Re: Link level Ethernet bridges (Modified)! does that mean joke?


Dan Ehrlich (blitz!ehrlich@psuvax1.psu.edu)
3 Mar 88 02:03:30 GMT


Once upon a time, in another job far, far, away I also encountered
these mysterious aa:0:4:0:xx:xx addresses. Read on. Maybe someone
can explain to me why DEC does what it does. :-)

In article <8802232312.AA02132@sneezy.lanl.gov> cpw%sneezy@LANL.GOV (C. Philip Wood) writes:
>...
>
> segment #1 segment #2
>
> 8:0:20:1:33:1e Ha--| |--Hz 8:0:2b:3:92:23
> | |
> 8:0:20:1:47:d2 Hb--| |--Hz-1 8:0:20:1:d7:e5
> |icmp icmp |--Hy 2:7:1:0:a6:59
> 8:0:9:0:65:3d Hp*--|echo-> echo->|--Hx aa:0:4:0:da:4 hummmm.
                                                         ^^^^^^^^^^^^^

This mystery number is from a VAX running DECnet, either under VMS or
Ultrix. It could also be from another vendor that has implemented
DECnet's host->ethernet address mapping. DEC in their infinite wisdom
uses one of the maintenance functions in the DE[QU]NA to change the
physical ethernet address from what is in the ROM to this magic
number. I do not know if they have gotten around to documenting how it
is computed, but if memory serves me correctly it goes something like
this:

        aa:0:4:0 is a magic constant that appears on any VAX
                        running DECnet.

        da:4 is, get ready, the DECnet host and area number
                        (byte swapped of course). Educated guess says
                        that this is DECnet host 1.218. (Area number)
                        * 1024 + (Host number).

When I asked DEC why they do this I got numerous excuses.

    "Keeping a table mapping DECnet host numbers to Ethernet addresses
     would soak up too much memory"
     "We do not have anything like ARP and do not expect to in the
      near future"
     "Whoes name is on the fron of the 'Blue Book' anyway?".

All of these are from the 1986 time frame and as I have not had reason
to ask the question of late do know know if the responses are still
valid.

> nada <-reply |
> . .
> . .
> . .
> 8:0:20:1:66:6a Hq--| |--Hr 8:0:2b:3:91:5c
> |----?[DEC lanBridge]?---|
>
> SUBNET 1 of Class B network 128.165, mask==0xfffffc00
>
>There is probably alot of irreverent/irrelevant, but, it was fun.
>...
Dan Ehrlich <Dan Ehrlich <ehrlich@psuvax1.{psu.edu,bitnet,uucp}>
The Pennsylvania State University, Department of Computer Science
333 Whitmore Laboratory, University Park, PA 16802
+1 814 863 1142 or +1 814 865 9723



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