Date   : Tue, 14 Jul 2009 14:44:13 +0100
From   : philb@... (Phil Blundell)
Subject: Is it a bridge? Was: Master Ethernet upgrade

On Tue, 2009-07-14 at 12:55 +0000, Ian Tonks wrote:
> But, what's being proposed between Ethernet and Econet doesn't fit
> into any of the above definitions, but I don't think I've seen a
> description of what the Ethernet to Econet "bridge" actually does in a
> low level techie way. Please correct me on this :) . 

I'm not sure exactly how low-level you were looking for, but I'll have a
go.

The main function is to act as a gateway between a native Econet and an
AUN (essentially Econet frames encapsulated in UDP) network.  The
"Ethernet" part is more or less an irrelevance: it's only used as a
transport for IP and the bridge doesn't do anything interesting with
Ethernet at the raw frame level.

There are three scenarios that come to mind, all of which are slightly
different:

(a) linking an Econet network to an AUN subnet on a LAN.  In this
situation the bridge would talk directly to the other AUN clients across
the LAN using some variant of the AUN address mapping system (e.g.
Econet address 5.73 equates to IP address 10.2.5.73).  AUN addresses
are, obviously, not globally routable so this arrangement doesn't work
over a WAN.

(b) linking two Econets across an IP backbone (e.g. the Internet, or a
wifi LAN), using two back-to-back bridges with, effectively, a point to
point tunnel configured between them.

(c) linking one or more individual IP-based hosts to an Econet.  In this
situation each host would have a specific address mapping (e.g.
209.85.229.147 -> 2.254) and hence, like (b), this arrangement will work
over a WAN.

There's also a secondary function which is to operate as an IP router
between the two networks, i.e. allow Econet-based hosts to access the
Internet.  This is rather less involved since it's just a straight IP
datagram forwarding arrangement with an Econet transport on one side and
an Ethernet transport on the other.

As far as the terminology goes I tend to call it a bridge since, viewed
from the Econet side, it acts in the same way as a traditional Econet
bridge does.  But you're right that both of these would probably be
called "routers" in more modern parlance.  Personally I find it hard to
get too worked up about the name either way.

p.