Ignore that, UDP are used for the initial hello. Session Establishment After two LSRs exchange LDP discovery Hello messages, they start the process of session establishment, which proceeds in two sequential phases: Transport connection establishment Session initialization The objective of the transport connection establishment phase is to establish a reliable TCP connection between two LDP peers. If both LDP peers initiate an LDP TCP connection, it might result in two concurrent TCP connections. To avoid this situation, an LSR first determines whether it should play the active or passive role in session establishment by comparing its own transport address with the transport address it obtains through the exchange of LDP Hellos. If its address has a higher value, it assumes the active role. Otherwise, it is passive. When an LSR plays the active role, it initiates a TCP connection to the LDP peer on the well-known LDP TCP port 646.