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The Session Initiation Protocol (SIP) is a signalling protocol used for controlling communication sessions such as Voice over IP telephone calls. SIP is based around request/response transactions, in a similar manner to the Hypertext Transfer Protocol (HTTP). Each transaction consists of a SIP request (which will be one of several request methods), and at least one response.〔 SIP requests and responses may be generated by any SIP user agent; user agents are divided into clients (UACs), which initiate requests, and servers (UASes), which respond to them.〔 A single user agent may act as both UAC and UAS for different transactions:〔 for example, a SIP phone is a user agent that will be a UAC when making a call, and a UAS when receiving one. Additionally, some devices will act as both UAC and UAS for a single transaction; these are called Back-to-Back User Agents (B2BUAs).〔 SIP responses specify a three-digit integer response code, which is one of a number of defined codes that detail the status of the request. These codes are grouped according to their first digit as "provisional", "success", "redirection", "client error", "server error" or "global failure" codes, corresponding to a first digit of 1–6; these are expressed as, for example, "1xx" for provisional responses with a code of 100–199.〔 The SIP response codes are an extension to the HTTP response codes, although not all HTTP response codes are valid in SIP.〔 SIP responses also specify a "reason phrase", and a default reason phrase is defined with each response code.〔 These reason phrases can be varied, however, such as to provide additional information〔 or to provide the text in a different language.〔 SIP, including the response codes and corresponding reason phrases, is defined in Internet Engineering Task Force (IETF) Requests for Comments (RFCs), namely RFC 3261. That RFC includes provisions for later RFCs to update the specification.〔 Specific parts of the SIP protocol, including response codes and their default reason phrases, are registered with the Internet Assigned Numbers Authority (IANA).〔〔 This list details all the SIP response codes defined in IETF RFCs and registered with the IANA . It also includes SIP response codes defined in obsolete SIP RFCs (specifically, RFC 2543), which are therefore not registered with the IANA; these are explicitly noted as such. ==1xx—Provisional Responses== ; 100 Trying : Extended search being performed may take a significant time so a forking proxy must send a 100 Trying response.〔 ; 180 Ringing : Destination user agent received INVITE, and is alerting user of call. ; 181 Call is Being Forwarded : Servers can optionally send this response to indicate a call is being forwarded.〔 ; 182 Queued : Indicates that the destination was temporarily unavailable, so the server has queued the call until the destination is available. A server may send multiple 182 responses to update progress of the queue.〔 ; 183 Session in Progress : This response may be used to send extra information for a call which is still being set up.〔 ; 199 Early Dialog Terminated : Can be used by User Agent Server to indicate to upstream SIP entities (including the User Agent Client (UAC)) that an early dialog has been terminated. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「List of SIP response codes」の詳細全文を読む スポンサード リンク
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