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From: willis@cs.tamu.edu (Willis Marti)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Packet Sizes
Message-ID: <9103281948.AA04740@neuron>
Date: 28 Mar 91 19:48:49 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 32

.In article <17724@sdcc6.ucsd.edu> jclark@sdcc6.ucsd.edu (John Clark) writes:
.>In article <9103151236.AA05472@asylum.sf.ca.us> romkey@asylum.sf.ca.us writes:
.>+If you're looking at raw ethernet packet lengths (as opposed to IP
.>+datagram lengths), you'll see lots of 60 byte packets on the net
.>+because ethernet has a minimum packet length of 60 bytes. Any packets
.>+that are shorter are padded out to 60. IP can tell how many bytes it
.>
.>Sometime ago I had a ethernet analyzer on a line with both TCP/IP
.>and DECNET traffic. It seems to me that there were some DECNET
.>packets shorter than the minimum. It could have been a halucination
.>or does DEC violate the standard.
.>-- 
.>
.>John Clark
.>jclark@ucsd.edu
.
.Is it possible that your ethernet analyzer was reporting the length of the
.DECNET packet?  DECNET uses a length field immediately after the Ethertype
.to define the size of the encapsulated DECNET packet.  The Ethernet packet
.is always supposed to be padded out to minimum size (if neccessary).
.
.Art

To clarify, things, the minimum ethernet frame is 64 octets -- which
translates into 6 bytes dest., 6 bytes src, 2 bytes (type/length), 4 btyes
frame check sequence & 56 bytes data+pad.  Preamble and SFD excluded.

And, yes, DEC does violate this minimum length (in loopback packets).
-------------------------------------------------------------------------------
 Willis F. Marti		Internet: willis@cs.tamu.edu
 Director, Computer Services Group, Dept of Computer Science, Texas A&M Univ.
 	---Not an official document of Texas A&M---