[Planetlab-users] question about packet loss

[Daekyeong Moon]

Hi Andy,

Yeah, all nodes are planetlab nodes.

Unfortunately, it does not always happen, and thus difficult to  
reproduce manually.

Then, I'm wondering what if the receiver is highly overloaded. (say, w  
command show around 8.0)
Can it drop packets for a long time? (e.g., more than a second)

I did another very simple experiment.
I used 216.165.109.79 as the receiver and 216.165.109.81 as the sender.
(I guess they're on the same network.)

The load average of the receiver was around 8, and the receiver  
dropped about 700 packets out of 6000, when the sender injected a 4  
byte udp packet every ms.
Especially, it dropped consecutive packets. Here's a log snippet from  
the receiver.
The first column is the seq number and the second one is the received  
payload size.

...
58 4
59 4
770 4  (big gap occurs here!! what happened!?)
771 4
772 4
...

I wiresharked on the sender node and I could see the missing sequence  
numbers.
(For some reason, wireshark on the receiver node didn't show packets  
at all)

This is normal despite the same network?

This experiment is fortunately reproduciable at this moment.
The program I used is fairly simple udp server/client program with  
just 88 LOC.
If you need it, I can send it to you (not to this group)


Thanks,

DK


On Aug 7, 2008, at 11:46 PM, Andy Bavier wrote:

> Hi,
>
> In your experiment, are both the sender *and* receiver PlanetLab
> nodes?  If so, can you retry your experiment with only the sender or
> receiver being a PL node?  I would expect the packet loss to occur on
> the sending side, since PL doesn't do any limiting of receive
> bandwidth.
>
> If you can give me a way of reproducing your problem on some nodes,
> I'll look into it more closely.
>
> Andy
>
> On Fri, Aug 8, 2008 at 8:02 AM, Daekyeong Moon  
> <dkmoon at cs.berkeley.edu> wrote:
>> Hi,
>>
>> I'm seeing weird udp loss.
>> I'm replaying traffic traces from some Internet application.
>> The replayer is a very simple udp application injecting packets  
>> from a pcap
>> file according to the packet interval recorded in the file.
>>
>> Each sender generates at most 50 udp packets per second for 2  
>> minutes, and
>> each packet is 300 bytes on average.
>> So, each sender consumes about 120Kbps, which is not much.
>>
>> A receiver receives packets from 7 senders (approximately 840Kbps),  
>> and
>> reports timestamp using SO_TIMESTAMP.
>> But, I found that the receiver sometimes doesn't receive any packet  
>> for more
>> than 10 seconds.
>> (select() call with 10 second timeout also fires.)
>>
>> Since, the traces don't have such big gap and the receiver receives  
>> from the
>> 7 senders, this is quite abnormal.
>> I repeated 200 times with random planet lab nodes in the US, and I  
>> don't
>> think it's the issue of particular nodes.
>>
>> I'm just guessing it could be related to the receiver's bandwidth
>> limitation.
>> (e.g. dropping all packets until the next time window if bandwidth  
>> usage in
>> a certain time window exceeds the Shared BW limit)
>>
>> Anybody experienced the same issue?
>>
>>
>> Thanks,
>>
>> DK
>>
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