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It is important to note that the tester does not replay messages in timer order alone. The tester is event driven. It constructs a Directed Acyclic Event Graph (DAG) and a schedule in memory before running the test. Each DAG node represents a message to be sent (SIP request or response ) to be sent. A node of the DAG becomes ready when all events that are supposed to have happened before that node have been observed at the tester. The DAG represents a partial ordering of events and actions.
The tester can emulate proxy as well as Use Agent. Requests sent from the proxy server can have multiple Via headers. The tester preserves via header orderings and parameters while re-writing the parameters of this header.