dispatched downstream following the path of a Path message, while a PathErr message is sent upstream following the path of a Resv message. As these messages are processed by GLSRs they immediately clear, or partially clear, the GLSP state. This enhancement is specific to GMPLS and enables the GLSP egress and intermediate GLSRs to initiate GLSP state removal. Using the PathErr message for clear- ing state, a flag introduced by GMPLS is set to indicate that path state is no longer valid. This means that GMPLS can tear down GLSP state in both directions (both upstream Convergent Network Management and Control Plane Path tear Path tear Path tear Remove down time state Path tear Remove state Remove state FIGURE 8.18 GLSP Tear down procedure with RSVP-TE. and downstream). Additionally, GMPLS provides rapid error notification via the Notify message (Figure 8.18). The Notify message can be used to inform an GLSP ingress or egress of errors, enabling them to initiate state removal in the place of an intermediate GLSR. Although the PathErr message is, strictly speaking, not needed, it can increase signaling efficiency by eliminat- ing the need for notification. 8.3.2.5 Constrained Routing Label Distribution Protocol In the GMPLS network, GLSR must agree on the meaning of the labels used to for- ward traffic between and through them. This signaling operation can be performed by RSVP-TE, but a group of equipment vendors has pushed another protocol. The CR-LDP is an extension of the protocol used for the same scope in MPLS: the Label Distribution Protocol (LDP). CR-LDP is a new protocol that defines a set of procedures and messages by which one GLSR informs another of the label bindings it has made. This implicitly advertises the GLSP setup since it is performed by assigning label bindings across GLSRs. The idea below the introduction in the GMPLS protocol suite of this extended protocol is to incorporate traffic engineering capabilities into a well-known protocol widely used in MPLS networks. The traffic engineering requirem