On Thu, Apr 23, 2020 at 8:06 AM Nick Zurku <nzurku@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@herrin.us https://bill.herrin.us/