It goes without saying that SIP is a protocol. After all, it is Session Initiation Protocol. It’s a series of request methods, responses, headers, and documented call flows. You send an INVITE and expect to receive a 100 Trying, 180 Ringing, and eventually a 200 OK. That’s the way I was taught SIP and that is what most people assume they will see when they trace SIP calls.
However, SIP as a solution can be quite different. Yes, you will see an INVITE to make a call (most of the time), but you may also see one, two, three, or even more subsequent INVITE requests for that one call.
Case in point is an Avaya Aura system and how SIP messages are processed by Session Manager and Communication Manager. It’s not that Avaya is doing anything wrong. There is no huge diversion from SIP as a protocol. It’s just that SIP as a solution requires network elements to take an active role in how call processing occurs. The value that Avaya adds to SIP calls leads to something more complicated than INVITE – 100 Trying – 180 Ringing – 200 OK – ACK.
In my latest article for the Avaya Connected Blog, I present Part One of my deep dive into Avaya Aura call processing.
Tao, Zen, and Tomorrow 
Hi Andrew Prokop,… am very glad to have you for sharing your knowledge with us..and clearing our doubts ….
am having one doubt..will you please tell me what is port forwarding and how to use port forwarding in sip testing.please clear this.
Port forwarding is used with NAT. Google the phrase and you should be able to find some good information about it.
Thank you so much Andrew Prokop.And one more issue…..For every transaction cseq number will increase right?
In basic call, Ack is considered as seperate transaction but the cseq num is same as of transaction 1.how its possible?
It’s because the two aren’t necessarily related. The cseq in the ACK tells the UAS which transaction is being acknowledged. It doesn’t matter that it’s a different transaction as the one for the ACK.
Thank you so much Andrew Prokop for clearing all my doubts….your blog is soo good and useful for gaining knowledge on sip.following your blog and checking updates in your blog became part in my daily tasks.once again thank you soo much.
I am happy to hear that you find my little hobby useful.