Internet Engineering Task Force SIP WG Internet Draft S.Donovan,J.Rosenberg draft-ietf-sip-session-timer-01.txt MCI Worldcom,dynamicsoft March 9, 2000 Expires: September, 2000 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 in 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 March 9, 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. 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. 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" [3]. 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. Proxies on the signaling path may have their own requirements for the refresh interval of the session. If the Supported header in the S.Donovan,J.Rosenberg [Page 2]
Internet Draft Session Timer March 9, 2000 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. If, however, the Supported header was absent, or was present but didn't include the tag "timer", the proxy MAY reject the request with a 421, and indicate in the Require header that the feature "timer" is needed. As an alternative, the proxy MAY forward the request, but MUST NOT add the Session-Expires header. This will allow the call to proceed without a timer. Eventually, the initial INVITE reaches the UAS. Like proxies, the UAS MAY add a session timer, reduce the session timer, or reject the request if the session timer is needed but not supported. If the UAS accepts the call, it places the final value of the session timer in the Session-Expires in the 200 OK response, and includes a Require header in the response indicating that the feature "timer" was applied to the response. As the 200 OK traverses the path back towards the UAC, proxies make note of the expiration time in the Session-Expires header in the response. The proxy is safe in destroying call state if a refresh is not received by this expiration time. The UAC then ACKs the INVITE. The Session-Expires need not be included in the ACK. If the UAC 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 UAS does not receive a re-INVITE refreshing the session, it SHOULD send a BYE to terminate the call. If the UAC 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 session. It is placed in only in INVITE requests, and is allowed in a 200 OK 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. S.Donovan,J.Rosenberg [Page 3]
Internet Draft Session Timer March 9, 2000 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 200 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" [3]. 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 being authenticated, the Session-Expires header MUST appear before the Authorization or Proxy-Authorization headers. When the response to the initial INVITE request arrives, it may or may not contain a Session-Expires header. UACs MUST be prepared to receive a Session-Expires header in a 200 OK response even if none were present in the request. The presence of this header in response S.Donovan,J.Rosenberg [Page 4]
Internet Draft Session Timer March 9, 2000 to an initial INVITE is only relevant in a 200 OK response. Assuming this is the case, 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 200 OK plus that delta time. Let the difference in time between the reception of the 200 OK and the session expiration time be called the refresh interval. Note that this expiration applies to the call leg created as a result of that 200 OK. It is explicitly allowed for there to be differing session timers (or none at all) on differing call legs. If the 200 OK response to the initial INVITE contained a Session- Expires, and the UAC wishes to continue with the session beyond the expiration, it MUST generate a refresh before the expiration time. It is RECOMMENDED that this be 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. The UAC MAY include a Session- Expires in this re-INVITE, indicating the new desired expiration time of the session. If the UAC does not place a Session-Expires header in the request, this means the UAC wishes for the session to have no expiration time going forward. The 200 OK response to the re-INVITE contains the actual expiration time. If the 200 OK response to the re-INVITE does not contain a Session-Expires header, the session no longer has an expiration. If the response to the re-INVITE was not a 200 OK, the call is still active (as per RFC 2543), but the expiration time remains unchanged. As with the original INVITE, the UAC MUST be prepared to receive a Session-Expires header in a 200 OK response even if it did not place one in the request. It is important to note that the processing rules for re-INVITEs parallels that of the initial INVITE. This helps maintain the idempotency of INVITE. A UAC 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 is unchanged. In the case of SDP [4], this is accomplished by using the same value for the origin field. If no 200 OK response to a refreshing re-INVITE is received before the expiration of the session, the UAC 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 S.Donovan,J.Rosenberg [Page 5]
Internet Draft Session Timer March 9, 2000 session. It is for this reason that the BYE should be sent before the expiration. 5 Proxy Behavior Session expirations are mostly of interest to call stateful proxy servers. However, any proxy server may follow the rules described here. 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 (1) none was present in the request, and (2) if the request contained a Supported header with the option tag "timer". This Session-Expires header may contain any desired expiration time the proxy would like. If the initial INVITE had both a Session-Expires header and a Supported header containing the option tag "timer", the proxy MAY reduce the value in the Session-Expires header before forwarding the request, but MUST NOT increase it. If the initial INVITE did not contain a Supported header with the option tag "timer", the proxy MUST NOT insert the Session-Expires header into the forwarded request. It MAY, however, follow the procedures defined in [3] and reject the request with a 421. When a 200 OK response arrives for the call, the actual expiration time for the call leg is contained in the Session-Expires header. The proxy MUST NOT modify the value of the Session-Expires header when forwarding the response 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 200 OK response, plus that delta time. Re-INVITE requests may arrive from the UAC, 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 200 OK 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- S.Donovan,J.Rosenberg [Page 6]
Internet Draft Session Timer March 9, 2000 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 initial INVITE did not contain a Supported header with the option tag "timer", the UAS MUST NOT insert the Session-Expires header into the 200 OK response. It MAY, however, follow the procedures defined in [3] and reject the request with a 421. If the UAS sends a 200 OK to the initial INVITE, and that 200 OK had a Sesion-Expires header, the UAS 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 transmission of the 200 OK plus that delta time. As described in Section 4, the UAC will send periodic re-INVITEs to refresh the session. The 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 no Session-Expires, no expiration time exists for the session. Since the re-INVITE is a standard re-INVITE, it is possible, for whatever reason, that the UAS might reject the request. The processing rules for session timer only apply to INVITEs with a 200 OK response. If no re-INVITE is received before expiration of the timer, the UAS SHOULD send a BYE for the call leg. 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. S.Donovan,J.Rosenberg [Page 7]
Internet Draft Session Timer March 9, 2000 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 60 seconds later: 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 120 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 S.Donovan,J.Rosenberg [Page 8]
Internet Draft Session Timer March 9, 2000 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 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 S.Donovan,J.Rosenberg [Page 9]
Internet Draft Session Timer March 9, 2000 8.3 No Session-Expires Header Rejection by SPS In this scenario, the UA sends an 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 rejects the INVITE. Calling UA -> SPS INVITE No Session-Expires or Supported header Calling UA <- SPS 421 Extension Required Require: timer Calling UA -> SPS ACK 8.4 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 Session-Expires: 120 Called UA starts timer on send Calling UA starts timer on receipt Calling UA -> Called UA ACK 8.5 Addition of Session-Expires by Proxy In this scenario, the calling UA indicates that it supports the S.Donovan,J.Rosenberg [Page 10]
Internet Draft Session Timer March 9, 2000 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 result is that the call is set up without a session timer. 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 Calling UA -> SPS ACK SPS -> Called UA ACK 9 Changes since -00 o Changed to make use of the Supported header. o Conveyance of Session Expires in ACK is no longer needed (because of usage of the Supported header), and has been removed. o Session Expires only allowed in INVITE, not in BYE as previously specified. Usage in BYE doesn't accomplish anything, as the call is terminated. o Specified that only UAC sends re-INVITEs for keepalives. o Session Expires only present in 200 OK responses. Doesn't appear to be needed in any other. o Session-Expires no longer mandatory in re-INVITEs. This is S.Donovan,J.Rosenberg [Page 11]
Internet Draft Session Timer March 9, 2000 done to unify the processing of INVITE and re-INVITE for session timer. Also means that a previously timed session can become untimed. o Changed semantics of delta-time to be relative to the time of message arrival, rather than the time of previous expiration. This is in line with normal Expires processing. o The original mechanism allowed a proxy to reject the request if the timer was too long, or to reduce the timer. There seems to be no clear reason to have both. Rejecting will just cause the client to resubmit with the shorter expiration, resulting in the same net effect with additional complexity. Thus, the notion of rejecting session timers is removed, and with it, the 488 response code, which is then uneccesary. This seems to simplify the protocol. o A mechanism had existed to reject a session timer with a Session-Expires of 0, meaning the expiration remains unchanged. With the new definition of the relative offset for Expires, this no longer requires special treatment. A proxy or UAS can set the session timer with an absolute time equal to the current expiration. 10 Open Issues o Should we allow Session-Expires in non-INVITE requests, perhaps INFO (I think no) o Should we allow the UAC to insert a Require header in the request, indicating that the UAS must support the session timer? (I think no) o With this draft (and the previous versions), there was no way for the proxy to reject the call if the calling UA supported the session timer, but the called UA didn't. Is this a problem? If so, what can be done about it? (I think there is nothing we can do, and we should not worry about it) 11 Author's Addresses Steven R. Donovan MCI Worldcom 1493/678 901 International Parkway Richardson, Texas 75081 S.Donovan,J.Rosenberg [Page 12]
Internet Draft Session Timer March 9, 2000 email: steven.r.donovan@wcom.com Jonathan Rosenberg dynamicsoft 200 Executive Drive Suite 120 West Orange, NJ 07052 email: jdrosen@dynamicsoft.com 12 Bibliography [1] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP: session initiation protocol," Request for Comments (Proposed Standard) 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 (Proposed Standard) 1889, Internet Engineering Task Force, Jan. 1996. [3] J. Rosenberg and H. Schulzrinne, "Mandating SIP extension support by servers," Internet Draft, Internet Engineering Task Force, Jan. 2000. Work in progress. [4] M. Handley and V. Jacobson, "SDP: session description protocol," Request for Comments (Proposed Standard) 2327, Internet Engineering Task Force, Apr. 1998. Full Copyright Statement Copyright (c) The Internet Society (2000). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than S.Donovan,J.Rosenberg [Page 13]
Internet Draft Session Timer March 9, 2000 English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Table of Contents 1 Introduction ........................................ 1 2 Protocol Overview ................................... 2 3 Session-Expires Header Field Definition ............. 3 4 UAC Behavior ........................................ 4 5 Proxy Behavior ...................................... 6 6 UAS Behavior ........................................ 6 7 Security Considerations ............................. 7 8 Examples ............................................ 8 8.1 Basic session-timer setup with UAS detected timeout ........................................................ 8 8.2 Basic negotiation of Session Time ................... 9 8.3 No Session-Expires Header Rejection by SPS .......... 10 8.4 Addition of Session-Expires by UAS .................. 10 8.5 Addition of Session-Expires by Proxy ................ 10 9 Changes since -00 ................................... 11 10 Open Issues ......................................... 12 11 Author's Addresses .................................. 12 12 Bibliography ........................................ 13 S.Donovan,J.Rosenberg [Page 14]
