SIP                                                           S. Donovan
Internet-Draft                                              J. Rosenberg
Expires: March 31, July 14, 2004                                       dynamicsoft
                                                         October 1, 2003
                                                        January 14, 2004

        Session Timers in the Session Initiation Protocol (SIP)
                    draft-ietf-sip-session-timer-12
                    draft-ietf-sip-session-timer-13

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   This Internet-Draft will expire on March 31, July 14, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2003). (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.

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  . . . . . . . . . . . . . . . . .  13
   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

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
   for the session expiration. The 422 response code indicates that the
   session timer duration was too small.

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.

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 of them can insert a
   Session-Expires header field into the request if none is already
   there, containing the desired interval. If one is already there, a
   proxy can reduce the interval, but not to a value lower than the
   Min-SE header field. The Min-SE header field contains the minimum
   allowed value of the session interval. Its purpose, explained below,
   is to ensure that the session interval is not lower than any proxy's
   configured minimum.

   If the Session-Expires interval is too low for a proxy, it can reject
   the request with a 422 response. That response contains the 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
   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.

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.

   There is no absolute minimum value for the Session-Expires header
   field. However, 1800 seconds (30 minutes) is RECOMMENDED. In other
   words, SIP entities MUST be prepared to handle Session-Expires header
   field values of any duration, 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, when not
   present, is infinity. This means that absence of the Session-Expires
   header field implies no expiration.

   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')

   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

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 not present, the default value for this header field is zero.

   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)

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.

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 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
   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.

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 zero) 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
   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.

8. Proxy Behavior

   Session timers are mostly of interest to call stateful proxy servers. 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 zero 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
   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.

   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
   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.

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.

   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. 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 zero when the Min-SE header field is not
   present), the UAS 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 a
   proxy increased the value of the Min-SE header field), the UAS MUST
   increase the value of the Session-Expires header field to make it
   equal to Min-SE header field value.

   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. The value MUST NOT be set it 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. Table 2 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.

   +----------------------+----------------------+---------------------+
   |

       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         |
   +----------------------+----------------------+---------------------+

   Table 2:

                         Figure 3: UAS Behavior

   The fourth row of Table 2 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
   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.

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 no 2xx response to a session refresh request is received before
   the session expiration, the UA SHOULD send a BYE request to terminate
   the session. It SHOULD send this BYE slightly before session
   expiration. The minimum of ten seconds and one third the session
   interval is RECOMMENDED.

      For example, if the session interval is 120 seconds, one third of
      this is 40 seconds. Since the minimum of 10 seconds and 40 seconds
      is 10 seconds, the BYE would be sent 10 seconds before the session
      expires.

   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 ten 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.

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.

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 IPSec or TLS. Since adequate protection is
   obtained only if TLS or IPSec 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.

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:

   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.

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     |            |
         |            |<-----------|            |
         |(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 sip:bob@biloxi.example.com sips:bob@biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
   Supported: timer
   Session-Expires: 50
   Max-Forwards: 70
   To: Bob <sip:bob@biloxi.example.com> <sips:bob@biloxi.example.com>
   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774 <sips:alice@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 INVITE
   Contact: <sip:alice@pc33.atlanta.example.com> <sips:alice@pc33.atlanta.example.com>
   Content-Type: application/sdp
   Content-Length: 142

   (Alice's SDP not shown)
   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/UDP SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
     ;received=192.0.2.1
   Min-SE: 3600
   To: Bob <sip:bob@biloxi.example.com>;tag=9a8kz <sips:bob@biloxi.example.com>;tag=9a8kz
   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774 <sips:alice@atlanta.example.com>;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 sip:bob@biloxi.example.com sips:bob@biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds9
   Supported: timer
   Session-Expires: 3600
   Min-SE: 3600
   Max-Forwards: 70
   To: Bob <sip:bob@biloxi.example.com> <sips:bob@biloxi.example.com>
   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774 <sips:alice@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314160 INVITE
   Contact: <sip:alice@pc33.atlanta.example.com> <sips:alice@pc33.atlanta.example.com>
   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:

   INVITE sip:bob@biloxi.example.com sips:bob@biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds10
   Supported: timer
   Session-Expires: 4000
   Min-SE: 4000
   Max-Forwards: 70
   To: Bob <sip:bob@biloxi.example.com> <sips:bob@biloxi.example.com>
   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774 <sips:alice@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314161 INVITE
   Contact: <sip:alice@pc33.atlanta.example.com> <sips:alice@pc33.atlanta.example.com>
   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/UDP SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds10
    ;received=192.0.2.1
   Require: timer
   Supported: timer
   Record-Route: sip:p1.atlanta.example.com;lr sips:p1.atlanta.example.com;lr
   Session-Expires: 4000;refresher=uac
   To: Bob <sip:bob@biloxi.example.com>;tag=9as888nd <sips:bob@biloxi.example.com>;tag=9as888nd
   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774 <sips:alice@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314161 INVITE
   Contact: <sip:bob@192.0.2.4> <sips:bob@192.0.2.4>
   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
   header field in her request (message 18):

   UPDATE sip:bob@192.0.2.4 sips:bob@192.0.2.4 SIP/2.0
   Via: SIP/2.0/UDP SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds12
   Route: sip:p1.atlanta.example.com;lr sips:p1.atlanta.example.com;lr
   Supported: timer
   Session-Expires: 4000;refresher=uac
   Max-Forwards: 70
   To: Bob <sip:bob@biloxi.example.com>;tag=9as888nd <sips:bob@biloxi.example.com>;tag=9as888nd
   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774 <sips:alice@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314162 UPDATE
   Contact: <sip:alice@pc33.atlanta.example.com> <sips:alice@pc33.atlanta.example.com>

   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/UDP SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds12
     ;received=192.0.2.1
   Require: timer
   Session-Expires: 4000;refresher=uac
   To: Bob <sip:bob@biloxi.example.com>;tag=9as888nd <sips:bob@biloxi.example.com>;tag=9as888nd
   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774 <sips:alice@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314162 UPDATE
   Contact: <sip:bob@192.0.2.4> <sips:bob@192.0.2.4>

   Shortly afterwards, Alice's UA crashes. As a result, she never sends
   a session refresh request. 3990 seconds later, Bob gives up, 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.

14. Acknowledgements

   The authors wish to thank Brett Tate for his contributions to this
   work. Brian Rosen completed the editing of the document.

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.

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
   72 Eagle Rock Avenue
   East Hanover,
   600 Lanidex Plaza
   Parsippany, NJ  07936  07054
   US

   EMail: jdrosen@dynamicsoft.com

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