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Re: sell shell accounts?
- From: Tim Salo
- Date: Tue Jul 23 18:10:09 1996
> From: Avi Freedman <firstname.lastname@example.org>
> Subject: Re: sell shell accounts?
> To: email@example.com (Jim Van Baalen)
> Date: Fri, 19 Jul 1996 17:17:19 -0400 (EDT)
> Cc: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
> 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,
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.)
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