The INVITE client transaction is created by the transaction user

The INVITE client transaction is created by the transaction user(TU) at the client side. When the transaction user wants to initiate the session, it creates the INVITE client transaction. Then the transaction user send this INVITE request to the transaction layer. As shown in Fig.3(a), there are five different states that INVITE client transaction can be entered:\\
1.~Calling~~~~~~~~~~~~~~~~2.~Proceeding~~~~~~~~~~~3.~Accepted\\
4.~Completed~~~~~~~~~~~5.~Terminated\\
When a INVITE client transaction is initiated we entered the \textit{initial} \textbf{\textit{Calling}} state. The transaction layer passes this INVITE request created by the transaction user(TU) to the transport layer. Transport layer then passes this INVITE request to the server side and start \textit{timer B}. If an unreliable transport is used start \textit{timer A}. In the \textit{initial} \textbf{\textit{Calling}} state six events take place for the INVITE client transaction. When \textit{timer A} fires it reset the timer by pushing the transaction and retransmit the INVITE request to the server. The client transaction change the state from \textit{initial} to \textit{terminated} when \textit{timer B} fires. The \textit{transport error} occur when the transport layer is unsuccessful to forward the INVITE request across the network, then the transaction entered the \textit{terminated} state and update the transaction user(TU). If the \textit{provisional response}(1XX) is received, the client transaction deliver this request to the transaction user(TU) and enter into the \textbf{\textit{Proceeding}} state. In case of receiving the final \textit{successful response} (2XX), i.e.server accept the INVITE request successfully, the transaction notify its transaction user(TU) regrading the request and entered into the \textbf{\textit{Accepted}} state. When the transaction receives the \textit{non-success response} (3XX-6XX), i.e. a server receive the INVITE request but does not accept it, the transaction passes the response to transaction user(TU) and generate an acknowledgment(ACK), forward this ACK to the transport layer, and enter into the \textbf{\textit{Completed}} state.\\
If the transaction is in the \textbf{\textit{Proceeding}} state, the transaction can receive one or more \textit{provisional responses} (1XX), notify its transaction user(TU) and continue to exits in the \textbf{\textit{Proceeding}} state. It can also receive a final \textit{successful response}(2XX),passes this response to the transaction user(TU) and enter into the \textbf{\textit{Accepted }} state, or it can also receive a \textit{non-success response}(3XX-6XX) forward the response to the transaction user(TU) and generate an ACK, passes the ACK to the transport layer and enter into the \textbf{\textit{Completed}} state.\\