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Removing a Restriction on the use of MPLS Explicit NULL
RFC 4182

Document type: RFC - Proposed Standard (September 2005; Errata)
Updated by RFC 5462
Updates RFC 3032
Document stream: IETF
Last updated: 2013-03-02
Other versions: plain text, pdf, html

IETF State: WG Document
Consensus: Unknown
Document shepherd: No shepherd assigned

IESG State: RFC 4182 (Proposed Standard)
Responsible AD: Alex Zinin
Send notices to: swallow@cisco.com, loa@pi.se

Network Working Group                                           E. Rosen
Request for Comments: 4182                           Cisco Systems, Inc.
Updates: 3032                                             September 2005
Category: Standards Track

        Removing a Restriction on the use of MPLS Explicit NULL

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.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   The label stack encoding for Multi-protocol Label Switching (MPLS)
   defines a reserved label value known as "IPv4 Explicit NULL" and a
   reserved label value known as "IPv6 Explicit NULL".  Previously,
   these labels were only legal when they occurred at the bottom of the
   MPLS label stack.  This restriction is now removed, so that these
   label values may legally occur anywhere in the stack.

   This document updates RFC 3032.

Table of Contents

   1. Introduction ....................................................2
   2. Detail of Change ................................................2
   3. Reasons for Change ..............................................3
   4. Deployment Considerations .......................................5
   5. Security Considerations .........................................5
   6. Acknowledgments .................................................5
   7. Normative References ............................................5
   8. Informative References ..........................................5

Rosen                       Standards Track                     [Page 1]
RFC 4182       Removing a Restriction on the use of MPLS  September 2005

1.  Introduction

   RFC 3032 defines a reserved label value known as "IPv4 Explicit NULL"
   and a reserved label value known as "IPv6 Explicit NULL" [RFC3032].
   It states that these label values are only legal at the bottom of the
   MPLS label stack.  However, no reason is given for this restriction.

   It has turned out that in practice there are some situations in which
   it is useful to send MPLS packets that have Explicit NULL occur
   somewhere other than at that bottom of the label stack.  While the
   intended semantics are obvious enough, the fact that such packets are
   gratuitously declared by RFC 3032 to be illegal has made it difficult
   to handle these situations in an interoperable manner.

   This document updates RFC 3032 by removing the unnecessary
   restriction, so that the two aforementioned label values are legal
   anywhere in the label stack.

2.  Detail of Change

   RFC 3032 states on page 4:

   There are several reserved label values:

       i. A value of 0 represents the "IPv4 Explicit NULL Label".  This
          label value is only legal at the bottom of the label stack.
          It indicates that the label stack must be popped, and the
          forwarding of the packet must then be based on the IPv4
          header.

     iii. A value of 2 represents the "IPv6 Explicit NULL Label".  This
          label value is only legal at the bottom of the label stack.
          It indicates that the label stack must be popped, and the
          forwarding of the packet must then be based on the IPv6
          header.

   Paragraph i is hereby changed to read:

       i. A value of 0 represents the "IPv4 Explicit NULL Label".

          An IPv4 Explicit NULL at the top of the label stack means that
          the stack must be popped.

          If the NULL was not the only label on the stack, this will
          cause the label beneath it to rise to the top of the stack.
          The disposition of the packet is based on the label that has
          now risen to the top.

Rosen                       Standards Track                     [Page 2]
RFC 4182       Removing a Restriction on the use of MPLS  September 2005

          If, on the other hand, the NULL was the only label on the
          stack, then the stack is now empty.  The resulting packet is
          treated as an IPv4 packet, and its disposition is based on the
          IPv4 header.

   Paragraph iii is hereby changed to read:

       iii. A value of 2 represents the "IPv6 Explicit NULL Label".

          An IPv6 Explicit NULL at the top of the label stack means that
          the stack must be popped.

          If the NULL was not the only label on the stack, this will

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