[cesarb]

> most routers can and do read info in tcp/udp headers.

Do most routers really do that, or just the ones which are also trying to act as a firewall?

[wmf]

For example, IP routers often peek at UDP/TCP port numbers to calculate ECMP flow hashing. This is technically naughty but it's read-only and it's only an optimization that isn't required for correct forwarding.

[Hikikomori]

Yes. I doubt you can find one that is not capable.