Reducing Network Traffic
`
`Caching
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`O'REILLY e
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`Duane Wessels
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`Web Caching
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`Duane Wessels
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`O’REILLY*
`Beijing - Cambridge » Farnbam « Kéin » Paris » Sebastopol - Taipei « Tokyo
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`

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`Web Caching
`by Duane Wessels
`Copyright © 2001 O'Reilly & Associates, Inc. All rights reserved.
`Printed in the United States of America.
`
`Published by O'Reilly & Associates, Inc., 101 Morris Street, Sebastopol, CA 95472.
`
`Editors: Nathan Torkingion and Paula Ferguson
`
`Production Editor: Leanne Clarke Soylemez
`
`Cover Designer: Edie Freedman
`
`Printing History:
`June 20001:
`
`First Edition.
`
`Nutshell Handbook, the Nutshell Handbook logo, and the O'Reilly logo are registered
`trademarks of O'Reilly & Associates, Inc. Many of the designations used by manufacturers
`andsellers to distinguish their products are claimed as trademarks. Where those designations
`appearin this book, and O'Reilly & Associates, Inc, was aware of a trademark claim, the
`designations have been printed in caps or initial caps. The association berween the image of
`areck thrush and web caching is a rademark of O'Reilly & Associates, Inc.
`
`While every precaution has been taken in the preparation of this book, the publisher assumes
`no responsibility for errors or omissions, or for damages resulting from the use of the
`information contained herein,
`
`Jibrary ofCompress Cotaloging-in-Publication Data
`Wessels, Duane.
`Web Caching/Duane Wessels
`pcm,
`ISBN 1-56592-546-X
`1, Cache memory. 2. Browsers (Computer programs) 3. Software configuration
`Management. 4. World Wide Web. L Title.
`TRISS.M4 Wa 2001
`O4.5'3—ce21
`
`2001033173
`
`ISBN; 1-56502-536-X
`1c]
`
`

`

`ia Why Cache the Web?
`
`if
`
`they access it. It's much more efficient to transfer the page once, cache it, and then
`serve future requests directly from the cache.
`In order for caching to be effective, the following conditions must be met:
`
`* Client requests must exhibit locality of reference.
`*
`The cost of caching must be less than the cost of direct retrieval,
`
`We can intuitively conclude that the first requirement is true. Certain web sites are
`very popular, Classic examples are the starting pages for Netscape and Microsoft
`browsers, Others include searching and indexing sites such as Yahoo! and
`Altavista, Event-based sites,
`such as
`those for
`the Olympics, NASA's Mars
`Pathfinder mission, and World Cup Soccer, become extrenrely popular for days or
`weeks ata time. Finally, every individual has a few favorite pages that he or she
`visils on a regular bases,
`
`It's not always obvious that the second requirement is true. We need to compare
`the costs of caching to the costs of not caching. Numerous factors enter into the
`analysis, some of which are easier to measure than others. To calculate the cost of
`caching, we can add up the costs for hardware, software, and staff time to admin-
`ister the system. We also need to consider the time users save waiting for pages to
`load (latency) and the cost of Internet bandwidth.
`Let's take a closer look at the three primary benefits of caching web content:
`
`To make web pages load faster (reduce latency)
`*
`* To reduce wide area bandwidth usage
`* To reduce the load placed on origin servers
`
`1.3.1 Latency
`Latency refers to delays in the transmission of data from one point to another, The
`transmission of data over electrical or optical circuits is limited by the speed of
`light. In fact, electrical and optical pulses travel at approximately two-thirds the
`speed of light in wires and fibers. Theoretically, it takes at least 25 milliseconds to
`send a packet across the U.S. In practice, it takes a linle longer, say about 30 mil-
`liseconds, Transoceanic delays are in the 100-millisecond range.
`
`Another source of latency is network congestion, When network links are close to
`full utilization, packets experience queuing delays inside rowters and switches.
`Queuing, which can occur at any number of points along a path, is occasionally a
`source of significant delay, When a device's queue is full, it is forced to discard
`incoming (or outgoing) packets. With reliable protocols, such as TCP, lest packets
`
`