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

Nick Zurku nzurku at
Thu Apr 23 15:06:36 UTC 2020

Hello all,

I would appreciate if someone from Comcast could contact me about this.

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. Traces seem reasonable
and currently we’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.

This issue does not affect v6 and that is full speed on every attempt. This
may be regionalized to the Comcast Pittsburgh market.

This is most widely affecting our linux mirror repository server:
Our colocation customers who are hosting VPN systems are also noticing
bottlenecks have started recently for their Comcast employees.

Nick Zurku
Systems Engineer
TeraSwitch, Inc.
nzurku at
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