netdev - Re: ip_no_pmtu

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Date:	Mon, 26 Oct 2015 15:35:57 -0700
From:	Vincent Li <vincent.mc.li@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: ip_no_pmtu_disc and UDP

I test again and i did see DF bit now, it is weird. I am going to do
more test, sorry for the noise.

On Mon, Oct 26, 2015 at 3:12 PM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> Hello,
>
> On Mon, Oct 26, 2015, at 23:00, Vincent Li wrote:
>> the UDP packet size is about 768, here is how packet path  like:
>>
>> client
>> <----------------------------------------router<-------------------------------------------------->server
>> (eth0 mtu 1500 ip 10.3.72.69)     (eth0 mtu 1500 ip 10.3.72.1,
>>           (eth0 mtu 1500 ip 10.2.72.99)
>>                                                       eth1.1102 mtu
>> 567 ip 10.2.72.139)
>>
>>
>> UDP client test script:
>>
>> [...]
>>
>> so I am hoping if I echo 0, 1, 2, 3 respectively to
>> /proc/sys/net/ipv4/ip_no_pmtu_disc, I am expected to see DF bit
>> set/unset from the client and should have shown me on the router eth0
>> interface tcpdump, but instead, DF bit never set on the client. am I
>> misunderstanding something?
>
> This is strange...
>
> Can you please capture traffic on eth0 on the client?
>
> For outgoing packets only zero or non-zero matter. A '0' definitely
> generates a UDP packet with a DF bit on my side, anything else a frame
> with DF bit cleared. I just verified this on net-next with your script.
> It also does not cause any setsockopts but uses the default.
>
>> for example:
>>
>>  two concurrent tcpdump on router eth0 (mtu 1500) and eth1.1102 (mtu
>> 576) interface:
>>
>> 1 #tcpdump -nn -i eth0 -v udp and host 10.3.72.69 &
>>
>> 14:51:11.946143 IP (tos 0x0, ttl 64, id 7193, offset 0, flags [none],
>> proto UDP (17), length 796)
>>     10.3.72.69.43748 > 10.2.72.99.9999: UDP, length 768
>>
>
> As I said, I cannot reproduce that. :( Please test on eth0 directly so
> we can be sure the packet does not get mangled.
>
> Can you also show me the output of
> ip route get 10.2.72.139
> on the client after you maybe already received a icmp pkt-too-big
> packet?
>
> Thanks,
> Hannes
>
>
>
>
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