Receiving Messages

The application sends the incoming message to the stack along with the connection object. If the transport is TCP, the message formatting layer breaks the byte stream at message boundaries. The SIP message's Content-Length header determines the message boundaries. A Content-Length header must be present in every message delivered over TCP in order to comply with RFC 3261. If the stack receives a message over TCP that does not contain a Content-Length header, the stack's behavior is unspecified.

The message formatting layer holds excess data that is not part of the current message for use with the next packet. When the system closes or reuses a connection, the connection manager must notify the stack. After receiving the notification, the stack frees the data and resources that the stack allocated to that connection. The stack delivers the message to the application by calling the receive function. The application registers the receive function with the stack during stack initialization.