MPLS Upstream Label Assignment and Context-Specific Label Space
RFC 5331

Network Working Group                                        R. Aggarwal
Request for Comments: 5331                              Juniper Networks
Category: Standards Track                                     Y. Rekhter
                                                        Juniper Networks
                                                                E. Rosen
                                                     Cisco Systems, Inc.
                                                             August 2008

    MPLS Upstream Label Assignment and Context-Specific Label Space

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   RFC 3031 limits the MPLS architecture to downstream-assigned MPLS
   labels.  This document introduces the notion of upstream-assigned
   MPLS labels.  It describes the procedures for upstream MPLS label
   assignment and introduces the concept of a "Context-Specific Label
   Space".

Table of Contents

   1. Introduction ....................................................2
   2. Specification of Requirements ...................................2
   3. Context-Specific Label Space ....................................2
   4. Upstream Label Assignment .......................................3
      4.1. Upstream-Assigned and Downstream-Assigned Labels ...........4
   5. Assigning Upstream-Assigned Labels ..............................5
   6. Distributing Upstream-Assigned Labels ...........................5
   7. Upstream Neighbor Label Space ...................................6
   8. Context Label on LANs ...........................................9
   9. Usage of Upstream-Assigned Labels ..............................10
   10. Security Considerations .......................................10
   11. Acknowledgements ..............................................11
   12. References ....................................................11
      12.1. Normative References .....................................11
      12.2. Informative References ...................................11

Aggarwal, et al.            Standards Track                     [Page 1]
RFC 5331                                                     August 2008

1.  Introduction

   RFC 3031 [RFC3031] limits the MPLS architecture to downstream-
   assigned MPLS labels.  To quote from RFC 3031:

   "In the MPLS architecture, the decision to bind a particular label L
   to a particular Forwarding Equivalence Class (FEC) F is made by the
   Label Switching Router (LSR) which is DOWNSTREAM with respect to that
   binding.  The downstream LSR then informs the upstream LSR of the
   binding.  Thus labels are "downstream-assigned", and label bindings
   are distributed in the "downstream to upstream" direction."

   This document introduces the notion of upstream-assigned MPLS labels
   to the MPLS architecture.  The procedures for upstream assignment of
   MPLS labels are described.

   RFC 3031 describes per-platform and per-interface label space.  This
   document generalizes the latter to a "Context-Specific Label Space"
   and describes a "Neighbor Label Space" as an example of this.
   Upstream-assigned labels are always looked up in a context-specific
   label space.

2.  Specification of Requirements

   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].

3.  Context-Specific Label Space

   RFC 3031 describes per-platform and per-interface label spaces.  This
   document introduces the more general concept of a "Context-Specific
   Label Space".  An LSR may maintain one or more context-specific label
   spaces.  In general, labels MUST be looked up in the per-platform
   label space unless something about the context determines that a
   label be looked up in a particular context-specific label space.

   One example of a context-specific label space is the per-interface
   label space discussed in RFC 3031.  When an MPLS packet is received
   over a particular interface, the top label of the packet may need to
   be looked up in the receiving interface's per-interface label space.
   In this case, the receiving interface is the context of the packet.
   Whether MPLS packets received over a particular interface need to