ork layer protocol considered is IP. However, it is possible to encapsulate many higher-layer protocols. There are also MPLS implementations that allow lower-layer protocols to be encapsulated over MPLS, and this is often termed layer-2 encapsulation, pseudowire, or "Any Transport over MPLS" (AToM); see De Ghein (2006) Chapter 13 Multiprotocol Label Switching for more information on the latter. Those familiar with the OSI layered network models might notice that this description does not fit the standard model because MPLS often sits in a nonexistent OSI layer between layers 2 and 3. By claiming that MPLS supports higher network layers over many link layers, some say that it is a layer-"2.5" protocol. However, it might be better to think of MPLS as breaking the OSI model rather than trying to make it fit. The Principle of Label Switching MPLS switches data using label swapping. This is not a new idea; Asynchronous Transfer Mode (ATM), as shown in Figure 13.1, is a technology that uses label swapping and that preceded MPLS. The basic principle in label swapping is that data is carried in packets with a header that is changed at each switch. The header contains a single identifier (the label) that is only relevant to each pair of neighboring switches. The label tells the switch how to forward the packet data and how to treat the packet in terms of Quality of Service (QoS). Note that this is quite different from IP, which uses an identifier (IP address) that has global significance and is not changed at each "switch" (IP router). While IP can support QoS, it should be noted that IP QoS mechanisms have had mixed reactions from operators and have a number of difficulties. In the case of ATM, the "packet" is a fixed-length 53-byte cell with a 5-byte header. The header contains two fields: the Virtual Path Identifier (VPI) and the Virtual Circuit Identifier (VCI). In Figure 13.1 it can be seen that the VPI field is swapped at each switch but that the VCI field remains the same throughout the circuit. This is a feature o