Google doesn't do any 'black magic' even though they do presentations and publish papers. Their edge infra is very boring, and yes, they shuffle edge a log and their edge routers like everybody's' else - off the shelf Juniper/Cisco without any tcp session preservation.
Migrating an in-use TCP session from one host to another is far from easy.
I did it for a project and the number of corner cases is insane - both in the TCP protocol (what of the window has holes in? What if the connection is half closed?), but also in the OS's handling of the TCP state and interactions with userspace (will be correctly wake up a process poll()'ing a socket if we migrate the socket after a packet is received but before the kernel wakes the poll()ING process?)
I’m not saying it’s easy, but a company like Google should have no problem implementing this. If you ever used Vmotion it does a good job of migrating a live VM to another physical host. Also enterprise firewalls have no problem moving TCP/NAT state from an active to passive firewall.
