Hi Santosh,
491 response for re-INVITE is sent when a request is already pending with the
UAC. Different values of timer are required so as to prevent the race condition
so that UAC and UAS do not keep sending re-INVITE to each other again & again.
The owner of call id is the person who initiates a call i.e. sends initial
INVITE request. For more details on this, you can refer to section 3.3.1 of RFC
5407.
Consider the following scenario:
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.
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.
Thanks and Regards,
Mangal Singh
Aricent
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of
[email protected]
Sent: Wednesday, July 11, 2012 12:10 PM
To: [email protected]
Subject: [Sip-implementors] UAC handling of 491 response to re-INVITE
Hi All,
Per section 14.1 (UAC Behavior) of RFC 3261, upon receiving 491 response to a
re-INVITE, UAC should start a timer with different values. Value of this timer
is determined based on the Call-Id owner.
Here I got 2 questions:
1. Why do we need to have different values of this timer?
2. Do we have SIP way of identifying owner of Call-ID?
Thanks in advance,
Santosh Kalsangrah
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