Comcast - Significant v4 vs v6 throughput differences, almost stateful.

Thu Apr 23 16:14:31 UTC 2020

On Thu, Apr 23, 2020 at 8:06 AM Nick Zurku <nzurku at teraswitch.com> wrote:
> We’re having serious throughput issues with our AS20326 pushing packets to Comcast over v4. Our transfers are either the full line-speed of the Comcast customer modem, or they’re seemingly capped at 200-300KB/s. This behavior appears to be almost stateful, as if the speed is decided when the connection starts. As long as it starts fast it will remain fast for the length of the transfer and slow if it starts slow.

Hi Nick,

That's actually kinda normal for TCP behavior. The two most dominating
factors in TCP throughput are the round-trip time (RTT) and how large
the congestion window has grown prior to the first lost packet. Other
factors (including later mild packet loss) tend to move the needle so
slowly you might not notice it moving at all.

One of the interesting patterns with TCP is that the sender tends to
shove out all the packets it can in the first few percent of the RTT
and then sits idle. When the bandwidths are relatively fast, the
receiver receives and acks them all in a short time window as well. As
a result you get these high-bandwidth spurts where packet loss due to
full buffers is likely even though for most of the RTT there are no
packets being transmitted at all. It can take several minutes for
packets to spread out within the RTT, and by then the congestion
window (hence throughput) is firmly established.

Regards,
Bill Herrin

-- 
William Herrin
bill at herrin.us
https://bill.herrin.us/