ction to SIP The Via header field in this example is populated with Marconi's host address and contains a new transaction identifier since the BYE is considered a separate transaction from the INVITE or ACK transactions shown previously. The To and From header fields reflect that this request is originated by Marconi, as they are reversed from the messages in the previous transaction. Tesla, how- ever, is able to identify the dialog using the presence of the same local and remote tags and Call-II as the INVITE, and tear down the correct media session. Notice that all the branch IDs shown in the example SO far begin with the string z9hG4bK. This is a special string that indicates that the branch ID has been calculated using strict rules defined in RFC 3261 and is as a result usable as a transaction identifier. The confirmation response to the BYE is a 200 OK: SIP/2.0 200 OK Via: SIP/2.0/UDE tower.radio.org:5060;branch=z9hG4bk392kf received=200.201.202.203 To: Nikola Tesla ksip:n.tesla@high-voltage.org>; tag=76341 From: G. Marconi ksip:marconi@radio.org>;tag=a53e42 Call-ID: 123456789@lab.high-voltage.org CSeq: 1 BYE Content-Length: 0 The response echoes the CSeq of the original request: 1 BYE. 2.2 SIP Call with Proxy Server In the SIP message exchange of Figure 2.1, Tesla knew the IP address of Mar- coni and was able to send the INVITE directly to that address. This will not be the case in general-an IP address cannot be used like a telephone number. One reason is that IP addresses are often dynamically assigned due to the shortage of IPv4 addresses. For example, when a PC dials in to an Internet service provider (ISP) modem bank, an IP address is assigned using DHCP to the PC from a pool of available addresses allocated to the ISP. For the duration of the session, the IP address does not change, but it is different for each dial-in session. Even for an "always on" Internet connection such as a DSL line, a different IP address can be assigned after each reboot of the PC. Also, an IP address does not uniquely identi