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TCP/IP Illustrated, Volume 1: The Protocols by W. Richard Stevens

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Chapter 23. TCP Keepalive Timer

Introduction

Many newcomers to TCP/IP are surprised to learn that no data whatsoever flows across an idle TCP connection. That is, if neither process at the ends of a TCP connection is sending data to the other, nothing is exchanged between the two TCP modules. There is no polling, for example, as you might find with other networking protocols. This means we can start a client process that establishes a TCP connection with a server, and walk away for hours, days, weeks or months, and the connection remains up. Intermediate routers can crash and reboot, phone lines may go down and back up, but as long as neither host at the ends of the connection reboots, the connection remains established.

This assumes that neither ...

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