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I'm interested specifically in implementations of the FIX protocol over TCP, but I'm happy to discuss the problem in more generality.

Suppose that a client sends TCP segments to a server, and some of the IP packets involved get lost. Then for the TCP segments that did not arrive fully, the client will not receive an ACK. Now normally this is a cue for the client to retransmit the TCP segments, after the timeout period has elapsed.

Now let's assume instead that the client decides not to send those missing TCP segments after all. What will then happen with the incomplete set of segments that the server has correctly received?

Does the server hope for the best and assume that eventually the missing segments will arrive? Is there a time-out whereby the server clears the incomplete information from his buffer? If so, generally how long is that timeout?

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TCP Keep-alive is discussed in section 4.2.3.6 of RFC-1122 (http://www.rfc-editor.org/std/std3.txt).

Without keepalive retransmissions, TCP connections can stay open indefinitely without exchanging any segments. If the connection is in fact dead, this isn't usually discovered until one side tries to send new data and gets a timeout or an ICMP error.

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  • Thank you. So it's the client, as opposed to the server, that sends a keep-alive, right?
    – Randomblue
    Oct 18, 2012 at 15:08
  • @Justin Either end can send a keep-alive.
    – Michael Hampton
    Oct 18, 2012 at 15:10

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