draft-ietf-sip-session-timer-11.txt   draft-ietf-sip-session-timer-12.txt 
SIP Working Group S.Donovan SIP S. Donovan
Internet Draft dynamicsoft Internet-Draft J. Rosenberg
Document: draft-ietf-sip-session-timer-11.txt J. Rosenberg Expires: March 31, 2004 dynamicsoft
Category: Informational Dynamicsoft October 1, 2003
Expires: January, 2004 July 1, 2003
Session Timers in the Session Initiation Protocol (SIP) Session Timers in the Session Initiation Protocol (SIP)
draft-ietf-sip-session-timer-12
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1]. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that other
other groups may also distribute working documents as Internet- groups may also distribute working documents as Internet-Drafts.
Drafts. Internet-Drafts are draft documents valid for a maximum of
six months and may be updated, replaced, or obsoleted by other Internet-Drafts are draft documents valid for a maximum of six months
documents at any time. It is inappropriate to use Internet- Drafts and may be updated, replaced, or obsoleted by other documents at any
as reference material or to cite them other than as "work in time. It is inappropriate to use Internet-Drafts as reference
progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at http://
www.ietf.org/ietf/1id-abstracts.txt.
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on March 31, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract Abstract
This document defines an extension to the Session Initiation This document defines an extension to the Session Initiation Protocol
Protocol (SIP). This extension allows for a periodic refresh of SIP (SIP). This extension allows for a periodic refresh of SIP sessions
sessions through a re-INVITE or UPDATE request. The refresh allows through a re-INVITE or UPDATE request. The refresh allows both user
both user agents and proxies to determine if the SIP session is agents and proxies to determine if the SIP session is still active.
still active. The extension defines two new header fields, Session- The extension defines two new header fields, Session-Expires, which
Expires, which conveys the lifetime of the session, and Min-SE, conveys the lifetime of the session, and Min-SE, which conveys the
which conveys the minimum allowed value for the session timer. minimum allowed value for the session timer.
Table of Contents 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 1. Introduction
The Session Initiation Protocol (SIP) [1], does not define a The Session Initiation Protocol (SIP) [1], does not define a
keepalive mechanism for the sessions it establishes. Although the keepalive mechanism for the sessions it establishes. Although the
user agents may be able to determine if the session has timed out user agents may be able to determine if the session has timed out
using session specific mechanisms, proxies will not be able to do using session specific mechanisms, proxies will not be able to do so.
so. The result is that call stateful proxies will not always be able The result is that call stateful proxies will not always be able to
to determine whether a session is still active or not. For instance, 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 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 session, or the BYE message gets lost due to network problems, a call
call stateful proxy will not know when the session has ended. In stateful proxy will not know when the session has ended. In this
this situation, the call stateful proxy will retain state for the situation, the call stateful proxy will retain state for the call and
call and has no deterministic method of determining when the call has no deterministic method of determining when the call state
state information no longer applies. information no longer applies.
<Lastname> Category - Expiration 1 To resolve this problem, this extension defines a keepalive mechanism
To resolve this problem, this extension defines a keepalive for SIP sessions. UAs send periodic re-INVITE or UPDATE [2] requests
mechanism for SIP sessions. UAs send periodic re-INVITE or UPDATE (referred to as session refresh requests) to keep the session alive.
[2] requests (referred to as session refresh requests) to keep the The interval for the session refresh requests is determined through a
session alive. The interval for the session refresh requests is negotiation mechanism defined here. If a session refresh request is
determined through a negotiation mechanism defined here. If a not received before the interval passes, the session is considered
session refresh request is not received before the interval passes, terminated. Both UAs are supposed to send a BYE, and call stateful
the session is considered terminated. Both UAs are supposed to send proxies can remove any state for the call.
a BYE, and call stateful proxies can remove any state for the call.
This refresh mechanism has additional applications. For the same This refresh mechanism has additional applications. For the same
reasons a call stateful proxy server would like to determine whether reasons a call stateful proxy server would like to determine whether
the session is still active, a user agent would like to make this the session is still active, a user agent would like to make this
determination. This determination can be made at a user agent determination. This determination can be made at a user agent without
without the use of SIP level mechanisms; for audio sessions, the use of SIP level mechanisms; for audio sessions, periodic RTCP
periodic RTCP packets serve as an indication of liveness [5]. packets serve as an indication of liveness [5]. However, it is
However, it is desirable to separate SIP session liveness from the desirable to separate SIP session liveness from the details of the
details of the particular session. particular session.
Another application of the session timer is in the construction of a Another application of the session timer is in the construction of a
SIP Network Address Translator (NAT) Application Level Gateway (ALG) SIP Network Address Translator (NAT) Application Level Gateway (ALG)
[6]. The ALG embedded in a NAT will need to maintain state for the [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 duration of a call. This state must eventually be removed. Relying on
on a BYE to trigger the removal of state, besides being unreliable, a BYE to trigger the removal of state, besides being unreliable,
introduces a potential denial of service attack. introduces a potential denial of service attack.
This document provides an extension to SIP that defines a session This document provides an extension to SIP that defines a session
expiration mechanism. Periodic refreshes, through re-INVITEs or expiration mechanism. Periodic refreshes, through re-INVITEs or
UPDATEs, are used to keep the session active. The extension is UPDATEs, are used to keep the session active. The extension is
sufficiently backwards compatible with SIP that it works so long as sufficiently backwards compatible with SIP that it works so long as
either one of the two participants in a dialog understand the either one of the two participants in a dialog understand the
extension. Two new header fields, Session-Expires and Min-SE, and a extension. Two new header fields, Session-Expires and Min-SE, and a
new response code, 422, are defined. Session-Expires conveys the new response code, 422, are defined. Session-Expires conveys the
duration of the session, and Min-SE conveys the minimum allowed duration of the session, and Min-SE conveys the minimum allowed value
value for the session expiration. The 422 response code indicates for the session expiration. The 422 response code indicates that the
that the session timer duration was too small. session timer duration was too small.
2. Terminology 2. Terminology
In this document, the key words æMUSTÆ, æMUSTNOTÆ, æREQUIREDÆ, In this document, the key words 'MUST', 'MUST NOT', 'REQUIRED',
æSHALLÆ, æSHALLNOTÆ, æSHOULDÆ, æSHOULDNOTÆ, æRECOMMENDEDÆ, æMAYÆ, 'SHALL', 'SHALLNOT', 'SHOULD', 'SHOULDNOT', 'RECOMMENDED', 'MAY', and
and æOPTIONALÆ are to be interpreted as described in RFC 2119 [3]} 'OPTIONAL' are to be interpreted as described in RFC 2119 [3] and
and indicate requirement levels for compliant SIP implementations. indicate requirement levels for compliant SIP implementations.
Additionally, we define the following terms: Additionally, we define the following terms:
Session Interval: The largest amount of time that can occur Session Interval: The largest amount of time that can occur between
between session refresh requests in a dialog before the session session refresh requests in a dialog before the session will be
will be considered timed out. The session interval is conveyed considered timed out. The session interval is conveyed in the
in the Session-Expires header field defined here. The UAS Session-Expires header field defined here. The UAS obtains this
obtains this value from the Session-Expires header field in a value from the Session-Expires header field in a 2xx response to a
2xx response to a session refresh request that it sends. session refresh request that it sends. Proxies and UACs determine
Proxies and UACs determine this value from the Session-Expires this value from the Session-Expires header field in a 2xx response
to a session refresh request that they receive.
S. Donovan et. Al. Informational 2
header field in a 2xx response to a session refresh request
that they receive.
Minimum Timer: Because of the processing load of mid-dialog Minimum Timer: Because of the processing load of mid-dialog requests,
requests, all elements (proxy, UAC, UAS) can have a configured all elements (proxy, UAC, UAS) can have a configured minimum value
minimum value for the session interval that they are willing to for the session interval that they are willing to accept. This
accept. This value is called the minimum timer. value is called the minimum timer.
Session Expiration: The time at which an element will consider Session Expiration: The time at which an element will consider the
the session timed out, if no successful session refresh session timed out, if no successful session refresh transaction
transaction occurs beforehand. occurs beforehand.
Session Refresh Request: An INVITE or UPDATE request within a Session Refresh Request: An INVITE or UPDATE request within a dialog.
dialog. If the request generates a 2xx response, the session If the request generates a 2xx response, the session expiration is
expiration is increased to the current time plus the session increased to the current time plus the session interval obtained
interval obtained from the response. A session refresh request from the response. A session refresh request is not to be confused
is not to be confused with a target refresh request, defined in with a target refresh request, defined in Section 6 of [1], which
Section 6 of [1], which is a request that can update the remote is a request that can update the remote target of a dialog.
target of a dialog.
Refresh: Same as a session refresh request. Refresh: Same as a session refresh request.
3. Overview of Operation 3. Overview of Operation
This section provides a brief overview of operation of the This section provides a brief overview of operation of the extension.
extension. It is tutorial in nature and should not be considered as It is tutorial in nature and should not be considered as normative.
normative.
This extension has the property that it works even when only one UA This extension has the property that it works even when only one UA
in a dialog supports it. The processing steps differ for handling in a dialog supports it. The processing steps differ for handling
each of the four cases (UAC supports it, or doesn't, and UAS each of the four cases (UAC supports it, or doesn't, and UAS supports
supports it, or doesn't). For simplicity's sake, this section will it, or doesn't). For simplicity's sake, this section will describe
describe basic operation in the case where both sides support the basic operation in the case where both sides support the extension.
extension.
A UAC starts by sending an INVITE. It includes a Supported header A UAC starts by sending an INVITE. It includes a Supported header
with the option tag ætimerÆ, indicating support for this extension. with the option tag 'timer', indicating support for this extension.
This request passes through proxies and B2BUAs, any one of which may This request passes through proxies, any one of which may have an
have an interest in establishing a session timer. Each of them can interest in establishing a session timer. Each of them can insert a
insert a Session-Expires header field into the request if none is Session-Expires header field into the request if none is already
already there, containing the desired interval. If one is already there, containing the desired interval. If one is already there, a
there, a proxy or B2BUA can reduce the interval, but not to a value proxy can reduce the interval, but not to a value lower than the
lower than the Min-SE header field. The Min-SE header field contains Min-SE header field. The Min-SE header field contains the minimum
the minimum allowed value of the session interval. Its purpose, allowed value of the session interval. Its purpose, explained below,
explained below, is to ensure that the session interval is not lower is to ensure that the session interval is not lower than any proxy's
than any proxy's configured minimum. 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
S. Donovan et. Al. Informational 3 If the Session-Expires interval is too low for a proxy, it can reject
Min-SE header field it observed in all 422 responses received the request with a 422 response. That response contains the Min-SE
previously. This way, the minimum timer meets the constraints of all header field, identifying the minimum session interval it is willing
proxies along the path. 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 After several INVITE/422 iterations, the request eventually arrives
at the UAS. The UAS can adjust the value of the session interval as 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 if it was a proxy, and when done, it places the final session
interval into the Session-Expires header field in a 2xx response. interval into the Session-Expires header field in a 2xx response. The
The Session-Expires header field also contains a ærefresherÆ Session-Expires header field also contains a 'refresher' parameter,
parameter, which indicates who is doing the refreshing - the UA that which indicates who is doing the refreshing - the UA that is
is currently the UAC, or the UA that is currently the UAS. As the currently the UAC, or the UA that is currently the UAS. As the 2xx
2xx response travels back through the proxy chain, each proxy or response travels back through the proxy chain, each proxy can observe
B2BUA can observe the final session interval, but they can't change the final session interval, but they can't change it.
it.
From the Session-Expires header field in the response, both UAs know 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 that a session timer is active, they know when it will expire, and
they know who is refreshing. At some point before the expiration, they know who is refreshing. At some point before the expiration, the
the currently active refresher generates a session refresh request, currently active refresher generates a session refresh request, which
which is a re-INVITE or UPDATE [2] request. If the refresher never is a re-INVITE or UPDATE [2] request. If the refresher never gets a
gets a response to that session refresh request, it sends a BYE to response to that session refresh request, it sends a BYE to terminate
terminate the session. Similarly, if the other side never gets the the session. Similarly, if the other side never gets the session
session refresh request before the session expires, it sends a BYE. refresh request before the session expires, it sends a BYE.
The refresh requests sent once the session is established are The refresh requests sent once the session is established are
processed identically to the initial requests, as described above. processed identically to the initial requests, as described above.
This means that a successful session refresh request will extend the This means that a successful session refresh request will extend the
session, as desired. session, as desired.
The extension introduces additional complications beyond this basic The extension introduces additional complications beyond this basic
flow to support cases where only one of the UAs supports it. One flow to support cases where only one of the UAs supports it. One such
such complication is that a proxy may need to insert the Session- complication is that a proxy may need to insert the Session-Expires
Expires header into the response, in the event that the UAS doesn't header into the response, in the event that the UAS doesn't support
support the extension. The negotiation of the role of refresher is the extension. The negotiation of the role of refresher is also
also affected by this capability; it takes into consideration which affected by this capability; it takes into consideration which
participants support the extension. participants support the extension.
It is worth noting that the session timer refreshes the session, not It is worth noting that the session timer refreshes the session, not
the dialog used to establish the session. Of course, the two are the dialog used to establish the session. Of course, the two are
related. If the session expires, a BYE is sent, which terminates the related. If the session expires, a BYE is sent, which terminates the
session and generally, the dialog. session and generally, the dialog.
4. Session-Expires Header Field Definition 4. Session-Expires Header Field Definition
The Session-Expires header field conveys the session interval for a The Session-Expires header field conveys the session interval for a
SIP session. It is placed only in INVITE or UPDATE requests, as well 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 as in any 2xx response to an INVITE or UPDATE. Like the SIP Expires
header field, it contains a delta-time. header field, it contains a delta-time.
There is no absolute minimum value for the Session-Expires header There is no absolute minimum value for the Session-Expires header
field. However, 1800 seconds (30 minutes) is RECOMMENDED. In other field. However, 1800 seconds (30 minutes) is RECOMMENDED. In other
words, SIP entities MUST be prepared to handle Session-Expires words, SIP entities MUST be prepared to handle Session-Expires header
field values of any duration, but entities that insert the
S. Donovan et. Al. Informational 4
header field values of any duration, but entities that insert the
Session-Expires header field SHOULD NOT choose values less than 30 Session-Expires header field SHOULD NOT choose values less than 30
minutes. minutes.
Small session intervals can be destructive to the network. They Small session intervals can be destructive to the network. They cause
cause excessive messaging traffic that affects both user agents and excessive messaging traffic that affects both user agents and proxy
proxy servers. They increase the possibility of æglareÆ which can servers. They increase the possibility of 'glare' that can occur when
occur when both user agents send a re-INVITE or UPDATE at the same both user agents send a re-INVITE or UPDATE at the same time. Since
time. Since the primary purpose of the session timer is to provide a the primary purpose of the session timer is to provide a means to
means to time out state in SIP elements, very small values won't time out state in SIP elements, very small values won't generally be
generally be needed. 30 minutes was chosen since 95% of phone calls needed. 30-minutes was chosen since 95% of phone calls are less than
are less than this duration. However, the 30 minute minimum is this duration. However, the 30 minute minimum is listed as a SHOULD,
listed as a SHOULD, and not a MUST, since the exact value for this and not a MUST, since the exact value for this number is dependent on
number is dependent on many network factors, including network many network factors, including network bandwidths and latencies,
bandwidths and latencies, computing power, memory availability, computing power, memory availability, network topology, and of
network topology, and of course, the application scenario. After course, the application scenario. After all, SIP can set up any kind
all, SIP can set up any kind of session, not just a phone call. At of session, not just a phone call. At the time of publication of this
the time of publication of this document, 30 minutes seems document, 30 minutes seems appropriate. Advances in technologies may
appropriate. Advances in technologies may result in the number being result in the number being excessively large five years in the
excessively large five years in the future. future.
The default value of the Session-Expires header field, when not The default value of the Session-Expires header field, when not
present, is infinity. This means that absence of the Session-Expires present, is infinity. This means that absence of the Session-Expires
header field implies no expiration. header field implies no expiration.
The syntax of the Session-Expires header field is: The syntax of the Session-Expires header field is:
Session-Expires = ('Session-Expires' / 'x') HCOLON delta-seconds Session-Expires = ('Session-Expires' / 'x') HCOLON delta-seconds
*(SEMI se-params) *(SEMI se-params)
se-params = refresher-param / generic-param se-params = refresher-param / generic-param
refresher-param = 'refresher' EQUAL ('uas' / 'uac') refresher-param = 'refresher' EQUAL ('uas' / 'uac')
Note that a compact form, the letter x, has been reserved for Note that a compact form, the letter x, has been reserved for
Session-Expires. The BNF for delta-seconds and generic-param is Session-Expires. The BNF for delta-seconds and generic-param is
defined in Section 25 of RFC 3261 [1]. defined in Section 25 of RFC 3261 [1].
Table 1 is an extension of Tables 2 and 3 in [1] for the Session- Table 1 is an extension of Tables 2 and 3 in [1] for the
Expires and Min-SE header fields. The column 'PRA' is for the PRACK Session-Expires and Min-SE header fields. The column 'PRA' is for the
method [7], æUPDÆ is for the UPDATE method [2], æSUBÆ is for the PRACK method [7], 'UPD' is for the UPDATE method [2], 'SUB' is for
SUBSCRIBE method [8], and æNOTÆ is for the NOTIFY method [8]. 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} +---------------+-----+-----+---+---+---+---+---+---+---+---+---+---+
| 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 5. Min-SE Header Field Definition
S. Donovan et. Al. Informational 5
The Min-SE header field indicates the minimum value for the session The Min-SE header field indicates the minimum value for the session
interval, in units of delta-seconds. When used in an INVITE or interval, in units of delta-seconds. When used in an INVITE or UPDATE
UPDATE request, it indicates the smallest value of the session request, it indicates the smallest value of the session interval that
interval that can be used for that session. can be used for that session.
When not present, the default value for this header field is zero. When not present, the default value for this header field is zero.
The Min-SE header field {MUSTNOT} be used in responses except those 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 with a 422 response code. It indicates the minimum value of the
session interval that the server is willing to accept. session interval that the server is willing to accept.
The syntax of the Min-SE header field is: The syntax of the Min-SE header field is:
Min-SE = 'Min-SE' HCOLON delta-seconds *(SEMI generic-param) Min-SE = 'Min-SE' HCOLON delta-seconds *(SEMI generic-param)
6. 422 Response Code Definition 6. 422 Response Code Definition
This extension introduces the 422 (Session Interval Too Small) This extension introduces the 422 (Session Interval Too Small)
response code. It is generated by a UAS or proxy when a request response code. It is generated by a UAS or proxy when a request
contains a Session-Expires header field with a duration that is contains a Session-Expires header field with a duration that is below
below the minimum timer for the server. The 422 response MUST the minimum timer for the server. The 422 response MUST contain a
contain a Min-SE header field with the minimum timer for that Min-SE header field with the minimum timer for that server.
server.
7. UAC Behavior 7. UAC Behavior
7.1 Generating an Initial Session Refresh Request 7.1 Generating an Initial Session Refresh Request
A UAC that supports the session timer extension defined here MUST A UAC that supports the session timer extension defined here MUST
include a Supported header field in each request (except ACK), 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 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. not requesting usage of the session timer for this session.
The UAC MAY include a Require header field in the request with the The UAC MAY include a Require header field in the request with the
value ætimerÆ to indicate that the UAS must support the session value 'timer' to indicate that the UAS must support the session timer
timer to participate in the session. This does not mean that the to participate in the session. This does not mean that the UAC is
UAC is requiring the UAS to perform the refreshes, just that it is requiring the UAS to perform the refreshes, just that it is requiring
requiring the UAS to support the extension. In addition, the UAC MAY the UAS to support the extension. In addition, the UAC MAY include a
include a Proxy-Require header field in the request with the value Proxy-Require header field in the request with the value 'timer' to
'timer' to indicate that proxies must support session timer in order indicate that proxies must support session timer in order to
to correctly process the request. However, usage of either Require correctly process the request. However, usage of either Require or
or Proxy-Require by the UAC is NOT RECOMMENDED. They are not needed, Proxy-Require by the UAC is NOT RECOMMENDED. They are not needed,
since the extension works even when only the UAC supports the since the extension works even when only the UAC supports the
extension. The Supported header field containing 'timer' MUST still extension. The Supported header field containing 'timer' MUST still
be included even if the Require or Proxy-Require header fields are be included even if the Require or Proxy-Require header fields are
present containing 'timer'. present containing 'timer'.
A UAC MAY include the Min-SE header field in the initial INVITE A UAC MAY include the Min-SE header field in the initial INVITE
request. request.
A UAC MAY include a Session-Expires in an initial session refresh 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 request if it wishes for a session timer to be applied to the
session. The value of this header field indicates the session session. The value of this header field indicates the session
interval desired by the UAC. If a Min-SE header is included in the interval desired by the UAC. If a Min-SE header is included in the
S. Donovan et. Al. Informational 6
initial session refresh request, the value of the Session-Expires initial session refresh request, the value of the Session-Expires
MUST be equal to the value in Min-SE. MUST be equal to the value in Min-SE.
The UAC MAY include the refresher parameter with value 'uac' if it The UAC MAY include the refresher parameter with value 'uac' if it
wishes to perform the refreshes. However, it is RECOMMENDED that the wishes to perform the refreshes. However, it is RECOMMENDED that the
parameter be omitted, so that it can be selected by the negotiation parameter be omitted, so that it can be selected by the negotiation
mechanisms described below. mechanisms described below.
7.2 Processing a 2xx Response 7.2 Processing a 2xx Response
Session timer requires a UA to create and maintain state. This state Session timer requires a UA to create and maintain state. This state
includes the session interval, the session expiration, and the includes the session interval, the session expiration, and the
identity of the refresher. This state is associated with the dialog identity of the refresher. This state is associated with the dialog
on which the session has been negotiated. on which the session has been negotiated.
When a 2xx response to a session refresh request arrives, it may or When a 2xx response to a session refresh request arrives, it may or
may not contain a Require header field with the value ætimerÆ. If it 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 does, the UAC MUST look for the Session-Expires header field to
process the response. process the response.
If there was a Require header field in the response with the value If there was a Require header field in the response with the value
ætimerÆ, the Session-Expires header field will always be present. 'timer', the Session-Expires header field will always be present.
UACs MUST be prepared to receive a Session-Expires header field in a UACs MUST be prepared to receive a Session-Expires header field in a
response even if none were present in the request. The ærefresherÆ response even if none were present in the request. The 'refresher'
parameter will be present in the Session-Expires header field, parameter will be present in the Session-Expires header field,
indicating who will be performing the refreshes. The UAC MUST set indicating who will be performing the refreshes. The UAC MUST set the
the identity of the refresher to the value of this parameter. If the identity of the refresher to the value of this parameter. If the
parameter contains the value æuacÆ, the UAC will perform them. It is parameter contains the value 'uac', the UAC will perform them. It is
possible that the UAC requested session timer (and thus included a possible that the UAC requested session timer (and thus included a
Session-Expires header field in the request), but there was no Session-Expires header field in the request), but there was no
Require or Session-Expires header field in the 2xx response. This Require or Session-Expires header field in the 2xx response. This
will happen when the UAS doesn't support the session timer will happen when the UAS doesn't support the session timer extension,
extension, and only the UAC has asked for a session timer (no and only the UAC has asked for a session timer (no proxies have
proxies have requested it). In this case, if the UAC still wishes to requested it). In this case, if the UAC still wishes to use the
use the session timer (they are purely for its benefit alone), it session timer (they are purely for its benefit alone), it has to
has to perform them. To do this, the UAC follows the procedures perform them. To do this, the UAC follows the procedures defined in
defined in this specification as if the Session-Expires header field this specification as if the Session-Expires header field were in the
were in the 2xx response, and its value was the same as the one in 2xx response, and its value was the same as the one in the request,
the request, but with a refresher parameter of æuacÆ. but with a refresher parameter of 'uac'.
If the 2xx response did not contain a Session-Expires header field, If the 2xx response did not contain a Session-Expires header field,
there is no session expiration. In this case, no refreshes need to there is no session expiration. In this case, no refreshes need to be
be sent. A 2xx without a Session-Expires can come for both initial sent. A 2xx without a Session-Expires can come for both initial and
and mid-dialog session refresh requests. This means that the session mid-dialog session refresh requests. This means that the session
timer can be æturned-offÆ mid dialog by receiving a response without timer can be 'turned-off' mid dialog by receiving a response without
a Session-Expires header. a Session-Expires header.
The UAC remembers the session interval for a session as the value of 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 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 recent 2xx response to a session refresh request on a dialog. It is
explicitly allowed for there to be differing session intervals (or explicitly allowed for there to be differing session intervals (or
none at all) on differing dialogs established as a result of a 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
S. Donovan et. Al. Informational 7 on for the session.
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 If the UAC must perform the refreshes, it computes the session
expiration for that session. The session expiration is the time of expiration for that session. The session expiration is the time of
reception of the last 2xx response to a session refresh request on reception of the last 2xx response to a session refresh request on
that dialog plus the session interval for that session. If UA wishes that dialog plus the session interval for that session. If UA wishes
to continue with the session beyond the session expiration, it MUST to continue with the session beyond the session expiration, it MUST
generate a refresh before the session expiration. It is RECOMMENDED generate a refresh before the session expiration. It is RECOMMENDED
that this refresh be sent once half the session interval has lapsed. that this refresh be sent once half the session interval has elapsed.
Additional procedures for this refresh are described in Section Additional procedures for this refresh are described in Section 10.
Section 10.
7.3 Processing a 422 Response 7.3 Processing a 422 Response
If the response to a session refresh request is a 422 (Session If the response to a session refresh request is a 422 (Session
Interval Too Small) response message, then the UAC MAY retry the Interval Too Small) response message, then the UAC MAY retry the
request. The procedures for retrying are described in Section request. The procedures for retrying are described in Section 7.4.
Section 7.1. This new request constitutes a new transaction and This new request constitutes a new transaction and SHOULD have the
SHOULD have the same value of the Call-ID, To, and From of the same value of the Call-ID, To, and From of the previous request, but
previous request, but the CSeq should contain a new sequence number the CSeq should contain a new sequence number that is one higher than
that is one higher than the previous. the previous.
7.4 Generating Subsequent Session Refresh Requests 7.4 Generating Subsequent Session Refresh Requests
The values of Supported, Require and Proxy-Require used in the The values of Supported, Require and Proxy-Require used in the
initial Session refresh request MUST be used. initial Session refresh request MUST be used.
The UAC MUST insert the Min-SE header field into a session refresh 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 request for a particular dialog if it has ever received a 422
response to a previous session refresh request on the same dialog, response to a previous session refresh request on the same dialog, or
or if it has received a session refresh request on that dialog which if it has received a session refresh request on that dialog which
contained a Min-SE header field. Similarly, if no dialog has been 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 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 if it has ever received a 422 response to a previous
INVITE request with the same Call-ID. INVITE request with the same Call-ID.
The value of the Min-SE header field present in a session refresh 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 request MUST be the largest value amongst all Min-SE header field
values returned in all 422 responses, or received in session refresh values returned in all 422 responses, or received in session refresh
requests, on the same dialog, if a dialog has been established. If requests, on the same dialog, if a dialog has been established. If no
no dialog has been established, the Min-SE header field value is set dialog has been established, the Min-SE header field value is set to
to the largest value amongst all Min-SE header field values returned the largest value amongst all Min-SE header field values returned in
in all 422 responses for an INVITE request with the same Call-ID. A 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 result of this rule is that the maximum value of the Min-SE is
effectively æclearedÆ once the dialog is established, and from that effectively 'cleared' once the dialog is established, and from that
point on, only the values from proxies known to be on the proxy path point on, only the values from proxies known to be on the proxy path
will end up being used. will end up being used.
The UAC may have its own opinions about the minimum session The UAC may have its own opinions about the minimum session interval.
interval. In that case, if the value above is too small, the UAC MAY In that case, if the value above is too small, the UAC MAY increase
increase it. it.
S. Donovan et. Al. Informational 8
In a session refresh request sent within a dialog with an active In a session refresh request sent within a dialog with an active
session timer, the Sesssion-Expires header field SHOULD be present. session timer, the Sesssion-Expires header field SHOULD be present.
When present, it MUST be equal to the maximum of the Min-SE header 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 field (recall that its default value when not present is zero) and
the current session interval. the current session interval.
If the session refresh request is not the initial one, it is If the session refresh request is not the initial one, it is
RECOMMENDED that the refresher parameter be set to æuacÆ if the RECOMMENDED that the refresher parameter be set to 'uac' if the
element sending the request is currently performing refreshes, else element sending the request is currently performing refreshes, else
æuasÆ if its peer is performing the refreshes. This way, the role of '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 refresher does not change on each refresh. However, if it wishes to
explicitly change the roles, it MAY use a value of æuasÆ if it knows 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 that the other side supports session timer. It could know this by
having received a request from its peer with a Supported header having received a request from its peer with a Supported header field
field containing the value ætimerÆ. If it wishes to reselect the containing the value 'timer'. If it wishes to reselect the roles, it
roles, it MAY omit the parameter. MAY omit the parameter.
A re-INVITE generated to refresh the session is a normal re-INVITE, 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 and an UPDATE generated to refresh a session is a normal UPDATE. If a
a UAC knows that its peer supports the UPDATE method, it is UAC knows that its peer supports the UPDATE method, it is RECOMMENDED
RECOMMENDED that UPDATE be used instead of a re-INVITE. A UA can that UPDATE be used instead of a re-INVITE. A UA can make this
make this determination if it has seen an Allow header field from determination if it has seen an Allow header field from its peer with
its peer with the value æUPDATEÆ, or through a mid-dialog OPTIONS the value 'UPDATE', or through a mid-dialog OPTIONS request. It is
request. It is RECOMMENDED that the UPDATE request not contain an RECOMMENDED that the UPDATE request not contain an offer [4], but a
offer [4], but a re-INVITE SHOULD contain one, even if the details re-INVITE SHOULD contain one, even if the details of the session have
of the session have not changed. In that case, the offer MUST not changed. In that case, the offer MUST indicate that it has not
indicate that it has not changed. In the case of SDP, this is changed. In the case of SDP, this is accomplished by including the
accomplished by including the same value for the origin field as same value for the origin field as previous SDP messages to its peer.
previous SDP messages to its peer. The same is true for an answer The same is true for an answer exchanged as a result of a session
exchanged as a result of a session refresh request; if it has not refresh request; if it has not changed, that MUST be indicated.
changed, that MUST be indicated.
8. Proxy Behavior 8. Proxy Behavior
Session timers are mostly of interest to call stateful proxy servers Session timers are mostly of interest to call stateful proxy servers.
and B2BUAs. However, a stateful proxy server MAY also follow the However, a stateful proxy server MAY also follow the rules described
rules described here. Stateless proxies MUST NOT attempt to request here. Stateless proxies MUST NOT attempt to request session timers.
session timers. Proxies which ask for session timers SHOULD record- Proxies that ask for session timers SHOULD record-route, since they
route, since they won't receive refreshes if they don't. won't receive refreshes if they don't.
The proxy processing rules require the proxy to remember The proxy processing rules require the proxy to remember
information between the request and response, ruling out information between the request and response, ruling out stateless
stateless proxies. proxies.
8.1 Processing of Requests 8.1 Processing of Requests
Processing of requests is identical for all session refresh Processing of requests is identical for all session refresh requests.
requests.
To request a session timer for a session, a proxy makes sure that a 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 Session-Expires header field is present in a session refresh request
for that session. A proxy MAY insert a Session-Expires header field for that session. A proxy MAY insert a Session-Expires header field
S. Donovan et. Al. Informational 9
in the request before forwarding it, if none was present in the in the request before forwarding it, if none was present in the
request. This Session-Expires header field may contain any desired request. This Session-Expires header field may contain any desired
expiration time the proxy would like, but not with a duration lower 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 than the value in the Min-SE header field in the request, if present.
present. The proxy MUST NOT include a refresher parameter in the The proxy MUST NOT include a refresher parameter in the header field
header field value. value.
If the request already had a Session-Expires header field, the proxy If the request already had a Session-Expires header field, the proxy
or B2BUA MAY reduce its value, but MUST NOT set it to a duration MAY reduce its value, but MUST NOT set it to a duration lower than
lower than the value in the Min-SE header field in the request, if the value in the Min-SE header field in the request, if present. If
present. If the value of the Session-Expires header field is greater the value of the Session-Expires header field is greater than or
than or equal to the value in the Min-SE header field (recall that equal to the value in the Min-SE header field (recall that the
the default is zero when the Min-SE header field is not present), default is zero when the Min-SE header field is not present), the
the proxy MUST NOT increase the value of the Session-Expires header proxy MUST NOT increase the value of the Session-Expires header
field. If the value of the Session-Expires header field is lower field. If the value of the Session-Expires header field is lower than
than the value of the Min-SE header field (possibly because the the value of the Min-SE header field (possibly because the proxy
proxy increased the value of the Min-SE header field, as described increased the value of the Min-SE header field, as described below),
below), the proxy MUST increase the value of the Session-Expires the proxy MUST increase the value of the Session-Expires header field
header field to make it equal to Min-SE header field value. The to make it equal to Min-SE header field value. The proxy MUST NOT
proxy MUST NOT insert or modify the value of the ærefresherÆ insert or modify the value of the 'refresher' parameter in the
parameter in the Session-Expires header field. Session-Expires header field.
If the request contains a Supported header field with a value If the request contains a Supported header field with a value
ætimerÆ, the proxy MAY reject the INVITE request with a 422 Session 'timer', the proxy MAY reject the INVITE request with a 422 (Session
Interval Too Small) response if the session interval in the Session- Interval Too Small) response if the session interval in the
Expires header field is smaller than the minimum interval defined by Session-Expires header field is smaller than the minimum interval
the proxy's local policy. When sending the 422 response, the proxy defined by the proxy's local policy. When sending the 422 response,
MUST include a Min-SE header field with the value of its minimum the proxy MUST include a Min-SE header field with the value of its
interval. minimum interval.
If the request doesn't indicate support for session timer, but the If the request doesn't indicate support for session timer, but the
request contains a session interval that is too small, the proxy request contains a session interval that is too small, the proxy
cannot usefully reject the request, as this would result in a call cannot usefully reject the request, as this would result in a call
failure. Rather, the proxy SHOULD insert a Min-SE header field failure. Rather, the proxy SHOULD insert a Min-SE header field
containing its minimum interval. If a Min-SE header field is already containing its minimum interval. If a Min-SE header field is already
present, the proxy SHOULD increase (but MUST NOT decrease) the value present, the proxy SHOULD increase (but MUST NOT decrease) the value
to equal its minimum interval. The proxy MUST then increase the to equal its minimum interval. The proxy MUST then increase the
session-Expires header field value to be equal to the value in 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, as described above. A proxy MUST NOT insert a
Min-SE header field, or modify the value of an existing header Min-SE header field, or modify the value of an existing header field,
field, in a proxied request if that request contains a Supported in a proxied request if that request contains a Supported header
header field with the value ætimerÆ. This is needed to protect field with the value 'timer'. This is needed to protect against
against certain denial of service attacks, described in Section certain denial of service attacks, described in Section 11.
Section 11.
Assuming the proxy has requested a session timer (and thus has Assuming the proxy has requested a session timer (and thus has
possibly inserted the Session-Expires header field or reduced it), possibly inserted the Session-Expires header field or reduced it),
the proxy MUST remember that it is using a session timer, and also the proxy MUST remember that it is using a session timer, and also
remember the value of the Session-Expires header field from the remember the value of the Session-Expires header field from the
proxied request. This MUST be remembered for the duration of the proxied request. This MUST be remembered for the duration of the
transaction. The proxy MUST remember, for the duration of the transaction. The proxy MUST remember, for the duration of the
transaction, whether the request contained the Supported header transaction, whether the request contained the Supported header field
field with the value ætimerÆ. with the value 'timer'.
S. Donovan et. Al. Informational 10
If the request did not contain a Supported header field with the If the request did not contain a Supported header field with the
value ætimerÆ, the proxy MAY insert a Require header field into the value 'timer', the proxy MAY insert a Require header field into the
request, with the value ætimerÆ. However, this is NOT RECOMMENDED. request, with the value 'timer'. However, this is NOT RECOMMENDED.
This allows the proxy to insist on session timer for the session. 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 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 the request; in this case, the proxy can already be sure that the
session timer can be used for the session. session timer can be used for the session.
8.2 Processing of Responses 8.2 Processing of Responses
When the final response to the request arrives, it is examined by When the final response to the request arrives, it is examined by the
the proxy. proxy.
If the response does not contain a Session-Expires header field, but If the response does not contain a Session-Expires header field, but
the proxy remembers that it requested a session timer in the request the proxy remembers that it requested a session timer in the request
(by inserting, modifying, or examining and accepting the Session- (by inserting, modifying, or examining and accepting the
Expires header field in the proxied request), this means that the Session-Expires header field in the proxied request), this means that
UAS did not support the session timer. If the proxy remembers that the UAS did not support the session timer. If the proxy remembers
the UAC did not support session timer either, the proxy forwards the that the UAC did not support session timer either, the proxy forwards
response upstream normally. There is no session expiration for this the response upstream normally. There is no session expiration for
session. If, however, the proxy remembers that the UAC did support this session. If, however, the proxy remembers that the UAC did
session timer, additional processing is needed. support session timer, additional processing is needed.
Because there is no Session-Expires or Require header field in the Because there is no Session-Expires or Require header field in the
response, the proxy knows it is the first session-timer-aware proxy response, the proxy knows it is the first session-timer-aware proxy
to receive the response. This proxy MUST insert a Session-Expires to receive the response. This proxy MUST insert a Session-Expires
header field into the response with the value it remembered from the header field into the response with the value it remembered from the
forwarded request. It MUST set the value of the ærefresherÆ forwarded request. It MUST set the value of the 'refresher' parameter
parameter to æuacÆ. The proxy MUST insert the Require header field to 'uac'. The proxy MUST insert the Require header field into the
into the response, with the value ætimerÆ, before forwarding it response, with the value 'timer', before forwarding it upstream.
upstream.
If the received response contains a Session-Expires header field, no If the received response contains a Session-Expires header field, no
modification of the response is needed. modification of the response is needed.
In all cases, if the 2xx response forwarded upstream by the proxy In all cases, if the 2xx response forwarded upstream by the proxy
contains a Session-Expires header field, its value represents the contains a Session-Expires header field, its value represents the
session interval for the session associated with that response. The session interval for the session associated with that response. The
proxy computes the session expiration as the time when the 2xx proxy computes the session expiration as the time when the 2xx
response is forwarded upstream, plus the session interval. This response is forwarded upstream, plus the session interval. This
session expiration MUST update any existing session expiration for session expiration MUST update any existing session expiration for
the session. The refresher parameter in the Session-Expires header the session. The refresher parameter in the Session-Expires header
field in the 2xx response forwarded upstream will be present, and it field in the 2xx response forwarded upstream will be present, and it
indicates which UA is performing the refreshes. There can be indicates which UA is performing the refreshes. There can be multiple
multiple 2xx responses to a single INVITE, each representing a 2xx responses to a single INVITE, each representing a different
different dialog, resulting in multiple session expirations, one for dialog, resulting in multiple session expirations, one for each
each session associated with each dialog. session associated with each dialog.
The proxy MUST NOT modify the value of the Session-Expires header The proxy MUST NOT modify the value of the Session-Expires header
field received in the response (assuming one was present) before field received in the response (assuming one was present) before
forwarding it upstream. forwarding it upstream.
S. Donovan et. Al. Informational 11
8.3 Session Expiration 8.3 Session Expiration
When the current time equals or passes the session expiration for a When the current time equals or passes the session expiration for a
session, the proxy MAY remove associated call state, and MAY free session, the proxy MAY remove associated call state, and MAY free any
any resources associated with the call. Unlike the UA, it MUST NOT resources associated with the call. Unlike the UA, it MUST NOT send a
send a BYE. BYE.
9. Proxy UAS Behavior 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 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. Timer be used itself.
If an incoming request contains a Supported header field with a If an incoming request contains a Supported header field with a value
value ætimerÆ and a Session Expires header, the UAS MAY reject the 'timer' and a Session Expires header, the UAS MAY reject the INVITE
INVITE request with a 422 (Session Interval Too Small) response if request with a 422 (Session Interval Too Small) response if the
the session interval in the Session-Expires header field is smaller session interval in the Session-Expires header field is smaller than
than the minimum interval defined by the UAS' local policy. When the minimum interval defined by the UAS' local policy. When sending
sending the 422 responses, the UAS MUST include a Min-SE header the 422 response, the UAS MUST include a Min-SE header field with the
field with the value of its minimum interval. value of its minimum interval.
If the UAS wishes to accept the request, it copies the value of the 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. 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 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 duration lower than the value in the Min-SE header field in the
request, if present. If the value of the Session-Expires header request, if present. If the value of the Session-Expires header field
field is greater than or equal to the value in the Min-SE header is greater than or equal to the value in the Min-SE header field
field (recall that the default is zero when the Min-SE header field (recall that the default is zero when the Min-SE header field is not
is not present), the UAS MUST NOT increase the value of the Session- present), the UAS MUST NOT increase the value of the Session-Expires
Expires header field. If the value of the Session-Expires header header field. If the value of the Session-Expires header field is
field is lower than the value of the Min-SE header field (possibly lower than the value of the Min-SE header field (possibly because a
because a proxy increased the value of the Min-SE header field), proxy increased the value of the Min-SE header field), the UAS MUST
the UAS MUST increase the value of the Session-Expires header field increase the value of the Session-Expires header field to make it
to make it equal to Min-SE header field value. equal to Min-SE header field value.
If the incoming request contains a Supported header field with a If the incoming request contains a Supported header field with a
value ætimerÆ but does not contain a Session-Expires header, the UAC value 'timer' but does not contain a Session-Expires header, the UAC
indicated support for timers, but did not request one. The UAS may indicated support for timers, but did not request one. The UAS may
request a session timer in the 2XX response by including a Session- request a session timer in the 2XX response by including a
Expires header. The value MUST NOT be set it to a duration lower Session-Expires header. The value MUST NOT be set it to a duration
than the value in the Min-SE header field in the request, if lower than the value in the Min-SE header field in the request, if
present. present.
The UAS MUST set the value of the refresher parameter in the The UAS MUST set the value of the refresher parameter in the
Session-Expires header field in the 2xx response. This value Session-Expires header field in the 2xx response. This value
specifies who will perform refreshes for the dialog. The value is specifies who will perform refreshes for the dialog. The value is
based on the value of this parameter in the request, and on whether based on the value of this parameter in the request, and on whether
the UAC supports the session timer extension. The UAC supports the the UAC supports the session timer extension. The UAC supports the
extension if the ætimerÆ option tag was present in a Supported extension if the 'timer' option tag was present in a Supported header
header field in the request. Table 1 defines how the value in the field in the request. Table 2 defines how the value in the response
response is set. A value of 'none' in the 2nd column means that is set. A value of 'none' in the 2nd column means that there was no
there was no refresher parameter in the request. A value of 'NA' in refresher parameter in the request. A value of 'NA' in the third
the third column means that this particular combination shouldn't column means that this particular combination shouldn't happen, as
happen, as it's disallowed by the protocol. it's disallowed by the protocol.
S. Donovan et. Al. Informational 12 +----------------------+----------------------+---------------------+
UAC supports? refresher parameter refresher parameter | UAC supports? | refresher parameter | refresher parameter |
in request in response | | in request | in response |
N none uas +----------------------+----------------------+---------------------+
N uac NA | N | none | uas |
N uas NA | | | |
Y none uas or uac | N | uac | NA |
Y uac uac | | | |
Y uas uas | N | uas | NA |
| | | |
| Y | none | uas or uac |
| | | |
| Y | uac | uac |
| | | |
| Y | uas | uas |
+----------------------+----------------------+---------------------+
Table 2: UAS Behavior Table 2: UAS Behavior
The fourth row of Table 2 describes a case where both the UAC and The fourth row of Table 2 describes a case where both the UAC and UAS
UAS support the session timer extension, and the UAC did not select support the session timer extension, and the UAC did not select who
who will perform refreshes. This allows the UAS to decide whether will perform refreshes. This allows the UAS to decide whether it, or
it, or the UAC, will perform the refreshes. However, as the table the UAC, will perform the refreshes. However, as the table indicates,
indicates, the UAS cannot overried the UAC's choice of refresher, if the UAS cannot override the UAC's choice of refresher, if it made
it made one. one.
If the refresher parameter in the Session-Expires header field in If the refresher parameter in the Session-Expires header field in the
the 2xx response has a value of æuacÆ, the UAS MUST place a Require 2xx response has a value of 'uac', the UAS MUST place a Require
header field into the response with the value ætimerÆ. This is header field into the response with the value 'timer'. This is
because the uac is performing refreshes and the response has to be because the uac is performing refreshes and the response has to be
processed for the UAC to know this. If the refresher parameter in processed for the UAC to know this. If the refresher parameter in the
the 2xx response has a value of æuasÆ, and the Supported header 2xx response has a value of 'uas', and the Supported header field in
field in the request contained the value ætimerÆ, the UAS SHOULD the request contained the value 'timer', the UAS SHOULD place a
place a Require header field into the response with the value Require header field into the response with the value 'timer'. In
ætimerÆ. In this case, the UAC is not refreshing, but it is supposed this case, the UAC is not refreshing, but it is supposed to send a
to send a BYE if it never receives a refresh. Since the call will BYE if it never receives a refresh. Since the call will still succeed
still succeed without the UAC doing this, insertion of the Require without the UAC doing this, insertion of the Require is a SHOULD
is a SHOULD here, rather than a MUST. here, rather than a MUST.
The UAS, just like the UAC, stores state for the session timer. This The UAS, just like the UAC, stores state for the session timer. This
state includes the session interval, the session expiration, and the state includes the session interval, the session expiration, and the
identity of the refresher. This state is bound to the dialog used to 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 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 delta-time from the Session-Expires header field in the most recent
2xx response to a session refresh request on that dialog. It also 2xx response to a session refresh request on that dialog. It also
remembers whether it, or its peer, is the refresher on the leg, remembers whether it, or its peer, is the refresher on the leg, based
based on the value of the refresher parameter from the most recent on the value of the refresher parameter from the most recent 2xx
2xx response to a session refresh request on that dialog. If the response to a session refresh request on that dialog. If the most
most recent 2xx response had no Session-Expires header field, there recent 2xx response had no Session-Expires header field, there is no
is no session expiration, and no refreshes need to be performed. session expiration, and no refreshes need to be performed.
If the UAS must refresh the session, it computes the session If the UAS must refresh the session, it computes the session
expiration. The session expiration is the time of transmission of expiration. The session expiration is the time of transmission of the
the last 2xx response to a session refresh request on that dialog last 2xx response to a session refresh request on that dialog plus
plus the session interval. If UA wishes to continue with the session the session interval. If UA wishes to continue with the session
beyond the session expiration, it MUST generate a refresh before the beyond the session expiration, it MUST generate a refresh before the
session expiration. It is RECOMMENDED that this refresh be sent once session expiration. It is RECOMMENDED that this refresh be sent once
half the session interval has elapsed. Additional procedures for half the session interval has elapsed. Additional procedures for this
this refresh are described in Section ref{sec:refresh}. refresh are described in Section 10.
S. Donovan et. Al. Informational 13
10. Performing Refreshes 10. Performing Refreshes
The side generating a refresh does so according to the UAC The side generating a refresh does so according to the UAC procedures
procedures defined in Section 7. Note that only a 2xx response to a defined in Section 7. Note that only a 2xx response to a session
session refresh request extends the session expiration. This means refresh request extends the session expiration. This means that a UA
that a UA could attempt a refresh, and receive a 422 response with a could attempt a refresh, and receive a 422 response with a Min-SE
Min-SE header field that contains a value much larger than the header field that contains a value much larger than the current
current session interval. The UA will still need to send a session session interval. The UA will still need to send a session refresh
refresh request before the session expiration (which has not request before the session expiration (which has not changed), even
changed), even though this request will contain a value of the though this request will contain a value of the Session-Expires that
Session-Expires that is much larger than the current session is much larger than the current session interval.
interval.
If no 2xx response to a session refresh request is received before If no 2xx response to a session refresh request is received before
the session expiration, the UA SHOULD send a BYE request to the session expiration, the UA SHOULD send a BYE request to terminate
terminate the session. It SHOULD send this BYE slightly before the session. It SHOULD send this BYE slightly before session
session expiration. The minimum of ten seconds and one third the expiration. The minimum of ten seconds and one third the session
session interval is RECOMMENDED. interval is RECOMMENDED.
For example, if the session interval is 120 seconds, one third For example, if the session interval is 120 seconds, one third of
of this is 40 seconds. Since the minimum of 10 seconds and 40 this is 40 seconds. Since the minimum of 10 seconds and 40 seconds
seconds is 10 seconds, the BYE would be sent 10 seconds before is 10 seconds, the BYE would be sent 10 seconds before the session
the session expires. expires.
Similarly, if the side not performing refreshes does not receive a Similarly, if the side not performing refreshes does not receive a
session refresh request before the session expiration, they SHOULD session refresh request before the session expiration, they SHOULD
send a BYE to terminate the session, slightly before the session send a BYE to terminate the session, slightly before the session
expiration. The minimum of ten seconds and one third the session expiration. The minimum of ten seconds and one third the session
interval is RECOMMENDED. interval is RECOMMENDED.
Firewalls and NAT ALGs may be very unforgiving about allowing Firewalls and NAT ALGs may be very unforgiving about allowing SIP
SIP traffic to pass after the expiration time of the session. traffic to pass after the expiration time of the session. It is
It is for this reason that the BYE should be sent before the for this reason that the BYE should be sent before the expiration.
expiration.
11. Security Considerations 11. Security Considerations
The session timer introduces the capability of a proxy or UA element 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 to force compliant UAs to send refreshes at a rate of the element's
choosing. This introduces the possibility of denial-of-service choosing. This introduces the possibility of denial-of-service
attacks with significant amplification properties. These attacks can attacks with significant amplification properties. These attacks can
be launched from æoutsidersÆ - elements which attempt to modify be launched from 'outsiders' - elements that attempt to modify
messages in transit, or by æinsidersÆ - elements which are messages in transit, or by 'insiders' - elements which are
legitimately in the request path, but are intent on doing harm. legitimately in the request path, but are intent on doing harm.
Fortunately, both cases are adequately handled by this Fortunately, both cases are adequately handled by this specification.
specification.
11.2 Inside Attacks 11.1 Inside Attacks
This introduces the possibility of rogue proxies or UAs introducing This introduces the possibility of rogue proxies or UAs introducing
denial-of-service attacks. However, the mechanisms in this denial-of-service attacks. However, the mechanisms in this
specification prevent that from happening. specification prevent that from happening.
S. Donovan et. Al. Informational 14 First, consider the case of a rogue UAC that wishes to force a UAS to
First, consider the case of a rogue UAC that wishes to force a UAS generate refreshes at a rapid rate. To do so, it inserts a
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
Session-Expires header field into an INVITE with a low duration and refresher parameter equal to uas. Assume it places a Supported header
a refresher parameter equal to uas. Assume it places a Supported field into the request. Any proxy, or the UAS, which objects to this
header field into the request. Any proxy, or the UAS, which objects low timer will reject the request with a 422, therefore preventing
to this low timer will reject the request with a 422, therefore the attack. If no Supported header field was present, the proxies
preventing the attack. If no Supported header field was present, the will insert a Min-SE header field into the request before forwarding
proxies will insert a Min-SE header field into the request before it. As a result, the UAS will not choose a session timer lower than
forwarding it. As a result, the UAS will not choose a session timer the minimum acceptable one to all elements on the path. This too
lower than the minimum acceptable one to all elements on the path. prevents the attack.
This too prevents the attack.
Next, consider the case of a rogue UAS that wishes to force a UAC to 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 generate refreshes at a rapid rate. In that case, the UAC has to
support session timer. The initial INVITE arrives at the rogue UAS, support session timer. The initial INVITE arrives at the rogue UAS,
which returns a 2xx with a very small session interval. The UAC uses 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 this timer, and quickly sends a refresh. Section 7.1 requires the UAC
UAC to copy the current session interval into the Session-Expires to copy the current session interval into the Session-Expires header
header field in the request. This enables the proxies to see the field in the request. This enables the proxies to see the current
current value. The proxies will reject this request, and provide a value. The proxies will reject this request, and provide a Min-SE
Min-SE with a higher minimum. The UAC will then use this higher with a higher minimum. The UAC will then use this higher minimum.
minimum. Note, that if the proxies did not eject the request, but Note, that if the proxies did not reject the request, but rather
rather proxied the request with a Min-SE header field, an attack proxied the request with a Min-SE header field, an attack would still
would still be possible. The UAS could discard this header field in be possible. The UAS could discard this header field in a 2xx
a 2xx response, and force the UAC to continue to generate rapid response, and force the UAC to continue to generate rapid requests.
requests.
In a similar fashion, a rogue proxy cannot force either the UAC or In a similar fashion, a rogue proxy cannot force either the UAC or
UAS to generate refreshes unless the proxy remains on the signaling UAS to generate refreshes unless the proxy remains on the signaling
path, and sees every request and response. path, and sees every request and response.
11.2 Outside Attacks 11.2 Outside Attacks
An element that can observe and modify a request or response in An element that can observe and modify a request or response in
transit can force rapid session refreshes. To prevent that, requests transit can force rapid session refreshes. To prevent that, requests
and responses need to be protected by message integrity. Since the and responses need to be protected by message integrity. Since the
session timer headers are not end-to-end, and are manipulated by session timer headers are not end-to-end, and are manipulated by
proxies, the SIP S/MIME capabilities are not suitable for this task. proxies, the SIP S/MIME capabilities are not suitable for this task.
Rather, integrity needs to be protected using hop-by-hop mechanisms. Rather, integrity needs to be protected using hop-by-hop mechanisms.
As a result, it is RECOMMENDED that an element that sends a request 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 a Session-Expires header field, or a Supported header field with
with the value ætimerÆ, do so using IPSec or TLS. Since adequate the value 'timer', do so using IPSec or TLS. Since adequate
protection is obtained only if TLS or IPSec is applied on each hop, 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 it is RECOMMENDED that the SIPS URI scheme be used in conjunction
with this extension. This means that proxies that record-route and with this extension. This means that proxies that record-route and
request session timer, SHOULD record-route with a SIPS URI. A UA request session timer, SHOULD record-route with a SIPS URI. A UA that
that inserts a Session-Expires header into a request or response inserts a Session-Expires header into a request or response SHOULD
SHOULD include a Contact URI that is a SIPS URI. include a Contact URI that is a SIPS URI.
12. IANA Considerations 12. IANA Considerations
S. Donovan et. Al. Informational 15
This extension defines two new header fields, a new response code, This extension defines two new header fields, a new response code,
and a new option tag. SIP [1] defines IANA procedures for and a new option tag. SIP [1] defines IANA procedures for registering
registering these. these.
12.1 IANA Registration of Min-SE and Session-Expires Header Fields 12.1 IANA Registration of Min-SE and Session-Expires Header Fields
The following is the registration for the Min-SE header field: The following is the registration for the Min-SE header field:
RFC Number: RFC XXXX [Note to IANA: Fill in with the RFC number RFC Number: RFC XXXX [Note to IANA: Fill in with the RFC number of
of this specification.] this specification.]
Header Name: Min-SE Header Name: Min-SE
Compact Form: none Compact Form: none
The following is the registration for the Session-Expires header The following is the registration for the Session-Expires header
field: field:
RFC Number: RFC XXXX [Note to IANA: Fill in with the RFC number RFC Number: RFC XXXX [Note to IANA: Fill in with the RFC number of
of this specification.] this specification.]
Header Name: Session-Expires Header Name: Session-Expires
Compact Form: x Compact Form: x
12.2 IANA Registration of the 422 (Session Interval Too Small) Response 12.2 IANA Registration of the 422 (Session Interval Too Small) Response
Code} Code
The following is the registration for the 422 (Session Interval Too The following is the registration for the 422 (Session Interval Too
Small) response code: Small) response code:
Response Code: 422 Response Code: 422
Default Reason Phrase: Session Interval Too Small Default Reason Phrase: Session Interval Too Small
RFC Number: RFC XXXX [Note to IANA: Fill in with the RFC number RFC Number: RFC XXXX [Note to IANA: Fill in with the RFC number of
of this specification.] this specification.]
12.3 IANA Registration of the ætimerÆ Option Tag} 12.3 IANA Registration of the 'timer' Option Tag
The following is the registration for the ætimerÆ option tag: The following is the registration for the 'timer' option tag:
Name: timer Name: timer
Description: This option tag is for support of the session Description: This option tag is for support of the session timer
timer extension. Inclusion in a Supported header field in a extension. Inclusion in a Supported header field in a request or
request or response indicates that the UA is capable of response indicates that the UA is capable of performing refreshes
performing refreshes according to that specification. Inclusion according to that specification. Inclusion in a Require header in
in a Require header in a request means that the UAS must a request means that the UAS must understand the session timer
understand the session timer extension to process the request. extension to process the request. Inclusion in a Require header
Inclusion in a Require header field in a response indicates field in a response indicates that the UAC must look for the
Session-Expires header field in the response, and process
S. Donovan et. Al. Informational 16 accordingly.
that the UAC must look for the Session-Expires header field in
the response, and process accordingly.
13. Example Call Flow 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 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. In this example, both the UAC and UAS support the
session timer extension. The initial INVITE request generated by the session timer extension. The initial INVITE request generated by the
UAC, Alice (message 1), might look like: UAC, Alice (message 1), might look like:
INVITE sip:bob@biloxi.com SIP/2.0 INVITE sip:bob@biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8 Via: SIP/2.0/UDP pc33.atlanta.example.com;branch=z9hG4bKnashds8
Supported: timer Supported: timer
Session-Expires: 50 Session-Expires: 50
Max-Forwards: 70 Max-Forwards: 70
To: Bob <sip:bob@biloxi.com> To: Bob <sip:bob@biloxi.example.com>
From: Alice <sip:alice@atlanta.com>;tag=1928301774 From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710 Call-ID: a84b4c76e66710
CSeq: 314159 INVITE CSeq: 314159 INVITE
Contact: <sip:alice@pc33.atlanta.com> Contact: <sip:alice@pc33.atlanta.example.com>
Content-Type: application/sdp Content-Type: application/sdp
Content-Length: 142 Content-Length: 142
(Alice's SDP not shown) (Alice's SDP not shown)
This request indicates that Alice supports the session timer, and is This request indicates that Alice supports the session timer, and is
request session refreshes every 50 seconds. This arrives at the request session refreshes every 50 seconds. This arrives at the first
first proxy, P1. This session interval is below the minimum allowed proxy, P1. This session interval is below the minimum allowed value
value of 3600. So, P1 rejects the request with a 422 (message 2): of 3600. So, P1 rejects the request with a 422 (message 2):
SIP/2.0 422 Session Interval Too Small SIP/2.0 422 Session Interval Too Small
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8 Via: SIP/2.0/UDP pc33.atlanta.example.com;branch=z9hG4bKnashds8
;received=192.0.2.1 ;received=192.0.2.1
Min-SE: 3600 Min-SE: 3600
To: Bob <sip:bob@biloxi.com>;tag=9a8kz To: Bob <sip:bob@biloxi.example.com>;tag=9a8kz
From: Alice <sip:alice@atlanta.com>;tag=1928301774 From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710 Call-ID: a84b4c76e66710
CSeq: 314159 INVITE CSeq: 314159 INVITE
This response contains an Min-SE header field with the value of This response contains a Min-SE header field with the value of 3600.
3600. Alice then retries the request. This time, the request Alice then retries the request. This time, the request contains a
contains a Min-SE header, since Alice has received a 422 for other Min-SE header, since Alice has received a 422 for other INVITE
INVITE requests with the same Call-ID. The new request (message 4) requests with the same Call-ID. The new request (message 4) might
might look like: look like:
INVITE sip:bob@biloxi.com SIP/2.0 INVITE sip:bob@biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds9 Via: SIP/2.0/UDP pc33.atlanta.example.com;branch=z9hG4bKnashds9
Supported: timer Supported: timer
Session-Expires: 3600 Session-Expires: 3600
Min-SE: 3600 Min-SE: 3600
Max-Forwards: 70 Max-Forwards: 70
To: Bob <sip:bob@biloxi.com> To: Bob <sip:bob@biloxi.example.com>
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
S. Donovan et. Al. Informational 17
From: Alice <sip:alice@atlanta.com>;tag=1928301774
Call-ID: a84b4c76e66710 Call-ID: a84b4c76e66710
CSeq: 314160 INVITE CSeq: 314160 INVITE
Contact: <sip:alice@pc33.atlanta.com> Contact: <sip:alice@pc33.atlanta.example.com>
Content-Type: application/sdp Content-Type: application/sdp
Content-Length: 142 Content-Length: 142
(Alice's SDP not shown) (Alice's SDP not shown)
Proxy P1 record-routes. Since the session interval is now acceptable Proxy P1 record-routes. Since the session interval is now acceptable
to it, it forwards the request to P2 (message 5). However, the to it, it forwards the request to P2 (message 5). However, the
session interval is below its minimum configured amount of 4000. So, session interval is below its minimum configured amount of 4000. So,
it rejects the request with a 422 response code (message 6), and 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, 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 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 INVITE is the maximum of all Min-SE she has received (3600 and 4000).
4000). Message 10 might look like: Message 10 might look like:
INVITE sip:bob@biloxi.com SIP/2.0 INVITE sip:bob@biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds10 Via: SIP/2.0/UDP pc33.atlanta.example.com;branch=z9hG4bKnashds10
Supported: timer Supported: timer
Session-Expires: 4000 Session-Expires: 4000
Min-SE: 4000 Min-SE: 4000
Max-Forwards: 70 Max-Forwards: 70
To: Bob <sip:bob@biloxi.com> To: Bob <sip:bob@biloxi.example.com>
From: Alice <sip:alice@atlanta.com>;tag=1928301774 From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710 Call-ID: a84b4c76e66710
CSeq: 314161 INVITE CSeq: 314161 INVITE
Contact: <sip:alice@pc33.atlanta.com> Contact: <sip:alice@pc33.atlanta.example.com>
Content-Type: application/sdp Content-Type: application/sdp
Content-Length: 142 Content-Length: 142
(Alice's SDP not shown) (Alice's SDP not shown)
P1 record-routes once again, but P2 does not (this wouldn't normally P1 record-routes once again, but P2 does not (this wouldn't normally
happen; presumably, if it asked for session timer, it would record- happen; presumably, if it asked for session timer, it would
route the subsequent request). The UAS receives the request. It record-route the subsequent request). The UAS receives the request.
copies the Session-Expires header from the request to the response, It copies the Session-Expires header from the request to the
and adds a refresher parameter with value æuacÆ. This 200 OK is response, and adds a refresher parameter with value 'uac'. This 200
forwarded back to Alice. The response she receives (message 15) OK is forwarded back to Alice. The response she receives (message 15)
might look like: might look like:
SIP/2.0 200 OK SIP/2.0 200 OK
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds10 Via: SIP/2.0/UDP pc33.atlanta.example.com;branch=z9hG4bKnashds10
;received=192.0.2.1 ;received=192.0.2.1
Require: timer Require: timer
Supported: timer Supported: timer
Record-Route: sip:p1.atlanta.com;lr Record-Route: sip:p1.atlanta.example.com;lr
Session-Expires: 4000;refresher=uac Session-Expires: 4000;refresher=uac
To: Bob <sip:bob@biloxi.com>;tag=9as888nd To: Bob <sip:bob@biloxi.example.com>;tag=9as888nd
From: Alice <sip:alice@atlanta.com>;tag=1928301774 From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710 Call-ID: a84b4c76e66710
CSeq: 314161 INVITE CSeq: 314161 INVITE
S. Donovan et. Al. Informational 18
Contact: <sip:bob@192.0.2.4> Contact: <sip:bob@192.0.2.4>
Content-Type: application/sdp Content-Type: application/sdp
Content-Length: 142 Content-Length: 142
(Bob's SDP not shown) (Bob's SDP not shown)
Alice generates an ACK (message 16), which is routed through P1 and Alice generates an ACK (message 16), which is routed through P1 and
then to Bob. Since Alice is the refresher, around 3000 seconds then to Bob. Since Alice is the refresher, around 3000 seconds later,
later, Alice sends an UPDATE request to refresh the session. Since Alice sends an UPDATE request to refresh the session. Since this
this request is part of an established dialog, and Alice has not request is part of an established dialog, and Alice has not received
received any 422 responses or requests on that dialog, there is no any 422 responses or requests on that dialog, there is no Min-SE
Min-SE header field in her request (message 18): header field in her request (message 18):
UPDATE sip:bob@192.0.2.4 SIP/2.0 UPDATE sip:bob@192.0.2.4 SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds12 Via: SIP/2.0/UDP pc33.atlanta.example.com;branch=z9hG4bKnashds12
Route: sip:p1.atlanta.com;lr Route: sip:p1.atlanta.example.com;lr
Supported: timer Supported: timer
Session-Expires: 4000;refresher=uac Session-Expires: 4000;refresher=uac
Max-Forwards: 70 Max-Forwards: 70
To: Bob <sip:bob@biloxi.com>;tag=9as888nd To: Bob <sip:bob@biloxi.example.com>;tag=9as888nd
From: Alice <sip:alice@atlanta.com>;tag=1928301774 From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710 Call-ID: a84b4c76e66710
CSeq: 314162 UPDATE CSeq: 314162 UPDATE
Contact: <sip:alice@pc33.atlanta.com> Contact: <sip:alice@pc33.atlanta.example.com>
This is forwarded through P1 to Bob. Bob generates a 200 OK, copying This is forwarded through P1 to Bob. Bob generates a 200 OK, copying
the Session-Expires header field into the response. This is the Session-Expires header field into the response. This is forwarded
forwarded through P1, and arrives at Alice. The response she through P1, and arrives at Alice. The response she receives (message
receives (message 21) might look like: 21) might look like:
SIP/2.0 200 OK SIP/2.0 200 OK
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds12 Via: SIP/2.0/UDP pc33.atlanta.example.com;branch=z9hG4bKnashds12
;received=192.0.2.1 ;received=192.0.2.1
Require: timer Require: timer
Session-Expires: 4000;refresher=uac Session-Expires: 4000;refresher=uac
To: Bob <sip:bob@biloxi.com>;tag=9as888nd To: Bob <sip:bob@biloxi.example.com>;tag=9as888nd
From: Alice <sip:alice@atlanta.com>;tag=1928301774 From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710 Call-ID: a84b4c76e66710
CSeq: 314162 UPDATE CSeq: 314162 UPDATE
Contact: <sip:bob@192.0.2.4> Contact: <sip:bob@192.0.2.4>
Shortly afterwards, Alice's UA crashes. As a result, she never sends Shortly afterwards, Alice's UA crashes. As a result, she never sends
an session refresh request. 3990 seconds later, Bob gives up, and 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 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 deliver it, but fails (since Alice's UA has crashed). P1 then returns
returns a 408 (Request Timeout) to Bob. a 408 (Request Timeout) to Bob.
14. Acknowledgements 14. Acknowledgements
The authors wish to thank Brett Tate for his contributions to this The authors wish to thank Brett Tate for his contributions to this
work. Brian Rosen completed the editing of this latest draft. work. Brian Rosen completed the editing of the document.
15. Author's Addresses Normative References
S. Donovan et. Al. Informational 19 [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Steven R. Donovan Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
dynamicsoft Session Initiation Protocol", RFC 3261, June 2002.
5100 Tennyson Parkway
Suite 1200
Plano, Texas 75024
email: sdonovan@dynamicsoft.com
Jonathan Rosenberg [2] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
dynamicsoft Method", RFC 3311, October 2002.
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
email: jdrosen@dynamicsoft.com
16. Normative References [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[1] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J. [4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session Session Description Protocol (SDP)", RFC 3264, June 2002.
initiation protocol," RFC 3261, Internet Engineering Task Force,
June 2002.
[2] J. Rosenberg, "The session initiation protocol (SIP) UPDATE [5] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
method," RFC 3311, Internet Engineering Task Force, Oct. 2002. "RTP: A Transport Protocol for Real-Time Applications", RFC
1889, January 1996.
[3] S. Bradner, "Key words for use in RFCs to indicate requirement Informative References
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
[4] J. Rosenberg and H. Schulzrinne, "An offer/answer model with [6] Srisuresh, P. and M. Holdrege, "IP Network Address Translator
session description protocol (SDP)," RFC 3264, Internet Engineering (NAT) Terminology and Considerations", RFC 2663, August 1999.
Task Force, June 2002.
17. Informative References [7] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
Responses in Session Initiation Protocol (SIP)", RFC 3262, June
2002.
[5] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: [8] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
a transport protocol for real-time applications," RFC 1889, Internet Notification", RFC 3265, June 2002.
Engineering Task Force, Jan. 1996.
[6] P. Srisuresh and M. Holdrege, "IP network address translator Authors' Addresses
(NAT) terminology and considerations," RFC 2663, Internet
Engineering Task Force, Aug. 1999.
[7] J. Rosenberg and H. Schulzrinne, "Reliability of provisional Steven Donovan
responses in session initiation protocol (SIP)," RFC 3262, Internet dynamicsoft
Engineering Task Force, June 2002. 5100 Tennyson Parkway
Suite 1200
Plano, TX 75024
US
[8] A. B. Roach, "Session initiation protocol (sip)-specific event EMail: sdonovan@dynamicsoft.com
notification," RFC 3265, Internet Engineering Task Force, June 2002.
S. Donovan et. Al. Informational 20 Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
East Hanover, NJ 07936
US
EMail: jdrosen@dynamicsoft.com
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Full Copyright Statement Full Copyright Statement
"Copyright (C) The Internet Society (date). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph kind, provided that the above copyright notice and this paragraph are
are included on all such copies and derivative works. However, this included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be copyrights defined in the Internet Standards process must be
followed, or as required to translate it into followed, or as required to translate it into languages other than
English.
S. Donovan et. Al. Informational 21 The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
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.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
 End of changes. 

This html diff was produced by rfcdiff 1.23, available from http://www.levkowetz.com/ietf/tools/rfcdiff/