'Multi-Protocol Label Switching (MPLS)' [1,2,9] requires a set of procedures for augmenting network layer packets with 'label stacks' (sometimes called 'tag 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 PPP data links and on LAN data links. This document also specifies rules and procedures for processing the various fields of the label stack encoding.