The transaction management layer creates and maintains transaction states for both clients and servers. The transaction management layer complies with Section 17 of RFC 3261.
The stack maintains a hash table of transactions. The index of this hash table is an MD5 hash. For messages that comply with RFC 3261, the MD5 hash is the hash of the branch ID. For messages that comply with RFC 2543, the MD5 hash is of a combination of the branch ID, Call-ID, From, To, and Cseq fields. The stack uses the branch ID in the topmost VIA header to identify the RFC that a message complies with. Messages that comply with RFC 3261 prefix their branch ID with the string z9hG4bK.
The library initializes the transaction layer as part of the stack initialization. The library also initializes the transaction error and state transition callback functions if the application provides them.