Path: utzoo!attcan!uunet!zephyr.ens.tek.com!orca.wv.tek.com!nevermore!alanj
From: alanj@nevermore.WV.TEK.COM (Alan Jeddeloh;685-2991;61-201;292-9740;orca)
Newsgroups: comp.dcom.lans
Subject: Re: How many physical addresses belong to 1 Ethernet station?
Message-ID: <9380@orca.wv.tek.com>
Date: 22 Oct 90 21:39:45 GMT
References: <1990Oct18.172918.24929@forwiss.uni-passau.de>
Sender: nobody@orca.wv.tek.com
Reply-To: alanj@nevermore.WV.TEK.COM (Alan Jeddeloh)
Organization: Tektronix, Inc., Wilsonville, OR
Lines: 38

In article <1990Oct18.172918.24929@forwiss.uni-passau.de> grass@unipas.fmi.uni-passau.de (Grass) writes:

>Here we are back to our question: is hardware address decoding done in
>practice? If yes, how many address pattern have to be buffered (adjusted by
>switches??? 48-bit?) on average/maximum?

All Ethernet controller chips can filter incoming packets on the basis of
the destination address.  All can filter directly addressed packets (leading
address bit == 0).  All (that I am familiar with) turn on or off receipt of
broadcast packets (all address bits 1's).  Chips vary in the scheme used
to filter multicast addresses.  The AMD 7990 "LANCE" and the Intel 80586
use a hash scheme:  Multicast addresses are hashed into some small number
(e.g. 64) and a corresponding bit vector loaded into the chip determine
whether the packet is accepted.  Software then must double check the received
packet address, but this is a low-overhead check since the multicast address
space is sparse and the chance of two different multicast addresses appearing
on the same wire having the same hash value is small.  I am not familiar with
the SEEQ chip.  I believe the National NIC (number?) chip uses a similar scheme.

The second generation chips have a little more power.  The Intel 80596 can
hash physical addresses as well as multicast addresses.  (I found that buried
in a preliminary data sheet.)  This allows efficient filtering on multiple
physical addresses.  This is useful for network monitors and diagnostic
equipment, a possibly multi-protocol routers.

The National 83932 SONIC uses a 16-entry content-addressable memory to filter
incoming packets.  The enties in the CAM can be any combination of multicast
addresses or physical addresses.  This limits you to 16 different addresses,
but eliminates the need to do the software check.  Broadcast address filtering
is a separate control.

Most (all?) chips also allow a "promiscuous" mode where all packets are
accepted.  Most (all?) allow a mode to accept all multicast packets as well.

    -Alan Jeddeloh      (503) 685-2991
    Tektronix Network (Yes, we're still here) Displays
    D/S 60-180; PO Box 1000; Wilsonville, OR 97070
    alanj@nevermore.wv.tek.com       Quoth the printer, "Nevermore!"