This is my complicated and long question. First let's talk about the context.

Network topology:

PC A --- RT A --- RT C --- RT B --- PC B

(RT C has a WAN NIC connected to "the cloud")

With this situation :

• PC A must send a packet to PC B
• Default routes direct packets to the cloud
• We haven't access to RT C's configuration
• RT C only knows how to join network A, not network B
• RT A knows about network B
• RT B knows about network A

RT C's routing table:

Destination NIC     Gateway
0.0.0.0     WAN     Cloud
Network A   LAN A   RT A's WAN

RT A's routing table:

Destination NIC     Gateway
0.0.0.0     WAN     LAN A
Network B   WAN     LAN A

RT B's routing table:

Destination NIC     Gateway
0.0.0.0     WAN     LAN B
Network A   WAN     LAN B

I would like to permit PC A and PC B to communicate, but I don't have access to RT C. Networks B and BC are new.

Can PC A send a packet to RT B's WAN NIC (which is possible) and "ask RT B to direct the packet to PC B" ?

I believe replacing RT B with a VPN server should do the trick, but I would like to know if it is possible to make it without establishing a new connection.