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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
Intended status: Standards Track                           June 11, 2008
Expires: December 13, 2008


                   "EXP field" renamed to "CoS Field"
                  draft-ietf-mpls-cosfield-def-02.txt

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Abstract

   The early MPLS documents defined the form of a the MPLS Label Stack
   entry.  This include 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 is to be "reserved for experimental use".

   Although the intended use of the EXP field was as a "Class of
   Service" field, it was not named the "Class of Service" (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 this
   document re-introduces the name "CoS field" for this field.  In doing
   so it also updates documents that define the current usee of the EXP
   this field.


































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Details of change  . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  RFC 3032 . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  RFC 3270 . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  RFC 5129 . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Use of the CoS field . . . . . . . . . . . . . . . . . . . . .  8
   4.  IANA considerations  . . . . . . . . . . . . . . . . . . . . .  9
   5.  Security considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     7.2.  Informative references . . . . . . . . . . . . . . . . . . 12
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 14



































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1.  Introduction

   The format of a MPLS label stack entry is defined by RFC 3032
   [RFC3032], includes three bit field called "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" information, the field was actually called the "Class of
   Service field" in the early versions of the working group document
   that was publshed 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".

   The designation "for Experimental use" has lead other Standards
   Development Organizations (SDO) and implementors to the assume that
   it possible to use the field for other purposes than Class of
   Service.  This document changes the name of the field to clearly
   indicate its use.

   The use of the EXP field was first defined in RFC 3270 [RFC3270]
   where a method to define a variant of DiffServ LSPs called EXP-
   Inferred-PSC LSP (E-LSPs) were specified.

   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.

   The defintions of how the EXP field are used are perfectly clear in
   RFC 3270 and RFC 5129.  However, these RFCs do not explicitly state
   they update 3032, and it is not captured in the RFC respository.
   This document updates RFC 3032, RFC 3270 and RFC 5129 to clarify the
   intended usage of the CoS 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].














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2.  Details of change

   The three RFCs are now updated according to the following.

2.1.  RFC 3032

   The RFC 3032 states on page 3:

      3.  Experimental Use

      This three-bit field is reserved for experimental use.

   This paragraph is now changed to:

      3.  Class of Service (CoS) field

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

      The definition of how to use the CoS field has been updated by RFC
      3270 and RFC 5129.

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.

   Section 1.2 on page 5 in RFC 3270 is now changed to:




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

      The EXP field has been renamed to the CoS field, and thus all
      references in RFC 3270 to EXP field SHOULD be taken to refer to
      the CoS field.  However, we retain the term E-LSP (EXP-Inferred-
      PSC LSP) as it 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 CoS 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 CoS
      field (previously called the EXP field) value for that packet.

      The mapping from the CoS 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 CoS field).  The RFC 3270 has itself been updated by RFC 5129
      [RFC5129].

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

2.3.  RFC 5129

   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.

   RFC 5219 is now updated like this:



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

      The EXP field has been renamed to the CoS field, and thus all
      references in RFC 5219 to EXP field SHOULD be taken to refer to
      the CoS field.

   Section 2 (bullet 3) on page 6 ofis now 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 CoS field of the shim
      header, but the IP header says the endpoints are not CoS-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].






























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3.  Use of the CoS field

   Due to the limited number of bits the particular use of the bits is
   intended to be flexible - including the definition of various QoS and
   ECN functions.

   Current implementations look at the CoS field with and without label
   context and the CoS field may be copied to the labels that are pushed
   onto the label stack.  This is to avoid the pushed labels having a
   different CoS field.

   CoS and ECN funtions may rewrite all or some of the bits.







































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4.  IANA considerations

   There are no request 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.











































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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.

   [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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Author's Address

   Loa Andersson
   Acreo AB

   Email: loa@pi.nu













































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