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SIP RFC 3261 - SIP requests and responses - Russell Zachary Feeser
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 …