As many of my readers know, every few months I teach a two and a half day class on “all things SIP.” My students are exposed to everything from “why SIP” to the nitty-gritty of SIP requests, responses, and call flows. I even speak about some of the more esoteric topics such as To and From tags, the Replaces header, nonce values, and TR-87.
Included in the esoteric list is the PRACK (Provisional Response Acknowledgement) method. PRACK wasn’t in the original SIP specification and was introduced later in RFC 3262. It came about after it was realized that some user agent servers need to know that a provisional response was received by a user agent client. Before PRACK, 1xx responses sent using UDP might get lost and the sender would never know. PRACK adds a layer of reliability to an otherwise unreliable call flow.
My latest addition to Avaya Connected presents an intimate look at the SIP PRACK method.
Tao, Zen, and Tomorrow 
[…] Known for his in depth look at the finer details of SIP, UC, and VoIP; Prokop recently posted A Detailed Look at the SIP PRACK Method. […]
broken link to understanding avaya sip prack. are oyu able to repost. i think it’s because avaya changed their site yet again.
I will keep looking, but I cannot find the old article. Sorry.