such as Frame Relay and ATM, already support label swapping. For example, with ATM the VCI and/or VPI fields can be used as labels. However, some layer-2 technologies, such as Ethernet, do not provide a label, and for these types of tech- nologies MPLS defines a shim label, as shown in Figure 13.5. In practice, with Ethernet becoming a more common choice for connectivity, the shim label is most widely used. The shim label is between the link layer header and the transported packet. Thus for Ethernet the shim label and IP packet all appear as one data item. Note that the shim label does not contain just the label value; it also contains three other fields: Experimental (Exp): a 3-bit field originally reserved for experimental use but now used to support IP differentiated services Stack bit: a 1-bit field that marks the last in a collection of labels termed a label stack Time-To-Live (TTL): an 8-bit field used to provide compatibility with the IP TTL field While the whole of the shim label is swapped at each LSR, the label field in the shim label is the value used to identify a packet as belonging to a unique Label-Switched Path (LSP). ATM provides two levels of label (VPI and VCI) to create the notion of traffic trunks that can be switched as a single entity, as shown in Figure 13.1. MPLS can achieve the same Shim Label Label Stack Link Layer Header (20 bits) (3 bits) (1 bit) (8 bits) IP Datagram Fig. 13.5 MPLS shim label. The MPLS Architecture Link Layer Header Label S = 0 Label S = 0 Label S = 0 Label S = 1 Fig. 13.6 Label stack, allowing Higher-Layer Protocol Data Unit multiple labels to be used. Packet Flow 1 41/72 26/35/43 Packet Flow 2 Fig. 13.7 Aggregation of two flows into nested LSP tunnels. Note that the label stack grows by 1 each time the flow enters a new LSP tunnel, and that only the innermost (lowest-level) label needs to be unique. The label values are arbitrary except that labels with the same value intentionally show how the higher-level labels can be the same to create aggregated tunnels. (and m