orwarding along normally routed paths. RFC 3037-LDP Applicability Describes the applicability of LDP by which LSRs distribute labels to support MPLS forwarding along normally routed paths. RFC 3038-VCID Notification over ATM Link for LDP Specifies the procedures for the communication of virtual channel ID (VCID) values between neighboring ATM-LSRs. RFC 3033-Th Assignment of the Information Field and Protocol Identifier in the Q.2941 Generic Identifier and Q.2957 User-to-User Signaling for the Internet Protocol Specifies the assignment of the information field and protocol identifier in the Q.2941 generic identifier and Q.2957 user-to-user signaling for the Internet protocol. RFC 3107-Carrying Label Information in BGP-4 Specifies the way in which the label-mapping information for a particular route is piggybacked in the same Border Gateway Protocol (BGP) update message that is used to distribute the route itself. When BGP is used to distribute a particular route, it can be also be used to distribute an MPLS label that is mapped to that route. 7.3.1 MPLS Elements MPLS utilizes label switching to forward data through the network. A label is a small fixed-format field that is inserted in front of each data packet upon entry into the MPLS network. At each hop across the network, the packet is routed based on the value of the incoming interface and label, and dispatched to an outgoing interface with a new label value. In effect, this is similar to virtual channel (VC) switching in ATM. The path that data fol- lows through a network is defined by the transition in label values as the label is swapped at each LSR. Because the mapping between labels is con- All-Optical Networks stant at each LSR, the complete path is determined by the initial label value. This is an LSP (see Figure 7-13). A set of packets that should be labeled with the same label value on entry to the MPLS network and that will therefore follow the same LSP is a FEC. 16 Figure 7-14 takes a snapshot view of MPLS, and Figure 7-15 depicts the operation of M