Session Initiation Protocol, or SIP, is a popular VoIP telecom protocol used to setup and manage calls and messages. There are a ton of RFCs out there related to SIP (RFCs 3261, 3263, 3264, etc.) so in came RFC 5411 to be your guide to all of the SIP related RFCs.
It's known as: A Hitchhiker's Guide to the Session Initiation Protocol (SIP)
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