Session Initiation Protocol (SIP) is the IETF's standard for establishing VOIP connections. SIP is an application layer control protocol for creating, modifying and terminating sessions with one or more participants. The architecture of SIP is similar to that of HTTP (client-server protocol). Requests are generated by the client and sent to the server. The server processes the requests and then sends a response to the client. A request and the responses for that request make a transaction.

The Session Initiation Protocol (SIP) supports a Peer-to-Peer VOIP network. The following are the key elements in a SIP network.

SIP Network Elements

• User Agents: A software program installed in a user's terminal or an IP phone to initiate and terminate phone calls, plus data and video communications. There are two logic parts in the user agents: User Agent Server (UAS) and User Agent Client (UAC). UAC sends requests and receives responses. UAS receives requests and sends responses.
• Proxy Server: Performs routing of a session invitations according to invitee's profile. There are two basic types of SIP proxy servers--stateless and stateful. Stateless servers are simple message forwarders. Stateful proxies, upon reception of a request, create a state and keep the state until the transaction finishes.
• Redirect Server Receives a request and sends back a reply containing a list of the current location of a particular user, by looking up the intended recipient of the request in the location database created by a registrar.
• Registrar Server: A registrar is a server that accepts REGISTER requests and places the information it receives in those requests into the location service for the domain in handles.

Main IETF SIP Standards

SIP Architecture

Related Terms: SIP Architecture, SIP network, SIP VOIP

Reference Links: http://www.javvin.com/protocolVOIP.html: VOIP Protocols