Originally designed to improve the efficiency of packets forwarding, MPLS provides support for Traffic Engineering and network resilience. Constrained-based path computation is a key building block for Traffic Engineering in MPLS networks, since it allows to set-up LSPs along paths that satisfy QoS constraints. This paper deals with two distinct categories of path computation algorithms: on-line path computation algorithms adn on-line multi path algorithms. All these algorithms have been implemented in NS2 as an extension of OSPF-TE\ns and integrated with RSVP-TE\ns.