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    <p>I do believe the second statement refers to payload (The shortest possible TCP/IP packet is 40 bytes long - 20 bytes for TCP header + 20 bytes for IPv4 header).</p> <p>Regarding the first, here's a quote from RFC 2663:</p> <blockquote> <p><strong>End of session for TCP, UDP and others</strong></p> <p>The end of a TCP session is detected when FIN is acknowledged by both halves of the session or when either half receives a segment with the RST bit in TCP flags field. However, because it is impossible for a NAT device to know whether the packets it sees will actually be delivered to the destination [...] the NAT device cannot safely assume that the segments containing FINs or SYNs will be the last packets of the session [...] Consequently, a session can be assumed to have been terminated only after a period of 4 minutes subsequent to this detection. The need for this extended wait period is described in RFC 793 [Ref 7], which suggests a TIME-WAIT duration of 2 * MSL (Maximum Segment Lifetime) or 4 minutes.</p> </blockquote> <p>Reference: <a href="http://tools.ietf.org/html/rfc2663" rel="nofollow">http://tools.ietf.org/html/rfc2663</a></p> <p>To my understanding, any packets that identifies a session would reset the TTL counter - but that depends heavily on implementation, since 'data' can be understood as 'packet' (minimum 40 bytes) or 'packet payload'. Nonetheless, @CodeCaster is spot-on; never assume that a connection is alive, make sure it is before sending (and, if possible and depending on criticality, acknowledge receipt.)</p>
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    1. PODoes TCP keepalive refresh the timeout on a NAT?
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    1. PODoes TCP keepalive refresh the timeout on a NAT?
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    1. COThanks for the reference. Now I'm wondering why to use TCP keepalive anyway but I think that's another question ...
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    2. COI think keep-alive's raison d'être lies on preventing unnecessary sequential handshakes - there's a cost associated to that, so if you plan on sending several small packets then keeping the connection alive is better than doing it all over again.
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    3. COYes of course it is. What I meant was the *built-in* keepalive in comparison to a selfmade implementation that sends ping-pong-packets every x minutes/seconds ;)
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