The proposed technique assumes that the client has somehow learned the current external (globally routable) IP address of the server's NAT.
The key idea for enabling the server to learn the client's
IP address is for the server to periodically send a message to
a fixed, known IP address. The simplest approach uses ICMP
ECHO REQUEST messages to an unallocated IP address, such
as 1.2.3.4. Since 1.2.3.4 is not allocated, the ICMP REQUEST
will not be routed by routers without a default route;
ICMP DESTINATION UNREACHABLE messages that may
be created by those routers can just be ignored by the server.
As a result of the messages sent to 1.2.3.4, the NAT
will enable routing of replies in response to this request.
The connecting client will then fake such a reply. Specifically,
the client will transmit an ICMP message indicating
TTL_EXPIRED. Such a message could legitimately
be transmitted by any Internet router and the sender address
would not be expected to match the server's target IP.
The server listens for (fake) ICMP replies and upon receipt
initiates a connection to the sender IP specified in the ICMP reply.