e can use the terms "ini- tiator" and "terminator" without ambiguity. 8.5.1 Upstream Labels An upstream label is a new object introduced to LSP Setup Requests by GMPLS. It enables an upstream LSR to signal the label that should be used GMPLS in Optical Networks by the adjacent downstream LSR to forward data in the direction from ter- minator to initiator. Figure 8-7 shows the exchange of signaling messages at an individual LSR. Again, RSVP is used as the example protocol: A Path message is received at the LSR. As described in a previous section, this message carries a suggested label, which is used to preprogram the switch for the forward data path. The Path message also carries an upstream label. If the indicated label (Ax) is not suitable for use at the LSR, it must reject the Path message (PathErr) at once. In this case, the PathErr message may include an acceptable Label Set object to tell the upstream LSR which labels would have been successful. The upstream LSR may retry the Path message using a different upstream label chosen from the acceptable label set or may fail the LSP setup by propagating the PathErr farther upstream. If the upstream label value is acceptable, the LSR selects a label (xy) to use as an upstream label on the outgoing interface and signals this in the upstream label of the Path message it sends out. If the switch is not capable of label modification, it will map the incoming upstream label value to the outgoing one (Ay=Ax). When the Path message reaches the LSP terminator and the switch there has been programmed, the reverse data path is complete and may be used immediately. The forward data path still has to be confirmed or reprogrammed as described previously, and the Resv message is used to carry the forward Figure 8-7 Bidirectional LSP Path (Suggested Label = 1a, Path (Suggested Label = 1b, setup. Upstream Label = 1x) Upstream Label 1y) Resv (Label = 1d) Resv (Label = 1c) Chapter 8 8.5 Bidirectionality Trunks through the core of an optical network often need to be bidirec- tional. 