[SIPForum-discussion] Why ACK??

[sreekant nair]

The INVITE can traverse through multiple proxies or hops before arriving at the final Server(end user). Along the way, the proxies will repond with a 100 trying message to its previous hop to stop the re-transmission of the INVITE. 

When the INVITE transaction ends (success/failure) the server response must again traverse the same path towards the client. The server needs to know that the final response actually made it to the client. It will re-transmit the response untill it times out or receives an ACK. 

In the success cases, it is mostly the ACK message that starts Billing, CDR generation and actual bi-directional media. 

Another aspect is that INVITE creates a dialog and the state needs to be accurately maintained. UPDATE is a transaction within an existing Dialog and is mostly used during Early Dialogs. We do not need an ACK for the UPDATE because an ACK for the INVITE will definitely be present. If you still require additional confirmation for Offer-Answer negotiation, then you should implement the PRACK method. 

Hope this helps. 

Regards
Sreekant 



________________________________
From: Dileep Ravindran <dileepravindran at tataelxsi.co.in>
To: Vijay Tiwari <vijay11tiwari at gmail.com>; Mark Lindsey <lindsey at e-c-group.com>; "Lee, Kao" <Kao.Lee at tdstelecom.com>; narasimham.settipalli at wipro.com; lalit sahu <lalitbtech at gmail.com>
Cc: discussion at sipforum.org
Sent: Wed, February 24, 2010 4:02:14 AM
Subject: Re: [SIPForum-discussion] Why ACK??


Thanks Every one for your valuable answers!
 
 
 I have few more doubts
 
1)If INVITE has 3 way handshake how come other methods like SUBSCRIBE does'nt have 3 way handshake.if we say that SDP negotiation is done using INVITE , we can also negotiate SDP in UPDATE method for which we never send ACK

2)Why a non-2XX response should have an ACK?

Thanks
Dileep 



      
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