K and RSeq headers are described). The PRACK method always increments the CSeq. A PRACK may contain a message body. An example exchange contains: SIP/2 0 180 Ringing Via: SIP/2.0/UDP lucasian.trinity.cambridge.edu.uk ; branch=z9hG4bK452352 ; received=1.2.3. To: Descartes <sip:rene.descartes@metaphysics.org>;tag=12323 From: Newton <sip:newton@kings.cambridge.edu.uk>;tag=98 INVITE Supported: 100rel CSeq: 1 INVITE 100 Trying CSeq: 1 INVITE 180 Ringing CSeq: 1 INVITE RSeq: 314 180 Ringing CSeq: 1 INVITE RSeq: 314 PRACK CSeq: 2 PRACK RAck: 314 1 INVITE 200 OK CSeq: 2 PRACK 200 OK CSeq: 1 INVITE CSeq: 1 ACK Media session Figure 4.11 Use of reliable provisional responses. SIP: Understanding the Session Initiation Protocol Call-ID: 5@lucasian.trinity.cambridge.edu.uk RSeq: 314 CSeq: 1 INVITE Content-Length: 0 PRACK sip:rene.descartes@metaphysics.org SIP/2.0 Via: SIP/2.0/UDP lucasian.trinity.cambridge.edu.uk branch=z9hG4bkdtyw Max-Forwards: 70 To: Descartes <sip:rene.descartes@metaphysics.org>;tag=12323 From: Newton <sip:newton@kings.cambridge.edu.uk>;tag=981 Call-ID: 5@lucasian.trinity.cambridge.edu.uk CSeq: 2 PRACK RACK: 314 1 INVITE Content-Length: 0 SIP/2.0 200 OK Via: SIP/2.0/UDP lucasian.trinity.cambridge.edu.uk ;branch=z9hG4bkdtyw ;received=1.2.3.4 To: Descartes sip:rene.descartes@metaphysics.org>;tag=12323 From: Newton <sip:newton@kings.cambridge.edu.uk>;tag=981 Call-ID: 5@lucasian.trinity.cambridge.edu.uk CSeq: 2 PRACK Content-Length: 0 The mandatory header fields in a PRACK request are shown in Table 4.13. 4.1.13 UPDATE The UPDATE method [18] is used to modify the state of a session without changing the state of the dialog. A session is established in SIP using an INVITE request (see Section 4.1.1) in an offer/answer manner (see Section 7.1.12). Typi- cally, a session offer is made in the INVITE and an answer made in a response to Table 4.13 Mandatory Header Fields in a PRACK Request Mandatory Header Fields Call-ID Max-Forwards SIP Request Messages the INVITE. In an established session, a re-INVITE is used to up