ng in MPLS), G might choose the solid wave- length (blue). But the solid wavelength (blue) is already in use between D and E SO it cannot be used. If the choice is given to LSR C, a similar prob- lem may occur. There is therefore a need to allow all OXCs along the path to constrain and/or influence the choice of labels to ensure that appropriate labels are chosen. 8.2.1 Label Set GMPLS introduces the concept of a label set. An upstream LSR includes a label set on its signaling request to restrict the downstream LSR's choice of label for the link between them. The downstream LSR must select a label from within the label set or else it must fail the LSP setup. This is useful in the optical domain when, for example, An LSR is incapable of converting between wavelengths, as shown in the previous example. An LSR can only generate and receive a subset of the wavelengths that can be switched by neighboring LSRs. It is desirable for an LSR to limit the amount of wavelength conversion that it has to perform in order to reduce the distortion on the optical signals. The label set is constructed by including and/or excluding an arbitrary number of label lists and/or ranges. If no labels are explicitly included, the GMPLS in Optical Networks set consists of all of the valid labels not explicitly excluded. If no label set is present, the downstream LSR is not constrained in its choice of label. As the label set is propagated on the Path message, each LSR may gener- ate a new outgoing label set based on its hardware capabilities and possibly on the incoming label set. For example, an LSR that is incapable of wave- length conversion generates an outgoing label set by forwarding the incom- ing label set minus any labels that it cannot generate or receive. In contrast, an LSR that is capable of converting all wavelengths in an incoming label set may choose to remove the label set when sending the signaling request down- stream to indicate that it will accept any label for use on its downstream link. Consider the example in Figure 