Re: [PATCH] xfrm:fragmented ipv4 tunnel packets in inner interface

From: "Maciej Żenczykowski" <maze@google.com>
To: "lina.wang" <lina.wang@mediatek.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	Linux NetDev <netdev@vger.kernel.org>,
	Kernel hackers <linux-kernel@vger.kernel.org>,
	linux-mediatek@lists.infradead.org,
	Jakub Kicinski <kuba@kernel.org>,
	Matthias Brugger <matthias.bgg@gmail.com>,
	Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
	Greg Kroah-Hartman <gregkh@google.com>,
	"David S . Miller" <davem@davemloft.net>,
	linux-arm-kernel@lists.infradead.org,
	Lorenzo Colitti <lorenzo@google.com>
Subject: Re: [PATCH] xfrm:fragmented ipv4 tunnel packets in inner interface
Date: Wed, 4 Nov 2020 20:41:43 -0800	[thread overview]
Message-ID: <CANP3RGfvadfLabY9vVFxcWioEBUURRSx3GHgbO2KHO180wX=bw@mail.gmail.com> (raw)
In-Reply-To: <1604547381.23648.14.camel@mbjsdccf07>

On Wed, Nov 4, 2020 at 7:53 PM lina.wang <lina.wang@mediatek.com> wrote:
>
> Besides several operators donot fragment packets even DF bit is not set,
> and instead just drop packets which they think big, maybe they have a
> consideration--- fragmentation is expensive for both the router that
> fragments and for the host that reassembles.
>
> BTW, ipv6 has the different behaviour when it meets such scenary, which
> is what we expected,ipv4 should follow the same thing. otherwise,
> packets are drop, it is a serious thing, and there is no hints. What we
> do is just fragmenting smaller packets, or is it possible to give us
> some flag or a sysctl to allow us to change the behaviour?
>
> On Thu, 2020-09-17 at 19:19 +0800, lina.wang wrote:
> > But it is not just one operator's broken router or misconfigured
> > router.In the whole network, it is common to meet that router will drop
> > bigger packet silently.I think we should make code more compatible.and
> > the scenary is wifi calling, which mostly used udp,you know, udp
> > wouldnot retransmit.It is serious when packet is dropped
> >
> > On Thu, 2020-09-17 at 09:46 +0200, Steffen Klassert wrote:
> > > On Tue, Sep 15, 2020 at 08:17:40PM +0800, lina.wang wrote:
> > > > We didnot get the router's log, which is some operator's.Actually, after
> > > > we patched, there is no such issue. Sometimes,router will return
> > > > packet-too-big, most of times nothing returned,dropped silently
> > >
> > > This looks like a broken router, we can't fix that in the code.
> > >
> > > > On Tue, 2020-09-15 at 11:32 +0200, Steffen Klassert wrote:
> > > > > On Tue, Sep 15, 2020 at 05:05:22PM +0800, lina.wang wrote:
> > > > > >
> > > > > > Yes, DF bit is not set.
> > > > >
> > > > > ...
> > > > >
> > > > > > Those two packets are fragmented to one big udp packet, which payload is 1516B.
> > > > > > After getting rid of tunnel header, it is also a udp packet, which payload is
> > > > > > 1466 bytes.It didnot get any response for this packet.We guess it is dropped
> > > > > > by router. Because if we reduced the length, it got response.
> > > > >
> > > > > If the DF bit is not set, the router should fragment the packet. If it
> > > > > does not do so, it is misconfigured. Do you have access to that router?

I'm afraid I don't really understand the exact scenario from the
description of the patch.

However... a few questions come to mind.
(a) why not set the DF flag, does the protocol require > mtu udp frames
(b) what is the mtu of the inner tunnel device, and what is the mtu on
the route through the device?
- shouldn't we be udp fragmenting to the route/device mtu?
maybe stuff is simply misconfigured...

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