Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!MITRE.MITRE.ORG!mckee
From: mckee@MITRE.MITRE.ORG (H. Craig McKee)
Newsgroups: comp.protocols.tcp-ip
Subject: minimum length ethernet packet
Message-ID: <8905151510.AA00477@mitre.arpa>
Date: 15 May 89 15:10:37 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The MITRE Corp., Washington, D.C.
Lines: 44

DEC has established a facility in St. Louis to test and integrate
various security components into a system for the USAF Military
Airlift Command.  (The end result of this effort is to be a system that
provides Multilevel Security.)  One of the components being tested is
the Xerox Encryption Unit (XEU).  The XEU is designed to be installed
in the drop cable between a processor and the transceiver, as
illustrated below.

      Thicknet or Thinnet ----| MAU |----------              
                                 |<----802.3                     
                                 |                               
                              | XEU |                            
                                 |<----802.3
                                 |
                          | DECserver 100 |

One of the problems discovered by DEC is that the XEU will not forward
short packets; as reported by DEC: "The XEU's will not forward a short
ethernet packet.  During power-up and self-testing, many devices
(including DECserver 100's) create a 32 byte packet (36 bytes with
CRC).  This packet is transmitted to the network.  The device expects
to receive the packet back from the network.  This is how the device
determines that the network is operational.  Although the Ethernet
Version 2 specification and the  IEEE 802.3 specification state that
the minimum length of a data packet is 64 bytes, it is quite common in
the industry to use a smaller packet for network verification
testing.  Actually, the LANCE chip set which is one of the most
common ethernet chips in the world implements this test.  Most network
vendors using the LANCE chip perform this minimum length packet test of
the network. [...]  For the terminal server to properly start up, this
packet must be passed through the XEU and back to the terminal
server."

Questions/Comments

I don't have the LANCE spec sheet, but it seems to me the chip set
should be passive with respect to packet length; that it is the 
responsibility of the driver routine to insure proper packet length.

"Most network vendors ... perform this minimum length packet test ..."
Is that true? It would imply Xerox should have done a better job
checking the compatibility of the XEU.

Any advice or comments would be appreciated.  Regards - Craig