SIP is an application layer signaling protocol standardized by the IETF in RFC 3261. That word ‘signaling’ means that it is a protocol used to create a kind of ‘control channel’ for an exchange of media (in telephony this is Real-time Transport Protocol or RTP). That’s not to say that we don’t break the rules occasionally, and stuff some text (media) into a SIP MESSAGE, but for the most part, SIP is strictly for control.

SIP messages fall into two categories: requests and responses. SIP requests are identifiable as all CAPS English words (INVITE, ACK, BYE, REFER, INFO, etc.), whereas […]