ECN

Thu Nov 14 13:45:00 UTC 2019

Owen DeLong <owen at delong.com> writes:

> Like it or not (and I really don’t), the majority of modern CDNs are
> using TCP over Anycast.
>
> It’s ugly and it’s prone to problems like this. It’s nice to see a
> customer with know-how actually publicizing and digging into the
> problem.

Thanks. I do plan to write this whole story up as a blog post, BTW.
Apart from just being a nice "battle story" I also think it's important
to get more visibility into these kinds of issues. I've mostly been
interested in issues related to ECN in general, but its interaction with
anycast is certainly... interesting :)

> Until now, I believe an unknown number of customers have been
> suffering in silence or relegated to the ISPs “We can’t reproduce you
> problem” bin without resolution.
>
> I’ve had lots of discussions on the subject and the usual end result
> is “It’s too hard to measure or quantify and there’s no visible
> contingent of impacted users”.
>
> Now we at least have one visible impacted user.

As I said, happy to be an exponent if it can help others resolve these
kinds of problems.

Incidentally, in case you're not aware, there are currently two
competing schemes being discussed at the IETF to re-purpose the ECT(1)
code point in the IP header. One proposal[0] is to use it as an
additional high-fidelity congestion indicator, while the other[1] is to
use it as an identifier for a new type of traffic that should get
special treatment (which almost, but not quite, amounts to priority
queueing). So if either proposal gains traction, expect more ECN-marked
traffic coming to a network near you in the maybe-not-so-distant future;
with all the interesting issues that can bring with it.

If someone feels like introducing some operational considerations into
the IETF discussions, I do believe both drafts will be discussed at the
tsvwg working group meetings at the Singapore IETF next week.

-Toke

[0] https://datatracker.ietf.org/doc/draft-morton-tsvwg-sce/
[1] https://datatracker.ietf.org/doc/draft-ietf-tsvwg-ecn-l4s-id/