[HECnet] Multinet Tunnel Connections to SG1::

[Peter Lothberg]

Forgive my ignorance, but when you say "actual DECnet routing", it must be
transporting it over something, what is that something? I assume it
is IP, in which case it would still need to encapsulate the DECnet
in IP wouldn't it? Have I misunderstood something here?


A bridge connects two ethernet segments together at the Ethernet
layer, they might have a learning mechanism so they knew what MAC
address is on what port, so if you have a multipoint network, all
packets are not sent everywhere.

Johnnys bridge takes a Etherent frame, puts it in a IP packet, sends
it to the other side, removes the outer iP header and sends the
Ethernet fram out on the physical ethernet on the other side.


A DECnet router is a L3 device, it will look at the packets and figure
out where it needs to go based on the DECnet address, not the MAC
address. It also speaks a routing protocol to figure out what is on
the other side, the same way IP has routing protocols. The closest IP
routing we have to DECnet is ISIS. OSPF is based on the same
principle, but (long rant..) it's backwards in many ways.

A L3 router can have many interfaces, and the interface type can be
anything, point-to-point, HDLC, DDCMP, POS (did anyone figure POS is 
kludge DDCMP clone?) and the L2 header is stripped off.

Now, in DECnet on a LAN segment there is a kludge, there is no ND or
ARP, instead the DECnet node address is "hard coded" in to the MAC
address..   aa00.0400.xxxx

To get xxxx multiplie the area number by 1,024 and add the node
number to the product. The resulting 16-bit decimal address is
converted to a hexadecimal number and appended to the address
AA00.0400 in byte-swapped order, with the least significant byte
first. For example, DECnet address 12.75 becomes 12363 (base 10),
which equals 304B (base 16). 

Now, so think of a L3 router with a physical Ethernet on one side and
a logical (GRE or other tunnel interface) on the other side. The
traffic across the IP network in this case will be the GRE encapsulate
Ethernet frame with address as above and ethertype 0x6003. 

This is DECnet Phase 4, on DEcnet Phase 5 it's all OSI-IP/CLNP and I
have forgotten all of it as OSI died..

And dept of useless knowledge, Tops10 ANF10 uses 0x6016

--Peter