netdev - RE: [PATCH 1/1] eth: Transparently receive IP over LLC/SNAP

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Message-ID: <4baebbcb95d84823a7f4ecbe18cbbc3c@AcuMS.aculab.com>
Date:   Sat, 5 Mar 2022 07:02:26 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Dimitrios P. Bouras'" <dimitrios.bouras@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH 1/1] eth: Transparently receive IP over LLC/SNAP

From: Dimitrios P. Bouras
> Sent: 05 March 2022 00:33
> 
> Practical use cases exist where being able to receive Ethernet packets
> encapsulated in LLC SNAP is useful, while at the same time encapsulating
> replies (transmitting back) in LLC SNAP is not required.

I think you need to be more explicit.
If received frames have the SNAP header I'd expect transmitted ones
to need it as well.

> Accordingly, this
> is not an attempt to add full-blown support for IP over LLC SNAP, only a
> "hack" that "just works" -- see Alan's comment on the the Linux-kernel
> list on this subject ("Linux supports LLC/SNAP and various things over it
> (IPX/Appletalk DDP etc) but not IP over it, as it's one of those standards
> bodies driven bogosities which nobody ever actually deployed" --
> http://lkml.iu.edu/hypermail/linux/kernel/1107.3/01249.html).

IP over SNAP is needed for Token ring networks (esp. 16M ones) where the
mtu is much larger than 1500 bytes.

It is all too long ago though, I can't remember whether token ring
tends to bit-reverse the MAC address (like FDDI does) which means you
can't just bridge ARP packets.
So you need a better bridge - and that can add/remove some SNAP headers.

...

	David

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