    Network Working Group                                       L. Andersson
    Request for Comments: 5462                                      Acreo AB
    Updates: 3032, 3270, 3272, 3443, 3469,                          R. Asati
             3564, 3985, 4182, 4364, 4379,                     Cisco Systems
             4448, 4761, 5129                                  February 2009
    Category: Standards Track

            Multiprotocol Label Switching (MPLS) Label Stack Entry:
                  "EXP" Field Renamed to "Traffic Class" Field

    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) 2009 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents in effect on the date of
       publication of this document (http://trustee.ietf.org/license-info).
       Please review these documents carefully, as they describe your rights
       and restrictions with respect to this document.

    Abstract

       The early Multiprotocol Label Switching (MPLS) documents defined the
       form of the MPLS label stack entry.  This includes a three-bit field
       called the "EXP field".  The exact use of this field was not defined
       by these documents, except to state that it was to be "reserved for
       experimental use".

       Although the intended use of the EXP field was as a "Class of
       Service" (CoS) field, it was not named a CoS field by these early
       documents because the use of such a CoS field was not considered to
       be sufficiently defined.  Today a number of standards documents
       define its usage as a CoS field.

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    RFC 5462                MPLS TC Field Definition           February 2009

       To avoid misunderstanding about how this field may be used, it has
       become increasingly necessary to rename this field.  This document
       changes the name of the field to the "Traffic Class field" ("TC
       field").  In doing so, it also updates documents that define the
       current use of the EXP field.

    Table of Contents

       1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
       2.  Details of Change  . . . . . . . . . . . . . . . . . . . . . .  3
         2.1.  RFC 3032 . . . . . . . . . . . . . . . . . . . . . . . . .  3
         2.2.  RFC 3270 . . . . . . . . . . . . . . . . . . . . . . . . .  5
         2.3.  RFC 5129 . . . . . . . . . . . . . . . . . . . . . . . . .  6
         2.4.  The Scope of This Change . . . . . . . . . . . . . . . . .  7
       3.  Use of the TC field  . . . . . . . . . . . . . . . . . . . . .  7
       4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
       5.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  8
       6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
         6.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
         6.2.  Informative References . . . . . . . . . . . . . . . . . .  9

    1.  Introduction

       The format of an MPLS label stack entry is defined by RFC 3032
       [RFC3032] to include a three-bit field called the "EXP field".  The
       exact use of this field is not defined by RFC 3032, except to state
       that it is to be "reserved for experimental use".

       The EXP field, from the start, was intended to carry "Class of
       Service" (CoS) information.  The field was actually called the "Class
       of Service field" in early versions of the working group document
       that was published as RFC 3032.  However, at the time that RFC 3032
       was published, the exact usage of this "Class of Service" field was
       not agreed upon and the field was designated as "experimental use";
       hence, the name has since been the "EXP field".

       The designation "for experimental use" has led other Standards
       Development Organizations (SDOs) and implementors to assume that it
       is possible to use the field for other purposes.  This document
       changes the name of the field to clearly indicate its use as a
       traffic classification field.

       At first, we discussed using the original "CoS field" as the name for
       the field, but it has been pointed out that this name does not cover
       the following changes that have occurred with respect to its usage
       since RFC 3032 was published.

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    RFC 5462                MPLS TC Field Definition           February 2009

       1.  The use of the EXP field was first defined in RFC 3270 [RFC3270],
           where a method to define a variant of Diffserv Label Switched
           Paths (LSP), called EXP-Inferred-PSC LSP (E-LSPs), was specified.
           PSC is a two-stage acronym that is expanded as PHB (Per Hop
           Behavior) Scheduling Class (PSC).

       2.  The use of the EXP field as defined in RFC 3270 has been further
           extended in RFC 5129 [RFC5129], where methods for explicit
           congestion marking in MPLS are defined.

       This document, hence, uses the name "Traffic Class field (TC field)",
       which better covers the potential use.  The MPLS TC field relates to
       an MPLS encapsulated packet the same way as the IPv6 TC field relates
       to an IPv6 encapsulated packet or the IPv4 Precedence field relates
       to an IPv4 encapsulated packet.

       The definitions of how the EXP field is used are perfectly clear in
       RFC 3270 and RFC 5129.  However, these RFCs do not explicitly state
       they update RFC 3032, and this fact was not captured in the RFC
       repository until after work on this document was started.

       This document updates RFC 3032, RFC 3270, and RFC 5129 to clarify the
       intended usage of the TC field.  The changes to these RFCs requires
       some changes to the actual text in those documents; Section 2
       explains the changes.

       This document also updates several other RFCs; see Section 2.4.  For
       these documents, the change is limited to changing the name of the
       Label Stack entry field.

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
       document are to be interpreted as described in RFC 2119 [RFC2119].

    2.  Details of Change

       The three RFCs 3032, 3270, and 5129 are now updated according to the
       following.

    2.1.  RFC 3032

       RFC 3032 states on page 4:

          3.  Experimental Use

          This three-bit field is reserved for experimental use.

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    RFC 5462                MPLS TC Field Definition           February 2009

       This paragraph is now changed to:

          3.  Traffic Class (TC) field

          This three-bit field is used to carry traffic class information,
          and the change of the name is applicable to all places it occurs
          in IETF RFCs and other IETF documents.

          RFC 3270 and RFC 5129 update the definition of the TC field and
          describe how to use the field.

       In Figure 1 on page 3 in RFC 3032, the format of a label stack entry
       is specified as:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Label
     |                Label                  | Exp |S|       TTL     | Stack
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Entry

                         Label:  Label Value, 20 bits
                         Exp:    Experimental Use, 3 bits
                         S:      Bottom of Stack, 1 bit
                         TTL:    Time to Live, 8 bits

                                  Figure 1

       Figure 1 in RFC 3032 is now changed to match the change of name to TC
       field:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Label
     |                Label                  | TC  |S|       TTL     | Stack
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Entry

                         Label:  Label Value, 20 bits
                         TC:     Traffic Class field, 3 bits
                         S:      Bottom of Stack, 1 bit
                         TTL:    Time to Live, 8 bits

                                   Figure 1 (new)

       Note: The designation of the picture above as "Figure 1 (new)" is
       introduced as a way to distinguish the figures in this document.  It
       will still be "Figure 1" in RFC 3032.

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    RFC 5462                MPLS TC Field Definition           February 2009

    2.2.  RFC 3270

       RFC 3270 says on page 6:

       1.2 EXP-Inferred-PSC LSPs (E-LSP)

          A single LSP can be used to support one or more OAs.  Such LSPs
          can support up to eight BAs of a given FEC, regardless of how many
          OAs these BAs span.  With such LSPs, the EXP field of the MPLS
          Shim Header is used by the LSR to determine the PHB to be applied
          to the packet.  This includes both the PSC and the drop
          preference.

          We refer to such LSPs as "EXP-inferred-PSC LSPs" (E-LSP), since
          the PSC of a packet transported on this LSP depends on the EXP
          field value for that packet.

          The mapping from the EXP field to the PHB (i.e., to PSC and drop
          precedence) for a given such LSP, is either explicitly signaled at
          label set-up or relies on a pre-configured mapping.

          Detailed operations of E-LSPs are specified in section 3 below.

       RFC 3270 is now updated like this:

       a.  A new paragraph is added at the end of Section 1 "Introduction":

              The EXP field has been renamed the TC field, and thus all
              references in RFC 3270 to the EXP field now refer to the TC
              field.

       b.  A new term is added to Section 1.1 "Terminology":

              TC        Traffic Class (replaces the term EXP)

       c.  In Section 1.1 "Terminology", the acronym E-LSP is now understood
           to mean:

              E-LSP     Explicitly TC-encoded-PSC LSP

       Section 1.2 on page 6 in RFC 3270 is now changed to:

       1.2 Explicitly TC-encoded-PSC LSPs (E-LSP)

          The EXP field has been renamed to the TC field, and thus all
          references in RFC 3270 to EXP field now refer to the TC field.
          However, we retain the acronym E-LSP (Explicitly TC-encoded-PSC
          LSP) as the acronym is in widespread use.

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    RFC 5462                MPLS TC Field Definition           February 2009

          A single LSP can be used to support one or more OAs.  Such LSPs
          can support up to eight BAs of a given FEC, regardless of how many
          OAs these BAs span.  With such LSPs, the TC field of the MPLS Shim
          Header is used by the LSR to determine the PHB to be applied to
          the packet.  This includes both the PSC and the drop preference.

          We refer to such LSPs as "Explicitly TC-encoded-PSC LSPs"
          (E-LSPs), since the PSC of a packet transported on this LSP
          depends on the TC field (previously called the EXP field) value
          for that packet.

          The mapping from the TC field to the PHB (i.e., to PSC and drop
          precedence) for a given such LSP is either explicitly signaled at
          label set-up or relies on a pre-configured mapping.

          This is an update to RFC 3032 [RFC3032], in line with the original
          intent of how this field in the MPLS Shim Header should be used
          (as a TC field).  RFC 3270 has itself been updated by RFC 5129
          [RFC5129].

          Detailed operations of E-LSPs are specified in Section 3 of RFC
          3270.

    2.3.  RFC 5129

       RFC 5129 is now updated like this:

       A new paragraph is added at the end of Section 1.1 "Background":

          The EXP field has been renamed to the TC field, and thus all
          references in RFC 5129 to the EXP field now refer to the TC field.

       Section 2 (bullet 5) on page 7 of RFC 5129 says:

       o  A third possible approach was suggested by [Shayman].  In this
          scheme, interior LSRs assume that the endpoints are ECN-capable,
          but this assumption is checked when the final label is popped.  If
          an interior LSR has marked ECN in the EXP field of the shim
          header, but the IP header says the endpoints are not ECN-capable,
          the edge router (or penultimate router, if using penultimate hop
          popping) drops the packet.  We recommend this scheme, which we
          call `per-domain ECT checking', and define it more precisely in
          the following section.  Its chief drawback is that it can cause
          packets to be forwarded after encountering congestion only to be
          dropped at the egress of the MPLS domain.  The rationale for this
          decision is given in Section 8.1.

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    RFC 5462                MPLS TC Field Definition           February 2009

       Section 2 (bullet 5) of RFC 5129 is now updated to:

       o  A third possible approach was suggested by [Shayman].  In this
          scheme, interior LSRs assume that the endpoints are ECN-capable,
          but this assumption is checked when the final label is popped.  If
          an interior LSR has marked ECN in the TC field of the shim header,
          but the IP header says the endpoints are not TC-capable, the edge
          router (or penultimate router, if using penultimate hop popping)
          drops the packet.  We recommend this scheme, which we call `per-
          domain ECT checking', and define it more precisely in the
          following section.  Its chief drawback is that it can cause
          packets to be forwarded after encountering congestion only to be
          dropped at the egress of the MPLS domain.  The rationale for this
          decision is given in Section 8.1.  This scheme is an update to RFC
          3032 [RFC3032] and RFC 3270 [RFC3270].

    2.4.  The Scope of This Change

       There are several places in the RFCs that are explicitly updated by
       this document that reference the "Exp field", sometimes they refer to
       the field as "Exp bits", "EXP bits", or "EXP".  In all those
       instances, the references now reference the TC field.

       There are also other RFCs (e.g., RFC 3272 [RFC3272], RFC 3443
       [RFC3443], RFC 3469 [RFC3469], RFC 3564 [RFC3564], RFC 3985
       [RFC3985], RFC 4182 [RFC4182], RFC 4364 [RFC4364], RFC 4379
       [RFC4379], RFC 4448 [RFC4448], and RFC 4761 [RFC4761]) that reference
       the "Exp field"; sometimes they refer to the field as "Exp bits",
       "EXP bits", and "EXP".  For all RFCs, including but not limited to
       those mentioned in this paragraph, such references now reference the
       TC field.

    3.  Use of the TC field

       Due to the limited number of bits in the TC field, their use for QoS
       and ECN (Explicit Congestion Notification) functions is intended to
       be flexible.  These functions may rewrite all or some of the bits in
       the TC field.

       Current implementations look at the TC field with and without label
       context, and the TC field may be copied to the label stack entries
       that are pushed onto the label stack.  This is done to avoid label
       stack entries that are pushed onto an existing label stack having
       different TC fields from the rest of the label stack entries.

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    RFC 5462                MPLS TC Field Definition           February 2009

    4.  Security Considerations

       This document only changes the name of one field in the MPLS shim
       header, and thus does not introduce any new security considerations.

    5.  Acknowledgments

       The authors would like to thank Stewart Bryant, Bruce Davie, George
       Swallow, and Francois Le Faucheur for their input to and review of
       the current document.

       The authors would also like to thank George Swallow, Khatri Paresh,
       and Phil Bedard for their help with grammar and spelling; a special
       thanks to Adrian Farrel for his careful review and help trawling the
       RFC-sea for RFCs that reference the EXP field.

    6.  References

    6.1.  Normative References

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

       [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
                  Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
                  Encoding", RFC 3032, January 2001.

       [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.

       [RFC3272]  Awduche, D., Chiu, A., Elwalid, A., Widjaja, I., and X.
                  Xiao, "Overview and Principles of Internet Traffic
                  Engineering", RFC 3272, May 2002.

       [RFC3443]  Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
                  in Multi-Protocol Label Switching (MPLS) Networks",
                  RFC 3443, January 2003.

       [RFC3469]  Sharma, V. and F. Hellstrand, "Framework for Multi-
                  Protocol Label Switching (MPLS)-based Recovery", RFC 3469,
                  February 2003.

       [RFC3564]  Le Faucheur, F. and W. Lai, "Requirements for Support of
                  Differentiated Services-aware MPLS Traffic Engineering",
                  RFC 3564, July 2003.

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    RFC 5462                MPLS TC Field Definition           February 2009

       [RFC3985]  Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
                  Edge (PWE3) Architecture", RFC 3985, March 2005.

       [RFC4182]  Rosen, E., "Removing a Restriction on the use of MPLS
                  Explicit NULL", RFC 4182, September 2005.

       [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
                  Networks (VPNs)", RFC 4364, February 2006.

       [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
                  Label Switched (MPLS) Data Plane Failures", RFC 4379,
                  February 2006.

       [RFC4448]  Martini, L., Rosen, E., El-Aawar, N., and G. Heron,
                  "Encapsulation Methods for Transport of Ethernet over MPLS
                  Networks", RFC 4448, April 2006.

       [RFC4761]  Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
                  (VPLS) Using BGP for Auto-Discovery and Signaling",
                  RFC 4761, January 2007.

       [RFC5129]  Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
                  Marking in MPLS", RFC 5129, January 2008.

    6.2.  Informative References

       [Shayman]  Shayman, M. and R. Jaeger, "Using ECN to Signal Congestion
                  Within an MPLS Domain", Work in Progress, November 2000.

    Authors' Addresses

       Loa Andersson
       Acreo AB

       EMail: loa@pi.nu

       Rajiv Asati
       Cisco Systems

       EMail: rajiva@cisco.com

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