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Network Working Group                                      Eric C. Rosen
Internet Draft                                       Cisco Systems, Inc.
Expiration Date: June 2004

                                                           December 2003


        Removing a Restriction on the use of MPLS Explicit NULL


                 draft-rosen-mpls-explicit-null-01.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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Abstract

   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.  This restriction is now removed, so that those label
   values are legal anywhere in the stack.












Rosen                                                           [Page 1]


Internet Draft   draft-rosen-mpls-explicit-null-01.txt     December 2003




Contents

    1        Introduction  .........................................   2
    2        Detail of Change  .....................................   2
    3        Reasons for Change  ...................................   3
    4        Acknowledgments  ......................................   5
    5        References  ...........................................   5
    6        Author's Address  .....................................   5





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:

     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.







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     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".  This
        label indicates that the label stack must be popped.  If the
        IPv4 Explicit NULL label was not at the bottom of the label
        stack, then the forwarding of the packet must then be based on
        the subsequent label.  The IPv4 Explicit NULL label is only
        legal at the bottom of the label stack if the label stack is
        immediately followed by an IPv4 header.  In this case, the
        forwarding of the packet must be based on the IPv4 header.

   Paragraph iii is hereby changed to read:

     iii. A value of 2 represents the "IPv6 Explicit NULL Label".  This
          label indicates that the label stack must be popped.  If the
          IPv6 Explicit NULL label was not at the bottom of the label
          stack, then the forwarding of the packet must then be based on
          the subsequent label.  The IPv6 Explicit NULL label is only
          legal at the bottom of the label stack if the label stack is
          immediately followed by an IPv6 header.  In this case, the
          forwarding of the packet must be based on the IPv6 header.


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



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   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
    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.

    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



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Internet Draft   draft-rosen-mpls-explicit-null-01.txt     December 2003


    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. Acknowledgments

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


5. References

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

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


6. Author's Address


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





















Rosen                                                           [Page 5]


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