draft-ietf-sipping-race-examples-02.txt   draft-ietf-sipping-race-examples-03.txt 
Internet Engineering Task Force M. HASEBE sipping M. Hasebe
Internet-Draft J. KOSHIKO Internet-Draft J. Koshiko
Intended status: Best Current Practice Y. SUZUKI Intended status: Best Current Y. Suzuki
Expires: Dec 30, 2007 T. YOSHIKAWA Practice T. Yoshikawa
NTT-East Expires: February 11, 2008 NTT-east Corporation
P. Kyzivat P. Kyzivat
Cisco Systems, Inc. Cisco Systems, Inc.
Jun 30th, 2007 August 10, 2007
Examples call flow in race condition on Session Initiation Protocol Examples call flow in race condition on Session Initiation Protocol
draft-ietf-sipping-race-examples-02.txt draft-ietf-sipping-race-examples-03
Status of this Memo Status of this Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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skipping to change at page 1, line 38 skipping to change at page 1, line 38
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This Internet-Draft will expire on December 30, 2007. This Internet-Draft will expire on February 11, 2008.
Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
Abstract Abstract
This document gives examples of the Session Initiation Protocol (SIP) This document gives examples of the Session Initiation Protocol (SIP)
call flows in race condition. Call flows in race condition are call flows in race condition. Call flows in race condition are
confusing, and this document shows the best practice to handle them. confusing, and this document shows the best practice to handle them.
The elements in these call flows include SIP User Agents and SIP The elements in these call flows include SIP User Agents and SIP
Proxies. Call flow diagrams and message details are shown. Proxies. Call flow diagrams and message details are shown.
Table of Contents Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 General Assumptions. . . . . . . . . . . . . . . . . . . . .3 1.1. General Assumptions . . . . . . . . . . . . . . . . . . . 3
1.2 Legend for Message Flows . . . . . . . . . . . . . . . . . .3 1.2. Legend for Message Flows . . . . . . . . . . . . . . . . . 3
1.3 SIP Protocol Assumptions . . . . . . . . . . . . . . . . . .3 1.3. SIP Protocol Assumptions . . . . . . . . . . . . . . . . . 4
2. The Dialog State Machine for INVITE dialog usage . . . . . . . .4 2. The Dialog State Machine for INVITE dialog usage . . . . . . . 4
3. Race Conditions. . . . . . . . . . . . . . . . . . . . . . . . .9 3. Race Conditions . . . . . . . . . . . . . . . . . . . . . . . 10
3.1 Receiving message in the Moratorium State. . . . . . . . . .9 3.1. Receiving message in the Moratorium State . . . . . . . . 11
3.1.1 Receiving Initial INVITE retransmission. . . . . . . . .10 3.1.1. Receiving Initial INVITE retransmission
3.1.2 Receiving CANCEL(Early state). . . . . . . . . . . . . .11 (Preparative state) in Moratorium state . . . . . . . 11
3.1.3 Receiving BYE (Early state). . . . . . . . . . . . . . .13 3.1.2. Receiving CANCEL (Early state) in Moratorium state . . 13
3.1.4 Receiving re-INVITE (Established state)(case 1). . . . .15 3.1.3. Receiving BYE (Early state) in Moratorium state . . . 15
3.1.5 Receiving re-INVITE (Established state)(case 2). . . . .19 3.1.4. Receiving re-INVITE (Established state) in
3.1.6 Receiving BYE (Established state). . . . . . . . . . . .22 Moratorium state (case 1) . . . . . . . . . . . . . . 17
3.2 Receiving message in the Mortal State. . . . . . . . . . . .24 3.1.5. Receiving re-INVITE (Established state) in
3.2.1 Receiving BYE(Established state) . . . . . . . . . . . .24 Moratorium state (case 2) . . . . . . . . . . . . . . 22
3.2.2 Receiving re-INVITE(Established state) . . . . . . . . .26 3.1.6. Receiving BYE (Established state) in Moratorium
3.2.3 Receiving 200 OK for re-INVITE(Established state). . . .28 state . . . . . . . . . . . . . . . . . . . . . . . . 26
3.2.4 Receiving ACK (Moratorium state) . . . . . . . . . . . .31 3.2. Receiving message in the Mortal State . . . . . . . . . . 28
3.3 Other race conditions. . . . . . . . . . . . . . . . . . . .32 3.2.1. Receiving BYE (Establish state) in Mortal state . . . 28
3.3.1 Re-INVITE crossover. . . . . . . . . . . . . . . . . . .32 3.2.2. Receiving re-INVITE (Establish state) in Mortal
3.3.2 UPDATE and re-INVITE crossover . . . . . . . . . . . . .36 state . . . . . . . . . . . . . . . . . . . . . . . . 31
3.3.3 Receiving REFER(Established state) . . . . . . . . . . .40 3.2.3. Receiving 200 OK for re-INVITE (Established state)
4. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . .42 in Mortal state . . . . . . . . . . . . . . . . . . . 33
5. Security Considerations. . . . . . . . . . . . . . . . . . . . .42 3.2.4. Receiving ACK (Moratorium state) in Mortal state . . . 36
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . .42 3.3. Other race conditions . . . . . . . . . . . . . . . . . . 37
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . .42 3.3.1. Re-INVITE crossover . . . . . . . . . . . . . . . . . 37
7.1 Normative References . . . . . . . . . . . . . . . . . . . .42 3.3.2. UPDATE and re-INVITE crossover . . . . . . . . . . . . 43
7.2 Informative References . . . . . . . . . . . . . . . . . . .42 3.3.3. Receiving REFER (Establish state) in Mortal state . . 47
Appendix A. BYE on the Early Dialog . . . . . . . . . . . . . . . .43 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48
Appendix B. BYE request overlapped on re-INVITE . . . . . . . . . .44 5. Security Considerations . . . . . . . . . . . . . . . . . . . 49
Appendix C. UA's behavior for CANCEL. . . . . . . . . . . . . . . .46 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 49
Appendix D. Notes on the request in Mortal state. . . . . . . . . .48 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 49
Appendix E. Forking and receiving new To tags . . . . . . . . . . .49 7.1. Normative References . . . . . . . . . . . . . . . . . . . 49
Author's Addresses. . . . . . . . . . . . . . . . . . . . . . . . .53 7.2. Informative References . . . . . . . . . . . . . . . . . . 49
Intellectual Property and Copyright Statements. . . . . . . . . . .54 Appendix A. BYE on the Early Dialog . . . . . . . . . . . . . . . 49
Appendix B. BYE request overlapped on re-INVITE . . . . . . . . . 51
Appendix C. UA's behavior for CANCEL . . . . . . . . . . . . . . 53
Appendix D. Notes on the request in Mortal state . . . . . . . . 55
Appendix E. Forking and receiving new To tags . . . . . . . . . . 56
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 60
Intellectual Property and Copyright Statements . . . . . . . . . . 62
1. Overview 1. Overview
The call flows shown in this document were developed in the design of The call flows shown in this document were developed in the design of
a SIP IP communications network. These examples are of race a SIP IP communications network. These examples are of race
condition, which stems from the dialog state transition mainly condition, which stems from the dialog state transition mainly
established by INVITE. established by INVITE.
When implementing SIP, various complex situations may arise. When implementing SIP, various complex situations may arise.
Therefore, it will be helpful to provide implementors of the protocol Therefore, it will be helpful to provide implementors of the protocol
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SIP implementors, designers, and protocol researchers and will help SIP implementors, designers, and protocol researchers and will help
them achieve the goal of a standard implementation of RFC 3261 [1]. 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 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]. 3261 [1] with SDP usage described in RFC 3264 [2].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 [3]. document are to be interpreted as described in BCP 14, RFC 2119 [3].
1.1 General Assumptions 1.1. General Assumptions
A number of architecture, network, and protocol assumptions underlie A number of architecture, network, and protocol assumptions underlie
the call flows in this document. Note that these assumptions are not the call flows in this document. Note that these assumptions are not
requirements. They are outlined in this section so that they may be requirements. They are outlined in this section so that they may be
taken into consideration and help understanding the call flow taken into consideration and help understanding the call flow
examples. examples.
These flows do not assume specific underlying transport protocols These flows do not assume specific underlying transport protocols
such as TCP, TLS, and UDP. See the discussion in RFC 3261 [1] for such as TCP, TLS, and UDP. See the discussion in RFC 3261 [1] for
details on the transport issues for SIP. details on the transport issues for SIP.
1.2 Legend for Message Flows 1.2. Legend for Message Flows
Dashed lines (---) and slash lines (/, \) represent signaling Dashed lines (---) and slash lines (/, \) represent signaling
messages that are mandatory to the call scenario. (X) represents messages that are mandatory to the call scenario. (X) represents
crossover of signaling messages. (->x, x<-) indicate that the packet crossover of signaling messages. (->x, x<-) indicate that the packet
is lost. The arrow indicates the direction of message flow. Double is lost. The arrow indicates the direction of message flow. Double
dashed lines (===) represent media paths between network elements. dashed lines (===) represent media paths between network elements.
Messages are identified in the figures as F1, F2, etc. These numbers Messages are identified in the figures as F1, F2, etc. These numbers
are used for references to the message details that follow the are used for references to the message details that follow the
Figure. Figure. Comments in the message details are shown in the following
Comments in the message details are shown in the following form: form:
/* Comments. */ /* Comments. */
1.3 SIP Protocol Assumptions 1.3. SIP Protocol Assumptions
This document does not prescribe the flows precisely as they are This document does not prescribe the flows precisely as they are
shown, but rather illustrates the principles for best practice. shown, but rather illustrates the principles for best practice. They
are best practice usages (orderings, syntax, selection of features
They are best practice usages (orderings, syntax, selection of for the purpose, or handling of error) of SIP methods, headers and
features for the purpose, or handling of error) of SIP methods, parameters. Note: The flows in this document must not be copied as
headers and parameters. Note: The flows in this document must not they are by implementors because additional characteristics were
be copied as they are by implementors because additional incorporated into the document for ease of explanation. To sum up,
characteristics were incorporated into the document for ease of the procedures described in this document represent well-reviewed
explanation. To sum up, the procedures described in this document examples of SIP usage, which are best common practice according to
represent well-reviewed examples of SIP usage, which are best common IETF consensus.
practice according to IETF consensus.
For simplicity in reading and editing the document, there are a For simplicity in reading and editing the document, there are a
number of differences between some of the examples and actual SIP number of differences between some of the examples and actual SIP
messages. For instance, Call-IDs are often repeated, CSeq often messages. For instance, Call-IDs are often repeated, CSeq often
begins at 1, header fields are usually shown in the same order, begins at 1, header fields are usually shown in the same order,
usually only the minimum required header field set is shown, and usually only the minimum required header field set is shown, and
other headers which would usually be included such as Accept, Allow, other headers which would usually be included such as Accept, Allow,
etc. are not shown. etc. are not shown.
Actors: Actors:
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Proxy Server ss.atlanta.example.com 192.0.2.111 Proxy Server ss.atlanta.example.com 192.0.2.111
2. The Dialog State Machine for INVITE dialog usage 2. The Dialog State Machine for INVITE dialog usage
Race conditions are generated when the dialog state of the receiving Race conditions are generated when the dialog state of the receiving
side differs from that of the sending side. side differs from that of the sending side.
For instance, a race condition occurs when UAC (User Agent Client) For instance, a race condition occurs when UAC (User Agent Client)
sends a CANCEL in the Early state while UAS (User Agent Server) is 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 transiting from the Early state to the Confirmed state by sending a
200 OK to initial INVITE (indicated as "ini-INVITE" hereafter). 200 OK to initial INVITE (indicated as "ini-INVITE" hereafter). The
The DSM (dialog state machine) for the INVITE dialog usage is DSM (dialog state machine) for the INVITE dialog usage is presented
presented as follows to help understanding UA's behavior in race as follows to help understanding UA's behavior in race conditions.
conditions.
The DSM clarifies UA's behavior by subdividing the dialog state shown The DSM clarifies UA's behavior by subdividing the dialog state shown
in RFC 3261 [1] into some internal states. We call the state before in RFC 3261 [1] into some internal states. We call the state before
a dialog establishment the Preparative state. The Confirmed state is a dialog establishment the Preparative state. The Confirmed state is
subdivided into two substates, the Moratorium and Established states, subdivided into two substates, the Moratorium and Established states,
and the Terminated state is subdivided into the Mortal and Morgue and the Terminated state is subdivided into the Mortal and Morgue
states. Messages which are the triggers of the state transitions states. Messages which are the triggers of the state transitions
between these states are indicated with arrows. In this figure, between these states are indicated with arrows. In this figure,
messages which are not related to state transition are omitted. messages which are not related to state transition are omitted.
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| +---------------+ | | +-----------+ | | +---------------+ | | +-----------+ |
| | | | | | | |
+------------------------+ +------------------+ +------------------------+ +------------------+
(sr): indicates that both sending and reception are allowed. (sr): indicates that both sending and reception are allowed.
Where (sr) is not indicated, response means send, Where (sr) is not indicated, response means send,
request means receive. request means receive.
Figure 2. DSM for INVITE dialog usage (Callee) Figure 2. DSM for INVITE dialog usage (Callee)
Figure 2 represents callee's DSM for the INVITE dialog usage. Figure 2 represents callee's DSM for the INVITE dialog usage. The
The figure does not illustrate the state transition related to figure does not illustrate the state transition related to CANCEL
CANCEL request. CANCEL request does not cause a dialog state request. CANCEL request does not cause a dialog state transition.
transition. However, the callee terminates the dialog and triggers However, the callee terminates the dialog and triggers the dialog
the dialog transition by sending 487 immediately after the reception transition by sending 487 immediately after the reception of the
of the CANCEL. Considering this, the behavior upon the reception of CANCEL. Considering this, the behavior upon the reception of the
the CANCEL request is further explained in Appendix C. CANCEL request is further explained in Appendix C.
Following are UA's behaviors in each state. Following are UA's behaviors in each state.
Preparative (Pre): The Preparative state is a state until the Preparative (Pre): The Preparative state is a state until the early
early dialog is established by sending or receiving a dialog is established by sending or receiving a provisional
provisional response with To tag after an ini-INVITE is sent or response with To tag after an ini-INVITE is sent or received. The
received. The dialog has not existed yet in the Preparative dialog has not existed yet in the Preparative state. If UA sends
state. If UA sends or receives a 2xx response, the dialog or receives a 2xx response, the dialog state transit from the
state transit from the Preparative to the Moratorium state Preparative to the Moratorium state which is a substate of the
which is a substate of the Confirmed state. Confirmed state. In addition, if UA sends or receives a 3xx-6xx
In addition, if UA sends or receives a 3xx-6xx response the response the dialog state transit to the Morgue state which is a
dialog state transit to the Morgue state which is a substate of substate of the Terminated state. Sending an ACK for a 3xx-6xx
the Terminated state. Sending an ACK for a 3xx-6xx response response and retransmissions of 3xx-6xx are not expressed on the
and retransmissions of 3xx-6xx are not expressed on the DSMs DSMs because they are sent by the INVITE transaction.
because they are sent by the INVITE transaction.
Early (Ear): The early dialog is established by sending or Early (Ear): The early dialog is established by sending or receiving
receiving a provisional response with To tag. The early dialog a provisional response with To tag. The early dialog exists
exists though the dialog does not exist in this state yet. though the dialog does not exist in this state yet. The dialog
The dialog state transits from the Early to Moratorium state, a state transits from the Early to Moratorium state, a substate of
substate of the Confirmed state, by sending or receiving a 2xx the Confirmed state, by sending or receiving a 2xx response. In
response. In addition, the dialog state transits to the Morgue addition, the dialog state transits to the Morgue state, a
state, a substate of the Terminated state, by sending or substate of the Terminated state, by sending or receiving a 3xx-
receiving a 3xx-6xx response. Sending an ACK for a 3xx-6xx 6xx response. Sending an ACK for a 3xx-6xx response and
response and retransmissions of 3xx-6xx are not expressed on retransmissions of 3xx-6xx are not expressed on this DSM because
this DSM because they are automatically processed on they are automatically processed on transaction layer and don't
transaction layer and don't influence the dialog state. UAC influence the dialog state. UAC may send CANCEL in the Early
may send CANCEL in the Early state. UAC may send BYE state. UAC may send BYE (although it is not recommended). UAS
(although it is not recommended). UAS may send a 1xx-6xx may send a 1xx-6xx response. Sending or receiving of a CANCEL
response. Sending or receiving of a CANCEL request does not request does not have direct influences on dialog state. The UA's
have direct influences on dialog state. The UA's behavior upon behavior upon the reception of the CANCEL request is further
the reception of the CANCEL request is further explained in explained in Appendix C.
Appendix C.
Confirmed (Con): Sending or receiving of a 2xx final response Confirmed (Con): Sending or receiving of a 2xx final response
establishes a dialog. Dialog exists in this state. The establishes a dialog. Dialog exists in this state. The Confirmed
Confirmed state transits to the Mortal state, a substate of the state transits to the Mortal state, a substate of the Terminated
Terminated state, by sending or receiving a BYE request. The state, by sending or receiving a BYE request. The Confirmed state
Confirmed state has two substates, the Moratorium and has two substates, the Moratorium and Established state, which are
Established state, which are different in messages UAs are different in messages UAs are allowed to send.
allowed to send.
Moratorium (Mora): The Moratorium state is a substate of the Moratorium (Mora): The Moratorium state is a substate of the
Confirmed state and inherits the behavior of the superstate. Confirmed state and inherits the behavior of the superstate. The
The Moratorium state transits to the Established state by Moratorium state transits to the Established state by sending or
sending or receiving an ACK request. UAC may send ACK and UAS receiving an ACK request. UAC may send ACK and UAS may send a 2xx
may send a 2xx final response. final response.
Established (Est): The Established state is a substate of the Established (Est): The Established state is a substate of the
Confirmed state and inherits the behavior of superstate. Both Confirmed state and inherits the behavior of superstate. Both
caller and callee may send various messages which influence a caller and callee may send various messages which influence a
dialog. Caller supports the transmission of ACK in response to dialog. Caller supports the transmission of ACK in response to
retransmission of a 2xx response to an ini-INVITE. retransmission of a 2xx response to an ini-INVITE.
Terminated (Ter): The Terminated state is divided into two Terminated (Ter): The Terminated state is divided into two
substates, the Mortal and Morgue states, to cover the behavior substates, the Mortal and Morgue states, to cover the behavior
when a dialog is being terminated. In this state, UAs hold when a dialog is being terminated. In this state, UAs hold
information about the dialog which is being terminated. information about the dialog which is being terminated.
Mortal (Mort): Caller and callee enter the Mortal state by sending Mortal (Mort): Caller and callee enter the Mortal state by sending
or receiving a BYE. UA MUST NOT send any new requests within or receiving a BYE. UA MUST NOT send any new requests within the
the dialog because there is no dialog. (Here the new requests dialog because there is no dialog. (Here the new requests do not
do not include ACK for 2xx and BYE for 401 or 407 as further include ACK for 2xx and BYE for 401 or 407 as further explained in
explained in Appendix D below.) Appendix D below.) In this state, only BYE or its response can be
In this state, only BYE or its response can be handled, and no handled, and no other messages can be received. This addresses
other messages can be received. This addresses the case where the case where BYE is sent by both a caller and a callee to
BYE is sent by both a caller and a callee to exchange reports exchange reports about the session when it is being terminated.
about the session when it is being terminated. Therefore the Therefore the UA possesses dialog information for internal
UA possesses dialog information for internal processing but the processing but the dialog shouldn't be externally visible. The UA
dialog shouldn't be externally visible. The UA stops managing stops managing its dialog state and changes it to the Morgue
its dialog state and changes it to the Morgue state, when the state, when the BYE transaction is terminated.
BYE transaction is terminated.
Morgue (Morg): The dialog no longer exists in this state. Morgue (Morg): The dialog no longer exists in this state. Sending
Sending or receiving of signaling which influences a dialog is or receiving of signaling which influences a dialog is not
not performed. (A dialog is literally terminated.) performed. (A dialog is literally terminated.) Caller and callee
Caller and callee enter the Morgue state via the termination of enter the Morgue state via the termination of the BYE or INVITE
the BYE or INVITE transaction. transaction.
3. Race conditions 3. Race Conditions
This section details race condition between two SIP UAs, Alice and This section details race condition between two SIP UAs, Alice and
Bob. Alice (sip:alice@atlanta.example.com) and Bob Bob. Alice (sip:alice@atlanta.example.com) and Bob
(sip:bob@biloxi.example.com) are assumed to be SIP phones or (sip:bob@biloxi.example.com) are assumed to be SIP phones or SIP-
SIP-enabled devices. Only significant signaling is illustrated. enabled devices. Only significant signaling is illustrated. Dialog
Dialog state transitions caused by sending or receiving of SIP state transitions caused by sending or receiving of SIP messages as
messages as well as '*race*', which indicates race condition, are well as '*race*', which indicates race condition, are shown. (For
shown. (For abbreviations for the dialog state transitions, refer to abbreviations for the dialog state transitions, refer to Section 2.)
Section 2.)
'*race*' indicates the moment when a race condition occurs. '*race*' indicates the moment when a race condition occurs.
Examples of race conditions are shown below. Examples of race conditions are shown below.
3.1 Receiving message in the Moratorium State 3.1. Receiving message in the Moratorium State
This section shows some examples of call flow in race condition when This section shows some examples of call flow in race condition when
receiving the message from other states in the Moratorium state. receiving the message from other states in the Moratorium state.
3.1.1 Receiving Initial INVITE retransmission (Preparative state) 3.1.1. Receiving Initial INVITE retransmission (Preparative state) in
in Moratorium state Moratorium state
State Alice Bob State State Alice Bob State
| | | |
| ini-INVITE F1 | | ini-INVITE F1 |
|------------------------------------>| |------------------------------------>|
Pre | 180 F2(Packet loss) | Pre Pre | 180 F2(Packet loss) | Pre
| x<-----------------------| | x<-----------------------|
| | Ear | | Ear
| ini-INVITE F4(=F1) 200 F3 | | ini-INVITE F4(=F1) 200 F3 |
|------------------ --------------| |------------------ --------------|
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Est | | Est Est | | Est
| | | |
This scenario illustrates the race condition which occurs when UAS This scenario illustrates the race condition which occurs when UAS
receives a Preparative message in the Moratorium state. All receives a Preparative message in the Moratorium state. All
provisional responses to the initial INVITE (ini-INVITE F1) are lost, provisional responses to the initial INVITE (ini-INVITE F1) are lost,
and UAC retransmits an ini-INVITE (F4). At the same time as 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 retransmission, UAS generates a 200 OK (F3) to the ini-INVITE and it
terminates an INVITE server transaction, according to Section terminates an INVITE server transaction, according to Section
13.3.1.4 of RFC 3261 [1]. 13.3.1.4 of RFC 3261 [1].
However, it is reported that terminating an INVITE server transaction However, it is reported that terminating an INVITE server transaction
by 200 OK is a SIP bug. (http://bugs.sipit.net/, #769) by 200 OK is a SIP bug. (http://bugs.sipit.net, #769) Therefore, the
Therefore, the INVITE server transaction is not terminated at F3, and INVITE server transaction is not terminated at F3, and the F4 MUST be
the F4 MUST be properly handled as a retransmission. properly handled as a retransmission. (UAs that do not deal with
(UAs that do not deal with this bug still need to recognize the this bug still need to recognize the dialog relying on its From tag
dialog relying on its From tag and Call-ID, and the retransmitted and Call-ID, and the retransmitted request relying on the CSeq header
request relying on the CSeq header field value even though it does field value even though it does not match the transaction.)
not match the transaction.)
In RFC 3261 [1], it is not specified whether UAS retransmits 200 to In RFC 3261 [1], it is not specified whether UAS retransmits 200 to
the retransmission of ini-INVITE. Considering the retransmission of the retransmission of ini-INVITE. Considering the retransmission of
200 triggered by timer (TU keeps retransmitting 200 based on T1 and 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 RFC T2 until it receives an ACK), according to Section 13.3.1.4 of RFC
3261 [1], it seems unnecessary to retransmit 200 when the UAS 3261 [1], it seems unnecessary to retransmit 200 when the UAS
receives the retransmission of ini-INVITE. (For implementation, it receives the retransmission of ini-INVITE. (For implementation, it
does not matter if the UAS sends the retransmission of 200, since the does not matter if the UAS sends the retransmission of 200, since the
200 does not cause any problem.) 200 does not cause any problem.)
Message Details Message Details
F1 INVITE Alice -> Bob F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice F2 180 Ringing Bob -> Alice
/* 180 response is lost and does not reach Alice. */ /* 180 response is lost and does not reach Alice. */
F3 200 OK Bob -> Alice F3 200 OK Bob -> Alice
skipping to change at page 11, line 14 skipping to change at page 12, line 17
Message Details Message Details
F1 INVITE Alice -> Bob F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice F2 180 Ringing Bob -> Alice
/* 180 response is lost and does not reach Alice. */ /* 180 response is lost and does not reach Alice. */
F3 200 OK Bob -> Alice F3 200 OK Bob -> Alice
/* According to 13.3.1.4 of RFC 3261, an INVITE server transaction is /* According to Section 13.3.1.4 of RFC 3261 [1], an INVITE server
terminated at this point. However, this has been reported as a transaction is terminated at this point. However, this has been
SIP bug, and the UAS MUST correctly recognize the ini-INVITE (F4) reported as a SIP bug, and the UAS MUST correctly recognize the
as a retransmission. */ ini-INVITE (F4) as a retransmission. */
F4 INVITE (retransmission) Alice -> Bob F4 INVITE (retransmission) Alice -> Bob
/* F4 is a retransmission of F1. They are exactly the same INVITE /* 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 request. For UAs do not deal with the bug reported in #769 (an
INVITE server transaction is terminated by 200 to INVITE), this INVITE server transaction is terminated by 200 to INVITE), this
request does not match the transaction as well as the dialog request does not match the transaction as well as the dialog since
since it does not have a To tag. it does not have a To tag. However, Bob have to recognize the
However, Bob have to recognize the retransmitted INVITE correctly, retransmitted INVITE correctly, without treating it as a new
without treating it as a new INVITE. */ INVITE. */
F5 ACK Alice -> Bob F5 ACK Alice -> Bob
3.1.2 Receiving CANCEL (Early state) 3.1.2. Receiving CANCEL (Early state) in Moratorium state
in Moratorium state
State Alice Bob State State Alice Bob State
| | | |
| INVITE F1 | | INVITE F1 |
|----------------------------->| |----------------------------->|
Pre | 180 Ringing F2 | Pre Pre | 180 Ringing F2 | Pre
|<-----------------------------| |<-----------------------------|
Ear | | Ear Ear | | Ear
|CANCEL F3 200(INVITE) F4| |CANCEL F3 200(INVITE) F4|
|------------ -------------| |------------ -------------|
skipping to change at page 12, line 25 skipping to change at page 13, line 47
| V | | | V | |
Morg | Timer J | | Morg | Timer J | |
| V | | V |
| | Morg | | Morg
| | | |
This scenario illustrates the race condition which occurs when UAS This scenario illustrates the race condition which occurs when UAS
receives an Early message, CANCEL, in the Moratorium state. Alice receives an Early message, CANCEL, in the Moratorium state. Alice
sends a CANCEL and Bob sends a 200 OK response to the initial INVITE 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, message at the same time. As described in the previous section,
according to RFC 3261, an INVITE server transaction is supposed to be according to RFC 3261 [1], an INVITE server transaction is supposed
terminated by a 200 response, but this has been reported as a bug to be terminated by a 200 response, but this has been reported as a
#769. bug #769.
This section describes a case in which an INVITE server transaction This section describes a case in which an INVITE server transaction
is not terminated by a 200 response to the INVITE request. In this is not terminated by a 200 response to the INVITE request. In this
case, there is an INVITE transaction which the CANCEL request case, there is an INVITE transaction which the CANCEL request
matches, so a 200 response is sent to the request. This 200 response matches, so a 200 response is sent to the request. This 200 response
simply means that the next hop received the CANCEL request simply means that the next hop received the CANCEL request
(Successful CANCEL (200) does not mean an INVITE failure). When UAS (Successful CANCEL (200) does not mean an INVITE failure). When UAS
does not deal with #769, UAC MAY receive a 481 response for CANCEL does not deal with #769, UAC MAY receive a 481 response for CANCEL
since there is no transaction which the CANCEL request matches. This since there is no transaction which the CANCEL request matches. This
481 simply means that there is no matching INVITE server transaction 481 simply means that there is no matching INVITE server transaction
and CANCEL is not sent to the next hop. and CANCEL is not sent to the next hop. Regardless of the success/
Regardless of the success/failure of the CANCEL, Alice checks the failure of the CANCEL, Alice checks the final response to INVITE, and
final response to INVITE, and if she receives 200 to the INVITE if she receives 200 to the INVITE request she immediately sends a BYE
request she immediately sends a BYE and terminates the dialog. and terminates the dialog. (Section 15, RFC 3261 [1])
(Section 15, RFC 3261 [1])
From the time F1 is received by Bob until the time that F8 is sent by 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 Bob, media may be flowing one way from Bob to Alice. From the time
that an answer is received by Alice from Bob there is the possibility that 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 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, has decided to cancel the call, she presumably will not send media,
so practically speaking the media stream will remain one way. so practically speaking the media stream will remain one way.
Message Details Message Details
skipping to change at page 13, line 19 skipping to change at page 14, line 37
F2 180 Ringing Bob -> Alice F2 180 Ringing Bob -> Alice
F3 CANCEL Alice -> Bob F3 CANCEL Alice -> Bob
/* Alice sends a CANCEL in the Early state. */ /* Alice sends a CANCEL in the Early state. */
F4 200 OK (INVITE) Bob -> Alice F4 200 OK (INVITE) Bob -> Alice
/* Alice receives a 200 to INVITE (F1) in the Moratorium state. /* Alice receives a 200 to INVITE (F1) in the Moratorium state.
Alice has the potential to send as well as receive media, but in 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 practice will not send because there is an intent to end the call.
call. */ */
F5 200 OK (CANCEL) Bob -> Alice F5 200 OK (CANCEL) Bob -> Alice
/* 200 to CANCEL simply means that the CANCEL was received. /* 200 to CANCEL simply means that the CANCEL was received. The 200
The 200 response is sent, since this document deals with the bug response is sent, since this document deals with the bug reported
reported in #769. When an INVITE server transaction is terminated in #769. When an INVITE server transaction is terminated as the
as the procedure stated in RFC 3261, UAC MAY receive 481 response procedure stated in RFC 3261 [1], UAC MAY receive 481 response
instead of 200. */ instead of 200. */
F6 ACK Alice -> Bob F6 ACK Alice -> Bob
/* INVITE is successful, and the CANCEL becomes invalid. Bob /* INVITE is successful, and the CANCEL becomes invalid. Bob
establishes RTP streams. establishes RTP streams. However, the next BYE request
However, the next BYE request immediately terminates the dialog immediately terminates the dialog and session. */
and session. */
F7 BYE Alice -> Bob F7 BYE Alice -> Bob
F8 200 OK Bob -> Alice F8 200 OK Bob -> Alice
3.1.3 Receiving BYE (Early state) 3.1.3. Receiving BYE (Early state) in Moratorium state
in Moratorium state
State Alice Bob State State Alice Bob State
| | | |
| ini-INVITE F1 | | ini-INVITE F1 |
|------------------------------->| |------------------------------->|
Pre | 180 F2 | Pre Pre | 180 F2 | Pre
|<-------------------------------| |<-------------------------------|
Ear | | Ear Ear | | Ear
| BYE F4 200(INVITE) F3| | BYE F4 200(INVITE) F3|
|------------- --------------| |------------- --------------|
skipping to change at page 14, line 38 skipping to change at page 16, line 15
RECOMMENDED). When a proxy is performing forking, the BYE is only RECOMMENDED). When a proxy is performing forking, the BYE is only
able to terminate the early dialog with a particular UA. If caller able to terminate the early dialog with a particular UA. If caller
wants to terminate all early dialogs instead of that with a wants to terminate all early dialogs instead of that with a
particular UA, it needs to send CANCEL, not BYE. However, it is not particular UA, it needs to send CANCEL, not BYE. However, it is not
illegal to send BYE in the Early state to terminate a specific early illegal to send BYE in the Early state to terminate a specific early
dialog according to the caller's intent. dialog according to the caller's intent.
The BYE functions normally even if it is received after the INVITE The BYE functions normally even if it is received after the INVITE
transaction termination because BYE differs from CANCEL, and is sent transaction termination because BYE differs from CANCEL, and is sent
not to the request but to the dialog. Alice gets into the Mortal not to the request but to the dialog. Alice gets into the Mortal
state on sending the BYE request, and remains Mortal until the state on sending the BYE request, and remains Mortal until Timer K
Timer K timeout occurs. In the Mortal state, UAC does not establish timeout occurs. In the Mortal state, UAC does not establish a
a session, even though it receives a 200 response to INVITE. Even session, even though it receives a 200 response to INVITE. Even so,
so, the UAC sends an ACK to 200 for the completion of INVITE the UAC sends an ACK to 200 for the completion of INVITE transaction.
transaction. The ACK is always sent to complete the three-way The ACK is always sent to complete the three-way handshake of INVITE
handshake of INVITE transaction (Further explained in Appendix D transaction (Further explained in Appendix D below).
below).
Message Details Message Details
F1 INVITE Alice -> Bob F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice F2 180 Ringing Bob -> Alice
F3 200 OK (ini-INVITE) Bob -> Alice F3 200 OK (ini-INVITE) Bob -> Alice
F4 BYE Alice -> Bob F4 BYE Alice -> Bob
/* Alice transits to the Mortal state upon sending BYE. /* Alice transits to the Mortal state upon sending BYE. Therefore,
Therefore, after this, she does not begin a session even though after this, she does not begin a session even though she receives
she receives a 200 response with an answer. */ a 200 response with an answer. */
F5 ACK Alice -> Bob F5 ACK Alice -> Bob
F6 200 OK (BYE) Bob -> Alice F6 200 OK (BYE) Bob -> Alice
3.1.4 Receiving re-INVITE (Established state) 3.1.4. Receiving re-INVITE (Established state) in Moratorium state
in Moratorium state (case 1) (case 1)
State Alice Bob State State Alice Bob State
| | | |
| ini-INVITE w/offer1 F1 | | ini-INVITE w/offer1 F1 |
|------------------------------->| |------------------------------->|
Pre | 180 F2 | Pre Pre | 180 F2 | Pre
|<-------------------------------| |<-------------------------------|
Ear | | Ear Ear | | Ear
| 200(ini-INV) w/answer1 F3 | | 200(ini-INV) w/answer1 F3 |
|<-------------------------------| |<-------------------------------|
skipping to change at page 16, line 7 skipping to change at page 17, line 43
|<------------ ------------->| |<------------ ------------->|
| ACK (re-INV) F9 | Est | ACK (re-INV) F9 | Est
|------------------------------->| |------------------------------->|
| | | |
| | | |
This scenario illustrates the race condition which occurs when UAS This scenario illustrates the race condition which occurs when UAS
receives re-INVITE request sent from the Established state, in the receives re-INVITE request sent from the Established state, in the
Moratorium state. Moratorium state.
UAS receives a re-INVITE (w/offer2) before receiving an ACK for UAS receives a re-INVITE (w/offer2) before receiving an ACK for ini-
ini-INVITE (w/offer1). UAS sends a 200 OK (w/answer2) to the INVITE (w/offer1). UAS sends a 200 OK (w/answer2) to the re-INVITE
re-INVITE (F8) because it has sent a 200 OK (w/answer1) to the (F8) because it has sent a 200 OK (w/answer1) to the ini-INVITE (F3,
ini-INVITE (F3, F5) and the dialog has already been established. F5) and the dialog has already been established. (Because F5 is a
(Because F5 is a retransmission of F3, SDP negotiation is not retransmission of F3, SDP negotiation is not performed here.)
performed here.)
If a 200 OK to the ini-INVITE has an offer and the answer is in the If a 200 OK to the ini-INVITE has an offer and the answer is in the
ACK, it is recommended that UA return a 491 to the re-INVITE (refer ACK, it is recommended that UA return a 491 to the re-INVITE (refer
to 3.1.5). (Note: 500 with Retry-After header may be returned, if to Section 3.1.5). (Note: 500 with Retry-After header may be
the 491 response is understood to indicate request collision. returned, if the 491 response is understood to indicate request
However, 491 is recommended here because 500 applies to so many cases collision. However, 491 is recommended here because 500 applies to
that it is difficult to determine what the real problem was.) so many cases that it is difficult to determine what the real problem
As it can be seen in Section 3.3.2 below, the 491 response seems to was.) As it can be seen in Section 3.3.2 below, the 491 response
be closely related to session establishment, even in cases other than seems to be closely related to session establishment, even in cases
INVITE cross-over. This example recommends 200 be sent instead of other than INVITE cross-over. This example recommends 200 be sent
491 because it does not have influence on session. However, a 491 instead of 491 because it does not have influence on session.
response can also lead to the same outcome, so the either response However, a 491 response can also lead to the same outcome, so the
can be used. either response can be used.
Moreover, if UAS doesn't receive an ACK for a long time, it should Moreover, if UAS doesn't receive an ACK for a long time, it should
send a BYE and terminate the dialog. Note that ACK F7 has the same send a BYE and terminate the dialog. Note that ACK F7 has the same
CSeq number as ini-INVITE F1 (See Section 13.2.2.4 of RFC 3261). CSeq number as ini-INVITE F1 (See Section 13.2.2.4 of RFC 3261 [1]).
The UA should not reject or drop the ACK on grounds of CSeq number. The UA should not reject or drop the ACK on grounds of CSeq number.
Note: There is a hint for implementation to avoid the race conditions Note: There is a hint for implementation to avoid the race conditions
of this type. That is for the caller to delay sending re-INVITE F6 of this type. That is for the caller to delay sending re-INVITE F6
for some period of time (2 seconds, perhaps), from which the caller for some period of time (2 seconds, perhaps), from which the caller
is reasonably able to assume that its ACK has been received. is reasonably able to assume that its ACK has been received.
Implementors can decouple the actions of the user (e.g. pressing the Implementors can decouple the actions of the user (e.g. pressing the
hold button) from the actions of the protocol (the sending of hold button) from the actions of the protocol (the sending of re-
re-INVITE F6), so the UA can behave as such. In this case, it is INVITE F6), so that the UA can behave as such. In this case, it is
dependent on the implementor's choice how long it waits. In most dependent on the implementor's choice as to how long it waits. In
cases, such implementation may be useful to prevent a type of race most cases, such implementation may be useful to prevent a type of
condition shown in this section. race condition shown in this section.
Message Details Message Details
F1 INVITE Alice -> Bob F1 INVITE Alice -> Bob
INVITE sip:bob@biloxi.example.com SIP/2.0 INVITE sip:bob@biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70 Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com> To: Bob <sip:bob@biloxi.example.com>
skipping to change at page 19, line 28 skipping to change at page 22, line 5
ACK sip:sip:bob@client.biloxi.example.com SIP/2.0 ACK sip:sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK230f21 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK230f21
Max-Forwards: 70 Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356 To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 ACK CSeq: 2 ACK
Content-Length: 0 Content-Length: 0
3.1.5 Receiving re-INVITE (Established state) 3.1.5. Receiving re-INVITE (Established state) in Moratorium state
in Moratorium state (case 2) (case 2)
State Alice Bob State State Alice Bob State
| | | |
| ini-INVITE (no offer) F1 | | ini-INVITE (no offer) F1 |
|------------------------------->| |------------------------------->|
Pre | 180 F2 | Pre Pre | 180 F2 | Pre
|<-------------------------------| |<-------------------------------|
Ear | | Ear Ear | | Ear
| 200(ini-INV) w/offer1 F3 | | 200(ini-INV) w/offer1 F3 |
|<-------------------------------| |<-------------------------------|
skipping to change at page 20, line 23 skipping to change at page 22, line 48
This scenario is basically the same as that of Section 3.1.4, but 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 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 to the previous case, the offer in the 200 (F3) and the offer in the
re-INVITE (F6) collide with each other. re-INVITE (F6) collide with each other.
Bob sends 491 to re-INVITE (F6) since he is not able to properly Bob sends 491 to re-INVITE (F6) since he is not able to properly
handle a new request until he receives an answer. (Note: 500 with handle a new request until he receives an answer. (Note: 500 with
Retry-After header may be returned, if the 491 response is understood Retry-After header may be returned, if the 491 response is understood
to indicate request collision. However, 491 is recommended here to indicate request collision. However, 491 is recommended here
because 500 applies to so many cases that it is difficult to because 500 applies to so many cases that it is difficult to
determine what the real problem was.) determine what the real problem was.) Even if F6 is UPDATE with
Even if F6 is UPDATE with offer, it will reach the same result. offer, it will reach the same result.
Note: As noted in Section 3.1.4, it may be useful for the caller to Note: As noted in Section 3.1.4, it may be useful for the caller to
delay sending re-INVITE F6 for some period of time (2 seconds, delay sending re-INVITE F6 for some period of time (2 seconds,
perhaps), from which the caller is reasonably able to assume that its perhaps), from which the caller is reasonably able to assume that its
ACK has been received, to prevent a type of race condition shown in ACK has been received, to prevent a type of race condition shown in
this section. this section.
Message Details Message Details
F1 INVITE Alice -> Bob F1 INVITE Alice -> Bob
skipping to change at page 20, line 47 skipping to change at page 23, line 23
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70 Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com> To: Bob <sip:bob@biloxi.example.com>
Call-ID: 3848276298220188511@atlanta.example.com Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE CSeq: 1 INVITE
Contact: <sip:alice@client.atlanta.example.com;transport=udp> Contact: <sip:alice@client.atlanta.example.com;transport=udp>
Content-Length: 0 Content-Length: 0
/* The request does not contain an offer. Detailed messages are /* The request does not contain an offer. Detailed messages are
shown for the sequence to illustrate offer and answer shown for the sequence to illustrate offer and answer examples.
examples. */ */
F2 180 Ringing Bob -> Alice F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice F3 200 OK Bob -> Alice
SIP/2.0 200 OK SIP/2.0 200 OK
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101 ;received=192.0.2.101
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356 To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE CSeq: 1 INVITE
Contact: <sip:bob@client.biloxi.example.com;transport=udp> Contact: <sip:bob@client.biloxi.example.com;transport=udp>
skipping to change at page 22, line 36 skipping to change at page 25, line 17
/* The request contains an offer. */ /* The request contains an offer. */
F7(=F4) ACK Alice -> Bob (retransmission) F7(=F4) ACK Alice -> Bob (retransmission)
/* A retransmission triggered by the reception of a retransmitted /* A retransmission triggered by the reception of a retransmitted
200. */ 200. */
F8 491 (re-INVITE) Bob -> Alice F8 491 (re-INVITE) Bob -> Alice
/* Bob sends 491 (Request Pending), since Bob has a pending /* Bob sends 491 (Request Pending), since Bob has a pending offer.
offer. */ */
F9 ACK (re-INVITE) Alice -> Bob F9 ACK (re-INVITE) Alice -> Bob
3.1.6 Receiving BYE (Established state) 3.1.6. Receiving BYE (Established state) in Moratorium state
in Moratorium state
State Alice Bob State State Alice Bob State
| | | |
| INVITE F1 | | INVITE F1 |
|-------------------------->| |-------------------------->|
Pre | 180 Ringing F2 | Pre Pre | 180 Ringing F2 | Pre
|<--------------------------| |<--------------------------|
Ear | | Ear Ear | | Ear
| 200 OK F3 | | 200 OK F3 |
|<--------------------------| |<--------------------------|
skipping to change at page 23, line 32 skipping to change at page 26, line 41
|<---------- ---------->| |<---------- ---------->|
| ^ ^ | | ^ ^ |
| | Timer K | | | | Timer K | |
| V | | | V | |
Morg | Timer J | | Morg | Timer J | |
| V | | V |
| | Morg | | Morg
| | | |
This scenario illustrates the race condition which occurs when UAS This scenario illustrates the race condition which occurs when UAS
receives an Established message, BYE, in the Moratorium state. receives an Established message, BYE, in the Moratorium state. An
An ACK request for a 200 OK response is lost (or delayed). ACK request for a 200 OK response is lost (or delayed). Immediately
Immediately after Bob retransmits the 200 OK to ini-INVITE, and Alice after Bob retransmits the 200 OK to ini-INVITE, and Alice sends a BYE
sends a BYE request at the same time. Depending on the request at the same time. Depending on the implementation of a SIP
implementation of a SIP UA, Alice may start a session again by the UA, Alice may start a session again by the reception of the
reception of the retransmitted 200 OK with SDP since she has already retransmitted 200 OK with SDP since she has already terminated a
terminated a session by sending a BYE. In that case, when UAC session by sending a BYE. In that case, when UAC receives a
receives a retransmitted 200 OK after sending a BYE, a session should retransmitted 200 OK after sending a BYE, a session should not be
not be started again since the session which is not associated with started again since the session which is not associated with dialog
dialog still remains. Moreover, in the case where UAS sends an offer still remains. Moreover, in the case where UAS sends an offer in a
in a 200 OK , UAS should not start a session again for the same 200 OK, UAS should not start a session again for the same reason if
reason if UAS receives a retransmitted ACK after receiving a BYE. UAS receives a retransmitted ACK after receiving a BYE.
Note: As noted in Section 3.1.4, there is a hint for implementation Note: As noted in Section 3.1.4, there is a hint for implementation
to avoid the race conditions of this type. That is for the caller to to avoid the race conditions of this type. That is for the caller to
delay sending BYE F6 for some period of time (2 seconds, perhaps), delay sending BYE F6 for some period of time (2 seconds, perhaps),
from which the caller is reasonably able to assume that its ACK has from which the caller is reasonably able to assume that its ACK has
been received. Implementors can decouple the actions of the user been received. Implementors can decouple the actions of the user
(e.g. hanging up) from the actions of the protocol (the sending of (e.g. hanging up) from the actions of the protocol (the sending of
BYE F6), so the UA can behave as such. In this case, it is dependent BYE F6), so that the UA can behave as such. In this case, it is
on the implementor's choice how long it waits. In most cases, such dependent on the implementor's choice as to how long it waits. In
implementation may be useful to prevent a type of race condition most cases, such implementation may be useful to prevent a type of
shown in this section. race condition shown in this section.
Message Details Message Details
F1 INVITE Alice -> Bob F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob F4 ACK Alice -> Bob
skipping to change at page 24, line 28 skipping to change at page 27, line 37
F5(=F3) 200 OK Bob -> Alice F5(=F3) 200 OK Bob -> Alice
/* UAS retransmits a 200 OK to the ini-INVITE since it has not /* UAS retransmits a 200 OK to the ini-INVITE since it has not
received an ACK. */ received an ACK. */
F6 BYE Alice -> Bob F6 BYE Alice -> Bob
/* Bob retransmits a 200 OK and Alice sends a BYE at the same time. /* 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 Alice transits to the Mortal state, so she does not begin a
session after this even though she receives a 200 response to session after this even though she receives a 200 response to the
the re-INVITE. */ re-INVITE. */
F7(=F4) ACK Alice -> Bob F7(=F4) ACK Alice -> Bob
F8 200 OK (BYE) Bob -> Alice F8 200 OK (BYE) Bob -> Alice
/* Bob sends a 200 OK to the BYE. */ /* Bob sends a 200 OK to the BYE. */
3.2 Receiving message in the Mortal State 3.2. Receiving message in the Mortal State
This section shows some examples of call flow in race condition This section shows some examples of call flow in race condition when
when receiving the message from other states in the Mortal state. receiving the message from other states in the Mortal state.
3.2.1 Receiving BYE (Establish state) 3.2.1. Receiving BYE (Establish state) in Mortal state
in Mortal state
State Alice Bob State State Alice Bob State
| | | |
| INVITE F1 | | INVITE F1 |
|----------------------->| |----------------------->|
Pre | 180 Ringing F2 | Pre Pre | 180 Ringing F2 | Pre
|<-----------------------| |<-----------------------|
Ear | | Ear Ear | | Ear
| 200 OK F3 | | 200 OK F3 |
|<-----------------------| |<-----------------------|
skipping to change at page 25, line 36 skipping to change at page 28, line 49
|<-------- --------->| |<-------- --------->|
| ^ ^ | | ^ ^ |
| | Timer K | | | | Timer K | |
| V | | | V | |
Morg | Timer J | | Morg | Timer J | |
| V | | V |
| | Morg | | Morg
| | | |
This scenario illustrates the race condition which occurs when UAS This scenario illustrates the race condition which occurs when UAS
receives an Established message, BYE, in the Mortal state. receives an Established message, BYE, in the Mortal state. Alice and
Alice and Bob send a BYE at the same time. A dialog and session are Bob send a BYE at the same time. A dialog and session are ended
ended shortly after a BYE request is passed to a client transaction. shortly after a BYE request is passed to a client transaction. As
As shown in Section 2, UA remains in the Mortal state. shown in Section 2, UA remains in the Mortal state.
UAs in the Mortal state return error responses to the requests that UAs in the Mortal state return error responses to the requests that
operate dialog or session, such as re-INVITE, UPDATE, or REFER. 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 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 consideration where BYE request is sent by both a caller and a callee
to exchange reports about the session when it is being terminated. to exchange reports about the session when it is being terminated.
(Since the dialogue and the session both terminate when a BYE is (Since the dialogue and the session both terminate when a BYE is
sent, the choice of sending 200 or an error response upon receiving sent, the choice of sending 200 or an error response upon receiving
BYE in the Mortal state does not affect the resulting termination. BYE in the Mortal state does not affect the resulting termination.
Therefore, even though this example uses a 200 response, other Therefore, even though this example uses a 200 response, other
skipping to change at page 26, line 16 skipping to change at page 29, line 30
F1 INVITE Alice -> Bob F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob F4 ACK Alice -> Bob
F5 BYE Alice -> Bob F5 BYE Alice -> Bob
/* The session is terminated at the moment Alice sends a BYE. /* The session is terminated at the moment Alice sends a BYE. The
The dialog still exists then, but it is certain to be terminated dialog still exists then, but it is certain to be terminated in a
in a short period of time. The dialog is completely short period of time. The dialog is completely terminated when
terminated when the timeout of the BYE request occurs. */ the timeout of the BYE request occurs. */
F6 BYE Bob -> Alice F6 BYE Bob -> Alice
/* Bob has also transmitted a BYE simultaneously with Alice. /* Bob has also transmitted a BYE simultaneously with Alice. Bob
Bob terminates the session and the dialog. */ terminates the session and the dialog. */
F7 200 OK Bob -> Alice F7 200 OK Bob -> Alice
/* Since the dialog is in the Moratorium state, Bob responds with a
/* Since the dialog is in the Moratorium state, Bob responds with 200 to the BYE request. */
a 200 to the BYE request. */
F8 200 OK Alice -> Bob F8 200 OK Alice -> Bob
/* Since Alice has transited from the Established state to the Mortal /* Since Alice has transited from the Established state to the Mortal
state by sending BYE, Alice responds with a 200 to the BYE state by sending BYE, Alice responds with a 200 to the BYE
request. */ request. */
3.2.2 Receiving re-INVITE (Establish state) 3.2.2. Receiving re-INVITE (Establish state) in Mortal state
in Mortal state
State Alice Bob State State Alice Bob State
| | | |
| INVITE F1 | | INVITE F1 |
|----------------------->| |----------------------->|
Pre | 180 Ringing F2 | Pre Pre | 180 Ringing F2 | Pre
|<-----------------------| |<-----------------------|
Ear | | Ear Ear | | Ear
| 200 OK F3 | | 200 OK F3 |
|<-----------------------| |<-----------------------|
skipping to change at page 27, line 30 skipping to change at page 31, line 44
|<-------- --------->|| |<-------- --------->||
^| ACK (re-INV) F9 || ^| ACK (re-INV) F9 ||
||<-----------------------|| ||<-----------------------||
Timer K|| || Timer K|| ||
V| || V| ||
Morg | |V Morg | |V
| | Morg | | Morg
| | | |
This scenario illustrates the race condition which occurs when UAS This scenario illustrates the race condition which occurs when UAS
receives an Established message, re-INVITE, in the Mortal state. receives an Established message, re-INVITE, in the Mortal state. Bob
Bob sends a re-INVITE, and Alice sends a BYE at the same time. sends a re-INVITE, and Alice sends a BYE at the same time. The re-
The re-INVITE is responded by a 481, since the TU of Alice has INVITE is responded by a 481, since the TU of Alice has transited
transited from the Established state to the Mortal state by sending from the Established state to the Mortal state by sending BYE. Bob
BYE. Bob sends ACK for the 481 response, because the ACK for error sends ACK for the 481 response, because the ACK for error responses
responses is handled by the transaction layer and at the point of is handled by the transaction layer and at the point of receiving the
receiving the 481 the INVITE client transaction still remains (even 481 the INVITE client transaction still remains (even though the
though the dialog has been terminated). dialog has been terminated).
Message Details Message Details
F1 INVITE Alice -> Bob F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob F4 ACK Alice -> Bob
F5 BYE Alice -> Bob F5 BYE Alice -> Bob
/* Alice sends a BYE and terminates the session, and transits from /* Alice sends a BYE and terminates the session, and transits from
the Established state to the Mortal state. */ the Established state to the Mortal state. */
F6 re-INVITE Bob -> Alice F6 re-INVITE Bob -> Alice
/* Alice sends a BYE, and Bob sends a re-INVITE at the same time. /* 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 The dialog state transits to the Mortal state at the moment Alice
Alice sends the BYE, but Bob does not know it until he receives sends the BYE, but Bob does not know it until he receives the BYE.
the BYE. Therefore, the dialog is in the Terminated state from Therefore, the dialog is in the Terminated state from Alice's
Alice's point of view, but it is the Confirmed state point of view, but it is the Confirmed state from Bob's point of
from Bob's point of view. A race condition occurs. */ view. A race condition occurs. */
F7 200 OK (BYE) Bob -> Alice F7 200 OK (BYE) Bob -> Alice
F8 481 Call/Transaction Does Not Exist (re-INVITE) Alice -> Bob F8 481 Call/Transaction Does Not Exist (re-INVITE) Alice -> Bob
/* Since Alice is in the Mortal state, she responds with a 481 to the /* Since Alice is in the Mortal state, she responds with a 481 to the
re-INVITE. */ re-INVITE. */
F9 ACK (re-INVITE) Bob -> Alice F9 ACK (re-INVITE) Bob -> Alice
/* ACK for an error response is handled by Bob's INVITE client /* ACK for an error response is handled by Bob's INVITE client
transaction. */ transaction. */
3.2.3 Receiving 200OK for re-INVITE (Established state) 3.2.3. Receiving 200 OK for re-INVITE (Established state) in Mortal
in Mortal state state
State Alice Bob State State Alice Bob State
| | | |
| INVITE F1 | | INVITE F1 |
|----------------------->| |----------------------->|
Pre | 180 Ringing F2 | Pre Pre | 180 Ringing F2 | Pre
|<-----------------------| |<-----------------------|
Ear | | Ear Ear | | Ear
| 200 OK F3 | | 200 OK F3 |
|<-----------------------| |<-----------------------|
skipping to change at page 29, line 22 skipping to change at page 33, line 47
| | ^ | | | ^ |
| | Timer K | | | | Timer K | |
| | V | | | V |
| | Timer J | Morg | | Timer J | Morg
| V | | V |
Morg | | Morg | |
| | | |
This scenario illustrates the race condition which occurs when UAS This scenario illustrates the race condition which occurs when UAS
receives an Established message, 200 to re-INVITE, in the Mortal receives an Established message, 200 to re-INVITE, in the Mortal
state. Bob sends a BYE immediately after sending a re-INVITE. state. Bob sends a BYE immediately after sending a re-INVITE. (A
(A user is not conscious that refresher sends re-INVITE user is not conscious that refresher sends re-INVITE automatically.
automatically. For example, in the case of a telephone application, For example, in the case of a telephone application, it is possible
it is possible that a user places a receiver immediately after that a user places a receiver immediately after refresher.) Bob
refresher.) sends ACK for a 200 response to INVITE in the Mortal state, so that
Bob sends ACK for a 200 response to INVITE in the Mortal state, so he completes the INVITE transaction.
that he completes the INVITE transaction.
Note: As noted in Section 3.1.4, there is a hint for implementation Note: As noted in Section 3.1.4, there is a hint for implementation
to avoid the race conditions of this type. That is for the UAC to to avoid the race conditions of this type. That is for the UAC to
delay sending BYE F6 until re-INVITE F5 transaction completes. delay sending BYE F6 until re-INVITE F5 transaction completes.
Implementors can decouple the actions of the user (e.g. hanging up) Implementors can decouple the actions of the user (e.g. hanging up)
from the actions of the protocol (the sending of BYE F6), so the UA from the actions of the protocol (the sending of BYE F6), so that the
can behave as such. In this case, it is dependent on the UA can behave as such. In this case, it is dependent on the
implementor's choice how long it waits. In most cases, such implementor's choice as to how long it waits. In most cases, such
implementation may be useful to prevent a type of race condition implementation may be useful to prevent a type of race condition
shown in this section. shown in this section.
Message Details Message Details
F1 INVITE Alice -> Bob F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice F3 200 OK Bob -> Alice
skipping to change at page 30, line 19 skipping to change at page 35, line 4
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE CSeq: 1 INVITE
Content-Length: 0 Content-Length: 0
/* Some detailed messages are shown for the sequence to illustrate /* Some detailed messages are shown for the sequence to illustrate
that the re-INVITE is handled in the usual manner in the Mortal that the re-INVITE is handled in the usual manner in the Mortal
state. */ state. */
F6 BYE Bob -> Alice F6 BYE Bob -> Alice
/* Bob sends BYE immediately after sending the re-INVITE. Bob
/* Bob sends BYE immediately after sending the re-INVITE. terminates the session and transits from the Established state to
Bob terminates the session and transits from the Established the Mortal state. */
state to the Mortal state. */
F7 200 OK (re-INVITE) Alice -> Bob F7 200 OK (re-INVITE) Alice -> Bob
SIP/2.0 200 OK SIP/2.0 200 OK
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashd7 Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashd7
;received=192.0.2.201 ;received=192.0.2.201
Require: timer Require: timer
Session-Expires: 300;refresher=uac Session-Expires: 300;refresher=uac
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356 From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
skipping to change at page 31, line 9 skipping to change at page 36, line 5
Max-Forwards: 70 Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356 From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 ACK CSeq: 2 ACK
Content-Length: 0 Content-Length: 0
/* Bob sends ACK in the Mortal state to complete the three-way /* Bob sends ACK in the Mortal state to complete the three-way
handshake of the INVITE transaction. */ handshake of the INVITE transaction. */
3.2.4 Receiving ACK (Moratorium state) 3.2.4. Receiving ACK (Moratorium state) in Mortal state
in Mortal state
State Alice Bob State State Alice Bob State
| | | |
| ini-INVITE F1 | | ini-INVITE F1 |
|------------------------------->| |------------------------------->|
Pre | 180 F2 | Pre Pre | 180 F2 | Pre
|<-------------------------------| |<-------------------------------|
Ear | 200 F3 | Ear Ear | 200 F3 | Ear
|<-------------------------------| |<-------------------------------|
Mora | | Mora Mora | | Mora
skipping to change at page 31, line 40 skipping to change at page 36, line 35
| | Timer K | | | | Timer K | |
| | V | | | V |
| | Timer J | Morg | | Timer J | Morg
| V | | V |
Morg | | Morg | |
| | | |
This scenario illustrates the race condition which occurs when UAS This scenario illustrates the race condition which occurs when UAS
receives an Established message, ACK to 200, in the Mortal state. 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 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 offer is in a 2xx, and the answer is in an ACK, this example is in a
a race condition. The session is not started by receiving the ACK race condition. The session is not started by receiving the ACK
because Bob has already terminated the session by sending the BYE. because Bob has already terminated the session by sending the BYE.
The answer in the ACK request is just ignored. The answer in the ACK request is just ignored.
Note: As noted in Section 3.1.4, there is a hint for implementation Note: As noted in Section 3.1.4, there is a hint for implementation
to avoid the race conditions of this type. Implementors can decouple to avoid the race conditions of this type. Implementors can decouple
the actions of the user (e.g. hanging up) from the actions of the the actions of the user (e.g. hanging up) from the actions of the
protocol (the sending of BYE F5), so the UA can behave as such. In protocol (the sending of BYE F5), so that the UA can behave as such.
this case, it is dependent on the implementor's choice how long it In this case, it is dependent on the implementor's choice as to how
waits. In most cases, such implementation may be useful to prevent a long it waits. In most cases, such implementation may be useful to
type of race condition shown in this section. prevent a type of race condition shown in this section.
Message Details Message Details
F1 INVITE Alice -> Bob F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob F4 ACK Alice -> Bob
/* RTP streams are established between Alice and Bob */ /* RTP streams are established between Alice and Bob */
F5 BYE Alice -> Bob F5 BYE Alice -> Bob
skipping to change at page 32, line 23 skipping to change at page 37, line 18
F4 ACK Alice -> Bob F4 ACK Alice -> Bob
/* RTP streams are established between Alice and Bob */ /* RTP streams are established between Alice and Bob */
F5 BYE Alice -> Bob F5 BYE Alice -> Bob
F6 200 OK Bob -> Alice F6 200 OK Bob -> Alice
/* Alice sends a BYE and terminates the session and dialog. */ /* Alice sends a BYE and terminates the session and dialog. */
3.3 Other race conditions 3.3. Other race conditions
This section shows the examples in race condition that are not This section shows the examples in race condition that are not
directly related to the dialog state transition. In SIP, processing directly related to the dialog state transition. In SIP, processing
functions are deployed in three layers, dialog, session, and functions are deployed in three layers, dialog, session, and
transaction. There are related each other, but have to be treated transaction. There are related each other, but have to be treated
separately. Section 17 of RFC 3261 [1] details the processing on separately. Section 17 of RFC 3261 [1] details the processing on
transactions. This draft has tried so far to clarify the processing transactions. This draft has tried so far to clarify the processing
on dialogs. This section explains race conditions which are related on dialogs. This section explains race conditions which are related
to sessions established by SIP. to sessions established by SIP.
3.3.1 Re-INVITE crossover 3.3.1. Re-INVITE crossover
Alice Bob Alice Bob
| | | |
| INVITE F1 | | INVITE F1 |
|--------------------------->| |--------------------------->|
| 180 Ringing F2 | | 180 Ringing F2 |
|<---------------------------| |<---------------------------|
| 200 OK F3 | | 200 OK F3 |
|<---------------------------| |<---------------------------|
| ACK F4 | | ACK F4 |
skipping to change at page 33, line 41 skipping to change at page 38, line 42
|--------------------------->| |--------------------------->|
| 200 OK F15 | | 200 OK F15 |
|<---------------------------| |<---------------------------|
| ACK F16 | | ACK F16 |
|--------------------------->| |--------------------------->|
| | | |
| | | |
In this scenario, Alice and Bob send re-INVITE at the same time. 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 When two re-INVITEs cross in the same dialog, they resend re-INVITEs
after different interval for each, according to Section 14.1, of after different interval for each, according to Section 14.1, of RFC
RFC 3261 [1]. When Alice sends the re-INVITE and it crosses, the 3261 [1]. When Alice sends the re-INVITE and it crosses, the re-
re-INVITE will be sent again after 2.1-4.0 seconds because she owns INVITE will be sent again after 2.1-4.0 seconds because she owns the
the Call-ID (she generated it). Bob will send an INVITE again after 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. 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 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 Call-ID of the dialog or not, in case an INVITE may cross with
another INVITE. another INVITE.
In this example, Alice's re-INVITE is for session modification and 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 Bob's re-INVITE is for session refresh. In this case, after the 491
responses, Bob retransmits the re-INVITE for session refresh earlier responses, Bob retransmits the re-INVITE for session refresh earlier
than Alice. If Alice was to retransmit her re-INVITE (that is, if 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 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 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. would not need to retransmit his re-INVITE to refresh the session.
In another instance where two re-INVITEs for session modification, In another instance where two re-INVITEs for session modification,
cross over, retransmitting the same re-INVITE again after 491 by the 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 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 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 intended to, so the UA needs to decide if the retransmission of the
re-INVITE is necessary. re-INVITE is necessary. (For example, when a call hold and an
(For example, when a call hold and an addition of video media cross addition of video media cross over, mere retransmission of the re-
over, mere retransmission of the re-INVITE at the firing of the timer INVITE at the firing of the timer may result in the situation where
may result in the situation where the video is transmitted the video is transmitted immediately after the holding of the audio.
immediately after the holding of the audio. This behavior is This behavior is probably not intended by the users.)
probably not intended by the users.)
Message Details Message Details
F1 INVITE Alice -> Bob F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob F4 ACK Alice -> Bob
skipping to change at page 36, line 32 skipping to change at page 42, line 4
o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
s=- s=-
c=IN IP4 192.0.2.101 c=IN IP4 192.0.2.101
t=0 0 t=0 0
m=audio 49172 RTP/AVP 0 m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=sendonly a=sendonly
/* Since Alice is the owner of the Call-ID, Alice sends a re-INVITE /* Since Alice is the owner of the Call-ID, Alice sends a re-INVITE
again after 2.1-4.0 seconds. */ again after 2.1-4.0 seconds. */
F15 200 OK Bob -> Alice F15 200 OK Bob -> Alice
F16 ACK Alice -> Bob F16 ACK Alice -> Bob
3.3.2 UPDATE and re-INVITE crossover 3.3.2. UPDATE and re-INVITE crossover
Alice Bob Alice Bob
| | | |
| INVITE F1 | | INVITE F1 |
|--------------------------->| |--------------------------->|
| 180 Ringing F2 | | 180 Ringing F2 |
|<---------------------------| |<---------------------------|
| | | |
| 200 OK F3 | | 200 OK F3 |
|<---------------------------| |<---------------------------|
skipping to change at page 37, line 40 skipping to change at page 44, line 6
| | | | | |
| |2.1-4.0 sec | |2.1-4.0 sec
| | | | | |
| UPDATE F13 v | | UPDATE F13 v |
|--------------------------->| |--------------------------->|
| 200 OK F14 | | 200 OK F14 |
|<---------------------------| |<---------------------------|
| | | |
| | | |
In this scenario, the UPDATE contains a SDP offer, therefore the In this scenario, the UPDATE contains an SDP offer, therefore the
UPDATE and re-INVITE are responded with 491 as in the case of UPDATE and re-INVITE are responded with 491 as in the case of "re-
"re-INVITE crossover". When an UPDATE for refresher which doesn't INVITE crossover". When an UPDATE for refresher which doesn't
contain a session description and a re-INVITE crossed each other, contain a session description and a re-INVITE crossed each other,
both requests succeed with 200 (491 means that UA have a pending both requests succeed with 200 (491 means that UA have a pending
request). Moreover, the same is equally true for UPDATE crossover. request). Moreover, the same is equally true for UPDATE crossover.
In the former case where either UPDATE contains a session description In the former case where either UPDATE contains a session description
the requests fail with 491, and in the latter cases succeed with 200. the requests fail with 491, and in the latter cases succeed with 200.
Editor's Note: Note:
A 491 response is sent because SDP offer is pending, and 491 is an A 491 response is sent because SDP offer is pending, and 491 is an
error which is related to matters that behave on the session error which is related to matters that behave on the session
established by SIP. established by SIP.
Message Details Message Details
F1 INVITE Alice -> Bob F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice F2 180 Ringing Bob -> Alice
skipping to change at page 40, line 17 skipping to change at page 47, line 5
t=0 0 t=0 0
m=audio 49172 RTP/AVP 0 m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=sendonly a=sendonly
/* Since Alice is the owner of the Call-ID, Alice sends the UPDATE /* Since Alice is the owner of the Call-ID, Alice sends the UPDATE
again after 2.1-4.0 seconds. */ again after 2.1-4.0 seconds. */
F14 200 OK Bob -> Alice F14 200 OK Bob -> Alice
3.3.3 Receiving REFER (Establish state) 3.3.3. Receiving REFER (Establish state) in Mortal state
in Mortal state
State Alice Bob State State Alice Bob State
| | | |
| INVITE F1 | | INVITE F1 |
|----------------------->| |----------------------->|
Pre | 180 Ringing F2 | Pre Pre | 180 Ringing F2 | Pre
|<-----------------------| |<-----------------------|
Ear | | Ear Ear | | Ear
| 200 OK F3 | | 200 OK F3 |
|<-----------------------| |<-----------------------|
skipping to change at page 41, line 7 skipping to change at page 47, line 43
| / \ | | | / \ | |
|<-------- --------->| |<-------- --------->|
| ^ | | | ^ | |
| | Timer K | | | | Timer K | |
| V Timer J | | | V Timer J | |
Morg | V | Morg | V |
| | Morg | | Morg
| | | |
This scenario illustrates the race condition which occurs when UAS This scenario illustrates the race condition which occurs when UAS
receives an Established message, REFER, in the Mortal state. receives an Established message, REFER, in the Mortal state. Bob
Bob sends a REFER, and Alice sends a BYE at the same time. Bob send sends a REFER, and Alice sends a BYE at the same time. Bob sends the
the REFER in the same dialog. Alice's dialog state moves to the REFER in the same dialog. Alice's dialog state moves to the Mortal
Mortal state at the point of sending BYE. In the Mortal state, UA state at the point of sending BYE. In the Mortal state, UA possesses
possesses dialog information for internal process but dialog dialog information for internal process but dialog shouldn't exist
shouldn't exist outwardly. Therefore, UA sends an error response to outwardly. Therefore, UA sends an error response to a REFER which is
a REFER which is transmitted as a mid-dialog request. So, Alice in transmitted as a mid-dialog request. So, Alice in the Mortal state
the Mortal state sends an error response to the REFER. sends an error response to the REFER. However, Bob has already
However, Bob has already started the SUBSCRIBE usage with REFER, so started the SUBSCRIBE usage with REFER, so the dialog continues until
the dialog continues until the SUBSCRIBE usage terminates, even the SUBSCRIBE usage terminates, even though the INVITE dialog usage
though the INVITE dialog usage terminates by receiving BYE. Bob's terminates by receiving BYE. Bob's behavior in this case needs to
behavior in this case needs to follow the procedure in the dialog follow the procedure in I-D.ietf-sipping-dialogusage [6]. (For
usage draft [6]. (For handling of dialogs with multiple usages see handling of dialogs with multiple usages see I-D.ietf-sipping-
the dialog usage draft [6].) dialogusage [6].)
Message Details Message Details
F1 INVITE Alice -> Bob F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob F4 ACK Alice -> Bob
skipping to change at page 42, line 11 skipping to change at page 49, line 7
F8 481 Call/Transaction Does Not Exist (REFER) Alice -> Bob F8 481 Call/Transaction Does Not Exist (REFER) Alice -> Bob
/* Alice in the Mortal state sends a 481 to the REFER. */ /* Alice in the Mortal state sends a 481 to the REFER. */
4. IANA Considerations 4. IANA Considerations
This document has no actions for IANA. This document has no actions for IANA.
5. Security Considerations 5. Security Considerations
This document contains clarifications of behavior specified in RFCs This document contains clarifications of behavior specified in RFC
3261 [1], 3264 [2], and 3515 [4]. The security considerations of 3261 [1], RFC 3264 [2] and RFC 3515 [4]. The security considerations
those documents continue to apply after the application of these of those documents continue to apply after the application of these
clarifications. clarifications.
6. Acknowledgements 6. Acknowledgements
The authors would like to thank Robert Sparks, Dean Willis, The authors would like to thank Robert Sparks, Dean Willis, Cullen
Cullen Jennings, James M. Polk, Gonzalo Camarillo, Kenichi Ogami, Jennings, James M. Polk, Gonzalo Camarillo, Kenichi Ogami, Akihiro
Akihiro Shimizu, Mayumi Munakata, Yasunori Inagaki, Shimizu, Mayumi Munakata, Yasunori Inagaki, Tadaatsu Kidokoro,
Tadaatsu Kidokoro, Kenichi Hiragi, Dale Worley, Vijay K. Gurbani Kenichi Hiragi, Dale Worley, Vijay K. Gurbani and Anders Kristensen
and Anders Kristensen for their comments on this document. for their comments on this document.
7. References 7. References
7.1 Normative References 7.1. Normative References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Peterson, J., Sparks, R., Handley, M., and E. Scooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002. Session Initiation Protocol", RFC 3261, June 2002.
[2] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with [2] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
the Session Description Protocol (SDP)", RFC 3264, June 2002. the Session Description Protocol (SDP)", RFC 3264, June 2002.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[4] Sparks, R., "The Session Initiation Protocol (SIP) Refer [4] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003. Method", RFC 3515, April 2003.
[5] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional [5] Sparks, R. and H. Schulzrinne, "Reliability of Provisional
Responses in the Session Initiation Protocol (SIP)", RFC 3262, Responses in the Session Initiation Protocol (SIP)", RFC 3262,
June 2002. June 2002.
7.2 Informative References 7.2. Informative References
[6] Sparks, R., "Multiple Dialog Usages in the Session Initiation [6] Sparks, R., "Multiple Dialog Usages in the Session Initiation
Protocol", draft-ietf-sipping-dialogusage-06 (work in progress), Protocol", I-D draft-ietf-sipping-dialogusage-06 (work in
February 16, 2007. progress), February 2007.
Appendix A - BYE on the Early Dialog Appendix A. BYE on the Early Dialog
This section, related to Section 3.1.3, explains why BYE is not 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 recommended in the Early state, illustrating the case in which BYE in
the early dialog triggers confusion. the early dialog triggers confusion.
Alice Proxy Bob Carol Alice Proxy Bob Carol
| | | | | | | |
| INVITE F1 | | | | INVITE F1 | | |
|--------------->| INVITE F2 | | |--------------->| INVITE F2 | |
| 100 F3 |----------------->| | | 100 F3 |----------------->| |
skipping to change at page 44, line 7 skipping to change at page 51, line 6
Care is advised in sending BYE in the Early state when forking by a Care is advised in sending BYE in the Early state when forking by a
proxy is expected. In this example, the BYE request progresses proxy is expected. In this example, the BYE request progresses
normally, and it succeeds in correctly terminating the dialog with normally, and it succeeds in correctly terminating the dialog with
Bob. After Bob terminates the dialog by receiving the BYE, he sends Bob. After Bob terminates the dialog by receiving the BYE, he sends
487 to the ini-INVITE. According to Section 15.1.2 of RFC 3261 [1], 487 to the ini-INVITE. According to Section 15.1.2 of RFC 3261 [1],
it is RECOMMENDED for UAS to generate 487 to any pending requests it is RECOMMENDED for UAS to generate 487 to any pending requests
after receiving BYE. In the example, Bob sends 487 to ini-INVITE after receiving BYE. In the example, Bob sends 487 to ini-INVITE
since he receives BYE while the ini-INVITE is in pending state. since he receives BYE while the ini-INVITE is in pending state.
However, Alice receives a final response to INVITE (a 200 from Carol) However, Alice receives a final response to INVITE (a 200 from Carol)
even though she has successfully terminated the dialog with Bob. even though she has successfully terminated the dialog with Bob. This
This means that, regardless of the success/failure of the BYE in the means that, regardless of the success/failure of the BYE in the Early
Early state, Alice MUST be prepared for the establishment of a new state, Alice MUST be prepared for the establishment of a new dialog
dialog until receiving the final response for the INVITE and until receiving the final response for the INVITE and terminating the
terminating the INVITE transaction. INVITE transaction.
It is not illegal to send BYE in the Early state to terminate a It is not illegal to send BYE in the Early state to terminate a
specific early dialog according to the caller's intent. However, the specific early dialog according to the caller's intent. However, the
choice of BYE or CANCEL in the Early state must be made carefully. 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 CANCEL is appropriate when the goal is to abandon the call attempt
entirely. BYE is appropriate when the goal is to abandon a entirely. BYE is appropriate when the goal is to abandon a
particular early dialog while allowing the call to be completed with particular early dialog while allowing the call to be completed with
other destinations. When using either BYE or CANCEL the UAC must be other destinations. When using either BYE or CANCEL the UAC must be
prepared for the possibility that a call may still be established to prepared for the possibility that a call may still be established to
one (or more) destinations. one (or more) destinations.
Appendix B - BYE request overlapped on re-INVITE Appendix B. BYE request overlapped on re-INVITE
UAC UAS UAC UAS
| | | |
The session has been already established The session has been already established
========================== ==========================
| re-INVITE F1 | | re-INVITE F1 |
|--------------------->| |--------------------->|
| BYE F2 | | BYE F2 |
|--------------------->| |--------------------->|
| 200(BYE) F3 | | 200(BYE) F3 |
skipping to change at page 45, line 39 skipping to change at page 52, line 39
| | | | | | | | | |
o | o | | o | o | |
| | | | | |
o | | o | |
| | | |
For UAC, the INVITE client transaction begins at the point F1 is For UAC, the INVITE client transaction begins at the point F1 is
sent. The UAC sends BYE (F2) immediately after F1. This is a sent. The UAC sends BYE (F2) immediately after F1. This is a
legitimate behavior. (Usually the usage of each SIP method is legitimate behavior. (Usually the usage of each SIP method is
independent, for BYE and others. However, it should be noted that it 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 is prohibited to send a request with an SDP offer while the previous
offer is in progress.) offer is in progress.)
After that, F2 triggers the BYE client transaction. At the same After that, F2 triggers the BYE client transaction. At the same
time, the dialog state transits to the Mortal state and then only a time, the dialog state transits to the Mortal state and then only a
BYE or its response can be handled. BYE or its response can be handled.
It is permitted to send F4 (a retransmission of INVITE) in the Mortal It is permitted to send F4 (a retransmission of INVITE) in the Mortal
state, because the retransmission of F1 is handled by the transaction state, because the retransmission of F1 is handled by the transaction
layer, and the INVITE transaction has not yet transited to the layer, and the INVITE transaction has not yet transited to the
Terminated state. As it is mentioned above, the dialog and the Terminated state. As it is mentioned above, the dialog and the
transaction behave independently each other. transaction behave independently each other. Therefore the
Therefore the transaction handling has to be continued even though transaction handling has to be continued even though the dialog moved
the dialog moved to the Terminated state. to the Terminated state.
Next, UAS's state is shown below. Next, UAS's state is shown below.
UAC UAS dialog INV BYE UAC UAS dialog INV BYE
| | : | | :
| | : | | :
| re-INVITE F1 | | | re-INVITE F1 | |
|-------------->x | | |-------------->x | |
| BYE F2 | | | BYE F2 | |
|--------------------->| | o |--------------------->| | o
skipping to change at page 46, line 30 skipping to change at page 53, line 31
| ACK(INV) F6 | | | ACK(INV) F6 | |
|--------------------->| |<-Start TimerI |--------------------->| |<-Start TimerI
| | | | | |
| | | | | |
| | o | | o
| | | |
For UAS, it can be regarded that F1 packet is lost or delayed (Here 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 the behavior is explained for the case UAS receives F2 BYE before F1
INVITE). Therefore, F2 triggers the BYE transaction for UAS, and INVITE). Therefore, F2 triggers the BYE transaction for UAS, and
simultaneously the dialog moves to the Mortal state. simultaneously the dialog moves to the Mortal state. Then, upon the
Then, upon the reception of F4 the INVITE server transaction begins. reception of F4 the INVITE server transaction begins. (It is allowed
(It is allowed to start the INVITE server transaction in the Mortal to start the INVITE server transaction in the Mortal state. The
state. The INVITE server transaction begins to handle received SIP INVITE server transaction begins to handle received SIP request
request regardless of the dialog state.) regardless of the dialog state.) UAS's TU sends an appropriate error
UAS's TU sends an appropriate error response for F4 INVITE, either response for F4 INVITE, either 481 (because the TU knows that the
481 (because the TU knows that the dialog which matches to the dialog which matches to the INVITE is in the Terminated state) or 500
INVITE is in the Terminated state) or 500 (because the re-sent F4 has (because the re-sent F4 has an out-of-order CSeq). (It is mentioned
an out-of-order CSeq). above that F4 (and F1) INVITE is a mid-dialog request. Mid-dialog
(It is mentioned above that F4 (and F1) INVITE is a mid-dialog requests have a To tag. It should be noted that UAS's TU does not
request. Mid-dialog requests have a To tag. It should be noted that begin a new dialog upon the reception of INVITE with a To tag.)
UAS's TU does not begin a new dialog upon the reception of INVITE
with a To tag.)
Appendix C - UA's behavior for CANCEL Appendix C. UA's behavior for CANCEL
This section explains the CANCEL behaviors which indirectly involve This section explains the CANCEL behaviors which indirectly involve
in the dialog state transition in the Early state. CANCEL does not in the dialog state transition in the Early state. CANCEL does not
have any influence on UAC's dialog state. However, the request has have any influence on UAC's dialog state. However, the request has
indirect influence on the dialog state transition because it has a indirect influence on the dialog state transition because it has a
significant effect on ini-INVITE. For UAS the CANCEL request has significant effect on ini-INVITE. For UAS the CANCEL request has
more direct effects on the dialog than the sending of CANCEL by UAC, more 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 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 explains UAS's behavior in the Early state. This flow diagram is
only an explanatory figure, and the actual dialog state transition is only an explanatory figure, and the actual dialog state transition is
skipping to change at page 48, line 6 skipping to change at page 55, line 10
There are two behaviors for UAS depending on the state when it There are two behaviors for UAS depending on the state when it
receives CANCEL. receives CANCEL.
One is when UAS receives CANCEL in the Early states. In this case One is when UAS receives CANCEL in the Early states. In this case
the UAS immediately sends 487 for the INVITE, and the dialog transits the UAS immediately sends 487 for the INVITE, and the dialog transits
to the Terminated state. to the Terminated state.
The other is the case in which UAS receives CANCEL in the Confirmed The other is the case in which UAS receives CANCEL in the Confirmed
state. In this case the dialog state transition does not occur state. In this case the dialog state transition does not occur
because UAS has already sent a final response to the INVITE to which because UAS has already sent a final response to the INVITE to which
the CANCEL is targeted. the CANCEL is targeted. (Note that, because of UAC's behavior, a UAS
(Note that, because of UAC's behavior, a UAS that receives a CANCEL that receives a CANCEL in the Confirmed state can expect to receive a
in the Confirmed state can expect to receive a BYE immediately and BYE immediately and move to the Terminated state. However, the UAS's
move to the Terminated state. However, the UAS's state does not state does not transit until it actually receives BYE.)
transit until it actually receives BYE.)
Appendix D - Notes on the request in Mortal state Appendix D. Notes on the request in Mortal state
This section describes UA's behavior in the Mortal state which needs This section describes UA's behavior in the Mortal state which needs
careful attention. Note that every transaction completes independent careful attention. Note that every transaction completes independent
of others, following the principle of RFC 3261 [1]. of others, following the principle of RFC 3261 [1].
In the Mortal state, BYE can be accepted, and the other messages in In the Mortal state, BYE can be accepted, and the other messages in
the INVITE dialog usage are responded with an error. However, the INVITE dialog usage are responded with an error. However,
sending of ACK and the authentication procedure for BYE are conducted sending of ACK and the authentication procedure for BYE are conducted
in this state. (The handling of messages concerning multiple dialog in this state. (The handling of messages concerning multiple dialog
usages is out of the scope of this document. Refer to [6] for usages is out of the scope of this document. Refer to I-D.ietf-
further information.) sipping-dialogusage [6] for further information.)
ACK for error responses is handled by the transaction layer, so the ACK for error responses is handled by the transaction layer, so the
handling is not related to the dialog state. Unlike the ACK for handling is not related to the dialog state. Unlike the ACK for
error responses, ACK for 2xx responses is a request newly generated 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 by TU. However, the ACK for 2xx and the ACK for error responses are
both a part of the INVITE transaction, even though their handling both a part of the INVITE transaction, even though their handling
differs (Section 17.1.1.1, RFC 3261 [1]). differs (Section 17.1.1.1, RFC 3261 [1]). Therefore, the INVITE
Therefore, the INVITE transaction is completed by the three-way transaction is completed by the three-way handshake, which includes
handshake, which includes ACK, even in the Mortal state. ACK, even in the Mortal state.
Considering actual implementation, UA needs to keep the INVITE dialog Considering actual implementation, UA needs to keep the INVITE dialog
usage until the Mortal state finishes, so that it is able to ACK for usage until the Mortal state finishes, so that it is able to send ACK
a 2xx response in the Mortal state. for a 2xx response in the Mortal state. If a 2xx to INVITE is
If a 2xx to INVITE is received in the Mortal state, the duration of received in the Mortal state, the duration of the INVITE dialog usage
the INVITE dialog usage will be extended to 64*T1 seconds after the will be extended to 64*T1 seconds after the receiving of the 2xx, to
receiving of the 2xx, to cope with the possible 2xx retransmission. cope with the possible 2xx retransmission. (The duration of the 2xx
(The duration of the 2xx retransmission is 64*T1, so the UA need to retransmission is 64*T1, so the UA need to be prepared to handle the
be prepared to handle the retransmission for this duration.) retransmission for this duration.) However, the UA shall send error
However, the UA shall send error response to other requests, since response to other requests, since the INVITE dialog usage in the
the INVITE dialog usage in the Mortal state is kept only for the Mortal state is kept only for the sending of ACK for 2xx.
sending of ACK for 2xx.
BYE authentication procedure shall be processed in the Mortal state. BYE authentication procedure shall be processed in the Mortal state.
When authentication is requested by 401 or 407 response, UAC resends When authentication is requested by 401 or 407 response, UAC resends
BYE with an appropriate credentials. Also UAS handles the BYE with appropriate credentials. Also UAS handles the
retransmission of the BYE which it requested authentication itself. retransmission of the BYE which it requested authentication itself.
Appendix E - Forking and receiving new To tags Appendix E. Forking and receiving new To tags
This section details the behavior of TU when it receives multiple This section details the behavior of TU when it receives multiple
responses with a different To tag to ini-INVITE. responses with a different To tag to ini-INVITE.
When an INVITE is forked inside a SIP network, there is a possibility 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 that the TU receives multiple responses with a different To tag to
ini-INVITE (See Section 12.1, 13.1, 13.2.2.4, 16.7, 19.3, etc. of RFC ini-INVITE (See Section 12.1, 13.1, 13.2.2.4, 16.7, 19.3, etc. of RFC
3261). If the TU receives multiple 1xx responses with a different 3261 [1]). If the TU receives multiple 1xx responses with a
To tag, the original DSM forks and a new DSM instance is created. As different To tag, the original DSM forks and a new DSM instance is
a consequence multiple early dialogs are generated. created. As a consequence multiple early dialogs are generated.
If one of the multiple early dialogs receives a 2xx response, it If one of the multiple early dialogs receives a 2xx response, it
naturally transits to the Confirmed state. No DSM state transition naturally transits to the Confirmed state. No DSM state transition
occurs for the other early dialogs, and their sessions (early media) occurs for the other early dialogs, and their sessions (early media)
terminate. The TU of the UAC terminates the INVITE transaction after terminate. The TU of the UAC terminates the INVITE transaction after
64*T1 seconds starting at the point of receiving the first 2xx 64*T1 seconds starting at the point of receiving the first 2xx
response. Moreover, all mortal early dialogs which do not transit to response. Moreover, all mortal early dialogs which do not transit to
the Established state are terminated (See Section 13.2.2.4 of the Established state are terminated (See Section 13.2.2.4 of RFC
RFC 3261). By "mortal early dialog" we mean any early dialog that 3261 [1]). By "mortal early dialog" we mean any early dialog that
the UA will terminate when another early dialog is confirmed. the UA will terminate when another early dialog is confirmed.
Below is an example sequence in which two 180 responses with a 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 different To tag are received, and then a 200 response for one of the
early dialogs (dialog A) is received. Dot lines (..) in sequences early dialogs (dialog A) is received. Dotted lines (..) in sequences
are auxiliary lines to represent the influence on dialog B. are auxiliary lines to represent the influence on dialog B.
UAC UAC
dialog(A) | INVITE F1 dialog(A) | INVITE F1
Pre o |-------------------------> Pre o |------------------------->
| | 100 F2 | | 100 F2
| |<------------------------- | |<-------------------------
| | 180(To tag=A) F3 | | 180(To tag=A) F3
Ear | |<------------------------- Ear | |<-------------------------
dialog(B) | | dialog(B) | |
forked new DSM | | 180(To tag=B) F4 forked new DSM | | 180(To tag=B) F4
Ear o..........|..........|<------------------------- Ear o..........|..........|<-------------------------
| | | | | |
| | | 200(A) F5 | | | 200(A) F5
terminate->|.....Mora |..........|<------------------------- terminate->|.....Mora |..........|<-------------------------
early | | ^ | ACK(A) F6 early | | ^ | ACK(A) F6
media | Est | | |-------------------------> media | Est | | |------------------------->
| | | | | | | |
| | |64*T1 | | | |64*T1 |
| | |(13.2.2.4 of RFC 3261) | | |(13.2.2.4 of RFC 3261 [1])
| | | | | | | |
| | | | | | | |
| | V | | | V |
o..........|.(terminate INVITE transaction) o..........|.(terminate INVITE transaction)
terminated | | terminated | |
dialog(B) | | dialog(B) | |
| | | |
Figure 4. Receiving 1xx responses with different To tags Figure 4. Receiving 1xx responses with different To tags
The figure above shows the DSM inside a SIP TU. Triggered by the The figure above shows the DSM inside a SIP TU. Triggered by the
reception of a provisional response with a different To tag reception of a provisional response with a different To tag (F4
(F4 180(To tag=B)), DSM forks and the early dialog B is generated. 180(To tag=B)), DSM forks and the early dialog B is generated. After
After 64*T1 seconds after the dialog A receives 200 OK response, the 64*T1 seconds after the dialog A receives 200 OK response, the dialog
dialog B, which does not transit to the Established state, B, which does not transit to the Established state, terminates.
terminates.
Next, the behavior of a TU which receives multiple 2xx responses with Next, the behavior of a TU which receives multiple 2xx responses with
a different To tag is explained. When a mortal early dialog, which a different To tag is explained. When a mortal early dialog, which
did not match the first 2xx response the TU received, receives did not match the first 2xx response the TU received, receives
another 2xx response which matches its To tag before 64*T1 INVITE another 2xx response which matches its To tag before 64*T1 INVITE
transaction timeout, its DSM state transits to the Confirmed state. transaction timeout, its DSM state transits to the Confirmed state.
However, the session on the mortal early dialog is terminated when However, the session on the mortal early dialog is terminated when
the TU receives the first 2xx to establish a dialog, so no session is the TU receives the first 2xx to establish a dialog, so no session is
established for the mortal early dialog. Therefore, when the mortal established for the mortal early dialog. Therefore, when the mortal
early dialog receives a 2xx response, the TU send an ACK and, early dialog receives a 2xx response, the TU send an ACK and,
skipping to change at page 51, line 26 skipping to change at page 58, line 50
| | | V | | | | V |
| | | (terminate INVITE transaction) | | | (terminate INVITE transaction)
V | | | V | | |
Morg o | | Morg o | |
| | | |
Figure 5. Receiving 1xx and 2xx responses with different To tags Figure 5. Receiving 1xx and 2xx responses with different To tags
Below is an example sequence when a TU receives multiple 200 Below is an example sequence when a TU receives multiple 200
responses with a different To tag before 64*T1 timeout of the INVITE responses with a different To tag before 64*T1 timeout of the INVITE
transaction, even though a TU does not receive provisional response, transaction in the absence of a provisional response. Even though a
the TU needs to respond to the 2xx responses (See Section 13.2.2.4 of TU does not receive provisional response, the TU needs to process the
RFC 3261). In that case, the DSM state is forked at the Confirmed 2xx responses (See Section 13.2.2.4 of RFC 3261 [1]). In that case,
state, and then the TU sends an ACK for the 2xx response and, the DSM state is forked at the Confirmed state, and then the TU sends
immediately after, the TU usually sends a BYE. (In special cases, an ACK for the 2xx response and, immediately after, the TU usually
e.g. a UA intends to establish multiple dialogs, the TU may not send sends a BYE. (In special cases, e.g. a UA intends to establish
the BYE.) multiple dialogs, the TU may not send the BYE.)
UAC UAC
dialog(A) | INVITE F1 dialog(A) | INVITE F1
Pre o |-----------------------> Pre o |----------------------->
| | 100 F2 | | 100 F2
| |<----------------------- | |<-----------------------
| | 180(To tag=A) F3 | | 180(To tag=A) F3
Ear | |<----------------------- Ear | |<-----------------------
| | | |
| | 200(A) F4 | | 200(A) F4
skipping to change at page 52, line 21 skipping to change at page 59, line 47
| | | |
Figure 6. Receiving 2xx responses with different To tags Figure 6. Receiving 2xx responses with different To tags
Below is an example sequence in which the option tag 100rel (RFC 3262 Below is an example sequence in which the option tag 100rel (RFC 3262
[5]) is required by a 180. [5]) is required by a 180.
If a forking proxy supports 100rel, it transparently transmits to the If a forking proxy supports 100rel, it transparently transmits to the
UAC a provisional response which contains a Require header with the UAC a provisional response which contains a Require header with the
value of 100rel. Upon receiving a provisional response with 100rel, value of 100rel. Upon receiving a provisional response with 100rel,
the UAC establishes the early dialog (B) and send PRACK. (Here, also the UAC establishes the early dialog (B) and sends PRACK. (Here,
every transaction completes independent of others".) also, every transaction completes independent of others.)
As Figure. 4, the early dialog (B) terminates at the same time the As Figure. 4, the early dialog (B) terminates at the same time the
INVITE transaction terminates. In the case where a proxy does not INVITE transaction terminates. In the case where a proxy does not
support 100rel, the provisional response will be handled in the usual support 100rel, the provisional response will be handled in the usual
way (a provisional response with 100rel is discarded by the proxy, way (a provisional response with 100rel is discarded by the proxy,
not to be transmitted to the UAC). not to be transmitted to the UAC).
UAC UAC
dialog(A) | INVITE F1 dialog(A) | INVITE F1
Pre o |-------------------------> Pre o |------------------------->
skipping to change at page 52, line 49 skipping to change at page 60, line 27
| ^ | ACK(A) F5 | ^ | ACK(A) F5
Est | | |-------------------------> Est | | |------------------------->
dialog(B) | | | dialog(B) | | |
forked new DSM | | | 180(To tag=B) w/100rel F6 forked new DSM | | | 180(To tag=B) w/100rel F6
Ear o..........|.|........|<------------------------- Ear o..........|.|........|<-------------------------
| | | | PRACK(B) F7 | | | | PRACK(B) F7
| | | |-------------------------> | | | |------------------------->
| | | | 200(B,PRACK) F8 | | | | 200(B,PRACK) F8
| | | |<------------------------- | | | |<-------------------------
| | |64*T1 | | | |64*T1 |
| | |(13.2.2.4 of RFC 3261) | | |(13.2.2.4 of RFC 3261 [1])
| | | | | | | |
| | | | | | | |
| | | | | | | |
| | V | | | V |
o..........|.(terminate INVITE transaction) o..........|.(terminate INVITE transaction)
terminated | | terminated | |
dialog(B) | | dialog(B) | |
| | | |
Figure 7. Receiving 1xx responses with different To tags (with 100rel) Figure 7. Receiving 1xx responses with different To tags when using
the mechanism for reliable provisional responses (100rel)
Author's Addresses
All listed authors actively contributed large amounts of text to this Authors' Addresses
document.
Miki Hasebe Miki Hasebe
NTT-east Corporation NTT-east Corporation
19-2 Nishi-shinjuku 3-chome Shinjuku-ku Tokyo 163-8019 Japan 19-2 Nishi-shinjuku 3-chome
Shinjuku-ku, Tokyo 163-8019
EMail: hasebe.miki@east.ntt.co.jp JP
Email: hasebe.miki@east.ntt.co.jp
Jun Koshiko Jun Koshiko
NTT-east Corporation NTT-east Corporation
19-2 Nishi-shinjuku 3-chome Shinjuku-ku Tokyo 163-8019 Japan 19-2 Nishi-shinjuku 3-chome
Shinjuku-ku, Tokyo 163-8019
JP
EMail: j.koshiko@east.ntt.co.jp Email: j.koshiko@east.ntt.co.jp
Yasushi Suzuki Yasushi Suzuki
NTT-east Corporation NTT-east Corporation
19-2 Nishi-shinjuku 3-chome Shinjuku-ku Tokyo 163-8019 Japan 19-2 Nishi-shinjuku 3-chome
Shinjuku-ku, Tokyo 163-8019
JP
EMail: suzuki.yasushi@east.ntt.co.jp Email: suzuki.yasushi@east.ntt.co.jp
Tomoyuki Yoshikawa Tomoyuki Yoshikawa
NTT-east Corporation NTT-east Corporation
19-2 Nishi-shinjuku 3-chome Shinjuku-ku Tokyo 163-8019 Japan 19-2 Nishi-shinjuku 3-chome
Shinjuku-ku, Tokyo 163-8019
JP
EMail: tomoyuki.yoshikawa@east.ntt.co.jp Email: tomoyuki.yoshikawa@east.ntt.co.jp
Paul H. Kyzivat Paul H. Kyzivat
Cisco Systems, Inc. Cisco Systems, Inc.
1414 Massachusetts Avenue 1414 Massachusetts Avenue
Boxborough, MA 01719 Boxborough, MA 01719
USA US
Email: pkyzivat@cisco.com Email: pkyzivat@cisco.com
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