karl@TRWIND.IND.TRW.COM (Karl Auerbach) (07/26/88)
I have recently built a "device driver" for PC-NFS in which the "device" is the much discussed packet driver interface. It works. However, because the packet driver specification uses the ether-type field as the demultiplexing tool, PC-NFS preempts other software from using IP, ARP, and RARP packets. The packet driver spec as it now exists does not support Novell. Nor does it support 802.3. Some folks have added their own extensions to fix these weaknesses. The trouble with 802.3 is that it, by itself, does not have a packet demultiplexing field -- the old ethernet type field being replaced with a length field. Normally 802.2 and SNAP would help demultiplex 802.3 packets and a packet driver service could be designed to deal with these. Novell on the other hand uses 802.3, but not 802.2. Rather they simply dump their IPX packets directly into the data part of the 802.3 packet. So, to demultiplex Netware packets the driver must do some work to figure out whether the data in an 802.3 packet is a Novell-form packet or an 802.2 packet. One of the tricks there is that Netware does not use checksums, but rather uses -1. So the first two bytes of the data is 0xFF 0xFF for Netware (and is a rather unlikely value for legal 802.2). --karl--