Hierarchical cache relations
The clients A-D have two levels of hierarchy, while client E has only one level
of hierarchy, which means that if he sends a HTTP request, it goes directly
to proxy A and proxies B-C are not involved at all. If one of the clients marked
as A-D sends a HTTP request, it goes through the relevant proxy, and if there is
a "cache miss" it sends a ICP request to the sibling proxy and the
parent proxy.