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Re: sell shell accounts?

  • From: Tim Salo
  • Date: Tue Jul 23 18:10:09 1996

> From: Avi Freedman <freedman@netaxs.com>
> Subject: Re: sell shell accounts?
> To: vansax@atmnet.net (Jim Van Baalen)
> Date: Fri, 19 Jul 1996 17:17:19 -0400 (EDT)
> Cc: richards@netrex.com, agislist@interstice.com, nanog@merit.edu
> 	[...]
> Nope, remember - there is no magic.  Any mesh of PVCs that one makes
> over a switched network must reflect the toplogy of that network, and
> one can set up a matching set of active routing sessions and route
> weights which will cause traffic to flow the same way.
> 	[...]

If I understand what you are asserting, I probably don't agree with it.

I assume that we are talking about using wide-area ATM networks, (or
more specifically, wide-area ATM services provided by, for example,
carriers).

Consider the following configuration

   ________           _______________________          ________
  | Router | loop A  |                       | loop C | Router |
  |   A    |=========| Wide-Area ATM Service |========|   C    |
  |________|         |_______________________|        |________|
                                 |
                                 | local loop B
                                 |
                             ________
                            | Router |
                            |   B    |
                            |________|


The "long" path in this diagram between Router A and Router C is
through Router B, with a VC between (A,B) and a VC between (B,C).

The "shortcut" path in this diagram is a VC between (A,C).

The biggest disadvantage of routing packets between A and C via B is
that every packet (needlessly) traverses local loop B twice.  
Furthermore, this local loop is perhaps the most expensive component
in the system.

The desire for a full mesh of VCs between routers becomes more compelling
if you have more than three routers.  The diagram is left as an exercise
to the reader; I rather dislike having to draw in ASCII.

Note that the two paths between A and C, (directly versus via B), seem
to take distinctly different paths.  (Perhaps, I missed your point).

It is not clear to me that being able to ping router B along the
path between A and C buys you much.  It sounds a bit (well, maybe only a
bit) like wanting to cut a T1 in half to install a router in the middle so
that you can tell which half of the T1 went down.  (Again, I am assuming
that the network in question is using ATM services, so the carrier
is responsible for keeping the VCs up.)

-tjs
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