SIP S. Donovan Internet-Draft J. Rosenberg Expires: August 13, 2004 dynamicsoft February 13, 2004 Session Timers in the Session Initiation Protocol (SIP) draft-ietf-sip-session-timer-14 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. This Internet-Draft will expire on August 13, 2004. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This document defines an extension to the Session Initiation Protocol (SIP). This extension allows for a periodic refresh of SIP sessions through a re-INVITE or UPDATE request. The refresh allows both user agents and proxies to determine if the SIP session is still active. The extension defines two new header fields, Session-Expires, which conveys the lifetime of the session, and Min-SE, which conveys the minimum allowed value for the session timer. Donovan & Rosenberg Expires August 13, 2004 [Page 1] Internet-Draft Session Timer February 2004 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Overview of Operation . . . . . . . . . . . . . . . . . . . 6 4. Session-Expires Header Field Definition . . . . . . . . . . 8 5. Min-SE Header Field Definition . . . . . . . . . . . . . . . 10 6. 422 Response Code Definition . . . . . . . . . . . . . . . . 11 7. UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . . 12 7.1 Generating an Initial Session Refresh Request . . . . . . . 12 7.2 Processing a 2xx Response . . . . . . . . . . . . . . . . . 12 7.3 Processing a 422 Response . . . . . . . . . . . . . . . . . 14 7.4 Generating Subsequent Session Refresh Requests . . . . . . . 14 8. Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . 16 8.1 Processing of Requests . . . . . . . . . . . . . . . . . . . 16 8.2 Processing of Responses . . . . . . . . . . . . . . . . . . 17 8.3 Session Expiration . . . . . . . . . . . . . . . . . . . . . 18 9. UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . . 19 10. Performing Refreshes . . . . . . . . . . . . . . . . . . . . 22 11. Security Considerations . . . . . . . . . . . . . . . . . . 23 11.1 Inside Attacks . . . . . . . . . . . . . . . . . . . . . . . 23 11.2 Outside Attacks . . . . . . . . . . . . . . . . . . . . . . 24 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . 25 12.1 IANA Registration of Min-SE and Session-Expires Header Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 12.2 IANA Registration of the 422 (Session Interval Too Small) Response Code . . . . . . . . . . . . . . . . . . . . . . . 25 12.3 IANA Registration of the 'timer' Option Tag . . . . . . . . 25 13. Example Call Flow . . . . . . . . . . . . . . . . . . . . . 27 14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32 Normative References . . . . . . . . . . . . . . . . . . . . 33 Informative References . . . . . . . . . . . . . . . . . . . 34 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 34 Intellectual Property and Copyright Statements . . . . . . . 35 Donovan & Rosenberg Expires August 13, 2004 [Page 2] Internet-Draft Session Timer February 2004 1. Introduction The Session Initiation Protocol (SIP) [1], does not define a keepalive mechanism for the sessions it establishes. Although the user agents may be able to determine if the session has timed out using session specific mechanisms, proxies will not be able to do so. The result is that call stateful proxies will not always be able to determine whether a session 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 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-INVITE or UPDATE [2] requests (referred to as session refresh requests) to keep the session alive. The interval for the session refresh requests is determined through a negotiation mechanism defined here. If a session refresh request 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 [5]. However, it is desirable to separate SIP session liveness from the details of the particular session. Another application of the session timer is in the construction of a SIP Network Address Translator (NAT) Application Level Gateway (ALG) [6]. The ALG embedded in a NAT will need to maintain state for the duration of a call. This state must eventually be removed. Relying on a BYE to trigger the removal of state, besides being unreliable, introduces a potential denial of service attack. This document provides an extension to SIP that defines a session expiration mechanism. Periodic refreshes, through re-INVITEs or UPDATEs, are used to keep the session active. The extension is sufficiently backwards compatible with SIP that it works so long as either one of the two participants in a dialog understand the extension. Two new header fields, Session-Expires and Min-SE, and a new response code, 422, are defined. Session-Expires conveys the duration of the session, and Min-SE conveys the minimum allowed value Donovan & Rosenberg Expires August 13, 2004 [Page 3] Internet-Draft Session Timer February 2004 for the session expiration. The 422 response code indicates that the session timer duration was too small. Donovan & Rosenberg Expires August 13, 2004 [Page 4] Internet-Draft Session Timer February 2004 2. Terminology In this document, the key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALLNOT', 'SHOULD', 'SHOULDNOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' are to be interpreted as described in RFC 2119 [3] and indicate requirement levels for compliant SIP implementations. Additionally, we define the following terms: Session Interval: The largest amount of time that can occur between session refresh requests in a dialog before the session will be considered timed out. The session interval is conveyed in the Session-Expires header field defined here. The UAS obtains this value from the Session-Expires header field in a 2xx response to a session refresh request that it sends. Proxies and UACs determine this value from the Session-Expires header field in a 2xx response to a session refresh request that they receive. Minimum Timer: Because of the processing load of mid-dialog requests, all elements (proxy, UAC, UAS) can have a configured minimum value for the session interval that they are willing to accept. This value is called the minimum timer. Session Expiration: The time at which an element will consider the session timed out, if no successful session refresh transaction occurs beforehand. Session Refresh Request: An INVITE or UPDATE request within a dialog. If the request generates a 2xx response, the session expiration is increased to the current time plus the session interval obtained from the response. A session refresh request is not to be confused with a target refresh request, defined in Section 6 of [1], which is a request that can update the remote target of a dialog. Refresh: Same as a session refresh request. Donovan & Rosenberg Expires August 13, 2004 [Page 5] Internet-Draft Session Timer February 2004 3. Overview of Operation This section provides a brief overview of operation of the extension. It is tutorial in nature and should not be considered as normative. This extension has the property that it works even when only one UA in a dialog supports it. The processing steps differ for handling each of the four cases (UAC supports it, or doesn't, and UAS supports it, or doesn't). For simplicity's sake, this section will describe basic operation in the case where both sides support the extension. A UAC starts by sending an INVITE. It includes a Supported header with the option tag 'timer', indicating support for this extension. This request passes through proxies, any one of which may have an interest in establishing a session timer. Each proxy can insert a Session-Expires header field and a Min-SE header field into the request if none is already there or alter the value of existing Session-Expires and Min-SE header fields as described below. The Min-SE header field establishes the lower bound for the session refresh interval, i.e. the fastest rate that any proxy servicing this request will be allowed to require. The purpose of this header field is to prevent hostile proxies from setting arbitrarily short refresh intervals such that their neighbors are overloaded. Each proxy processing the request can raise this lower bound (increase the period between refreshes) but is not allowed to lower it. The Session-Expires header field establishes the upper bound for the session refresh interval, i.e., the time period after processing a request for which any session-stateful proxy must retain its state for this session. Any proxy servicing this request can lower this value, but is not allowed to decrease it below the value specified in the Min-SE header field. If the Session-Expires interval is too low for a proxy (i.e, lower than the value of Min-SE that the proxy would wish to assert), the proxy rejects the request with a 422 response. That response contains a Min-SE header field, identifying the minimum session interval it is willing to support. The UAC will try again, this time including the Min-SE header in the request. The header field contains the largest Min-SE header field it observed in all 422 responses received previously. This way, the minimum timer meets the constraints of all proxies along the path. After several INVITE/422 iterations, the request eventually arrives at the UAS. The UAS can adjust the value of the session interval as if it was a proxy, and when done, it places the final session Donovan & Rosenberg Expires August 13, 2004 [Page 6] Internet-Draft Session Timer February 2004 interval into the Session-Expires header field in a 2xx response. The Session-Expires header field also contains a 'refresher' parameter, which indicates who is doing the refreshing - the UA that is currently the UAC, or the UA that is currently the UAS. As the 2xx response travels back through the proxy chain, each proxy can observe the final session interval, but they can't change it. From the Session-Expires header field in the response, both UAs know that a session timer is active, they know when it will expire, and they know who is refreshing. At some point before the expiration, the currently active refresher generates a session refresh request, which is a re-INVITE or UPDATE [2] request. If the refresher never gets a response to that session refresh request, it sends a BYE to terminate the session. Similarly, if the other side never gets the session refresh request before the session expires, it sends a BYE. The refresh requests sent once the session is established are processed identically to the initial requests, as described above. This means that a successful session refresh request will extend the session, as desired. The extension introduces additional complications beyond this basic flow to support cases where only one of the UAs supports it. One such complication is that a proxy may need to insert the Session-Expires header into the response, in the event that the UAS doesn't support the extension. The negotiation of the role of refresher is also affected by this capability; it takes into consideration which participants support the extension. It is worth noting that the session timer refreshes the session, not the dialog used to establish the session. Of course, the two are related. If the session expires, a BYE is sent, which terminates the session and generally, the dialog. Donovan & Rosenberg Expires August 13, 2004 [Page 7] Internet-Draft Session Timer February 2004 4. Session-Expires Header Field Definition The Session-Expires header field conveys the session interval for a SIP session. It is placed only in INVITE or UPDATE requests, as well as in any 2xx response to an INVITE or UPDATE. Like the SIP Expires header field, it contains a delta-time. The absolute minimum for the Session-Expires header field is 90 seconds. This value represents a bit more than twice the duration that a SIP transaction can take in the event of a timeout. This allows sufficient time for a UA to attempt a refresh at the halfpoint of the session interval, and for that transaction to complete normally before the session expires. However, 1800 seconds (30 minutes) is RECOMMENDED as the value for the Session-Expires header field. In other words, SIP entities MUST be prepared to handle Session-Expires header field values of any duration greater than 90 seconds, but entities that insert the Session-Expires header field SHOULD NOT choose values less than 30 minutes. Small session intervals can be destructive to the network. They cause excessive messaging traffic that affects both user agents and proxy servers. They increase the possibility of 'glare' that can occur when both user agents send a re-INVITE or UPDATE at the same time. Since the primary purpose of the session timer is to provide a means to time out state in SIP elements, very small values won't generally be needed. 30-minutes was chosen since 95% of phone calls are less than this duration. However, the 30 minute minimum is listed as a SHOULD, and not a MUST, since the exact value for this number is dependent on many network factors, including network bandwidths and latencies, computing power, memory availability, network topology, and of course, the application scenario. After all, SIP can set up any kind of session, not just a phone call. At the time of publication of this document, 30 minutes seems appropriate. Advances in technologies may result in the number being excessively large five years in the future. The default value of the Session-Expires header field is undefined. This means that absence of the Session-Expires header field implies no expiration of the session using the mechanism defined in this specification. Note that other mechanisms not defined in this specification, such as locally configured timers, may apply. The syntax of the Session-Expires header field is: Session-Expires = ('Session-Expires' / 'x') HCOLON delta-seconds *(SEMI se-params) se-params = refresher-param / generic-param refresher-param = 'refresher' EQUAL ('uas' / 'uac') Donovan & Rosenberg Expires August 13, 2004 [Page 8] Internet-Draft Session Timer February 2004 Note that a compact form, the letter x, has been reserved for Session-Expires. The BNF for delta-seconds and generic-param is defined in Section 25 of RFC 3261 [1]. Table 1 is an extension of Tables 2 and 3 in [1] for the Session-Expires and Min-SE header fields. The column 'PRA' is for the PRACK method [7], 'UPD' is for the UPDATE method [2], 'SUB' is for the SUBSCRIBE method [8], and 'NOT' is for the NOTIFY method [8]. +---------------+-----+-----+---+---+---+---+---+---+---+---+---+---+ | Header |where|proxy|ACK|BYE|CAN|INV|OPT|REG|PRA|UPD|SUB|NOT| +---------------+-----+-----+---+---+---+---+---+---+---+---+---+---+ |Session-Expires| R | amr | - | - | - | o | - | - | - | o | - | - | | | | | | | | | | | | | | | |Session-Expires| 2xx | ar | - | - | - | o | - | - | - | o | - | - | | | | | | | | | | | | | | | |Min-SE | R | amr | - | - | - | o | - | - | - | o | - | - | | | | | | | | | | | | | | | |Min-SE | 422 | | - | - | - | m | - | - | - | m | - | - | +---------------+-----+-----+---+---+---+---+---+---+---+---+---+---+ Table 1 Session-Expires and Min-SE Header Fields Donovan & Rosenberg Expires August 13, 2004 [Page 9] Internet-Draft Session Timer February 2004 5. Min-SE Header Field Definition The Min-SE header field indicates the minimum value for the session interval, in units of delta-seconds. When used in an INVITE or UPDATE request, it indicates the smallest value of the session interval that can be used for that session. When present in a request or response, its value MUST NOT be less than 90 seconds. When not present, the default value for this header field is 90 seconds. The Min-SE header field MUST NOT be used in responses except those with a 422 response code. It indicates the minimum value of the session interval that the server is willing to accept. The syntax of the Min-SE header field is: Min-SE = 'Min-SE' HCOLON delta-seconds *(SEMI generic-param) Donovan & Rosenberg Expires August 13, 2004 [Page 10] Internet-Draft Session Timer February 2004 6. 422 Response Code Definition This extension introduces the 422 (Session Interval Too Small) response code. It is generated by a UAS or proxy when a request contains a Session-Expires header field with a duration that is below the minimum timer for the server. The 422 response MUST contain a Min-SE header field with the minimum timer for that server. Donovan & Rosenberg Expires August 13, 2004 [Page 11] Internet-Draft Session Timer February 2004 7. UAC Behavior 7.1 Generating an Initial Session Refresh Request A UAC that supports the session timer extension defined here MUST include a Supported header field in each request (except ACK), listing the option tag 'timer' [1]. It MUST do so even if the UAC is not requesting usage of the session timer for this session. The UAC MAY include a Require header field in the request with the value 'timer' to indicate that the UAS must support the session timer to participate in the session. This does not mean that the UAC is requiring the UAS to perform the refreshes, just that it is requiring the UAS to support the extension. In addition, the UAC MAY include a Proxy-Require header field 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. The Supported header field containing 'timer' MUST still be included even if the Require or Proxy-Require header fields are present containing 'timer'. A UAC MAY include the Min-SE header field in the initial INVITE request. A UAC MAY include a Session-Expires header field in an initial session refresh request if it wishes for a session timer to be applied to the session. The value of this header field indicates the session interval desired by the UAC. If a Min-SE header is included in the initial session refresh request, the value of the Session-Expires MUST be equal to the value in Min-SE. The UAC MAY include the refresher parameter with value 'uac' if it wishes to perform the refreshes. However, it is RECOMMENDED that the parameter be omitted, so that it can be selected by the negotiation mechanisms described below. 7.2 Processing a 2xx Response Session timer requires a UA to create and maintain state. This state includes the session interval, the session expiration, and the identity of the refresher. This state is associated with the dialog on which the session has been negotiated. When a 2xx response to a session refresh request arrives, it may or may not contain a Require header field with the value 'timer'. If it does, the UAC MUST look for the Session-Expires header field to Donovan & Rosenberg Expires August 13, 2004 [Page 12] Internet-Draft Session Timer February 2004 process the response. If there was a Require header field in the response with the value 'timer', the Session-Expires header field will always be present. UACs MUST be prepared to receive a Session-Expires header field in a response even if none were present in the request. The 'refresher' parameter will be present in the Session-Expires header field, indicating who will be performing the refreshes. The UAC MUST set the identity of the refresher to the value of this parameter. If the parameter contains the value 'uac', the UAC will perform them. It is possible that the UAC requested session timer (and thus included a Session-Expires header field in the request), but there was no Require or Session-Expires header field in the 2xx response. This will happen when the UAS doesn't support the session timer extension, and only the UAC has asked for a session timer (no proxies have requested it). In this case, if the UAC still wishes to use the session timer (they are purely for its benefit alone), it has to perform them. To do this, the UAC follows the procedures defined in this specification as if the Session-Expires header field were in the 2xx response, and its value was the same as the one in the request, but with a refresher parameter of 'uac'. If the 2xx response did not contain a Session-Expires header field, there is no session expiration. In this case, no refreshes need to be sent. A 2xx without a Session-Expires can come for both initial and mid-dialog session refresh requests. This means that the session timer can be 'turned-off' mid dialog by receiving a response without a Session-Expires header. The UAC remembers the session interval for a session as the value of the delta-time from the Session-Expires header field in the most recent 2xx response to a session refresh request on a dialog. It is explicitly allowed for there to be differing session intervals (or none at all) on differing dialogs established as a result of a single INVITE. It also remembers whether it, or its peer, is the refresher on for the session. If the UAC must perform the refreshes, it computes the session expiration for that session. The session expiration is the time of reception of the last 2xx response to a session refresh request on that dialog plus the session interval for that session. If UA wishes to continue with the session beyond the session expiration, it MUST generate a refresh before the session expiration. It is RECOMMENDED that this refresh be sent once half the session interval has elapsed. Additional procedures for this refresh are described in Section 10. Donovan & Rosenberg Expires August 13, 2004 [Page 13] Internet-Draft Session Timer February 2004 7.3 Processing a 422 Response If the response to a session refresh request is a 422 (Session Interval Too Small) response message, then the UAC MAY retry the request. The procedures for retrying are described in Section 7.4. This new request constitutes a new transaction and SHOULD have the same value of the Call-ID, To, and From of the previous request, but the CSeq should contain a new sequence number that is one higher than the previous. 7.4 Generating Subsequent Session Refresh Requests The values of Supported, Require and Proxy-Require used in the initial Session refresh request MUST be used. The UAC MUST insert the Min-SE header field into a session refresh request for a particular dialog if it has ever received a 422 response to a previous session refresh request on the same dialog, or if it has received a session refresh request on that dialog which contained a Min-SE header field. Similarly, if no dialog has been established yet, a UAC MUST insert the Min-SE header field into an INVITE request if it has ever received a 422 response to a previous INVITE request with the same Call-ID. The value of the Min-SE header field present in a session refresh request MUST be the largest value amongst all Min-SE header field values returned in all 422 responses, or received in session refresh requests, on the same dialog, if a dialog has been established. If no dialog has been established, the Min-SE header field value is set to the largest value amongst all Min-SE header field values returned in all 422 responses for an INVITE request with the same Call-ID. A result of this rule is that the maximum value of the Min-SE is effectively 'cleared' once the dialog is established, and from that point on, only the values from proxies known to be on the proxy path will end up being used. The UAC may have its own opinions about the minimum session interval. In that case, if the value above is too small, the UAC MAY increase it. In a session refresh request sent within a dialog with an active session timer, the Sesssion-Expires header field SHOULD be present. When present, it MUST be equal to the maximum of the Min-SE header field (recall that its default value when not present is 90 seconds) and the current session interval. If the session refresh request is not the initial one, it is RECOMMENDED that the refresher parameter be set to 'uac' if the Donovan & Rosenberg Expires August 13, 2004 [Page 14] Internet-Draft Session Timer February 2004 element sending the request is currently performing refreshes, else 'uas' if its peer is performing the refreshes. This way, the role of refresher does not change on each refresh. However, if it wishes to explicitly change the roles, it MAY use a value of 'uas' if it knows that the other side supports session timer. It could know this by having received a request from its peer with a Supported header field containing the value 'timer'. If it wishes to reselect the roles, it MAY omit the parameter. A re-INVITE generated to refresh the session is a normal re-INVITE, and an UPDATE generated to refresh a session is a normal UPDATE. If a UAC knows that its peer supports the UPDATE method, it is RECOMMENDED that UPDATE be used instead of a re-INVITE. A UA can make this determination if it has seen an Allow header field from its peer with the value 'UPDATE', or through a mid-dialog OPTIONS request. It is RECOMMENDED that the UPDATE request not contain an offer [4], but a re-INVITE SHOULD contain one, even if the details of the session have not changed. In that case, the offer MUST indicate that it has not changed. In the case of SDP, this is accomplished by including the same value for the origin field as previous SDP messages to its peer. The same is true for an answer exchanged as a result of a session refresh request; if it has not changed, that MUST be indicated. Donovan & Rosenberg Expires August 13, 2004 [Page 15] Internet-Draft Session Timer February 2004 8. Proxy Behavior Session timers are mostly of interest to call stateful proxy servers (that is, servers that maintain the state of calls and dialogs established through them). However, a stateful proxy server (that is, a server which is aware of transaction state, but does not retain call or dialog state) MAY also follow the rules described here. Stateless proxies MUST NOT attempt to request session timers. Proxies that ask for session timers SHOULD record-route, since they won't receive refreshes if they don't. The proxy processing rules require the proxy to remember information between the request and response, ruling out stateless proxies. 8.1 Processing of Requests Processing of requests is identical for all session refresh requests. To request a session timer for a session, a proxy makes sure that a Session-Expires header field is present in a session refresh request for that session. A proxy MAY insert a Session-Expires header field in the request before forwarding it, if none was present in the request. This Session-Expires header field may contain any desired expiration time the proxy would like, but not with a duration lower than the value in the Min-SE header field in the request, if present. The proxy MUST NOT include a refresher parameter in the header field value. If the request already had a Session-Expires header field, the proxy MAY reduce its value, but MUST NOT set it to a duration lower than the value in the Min-SE header field in the request, if present. If the value of the Session-Expires header field is greater than or equal to the value in the Min-SE header field (recall that the default is 90 seconds when the Min-SE header field is not present), the proxy MUST NOT increase the value of the Session-Expires header field. If the value of the Session-Expires header field is lower than the value of the Min-SE header field (possibly because the proxy increased the value of the Min-SE header field, as described below), the proxy MUST increase the value of the Session-Expires header field to make it equal to Min-SE header field value. The proxy MUST NOT insert or modify the value of the 'refresher' parameter in the Session-Expires header field. If the request contains a Supported header field with a value 'timer', the proxy MAY reject the INVITE request with a 422 (Session Interval Too Small) response if the session interval in the Donovan & Rosenberg Expires August 13, 2004 [Page 16] Internet-Draft Session Timer February 2004 Session-Expires header field is smaller than the minimum interval defined by the proxy's local policy. When sending the 422 response, the proxy MUST include a Min-SE header field with the value of its minimum interval. That minimum MUST NOT be lower than 90 seconds. If the request doesn't indicate support for session timer, but the request contains a session interval that is too small, the proxy cannot usefully reject the request, as this would result in a call failure. Rather, the proxy SHOULD insert a Min-SE header field containing its minimum interval. If a Min-SE header field is already present, the proxy SHOULD increase (but MUST NOT decrease) the value to equal its minimum interval. The proxy MUST then increase the Session-Expires header field value to be equal to the value in the Min-SE header field, as described above. A proxy MUST NOT insert a Min-SE header field, or modify the value of an existing header field, in a proxied request if that request contains a Supported header field with the value 'timer'. This is needed to protect against certain denial of service attacks, described in Section 11. Assuming the proxy has requested a session timer (and thus has possibly inserted the Session-Expires header field or reduced it), the proxy MUST remember that it is using a session timer, and also remember the value of the Session-Expires header field from the proxied request. This MUST be remembered for the duration of the transaction. The proxy MUST remember, for the duration of the transaction, whether the request contained the Supported header field with the value 'timer'. If the request did not contain a Supported header field with the value 'timer', the proxy MAY insert a Require header field into the request, with the value 'timer'. However, this is NOT RECOMMENDED. This allows the proxy to insist on session timer for the session. This header field is not needed if a Supported header field was in the request; in this case, the proxy can already be sure that the session timer can be used for the session. 8.2 Processing of Responses When the final response to the request arrives, it is examined by the proxy. If the response does not contain a Session-Expires header field, but the proxy remembers that it requested a session timer in the request (by inserting, modifying, or examining and accepting the Session-Expires header field in the proxied request), this means that the UAS did not support the session timer. If the proxy remembers that the UAC did not support session timer either, the proxy forwards the response upstream normally. There is no session expiration for Donovan & Rosenberg Expires August 13, 2004 [Page 17] Internet-Draft Session Timer February 2004 this session. If, however, the proxy remembers that the UAC did support session timer, additional processing is needed. Because there is no Session-Expires or Require header field in the response, the proxy knows it is the first session-timer-aware proxy to receive the response. This proxy MUST insert a Session-Expires header field into the response with the value it remembered from the forwarded request. It MUST set the value of the 'refresher' parameter to 'uac'. The proxy MUST insert the Require header field into the response, with the value 'timer', before forwarding it upstream. If the received response contains a Session-Expires header field, no modification of the response is needed. In all cases, if the 2xx response forwarded upstream by the proxy contains a Session-Expires header field, its value represents the session interval for the session associated with that response. The proxy computes the session expiration as the time when the 2xx response is forwarded upstream, plus the session interval. This session expiration MUST update any existing session expiration for the session. The refresher parameter in the Session-Expires header field in the 2xx response forwarded upstream will be present, and it indicates which UA is performing the refreshes. There can be multiple 2xx responses to a single INVITE, each representing a different dialog, resulting in multiple session expirations, one for each session associated with each dialog. The proxy MUST NOT modify the value of the Session-Expires header field received in the response (assuming one was present) before forwarding it upstream. 8.3 Session Expiration When the current time equals or passes the session expiration for a session, the proxy MAY remove associated call state, and MAY free any resources associated with the call. Unlike the UA, it MUST NOT send a BYE. Donovan & Rosenberg Expires August 13, 2004 [Page 18] Internet-Draft Session Timer February 2004 9. UAS Behavior The UAS must respond to a request for a session timer by the UAC or a proxy in the path of the request, or it may request that a session Timer be used itself. If an incoming request contains a Supported header field with a value 'timer' and a Session Expires header, the UAS MAY reject the INVITE request with a 422 (Session Interval Too Small) response if the session interval in the Session-Expires header field is smaller than the minimum interval defined by the UAS' local policy. When sending the 422 response, the UAS MUST include a Min-SE header field with the value of its minimum interval. This minimum interval MUST NOT be lower than 90 seconds. If the UAS wishes to accept the request, it copies the value of the Session-Expires header field from the request into the 2xx response. The UAS response MAY reduce its value, but MUST NOT set it to a duration lower than the value in the Min-SE header field in the request, if present, else 90s if not. The UAS MUST NOT increase the value of the Session-Expires header field. If the incoming request contains a Supported header field with a value 'timer' but does not contain a Session-Expires header, the UAC indicated support for timers, but did not request one. The UAS may request a session timer in the 2XX response by including a Session-Expires header field. The value MUST NOT be set to a duration lower than the value in the Min-SE header field in the request, if present. The UAS MUST set the value of the refresher parameter in the Session-Expires header field in the 2xx response. This value specifies who will perform refreshes for the dialog. The value is based on the value of this parameter in the request, and on whether the UAC supports the session timer extension. The UAC supports the extension if the 'timer' option tag was present in a Supported header field in the request. Figure 3 defines how the value in the response is set. A value of 'none' in the 2nd column means that there was no refresher parameter in the request. A value of 'NA' in the third column means that this particular combination shouldn't happen, as it's disallowed by the protocol. Donovan & Rosenberg Expires August 13, 2004 [Page 19] Internet-Draft Session Timer February 2004 UAC supports? refresher parameter refresher parameter in request in response ------------------------------------------------------- N none uas N uac NA N uas NA Y none uas or uac Y uac uac Y uas uas Figure 3: UAS Behavior The fourth row of Figure 3 describes a case where both the UAC and UAS support the session timer extension, and the UAC did not select who will perform refreshes. This allows the UAS to decide whether it, or the UAC, will perform the refreshes. However, as the table indicates, the UAS cannot override the UAC's choice of refresher, if it made one. If the refresher parameter in the Session-Expires header field in the 2xx response has a value of 'uac', the UAS MUST place a Require header field into the response with the value 'timer'. This is because the uac is performing refreshes and the response has to be processed for the UAC to know this. If the refresher parameter in the 2xx response has a value of 'uas', and the Supported header field in the request contained the value 'timer', the UAS SHOULD place a Require header field into the response with the value 'timer'. In this case, the UAC is not refreshing, but it is supposed to send a BYE if it never receives a refresh. Since the call will still succeed without the UAC doing this, insertion of the Require is a SHOULD here, rather than a MUST. The UAS, just like the UAC, stores state for the session timer. This state includes the session interval, the session expiration, and the identity of the refresher. This state is bound to the dialog used to set up the session. The session interval is set to the value of the delta-time from the Session-Expires header field in the most recent 2xx response to a session refresh request on that dialog. It also remembers whether it, or its peer, is the refresher on the leg, based on the value of the refresher parameter from the most recent 2xx response to a session refresh request on that dialog. If the most recent 2xx response had no Session-Expires header field, there is no session expiration, and no refreshes need to be performed. If the UAS must refresh the session, it computes the session expiration. The session expiration is the time of transmission of the last 2xx response to a session refresh request on that dialog plus the session interval. If UA wishes to continue with the session Donovan & Rosenberg Expires August 13, 2004 [Page 20] Internet-Draft Session Timer February 2004 beyond the session expiration, it MUST generate a refresh before the session expiration. It is RECOMMENDED that this refresh be sent once half the session interval has elapsed. Additional procedures for this refresh are described in Section 10. Donovan & Rosenberg Expires August 13, 2004 [Page 21] Internet-Draft Session Timer February 2004 10. Performing Refreshes The side generating a refresh does so according to the UAC procedures defined in Section 7. Note that only a 2xx response to a session refresh request extends the session expiration. This means that a UA could attempt a refresh, and receive a 422 response with a Min-SE header field that contains a value much larger than the current session interval. The UA will still need to send a session refresh request before the session expiration (which has not changed), even though this request will contain a value of the Session-Expires that is much larger than the current session interval. If the session refresh request transaction times out or generates a 408 or 481 response, then the UAC sends a BYE request per the rules in Section 12.2.1.2 of RFC 3261 [1]. If the session refresh request does not generate a 2xx response (and, as a result, the session is not refreshed), and a response other than 408 or 481 is received, the UAC SHOULD follow the rules specific to that response code, and retry if possible. For example, if the response is a 401, the UAC would retry the request with new credentials. However, the UAC SHOULD NOT continuously retry the request if the server indicates the same error response. Similarly, if the side not performing refreshes does not receive a session refresh request before the session expiration, they SHOULD send a BYE to terminate the session, slightly before the session expiration. The minimum of 32 seconds and one third the session interval is RECOMMENDED. Firewalls and NAT ALGs 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. Donovan & Rosenberg Expires August 13, 2004 [Page 22] Internet-Draft Session Timer February 2004 11. Security Considerations The session timer introduces the capability of a proxy or UA element to force compliant UAs to send refreshes at a rate of the element's choosing. This introduces the possibility of denial-of-service attacks with significant amplification properties. These attacks can be launched from 'outsiders' - elements that attempt to modify messages in transit, or by 'insiders' - elements which are legitimately in the request path, but are intent on doing harm. Fortunately, both cases are adequately handled by this specification. 11.1 Inside Attacks This introduces the possibility of rogue proxies or UAs introducing denial-of-service attacks. However, the mechanisms in this specification prevent that from happening. First, consider the case of a rogue UAC that wishes to force a UAS to generate refreshes at a rapid rate. To do so, it inserts a Session-Expires header field into an INVITE with a low duration and a refresher parameter equal to uas. Assume it places a Supported header field into the request. Any proxy, or the UAS, which objects to this low timer will reject the request with a 422, therefore preventing the attack. If no Supported header field was present, the proxies will insert a Min-SE header field into the request before forwarding it. As a result, the UAS will not choose a session timer lower than the minimum acceptable one to all elements on the path. This too prevents the attack. Next, consider the case of a rogue UAS that wishes to force a UAC to generate refreshes at a rapid rate. In that case, the UAC has to support session timer. The initial INVITE arrives at the rogue UAS, which returns a 2xx with a very small session interval. The UAC uses this timer, and quickly sends a refresh. Section 7.1 requires the UAC to copy the current session interval into the Session-Expires header field in the request. This enables the proxies to see the current value. The proxies will reject this request, and provide a Min-SE with a higher minimum. The UAC will then use this higher minimum. Note, that if the proxies did not reject the request, but rather proxied the request with a Min-SE header field, an attack would still be possible. The UAS could discard this header field in a 2xx response, and force the UAC to continue to generate rapid requests. In a similar fashion, a rogue proxy cannot force either the UAC or UAS to generate refreshes unless the proxy remains on the signaling path, and sees every request and response. Donovan & Rosenberg Expires August 13, 2004 [Page 23] Internet-Draft Session Timer February 2004 11.2 Outside Attacks An element that can observe and modify a request or response in transit can force rapid session refreshes. To prevent that, requests and responses need to be protected by message integrity. Since the session timer headers are not end-to-end, and are manipulated by proxies, the SIP S/MIME capabilities are not suitable for this task. Rather, integrity needs to be protected using hop-by-hop mechanisms. As a result, it is RECOMMENDED that an element that sends a request with a Session-Expires header field, or a Supported header field with the value 'timer', do so using TLS. Since adequate protection is obtained only if security is applied on each hop, it is RECOMMENDED that the SIPS URI scheme be used in conjunction with this extension. This means that proxies that record-route and request session timer, SHOULD record-route with a SIPS URI. A UA that inserts a Session-Expires header into a request or response SHOULD include a Contact URI that is a SIPS URI. Donovan & Rosenberg Expires August 13, 2004 [Page 24] Internet-Draft Session Timer February 2004 12. IANA Considerations This extension defines two new header fields, a new response code, and a new option tag. SIP [1] defines IANA procedures for registering these. 12.1 IANA Registration of Min-SE and Session-Expires Header Fields The following is the registration for the Min-SE header field: RFC Number: RFC XXXX [Note to IANA: Fill in with the RFC number of this specification.] Header Name: Min-SE Compact Form: none The following is the registration for the Session-Expires header field: RFC Number: RFC XXXX [Note to IANA: Fill in with the RFC number of this specification.] Header Name: Session-Expires Compact Form: x 12.2 IANA Registration of the 422 (Session Interval Too Small) Response Code The following is the registration for the 422 (Session Interval Too Small) response code: Response Code: 422 Default Reason Phrase: Session Interval Too Small RFC Number: RFC XXXX [Note to IANA: Fill in with the RFC number of this specification.] 12.3 IANA Registration of the 'timer' Option Tag The following is the registration for the 'timer' option tag: Donovan & Rosenberg Expires August 13, 2004 [Page 25] Internet-Draft Session Timer February 2004 Name: timer Description: This option tag is for support of the session timer extension. Inclusion in a Supported header field in a request or response indicates that the UA is capable of performing refreshes according to that specification. Inclusion in a Require header in a request means that the UAS must understand the session timer extension to process the request. Inclusion in a Require header field in a response indicates that the UAC must look for the Session-Expires header field in the response, and process accordingly. Donovan & Rosenberg Expires August 13, 2004 [Page 26] Internet-Draft Session Timer February 2004 13. Example Call Flow Example Session Timer Flow Alice Proxy P1 Proxy P2 Bob |(1) INVITE | | | |SE: 50 | | | |----------->| | | |(2) 422 | | | |MSE: 3600 | | | |<-----------| | | |(3) ACK | | | |----------->| | | |(4) INVITE | | | |SE:3600 | | | |MSE:3600 | | | |----------->| | | | |(5) INVITE | | | |SE:3600 | | | |MSE:3600 | | | |----------->| | | |(6) 422 | | | |MSE:4000 | | | |<-----------| | | |(7) ACK | | | |----------->| | |(8) 422 | | | |MSE:4000 | | | |<-----------| | | |(9) ACK | | | |----------->| | | |(10) INVITE | | | |SE:4000 | | | |MSE:4000 | | | |----------->| | | | |(11) INVITE | | | |SE:4000 | | | |MSE:4000 | | | |----------->| | | | |(12) INVITE | | | |SE:4000 | | | |MSE:4000 | | | |----------->| | | |(13) 200 OK | | | |SE:4000 | | | |<-----------| | |(14) 200 OK | | | |SE:4000 | | Donovan & Rosenberg Expires August 13, 2004 [Page 27] Internet-Draft Session Timer February 2004 | |<-----------| | |(15) 200 OK | | | |SE:4000 | | | |<-----------| | | |(16) ACK | | | |----------->| | | | |(17) ACK | | | |------------------------>| |(18) UPDATE | | | |SE:4000 | | | |----------->| | | | |(19) UPDATE | | | |SE:4000 | | | |------------------------>| | |(20) 200 OK | | | |SE:4000 | | | |<------------------------| |(21) 200 OK | | | |SE:4000 | | | |<-----------| | | | |(22) BYE | | | |<------------------------| |(23) BYE | | | |<-----------| | | | |(24) 408 | | | |------------------------>| Figure 1: Example Session Timer Flow Figure 1 gives an example of a call flow that makes use of the session timer. In this example, both the UAC and UAS support the session timer extension. The initial INVITE request generated by the UAC, Alice (message 1), might look like: INVITE sips:bob@biloxi.example.com SIP/2.0 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8 Supported: timer Session-Expires: 50 Max-Forwards: 70 To: Bob From: Alice ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: Content-Type: application/sdp Content-Length: 142 (Alice's SDP not shown) Donovan & Rosenberg Expires August 13, 2004 [Page 28] Internet-Draft Session Timer February 2004 This request indicates that Alice supports the session timer, and is request session refreshes every 50 seconds. This arrives at the first proxy, P1. This session interval is below the minimum allowed value of 3600. So, P1 rejects the request with a 422 (message 2): SIP/2.0 422 Session Interval Too Small Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8 ;received=192.0.2.1 Min-SE: 3600 To: Bob ;tag=9a8kz From: Alice ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE This response contains a Min-SE header field with the value of 3600. Alice then retries the request. This time, the request contains a Min-SE header, since Alice has received a 422 for other INVITE requests with the same Call-ID. The new request (message 4) might look like: INVITE sips:bob@biloxi.example.com SIP/2.0 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds9 Supported: timer Session-Expires: 3600 Min-SE: 3600 Max-Forwards: 70 To: Bob From: Alice ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314160 INVITE Contact: Content-Type: application/sdp Content-Length: 142 (Alice's SDP not shown) Proxy P1 record-routes. Since the session interval is now acceptable to it, it forwards the request to P2 (message 5). However, the session interval is below its minimum configured amount of 4000. So, it rejects the request with a 422 response code (message 6), and includes a Min-SE header field with the value of 4000. Once more, Alice retries the INVITE. This time, the Min-SE header field in her INVITE is the maximum of all Min-SE she has received (3600 and 4000). Message 10 might look like: Donovan & Rosenberg Expires August 13, 2004 [Page 29] Internet-Draft Session Timer February 2004 INVITE sips:bob@biloxi.example.com SIP/2.0 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds10 Supported: timer Session-Expires: 4000 Min-SE: 4000 Max-Forwards: 70 To: Bob From: Alice ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314161 INVITE Contact: Content-Type: application/sdp Content-Length: 142 (Alice's SDP not shown) P1 record-routes once again, but P2 does not (this wouldn't normally happen; presumably, if it asked for session timer, it would record-route the subsequent request). The UAS receives the request. It copies the Session-Expires header from the request to the response, and adds a refresher parameter with value 'uac'. This 200 OK is forwarded back to Alice. The response she receives (message 15) might look like: SIP/2.0 200 OK Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds10 ;received=192.0.2.1 Require: timer Supported: timer Record-Route: sips:p1.atlanta.example.com;lr Session-Expires: 4000;refresher=uac To: Bob ;tag=9as888nd From: Alice ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314161 INVITE Contact: Content-Type: application/sdp Content-Length: 142 (Bob's SDP not shown) Alice generates an ACK (message 16), which is routed through P1 and then to Bob. Since Alice is the refresher, around 3000 seconds later, Alice sends an UPDATE request to refresh the session. Since this request is part of an established dialog, and Alice has not received any 422 responses or requests on that dialog, there is no Min-SE Donovan & Rosenberg Expires August 13, 2004 [Page 30] Internet-Draft Session Timer February 2004 header field in her request (message 18): UPDATE sips:bob@192.0.2.4 SIP/2.0 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds12 Route: sips:p1.atlanta.example.com;lr Supported: timer Session-Expires: 4000;refresher=uac Max-Forwards: 70 To: Bob ;tag=9as888nd From: Alice ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314162 UPDATE Contact: This is forwarded through P1 to Bob. Bob generates a 200 OK, copying the Session-Expires header field into the response. This is forwarded through P1, and arrives at Alice. The response she receives (message 21) might look like: SIP/2.0 200 OK Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds12 ;received=192.0.2.1 Require: timer Session-Expires: 4000;refresher=uac To: Bob ;tag=9as888nd From: Alice ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314162 UPDATE Contact: Shortly afterwards, Alice's UA crashes. As a result, she never sends a session refresh request. 3968 seconds later, Bob times out, and sends a BYE request (message 22). This is sent to P1. P1 attempts to deliver it, but fails (since Alice's UA has crashed). P1 then returns a 408 (Request Timeout) to Bob. Donovan & Rosenberg Expires August 13, 2004 [Page 31] Internet-Draft Session Timer February 2004 14. Acknowledgements The authors wish to thank Brett Tate for his contributions to this work. Brian Rosen completed the editing of the document. Donovan & Rosenberg Expires August 13, 2004 [Page 32] Internet-Draft Session Timer February 2004 Normative References [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [2] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE Method", RFC 3311, October 2002. [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [5] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996. Donovan & Rosenberg Expires August 13, 2004 [Page 33] Internet-Draft Session Timer February 2004 Informative References [6] Srisuresh, P. and M. Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", RFC 2663, August 1999. [7] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional Responses in Session Initiation Protocol (SIP)", RFC 3262, June 2002. [8] Roach, A., "Session Initiation Protocol (SIP)-Specific Event Notification", RFC 3265, June 2002. Authors' Addresses Steven Donovan dynamicsoft 5100 Tennyson Parkway Suite 1200 Plano, TX 75024 US EMail: sdonovan@dynamicsoft.com Jonathan Rosenberg dynamicsoft 600 Lanidex Plaza Parsippany, NJ 07054 US EMail: jdrosen@dynamicsoft.com Donovan & Rosenberg Expires August 13, 2004 [Page 34] Internet-Draft Session Timer February 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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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 Donovan & Rosenberg Expires August 13, 2004 [Page 35] Internet-Draft Session Timer February 2004 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Donovan & Rosenberg Expires August 13, 2004 [Page 36]