netdev - Re: EoGRE sends undersized frames without padding

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Date:   Wed, 5 Jun 2019 18:17:51 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     "Steinar H. Gunderson" <steinar+kernel@...derson.no>
Cc:     Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: EoGRE sends undersized frames without padding

On Wed, Jun 5, 2019 at 12:27 AM Steinar H. Gunderson
<steinar+kernel@...derson.no> wrote:
>
> On Tue, Jun 04, 2019 at 09:31:42PM -0700, Cong Wang wrote:
> >> This works fine for large packets, but the system in the other end
> >> drops smaller packets, such as ARP requests and small ICMP pings.
> > Is the other end Linux too?
>
> Yes and no. The other end is Linux with Open vSwitch, which sends the packet
> on to a VM. The VM is an appliance which I do not control, and while the
> management plane runs Linux, the data plane is as far as I know implemented
> in userspace.

Hmm, sounds like openvswitch should pad the packets in this scenario,
like hardware switches padding those on real wires.


>
> The appliance itself can also run EoGRE directly, but I haven't gotten it to
> work.
>
> > If the packet doesn't go through any real wire, it could still be accepted
> > by Linux even when it is smaller than ETH_ZLEN, I think.
>
> Yes, but that's just Linux accepting something invalid, no? It doesn't mean
> it should be sending it out.

Well, we can always craft our own ill-formatted packets, right? :) Does
any standard say OS has to drop ethernet frames shorter than the
minimum?


>
> > Some hardware switches pad for ETH_ZLEN when it goes through a real wire.
>
> All hardware switches should; it's a 802.1Q demand. (Some have traditionally
> been buggy in that they haven't added extra padding back when they strip the
> VLAN tag.)

If so, so is the software switch, that is openvswitch?

>
> > It is still too early to say it is a bug. Is this a regression?
>
> I haven't tried it with earlier versions; I would suspect it's not a
> regression.

Good to know.

Thanks.