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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 consistent with 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 [1] :§21.4.18 or to provide the text in a different language.

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.

This list includes all the SIP response codes defined in IETF RFCs and registered in the SIP Parameters IANA registry as of 14 July 2017. 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.

List of SIP status codes

1xx – Provisional

  • Preliminary information that the server is still performing further actions and therefore cannot yet send a final response.

2xx – Successful

  • The request was successful.

3xx – Redirection

  • These messages inform about a new contact address of the called party or about other services that enable the connection to be established successfully.

4xx – Request Failures

  • Request failures are negative confirmations. The previous message could not be processed.

5xx – Server Failures

  • A server involved in the transmission could not process a message.

6xx – Global Failures

General errors: The server has been contacted successfully, but the transaction does not take place.

7xx – Fehlercodes des SIP-Stacks

100TryingAn attempt is made to transfer the call.
180RingingAn attempt is made to ring from the called party.
181Call Is Being ForwardedThe call is forwarded.

The call is on hold.

183Session ProgressThe connection is established.
199Early Dialog TerminatedThe dialog was closed during connection setup.
200OKThe request has been processed successfully and the result of the request is transferred in the response.
202AcceptedThe request has been accepted, but will be executed at a later time.
204No NotificationThe request was executed successfully, but the corresponding response is deliberately not sent.
300Multiple ChoicesThere is no unique destination address for the remote terminal.
301Moved PermanentlyThe called party is permanently reachable somewhere else.
302Moved TemporarilyThe called party is temporarily reachable somewhere else.
305Use ProxyThe specified proxy must be used.
380Alternative ServiceThe call was not successful, but alternative services are available.

400Bad RequestThe SIP request is incorrect.
401UnauthorizedThe authorization is incorrect.
402Payment RequiredNot yet defined; intended for "not enough credit available".
403ForbiddenThe request was invalid.
404Not FoundThe remote terminal was not found or does not exist.
405Method Not AllowedThe method of the request (for example, SUBSCRIBE or NOTIFY) is not allowed.
406Not AcceptableThe call options are not allowed.
407Proxy Authentication RequiredThe proxy needs authorization.
408Request TimeoutTimeout - The remote peer does not respond within a reasonable time.
410GoneThe desired subscriber can no longer be reached at the specified address.
412Conditional Request FailedThe prerequisites for processing the request could not be met because a request required for this failed.
413Request Entity Too LargeThe message content is too large.
414Request URI Too LongThe SIP address (URI) of the request is too long.
415Unsupported Media TypeThe codec is not supported.
416Unsupported URI SchemeThe SIP address is incorrect.
417Unknown Resource-PriorityThe request should be treated with a certain priority, but the server does not understand the details.
420Bad ExtensionThe server does not understand a protocol extension.
421Extension RequiredThe server needs a protocol extension.
422Session Interval Too SmallThe Session Expires value is too low for the server.
423Interval Too BriefThe value of the desired machining time is too short.
428Use Identity HeaderThe identity header is missing.
429Provide Referrer IdentityNo valid referred by token is specified.
430Flow FailedThe particular routing failed (proxy internal, endpoints should treat the response like code 400).
433Anonymity DisallowedThe server refuses to process anonymous requests.
436Bad Identity-InfoThe SIP address contained in the identity header is invalid, unavailable, or not supported.
437Unsupported CertificateThe verifier cannot verify the certificate in the identity header.
438Invalid Identity HeaderThe certificate in the identity header is invalid.
439First Hop Lacks Outbound SupportThe registrar supports outbound feature, but the proxy used does not.
440Max-Breadth ExceededIt is no longer possible to derive concurrent forks from the query.
469Bad Info PackageUnsuitable Info-Package - Transmission error, resend.
470Consent NeededThe server has no access rights to at least one of the specified SIP addresses.
480Temporarily UnavailableThe called party is currently not reachable.
481Call/Transaction Does Not ExistThis connection does not exist (anymore).
482Loop DetectedA forwarding loop has been detected.
483Too Many HopsToo many forwarding steps were identified.
484Address IncompleteThe SIP address is incomplete.
485AmbiguousThe SIP address cannot be uniquely resolved.
486Busy HereThe called party is busy.
487Request TerminatedThe call attempt was aborted.
488Not Acceptable HereIllegal call attempt.
489Bad EventThe server does not know the specified event.
491Request PendingA request from the same dialog is still being processed.
493UndecipherableThe request contains an encrypted MIME body that the recipient cannot decrypt.
494Security Agreement RequiredThe request requires a security agreement, but does not include a security mechanism supported by the server.

500Server Internal ErrorInternal server error.
501Not ImplementedThe server does not support the SIP request.
502Bad GatewayThe gateway in the SIP request is corrupted.
503Service UnavailableThe server's SIP service is temporarily unavailable.
504Server Time-outThe server cannot reach another server in a reasonable time.
505Version Not SupportedThe SIP protocol version is not supported by the server.
513Message Too LargeThe SIP message is too large for UDP; TCP must be used.
580Precondition FailureThe server cannot or does not want to meet the requirements for processing the request.
600Busy EverywhereAll terminal devices of the called subscriber are occupied.
603DeclinedThe called party has rejected the call attempt.
604Does Not Exist AnywhereThe called party no longer exists.
606Not AcceptableThe terminal device of the called party rejects the SIP request as invalid.
701Party Hangs UpThe called party has hung up.