SIP: Session Initiation Protocol is an IP telephony signalling protocol used to establish, modify and terminate VOIP telephone calls. The protocol resembles the HTTP protocol, is text based, open and flexible. SIP uses Methods and Requests to establish a call session. These can be seen below and make troubleshooting with SIP easier.
SIP Requests:
| Command | Use |
| INVITE | Establishes a session |
| ACK | Confirms an INVITE request |
| BYE | Ends a session |
| CANCEL | Cancels establishing of a session |
| REGISTER | Communicates user location (host name, IP) |
| OPTIONS | Communicates information about the capabilities of the calling and receiving SIP phones |
SIP responses:
1xx = informational responses
- 100 Trying
- 180 Ringing
- 181 Call Is Being Forwarded
- 182 Queued
- 183 Session Progress
2xx = success responses
- 200 OK
- 202 accepted: Used for referrals
3xx = redirection responses
- 300 Multiple Choices
- 301 Moved Permanently
- 302 Moved Temporarily
- 305 Use Proxy
- 380 Alternative Service
4xx = request failures
- 400 Bad Request
- 401 Unauthorized: Used only by registrars. Proxys should use proxy authorization 407.
- 402 Payment Required (Reserved for future use)
- 403 Forbidden
- 404 Not Found: User not found
- 405 Method Not Allowed
- 406 Not Acceptable
- 407 Proxy Authentication Required
- 408 Request Timeout: Couldn't find the user in time
- 410 Gone: The user existed once, but is not available here any more
- 413 Request Entity Too Large
- 414 Request-URI Too Long
- 415 Unsupported Media Type
- 416 Unsupported URI Scheme
- 420 Bad Extension: Bad SIP Protocol Extension used, not understood by the server
- 421 Extension Required
- 423 Interval Too Brief
- 480 Temporarily Unavailable
- 481 Call/Transaction Does Not Exist
- 482 Loop Detected
- 483 Too Many Hops
- 484 Address Incomplete
- 485 Ambiguous
- 486 Busy Here
- 487 Request Terminated
- 488 Not Acceptable Here
- 491 Request Pending
- 493 Undecipherable: Could not decrypt S/MIME body part
5xx = server errors
- 500 Server Internal Error
- 501 Not Implemented: The SIP request method is not implemented here
- 502 Bad Gateway
- 503 Service Unavailable
- 504 Server Time-out
- 505 Version Not Supported: The server does not support this version of the SIP protocol
- 513 Message Too Large
6xx = global failures
- 600 Busy Everywhere
- 603 Decline
- 604 Does Not Exist Anywhere
- 606 Not Acceptable
For more information: https://en.wikipedia.org/wiki/Session_Initiation_Protocol
