Perhaps there's hope for me yet.
I'm about halfway through my Introduction to Computer Networking class, and although I'm still a newb dunce, I'm at least learning enough so that when I read a story like this, most of the article actually makes sense!
Going back to validate, things started to make sense. I also noticed a little hint in tcpdump verbose mode (tcp cksum bad) which was missed before. A Montreal machine receiving this packet discarded it at the kernel level after realizing it’s corrupt, never passing it to the userland ssh daemon. London then re-transmitted it, going through the same corruption, getting the same silent treatment. From ssh and sshd’s perspective, the connection was at a stalemate.
In some ways, what's been best about the networking course is that I'm starting to understand what an incredible achievement it is that the Internet works at all, not to mention how incredibly well it actually works.
With luck, given the current syllabus, by Thanksgiving I will have learned enough to understand this story.
the internet is a collection of networks called autonomous systems. Autonomous systems communicate with each other through the Border Gateway Protocol, or BGP, which is a system for exchanging information about routes from one location in the network to another. If you want to access Google, your ISP needs to have a route from your computer to Google’s servers.
Patience grasshopper. Hold the course, watch the videos, it will slowly continue to make sense.