TAILIEUCHUNG - Chapter 4 - Session Control on the Internet

Many think that the most important component of the signaling plane is the protocol that performs session control. The protocol chosen to perform this task in the IMS is the Session Initiation Protocol (SIP) (defined in RFC 3261 [286]). SIP was originally developed within the SIP working group in the IETF. Even though SIP was initially designed to invite users to existing multimedia conferences, today it is mainly used to create, modify and terminate multimedia sessions. | Chapter 4 Session Control on the Internet Many think that the most important component of the signaling plane is the protocol that performs session control. The protocol chosen to perform this task in the IMS is the Session Initiation Protocol SIP defined in RFC 3261 286 . SIP was originally developed within the SIP working group in the IETF. Even though SIP was initially designed to invite users to existing multimedia conferences today it is mainly used to create modify and terminate multimedia sessions. In addition there exist SIP extensions to deliver instant messages and to handle subscriptions to events. We will first look at the core protocol used to manage multimedia sessions and then we will deal with the most important extensions. SIP Functionality Protocols developed by the IETF have a well-defined scope. The functionality to be provided by a particular protocol is carefully defined in advance before any working group starts working on it. In our case the main goal of SIP is to deliver a session description to a user at their current location. Once the user has been located and the initial session description delivered SIP can deliver new session descriptions to modify the characteristics of the ongoing sessions and terminate the session whenever the user wants. Session Descriptions and SDP A session description is as its name indicates a description of the session to be established. It contains enough information for the remote user to join the session. In multimedia sessions over the Internet this information includes the IP address and port number where the media needs to be sent and the codecs used to encode the voice and the images of the participants. Session descriptions are created using standard formats. The most common format for describing multimedia sessions is the Session Description Protocol SDP defined in RFC 2327 160 . Note that although the P in SDP stands for Protocol SDP is simply a textual format to describe multimedia .

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