netdev - Re: [PATCH v2 1/2] net: Added mtu parameter to dev_forward

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Date:   Thu, 11 May 2017 12:44:54 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Fredrik Markström <fredrik.markstrom@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, bridge@...ts.linux-foundation.org
Subject: Re: [PATCH v2 1/2] net: Added mtu parameter to dev_forward_skb
 calls

On Thu, 11 May 2017 21:10:11 +0200
Fredrik Markström <fredrik.markstrom@...il.com> wrote:

> On Thu, May 11, 2017 at 6:01 PM, Stephen Hemminger
> <stephen@...workplumber.org> wrote:
> > On Thu, 11 May 2017 15:46:27 +0200
> > Fredrik Markstrom <fredrik.markstrom@...il.com> wrote:
> >  
> >> From: Fredrik Markström <fredrik.markstrom@...il.com>
> >>
> >> is_skb_forwardable() currently checks if the packet size is <= mtu of
> >> the receiving interface. This is not consistent with most of the hardware
> >> ethernet drivers that happily receives packets larger then MTU.  
> >
> > Wrong.  
> 
> What is "Wrong" ? I was initially skeptical to implement this patch,
> since it feels odd to have different MTU:s set on the two sides of a
> link. After consulting some IP people and the RFC:s I kind of changed
> my mind and thought I'd give it a shot. In the RFCs I couldn't find
> anything that defined when and when not a received packet should be
> dropped.
> 
> >
> > Hardware interfaces are free to drop any packet greater than MTU (actually MTU + VLAN).
> > The actual limit is a function of the hardware. Some hardware can only limit by
> > power of 2; some can only limit frames larger than 1500; some have no limiting at all.  
> 
> Agreed. The purpose of these patches is to be able to configure an
> veth interface to mimic these different behaviors. Non of the Ethernet
> interfaces I have access to drops packets due to them being larger
> then the configured MTU like veth does.
> 
> Being able to mimic real Ethernet hardware is useful when
> consolidating hardware using containers/namespaces.
> 
> In a reply to a comment from David Miller in my previous version of
> the patch I attached the example below to demonstrate the case in
> detail.
> 
> This works with all ethernet hardware setups I have access to:
> 

Why not just use an iptables rule to enforce what ever semantic you
want?