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Re: multi-homing

  • From: Joel Baker
  • Date: Wed Aug 29 11:23:53 2001

On Tue, Aug 28, 2001 at 09:06:16PM -0400, Leo Bicknell wrote:
> I know someone will want to argue that it is not possible to allow
> _everyone_ to multihome, so I will go ahead and suggest an example
> of a network very close to that goal, in fact one you can run IP
> over (if a tad slowly).  The cellular telephone network for the
> most part has this property.  Every phone has a number.  It can
> dynamically connect to multiple providers, and can change providers
> as conditions change, or the device moves.  The only property it
> doesn't have that IP multihoming doesn't have is the ability to
> use two providers at the same time; however the technology for a
> handset that could make a CDMA and a TMDA call at the same time on
> two different networks definitely exists.
> This is not to say I think 'cellular phone routing' would solve
> the IP issues if directly translated, in fact I think quite the
> opposite.  That said, I do belive large scale systems that allow
> individual users to move, or multihome (I think those two items
> are more closely related than people think) while keeping their
> "address" exist.

The closest analog in the networking world that comes to mind for matching
the properties of how both cell-phone portability and LNP work isn't IP.
It's DNS. Consider the following points, each of which applies to both
telco routing and DNS, but *not* to IP:

1) 'Routing' lookup done once, at the start of the transaction.
2) Relatively low overhead compared to the transaction.
3) Handled by a distributed system using local *external* query points
   whose primary job is providing routing information, not actually doing
   the routing.
4) Every address is portable between nearly arbitrary providers (and, in
   the case of the telco, I believe is now required to be by law).

I'm far from the first person to bring this up; please see some of the
excellent papers from the ATM, VOIP, and MPLS arenas about "why routing
telco traffic is completely different from routing IP traffic", many of
them authored by members of this forum, for further details.

The primary failure points in this is that DNS recovery times are nowhere
near as good as most telco re-routing times, can't occur mid-stream, and
because of this combination, tends to be far more problematic when there
is a failure, because people *notice* - both in that their session goes
away, and that they then can't even just "redial" and blame it on a cosmic
ray at the wrong time.
Joel Baker                           System Administrator -
[email protected]