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Briscoe 5 Expires: January 12, 2009 BT & UCL 6 M. Menth 7 University of Wuerzburg 8 July 11, 2008 10 Baseline Encoding and Transport of Pre-Congestion Information 11 draft-moncaster-pcn-baseline-encoding-02 13 Status of this Memo 15 By submitting this Internet-Draft, each author represents that any 16 applicable patent or other IPR claims of which he or she is aware 17 have been or will be disclosed, and any of which he or she becomes 18 aware will be disclosed, in accordance with Section 6 of BCP 79. 20 Internet-Drafts are working documents of the Internet Engineering 21 Task Force (IETF), its areas, and its working groups. Note that 22 other groups may also distribute working documents as Internet- 23 Drafts. 25 Internet-Drafts are draft documents valid for a maximum of six months 26 and may be updated, replaced, or obsoleted by other documents at any 27 time. It is inappropriate to use Internet-Drafts as reference 28 material or to cite them other than as "work in progress." 30 The list of current Internet-Drafts can be accessed at 31 http://www.ietf.org/ietf/1id-abstracts.txt. 33 The list of Internet-Draft Shadow Directories can be accessed at 34 http://www.ietf.org/shadow.html. 36 This Internet-Draft will expire on January 12, 2009. 38 Copyright Notice 40 Copyright (C) The IETF Trust (2008). 42 Abstract 44 Pre-congestion notification (PCN) provides information to support 45 admission control and flow termination in order to protect the 46 Quality of Service of inelastic flows. It does this by marking 47 packets when traffic load on a link is approaching or has exceeded a 48 threshold below the physical link rate. This document specifies how 49 such marks are to be encoded into the IP header. The baseline 50 encoding described here provides for only two PCN encoding states. 51 Other documents describe extended encoding schemes that allow for 52 three encoding states. 54 Status 56 This memo is posted as an Internet-Draft with an intent to eventually 57 progress to standards track. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 62 2. Requirements notation . . . . . . . . . . . . . . . . . . . . 3 63 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 64 4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 4 65 4.1. Rationale for Encoding . . . . . . . . . . . . . . . . . . 5 66 4.2. PCN-Enabled DiffServ Codepoints . . . . . . . . . . . . . 5 67 4.2.1. Implications of re-using a DiffServ Codepoint . . . . 6 68 4.3. Valid and Invalid Encoding Transitions at a PCN Node . . . 6 69 5. Backwards Compatability . . . . . . . . . . . . . . . . . . . 7 70 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 71 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 72 8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 8 73 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 74 10. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 8 75 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 76 11.1. Normative References . . . . . . . . . . . . . . . . . . . 8 77 11.2. Informative References . . . . . . . . . . . . . . . . . . 9 78 Appendix A. Tunnelling Constraints . . . . . . . . . . . . . . . 9 79 Appendix B. Deployment Scenarios for PCN Using Baseline 80 Encoding . . . . . . . . . . . . . . . . . . . . . . 10 81 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 82 Intellectual Property and Copyright Statements . . . . . . . . . . 13 84 1. Introduction 86 Pre-congestion notification (PCN) provides information to support 87 admission control and flow termination in order to protect the 88 quality of service (QoS) of inelastic flows. This is achieved by 89 marking packets according to the level of pre-congestion at nodes 90 within the PCN-domain. Two algorithms exist for that purpose. 91 Excess traffic marking marks all PCN packets exceeding a certain 92 reference rate on a link while threshold marking marks all PCN 93 packets on a link when the PCN traffic rate exceeds the reference 94 rate. These markings are evaluated by the egress nodes of the PCN- 95 domain. [PCN-arch] describes how PCN packet markings can be used to 96 assure the QoS of inelastic flows within a single DiffServ domain. 98 This document specifies how these PCN marks are encoded into the IP 99 header. It also describes how packets are identified as belonging to 100 a PCN flow. Some deployment models require two PCN encoding states, 101 others require three. The baseline encoding described here only 102 provides for two PCN encoding states. An extended encoding described 103 in [PCN-3-enc-state] provides for three PCN encoding states. 105 Changes from previous drafts (to be removed by the RFC Editor) 107 From -01 to -02: 109 Minor changes throughout including tightening up language to 110 remain consistent with the PCN Architecture terminology 112 From -00 to -01: 114 Change of title from "Encoding and Transport of (Pre-)Congestion 115 Information from within a DiffServ Domain to the Egress" 117 Extensive changes to Introduction and abstract. 119 Added a section on the implications of re-using a DSCP. 121 Added appendix listing possible operator scenarios for using this 122 baseline encoding. 124 Minor changes throughout. 126 2. Requirements notation 128 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 129 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 130 document are to be interpreted as described in [RFC2119]. 132 3. Terminology 134 The following terms are used in this document: 136 o not-PCN - packets that are not PCN capable. 138 o PCN-marked - codepoint indicating packets that have been marked at 139 a PCN-interior-node using some PCN marking behaviour. Also PM. 141 o not-Marked - codepoint indicating packets that are PCN capable but 142 are not PCN-marked. Also NM. 144 o PCN-Capable codepoints - collective term for all the NM and PM 145 codepoints. 147 o PCN-enabled Diffserv codepoint - a Diffserv codepoint for which 148 PCN has been enabled on a particular machine. 150 In addition the document uses the terminology defined in [PCN-arch]. 152 4. Encoding two PCN States in IP 154 The PCN encoding states are defined using the Type of Service field 155 of the IP header which is a combination of the DSCP field and ECN 156 field. The baseline PCN encoding closely follows the semantics of 157 ECN [RFC3168]. It allows the encoding of two PCN states: Not Marked 158 and PCN-Marked. It also allows for traffic that is not PCN capable 159 to be marked as such (not-PCN). The following table defines how to 160 encode these states in IP: 162 +--------+--------------+-------------+-------------+---------+ 163 | DSCP | not-ECT (00) | ECT(0) (10) | ECT(1) (01) | CE (11) | 164 +--------+--------------+-------------+-------------+---------+ 165 | DSCP n | not-PCN | NM | NM | PM | 166 +--------+--------------+-------------+-------------+---------+ 168 Where DSCP n is a PCN-enabled DiffServ codepoint (see Section 4.2) 170 Table 1: Encoding PCN in IP 172 The following rules apply to all PCN traffic: 174 o PCN traffic MUST be marked with a PCN-enabled DiffServ Codepoint. 175 That is a DiffServ codepoint that indicates that PCN is enabled. 176 To conserve DSCPs, DiffServ Codepoints SHOULD be chosen that are 177 already defined for use with admission controlled traffic, such as 178 the Voice-Admit codepoint defined in [voice-admit]. 180 o Any packet that is not PCN capable (not-PCN) but which shares the 181 same DiffServ codepoint as PCN capable traffic MUST have the ECN 182 field set to 00. 184 o Any packet that belongs to a PCN capable flow MUST have the ECN 185 field set to one of the two ECT codepoints 10 or 01 at the PCN- 186 ingress-node. 188 o Any packet that is PCN capable and has been PCN-marked by a PCN- 189 interior-node MUST have the ECN field set to 11. 191 4.1. Rationale for Encoding 193 The exact choice of encoding was dictated by the constraints imposed 194 by existing IETF RFCs, in particular [RFC3168] and [RFC4774]. Full 195 details are contained in [pcn-enc-compare]. One of the tightest 196 constraints was the need for any PCN encoding to survive being 197 tunnelled through either an IP in IP tunnel or an IPSec Tunnel. 198 Appendix A explains this in detail. The main effect of this 199 constraint is that any PCN marking has to use the ECN field set to 11 200 (CE codepoint). If the packet is being tunneled then only the CE 201 codepoint gets copied into the inner header upon decapsulation. An 202 additional constraint is the need to minimise the use of DiffServ 203 codepoints as these are in increasingly short supply. Section 4.2 204 explains how we have minimised this still further by reusing pre- 205 existing Diffserv codepoint(s) such that non-PCN traffic can still be 206 distinguished from PCN traffic. 208 The encoding scheme (Table 1) that best addresses the above 209 constraints ends up looking very similar to ECN. This is perhaps not 210 surprising given the similarity in architectural intent between PCN 211 and ECN. 213 4.2. PCN-Enabled DiffServ Codepoints 215 Equipment complying with the baseline PCN encoding MUST allow PCN to 216 be enabled for a certain Diffserv codepoint or codepoints. This 217 document defines the term "PCN-Enabled Diffserv Codepoint" for such a 218 DSCP. Enabling PCN for a DSCP switches on PCN marking behaviour for 219 packets with that DSCP, but only if those packets also have their ECN 220 field set to indicate a codepoint other than not-PCN. 222 Enabling PCN marking behaviour disables any other marking behaviour 223 (e.g. enabling PCN disables the default ECN marking behaviour 224 introduced in [RFC3168]). The scheduling behaviour used for a packet 225 does not change whether PCN is enabled for a DSCP or not and whatever 226 the setting of the ECN field. 228 4.2.1. Implications of re-using a DiffServ Codepoint 230 [RFC4774] requires that packets for which alternate ECN semantics 231 (PCN semantics) are used are clearly distinguished from packets to 232 which the default ECN semantics [RFC3168] apply. One means of doing 233 this is using a DSCP to indicate that the ECN field is to be 234 interpreted in a different manner. We have chosen to use this 235 approach for PCN. Non-PCN-enabled forwarding nodes treat packets 236 with a PCN-enabled DSCP like ECN traffic if appropriate ECN 237 codepoints are set in the IP header. This has several consequences. 239 o Care must be taken to ensure that forwarding nodes do not 240 interpret PCN encodings as ECN encodings, and that no harm is done 241 if this were to happen. To that end, appropriate marking and re- 242 marking is performed at the ingress and the egress of a PCN- 243 domain. 245 o The re-used DSCP should be able to serve its original purpose 246 which was not PCN support. This is achieved by marking the 247 packets of such flows with a not-PCN codepoint. 249 o The scheduling behaviour is coupled with the DSCP only. 250 Therefore, the same scheduling and buffer management rules are 251 applied for non-PCN-capable and PCN-capable traffic using the same 252 PCN-enabled DSCP. 254 o Once the ECN field of a packet is used for PCN encoding, it has 255 lost its previous information unless this information is tunnelled 256 through the PCN domain. Therefore, the baseline PCN encoding 257 disables ECN for PCN-enabled DSCPs. [PCN-3-enc-state] provides 258 end-to-end ECN support where this is needed. 260 4.3. Valid and Invalid Encoding Transitions at a PCN Node 262 PCN-boundary-node behaviour compliant with the PCN baseline encoding: 264 o Any packet with the ECN field already marked as CE or ECT arriving 265 at a PCN-ingress-node SHOULD be dropped or downgraded to a lower 266 class of service. Alternatively it MAY be tunnelled through the 267 PCN-domain. It MUST NOT be admitted to the PCN-domain directly. 269 o On leaving the PCN-domain the ECN bits of every PCN-packet MUST be 270 set to 00 (not-ECT). 272 PCN-interior-node behaviour compliant with the PCN baseline encoding: 274 o PCN-interior-nodes MUST NOT change not-PCN to another codepoint 275 and they MUST NOT change a PCN-Capable codepoint to not-PCN. 277 o PCN-interior-nodes that are in a pre-congestion state above the 278 configured level MUST set the PM codepoint by changing the ECN 279 bits of NM marked packets to 11. 281 o The PM codepoint MUST NOT be changed to NM. 283 5. Backwards Compatability 285 BCP 124 [RFC4774] gives guidelines for specifying alternative 286 semantics for the ECN field. It sets out a number of factors that 287 must be taken into consideration. It also suggests various 288 techniques to allow the co-existence of default ECN and alternative 289 ECN semantics. The alternative semantics specified here are 290 compliant with this BCP: 292 o they use a DSCP to allow routers to distinguish that traffic uses 293 the alternate ECN semantics; 295 o these semantics are defined for use within a controlled domain; 297 o ECN marked traffic is blocked from entering the PCN-domain 298 directly (though it might be tunnelled through the PCN-domain). 300 o All traffic leaving the controlled domain is re-marked as not-ECT. 302 6. IANA Considerations 304 This document makes no request to IANA. It does however suggest a 305 change to the default ([RFC3168]) behaviour for the ECN field for the 306 Voice-Admit [voice-admit] DSCP. 308 7. Security Considerations 310 Packets claim entitlement to be PCN marked by carrying a PCN-enabled 311 DSCP and a PCN-Capable ECN codepoint. This encoding document is 312 intended to stand independently of the architecture used to determine 313 whether specific packets are authorised to be PCN marked, which will 314 be described in a future separate document on PCN edge-node 315 behaviour. The PCN working group has initially been chartered to 316 only consider a PCN-domain to be entirely under the control of one 317 operator, or a set of operators who trust each other [PCN-charter]. 318 However there is a requirement to keep inter-domain scenarios in mind 319 when defining the PCN encoding. One way to extend to multiple 320 domains would be to concatenate PCN-domains and use PCN-boundary- 321 nodes back to back at borders. Then any one domain's security 322 against its neighbours would be described as part of the edge-node 323 behaviour document as above. One proposal on the table allows one to 324 extend PCN across multiple domains without PCN-boundary-nodes back- 325 to-back at borders [re-PCN]. It is believed that the encoding 326 described here would be compatible with the security framework 327 described there. 329 8. Conclusions 331 This document defines the baseline PCN encoding utilising a 332 combination of a PCN-enabled DSCP and the ECN field in the IP header. 333 This baseline encoding allows the existence of two PCN encoding 334 states, not-Marked and PCN-Marked. It also allows for the co- 335 existence of traffic that is not PCN-capable within the same DSCP so 336 long as theat traffic doesn't require end-to-end ECN support. The 337 encoding scheme is conformant with [RFC4774]. 339 9. Acknowledgements 341 This document builds extensively on work done in the PCN working 342 group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley and 343 others. Full details of the alternative schemes that were considered 344 for adoption can be found in the document [pcn-enc-compare]. Thanks 345 to Ruediger Geib for providing detailed comments on this document. 347 10. Comments Solicited 349 Comments and questions are encouraged and very welcome. They can be 350 addressed to the IETF congestion and pre-congestion working group 351 mailing list , and/or to the authors. 353 11. References 355 11.1. Normative References 357 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 358 Requirement Levels", BCP 14, RFC 2119, March 1997. 360 [RFC4774] Floyd, S., "Specifying Alternate Semantics for the 361 Explicit Congestion Notification (ECN) Field", BCP 124, 362 RFC 4774, November 2006. 364 11.2. Informative References 366 [PCN-3-enc-state] 367 Moncaster, T., Briscoe, B., and M. Menth, "A three state 368 extended PCN encoding scheme", 369 draft-moncaster-pcn-3-state-encoding-00 (work in 370 progress), June 2008. 372 [PCN-arch] 373 Eardley, P., "Pre-Congestion Notification Architecture", 374 draft-ietf-pcn-architecture-03 (work in progress), 375 February 2008. 377 [PCN-charter] 378 "IETF Charter for Congestion and Pre-Congestion 379 Notification Working Group". 381 [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition 382 of Explicit Congestion Notification (ECN) to IP", 383 RFC 3168, September 2001. 385 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 386 Internet Protocol", RFC 4301, December 2005. 388 [pcn-enc-compare] 389 Chan, K., Karagiannis, G., Moncaster, T., Menth, M., 390 Eardley, P., and B. Briscoe, "Pre-Congestion Notification 391 Encoding Comparison", 392 draft-chan-pcn-encoding-comparison-03 (work in progress), 393 February 2008. 395 [re-PCN] Briscoe, B., "Emulating Border Flow Policing using Re-ECN 396 on Bulk Data", draft-briscoe-re-pcn-border-cheat-00 (work 397 in progress), July 2007. 399 [voice-admit] 400 Baker, F., Polk, J., and M. Dolly, "DSCPs for Capacity- 401 Admitted Traffic", 402 draft-ietf-tsvwg-admitted-realtime-dscp-04 (work in 403 progress), February 2008. 405 Appendix A. Tunnelling Constraints 407 The rules that govern the behaviour of the ECN field for IP-in-IP 408 tunnels were defined in [RFC3168]. This allowed for two tunnel 409 modes. The limited functionality mode sets the outer header to not- 410 ECT, regardless of the value of the inner header, in other words 411 disabling ECN within the tunnel. The full functionality mode copies 412 the inner ECN field into the outer header if the inner header is not- 413 ECT or either of the 2 ECT codepoints. If the inner header is CE 414 then the outer header is set to ECT(0). On decapsulation, if the CE 415 codepoint is set on the outer header then this is copied into the 416 inner header. Otherwise the inner header is left unchanged. The 417 stated reason for blocking CE from being copied to the outer header 418 was to prevent this from being used as a covert channel through IPSec 419 tunnels. 421 The IPSec protocol [RFC4301] changed the ECN tunnelling rule to allow 422 IPSec tunnels to simply copy the inner header into the outer header. 423 On decapsulation the outer header is discarded and the ECN field is 424 only copied down if it is set to CE. 426 Because of the possible existence of tunnels, only CE (11) can be 427 used as a PCN marking as it is the only mark that will survive 428 decapsulation. However there is a need for caution with all 429 tunneling within the PCN-domain. RFC3168 full functionality IP in IP 430 tunnels are expected to set the ECN field to ECT(0) if the inner ECN 431 field is set to CE. This leads to the possibility that some packets 432 within the PCN-domain that have already been marked may have that 433 mark concealed further into the domain. This is undesirable for many 434 PCN schemes and thus standard IP in IP tunnels SHOULD NOT be used 435 within a PCN-domain. Further work is needed within the Transport 436 Area to rationalise the behaviour of tunnels in respect to the ECN 437 field. 439 Appendix B. Deployment Scenarios for PCN Using Baseline Encoding 441 This appendix illustrates possible PCN deployment scenarios where the 442 baseline encoding can be used and also explain a case for which 443 baseline encoding is not sufficient. {Note this appendix is provided 444 for information only}. 446 1. An operator may wish to use PCN-based admission control only. To 447 that end, threshold marking based on admissible rates might be 448 used as the only PCN metering and marking algorithm. As a 449 consequence, the PM marks on the packets are interpreted as 450 admission-stop (AS) marks. The admission-control algorithm is 451 based on "admissible-rate overload". 453 2. An operator may wish to use PCN-based flow termination only. To 454 that end, excess rate marking based on supportable rates might be 455 used as the only PCN metering and marking algorithm. As a 456 consequence, the PM marks on the packets are interpreted as 457 excess-traffic (ET) marks. The flow termination algorithm is 458 based on "supportable-rate overload". 460 3. An operator may wish to use both PCN-based admission control and 461 flow termination. To that end, excess rate marking based on 462 admissible rates may be used as the only PCN metering and marking 463 algorithm. As a consequence, the PM marks on the packets are 464 interpreted as admission-stop (AS) marks. Both the admission 465 control and the flow termination algorithm are based on 466 "admissible-rate overload". 468 4. An operator may wish to implement admission control based on 469 threshold marking at admissible rates and flow termination based 470 on excess rate marking at supportable rates because these methods 471 are believed to work better with small ingress-egress aggregates. 472 Then two different markings are needed. Such a deployment 473 scenario is not supported by the PCN baseline encoding. 475 Authors' Addresses 477 Toby Moncaster 478 BT 479 B54/70, Adastral Park 480 Martlesham Heath 481 Ipswich IP5 3RE 482 UK 484 Phone: +44 1473 648734 485 Email: toby.moncaster@bt.com 486 URI: http://www.cs.ucl.ac.uk/staff/B.Briscoe/ 488 Bob Briscoe 489 BT & UCL 490 B54/77, Adastral Park 491 Martlesham Heath 492 Ipswich IP5 3RE 493 UK 495 Phone: +44 1473 645196 496 Email: bob.briscoe@bt.com 497 Michael Menth 498 University of Wuerzburg 499 room B206, Institute of Computer Science 500 Am Hubland 501 Wuerzburg D-97074 502 Germany 504 Phone: +49 931 888 6644 505 Email: menth@informatik.uni-wuerzburg.de 507 Full Copyright Statement 509 Copyright (C) The IETF Trust (2008). 511 This document is subject to the rights, licenses and restrictions 512 contained in BCP 78, and except as set forth therein, the authors 513 retain all their rights. 515 This document and the information contained herein are provided on an 516 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 517 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 518 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 519 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 520 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 521 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 523 Intellectual Property 525 The IETF takes no position regarding the validity or scope of any 526 Intellectual Property Rights or other rights that might be claimed to 527 pertain to the implementation or use of the technology described in 528 this document or the extent to which any license under such rights 529 might or might not be available; nor does it represent that it has 530 made any independent effort to identify any such rights. 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