Re: Linux TCP's Robustness to Multipath Packet Reordering

Dominik Kaspar
2011-04-26T21:27:52Z

Hi John,

Thanks for your advice. I am very well aware that TCP is not designed
to work under such conditions. I am still surprised how well Linux TCP
handles many situations of excessive, persistent packet reordering. In
scenarios of fairly heterogeneous path characteristics, Linux TCP
aggregates multiple paths close to ideally :-)

If I'm not mistaken, cwnd moderation is a measure to prevent TCP from
sending large bursts if a single ACK covers many segments. In what way
can cwnd moderation prevent TCP from increasing its estimate of packet
reordering?

Greetings,
Dominik

On Tue, Apr 26, 2011 at 10:16 PM, John Heffner <johnwheffner@gmail.com> wrote:
> First, TCP is definitely not designed to work under such conditions.
> For example, assumptions behind RTO calculation and fast retransmit
> heuristics are violated.  However, in this particular case my first
> guess is that you are being limited by "cwnd moderation," which was
> the topic of recent discussion here.  Under persistent reordering,
> cwnd moderation can inhibit the ability of cwnd to grow.
>
> Thanks,
>  -John
>
>
> On Tue, Apr 26, 2011 at 2:00 PM, Dominik Kaspar <dokaspar.ietf@gmail.com> wrote:
>> Hi Eric,
>>
>> Here are the tcpdump files for the first TSO-disabled experiment, in a
>> full version and a short version with only the first 10000 packets:
>>
>> http://home.simula.no/~kaspar/static/mptcp-emu-wlan-hspa-01-tos0-exp1-full.pcap
>> http://home.simula.no/~kaspar/static/mptcp-emu-wlan-hspa-01-tos0-exp1-short.pcap
>>
>> By the way, the packets are sent from the server (x.x.x.189) to the
>> client interfaces (x.x.x.74) and (x.x.x.216) with the following
>> pattern (which is a non-bursty 128-bit approximation of scheduling
>> with a 600:400 ratio over primary path 0 and secondary path 1):
>>
>> 0010010100101001010010100101001010010100101001010010100101001010
>> 0101001010010100101001010010100101001010010100101001010010100101
>>
>> Greetings,
>> Dominik
>>
>> On Tue, Apr 26, 2011 at 7:10 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
>>> Le mardi 26 avril 2011 à 18:58 +0200, Dominik Kaspar a écrit :
>>>> Hi Eric,
>>>>
>>>> On Mon, Apr 25, 2011 at 5:38 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
>>>> >
>>>> > Since you have at sender a rule to spoof destination address of packets,
>>>> > you should make sure you dont send "super packets (up to 64Kbytes)",
>>>> > because it would stress the multipath more than you wanted to. This way,
>>>> > you send only normal packets (1500 MTU).
>>>> >
>>>> > ethtool -K eth0 tso off
>>>> > ethtool -K eth0 gso off
>>>> >
>>>> > I am pretty sure it should help your (atypic) workload.
>>>>
>>>> I made new experiments with the exact same multipath setup as before,
>>>> but disabled TSO and GSO on all involved Ethernet interfaces. However,
>>>> this did not seem to change much about TCP's behavior when packets are
>>>> striped over heterogeneous paths. You can see the results of four
>>>> 20-minute experiments on this plot:
>>>>
>>>> http://home.simula.no/~kaspar/static/mptcp-emu-wlan-hspa-01-tos0.png
>>>>
>>>> Cheers,
>>>> Dominik
>>>
>>> Hi Dominik
>>>
>>> Any chance to have a pcap file from sender side, of say first 10.000
>>> packets ?
>>>
>>>
>>>
>>>
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>