Can you elaborate a little?
I mean any ISP can check if a packet leaving their network is actually a network they have under their control, routing doesn't have anything to do with that?
I mean any ISP can check if a packet leaving their network is actually a network they have under their control, routing doesn't have anything to do with that?
It doesn't work in the "Internet core". If you're Verizon and you got a packet from Comcast that says it's originally from Cogent, how do you validate that? This router that happens to be checking the packet prefers to send packets to Cogent via Sprint, but that doesn't mean Cogent also prefers to send packets to you via Sprint. Each router can have a different preference, too. (Example scenario only)
I think many providers now also limit UDP which might become an issue when http/3 gets more adopted.
(Tier 2 providers are those with lots of interconnections, but not the whole world, just regionally. Tier 3 providers are those who just buy wholesale internet service from another provider and don't have many if any other interconnections.)
No sane provider restricts UDP. You might be thinking of one of the more obscure protocols like SCTP.