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

Nick Zurku nzurku at
Thu Apr 23 17:58:16 UTC 2020

We started getting the wave of complaints over the last two weeks or so.
Perhaps up to a month ago was when the initial few issues that were
reported but were chalked up to being “an issues out on the internet.”

Did your issues in CT start on a certain date?

Nick Zurku
Systems Engineer
TeraSwitch, Inc.
Office: 412-945-7048
nzurku at

On April 23, 2020 at 11:24:59 AM, Dovid Bender (dovid at wrote:

We have customers in CT with the same issues. When did this start?

On Thu, Apr 23, 2020 at 11:07 AM Nick Zurku <nzurku at> wrote:

> 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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