SIP session messaging protocol
SIP-based communication uses a text-based request/reply structure which is very close to the web HTTP protocol. The client device makes a request to a server, which replies to the client with a response code. The client can then take further action based on this. The SIP protocol defines six types of requests and six types of replies.
The range of requests are:
| SIP request | Description |
|---|---|
| REGISTER | Register's a client ( user ) via their IP location. |
| INVITE | Starts a communication session and invites a contact to join the session. |
| ACK | ACKnowledges the last message exchange from a user. |
| BYE | Sent from a user to leave the session or decline the session. |
| CANCEL | Used to CANCEL a request to a user. |
| OPTIONS | Requests information about the server's features and offerings. |
| INFO | Used to send messages during a session. |
And the range of replies are:
| SIP Reply | Description |
|---|---|
| 1xx (Information) | Provides progress response to the caller, eg 180 = Ringing. |
| 2xx (Success) | Returns that the request has been accepted. Eg 200 = ok. |
| 3xx (Redirect) | Replies with a new location to where the request should be sent. Eg 302 = Moved temporarily. |
| 4xx (Request Failure) | Gives an error message where a request has not been processed successfully; eg: 404 = Not Found. |
| 5xx (Server Failure) | Replies that there is a specific server problem, eg 501 = Not implemented. |
| 6xx (Global Failure) | Indicates that there has been a global failure, eg 603 = Decline. |
