[Simh] Ultrix network issues

[Douglas Hall]

I've been successfully running Ultrix 4.5 on a Microvax3900 sim , but I've had problems with networking since migrating the sim to a Xen DomU host. 

TCP network performance for file transfers is painfully slow. I've reproduced the problem with ftp and rcp. After an initial flurry of traffic Ultrix only responds to incoming tcp packets after, in some cases, as much as 2 minutes later.  Interactive sessions seem to have no problems.

The only thing that has changed has been the migration to Xen (it was previously running on a Vmware ESxi host) and a change from tap to raw ethernet interfaces.  The underlying OS (Debian Stretch) and the Ultrix install is identical between systems. 

I've got other OSs working fine with no network issues, various VMS versions.

Anyone got any pointers, or have also seen similar issues with Ultrix?

Running latest simh code from GitHub.

-Douglas