This is the UDP port Squid uses for ICP messages. In particular, it is used both for sending and receiving queries and replies. Your Squid receives ICP queries from other caches on this port. It also receives ICP replies from other caches, in response to its own queries, on this port.
Unlike http_port, you can’t specify a list of ICP port numbers. Furthermore, you must use the udp_incoming_address and udp_outgoing_address directives if you want to restrict ICP traffic to a specific interface address.
Setting icp_port to
0 disables ICP.
icp_query_timeout, icp_access, log_icp_queries, icp_hit_stale, udp_incoming_address, htcp_port, http_port, cache_peer