RE: [PATCH net-next] net: x25: Remove unimplemented X.25-over-LLC code stubs

[David Laight]

From: Xie He
> Sent: 09 December 2020 22:54
> 
> On Wed, Dec 9, 2020 at 1:21 PM David Laight <David.Laight@xxxxxxxxxx> wrote:
> >
> > I always wondered about running Class 2 transport directly over LLC2
> > (rather than Class 4 over LLC1).
> > But the only LLC2 user was netbios - and microsoft's LLC2 was broken.
> > Not to mention the window probing needed to handle systems that
> > said they supported a window of (IIRC) 15 but would discard the
> > 5th back to back frame.
> 
> To me, LLC1 and LLC2 are to Ethernet what UDP and TCP are to IP
> networks. I think we can use LLC1 and LLC2 wherever UDP and TCP can be
> used, as long as we are in the same LAN and are willing to use MAC
> addresses as the addresses.

Except that you don't have any where near enough 'ports' so you need
something to demultiplex messages to different applications.

We (ICL) always ran class 4 transport (which does error recovery)
directly over LLC1 using MAC address (a NUL byte for the network layer).
This requires a bridged network and globally unique MAC addresses.
Sending out an LLC reflect packet to the broadcast MAC address used to
generate a couple of thousand responses (many would get discarded
because the bridges got overloaded).

> X.25 layer 3 certainly can also run over LLC2.

You don't need X.25 layer 3.
X.25 layer 2 does error recovery over a point-to-point link.
X.25 layer 3 does switching between machines.
Class 2 transport does multiplexing over a reliable lower layer.
So you normally need all three.

However LLC2 gives you a reliable connection between two machines
(selected by MAC address).
So you should be able to run Class 2 transport (well one of its
4 variants!) directly over LL2.

The advantage over Class 4 transport over LLC1 is that there is
only one set of retransmit buffers (etc) regardless of the number
of connections.

But this is all 30 year old history...

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)