Ethereal-dev: Re: [Ethereal-dev] incorrect packet length

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From: Guy Harris <guy@xxxxxxxxxx>
Date: Fri, 6 Dec 2002 15:44:11 -0800
On Fri, Dec 06, 2002 at 10:36:44PM +0100, J.Smith wrote:
> I've tried to verify this behaviour by loading the attached file
> 'incorrect-packet-length.single-packet.trace' in both ethereal/tethereal and
> in tcpdump (using the '-r' option). Tcpdump then correctly states the total
> packet size as 'len 1500', while (t)ethereal incorrectly reports the total
> packet size as '1514'.

Incorrectly?  The packet *is* 1514 bytes long - 14 bytes of Ethernet
header, 20 bytes of IP header, 20 bytes of TCP header, and 1460 bytes of
TCP payload.

There are 1500 bytes of Ethernet *payload*; that's what tcpdump is
reporting.  That's the maximum amount of MAC client data according to
3.1.1 "MAC frame format" in IEEE Std 802.3-2002 - but that doesn't
include the destination address, source address, and length/type fields
in the Ethernet header, nor does it include the FCS at the end.