Final answer:
LDP sessions are formed using TCP, which ensures reliable communication and synchronization of label information in MPLS networks.
Step-by-step explanation:
The transport layer protocol used to form LDP (Label Distribution Protocol) sessions is TCP (Transmission Control Protocol). LDP is a protocol used in MPLS (Multiprotocol Label Switching) networks for distributing labels that determine how packets should be forwarded through the network. LDP establishes sessions between neighboring routers to exchange label information and uses TCP as its underlying transport mechanism to ensure reliable communication. The choice of TCP provides error checking, guaranteed delivery of LDP messages, and controls the flow of LDP traffic, which are all crucial for the synchronization of label information and the stability of the MPLS network.