[R-C] Simplifying the TCP throughput algorithm

[Harald Alvestrand]

On 03/29/2012 01:15 PM, Luca De Cicco wrote:
> Dear Harald,
>
> it's not that safe modifying the PFTK formula without breaking
> tcp-friendliness (if we want to be tcp-friendly, and I assume we want
> it).  That formula mimics the long term behaviour of a TCP flow
> experiencing a specific RTT and packet loss ratio p.
> Not taking into account the RTT of the connection to estimate TCP
> throughput is quite dangerous.
My point is that taking the RTT into account gives a change of less than 
1% in the calculated value, for all the values I've tried; if you have a 
specific parameter combination in mind where the RTT makes a huge 
difference, please give the numbers you are thinking of.

Given the lack of knowledge we have of so many other factors that enter 
into the system (video rate, for instance, will jump by quite many 
percent when someone shakes the camera), an estimate with an error of 
less than 1% is a spectacularly good estimate for most purposes.

> Cheers,
> Luca
>
>
> On Thu, Mar 29, 2012 at 1:07 PM, Ali C. Begen (abegen)<abegen at cisco.com>  wrote:
>> I suppose these were random losses?
>>
>>> -----Original Message-----
>>> From: rtp-congestion-bounces at alvestrand.no [mailto:rtp-congestion-bounces at alvestrand.no] On Behalf Of Harald
>>> Alvestrand
>>> Sent: Thursday, March 29, 2012 1:04 PM
>>> To: rtp-congestion at alvestrand.no
>>> Subject: [R-C] Simplifying the TCP throughput algorithm
>>>
>>> I did some extremely rough numerical experiments with the TCP throughput
>>> algorithm.
>>>
>>> Results for TCP throughput in bits/sec at MSS=1440 bytes at various loss
>>> rates:
>>>
>>> 0.001% loss: Throughput 4.5 Mbits/sec (HD can survive)
>>> 0.01% loss: Throughput 1.4 Mbits/sec
>>> 0.1% loss: Throughput 458 Kbits/sec (VGA can survive)
>>> 1% loss: Throughput 144 Kbits/sec (good audio can survive)
>>> 10% loss: Throughput 43 Kbits/sec (crappy audio can survive)
>>>
>>> These are stable in the first 2 digits over a large range of RTT (1 ms
>>> to 100 ms).
>>>
>>> Interesting numerical result: In all cases, the second term of the TCP
>>> throughput denominator is less than 1% of the first term, so a
>>> reasonable approximation is:
>>>
>>> T = s / sqrt(p * 2/3)
>>>
>>> That's a simple formula.
>>>
>>>                    Harald
>>>
>>> _______________________________________________
>>> Rtp-congestion mailing list
>>> Rtp-congestion at alvestrand.no
>>> http://www.alvestrand.no/mailman/listinfo/rtp-congestion
>> _______________________________________________
>> Rtp-congestion mailing list
>> Rtp-congestion at alvestrand.no
>> http://www.alvestrand.no/mailman/listinfo/rtp-congestion