There would be so much to ask to properly answer, and so many possible implmentations, I'll just make some asumptions and propose one solution you could modify later.
You could use TCP flags and conntrack, the connection machine in the linux kernel, to find which connections are B to A tcp ones.
Then, you could mark them in the connection machine with
This way, and with
-j CONNMARK --restore-mark target, when a packet in this connection is going in the A to B direction, it will be marked 100.
TC side, I'm using htb as a classful QDISC. HTB class will drop packets if there is too much traffic.
fq_codel is used as a queue. fq_codel does organize queues in each class.
The root htb qdisc send packet which are not marked in class 1:1000.
Marked packets will go in 1:100.
tc filter is used to recognize marked 100 packets and send them into class 1:100.
tc class is used to set the limit bandwidth.
The following code is assuming your max bandwidth is 1Mb/s, your network B interface is eth0, your A network is eth1, and your layer to is ethernet.
# Spread connmark to mark
iptables -t mangle -A POSTROUTING -o eth0 -j CONNMARK --restore-mark
# New TCP connections from B to A : connmark 100
iptables -t mangle -A POSTROUTING -o eth1 -p tcp --syn -m state --state NEW -j CONNMARK --set-mark 100
# Interface : eth0, qdisc : htb, default leaf : 1000
tc qdisc add dev eth0 root handle 1:0 htb default 1000
# Interface eth0, parent branch node : 1:0, branch id : 1:10
tc class add dev eth0 parent 1:0 classid 1:10 htb rate 1000kbit ceil 1000kbit
# CLASS - B to A TCP branch
tc class add dev eth0 parent 1:10 classid 1:100 htb rate 700kbit ceil 1000kbit burst 5k prio 0 linklayer ethernet
# QDISC - Queue
tc qdisc add dev eth0 parent 1:100 handle 110: fq_codel
# FILTER - Filter
tc filter add dev eth0 parent 1:0 protocol ip prio 0 handle 100 fw flowid 1:100
# CLASS - Default branch
tc class add dev eth0 parent 1:10 classid 1:200 htb rate 300kbit ceil 500kbit burst 5k prio 7 linklayer ethernet
# QDISC - default branch queue
tc qdisc add dev eth0 parent 1:1000 handle 1010: fq_codel
I might not be as clear as I would want to B, ask me anything you don't understand.