en Bad gateway Not found Service unavailable Method not allowed Gateway time-out Not acceptable SIP version not supported Proxy authentication required Busy everywhere Request time-out Decline Conflict Does not exist anywhere Not acceptable Length required SIP: Protocol Operation INVITE OPTIONS CANCEL REGISTER We will see in subsequent chapters how some extensions to the core SIP specification define additional methods. Both requests and responses can contain SIP bodies. The body of a mes- sage is its payload. SIP bodies usually consist of a session description. INVITE INVITE requests invite users to participate in a session. The body of INVITE requests contains the description of the session. For instance, when Bob calls Laura, his UA sends an INVITE with a session description to Laura's UA. Let us assume that Bob's UA uses Session Description Protocol (SDP) to describe the session. Her UA receives the INVITE with the following session description: o=Bob 2890844526 2890842807 IN IP4 131.160.1.112 s=I want to know how you are doing C=IN IP4 131.160.1.112 t=0 0 m=audio 49170 RTP/AVP 0 The INVITE received by Laura's UA means that Bob is inviting Laura to join an audio session. From the session description carried in the INVITE, Laura's UA knows that Bob wants to receive Real-time Transport Protocol (RTP) packets containing Laura's voice on 131.160.1.112 User Datagram Protocol (UDP) port number 49170. Her UA also knows that Bob can receive Pulse Code Modulation (PCM) encoded voice. (RTP/AVP0 in the m line indicates PCM.) Laura's UA begins alerting Laura and returns a "180 Ringing" response to Bob's UA. When Laura finally accepts the call, her UA will return a "200 OK" response with a session description in it. Chapter 5 o=Laura 2891234526 2812342807 IN IP4 138.85.27.10 s=I want to know how you are doing c=IN IP4 138.85.27.10 t=0 0 m=audio 20000 RTP/AVP 0 At this point, Laura accepts the call and informs Bob that she will receive RTP packets on 138.85.27.10 UDP port 20000 (Figure 5-1). If, when Laura and Bob are in t