Ok so apparently you can specify a source IP when doing a SYN request or something. Besides just asking why in the world that would even be allowed, I will move past that because I want to understand how these things work, like SYN floods.
When a packet of any kind enters a WAN, the service provider knows where the IP came from when it hits the edge router/gateway. I would think. I mean without needing to read packet info. They've got the MAC of the modem or know the interface of the line etc.
So it hits their gateway, and somehow SYNs with fake IPs are allowed to continue their journey?
Why? Of all things that are possible but cause issues, this seems most needless to me. Is there no encapsulation that happens along the way to record the actual origin IP to the target address?
If not, why not? Is there some logistical issue that makes this unfeasible?