The transport to use, transport flags, remote ip and port the block is to be used to override the T1, T2, even TimerA etc. timer value. The block can for example have { self.t1 = 200; self.te=100 } to override the T1, E on a transaction by transcation basis, otherwise the default is taken from SipperConfigurator or SIP::Transaction::BaseTransaction::T1 in that order. Usage: TU creates transaction and starts invoking "Invocation Action" methods. Also for responses received, checks whether to consume them or not based on consume? method return value. There are four call backs into TU from NICT and they are notification on transport error and transaction timeout. There are 4 TU callbacks.

  (a) transaction_transport_err() gets called should NICT get a transport failure
  (b) transaction_timeout() gets called on timeout of transaction
  (c) transaction_transaction_cleanup() gets called on transaction termination.
  (d) transaction_wrong_state() if the transaction transition is tried in a wrong state
      i.e a message is either received or being tried to send out in a state where it is
      illegal to do so.

The NICT receives responses (provisional and final)

These method is to be invoked by the TU to send the non-invite The state machine drives the whole interaction by calling the callbacks.