Routing Through ICP Neighborhoods

ICP can be used for communication among proxies located in different administrative domains. It enables a proxy cache in one administrative domain to communicate with a proxy cache in another administrative domain. It is effective for situations in which several proxy servers want to communicate, but cannot all be configured from one master proxy as they are in a proxy array. Figure 12–3 shows an ICP exchange between proxies in different administrative domains.

The proxies that communicate with each other through ICP are called neighbors. You cannot have more than 64 neighbors in an ICP neighborhood. The two types of neighbors in an ICP neighborhood are parents and siblings. Only parents can access the remote server if no other neighbors have the requested URL. Your ICP neighborhood can have no parents or it can have more than one parent. Any neighbor in an ICP neighborhood that is not a parent is considered a sibling. Siblings cannot retrieve documents from remote servers unless the sibling is marked as the default route for ICP, and ICP uses the default.

You can use polling rounds to determine the order in which neighbors receive queries. A polling round is an ICP query cycle. For each neighbor, you must assign a polling round. If you configure all neighbors to be in polling round one, then all neighbors will be queried in one cycle at the same time. If you configure some of the neighbors to be in polling round 2, then all of the neighbors in polling round one are queried first and if none of them return a Hit, all round two proxies will be queried. The maximum number of polling rounds is two.

Since ICP parents are likely to be network bottlenecks, you can use polling rounds to lighten their load. A common setup is to configure all siblings to be in polling round one and all parents to be in polling round two. That way, when the local proxy requests a URL, the request goes to all of the siblings in the neighborhood first. If none of the siblings have the requested URL, the request goes to the parent. If the parent does not have the URL, the URL will retrieve it from a remote server.

Each neighbor in an ICP neighborhood must have at least one ICP server running. If a neighbor does not have an ICP server running, it cannot answer the ICP requests from their neighbors. Enabling ICP on your proxy server starts the ICP server if it is not already running.

Figure 12–3 ICP Exchange