Constraint-based Routing Label Distribution Protocol (CR-LDP) is a control protocol used in some computer networks.
As of February 2003, the IETF MPLS working group deprecated CR-LDP and decided to focus purely on RSVP-TE

RSVP-TE

Resource Reservation Protocol - Traffic Engineering is an extension of the resource reservation protocol for traffic engineering. It supports the reservation of resources across an IP network. Applications running on IP end systems can use RSVP to indicate to other nodes the nature of the packet...

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It is an extension of the Label Distribution Protocol

Label Distribution Protocol

Label Distribution Protocol is a protocol in which two Label Edge Routers exchange label mapping information. The two LERs are called LDP peers and the exchange of information is bi-directional....

 (LDP), one of the protocols in the Multiprotocol Label Switching

Multiprotocol Label Switching

Multiprotocol Label Switching is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links between...

 architecture. CR-LDP contains extensions for LDP to extend its capabilities such as setup paths beyond what is available for the routing

Routing

Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...

 protocol. For instance, a Label Switched Path

Label Switched Path

In MPLS networking, a Label Switched Path is a path through an MPLS network, set up by a signaling protocol such as LDP, RSVP-TE, BGP or CR-LDP. The path is set up based on criteria in the forwarding equivalence class ....

 can be set up based on explicit route constraints, quality of service

Quality of service

The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...

 constraints, and other constraints. 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 network

Virtual private network

A virtual private network is a network that uses primarily public telecommunication infrastructure, such as the Internet, to provide remote offices or traveling users access to a central organizational network....

s.
CR-LDP is almost same as basic LDP, in packet structure, but it contains some extra TLVs which basically set up the constraint-based LSP.