Session Initiation Protocol (SIP) is an application-layer control protocol for creating, modifying and terminating sessions with one or more participants. Transactions are a fundamental component of SIP. A transaction is a request sent by a client transaction to a server transaction, along with all responses to that request sent from the server transaction back to the client. Any task that a user agent client accomplishes will take place using a series of transactions. A client transaction sends requests, whereas a server transaction responds to such requests.