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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.
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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]