 SIP can be used either in a client-server or in a peer-to-peer manner. Generally, SIP is a text-based protocol similar to HTTP (and HTTP was similar to SMTP for email communication). SIP uses messages in a request/response manner sim- ilar to HTTP. For such a purpose SIP defines six main messages used for session setup, management, and termination, as given in Table 3.1. The responses for the SIP messages are used in the same format as responses for HTTP (which appeared before SIP). So, similar to the tradition already established by the IETF Table 3.1 Main SIP messages. SIP message Description INVITE SIP session initialization message. Acknowledgment response upon receipt of successful SIP request message (e.g. INVITE message). SIP session termination message. CANCEL To cancel initialization process already started (e.g. it is used when a client sends an INVITE and then changes its decision to call). REGISTER Used by SIP user agent (in user equipment) to register its current IP address and the SIP URIs for which the user wants to receive calls. OPTIONS Used to request information about capabilities of the caller, while this message does not set up a session by itself (a session is set up only with the INVITE message). 3 QoS in NGN and Future Networks for its standardized protocols on the application layer, SIP does not reinvent the wheel when it is not needed (e.g. for SIP response codes and messages). All response codes are grouped into six groups, where each group of codes is identified by the first digit (response code groups are given in Table 3.2). To invite a given party to a session, SIP uses its naming/addressing schemes. Since the most used addressing form on the Internet at the time of SIP standardization (i.e. year 2002) was the form of email addresses (e.g. user@FullyQualifiedDomainName) and URI (Uniform Resource Identifier) schemes (e.g. http://www.example.com for WWW), SIP also adopts the URI scheme and user addressing scheme based on general standard syntax (as used for WWW and email). The host n