ACK

ACK sip:bob@biloxi.com SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com:5060;branch=z9hG4bK776asdhds
To: Bob <sip:bob@biloxi.com>
From: Alice <sip:alice@atlanta.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 ACK
Content-Length: 0

A successful set-up sequence was: INVITE/200/ACK

A set-up failure would be a sequence such as: INVITE/4xx^†/ACK

NB: INVITE is the only method in SIP that involves a 3-way handshake with ACK

The further setup of the call can proceed directly between Alice and Bob, based on the the information (especially that in SDP) which they have exchanged.

Now we will examine the details of these initial SIP messages!

^† or 5xx or 6xx

Transcript

[slide129] And then Alice's ACK, she sends it To Bob, From Alice, we're all set. This is the only one that's a three-way handshake.