Cache hierarchies

A cache hierarchy is the name given to an arrangement of proxy servers which can communicate with each other to forward requests. The arrangement is typically a tree structure in which the proxy servers have a parent-child or sibling relationship. Parent proxy servers are closer to the remote servers, compared to the child servers, and the child servers typically use the parent servers to fetch content for their clients. Child servers can act as a parent server to other proxy servers. Let's have a look at the following diagram:

Siblings are the proxy servers at the same level in the tree structure. In a cache hierarchy, proxy servers use protocols like ICP, HTCP, Cache Digests, and CARP to identify a useful source. The other peering types are origin server, which is generally a special type of parent and multicast, which in essence is a special type of sibling.