[Simh] Using TAP/TUN

[Mark Pizzolato - Info Comm]

On Saturday, March 03, 2012 at 4:05 PM, Michael Bloom wrote:
> I just wanted to comment that the instructions for networking in
> 00readme_ethernet can be simplified for linux.  The bridge utilities
> are not really necessary after using tunctl, unless you need to do
> something more complex.
> 
> [...] example details snipped.

> After that,  the emulated system and the host were both able to see
> each other,  and both could access the outside world.

That is completely true for IP traffic.

However, the key goal for the simulator level networking is to have 
these simulated systems be able to talk to other real or simulated systems
in all the ways that they did when they were natively networking.  When
these systems were in their prime, IP was not the dominant networking 
protocol.  

These systems spoke DECnet, LAT, SCS(Vax Cluster Communications), etc.
All of these protocols were designed around communications on a LAN.
The 0readme_ethernet.txt document's goal is to try and get a simulator 
to BOTH be able to participate with these protocols on the local LAN
AND to have the host system be able to also communicate with the host 
System with whatever protocols they may actually share (usually only
IP these days).

In the earliest days of simh networking, the only strategy we could
come up with which would achieve the full networking goal was to 
install an additional NIC in the hosting system which was dedicated 
to the simh instance and connect that NIC to the same LAN as the
primary host NIC.  The host's network stack would be configured to
not use this additional NIC for anything and things would work just
fine.  This strategy was one which also worked for essentially any
host platform.

Meanwhile, many folks either had host systems which couldn't easily
accommodate the addition of additional NICs or they merely wanted
to come up with ways to achieve the full set of goals without the 
addition of extra hardware.  The current simh networking document
(0readme_ethernet.txt) describes how these combined goals can be 
achieved in various host specific ways.  On Linux the bridging 
approach achieves this functionality.  On Windows, the native stack 
(combined with some extra code in sim_ether.c) can achieve the goal 
Without any extra hardware or any special host configuration steps.

If your only goal as to use IP to communicate between a simulator
and other places (including the hosting system), then your recipe
will be sufficient.  I'm not sure how well it would achieve this
goal if you happened to want to run multiple simulators on the 
same host.  The bridging recipe works for this extended case as 
well.

- Mark Pizzolato