[Info-vax] DECnet Phase IV and VMS code comments

[Johnny Billquist]

On 2016-11-22 13:30, Dirk Munk wrote:
> Johnny Billquist wrote:
>> I don't know about VMS much, but I do know that RSX will check incoming
>> packets so that the MAC address agrees with the claimed node address.
>
> How do you mean? The original MAC address of a NIC is replaced by a
> DECnet generated MAC address, AA-00-04-00-aa-bb, where aa-bb contains
> the DECnet Phase IV address. So if you give some node on your LAN such a
> MAC address by hand, DECnet will see it as a DECnet node. Furthermore
> you can't uses two NIC's of the same computer on one network segment.

The original MAC of the NIC is not something visible to other machines 
on the network, so that is never usable. The DECnet MAC address is what 
other machines will see. And DECnet in RSX will check that the claimed 
DECnet address matches the source MAC address as computed.

Obviously any machine can have that MAC address, and if several machines 
on a network have the same MAC address, many funny things can and will 
happen.

DECnet will see another machine as a DECnet node if it has an 
appropriate MAC address, and are speaking the appropriate protocols. 
Which should also be very obvious. :-)

	Johnny