HP-UX Java SIP Stack Programmer's Guide (February 2008)

Figure 1-1 SIP Protocol Structure
Figure 1-1 discusses the following layers:
• The application layer provides application programs with an interface to
communicate and transfer data across the network.
• Following are the SIP protocol layers:
— A transaction user (TU) can be any SIP entity except a stateless proxy. A
transaction user uses transactions to send a request to the peer. It creates a
client transaction and sends the request, the destination IP address, port number,
and transport service to which the request must be sent.
— The transaction layer handles application layer retransmissions and timeouts,
and matches responses to requests. Any task that a user agent client
accomplishes takes place using a series of transactions.
— All SIP components contain a transport layer to send requests and responses
over network transports. For connection-oriented transports, the transport layer
determines the type of connection to use for a request or response. The network
transport can be Transmission Control Protocol (TCP), User Datagram Protocol
(UDP), Transport Layer Security (TLS), or Stream Control Transmission Protocol
(SCTP). The transport layer also determines how a client can send requests and
responses, and how a server can receive requests and responses over the
network.
SIP Architecture 21