Internet Draft 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] Internet Draft Session Timer July 14, 2000 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] Internet Draft Session Timer July 14, 2000 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] Internet Draft Session Timer July 14, 2000 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] Internet Draft Session Timer July 14, 2000 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] Internet Draft Session Timer July 14, 2000 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] Internet Draft Session Timer July 14, 2000 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] Internet Draft Session Timer July 14, 2000 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] Internet Draft Session Timer July 14, 2000 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] Internet Draft Session Timer July 14, 2000 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] Internet Draft Session Timer July 14, 2000 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] Internet Draft Session Timer July 14, 2000 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] Internet Draft Session Timer July 14, 2000 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] Internet Draft Session Timer July 14, 2000 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] Internet Draft Session Timer July 14, 2000 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 S.Donovan,J.Rosenberg [Page 15] Internet Draft Session Timer July 14, 2000 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 S.Donovan,J.Rosenberg [Page 16] Internet Draft Session Timer July 14, 2000 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] Internet Draft Session Timer July 14, 2000 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 S.Donovan,J.Rosenberg [Page 18] Internet Draft Session Timer July 14, 2000 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]