Overview

For the past few days I noticed that many websites that were previously fast were now running super slowly if they loaded at all. I've got Gb Verizon Fios service and my speed tests kept telling me that my download was still in the high 100s mbps - so it was baffling that simple sites wouldn't load at all.

In this post I'm going to share some resources I used to identify ipv6 checksums as my root cause issue, how I resolved this issue, and why I think it's a decent long-term fix.

Context

First let's check that the problem you're facing is similar to the problem I was facing. If not, this fix probably won't work for you.

From my research the simplest way to check whether you 1) are having issues and 2) whether it's likely due to this ipv6 checksum issue is to use the Verizon Internet Speed Test.

With the ipv6 checksum issue, you should see a result where:

• Download speed looks okay
• Upload speed is either 0 or times out

Example:

What's the core problem?

From my research (references below) this symptom is most often caused by incompatibilities in a feature called the ipv6 checksum and typically reveals itself when your network upgrades from ipv4 -> ipv6.

ipv4 vs ipv6

• ipv4 is how IP addresses (kinda like phone numbers / street addresses for individual computers / networks) have worked since the beginning of the internet. The problem is that it was created at a time where we didn't think there'd be that many connected devices. Now we have so many devices that we need to have more available addresses to connect them all.
  • Available addresses: 4.29x10^9
• ipv6 is the next generation of IP addresses
  • Available addresses: 3.4x10^38

So the important thing to note is that the format of these addresses and how they work is materially different. So even if a network entity / agent works well on one, it doesn't necessarily mean it will work well on another.

It seems that Network Interface Cards (NICs) have common problems with these upgrades - some have edge cases where they perform poorly or simply don't work well with one protocol vs another.

So typically when we see this 0 mbps upload thing it's caused by a NIC incompatibility and this is often built into the hardware so there's often no upgrade path besides replacement.

References

• Differences between IPv4 and IPv6
• Solved: IPv6 Causing 0 mbps upload speed
• Little or no upload speed with Fios

How do we solve this?

Based on my research the simplest method is to turn off ipv6 checksum. While it sounds like this would be bad because it would remove a checksum security feature, in reality what this does is turn off delegation of this functionality to the NIC. In other words, this feature still runs, it just doesn't run in the hardware-optimized NIC and instead will run on your machine - typically using CPU.

To do this on Windows 10, run PowerShell as administrator and run:

Disable-NetAdapterChecksumOffload -Name "*" -TcpIPv6

Command source: Disable-NetAdapterChecksumOffload

Pros:

• Fixes the problem quickly
• Does not require replacement of your NIC

Cons:

• May cause a small performance degradation in network speed (<10%) since not running on hardware
• May cause a small increase in CPU (<1%) since we are now doing computation on machine

References:

• Does it make sense to generally "Disable TCP Checksum Offload"?

Conclusion

After running the above command, my network speed seemed to go back to normal! I was again able to upload to YouTube, search Google, make Google Slides, etc.