Cover page by W. Richard Stevens, Kevin R. Fall

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17. TCP Keepalive

17.1. 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 endpoints. There is no polling, for example, as you might find with other networking protocols. This means that 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 should remain up. In theory, intermediate routers can crash and reboot, data lines may go down and back up, but as long as neither host at the ends of the connection reboots (or changes its IP address), the connection ...

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