![]() ![]() 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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