 class" response in a request to an unknown user. The other classes of SIP responses are covered in Chapter 5. The response code number in SIP alone determines the way the response is interpreted by the server or the user. The reason phrase, Ringing in this case, is suggested in the standard, but any text can be used to convey more informa- tion. For instance, 180 Hold your horses, I'm trying to wake him up! is a perfectly valid SIP response and has the same meaning as a 180 Ringing response. The 180 Ringing response has the following structure: SIP/2.0 180 Ringing Via: SIP/2.0/UDP lab.high-voltage.org:5060;branch=z9hG4bkfw19b ;received=100.101.102 To: G. Marconi <sip:marconi@radio.org>;tag=a53e4 From: Nikola Tesla <sip:n.tesla@high-voltage.org>>;tag=76341 Call-ID: 123456789@lab.high-voltage.org CSeq: 1 INVITE Contact: <sip:marconi@tower.radio.org> Content-Length: 0 The message was created by copying many of the header fields from the INVITE message, including the Via, To, From, Call-ID, and CSeq, then adding a response start line containing the SIP version number, the response code, and the reason phrase. This approach simplifies the message processing for responses. The Via header field contains the original branch parameter but also has an additional received parameter. This parameter contains the literal IP address that the request was received from (100.101.102.103), which typically is the same address that the URI in the Via resolves using DNS (lab.high-voltage.org) Note that the To and From header fields are not reversed in the response message as one might expect them to be. Even though this message is sent to Marconi from Tesla, the header fields read the opposite. This is because the To and From header fields in SIP are defined to indicate the direction of the request, not the direction of the message. Since Tesla initiated this request, all responses will read To: Marconi From: Tesla. Introduction to SIP The To header field now contains a tag that was generated by Marconi. All future requests and respo