Removing a Restriction on the use of MPLS Explicit NULL
draft-ietf-mpls-explicit-null-02

Network Working Group                                      Eric C. Rosen
Internet Draft                                       Cisco Systems, Inc.
Expiration Date: August 2005
Updates RFC 3032

                                                           February 2005

        Removing a Restriction on the use of MPLS Explicit NULL

                  draft-ietf-mpls-explicit-null-02.txt

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Abstract

   The label stack encoding for MPLS (Multi-protocol Label Switching)
   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.

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Contents

    1        Introduction  .........................................   2
    2        Detail of Change  .....................................   2
    3        Reasons for Change  ...................................   4
    4        Deployment Considerations  ............................   5
    5        Security Considerations  ..............................   6
    6        Acknowledgments  ......................................   6
    7        Normative References  .................................   6
    8        Informative References  ...............................   6
    9        Author's Address  .....................................   6
   10        Intellectual Property Statement  ......................   6
   11        Full Copyright Statement  .............................   7

1. Introduction

   RFC 3032 defines a reserved label value known as "IPv4 Explicit NULL"
   and a reserved label value known as "IPv6 Explicit NULL".  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 which have Explicit NULL occur
   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:

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       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.

          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
          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.

          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 IPv6 packet, and its disposition is based on the
          IPv6 header.

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3. Reasons for Change

   Restricting Explicit NULL to the bottom of the stack has caused some
   problems in practice.

   With this restriction in place, one should not distribute, to a
   particular label distribution peer, a binding of Explicit NULL to a
   particular FEC, unless the following condition (call it "Condition
   L") holds:  all MPLS packets received by that peer with an incoming
   label corresponding to that FEC contain only a single label stack
   entry.  If Explicit NULL is bound to the FEC, but Condition L doesn't
   hold, the peer is being requested to create illegal packets.  None of
   the MPLS specifications say what the peer is actually supposed to do
   in this case.  This situation is made more troublesome by the facts
   that, in practice, Condition L rarely holds, and it is not possible
   in general to determine whether it holds or not.

   Further, if one is supporting the Pipe Model of RFC3270, there are
   good reasons to create label stacks in which Explicit NULL is at the
   top of the label stack, but a non-null label is at the bottom.

   RFC3270 specifies the procedures for MPLS support of Differentiated
   Services.  In particular, it defines a "Pipe Model", in which
   (quoting from RFC3270, section 2.6.2):

    "tunneled packets must convey two meaningful pieces of Diff-Serv
    information:

      - the Diff-Serv information which is meaningful to intermediate
        nodes along the LSP span including the LSP Egress (which we
        refer to as the 'LSP Diff-Serv Information').  This LSP Diff-
        Serv Information is not meaningful beyond the LSP Egress:
        Whether Traffic Conditioning at intermediate nodes on the LSP
        span affects the LSP Diff-Serv information or not, this updated
        Diff-Serv information is not considered meaningful beyond the
        LSP Egress and is ignored.

      - the Diff-Serv information which is meaningful beyond the LSP
        Egress (which we refer to as the 'Tunneled Diff-Serv
        Information').  This information is to be conveyed by the LSP
        Ingress to the LSP Egress.  This Diff-Serv information is not
        meaningful to the intermediate nodes on the LSP span."

    When the Pipe Model is in use, it is common practice for the LSP
    Egress to bind Explicit Null to the tunnel's FEC.  The intention is
    that the LSP Diff-Serv information will be carried in the EXP bits
    of the Explicit Null label stack entry, and the tunneled Diff-Serv
    information will be carried in whatever is "below" the Explicit Null

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    label stack entry, i.e., in the IP header DS bits or in the EXP bits
    of the next entry on the MPLS label stack.

    Naturally, this practice causes a problem if the Pipe Model LSP is
    being used to tunnel MPLS packets (i.e., if Condition L does not
    hold).  With strict adherence to RFCs 3031 and 3036, this practice
    results in an MPLS packet where Explicit NULL is at the top of the
    label stack, even though it is not the only entry in the label
    stack.  However, RFC 3032 makes this packet illegal.

    Some implementations simply transmit the illegal packet.  Others try
    to convert it to a legal packet by stripping off the Explicit NULL
    before transmitting it.  However, that breaks the Pipe Model by
    discarding the LSP Diff-Serv information.  It is conceivable that
    there may be an implementation which drops the illegal packet
    entirely; this would also break the Pipe Model, as it would lose not
    only the LSP Diff-Serv information but the entire packet.

    Of course the LSP egress is not compelled to bind Explicit NULL to
    the tunnel's FEC; an ordinary label could be used instead.  However,
    using Explicit NULL enables the egress to determine immediately
    (i.e., without need for lookup in the Label Information Base) that
    the further forwarding of the packet is to be determined by whatever
    is below the label.  Avoiding this lookup can have favorable
    implications on forwarding performance.

    Removing the restriction that Explicit Null only occur at the bottom
    of the stack is the simplest way to facilitate the proper operation
    of the Pipe Model.

4. Deployment Considerations

   Implementations which adhere to this specification will interoperate
   correctly, and will correctly support the Pipe Model.

   Implementations which do not adhere to this specification may not
   interoperate.  In particular if a router advertises a binding of
   Explicit NULL, and if that router has an upstream LDP peer which will
   not transmit a packet that has multiple label stack entries with
   Explicit Null at top of the stack, then it will not be possible to
   use Explicit NULL to support the Pipe Model until the upstream LDP
   peer is brought into compliance with this specification.

   It is possible that there may be a router implementation, preceding
   this specification, which will discard any received packet with
   multiple label stack entries and a top label value of Explicit Null.
   It is advisable to configure any such routers so that they do not

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   advertise any bindings to Explicit Null.

5. Security Considerations

   This document updates RFC 3032 by allowing Explicit NULL to occur at
   any position in the label stack.  This modification does not impose
   any new security considerations beyond those discussed in RFC 3032.

6. Acknowledgments

   Thanks to Rahul Aggarwal, Francois LeFaucheur, Yakov Rekhter, and Dan
   Tappan for their helpful comments.

7. Normative References

   [RFC3032] "MPLS Label Stack Encoding", Rosen, et. al., January 2001

8. Informative References

   [RFC3270] "Multi-Protocol Label Switching (MPLS) Support of
   Differentiated Services", Le Faucheur, et. al., May 2002

9. Author's Address

      Eric C. Rosen
      Cisco Systems, Inc.
      1414 Massachusetts Avenue
      Boxborough, MA 01719
      Email: erosen@cisco.com

10. Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
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   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be

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11. Full Copyright Statement

   Copyright (C) The Internet Society (2005).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78 and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an
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Rosen                                                           [Page 7]