netdev - Re: EoGRE sends undersized frames without padding

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Message-ID: <20190605072712.avp3svw27smrq2qx@sesse.net>
Date:   Wed, 5 Jun 2019 09:27:12 +0200
From:   "Steinar H. Gunderson" <steinar+kernel@...derson.no>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: EoGRE sends undersized frames without padding

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.

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.

> 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.)

> 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.

/* Steinar */
-- 
Homepage: https://www.sesse.net/