UA <- SPS
          200 OK




8.5 Addition of Session-Expires by UAS

   In this scenario, the calling UA indicates that it supports the
   session timer, but does not add the Session-Expires into the INVITE.
   The UAS, however, adds the header.



   Calling UA -> Called UA
          INVITE
          Supported: timer

   Calling UA <- Called UA
          200 OK
          Require: timer



S.Donovan,J.Rosenberg                                        [Page 14]


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          Session-Expires: 120   Called UA starts timer on send
                                 Calling UA starts timer on receipt

   Calling UA -> Called UA
          ACK




8.6 Session Timer without Called UA Support

   In this scenario, the calling UA indicates that it supports the
   session timer, but does not add the Session-Expires header into the
   INVITE. The proxy adds it, but session timer is not supported by the
   UAS. The call is still set up with a session timer, as all that is
   required is for one of the user agents involved in the call leg to
   understand the "timer" feature.



   Calling UA -> SPS
          INVITE
          k: timer

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

   SPS <- Called UA
          200 OK                 Called UA doesn't understand session timer

   Calling UA <- SPS
          200 OK
          Session-Expires: 180   SPS adds Session-Expires and Require
          Require: timer

   Calling UA -> SPS
          ACK

   SPS -> Called UA
          ACK




8.7 Proxy insists on session timer




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   Calling UA -> SPS
           INVITE

   SPS -> Called UA
           INVITE                SPS adds session expires
           Require: timer        SPS adds Require
           Session-Expires: 180

   SPS <- Called UA
           420 Bad Extension
           Unsupported: timer

   Calling UA <- SPS
           420 Bad Extension
           Unsupported: timer

   Calling UA -> SPS
          ACK

   SPS -> Called UA
          ACK




8.8 Neither UA Supports Session Timer

   In this case, the proxy does not insist on session timer, so the call
   is set up without it.



   Calling UA -> SPS
          INVITE

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

   SPS <- Called UA
          200 OK                 Called UA doesn't understand session timer

   Calling UA <- SPS         SPS doesn't add S-E since it knows Calling UA
          200 OK               doesn't support it

   Calling UA -> SPS
          ACK




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   SPS -> Called UA
          ACK




9 Changes since -01

        o Added wording indicating that the UAC can add the Require and
          Proxy-Require headers with the "timer" header.

        o Added the ability for proxy to include Session-Timer header
          when the UAC did not include the Supported with the "timer"
          tag.

        o Added the ability for a proxy to insert the Session-Timer
          header into a response, in the case where the called UA didn't
          support session timer, but the calling UA did.

        o Added the ability for the called UA to refresh the session
          timer.  This is only in the case that the calling UA does not
          support the "timer" feature.

        o Session-Expires can be present in non-200 OK responses to
          reINVITEs

        o Added wording about getting Session-Expires of zero in non-200
          OK response to initial INVITE, handling crash and reboot
          cycles.

        o Discuss re-INVITEs changing the role of who is doing
          refreshes.

10 Author's Addresses



   Steven R. Donovan
   dynamicsoft
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936
   email: sdonovan@dynamicsoft.com

   Jonathan Rosenberg
   dynamicsoft
   72 Eagle Rock Avenue
   First Floor



S.Donovan,J.Rosenberg                                        [Page 17]


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   East Hanover, NJ 07936
   email: jdrosen@dynamicsoft.com




11 Bibliography

   [1] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
   session initiation protocol," Request for Comments 2543, Internet
   Engineering Task Force, Mar. 1999.

   [2] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: a
   transport protocol for real-time applications," Request for Comments
   1889, Internet Engineering Task Force, Jan. 1996.

   [3] J. Rosenberg, D. Drew, and H. Schulzrinne, "Getting SIP through
   firewalls and NATs," Internet Draft, Internet Engineering Task Force,
   Feb. 2000.  Work in progress.

   [4] J. Rosenberg and H. Schulzrinne, "The SIP supported header,"
   Internet Draft, Internet Engineering Task Force, Mar. 2000.  Work in
   progress.

   [5] M. Handley and V. Jacobson, "SDP: session description protocol,"
   Request for Comments 2327, Internet Engineering Task Force, Apr.
   1998.





                           Table of Contents



   1          Introduction ........................................    1
   2          Protocol Overview ...................................    2
   3          Session-Expires Header Field Definition .............    4
   4          UAC Behavior ........................................    5
   5          Proxy Behavior ......................................    7
   6          UAS Behavior ........................................    9
   7          Security Considerations .............................   10
   8          Examples ............................................   10
   8.1        Basic session-timer setup with UAS detected
   timeout ........................................................   10
   8.2        Basic negotiation of Session Time ...................   11
   8.3        No Session-Expires Header in INVITE .................   12



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   8.4        Session timer without Calling UA Support ............   13
   8.5        Addition of Session-Expires by UAS ..................   14
   8.6        Session Timer without Called UA Support .............   15
   8.7        Proxy insists on session timer ......................   15
   8.8        Neither UA Supports Session Timer ...................   16
   9          Changes since -01 ...................................   17
   10         Author's Addresses ..................................   17
   11         Bibliography ........................................   18











































S.Donovan,J.Rosenberg                                        [Page 19]


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