To reduce wide-area network bandwidth demand and to reduce the load on Internet information servers, caches resolve misses through other caches higher in a hierarchy, as illustrated in Figure 1. In addition to the parent-child relationships illustrated in this figure, the cache supports a notion of siblings. These are caches at the same level in the hierarchy, provided to distribute cache server load.
Each cache in the hierarchy independently decides whether to fetch the reference from the object's home site or from its parent or sibling caches, using a simple resolution protocol that works as follows.
If the URL contains any of a configurable list of substrings, then the object is fetched directly from the object's home, rather than through the cache hierarchy. This feature is used to force the cache to resolve non-cacheable (``cgi-bin'') URLs and local URLs directly from the object's home. Similarly, if the URL's domain name matches a configurable list of substrings, then the object is resolved through the particular parent bound to that domain.
Otherwise, when a cache receives a request for a URL that misses, it performs a remote procedure call to all of its siblings and parents, checking if the URL hits any sibling or parent. The cache retrieves the object from the site with the lowest measured latency.
Additionally, a cache option can be enabled that tricks the referenced URL's home site into implementing the resolution protocol. When this option is enabled, the cache sends a ``hit'' message to the UDP echo port of the object's home machine. When the object's home echos this message, it looks to the cache like a hit, as would be generated by a remote cache that had the object. This option allows the cache to retrieve the object from the home site if it happens to be closer than any of the sibling or parent caches.
A cache resolves a reference through the first sibling, parent, or home site to return a UDP ``Hit'' packet or through the first parent to return a UDP ``Miss'' message if all caches miss and the home's UDP ``Hit'' packet fails to arrive within two seconds. However, the cache will not wait for a home machine to time out; it will begin transmitting as soon as all of the parent and sibling caches have responded. The resolution protocol's goal is for a cache to resolve an object through the source (cache or home) that can provide it most efficiently. This protocol is really a heuristic, as fast response to a ping indicates low latency. We plan to evolve to a metric that combines both response time and available bandwidth.
As will be shown in Section 3.5, hierarchies as deep as three caches add little noticeable access latency. The only case where the cache adds noticeable latency is when one of its parents fail, but the child cache has not yet detected it. In this case, references to this object are delayed by two seconds, the parent-to-child cache timeout. As the hierarchy deepens, the root caches become responsible for more and more clients. To keep root caches servers from becoming overloaded, we recommend that the hierarchy terminate at the first place in the regional or backbone network where bandwidth is plentiful.