Morgan Kaufmann Publishers is an imprint of Elsevier. 30 Corporate Drive, Suite 400, Burlington, MA 01803, USA This book is printed on acid-free paper. Copyright © 2009 by Jerome H. Saltzer and M. Frans Kaashoek. All rights reserved. Designations used by companies to distinguish their products are often claimed as trademarks or registered trademarks. In all instances in which Morgan Kaufmann Publishers is aware of a claim, the product names appear in initial capital or all capital letters. All trademarks that appear or are otherwise referred to in this work belong to their respective owners. Morgan Kaufmann Publishers does not have any relationship or affiliation with such trademark owners nor do such trademark owners confirm, endorse or approve the contents of this work. Readers, however, should contact the appropriate companies for more information regarding trade- marks and any related registrations. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means--electronic, mechanical, photocopying, scanning, or otherwise-- without prior written permission of the publisher. Permissions may be sought directly from Elsevier's Science & Technology Rights Department in Oxford, UK: phone: (+44) 1865 843830, fax: (+44) 1865 853333, E-mail: permissions@ elsevier.com. You may also complete your request online via the Elsevier homepage (http:// elsevier.com), by selecting "Support & Contact" then "Copyright and Permission" and then "Obtaining Permissions." Suggestions, comments, and corrections: Please send correspondence by e-mail to Saltzer@mit.edu and kaashoek@mit.edu Library of Congress Cataloging-in-Publication Data Application Submitted ISBN: 978-0-12-374957-4 For information on all Morgan Kaufmann publications, visit our Web site at www.mkp.com or www.elsevierdirect.com Printed in the United States of America 09 10 11 12 13 10 9 8 7 6 5 4 3 2 1 Typeset by: diacriTech, Chennai, India