What is SIP?

SIP (Session Initiation Protocol) is an IETF standard multimedia conferencing protocol, which includes voice, video, and data conferencing, for use over packet-switched networks.

SIP is an open standard replacement for the ITU's H.323.

SIP is described in RFC 3621 - SIP: Session Initiation Protocol.

SIP is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences.

SIP invitations used to create sessions carry session descriptions that allow participants to agree on a set of compatible media types. SIP makes use of elements called proxy servers to help route requests to the user's current location, authenticate and authorize users for services, implement provider call-routing policies, and provide features to users. SIP also provides a registration function that allows users to upload their current locations for use by proxy servers. SIP runs on top of several different transport protocols.

Other RFC's which document SIP include:

• RFC 3262 - Reliability of Provisional Responses in the Session Initiation Protocol (SIP)
• RFC 3263 - Session Initiation Protocol (SIP): Locating SIP Servers
• RFC 3264 - An Offer/Answer Model with the Session Description Protocol (SDP)
• RFC 3265 - Session Initiation Protocol (SIP)-Specific Event Notification

Books on SIP

SIP Demystified	State-of-the-art SIP primer SIP (Session Initiation Protocol) is the open standard that will make IP telephony an irresistible force in communications, doing for converged services what http does for the Web. SIP Demystified - authored by Gonzalo Camarillo, one of the contributors to SIP development in the IETF-;gives you the tools to keep your company and career competitive. This guide tells you why the standard is needed, what architectures it supports, and how it interacts with other protocols. As a bonus, you even get a context-setting background in data networking. Perfect if you're moving from switched voice into a data networking environment, here's everything you need to understand: Where, why, and how SIP is used What SIP can do and deliver SIP's fit with other standards and systems How to plan implementations of SIP-enabled services How to size up and choose from available SIP products.
SIP: Understanding the Session Initiation Protocol	This newly revised edition of the ground-breaking Artech House bestseller, SIP: Understanding the Session Initiation Protocol offers a thorough and up-to-date understanding of this revolutionary technology for IP Telephony. Essential reading for anyone involved in the development and operation of voice or data networks, the second edition includes brand new discussions on the use of SIP as a wireless communications protocol and mobility technology. Professionals find details on the latest application areas such as instant messaging. The book explains how SIP is a highly-scalable and cost-effective way to offer new and exciting telecommunication feature sets. From an examination of SIP as a key component in the Internet multimedia conferencing architecture to a look at the future direction of SIP, practitioners get the knowledge they need to design "next generation" networks and develop new applications and software stacks.
Internet Communications Using SIP	Session Initiation Protocol (SIP) has gained tremendous market acceptance since it became an official IETF Internet communications standard in 1999. SIP is the technology that makes it possible for multimedia communications sessions on the Web--ones that allow voice, video, chat, interactive games, and others to run all at the same time. Now that the deployment of real SIP networks is about to take off, two leaders of the commercial rollout deliver complete guidance on this exciting new technology. Geared to IT and networking professionals and decision-makers at Internet service providers (ISPs), as well as networking (NSPs) and application (ASPs) service providers, this book helps readers sort through the available vendor offerings and services to discover how to integrate and maximize SIP's power across their networks.
SIP Beyond VoIP: The Next Step in the IP Communications Revolution	VON Publishing's latest effort is SIP Beyond VoIP, an extraordinary 333-page effort that picks up where previous books have left off about SIP (Session Initiation Protocol), the protocol that has revolutionized the world of VoIP. The book's three distinguished authors relate in great detail how this versatile and extensible protocol has truly "moved beyond VoIP" and is now starting to have an impact on the whole telecommunication industry, including wireless and enterprise communications. Anyone who thinks that SIP has any real competitors will come away from this book in astonishment. "SIP Events" are the glue that even now integrates communications and applications. And "SIP Presence" may well be the "dial tone" of the 21st century. The book's advanced discussion of SIP interleaves with such associated topics as DNS (the Domain Name Service), ENUM (electronic numbering), NAT (Network Address Translation) and firewall traversal, security, Peer-to-Peer SIP (P2P SIP) networks, SIP-based conferencing/collaboration and even accessibility to communications for disabled people. This heavily illustrated, footnoted and fully-indexed book also has a foreword by Vinton Cerf, who writes: "It is my honest opinion that we have barely scratched the surface of the various applications to which SIP may be adapted. If we have seen 1% of the applications of SIP so far, then there are still 99% waiting to be invented, developed or deployed. The generality of SIP will make it a major workhorse the Internet of this century." If you think you know SIP, think again. Get this book its authors will set you straight about SIP, once and for all!