Linux Router: TCP slow, UDP fast

[fw at deneb.enyo.de (Florian Weimer)]

> I'm looking at TSO too as above, mentioned by Nickola. I'll maybe risk
> changing it with ethtool during a quiet network moment.

Turning off offloading might be something to try indeed.

Regarding the negotation issue, can you look at the other end of the
link and check what it's saying?

Looking at "netstat -s" statistics at the endpoint (not the router)
could be illuminating, too.  I haven't got any expertise in this area,
but TCP problems can often be diagnosed by looking at tcpdump/packet
captures and analyzing them using tcptrace (and the special xplot
variant which can plot tcptrace output).