SIP

Session Initiation Protocol

Protocol
Introduced in R99
SIP is an IETF-defined application-layer signaling protocol used to create, modify, and terminate multimedia sessions such as voice and video calls over IP networks. In 3GPP, it is the core protocol for IP Multimedia Subsystem (IMS) services, enabling rich communication services over cellular and fixed networks.

Description

The Session Initiation Protocol (SIP) is a text-based, application-layer control protocol standardized by the IETF (RFC 3261 and others). Its primary function is to establish, manage, and tear down interactive communication sessions between participants. These sessions can include voice, video, instant messaging, online games, and virtual reality—essentially any real-time data exchange. SIP operates on a client-server model but is inherently peer-to-peer in its session management. It is independent of the underlying transport layer, commonly running over UDP, TCP, or TLS.

SIP works through the exchange of request and response messages between User Agents (UAs), which can be clients (UAC) initiating requests or servers (UAS) responding to them. A typical SIP transaction for establishing a voice call involves several key messages: the caller's UA sends an INVITE request to the callee's address (a SIP URI). This request is routed through SIP proxy servers, which help locate the callee and enforce routing policies. The callee's UA responds with provisional responses (e.g., 180 Ringing) and a final success response (200 OK), which includes session description parameters typically formatted using the Session Description Protocol (SDP). The caller confirms with an ACK, and the media session (using RTP/RTCP) is established directly between the endpoints. To terminate the session, a BYE request is sent.

Within the 3GPP architecture, SIP is the cornerstone signaling protocol of the IP Multimedia Subsystem (IMS). The IMS provides a standardized framework for delivering multimedia services over IP. Key 3GPP network elements that act as SIP servers include the Proxy-Call Session Control Function (P-CSCF), which is the first contact point for the UE; the Serving-CSCF (S-CSCF), which performs session control and interacts with application servers; and the Interrogating-CSCF (I-CSCF), which interfaces with other networks. SIP in IMS is used for registration, authentication (often using IMS Authentication and Key Agreement (AKA)), and session control for services like Voice over LTE (VoLTE), Voice over NR (VoNR), Rich Communication Services (RCS), and Video Telephony. The protocol is extended with 3GPP-specific headers and parameters to support cellular requirements such as emergency calling, access network information, and policy control integration.

Purpose & Motivation

SIP was created to solve the problem of establishing and managing multimedia sessions in the evolving internet landscape. Prior to SIP, proprietary signaling protocols were common, hindering interoperability between different vendors' equipment and service providers. The growth of IP networks created a need for a standardized, flexible, and extensible signaling protocol that could support the burgeoning variety of real-time applications beyond simple voice. SIP was designed to be simple, modular, and text-based (like HTTP), making it easier to implement, debug, and extend compared to binary protocols like H.323.

3GPP adopted SIP as the signaling protocol for IMS to enable the convergence of cellular telephony with Internet multimedia services. This addressed a critical strategic need: to evolve circuit-switched voice services to a more efficient, feature-rich, all-IP architecture. SIP allowed mobile operators to offer standardized, interoperable voice and video services over their LTE and 5G data networks (VoLTE/VoNR), ensuring service continuity and quality. It also provided the foundation for new revenue-generating services like RCS (branded as Chat) and seamless integration with over-the-top (OTT) applications. By leveraging an IETF standard, 3GPP ensured that IMS-based services could interoperate with fixed-line VoIP networks and the broader internet, facilitating fixed-mobile convergence and fostering a rich ecosystem of application developers and service enablers.

Key Features

  • Text-based, application-layer signaling protocol for initiating, modifying, and terminating sessions
  • Uses a request/response transaction model similar to HTTP
  • Transport-independent, operating over UDP, TCP, or TLS
  • Employs SIP URIs (e.g., sip:[email protected]) for addressing
  • Works in conjunction with SDP for negotiating media session parameters
  • Core protocol within the 3GPP IMS architecture for services like VoLTE and RCS

Evolution Across Releases

R99 Initial

SIP was first introduced into the 3GPP architecture in Release 1999 as part of the initial definition of the IP Multimedia Subsystem (IMS). This established SIP as the core signaling protocol for enabling multimedia services over the PS domain, defining basic registration and session control procedures between the UE and early CSCF elements.

TS 21.905TS 21.978TS 22.228TS 22.940TS 22.949TS 22.953TS 22.977TS 23.125TS 23.141TS 23.179TS 23.207TS 23.218TS 23.221TS 23.228TS 23.231TS 23.271TS 23.278TS 23.280TS 23.281TS 23.333TS 23.379TS 23.392TS 23.417TS 23.517TS 23.700TS 23.701TS 23.802TS 23.815TS 23.849TS 23.976TS 23.979TS 23.981TS 24.103TS 24.141TS 24.147TS 24.166TS 24.167TS 24.173TS 24.196TS 24.206TS 24.216TS 24.228TS 24.229TS 24.247TS 24.259TS 24.281TS 24.282TS 24.315TS 24.323TS 24.379TS 24.391TS 24.404TS 24.405TS 24.406TS 24.407TS 24.408TS 24.410TS 24.416TS 24.428TS 24.429TS 24.447TS 24.454TS 24.484TS 24.504TS 24.505TS 24.508TS 24.516TS 24.523TS 24.524TS 24.525TS 24.528TS 24.529TS 24.604TS 24.605TS 24.606TS 24.607TS 24.608TS 24.610TS 24.615TS 24.616TS 24.628TS 24.629TS 24.642TS 24.647TS 24.654TS 24.803TS 24.819TS 24.841TS 24.930TS 25.993TS 26.114TS 26.235TS 26.236TS 26.862TS 26.914TS 26.928TS 26.982TS 26.998TS 27.060TS 29.162TS 29.199TS 29.228TS 29.229TS 29.328TS 29.332TS 29.412TS 29.421TS 29.424TS 29.433TS 29.458TS 29.514TS 29.562TS 29.658TS 31.103TS 31.829TS 32.260TS 32.272TS 32.280TS 32.281TS 32.296TS 32.808TS 33.106TS 33.107TS 33.108TS 33.126TS 33.127TS 33.128TS 33.141TS 33.203TS 33.210TS 33.790TS 33.804TS 33.838TS 33.871TS 33.978TS 36.579TS 37.579TS 38.291TS 43.129

Defining Specifications

SpecificationTitle
TS 21.905 3GPP TS 21.905
TS 21.978 3GPP TS 21.978
TS 22.228 3GPP TS 22.228
TS 22.940 3GPP TS 22.940
TS 22.949 3GPP TS 22.949
TS 22.953 3GPP TS 22.953
TS 22.977 3GPP TS 22.977
TS 23.125 3GPP TS 23.125
TS 23.141 3GPP TS 23.141
TS 23.179 3GPP TS 23.179
TS 23.207 3GPP TS 23.207
TS 23.218 3GPP TS 23.218
TS 23.221 3GPP TS 23.221
TS 23.228 3GPP TS 23.228
TS 23.231 3GPP TS 23.231
TS 23.271 3GPP TS 23.271
TS 23.278 3GPP TS 23.278
TS 23.280 3GPP TS 23.280
TS 23.281 3GPP TS 23.281
TS 23.333 3GPP TS 23.333
TS 23.379 3GPP TS 23.379
TS 23.392 3GPP TS 23.392
TS 23.417 3GPP TS 23.417
TS 23.517 3GPP TS 23.517
TS 23.700 3GPP TS 23.700
TS 23.701 3GPP TS 23.701
TS 23.802 3GPP TS 23.802
TS 23.815 3GPP TS 23.815
TS 23.849 3GPP TS 23.849
TS 23.976 3GPP TS 23.976
TS 23.979 3GPP TS 23.979
TS 23.981 3GPP TS 23.981
TS 24.103 3GPP TS 24.103
TS 24.141 3GPP TS 24.141
TS 24.147 3GPP TS 24.147
TS 24.166 3GPP TS 24.166
TS 24.167 3GPP TS 24.167
TS 24.173 3GPP TS 24.173
TS 24.196 3GPP TS 24.196
TS 24.206 3GPP TS 24.206
TS 24.216 3GPP TS 24.216
TS 24.228 3GPP TS 24.228
TS 24.229 3GPP TS 24.229
TS 24.247 3GPP TS 24.247
TS 24.259 3GPP TS 24.259
TS 24.281 3GPP TS 24.281
TS 24.282 3GPP TS 24.282
TS 24.315 3GPP TS 24.315
TS 24.323 3GPP TS 24.323
TS 24.379 3GPP TS 24.379
TS 24.391 3GPP TS 24.391
TS 24.404 3GPP TS 24.404
TS 24.405 3GPP TS 24.405
TS 24.406 3GPP TS 24.406
TS 24.407 3GPP TS 24.407
TS 24.408 3GPP TS 24.408
TS 24.410 3GPP TS 24.410
TS 24.416 3GPP TS 24.416
TS 24.428 3GPP TS 24.428
TS 24.429 3GPP TS 24.429
TS 24.447 3GPP TS 24.447
TS 24.454 3GPP TS 24.454
TS 24.484 3GPP TS 24.484
TS 24.504 3GPP TS 24.504
TS 24.505 3GPP TS 24.505
TS 24.508 3GPP TS 24.508
TS 24.516 3GPP TS 24.516
TS 24.523 3GPP TS 24.523
TS 24.524 3GPP TS 24.524
TS 24.525 3GPP TS 24.525
TS 24.528 3GPP TS 24.528
TS 24.529 3GPP TS 24.529
TS 24.604 3GPP TS 24.604
TS 24.605 3GPP TS 24.605
TS 24.606 3GPP TS 24.606
TS 24.607 3GPP TS 24.607
TS 24.608 3GPP TS 24.608
TS 24.610 3GPP TS 24.610
TS 24.615 3GPP TS 24.615
TS 24.616 3GPP TS 24.616
TS 24.628 3GPP TS 24.628
TS 24.629 3GPP TS 24.629
TS 24.642 3GPP TS 24.642
TS 24.647 3GPP TS 24.647
TS 24.654 3GPP TS 24.654
TS 24.803 3GPP TS 24.803
TS 24.819 3GPP TS 24.819
TS 24.841 3GPP TS 24.841
TS 24.930 3GPP TS 24.930
TS 25.993 3GPP TS 25.993
TS 26.114 3GPP TS 26.114
TS 26.235 3GPP TS 26.235
TS 26.236 3GPP TS 26.236
TS 26.862 3GPP TS 26.862
TS 26.914 3GPP TS 26.914
TS 26.928 3GPP TS 26.928
TS 26.982 3GPP TS 26.982
TS 26.998 3GPP TS 26.998
TS 27.060 3GPP TS 27.060
TS 29.162 3GPP TS 29.162
TS 29.199 3GPP TS 29.199
TS 29.228 3GPP TS 29.228
TS 29.229 3GPP TS 29.229
TS 29.328 3GPP TS 29.328
TS 29.332 3GPP TS 29.332
TS 29.412 3GPP TS 29.412
TS 29.421 3GPP TS 29.421
TS 29.424 3GPP TS 29.424
TS 29.433 3GPP TS 29.433
TS 29.458 3GPP TS 29.458
TS 29.514 3GPP TS 29.514
TS 29.562 3GPP TS 29.562
TS 29.658 3GPP TS 29.658
TS 31.103 3GPP TR 31.103
TS 31.829 3GPP TR 31.829
TS 32.260 3GPP TR 32.260
TS 32.272 3GPP TR 32.272
TS 32.280 3GPP TR 32.280
TS 32.281 3GPP TR 32.281
TS 32.296 3GPP TR 32.296
TS 32.808 3GPP TR 32.808
TS 33.106 3GPP TR 33.106
TS 33.107 3GPP TR 33.107
TS 33.108 3GPP TR 33.108
TS 33.126 3GPP TR 33.126
TS 33.127 3GPP TR 33.127
TS 33.128 3GPP TR 33.128
TS 33.141 3GPP TR 33.141
TS 33.203 3GPP TR 33.203
TS 33.210 3GPP TR 33.210
TS 33.790 3GPP TR 33.790
TS 33.804 3GPP TR 33.804
TS 33.838 3GPP TR 33.838
TS 33.871 3GPP TR 33.871
TS 33.978 3GPP TR 33.978
TS 36.579 3GPP TR 36.579
TS 37.579 3GPP TR 37.579
TS 38.291 3GPP TR 38.291
TS 43.129 3GPP TR 43.129