As of February 2003, the IETF MPLS working group deprecated CR-LDP and decided to focus purely on RSVP-TE.
CR-LDP contains extensions for LDP to extend its capabilities such as setup paths beyond what is available for the routing protocol.
Constraint-based routing (CR) is a mechanism used to meet traffic engineering requirements.
These requirements are met by extending LDP for support of constraint-based routed label-switched paths (CR-LSPs).
Other uses for CR-LSPs include MPLS-based virtual private networks.