label distribution protocol as the name implies, is used for distributing labels between routers. it have two operation, the interface mode which is used to distribute labels for the isis or ospf calculated (or static) routes. this uses hellos for discovery mechanism similar to igp's one over the links where the igp run too. this is the most common mode, and it must be run on all the links in order to every router have a label for all the loopbacks within the domain. this is a requirement that if not satisfied, blackholing may happen. the protocol itself does not check if a path is end to end mpls capable, but isis and ospf both have the configuration knobs to advertise a neighbor with maximum metric until ldp comes up with the given peer. the other mode is a targeted mode which could be used instead of intreface mode over non-broadcast links (uncommon) or to exchange layer2 vpn service information and states between pe routers. one given layer2 vpn is considered down until both ends send an identical set of parameters and an up status of the service. the operation of ldp is similar to what bgp does. after the initialization and address exchange, it floods labels for all the igp routes, later it sends out the only the changed information to the neighbor. the interface mode is later extended with point to multipoint and multipoint to multipoint capabilities under the mldp name for efficient multicast distribution. both build the tree toward a root node, and the latter builds the backwards path too for bidirectional communication. reachable paths are sent in label mapping, disappearing paths are sent in label withdrawals which are acknowledged with label release, and for label space constrained operation (atm and frame relay) there is label request. paths for which ldp exchanges labels are called forwarding equialent classes which is analogous with lsp. here are the captures for interface, atom, p2mp and mp2mp.
interestingly, ldp is the ietf successor of tag distribution protocol from the days when mpls was a proprietary solution of cisco. they seems to prefer ldp over rsvp automesh.