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Being WebSocket-based only has the unfortunate consequence of not working well on cellular networks, because some like Verizon will often blackhole WS connections.

This means that if you are building a real time app that has any chance to be used on mobile, you'll always need a fallback.

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While it MAY be true that WS is blocked by some mobile carriers (though I haven't seen it myself .. at least in Germany), providers will have more problems blocking WSS since it runs over TLS. Since WSS only starts after a TLS connection is established, identifying the traffic as WebSocket requires intercepting TLS using MITM proxy techniques. This in turn requires the ability to inject ad-hoc "fake" certificates into browsers .. which is possible if there is an appropriate CA cert for the proxy installed in the browser. This can and is be done within controlled envionments like corporate networks where you can force user browsers to incorporate such CA certs. I never have seen it on public networks. It would be a major security and privacy issue. My provider reading my online banking traffic? Yes, please! I sue them to hell and make some bucks;)

Ah, interesting. But what if the app communicates via https, how could verizon detect any inner protocol?

Or does it happen only when using the ISP as http proxy?

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