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BT Infinity and AWS CloudFront IPv6 MTU

Late last year I converted this blog from Octopress to Hugo and migrated it from a Linode VM to AWS S3, in an effort to spend less of my free time maintaining dependencies and virtual machines. It also reduced the runtime costs from $5/mo to $0.2/mo.

I wanted to keep IPv6 support and enable TLS, so I put it behind AWS CloudFront. When testing the new site from my home connection, which has native IPv6, I noticed that initial page loads were extremely slow. I debugged it with a combination of:

These showed that three seconds were spent on the TLS handshake. Since MTU related problems seem to have become “my thing” I had a hunch about where this was going.

Thankfully a James Dobson has written a really good blog post describing an identical problem with another ISP. In summary: PPPoE requires an extra 8 bytes, IPv6 doesn’t allow fragmentation, and PMTUD is nearly always broken.

Being a good netizen I wanted to make this better..

AWS CloudFront

This wasn’t CloudFront’s fault but as a popular CDN provider they could do a better job of dealing with broken ISPs, because there will always be broken ISPs.

I’m fortunate to have access to an AWS Premium Support account through my job and I realised that the same problem affected a number of sites on our project. I raised a feature request for CloudFront to clamp the MSS in their SYN-ACK to 1420 bytes.

They took my request seriously and the change was deployed in December, which fixed my immediate problem and possibly many other affected people.

BT Infinity

The root of the problem was with my BT broadband connection. Initially I tried contacting customer support to report the problem, but had real trouble getting first line to triage or escalate it.

I eventually took to Twitter in frustration. A kind person referred my tweet to an amazingly helpful engineer at BT who arranged a call to debug the problem. It turns out that my router was a new variant (Smart Hub 6B) that didn’t have the workarounds from previous models applied to it.

They contacted me again in January to confirm that a new firmware has been rolled out which uses IPv6 Router Advertisement options to reduce the MTU to 1492.


It’s worth trying to make the Internet a better place no matter how futile it may seem at times. It feels good that a couple of relatively small changes will have had a positive impact on many people, even if they don’t realise it.

I originally joined Twitter because it allowed me to talk to clever people in good places and that still stands true today. There are lots of helpful people out there, you just need to find them.