Internet Engineering Task Force SIP WG
Internet Draft S.Donovan,J.Rosenberg
draft-ietf-sip-session-timer-04.txt dynamicsoft
November 22, 2000
Expires: May 2001
The SIP Session Timer
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document proposes an extension to the Session Initiation
Protocol (SIP). This extension allows for a periodic refresh of SIP
sessions through a re-INVITE. The refresh allows both user agents and
proxies to determine if the SIP session is still active. The
extension defines a new general header, Session-Expires, which
conveys the lifetime of the session.
1 Introduction
The Session Initiation Protocol (SIP) [1], does not currently define
a keepalive mechanism. The result is that call stateful proxies will
not always be able to determine whether a call is still active or
not. For instance, when a user agent fails to send a BYE message at
the end of a session, or the BYE message gets lost due to network
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problems, a call stateful proxy will not know when the session has
ended. In this situation, the call stateful proxy will retain state
for the call and has no deterministic method of determining when the
call state information no longer applies.
To resolve this problem, this extension defines a keepalive mechanism
for SIP sessions. UAs send periodic re-INVITEs to keep the session
alive. The interval for the re-INVITEs is determined through a
negotiation mechanism defined here. If a re-INVITE is not received
before the interval passes, the session is considered terminated.
Both UAs are supposed to send a BYE, and call stateful proxies can
remove any state for the call.
INVITE is used as a refresh, as opposed to another method,
to allow sessions to be recovered after a crash and restart
of one of the UAs. It makes SIP sessions soft state.
This refresh mechanism has additional applications. For the same
reasons a call stateful proxy server would like to determine whether
the session is still active, a user agent would like to make this
determination. This determination can be made at a user agent without
the use of SIP level mechanisms; for audio sessions, periodic RTCP
packets serve as an indication of liveness [2]. However, it is
desirable to separate SIP call liveness from the details of the
particular session.
Another important application of the session timer is in NAT and
firewall control [3]. In order for SIP to flow through a NAT or
firewall, holes and/or address bindings must be dynamically created
to allow the media for the session to flow. These holes and/or
address bindings represent state which must be eventually removed.
Relying on a BYE to trigger the removal of state, besides being
unreliable, introduces a potential denial of service attack.
This document proposes an extension to SIP that defines a session
expiration mechanism. Periodic refreshes, through re-INVITEs, are
used to keep the session active. The extension is sufficiently
backwards compatible with SIP that it works so long as either one of
the two participants in a call leg understand the extension. A new
general header, the Session-Expires header, is defined. It conveys
the expiration time of the session.
2 Protocol Overview
UACs which support the session timer extension defined here MUST
include a Supported header in all requests, excepting ACK, containing
the option tag "timer" [4]. When a UAC makes a call, it MAY include a
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Session-Expires header in the INVITE request. The presence of the
Session-Expires header indicates that the UAC wishes to use the
session timer for this call. Its value indicates the desired
expiration time of the session.
Proxies on the signaling path may have their own requirements for the
refresh interval of the session. If the Supported header in the
request lists the option tag "timer", a proxy can be certain the UAC
understands the session timer. In this case, if no Session-Expires
was present, the proxy can insert one if it so desires. If one was
present, the proxy can lower, but not increase, the expiration time
of the session. The proxy remembers the value of Session-Expires it
placed into the request, and also remembers that the UAC supported
session timer. The UAC will ultimately be responsible for sending the
refreshes for this call leg.
If the Supported header was absent from the request, or was present
but didn't include the tag "timer", the proxy knows the UAC cannot
generate refreshes, but the called party may be able to. If no
Session-Expires was present, the proxy can insert one if it so
desires. If one was present, the proxy can lower, but not increase,
the expiration time of the session. The proxy MUST remember the value
of Session-Expires it placed into the request, and also MUST remember
that the UAC did not support the session timer. The UAS may be
responsible for sending the refreshes for this call leg. If the proxy
wishes to insist that the call is only established if the UAS
supports session timer, it MAY insert a Require header into the
request, with the value "timer", although this is NOT RECOMMENDED,
since it eliminates the possibility of call establishment if the UAS
does not support session timers.
Eventually, the initial INVITE reaches the UAS. There are two cases -
the UAS supports session timer, or it does not. If it does, and the
response is otherwise acceptable to the UAS, the UAS MUST copy the
Session-Expires into the 200 class response, or MAY add one into the
response if none was present in the request. They UAS MAY reduce the
session timer in the 200 class response. If the request also
contained the Supported header with the value "timer", the UAS knows
that the UAC can do refreshes. To make sure the UAC is aware it must
actually do them, the UAS MUST add a Require header to the 200 class
response, with the tag "timer", when the request contains a Require
header with tag "timer". The UAS does not perform the refreshes in
this case. If the request did not contain the Supported header with
the value "timer", the UAS knows the UAC cannot perform refreshes.
So, it assumes the responsibility. In this case, it MUST add the
value of the Session-Expires to the response, but it MUST NOT add a
Require header with value "timer". This is because the UAC does not
support the extension; the UAS cannot insist on its usage at the UAC,
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which is what a Require header in the response would accomplish.
If the UAS does not support session timer, it behaves as a normal
UAS. It will, in this case, neither insert Session-Expires in the
response nor a Require header with value "timer".
This response travels backwards through the proxies. When it reaches
a proxy which remembers that it asked for session timer, the proxy
examines the response. If the response did not contain a Session-
Expires header, but the proxy remembers that the UAC supported
session timer, the proxy knows that the UAC supports session timer,
but the UAS does not. So, it inserts the Session-Expires header into
the response and also adds a Require header with a value of "timer".
If the response the proxy received did have a Session-Expires header,
but no Require header with value "timer", the proxy knows that the
UAS understands session timer, but not the UAC. It simply forwards
this request upstream. If the proxy gets a response without Session-
Expires, and the proxy remembers that the UAC did not support session
timer, the proxy knows that session timer cannot be used, since
neither UAS nor UAC support it. Finally, if the response contains the
Session-Expires and Require header with the value "timer", the proxy
knows that both UAC and UAS support session timer, and that the UAC
will be performing refreshes.
The response then arrives at the UAC. If it contains a Require header
with the value "timer", the UAC knows it is responsible for
refreshes. The response will also contain a Session-Expires header,
and the value of that header is used as the interval for refreshes.
If the response contains no Session-Expires header or Require header,
but the UAC had inserted a Session-Expires header into the request
(since it wanted a session timer), the UAC will also be responsible
for refreshes.
The UAC then ACKs the INVITE. The Session-Expires MUST NOT be
included in the ACK.
If the calling UA is supposed to perform refreshes and wishes to keep
the session alive, it MUST send a re-INVITE before the expiration
time. This re-INVITE MAY contain a Session-Expires header. The
processing of this re-INVITE by proxies and UAS is identical to that
of the initial INVITE, excepting that processing may also take place
for non-200 final responses.
If the called UA is supposed to perform refreshes and wishes to keep
the session alive, it MUST send a re-INVITE before the expiration
time. This re-INVITE MAY contain a Session-Expires header. The
processing of this re-INVITE by proxies and UAS is identical to that
of the initial INVITE, excepting that processing may also take place
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for non-200 final responses.
If the calling UA or the called UA is not performing refreshes, and
does not receive a re-INVITE refreshing the session before the
session expires, they SHOULD send a BYE to terminate the call. If a
refreshing UA does not receive a response to the re-INVITE used to
refresh the session, it SHOULD send a BYE to terminate the call.
Similarly, if a proxy doesn't receive a re-INVITE before expiration
of the session, it MAY remove associated call state, and MAY free any
resources associated with the call. Unlike the UA, it MUST NOT send a
BYE.
3 Session-Expires Header Field Definition
The Session-Expires header conveys the expiration time for a SIP
session described in the body of the message. It is placed only in
INVITE requests, and is allowed in any final response to an INVITE.
Like the SIP Expires header, it can contain either an absolute time
or a delta-time. If it contains an absolute time, this time indicates
the time at which a proxy or UA may safely destroy any state
associated with the call. If it contains a delta time, the expiration
time of the session is defined as that delta plus the time at which
the header is observed in a final response. For example, if a UAS
generates a 200 OK response to a re-INVITE that contained a Session-
Expires header with a value of 3600, the UAS computes the expiration
time of the session as one hour after the time when the 200 OK was
sent. For each proxy, the expiration time is one hour after the time
when the 200 OK was received or sent (assuming these two are
sufficiently close together). For the UAC, the expiration time is one
hour after the receipt of the final response. Any entity that inserts
a Session-Expires header with an absolute time MUST also insert a
Date header into the message. This ensures proper operation for
devices that do not have access to absolute time.
There is no absolute minimum session refresh interval. However, 30
seconds is RECOMMENDED. In other words, SIP entites MUST be prepared
to handle session refresh intervals of any duration, but entities
that insert the Session-Expires header SHOULD NOT choose times that
result in intervals less than 30 seconds.
The syntax of the Session-Expires header is:
Session-Expires = ("Session-Expires" | "x") ":" ( SIP-date |
delta-seconds )
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Note that a compact form, the letter 'x', has been reserved for
Session-Expires. Both SIP-Date and delta-seconds are defined in
Section 6.20 of RFC 2543 [1].
Table 1 is an extension of tables 4 and 5 in [1] for the Session-
Expires header:
where enc e-e ACK BYE CAN INV OPT REG
_____________________________________________________
Session-Expires R n h - - - o - -
Session-Expires r n h - - - o - -
Table 1: Summary of header fields. "o": optional "-": not applicable,
"R': request header, "r": response header, "g": general header, "*":
needed if message body is not empty. A numeric value in the "type"
column indicates the status code the header field is used with.
4 UAC Behavior
A UAC which supports the session timer extension defined here MUST
include a Supported header in each request (excepting ACK), listing
the option tag "timer" [4]. It MUST do so even if the UAC is not
requesting keepalives for the call.
If the UAC wishes to request keepalives for this call, it MUST
include a Session-Expires in the INVITE request used to initiate the
call. The value of this header indicates the time when the UAC will
consider the call expired if no refresh is sent. If the request is
being authenticated, the Session-Expires header MUST appear before
the Authorization or Proxy-Authorization headers.
The UAC MAY include a Require in the request with the value "timer"
to indicate that the UAS must support the session timer to
participate in the session. In addition, the UAC MAY include a
Proxy-Require header in the request with the value "timer" to
indicate that proxies must support session timer in order to
correctly process the request. However, usage of either Require or
Proxy-Require by the UAC is NOT RECOMMENDED. They are not needed,
since the extension works even when only the UAC supports the
extension.
When the response to the initial INVITE request arrives, it may or
may not contain a Session-Expires header, and may or may not contain
a Require header with the value "timer". UACs MUST be prepared to
receive a Session-Expires header in a response even if none were
present in the request.
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Table 2 describes the behavior of the UAC (in terms of handling
refreshes) after receiving a 200 class response to an initial INVITE,
or any final response to a re-INVITE, based on the headers present in
that response.
Require Session-Expires Behavior
N N If request contained Session-Expires,
UAC is performing refreshes. Otherwise, do nothing.
N Y UAS is performing refreshes. Do nothing.
Y N Should never happen. Do nothing.
Y Y UAC MUST perform refreshes.
Table 2: UAC Behavior
If the UAC must refresh, it follows the following procedure. The UAC
computes the expiration time of the session. This is based on the
value of the Session-Expires in the response. If there is no
Session-Expires in the response, but the UAC is refreshing, the value
from the request is used (this only happens when the UAC wants
session timers, but none of the proxies, nor the UAS, want or support
it). If the Session-Expires contains an absolute time, that is the
time of expiration. If it contains a delta-time, the expiration time
is the time of reception of the response plus that delta time. Let
the difference in time between the reception of the response and the
session expiration time be called the refresh interval. Note that
this expiration applies only to the call leg associated with the
response. It is explicitly allowed for there to be differing session
timers (or none at all) on differing call legs, in the case where
there are multiple 2xx OK responses to an initial INVITE with
different tags in the To field.
If UA wishes to continue with the session beyond the expiration, it
MUST generate a refresh before the expiration time. It is RECOMMENDED
that this refresh be sent once half the refresh interval has elapsed.
This refresh is accomplished by sending a re-INVITE request on the
given call leg. Sending of the refresh (in terms of this extension),
and processing the response are exactly identical to the rules above,
excepting that the processing applies to non-200 final responses, as
well as 200 class final responses.
A UA MAY use the refreshing re-INVITE as a normal SIP re-INVITE; that
is, this re-INVITE MAY contain an updated session description. In the
case where the re-INVITE contains an updated session description, the
session description MUST somehow indicate that it has changed. In the
case of SDP [5], this is accomplished by using a different value for
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the origin field.
If the refreshing re-INVITE is used solely for refreshing, it MUST
still contain a session description, unchanged from the previous
INVITE. The session description MUST somehow indicate that it has not
changed. In the case of SDP, this is accomplished by including the
same value for the origin field as previous messages to its peer.
If no response to a refreshing re-INVITE is received before the
expiration of the session, the UA SHOULD send a BYE request to
terminate the call. It SHOULD send this BYE slightly before
expiration of the session. The minimum of ten seconds and one third
the session interval is RECOMMENDED.
For example, if the session interval is 120 seconds, one
third of this is 40 seconds. Since the minimum of 10
seconds and 40 seconds is 10 seconds, the BYE would be sent
10 seconds before the session expires.
Firewalls and NATs may be very unforgiving about allowing
SIP traffic to pass after the expiration time of the
session. It is for this reason that the BYE should be sent
before the expiration.
Note that it is possible that the calling UA is generating refreshes,
and then it receives a re-INVITE. After following the rules for UAS
described below, the calling UA now determines it is not supposed to
generate refreshes. The UA SHOULD cease generating refreshes in this
case, and let the other UA perform them. This also implies that the
responsibility for generating refreshes may change during a call.
This happens commonly in one specific case - both caller and callee
support session timer. The caller will be doing re-INVITEs initially.
If the callee re-INVITEs, it becomes the UAC for this transaction,
and the rules defined in this specification will result in a change
in refresh responsibility to the called party. In general, when both
parties support session timer, refreshes become the responsibility of
the party which performed the last INVITE transaction.
Note that it is possible for a UAS to believe that an INVITE is an
initial INVITE, and for the UAC to believe it is a re-INVITE. This
happens when a UA crashes and reboots between refreshes. When the
refresh arrives at the rebooted UA, it decides to reject the call.
The UAS can detect that the re-INVITE is for an existing call by the
existence of the tag in the To field of the re-INVITE. In order to
alert the UAC that it believes the call is now down (the UAC believes
this request to be a re-INVITE, and so a non-200 OK final response
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will not cause it to destroy the call), the UAS SHOULD include a
Session-Expires and Require into the non-200 response (assuming
session timer is supported by the UAC), with an immediate expiration
time.
5 Proxy Behavior
Session expirations are mostly of interest to call stateful proxy
servers. However, a stateful proxy server MAY also follow the rules
described here. Stateless proxies MUST NOT attempt to request session
timers. In general, proxies which ask for session timers will
record-route, since they won't receive refreshes if they don't.
The proxy processing rules require the proxy to remember
information between the request and response, ruling out
stateless proxies.
5.1 Processing of requests
Due to local policy, a proxy may have guidelines about the desired
maximum lifetime for a call initiated through it. When an initial
INVITE is received to establish a call, a proxy MAY insert a
Session-Expires header in the request before forwarding it, if none
was present in the request. This Session-Expires header may contain
any desired expiration time the proxy would like. If the request
already had a Session-Expires header, the proxy MAY reduce the value
in the Session-Expires header before forwarding the request, but MUST
NOT increase it.
Assuming the proxy wishes to use session timer (and thus has possibly
inserted the Session-Expires header or reduced it), the proxy MUST
remember that it is using session timer, and also remember the value
in the request it forwarded, until the final response to the request
arrives, or the transaction times out. If the request also contained
the Supported header with the value "timer", the proxy MUST remember
this as well.
If the request did not contain a Supported header with the value
"timer", the proxy MAY insert a Require header into the request, with
the value "timer". However, this is NOT RECOMMENDED. This allows the
proxy to insist on session timer for the session. This header is not
needed if a Supported header was in the request; in this case, the
proxy can already be sure that the session timer can be used for the
session.
5.2 Processing of Responses
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When the final response to the request arrives, it is examined by the
proxy. There are four cases.
CASE I: UAC supports, UAS doesn't. In this case, the request
forwarded by the proxy contained a Session-Expires header
(possibly inserted by the proxy). Recall that all proxies
interested in session timer MUST remember the value of the
timer in the forwarded request, in addition to whether the
UAC supports it. Handling of the response for this scenario
depends on whether the session timer aware proxy is the one
closest to the UAS. For the proxy closest to the UAS, the
final response to the INVITE does not contain a Session-
Expires header, nor does it contain a Require header with
the option tag "timer". The proxy MUST insert a Session-
Expires header into the response with the value it
remembered from the forwarded request. The proxy MUST also
insert the Require header into the response, with the value
"timer", before forwarding it upstream. The value of the
Session-Expires in the forwarded response represents the
expiration time of the session. For other proxies, the
response will already contain the Session-Expires and
Require header, so that this cae is indistinguishable from
CASE IV.
CASE II: UAC doesn't support, UAS doesn't support. In this case,
the final response has no Session-Expires header, and the
proxy remembers that the UAC did not support the session
timer. The proxy forwards the response upstream normally.
There are no session timers for this call leg.
CASE III: UAS supports, UAC doesn't. In this case, the final
response contains a Session-Expires header, but no Require
header with the tag "timer". The proxy remembers that the
UAC did not support session timers. The UAS will perform
refreshes in this case. The proxy forwards the response
upstream.
CASE IV: UAC supports, UAS supports. In this case, the final
response contains a Session-Expires header and a Require
header with the tag "timer". The UAC performs refreshes in
this case. This case will also occur when the UAC supports
session timer, and the UAS doesn't, but a downstream proxy
had recognized this as CASE I and inserted the Session-
Expires and Require headers into the response. The proxy
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forwards the response upstream.
In all cases, if the final response forwarded upstream by the proxy
contains a Session-Expires header, its value represents the
expiration time of the session for the call leg associated with that
response. Note that there can be multiple 200 class responses to a
single INVITE, each representing a different call leg, resulting in
multiple session timers, one for each call leg.
In all cases, the proxy MUST NOT modify the value of the Session-
Expires header received in the response (assuming one was present)
before forwarding it upstream.
The expiration of the call leg will occur at the time indicated in
the Session-Expires header. If the Session-Expires header contains a
delta-time, the expiration time is the time of receipt of the final
response, plus that delta time.
Re-INVITE requests may arrive from either UA, refreshing the session
and extending the expiration time. Processing of these re-INVITEs by
a proxy is identical to the procedure for processing the initial
INVITE, except for the fact that processing includes any final
response, not just 200 class.
If the session expires without having seen a response to a re-INVITE,
the proxy MAY consider the call leg terminated. This means it MAY
flush any state associated with that call leg.
Note that a proxy MUST NOT send a BYE request once the session
expires.
6 UAS Behavior
When a UAS receives an INVITE for a new call, and that INVITE
contains a Session-Expires header, the UAS MUST place a Session-
Expires header in a 200 OK response (assuming the UAS accepts the
call). The UAS MAY reduce the expiration time when it places this
header into the response, but it MUST NOT increase it. If the inital
INVITE did not contain a Session-Expires header, but it did contain a
Supported header containing the option tag "timer", the UAS MAY
insert a Session-Expires header into the response. This header MAY
have any desired expiration time.
If the UAS places a Session-Expires header into the response, and the
request contained a Supported header with the value "timer", the UAS
MUST place a Require header into the response with the value "timer".
In this case, the UAC will generate the refreshes.
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If the UAS places a Session-Expires header into the response, and the
request did not contain a Supported header with the value "timer",
the UAS MUST NOT place a Require header into the response with the
value "timer". In this case, the UAS will generate the refreshes.
These cases are summarized in Table 3. The table indicates the
behavior of the UAS, assuming it supports session timers, as a
function of the presence of the headers in the received request.
Supported Session-Expires Behavior
N N If response contains Session-Expires, UAS refreshing.
Otherwise, do nothing.
N Y Copy Session-Expires to response.
May reduce. UAS refreshing.
Y N MAY add Session-Expires to response.
If added, MUST add Require to response.
UAC refreshing.
Y Y Copy Session-Expires to response.
May reduce. Add Require to response.
UAC refreshing.
Table 3: UAS Behavior
If the UAS is generating refreshes, it computes the expiration time
of the session based on the value of the Session-Expires header in
the response it sent. This expiration applies only to the call leg
associated with that final response. The processing from this point
is as described in section 4 once the UAC determined it was
performing refreshes.
As described in Section 4, the refreshing UA will send periodic re-
INVITEs to refresh the session. A UAS MUST be prepared to receive and
process these re-INVITEs. Processing of the re-INVITE, as far as the
session timer is concerned, is identical to the rules for the initial
INVITE, described above, except that processing applies to any final
response, not just 200 OK. Note that if the 200 OK to the re-INVITE
has no Session-Expires, no expiration time exists for the session.
This can happen if the proxies and/or UAS change their mind about
session timers, and decide they no longer wish to use them. Since
each INVITE is treated independently, the proxies or UAS do not need
to continue requesting session timer event though they did so the
first time.
7 Security Considerations
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The session timer introduces the capability of a proxy to effectively
force clients to send refreshes at a rate of the proxies choosing. It
can also force the clients to send a BYE by setting the expirations
to times that are too short. This introduces the possibility of rogue
proxies introducing denial-of-service attacks. Use of short refresh
intervals allows the proxies to create network load. The session
timer mechanism allows the proxy to be able to terminate established
calls - a capability a normal proxy doesn't have in [1].
As a result of these potential attacks, it is RECOMMENDED that IP or
transport level security is used when communicating between proxies,
and that requests with Session-Expires headers only be accepted over
these secure transports.
8 Examples
The following examples are meant to illustrate the functionality
associated with the session timer. In the interest of brevity, all
headers excepting Supported, Session-Expires and Require are
intentionally left out of the SIP messages.
8.1 Basic session timer
In this case, two UAs communicate directly, with no proxies. Both
support the session timer. The call is setup with a two minute
session expiration. One minute later, the UAC refreshes the session.
Calling UA -> Called UA
INVITE
Supported: timer
Session-Expires: 120
Calling UA <- Called UA
200 OK
Require: timer
Session-Expires: 120 Called UA starts session timer on send
Calling UA starts session timer on receipt
Calling UA -> Called UA
ACK
60 seconds later:
Calling UA -> Called UA
INVITE
Supported: timer
Session-Expires: 120
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Calling UA <- Called UA
200 OK
Require: timer
Session-Expires: 120 Called UA starts session timer on send
Calling UA starts session timer on receipt
Calling UA -> Called UA
ACK
110 seconds later the called UA did not receive a re-INVITE. It
therefore considers the call terminated and sends a BYE:
Calling UA <- Called UA
BYE
Calling UA -> Called UA
200 OK
8.2 Basic negotiation of Session Time
In this configuration, two UAs talk through a single stateful proxy
server (SPS). Both the SPS and the UAS reduce the session timer.
Calling UA -> SPS
INVITE
Supported: timer
Session-Expires: 240
SPS -> Called UA
INVITE SPS wants a shorter timer
Supported: timer
Session-Expires: 180
SPS <- Called UA
200 OK Called UA wants a shorter timer
Session-Expires: 120 Called UA starts timer
Require: timer
Calling UA <- SPS
200 OK
Session-Expires: 120 Proxy starts timer on send
Require: timer Calling UA starts timer on receipt
Calling UA -> SPS
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ACK
SPS -> Called UA
ACK
For whatever reason, the calling UA decides not to refresh. So, after
110 seconds, it sends a BYE.
Calling UA -> SPS
BYE
SPS -> Called UA
BYE
SPS <- Called UA
200 OK
Calling UA <- SPS
200 OK
8.3 No Session-Expires Header in INVITE
In this scenario, the UA sends an INVITE without a Session-Expires
header and with a Supported header containing the option tag "timer".
Since the proxy wants session timer for the call, it adds the
Session-Expires header.
Calling UA -> SPS
INVITE No Session-Expires
Supported: timer
SPS -> Called UA
INVITE
Supported: timer
Session-Expires: 120 Proxy added Session-Expires
SPS <- Called UA
200 OK
Session-Expires: 120
Require: timer
Calling UA <- SPS
200 OK
Session-Expires: 120
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Require: timer
Calling UA -> SPS
ACK
SPS -> Called UA
ACK
8.4 Session timer without Calling UA Support
In this scenario, the calling UA sends and INVITE without a Session-
Expires header and without a Supported header containing the option
tag "timer". Since the proxy wants session timer for the call it adds
Session-Expires header before proxying the INVITE to the called UA.
Calling UA -> SPS
INVITE
SPS -> Called UA
INVITE SPS adds session expires
Session-Expires: 180
SPS <- Called UA
200 OK Called UA wants a shorter timer
Session-Expires: 120 Called UA starts timer
Calling UA <- SPS
200 OK
Session-Expires: 120 Proxy starts timer on send
Calling UA -> SPS
ACK
SPS -> Called UA
ACK
Sixty seconds later, the called UA sends a re-INVITE. Note that the
called UA does support session timer, so it includes a
header{Supported} header in the request. The SPS adds the
Session-Expires and Require headers into the response from the calling
UA.
SPS <- Called UA
INVITE
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Supported: timer
Session-Expires: 120
Calling UA <- SPS
INVITE
Supported: timer
Session-Expires:120
Calling UA -> SPS
200 OK
SPS -> Called UA
Session-Expires: 120
Require: timer
200 OK
SPS <- Called UA
ACK
Calling UA <- SPS
ACK
The Calling UA terminates the session for non timer
related reasons:
Calling UA -> SPS
BYE
SPS -> Called UA
BYE
SPS <- Called UA
200 OK
Calling UA <- SPS
200 OK
8.5 Session Timer without Called UA Support
In this scenario, the calling UA indicates that it supports the
session timer, but does not add the Session-Expires header into the
INVITE. The proxy adds it, but session timer is not supported by the
UAS. The call is still set up with a session timer, as all that is
required is for one of the user agents involved in the call leg to
understand the "timer" feature.
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Calling UA -> SPS
INVITE
k: timer
SPS -> Called UA
INVITE SPS adds S-E header
k: timer
Session-Expires: 180
SPS <- Called UA
200 OK Called UA doesn't understand session timer
Calling UA <- SPS
200 OK
Session-Expires: 180 SPS adds Session-Expires and Require
Require: timer
Calling UA -> SPS
ACK
SPS -> Called UA
ACK
The UAC then re-invites, which is responded to with a 400 because the
new media streams were rejected. Since this is a re-INVITE, the
session is still active, and thus the processing of the response to
re-INVITE still occurs in the proxy.
Calling UA -> SPS
INVITE
k: timer
Session-Expires: 180 UA asks for specific timer this time
This is not mandatory
SPS -> Called UA
INVITE SPS does not reduce Session-Expires header
k: timer
Session-Expires: 180
SPS <- Called UA
400 Rejected Media Called UA doesn't understand session timer
Calling UA <- SPS
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400 Rejected Media
Session-Expires: 180 SPS adds Session-Expires and Require
Require: timer
Calling UA -> SPS
ACK
SPS -> Called UA
ACK
8.6 Proxy insists on session timer
In this scenario, the calling UA does not support the session timer,
but the SPS on the setup path insists on it by inserting a Require
header. The UAS does not support the session timer either, so it
rejects the request with a 420 as per standard RFC2543 extension
handling. This response is forwarded upstream towards the UAC. The
UAC treats it as a normal 400 class response, and then ACKs it. In
this case, the call cannot be established.
It is slightly odd that a UAC will send a request without a
Require header, and yet get a 420 response back with an
Unsupported header. Normal UAs that don't support session
timer should handle this case correctly, treating it just
as a normal 400 class response.
Note that proxy insertion of Require is NOT RECOMMENDED.
Calling UA -> SPS
INVITE
SPS -> Called UA
INVITE SPS adds session expires
Require: timer SPS adds Require
Session-Expires: 180
SPS <- Called UA
420 Bad Extension
Unsupported: timer
Calling UA <- SPS
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420 Bad Extension
Unsupported: timer
Calling UA -> SPS
ACK
SPS -> Called UA
ACK
8.7 Neither UA Supports Session Timer
In this case, neither UA supports the session timer. However, one of
the proxies on the call setup path requests (but does not require)
it. The call completes without session timers.
Calling UA -> SPS
INVITE
SPS -> Called UA
INVITE SPS adds S-E header compact form
x: 180
SPS <- Called UA
200 OK Called UA doesn't understand session timer
Calling UA <- SPS SPS doesn't add S-E since it knows Calling UA
200 OK doesn't support it
Calling UA -> SPS
ACK
SPS -> Called UA
ACK
8.8 Both UAs Support, Change in Roles
In this case, both user agents support session timer. The initial
INVITE from caller to callee results in refreshes being generated by
the caller. A re-INVITE sent from the callee changes that role so
that the callee refreshes.
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Calling UA -> SPS
INVITE
Supported: timer
Session-Expires: 240
SPS -> Called UA
INVITE SPS wants timer, no change in value
Supported: timer
Session-Expires: 240
SPS <- Called UA
200 OK Called UA supports timer
Session-Expires: 240 Inserts Require, Session-Expires
Require: timer
Calling UA <- SPS Calling UA gets response with Require:timer
200 OK It is refreshing
Session-Expires: 240
Require: timer
Calling UA -> SPS
ACK
SPS -> Called UA
ACK
The called UA (which is a UAC for this transaction) now sends a re-
INVITE:
SPS <- Called UA
INVITE
Supported: timer
Session-Expires: 240
Calling UA <- SPS
INVITE SPS wants timer, no change in value
Supported: timer
Session-Expires: 240
Calling UA -> SPS
200 OK Calling UA supports timer
Session-Expires: 240 Inserts Require, Session-Expires
Require: timer
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SPS -> Called UA Called UA gets response with Require:timer
200 OK It is refreshing
Session-Expires: 240
Require: timer
SPS <- Called UA
ACK
Calling UA <- SPS
ACK
9 Changes since -03
o Now handle the case where the UAC wants refreshes, but none of
the proxies, nor the UAS, support it. Same in the reverse -
case where UAS wants it, but the UAC nor any of the proxies
want it.
o Added note about proxy insertion of Require resulting in a 420
even though UAC didn't ask for any extensions.
o Added compact form
o Specified conditions under which refresh responsibilities
change. Also added an example showing this case.
10 Author's Addresses
Steven R. Donovan
dynamicsoft
5100 Tennyson Parkway, Suite 1200
Plano, Texas 75024
email: sdonovan@dynamicsoft.com
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
email: jdrosen@dynamicsoft.com
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11 Bibliography
[1] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
session initiation protocol," Request for Comments 2543, Internet
Engineering Task Force, Mar. 1999.
[2] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: a
transport protocol for real-time applications," Request for Comments
1889, Internet Engineering Task Force, Jan. 1996.
[3] J. Rosenberg, D. Drew, and H. Schulzrinne, "Getting SIP through
firewalls and NATs," Internet Draft, Internet Engineering Task Force,
Feb. 2000. Work in progress.
[4] J. Rosenberg and H. Schulzrinne, "The SIP supported header,"
Internet Draft, Internet Engineering Task Force, Mar. 2000. Work in
progress.
[5] M. Handley and V. Jacobson, "SDP: session description protocol,"
Request for Comments 2327, Internet Engineering Task Force, Apr.
1998.
Table of Contents
1 Introduction ........................................ 1
2 Protocol Overview ................................... 2
3 Session-Expires Header Field Definition ............. 5
4 UAC Behavior ........................................ 6
5 Proxy Behavior ...................................... 9
5.1 Processing of requests .............................. 9
5.2 Processing of Responses ............................. 9
6 UAS Behavior ........................................ 11
7 Security Considerations ............................. 12
8 Examples ............................................ 13
8.1 Basic session timer ................................. 13
8.2 Basic negotiation of Session Time ................... 14
8.3 No Session-Expires Header in INVITE ................. 15
8.4 Session timer without Calling UA Support ............ 16
8.5 Session Timer without Called UA Support ............. 17
8.6 Proxy insists on session timer ...................... 19
8.7 Neither UA Supports Session Timer ................... 20
8.8 Both UAs Support, Change in Roles ................... 20
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9 Changes since -03 ................................... 22
10 Author's Addresses .................................. 22
11 Bibliography ........................................ 23
S.Donovan,J.Rosenberg [Page 24]
