Path: utzoo!mnetor!uunet!husc6!mailrus!ames!pasteur!ucbvax!imagen.UUCP!geof
From: geof@imagen.UUCP (Geof Cooper)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: packet lengths
Message-ID: <8804191854.AA00089@apolling.imagen.uucp>
Date: 19 Apr 88 18:54:48 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 14


 > I have been comparing the lengths of packets specified in IP headers
 > against actual packet lengths.  What I am seeing, ignoring IP packets
 > smaller than the minimum packet size, is that a fair number of machines
 > send out packets that are 1-3 bytes longer than is specified by the
 > IP length.

The Ethernet requires that packets be an integral number of 16-bit words
long.  The Ethernet also has a minimum packet size of 60 bytes.  Any IP
packet that is less than 60 bytes in length (including ethernet header)
will be padded to 60 bytes.  The IEEE 802.3 length field would allow
you to explicitly set the ethernet length, but Ethernet doesn't.

- Geof Cooper