0-699 Global failure A response with a status code from 100 to 199 is considered provisional. Responses from 200 to 699 are final responses. A SIP transaction between a client and a server comprises a request from the client, one or more pro- visional responses, and one final response. (This rule has one exception, as we'll see in the next section.) Together with the status code, SIP responses carry a reason phrase. The latter contains human-readable information about the status code. For instance, a status code of 180 means that the user invited to a session is being alerted. Therefore, the reason phrase might contain "Ringing." The reason phrase can, of course, be written in a language other than English because it will be read by a human. Accordingly, a computer processing a SIP response ignores the reason phrase. It finds sufficient information in the response code. However, it can display the reason phrase to the user, who will certainly find it more useful to know that the remote SIP phone is "Ringing" than to know a 180 response has been received. (From now on, we will cite responses using the status code followed by the reason phrase: for example, "180 Ringing.") Table 5-2 contains all the status codes currently defined with their associated default reason phrases. SIP Requests The core SIP specification defines six types of SIP requests, each of them with a different purpose. Every SIP request contains a field, called a method, which denotes its purpose. The list shows the six methods. Chapter 5 Trying Request entity too large Table 5-2 Ringing Request-URI too large SIP Response Codes Call is being forwarded Unsupported media Queued Bad extension Session progress Temporarily not available Call leg/transaction does not exist Accepted Loop detected Multiple choices Too many hops Moved permanently Address incomplete Moved temporarily Ambiguous Use proxy Busy here Alternative service Request cancelled Bad request Not acceptable here Unauthorized Internal servere error Payment required Not implemented Forbidd