[happyopossum]

> I don't understand how a product as popular as Tailscale can get this far while dropping certain ordinary types of packets.

I’d venture to guess based on this outcome that fragmented UDP over IPv6 isn’t really an ordinary occurrence. Given the preponderance of HTTPS traffic, the aversion to fragmentation in IPv6, and the weird corner case of there being a hardcoded packet size in webrtc, it’s reasonable to assume that this is a corner case.

A good one to be aware of, but not common.

[syllogistic]

Would agree it's uncommon in general traffic. Rare conditions [webrtc-rs, 1280 class tunnel / tailscale, and ipv6 pair] but deadly when they are met since every connection silently fails. That's what made it worth chasing down for 2 weeks [and good for sleuthing :)].

[inigyou]

It's a corner case of ordinary traffic, since all TCP apps and most UDP apps adapt to PMTU, but fragmentation is there for those that don't. It's not like something you can only get by generating malicious traffic intentionally.

Welcome to networking mistakes, I guess. I can't remember the specifics but I once encountered a router that would drop traffic that looked like encapsulated TCP at a certain offset, or something like that. They couldn't fix it because the behavior was hardwired. I knew of it because I worked with the firmware team.

Factorio discovered that UDP packets with a checksum of 0x0000 get dropped by some devices.