The general structure of a SIP message is based on Internet Message Format (RFC5322, RFC2822). With some minor differences in character sets the SIP message syntax is identical to HTTP 1.1 (but surely the SIP is not an extension of HTTP).
The SIP messages start with the Start-line, which is followed by a number of header fields in the format name:value. An empty line separates the header fields from the optional message body. As the SIP is a textual protocol, with just a little practice with Wireshark or a similar tool, it is not that difficult to analyze the massages. But to truly understand the headers in the context of IMS requires some experience. In this post we will highlight the most important message parameters.