[HECnet] Revisiting circuit costs

[Johnny Billquist]

Ok, so this might be a sensitive subject, after all out mails the last 
month.

I figured I should at least try to document and explain how I am setting 
costs on my circuits. Others are, of course, free to choose any other 
ideas. And DECnet will deal with it, so this only might under some 
circumstances make life a little bit better, or worse...

To start with, let me tell you that I run RSX. What I'm writing below 
sometimes reflects things under RSX as well, which might not be the same 
for other systems.

Now, DECnet have costs. These are "arbitrary" numbers, which DECnet use 
to pick which path to prefer when there are several to choose between. 
And DECnet do calculations based on the costs of many hops as well, so 
the costs I put locally are not the final decider all the time. The 
further away the destination is, the more the costs of intermediate hops 
matter.

Second, area routing obscures a lot of this. Area routing is separate, 
and also works on the same principles.

Now, in RSX, for an area router, I can actually set the cost for both 
level one routing, and level two routing, separately. This is not 
possible in VMS (as far as I know), so some of the stuff I do will not 
be possible in VMS.

Ok, so how do my network look like, and how am I thinking?
Well, first of all, ethernet is mainly a local transport. So, I want to 
keep the cost at a level that reflects this. It's cheap and fast. So I 
do not want a high cost on it. RSX defaults the cost to 3, and I'm happy 
to keep it at that. (I know that VMS defaults to 4.)
Now, this is for level one routing we're talking about.
When it comes to area routing, ethernet is not my preferred choice. It 
always means going over the bridge to get to whatever area. If I have a 
point-to-point link to that area, I think it is a better choice. So I 
want the level two cost to be higher than any direct links. At the 
moment, I've set the level two cost on ethernet at Mim to 6.

Point-to-point links will thus have a cost of less than 6 here. And the 
actual cost is relative to the physical distance between Mim and that 
location. Let's say that other places in Europe might be 1 or 2. US easy 
coast 3, and US west coast 5. That should give a fair relative cost of 
these links as far as I am concerned.

It also means that unless the US east and west coast have a cost of less 
than 2 between them, I should go directly to the east coast, without 
hopping through the west coast first. I could, of course, set the costs 
equal for all of the US, but then we get to the question, what if I want 
to get to a node in the central US? If people in the US put reasonable 
costs on their links, the cost should be about equal from the west or 
the east to central. If I said my cost to the west and east were equal, 
then I could end up going to the west to get to central, which seems the 
long way around. And in honesty, the cost from Sweden should be greater 
to the west coast than the east coast... it just makes sense to put 
honest numbers in there that makes sense.
And if the US east and west puts a cost of 1 to the other end, what cost 
is there to central? Also 1. Do they not think one is more expensive 
than the other? And if they don't differentiate the costs for such 
distances, then either they honestly are just as fast, or else they 
might end up jumping through extra hops that do cost, that they maybe 
didn't want to.

So my thinking is to put a cost on my point-to-point links that roughly 
gives some reflection on how far or slow that link is.
Ethernet is something I mostly want to use for traffic inside my area, 
but if someone else only have the bridge, then I'll use ethernet for 
that one too.

Remember, no matter how high I put the cost on the ethernet, if it is 
traffic within the area, no links to any node in another area will even 
be considered, no matter what the cost is.
If you have both the bridge, and ptp links inside the area, then it matters.

If you have both ptp links and bridge to other areas, then cost matters. 
Set the cost so that it makes sense. If the bridge generally is the 
cheaper/faster/better path, then set the cost lower there. If the ptp 
links are better, set the cost accordingly.

With all that said, I've also implicitly given some measurements of what 
the costs (for me) reflect. Across all the US costs me about 2. Across 
the atlantic about 3. Europe is about 2 across as well.

And for me, ethernet to another area is more expensive than that. But in 
most cases, if the hop means a ptp, and then back on the bridge anyway, 
then I would hope the cost of the bridge is less. But it will depend, of 
course...

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol