Debugging a WebRTC Data Channel Issue: How Tailscale VPN and WebRTC Interacted on iPad
A developer debugged why a web application loaded on Mac, Linux, and phone but hung on iPad, despite identical network conditions and browser engines. The root cause involved two separate bugs: a hardcoded constant in the webrtc-rs library and a design decision in Tailscale's VPN that reduced packet size, causing WebKit's data channel reassembly to fail. The incident illustrates how subtle interactions between networking layers and browser implementations can create difficult-to-diagnose issues.
A developer encountered a heisenbug where their p2claw application loaded successfully on Mac, Linux, and phone but displayed a blank page on iPad, despite all devices using the same WiFi, browser engine (WebKit), and network. Through systematic instrumentation and logging, they discovered that the iPad's outbound WebRTC data channel buffer was filling up with unconfirmed data—specifically, a 7,874-byte message body that never reached the browser. Initial suspects included message size limits, WiFi instability, and WebKit bugs, but all were ruled out. The breakthrough came when they realized the iPad had Tailscale VPN enabled, which wraps traffic in an additional layer, reducing available packet size. This caused large responses to be fragmented into smaller pieces, and WebKit's userspace data channel implementation failed to properly reassemble them, creating a deadlock. A workaround was deployed the same day, though understanding the full interaction took two additional weeks.
What different sources said
- Hacker NewsCenter
The iPad was on Tailscale: a WebRTC debugging story
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