What is SIP?

What is SIP

The Session Initiation Protocol (SIP) is a signaling protocol used to establish sessions in an IP network. An SIP session could be a simple two-way telephone call or it could be a multimedia conference session. The capability to establish SIP sessions means that a host of innovative services become possible, such as voice-enriched e-commerce, web page, instant audio-video messaging, and other IP related services.
The Voice over IP has adopted SIP as its protocol for signaling. SIP is an RFC–3261 standard from the IETF. SIP is still developing and being extended as technology matures and SIP products are socialized in the marketplace.
SIP is a request-response protocol that closely resembles two other Internet protocols, HTTP and SMTP; consequently, SIP sits comfortably alongside Internet applications. Using SIP, telephony becomes another web application and integrates easily into many other Internet services. SIP is a simple toolkit that service providers can use to build converged voice and multimedia services.
One of the amazing things about SIP is that it is a text-based protocol modeled on the request/response model used in HTTP.  This makes it easy to debug because the messages are easy to construct and easy to see.  Contrasted with H.323, SIP is an exceedingly simple protocol.  Nevertheless, it has enough powerful features to model the behavior of a very complex traditional telephone PBX.