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    <p>Let's start by pruning some of that Wireshark output. We can toss the ARPs in packets 2, 3, 5 and 6. Looking at the rest, you have two sets of traffic in there. Packets 8 and 9 are two different connections, so you can't compare them. 7, 8 and 10, however, are part of one connection so let's examine those.</p> <p>Packet 7 is 458 bytes of data being sent to the Linux box with a TCP sequence number of 1. However, the ACK that the Linux box returns is 4294964377. This means that Wireshark is showing relative TCP values and that the Linux box is not sending an ACK for packet 7, but for an earlier packet. Your PC then waits for a follow-up ACK and, when it doesn't get one, retransmits the required data. In this case the 458 bytes from packet 7 along with a previous 1002 bytes. That's why the sequence number from packet 10 matches the ACK from packet 8.</p> <p>Unfortunately this doesn't tell you why data is being dropped. Packet 8 shows the Linux box indicating it still has a full 32k of input buffer available for this connection ("Win=32767").</p>
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    1. COThanks so much. My mistake was forgetting that I was sending two packets at the same time to that machine, one on port 20600 and one on 20700. So I had confused the two conversations. Now I guess my question is: how can I make the re-transmission faster? Why the long pause between the duplicate ack for an earlier packet and the re-transmission from my end? How can I make this faster?
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      1. USMichael Covelli
    2. COCan you run a capture on both your PC and the Linux box at the same time? This looks like a classic case of a packet not reaching its destination. I assume this connection is over a LAN with some sort of switch. If that's the case, maybe you have some other tools to monitor overall network congestion. I'm not sure if you can change the retransmit timeout, but in any event that seems to be treating symptoms and not the cause.
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    3. COActually, I did manage to drastically reduce the number of transmissions today by increasing the ARP cache timeout. I noticed whenever more than 2 minutes elapsed between sends on the socket, the probability of a re-transmit increased dramatically. I looked for anything with a period of 2 minutes and found the ARP cache and increased it and there weren't any issues for the past several hours. But that still leaves two question - 1) Why does increasing the ARP cache timeout fix it? 2) Is there a way to shorten the re-transmit time for those times when it still does happen.
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