Subject: [Sipping] Question Re 200 OK retransmission and ACK

From: "Chaney, Charles (SNL US)" <[email-masked]>
Date: Thu, 25 Oct 2007 10:09:19 -0700

Hello,

A question has been raised regarding resending of ACK due to 200 OK
retransmission and how this is shown in the draft. I don't think the
draft has changed in this area from the initial version.

Is the ACK as shown (ACK F7(=F4)) in 3.1.4 really equivalent?
Specifically is it allowed that the Via branch-id in the ACK be the same
or must it be different?

RFC3261 13.2.2 implies the ACK is generated each time an ACK is sent.
This is interpreted by some as build from scratch which means that the
branch-id must be recreated to be unique across space and time. As a
result, it seems that ACK is not considered a retransmission as it does
not create any transaction itself and is sent as a unique request
(generated) every time a 200 OK response is received.

Is this understanding correct?

Please clarify the draft if there is still some ambiguity.

Thank You,
Chuck Chaney

3.1.4.  Receiving re-INVITE (Established state) in 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/answeFrom [email-masked] Thu Oct 25 13:14:23 2007
Content-class: urn:content-classes:message
Date: Thu, 25 Oct 2007 10:09:19 -0700
From: "Chaney, Charles (SNL US)" <[email-masked]>
To: "Miki HASEBE" <[email-masked]>
Cc: [email-masked]
Subject: [Sipping] Question Re 200 OK retransmission and ACK 

Hello,

A question has been raised regarding resending of ACK due to 200 OK
retransmission and how this is shown in the draft. I don't think the
draft has changed in this area from the initial version.

Is the ACK as shown (ACK F7(=F4)) in 3.1.4 really equivalent?
Specifically is it allowed that the Via branch-id in the ACK be the same
or must it be different?

RFC3261 13.2.2 implies the ACK is generated each time an ACK is sent.
This is interpreted by some as build from scratch which means that the
branch-id must be recreated to be unique across space and time. As a
result, it seems that ACK is not considered a retransmission as it does
not create any transaction itself and is sent as a unique request
(generated) every time a 200 OK response is received.

Is this understanding correct?

Please clarify the draft if there is still some ambiguity.

Thank You,
Chuck Chaney

3.1.4.  Receiving re-INVITE (Established state) in 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
          |------------------------------->|
          |                                |
          |                                |

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