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Asymmetric Routing Around the Firewall

This is a follow-up to my previous post about some mysterious TCP connection timeouts in the UC Berkeley wired network. I received many thoughtful emails in response to that post, and there was an excellent discussion on Hacker News. I’m very grateful to everyone who spent time helping with my ridiculous home networking issues. Since then, I’ve learned some new information that (mostly) solves the mystery. In this post, I’ll first summarize the main theories people suggested, then tell the story of how a Bunny CDN engineer came to the rescue, and finally describe the root cause.

In this post, I’ll first summarize the main theories people suggested, then tell the story of how a Bunny CDN engineer came to the rescue, and finally describe the root cause. Unfortunately, I didn’t find any evidence in the IP TTL field or traceroute output to prove that the timeouts were caused by asymmetric routing. Update (2024-04-12): Today, Berkeley IT told me, “It appears that the Xfinity link present only on the Reshall networks has introduced a condition that has contributed to the asymmetrical routing situation,” and they’re working on fixing it.

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