Q: What is the difference between SIP Transaction, Dialog, and Session ?

A: SIP Transaction is the most basic unit of communication between two hops. Each transaction is identified by a unique branch parameter in the Via header field.   This branch parameter is generated by the sender hop in the communication between two hops.

While SIP Dialogue is a peer-to-peer relationship . It contains one or more transactions. A dialogue is identified by a combination of the Call-ID, local tag, and remote tag in From and To header fields respectively.

The Call-ID is generated by the calling UE. Also the local tag is generated by the calling UE, while the remote tag is generated by the called UE.

Finally, SIP Session encompasses the entire multimedia interaction between two peers. This includes the actual transfer of data like voice, video, or messages.