Datasheet

“main” (Installation and Administration) — 2004/6/25 — 13:29 — page 601 — #627
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Internet
Multiple Caches
Several proxies can be configured in such a way that objects can be ex-
changed between them. This reduces the total system load and increases
the chances of finding an object already existing in the local network. It is
also possible to configure cache hierarchies, so a cache is able to forward
object requests to sibling caches or to a parent cache — causing it to get ob-
jects from another cache in the local network or directly from the source.
Choosing the appropriate topology for the cache hierarchy is very impor-
tant, because it is not desirable to increase the overall traffic on the net-
work. For a very large network, it would make sense to configure a proxy
server for every subnetwork and connect them to a parent proxy, which in
turn is connected to the proxy cache of the ISP.
All this communication is handled by ICP (Internet cache protocol) running
on top of the UDP protocol. Data transfers between caches are handled us-
ing HTTP (hypertext transmission protocol) based on TCP.
To find the most appropriate server from which to get the objects, one
cache sends an ICP request to all sibling proxies. These answer the requests
via ICP responses with a HIT code if the object was detected or a MISS if
it was not. If multiple HIT responses were found, the proxy server decides
from which server to download, depending on factors such as which cache
sent the fastest answer or which one is closer. If no satisfactory responses
are received, the request is sent to the parent cache.
Note
To avoid duplication of objects in different caches in the network,
other ICP protocols are used, such as CARP (cache array routing
protocol) or HTCP (hypertext cache protocol). The more objects
maintained in the network, the greater the possibility of finding
the desired one.
Note
Caching Internet Objects
Not all objects available in the network are static. There are a lot of dynam-
ically generated CGI pages, visitor counters, and encrypted SSL content
documents. Objects like this are not cached because they change each time
they are accessed.
601SUSE LINUX Enterprise Server