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 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 September 11, 2008. Andersson Expires September 11, 2008 [Page 1] Internet-Draft MPLS EXP bits defintion March 2008 Abstract - 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 Andersson Expires September 11, 2008 [Page 2] Internet-Draft MPLS EXP bits defintion March 2008 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]. Andersson Expires September 11, 2008 [Page 3] Internet-Draft MPLS EXP bits defintion March 2008 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: Andersson Expires September 11, 2008 [Page 4] Internet-Draft MPLS EXP bits defintion March 2008 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: Andersson Expires September 11, 2008 [Page 5] Internet-Draft MPLS EXP bits defintion March 2008 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]. Andersson Expires September 11, 2008 [Page 6] Internet-Draft MPLS EXP bits defintion March 2008 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. Andersson Expires September 11, 2008 [Page 7] Internet-Draft MPLS EXP bits defintion March 2008 4. IANA considerations TBD Andersson Expires September 11, 2008 [Page 8] Internet-Draft MPLS EXP bits defintion March 2008 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. Andersson Expires September 11, 2008 [Page 9] Internet-Draft MPLS EXP bits defintion March 2008 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.", . Andersson Expires September 11, 2008 [Page 10] Internet-Draft MPLS EXP bits defintion March 2008 Author's Address Loa Andersson Acreo AB Email: loa@pi.se Andersson Expires September 11, 2008 [Page 11] Internet-Draft MPLS EXP bits defintion March 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. 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