Chapter 23 WAN Optimization, Web Cache, Explicit Proxy, and WCCP : FortiGate WCCP : WCCP Messages
  
WCCP Messages
When the WCCP service is active on a web cache server it periodically sends a WCCP HERE I AM broadcast or unicast message to the FortiGate unit operating as a WCCP router. This message contains the following information:
Web cache identity (the IP address of the web cache server).
Service info (the service group to join).
If the information received in the previous message matches what is expected, the FortiGate unit replies with a WCCP I SEE YOU message that contains the following details:
• Router identity (the FortiGate unit’s IP address.
• Sent to IP (the web cache IP addresses to which the packets are addressed)
When both ends receive these two messages the connection is established, the service group is formed and the designated web cache is elected.