[pfSense Support] Strange TCP connection behavior 2.0 RC2 (+3)

[andreas at naund.org (Andreas Ott)]

Hi,

On Tue, Jun 28, 2011 at 10:52:55AM +0200, Eugen Leitl wrote:
> ----- Forwarded message from William Salt <williamejsalt at googlemail.com> -----
> From: William Salt <williamejsalt at googlemail.com>
> Date: Tue, 28 Jun 2011 08:03:25 +0100
> To: support at pfsense.com
> Subject: [pfSense Support] Strange TCP connection behavior 2.0 RC2 (+3)
> Reply-To: support at pfsense.com

> Each TCP connection starts very slowly, and will max out at around 190mbps,
> taking nearly 2 minutes to climb to this speed before *plateauing*.
> 
> We have to initiate many (5+) connections to saturate the link with tcp
> connections with iperf.
> ----- End forwarded message -----

You pretty much solved your own puzzle right there: the throughput on a
single TCP connection will max out at the value determined by the bandwidth 
delay product (excluding other strange conditions, such as deep buffers).

Here is a calculator online:
http://www.switch.ch/network/tools/tcp_throughput/

-andreas
[who has to explain this about once a week to customers who think
that they bought a GigE connection but then can't "ftp" a file from
coast to coast at 1Gbps throughput. Use multiple TCP streams!]