Session Initiation ProtocolIntroductionSession Initiation Protocol (SIP) is a signaling protocol used to establish a multimedia communications session over an Internet Protocol (IP) network. SIP was originally designed by Mark Handley and Henning Schulzrinne in 1996. The latest version of SIP is specified in RFC 3261 of the Internet Engineering Task Force (IETF) Network Working Group. SIP is a request response protocol, that is, it processes client requests and server responses. It is very similar to two other Internet protocols like SMTP (Simple Mail Transfer Protocol) and HTTP (Hyper Text Transfer Protocol). SIP is used in instant messaging (IM), streaming media distribution, file transfer, video conferencing, online gaming, and presence information. Session Initiation Protocol is an application layer protocol that can run over User Datagram Protocol (UDP), the flow control transmission protocol. (SCTP) and Transmission Control Protocol (TCP). It facilitates the initiation, termination and modification of a communication session between individual or multiple recipients (unicast or multicast session), it does not know the details of a session. SIP also supports redirection services, which allow users to initiate and receive communication services from any location or network. In November 2000, SIP was accepted as a permanent part of the IP Multimedia System (IMS) architecture and 3GPP signaling protocol for IP-based streaming media services in cellular systems. Function or Features SIP has four key functions that facilitate various interaction capabilities: User name and location translation: This involves the translation of user name information into SIP location information, which ensures that the middle of the paper allows customers to bring their own SIP devices such as softphones or telephone extensions. SIP is used by web developers to develop services and interfaces such as SIP-CGI, SIP servlets, JAIN™ API, etc. Session Initiation Protocol for Instant Messaging and Presence Extensions, also known as SIMPLE, is a SIP-based suite used for presence information and instant messaging. It uses Message Session Relay Protocol (MSRP) to transfer files during an instant messaging session. SIP services are also used in mobile technology like 3G, WAP, etc. Description – This article contains information about the SIP protocol used in VoIP. It also provides information about SIP architecture, SIP components, SIP applications and the difference between SIP and H.323 protocols. Keywords – voip, sip, sip and h.323, sip architecture, sip applications, sip functions.