e OPTIONS method is used to query an SIP server/client default ports for SIP, although any number above 49172 capability. In addition to the six methods defined in RFC may be used. The protocol stack for SIP-based IP phone 3261, other methods were added later as SIP extensions service is shown in Figure 3.6-3. and specified in different RFCs. Examples include INFO Video I/O Audio I/O Capturing/Display Recording/Playback Video Codec Audio Codec Network QoS VT Call A/V Sync Control RTP with Payload Format UDP or TCP IPv4, IPv6 IP Networks Figure 3.6-2 A typical architecture for an IP-based video and audio communication system. Real-time communication over networks CHAPTER 3.6 Signaling INVITE Video/Audio 180 RINGING Call setup 200 OK Figure 3.6-3 Protocol stack (signaling flow and data flow). (RFC 2976 [2]), MESSAGE (RFC 3428 [3]), NOTIFY (RFC 3265 [4]), PRACK (RFC 3262 [5]), REFER (RFC conversation conversation 3515 [6]), SUBSCRIBE (RFC 3265 [4]), and UPDATE (RFC 3311 [7]). The response message is called the "response code" in SIP. The SIP response codes are inherited from HTTP/ Call tear down 200 OK 1.1, except for the 6xx class, which is defined by SIP itself (RFC 3261). The six classes of SIP response codes are described briefly here. 1xx (provisional response): information to indicate Figure 3.6-4 Simple message flow for call setup and tear down. current status before a definitive response. The 1xx re- sponse is designed such that an ACK is never triggered by it and thus the reliability for 1xx transmission is not 3.6.2.2.1 Address critical. 180 Ringing is an example of a 1xx response, SIP supports a variety of addressing schemes, including which is used to inform the originator that the UA has SIP URI (Uniform Resource Identifiers), secure SIP already received the INVITE request. URI, telephone URI, and e-mail URL (Uniform Re- 2xx (successful response): a response used to indicate source Locator). SIP URI is usually represented as sip: that the request has been successfully received. Example: P<userinfo>@<hos