[HECnet] Attaching to hecnet

[Johnny Billquist]

H Vlems wrote:
Johnny, the original DECnet manuals showed pictures of area routers that
were interconnected by WAN links. Each site had its own area number. In fact
the name "area routing" implies clearly that the concept was meant to set up
DECnet networks that were geographically separated.

Certainly. But that does not imply that just because you have physically different locations that they must be in different areas. And areas don't necessarily imply geography... :-)

Level 1 routers does exactly the same job, the same way, but keeps it within one area.

But basically, the reasons for the different "levels" are originally technical. You have segments, which more or less means directly connected machines.
Then you have an area, which connects these segments. Traffic within a segment can be fairly high. Between segments there is much less traffic, since only traffic actually meant for a destination at the other end is going through, and then of course the routing messages of the level 1 routers.
Level 1 routers knows where all machines in the same area are located, and knows the most efficient way to any node within the same area. However, it only keeps track of the closest area router, and knows nothing about any other area.
Area routers work just like level 1 routers, but they also have an area routing table, so they know the most efficient path to an area router for any area.

So, an end node only knows what machines are on the same segment, and which is the closest level 1 router. Level 1 routers knows where all machines are in the same area, and knows where the closest area router is.
Area routers knows where every machine is on the local area, and also where all area routers are.

Now, this means that a machine that sits in one area will not neccesarily take the shortest path to a machine in another area. In fact, it will not neccesarily take the shortest path even to another machine in the same area. But that's another story. :-)

For most points and purposes, a level 1 router and an area router will give you exactly the same effect.
So, just because you have machines that are physically remote is by no mean a reason for them to be in separate areas. A level 1 router will solve that just as well.

What areas bring to the table is essentially just that you expand the address space, and add another level of hierarchy to the network. It is (obviously) up to you how you want to interpret that extra level. I'm not saying that you cannot use it to match network hierarchy to geography, just that there is no technical reason to align things that way.

I ran a fairly large DECnet environment 20 years ago with >20 PDP-11's, >80
VAX systems >600 pc's and several alpha's, plus a few unix systems that ran
DECnet too (in those days DECnet was the multiplatform protocol of choice).
I seprataed each factory in its own DECnet area. DEC Holland got midly
interested in the design because they wondered whether the level 2 routing
overhead wouldn't hurt network performance. Which it didn't :-)

Can't see why it should. If you have a fairly busy network, the overhead of the level 2 routing packets is very low. And if the network is mostly idle, you have the capacity to spare anyway.
I'm surprised if DEC Holland was that curious, or didn't know better, since DEC's internal network was way larger at that time.
They had already passed 60.000 nodes on Easynet (which was the name of the internal network). I worked at DEC for a while in the 80s, and Easynet worked just fine, and was very nice.
If I remember right, areas were sortof allocated by country, although some places (like the US) used several, while some places shared one between several countries.

One reason for separating systems in different areas was that rebooting the
pc's would generate so many DECnet state up and down messages that PDP-11's
and the older VAX systems choked in their console output.

Yeah, that can be an annoyance. But that's what the logging filters are there for.

	Johnny

Hans
-----Oorspronkelijk bericht-----
Van: owner-hecnet at Update.UU.SE [mailto:owner-hecnet at Update.UU.SE] Namens
Johnny Billquist
Verzonden: maandag, juni 2010 21:11
Aan: hecnet at Update.UU.SE
Onderwerp: Re: [HECnet] Attaching to hecnet
Brian Hechinger wrote:
Ok, so I should be getting to my plans soon enough here and had some
questions
about how I should set this up.

I'm going to install SIMH on the machine in colo (100mbit connection here
in
the US, so might be useful as a "hub") as well as a small handful of boxes
here at home.
Sounds good.
Should I just use one area?   What's the best way to set that up?
Areas in DECnet are not really meant to be associated with physical location, but rather with organizational. So keep it within one area.
For physically different locations, you might want to use level 1 routers, though. But as the internet nowadays is so fast, using just a bridge, and pretend that it's all one ethernet segment also works just fine.
	Johnny
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