TCP Connection Maintenance
A TCP connection can optionally be maintained through the periodic exchange of a TCP keepalive segment, which is an ACK segment containing no data. The Sequence Number field in the TCP header of the keepalive segment is set to 1 less than the current sequence number for the outbound data stream. For example, if a TCP peer’s next byte of data is 18745323, the TCP keepalive sent by the TCP peer has the Sequence Number field set to 18745322.
After receiving this ACK segment, the other TCP peer sends back an ACK segment with the Acknowledgment Number field set to the next byte that it expects to receive. In this example, the TCP peer sends an ACK segment with the Acknowledgment Number field set to 18745323. This simple exchange ...
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