[ih] "The First Router" on Jeopardy

[Clem Cole]

On Tue, Nov 23, 2021 at 10:26 AM Dave Crocker via Internet-history <
internet-history at elists.isoc.org> wrote:

> On 11/23/2021 6:59 AM, Steve Crocker via Internet-history wrote:
> > That said, I'll acknowledge that a bridge, which simply flooded the
> > channels with whatever came in,
>
>
> What you've described is a repeater.
>
> A bridge is a repeater that learns where hosts are and becomes selective
> about the packets it forwards.  (I assume Radia is on this list.)
>
> A repeater does not deal with loops.  A bridge does.
>
FWIW:  I describe the difference to students as an analog process vs. a
digital one.  The repeater function (like  devices used in radio) listen at
a specific frequency and other than the time delay inherent within, what
goes in, goes out, but the signal strength as been improved (*i.e.* the S/N
is corrected), but it's the same information good or bad on both sides of
the repeater.  Which means that, if the packet is ill formed coming to the
circuit, it will be repeated ill formed going out of the circuit.  As you
point out - thus a repeater can create loops.

A bridge (and a router) has (have) some amount of logic inside of it and at
least stores and examines some of the bits before copying the information
(if not the entire packet before forwarding them).  Moreover the
information in does not have to be exactly the same what is out.  As was
said, some filtering of some packets such as broadcast packets may be
tossed.  But ill formed packets are not forwarded.    Plus, the src address
of the packet is not necessarily the same for a packet going in as the
source going out of a bridge. Which also means that the CRC must be
recalculated if any of the information is changed from input to output.
Obviously because a new packet is created, the bridge will also improve the
S/N in the electrical signal, but the key is that it may have changed the
information content as it did it and not all of the information is passed
through it and as you importantly point out, because it can filter, it can
check for loops and remove packets that are causing same.

Finally, at a higher level yet is a router which adds more logic in that it
inspects deeper into the packet and understands more about the contents
other than the raw bit level of a bridge [ src/dest/len/crc *vs.* actual
protocol knowledge ] and may filter and/or modify the contents of the data
within the packet itself.  The types of filtering and type of packet
modifications are even more robust than that of the bridge.  That is to
say, more data tends to get filtered and thrown away and /or modified as
appropriate.

This of course is why some of the early products like what 3Com produced
were called 'BROUTERS' that did both of the lower (bridge) higher (router)
level functions in the same box, since some of the protocols it might
not know how to route, and for those packets, it acts like a bridge -- IIRC
it bridged 'NetBIOS' packets and could route Netware and IP.