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Network Working Group                                           E. RosenRequest for Comments: 4182                           Cisco Systems, Inc.Updates:3032                                             September 2005Category: Standards TrackRemoving a Restriction on the use of MPLS Explicit NULLStatus 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 updatesRFC 3032.Table of Contents1. Introduction ....................................................22. Detail of Change ................................................23. Reasons for Change ..............................................34. Deployment Considerations .......................................55. Security Considerations .........................................56. Acknowledgments .................................................57. Normative References ............................................58. Informative References ..........................................5Rosen                       Standards Track                     [Page 1]

RFC 4182       Removing a Restriction on the use of MPLS  September 20051.  IntroductionRFC 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 byRFC 3032 to be illegal has made it difficult   to handle these situations in an interoperable manner.   This document updatesRFC 3032 by removing the unnecessary   restriction, so that the two aforementioned label values are legal   anywhere in the label stack.2.  Detail of ChangeRFC 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          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 ofRFC 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   fromRFC 3270, Section 2.6.2):Rosen                       Standards Track                     [Page 3]

RFC 4182       Removing a Restriction on the use of MPLS  September 2005    "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.Rosen                       Standards Track                     [Page 4]

RFC 4182       Removing a Restriction on the use of MPLS  September 2005   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 updatesRFC 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 inRFC 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.Rosen                       Standards Track                     [Page 5]

RFC 4182       Removing a Restriction on the use of MPLS  September 2005Author's Address   Eric C. Rosen   Cisco Systems, Inc.   1414 Massachusetts Avenue   Boxborough, MA 01719   EMail: erosen@cisco.comRosen                       Standards Track                     [Page 6]

RFC 4182       Removing a Restriction on the use of MPLS  September 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   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   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Rosen                       Standards Track                     [Page 7]