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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 6MAN S. Amante 3 Internet-Draft Level 3 4 Intended status: Informational B. Carpenter 5 Expires: November 3, 2011 Univ. of Auckland 6 S. Jiang 7 Huawei Technologies Co., Ltd 8 May 2, 2011 10 Rationale for update to the IPv6 flow label specification 11 draft-ietf-6man-flow-update-05 13 Abstract 15 Various published proposals for use of the IPv6 flow label are 16 incompatible with its original specification in RFC 3697. 17 Furthermore, very little practical use is made of the flow label, 18 partly due to some uncertainties about the correct interpretation of 19 the specification. This document discusses and motivates changes to 20 the specification in order to clarify it, and to introduce some 21 additional flexibility. 23 Status of this Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at http://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on November 3, 2011. 40 Copyright Notice 42 Copyright (c) 2011 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (http://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 58 2. Impact of current specification . . . . . . . . . . . . . . . 3 59 3. Changes to specification . . . . . . . . . . . . . . . . . . . 6 60 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 8 61 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 62 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 63 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 64 8. Change log [RFC Editor: please remove] . . . . . . . . . . . . 10 65 9. Informative References . . . . . . . . . . . . . . . . . . . . 10 66 Appendix A. Alternative Approaches . . . . . . . . . . . . . . . 11 67 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 69 1. Introduction 71 The flow label field in the IPv6 header was reserved but left 72 experimental by [RFC2460], which mandates only that "Hosts or routers 73 that do not support the functions of the Flow Label field are 74 required to set the field to zero when originating a packet, pass the 75 field on unchanged when forwarding a packet, and ignore the field 76 when receiving a packet." 78 The flow label field was normatively specified by [RFC3697]. In 79 particular, we quote three rules from that RFC: 80 a. "The Flow Label value set by the source MUST be delivered 81 unchanged to the destination node(s)." 82 b. "IPv6 nodes MUST NOT assume any mathematical or other properties 83 of the Flow Label values assigned by source nodes." 84 c. "Router performance SHOULD NOT be dependent on the distribution 85 of the Flow Label values. Especially, the Flow Label bits alone 86 make poor material for a hash key." 88 Additionally, RFC 3697 leaves it undefined what method a host should 89 adopt by default to choose the value of the flow label, if no 90 specific method is in use. It was expected that various signaling 91 methods might be defined for agreeing on values of the flow label, 92 but no such methods have been standardised, except a pre-existing 93 option in RSVP [RFC2205]. 95 The flow label is hardly used in practice in widespread IPv6 96 implementations, although some operating systems do set it 97 [McGann05]. To some extent this is due to the main focus being on 98 basic deployment of IPv6, but the absence of a default method of 99 choosing the flow label value means that most host implementations 100 simply set it to zero. There is also anecdotal evidence that the 101 rules quoted above have led to uncertainty about exactly what is 102 possible. Furthermore, various use cases have been proposed that 103 infringe one or another of the rules. None of these proposals has 104 been accepted as a standard and in practice there is no significant 105 deployment of any mechanism to set the flow label. 107 The intention of this document is to explain this situation in more 108 detail and to motivate changes to RFC 3697 intended to remove the 109 uncertainties and encourage active usage of the flow label. It does 110 not formally update RFC 3697, but it serves as background material 111 for [I-D.ietf-6man-flow-3697bis]. 113 2. Impact of current specification 115 Rule (a) makes it impossible for the routing system to use the flow 116 label as any form of dynamic routing tag. This was a conscious 117 choice in the early design of IPv6 and there appears to be no 118 practical possibility of revisiting this choice at this stage in the 119 deployment of IPv6, which uses conventional routing mechanisms like 120 those used for IPv4. However, this rule also makes it impossible to 121 make any use at all of the flow label unless hosts choose to set it. 122 It also forbids clearing the flow label for security reasons. 124 This last point highlights the security properties, or rather the 125 lack of them, of the flow label. The flow label field is always 126 unprotected as it travels through the network, because there is no 127 IPv6 header checksum, and the flow label is not included in transport 128 pseudo-header checksums, nor in IPsec checksums. As a result, 129 intentional and malicious changes to its value cannot be detected. 130 Also, it could be used as a covert data channel, since apparently 131 pseudo-random flow label values could in fact consist of covert data. 132 If the flow label were to carry quality of service semantics, then 133 like the diffserv code point [RFC2474], it would not be intrinsically 134 trustworthy across domain boundaries. As a result, some security 135 specialists believe that flow labels should be cleared for safety 136 [I-D.gont-6man-flowlabel-security]. These points must be considered 137 when discussing the immutability of the flow label across domain 138 boundaries. 140 Rule (b) appears to forbid any usage in which the bits of the flow 141 label are encoded with a specific semantic meaning. However, the 142 words "MUST NOT assume" are to be interpreted precisely - if a router 143 knows by configuration or by signaling that the flow label has been 144 assigned in a certain way, it can make use of that knowledge. It is 145 not made clear by the rule that there is an implied distinction 146 between stateless models (in which case no assumption may be made) 147 and stateful models (in which the router has explicit knowledge). 149 If the word "alone" is overlooked, rule (c) has sometimes been 150 interpreted to forbid the use of the flow label as part of a hash 151 used by load distribution mechanisms. In this case too, the word 152 "alone" is to be interpreted precisely - a router is allowed to 153 combine the flow label value with other data in order to produce a 154 uniformly distributed hash. 156 Both before and after these rules were laid down, a considerable 157 number of proposals for use of the flow label were published that 158 seem incompatible with them. Numerous examples and an analysis are 159 presented in [I-D.hu-flow-label-cases]. Those examples propose use 160 cases in which some or all of the following apply: 161 o The flow label may be changed by intermediate systems. 163 o It doesn't matter if the flow label is changed, because the 164 receiver doesn't use it. 165 o Some or all bits of the flow label are encoded: they have specific 166 meanings understood by routers and switches along the path. 167 o The encoding is related to the required quality of service, as 168 well as identifying a flow. 169 o The flow label is used to control forwarding or switching in some 170 way. 172 These proposals all require either some form of encoding of semantics 173 in the bits of the flow label, or the ability for routers to modify 174 the flow label, or both. Thus they appear to infringe the rules from 175 RFC 3697 quoted above. 177 We can conclude that a considerable number of researchers and 178 designers have been stymied by RFC 3697. On the other hand, some 179 other proposals discussed in [I-D.hu-flow-label-cases] appear to be 180 compatible with RFC 3697. Several are based on the originator of a 181 packet choosing a pseudo-random flow label for each flow, which is 182 one option suggested in RFC 3697. Thus, we can also conclude that 183 there is a useful role for this approach. 185 If our goal is for the flow label to be used in practice, the 186 conflict between the various approaches creates a dilemma. There 187 appear to be two major options: 188 1. Discourage locally defined and/or stateful use of the flow label. 189 Strengthen RFC 3697 to say that hosts should set a label value, 190 without necessarily creating state, which would clarify and limit 191 its possible uses. In particular, its use for load distribution 192 and balancing would be encouraged. 193 2. Relax the rules to encourage locally defined and/or stateful use 194 of the flow label. This approach would make the flow label 195 completely mutable and would exclude use cases depending on 196 strict end-to-end immutability. It would encourage applications 197 of a pseudo-random flow label, such as load distribution, on a 198 local basis, but it would exclude end-to-end applications. 200 During 2010/2011 there was considerable debate about these options 201 and variants of them, with a variety of proposals in previous 202 versions of this document and in mailing list discussions. After 203 these discussions, there appears to be a view that simplicity should 204 prevail, and that complicated proposals such as defining quality of 205 service semantics in the flow label, or sub-dividing the flow label 206 field into smaller sub-fields, will not prove efficient or 207 deployable, especially in high speed routers. There is also a 208 clearly expressed view that using the flow label for various forms of 209 stateless load distribution is the best simple application for it. 210 At the same time, it is necessary to recognize that the strict 211 immutability rule has drawbacks as noted above. 213 Even under the rules of RFC 3697, the flow label is intrinsically 214 untrustworthy, because modifications en route cannot be detected. 215 For this reason, even with the current strict immutability rule, 216 downstream nodes cannot rely mathematically on the value being 217 unchanged. In this sense, any use of the flow label must be viewed 218 as an optimisation on a best effort basis; a packet with a changed 219 (or zero) flow label value should never cause a hard failure. 221 The remainder of this document discusses specific modifications to 222 the standard, which are defined normatively in a companion document 223 [I-D.ietf-6man-flow-3697bis]. 225 3. Changes to specification 227 Although RFC 3697 requires the flow label to be delivered unchanged, 228 as noted above, it is not included in any transport layer pseudo- 229 header checksums nor in IPsec authentication [RFC4302]. Both RFC 230 2460 and RFC 3697 define the default flow label to be zero. At the 231 time of writing, this is the observed value in an overwhelming 232 proportion of IPv6 packets; the most widespread operating systems and 233 applications do not set it, and routers do not rely on it. Thus 234 there is no reason to expect operational difficulties if a careful 235 change is made to the rules of RFC 3697. 237 In particular, the facts that the label is not checksummed and rarely 238 used mean that the current strict immutability of the label can be 239 moderated without serious operational consequences. 241 The purposes of the proposed changes are to remove the uncertainties 242 left by RFC 3697, in order to encourage setting of the flow label by 243 default, and to enable its generic use. The proposed generic use is 244 to encourage uniformly distributed flow labels that can be used to 245 assist load distribution balancing. There should be no impact on 246 existing IETF specifications other than RFC 3697 and no impact on 247 currently operational software and hardware. 249 A secondary purpose is to modify the immutability of the flow label 250 in a limited way, to allow hosts that do not set the flow label to 251 benefit from it nevertheless. The fact that the flow label may in 252 practice be changed en route is also reflected in the reformulation 253 of the rules. 255 A general description of the changes follows. The normative text is 256 to be found in [I-D.ietf-6man-flow-3697bis]. 258 The definition of a flow is subtly changed from RFC 3697 to allow any 259 node, not just the source node, to set the flow label value. 260 However, it is recommended that sources should set a uniformly 261 distributed flow label value in all flows, replacing the less precise 262 recommendation made in Section 3 of RFC 3697. Both stateful and 263 stateless methods of assigning a uniformly distributed value could be 264 used. 266 Flow label values should be chosen such that their bits exhibit a 267 high degree of variability, making them suitable for use as part of 268 the input to a hash function used in a load distribution scheme. At 269 the same time, third parties should be unlikely to be able to guess 270 the next value that a source of flow labels will choose. 272 In statistics, a discrete uniform distribution is defined as a 273 probability distribution in which each value in a given range of 274 equally spaced values (such as a sequence of integers) is equally 275 likely to be chosen as the next value. The values in such a 276 distribution exhibit both variability and unguessability. Thus, an 277 approximation to a discrete uniform distribution is preferable as the 278 source of flow label values. In contrast, an implementation in which 279 flow labels are assigned sequentially is definitely not recommended. 281 In practice it is expected that a uniform distribution of flow label 282 values will be approximated by use of a hash function or a pseudo- 283 random number generator. Either approach will produce values which 284 will appear pseudo-random to an external observer. 286 Section 3 of RFC 3697 also allows nodes to participate in an 287 unspecified stateful method of flow state establishment. The changes 288 do not remove that option, but it is made clear that stateless models 289 are also possible and are the recommended default. The specific text 290 about requirements for stateful models has been reduced to a bare 291 minimum requirement that they do not interfere with the stateless 292 model. To enable stateless load distribution at any point in the 293 Internet, a network domain using a stateful model should never export 294 packets originating within the domain whose flow label values do not 295 conform to a uniform distribution. 297 The main novelty is that a forwarding node (typically a first-hop or 298 ingress router) may set the flow label value if the source has not 299 done so, according to the same recommendations that apply to the 300 source. This might place a considerable processing load on ingress 301 routers, even if they adopted a stateless method of flow 302 identification and label assignment. 304 The immutability of the flow label, once it has been set, is not 305 changed. However, some qualifications are placed on this property, 306 to allow for the fact that the flow label is an unprotected field and 307 might be changed undetectably. No Internet-wide mechanism can depend 308 mathematically on immutable flow labels. The new rules require that 309 flow labels exported to the Internet should always be either zero or 310 uniformly distributed, but even this cannot be relied on 311 mathematically. Use cases need to be robust against non-conforming 312 flow label values. This will also enhance compatibility with any 313 legacy hosts that set the flow label according to RFC 2460 or RFC 314 3697. 316 A complication that led to much discussion is the possibility that 317 hosts inside a particular domain might use a stateful method of 318 setting the flow label, and that packets bearing stateful labels 319 might then erroneously escape the domain and be received by nodes 320 performing stateless processing such as load balancing. This might 321 result in undesirable operational implications (e.g., congestion, 322 reordering) for not only the inappropriately flow-labelled packets, 323 but also well-behaved flow-labelled packets, during forwarding at 324 various intermediate devices. It was proposed to suggest that border 325 routers might "correct" this problem by overwriting such labels in 326 packets leaving the domain. However, neither domain border egress 327 routers nor intermediate routers/devices (using a flow label, for 328 example, as a part of an input key for a load-distribution hash) can 329 determine by inspection that a value is not part of a uniform 330 distribution. Therefore, there is no way that such values can be 331 detected and "corrected". 333 4. Discussion 335 The following are some practical consequences of the above changes: 336 o Sending hosts that are not updated will in practice continue to 337 send all-zero labels. If there is no label-setting router along 338 the path taken by a packet, the label will be delivered as zero. 339 o Sending hosts conforming to the new specification will by default 340 choose uniformly distributed labels between 1 and 0xFFFFF. 341 o Sending hosts may continue to send all-zero labels, in which case 342 an ingress router may set uniformly distributed labels between 1 343 and 0xFFFFF. 344 o The flow label is no longer unrealistically asserted to be 345 strictly immutable; it is recognised that it may, incorrectly, be 346 changed en route. In some circumstances this will break end-to- 347 end usage, e.g. potential detection of third-party spoofing 348 attacks [I-D.gont-6man-flowlabel-security]. 349 o The expected default usage of the flow label is some form of 350 stateless load distribution, such as the ECMP/LAG usage defined in 351 [I-D.carpenter-flow-ecmp]. 353 o If the new rules are followed, all IPv6 traffic flows on the 354 Internet should have zero or uniformly distributed flow label 355 values. 357 From an operational viewpoint, existing IPv6 hosts that set a default 358 (zero) flow label value and ignore the flow label on receipt will be 359 unaffected by implementations of the new specification. In general, 360 it is assumed that hosts will ignore the value of the flow label on 361 receipt; it cannot be relied on as an end-to-end signal. However, 362 this doesn't apply if a cryptographically generated label is being 363 used to detect attackers [I-D.gont-6man-flowlabel-security]. 365 Similarly, routers that ignore the flow label will be unaffected by 366 implementations of the specification. 368 Hosts that set a default (zero) flow label but are in a domain where 369 routers set a label as recommended in Section 3 will benefit from 370 whatever flow label handling is used on the path. 372 Hosts and routers that adopt the recommended mechanism will enhance 373 the performance of any load balancing devices that include the flow 374 label in the hash used to select a particular path or server, even 375 when packets leave the local domain. 377 5. Security Considerations 379 See [I-D.ietf-6man-flow-3697bis] and 380 [I-D.gont-6man-flowlabel-security] for full discussion. 382 6. IANA Considerations 384 This document requests no action by IANA. 386 7. Acknowledgements 388 The authors are grateful to Qinwen Hu for general discussion about 389 the flow label and for his work in searching the literature. 390 Valuable comments and contributions were made by Fred Baker, Steve 391 Blake, Remi Despres, Alan Ford, Fernando Gont, Brian Haberman, Tony 392 Hain, Joel Halpern, Chris Morrow, Thomas Narten, Pekka Savola, Mark 393 Smith, Pascal Thubert, Iljitsch van Beijnum, and other participants 394 in the 6man working group. 396 This document was produced using the xml2rfc tool [RFC2629]. 398 8. Change log [RFC Editor: please remove] 400 draft-ietf-6man-flow-update-05: updated again to be in step with RFC 401 3697bis, 2011-05-02 403 draft-ietf-6man-flow-update-04: updated again to be in step with RFC 404 3697bis, 2011-03-13 406 draft-ietf-6man-flow-update-03: updated to be in step with RFC 407 3697bis, 2011-02-26 409 draft-ietf-6man-flow-update-02: repurposed as rationale for update of 410 RFC 3697, 2011-01-31 412 draft-ietf-6man-flow-update-01: clarified that this is not a formal 413 update of RFC 3697, clarified text about domains exporting 414 inappropriate labels, 2011-01-10 416 draft-ietf-6man-flow-update-00: adopted as WG document at IETF 79, 417 mutability rules adjusted according to WG discussion, 2010-12-03 419 draft-carpenter-6man-flow-update-04: even more simplified according 420 to WG discussion, 2010-09-16 422 draft-carpenter-6man-flow-update-03: futher simplified according to 423 WG discussion, 2010-05-07 425 draft-carpenter-6man-flow-update-02: revised and simplified according 426 to WG discussion, 2010-04-13 428 draft-carpenter-6man-flow-update-01: revised according to mail list 429 discussion, 2010-03-05 431 draft-carpenter-6man-flow-update-00: original version, 2010-02-18 433 9. Informative References 435 [I-D.carpenter-flow-ecmp] 436 Carpenter, B. and S. Amante, "Using the IPv6 flow label 437 for equal cost multipath routing and link aggregation in 438 tunnels", draft-carpenter-flow-ecmp-03 (work in progress), 439 October 2010. 441 [I-D.gont-6man-flowlabel-security] 442 Gont, F., "Security Assessment of the IPv6 Flow Label", 443 draft-gont-6man-flowlabel-security-01 (work in progress), 444 November 2010. 446 [I-D.hu-flow-label-cases] 447 Hu, Q. and B. Carpenter, "Survey of proposed use cases for 448 the IPv6 flow label", draft-hu-flow-label-cases-03 (work 449 in progress), February 2011. 451 [I-D.ietf-6man-flow-3697bis] 452 Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme, 453 "IPv6 Flow Label Specification", 454 draft-ietf-6man-flow-3697bis-02 (work in progress), 455 March 2011. 457 [I-D.martinbeckman-ietf-ipv6-fls-ipv6flowswitching] 458 Beckman, M., "IPv6 Dynamic Flow Label Switching (FLS)", 459 draft-martinbeckman-ietf-ipv6-fls-ipv6flowswitching-03 460 (work in progress), March 2007. 462 [McGann05] 463 McGann, O. and D. Malone, "Flow Label Filtering 464 Feasibility", European Conference on Computer Network 465 Defence , 2005. 467 [RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S. 468 Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1 469 Functional Specification", RFC 2205, September 1997. 471 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 472 (IPv6) Specification", RFC 2460, December 1998. 474 [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, 475 "Definition of the Differentiated Services Field (DS 476 Field) in the IPv4 and IPv6 Headers", RFC 2474, 477 December 1998. 479 [RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629, 480 June 1999. 482 [RFC3697] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering, 483 "IPv6 Flow Label Specification", RFC 3697, March 2004. 485 [RFC4302] Kent, S., "IP Authentication Header", RFC 4302, 486 December 2005. 488 Appendix A. Alternative Approaches 490 A model was discussed in an earlier version of this document which 491 defined a notion of 'flow label domain' analogous to a differentiated 492 services domain [RFC2474]. This model would have encouraged local 493 usage of the flow label as an alternative to any form of generic use, 494 but it required complex rules for the behaviour of domain boundary 495 routers, and proved controversial in discussion. 497 Two even more complex alternative approaches were also considered and 498 rejected. 500 The first was to distinguish locally significant flow labels from 501 those conforming to RFC 3697 by setting or clearing the most 502 significant bit (MSB) of the flow label. This led to quite 503 complicated rules, seems impossible to make fully self-consistent, 504 and was not considered practical. 506 The second was to use a specific differentiated services code point 507 (DSCP)[RFC2474] in the Traffic Class octet instead of the MSB of the 508 flow label itself, to flag a locally defined behaviour. A more 509 elaborate version of this was proposed in 510 [I-D.martinbeckman-ietf-ipv6-fls-ipv6flowswitching]. There are two 511 issues with this approach. One is that DSCP values are themselves 512 only locally significant, inconsistent with the end-to-end nature of 513 the original flow label definition. Secondly, it seems unwise to 514 meld the semantics of differentiated services, which are currently 515 deployed, with the unknown future semantics of flow label usage. 516 However, this approach, while not recommended, does not appear to 517 violate any basic principles if applied strictly within a single 518 differentiated services domain. 520 Authors' Addresses 522 Shane Amante 523 Level 3 Communications, LLC 524 1025 Eldorado Blvd 525 Broomfield, CO 80021 526 USA 528 Email: shane@level3.net 530 Brian Carpenter 531 Department of Computer Science 532 University of Auckland 533 PB 92019 534 Auckland, 1142 535 New Zealand 537 Email: brian.e.carpenter@gmail.com 538 Sheng Jiang 539 Huawei Technologies Co., Ltd 540 Huawei Building, No.3 Xinxi Rd., 541 Shang-Di Information Industry Base, Hai-Dian District, Beijing 542 P.R. China 544 Email: shengjiang@huawei.com