SIP request messages

There are several types of message requests. SIP is transactional, communicating through requests and replies. The most important types of requests are described in the following table:

Most of the time, you will use REGISTER, INVITE, ACK, BYE, and CANCEL. Some messages are used for other features. For example, INFO is used for Dual-tone Multi-frequency (DTMF) relay and mid-call signaling information. PUBLISH, NOTIFY, and SUBSCRIBE give support to the presence systems. REFER is used for call transfer and MESSAGE for chat applications. Newer requests can appear depending on the protocol standardization process. Responses to these requests are in the text format as in the HTTP protocol. Some of the most important replies are shown as follows: