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)
