SIP Messages

A SIP message usually contains a few more items, including the following:

To Field The recipient of the original SIP message
From Field The sender of the SIP message
Contact Field The IP address of the SIP User Agent
Call-ID Field A number that uniquely identifies a given call between two User Agents; all SIP messages that belong to a single communication stream (a single phone call) use the same Call-ID so that the packets will be grouped correctly
CSeq Field Sequence number of SIP messages; a sequence number is a value that shows the order of packets when several packets are sent between entities, and it usually increments by one
Content-Type Field The MIME type for the payload, such as application/sdp
Content-Length Field The size of ...

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