RFC 5407
Example Call Flows of Race Conditions in the Session Initiation Protocol (SIP)
Pages: 60
Best Current Practice: 147
Best Current Practice: 147
Part 2 of 2 – Pages 28 to 60
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 | |
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. */
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
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. */
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.
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. */
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. */
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.
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 | |------------ -------------|
| \ / |
| 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.
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
/* 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
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 |
|<---------------------------|
| 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.
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
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. */
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
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
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.
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.
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
[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
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.
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
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.
+-------------+ | 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.)
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,
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.
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.)
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
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.)
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)
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