[HECnet] Attaching to hecnet

Paul Koning wrote: Paul Koning wrote: ... The way this works is that there are two multicast addresses used on Ethernet segments. (Well, three later on, a separate one for all L2 routers.) One is "all routers", one is "all endnodes". Routers listen to the first, endnodes to the second. Hellos are sent to "all routers" so ONLY routers hear hellos. Both endnodes and routers send hellos (different types). So routers build a list of all the nodes on each interface. The routers on an Ethernet pick one to be the designated router, and only that router sends a hello to "all endnodes". That's how endnodes know the DR. Not really. end-nodes send endnode-hello messages to ab:00:00:03:00:00 l1 routers send router-hello messages to ab:00:00:03:00:00 l2 routers send router-hello messages to ab:00:00:03:00:00, ab:00:00:04:00:00 and 09:00:2b:02:00:00 (exactly which of these last two addresses are sent to seems to differ from node to node, and I haven't figured out exactly how yet). That fits what I said. Ab:00:00:03:00:00 is "all routers". So the hellos sent to that address are received by routers but not by endnodes. Ab:00:00:04:00:00 is "all endnodes". The hello that goes to that address is from the designated router. In your example I guess that was an L2 router but that's just a coincidence. And there will only be one router (L1 or L2) sending to that address, the designated router. Ah. I was sampling from a too short time. Eventually I received a router hello message on ab:00:00:04:00:00 from an L1 router as well. Ok. I was making the wrong assumptions because of this. Now you made me read the spec instead, which was good. Thanks. :-) 09:00:2b:02:00:00 was a later addition. It's not in the routing 2.0.0 spec, it would be in the 2.1.0 spec if we could find one. That would be the "all L2 routers" address. The idea for adding that address was to allow L2 routing messages to be sent to that address instead of the "all routers" address, so L2 route updates would not bother L1 routers. Makes sense. ... So, obviously, routers and endnodes both send, and listen to the same messages. No, you're making an incorrect assumption. The multicast addresses you send to, and the ones you listen to, are not necessarily the same. In IP, yes (ARPs go to broadcast and everyone listens to that). Not so in DECnet. End nodes listen to all-endnodes and send to all-routers. Routers listen to all-routers and send to all-routers and (if DR) to all-endnodes. Yes. I made the wrong assumption. Looked at too little data. DECnet very deliberately used multicast rather than broadcast because control packets are of interest only to some of the nodes, so by using a specific multicast address, you can enable that if you need to hear the traffic and disable it if you don't. Which is a good thing. In fact, it's clear that the existence of broadcast is a design error (and so is the fact that ARP uses it). Broadcast is defined as "traffic that everyone wants to hear" and there IS no such traffic. For example, you don't want to hear ARPs unless you speak IP, so clearly those should have used an "All IP nodes" multicast address. Yep. Not to mention some protocols designed be people using PC machines running DOS... Johnny