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Yourtchenko 5 Intended status: Standards Track Cisco 6 Expires: May 4, 2017 S. Krishnan 7 Ericsson 8 October 31, 2016 10 IPv6 Neighbor Discovery Optional RS/RA Refresh 11 draft-ietf-6man-rs-refresh-02 13 Abstract 15 IPv6 Neighbor Discovery relies on periodic multicast Router 16 Advertisement messages to update timer values and to distribute new 17 information (such as new prefixes) to hosts. On some links the use 18 of periodic multicast messages to all host becomes expensive, and in 19 some cases it results in hosts waking up frequently. Many 20 implementations of RFC 4861 also use multicast for solicited Router 21 Advertisement messages, even though that behavior is optional. 23 This specification provides an optional mechanism for hosts and 24 routers where instead of periodic multicast Router Advertisements the 25 hosts are instructed (by the routers) to use Router Solicitations to 26 request refreshed Router Advertisements. This mechanism is enabled 27 by configuring the router to include a new option in the Router 28 Advertisement in order to allow the network administrator to choose 29 host behavior based on whether periodic multicast are more efficient 30 on their link or not. The routers can also tell whether the hosts 31 are capable of the new behavior through a new flag in the Router 32 Solicitations. 34 Status of This Memo 36 This Internet-Draft is submitted in full conformance with the 37 provisions of BCP 78 and BCP 79. 39 Internet-Drafts are working documents of the Internet Engineering 40 Task Force (IETF). Note that other groups may also distribute 41 working documents as Internet-Drafts. The list of current Internet- 42 Drafts is at http://datatracker.ietf.org/drafts/current/. 44 Internet-Drafts are draft documents valid for a maximum of six months 45 and may be updated, replaced, or obsoleted by other documents at any 46 time. It is inappropriate to use Internet-Drafts as reference 47 material or to cite them other than as "work in progress." 48 This Internet-Draft will expire on May 4, 2017. 50 Copyright Notice 52 Copyright (c) 2016 IETF Trust and the persons identified as the 53 document authors. All rights reserved. 55 This document is subject to BCP 78 and the IETF Trust's Legal 56 Provisions Relating to IETF Documents 57 (http://trustee.ietf.org/license-info) in effect on the date of 58 publication of this document. Please review these documents 59 carefully, as they describe your rights and restrictions with respect 60 to this document. Code Components extracted from this document must 61 include Simplified BSD License text as described in Section 4.e of 62 the Trust Legal Provisions and are provided without warranty as 63 described in the Simplified BSD License. 65 Table of Contents 67 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 68 2. Goals and Requirements . . . . . . . . . . . . . . . . . . . 4 69 3. Definition Of Terms . . . . . . . . . . . . . . . . . . . . . 4 70 4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4 71 5. New Neighbor Discovery Flags and Options . . . . . . . . . . 5 72 5.1. Introducing a Router Solicitation Flag . . . . . . . . . 5 73 5.2. Refresh Time option . . . . . . . . . . . . . . . . . . . 6 74 6. Conceptual Data Structures . . . . . . . . . . . . . . . . . 6 75 7. Host Behavior . . . . . . . . . . . . . . . . . . . . . . . . 7 76 7.1. Sleep and Wakeup . . . . . . . . . . . . . . . . . . . . 8 77 7.2. Movement . . . . . . . . . . . . . . . . . . . . . . . . 8 78 8. Router Behavior . . . . . . . . . . . . . . . . . . . . . . . 8 79 8.1. Router and/or Interface Initialization . . . . . . . . . 9 80 8.2. Periodic Multicast RA for unmodified hosts . . . . . . . 9 81 8.3. Unsolicited RAs to share new information . . . . . . . . 9 82 9. Router Advertisement Consistency . . . . . . . . . . . . . . 10 83 10. Security Considerations . . . . . . . . . . . . . . . . . . . 10 84 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 85 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 86 13. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 10 87 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 88 14.1. Normative References . . . . . . . . . . . . . . . . . . 11 89 14.2. Informative References . . . . . . . . . . . . . . . . . 11 90 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 92 1. Introduction 94 IPv6 Neighbor Discovery [RFC4861] was defined at a time when local 95 area networks had different properties than today. A common link was 96 the yellow-coax shared wire Ethernet, where a link-layer multicast 97 and unicast worked the same - send the packet on the wire and the 98 interested receivers will pick it up. Thus the network cost 99 (ignoring any processing cost on the receivers that might not 100 completely filter out Ethernet multicast addresses that they did not 101 want) and the reliability of sending a link-layer unicast and 102 multicast was the same. Furthermore, the hosts at the time was 103 always on and connected. Powering on and off the workstation/PC 104 hosts at the time was slow and disruptive process. 106 Under the above assumptions it was quite efficient to maintain the 107 shared state of the link such as the prefixes and their lifetimes 108 using periodic multicast Router Advertisement messages. It was also 109 efficient to use multicast Neighbor Solicitations for address 110 resolution as a slight improvement over the broadcast use in ARP. 111 And finally, checking for a potential duplicate IPv6 address using 112 multicast was efficient and natural. 114 There are still links, such a satellite links, where periodic 115 multicast advertisements is the most efficient and reliable approach 116 to keep the hosts up to date. However other links have different 117 performance and reliability for multicast than for unicast (see for 118 instance [I-D.vyncke-6man-mcast-not-efficient] which discusses WiFi 119 links). On some of those links the performance and reliability is 120 dependent on the direction e.g., with host to network multicast 121 having the same characteristics as unicast, but network to host being 122 different. Cellular networks which employ paging and support 123 sleeping hosts have different issues (see e.g., 124 [I-D.garneij-6man-nd-m2m-issues] that would benefit from having the 125 hosts wake up and request information from the routers instead of the 126 routers periodically multicasting the information. 128 Since different links types and deployments have different needs, 129 this specification provides mechanism by which the routers can 130 determine whether all the hosts support the RS refresh, and the hosts 131 only employ the RS refresh when instructed by the routers using an 132 option in the Router Advertisement. 134 The operator retains the option to use unsolicited multicast Router 135 Advertisement to announce new or removed information. That can be 136 useful for uncommon cases while allowing using a higher refresh time 137 for normal network operations. 139 Hosts that sleep without waking up due to multicast Router 140 Advertisements need to send a RS refresh when they wake up in order 141 to receive configuration changes that took place while the host was 142 sleeping. 144 The specification does not assume that all hosts on the link 145 implement the new capability. As soon as there are router(s) on a 146 link which supports these optimizations, then the updated hosts on 147 the link can sleep better, while co-existing on the same link with 148 unmodified hosts. 150 2. Goals and Requirements 152 The key goal is to allow the operator to choose whether RS refresh is 153 more efficient than periodic multicast RAs, while preserving the 154 timely and scalable reconfiguration capabilities that a periodic RA 155 model provides. 157 The approach should allow for hosts that sleep on a schedule i.e., 158 that do not wake up due to unsolicited RA messages. 160 In general a link can have multiple routers hence the RS messages 161 should be multicast to find new routers. But for networks which do 162 not there operator should be able to choose unicast RS behavior. 164 In addition, an operator might want to be notified whether the link 165 includes hosts that do not support the new mechanism. Potential 166 router implementations can react dynamically to that information, or 167 can log events to system management when hosts appear which do not 168 implement this new capability. 170 The assumption is that host which implement this specification also 171 implement [I-D.ietf-6man-resilient-rs] as that ensures resiliency to 172 packet loss. 174 3. Definition Of Terms 176 The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 177 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 178 document are to be interpreted as described in [RFC2119]. 180 4. Protocol Overview 182 The hosts include a new flag in the Router Solicitation message, 183 which allows the routers to report to system management whether there 184 are hosts that do not support the RS refresh on the link. 186 If the network administrator has configured the routers to send the 187 new Refresh Time option, then the option will be included in all the 188 Router Advertisements. This option includes the time interval when 189 the hosts should send Router Solicitations refresh messages. 191 The host maintains the value of the Refresh Time option (RTO) by 192 recording it in the default router list. A value of zero can be used 193 to indicate that a router did not include a Refresh Time option. 195 The host calculates a timeout after it has received a RTO - either 196 per router or per link. If it is maintained per link then the host 197 SHOULD use the minimum Refresh Time it has received from the routers 198 on the link. The timeout is a random value uniformly distributed 199 between 0.5 and 1.5 times the Refresh Time value (in order to avoid 200 synchronization of the timers across hosts [SYNC].) When this timer 201 fires the host sends one Router Solicitation. 203 5. New Neighbor Discovery Flags and Options 205 This specification introduces a new option used in the RAs which both 206 indicates that the router can handle RS refresh by immediately 207 responding with a unicast RA, and a flag for the RS that indicates to 208 the router that the host will do RS refresh if the router so wishes. 210 5.1. Introducing a Router Solicitation Flag 212 A node which implements this specification sets the R flag in all the 213 Router Solicitation messages it sends. That allows the router to 214 determine whether there are legacy hosts on the link. 216 0 1 2 3 217 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 218 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 219 | Type | Code | Checksum | 220 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 221 |R| Reserved | 222 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 224 New fields: 226 R-flag: When set indicates that the sending node is capable of 227 doing unicast RS refresh. 229 Reserved: Field is reduced from 32 bits to 31 bits. It MUST be 230 initialized to zero by the sender and MUST be ignored 231 by the receiver. 233 5.2. Refresh Time option 235 A router which implements this specification can be configured to 236 instruct hosts to use RS refresh. When the operator configures this 237 mode of operation, then the router MUST include this new option in 238 the RA. If the operator has a single router (or single VRRP router) 239 on the link, then the operator MAY set the Unicast flag in the 240 option. 242 0 1 2 3 243 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 244 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 245 | Type | Length=1 | Refresh Time | 246 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 247 |U| Reserved | 248 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 250 Fields: 252 Type: TBD ND option code value (IANA) 254 Length: 8-bit unsigned integer. The length of the option 255 (including the type and length fields) in units of 8 256 bytes. The value 0 is invalid. Value is 1 for this 257 option. 259 Refresh Time: 16-bit unsigned integer. Units is seconds. The value 260 zero is invalid and make the receiver ignore the 261 option. 263 U-flag: 1 bit flag to indicate that the host should unicast 264 the RS refresh. 266 Reserved: 31 bits. This field is unused. It MUST be 267 initialized to zero by the sender and MUST be ignored 268 by the receiver. 270 6. Conceptual Data Structures 272 In addition to the Conceptual Data structures in [RFC4861] a host 273 records the received Refresh Time value and the Unicast flag in the 274 default router list. It also maintains a timeout - either per link 275 or per default router. If the timeout is per link it is set to the 276 minimum of the received Refresh Time values. 278 7. Host Behavior 280 See Protocol Overview section above. 282 A host implementing this specification SHOULD also implement 283 [I-D.ietf-6man-resilient-rs]. That ensures that if there is packet 284 loss and/or the periodic router advertisements are very infrequent, 285 the host will always receive a timely RA as part of its 286 initialization. 288 If there is no RTO in the received Router Advertisements or there is 289 an RTO with a zero Refresh Time, then the host behavior does not 290 change. However, if RTOs start appearing in RAs after the initial 291 RAs, the host SHOULD start performing RS refresh. As the last router 292 that included RTO options time out from the default router list, the 293 host SHOULD stop sending RS refresh messages. 295 The host MUST join the all-nodes multicast address as in [RFC4861] 296 since the routers MAY send multicast RAs for important changes. 298 Some links might have routers with different configuration where some 299 router includes RTO in the RA and others do not. Hosts MAY make the 300 simplifying assumption that if any router on the link includes RTO 301 then the host can use RS refresh to all the routers in the default 302 router list. Also, the routers might advertise different Refresh 303 Time, and hosts MAY use the minimum of the time received from any 304 router that remains in the default router list, or use a separate 305 timer for each router in the default router list. Note that 306 Section 9 says that routers SHOULD report such inconsistencies to 307 system management. 309 A RTO option with a Refresh Time value of zero is silently ignored, 310 that is, the RA is handled the same way as if it did not contain an 311 RTO option. 313 If the U-flag is zero for at least one of the routers in the default 314 router list, then the host will send each refresh RS to the all- 315 routers multicast address. Otherwise the host will unicast the RS 316 refresh to each router in the default router list. The host can 317 either maintain the Refresh Time and Unicast flag per router or per 318 link. If they are maintained per router then the host MUST NOT 319 multicast an RS for every default router list entry but instead 320 multicast once when the minimum (across the default router list for 321 the interface) Refresh Time expires. If they are maintained per 322 link, then the host would determine an effective Unicast bit for the 323 link; set if all the routers which sent RTO set the Unicast bit. 325 If there is no response to a refresh RS, the host follows the same 326 retransmit behavior as in resilient-rs [I-D.ietf-6man-resilient-rs]. 328 7.1. Sleep and Wakeup 330 The protocol allows the sleepy nodes to complete its sleep schedule 331 without waking up due to multicast Router Advertisement messages and 332 the host is not required to wake up solely for the purposes of 333 performing RS refresh. Such a host SHOULD send a RS refresh upon 334 wakeup even if the Refresh Time has not yet expired, in order to 335 receive any updated RA information. 337 Hosts that do wake up due to multicast RAs only needs to perform a 338 refresh on wakeup if the Refresh timeout has expired while the host 339 was sleeping. 341 7.2. Movement 343 When a host wakes up or thinks it might have moved to a different 344 link (new L2 association, lost and required L2 connectivity, etc) it 345 can combine DNA (Detecting Network Attachment - DNA [RFC6059]), NUD, 346 and refreshing its prefixes etc by sending a unicast RS to each of 347 its existing RTO default router(s). If it receives unicast RA from a 348 router, then it can mark the router as REACHABLE. 350 Note that DNA specifies using NS messages since many IPv6 routers 351 delay (and multicast) solicited RAs and DNA wants to avoid that 352 delay. Routers which implement this specification and send RTO 353 SHOULD unicast solicited RAs, hence if a router included the RTO then 354 the host can use RS for DNA without incurring additional delay. Thus 355 the host would not need to use a unicast NS as part of DNA for RTO 356 routers. For non-RTO routers the host MAY choose to use NS for DNA 357 as in [RFC6059]. 359 8. Router Behavior 361 See Protocol Overview section. 363 A router implementing this specification (and including the RTO in 364 the RAs) SHOULD also respond to unicast RS messages (that do not have 365 an unspecified source address) with unicast RAs. If a RS message has 366 an unspecified source address then the router MAY respond with a RA 367 unicast at layer 2 (sent to the link-layer source address of the RS), 368 or it MAY follow the rate-limited multicast RA procedure in 369 [RFC4861]. 371 The RECOMMENDED default configuration for routers is to have RTO 372 disabled. When RTO is enabled the RECOMMENDED default configuration 373 is to have the Unicast flag disabled. 375 8.1. Router and/or Interface Initialization 377 This specification does not change the initialization procedure. 378 Thus a router multicasts some initial Router Advertisements 379 (MAX_INITIAL_RTR_ADVERTISEMENTS) at system startup or interface 380 initialization as specified in [RFC4861] and its updates. 382 8.2. Periodic Multicast RA for unmodified hosts 384 By default a router MUST send periodic multicast RAs as specified in 385 [RFC4861]. A router can be configured to omit those, which can be 386 used in particular deployments. If they are omitted, then there MUST 387 be a mechanism to prevent or detect the existence of unmodified hosts 388 on the link. That could be performed at deployment time (e.g., only 389 hosts which are known to support RTO are configured with the layer 2 390 security keys), or the routers could either detect any RSs which do 391 not include the R-flag and report this to system management or 392 dynamically enable periodic multicast RAs when observing at least one 393 RS without the R-flag. 395 Note that such dynamic detection of "legacy" hosts is not bullet 396 proof, in particular when there is packet loss on the link. If a 397 host does not implement resilient RS [I-D.ietf-6man-resilient-rs], 398 then the host might receive a multicast RA (from router 399 initialization or the periodic multicast RAs) without the router ever 400 receiving a RS from the host. Such a host would function as long as 401 the routers are sending periodic multicast RAs. However, hosts 402 without resilent RS do not operate well in the presence of packet 403 loss. They might be without service (no default router and no 404 prefixes) for one or more multiples of the RA advertisement interval 405 (MaxRtrAdvInterval), which currently can be as high as 30 minutes. 407 8.3. Unsolicited RAs to share new information 409 When a router has new information to share (new prefixes, prefixes 410 that should be immediately deprecated, etc) it MAY multicast up to 411 MAX_INITIAL_RTR_ADVERTISEMENTS number of Router Advertisements. 413 On links where multicast is expensive the router MAY instead unicast 414 up to MAX_INITIAL_RTR_ADVERTISEMENTS number of Router Advertisements 415 to the hosts in its neighbor cache. 417 Note that such new information is not likely to reach hosts sleeping 418 on a schedule until those hosts refresh by sending a RS. However, as 419 such hosts are recommended to send a RS refresh when they wake up, 420 they will receive the updated information and not use the potentially 421 stale information to send packets. 423 9. Router Advertisement Consistency 425 The routers follows section 6.2.7 in [RFC4861] by receiving RAs from 426 other routers on the link. In addition to the checks in that 427 section, the routers SHOULD verify that the RTO have the same Refresh 428 Time, and report to system management if they differ. While the host 429 will pick the lowest time and operate correctly, it is not useful to 430 use different Refresh Times for different routers. 432 10. Security Considerations 434 These optimizations are not known to introduce any new threats 435 against Neighbor Discovery beyond what is already documented for IPv6 436 [RFC3756]. 438 Section 11.2 of [RFC4861] applies to this document as well. 440 The mechanisms in this document work with SeND [RFC3971]. 442 11. IANA Considerations 444 A new flag (R-flag) in the Router Solicitation message has been 445 introduced by carving out a bit from the Reserved field. There is 446 currently no IANA registry for RS flags. Perhaps one should be 447 created? 449 This document needs a new Neighbor Discovery option type for the RTO. 451 12. Acknowledgements 453 The original idea came up in a discussion with Suresh Krishnan. 454 Comments from Samita Chakrabarti, Lorenzo Colitti, and Erik Kline 455 have helped improve the document. 457 This document has been discussed in the efficient-nd design team. 459 13. Change Log 461 Changes since the draft-nordmark-6man-rs-refresh-00 version of the 462 draft: 464 o Removed any suggestion that periodic RAs would not be needed. The 465 remain required. 467 o Made Refresh Time zero be reserved and RTOs with this value 468 ignored by the receiver. 470 o Removed notion that all-ones refresh time means infinite lifetime. 471 It now means 65535 seconds. 473 o Changed default to be multicast RS refresh, with the option to 474 specify unicast in the RTO. This enables discovering new routers 475 on the link. 477 o Clarified the normative behavior for hosts that sleep on a 478 schedule. 480 o Clarified the updated DNA behavior. 482 o Editorial fixes. 484 14. References 486 14.1. Normative References 488 [I-D.ietf-6man-resilient-rs] 489 Krishnan, S., Anipko, D., and D. Thaler, "Packet loss 490 resiliency for Router Solicitations", draft-ietf-6man- 491 resilient-rs-06 (work in progress), April 2015. 493 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 494 Requirement Levels", BCP 14, RFC 2119, 495 DOI 10.17487/RFC2119, March 1997, 496 . 498 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 499 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, 500 December 1998, . 502 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 503 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 504 DOI 10.17487/RFC4861, September 2007, 505 . 507 14.2. Informative References 509 [I-D.garneij-6man-nd-m2m-issues] 510 Garneij, F., Chakrabarti, S., and S. Krishnan, "Impact of 511 IPv6 Neighbor Discovery on Cellular M2M Networks", draft- 512 garneij-6man-nd-m2m-issues-00 (work in progress), July 513 2014. 515 [I-D.vyncke-6man-mcast-not-efficient] 516 Vyncke, E., Thubert, P., Levy-Abegnoli, E., and A. 517 Yourtchenko, "Why Network-Layer Multicast is Not Always 518 Efficient At Datalink Layer", draft-vyncke-6man-mcast-not- 519 efficient-01 (work in progress), February 2014. 521 [RFC3756] Nikander, P., Ed., Kempf, J., and E. Nordmark, "IPv6 522 Neighbor Discovery (ND) Trust Models and Threats", 523 RFC 3756, DOI 10.17487/RFC3756, May 2004, 524 . 526 [RFC3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander, 527 "SEcure Neighbor Discovery (SEND)", RFC 3971, 528 DOI 10.17487/RFC3971, March 2005, 529 . 531 [RFC6059] Krishnan, S. and G. Daley, "Simple Procedures for 532 Detecting Network Attachment in IPv6", RFC 6059, 533 DOI 10.17487/RFC6059, November 2010, 534 . 536 [SYNC] Floyd, S. and V. Jacobson, "The Synchronization of 537 Periodic Routing Messages", IEEE/ACM Transactions on 538 Networking , April 1994, 539 . 541 Authors' Addresses 543 Erik Nordmark 544 Arista Networks 545 Santa Clara, CA 546 USA 548 Email: nordmark@acm.org 550 Andrew Yourtchenko 551 Cisco 552 7a de Kleetlaan 553 Diegem, 1831 554 Belgium 556 Phone: +32 2 704 5494 557 Email: ayourtch@cisco.com 558 Suresh Krishnan 559 Ericsson 560 8400 Decarie Blvd. 561 Town of Mount Royal, QC 562 Canada 564 Phone: +1 514 345 7900 x42871 565 Email: suresh.krishnan@ericsson.com