SIP（Session Initiation Protocol）是由IETF定义

				INVITE sip:user@company.com  SIP/2.0
				To: sip:user@company.com
				From: sip:caller@university.edu
				Call-ID: 3@10.0.0.1
				CSeq: 1 INVITE
				Via: SIP/2.0/UDP 135.180.130.133
				Content-Type: application/sdp
				Content-Length: 174
				
				v=0
				o=mhandley 29739 7272939 IN IP4 126.5.4.3
				s=SIP Call
				c=IN IP4 135.180.130.88
				t=3149328700 0
				m=audio 49210 RTP/AVP 0 12
				m=video 3227 RTP/AVP 31
				a=rtpmap:31 LPC/8000