Hi there, i'm struck in some brainstorming i'm doing.

I explain the scenario.

A is communicating via http to B with some GET request.
After some time
A request a specific web page
[Mark]
B answer with the page

My need is that at [Mark] i inject a packet, sending to machine A the page that I want.
Of course B will send the packet as well, but latency is on my side, cause i'm on the same machine (or lan) of A,
so the B real packet will be rejected.

Ok, hints now ?
How could perform such an attack?