The Session Initiation Protocol (SIP) is the signaling protocol used for initiating and terminating communication sessions such as IM communications and multimedia communications such as VOIP and video over an IP network.
This is the underlying protocol used by Office Communications Server 2007 R2, Office Communicator 2007 R2 and the platform SDKs when managing sessions.
While most of the details of SIP are abstracted by the SDKs, it’s important to understand the underlying protocols used by the products and SDKs when you need to debug an issue.
I picked this book up over the summer to better understand SIP. It’s a quick read and I find myself referring back to it often.