Abstract
This chapter describes the various in-built mechanisms defined by the core session initiation protocol (SIP) specification that allow protocol extension while not disrupting the core SIP behavior and presents some of the general-purpose SIP extensions. The SIP protocol can be extended by adding new headers, methods, parameters, or bodies and for this [RFC 3427] defines the change process for the SIP protocol. To handle the possible interoperability break caused by the introduction of new methods, the clients should either use the OPTIONS request before sending a new method or indicate in the outgoing requests new methods. [RFC 3265] defines a new SIP extension that provides a framework for asynchronous event notification in the context of SIP-based communication systems, which is SIP specific and focused on complementing the key communications functionalities enabled by the core SIP. The extension provides a framework that allows SIP nodes to request notification from remote nodes indicating that certain events have occurred. [RFC 4244] defines an SIP extension that allows the capturing of the request history information of a call, which is based on a new header field called history-Info that is updated every time retargeting occurs.
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