当ldp router之间的连接down的时候，mpls ldp session protection用来维持LDP绑定信息。因为LSR之间的信息是通过hello messages交互来得到的，当连接down了的时候，LSR之间不能在交互hell messages，所以LSR之间的信息在hold time到期的时候，LSR将自动清除关于neighbor的信息。MPLS LDP Session Protection uses LDP Targeted Hellos to protect LDP sessions，When MPLS LDP Session Protection is enabled, an LDP Targeted Hello Adjacency is also established for the LDP session. If the link between the two routers fails, the LDP Link Adjacency also fails. However, if the LDP peer is still reachable through IP, the LDP session stays up, because the LDP Targeted Hello Adjacency still exists between the routers. When the directly connected link recovers, the session does not need to be reestablished, and LDP bindings for prefixes do not need to be relearned.