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rfc:rfc5462

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.

Andersson & Asati Standards Track [Page 1] 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.

Andersson & Asati Standards Track [Page 2] 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.

Andersson & Asati Standards Track [Page 3] 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.

Andersson & Asati Standards Track [Page 4] 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.

Andersson & Asati Standards Track [Page 5] 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.

Andersson & Asati Standards Track [Page 6] 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.

Andersson & Asati Standards Track [Page 7] 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.

Andersson & Asati Standards Track [Page 8] 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

Andersson & Asati Standards Track [Page 9]

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