[SIPForum-discussion] Why ACK is separate transaction for INVITE

[Perttu Ahvenainen]

Hello all,

Founfd from RFC following statement about this:

*17 Transactions

   SIP is a transactional protocol: interactions between components take
   place in a series of independent message exchanges.  Specifically, a
   SIP transaction consists of a single request and any responses to
   that request, which include zero or more provisional responses and
   one or more final responses.  In the case of a transaction where the
   request was an INVITE (known as an INVITE transaction), the
   transaction also includes the ACK only if the final response was not
   a 2xx response.  If the response was a 2xx, the ACK is not considered
   part of the transaction.

      The reason for this separation is rooted in the importance of
      delivering all 200 (OK) responses to an INVITE to the UAC.  To
      deliver them all to the UAC, the UAS alone takes responsibility
      for retransmitting them (see Section 13.3.1.4), and the UAC alone
      takes responsibility for acknowledging them with ACK (see Section
      13.2.2.4).  Since this ACK is retransmitted only by the UAC, it is
      effectively considered its own transaction.*

-Perttu Ahvenainen

2011/6/22 Ravikannan Alagarsamy <ravikannanbe at gmail.com>

> Hi,
>
> Hope you know that for every new transaction Cseq should increment if it is
> not increment means that should be same transaction(non-successful call).
>
> So as per 3-way handshake policy once it gets 200ok response it sent ACK
> with increment Cseq id and it is separate transaction. if it is not
> incremented means the call is not successful we are sending ACK for some
> error message.
>
> Thanks
> Ravi
>
> On Tue, Jun 21, 2011 at 4:12 PM, Retesh <retesh.chadha at gmail.com> wrote:
>
>> Hi
>>
>> I think 1 of the reason is that ACK for 2xx can take a different network
>> path then INVITE or 2xx messages based on the contact and route set received
>> in 2xx and so it needs to be a separate transaction (different Via header
>> and branch parameters), which is not the case with ACK for non-2xx final
>> response.
>>
>> Hope this helps.
>>
>> Regards
>> Retesh
>>
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