Re: [lwip-users] Re: retransmission every 9 pacakets.

[Kieran Mansley]

On Thu, 2007-09-13 at 11:16 +0800, Yang wrote:
> Hi Kieran!
>   The trace is in attach file. It's a ftp server I wrote with lwip
> socket.
> You can see that if data is send both side, in control link, it work
> fine.
> but as data link send file to PC, retrans happends. 

Hmm, I see retransmissions in both parts (both control and data
transfer) of the connection.  Which end of the connection is lwIP, and
which end something else?

My guess from looking at the trace is that the receiving end is using
delayed ACKs (it only acknowledges every other packet) and this is fine
when the sender has enough data to send two packets.  However, if it
only sends one packet, the receiver is supposed to time out and send an
ACK anyway.  This isn't happening before the sender gives up waiting for
an ACK and retransmits.  The problem is I think with the sender as it's
not waiting long enough for the ACK, rather than with with the
receiver's delayed ACK.  It should be waiting at least 200ms before
retransmitting (or more like 1 second to be fully RFC compliant), but
they're all coming out faster than that - more like 150ms.

If lwIP is the sender, I would check that the TCP timers are being
called with the correct intervals.

Kieran