Subject: [Sipping] Question Re 200 OK retransmission and ACK <draft-ietf-sipping-race-examples-04>

From: "Chaney, Charles (SNL US)" <[email-masked]>
Date: Thu, 25 Oct 2007 10:16:12 -0700

Hello,

A question has been raised regarding resending of ACK due to 200 OK
retransmission and how this is shown in
<draft-ietf-sipping-race-examples-04>. 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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---

From: Miki HASEBE <[email-masked]>
Date: Fri, 26 Oct 2007 17:52:19 +0900

Hi,

"(=F4)" of ACK F7 shows that it is equal to the F4 on the point that it is an
ACK for F3.

As you pointed out, I think that it is not clear in RFC3261 whether
the Via-branch in retransmitted ACK to 200 MUST be changed or not.
However, since behaviour of UAS does not depend on whether a Via-branch in
retransmitted ACK is the same as the lost one, I don't think that it's
necessary to decide whether the Via-branch is changed or not.

Thank you,
Miki

---

From: "Chaney, Charles (SNL US)" <[email-masked]>
Date: Fri, 26 Oct 2007 06:25:26 -0700

Hi,

Thanks for the answer. Unfortunately there appear to be some downstream
servers having special behavior similar to a B2BUA forwarding requests
further downstream that do rely on the branch being different as a form
of state. I'm not going to dispute this UAS behavior even though its
questionable if the UAC behavior is supposed to be different. 

I hope you can provide a clarification in the next draft so that others
don't use the example as an RFC3261 clarifying statement since its not
what you intended.

Thanks Again,
Charles (Chuck) Chaney

> -----Original Message-----
> From: ext Miki HASEBE [mailto:[email-masked]] 
> Sent: Friday, October 26, 2007 4:52 AM
> To: Chaney, Charles (SNL US)
> Cc: [email-masked]
> Subject: Re: Question Re 200 OK retransmission and ACK 
> <draft-ietf-sipping-race-examples-04>
> 
> Hi,
>