NAT impact on peer to peer (p2p) networks

Question

Suppose a peer A discovers through query that peer B has a file it wants to download. Also, suppose that A and B are both behind a NAT. Is there a way to allow A to establish a TCP connection with B without application-specific NAT configuration? Discuss why

Answer 1

Lets use A for Arnold and B for Bernard 

It is not possible to devise such a technique. In order to establish a direct TCP connection between Arnold and Bernard, either Arnold or Bob must initiate a connection to the other. But the NATs covering Arnold and Bob drop SYN packets arriving from the WAN side. Thus neither Arnold nor Bob can initiate a TCP connection to the other if they are both behind NATs