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List:       sip-implementors
Subject:    Re: [Sip-implementors] ACKing 200 OKs
From:       praveen dandin <praveendandin () yahoo ! co ! in>
Date:       2007-12-20 10:53:46
Message-ID: 712541.20285.qm () web7603 ! mail ! in ! yahoo ! com
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  Hi Daniel,
   
  Please refer the beginning of the section 17 of 3261. It says
   
  "
  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), and the UAC alone \
takes responsibility for acknowledging them with ACK (see Section 13.2.2). Since this \
ACK is   retransmitted only by the UAC, it is effectively considered its own \
transaction."  
   
  On receipt of the 2xx the INVITE transaction has to be terminated (i.e, it should \
be removed ) and 2xx has to be passed to the UAC core. The UAC core is responsible \
for sending the ACK whenever 2xx received ( it includes retransmissions of 2xx). UAC \
core/TU will pass the ACK directly to the transport layer  (but not to transaction \
layer) for transmission on receipt of 2xx.  
  Hope the following paragraph from RFC 3261 help you in understanding more on this:
  “
  The 2xx response and its ACK receive special treatment. This response is \
retransmitted only by a UAS, and its ACK generated only by the UAC. This end-to-end \
treatment is needed so that a caller knows the entire set of users that have accepted \
the call. Because of this special handling, retransmissions of the 2xx response are \
handled by the UA core, not the transaction layer. Similarly, generation of the ACK \
for the 2xx is handled by the UA core. Each proxy along the path merely forwards each \
2xx response to INVITE and its corresponding ACK.”  
  Regards,
  Praveen Dandin
     
   


       
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