FTP stalls on tl0 on Challenge S

Hi,

I'm seeing a rather weird problem on a ThunderLAN 100Mb card (tl0) in my
R4400 Challenge S running 5.0.2. I'm in the process of updating various
packages from pkgsrc and noticed that FTP was stalling when fetching
the Vim 7.2 patch file 7.2.304. It didn't matter which server it tried,
it always stalled after 1448 bytes (the file is 1831 bytes). I got
round the problem by diverting the connection through the on-board 10Mb
interface (sq0) but I was curious as to why it failed.

I copied the file on to another local NetBSD box (an Ultra 5), did some
experimenting, and it appears that it's the length that is the
significant factor. If I add a byte, FTP works, if I remove a byte, FTP
works, but at exactly 1831 bytes FTP stalls. Apart from that, the
ThunderLAN card is working fine, I'm running xterm sessions and NFS
mounts through it no problem. Any ideas what's special about a length
of 1831?

George