Glossary

Understanding of the following terms is useful in understanding the Netra Proxy Cache Server product. Italicized terms in definitions are defined elsewhere in the glossary.

cache

A store of information used for repeated fast access. With respect to web pages, a cache is a repository of URLs that lies between clients (browsers) and origin web servers. Design issues for caches involve which pages to store, for how long. Caches are useful when the ratio of hits to misses is at least 30% (approximately). This threshold varies according to the types of objects being cached and the expense and bandwidth of the links between the cache and the origin web server. For example, a hit rate of 20% might be cost effective if the hits are for very large objects. Similarly, if the links between the cache and origin web server are very expensive, even a low hit rate might prove cost effective. Another important design issue is how expensive a miss is. For example, if a cache were of such size that an exhaustive search is a resource-intensive activity, it might be more cost effective not to have the cache.

hit

The finding of a requested URL in a cache, obviating the need to request the object from a parent or an origin web server. A hit is distinguished from a miss.

HTTPS

An advanced form of SSL tunneling supported by popular web browsers.

Inter Cache Protocol (ICP)

A lightweight protocol used by a proxy cache server to inquire of its siblings and ICP-capable parents as to whether they have a web object. The ICP contains a metric (response time) that enables a requestor to choose among multiple caches.

miss

The failure to find a requested URL in a cache, requiring further activity on the part of the cache server. This activity might include any or all of sending ICP requests to parents and siblings, sending requests to parents, and sending a request to an origin web server. A miss is distinguished from a hit. To a large degree, the success of a web cache depends on the expense associated with a miss.

neighbor

Refers to parents and siblings of a proxy cache server.

parent

An proxy cache server that is responsible for returning a requested object if a child server cannot retrieve the object from its own cache. The parent attempts to locate the requested object in its own cache. If it cannot, it requests the object from its parents and siblings, or, in the absence of parents and siblings, from the origin web server. A parent might be ICP-capable, in which case it receives ICP queries for objects along with a proxy cache server's siblings.

persistent connections

A feature of HTTP 1.1, as implemented by popular web browsers, wherein multiple, different HTTP requests can be carried on the same TCP connection. Sometimes referred to as "HTTP keepalive".

sibling

A proxy cache server that has a peer relationship with another proxy cache server. If a proxy cache server receives a request for an object that it cannot fulfill, it checks its parents and siblings to see whether they have the object. A sibling server checks only its local cache and no further. A parent checks its own cache and, if the object is not there, attempts to retrieve the object from its parent or from an origin web server.

Secure Sockets Layer (SSL) tunneling

Involves opening a direct socket between the client (browser) and a target web server for secure communication. A tunneled connection might pass through one or more proxy cache servers.

test object

A script or program that tests the functionality of a host resource. A test object might return, for a example, a boolean, indicating whether a network interface can transmit and receive.

web object

A web page, audio or video clip, graphic file, or other object that can be provided by a web server to a client (most often a browser) using the HyperText Transfer Protocol. A proxy cache server caches web objects, although not all proxy cache servers can cache all types of web objects.