Parent, Siblings, and the ICP

In the absence of siblings, upon a miss (an object not in its local cache) a proxy cache server issues a HTTP request for the object to its parents or to the origin web server.

In an environment in which the Inter Cache Protocol (ICP) is supported (as it is in the Netra Proxy Cache Server), upon a miss, a proxy cache server asks all of its parents and siblings if any of them has the requested object. If no parent or sibling responds within a certain period, the proxy cache server forwards the request to one its parents.

Note that a parent might be called upon to be responsible for returning the object to a requesting server. A request to a sibling never goes beyond that sibling; that is, a sibling only checks its local cache and does not forward a request.

You can specify the use of certain parents (or siblings) for certain domains, through the use of the Query Parent Cache for Domains property, described in "Proxy Cascade".

The following example illustrates the use of ordering in the parent/sibling table and the Query Parent Cache for Domains property. Assume the following table:

host1 ICP-capable parent
host2 non-ICP-capable parent
host3 ICP-capable parent
host4 sibling

Assume further the Query Parent Cache for Domains property is defined as follows:

host1 .edu 
host2 .com
host3 .com
host4 .com

Your server receives a request containing the domain acme.eng.com. The following sequence occurs:

1. Your server contacts host3 and host4. It does not contact host2 because that host is not ICP-capable; host1 is not contacted because you configured it to handle the .edu domain.

2. Both host3 and host4 return ICP misses

3. Your server fetches the URL from host2 because it is the first parent in the parent/sibling table that matches the .com domain.