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

----- On Apr 23, 2020, at 8:06 AM, Nick Zurku <nzurku at teraswitch.com> wrote: 

> Weâ??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 theyâ??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. Traces seem reasonable and currently weâ??ve
> influenced the path onto GTT both ways. If we prepend and reroute on our side,
> the same exact issue with happen on another transit provider.

Have you tried running a test to see if there may be ECMP issues? I wrote a rudimentary script once, [ https://pastebin.com/TTWEj12T | https://pastebin.com/TTWEj12T ] , that might help here. This script is written to detect packet loss on multiple ECMP paths, but you might be able to modify it for througput. 

The rationale behind my thinking is that if you have certain ECMP links that are oversubscribed, the TCP sessions following that path will stay "low" bandwidth. Sessions what win the ECMP lottery and pass through a non-congested ECMP path may show better performance. 


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