Network Working Group M. Hasebe
Request for Comments: 5407 J. Koshiko
BCP: 147 NTT-east Corporation
Category: Best Current Practice Y. Suzuki
NTT Corporation
T. Yoshikawa
NTT-east Corporation
P. Kyzivat
Cisco Systems, Inc.
December 2008
Example Call Flows of Race Conditions in the
Session Initiation Protocol (SIP)
Status of This Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2008 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (http://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
This document gives example call flows of race conditions in the
Session Initiation Protocol (SIP). Race conditions are inherently
confusing and difficult to thwart; this document shows the best
practices to handle them. The elements in these call flows include
SIP User Agents and SIP Proxy Servers. Call flow diagrams and
message details are given.
Hasebe, et al. Best Current Practice [Page 1]RFC 5407 Example Call Flows of Race Conditions December 2008
Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. General Assumptions . . . . . . . . . . . . . . . . . . . 3
1.2. Legend for Message Flows . . . . . . . . . . . . . . . . . 3
1.3. SIP Protocol Assumptions . . . . . . . . . . . . . . . . . 4
2. The Dialog State Machine for INVITE Dialog Usage . . . . . . . 5
3. Race Conditions . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. Receiving Message in the Moratorium State . . . . . . . . 11
3.1.1. Callee Receives Initial INVITE Retransmission
(Preparative State) While in the Moratorium State . . 11
3.1.2. Callee Receives CANCEL (Early State) While in the
Moratorium State . . . . . . . . . . . . . . . . . . . 13
3.1.3. Callee Receives BYE (Early State) While in the
Moratorium State . . . . . . . . . . . . . . . . . . . 15
3.1.4. Callee Receives re-INVITE (Established State)
While in the Moratorium State (Case 1) . . . . . . . . 17
3.1.5. Callee Receives re-INVITE (Established State)
While in the Moratorium State (Case 2) . . . . . . . . 22
3.1.6. Callee Receives BYE (Established State) While in
the Moratorium State . . . . . . . . . . . . . . . . . 26
3.2. Receiving Message in the Mortal State . . . . . . . . . . 28
3.2.1. UA Receives BYE (Established State) While in the
Mortal State . . . . . . . . . . . . . . . . . . . . . 28
3.2.2. UA Receives re-INVITE (Established State) While in
the Mortal State . . . . . . . . . . . . . . . . . . . 30
3.2.3. UA Receives 200 OK for re-INVITE (Established
State) While in the Mortal State . . . . . . . . . . . 32
3.2.4. Callee Receives ACK (Moratorium State) While in
the Mortal State . . . . . . . . . . . . . . . . . . . 35
3.3. Other Race Conditions . . . . . . . . . . . . . . . . . . 36
3.3.1. Re-INVITE Crossover . . . . . . . . . . . . . . . . . 36
3.3.2. UPDATE and re-INVITE Crossover . . . . . . . . . . . . 40
3.3.3. Receiving REFER (Established State) While in the
Mortal State . . . . . . . . . . . . . . . . . . . . . 45
4. Security Considerations . . . . . . . . . . . . . . . . . . . 46
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 46
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
6.1. Normative References . . . . . . . . . . . . . . . . . . . 47
6.2. Informative References . . . . . . . . . . . . . . . . . . 47
Appendix A. BYE in the Early Dialog . . . . . . . . . . . . . . . 48
Appendix B. BYE Request Overlapping with re-INVITE . . . . . . . 49
Appendix C. UA's Behavior for CANCEL . . . . . . . . . . . . . . 52
Appendix D. Notes on the Request in the Mortal State . . . . . . 54
Appendix E. Forking and Receiving New To Tags . . . . . . . . . . 54
Hasebe, et al. Best Current Practice [Page 2]RFC 5407 Example Call Flows of Race Conditions December 2008
1. Overview
The call flows shown in this document were developed in the design of
a SIP IP communications network. These examples are of race
conditions, which stem from transitions in dialog states -- mainly
transitions during session establishment after the sending of an
INVITE.
When implementing SIP, various complex situations may arise.
Therefore, it is helpful to provide implementors of the protocol with
examples of recommended terminal and server behavior.
This document clarifies SIP User Agent (UA) behaviors when messages
cross each other as race conditions. By clarifying the operation
under race conditions, inconsistent interpretations between
implementations are avoided and interoperability is expected to be
promoted.
It is the hope of the authors that this document will be useful for
SIP implementors, designers, and protocol researchers and will help
them achieve the goal of a standard implementation of RFC 3261 [1].
These call flows are based on version 2.0 of SIP, defined in RFC 3261
[1], with SDP usage as described in RFC 3264 [2].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 [3].
1.1. General Assumptions
A number of architectural, network, and protocol assumptions underlie
the call flows in this document. Note that these assumptions are not
requirements. They are outlined in this section so that they may be
taken into consideration and help understanding of the call flow
examples.
These flows do not assume specific underlying transport protocols
such as TCP, TLS, and UDP. See the discussion in RFC 3261 [1] for
details of the transport issues for SIP.
1.2. Legend for Message Flows
Dashed lines (---) and slash lines (/, \) represent signaling
messages that are mandatory to the call scenario. (X) represents the
crossover of signaling messages. (->x, x<-) indicate that the packet
is lost. The arrow indicates the direction of message flow. Double
dashed lines (===) represent media paths between network elements.
Hasebe, et al. Best Current Practice [Page 3]
RFC 5407 Example Call Flows of Race Conditions December 2008
Messages are identified in the figures as F1, F2, etc. These numbers
are used for references to the message details that follow the
figure. Comments in the message details are shown in the following
form:
/* Comments. */
1.3. SIP Protocol Assumptions
This document does not prescribe the flows precisely as they are
shown, but rather illustrates the principles for best practice. They
are best practice usages (orderings, syntax, selection of features
for the purpose, or handling of errors) of SIP methods, headers, and
parameters. Note: The flows in this document must not be copied
as-is by implementors because additional annotations have been
incorporated into this document for ease of explanation. To sum up,
the procedures described in this document represent well-reviewed
examples of SIP usage, which exemplify best common practice according
to IETF consensus.
For reasons of simplicity in reading and editing the document, there
are a number of differences between some of the examples and actual
SIP messages. For instance, Call-IDs are often replicated, CSeq
often begins at 1, header fields are usually shown in the same order,
usually only the minimum required header field set is shown, and
other headers that would usually be included, such as Accept, Allow,
etc., are not shown.
Actors:
Element Display Name URI IP Address
------- ------------ --- ----------
User Agent Alice sip:alice@atlanta.example.com 192.0.2.101
User Agent Bob sip:bob@biloxi.example.com 192.0.2.201
User Agent Carol sip:carol@chicago.example.com 192.0.2.202
Proxy Server ss.atlanta.example.com 192.0.2.111
The term "session" is used in this document in the same way it is
used in Sections 13-15 of RFC 3261 [1] (which differs somewhat from
the definition of the term in RFC 3261). RFC 5057 [6] introduces
another term, "invite dialog usage", which is more precisely defined.
The term "session" used herein is almost, but not quite, identical to
the term "invite dialog usage". The two have differing definitions
of when the state ends -- the session ends earlier, when BYE is sent
or received.
Hasebe, et al. Best Current Practice [Page 4]
RFC 5407 Example Call Flows of Race Conditions December 2008
2. The Dialog State Machine for INVITE Dialog Usage
Race conditions are generated when the dialog state of the receiving
side differs from that of the sending side.
For instance, a race condition occurs when a UAC (User Agent Client)
sends a CANCEL in the Early state while the UAS (User Agent Server)
is transitioning from the Early state to the Confirmed state by
sending a 200 OK to an initial INVITE (indicated as "ini-INVITE"
hereafter). The DSM (dialog state machine) for the INVITE dialog
usage is presented as follows to help understanding of the UA's
behavior in race conditions.
The DSM clarifies the UA's behavior by subdividing the dialog state
shown in RFC 3261 [1] into various internal states. We call the
state before the establishment of a dialog the Preparative state.
The Confirmed state is subdivided into two substates, the Moratorium
and the Established states, and the Terminated state is subdivided
into the Mortal and Morgue states. Messages that are the triggers
for the state transitions between these states are indicated with
arrows. In this figure, messages that are not related to state
transition are omitted.
Below are the DSMs, first for the caller and then for the callee.
Hasebe, et al. Best Current Practice [Page 5]
RFC 5407 Example Call Flows of Race Conditions December 2008
INV +-----------------------------------------------+
--->| Preparative |
+-----------------------------------------------+
| | |
| 3xx-6xx | 1xx-tag | 2xx
| | |
| | 1xx-tag |
| V w/new tag |
| +-----------------+ [new DSM] |
| 3xx-6xx | | | (new DSM |
+<--------| Early | | instance |
| | |<--+ created) |
| +-----------------+ |
| | | | 2xx w/new tag
| | BYE | 2xx | [new DSM]
| | +------------>+<-+ | (new DSM
| | | | instance
+-----C------------C-----+ +-----------C------+ | created)
| | Terminated | | | Confirmed | | |
| | +<----C---------| | | |
| | | | BYE(sr) | | | |
| | V | | V | |
| 2xx | +-----------+ | | +-----------+ | |
| +---C--| |---C-+ | | | | |
| | | | Mortal | | | BYE(r)| | Moratorium|<-C--+
| +---C->| |<--C-+ | | | |
| ACK | +-----------+ | | +-----------+ |
| | | | | | |
| | | Timeout | | | ACK |
| | | | | | |
| V V | | V |
| +---------------+ | | +-----------+ |
| | | | | | |--C-+
| | Morgue | | | |Established| | | 2xx,ACK
| | | | | | |<-C-+
| +---------------+ | | +-----------+ |
| | | |
+------------------------+ +------------------+
(r): indicates that only reception is allowed.
Where (r) is not used as an indicator, "response" means
receive, and "request" means send.
(sr): indicates that both sending and reception are allowed.
Figure 1: DSM for INVITE dialog usage (caller)
Hasebe, et al. Best Current Practice [Page 6]RFC 5407 Example Call Flows of Race Conditions December 2008
Figure 1 represents the caller's DSM for the INVITE dialog usage.
The caller MAY send a BYE in the Early state, even though this
behavior is not recommended. A BYE sent in the Early state
terminates the early dialog using a specific To tag. That is, when a
proxy is performing forking, the BYE is only able to terminate the
early dialog with a particular UA. If the caller wants to terminate
all early dialogs instead of that with a 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 dialog if this is the
caller's intent. Moreover, until the caller receives a final
response and terminates the INVITE transaction, the caller MUST be
prepared to establish a dialog by receiving a new response to the
INVITE even if it has already sent a CANCEL or BYE and terminated the
dialog (see Appendix A).
Hasebe, et al. Best Current Practice [Page 7]
RFC 5407 Example Call Flows of Race Conditions December 2008
INV +-----------------------------------------------+
--->| Preparative |
+-----------------------------------------------+
| | |
| 3xx-6xx | 1xx-tag | 2xx
| | |
| V |
| +------------------+ |
| 3xx-6xx | | |
+<--------| Early | |
| | | |
| +------------------+ |
| | | |
| |BYE/487(INV) | 2xx |
| | +------------>+<-+
| | |
+-----C------------C-----+ +-----------C------+
| | Terminated | | | Confirmed | |
| | +<----C---------| | |
| | | | BYE(sr) | | |
| | V | | V |
| | +------------+ | | +-----------+ |
| | | |---C-+ | | |--C-+
| | | Mortal | | | BYE | | Moratorium| | | 2xx
| | | |<--C-+ | | |<-C-+ if ACK not
| | +------------+ | | +-----------+ | received
| | | | | | |
| | | Timeout | | | ACK |
| | | | | | |
| V V | | V |
| +---------------+ | | +-----------+ |
| | | | | | | |
| | Morgue | | | |Established| |
| | | | | | | |
| +---------------+ | | +-----------+ |
| | | |
+------------------------+ +------------------+
(sr): indicates that both sending and reception are allowed.
Where (sr) is not used as an indicator, "response" means send,
and "request" means receive.
Figure 2: DSM for INVITE dialog usage (callee)
Figure 2 represents the callee's DSM for the INVITE dialog usage.
The figure does not illustrate the state transition related to CANCEL
requests. A CANCEL request does not cause a dialog state transition.
However, the callee terminates the dialog and triggers the dialog
Hasebe, et al. Best Current Practice [Page 8]RFC 5407 Example Call Flows of Race Conditions December 2008
transition by sending a 487 immediately after the reception of the
CANCEL. This behavior upon the reception of the CANCEL request is
further explained in Appendix C.
The UA's behavior in each state is as follows.
Preparative (Pre): The Preparative state is in effect until the
early dialog is established by sending or receiving a provisional
response with a To tag after an ini-INVITE is sent or received.
The dialog does not yet exist in the Preparative state. If the UA
sends or receives a 2xx response, the dialog state transitions
from the Preparative state to the Moratorium state, which is a
substate of the Confirmed state. In addition, if the UA sends or
receives a 3xx-6xx response, the dialog state transitions to the
Morgue state, which is a substate of the Terminated state.
Sending an ACK for a 3xx-6xx response and retransmissions of 3xx-
6xx are not shown on the DSMs because they are sent by the INVITE
transaction.
Early (Ear): The early dialog is established by sending or receiving
a provisional response except 100 Trying. The early dialog exists
even though the dialog does not exist in this state yet. The
dialog state transitions from the Early state to the Moratorium
state, a substate of the Confirmed state, by sending or receiving
a 2xx response. In addition, the dialog state transitions to the
Morgue state, a substate of the Terminated state, by sending or
receiving a 3xx-6xx response. Sending an ACK for a 3xx-6xx
response and retransmissions of 3xx-6xx are not shown on this DSM
because they are automatically processed on the transaction layer
and don't influence the dialog state. The UAC may send a CANCEL
in the Early state. The UAC may also send a BYE (although it is
not recommended). The UAS may send a 1xx-6xx response. The
sending or receiving of a CANCEL request does not have a direct
influence on the dialog state. The UA's behavior upon the
reception of the CANCEL request is explained further in Appendix
C.
Confirmed (Con): The sending or receiving of a 2xx final response
establishes a dialog. The dialog starts in this state. The
Confirmed state transitions to the Mortal state, a substate of the
Terminated state, by sending or receiving a BYE request. The
Confirmed state has two substates, the Moratorium and the
Established states, which are different with regard to the
messages that UAs are allowed to send.
Hasebe, et al. Best Current Practice [Page 9]RFC 5407 Example Call Flows of Race Conditions December 2008
Moratorium (Mora): The Moratorium state is a substate of the
Confirmed state and inherits its behavior. The Moratorium state
transitions to the Established state by sending or receiving an
ACK request. The UAC may send an ACK and the UAS may send a 2xx
final response.
Established (Est): The Established state is a substate of the
Confirmed state and inherits its behavior. Both caller and callee
may send various messages that influence a dialog. The caller
supports the transmission of ACK to the retransmission of a 2xx
response to an ini-INVITE.
Terminated (Ter): The Terminated state is subdivided into two
substates, the Mortal and Morgue states, to cover the behavior
when a dialog is being terminated. In this state, the UA holds
information about the dialog that is being terminated.
Mortal (Mort): The caller and callee enter the Mortal state by
sending or receiving a BYE. The UA MUST NOT send any new requests
within the dialog because there is no dialog. (Here, the new
requests do not include ACK for 2xx and BYE for 401 or 407, as
further explained in Appendix D below.) In the Mortal state, BYE
can be accepted, and the other messages in the INVITE dialog usage
are responded to with an error. This addresses the case where a
caller and a callee exchange reports about the session when it is
being terminated. Therefore, the UA possesses dialog information
for internal processing but the dialog shouldn't be externally
visible. The UA stops managing its dialog state and changes it to
the Morgue state when the BYE transaction is terminated.
Morgue (Morg): The dialog no longer exists in this state. The
sending or receiving of signaling that influences a dialog is not
performed. (A dialog is literally terminated.) The caller and
callee enter the Morgue state via the termination of the BYE or
INVITE transaction.
3. Race Conditions
This section details a race condition between two SIP UAs, Alice and
Bob. Alice (sip:alice@atlanta.example.com) and Bob
(sip:bob@biloxi.example.com) are assumed to be SIP phones or SIP-
enabled devices. Only significant signaling is illustrated. Dialog
state transitions caused by the sending or receiving of SIP messages
are shown, and race conditions are indicated by '*race*'. (For
abbreviations for the dialog state transitions, refer to Section 2.)
'*race*' indicates the moment when a race condition occurs.
Examples of race conditions are described below.
Hasebe, et al. Best Current Practice [Page 10]RFC 5407 Example Call Flows of Race Conditions December 2008
3.1. Receiving Message in the Moratorium State
This section shows some examples of call flow race conditions when
receiving messages from other states while in the Moratorium state.
3.1.1. Callee Receives Initial INVITE Retransmission (Preparative
State) While in the Moratorium State
State Alice Bob State
| |
| ini-INVITE F1 |
|------------------------------------>|
Pre | 180 F2(Packet loss) | Pre
| x<-----------------------|
| | Ear
| ini-INVITE F4(=F1) 200 F3 |
|------------------ --------------|
| \ / | Mora
| X |
| / \ |
|<----------------- ------------->| *race*
Mora | ACK F5 |
|------------------------------------>|
Est | | Est
| |
This scenario illustrates the race condition that occurs when the UAS
receives a Preparative message while in the Moratorium state. All
provisional responses to the initial INVITE (ini-INVITE F1) are lost,
and the UAC retransmits an ini-INVITE (F4). At the same time as this
retransmission, the UAS generates a 200 OK (F3) to the ini-INVITE and
terminates the INVITE server transaction, according to Section
13.3.1.4 of RFC 3261 [1].
However, it is reported that terminating an INVITE server transaction
when sending a 200 OK is an essential correction to SIP [7].
Therefore, the INVITE server transaction is not terminated by F3, and
F4 MUST be handled properly as a retransmission.
In RFC 3261 [1], it is not specified whether the UAS retransmits 200
to the retransmission of ini-INVITE. Considering the retransmission
of 200 triggered by a timer (the transaction user (TU) keeps
retransmitting 200 based on T1 and 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 receives the retransmission of the
ini-INVITE. (For implementation, it does not matter if the UAS sends
the retransmission of 200, since the 200 does not cause any problem.)
Hasebe, et al. Best Current Practice [Page 11]RFC 5407 Example Call Flows of Race Conditions December 2008
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
/* 180 response is lost and does not reach Alice. */
F3 200 OK Bob -> Alice
/* According to Section 13.3.1.4 of RFC 3261 [1], the INVITE server
transaction is terminated at this point. However, this has been
reported as an essential correction to SIP, and the UAS MUST
correctly recognize the ini-INVITE (F4) as a retransmission. */
F4 INVITE (retransmission) Alice -> Bob
/* F4 is a retransmission of F1. They are exactly the same INVITE
request. For UAs that have not dealt with the correction [7] (an
INVITE server transaction is terminated when sending 200 to
INVITE), this request does not match the transaction as well as
the dialog since it does not have a To tag. However, Bob must
recognize the retransmitted INVITE correctly, without treating it
as a new INVITE. */
F5 ACK Alice -> Bob
Hasebe, et al. Best Current Practice [Page 12]RFC 5407 Example Call Flows of Race Conditions December 2008
3.1.2. Callee Receives CANCEL (Early State) While in the Moratorium
State
State Alice Bob State
| |
| INVITE F1 |
|----------------------------->|
Pre | 180 Ringing F2 | Pre
|<-----------------------------|
Ear | | Ear
|CANCEL F3 200(INVITE) F4|
|------------ -------------|
| \ / | Mora
| X |
| / \ |
|<----------- ------------>| *race*
Mora | |
| ACK F6 200(CANCEL) F5|
|------------ -------------|
Est | \ / |
| X |
| / \ |
|<----------- ------------>|
| | Est
| One Way RTP Media |
| (Two Way RTP Media possible) |
|<=============================|
| BYE F7 |
|----------------------------->|
Mort | 200 F8 | Mort
|<-----------------------------|
| ^ ^ |
| | Timer K | |
| V | |
Morg | Timer J | |
| V |
| | Morg
| |
This scenario illustrates the race condition that occurs when the UAS
receives an Early message, CANCEL, while in the Moratorium state.
Alice sends a CANCEL, and Bob sends a 200 OK response to the initial
INVITE message at the same time. As described in the previous
section, according to RFC 3261 [1], an INVITE server transaction is
supposed to be terminated by a 200 response, but this has been
corrected in [7].
Hasebe, et al. Best Current Practice [Page 13]RFC 5407 Example Call Flows of Race Conditions December 2008
This section describes a case in which an INVITE server transaction
is not terminated by a 200 response to the INVITE request. In this
case, there is an INVITE transaction that the CANCEL request matches,
so a 200 response to the request is sent. This 200 response simply
means that the next hop receives the CANCEL request (successful
CANCEL (200) does not mean the INVITE was actually canceled). When a
UAS has not dealt with the correction [7], the UAC MAY receive a 481
response to the CANCEL since there is no transaction that the CANCEL
request matches. This 481 simply means that there is no matching
INVITE server transaction and CANCEL is not sent to the next hop.
Regardless of the success/failure of the CANCEL, Alice checks the
final response to the INVITE, and if she receives 200 to the INVITE
request she immediately sends a BYE and terminates the dialog. (See
Section 15, RFC 3261 [1].)
From the time F1 is received by Bob until the time that F8 is sent by
Bob, media may be flowing one way from Bob to Alice. From the time
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 has decided to cancel the call, she presumably will not
send media, so practically speaking the media stream will remain one
way.
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 CANCEL Alice -> Bob
/* Alice sends a CANCEL in the Early state. */
F4 200 OK (INVITE) Bob -> Alice
/* Alice receives a 200 to INVITE (F1) in the Moratorium state.
Alice has the potential to send as well as receive media, but in
practice will not send because there is an intent to end the
call. */
F5 200 OK (CANCEL) Bob -> Alice
/* 200 to CANCEL simply means that the CANCEL was received. The 200
response is sent, since this case assumes the correction [7] has
been made. If an INVITE server transaction is terminated
according to the procedure stated in RFC 3261 [1], the UAC MAY
receive a 481 response instead of 200. */
Hasebe, et al. Best Current Practice [Page 14]RFC 5407 Example Call Flows of Race Conditions December 2008
F6 ACK Alice -> Bob
/* INVITE is successful, and the CANCEL becomes invalid. Bob
establishes RTP streams. However, the next BYE request
immediately terminates the dialog and session. */
F7 BYE Alice -> Bob
F8 200 OK Bob -> Alice
3.1.3. Callee Receives BYE (Early State) While in the Moratorium State
State Alice Bob State
| |
| ini-INVITE F1 |
|------------------------------->|
Pre | 180 F2 | Pre
|<-------------------------------|
Ear | | Ear
| BYE F4 200(INVITE) F3|
|------------- --------------|
Mort | \ / | Mora
| X |
| / \ |
|<------------ ------------->| *race*
| | Mort
| ACK F5 200(BYE) F6 |
|------------- --------------|
| \ / ^ |
| X | |
| / \ | |
|<------------ ------------->|
| ^ | |
| | Timer K | |
| V | |
Morg | Timer J | |
| V |
| | Morg
| |
This scenario illustrates the race condition that occurs when the UAS
receives an Early message, BYE, while in the Moratorium state. Alice
sends a BYE in the Early state, and Bob sends a 200 OK to the initial
INVITE request at the same time. Bob receives the BYE in the
Confirmed dialog state although Alice sent the request in the Early
state (as explained in Section 2 and Appendix A, this behavior is not
recommended). When a proxy is performing forking, the BYE is only
able to terminate the early dialog with a particular UA. If the
Hasebe, et al. Best Current Practice [Page 15]RFC 5407 Example Call Flows of Race Conditions December 2008
caller wants to terminate all early dialogs instead of only that with
a 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 dialog if that is the caller's intent.
The BYE functions normally even if it is received after the INVITE
transaction termination because BYE differs from CANCEL, and is sent
not to the request but to the dialog. Alice enters the Mortal state
on sending the BYE request, and remains Mortal until the Timer K
timeout occurs. In the Mortal state, the UAC does not establish a
session even though it receives a 200 response to the INVITE. Even
so, the UAC sends an ACK to 200 in order to complete the INVITE
transaction. The ACK is always sent to complete the three-way
handshake of the INVITE transaction (further explained in Appendix D
below).
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK (ini-INVITE) Bob -> Alice
F4 BYE Alice -> Bob
/* Alice transitions to the Mortal state upon sending BYE.
Therefore, after this, she does not begin a session even though
she receives a 200 response with an answer. */
F5 ACK Alice -> Bob
F6 200 OK (BYE) Bob -> Alice
Hasebe, et al. Best Current Practice [Page 16]RFC 5407 Example Call Flows of Race Conditions December 2008
3.1.4. Callee Receives re-INVITE (Established State) While in the
Moratorium State (Case 1)
State Alice Bob State
| |
| ini-INVITE w/offer1 F1 |
|------------------------------->|
Pre | 180 F2 | Pre
|<-------------------------------|
Ear | | Ear
| 200(ini-INV) w/answer1 F3 |
|<-------------------------------|
Mora | ACK F4(packet loss) | Mora
|-------------------->x |
Est | |
| re-INVITE F6 200 F5(=F3) |
| w/offer2 w/answer1 |
|------------- --------------|
| \ / |
| X |
| / \ |
|<------------ ------------->| *race*
| 200(re-INV) F8|
| ACK F7(=F4) w/answer2 |
|------------- --------------|
| \ / |
| X |
| / \ |
|<------------ ------------->|
| ACK (re-INV) F9 | Est
|------------------------------->|
| |
| |
This scenario illustrates the race condition that occurs when a UAS
in the Moratorium state receives a re-INVITE sent by a UAC in the
Established state.
The UAS receives a re-INVITE (with offer2) before receiving an ACK
for the ini-INVITE (with offer1). The UAS sends a 200 OK (with
answer2) to the re-INVITE (F8) because it has sent a 200 OK (with
answer1) to the ini-INVITE (F3, F5) and the dialog has already been
established. (Because F5 is a retransmission of F3, SDP negotiation
is not performed here.)
As can be seen in Section 3.3.2 below, the 491 response seems to be
closely related to session establishment, even in cases other than
INVITE crossover. This example recommends that 200 be sent instead
Hasebe, et al. Best Current Practice [Page 17]RFC 5407 Example Call Flows of Race Conditions December 2008
of 491 because it does not have an influence on the session.
However, a 491 response can also lead to the same outcome, so either
response can be used.
Moreover, if the 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 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 the
CSeq number.
Note: Implementation issues are outside the scope of this document,
but the following tip is provided for avoiding race conditions of
this type. The caller can delay sending re-INVITE F6 for some period
of time (2 seconds, perhaps), after which the caller can reasonably
assume that its ACK has been received. Implementors can decouple the
actions of the user (e.g., pressing the hold button) from the actions
of the protocol (the sending of re-INVITE F6), so that the UA can
behave like this. In this case, it is the implementor's choice as to
how long to wait. In most cases, such an implementation may be
useful to prevent the type of race condition shown in this section.
This document expresses no preference about whether or not they
should wait for an ACK to be delivered. After considering the impact
on user experience, implementors should decide whether or not to wait
for a while, because the user experience depends on the
implementation and has no direct bearing on protocol behavior.
Message Details
F1 INVITE Alice -> Bob
INVITE sip:bob@biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:alice@client.atlanta.example.com;transport=udp>
Content-Type: application/sdp
Content-Length: 137
v=0
o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
Hasebe, et al. Best Current Practice [Page 18]
RFC 5407 Example Call Flows of Race Conditions December 2008
/* Detailed messages are shown for the sequence to illustrate the
offer and answer examples. */
F2 180 Ringing Bob -> Alice
SIP/2.0 180 Ringing
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:bob@client.biloxi.example.com;transport=udp>
Content-Length: 0
F3 200 OK Bob -> Alice
SIP/2.0 200 OK
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:bob@client.biloxi.example.com;transport=udp>
Content-Type: application/sdp
Content-Length: 133
v=0
o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
s=-
c=IN IP4 192.0.2.201
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F4 ACK Alice -> Bob
ACK sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashd8
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 ACK
Content-Length: 0
Hasebe, et al. Best Current Practice [Page 19]RFC 5407 Example Call Flows of Race Conditions December 2008
/* The ACK request is lost. */
F5(=F3) 200 OK Bob -> Alice (retransmission)
/* The UAS retransmits a 200 OK to the ini-INVITE since it has not
received an ACK. */
F6 re-INVITE Alice -> Bob
INVITE sip:sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf91
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Content-Length: 147
v=0
o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=sendonly
F7(=F4) ACK Alice -> Bob (retransmission)
/* "(=F4)" of ACK F7 shows that it is equivalent to F4 in that it is
an ACK for F3. This doesn't mean that F4 and F7 must be equal in
Via-branch value. Although it is ambiguous in RFC 3261 whether
the Via-branch of ACK F7 differs from that of F4, it doesn't
affect the UAS's behavior. */
F8 200 OK (re-INVITE) Bob -> Alice
SIP/2.0 200 OK
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf91
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Content-Length: 143
Hasebe, et al. Best Current Practice [Page 20]RFC 5407 Example Call Flows of Race Conditions December 2008
v=0
o=bob 2890844527 2890844528 IN IP4 client.biloxi.example.com
s=-
c=IN IP4 192.0.2.201
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=recvonly
F9 ACK (re-INVITE) Alice -> Bob
ACK sip:sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK230f21
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 ACK
Content-Length: 0
Hasebe, et al. Best Current Practice [Page 21]
RFC 5407 Example Call Flows of Race Conditions December 2008
3.1.5. Callee Receives re-INVITE (Established State) While in the
Moratorium State (Case 2)
State Alice Bob State
| |
| ini-INVITE (no offer) F1 |
|------------------------------->|
Pre | 180 F2 | Pre
|<-------------------------------|
Ear | | Ear
| 200(ini-INV) w/offer1 F3 |
|<-------------------------------|
Mora | ACK w/answer1 F4(packet loss) | Mora
|-------------------->x |
Est | |
| re-INVITE F6 200 F5(=F3) |
| w/offer2 w/offer1 |
|------------- --------------|
| \ / |
| X |
| / \ |
|<------------ ------------->|
| ACK F7(=F4) 491(re-INV) F8|
|------------- --------------|
| \ / |
| X |
| / \ |
|<------------ ------------->|
| ACK (re-INV) F9 | Est
|------------------------------->|
| |
| |
This scenario is basically the same as that of Section 3.1.4, but
differs in sending an offer in the 200 and an answer in the ACK. In
contrast to the previous case, the offer in the 200 (F3) and the
offer in the re-INVITE (F6) collide with each other.
Bob sends a 491 to the re-INVITE (F6) since he is not able to
properly handle a new request until he receives an answer. (Note:
500 with a Retry-After header may be returned if the 491 response is
understood to indicate request collision. However, 491 is
recommended here because 500 applies to so many cases that it is
difficult to determine what the real problem was.) The same result
will be reached if F6 is an UPDATE with offer.
Hasebe, et al. Best Current Practice [Page 22]RFC 5407 Example Call Flows of Race Conditions December 2008
Note: As noted in Section 3.1.4, the caller may delay sending a re-
INVITE F6 for some period of time (2 seconds, perhaps), after which
the caller may reasonably assume that its ACK has been received, to
prevent this type of race condition. This document expresses no
preference about whether or not they should wait for an ACK to be
delivered. After considering the impact on user experience,
implementors should decide whether or not to wait for a while,
because the user experience depends on the implementation and has no
direct bearing on protocol behavior.
Message Details
F1 INVITE Alice -> Bob
INVITE sip:bob@biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:alice@client.atlanta.example.com;transport=udp>
Content-Length: 0
/* The request does not contain an offer. Detailed messages are
shown for the sequence to illustrate offer and answer
examples. */
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
SIP/2.0 200 OK
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:bob@client.biloxi.example.com;transport=udp>
Content-Type: application/sdp
Content-Length: 133
v=0
o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
s=-
c=IN IP4 192.0.2.201
Hasebe, et al. Best Current Practice [Page 23]RFC 5407 Example Call Flows of Race Conditions December 2008
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
/* An offer is made in 200. */
F4 ACK Alice -> Bob
ACK sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashd8
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 ACK
Content-Type: application/sdp
Content-Length: 137
v=0
o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
/* The request contains an answer, but the request is lost. */
F5(=F3) 200 OK Bob -> Alice (retransmission)
/* The UAS retransmits a 200 OK to the ini-INVITE since it has not
received an ACK. */
F6 re-INVITE Alice -> Bob
INVITE sip:sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf91
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Content-Length: 147
v=0
o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
Hasebe, et al. Best Current Practice [Page 24]RFC 5407 Example Call Flows of Race Conditions December 2008
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=sendonly
/* The request contains an offer. */
F7(=F4) ACK Alice -> Bob (retransmission)
/* A retransmission triggered by the reception of a retransmitted
200. "(=F4)" of ACK F7 shows that it is equivalent to the F4 in
that it is an ACK for F3. This doesn't mean that F4 and F7 are
necessarily equal in Via-branch value. Although it is ambiguous
in RFC 3261 whether the Via-branch of ACK F7 differs from that of
F4, it doesn't affect the UAS's behavior. */
F8 491 (re-INVITE) Bob -> Alice
/* Bob sends 491 (Request Pending), since Bob has a pending
offer. */
F9 ACK (re-INVITE) Alice -> Bob
Hasebe, et al. Best Current Practice [Page 25]RFC 5407 Example Call Flows of Race Conditions December 2008
3.1.6. Callee Receives BYE (Established State) While in the Moratorium
State
State Alice Bob State
| |
| INVITE F1 |
|-------------------------->|
Pre | 180 Ringing F2 | Pre
|<--------------------------|
Ear | | Ear
| 200 OK F3 |
|<--------------------------|
Mora | ACK F4(packet loss) | Mora
|--------------->x |
Est | Both Way RTP Media |
|<=========================>|
| BYE F6 200 F5(=F3)|
|----------- -----------|
Mort | \ / |
| X |
| / \ |
|<---------- ---------->| *race*
|ACK F7(=F4) 200(BYE) F8| Mort
|----------- -----------|
| \ / |
| X |
| / \ |
|<---------- ---------->|
| ^ ^ |
| | Timer K | |
| V | |
Morg | Timer J | |
| V |
| | Morg
| |
This scenario illustrates the race condition that occurs when the UAS
receives an Established message, BYE, while in the Moratorium state.
An ACK request for a 200 OK response is lost (or delayed). Bob
retransmits the 200 OK to the ini-INVITE, and at the same time Alice
sends a BYE request and terminates the session. Upon receipt of the
retransmitted 200 OK, Alice's UA might be inclined to reestablish the
session. But that is wrong -- the session should not be
reestablished when the dialog is in the Mortal state. Moreover, in
the case where the UAS sends an offer in a 200 OK, the UAS should not
start a session again, for the same reason, if the UAS receives a
retransmitted ACK after receiving a BYE.
Hasebe, et al. Best Current Practice [Page 26]RFC 5407 Example Call Flows of Race Conditions December 2008
Note: As noted in Section 3.1.4, implementation issues are outside
the scope of this document, but the following tip is provided for
avoiding race conditions of this type. The caller can delay sending
BYE F6 for some period of time (2 seconds, perhaps), after which the
caller can reasonably assume that its ACK has been received.
Implementors can decouple the actions of the user (e.g., hanging up)
from the actions of the protocol (the sending of BYE F6), so that the
UA can behave like this. In this case, it is the implementor's
choice as to how long to wait. In most cases, such an implementation
may be useful to prevent the type of race condition shown in this
section. This document expresses no preference about whether or not
they should wait for an ACK to be delivered. After considering the
impact on user experience, implementors should decide whether or not
to wait for a while, because the user experience depends on the
implementation and has no direct bearing on protocol behavior.
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob
/* ACK request is lost. */
F5(=F3) 200 OK Bob -> Alice
/* The UAS retransmits a 200 OK to the ini-INVITE since it has not
received an ACK. */
F6 BYE Alice -> Bob
/* Bob retransmits a 200 OK and Alice sends a BYE at the same time.
Alice transitions to the Mortal state, so she does not begin a
session after this even though she receives a 200 response to the
re-INVITE. */
F7(=F4) ACK Alice -> Bob
/* "(=F4)" of ACK F7 shows that it is equivalent to the F4 in that it
is an ACK for F3. This doesn't mean that F4 and F7 must be equal
in Via-branch value. Although it is ambiguous in RFC 3261 whether
the Via-branch of ACK F7 differs from that of F4, it doesn't
affect the UAS's behavior. */
Hasebe, et al. Best Current Practice [Page 27]RFC 5407 Example Call Flows of Race Conditions December 2008
F8 200 OK (BYE) Bob -> Alice
/* Bob sends a 200 OK to the BYE. */
3.2. Receiving Message in the Mortal State
This section shows some examples of call flow race conditions when
receiving messages from other states while in the Mortal state.
3.2.1. UA Receives BYE (Established State) While in the Mortal State
State Alice Bob State
| |
| INVITE F1 |
|----------------------->|
Pre | 180 Ringing F2 | Pre
|<-----------------------|
Ear | | Ear
| 200 OK F3 |
|<-----------------------|
Mora | ACK F4 | Mora
|----------------------->|
Est | Both Way RTP Media | Est
|<======================>|
| |
| BYE F5 BYE F6 |
|--------- ----------|
Mort | \ / | Mort
| X |
| / \ |
|<-------- --------->| *race*
| |
| 200 F8 200 F7 |
|--------- ----------|
| \ / |
| X |
| / \ |
|<-------- --------->|
| ^ ^ |
| | Timer K | |
| V | |
Morg | Timer J | |
| V |
| | Morg
| |
Hasebe, et al. Best Current Practice [Page 28]RFC 5407 Example Call Flows of Race Conditions December 2008
This scenario illustrates the race condition that occurs when the UAS
receives an Established message, BYE, while in the Mortal state.
Alice and Bob send a BYE at the same time. A dialog and session are
ended shortly after a BYE request is passed to a client transaction.
As shown in Section 2, the UA remains in the Mortal state.
UAs in the Mortal state return error responses to the requests that
operate within a dialog or session, such as re-INVITE, UPDATE, or
REFER. However, the UA shall return a 200 OK to the BYE taking the
use case into consideration where a caller and a callee exchange
reports about the session when it is being terminated. (Since the
dialog and the session both terminate when a BYE is sent, the choice
of sending a 200 or an error response upon receiving a BYE while in
the Mortal state does not affect the resulting termination.
Therefore, even though this example uses a 200 response, other
responses can also be used.)
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob
F5 BYE Alice -> Bob
/* The session is terminated at the moment Alice sends a BYE. The
dialog still exists then, but it is certain to be terminated in a
short period of time. The dialog is completely terminated when
the timeout of the BYE request occurs. */
F6 BYE Bob -> Alice
/* Bob has also transmitted a BYE simultaneously with Alice. Bob
terminates the session and the dialog. */
F7 200 OK Bob -> Alice
/* Since the dialog is in the Moratorium state, Bob responds with a
200 to the BYE request. */
Hasebe, et al. Best Current Practice [Page 29]RFC 5407 Example Call Flows of Race Conditions December 2008
F8 200 OK Alice -> Bob
/* Since Alice has transitioned from the Established state to the
Mortal state by sending a BYE, Alice responds with a 200 to the
BYE request. */
3.2.2. UA Receives re-INVITE (Established State) While in the Mortal
State
State Alice Bob State
| |
| INVITE F1 |
|----------------------->|
Pre | 180 Ringing F2 | Pre
|<-----------------------|
Ear | | Ear
| 200 OK F3 |
|<-----------------------|
Mora | ACK F4 | Mora
|----------------------->|
Est | Both Way RTP Media | Est
|<======================>|
| |
| BYE F5 re-INVITE F6|
|--------- ----------|
Mort | \ / |
| X |
| / \ |
*race* |<-------- --------->|
| | Mort
| 481 F8 200 F7 |
| (re-INV) (BYE) |
|--------- ----------|
| \ / |^
| X ||
| / \ ||Timer J
|<-------- --------->||
^| ACK (re-INV) F9 ||
||<-----------------------||
Timer K|| ||
V| ||
Morg | |V
| | Morg
| |
This scenario illustrates the race condition that occurs when the UAS
receives an Established message, re-INVITE, while in the Mortal
state. Bob sends a re-INVITE, and Alice sends a BYE at the same
Hasebe, et al. Best Current Practice [Page 30]RFC 5407 Example Call Flows of Race Conditions December 2008
time. The re-INVITE receives a 481 response since the TU of Alice
has transitioned from the Established state to the Mortal state by
sending BYE. Bob sends an ACK for the 481 response because the ACK
for error responses is handled by the transaction layer and, at the
point of receiving the 481, the INVITE client transaction still
remains (even though the dialog has been terminated).
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob
F5 BYE Alice -> Bob
/* Alice sends a BYE and terminates the session, and transitions from
the Established state to the Mortal state. */
F6 re-INVITE Bob -> Alice
/* Alice sends a BYE, and Bob sends a re-INVITE at the same time.
The dialog state transitions to the Mortal state at the moment
Alice sends the BYE, but Bob does not know this until he receives
the BYE. Therefore, the dialog is in the Terminated state from
Alice's point of view, but in the Confirmed state from Bob's point
of view. A race condition occurs. */
F7 200 OK (BYE) Bob -> Alice
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
re-INVITE. */
F9 ACK (re-INVITE) Bob -> Alice
/* ACK for an error response is handled by Bob's INVITE client
transaction. */
Hasebe, et al. Best Current Practice [Page 31]RFC 5407 Example Call Flows of Race Conditions December 2008
3.2.3. UA Receives 200 OK for re-INVITE (Established State) While in
the Mortal State
State Alice Bob State
| |
| INVITE F1 |
|----------------------->|
Pre | 180 Ringing F2 | Pre
|<-----------------------|
Ear | | Ear
| 200 OK F3 |
|<-----------------------|
Mora | ACK F4 | Mora
|----------------------->|
Est | Both Way RTP Media | Est
|<======================>|
| |
| re-INVITE F5 |
|<-----------------------|
| 200 F7 BYE F6 |
|--------- ----------|
| \ / | Mort
| X |
| / \ |
|<-------- --------->| *race*
Mort | 200 F8 ACK F9 |
| (BYE) (re-INV) |
|--------- ----------|
| ^ \ / |
| | X |
| | / \ |
|<-------- --------->|
| | ^ |
| | Timer K | |
| | V |
| | Timer J | Morg
| V |
Morg | |
| |
This scenario illustrates the race condition that occurs when the UAS
receives an Established message, 200 to a re-INVITE, while in the
Mortal state. Bob sends a BYE immediately after sending a re-INVITE.
(For example, in the case of a telephone application, it is possible
that a user hangs up the phone immediately after refreshing the
session.) Bob sends an ACK for a 200 response to INVITE while in the
Mortal state, completing the INVITE transaction.
Hasebe, et al. Best Current Practice [Page 32]RFC 5407 Example Call Flows of Race Conditions December 2008
Note: As noted in Section 3.1.4, implementation issues are outside
the scope of this document, but the following tip is provided for
avoiding race conditions of this type. The UAC can delay sending a
BYE F6 until the re-INVITE transaction F5 completes. Implementors
can decouple the actions of the user (e.g., hanging up) from the
actions of the protocol (the sending of BYE F6), so that the UA can
behave like this. In this case, it is the implementor's choice as to
how long to wait. In most cases, such an implementation may be
useful in preventing the type of race condition described in this
section. This document expresses no preference about whether or not
they should wait for an ACK to be delivered. After considering the
impact on user experience, implementors should decide whether or not
to wait for a while, because the user experience depends on the
implementation and has no direct bearing on protocol behavior.
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob
F5 re-INVITE Bob -> Alice
INVITE sip:alice@client.atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashd7
Session-Expires: 300;refresher=uac
Supported: timer
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Content-Length: 0
/* Some detailed messages are shown for the sequence to illustrate
that the re-INVITE is handled in the usual manner in the Mortal
state. */
F6 BYE Bob -> Alice
/* Bob sends BYE immediately after sending the re-INVITE. Bob
terminates the session and transitions from the Established state
to the Mortal state. */
Hasebe, et al. Best Current Practice [Page 33]RFC 5407 Example Call Flows of Race Conditions December 2008
F7 200 OK (re-INVITE) Alice -> Bob
SIP/2.0 200 OK
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashd7
;received=192.0.2.201
Require: timer
Session-Expires: 300;refresher=uac
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Content-Length: 0
/* Bob sends BYE, and Alice responds with a 200 OK to the re-INVITE.
A race condition occurs. */
F8 200 OK (BYE) Alice -> Bob
F9 ACK (re-INVITE) Bob -> Alice
ACK sip:alice@client.atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bK74b44
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 ACK
Content-Length: 0
/* Bob sends ACK in the Mortal state to complete the three-way
handshake of the INVITE transaction. */
Hasebe, et al. Best Current Practice [Page 34]RFC 5407 Example Call Flows of Race Conditions December 2008
3.2.4. Callee Receives ACK (Moratorium State) While in the Mortal State
State Alice Bob State
| |
| ini-INVITE F1 |
|------------------------------->|
Pre | 180 F2 | Pre
|<-------------------------------|
Ear | 200 F3 | Ear
|<-------------------------------|
Mora | | Mora
| ACK F4 BYE F5 |
|------------- --------------|
Est | \ / | Mort
| X |
| / \ |
|<------------ ------------->| *race*
Mort | 200 F6 |
|------------------------------->|
| ^ ^ |
| | Timer K | |
| | V |
| | Timer J | Morg
| V |
Morg | |
| |
This scenario illustrates the race condition that occurs when the UAS
receives an Established message, ACK to 200, while in the Mortal
state. Alice sends an ACK and Bob sends a BYE at the same time.
When the offer is in a 2xx, and the answer is in an ACK, there is a
race condition. A session is not started when the ACK is received
because Bob has already terminated the session by sending a BYE. The
answer in the ACK request is just ignored.
Note: As noted in Section 3.1.4, implementation issues are outside
the scope of this document, but the following tip is provided for
avoiding race conditions of this type. Implementors can decouple the
actions of the user (e.g., hanging up) from the actions of the
protocol (the sending of BYE F5), so that the UA can behave like
this. In this case, it is the implementor's choice as to how long to
wait. In most cases, such an implementation may be useful in
preventing the type of race condition described in this section.
This document expresses no preference about whether or not they
should wait for an ACK to be delivered. After considering the impact
on user experience, implementors should decide whether or not to wait
for a while, because the user experience depends on the
implementation and has no direct bearing on protocol behavior.
Hasebe, et al. Best Current Practice [Page 35]RFC 5407 Example Call Flows of Race Conditions December 2008
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob
/* RTP streams are established between Alice and Bob. */
F5 BYE Alice -> Bob
F6 200 OK Bob -> Alice
/* Alice sends a BYE and terminates the session and dialog. */
3.3. Other Race Conditions
This section shows examples of race conditions that are not directly
related to dialog state transition. In SIP, processing functions are
deployed in three layers: dialog, session, and transaction. They are
related to each other, but have to be treated separately. Section 17
of RFC 3261 [1] details the processing of transactions. This
document has tried so far to clarify the processing on dialogs. This
section explains race conditions that are related to sessions
established with SIP.
3.3.1. Re-INVITE Crossover
Alice Bob
| |
| INVITE F1 |
|--------------------------->|
| 180 Ringing F2 |
|<---------------------------|
| 200 OK F3 |
|<---------------------------|
| ACK F4 |
|--------------------------->|
| Both Way RTP Media |
|<==========================>|
| |
|re-INVITE F5 re-INVITE F6 |
|------------ -------------|
Hasebe, et al. Best Current Practice [Page 36]RFC 5407 Example Call Flows of Race Conditions December 2008
| \ / |
| X |
| / \ |
|<----------- ------------>|
| 491 F8 491 F7 |
|------------ -------------|
| \ / |
| X |
| / \ |
|<----------- ------------>|
| ^ ACK F9 ^ ACK F10|
|--|--------- ----|--------|
| | \ / | |
| | X | |
| | / \ | |
|<-|---------- ---|------->|
| | | |
| |0-2.0 sec | |
| | | |
| v re-INVITE F11(=F6) |
|<------------------|--------|
| 200 OK F12 | |
|-------------------|------->|
| ACK F13 | |
|<------------------|--------|
| | |
| |2.1-4.0 sec
| | |
|re-INVITE F14(=F5) v |
|--------------------------->|
| 200 OK F15 |
|<---------------------------|
| ACK F16 |
|--------------------------->|
| |
| |
In this scenario, Alice and Bob send re-INVITEs at the same time.
When two re-INVITEs cross in the same dialog, they are retried, each
after a different interval, according to Section 14.1 of RFC 3261
[1]. When Alice sends the re-INVITE and it crosses with Bob's, the
re-INVITE will be retried after 2.1-4.0 seconds because she owns the
Call-ID (she generated it). Bob will retry his INVITE again after
0.0-2.0 seconds, because Bob isn't the owner of the Call-ID.
Therefore, each User Agent must remember whether or not it has
generated the Call-ID of the dialog, in case an INVITE may cross with
another INVITE.
Hasebe, et al. Best Current Practice [Page 37]RFC 5407 Example Call Flows of Race Conditions December 2008
In this example, Alice's re-INVITE is for session modification and
Bob's re-INVITE is for session refresh. In this case, after the 491
responses, Bob retries the re-INVITE for session refresh earlier than
Alice. If Alice was to retry her re-INVITE (that is, if she was not
the owner of Call-ID), the request would refresh and modify the
session at the same time. Then Bob would know that he does not need
to retry his re-INVITE to refresh the session.
In another instance, where two re-INVITEs for session modification
cross over, retrying the same re-INVITE again after a 491 by the
Call-ID owner (the UA that retries its re-INVITE after the other UA)
may result in unintended behavior, so the UA must decide if the retry
of the re-INVITE is necessary. (For example, when a call hold and an
addition of video media cross over, mere retry of the re-INVITE at
the firing of the timer may result in the situation where the video
is transmitted immediately after the holding of the audio. This
behavior is probably not intended by the users.)
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob
F5 re-INVITE Alice -> Bob
INVITE sip:sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Content-Length: 147
v=0
o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=sendonly
Hasebe, et al. Best Current Practice [Page 38]
RFC 5407 Example Call Flows of Race Conditions December 2008
/* Some detailed messages are shown for the sequence to illustrate
what sort of INVITE requests crossed over each other. */
F6 re-INVITE Bob -> Alice
INVITE sip:alice@client.atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashd7
Session-Expires: 300;refresher=uac
Supported: timer
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Content-Length: 0
/* A re-INVITE request for a session refresh and another for a call
hold are sent at the same time. */
F7 491 Request Pending Bob -> Alice
/* Since a re-INVITE is in progress, a 491 response is returned. */
F8 491 Request Pending Alice -> Bob
F9 ACK (INVITE) Alice -> Bob
F10 ACK (INVITE) Bob -> Alice
F11 re-INVITE Bob -> Alice
INVITE sip:alice@client.atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashd71
Session-Expires: 300;refresher=uac
Supported: timer
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Content-Type: application/sdp
Content-Length: 133
v=0
o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
s=-
c=IN IP4 192.0.2.201
Hasebe, et al. Best Current Practice [Page 39]RFC 5407 Example Call Flows of Race Conditions December 2008
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
/* Since Bob is not the owner of the Call-ID, he sends a re-INVITE
again after 0.0-2.0 seconds. */
F12 200 OK Alice -> Bob
F13 ACK Bob -> Alice
F14 re-INVITE Alice -> Bob
INVITE sip:sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf91
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 3 INVITE
Content-Length: 147
v=0
o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=sendonly
/* Since Alice is the owner of the Call-ID, Alice sends a re-INVITE
again after 2.1-4.0 seconds. */
F15 200 OK Bob -> Alice
F16 ACK Alice -> Bob
3.3.2. UPDATE and re-INVITE Crossover
Alice Bob
| |
| INVITE F1 |
|--------------------------->|
| 180 Ringing F2 |
|<---------------------------|
| |
| 200 OK F3 |
Hasebe, et al. Best Current Practice [Page 40]RFC 5407 Example Call Flows of Race Conditions December 2008
|<---------------------------|
| ACK F4 |
|--------------------------->|
| Both Way RTP Media |
|<==========================>|
| |
| UPDATE F5 re-INVITE F6 |
|------------ -------------|
| \ / |
| X |
| / \ |
|<----------- ------------>|
| 491 F8 491 F7 |
| (re-INVITE) (UPDATE) |
|------------ -------------|
| \ / |
| X |
| / \ |
|<----------- ------------>|
| ^ ACK F9 ^ |
|<-|----------------|--------|
| | | |
| |0-2.0 sec | |
| | | |
| v re-INVITE F10 | |
|<------------------|--------|
| 200 OK F11 | |
|-------------------|------->|
| ACK F12 | |
|<------------------|--------|
| | |
| |2.1-4.0 sec
| | |
| UPDATE F13 v |
|--------------------------->|
| 200 OK F14 |
|<---------------------------|
| |
| |
In this scenario, the UPDATE contains an SDP offer; therefore, the
UPDATE and re-INVITE are both responded to with 491 as in the case of
"re-INVITE crossover". When an UPDATE for session refresh that
doesn't contain a session description and a re-INVITE cross each
other, both requests succeed with 200 (491 means that a UA has a
pending request). The same is true for UPDATE crossover. In the
former case where either UPDATE contains a session description, the
requests fail with 491; in the latter cases, they succeed with 200.
Hasebe, et al. Best Current Practice [Page 41]RFC 5407 Example Call Flows of Race Conditions December 2008
Note: A 491 response is sent because an SDP offer is pending, and 491
is an error that is related to matters that impact the session
established by SIP.
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob
F5 UPDATE Alice -> Bob
UPDATE sip:sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 UPDATE
Content-Length: 147
v=0
o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=sendonly
/* Some detailed messages are shown for the sequence to illustrate
messages crossing over each other. */
F6 re-INVITE Bob -> Alice
INVITE sip:alice@client.atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashd7
Session-Expires: 300;refresher=uac
Supported: timer
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Hasebe, et al. Best Current Practice [Page 42]RFC 5407 Example Call Flows of Race Conditions December 2008
Content-Type: application/sdp
Content-Length: 133
v=0
o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
s=-
c=IN IP4 192.0.2.201
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
/* This is a case where a re-INVITE for a session refresh and an
UPDATE for a call hold are sent at the same time. */
F7 491 Request Pending (UPDATE) Bob -> Alice
/* Since a re-INVITE is in process, a 491 response is returned. */
F8 491 Request Pending (re-INVITE) Alice -> Bob
F9 ACK (re-INVITE) Alice -> Bob
F10 re-INVITE Bob -> Alice
INVITE sip:alice@client.atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashd71
Session-Expires: 300;refresher=uac
Supported: timer
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Content-Type: application/sdp
Content-Length: 133
v=0
o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
s=-
c=IN IP4 192.0.2.201
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
/* Since Bob is not the owner of the Call-ID, Bob sends an INVITE
again after 0.0-2.0 seconds. */
Hasebe, et al. Best Current Practice [Page 43]RFC 5407 Example Call Flows of Race Conditions December 2008
F11 200 OK Alice -> Bob
F12 ACK Bob -> Alice
F13 UPDATE Alice -> Bob
UPDATE sip:sip:bob@client.biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf91
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 3 UPDATE
Content-Length: 147
v=0
o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 192.0.2.101
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=sendonly
/* Since Alice is the owner of the Call-ID, Alice sends the UPDATE
again after 2.1-4.0 seconds. */
F14 200 OK Bob -> Alice
Hasebe, et al. Best Current Practice [Page 44]RFC 5407 Example Call Flows of Race Conditions December 2008
3.3.3. Receiving REFER (Established State) While in the Mortal State
State Alice Bob State
| |
| INVITE F1 |
|----------------------->|
Pre | 180 Ringing F2 | Pre
|<-----------------------|
Ear | | Ear
| 200 OK F3 |
|<-----------------------|
Mora | ACK F4 | Mora
|----------------------->|
Est | Both Way RTP Media | Est
|<======================>|
| |
| BYE F5 REFER F6 |
|--------- ----------|
Mort | \ / |
| X |
| / \ |
*race* |<-------- --------->|
| | Mort
| 481 F8 200 F7 |
| (REFER) (BYE) |
|--------- ----------|
| \ / ^ |
| X | |
| / \ | |
|<-------- --------->|
| ^ | |
| | Timer K | |
| V Timer J | |
Morg | V |
| | Morg
| |
This scenario illustrates the race condition that occurs when the UAS
receives an Established message, REFER, while in the Mortal state.
Bob sends a REFER, and Alice sends a BYE at the same time. Bob sends
the REFER in the same dialog. Alice's dialog state moves to the
Mortal state at the point of sending BYE. In the Mortal state, the
UA possesses dialog information for an internal process but the
dialog shouldn't exist outwardly. Therefore, the UA sends an error
response to the REFER, which is transmitted as a mid-dialog request.
So Alice, in the Mortal state, sends an error response to the REFER.
However, Bob has already started the SUBSCRIBE usage with REFER, so
Hasebe, et al. Best Current Practice [Page 45]RFC 5407 Example Call Flows of Race Conditions December 2008
the dialog continues until the SUBSCRIBE usage terminates, even
though the INVITE dialog usage terminates by receiving BYE. Bob's
behavior in this case needs to follow the procedures in RFC 5057 [6].
Message Details
F1 INVITE Alice -> Bob
F2 180 Ringing Bob -> Alice
F3 200 OK Bob -> Alice
F4 ACK Alice -> Bob
F5 BYE Alice -> Bob
/* Alice sends a BYE request and terminates the session, and
transitions from the Confirmed state to the Terminated state. */
F6 REFER Bob -> Alice
/* Alice sends a BYE, and Bob sends a REFER at the same time. Bob
sends the REFER on the INVITE dialog. The dialog state
transitions to the Mortal state at the moment Alice sends the BYE,
but Bob doesn't know this until he receives the BYE. A race
condition occurs. */
F7 200 OK (BYE) Bob -> Alice
F8 481 Call/Transaction Does Not Exist (REFER) Alice -> Bob
/* Alice in the Mortal state sends a 481 to the REFER. */
4. Security Considerations
This document contains clarifications of behavior specified in RFC
3261 [1], RFC 3264 [2], and RFC 3515 [4]. The security
considerations of those documents continue to apply after the
application of these clarifications.
5. Acknowledgements
The authors would like to thank Robert Sparks, Dean Willis, Cullen
Jennings, James M. Polk, Gonzalo Camarillo, Kenichi Ogami, Akihiro
Shimizu, Mayumi Munakata, Yasunori Inagaki, Tadaatsu Kidokoro,
Kenichi Hiragi, Dale Worley, Vijay K. Gurbani, and Anders Kristensen
for their comments on this document.
Hasebe, et al. Best Current Practice [Page 46]RFC 5407 Example Call Flows of Race Conditions December 2008
6. References
6.1. Normative References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[2] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[5] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
Responses in Session Initiation Protocol (SIP)", RFC 3262,
June 2002.
6.2. Informative References
[6] Sparks, R., "Multiple Dialog Usages in the Session Initiation
Protocol", RFC 5057, November 2007.
[7] Sparks, R., "Correct transaction handling for 200 responses to
Session Initiation Protocol INVITE requests", Work in Progress,
July 2008.
Hasebe, et al. Best Current Practice [Page 47]RFC 5407 Example Call Flows of Race Conditions December 2008
Appendix A. BYE in the Early Dialog
This section, related to Section 3.1.3, explains why BYE is not
recommended in the Early state, illustrating a case in which a BYE in
the early dialog triggers confusion.
Alice Proxy Bob Carol
| | | |
| INVITE F1 | | |
|--------------->| INVITE F2 | |
| 100 F3 |----------------->| |
|<---------------| 180(To tag=A) F4 | |
| 180(A) F5 |<-----------------| |
|<---------------| | |
| | INVITE(Fork) F6 |
| |------------------------>|
| | 100 F7 |
| BYE(A) F8 |<------------------------|
|--------------->| BYE(A) F9 | |
| |----------------->| |
| | 200(A,BYE) F10 | |
| 200(A,BYE) F11 |<-----------------| |
|<---------------| 487(A,INV) F12 | |
| |<-----------------| |
| | ACK(A) F13 | |
| |----------------->| |
| | | |
| | |
| | 200(To tag=B) F13 |
| 200(B) F14 |<------------------------|
|<---------------| |
| ACK(B) F15 | |
|--------------->| ACK(B) F16 |
| |------------------------>|
| BYE(B) F17 | |
|--------------->| BYE(B) F18 |
| |------------------------>|
| | 200(B) F19 |
| 200(B) F20 |<------------------------|
|<---------------| |
| | |
| | |
Care is advised in sending BYE in the Early state when forking by a
proxy is expected. In this example, the BYE request progresses
normally, and it succeeds in correctly terminating the dialog with
Bob. After Bob terminates the dialog by receiving the BYE, he sends
a 487 to the ini-INVITE. According to Section 15.1.2 of RFC 3261
Hasebe, et al. Best Current Practice [Page 48]RFC 5407 Example Call Flows of Race Conditions December 2008
[1], it is RECOMMENDED for the UAS to generate a 487 to any pending
requests after receiving a BYE. In this example, Bob sends a 487 to
the ini-INVITE since he receives the BYE while the ini-INVITE is in
pending state.
However, Alice receives a final response to the INVITE (a 200 from
Carol) even though she has successfully terminated the dialog with
Bob. This means that, regardless of the success/failure of the BYE
in the Early state, Alice MUST be prepared for the establishment of a
new dialog until receiving the final response for the INVITE and
terminating the INVITE transaction.
It is not illegal to send a BYE in the Early state to terminate a
specific early dialog -- it may satisfy the intent of some callers.
However, the choice of BYE or CANCEL in the Early state must be made
carefully. CANCEL is appropriate when the goal is to abandon the
call attempt entirely. BYE is appropriate when the goal is to
abandon a particular early dialog while allowing the call to be
completed with other destinations. When using either BYE or CANCEL,
the UAC must be prepared for the possibility that a call may still be
established to one or more destinations.
Appendix B. BYE Request Overlapping with re-INVITE
UAC UAS
| |
The session has been already established
==========================
| re-INVITE F1 |
|--------------------->|
| BYE F2 |
|--------------------->|
| 200(BYE) F3 |
|<---------------------|
| INVITE F4(=F1) |
|--------------------->|
| |
| |
This case could look similar to the one in Section 3.2.3. However,
it is not a race condition. This case describes the behavior when
there is no response to the INVITE for some reason. The appendix
explains the behavior in this case and its rationale, since this case
is likely to cause confusion.
First of all, it is important not to confuse the behavior of the
transaction layer and that of the dialog layer. RFC 3261 [1] details
the transaction layer behavior. The dialog layer behavior is
Hasebe, et al. Best Current Practice [Page 49]RFC 5407 Example Call Flows of Race Conditions December 2008
explained in this document. It has to be noted that these two
behaviors are independent of each other, even though both layers may
be triggered to change their states by sending or receiving the same
SIP messages. (A dialog can be terminated even though a transaction
still remains, and vice versa.)
In the sequence above, there is no response to F1, and F2 (BYE) is
sent immediately after F1. (F1 is a mid-dialog request. If F1 was
an ini-INVITE, BYE could not be sent before the UAC received a
provisional response to the request with a To tag.)
Below is a figure that illustrates the UAC's dialog state and the
transaction state.
BYE INV dialog UAC UAS
: | |
: | |
| | re-INVITE F1 |
o | |--------------------->|
| | | BYE F2 |
o | (Mortal) |--------------------->|
| | | | 200(BYE) F3 |
| | | |<---------------------|
| | | | INVITE F4(=F1) |
| | | |--------------------->|
| | | | 481(INV) F5 |
| | | |<---------------------|
| | | | ACK(INV) F6 |
| | | |--------------------->|
| | | | |
o | o | |
| | |
o | |
| |
For the UAC, the INVITE client transaction begins at the point F1 is
sent. The UAC sends BYE (F2) immediately after F1. This is a
legitimate behavior. (Usually, the usage of each SIP method is
independent, for BYE and others. However, it should be noted that it
is prohibited to send a request with an SDP offer while the previous
offer is in progress.)
After that, F2 triggers the BYE client transaction. At the same
time, the dialog state transitions to the Mortal state and then only
a BYE or a response to a BYE can be handled.
Hasebe, et al. Best Current Practice [Page 50]RFC 5407 Example Call Flows of Race Conditions December 2008
It is permitted to send F4 (a retransmission of INVITE) in the Mortal
state because the retransmission of F1 is handled by the transaction
layer, and the INVITE transaction has not yet transitioned to the
Terminated state. As is mentioned above, the dialog and the
transaction behave independently each other. Therefore, the
transaction handling has to be continued even though the dialog has
moved to the Terminated state.
Note: As noted in Section 3.1.4, implementation issues are outside
the scope of this document, but the following tip is provided for
avoiding race conditions of this type. The UAC can delay sending BYE
F2 until the re-INVITE transaction F1 completes. Implementors can
decouple the actions of the user (e.g., hanging up) from the actions
of the protocol (the sending of BYE F2), so that the UA can behave
like this. In this case, it is the implementor's choice as to how
long to wait. In most cases, such an implementation may be useful to
prevent this case. This document expresses no preference about
whether or not they should wait for an ACK to be delivered. After
considering the impact on user experience, implementors should decide
whether or not to wait for a while, because the user experience
depends on the implementation and has no direct bearing on protocol
behavior.
Next, the UAS's state is shown below.
UAC UAS dialog INV BYE
| | :
| | :
| re-INVITE F1 | |
|-------------->x | |
| BYE F2 | |
|--------------------->| | o
| 200(BYE) F3 | (Mortal) |
|<---------------------| | |<-Start Timer J
| INVITE F4(=F1) | | |
|--------------------->| | o |
| 4xx/5xx(INV) F5 | o | o
|<---------------------| |
| ACK(INV) F6 | |
|--------------------->| |<-Start Timer I
| | |
| | |
| | o
| |
For the UAS, it can be considered that packet F1 is lost or delayed
(here, the behavior is explained for the case that the UAS receives
F2 BYE before F1 INVITE). Therefore, F2 triggers the BYE transaction
Hasebe, et al. Best Current Practice [Page 51]RFC 5407 Example Call Flows of Race Conditions December 2008
for the UAS, and simultaneously the dialog moves to the Mortal state.
Then, upon the reception of F4, the INVITE server transaction begins.
(It is permitted to start the INVITE server transaction in the Mortal
state. The INVITE server transaction begins to handle the received
SIP request regardless of the dialog state.) The UAS's TU sends an
appropriate error response for the F4 INVITE, either 481 (because the
TU knows that the dialog that matches the INVITE is in the Terminated
state) or 500 (because the re-sent F4 has an out-of-order CSeq). (It
is mentioned above that INVITE message F4 (and F1) is a mid-dialog
request. Mid-dialog requests have a To tag. It should be noted that
the 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
This section explains the CANCEL behaviors that indirectly impact the
dialog state transition in the Early state. CANCEL does not have any
influence on the UAC's dialog state. However, the request has an
indirect influence on the dialog state transition because it has a
significant effect on ini-INVITE. For the UAS, the CANCEL request
has more direct effects on the dialog than on the sending of a CANCEL
by the UAC, because it can be a trigger to send the 487 response.
Figure 3 explains the UAS's behavior in the Early state. This flow
diagram is only an explanatory figure, and the actual dialog state
transition is as illustrated in Figures 1 and 2.
In the flow, full lines are related to dialog state transition, and
dotted lines are involved with CANCEL. (r) represents the reception
of signaling, and (s) means sending. There is no dialog state for
CANCEL, but here the Cancelled state is handled virtually just for
the ease of understanding of the UA's behavior when it sends and
receives CANCEL.
Hasebe, et al. Best Current Practice [Page 52]
RFC 5407 Example Call Flows of Race Conditions December 2008
+-------------+
| Preparative |---+
+-------------+ |
: | 1xx(s) |
: V |
: +-------+ | 2xx(s)
: | Early |-----+------+
: +-------+ |
: : V
: : +-----------+
: : | Confirmed |<...
:.....: +-----------+ :
: | : :
: BYE(r)| : :
: CANCEL(r) | :.......:
V | CANCEL(r)
............. |
: Cancelled : |
:...........: |
| 487(s) |
| |
+--------------------+
|
V
+------------+
| Terminated |
+------------+
Figure 3: CANCEL flow diagram for UAS
There are two behaviors for the UAS depending on the state when it
receives a CANCEL.
The first behavior is when the UAS receives a CANCEL in the Early
state. In this case, the UAS immediately sends a 487 for the INVITE,
and the dialog transitions to the Terminated state.
The other is the case in which the UAS receives a CANCEL while in the
Confirmed state. In this case, the dialog state transition does not
occur, because the UAS has already sent a final response to the
INVITE to which the CANCEL is targeted. (Note that, because of the
UAC's behavior, a UAS that receives a CANCEL in the Confirmed state
can expect to receive a BYE immediately and move to the Terminated
state. However, the UAS's state does not transition until it
actually receives a BYE.)
Hasebe, et al. Best Current Practice [Page 53]RFC 5407 Example Call Flows of Race Conditions December 2008
Appendix D. Notes on the Request in the Mortal State
This section describes the UA's behavior in the Mortal state, which
needs careful attention. Note that every transaction completes
independently of others, following the principle of RFC 3261 [1].
In the Mortal state, only a BYE can be accepted, and the other
messages in the INVITE dialog usage are responded to with an error.
However, sending of ACK and the authentication procedure for BYE are
conducted in this state. (The handling of messages concerning
multiple dialog usages is out of the scope of this document. Refer
to RFC 5057 [6] for further information.)
ACK for error responses is handled by the transaction layer, so the
handling is not related to the dialog state. Unlike the ACK for
error responses, ACK for 2xx responses is a request newly generated
by a TU. However, the ACK for 2xx and the ACK for error responses
are both part of the INVITE transaction, even though their handling
differs (Section 17.1.1.1, RFC 3261 [1]). Therefore, the INVITE
transaction is completed by the three-way handshake, which includes
ACK, even in the Mortal state.
Considering actual implementation, the UA needs to keep the INVITE
dialog usage until the Mortal state finishes, so that it is able to
send ACK for a 2xx response in the Mortal state. If a 2xx to INVITE
is received in the Mortal state, the duration of the INVITE dialog
usage will be extended to 64*T1 seconds after the receipt of the 2xx,
to cope with the possible 2xx retransmission. (The duration of the
2xx retransmission is 64*T1, so the UA needs to be prepared to handle
the retransmission for this duration.) However, the UA shall send an
error response to other requests, since the INVITE dialog usage in
the Mortal state is kept only for the sending of ACK for 2xx.
The BYE authentication procedure shall be processed in the Mortal
state. When authentication is requested by a 401 or 407 response,
the UAC resends BYE with appropriate credentials. Also, the UAS
handles the retransmission of the BYE for which it requested
authentication.
Appendix E. Forking and Receiving New To Tags
This section details the behavior of the TU when it receives multiple
responses with different To tags to the ini-INVITE.
When an INVITE is forked inside a SIP network, there is a possibility
that the TU receives multiple responses to the ini-INVITE with
differing To tags (see Sections 12.1, 13.1, 13.2.2.4, 16.7, 19.3,
Hasebe, et al. Best Current Practice [Page 54]
RFC 5407 Example Call Flows of Race Conditions December 2008
etc., of RFC 3261 [1]). If the TU receives multiple 1xx responses
with different To tags, the original DSM forks and a new DSM instance
is created. As a consequence, multiple early dialogs are generated.
If one of the multiple early dialogs receives a 2xx response, it
naturally transitions to the Confirmed state. No DSM state
transition occurs for the other early dialogs, and their sessions
(early media) terminate. The TU of the UAC terminates the INVITE
transaction after 64*T1 seconds, starting at the point of receiving
the first 2xx response. Moreover, all mortal early dialogs that do
not transition to the Established state are terminated (see Section
13.2.2.4 of RFC 3261 [1]). By "mortal early dialog", we mean any
early dialog that the UA will terminate when another early dialog is
confirmed.
Below is an example sequence in which two 180 responses with
different To tags are received, and then a 200 response for one of
the early dialogs (dialog A) is received. Dotted lines (..) in the
sequences are auxiliary lines to represent the influence on dialog B.
Hasebe, et al. Best Current Practice [Page 55]
RFC 5407 Example Call Flows of Race Conditions December 2008
UAC
dialog(A) | INVITE F1
Pre o |------------------------->
| | 100 F2
| |<-------------------------
| | 180(To tag=A) F3
Ear | |<-------------------------
dialog(B) | |
forked new DSM | | 180(To tag=B) F4
Ear o..........|..........|<-------------------------
| | |
| | | 200(A) F5
terminate->|.....Mora |..........|<-------------------------
early | | ^ | ACK(A) F6
media | Est | | |------------------------->
| | | |
| | |64*T1 |
| | |(13.2.2.4 of RFC 3261 [1])
| | | |
| | | |
| | V |
o..........|.(terminate INVITE transaction)
terminated | |
dialog(B) | |
| |
Figure 4: Receiving 1xx responses with different To tags
The figure above shows the DSM inside a SIP TU. Triggered by the
reception of a provisional response with a different To tag (F4
180(To tag=B)), the DSM forks and the early dialog B is generated.
64*T1 seconds later, dialog A receives a 200 OK response. Dialog B,
which does not transition to the Established state, terminates.
Next, the behavior of a TU that receives multiple 2xx responses with
different To tags is explained. When a mortal early dialog that did
not match the first 2xx response that the TU received receives
another 2xx response that matches its To tag before the 64*T1 INVITE
transaction timeout, its DSM transitions to the Confirmed state.
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
established for the mortal early dialog. Therefore, when the mortal
early dialog receives a 2xx response, the TU sends an ACK and,
immediately after, the TU usually sends a BYE to terminate the DSM.
(In special cases, e.g., if a UA intends to establish multiple
dialogs, the TU may not send the BYE.)
Hasebe, et al. Best Current Practice [Page 56]RFC 5407 Example Call Flows of Race Conditions December 2008
The handling of the second early dialog after receiving the 200 for
the first dialog is quite appropriate for a typical device, such as a
phone. It is important to note that what is being shown is a typical
useful action and not the only valid one. Some devices might want to
handle things differently. For instance, a conference focus that has
sent out an INVITE that forks may want to accept and mix all the
dialogs it gets. In that case, no early dialog is treated as mortal.
Below is an example sequence in which two 180 responses with a
different To tag are received and then a 200 response for each of the
early dialogs is received.
UAC
dialog(A) | INVITE F1
Pre o |----------------------->
| | 100 F2
| |<-----------------------
| | 180(To tag=A) F3
dialog(B) Ear | |<-----------------------
forked new DSM | | 180(To tag=B) F4
Ear o..........|..........|<-----------------------
| | |
| | | 200(A) F5
terminate->|.....Mora |..........|<-----------------------
early | | ^ | ACK(A) F6
media | Est | | |----------------------->
| | |64*T1 |
| | | | 200(B) F7
Mora |..........|.|........|<-----------------------
| | | | ACK(B) F8
Est |..........|.|........|----------------------->
| | | | BYE(B) F9
Mort |..........|.|........|----------------------->
^ | | | | 200(B) F10
| | | | |<-----------------------
|Timer K | | |
| | | V |
| | | (terminate INVITE transaction)
V | | |
Morg o | |
| |
Figure 5: Receiving 1xx and 2xx responses with different To tags
Below is an example sequence when a TU receives multiple 200
responses with different To tags before the 64*T1 timeout of the
INVITE transaction in the absence of a provisional response. Even
though a TU does not receive a provisional response, the TU needs to
Hasebe, et al. Best Current Practice [Page 57]RFC 5407 Example Call Flows of Race Conditions December 2008
process the 2xx responses (see Section 13.2.2.4 of RFC 3261 [1]). In
that case, the DSM state is forked at the Confirmed state, and then
the TU sends an ACK for the 2xx response and, immediately after, the
TU usually sends a BYE. (In special cases, e.g., if a UA intends to
establish multiple dialogs, the TU may not send the BYE.)
UAC
dialog(A) | INVITE F1
Pre o |----------------------->
| | 100 F2
| |<-----------------------
| | 180(To tag=A) F3
Ear | |<-----------------------
| |
| | 200(A) F4
Mora |..........|<-----------------------
| ^ | ACK(A) F5
Est | | |----------------------->
| | |
dialog(B) | |64*T1 |
forked new DSM | | | 200(To tag=B) F6
Mora o..........|.|........|<-----------------------
| | | | ACK(B) F7
Est |..........|.|........|----------------------->
| | | | BYE(B) F8
Mort |..........|.|........|----------------------->
^ | | | | 200(B) F9
| | | | |<-----------------------
| | | V |
|Timer K | (terminate INVITE transaction)
| | | |
V | | |
Morg o | |
| |
Figure 6: Receiving 2xx responses with different To tags
Below is an example sequence in which the option tag 100rel (RFC 3262
[5]) is required by a 180.
If a forking proxy supports 100rel, it transparently transmits to the
UAC a provisional response that contains a Require header with the
value of 100rel. Upon receiving a provisional response with 100rel,
the UAC establishes the early dialog (B) and sends PRACK (Provisional
Response Acknowledgement). (Here, also, every transaction completes
independently of others.)
Hasebe, et al. Best Current Practice [Page 58]RFC 5407 Example Call Flows of Race Conditions December 2008
As in Figure 4, the early dialog (B) terminates at the same time the
INVITE transaction terminates. In the case where a proxy does not
support 100rel, the provisional response will be handled in the usual
way (a provisional response with 100rel is discarded by the proxy,
not to be transmitted to the UAC).
UAC
dialog(A) | INVITE F1
Pre o |------------------------->
| | 100 F2
| |<-------------------------
| | 180(To tag=A) F3
Ear | |<-------------------------
| | 200(A) F4
Mora |..........|<-------------------------
| ^ | ACK(A) F5
Est | | |------------------------->
dialog(B) | | |
forked new DSM | | | 180(To tag=B) w/100rel F6
Ear o..........|.|........|<-------------------------
| | | | PRACK(B) F7
| | | |------------------------->
| | | | 200(B,PRACK) F8
| | | |<-------------------------
| | |64*T1 |
| | |(13.2.2.4 of RFC 3261 [1])
| | | |
| | | |
| | | |
| | V |
o..........|.(terminate INVITE transaction)
terminated | |
dialog(B) | |
| |
Figure 7: Receiving 1xx responses with different To tags
when using the mechanism for reliable provisional responses (100rel)
Hasebe, et al. Best Current Practice [Page 59]RFC 5407 Example Call Flows of Race Conditions December 2008
Authors' Addresses
Miki Hasebe
NTT-east Corporation
19-2 Nishi-shinjuku 3-chome
Shinjuku-ku, Tokyo 163-8019
JP
EMail: hasebe.miki@east.ntt.co.jp
Jun Koshiko
NTT-east Corporation
19-2 Nishi-shinjuku 3-chome
Shinjuku-ku, Tokyo 163-8019
JP
EMail: j.koshiko@east.ntt.co.jp
Yasushi Suzuki
NTT Corporation
9-11, Midori-cho 3-Chome
Musashino-shi, Tokyo 180-8585
JP
EMail: suzuki.yasushi@lab.ntt.co.jp
Tomoyuki Yoshikawa
NTT-east Corporation
19-2 Nishi-shinjuku 3-chome
Shinjuku-ku, Tokyo 163-8019
JP
EMail: tomoyuki.yoshikawa@east.ntt.co.jp
Paul H. Kyzivat
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
US
EMail: pkyzivat@cisco.com
Hasebe, et al. Best Current Practice [Page 60]
