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From: imp@jhereg.osa.com (Charles T. Lukaszewski)
Newsgroups: comp.dcom.lans
Subject: DECnet (or: My VAX's MAC-layer address identity crisis)
Summary: DECnet Protocols
Message-ID: <1990Oct25.004509.7608@jhereg.osa.com>
Date: 25 Oct 90 00:45:09 GMT
References: <1990Oct18.172918.24929@forwiss.uni-passau.de>
Sender: Chuck T. Lukaszewski
Followup-To: imp@osa.com
Organization: Open Systems Architects, Inc.
Lines: 37

In article <1990Oct18.172918.24929@forwiss.uni-passau.de> grass@
unipas.fmi.uni-passau.de (Grass) writes:
>
>This sounds that at least group address recognition is done by software: all
>messages with its first destination address bit set are received by every
>station, transferred to some place in memory and examined by some software
>routines. If the destination address does not match with the actual set of
>active group addresses the whole message is discarded from memory. What a
>waste of cpu/memory/bus ressources!

One implementation that has not been mentioned in this discussion is the
Digital VAX Ethernet controller and the DECnet Phase IV protocols.  Though
reliable information is hard to come by, I have studied the Phase IV proto-
cols at length with HP network analyzers.

It turns out that a VAX, especially a member of a Local-Area VAX Cluster,
actually uses a series of Ethernet addresses, with the DECnet software
"reprogramming" the board's MAC address regularly.  DEC makes extensive
use of multicasts, and you can distinguish between various types of con-
versations based on the first octet in the destination address (AA and AB
are the primary types).  In fact, Ethernet addresses beginning with AA
seem to be addressed to particular stations, as the DECnet node number
and area number are also encoded into the address.

Hewlett-Packard has a really excellent performance analysis package for
DECnet which offers the same kind of statistics as are available for
general Ethernet traffic - FYI.

I'd like to hear from someone more knowledgeable about DECnet internals -
reverse engineering is such a frustrating excercise.


-- 
_______________________________________________________________________________
Charles T. Lukaszewski            imp@osa.com                      612 525-0000
Managing Partner & Chairman                       Open Systems Architects, Inc.
 "Who needs a disclaimer? I liked the opinions so much, I bought the company!"