Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!deimos.cis.ksu.edu!unmvax!tut.cis.ohio-state.edu!ucbvax!agate!saturn!eshop
From: eshop@saturn.ucsc.edu (Jim Warner)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: minimum length ethernet packet
Keywords: runt LANCE Ethernet
Message-ID: <7522@saturn.ucsc.edu>
Date: 17 May 89 00:06:15 GMT
References: <8905151510.AA00477@mitre.arpa>
Reply-To: eshop@saturn.ucsc.edu (Jim Warner)
Organization: University of California, Santa Cruz
Lines: 40

In article <8905151510.AA00477@mitre.arpa> mckee@MITRE.MITRE.ORG (H. Craig McKee) writes:

>
>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.

Remember that Ethernet is half-duplex.  A station cannot send and receive
at the same time.  To be able to do so, Ethernet chips would have to be
more complicated than they are.  And their DMA circuitry would have to
be able to support twice the data rate.  The LANCE chip has enough ram 
in its Silo to receive the first 32 bytes of a packet when it is in 
loopback mode.  Basicly, the test is to send a runt 32 byte packet and 
then read what is in the Silo to see if it is what was sent.

A problem with this test is that the interface can't determine if a
failure might be due to faulty hardware or because the runt suffered
a collision.  Perhaps this is part of the reason that the spec sheet
encourages that the test be run several times in case of a failure
before declaring the hardware nonfunctional.

>"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.

I sorta doubt that this statement is true.  I have a handly little
transceiver cable breakout box that I can use to open the collision
sense pair in the transceiver cable.  What I've found is that most
vendors ignor a missing heartbeat.  I think that, except for DEC,
most vendors are pretty sloppy about looking for errors.

Still I think that if Xerox is going to stick something in the transceiver
cable, they need to provide the local echo "just like a transceiver."
I agree that it sounds like they didn't do their homework.

>Any advice or comments would be appreciated.  Regards - Craig

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