[lwip-users] Dup ACKs and window updates during FTP session

[Wilson, Clive]

Hello,

I'm using LwIP v1.1.0 and am having a few problems with retransmissions and duplicate ACKs, and hope someone might be able to offer some advice.

I'm using LwIP to drive an FTP client over a dial-up PPP connection. This is connected to a Windows 2000 server RAS machine, running Filezilla FTP server. However, I don't think the network interface is necessarily important here.

I am transferring files in both directions, but the area of concern is in retrieving files from the FTP server.

I have attached an Ethereal trace of the complete session. What I am seeing are packet retransmissions and duplicate ACKs between LwIP (IP address 192.168.123.5) and the FTP server (192.168.123.4). Interestingly, it seems to be fairly repeatable: I always seem to get the packet containing "226 Transfer OK" retransmitted after the 25KByte file transfer. I also get a duplicate ACK a few packets later. Also, if I make the transferred file larger (say 200KBytes) I will see multiple retransmits and dup ACKs - but only at the end of the file transfer, not during it.

Also, as you can see from the attached Ethereal trace there are a number of window updates. Is this normal?

I hope someone can suggest whether there is anything untoward with this behaviour, and if so, where I might start looking for the reason.

Many thanks in anticipation for any help you can give.

Kind regards,

Clive Wilson

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