Internet Engineering Task Force M. HASEBE
Internet-Draft J. KOSHIKO
Expires: Sep 6, 2007 Y. SUZUKI
T. YOSHIKAWA
NTT-East
P. Kyzivat
Cisco Systems, Inc.
Mar 5th, 2007
Examples call flow in race condition on Session Initiation Protocol
draft-ietf-sipping-race-examples-01.txt
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Abstract
This document gives examples of the Session Initiation Protocol (SIP)
call flows in race condition. Call flows in race condition are
confusing, and this document shows the best practice to handle
them. The elements in these call flows include SIP User Agents
and SIP Proxies. Call flow diagrams and message details are shown.
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
1.1 General Assumptions. . . . . . . . . . . . . . . . . . . . .3
1.2 Legend for Message Flows . . . . . . . . . . . . . . . . . .3
1.3 SIP Protocol Assumptions . . . . . . . . . . . . . . . . . .3
2. The Dialog State Machine for INVITE dialog usage . . . . . . . .4
3. Race Conditions. . . . . . . . . . . . . . . . . . . . . . . . .10
3.1 Receiving message in the Moratorium State. . . . . . . . . .10
3.1.1 Receiving Initial INVITE retransmission(Trying state). .10
3.1.2 Receiving CANCEL(Proceeding or Early state). . . . . . .12
3.1.3 Receiving BYE (Early state). . . . . . . . . . . . . . .14
3.1.4 Receiving re-INVITE (Established state)(case 1). . . . .15
3.1.5 Receiving re-INVITE (Established state)(case 2). . . . .19
3.1.6 Receiving BYE (Established state). . . . . . . . . . . .24
3.2 Receiving message in the Mortal State. . . . . . . . . . . .24
3.2.1 Receiving BYE(Established state) . . . . . . . . . . . .25
3.2.2 Receiving re-INVITE(Established state) . . . . . . . . .27
3.2.3 Receiving 200OK for re-INVITE(Established state) . . . .30
3.2.4 Receiving ACK (Moratorium state) . . . . . . . . . . . .32
3.3 Other race conditions. . . . . . . . . . . . . . . . . . . .34
3.3.1 Re-INVITE crossover. . . . . . . . . . . . . . . . . . .34
3.3.2 UPDATE and re-INVITE crossover . . . . . . . . . . . . .38
3.3.3 Receiving REFER(Established state) . . . . . . . . . . .42
Appendix A. BYE on the Early Dialog . . . . . . . . . . . . . . . .44
Appendix B. BYE request overlapped on re-INVITE . . . . . . . . . .45
Appendix C. UA's behaviour for CANCEL . . . . . . . . . . . . . . .47
Appendix D. Notes on the request in Mortal state. . . . . . . . . .49
Appendix E. Forking and receiving new To-tags . . . . . . . . . . .50
References . . . . . . . . . . . . . . . . . . . . . . . . . . . .53
Author's Addresses. . . . . . . . . . . . . . . . . . . . . . . . .53
Intellectual Property Statement . . . . . . . . . . . . . . . . . .54
Disclaimer of Validity. . . . . . . . . . . . . . . . . . . . . . .55
Copyright Statement . . . . . . . . . . . . . . . . . . . . . . . .55
Acknowledgment. . . . . . . . . . . . . . . . . . . . . . . . . . .55
1. Overview
The call flows shown in this document were developed in the design of
a SIP IP communications network. These examples are of race
condition, which stems from the dialog state transition mainly
established by INVITE.
When implementing SIP, various complex situations may arise.
Therefore, it will be helpful to provide implementors of the
protocol with examples of recommended terminal and server behavior.
This document clarifies SIP UA behaviors when messages cross each
other as race conditions. By clarifying operation under race
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conditions, inconsistent interpretations between implementations are
avoided and interoperability is expected to be promoted.
It is the hope of the authors that this document will be useful for
SIP implementors, designers, and protocol researchers and will help
them achieve the goal of a standard implementation of RFC 3261 [1].
These call flows are based on the version 2.0 of SIP defined in RFC
3261 [1] with SDP usage described in RFC 3264 [2].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 [4].
1.1 General Assumptions
A number of architecture, network, and protocol assumptions underlie
the call flows in this document. Note that these assumptions are not
requirements. They are outlined in this section so that they may be
taken into consideration and help understanding the call
flow examples.
These flows do not assume specific underlying transport protocols
such as TCP, TLS, and UDP. See the discussion in RFC 3261 [1] for
details on the transport issues for SIP.
1.2 Legend for Message Flows
Dashed lines (---) and slash lines (/,\) represent signaling messages
that are mandatory to the call scenario. (X) represents crossover of
signaling messages. (->x,x<-) indicate that the packet is lost.
The arrow indicate the direction of message flow. Double dashed
lines (===) represent media paths between network elements.
Messages are identified in the Figures as F1, F2, etc. These numbers
are used for references to the message details that follow the
Figure.
Comments in the message details are shown in the following form:
/* Comments. */
1.3 SIP Protocol Assumptions
This document does not prescribe the flows precisely as they are
shown, but rather illustrates the principles for best practice.
They are best practice usages (orderings, syntax, selection of
features for the purpose, or handling of error) of SIP methods,
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headers and parameters. NOTE: The flows in this document must not
be copied as they are by implementors because additional
characteristics were incorporated into the document for ease of
explanation. To sum up, the procedures described in this document
represent well-reviewed examples of SIP usage, which are best common
practice according to IETF consensus.
For simplicity in reading and editing the document, there are a
number of differences between some of the examples and actual SIP
messages. For instance, Call-IDs are often repeated, CSeq often
begins at 1, header fields are usually shown in the same order,
usually only the minimum required header field set is shown, and
other headers which would usually be included such as Accept, Allow,
etc are not shown.
Actors:
Element Display Name URI IP Address
------- ------------ --- ----------
User Agent Alice sip:alice@atlanta.example.com 192.0.2.101
User Agent Bob sip:bob@biloxi.example.com 192.0.2.201
User Agent Carol sip:carol@chicago.example.com 192.0.2.202
Proxy Server ss.atlanta.example.com 192.0.2.111
2. The Dialog State Machine for INVITE dialog usage
Race conditions are generated when the dialog state of the receiving
side differs from that of the sending side.
For instance, a race condition occurs when UAC (User Agent Client)
sends a CANCEL in the Early state while UAS (User Agent Server) is
transiting from the Early state to the Confirmed state by sending a
200 OK to ini-INVITE.
The DSM (dialog state machine) for the INVITE dialog usage is
presented as follows to help understanding UA's behavior in race
conditions.
The DSM clarifies UA's behavior by subdividing some internal states
shown in the FSM (Finite State Machine) for dialog state of the
dialog-package [7], without changing the states of the dialog,
"early", "confirmed", and "terminated" shown in RFC3261 [1].
The Preparative state is put before the Early state, which includes
the Trying and Proceeding states. The Confirmed state is subdivided
into two substates, the Moratorium and Established states and the
Terminated state is subdivided into the Mortal and Morgue states.
Messages which are the triggers of the state transitions between
these states are indicated with arrows. In this figure, messages
which are not related to state transition are omitted.
Below are the DSMs for UAC and UAS respectively.
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+-----------------------------------------------+
| Preparative |
| +----------+ +--------------+ |
| | | 100 | |-----C-+
| | Trying |---------->| Proceeding | | | 100
| | | | |<----C-+
| +----------+ +--------------+ |
| |
+-----------------------------------------------+
| | |
| 3xx-6xx | 1xx-tag | 2xx
| | |
| V |
| +------------------+ |
| 3xx-6xx | |--+ 1xx-tag |
+<--------| Early | | w/new tag |
| | |<-+ (new DSM |
| +------------------+ instance |
| | | created) |
| | BYE | 2xx |
| | +------------>+<-+
| | |
+-----C------------C-----+ +-----------C------+
| | Terminated | | | Confirmed | |
| | +<----C---------| | |
| | | | BYE | | |
| | V | | V |
| | +------------+ | | +-----------+ |
| | | |---C-+ | | |--C-+ 2xx
| | | Mortal | | | BYE(r)| | Moratorium| | | w/new tag
| | | |<--C-+ | | |<-C-+ (new DSM
| | +------------+ | | +-----------+ | instance
| | | | | | | created)
| | | Timeout | | | ACK |
| | | (Timer K) | | | |
| V V | | V |
| +---------------+ | | +-----------+ |
| | | | | | | |
| | Morgue | | | |Established| |
| | | | | | | |
| +---------------+ | | +-----------+ |
| | | |
+------------------------+ +------------------+
(r): indicates only reception is allowed.
Where (r) is not indicated, response means receive, request
means send.
Figure 1. DSM for INVITE dialog usage (UAC)
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Figure 1 represents the UAC's DSM for the INVITE dialog usage. UAC
MAY send a BYE in the Early state, even though this behavior is
NOT RECOMMENDED. The BYE sent in the Early state terminates the
Early dialog with a specific To-tag. That is, when a proxy is
performing forking, the BYE is only able to terminate the Early
dialog with a particular UA. If UAC wants to terminate all Early
dialogs instead of that with a particular UA, it needs to send
CANCEL, not BYE. Moreover, until UAC receives a final response and
terminates the INVITE transaction, the UAC MUST be prepared to
establish a dialog by receiving a new response to the INVITE even
though it had sent a BYE and terminated the dialog (see Appendix A).
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+-----------------------------------------------+
| Preparative |
| +----------+ +--------------+ |
| | | 100 | |-----C-+
| | Trying |---------->| Proceeding | | | 100
| | | | |<----C-+
| +----------+ +--------------+ |
| |
+-----------------------------------------------+
| | |
| 3xx-6xx | 1xx-tag | 2xx
| | |
| V |
| +------------------+ |
| 3xx-6xx | |--+ |
+<--------| Early | | 1xx-tag |
| | |<-+ |
| +------------------+ |
| | | |
| | BYE | 2xx |
| | +------------>+<-+
| | |
+-----C------------C-----+ +-----------C------+
| | Terminated | | | Confirmed | |
| | +<----C---------| | |
| | | | BYE(sr) | | |
| | V | | V |
| | +------------+ | | +-----------+ |
| | | |---C-+ | | |--C-+
| | | Mortal | | | BYE | | Moratorium| | | 2xx
| | | |<--C-+ | | |<-C-+
| | +------------+ | | +-----------+ |
| | | | | | |
| | | Timeout | | | ACK |
| | | (Timer J) | | | |
| V V | | V |
| +---------------+ | | +-----------+ |
| | | | | | | |
| | Morgue | | | |Established| |
| | | | | | | |
| +---------------+ | | +-----------+ |
| | | |
+------------------------+ +------------------+
(sr): indicates that both sending and reception are allowed.
Where (sr) is not indicated, response means send,
request means receive.
Figure 2. DSM for INVITE dialog usage (UAS)
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Figure 2 represents UAS's DSM for the INVITE dialog usage.
The figure does not illustrate the state transition related to
CANCEL request. CANCEL request does not cause a dialog state
transition. However, the UAS terminates the dialog and triggers the
dialog transition by sending 487 immediately after the reception of
the CANCEL. Considering this, the behavior upon the reception of the
CANCEL request is further explained in the Appendix C.
Following are UA's behaviors in each state.
Preparative (Pre): The Preparative state is a state until the
Early dialog is established by sending and receiving a
provisional response with To-tag after an ini-INVITE is sent
and received. The dialog has not existed yet in the
Preparative state. The dialog state transits from the
Preparative to the Early by sending or receiving a provisional
response with To-tag. Moreover, if UA sends or receives a 2xx
response, the dialog state transit to the Moratorium state
which is a substate of the Confirmed state.
In addition, if UA sends or receives a 3xx-6xx response the
dialog state transit to the Morgue state which is a substate of
the Terminated state. Sending an ACK for a 3xx-6xx response
and retransmissions of 3xx-6xx are not expressed on this DSM
because they are sent by the INVITE transaction.
Trying (Try): The Trying state is a substate of the Preparative
state and inherits the behavior of the superstate. The Trying
state is started by sending and receiving of an ini-INVITE.
It transits to the Proceeding state by sending or receiving a
1xx (usually 100 trying) without To-tag. UAC may retransmit an
INVITE on transaction layer and must not send a CANCEL request.
UAS may send a 1xx-6xx response.
Proceeding (Pro): The Proceeding state is a substate of the
Preparative state and inherits the behavior of the superstate.
Dialog becomes the Proceeding state if a dialog in the Trying
state sends or receives a 1xx without To-tag (usually 100
trying). UAC may send CANCEL, and UAS may send a 1xx-6xx
response in the Proceeding state.
Early (Ear): The early dialog is established by sending or
receiving a provisional response with To-tag. The early dialog
exists though the dialog does not existed in this state yet.
The dialog state transits from the Early to Moratorium state, a
substate of the Confirmed state, by sending or receiving a 2xx
response. In addition, the dialog state transits to the Morgue
state, a substate of the Terminated state, by sending and
receiving a 3xx-6xx response. Sending an ACK for a 3xx-6xx
response and retransmissions of 3xx-6xx are not expressed on
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this DSM because they are automatically processed on
transaction layer and don't influence the dialog state. UAC
may send CANCEL in the Early state. UAC may send BYE
(although it is not recommended). UAS may send a 1xx-6xx
response. Sending or receiving of a CANCEL request does not
have direct influences on dialog state. The UA's behavior upon
the reception of the CANCEL request is further explained in the
Appendix C.
Confirmed (Con): Sending or receiving of a 2xx final response
establishes a dialog. Dialog exists in this state. The BYE
request the changes state from the Confirmed to Mortal state,
a substate of the Terminated state. The Confirmed has two
substates, the Moratorium and Established state, which are
different in messages UAs are allowed to send.
Moratorium (Mora): The Moratorium state is a substate of the
Confirmed state and inherits the behavior of the superstate.
The Moratorium state transits to the Established state by
sending or receiving an ACK request. UAC may send ACK and UAS
may send a 2xx final response.
Established (Est): The Established state is a substate of the
Confirmed state and inherits the behavior of superstate. Both
caller and callee may send various messages which influences a
dialog. Caller supports the transmission of ACK for a
retransmission of a 2xx response to an ini-INVITE.
Terminated (Ter): The Terminated state is divided into two
substates, the Mortal and Morgue states, to cover the behavior
when a dialog is being terminated. In this state, UAs hold
information about the dialog which is being terminated. The
Confirmed state transits to the Mortal state, a substate of the
Terminated state, by sending or receiving a BYE request.
Mortal (Mort): Caller and callee becomes Mortal state by sending
or receiving a BYE. UA MUST NOT send any new requests since
there is no dialog. (Here the new requests do not include ACK
for 2xx and BYE for 401 or 407 as further explained in the
Appendix D below.)
In this state, only BYE or its response can be handled, and no
other messages can be received. This is because the use case
is taken into consideration that BYE is sent by both a caller
and a callee to exchange reports about the session when it is
being terminated. Therefore, UA possesses dialog information
for internal process but dialog shouldn't exist outwardly. The
UA stops managing its dialog state and changes it to the Morgue
state, when the BYE transaction is finished by timer
(Timer F or Timer K for UAC. Timer J for UAS).
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Morgue (Morg): Dialog does not exist any more in this state.
Sending or receiving of a signal which influences a dialog is
not performed. (A dialog is literally terminated.)
3. Race conditions
This section details race condition between two SIP UAs, Alice and
Bob. Alice (sip:alice@atlanta.example.com) and Bob
(sip:bob@biloxi.example.com) are assumed to be SIP phones or
SIP-enabled devices.
Only significant signals are illustrated. Dialog state transitions
caused by sending and receiving of SIP messages as well as '*race*',
which indicates race condition are shown. (For abbreviations for
the dialog state transitions, refer to Chapter 2.)
'*race*' indicates the moment when a race condition occurs.
Examples of race conditions are shown below.
3.1 Receiving message in the Moratorium State
This section shows some examples of call flow in race condition
when receiving the message from other states in the Moratorium state.
3.1.1 Receiving Initial INVITE retransmission (Trying state)
in Moratorium state
State Alice Bob State
| |
| ini-INVITE F1 |
|------------------------------------>|
Pre | 180 F2(Packet loss) | Pre
| x<-----------------------|
| | Ear
| ini-INVITE(=F1) F4 200 F3 |
|------------------ --------------|
| \ / | Mora
| X |
| / \ |
|<----------------- ------------->| *race*
Mora | ACK F5 |
|------------------------------------>|
Est | | Est
| |
This scenario illustrates the race condition which occurs when UAS
receives a Preparative message in the Moratorium state. All
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provisional responses to the initial INVITE (ini-INVITE F1) are lost,
and UAC retransmits an ini-INVITE (F4). At the same time as
retransmission, UAS generates a 200 OK (F3) to the ini-INVITE and it
terminate an INVITE server transaction, according to Section 13.3.1.4
of RFC3261 [1].
However, it is reported that terminating an INVITE server transaction
by 200 OK is a SIP bug. (http://bugs.sipit.net/, #769)
Therefore, the INVITE server transaction is not terminated at F3, and
the F4 MUST be properly handled as a retransmission.
(UAs that do not deal with this bug still need to recognize the
dialog relying on its From-tag and Call-ID, and the retransmitted
request relying on the CSeq header field value even though it does
not match the transaction.)
In RFC3261 [1], it is not specified whether UAS retransmits 200 to
the retransmission of ini-INVITE. Considering the retransmission of
200 triggered by timer (TU keeps retransmitting 200 based on T1
and T2 until it receives an ACK), according to Section 13.3.1.4 of
RFC3261 [1], it seems unnecessary to retransmit 200 when the UAS
receives the retransmission of ini-INVITE. (For implementation, it
does not matter if the UAS sends the retransmission of 200, since the
200 does not cause any problem.)
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
/* 180 response is lost and does not reach Alice. */
F3 200 OK Bob -> Alice
/* According to 13.3.1.4 of RFC3261, an INVITE server transaction
is terminated at this point. However, this has been reported as a
SIP bug, and the UAS MUST correctly recognize the ini-INVITE (F4) as
a retransmission. */
F4 INVITE (retransmission) Alice -> Bob
/* F4 is a retransmission of F1. They are exactly the same INVITE
request. For UAs do not deal with the bug reported in #769 (an
INVITE server transaction is terminated by 200 to INVITE), this
request does not match the transaction as well as the dialog
since it does not have a To-tag.
However, Bob have to recognize the retransmitted INVITE correctly,
without treating it as a new INVITE. */
F5 ACK Alice -> Bob
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3.1.2 Receiving CANCEL (Proceeding or Early state)
in Moratorium state
State Alice Bob State
| |
| INVITE F1 |
Pre |----------------------------->|
| 180 Ringing F2 | Pre
Ear |<-----------------------------|
| | Ear
|CANCEL F3 200(INVITE) F4|
|------------ -------------|
| \ / | Mora
| X |
| / \ |
|<----------- ------------>| *race*
Mora | |
| ACK F6 200(CANCEL) F5|
|------------ -------------|
Est | \ / |
| X |
| / \ |
|<----------- ------------>|
| | Est
| One Way RTP Media |
| (Two Way RTP Media possible) |
|<=============================|
| BYE F7 |
|----------------------------->|
Mort | 200 F8 | Mort
|<-----------------------------|
| ^ ^ |
| | Timer K | |
| V | |
Morg | Timer J | |
| V |
| | Morg
| |
This scenario illustrates the race condition which occurs when UAS
receives an Early message, CANCEL, in the Moratorium state. Alice
sends a CANCEL and Bob sends a 200 OK response to the initial INVITE
message at the same time. As described in the previous section,
according to RFC3261, an INVITE server transaction is supposed to be
terminated by a 200 response, but this has been reported as a bug
#769.
This section describes a case in which an INVITE server transaction
is not terminated by a response to the CANCEL request. In this case,
there is an INVITE transaction which the CANCEL request matches, so a
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200 response is sent to the request. This 200 response simply means
that the next hop received the CANCEL request (Successful CANCEL
(200) does not mean an INVITE failure). When UAS does not deal with
#769, UAC MAY receive a 481 response for CANCEL since there is no
transaction which the CANCEL request matches. This 481 simply means
that there is no matching INVITE server transaction and CANCEL is not
sent to the next hop.
Regardless of the success/failure of the CANCEL, Alice checks the
final response to INVITE, and if she receives 200 to the INVITE
request she immediately sends a BYE and terminates a dialog.
(Section 15, RFC3261 [1])
From the time F1 is received by Bob until the time that F8 is sent by
Bob, media may be flowing one way from Bob to Alice. From the time
than an answer is received by Alice from Bob there is the possibility
that media may flow from Alice to Bob as well. However, once Alice
has decided to cancel the call, she presumably will not send media,
so practically speaking the media stream will remain one way.
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 CANCEL Alice -> Bob
/* Alice sends a CANCEL in the Early state. */
F4 200 OK (INVITE) Bob -> Alice
/* Alice receives a 200 to INVITE (F1) in the Moratorium state.
Alice has the potential to send as well as receive media,
but in practice will not send because there is an intent
to end the call. */
F5 200 OK (CANCEL) Bob -> Alice
/* 200 to CANCEL simply means that the CANCEL was received.
The 200 response is sent, since this document deals with the
bug reported in #769. When an INVITE server transaction is
terminated as the procedure stated in RFC3261, UAC MAY receive
481 response instead of 200. */
F6 ACK Alice -> Bob
/* INVITE is successful, and the CANCEL becomes invalid. Bob
establishes RTP streams.
However, the next BYE request immediately terminates
the dialog and session. */
F7 BYE Alice -> Bob
F8 200 OK Bob -> Alice
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3.1.3 Receiving BYE (Early state)
in Moratorium state
State Alice Bob State
| |
| ini-INVITE F1 |
|------------------------------->|
Pre | 180 F2 | Pre
|<-------------------------------|
Ear | | Ear
| BYE F4 200(INVITE) F3|
|------------- --------------|
Mort | \ / | Mora
| X |
| / \ |
|<------------ ------------->| *race*
| | Mort
| ACK F5 200(BYE) F6 |
|------------- --------------|
| \ / ^ |
| X | |
| / \ | |
|<------------ ------------->|
| ^ | |
| | Timer K | |
| V | |
Morg | Timer J | |
| V |
| | Morg
| |
This scenario illustrates the race condition which occurs when UAS
receives an Early message, BYE, in the Moratorium state. Alice sends
a BYE in the Early state and Bob sends a 200 OK response to the
initial INVITE request at the same time. Bob receives the BYE in the
Confirmed dialog state though Alice sent the request in the Early
state (As explained in Section 2, this behavior is NOT RECOMMENDED).
The BYE functions normally even if it is received after the INVITE
transaction termination because BYE differs from CANCEL, and is sent
not to the request but to the dialog. Alice gets into the Mortal
state on receiving the BYE response, and remains Mortal until the
Timer K timeout occurs. In the Mortal state, UAC does not establish
a session, even though it receives a 200 response to INVITE. Even
so, the UAC sends an ACK to 200 for the completion of INVITE
transaction. The ACK is always sent to complete the three-way
handshake of INVITE transaction (Further explained in the Appendix D
below).
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Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK (ini-INVITE) Bob -> Alice
F4 BYE Alice -> Bob
/* Alice transits to the Mortal state upon sending BYE.
Therefore, after this, she does not begin a session even
though she receives a 200 response with an answer. */
F5 ACK Alice -> Bob
F6 200 OK (BYE) Bob -> Alice
3.1.4 Receiving re-INVITE (Established state)
in Moratorium state (case 1)
State Alice Bob State
| |
| ini-INVITE w/offer1 F1 |
|------------------------------->|
Pre | 180 F2 | Pre
|<-------------------------------|
Ear | | Ear
| 200(ini-INV) w/answer1 F3 |
|<-------------------------------|
Mora | ACK F4(packet loss) | Mora
|-------------------->x |
Est | |
| re-INVITE F6 200(=F3) F5 |
| w/offer2 w/answer1 |
|------------- --------------|
| \ / |
| X |
| / \ |
|<------------ ------------->| *race*
| 200(re-INV) F8|
| ACK(=F4) F7 w/answer2 |
|------------- --------------|
| \ / |
| X |
| / \ |
|<------------ ------------->|
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| ACK F9 | Est
|------------------------------->|
| |
| |
This scenario illustrates the race condition which occurs when UAS
receives re-INVITE request sent from the Established state, in the
Moratorium state.
UAS receives a re-INVITE (w/offer2) before receiving an ACK for
ini-INVITE (w/offer1). UAS sends a 200 OK (w/answer2) to the
re-INVITE (F8) because it has sent a 200 OK (w/answer1) to the
ini-INVITE (F3, F5) and the dialog has already been established.
(Because F5 is a retransmission of F3, SDP negotiation is not
performed here.) If a 200 OK to the ini-INVITE has an offer and the
answer is in the ACK, UA should return by a 491 to the
re-INVITE (refer to 3.1.5). As it can be seen in Section 3.3.2
below, the 491 response seems to be closely related to session
establishment, even in cases other than INVITE cross-over. This
example recommends 200 be sent instead of 491 because it does not
have influence on session. However, a 491 response can also lead to
the same outcome, so the either response can be used.
Moreover, if UAS doesn't receive an ACK for a long time,
it should send a BYE and terminate the dialog.
Message Details
F1 INVITE Alice -> Bob
INVITE sip:bob@biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:alice@client.atlanta.example.com;transport=udp>
Content-Type: application/sdp
Content-Length: 137
v=0
o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
/* ini-INVITE contains an offer. */
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F2 180 Ringing Bob -> Alice
SIP/2.0 180 Ringing
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:bob@client.biloxi.example.com;transport=udp>
Content-Length: 0
F3 200 OK Bob -> Alice
SIP/2.0 200 OK
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:bob@client.biloxi.example.com;transport=udp>
Content-Type: application/sdp
Content-Length: 133
v=0
o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
s=-
c=IN IP4 192.0.2.201
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F4 ACK Alice -> Bob
ACK sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds8
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 ACK
Content-Length: 0
/* ACK request is lost. */
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F5 200 OK (=F3) Bob -> Alice (retransmission)
/* UAS retransmits a 200 OK to the ini-INVITE since it has not
received an ACK. */
F6 re-INVITE Alice -> Bob
INVITE sip:sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9.1
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Content-Length: 147
v=0
o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=sendonly
F7 ACK (=F4) Alice -> Bob (retransmission)
F8 200 OK (re-INVITE) Bob -> Alice
SIP/2.0 200 OK
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9.1
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Content-Length: 143
v=0
o=bob 2890844527 2890844528 IN IP4 client.biloxi.example.com
s=-
c=IN IP4 192.0.2.201
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=recvonly
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F9 ACK Alice -> Bob
ACK sip:sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK230f2.1
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 ACK
Content-Length: 0
3.1.5 Receiving re-INVITE (Established state)
in Moratorium state (case 2)
State Alice Bob State
| |
| ini-INVITE (no offer) F1 |
|------------------------------->|
Pre | 180 F2 | Pre
|<-------------------------------|
Ear | | Ear
| 200(ini-INV) w/offer1 F3 |
|<-------------------------------|
Mora | ACK w/answer1 F4(packet loss) | Mora
|-------------------->x |
Est | |
| re-INVITE F6 200(=F3) F5 |
| w/offer2 w/offer1 |
|------------- --------------|
| \ / |
| X |
| / \ |
|<------------ ------------->|
| ACK(=F4) F7 491(re-INV) F8|
|------------- --------------|
| \ / |
| X |
| / \ |
|<------------ ------------->|
| ACK F9 | Est
|------------------------------->|
| |
| |
This scenario is basically the same as that of Section 3.1.4, but
differs in sending an offer in 200 and an answer in ACK. Different
to the previous case, the offer in the 200 (F3) and the offer in the
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re-INVITE (F6) collide with each other.
Bob sends 491 to re-INVITE (F6) since he is not able to properly
handle a new request until he receives an answer. Even if F6 is
UPDATE with offer, it will reach the same result.
Message Details
F1 INVITE Alice -> Bob
INVITE sip:bob@biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:alice@client.atlanta.example.com;transport=udp>
Content-Length: 0
/* The request does not contain an offer. */
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
SIP/2.0 200 OK
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:bob@client.biloxi.example.com;transport=udp>
Content-Type: application/sdp
Content-Length: 133
v=0
o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
s=-
c=IN IP4 192.0.2.201
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
/* An offer is made in 200 */
F4 ACK Alice -> Bob
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ACK sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds8
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 ACK
Content-Type: application/sdp
Content-Length: 137
v=0
o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
/* The request contains an answer, but the request is lost. */
F5 200 OK (=F3) Bob -> Alice (retransmission)
/* UAS retransmits a 200 OK to the ini-INVITE since it has not
received an ACK. */
F6 re-INVITE Alice -> Bob
INVITE sip:sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9.1
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Content-Length: 147
v=0
o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=sendonly
/* The request contains an offer. */
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F7 ACK (=F4) Alice -> Bob (retransmission)
/* A retransmission triggered by the reception of a retransmitted
200. */
F8 491 (re-INVITE) Bob -> Alice
/* Bob sends 491 (Request Pending), since Bob has a pending
offer. */
F9 ACK Alice -> Bob
3.1.6 Receiving BYE (Established state)
in Moratorium state
State Alice Bob State
| |
| INVITE F1 |
|-------------------------->|
Pre | 180 Ringing F2 | Pre
|<--------------------------|
Ear | | Ear
| 200 OK F3 |
|<--------------------------|
Mora | ACK F4(packet loss) | Mora
|--------------->x |
Est | Both Way RTP Media |
|<=========================>|
| BYE F6 200(=F3) F5|
|----------- -----------|
Mort | \ / |
| X |
| / \ |
|<---------- ---------->| *race*
|ACK(=F4) F7 200(BYE) F8| Mort
|----------- -----------|
| \ / |
| X |
| / \ |
|<---------- ---------->|
| ^ ^ |
| | Timer K | |
| V | |
Morg | Timer J | |
| V |
| | Morg
| |
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This scenario illustrates the race condition which occurs when UAS
receives an Established message, BYE, in the Moratorium state.
An ACK request for a 200 OK response is lost (or delayed).
Immediately after Bob retransmits the 200 OK to ini-INVITE, and Alice
sends a BYE request at the same time. Depending on the
implementation of a SIP UA, Alice may start a session again by the
reception of the retransmitted 200 OK with SDP since she has already
terminated a session by sending a BYE. In that case, when UAC
receives a retransmitted 200 OK after sending a BYE, a session should
not be started again since the session which is not associated with
dialog still remains. Moreover, in the case where UAS sends an offer
in a 200 OK , UAS should not start a session again for the same
reason if UAS receives a retransmitted ACK after receiving a BYE.
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob
ACK sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds8
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 ACK
Content-Length: 0
/* ACK request is lost. */
F5 200 OK (retransmission) Bob -> Alice
SIP/2.0 200 OK
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:bob@client.biloxi.example.com;transport=udp>
Content-Type: application/sdp
Content-Length: 133
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v=0
o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
s=-
c=IN IP4 192.0.2.201
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
/* UAS retransmits a 200 OK to the ini-INVITE since it has not
received an ACK. */
F6 BYE Alice -> Bob
BYE sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds9
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 BYE
Content-Length: 0
/* Bob retransmits a 200 OK and Alice sends a BYE at the same time.
Alice transits to the Mortal state, so she does not begin a
session after this even though she receives a 200 response to
the re-INVITE. */
F7 ACK(=F4) Alice -> Bob
F8 200 OK (BYE) Bob -> Alice
SIP/2.0 200 OK
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds9
;received=192.0.2.101
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 BYE
Content-Length: 0
/* Bob sends a 200 OK to the BYE. */
3.2 Receiving message in the Mortal State
This section shows some examples of call flow in race condition
when receiving the message from other states in the Mortal state.
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3.2.1 Receiving BYE (Establish state)
in Mortal state
State Alice Bob
| |
| INVITE F1 |
|----------------------->|
Pre | 180 Ringing F2 | Pre
|<-----------------------|
Ear | | Ear
| 200 OK F3 |
|<-----------------------|
Mora | ACK F4 | Mora
|----------------------->|
Est | Both Way RTP Media | Est
|<======================>|
| |
| BYE F5 BYE F6 |
|--------- ----------|
Mort | \ / | Mort
| X |
| / \ |
|<-------- --------->| *race*
| |
| 200 F8 200 F7 |
|--------- ----------|
| \ / |
| X |
| / \ |
|<-------- --------->|
| ^ ^ |
| | Timer K | |
| V | |
Morg | Timer J | |
| V |
| | Morg
| |
This scenario illustrates the race condition which occurs when UAS
receives an Established message, BYE, in the Mortal state.
Alice and Bob send a BYE at the same time. A dialog and session is
ended shortly after a BYE request is passed to a client transaction.
As shown in Section 2, UA remains in the Mortal state.
UAs in the Mortal state return error responses to the requests that
operate dialog or session, such as re-INVITE, UPDATE, or REFER.
However, UA shall return 200 OK to the BYE taking the use case into
consideration where BYE request is sent by both a caller and a callee
to exchange reports about the session when it is being terminated.
(Since the dialogue and the session both terminate when a BYE is
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sent, the choice of sending 200 or an error response upon receiving
BYE in the Mortal state does not affect the resulting termination.
Therefore, even though this example uses a 200 response, other
responses can also be used.)
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob
F5 BYE Alice -> Bob
BYE sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds8
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 BYE
Content-Length: 0
/* The session is terminated at the moment Alice sends a BYE.
The dialog still exists then, but it is certain to be terminated
in a short period of time. The dialog is completely
terminated when the timeout of the BYE request occurs. */
F6 BYE Bob -> Alice
BYE sip:alice@client.atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 BYE
Content-Length: 0
/* Bob has also transmitted a BYE simultaneously with Alice.
Bob terminates the session and the dialog. */
F7 200 OK Bob -> Alice
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SIP/2.0 200 OK
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds8
;received=192.0.2.201
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 BYE
Content-Length: 0
/* Since the dialog is in the Moratorium state, Bob responds with
a 200 to the BYE request. */
F8 200 OK Alice -> Bob
SIP/2.0 200 OK
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
;received=192.0.2.201
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 BYE
Content-Length: 0
/* Since Alice has transited from the Established state to the Mortal
state by sending BYE, Alice responds with a 200 to the BYE
request. */
3.2.2 Receiving re-INVITE (Establish state)
in Mortal state
State Alice Bob
| |
| INVITE F1 |
|----------------------->|
Pre | 180 Ringing F2 | Pre
|<-----------------------|
Ear | | Ear
| 200 OK F3 |
|<-----------------------|
Mora | ACK F4 | Mora
|----------------------->|
Est | Both Way RTP Media | Est
|<======================>|
| |
| BYE F5 re-INVITE F6|
|--------- ----------|
Mort | \ / |
| X |
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| / \ |
*race* |<-------- --------->|
| | Mort
| 481 F8 200 F7 |
|--------- ----------|
| \ / |^
| X ||
| / \ ||Timer J
|<-------- --------->||
^| ACK F9 ||
||<-----------------------||
Timer K|| ||
|| ||
V| ||
Morg | |V
| | Morg
| |
This scenario illustrates the race condition which occurs when UAS
receives an Established message, re-INVITE, in the Mortal state.
Bob sends a re-INVITE, and Alice sends a BYE at the same time.
The re-INVITE is responded by a 481, since the TU of Alice has
transited from the Established state to the Mortal state by sending
BYE. Bob sends ACK for the 481 response, because the ACK for error
responses is handled by the transaction layer and at the point of
receiving the 481 the INVITE client transaction still remains (even
though the dialog has been terminated).
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob
F5 BYE Alice -> Bob
BYE sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds8
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 BYE
Content-Length: 0
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/* Alice sends a BYE and terminates the session, and transits from
the Established state to the Mortal state. */
F6 re-INVITE Bob -> Alice
INVITE sip:alice@client.atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
Session-Expires: 300;refresher=uac
Supported: timer
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Content-Length: 0
/* Alice sends a BYE, and Bob sends a re-INVITE at the same time.
The dialog state transits to the Mortal state at the moment
Alice sends the BYE, but Bob does not know it until he receives
the BYE. Therefore, the dialog is in the Terminated state from
Alice's point of view, but it is the Confirmed state
from Bob's point of view. A race condition occurs. */
F7 200 OK Bob -> Alice
SIP/2.0 200 OK
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds8
;received=192.0.2.201
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 BYE
Content-Length: 0
F8 481 Call/Transaction Does Not Exist Alice -> Bob
SIP/2.0 481 Call/Transaction Does Not Exist
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
;received=192.0.2.201
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Content-Length: 0
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/* Since Alice is in the Mortal state, she responds with a 481 to the
re-INVITE. */
F9 ACK Bob -> Alice
/* ACK for an error response is handled by Bob's INVITE client
transaction. */
3.2.3 Receiving 200OK for re-INVITE (Established state)
in Mortal state
State Alice Bob
| |
| INVITE F1 |
|----------------------->|
Pre | 180 Ringing F2 | Pre
|<-----------------------|
Ear | | Ear
| 200 OK F3 |
|<-----------------------|
Mora | ACK F4 | Mora
|----------------------->|
Est | Both Way RTP Media | Est
|<======================>|
| |
| re-INVITE F5 |
|<-----------------------|
| 200 F7 BYE F6 |
|--------- ----------|
| \ / | Mort
| X |
| / \ |
|<-------- --------->| *race*
Mort | 200 F8 ACK F9 |
|--------- ----------|
| ^ \ / |
| | X |
| | / \ |
|<-------- --------->|
| | ^ |
| | Timer K | |
| | V |
| | Timer J | Morg
| V |
Morg | |
| |
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This scenario illustrates the race condition which occurs when UAS
receives an Established message, 200 to re-INVITE, in the Mortal
state. Bob sends a BYE immediately after sending a re-INVITE.
(A user is not conscious that refresher sends re-INVITE
automatically. For example, in the case of a telephone application,
it is possible that a user places a receiver immediately after
refresher.)
Bob sends ACK for a 200 response to INVITE in the Mortal state, so
that he completes the INVITE transaction.
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob
F5 re-INVITE Bob -> Alice
INVITE sip:alice@client.atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
Session-Expires: 300;refresher=uac
Supported: timer
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Content-Length: 0
F6 BYE Bob -> Alice
BYE sip:alice@client.atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds8
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 BYE
Content-Length: 0
/* Bob sends BYE immediately after sending the re-INVITE.
Bob terminates the session and transits from the Established
state to the Mortal state. */
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F7 200 OK (re-INVITE) Alice -> Bob
SIP/2.0 200 OK
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds7
;received=192.0.2.201
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Content-Length: 0
/* Bob sends BYE, and Alice responds with a 200 OK to the re-INVITE.
A race condition occurs. */
F8 200 OK (BYE) Alice -> Bob
SIP/2.0 200 OK
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds8
;received=192.0.2.201
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 BYE
Content-Length: 0
F9 ACK Bob -> Alice
/* Bob sends ACK in the Mortal state to complete the three-way
handshake of the INVITE transaction. */
3.2.4 Receiving ACK (Moratorium state)
in Mortal state
State Alice Bob State
| |
| ini-INVITE F1 |
|------------------------------->|
Pre | 180 F2 | Pre
|<-------------------------------|
Ear | 200 F3 | Ear
|<-------------------------------|
Mora | | Mora
| ACK F4 BYE F5 |
|------------- --------------|
Est | \ / | Mort
| X |
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| / \ |
|<------------ ------------->| *race*
Mort | 200 F6 |
|------------------------------->|
| ^ ^ |
| | Timer K | |
| | V |
| | Timer J | Morg
| V |
Morg | |
| |
This scenario illustrates the race condition which occurs when UAS
receives an Established message, ACK to 200, in the Mortal state.
Alice sends an ACK and Bob sends a BYE at the same time. When the
offer is in a 2xx, and the answer is in an ACK, this example is in
a race condition. The session is not started by receiving the ACK
because Bob has already terminated the session by sending the BYE.
The answer in the ACK request is just ignored.
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob
ACK sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bd5
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 ACK
Content-Length: 0
/* RTP streams are established between Alice and Bob */
F5 BYE Alice -> Bob
BYE sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds8
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
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CSeq: 2 BYE
Content-Length: 0
/* Alice sends a BYE and terminates the session and dialog. */
F6 200 OK Bob -> Alice
SIP/2.0 200 OK
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds8
;received=192.0.2.201
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 BYE
Content-Length: 0
3.3 Other race conditions
This section shows the examples in race condition that are not
directly related to the dialog state transition. Here explains
the way to handle the race condition generated when UAs treat
"What is established by SIP", which has some relation to dialog.
3.3.1 Re-INVITE crossover
Alice Bob
| |
| INVITE F1 |
|--------------------------->|
| 180 Ringing F2 |
|<---------------------------|
| |
| 200 OK F3 |
|<---------------------------|
| ACK F4 |
|--------------------------->|
| Both Way RTP Media |
|<==========================>|
| |
|re-INVITE F5 re-INVITE F6 |
|------------ -------------|
| \ / |
| X |
| / \ |
|<----------- ------------>|
| 491 F8 491 F7 |
|------------ -------------|
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| \ / |
| X |
| / \ |
|<----------- ------------>|
| ^ ACK F9 ^ ACK F10|
|--|--------- ----|--------|
| | \ / | |
| | X | |
| | / \ | |
|<-|---------- ---|------->|
| | | |
| |0-2.0 sec | |
| | | |
| v re-INVITE(=F6) F11 |
|<------------------|--------|
| 200 OK F12 | |
|-------------------|------->|
| ACK F13 | |
|<------------------|--------|
| | |
| |2.1-4.0 sec
| | |
|re-INVITE(=F5) F14 v |
|--------------------------->|
| 200 OK F15 |
|<---------------------------|
| ACK F16 |
|--------------------------->|
| |
| |
In this scenario, Alice and Bob send re-INVITE at the same time.
When two re-INVITEs cross in the same dialog, they resend re-INVITEs
after different interval for each, according to Section 14.1, of
RFC3261 [1]. When Alice sends the re-INVITE and it crosses, the
re-INVITE will be sent again after 2.1-4.0 seconds because she owns
the Call-ID (she generated it). Bob will send an INVITE again after
0.0-2.0 seconds, because Bob isn't the owner of the Call-ID.
Therefore, each user agent must remember whether it has generated the
Call-ID of the dialog or not, in case an INVITE may cross with
another INVITE.
In this example, Alice's re-INVITE is for session modification
and Bob's re-INVITE is for session refresh. In this case, after
the 491 responses, Bob retransmits the re-INVITE for session refresh
earlier than Alice. If Alice was to retransmit her re-INVITE (that
is, if she was not the owner of Call-ID), the request would refresh
and modify the session at the same time. Then Bob would know that
he would not need to retransmit his re-INVITE to refresh the session.
In another instance where two re-INVITEs for session modification,
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cross over, retransmitting the same re-INVITE again after 491 by the
Call-ID owner (the UA which retransmits its re-INVITE after the
other UA) may result in a behavior different from what the user
originally intended to, so the UA needs to decide if the
retransmission of the re-INVITE is necessary.
(For example, when a call hold and an addition of video media cross
over, mere retransmission of the re-INVITE at the firing of the timer
may result in the situation where the video is transmitted
immediately after the holding of the audio. This behavior is
probably not intended by the users.)
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob
F5 re-INVITE Alice -> Bob
INVITE sip:sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Content-Length: 147
v=0
o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=sendonly
/* re-INVITE for session modification (a=sendrecv -> sendonly). */
F6 re-INVITE Bob -> Alice
INVITE sip:alice@client.atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
Session-Expires: 300;refresher=uac
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Supported: timer
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Content-Length: 0
/* A re-INVITE request for a session refresh and that for
a call hold are sent at the same time. */
F7 491 Request Pending Bob -> Alice
/* Since a re-INVITE is in progress, a 491 response is returned. */
F8 491 Request Pending Alice -> Bob
F9 ACK (INVITE) Alice -> Bob
F10 ACK (INVITE) Bob -> Alice
F11 re-INVITE Bob -> Alice
INVITE sip:alice@client.atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7.1
Session-Expires: 300;refresher=uac
Supported: timer
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Content-Type: application/sdp
Content-Length: 133
v=0
o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
s=-
c=IN IP4 192.0.2.201
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
/* Since Bob is not the owner of the Call-ID, he sends a re-INVITE
again after 0.0-2.0 seconds. */
F12 200 OK Alice -> Bob
F13 ACK Bob -> Alice
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F14 re-INVITE Alice -> Bob
INVITE sip:sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9.1
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 3 INVITE
Content-Length: 147
v=0
o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=sendonly
/* Since Alice is the owner of the Call-ID, Alice sends a re-INVITE
again after 2.1-4.0 seconds. */
F15 200 OK Bob -> Alice
F16 ACK Alice -> Bob
3.3.2 UPDATE and re-INVITE crossover
Alice Bob
| |
| INVITE F1 |
|--------------------------->|
| 180 Ringing F2 |
|<---------------------------|
| |
| 200 OK F3 |
|<---------------------------|
| ACK F4 |
|--------------------------->|
| Both Way RTP Media |
|<==========================>|
| |
| UPDATE F5 re-INVITE F6 |
|------------ -------------|
| \ / |
| X |
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| / \ |
|<----------- ------------>|
| 491 F8 491 F7 |
|------------ -------------|
| \ / |
| X |
| / \ |
|<----------- ------------>|
| ^ ACK F9 ^ |
|<-|----------------|--------|
| | | |
| |0-2.0 sec | |
| | | |
| v re-INVITE F10 | |
|<------------------|--------|
| 200 OK F11 | |
|-------------------|------->|
| ACK F12 | |
|<------------------|--------|
| | |
| |2.1-4.0 sec
| | |
| UPDATE F13 v |
|--------------------------->|
| 200 OK F14 |
|<---------------------------|
| |
| |
In this scenario, the UPDATE contains a SDP offer, therefore the
UPDATE and re-INVITE are responded with 491 as in the case of
"re-INVITE crossover". When an UPDATE for refresher which doesn't
contain a session description and a re-INVITE crossed each other,
both requests succeed with 200 (491 means that UA have a pending
request). Moreover, the same is equally true for UPDATE crossover.
In the former case where either UPDATE contains a session description
the requests fail with 491, and in the latter cases succeed with 200.
Editor's Note:
A 491 response is considered as a result that UA judged the
effectiveness of request to "What is established by SIP".
Therefore, it is considered that 491 will be used in all the
requests that demand operation to "What is established by SIP".
Message Details
F1 INVITE Alice -> Bob
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F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob
F5 UPDATE Alice -> Bob
UPDATE sip:sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 UPDATE
Content-Length: 147
v=0
o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=sendonly
F6 re-INVITE Bob -> Alice
INVITE sip:alice@client.atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
Session-Expires: 300;refresher=uac
Supported: timer
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Content-Length: 0
/* A case where a re-INVITE for a session refresh and a re-INVITE for
a call hold are sent at the same time. */
F7 491 Request Pending Bob -> Alice
/* Since a re-INVITE is in process, a 491 response are returned. */
F8 491 Request Pending Alice -> Bob
F9 ACK (re-INVITE) Alice -> Bob
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F10 re-INVITE Bob -> Alice
INVITE sip:alice@client.atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7.1
Session-Expires: 300;refresher=uac
Supported: timer
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Content-Type: application/sdp
Content-Length: 133
v=0
o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
s=-
c=IN IP4 192.0.2.201
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
/* Since Bob is not the owner of Call-ID, Bob sends an INVITE again
after 0.0-2.0 seconds. */
F11 200 OK Alice -> Bob
F12 ACK Bob -> Alice
F13 UPDATE Alice -> Bob
UPDATE sip:sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9.1
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 3 UPDATE
Content-Length: 147
v=0
o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=sendonly
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/* Since Alice is the owner of the Call-ID, Alice sends the UPDATE
again after 2.1-4.0 seconds. */
F14 200 OK Bob -> Alice
3.3.3 Receiving REFER (Establish state)
in Mortal state
State Alice Bob State
| |
| INVITE F1 |
|----------------------->|
Pre | 180 Ringing F2 | Pre
|<-----------------------|
Ear | | Ear
| 200 OK F3 |
|<-----------------------|
Mora | ACK F4 | Mora
|----------------------->|
Est | Both Way RTP Media | Est
|<======================>|
| |
| BYE F5 REFER F6 |
|--------- ----------|
Mort | \ / |
| X |
| / \ |
*race* |<-------- --------->|
| | Mort
| 481 F8 200 F7 |
|--------- ----------|
| \ / ^ |
| X | |
| / \ | |
|<-------- --------->|
| ^ | |
| | Timer K | |
| V | |
Morg | Timer J | |
| V |
| | Morg
| |
This scenario illustrates the race condition which occurs when UAS
receives an Established message, REFER, in the Mortal state.
Bob sends a REFER, and Alice sends a BYE at the same time. Bob
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send the REFER in the same dialog. Alice sends an error response
to the requests which operates the session, such as REFER, because
by sending the BYE, Alice had terminated the session which would
have corresponded to the REFER. For handling of dialogs with
multiple usages, as can be seen in the use of REFER method,
see the draft on dialog usage [8].
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob
F5 BYE Alice -> Bob
/* Alice sends a BYE request and terminates the session, and transits
from the Confirmed state to Terminated state. */
F6 REFER Bob -> Alice
REFER sip:alice@client.atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
Refer-To: sip:carol@cleveland.example.org
Contact: <sip:bob@client.biloxi.example.com;transport=udp>
CSeq: 1 REFER
Content-Length: 0
/* Alice sends a BYE, and Bob sends a REFER at the same time.
Bob sends the REFER on the INVITE dialog.
The dialog state transits to the Mortal state at the moment
Alice sends the BYE, but Bob doesn't know it until he receives
the BYE. A race condition occurs. */
F7 200 OK Bob -> Alice
F8 481 Call/Transaction Does Not Exist Alice -> Bob
/* Since Alice has terminated the session, she responds with a 481
to the REFER. */
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Appendix A - BYE on the Early Dialog
This section, related to Section 3.1.3, explains why BYE is not
recommended in the Early state, illustrating the case in which BYE
in the Early dialog triggers confusion.
Alice Proxy Bob Carol
| | | |
| INVITE F1 | | |
|--------------->| INVITE F2 | |
| 100 F3 |----------------->| |
|<---------------| 180(To-tag=1) F4 | |
| 180(1) F5 |<-----------------| |
|<---------------| | |
| | INVITE(Fork) F6 |
| |------------------------>|
| | 100 F7 |
| BYE(1) F8 |<------------------------|
|--------------->| BYE(1) F9 | |
| |----------------->| |
| | 200(1) F10 | |
| 200(1) F11 |<-----------------| |
|<---------------| 487(1) F12 | |
| |<-----------------| |
| | ACK(1) F13 | |
| |----------------->| |
| | | |
| | |
| | 200(To-tag=2) F13 |
| 200(2) F14 |<------------------------|
|<---------------| |
| ACK(2) F15 | |
|--------------->| ACK(2) F16 |
| |------------------------>|
| BYE(2) F17 | |
|--------------->| BYE(2) F18 |
| |------------------------>|
| | 200(2) F19 |
| 200(2) F20 |<------------------------|
|<---------------| |
| | |
| | |
Care is advised in sending BYE in the Early state when forking by a
proxy is expected. In this example, the BYE request progresses
normally, and it succeeds in correctly terminating the dialog with
Bob. After Bob terminates the dialog by receiving the BYE, he sends
487 to the ini-INVITE. According to Section 15.1.2 of RFC3261 [1],
it is RECOMMENDED for UAS to generate 487 to any pending requests
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after receiving BYE. In the example, Bob sends 487 to ini-INVITE
since he receives BYE while the ini-INVITE is in pending state.
However, Alice receives a final response to INVITE (a 200 from Carol)
even though she has successfully terminated the dialog with Bob. This
means that, regardless of the success/failure of the BYE in the Early
state, Alice MUST be prepared for the establishment of a new dialog
until receiving the final response for the INVITE and terminating the
INVITE transaction.
The choice of BYE or CANCEL in the early state must be made
carefully. CANCEL is appropriate when the goal is to abandon
the call attempt entirely. BYE is appropriate when the goal is
to abandon a particular early dialog while allowing the call
to be completed with other destinations. When using either BYE
or CANCEL the UAC must be prepared for the possibility that
a call may still be established to one (or more) destinations.
Appendix B - BYE request overlapped on re-INVITE
UAC UAS
| |
The session has been already established
==========================
| F1 re-INVITE |
|--------------------->|
| F2 BYE |
|--------------------->|
| F3 200(BYE) |
|<---------------------|
| F4 INVITE(=F1) |
|--------------------->|
| |
| |
This case could look similar to the one in Section 3.2.3. However,
it is not a race condition. This case describes the behavior where
there is no response for INVITE for some reasons. The appendix
explains the behavior in such case and its rationale behind, since
this case is likely to cause confusion.
First of all, it is important not to confuse the behavior of the
transaction layer and that of the dialog layer. RFC3261 [1] details
the Transaction layer behavior. The dialog layer behavior is
explained in this document.
It has to be noted that these behaviors are independent of each
other, even though the both layers change their states triggered by
sending or receiving of the same SIP messages (A dialog can be
terminated even though a transaction still remain, and vice versa).
In the sequence above, there is no response for F1, and F2 (BYE) is
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sent immediately after F1 (F1 is a mid-dialog request. If F1 was
ini-INVITE, BYE could not be sent before UAC received a provisional
response to the request with To-tag).
Below is a figure which illustrates UAC's dialog state and
transaction state.
BYE INV dialog UAC UAS
: | |
: | |
| | F1 re-INVITE |
o | |--------------------->|
| | | F2 BYE |
o | (Mortal) |--------------------->|
| | | | F3 200(BYE) |
| | | |<---------------------|
| | | | F4 INVITE(=F1) |
| | | |--------------------->|
| | | | F5 481(INV) |
| | | |<---------------------|
| | | | F6 ACK(INV) |
| | | |--------------------->|
| | | | |
o | o | |
| | |
o | |
| |
For UAC, the INVITE client transaction begins at the point F1 is
sent. The UAC sends BYE (F2) immediately after F1. This is a
legitimate behavior. (Usually the usage of each SIP method is
independent, for BYE and others. However, it should be noted that
it is prohibited to send a request with a SDP offer while the
previous offer is in progress.)
After that, F2 triggers the BYE client transaction. At the same time,
the dialog state transits to the Mortal state and then only a BYE or
its response can be handled.
It is permitted to send F4 (a retransmission of INVITE) in the Mortal
state, because the retransmission of F1 is handled by the transaction
layer, and the INVITE transaction has not yet transited to the
Terminated state. As it is mentioned above, the dialog and the
transaction behave independently each other.
Therefore the transaction handling has to be continued even though
the dialog moved to the Terminated state.
Next, UAS's state is shown below.
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UAC UAS dialog INV BYE
| | :
| | :
| F1 re-INVITE | |
|-------------->x | |
| F2 BYE | |
|--------------------->| | o
| F3 200(BYE) | (Mortal) |
|<---------------------| | |<-Start TimerJ
| F4 INVITE(=F1) | | |
|--------------------->| | o |
| F5 4xx(INV) | o | o
|<---------------------| |
| F6 ACK(INV) | |
|--------------------->| |<-Start TimerI
| | |
| | |
| | o
| |
For UAS, it can be regarded that F1 packet is lost or delayed (Here
the behavior is explained for the case UAS receives F2 BYE before F1
INVITE). Therefore, F2 triggers the BYE transaction for UAS, and
simultaneously the dialog moves to the Mortal state.
Then, upon the reception of F4 the INVITE server transaction begins.
(It is allowed to start the INVITE server transaction in the Mortal
state. The INVITE server transaction begins to handle received SIP
request regardless of the dialog state.)
UAS's TU sends an appropriate error response for F4 INVITE, either
481 (because the TU knows that the dialog which matches to the
INVITE is in the Terminated state) or 500 (because the re-sent F4
has an out-of-order CSeq).
(It is mentioned above that F4 (and F1) INVITE is a mid-dialog
request. Mid-dialog requests have a To-tag. It should be noted that
UAS's TU does not begin a new dialog upon the reception of INVITE
with a To-tag.)
Appendix C - UA's behaviour for CANCEL
This section explains the CANCEL and the Expires header behaviors
which indirectly involve in the dialog state transition in the Early
state. CANCEL does not have any influence on UAC's dialog state.
However, the request has indirect influence on the dialog state
transition because it has a significant effect on ini-INVITE.
Similarly, the Expires header does not have direct influence on the
dialog state transition, but it indirectly affect the state
transition because its expiration triggers the sending of CANCEL.
For UAS the CANCEL request and the Expires header timeout have more
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direct effects on the dialog than the sending of CANCEL by UAC,
because they can be a trigger to send the 487 response. Figure 3
explains UAS's behavior in the Early state. This flow diagram is
only an explanatory figure, and the actual dialog state transition is
as illustrated in Figure 1 and 2.
In the flow, full lines are related to dialog state transition, and
dotted lines are involved with CANCEL. (r) represents the reception
of signals, and (s) means sending. There is no dialog state for
CANCEL, but here the Cancelled state is virtually handled just for
the ease of understanding of the UA's behavior when it sends and
receives CANCEL.
Below, UAS's flow is explained.
+------------+
| Proceeding |----+
+------------+ |
: | 1xx(s) |
: V |
: +-------+ | 2xx(s)
: | Early |-----+------+
: +-------+ |
: : V
: : +-----------+
: : | Confirmed |<...
:.....: +-----------+ :
: | : :
: BYE(r)| : :
: CANCEL(r) | :.......:
V | CANCEL(r)
............. |
: Cancelled : |
:...........: |
| 487(s) |
| |
+--------------------+
|
V
+------------+
| Terminated |
+------------+
Figure 3. CANCEL flow diagram for UAS
There are two behaviors for UAS depending on the state when it
receives CANCEL.
One is when UAS receives CANCEL in the Proceeding and Early states.
In this case the UAS immediately sends 487 for the INVITE, and the
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dialog transits to the Terminated state.
The other is the case in which UAS receives CANCEL in the Confirmed
state. In this case the dialog state transition does not occur
because UAS has already sent a final response to the INVITE to which
the CANCEL is targeted.
(Note that, because of UAC's behavior, a UAS that receives a CANCEL
in the Confirmed state can expect to receive a BYE immediately and
move to the Terminated state. However, the UAS's state does not
transit until it actually receives BYE.)
Appendix D - Notes on the request in Mortal state
This section describes UA's behavior in the Mortal state which need
careful attention.
In the Mortal state, BYE can be accepted, and the other messages in
the INVITE dialog usage are responded with an error. However,
sending of ACK and the authentication procedure for BYE are
conducted in this state.
(The handling of messages concerning multiple dialog usages is out of
the scope of this document. Refer to [8] for further information.)
ACK for error responses is handled by the transaction layer, so the
handling is not related to the dialog state. Unlike the ACK for
error responses, ACK for 2xx responses is a request newly generated
by TU. However, the ACK for 2xx and the ACK for error responses are
both a part of the INVITE transaction, even though their hadling
differs (Section 17.1.1.1, RFC3261 [1]).
Therefore, the INVITE transaction is completed by the three-way
handshake, which includes ACK, even in the Mortal state.
Considering actual implementation, UA needs to keep the INVITE dialog
usage until the Mortal state finishes, so that it is able to ACK for
a 2xx response in the Mortal state.
If a 2xx to INVITE is received in the Mortal state, the duration of
the INVITE dialog usage will be extended to 64*T1 seconds after the
receiving of the 2xx, to cope with the possible 2xx retransmission.
(The duration of the 2xx retransmission is 64*T1, so the UA need to
be prepared to handle the retransmission for this duration.)
However, the UA shall send error response to other requests, since
the INVITE dialog usage in the Mortal state is kept only for the
sending of ACK for 2xx.
BYE authentication procedure shall be processed in the Mortal state.
When authentication is requested by 401 or 407 response, UAC resends
BYE with an appropriate credentials. Also UAS handles the
retransmission of the BYE which it requested authentication itself.
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Appendix E - Forking and receiving new To-tags
This section details the behavior of TU when it receives multiple
responses with a different To tag to ini-INVITE.
When an INVITE is forked inside a SIP network, there is a possibility
that the TU receives multiple responses with a different To tag to
ini-INVITE (See Section 11.2, 13.1, 13.2.2.4, 16.7, 19.3 etc. of
RFC3261). If the TU receives multiple 1xx responses with a different
To tag, the original DSM forks and a new DSM instance is created. As
a consequence multiple early dialogs are generated.
If one of the multiple early dialogs receives a 2xx response, it
naturally transits to the Confirmed state. No DSM state transition
occurs for the other early dialogs, and their sessions (early media)
terminate. The TU of the UAC terminates the INVITE transaction after
64*T1 seconds starting at the point of receiving the first 2xx
response. Moreover, all mortal early dialog which do not transit to
the Established state are terminated (See Section 13.2.2.4 of
RFC3261).
Below is an example sequence in which two 180 responses with a
different To tag are received, and then a 200 response for one of the
early dialogs (dialog A) is received. Dot lines (..) in sequences
is auxiliary line to represent the influence on dialog B.
UAC
dialog(A) | F1 INVITE
Pre o |------------------------->
| | F2 100
| |<-------------------------
| | F3 180(To-tag=A)
Ear | |<-------------------------
dialog(B) | |
forked new DSM | | F4 180(To-tag=B)
Ear o..........|..........|<-------------------------
| | |
| | | F5 200(To-tag=A)
terminate->|.....Mora |..........|<-------------------------
early | | ^ | F6 ACK(To-tag=A)
media | Est | | |------------------------->
| | | |
| | |64*T1 |
| | |(13.2.2.4 of RFC3261)
| | | |
| | | |
| | | |
| | V |
o..........|.(terminate INVITE transaction)
terminated | |
dialog(B) | |
| |
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The figure above shows the DSM inside a SIP TU. Triggered by the
reception of a provisional response with a different To tag
(F4 180(To-tag=B)), DSM forks and the early dialog B is generated.
After 64*T1 seconds after the dialog A receives 200 OK response, the
dialog B, which does not transit to the Established state,
terminates.
Next, the behavior of a TU which receives multiple 2xx responses with
a different To tag is explained. When a mortal early dialog, which
did not match the first 2xx response the TU received, receives
another 2xx response which matches its To tag before 64*T1 INVITE
transaction timeout, its DSM state transits to the Confirmed state.
However, the seesion on the mortal early dialog is terminated when
the TU receives the first 2xx to establish a dialog, so no session is
established for the mortal early dialog. Therefore, when the mortal
early dialog receives a 2xx response, the TU should send an ACK and,
immediately after, BYE to terminate DSM.
The handling of the second early dialog after receiving the 200 for
the first dialog is quite appropriate for a typical device, such as a
phone. It is important to note that what is being shown is a
typically useful action and not the only valid one. Some devices
might want to handle things differently. For instance, a conference
focus that has sent out an invite that forks may want to accept and
mix all the dialogs it gets.
Below is an example sequence in which two 180 responses with a
different To tag are received and then a 200 response for each of the
early dialogs is received.
UAC
dialog(A) | F1 INVITE
Pre o |----------------------->
| | F2 100
| |<-----------------------
| | F3 180(To-tag=A)
dialog(B) Ear | |<-----------------------
forked new DSM | | F4 180(To-tag=B)
Ear o..........|..........|<-----------------------
| | |
| | | F5 200(To-tag=A)
terminate->|.....Mora |..........|<-----------------------
early | | ^ | F6 ACK(To-tag=A)
media | Est | | |----------------------->
| | |64*T1 |
| | | | F7 200(To-tag=B)
Mora |..........|.|........|<-----------------------
| | | | F8 ACK(To-tag=B)
Est |..........|.|........|----------------------->
| | | | F9 BYE(To-tag=B)
Mort |..........|.|........|----------------------->
^ | | | | F10 200(To-tag=B)
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| | | | |<-----------------------
|Timer K | | |
| | | V |
| | | (terminate INVITE transaction)
V | | |
Morg o | |
| |
Finally, when a TU receives multiple 200 responses with a different
To tag before 64*T1 timeout of the INVITE transaction, even though
a TU does not receive provisional response, the TU needs to respond
to the 2xx responses (See Section 13.2.2.4 of RFC3261). In that
case, the DSM state is forked at the Confirmed state, and then the
TU sends an ACK for the 2xx response and, immediately after, BYE.
UAC
dialog(A) | F1 INVITE
Pre o |----------------------->
| | F2 100
| |<-----------------------
| | F3 180(To-tag=A)
Ear | |<-----------------------
| |
| | F4 200(To-tag=A)
Mora |..........|<-----------------------
| ^ | F5 ACK(To-tag=A)
Est | | |----------------------->
| | |
dialog(B) | |64*T1 |
forked new DSM | | | F6 200(To-tag=B)
Mora o..........|.|........|<-----------------------
| | | | F7 ACK(To-tag=B)
Est |..........|.|........|----------------------->
| | | | F8 BYE(To-tag=B)
Mort |..........|.|........|----------------------->
^ | | | | F9 200(To-tag=B)
| | | | |<-----------------------
| | | V |
|Timer K | (terminate INVITE transaction)
| | | |
V | | |
Morg o | |
| |
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References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[2] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
SDP", RFC 3264, April 2002.
[3] Johnston, A., Donovan, S., Sparks, R., Cunningham, C. and K.
Summers, "Session Initiation Protocol (SIP) Basic Call Flow
Examples", BCP 75, RFC 3665, December 2003.
[4] Johnston, A., Donovan, S., Sparks, R., Cunningham, C. and K.
Summers, "Session Initiation Protocol (SIP) Public Switched
Telephone Network (PSTN) Call Flows", BCP 76, RFC 3666, December
2003.
[5] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[6] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
Responses in the Session Initiation Protocol (SIP)", RFC 3262,
June 2002.
[7] Rosenberg, J., Schulzrinne, H., Mahy, R., "An INVITE-Initiated
Dialog Event Package for the Session Initiation Protocol (SIP)",
RFC 4235, November 2005.
[8] Sparks, R., "Multiple Dialog Usages in the Session Initiation
Protocol", draft-ietf-sipping-dialogusage-06 (work in progress),
Febrary 20, 2007.
Author's Addresses
All listed authors actively contributed large amounts of text to this
document.
Miki Hasebe
NTT-east Corporation
19-2 Nishi-shinjuku 3-chome Shinjuku-ku Tokyo 163-8019 Japan
EMail: hasebe.miki@east.ntt.co.jp
Jun Koshiko
NTT-east Corporation
19-2 Nishi-shinjuku 3-chome Shinjuku-ku Tokyo 163-8019 Japan
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EMail: j.koshiko@east.ntt.co.jp
Yasushi Suzuki
NTT-east Corporation
19-2 Nishi-shinjuku 3-chome Shinjuku-ku Tokyo 163-8019 Japan
EMail: suzuki.yasushi@east.ntt.co.jp
Tomoyuki Yoshikawa
NTT-east Corporation
19-2 Nishi-shinjuku 3-chome Shinjuku-ku Tokyo 163-8019 Japan
EMail: tomoyuki.yoshikawa@east.ntt.co.jp
Paul H. Kyzivat
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
USA
Email: pkyzivat@cisco.com
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Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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Acknowledgment
The authors would like to thank Robert Sparks, Dean Willis,
Cullen Jennings, James M. Polk, Gonzalo Camarillo, Kenichi Ogami,
Akihiro Shimizu, Mayumi Munakata, Yasunori Inagaki,
Tadaatsu Kidokoro, Kenichi Hiragi and Dale Worley, for their comments
on this document.
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