I left in 2019, but when I was there, WhatsApp used port 5222, but the client would try port 443 if port 5222 didn't work. After it had tried those enough, it would try on port 80 with HTTP wrappers.
Really, the right model for a public service is what AOL did for AIM. Listen on all the ports. Clients should try on the 'proper' port, then 443, then 80, then random permutation. Skip certain ports because nobody likes it if you probe on smtp, smb, or chargen (etc)
Yeah. I'd be surprised if Apple didn't use similar logic. XMPP can do multiple ports (a deployment can specify the recommended order in DNS). I've not heard of an all-ports deployment, but it does sound like an interesting experiment :)
Apple has a bit of an easier time, because they can (and do) lean on cellular carriers that restrict access to their push servers. And they have 17.0.0.0/8
But they say port 5223 and 2197 with fallback to 443 [1]. Google says 5228-5230 and 443, with a couple outliers [2].
I left in 2019, but when I was there, WhatsApp used port 5222, but the client would try port 443 if port 5222 didn't work. After it had tried those enough, it would try on port 80 with HTTP wrappers.
Really, the right model for a public service is what AOL did for AIM. Listen on all the ports. Clients should try on the 'proper' port, then 443, then 80, then random permutation. Skip certain ports because nobody likes it if you probe on smtp, smb, or chargen (etc)