[HECnet] DECnet et al

[Paul Koning]

On Jul 17, 2011, at 1:38 PM, Johnny Billquist wrote:

On 2011-07-17 16.55, Bob Armstrong wrote:
Paul Koning wrote:

It is legal to have multiple areas on a single Ethernet;
the routing spec covers that case explicitly.   If that is done,
end nodes will communicate directly to a destination on the
Ethernet even if off-area ...

  Given that Johnny's bridge program effectively creates a big Ethernet,
does that imply that HECnet end nodes in different areas can communicate
without an area router?

Yes, at least partially.

They can comminucate directly with machines sitting on the same ethernet segment, even if those machines are in another area.

However, I think that an endnode will not pick any router in another area as its designated router.

Correct.

The endnode rule is:

1. If the destination is in the cache, send to it.
2. If there is a designated router (in my area), send to it.
3. Send directly.

"Directly" means to the MAC address formed by prefixing aa-00-04-00 onto the little endian node address.

The only difference between Phase IV and IV+ is how the cache works.   In phase IV, it's an "on Ethernet" cache.   A node sets a bit in the (long format, i.e., Ethernet) packet header when it originates a packet.   A router clears that bit if it forwards onto a different circuit, but leaves it alone if it forwards onto the same Ethernet as the packet arrived on.   The receiving end node makes a cache entry if the bit it set.

For Phase IV+, the cache is a "previous hop" cache.   It remembers the node address (or MAC address, same thing) from the Ethernet source address of incoming packets, and associates that with the node address of the arriving packet.   So if you have several routers, and the designated router is not the optimal path to destination X, phase IV will keep sending to the DR, but phase IV+ will send all packets after the first to the router that's best for X, because that's the router which sent the reply back to the endnode.

As for routers on a multi-area Ethernet, yes, the usual rules of the hierarchy apply: all areas must have L2 routers in them.   And traffic from an L1 router will visit at least two L2 routers if it goes out of area.   So in a multi-area Ethernet, out of area but on-Ethernet traffic from an L1 router will take 3 hops, but if the destination is an end node, the reply will go direct (Phase IV or IV+, either way).   And if the source is L2, it will take 2 hops.

	paul