8.5 Which Protocol to Use

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8.5 Which Protocol to Use

The four protocols and algorithms presented here each have unique features and characteristics. Which one you should use depends on numerous factors. The following guidelines may help to determine the best protocol for your particular situation.

ICP may be a good choice if you need to interoperate with products from different vendors . Since ICP has been around longer than the others, it is supported in most caching products. It is also a reasonable choice if you want to build or connect into a small mesh of caches. You want to avoid having too many neighbors with ICP; try to limit yourself to no more than five or six. ICP may not be a good choice if you are very concerned about security, and it may be all but useless on networks with high delays and/or a large amount of congestion. The protocol has no authentication mechanisms and may be susceptible to address spoofing. Finally, you probably cannot use ICP if there is a firewall between you and your neighbor caches because firewalls typically block UDP traffic by default.

CARP is a logical choice if you have multiple parent caches administered by a single organization. For example, some large service providers may have a cluster of proxy caches located where they connect to the rest of the Internet. If this applies to you, make sure you always have up-to-date configuration information. CARP is the only protocol that does not allow you to create sibling relationships.

HTCP has characteristics similar to ICP. You can use HTCP for small cache meshes and where network conditions are good. Unlike ICP, HTCP has relatively strong authentication. This may be particularly important if you need the object deletion features. HTCP should cause fewer false hits than ICP because hit/ miss decisions are based on full HTTP headers rather than only the URI. Note that HTCP messages are about five times larger than ICP, so it uses more bandwidth.

You should use Cache Digests if you can afford to trade increased memory usage for lower forwarding delays. The bandwidth tradeoffs of Cache Digests versus ICP depend on many factors. You should apply the formulas in Section 8.4.2, to your particular situation. You cannot really use Cache Digests over slow network connections because the transfer of a large digest saturates the link. Cache Digests probably result in a higher percentage of false hits compared to ICP. Even so, overall client response times should be lower, and false hits are only a concern for sibling relationships.

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