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Tangential, but to 1.: Isn't RCS just built upon TCP/IP and SIP? Why can't you just implement it in user space? Or does it need to send some magic packets through the modem? It is really hard to find details on the protocol.

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Another problem is that Google's RCS implementation has a Signal-protocol prekeys-exchange server in the loop that is not accessible for the public. So even if you'd implement RCS you could not send messages to android devices as you can't fetch the prekeys through anything but the Messages app.

So sounds like Google pretty much owns the RCS protocol? I know it's meant to be an open standard, but sounds like there's no point in any third party app implementing if it can't send messages to people with Google Messages.

Yes, TCP/ip and SIP, no magic packet, but: 1. You need to do challenges with the SIM card for authentication 2. On most carriers, "ims" is on a special IPv6 data connection that can't be used by apps.

The specifications are public, but yeah it requires a lot of work to get into.

That being said, I think neither of those limitations apply to Google's "RCS" (which bypasses carriers, so they can't get their own IPv6 connection and I don't think they can use SIM challenges for auth), so I think it should be possible in a 3rd party app.

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