[SIPForum-discussion] Why ACK??

[Vijay Tiwari]

Hello Dileep

may be this can help you to understand why we require Three-Way handshake in
invite message.


**

**INVITE is the only method that uses a three-way handshake as opposed to a
two-way handshake (METHOD-final response). Certain characteristics set the
INVITE method apart from other methods. When a client issues a request other
than INVITE, it expects a fast response from the server. However, the
response from an INVITE request might take a long time. When Bob calls
Laura, she may have to fish her SIP phone out of her coat pocket and press
buttons, so the “200 OK” response that will come will be more or less
delayed. Sending an ACK from the client to the server when the response is
received lets the server know that the client is still there and that the
session has been successfully established. The three-way handshake also
enables the implementation of forking proxies. When one of these forks a
request, the client who issued the request will obtain several responses
from different servers. Sending an ACK to every destination that has
responded is essential to ensuring SIP operation over unreliable protocols
such as UDP.



Besides the speedy session setup and forking, INVITE’s three-way handshake
also enables us to send an INVITE without a session description, which will
be sent later in the ACK. This feature is useful, for example, when SIP
interworks with other signaling protocols that use different message
sequencing.


On Tue, Feb 23, 2010 at 10:48 AM, Dileep Ravindran <
dileepravindran at tataelxsi.co.in> wrote:

>  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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-- 
They can because they think they can.
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