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Date:   Fri, 12 May 2017 10:05:16 +0200
From:   Teco Boot <teco@...-net.nl>
To:     Fredrik Markström <fredrik.markstrom@...il.com>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bridge@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [Bridge] [PATCH v2 1/2] net: Added mtu parameter to dev_forward_skb calls

IP MTU and L2 MTU are different animals.

IMHO IP MTU is for fragmentation at sender of a link. There is no need dropping IP packets at receiver with size > configured IP MTU. IP packets with size > receiver L2 MTU will be dropped at sub-IP layer.

For this patch: if veth has some notion on L2 MTU (e.g. buffer size limits), there has to be checks for it. I don't know why configuring MRU helps, more config, more mistakes. If there is no need for dropping the packet: don't.

Teco


> Op 11 mei 2017, om 21:10 heeft Fredrik Markström <fredrik.markstrom@...il.com> het volgende geschreven:
> 
> 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:
> 
> ---- 8< ------
> # Host A eth2 and Host B eth0 is on the same network.
> 
> # On HOST A
> % ip address add 1.2.3.4/24 dev eth2
> % ip link set eth2 mtu 300 up
> 
> % # HOST B
> % ip address add 1.2.3.5/24 dev eth0
> % ip link set eth0 mtu 1000 up
> % ping -c 1 -W 1 -s 400 1.2.3.4
> PING 1.2.3.4 (1.2.3.4) 400(428) bytes of data.
> 408 bytes from 1.2.3.4: icmp_seq=1 ttl=64 time=1.57 ms
> 
> --- 1.2.3.4 ping statistics ---
> 1 packets transmitted, 1 received, 0% packet loss, time 0ms
> rtt min/avg/max/mdev = 1.573/1.573/1.573/0.000 ms
> ---- 8< ------
> 
> 
> But it doesn't work with veth:
> 
> ---- 8< ------
> # veth0 and veth1 is a veth pair and veth1 has ben moved to a separate
> network namespace.
> % # NS A
> % ip address add 1.2.3.4/24 dev veth0
> % ip link set veth0 mtu 300 up
> 
> % # NS B
> % ip address add 1.2.3.5/24 dev veth1
> % ip link set veth1 mtu 1000 up
> % ping -c 1 -W 1 -s 400 1.2.3.4
> PING 1.2.3.4 (1.2.3.4) 400(428) bytes of data.
> 
> --- 1.2.3.4 ping statistics ---
> 1 packets transmitted, 0 received, 100% packet loss, time 0ms
> ---- 8< ------
> 
> -- 
> /Fredrik

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