[e2e] 10% packet loss stops TCP flow

Sireen Habib Malik s.malik at tuhh.de
Mon Feb 28 04:42:14 PST 2005

for w= avg_window,  is it w^2 * p =1.5?

sorry to the sorry to be trivial.


ireen Habib Malik wrote:

> Hi,
> ......sorry to be trivial but please correct me if i think the 
> expression should be (w^2)*p=2.25.
> I would have worked out the answer the same way but through the above 
> expression.
> thanks for the clarification.
> -- 
> Sireen Malik
> Christian Huitema wrote:
>>> There is another very simple way to look at it. The most
>>> "optimistic" model of TCP, that only looks at congestion
>>> avoidance (AIMD) and ignores slow start, timeouts etc., predicts
>>> the following relationship between the average window size (w)
>>> and loss probability (p):
>>>     (w^2)*p = 2
>>> Note that this is _independent_ of link bandwidth and RTT. So
>>> with a loss probability of 10%, the average window size is going
>>> to be of the order 4. If you factor in other protocol details,
>>> the number will be even smaller. With a window size of 4 or
>>> below, fast recovery will not work and any loss will lead to a
>>> timeout (for the average connection). Hence loss rates of the
>>> order 10% simply will not work, irrespective of RTT/link bandwidth.
>> The formula (w^2)*p = 2 assumes a connection in "congestion avoidance"
>> mode, which is not actually the case if the loss rate is high. If the
>> loss rate is high, the connection will be very often in "slow start"
>> mode, where the approximation is "w*p = 2". If the loss rate is still
>> higher, we move to "w=1", and the adaptation comes from increasing the
>> RTT. 
>> The classic rule of thumb is that a link exhibiting more than 1% packet
>> loss is considered broken.
>> By the way, Noel's challenge points to an interesting point. Long
>> transfers are more likely to break than short transfers. If we want to
>> actually transfer large files over poor quality networks, we cannot rely
>> on TCP alone, but we have to split them in small chunks. Much like what
>> BitTorrent does...
>> -- Christian Huitema


Sireen Malik, M.Sc.
PhD. Candidate,

Communication Networks
Hamburg University of  Technology,
FSP 4-06 (room 3008)
Denickestr. 17
21073 Hamburg, Deutschland

Tel: +49 (40) 42-878-3387
Fax: +49 (40) 42-878-2941
E-Mail: s.malik at tuhh.de

--Everything should be as simple as possible, but no simpler (Albert Einstein)

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