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Andersson 3 Internet-Draft Acreo AB 4 Intended status: Standards Track June 11, 2008 5 Expires: December 13, 2008 7 "EXP field" renamed to "CoS Field" 8 draft-ietf-mpls-cosfield-def-02.txt 10 Status of this Memo 12 By submitting this Internet-Draft, each author represents that any 13 applicable patent or other IPR claims of which he or she is aware 14 have been or will be disclosed, and any of which he or she becomes 15 aware will be disclosed, in accordance with Section 6 of BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt. 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 This Internet-Draft will expire on December 13, 2008. 35 Abstract 37 The early MPLS documents defined the form of a the MPLS Label Stack 38 entry. This include a three bit field called the "EXP field". The 39 exact use of this field was not defined by these documents, except to 40 state that it is to be "reserved for experimental use". 42 Although the intended use of the EXP field was as a "Class of 43 Service" field, it was not named the "Class of Service" (CoS) field 44 by these early documents because the use of such a CoS field was not 45 considered to be sufficiently defined. Today a number of standards 46 documents define its usage as a CoS field. . 48 To avoid misunderstanding about how this field may be used this 49 document re-introduces the name "CoS field" for this field. In doing 50 so it also updates documents that define the current usee of the EXP 51 this field. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 56 2. Details of change . . . . . . . . . . . . . . . . . . . . . . 5 57 2.1. RFC 3032 . . . . . . . . . . . . . . . . . . . . . . . . . 5 58 2.2. RFC 3270 . . . . . . . . . . . . . . . . . . . . . . . . . 5 59 2.3. RFC 5129 . . . . . . . . . . . . . . . . . . . . . . . . . 6 60 3. Use of the CoS field . . . . . . . . . . . . . . . . . . . . . 8 61 4. IANA considerations . . . . . . . . . . . . . . . . . . . . . 9 62 5. Security considerations . . . . . . . . . . . . . . . . . . . 10 63 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11 64 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 65 7.1. Normative References . . . . . . . . . . . . . . . . . . . 12 66 7.2. Informative references . . . . . . . . . . . . . . . . . . 12 67 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13 68 Intellectual Property and Copyright Statements . . . . . . . . . . 14 70 1. Introduction 72 The format of a MPLS label stack entry is defined by RFC 3032 73 [RFC3032], includes three bit field called "EXP field". The exact 74 use of this field is not defined by RFC 3032 leaves,, except to state 75 that it is to be "reserved for experimental use". 77 The EXP field, from the start, was intended to carry "Class of 78 Service" information, the field was actually called the "Class of 79 Service field" in the early versions of the working group document 80 that was publshed as RFC 3032. However at the time that RFC 3032 was 81 published the exact usage of this "Class of Service" field was not 82 agreed and the field was designated as "Experimental use". 84 The designation "for Experimental use" has lead other Standards 85 Development Organizations (SDO) and implementors to the assume that 86 it possible to use the field for other purposes than Class of 87 Service. This document changes the name of the field to clearly 88 indicate its use. 90 The use of the EXP field was first defined in RFC 3270 [RFC3270] 91 where a method to define a variant of DiffServ LSPs called EXP- 92 Inferred-PSC LSP (E-LSPs) were specified. 94 The use of the EXP field as defined in RFC 3270 has been further 95 extended in RFC 5129 [RFC5129], where methods for explicit congestion 96 marking in MPLS are defined. 98 The defintions of how the EXP field are used are perfectly clear in 99 RFC 3270 and RFC 5129. However, these RFCs do not explicitly state 100 they update 3032, and it is not captured in the RFC respository. 101 This document updates RFC 3032, RFC 3270 and RFC 5129 to clarify the 102 intended usage of the CoS field. 104 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 105 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 106 document are to be interpreted as described in RFC 2119 [RFC2119]. 108 2. Details of change 110 The three RFCs are now updated according to the following. 112 2.1. RFC 3032 114 The RFC 3032 states on page 3: 116 3. Experimental Use 118 This three-bit field is reserved for experimental use. 120 This paragraph is now changed to: 122 3. Class of Service (CoS) field 124 This three-bit field is used to carry Class of Service information 125 and the change of the name is applicable to all places it occurs 126 in IETF RFCs and other IETF documents. 128 The definition of how to use the CoS field has been updated by RFC 129 3270 and RFC 5129. 131 2.2. RFC 3270 133 RFC 3270 says on page 6: 135 1.2 EXP-Inferred-PSC LSPs (E-LSP) 137 A single LSP can be used to support one or more OAs. Such LSPs 138 can support up to eight BAs of a given FEC, regardless of how many 139 OAs these BAs span. With such LSPs, the EXP field of the MPLS 140 Shim Header is used by the LSR to determine the PHB to be applied 141 to the packet. This includes both the PSC and the drop 142 preference. 144 We refer to such LSPs as "EXP-inferred-PSC LSPs" (E-LSP), since 145 the PSC of a packet transported on this LSP depends on the EXP 146 field value for that packet. 148 The mapping from the EXP field to the PHB (i.e., to PSC and drop 149 precedence) for a given such LSP, is either explicitly signaled at 150 label set-up or relies on a pre-configured mapping. 152 Detailed operations of E-LSPs are specified in section 3 below. 154 Section 1.2 on page 5 in RFC 3270 is now changed to: 156 1.2 EXP-Inferred-PSC LSPs (E-LSP) 158 The EXP field has been renamed to the CoS field, and thus all 159 references in RFC 3270 to EXP field SHOULD be taken to refer to 160 the CoS field. However, we retain the term E-LSP (EXP-Inferred- 161 PSC LSP) as it is in widespread use. 163 A single LSP can be used to support one or more OAs. Such LSPs 164 can support up to eight BAs of a given FEC, regardless of how many 165 OAs these BAs span. With such LSPs, the CoS field of the MPLS 166 Shim Header is used by the LSR to determine the PHB to be applied 167 to the packet. This includes both the PSC and the drop 168 preference. 170 We refer to such LSPs as "EXP-inferred-PSC LSPs" (E-LSP), since 171 the PSC of a packet transported on this LSP depends on the CoS 172 field (previously called the EXP field) value for that packet. 174 The mapping from the CoS field to the PHB (i.e., to PSC and drop 175 precedence) for a given such LSP, is either explicitly signaled at 176 label set-up or relies on a pre-configured mapping. 178 This is an update to RFC 3032 [RFC3032] in line with the original 179 intent of how this field in the MPLS Shim Header should be used 180 (as CoS field). The RFC 3270 has itself been updated by RFC 5129 181 [RFC5129]. 183 Detailed operations of E-LSPs are specified in section 3 of 184 RFC3270. 186 2.3. RFC 5129 188 Section 2 (bullet 3) on page 6 of RFC 5129 says: 190 o A third possible approach was suggested by [Shayman]. In this 191 scheme, interior LSRs assume that the endpoints are ECN-capable, 192 but this assumption is checked when the final label is popped. If 193 an interior LSR has marked ECN in the EXP field of the shim 194 header, but the IP header says the endpoints are not ECN-capable, 195 the edge router (or penultimate router, if using penultimate hop 196 popping) drops the packet. We recommend this scheme, which we 197 call `per-domain ECT checking', and define it more precisely in 198 the following section. Its chief drawback is that it can cause 199 packets to be forwarded after encountering congestion only to be 200 dropped at the egress of the MPLS domain. The rationale for this 201 decision is given in Section 8.1. 203 RFC 5219 is now updated like this: 205 A new paragraph is added at the end of section 1.1 "Background": 207 The EXP field has been renamed to the CoS field, and thus all 208 references in RFC 5219 to EXP field SHOULD be taken to refer to 209 the CoS field. 211 Section 2 (bullet 3) on page 6 ofis now changed to: 213 o A third possible approach was suggested by [Shayman]. In this 214 scheme, interior LSRs assume that the endpoints are ECN-capable, 215 but this assumption is checked when the final label is popped. If 216 an interior LSR has marked ECN in the CoS field of the shim 217 header, but the IP header says the endpoints are not CoS-capable, 218 the edge router (or penultimate router, if using penultimate hop 219 popping) drops the packet. We recommend this scheme, which we 220 call `per-domain ECT checking', and define it more precisely in 221 the following section. Its chief drawback is that it can cause 222 packets to be forwarded after encountering congestion only to be 223 dropped at the egress of the MPLS domain. The rationale for this 224 decision is given in Section 8.1. This scheme is an update to RFC 225 3032 [RFC3032] and RFC 3270 [RFC3270]. 227 3. Use of the CoS field 229 Due to the limited number of bits the particular use of the bits is 230 intended to be flexible - including the definition of various QoS and 231 ECN functions. 233 Current implementations look at the CoS field with and without label 234 context and the CoS field may be copied to the labels that are pushed 235 onto the label stack. This is to avoid the pushed labels having a 236 different CoS field. 238 CoS and ECN funtions may rewrite all or some of the bits. 240 4. IANA considerations 242 There are no request for IANA allocation of code points in this 243 document. 245 5. Security considerations 247 This document only changes the name of one field in the MPLS Shim 248 Header and thus does not introduce any new security considerations. 250 6. Acknowledgments 252 The author would like to thank Stewart Bryant, Bruce Davie, George 253 Swallow, and Francois Le Faucheur for their input to and review of 254 the current document. 256 The author also like to thanks George Swallow, Khatri Paresh and Phil 257 Bedard for their help with grammar and spelling. 259 7. References 261 7.1. Normative References 263 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 264 Requirement Levels", BCP 14, RFC 2119, March 1997. 266 [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., 267 Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack 268 Encoding", RFC 3032, January 2001. 270 [RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen, 271 P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi- 272 Protocol Label Switching (MPLS) Support of Differentiated 273 Services", RFC 3270, May 2002. 275 [RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion 276 Marking in MPLS", RFC 5129, January 2008. 278 7.2. Informative references 280 [Shayman] Shayman, M. and R. Jaeger, University of Michigan, "Using 281 ECN to Signal Congestion Within an MPLS Domain", Work in 282 Progress, November 2000.", . 285 Author's Address 287 Loa Andersson 288 Acreo AB 290 Email: loa@pi.nu 292 Full Copyright Statement 294 Copyright (C) The IETF Trust (2008). 296 This document is subject to the rights, licenses and restrictions 297 contained in BCP 78, and except as set forth therein, the authors 298 retain all their rights. 300 This document and the information contained herein are provided on an 301 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 302 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 303 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 304 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 305 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 306 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 308 Intellectual Property 310 The IETF takes no position regarding the validity or scope of any 311 Intellectual Property Rights or other rights that might be claimed to 312 pertain to the implementation or use of the technology described in 313 this document or the extent to which any license under such rights 314 might or might not be available; nor does it represent that it has 315 made any independent effort to identify any such rights. 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