OLPC "iperf hang" notes

[Javier Cardona]

Hi Mitch,

I took a over-the-air capture of the transmitted frames and repeated
your analysis.  This is the sequence that I got:

2 4 1 3 5 7 9 6 10 8 11 12 14 16 13 17 15 18 19 20 21 * 23 24 25 26 27
(I uploaded the capture here http://dev.laptop.org/ticket/915, "*" is
for packet 23)

The TCP trace acknowledgments up to packet 20.  This last
acknowledgement is repeated 6 times and after that the connection is
frozen.

Javier



> I decoded the packets in the USB trace.  There is a lot of packet
> reordering going on - the sequence numbers don't increase monotonically.
>
> Subtracting out the first sequence number and dividing by the constant
> fixed length of the outgoing packets, the sequence is:
>
> 0 1  4  6  2  3  7  5  8  9  11  13  10 14 12  15  16  18  20  17  21
> 19  22  23  *  25 *  27  28  29  30
>
> The ACKs work as expected, i.e. when the sequence fills in, the ACKs
> catch up.
>
> "*" shows a sequence number that never showed up - packets "24" and "26"
> were not transmitted.  The ACK sequence stalled after "23", reflecting
> the fact that 24 never arrived.
>
> I killed the process 12 seconds after progress stalled (i.e. after point
> "30').  There were no retransmissions during that time.
>
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>


-- 
Javier Cardona
cozybit Inc.
p 415 974 6770
f 415 974 6771
c 415 630 0627
e javier at cozybit.com