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Versions: 00 01 02 03 04 05 06 07 08 RFC 5462

Network Working Group                                       L. Andersson
Internet-Draft                                                  Acreo AB
Updates: RFC 3032, RFC 3270, RFC                                R. Asati
5129, RFC 3272, RFC 3443, RFC                              Cisco Systems
3469, RFC 3564,  RFC 3985, RFC                          December 5, 2008
4182, RFC 4364, RFC 4379, RFC
4448, RFC 4761 (if approved)
Intended status: Standards Track
Expires: June 8, 2009


 Multi-Protocol Label Switching  (MPLS) label stack entry: "EXP" field
                    renamed to "Traffic Class" field
                  draft-ietf-mpls-cosfield-def-08.txt

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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 the 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. .

   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 this field.
































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Details of change  . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  RFC 3032 . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.2.  RFC 3270 . . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.3.  RFC 5129 . . . . . . . . . . . . . . . . . . . . . . . . .  9
     2.4.  The Scope of this Change . . . . . . . . . . . . . . . . .  9
   3.  Use of the TC field  . . . . . . . . . . . . . . . . . . . . . 11
   4.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 12
   5.  Security considerations  . . . . . . . . . . . . . . . . . . . 13
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     7.2.  Informative references . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
   Intellectual Property and Copyright Statements . . . . . . . . . . 18


































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1.  Introduction

   The format of a MPLS label stack entry is defined by RFC 3032
   [RFC3032], include a three bit field called the "EXP field".  The
   exact use of this field is not defined by RFC 3032 leaves, 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 the 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 and the field was designated as "Experimental use"; hence
   the name has since then been the "EXP Field".

   The designation "for Experimental use" has led other Standards
   Development Organizations (SDO) and implementors to the assume that
   it 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 to use the original "CoS field" as the name for
   the field, but it has been pointed that this name does not cover the
   following changes with respect to its usage, since RFC 3032 was
   published.

   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.
       The PSC is a two stage acroynym that is expanded as Per Hop
       Behavior (PHB) and PHB 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 encapsulted 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 the work on this document were started.  This
   document updates RFC 3032, RFC 3270 and RFC 5129 to clarify the
   intended usage of the TC field.  Section 2 explains the changes.



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   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].
















































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2.  Details of change

   The three RFCs 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.

   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 updates the definition of the TC field and
      describes how to use the field.

   In Figure 1 on page 3 in RFC3032 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 of
   the TC field to:




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  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
                     CoS:    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 draft.  It
   will still be "Figure 1." in RFC 3032.

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:



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

          The EXP field has been renamed to the TC field, and thus all
          references in RFC 3270 to EXP field SHOULD be taken to 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 5 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 SHOULD be taken to refer to
      the TC field.  However, we retain the acronym E-LSP (Explicitly
      TC-encoded-PSC LSP) as the acronym is in widespread use.

      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 LSP" (E-LSP),
      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 TC field).  The RFC 3270 has itself been updated by RFC 5129
      [RFC5129].

      Detailed operations of E-LSPs are specified in section 3 of
      RFC3270.






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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 EXP field SHOULD be taken to refer to
      the TC field.

   Section 2 (bullet 3) on page 6 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.

   Section 2 (bullet 3) of RFC 5129 is now updated changed 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 has explicitly updated by
   this document that reference the "Exp field", sometimes they refer to
   the field as "Exp bits", "EXP bits" and "EXP".  In all those
   instances the references SHOULD be taken to reference the TC field.




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   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 references
   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 SHOULD be taken to
   reference the TC field.











































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3.  Use of the TC field

   Due to the limited number of bits in the TC field, their use for QoS
   and ECN functions is intended to be flexible.  These funtions 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 that
   label stack entries that are pushed on to an existing label stack
   have different TF fields from the rest of the label stack entries.








































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4.  IANA considerations

   There are no requests for IANA allocation of code points in this
   document.















































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5.  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.















































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6.  Acknowledgments

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









































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7.  References

7.1.  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.

   [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.



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   [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.

7.2.  Informative references

   [Shayman]  Shayman, M. and R. Jaeger, University of Michigan, "Using
              ECN to Signal Congestion Within an MPLS Domain", Work in
              Progress, November 2000.", <http://www.watersprings.org/
              pub/id/draft-shayman-mpls-ecn-00.txt/>.






































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Authors' Addresses

   Loa Andersson
   Acreo AB

   Email: loa@pi.nu


   Rajiva Asati
   Cisco Systems

   Email: rajiva@cisco.com







































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