Session Initiation Protocol

Session Initiation Protocol

Session Initiation Protocol was introduced in the year 1999 and implemented in the Application layer of the OSI model to perform communication on Internet telephone networks. This telephony protocol is used for creating, maintaining, and concluding communication sessions such as audio, video, and other text messaging applications. It is applied in VoIP (Voice Over Internet Protocol) and VoLTE (Voice Over Long Term Evolution) in wireless mobile devices. This protocol uses a text-based approach, which works similarly to HTTP (Hyper Text Transfer Protocol) and SMTP (Simple Mail Transfer Protocol) elements.

SIP protocol can be used with TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) as both fall under transport protocol which is used to send data packets. This protocol establishes a session by sending data in form of packets to the target device which are identified by SIP addresses. It does not encode, or perform any error checking during the session so it is used in voice and video protocols over the telephone or mobile network devices.


  • The current location can be registered from the home servers located at the user end. Thereby SIP sends request connection messages that are redirected to the registered user location.

  • It provides authentication and enables end-to-end encryption process using Secure/Multipurpose Internet Mail Extensions (S/MIMS) which is a defined standard for sending digitally encrypted data in mail applications.

  • It can support various ranges of Multimedia applications as it is not restricted to any fixed media format.

  • SIP creates invitations to initiate a session and contains descriptions that are used by the participants to accept on their desired media devices.

  • Users are given control to communicate through unicast or multicast addressing methods.

  • Software applications that implement SIP elements are extendable for upgradation for any new media applications like smart IoT enables devices and automatic sensors.

Network Elements

  • SIP user agent − The user agent is an endpoint that sends, receives, and manages SIP messages. When the User-agent client sends SIP messages it is received at the server end and the SIP response is reverted to the client. As in HTTP protocol, a client sends a header field, description of the software, and product ID in the request message, and these fields are evaluated at the SIP server for configuration purposes.

  • Redirect server − It acts as a user agent server to produce responses to the request received from the client. It instructs the client to communicate with another set of Uniform Resource Identifiers (URI) for resources. This does not intimate requests from SIP and does not accept SIP calls.

  • Proxy server − The servers between the user-agent client and user-agent server send and receive a request which is also called routing of incoming calls as this server sends SIP requests to the nearest destination.

  • Registrar − This element provides the location service on accepting registration requests and it locates nearest peers on receiving the continuous request from SIP clients. This links IP addresses to the URI of the SIP agent.

SIP Messages

As said earlier this protocol works on HTTP requests and response messages. This uses various SIP request methods for signaling purposes which are given below −

Register: Provides information on current user location to the redirection server. It registers the User-agent with the registrar network element.

  • INVITE − This request message is forwarded to a server agent which is generated from a client agent to start a session.

  • ACK − Confirmation of the response that is received for the INVITE request.

  • BYE − Request message sent to terminate the current session or end the call.

  • CANCEL − On receiving a cancel request, SIP cancels the pending request if any.

  • REFER − This command asks the receiver to issue a request for call transfer services.

  • UPDATE − This is used for any modifications made at the session state.

  • OPTIONS − to know details about User agent abilities without making any external request to the user agent directly.


SIP works along with other protocols like Session Description Protocol (SDP) to make a Voice communication process (VoIP). The session, time, and media information are shared with the users to exchange their information and they are attached as payload options to SIP messages as SIP does not send this information itself.

SIP, a signaling protocol present in the Application layer supports VoIP communication to the devices that send request and response messages similar to the working of HTTP protocol. This also supports video, live sessions, and video conferencing applications. It can control Internet telephone calls, internet conferences, and multimedia calls of the communication network.

Updated on: 26-Apr-2023


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