I have two Dell boxes with two ethernet ports a piece. I have a cable directly connecting two of these ports, creating a tiny LAN with 10.3.3.x addresses. The other port on each box is hooked up to a switch and has a DHCP-supplied address to talk to the outside world.
I've noticed that when scp'ing large files from one box to the other over the private LAN, the transfers sometimes stall. It appears that any other network activity on either box will cause the transfer to resume.
Wake-on-LAN is disabled on all interfaces according to ethtool. What else could be causing these stalled transfers?