1.6 Caching Proxy Features

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1.6 Caching Proxy Features

The key feature of a caching proxy is its ability to store responses for later use. This is what saves you time and bandwidth. Caching proxies actually tend to have a wide range of additional features that many organizations find valuable . Most of these are things you can do only with a proxy but which have relatively little to do with caching. For example, if you want to authenticate your users, but don't care about caching, you might use a caching proxy product anyway. I'll introduce some of the features here, with detailed discussions to follow in later chapters of this book.

Authentication

A proxy can require users to authenticate themselves before it serves any requests . This is particularly useful for firewall proxies. When each user has a unique username and password, only authorized individuals can surf the Web from inside your network. Furthermore, it provides a higher quality audit trail in the event of problems.

Request filtering

Caching proxies are often used to filter requests from users. Corporations usually have policies that prohibit employees from viewing pornography at work. To help enforce the policy, the corporate proxy can be configured to deny requests to known pornographic sites. Request filtering is somewhat controversial . Some people equate it with censorship and correctly point out that filtering schemes are not perfect.

Response filtering

In addition to filtering requests, proxies can also filter responses. This usually involves checking the contents of an object as it is being downloaded. A filter that checks for software viruses is a good example. Some organizations use proxies to filter out Java and JavaScript code, even when it is embedded in an HTML file. I've also heard about software that attempts to prevent access to pornography by searching images for a high percentage of flesh-tone pixels. ^[3]

^[3] For example, see http://www.heartsoft.com, http://www.eye-t.com, and http://www.thebair.com.

Prefetching

Prefetching is the process of retrieving some data before it is actually requested . Disk and memory systems typically use prefetching, also known as "read ahead." For the Web, prefetching usually involves requesting images and hyperlinked pages referenced in an HTML file.

Prefetching represents a tradeoff between latency and bandwidth. A caching proxy selects objects to prefetch, assuming that a client will request them. Correct predictions result in a latency reduction; incorrect predictions, however, result in wasted bandwidth. So the interesting question is, how accurate are prefetching predictions ? Unfortunately, good measurements are hard to come by. Companies with caching products that use prefetching are secretive about their algorithms.

Translation and transcoding

Translation and transcoding both refer to processes that change the content of something without significantly changing its meaning or appearance. For example, you can imagine an application that translates text pages from English to German as they are downloaded.

Transcoding usually refers to low-level changes in digital data rather than high-level human languages. Changing an image file format from GIF to JPEG is a good example. Since the JPEG format results in a smaller file than GIF, they can be transferred faster. Applying general-purpose compression is another way to reduce transfer times. A pair of cooperating proxies can compress all transfers between them and uncompress the data before it reaches the clients .

Traffic shaping

A significant number of organizations use application layer proxies to control bandwidth utilization. In some sense, this functionality really belongs at the network layer, where it's possible to control the flow of individual packets. However, the application layer provides extra information that network administrators find useful. For example, the level of service for a particular request can be based on the user's identification, the agent making the request, or the type of data being requested (e.g., HTML, Postscript, MP3).

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