n or call that is requested. It could be a simple voice (audio) session, a multimedia session such as a video conference, or it could be a gaming session. The INVITE message contains the following fields: INVITE sip :marconi@radio.org SIP/2.0 Via: SIP/2.0/ UDP lab.high-voltage.org:5060;branch=z9hG4bkfw19b Max-Forwards: 70 To: G. Marconi <sip:Marconi@radio.org> From: Nikola Tesla <sip:n.tesla@high-voltage.org>;tag=7634 Call-ID: 123456789@lab.high-voltage.org CSeq: 1 INVITE Subject: About That Power Outage Contact: <sip:n.tesla@lab.high-voltage.org> Content-Type: application/sdp Content-Length: 158 o=Tesla 2890844526 2890844526 IN IP4 lab.high-voltage.org s=Phone Call C=IN IP4 100.101.102.103 m=audio 49170 RTP/AVP 0 a=rtpmap : 0 PCMU/8000 Introduction to SIP Since SIP is a text-encoded protocol, this is actually what the SIP message would look like "on the wire" as a UDP datagram being transported over, for example, Ethernet. The fields listed in the INVITE message are called header fields. They have the form Header: Value CRLF. The first line of the request message, called the start line, lists the method, which is INVITE, the Request-URI, then the SIP version number (2.0), all separated by spaces. Each line of a SIP message is terminated by a CRLF. The Request-URI is a special form of SIP URI and indicates the resource to which the request is being sent, also known as the request target. SIP URIs are discussed in more detail in later sections. The first header field following the start line shown is a Via header field. Each SIP device that originates or forwards a SIP message stamps its own address in a Via header field, usually written as a host name that can be resolved into an IP address using a DNS query. The Via header field contains the SIP version number (2.0), a "I", then UDP for UDP transport, a space, then the hostname or address, a colon, then a port number, in this example the "well-known" SIP port number 5060. Transport of SIP using TCP, UDP, TLS, and SCTP and the use of port numbers are covered late