Re: ACK is dropped.

Hello!

The ACK of 2XX responses must use a branch ID different from that of the 
INVITE. For Non-2xx responses, the ACK has the same branch id as the 
INVITE.  Note that for 2XX what you are ACKing is the dialog and not the 
INIVTE Transaction. Whereas for Non-2XX what you are ACKing is the error 
response and hence the branch ID is the SAME as the INVITE for this 
case. In this case according to the transaction state machine, (RFC 3261 
Figure 7) the ACK is not passed  up to the TU -- that is your Listener 
will not see the ACK. The ACK is not actually dropped -- its just that 
your listener will not see it.

Regards

Ranga

  Jie Shen wrote:

> Hello everyone,
>     i fell in a problem today. i developed a proxy based on 
> nist-sip1.2. today, i tested the proxy with a new sip phone and found 
> the ACK message is dropped by sip stack. by going through the related 
> code, i found the reason of dropping ACK is that the ACK and its 
> INVITE have the same branchids in via header. 
>     in the beginning, i supposed it's a problem of the new sip phone. 
> but when i looked up rfc3261, i found the following information from 
> rfc3261 section17.1.1.3: "The ACK MUST contain a single Via header 
> field, and this MUST be equal to the top Via header field of the 
> original request.". there is also an example:
>
>    For example, consider the following request:
>
>    INVITE sip:[email protected] <mailto:sip:[email protected]> SIP/2.0
>    Via: SIP/2.0/UDP pc33.atlanta.com 
> <http://pc33.atlanta.com>;branch=z9hG4bKkjshdyff
>    To: Bob < sip:[email protected] <mailto:sip:[email protected]>>
>    From: Alice <sip:[email protected] 
> <mailto:sip:[email protected]>>;tag=88sja8x
>    Max-Forwards: 70
>    Call-ID: 987asjd97y7atg
>    CSeq: 986759 INVITE
>
>    The ACK request for a non-2xx final response to this request would
>    look like this:
>
>    ACK sip:[email protected] <mailto:sip:[email protected]> SIP/2.0
>    Via: SIP/2.0/UDP pc33.atlanta.com 
> <http://pc33.atlanta.com>;branch=z9hG4bKkjshdyff
>    To: Bob < sip:[email protected] <mailto:sip:[email protected]>>;tag=99sa0xk
>    From: Alice <sip:[email protected] 
> <mailto:sip:[email protected]>>;tag=88sja8x
>    Max-Forwards: 70
>    Call-ID: 987asjd97y7atg
>    CSeq: 986759 ACK
>
>     so i am confused. anyone can help me?
>
> -- 
> Best Regards,
>
> Jie Shen
>
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-- 
M. Ranganathan 

Advanced Networking Technologies Division,
National Institute of Standards and Technology (NIST),
100 Bureau Drive, Stop 8920, Gaithersburg, MD 20899. 
tel:301 975 3664 , fax:301 590 0932 http://w3.antd.nist.gov/
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