Use of a single data structure for label forwarding and imposition

A label-switching router including:

• A) reception circuitry that receives incoming communications packets that comprise:
• i) network-layer datagrams including respective network-layer destination addresses;
• and
• ii) outer headers including link-layer headers, the outer header of each of at least some of the incoming packets additionally including at least one shim-header stack entry;
• B) a forwarding information base containing a plurality of forwarding entries, each of which is associated with a respective destination and specifies for the respective destination a link-layer header and a stack-entry sequence of zero or more shim-header stack entries containing respective labels, the stack-entry sequences associated with some destinations being empty, the stack-entry sequences associated with other destinations consisting of at least one stack entry;
• C) an output-packet assembler that constructs an output packet for each of a forwardable plurality of incoming packets, each of at least some of whose outer headers include at least one shim-header stack entry, the output-packet assembler constructing each such output packet by fetching a link-layer header and stack-entry sequence associated by the forwarding information base with the incoming packet's destination independently of the number of shim-header stack entries in the incoming packet's outer header and replacing therewith the part of the incoming packet's outer header down through its top shim-header stack entry;
• and
• D) output circuitry for transmitting the outgoing packets thus constructed.