Ethereal-dev: Re: [Ethereal-dev] Trying to dissect LAPBether ...

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From: Guy Harris <gharris@xxxxxxxxxxxx>
Date: Thu, 28 Dec 2000 19:13:49 -0800
On Wed, Dec 27, 2000 at 01:54:13AM +1000, Richard Sharpe wrote:
> There is a LAPB driver for Ethernet, lapbether, available for Linux, which
> allows Linux to Linux LAPB over ethernet, over which you can shove X.25 ...
> 
> I am trying to dissect it; it is carried in frames of type 0x6000, an
> unused type allocated to DEC.

Actually, it appears to be carried in frames of various different types.

There was an e-mail thread at the beginning of the year, the first
message of which was:

	From: Vogl <svogl@xxxxxx>
	To: "'ethereal-users@xxxxxxxx'" <ethereal-users@xxxxxxxx>
	Subject: [ethereal-users] Monitoring Linux-X.25
	Date: Wed, 12 Jan 2000 11:46:12 +0100

	Hello,
	I'm playing with Linux X.25-implementation and would like to
	monitor the X.25 traffic on the ethernet.  Although there seems
	to be an implementation of X.25 in ethereal I get no resolution
	of the X.25 protocol.  I've recompiled the lapbether module to
	use packet type 0x0805 (the original implementation uses DEC
	type 0x6000), but that does not help.

	Has anyone an idea how

	either:
	to get ethereal resolving the X.25 protocol under Linux
	or:
	how to hack ethereal to get the protocol resolved by using an
	offset of 14 bytes at the ethernet packet level?

	Thanks in advance,

	Siegfried

It turned out that what he really wanted was to get Ethereal to resolve
LAPB, which would then resolve X.25 - i.e., this is LAPB-over-Ethernet,
not X.25-Packet-Layer-Protocol-over-Ethernet.

0805 is described by

	http://www.isi.edu/in-notes/iana/assignments/ethernet-numbers

as being for "X.25 Level 3", which I'd expect to be the X.25 Packet
Layer Protocol, not LAPB; 6000 is, according to that document, a "DEC
Unassigned" protocol number.

I don't know what original implementation of lapbether used 6000 - at
least as of 2.2.13, it used ETH_P_X25, which is 0805 - but this suggests
that perhaps the Ethernet type used by packet-lapbether.c should be a
configurable preference, as it appears that there will be both ethertype
6000 and ethertype 0805 traffic with LAPB inside it.