One document matched: draft-andersson-mpls-expbits-def-00.txt
Network Working Group L. Andersson
Internet-Draft Acreo AB
Intended status: Standards Track March 10, 2008
Expires: September 11, 2008
MPLS EXP-bits definition
draft-andersson-mpls-expbits-def-00.txt
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Abstract
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Details of change . . . . . . . . . . . . . . . . . . . . . . 4
2.1. RFC 3032 . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. RFC 3270 . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. RFC 5129 . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Use of the CoS bits . . . . . . . . . . . . . . . . . . . . . 7
4. IANA considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security considerations . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . . 10
6.2. Informative references . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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1. Introduction
The format of the MPLS label is defined in RFC 3032 [RFC3032], that
definition includes three bits called "EXP bits". RFC 3032 leaves
the exact description of how the EXP bits should be used undefined,
they are said to be for "experimental use".
The EXP bits has from the start be intended to be used for "Class of
Service", the bits were actually called "Class of Service bits" in
the early versions of the working group document that was publshed as
RFC 3032.C However at the time that RFC 3032 were published the
"Class of Service" were considered not to be defined well enough and
the bit were left for "Experimental use".
The use of the EXP bits was first defined in RFC 3270 [RFC3270] where
a method to define a variant of DiffServ LSPs called EXP-Inferred-PSC
LSP (E-LSPs).
The use of the EXP bits as defined in RFC 3270 has been further
extended in RFC 5129 [RFC5129], where methods for explicit congestion
marking in MPLS is defined.
The defintions of how the EXP bits are used are perfectly clear in
RFC 3270 and RFC 5129. However it is never explicitly stated that
these RFCs updates RFC 3032, and it is not captured in the RFC
respository. This document changes RFC 3032, RFC 3270 and RFC 5129
to capture these updates.
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 state 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) bits
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 bits has been update 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 bits have been renamed to the CoS bits, and thus all
references in RFC 3270 to EXP bits should be taken to refer to the
CoS bits. 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 bits 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
bits (previously called the EXP bits) value for that packet.
The mapping from the CoS bits 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 bits). 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 bits have been renamed to the CoS bits, and thus all
references in RFC 5219 to EXP bits should be taken to refer to the
CoS bits.
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 bits
Due to the limited number of bits the particular use of the bits is
intended to be flexible - including the defininition of various QoS
and ECN functions.
Current implementations look at the CoS bits with and without label
context and the CoS bits may be copied to the labels that are pushed
onto the laabel stack. This is to avoid that the pushed labels have
a different set of CoS bits.
CoS and ECN funtions may rewrite all or some of the bits.
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4. IANA considerations
TBD
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5. Security considerations
This document only changes the name of one field in the MPLS Shim
Header and thus do not introduce any new security considerations.
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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.
[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, 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.se
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