From: vjs@rhyolite.wpd.sgi.com (Vernon Schryver)
Newsgroups: comp.dcom.lans.fddi
Subject: Re: FDDI and ethernet address spaces
Keywords: MAC Address, 48-bit, 16-bit, bit swap, FDDI
Date: 15 Apr 92 03:27:52 GMT
Organization: Silicon Graphics, Inc.  Mountain View, CA

In article <1992Apr14.054134.11113@berlioz.nsc.com>, my@berlioz.nsc.com (Mike Yip) writes:
> In article vjs@rhyolite.wpd.sgi.com (Vernon Schryver) writes:
> >station ID.  There is no profit for anyone in fiddling around with
> >locally administrated 16-bit addresses.  Very few people have so much time
> >on their hands that they like assigning MAC addresses to stations.

> Well, in a closed enviornment, say the space shuttle or an airplane,
> someone might want to use the 16 bits address on the equipment.  For example,
> the missile launch control board will always have the same MAC address
> on every plane so that a statistic collection board somewhere can dump
> and identify net traffic easily.  ;-)  I have no idea how useful the 16-bit
> address is, but someone somewhere will think of some strange reason to use it.

You think so?  There's no chance that anyone might happen to put two such
data gadgets on the same net while testing on the ground?  What if you
add a second G/72-Q4B thruster-aileron performance monitor when upgrading
the airframe?

What is the advantage of having a fixed MAC address?  The shuttle, space
station, and airplanes tend to have more than one data colletion box.
Using hard coded 16-bit addresses for them would be a literal disaster.

Then there is the question of higher level protocols.  

In my experience, most people who like allocating with MAC addresses also
think nothing of designing their own higher layer protocols, but have never
heard the phrases "fragmentation," "windows", "retransmission", or
"forwarding".  They regularly design wonderful start-stop protocols that
achieve as much as 200Kbit/sec over ethernet.  I assume present company
knows all that stuff, and many of us have encountered this common species
of "raw packet" customer.  A classic example is a high profile outfit I
call "So.FL Pyrotechnics," which has a wonderful protocol which requires
running ethernet stations in promiscuous mode and were proud of their
amazing network speed.  (Guess what I found amazing about it.)

Or the ANSI committee that designed MAC-layer network management frames
that can be bigger than the max packet size, but couldn't be bothered with
worrying about the implications, and collectively had never heard of
"fragmentation" not to mention the RFC with "fragmentation considered
harmful" in its title.  (guess which committee)

> >I don't think it's worth the silicon for a station.  It takes a 256-byte
> >table and about 24 instructions on your favorite RISC to bit-swap a 6-byte
> >MAC address.  Those 24 instructions can be hidden among the instructions
> >that you use to build the MAC header from the ARP table.

> Agree.  But someone somewhere might want to save that 24 instructions.
> Especially in bridge/router situations.  I think that some routers or 
> bridges that use one (combined) hardware CAM (Content addressable memory)
> to do big time address recongnition will like to use only one bit order.
> For example, a FDDI-Enet bridge might want to use the enet bit order in 
> the CAM only.

Yes, perhaps bridges.
Routers must do ARP, which means they must build & parse packets containing
the same addresses in both bit orders.

The bit reversal hardware may be handy for some.  I'd rather have more
intelligently designed FDDI DMA engines.  (NOTE:  not "smarter" DMA, but
better designed.)  The ones I've seen are all terrible.  Most could be
faster if they were more usable.  (I just LOVE hardware guys who insist on
doing me favors I didn't ask for.)


vjs