The iPad was on Tailscale: a WebRTC debugging story

5 hours ago

A p2claw app loaded on an iPad but showed a blank page while working on other devices, leading to a complex debugging process.
The problem was traced to WebRTC data channel communication failing, where data sent from the box agent was not being received by the iPad, causing the page to hang.
Initial suspects included the iPad, WebKit, and Tailscale, but the root cause was two combined issues: a hardcoded MTU constant in webrtc-rs and Tailscale dropping IPv6 fragments.
WeBRTC data channels guarantee in-order delivery; missing chunks block subsequent messages, which occurred due to packet loss.
Tailscale, a VPN, wraps traffic in an extra layer, reducing packet size and causing large packets to fragment. Tailscale's IPv6 parser drops IPv6 fragments by design, leading to packet loss.
Debugging revealed that IPv6 fragments were being dropped by Tailscale's ACL policy, while IPv4 fragments were handled correctly, explaining the intermittent issue.
The solution involved capping message sizes to avoid fragmentation and filing issues with webrtc-rs and Tailscale for fixes.
Key lessons: sender-side packet captures only show packets leaving, not arrival; and bugs specific to one device may involve unique network paths.
This case highlights challenges in peer-to-peer applications and the importance of considering path MTU and fragmentation in network programming.

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