





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










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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.




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          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.

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 Forwarding Equivalence Class (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 RFC 3270 [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.

   RFC 3270 specifies the procedures for MPLS support of Differentiated
   Services.  In particular, it defines a "Pipe Model" in which (quoting
   from RFC 3270, Section 2.6.2):



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    "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.  It is conceivable that
   there may be an implementation that 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.



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

   Implementations that 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 that 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
   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]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, January 2001.

8.  Informative References

   [RFC3270]  Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
              P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
              Protocol Label Switching (MPLS) Support of Differentiated
              Services", RFC 3270, May 2002.





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Author's Address

   Eric C. Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA 01719

   EMail: erosen@cisco.com











































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Full Copyright Statement

   Copyright (C) The Internet Society (2005).

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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