Types of hierarchies

Common configurations

Hierarchies control how NetCache forwards unresolved requests. They are configured on an appliance-by-appliance basis; that is, a hierarchy configuration provides instructions only for the appliance on which the hierarchy is configured. No other NetCache appliance, third-party proxy-cache server, origin server, or firewall is aware of any hierarchy that is configured on another NetCache appliance. Common hierarchy configurations include

Single host (a single parent cache or a single firewall)
Parent cluster or firewall cluster
Neighbor

Hierarchies that contain a single parent or firewall

The simplest type of hierarchy has one member in addition to the NetCache appliance being configured, that is, one proxy-cache server or one firewall. You would configure a single-member hierarchy if

Your NetCache appliance does not have direct connectivity to the Internet.
You want another proxy-cache server to provide content control for your NetCache appliance.

Cluster hierarchies

A cluster is a group of proxy-cache servers (parents) or firewalls over which the NetCache appliance distributes requests. Clusters do the following:

Load balance requests over multiple proxy-cache servers or firewalls that handle the same protocol
Fail over to maintain connectivity if any member of the cluster goes down

For more information see, Cluster hierarchies.

Neighbors

A neighbor is a logical role that can be assigned to hierarchy members on a Web cache. Neighbors cannot fetch objects directly from the Internet on behalf of another cache; however, they can accept a query for a requested object from another NetCache appliance or from a third-party proxy-cache server.

The communication protocol used to contact neighbors is always ICP (Internet Cache Protocol). ICP is a query-based communication that increases traffic without significantly increasing hit rate. ICP overhead can introduce network latency.

Note
Streaming media configurations do not support neighbors.

Neighbors as hierarchy members

The following illustration shows a hierarchy that includes a neighbor. In this case, one of the hierarchy members is identified as a parent and the other is identified as a neighbor. If your hierarchy includes a neighbor, ICP is used to query each hierarchy member to resolve a request.

About ICP

ICP is a method of distributing requests within a cache hierarchy. A derivative of the User Datagram Protocol (UDP), ICP enables NetCache to query other caches for individual Web pages. If ICP is configured as a distribution method, the only UDP packets that NetCache sends or receives are ICP packets. NetCache accepts all UDP packets and then drops any UDP packets that are not ICP.

Note
ICP is used for querying for availability of objects only, not for the actual fetching of objects. Fetching of objects is done using the protocol specified in the URL.

ICP can be configured to support multiple parent caches, a parent and a neighbor, or multiple firewalls. Before you enable ICP as a distribution method, consider the following:

ICP is a query-based communication.
When ICP is enabled, the child cache queries each hierarchy member to determine if a member of the hierarchy has the requested object. This method increases traffic without significantly increasing hit rate; that is, ICP can introduce latency.
Load balancing and failover are not supported.
Unlike a cluster configuration, if the hierarchy consists of multiple parents, the child cannot balance requests over the two parent caches.

Note
Network Appliance recommends using ICP only for backward compatibility with squid-based caches.