LDP periodically sends hello messages (every 5 seconds).
The Label Switch Router (LSR) sends LDP link hello messages to all the routers on the subnet as User Datagram Protocol (UDP) packets with a multicast destination address of 188.8.131.52 (“all routers on a subnet”) and destination port number of 646. (TDP uses destination port 711.)
The hello message, which may optionally contain a transport address type, length, value (TLV) to instruct the peer to open the TCP session to the transport address instead of the source address found in the IP header, The LDP identifier (LDP ID) is used to uniquely identify the neighbor and the label space.
since R1 has no reachability to R2 Transport Address (184.108.40.206)
lets inspect the LDP Hello Packet again:
having now reachability to the transport-address, a neighboring LSR will respond by opening a TCP session with the same destination port number 646, and the two routers begin to establish an LDP session through unicast TCP.
if we follow the TCP Stream