From: brian@XLNVAX.NOVELL.COM (Brian Meek)
Subject: Re: Packet Drivers Reply Some More
Reply-To: Novell LAN Interest Group <NOVELL%SUVM@clvm.clarkson.edu>
Organization: Clarkson
Date: 27 Feb 90 03:03:24 GMT

> >   1.  What are packet drivers, what are they used for, why?
>
> >Packet drivers serve two puposes: they hide the difference between PC
> >Ethernet cards behind a software interface, and they allow multiple
> >protocols (most often TCP/IP and Novell) access to the same Ethernet card.
>
> >   2.  What in the !@#$%^^&* is ECONFIG used for (e.g. WHY is it used)?
>
> >ECONFIG changes the format of the packet sent out on the wire.  Beyond
> >that, I'm hazy of the details.
>
>    Let's fill in a couple of more details...
>
>    NetWare normally sends 802.3 formatted packets accross the network.  The
>    packet driver and the public domain TCP/IP packages want an Ethernet
>    Type 2 packet to be compatible.  There is
>    no problems with different packet types sharing the network.  It is only
>    a problem when they have to communicate with one another.
>
>    The ECONFIG utility modifies the NetWare packets to Ethernet Type 2
>    packets.

This is essentially correct, but the main problem with Novell's standard
"802.3" packet format is that it does not contain an 802.2 LLC header.
This packet type will be ignored by protocols that use an ethernet packet
format (such as TCP/IP) and can therefore peacefully coexist.  However,
when a protocol that uses 802.3 packet format expects to see the 802.2
LLC info, the standard Novell packet can cause problems because the IPX
header (immediately following the length/type field) is not a legal 802.2
packet format.

Generic Novell ethernet packet -

   |802.3 MAC Header  | XNS / IPX Header|
   --------------------------------------
   |dadr | sadr | len | csum | len | etc.
   --------------------------------------
                        ^^^^ Checksum is not used in IPX, FFFF is placed here.
802.2 LLC format -

   |802.3 MAC Header  |  802.2 LLC Header  |  SNAP Ext.  |
   =-----=------=-----=------=------=------=-----=-------=
   |dadr | sadr | len | dsap | ssap | cntl | pid | etype |
   =-----=------=-----=------=------=------=-----=-------=

When an IPX packet is sent to a broadcast MAC address, the 802.2 devices
examine the packet further...  they see the nul-checksum "FFFF" and believe
it represents a "Global DSAP" (Destination Service Access Point) packet.
>From that point on, the IPX packet makes no sense to the 802.2 device.

When Excelan (now Novell) first wrote NetWare drivers for the LAN WorkPlace
for DOS, we recognized the problem that 802.2 devices would have with the
regular IPX packet format and included an optional packet format that provided
an 802.2 header and SNAP extension... this was before the "ECONFIG" option
was available.  ECONFIG became a necessity with the release of NetWare VMS,
where NetWare traffic could not be retrieved by DEC's ethernet drivers without
an 802.2 header or ethernet type field and might be required to share media
with 802.3 devices.  With the availability of ECONFIG, Excelan dropped the
pseudo 802.2 packet format option and made the EXOS 205T drivers
econfigurable.

The "ECONFIGed" packet looks like this:

   |802.3 MAC Header  |  IPX Header...   |
   =-----=------=-----=------=-----=-----=
   |dadr | sadr |type | csum | len | etc.|
   =-----=------=-----=------=-----=-----=
                (8137)

Please note that only drivers that are specifically designed to work
with ECONFIG are usable with ECONFIG.  As Glenn said, there is not a
problem (as far as NetWare is concerned) having both ECONFIGed and
regular NetWare packets on the same cable.  I encourage the use of econfig
at all times.

brian
____________________________________________________________________________
         Brian Meek             Novell - 2180 Fortune Dr. San Jose, CA 95131
Internet Mail: brian@novell.COM                        Phone: (408) 473-8375
   CompuServe: 76702,346                                 FAX: (408) 433-0775
