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Directory Structures for Collective Internet Caches
Efficient directory management is the key design problem for any
Internet cache.
Several considerations influence the design choices:
- Load on the internal network.
Important to almost all configurations, internal network usage
affects link contention and congestion and the potential for lost packets
on the network connecting the caching servers, e.g., an ISP's internal
network.
- Costs of external network usage. Web caches exist in part
to reduce bandwidth demands on the external network. One reason to
deploy Web caches is altruistic:
if everyone is a ``good citizen'', then everyone benefits
from reduced congestion on backbone links and Web servers. A second reason
is that caches can directly affect access
costs when Internet access charges are metered
by traffic volume.
- Object fetch latency. From the
perspective of an individual client, object fetch latency is the
most important motivation for using a Web cache, and the only real
measure of its success.
- Latency of the internal network.
As the network distance between the servers in a collective cache
increases, synchronous network exchanges with remote parts of the cache
are no longer practical. Geographically dispersed caches
must localize
query traffic by replicating the directory or reducing the ``coverage''
of queries.
The following subsections explore several directory structures
and evaluate them with respect to these considerations.
Next: Hierarchical ICP Caches
Up: A Taste of Crispy
Previous: Introduction
Syam Gadde
1998-05-19