ame in the URI contains either a fully qualified domain name (FQDN) or an IP address (IPv4 or IPv6 address). The port num- ber is used where necessary. If it is omitted, the default SIP port numbers are used, which are ports 5060 and/or 5061 (for both, TCP and UDP as transport layer protocols). The general form of the SIP URI is the following: sip:{user[:password]@}host[:port][;uri-parameters][?headers] sips:{user[:password]@}host[:port][;uri-parameters][?headers) Generally, there are two types of SIP network elements, i.e. two types of user agents (UA): User agent client (UAC). This makes SIP requests and sends them to servers. User agent server (UAS). This receives the requests, processes them, and returns SIP responses. Regarding SIP UAs, a single UA can function as both, UAC and UAS, where the roles of either UAC or UAS last only during a given SIP transaction. SIP UA is a logical endpoint in the SIP network architecture, which is used to create or receive SIP messages, used for management of the SIP sessions. On the SIP servers' side, there are several server types defined by the IETF, which include redirect server (used for redirection of SIP client Table 3.2 SIP response codes and messages. SIP response Description Provisional response, used by SIP servers to indicate a progress, but it does not terminate a SIP transaction (e.g. 100 Trying). Success response, which means that action was successfully received, understood, and accepted (e.g. 200 OK). Redirection response, which indicates that further action is needed for completion of a given request (e.g. 302 Moved Temporarily). Client error, which usually means that the request contains bad syntax or cannot be completed by the server (e.g. 404 Not Found). Server error, which is server-side error on a given valid request from the client side (e.g. 504 Server Time-out). Global failure, which means that the request cannot be fulfilled by any server (e.g. 603 Decline). 3.3 Service Architecture in NGN Location server server Registrar Outgoing (trapezoid) Incomi