Caching the Web, Part 1
ICP, Internet Cache Protocol, is a protocol used for communicating among web caches. A lightweight protocol built on top of UDP, ICP is used to locate specific web objects in neighboring caches. Most transfers of objects between caches are done with the TCP-based HTTP protocol, but making the decision of where to retrieve an object must be done with a simpler and faster mechanism. Other information needed is which caches are down or have congested links.
One cache, in order to find the best location from which to download an object, sends an ICP request packet to all of its siblings and parent caches, and they send back ICP replies with a HIT or MISS code. A HIT code means this cache has the object and agrees to serve it. A MISS code means it doesn't have the object. Thus, the cache now knows who has the object it needs, and, combining this information with other factors such as round-trip times of each response, can perform the cache selection and make the request via HTTP to its choice. If all the caches reply with MISS packets, it requests the document from its parent cache. An ICP request/reply exchange should occur in a second or two, so the latency increases this time for the browser, but this is usually not noticed by the end user.
If the object requested via ICP is small enough, it can be included in the ICP HIT reply, like an HTTP redirect, but this is not a very common situation. Of course, ICP is needed only in a multi-level cache environment with multiple siblings and parent caches. Using ICP is not necessary in situations like the ones in Figures 1 and 2. When only one cache is involved, or when one cache always requests documents from the same higher-level cache, ICP would only add unwanted overhead.
At this point, we must realize that not all objects in the web are cacheables. Most FTP files are, as well as most static web pages, but a large number of CGI-generated web pages (dynamic documents) are not. This kind of document is non-cacheable, because it is different each time you request it. Two good examples of this kind of object are access counters and live database queries. Caching a reply from a flight reservation system is senseless, since the next query will most likely return more up-to-date values. Other kinds of documents which should not be cached include SSL documents (securely transmitted documents).
Even if you do not have a proxy-cache server, you must be aware of the effects other proxy-cache servers are causing on the Internet. You may be publishing information on your web server that other caches are storing and serving for more time than you probably want. This is particularly true if you periodically update your site and it's important to you that a final user never gets out-of-date pages or graphics.
A document in a cache server can have three different states: FRESH, NORMAL and STALE. When an object is FRESH, it is served normally when a request for it arrives without checking the source to see if the object has been modified since its last retrieval. If it's in NORMAL state, an If-Modified-Since GET request is sent to the source, so the cache server downloads the object from the source only if it has changed since its last retrieval. A STALE document is no longer valid, and it's retrieved from the source again.
Normally, when a web server sends a document, it adds an HTTP header called Last-Modified containing the date the object was created or last modified. This data is used by cache servers to heuristically calculate how much time may pass for the object to still be considered FRESH. Usually, a proportion of the time elapsed between the date the document was last modified and the date when the document was received is used. A normal proportion is 10% to 30% of this time. If this proportion is set to 20%, a document modified 10 days ago will remain in the cache only two days before being checked for changes.
Webmasters who frequently update their information need more control over the time their documents remain unchecked in web caches. In this case, the Expires HTTP header in the documents served by your server can be used to indicate when this document must be dropped by any cache server. This header explicitly gives the caches the expiration date of a document. A valid RFC1123 time format should be used with this header, for example:
Expires: Mon, 25 Aug 1997 10:00:00 GMT
This header can be generated easily in CGI scripts or the mod_expires module included in Apache 1.2. For example, the following Apache directives (in a <Directory> </Directory> or a .htaccess) would do it:
ExpiresActive On ExpiresByType image/gif A432000 ExpiresByType image/jpeg A432000 ExpiresByType text/html A10800The Expires header is activated for all subsequent documents with a value of five days for JPEG and GIF images and three hours for HTML documents.
If you have documents which should never be cached in any server or browser, use the HTTP header called:
Of course, a cache may expire an object sooner, based on site configuration, lack of free disk space, LRU (less recently used) policies, etc., but it can never cache an object beyond its Expires time.
Next month, we will discuss Squid, the best free software solution for building proxy-cache servers.
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