netdev - Re: ip_no_pmtu

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Date:	Mon, 26 Oct 2015 15:53:06 -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

ok, I observed  if i increase the UDP client packet size > local
interface  MTU 1500, the client will fragment the packet first and
then send it out, if the UDP client packet size < local interface MTU
1500, the DF bit will be set when ip_no_pmtu_disc set to 0, is this
expected behavior ?



On Mon, Oct 26, 2015 at 3:35 PM, Vincent Li <vincent.mc.li@...il.com> wrote:
> 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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