| INVITE | A SIP device is being invited to participate in a call. |
| ACK | Confirms that the client has received a final response to an INVITE request. |
| BYE | Terminates a SIP call. Can be sent by any party involved. |
| CANCEL | Cancels any pending call but does not terminate a call that has already been connected. |
| OPTIONS | Queries the capabilities of servers without requesting to establish a call. |
| REGISTER | Registers an IP with a SIP registrar. |
| PRACK | Insures reliability of provisional 1xx responses if a UAS offers them. |
| UPDATE | Updates a previously made offer for a not-yet-established session. |
| REFER | Initiates a call transfer by telling the recipient (specified by URI) to contact a third party using the contact information provided in the request. |
| SUBSCRIBE | Subscribes to be notified of an event occurrence; for example a user presence update. |
| NOTIFY | Used to notify that an event has occurred. |
| MESSAGE | A method signifying the payload is an instant message. |