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
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on December 13, 2008.
Andersson Expires December 13, 2008 [Page 1]
Internet-Draft MPLS CoS field defintion June 2008
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.
Andersson Expires December 13, 2008 [Page 2]
Internet-Draft MPLS CoS field defintion June 2008
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
Andersson Expires December 13, 2008 [Page 3]
Internet-Draft MPLS CoS field defintion June 2008
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].
Andersson Expires December 13, 2008 [Page 4]
Internet-Draft MPLS CoS field defintion June 2008
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:
Andersson Expires December 13, 2008 [Page 5]
Internet-Draft MPLS CoS field defintion June 2008
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:
Andersson Expires December 13, 2008 [Page 6]
Internet-Draft MPLS CoS field defintion June 2008
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].
Andersson Expires December 13, 2008 [Page 7]
Internet-Draft MPLS CoS field defintion June 2008
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.
Andersson Expires December 13, 2008 [Page 8]
Internet-Draft MPLS CoS field defintion June 2008
4. IANA considerations
There are no request for IANA allocation of code points in this
document.
Andersson Expires December 13, 2008 [Page 9]
Internet-Draft MPLS CoS field defintion June 2008
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.
Andersson Expires December 13, 2008 [Page 10]
Internet-Draft MPLS CoS field defintion June 2008
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.
Andersson Expires December 13, 2008 [Page 11]
Internet-Draft MPLS CoS field defintion June 2008
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/>.
Andersson Expires December 13, 2008 [Page 12]
Internet-Draft MPLS CoS field defintion June 2008
Author's Address
Loa Andersson
Acreo AB
Email: loa@pi.nu
Andersson Expires December 13, 2008 [Page 13]
Internet-Draft MPLS CoS field defintion June 2008
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Andersson Expires December 13, 2008 [Page 14]
