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. : §8.3.1 204 No Notification Indicates the request was successful, but the corresponding response will not be received. : §21.2.1 202 Accepted Indicates that the request has been accepted for processing, but the processing has not been completed. 2xx-Successful Responses 200 OK Indicates that the request was successful. : §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. : §21.1.4 183 Session Progress This response may be used to send extra information for a call which is still being set up. A server may send multiple 182 responses to update progress of the queue. : §21.1.3 182 Queued Indicates that the destination was temporarily unavailable, so the server has queued the call until the destination is available. : §21.1.2 181 Call is Being Forwarded Servers can optionally send this response to indicate a call is being forwarded. : §21.1.1 180 Ringing Destination user agent received INVITE, and is alerting user of call. : §20.43 The current list of official warnings is registered in the SIP Parameters IANA registry.ġxx-Provisional Responses 100 Trying Extended search being performed may take a significant time so a forking proxy must send a 100 Trying response. The Warning contains a separate three-digit code followed by text with more details about the warning. SIP responses may also include an optional Warning header, containing additional details about the response. 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. 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. That RFC also defines a SIP Parameters Internet Assigned Numbers Authority (IANA) registry to allow other RFC to provide more response codes. The SIP response codes and corresponding reason phrases were initially defined in RFC 3261. : §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. SIP responses also specify a "reason phrase", and a default reason phrase is defined with each response code. : §7.2 The SIP response codes are consistent with the HTTP response codes, although not all HTTP response codes are valid in SIP. 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. 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. Additionally, some devices will act as both UAC and UAS for a single transaction these are called Back-to-Back User Agents (B2BUAs). : §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. 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. Each transaction consists of a SIP request (which will be one of several request methods), and at least one response. SIP is based on request/response transactions, in a similar manner to the Hypertext Transfer Protocol (HTTP). The Session Initiation Protocol (SIP) is a signalling protocol used for controlling communication sessions such as Voice over IP telephone calls.
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