in order to be switched in the same way, the label is a wave- band ID and a pair of numbers (channel identifiers) that indicates the lower and upper wavelengths of the selected waveband. Time Slot Labels When the bandwidth of an optical fiber is subdivided into time slots by TDM, an optical switch may satisfy a particular data flow request by allocating one or more time slots to that flow. In general, there- fore, a TDM label value must be sufficient to specify the allocated time slot(s). The exact details of TDM label representation depends upon the TDM hierarchy in use-for example, SONET or Synchronous Digital Hier- archy (SDH). SONET/SDH Labels A SONET/SDH label is represented as a sequence of five terms, known as S, U, K, L, and M, which select branches of the SONET/SDH TDM hierarchy at increasingly fine levels of detail. Bandwidth Allocations For all the types of GMPLS labels described here, the label value directly implies the bandwidth that is available for the corresponding data flow. For example, if a label denotes a single SONET VT-6 time slot, the available bandwidth is, inextricably, the bandwidth of a VT-6 time slot. The same holds true for other TDM labels and lambda, waveband, or fiber labels. This is quite different from the case for non- generalized labels and is a fundamental reflection of the nature of optical networks. Chapter 8 8.1.3 Requesting Generalized Labels The basic MPLS protocol for agreeing a label value across a link is unchanged for optical networks: The upstream LSR sends a request to the downstream LSR (a Path message in RSVP and a Label Request message in CR-LDP). The request contains enough information about the requested bandwidth and QoS for the downstream LSR to make a sensible label choice. The downstream LSR receives the request and allocates a label value that meets the requirements specified by the request. The downstream LSR sends a response to the upstream LSR (the Resv message in RSVP and label mapping in CR-LDP) that communicates the selected label value. GMPLS gener