First ISP-hosted "transparent" mirror

Jason Fesler jfesler at
Sat May 17 04:51:36 UTC 2014

TL:DR? “Thanks, Comcast!” and “Who’s Next?”

The site started out 4 years ago, at a table in Seattle,
after an IPv6 round table meeting hosted by Internet Society. John
Brzozowski and myself were each trying to come up with a way to help
end users figure out that their IPv6 internet was good or bad.
Ultimately I kept plugging away at it, as John was distracted with
some kind of broadband IPv6 rollout for his employer (Comcast). And
the site went live about a month later, with
solicitation to a few operations lists for feedback. All in all,
pretty successful.

I’ve had two concerns since deploying one, how to
scale; and two, how to ensure the user’s connectivity back to the
service is awesome (or at least, not bad). John was thinking the same
thing - worried about sending too many of his customers to my site,
and crushing it in the process. Not good for either of us.

Both of those are relatively easy to solve. Simply deploy tons of
mirrors around the world, problem solved - if you have the cash and/or
smart business plan to back it. I don’t monetize the site with
advertising; nor do I charge fees. Nor do I have a crack CFO who can
help me IPO, and make me rich in the process. I don’t really have the
time or energy to solicit for corporate handouts. As it turns out, it
appears that I’m bad when it comes to making money on this project. So
any solution has to be cheap.

Asking folks to run regional mirrors (such as “” or
“”) is great; it offers a community local resources
that are more immune to global connectivity issues. However, people
must explicitly decide to visit these mirrors; to chose the location
they want to test from. Those regional mirrors are mostly light duty
as a result. They are still invaluable - they provide the back end
that the global connectivity test uses, for any IPv6-validated
customer visiting any of the mirrors. With this global test, we
effectively crowd source getting IPv6 peering problems fixed.

John and I decided to take things a step further; something I’m happy
to see finally make it across the finish line after a fair bit of
upfront dev work.

Comcast is now running two mirrors and preparing a third - which
directly act as “”. Nothing changes for the user. John
has to worry less about transient (and transit!) connectivity back to

This is done with a poor-man’s GSLB (Global Server Load Balancer).
We’re using an in-house built DNS server that looks at the internet
routing table to see what ISP the DNS queries come from. Based on the
source BGP ASN, we can decide which ISP mirror gets the traffic. (PS:
thanks to and everyone who feeds data to it; that stuff
is great!)

In the end: we both get to worry less about Comcast traffic volume to; as well as ensure a good user experience for the
customers visiting.

What’s next? That’s where you come in :-).

If you’re ...

 * working at a large ISP
 * doing real IPv6 deployment
 * or considering using “” with customers

I’d love to help you set up a transparent mirror (acting as
“”). For you, it means controlling the user experience
using this site; as well as removing any capacity concerns. For me, it
means the same thing. Win, win. More info at

If you want to help, or have questions, don’t hesitate to ask.


(link for sharing, if you're inclined:

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