“Multi-Protocol Label Switching (MPLS)” [1] requires a set of
procedures for augmenting network layer packets with “label stacks”,
thereby turning them into “labeled packets”. Routers which support
MPLS are known as “Label Switching Routers”, or “LSRs”. In order to
transmit a labeled packet on a particular data link, an LSR must
support an encoding technique which, given a label stack and a
network layer packet, produces a labeled packet. This document
specifies the encoding to be used by an LSR in order to transmit
labeled packets on Point-to-Point Protocol (PPP) data links, on LAN
data links, and possibly on other data links as well. On some data
links, the label at the top of the stack may be encoded in a
different manner, but the techniques described here MUST be used to
encode the remainder of the label stack. This document also
specifies rules and procedures for processing the various fields of
the label stack encoding.