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`UNIX is a technology trademark of X/Open Company, Ltd.
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`The publisher offers discounts on this book when ordered in quantity for special sales.
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`Library of Congress Cataloging-in-Publication Data
`Stevens, W. Richard
`
`TCP/IP Illustrated: the protocols/W. Richard Stevens.
`p. cm. — (Addison-Wesley professional computing series)
`Includes bibliographical references and index.
`ISBN 0-201-63346-9 (v. 1)
`1.TCP/IP (Computer network protocol) I. Title. II. Series.
`TK5105.SSS74 1994
`004.6’2—dc20
`
`Copyright © 1994 Addison Wesley Longman, Inc.
`
`All rights reserved. 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, recording, or
`otherwise, without prior written permission of the publisher. Printed in the United States of
`America. Published simultaneously in Canada.
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`Text printed on recycled and acid-free paper
`ISBN 0-201—63346-9
`
`10111213141516 MA 00 99 98 97
`
`10th Printing July, 1997
`
`1
`
`
`
`
`226
`TCP: Transmission Control Protocol
`Chapter17
`
`This is therefore the sequence number plus 1 of the last successfully received byte II
`data. This field is valid only if the ACK flag (described below) is on.
`Sending an ACK costs nothing because the 32-bit acknowledgment number field i
`always part of the header, as is the ACK flag. Therefore we’ll see that once a connecti-
`is established, this field is always set and the ACK flag is always on.
`
`ETA
`
`