1. Introduction

   This document defines how to encapsulate an MPLS label stack and its
   payload inside the L2TPv3 tunnel payload.  After defining the MPLS
   over L2TPv3 encapsulation procedure, other MPLS over IP encapsulation
   options, including IP, Generic Routing Encapsulation (GRE), and IPsec
   are discussed in context with MPLS over L2TPv3 in an Applicability
   section.  This document only describes encapsulation and does not
   concern itself with all possible MPLS-based applications that may be
   enabled over L2TPv3.

1.1. Specification of Requirements

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.  The key
   words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
   "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document
   are to be interpreted as described in [RFC2119].