[SIPForum-discussion] Why ACK??

[narasimham.settipalli at wipro.com]

Hi,

 

There could be several reasons for three way handshake needed for
INVITE. Some of them are:

 

1. Three way handshake is a procedure to synchronize requests and
responses. It is used even in TCP as well. 

2. It may not be possible to establish VoIP call just with INVITE/200
OK. Some times mis-matches in SDP negotiation needs confirmation from
caller or calle. These can be sorted out by having ACK message. 

3. Media negotiaton offer/answer model needs both parties agreeing on
common media codecs and formats. There is a possibility that one party
sends it's capabilities and other sends in different format (With
priority). Caller may not agree and would like to have his own priority.
ACK message will help in this. 

 

My assumption is that caller and calle sync up may not be achieved just
with INVITE/18X/200OK. Needs final confirmation from Caller if there is
a disagreement. 

And also ACK is a response to 200 OK. It is kind of response to 200 OK. 

 

Rgds,

Narasimham.

 

________________________________

From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of Dileep Ravindran
Sent: Tuesday, February 23, 2010 10:49 AM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] Why ACK??

 

Hi Every one...

 

    Can any one tell why a 3 way handshake is required for INVITE
???even though SIP can be tramitted over TCP

 
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