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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 on 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.: p11  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.: §8  A single user agent may act as both UAC and UAS for different transactions:: p26  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).: p20  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.: §7.2  The SIP response codes are consistent with the HTTP response codes, although not all HTTP response codes are valid in SIP.: §21  SIP responses also specify a "reason phrase", and a default reason phrase is defined with each response code.: §7.2  These reason phrases can be varied, however, such as to provide additional information: §21.4.18  or to provide the text in a different language.: §20.3  The SIP response codes and corresponding reason phrases were initially defined in RFC 3261. That RFC also defines a SIP Parameters Internet Assigned Numbers Authority (IANA) registry to allow other RFC to provide more response codes.: §27  This list includes all the SIP response codes defined in IETF RFCs and registered in the SIP Parameters IANA registry as of 27 January 2023. This list 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. SIP responses may also include an optional Warning header, containing additional details about the response. The Warning contains a separate three-digit code followed by text with more details about the warning.: §20.43  The current list of official warnings is registered in the SIP Parameters IANA registry. 1xx—Provisional Responses 100 Trying Extended search being performed may take a significant time so a forking proxy must send a 100 Trying response.: §21.1.1  180 Ringing Destination user agent received INVITE, and is alerting user of call.: §21.1.2  181 Call is Being Forwarded Servers can optionally send this response to indicate a call is being forwarded.: §21.1.3  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.: §21.1.4  183 Session Progress This response may be used to send extra information for a call which is still being set up.: §21.1.5  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. 2xx—Successful Responses 200 OK Indicates that the request was successful.: §21.2.1  202 Accepted Indicates that the request has been accepted for processing, but the processing has not been completed.: §7.3.1  Deprecated.: §8.3.1  204 No Notification Indicates the request was successful, but the corresponding response will not be received. 3xx—Redirection Responses 300 Multiple Choices The address resolved to one of several options for the user or client to choose between, which are listed in the message body or the message's Contact fields.: §21.3.1  301 Moved Permanently The original Request-URI is no longer valid, the new address is given in the Contact header field, and the client should update any records of the original Request-URI with the new value.: §21.3.2  302 Moved Temporarily The client should try at the address in the Contact field. If an Expires field is present, the client may cache the result for that period of time.: §21.3.3  305 Use Proxy The Contact field details a proxy that must be used to access the requested destination.: §21.3.4  380 Alternative Service The call failed, but alternatives are detailed in the message body.: §21.3.5  4xx—Client Failure Responses 400 Bad Request The request could not be understood due to malformed syntax.: §21.4.1  401 Unauthorized The request requires user authentication. This response is issued by UASs and registrars.: §21.4.2  402 Payment Required Reserved for future use.: §21.4.3  403 Forbidden The server understood the request, but is refusing to fulfill it.: §21.4.4  Sometimes (but not always) this means the call has been rejected by the receiver. 404 Not Found The server has definitive information that the user does not exist at the domain specified in the Request-URI. This status is also returned if the domain in the Request-URI does not match any of the domains handled by the recipient of the request.: §21.4.5  405 Method Not Allowed The method specified in the Request-Line is understood, but not allowed for the address identified by the Request-URI.: §21.4.6  406 Not Acceptable The resource identified by the request is only capable of generating response entities that have content characteristics but not acceptable according to the Accept header field sent in the request.: §21.4.7  407 Proxy Authentication Required The request requires user authentication. This response is issued by proxies.: §21.4.8  408 Request Timeout Couldn't find the user in time. The server could not produce a response within a suitable amount of time, for example, if it could not determine the location of the user in time. The client MAY repeat the request without modifications at any later time.: §21.4.9  409 Conflict User already registered.: §7.4.10  Deprecated by omission from later RFCs and by non-registration with the IANA. 410 Gone The user existed once, but is not available here any more.: §21.4.10  411 Length Required The server will not accept the request without a valid Content-Length.: §7.4.12  Deprecated by omission from later RFCs and by non-registration with the IANA. 412 Conditional Request Failed The given precondition has not been met. 413 Request Entity Too Large Request body too large.: §21.4.11  414 Request-URI Too Long The server is refusing to service the request because the Request-URI is longer than the server is willing to interpret.: §21.4.12  415 Unsupported Media Type Request body in a format not supported.: §21.4.13  416 .... Discover the Jon Sparks popular books. Find the top 100 most popular Jon Sparks books.