 path state to enable correct signaling of returning Resv messages and saves the upstream label. The next hop then selects its own upstream label, creates state for the upstream direction, replaces the upstream label in the Path message, and passes it on downstream to the next hop. This procedure is repeated until the next hop is the receiver GLSR (GLSP egress). The GLSP has now been established in the upstream direction, but no state has been saved in the downstream direction. Consequently, the receiver GLSR selects a downstream label Telecommunication Networks Upstream set Set upstream Downstream set Set downstream FIGURE 8.14 Establishment of a bidirectional GLSPs employing RSVP-TE for signaling; full sets of Path and Reservation (Resv) messages are exchanged between two GLSRs. and returns a Resv message upstream. This Resv message mimics the Path message, but inserts a GENERALIZED_LABEL object defining the selected downstream label. If the Resv message is received successfully, the previous hop (the signaling direction has changed) then sets state for the downstream direction, replaces the downstream label with its own selected label, and passes the Resv message further upstream. This procedure is repeated until the sender GLSR successfully receives the Resv message corresponding to a dispatched Path message. Now, the request has been fully established and is ready to tunnel data in both directions. This procedure is shown in Figure 8.14 while the final aspect of the GLSP as a cascade of labels is sketched in Figure 8.15. It is possible using RSVP-TE to route a certain GLSP on a preassigner route. This is needed in order to traffic engineer the network. As a matter of fact, if a generic network is considered, the OSPF routing criterion used both by OSPF-TE and IS-IS is a suboptimum routing strategy. It is not difficult to find examples where routing a few GLSPs along a longer path allows a more efficient exploitation of the network resources. Generalized label assignments for a bidirectional GLSP In port labe