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Hierarchy of Procedure



Dialog/Session : this is concept (a call processing unit) in Signaling part and set up on a endpoint-to-endpoint basis. A dialog can be identified by the combination of To tag, From tag, and Call-ID. (For example, the whole SIP Registration process can be a Dialog and the whole VoLTE call setup process can be another Dialog.


Transation : Transaction is a concept in Signaling part and set up on a hop-by-hop basis. A transaction is identified by Branch value in Via Header.(branch value will change for every single Request-Response round trip). For instance, Request/ACK/BYE are regarded as different transactions. Beside, one single transaction is not terminated until a final response to initial request is received (2xx, 3xx response). Within each single transaction, the direction of transaction indicated by From & To Header Field will never change; but transaction direction might be different among transactions. (We often try to figure out the direction of a transaction of a message by looking at 'from', 'to', but it can easily mislead you or confuse you because 'from', 'to' header does not change within a transaction regardless of message direction)



Further Reading :