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List:       sip-implementors
Subject:    Re: [Sip-implementors] ACK - separate transaction
From:       "joegen () opensipstack ! org" <joegen ! baclor () gmail ! com>
Date:       2007-07-23 16:46:25
Message-ID: 46A4DB61.5080601 () opensipstack ! org
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An error response to an INVITE terminates a transaction.   The error MAY 
occur before the INVITE reaches the actual destination.  Example is a 
404 generated by a proxy because it cannot route the INVITE to an actual 
UA.   A 200 Ok for an INVITE, however, can only be generated by the 
destination UA.   The 200 Ok does not only terminate the transaction but 
confirms a dialog.  Thus, there should be a guaranty that the 200 Ok 
actually reaches the INVITE initiator (caller).  To guaranty this, the 
ACK should only come from the actual caller and not possible proxies in 
the middle.

Another way of looking at it is, when an error response is sent, the 
sender of the error response does not really care about a dialog getting 
established out of the response.  Thus, the only use for a transaction 
that ended with an error response for an ACK is to stop the transaction 
from retransmitting the error response.  However, for a 200 Ok final 
response, ACK is not merely intended to stop retransmission of the 200 
Ok, but to confirm that the dialog is bidirectionally established 
between the caller and the callee.

Joegen


Hema R wrote:
> Hi,
> 
> 
> 
> What is the reason for including ACK in an INVITE transaction for a
> non-2xx response and making it a separate transaction for a 2xx
> response?
> 
> Is there any specific reason from the design perspective?
> 
> 
> 
> Thanks,
> 
> Hema.
> 
> 
> 
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