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

[Kieran Mansley]

On Fri, 2007-11-23 at 09:11 -0600, Wilson, Clive wrote:
> 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.

The only thing that is particularly odd is the repeatability.  In the
trace you supply packet 165 is a retransmission of 160.  It shouldn't
have been necessary to retransmit because packet 161 ACKs packet 160.
However, if the ACK was lost or delayed then a single retransmission
like this is harmless and to be expected.  The duplicate ACK will be a
consequence of the retransmission being received, and is again OK.
Also, the retransmission is done by the non-lwIP end, so the only way
that lwIP could be at fault is if the ACK were somehow corrupt, which it
doesn't seem to be.  

Window updates are normal.

Kieran