Buffer emptying during TCP transmission

[Vineet]

Hello all,

I have a TCP Reno flow between two nodes. The channel capacity of the bottleneck link is 4 Mbps and the roundtrip propagation delay is 48 ms. It is well known that if the buffer size is less than bandwidth-delay (BDP) product, then the channel is going to be underutilized and the buffers empties every now and then. To avoid such scenario, I set the buffer size to be 40 kB, whereas the BDP is 24 kB. In spite of this, the buffer manifests emptiness during TCP multiplicative decrease. Please find below the buffer occupancy graph. Is there anything that I have missed out analyzing?

Thanks in advance!

[Nat P]

Il giorno martedì 14 giugno 2016 15:02:07 UTC+2, Vineet ha scritto:

Hello all,

I have a TCP Reno flow between two nodes. The channel capacity of the bottleneck link is 4 Mbps and the roundtrip propagation delay is 48 ms. It is well known that if the buffer size is less than bandwidth-delay (BDP) product, then the channel is going to be underutilized and the buffers empties every now and then. To avoid such scenario, I set the buffer size to be 40 kB, whereas the BDP is 24 kB.

Hi,
the point you are raising is on buffers along the path; not the TCP buffers. TCP buffers sizing is matter of current research, there is no fixed value. But yes, at least it should be able to provide enough data for one window of BDP bytes.

BDP in your scenario is 48 kB. Please consider as 'delay' the RTT, not the propagation delay, and redo your calculations based on this.
 

In spite of this, the buffer manifests emptiness during TCP multiplicative decrease. Please find below the buffer occupancy graph. Is there anything that I have missed out analyzing?

 

Can  you also explain how you get these values?

Nat

[Vineet Gokhale]

Hi Nat,

48 ms that I mentioned is the roundtrip propagation delay and not one-way. Hence RTT = 48 + 1.24 (transmission delay) = 49.24 ms.

BDP in this case will still be 24.62 kB. And path buffer > BDP. The buffer should still never empty, right?

Thanks in advance!

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