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Internet Engineering Task Force                                   SIP WG
Internet Draft                                                 S.Donovan
                                                             dynamicsoft
                                                             J.Rosenberg
                                                             dynamicsoft
draft-ietf-sip-session-timer-09.txt
July 1, 2002
Expires: January 2003


        Session Initiation Protocol Extension for Session Timer

STATUS OF THIS MEMO

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   To view the list Internet-Draft Shadow Directories, see
   http://www.ietf.org/shadow.html.


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.








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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 proposes 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



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   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", "MUSTNOT", "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.

   Session refreshes are accomplished using SIP INVITE or UPDATE [2]



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   requests. The initial session refresh request establishes the
   duration of the session, which is carried in the Session-Expires
   header field in the 2xx response to the session refresh request. The
   Session-Expires header field in the 2xx response also indicates which
   side will be responsible for generating these refreshes - the UAC or
   the UAS. The responsible side generates a refresh (using a re-INVITE
   or UPDATE) before the session expires. 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. All
   session refresh requests are processed identically to the initial
   session refresh request, so that the 2xx response to a session
   refresh request carries the new time at which the session will
   expire.

   It is an explicit goal of this extension to operate so long as one of
   the two UAs in a dialog support the extension. That side, of course,
   ends up performing the refreshes. The other side will merely see them
   as repetitive re-INVITE or UPDATE requests. This facilitates
   interoperability.

   The extension supports negotiation of a reasonable value for the
   session interval, and negotiation of which side of the dialog is
   performing the refreshes. Negotiation of the session interval is
   critical. The value must be small enough to provide a useful
   expiration, but not so small to overload the proxies with messages.
   The goal of the negotiation algorithm is to choose a session interval
   that is the smallest of all the values requested by all elements on
   the dialog path, but only if that value is larger than the largest
   minimum timer requested by all elements. Negotiation of the refresher
   role is simpler. If only one side supports the extension, that side
   acts as refresher. Otherwise, one side chooses who will refresh. This
   negotiation takes place as part of session refresh request
   processing, and due to the idempotency of that processing, is redone
   each time the session is refreshed.

   To negotiate the value of the refresh interval, the Min-SE and
   Session-Expires header fields are used. The UAC generates a session
   refresh request, and includes a Session-Expires header field if it
   wishes to use the session timer. The Session-Expires header field
   value contains the session interval. As the session refresh request
   traverses proxies, the proxies (and the UAS) can reduce the value of
   the session interval, but not lower than the value of the Min-SE
   header field. If a proxy or UAS receives a request with a Session-
   Expires header field value lower than a configured minimum, it can
   reject the request with a 422 response. This response contains a
   Min-SE header field with the minimum allowed value. The client
   retries the request, inserting a Min-SE header field containing the



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   maximum value of the Min-SE header fields returned in previous 422
   responses for the call. In the case where the UAC isn't aware of the
   session timer extension, a proxy that receives a session refresh
   request with a Session-Expires lower than the configured minimum
   places the Min-SE header field in the proxied request, and increases
   the Session-Expires header field value to that minimum. When the
   request eventually reaches the UAS (potentially after a few retries
   resulting from a 422), it will contain a Session-Expires header field
   whose value meets the design criteria described above. The UAS
   returns a 200 OK with a Session-Expires header field containing that
   value. The header field also contains a parameter, called refresher,
   that indicates which side is performing the refreshes.

   When only the UAC supports the session timer extension, one of the
   proxies "fills the shoes" of the UAS. Specifically, when the UAS does
   support session timer, it places the value of the session interval
   into the Session-Expires header field in the 2xx response. When the
   UAS doesn't support it, the proxy closest to the UAS that asked for a
   session expiration performs that job, and inserts the value of the
   session interval into the 2xx response as it passes by.

   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, and is
   allowed 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 entites 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" which 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



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   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 field     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
   which can be used for that session.


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

   The Min-SE header field MUSTNOT 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 a Session Refresh Request

   A UAC which 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".

   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



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   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.

   A UAC MAY include the Min-SE header field in an INVITE request
   outside of a dialog, even if it never received a 422 previously.

   A UAC MAY include a Session-Expires in an initial session refresh
   request 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. In a session refresh request sent within
   a dialog with an active session timer, the 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.

   In an initial session refresh request, 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. 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.




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   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.

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



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   sent. A 2xx without a Session-Expires can come for both initial and
   mid-dialog session refresh requests.

   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 7.1.

8 Proxy Behavior

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



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   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



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   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



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   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

   When a UAS receives a session refresh request, the processing of that
   request can logically be broken into two steps. In the first step,
   the UAS acts as a "virtual proxy", and follows the rules specified in
   Section 8.1 as if it were a proxy. This means that the same session
   timer manipulations that a proxy can do, can also be done by a UAS.
   Specifically, this means that it can insert or reduce the session
   interval (but not below Min-SE header field value, if present),
   reject the request with a 422, and insert/increase the Min-SE header
   field value, just as a proxy can. Of course, rather than proxying the
   request, the "modified" request is passed into the second step of
   processing, which we call the "virtual UAS" processing. Viewing the
   UAS as the concatenation of a proxy and a UAS-specific processing
   component simplifies the specification of behavior and guarantees
   consistency. This separation is for the purposes of defining
   behavior. It does not mandate that the implementation work this way.

   Once the request is received by the virtual UAS (assuming it is
   received; it may have been rejected with a 422 based on the rules in
   Section 8.1), virtual UAS processing begins. From this point forward,
   the term "UAS" refers to the "virtual UAS". If the UAS wishes to
   accept request, it copies the value of the Session-Expires header
   field from the request into the 2xx response. Of course, this request
   refers to the one received from the "virtual proxy".

   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 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



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   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: 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 overried 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.



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   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 an 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



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   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 which 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



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   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 which 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 is applied on each hop,
   it is RECOMMENDED that the SIPS URI scheme be used in conjunction
   with this extension. This means that proxies which record-route and
   request session timer, SHOULD record-route with a SIPS URI. A UA
   which inserts a Session-Expires header into a request or response
   SHOULD include a Contact URI thats 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



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        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


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

   (Alice's SDP not shown)





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   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 pc33.atlanta.com;branch=z9hG4bKnashds8
     ;received=192.0.2.1
   Min-SE: 3600
   To: Bob <sip:bob@biloxi.com>;tag=9a8kz
   From: Alice <sip:alice@atlanta.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 INVITE



   This response contains an 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.com SIP/2.0
   Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds9
   Supported: timer
   Session-Expires: 3600
   Min-SE: 3600
   Max-Forwards: 70
   To: Bob <sip:bob@biloxi.com>
   From: Alice <sip:alice@atlanta.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314160 INVITE
   Contact: <sip:alice@pc33.atlanta.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



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        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

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   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.com SIP/2.0
   Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds10
   Supported: timer
   Session-Expires: 4000
   Min-SE: 4000
   Max-Forwards: 70
   To: Bob <sip:bob@biloxi.com>
   From: Alice <sip:alice@atlanta.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314161 INVITE
   Contact: <sip:alice@pc33.atlanta.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 pc33.atlanta.com;branch=z9hG4bKnashds10
    ;received=192.0.2.1
   Require: timer
   Supported: timer
   Record-Route: sip:p1.atlanta.com;lr
   Session-Expires: 4000;refresher=uac
   To: Bob <sip:bob@biloxi.com>;tag=9as888nd
   From: Alice <sip:alice@atlanta.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314161 INVITE
   Contact: <sip:bob@192.0.2.4>
   Content-Type: application/sdp



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   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 SIP/2.0
   Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds12
   Route: sip:p1.atlanta.com;lr
   Supported: timer
   Session-Expires: 4000;refresher=uac
   Max-Forwards: 70
   To: Bob <sip:bob@biloxi.com>;tag=9as888nd
   From: Alice <sip:alice@atlanta.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314162 UPDATE
   Contact: <sip:alice@pc33.atlanta.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 pc33.atlanta.com;branch=z9hG4bKnashds12
     ;received=192.0.2.1
   Require: timer
   Session-Expires: 4000;refresher=uac
   To: Bob <sip:bob@biloxi.com>;tag=9as888nd
   From: Alice <sip:alice@atlanta.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314162 UPDATE
   Contact: <sip:bob@192.0.2.4>





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   Shortly afterwards, Alice's UA crashes. As a result, she never sends
   an 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.

15 Author's Addresses



   Steven R. Donovan
   dynamicsoft
   5100 Tennyson Parkway
   Suite 1200
   Plano, Texas 75024
   email: sdonovan@dynamicsoft.com

   Jonathan Rosenberg
   dynamicsoft
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936
   email: jdrosen@dynamicsoft.com



16 Normative References

   [1] J. Rosenberg, H. Schulzrinne, et al.  , "SIP: Session initiation
   protocol," Internet Draft, Internet Engineering Task Force, Feb.
   2002.  Work in progress.

   [2] J. Rosenberg, "The session initiation protocol UPDATE method,"
   Internet Draft, Internet Engineering Task Force, May 2002.  Work in
   progress.

   [3] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.

   [4] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
   SDP," Internet Draft, Internet Engineering Task Force, Feb. 2002.
   Work in progress.




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17 Informative References

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

   [6] P. Srisuresh and M. Holdrege, "IP network address translator
   (NAT) terminology and considerations," RFC 2663, Internet Engineering
   Task Force, Aug. 1999.

   [7] J. Rosenberg and H. Schulzrinne, "Reliability of provisional
   responses in SIP," Internet Draft, Internet Engineering Task Force,
   Feb. 2002.  Work in progress.

   [8] A. Roach, "SIP-specific event notification," Internet Draft,
   Internet Engineering Task Force, Mar. 2002.  Work in progress.


   Full Copyright Statement

   Copyright (c) The Internet Society (2002). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.






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                           Table of Contents



   1          Introduction ........................................    3
   2          Terminology .........................................    4
   3          Overview of Operation ...............................    4
   4          Session-Expires Header Field Definition .............    6
   5          Min-SE Header Field Definition ......................    7
   6          422 Response Code Definition ........................    8
   7          UAC Behavior ........................................    8
   7.1        Generating a Session Refresh Request ................    8
   7.2        Processing a 2xx Response ...........................   10
   7.3        Processing a 422 Response ...........................   11
   8          Proxy Behavior ......................................   11
   8.1        Processing of Requests ..............................   11
   8.2        Processing of Responses .............................   13
   8.3        Session Expiration ..................................   14
   9          UAS Behavior ........................................   14
   10         Performing Refreshes ................................   16
   11         Security Considerations .............................   16
   11.1       Inside Attacks ......................................   17
   11.2       Outside Attacks .....................................   17
   12         IANA Considerations .................................   18
   12.1       IANA Registration of Min-SE and Session-Expires
   Header Fields ..................................................   18
   12.2       IANA Registration of the 422 (Session Interval Too
   Small) Response Code ...........................................   18
   12.3       IANA Registration of the "timer" Option Tag .........   19
   13         Example Call Flow ...................................   19
   14         Acknowledgements ....................................   24
   15         Author's Addresses ..................................   24
   16         Normative References ................................   24
   17         Informative References ..............................   25














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