Current high-volume, high-availability, content distribution networks use clusters of Web servers to achieve scalability and reliability. To serve a large and diverse client population, content can be replicated across servers, or partitioned with a dedicated server for particular content or clients. In such environments a front-end dispatcher (usually called a “web switch” or a “reverse proxy” based on its functional usage) directs incoming client requests to one of the server machines. The request-routing decision can be based on a number of criteria, including requested content, server load, client request, or client identity.
Dispatchers are typically required to perform several functions related to the routing decision. These include:                distribute incoming requests as to balance the load across servers;        examine client requests to determine which server is appropriate to handle the request (i.e., content-based routing);        identify the client to maintain affinity with a particular server for e-business applications or to provide service differentiation.        
Dispatchers may be broadly categorized into two types: layer-4 dispatchers which base decisions on TCP and IP headers alone, and layer-7 dispatchers which use application layer information (e.g., HTTP headers) to direct clients. The use of layer-4 or layer-7 dispatchers depends on the request-routing goal. Load-balancing across replicated content servers, for example, typically does not require knowledge of the client request or identity, and thus is well-suited to a layer-4 approach. Content-based routing requires the dispatcher to terminate the incoming connection, examine the higher-layer headers, and either create a new connection with the appropriate server (using connection splicing), or transfer the connection to the appropriate server (using connection handoff). Layer-7 dispatchers, while more sophisticated, suffer from limitations on scalability and performance since they must perform connection termination and management for a large number of clients.
High-speed switching hardware is also commonplace in core ISP networks with a growing migration to Multiprotocol Label Switching (MPLS). MPLS provides a circuit-switching service in a hop-by-hop routed network and is presently used for flexible routing, traffic engineering and Virtual Private Networks (VPNs). It achieves this by grouping related packets by assigning them a common, fixed-size label. Packets sharing a label belong to the same forwarding equivalence class (FEC) and can be routed and treated the same way in the network. Standard usage of MPLS involves establishment of arbitrary label-switched paths (LSPs) for forwarding particular classes of traffic. LSPs may also be nested by stacking MPLS labels where an outer label might be used to assign traffic to a common network-wide path, while an inner label could be used to demultiplex traffic among classes of traffic on that path. An MPLS-enabled network includes of label-switched routers(LSRs) that implement the MPLS protocols.
MPLS labels do not have built-in semantics, i.e. labels are used only to map a packet from an LSR input port to an LSR output port. It would be advantageous to make use of this flexibility by mapping application-layer information onto labels to enable high-performance Web switching, rather than using labels to express routing and forwarding policies (as is customary).