AA’s quota will be measured by the sum of for all downstream IP packets regardless of what they are for or what type they are, and all upstream is free on fixed lines. (So using the terms download and upload doesn’t help you here, but it’s a fair question.) For 3G/4G both downstream and upstream is charged so I believe.
So when you are doing a download, if using eg TCP then the upstream TCP ACKs will be free. If you are doing a TCP upload then the downstream ACKs will be chargeable (as there is a small amount of downstream traffic in the reverse direction when doing a TCP upload).
What’s the ratio of forward traffic (that with payload) byte count to reverse ACK byte count in TCP ? Let’s assume hypothetically that a payload IP packet is 1500 bytes including IP headers. A IPv4 TCP ACK is what, 40 bytes ? Or an IPv6 TCP ACK is 60 bytes? Sanity-check please. And an ACK is sent for the nth payload tx, where n= what? =2.