Ethernet on recent FPGAs

Rich Seifert wrote:

(snip)

> We *did* consider using randomly-chosen addresses, but in a 64-bit 
> address space; the probability of a clash in a 48-bit space was 
> considered too high. I wrote a paper on the subject back in 1979-80, 
> which is probably lost to history.

Higher than the actual clash rate on the current system?

> Although properly-implemented random addresses would have worked, we 
> chose the administered-vendor-space 48-bit scheme of today as being more 
> "politically palatable"; I am sure we would have been hammered even 
> worse about letting randomness control network addressing than we were 
> about letting randomness control the backoff algorithm.

I have heard stories about NICs produced with the same address,
most likely in the same batch with a high probability of being
installed on the same net.

We once had a Sun system that the local computer hardware people
sent back to Sun to be fixed.  The rule at the time was that one
should remove the ROM (or battery backed RAM) before sending it
in.  In this case, it was reinstalled backwards destroying the
stored address.  Losing the stored address from battery backed
RAM can't be that uncommon.  (The battery life tends to be
less than the processor life.)

Also, I once knew a net with a device with address 00:00:00:00:00:00.
As there was only one, it was decided to leave it alone.

-- glen

Nico Coesel wrote:
(snip)

> Yes and no. Like others already pointed out, devices need a serial
> number anyway. Not just for administration but also to keep track of
> devices. So having a fixed number (MAC address) assigned to unit xyz
> generally saves a lot of trouble.

As the subject is FPGAs, I believe it isn't hard to change data in
a synthesized ROM after the bit file has been generated.  That should
be pretty convenient for generating the packet.  Getting the IP
address is a little harder.

> Besides, if you are going to generate random MAC addresses you may get
> intermittant errors because fixed MAC addresses are expected. Those
> kind of errors are the last you want on a network.

Not so much network errors as administration problems.  It is harder
to track down devices with variable MAC addresses.

-- glen

MikeShepherd564@btinternet.com wrote:
> 
> The options seem to be:
> 
> 1) Buy a batch of unique addresses (which isn't cheap and involves
> administrative hassle).  If you're designing a LAN for an airliner I'm
> going to fly on, I'd like you to use this method.
> 

Costs $1,650 for 16,777,216 unique addresses.

http://standards.ieee.org/regauth/oui/forms/

	-hpa

Nico Coesel wrote:
> 
> Yes and no. Like others already pointed out, devices need a serial
> number anyway. Not just for administration but also to keep track of
> devices. So having a fixed number (MAC address) assigned to unit xyz
> generally saves a lot of trouble.
> 
> Besides, if you are going to generate random MAC addresses you may get
> intermittant errors because fixed MAC addresses are expected. Those
> kind of errors are the last you want on a network.
> 

A bigger issue is that randomness is frequently a premium resource in 
real-life systems.

	-hpa

In article <1fednYDifbVWOh_anZ2dnUVZ_gKdnZ2d@comcast.com>,
 glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote:

> Nico Coesel wrote:
> (snip)
> 
> > Besides, if you are going to generate random MAC addresses you may get
> > intermittant errors because fixed MAC addresses are expected. Those
> > kind of errors are the last you want on a network.
> 
> Not so much network errors as administration problems.  It is harder
> to track down devices with variable MAC addresses.
> 

The intent was never to have MAC addresses generated at random *in the 
field*, i.e., upon interface initialization. The idea was to generate 
them at random in the *manufacturing process*; there would not be 
"variable" MAC address for a given device over time. We only wanted to 
avoid the OUI administration problem.



--
Rich Seifert              Networks and Communications Consulting
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