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Linkova 3 Internet-Draft Google 4 Updates: 4861 (if approved) September 15, 2020 5 Intended status: Standards Track 6 Expires: March 19, 2021 8 Gratuitous Neighbor Discovery: Creating Neighbor Cache Entries on First- 9 Hop Routers 10 draft-ietf-6man-grand-03 12 Abstract 14 Neighbor Discovery (RFC4861) is used by IPv6 nodes to determine the 15 link-layer addresses of neighboring nodes as well as to discover and 16 maintain reachability information. This document updates RFC4861 to 17 allow routers to proactively create a Neighbor Cache entry when a new 18 IPv6 address is assigned to a node. It also updates RFC4861 and 19 recommends nodes to send unsolicited Neighbor Advertisements upon 20 assigning a new IPv6 address. The proposed change will minimize the 21 delay and packet loss when a node initiate connections to off-link 22 destination from a new IPv6 address. 24 Status of This Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at https://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on March 19, 2021. 41 Copyright Notice 43 Copyright (c) 2020 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (https://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 59 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 60 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 61 2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4 62 3. Solution Requirements . . . . . . . . . . . . . . . . . . . . 6 63 4. Proposed Changes to Neighbor Discovery . . . . . . . . . . . 6 64 4.1. Nodes Sending Gratuitous Neighbor Advertisements . . . . 6 65 4.2. Routers Creating Cache Entries Upon Receiving Unsolicited 66 Neighbor Advertisements . . . . . . . . . . . . . . . . . 7 67 5. Avoiding Disruption . . . . . . . . . . . . . . . . . . . . . 8 68 5.1. Neighbor Cache Entry Exists in Any State Other That 69 INCOMPLETE . . . . . . . . . . . . . . . . . . . . . . . 8 70 5.2. Neighbor Cache Entry is in INCOMPLETE state . . . . . . . 8 71 5.3. Neighbor Cache Entry Does Not Exist . . . . . . . . . . . 9 72 5.3.1. The Rightful Owner Is Not Sending Packets From The 73 Address . . . . . . . . . . . . . . . . . . . . . . . 9 74 5.3.2. The Rightful Owner Has Started Sending Packets From 75 The Address . . . . . . . . . . . . . . . . . . . . . 10 76 6. Modifications to RFC-Mandated Behavior . . . . . . . . . . . 11 77 6.1. Modification to RFC4861 Neighbor Discovery for IP version 78 6 (IPv6) . . . . . . . . . . . . . . . . . . . . . . . . 11 79 6.1.1. Modification to the section 7.2.5 . . . . . . . . . . 11 80 6.1.2. Modification to the section 7.2.6 . . . . . . . . . . 12 81 7. Solution Limitations . . . . . . . . . . . . . . . . . . . . 13 82 8. Solutions Considered but Discarded . . . . . . . . . . . . . 13 83 8.1. Do Nothing . . . . . . . . . . . . . . . . . . . . . . . 14 84 8.2. Change to the Registration-Based Neighbor Discovery . . . 14 85 8.3. Host Sending NS to the Router Address from Its GUA . . . 14 86 8.4. Host Sending Router Solicitation from its GUA . . . . . . 15 87 8.5. Routers Populating Their Caches by Gleaning From Neighbor 88 Discovery Packets . . . . . . . . . . . . . . . . . . . . 16 89 8.6. Initiating Hosts-to-Routers Communication . . . . . . . . 16 90 8.7. Making the Probing Logic on Hosts More Robust . . . . . . 17 91 8.8. Increasing the Buffer Size on Routers . . . . . . . . . . 17 92 8.9. Transit Dataplane Traffic From a New Address Triggering 93 Address Resolution . . . . . . . . . . . . . . . . . . . 18 94 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 95 10. Security Considerations . . . . . . . . . . . . . . . . . . . 18 96 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19 97 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 98 12.1. Normative References . . . . . . . . . . . . . . . . . . 19 99 12.2. Informative References . . . . . . . . . . . . . . . . . 20 100 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 21 102 1. Introduction 104 The Neighbor Discovery state machine defined in [RFC4861] assumes 105 that communications between IPv6 nodes are in most cases bi- 106 directional and if a node A is trying to communicate to its neighbor, 107 node B, the return traffic flows could be expected. So when the node 108 A starts the address resolution process, the target node B would also 109 create an entry for A address in its neighbor cache. That entry will 110 be used for sending the return traffic to A. 112 In particular, section 7.2.5 of [RFC4861] states: "When a valid 113 Neighbor Advertisement is received (either solicited or unsolicited), 114 the Neighbor Cache is searched for the target's entry. If no entry 115 exists, the advertisement SHOULD be silently discarded. There is no 116 need to create an entry if none exists, since the recipient has 117 apparently not initiated any communication with the target." 119 While this approach is perfectly suitable for host-to-host on-link 120 communications, it does not work so well when a host sends traffic to 121 off-link destinations. After joining the network and receiving a 122 Router Advertisement the host populates its neighbor cache with the 123 default router IPv6 and link-layer addresses and is able to send 124 traffic to off-link destinations. At the same time the router does 125 not have any cache entries for the host global addresses yet and only 126 starts address resolution upon receiving the first packet of the 127 return traffic flow. While waiting for the resolution to complete 128 routers only keep a very small number of packets in the queue, as 129 recommended in Section 7.2.2 [RFC4861]. All subsequent packets 130 arriving before the resolution process finishes are likely to be 131 dropped. It might cause user-visible packet loss and performance 132 degradation. 134 1.1. Requirements Language 136 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 137 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 138 "OPTIONAL" in this document are to be interpreted as described in BCP 139 14 [RFC2119] [RFC8174] when, and only when, they appear in all 140 capitals, as shown here. 142 1.2. Terminology 144 Node: a device that implements IP, [RFC4861]. 146 Host: any node that is not a router, [RFC4861]. 148 ND: Neighbor Discovery, [RFC4861]. 150 SLAAC: IPv6 Stateless Address Autoconfiguration, [RFC4862]. 152 NS: Neighbor Solicitation, [RFC4861]. 154 NA: Neighbor Advertisement, [RFC4861]. 156 RS: Router Solicitation, [RFC4861]. 158 RA: Router Advertisement, [RFC4861]. 160 SLLA: Source link-layer Address, an option in the ND packets 161 containing the link-layer address of the sender of the packet 162 [RFC4861]. 164 TLLA: Target link-layer Address, an option in the ND packets 165 containing the link-layer address of the target [RFC4861]. 167 GUA: Global Unicast Address [RFC4291]. 169 DAD: Duplicate Address Detection, [RFC4862]. 171 Optimistic DAD: a modification of DAD, [RFC4429]. 173 2. Problem Statement 175 The most typical scenario when the problem may arise is a host 176 joining the network, forming a new address and using that address for 177 accessing the Internet: 179 1. A host joins the network and receives a Router Advertisement (RA) 180 packet from the first-hop router (either a periodic unsolicited 181 RA or a response to a Router Solicitation sent by the host). The 182 RA contains information the host needs to perform SLAAC and to 183 configure its network stack. The RA is send from the router's 184 link-local address to link-local destination and may contain the 185 link-layer address of the router. As a result the host can 186 populate its Neighbor Cache with the router's link-local and 187 link-layer addresses. 189 2. The host starts opening connections to off-link destinations. A 190 very common use case is a mobile device sending probes to detect 191 the Internet connectivity and/or the presence of a captive portal 192 on the network. To speed up that process many implementations 193 use Optimistic DAD which allows them to send probes before the 194 DAD process is completed. At that moment the device neighbor 195 cache contains all information required to send those probes 196 (such as the default router link-local the link-layer addresses). 197 The router neighbor cache, however, might contain an entry for 198 the device link-local address (if the device has been performing 199 the address resolution for the router link-local address), but 200 there are no entries for the device global addresses. 202 3. Return traffic is received by the first-hop router. As the 203 router does not have any cache entry for the host global address 204 yet, the router starts the neighbor discovery process by creating 205 an INCOMPLETE cache entry and then sending a Neighbor 206 Solicitation to the Solicited Node Multicast Address. As per 207 Section 7.2.2 of [RFC4861] Routers MUST buffer at least one data 208 packet and MAY buffer more, while resolving the packet 209 destination address. However most router implementations limit 210 the buffer size to a few packets only, so all subsequent packets 211 for the host global address are dropped, until the address 212 resolution process is completed. 214 4. If the host sends multiple probes in parallel, it would consider 215 all but one of them failed. That leads to user-visible delay in 216 connecting to the network, especially if the host implements some 217 form of backoff mechanism and does not retransmit the probes as 218 soon as possible. 220 This scenario illustrates the problem occurring when the device 221 connects to the network for the first time or after an inactivity 222 period long enough for the device address to be removed from the 223 router's neighbor cache. However, the same sequence of events happen 224 when the host starts using a new global address previously unseen by 225 the router, such as a new privacy address [RFC4941] or if the 226 router's Neighbor Cache has been flushed. 228 While in dual-stack networks this problem might be hidden by Happy 229 Eyeballs [RFC8305] it manifests quite clearly in IPv6-only 230 environments, especially wireless ones, leading to poor user 231 experience and contributing to a negative perception of IPv6-only 232 solutions as unstable and non-deployable. 234 3. Solution Requirements 236 It would be highly desirable to improve the Neighbor Discovery 237 mechanics so routers have a usable cache entry for a host address by 238 the time the router receives the first packet for that address. In 239 particular: 241 o If the router does not have a Neighbor Cache entry for the 242 address, a STALE entry needs to be created. 244 o The solution needs to work for Optimistic addresses as well. 245 Devices implementing the Optimistic DAD usually attempt to 246 minimize the delay in connecting to the network and therefore are 247 more likely to be affected by the problem described in this 248 document. 250 o In case of duplicate addresses present in the network, the 251 proposed solution MUST NOT override the existing entry. 253 o In topologies with multiple first-hop routers the cache needs to 254 be updated on all of them, as traffic might be asymmetric: 255 outgoing flows leaving the network via one router while the return 256 traffic enters the segment via another one. 258 In addition the solution MUST NOT exacerbate issues described in 259 [RFC6583] and MUST be compatible with the recommendations provided in 260 [RFC6583]. 262 4. Proposed Changes to Neighbor Discovery 264 The following changes are proposed to minimize the delay in creating 265 new entries in a router neighbor cache 267 o A node sends unsolicited NAs upon assigning a new IPv6 address to 268 its interface. 270 o A router creates a new cache entry upon receiving an unsolicited 271 NA from a host. 273 The following sections discuss these changes in more detail. 275 4.1. Nodes Sending Gratuitous Neighbor Advertisements 277 The section 7.2.6 of [RFC4861] discusses using unsolicited Neighbor 278 Advertisement to inform node neighbors of the new link-layer address 279 quickly. The same mechanism could be used to notify the node 280 neighbors about the new network-layer address as well: the node can 281 send gratuitous unsolicited Neighbor Advertisements upon assigning a 282 new IPv6 address to its interface. 284 To minimize the potential disruption in case of duplicate addresses 285 the node should not set the Override flag for a preferred address and 286 must not set the Override flag if the address is in Optimistic 287 [RFC4429] state. 289 As the main purpose of sending unsolicited NAs upon configuring a new 290 address is to proactively create a Neighbor Cache entry on the first- 291 hop routers, the gratuitous NAs are sent to all-routers multicast 292 address (ff02::2). Limiting the recipients to routers only would 293 help reduce the multicast noise level. If the link-layer devices are 294 performing MLD snooping [RFC4541] then those unsolicited NAs will be 295 only sent to onlink routers instead of being flooded to all nodes. 297 It should be noted that the proposed mechanism does not cause any 298 significant increase in the multicast traffic. The additional 299 multicast unsolicited NA would proactively create a STALE cache entry 300 on routers as discussed below. When the router receives the return 301 traffic flows it does not need to send multicast NSes to the 302 solicited node multicast address but would be sending unicast NSes 303 instead. Therefore total amount of multicast traffic should not 304 increase. 306 4.2. Routers Creating Cache Entries Upon Receiving Unsolicited Neighbor 307 Advertisements 309 The section 7.2.5 of [RFC4861] states: "When a valid Neighbor 310 Advertisement is received (either solicited or unsolicited), the 311 Neighbor Cache is searched for the target's entry. If no entry 312 exists, the advertisement SHOULD be silently discarded. There is no 313 need to create an entry if none exists, since the recipient has 314 apparently not initiated any communication with the target". 316 The reasoning behind dropping unsolicited Neighbor Advertisements 317 ("the recipient has apparently not initiated any communication with 318 the target") is valid for onlink host-to-host communication but, as 319 discussed above, it does not really apply for the scenario when the 320 host is announcing its address to routers. Therefore it would be 321 beneficial to allow routers creating new entries upon receiving an 322 unsolicited Neighbor Advertisement. 324 This document updates [RFC4861] so that routers create a new Neighbor 325 Cache entry upon receiving an unsolicited Neighbor Advertisement. 326 The proposed changes do not modify routers behaviour specified in 327 [RFC4861] for the scenario when the corresponding Neighbor Cache 328 entry already exists. 330 5. Avoiding Disruption 332 If nodes following the recommendations in this document are using the 333 DAD mechanism defined in [RFC4862], they would send unsolicited NA as 334 soon as the address changes the state from tentative to preferred 335 (after its uniqueness has been verified). However nodes willing to 336 minimize network stack configuration delays might be using optimistic 337 addresses, which means there is a possibility of the address not 338 being unique on the link. The section 2.2 of [RFC4429] discusses 339 measures to ensure that ND packets from the optimistic address do not 340 override any existing neighbor cache entries as it would cause 341 traffic interruption of the rightful address owner in case of address 342 conflict. As nodes willing to speed up their network stack 343 configuration are most likely to be affected by the problem outlined 344 in this document it seems reasonable for such hosts to advertise 345 their optimistic addresses by sending unsolicited NAs. The main 346 question to consider is the potential risk of overriding the cache 347 entry for the rightful address owner if the optimistic address 348 happens to be duplicated. 350 The following sections are discussing the address collision scenario 351 when a node sends an unsolicited NA for an address in the Optimistic 352 state, while another node has the same address assigned already. 354 5.1. Neighbor Cache Entry Exists in Any State Other That INCOMPLETE 356 If the router Neighbor Cache entry for the target address already 357 exists in any state other than INCOMPLETE, then as per section 7.2.5 358 of [RFC4861] an unsolicited NA with the Override flag cleared would 359 change the entry state from REACHABLE to STALE but would not update 360 the entry in any other way. Therefore even if the host sends an 361 unsolicited NA from the its Optimistic address the router cache entry 362 would not be updated with the new Link-Layer address and no impact to 363 the traffic for the rightful address owner is expected. 365 5.2. Neighbor Cache Entry is in INCOMPLETE state 367 Another corner case is the INCOMPLETE cache entry for the address. 368 If the host sends an unsolicited NA from the Optimistic address it 369 would update the entry with the host link-layer address and set the 370 entry to the STALE state. As the INCOMPLETE entry means that the 371 router has started the ND process for the address and the multicast 372 NS has been sent, the rightful owner is expected to reply with 373 solicited NA with the Override flag set. Upon receiving a solicited 374 NA with the Override flag the cache entry will be updated with the 375 TLLA supplied and (as the NA has the Solicited flag set), the entry 376 state will be set to REACHABLE. It would recover the cache entry and 377 set the link-layer address to the one of the rightful owner. The 378 only potential impact would be for packets arriving to the router 379 after the unsolicited NA from the host but before the rightful owner 380 responded with the solicited NA. Those packets would be sent to the 381 host with the optimistic address instead of its rightful owner. 382 However those packets would have been dropped anyway as until the 383 solicited NA is received the router can not send the traffic. 385 5.3. Neighbor Cache Entry Does Not Exist 387 There are two distinct scenarios which can lead to the situation when 388 the router does not have a NC entry for the IPv6 address: 390 1. The rightful owner of the address has not been using it for 391 communication. 393 2. The rightful owner just started sending packets from that address 394 but the router has not received any return traffic yet. 396 The impact on the rightful owner's traffic flows would be different 397 in those cases. 399 5.3.1. The Rightful Owner Is Not Sending Packets From The Address 401 In this scenario the following events are expected to happen: 403 1. The host configures the address and sets its state to Optimistic. 405 2. The host sends an unsolicited NA with the Override flag set to 406 zero and starts sending traffic from the Optimistic address. 408 3. The router creates a STALE entry for the address and the host 409 link-layer address. 411 4. The host starts DAD and detects the address duplication. 413 5. The router receives the return traffic for the duplicated 414 address. As the NC entry is STALE it sends traffic using that 415 entry, changes it to DELAY and wait up to DELAY_FIRST_PROBE_TIME 416 ([RFC4861]) seconds. 418 6. The router changes the NC entry state to PROBE and sends up to 419 MAX_UNICAST_SOLICIT ([RFC4861]) unicast NSes separated by 420 RetransTimer milliseconds ([RFC4861]) to the host link-layer 421 address. 423 7. As the host has detected the address conflict already it does not 424 respond to the unicast NSes. 426 8. The router sends a multicast NS to the solicited node multicast 427 address, the rightful owner responds and the router NC entry is 428 updated with the rightful owner link-local address. 430 The rightful owner is not experiencing any disruption as it does not 431 send/receive any traffic. If after step 7 the router keeps receiving 432 any return traffic for communication initiated at step 2, those 433 packets would be forwarded to the rightful owner. However the same 434 behaviour would be observed if changes proposed in this document are 435 implemented: if the host starts sending packets from its Optimistic 436 address but then changed the address state to Duplicated, almost all 437 return traffic would be forwarded to the rightful owner of the said 438 address. Therefore it's safe to conclude that the proposed changes 439 do not cause any disruption for the rightful owner. 441 5.3.2. The Rightful Owner Has Started Sending Packets From The Address 443 In this scenario the following events are happening: 445 1. The rightful owner starts sending traffic from the address (e.g. 446 the address has just been configured or has not been recently 447 used). 449 2. The host configures the address and sets its state to Optimistic. 451 3. The host sends an unsolicited NA with the Override flag set to 452 zero and starts sending traffic from the Optimistic address. 454 4. The router creates a STALE entry for the address and the host 455 link-layer address. 457 5. The host starts DAD and detects the address duplication. 459 6. The router receives the return traffic flows for both the 460 rightful owner of the duplicated address and the new host. As 461 the NC entry is STALE it sends traffic using that entry, changes 462 it to DELAY and wait up to DELAY_FIRST_PROBE_TIME ([RFC4861]) 463 seconds. 465 7. The router changes the NC entry state to PROBE and sends up to 466 MAX_UNICAST_SOLICIT ([RFC4861]) unicast NSes separated by 467 RetransTimer milliseconds ([RFC4861]) to the host link-layer 468 address. 470 8. As the host has detected the address conflict already it does not 471 respond to the unicast NSes. 473 9. The router sends a multicast NS to the solicited node multicast 474 address, the rightful owner responds and the router NC entry is 475 updated with the rightful owner link-local address. 477 As a result the traffic for the address rightful owner would be sent 478 to the host with the duplicated address instead. The duration of the 479 disruption can be estimated as DELAY_FIRST_PROBE_TIME*1000 + 480 (MAX_UNICAST_SOLICIT - 1)*RetransTimer milliseconds. As per the 481 constants defined in Section 10 of [RFC4861] this interval is equal 482 to 5*1000 + (3 - 1)*1000 = 7000ms or 7 seconds. 484 However it should be noted that the probability of such scenario is 485 rather low as it would require the following things to happen almost 486 simultaneously (within tens of milliseconds): 488 o One host starts using a new IPv6 address and sending traffic. 490 o Another host configures the same IPv6 address in Optimistic mode 491 before the router receives the return traffic for the first host. 493 6. Modifications to RFC-Mandated Behavior 495 All normative text in this memo is contained in this section. 497 6.1. Modification to RFC4861 Neighbor Discovery for IP version 6 (IPv6) 499 6.1.1. Modification to the section 7.2.5 501 This document proposes the following changes to the section 7.2.5 of 502 [RFC4861]: 504 ------------------------------------------------------------------ 506 OLD TEXT: 508 ------------------------------------------------------------------ 510 When a valid Neighbor Advertisement is received (either solicited or 511 unsolicited), the Neighbor Cache is searched for the target's entry. 512 If no entry exists, the advertisement SHOULD be silently discarded. 513 There is no need to create an entry if none exists, since the 514 recipient has apparently not initiated any communication with the 515 target. 517 ------------------------------------------------------------------ 519 NEW TEXT: 521 ------------------------------------------------------------------ 523 When a valid Neighbor Advertisement is received (either solicited or 524 unsolicited), the Neighbor Cache is searched for the target's entry. 525 If no entry exists, hosts SHOULD silently discard the advertisement. 526 There is no need to create an entry if none exists, since the 527 recipient has apparently not initiated any communication with the 528 target. Routers SHOULD create a new entry for the target address 529 with the link-layer address set to the Target link-layer address 530 option (if supplied). The entry its reachability state MUST also be 531 set to STALE. If the received Neighbor Advertisement does not 532 contain the Target link-layer address option the advertisement SHOULD 533 be silently discarded. 535 ------------------------------------------------------------------ 537 6.1.2. Modification to the section 7.2.6 539 This document proposes the following changes to the section 7.2.6 of 540 [RFC4861]: 542 OLD TEXT: 544 ------------------------------------------------------------------ 546 Also, a node belonging to an anycast address MAY multicast 547 unsolicited Neighbor Advertisements for the anycast address when the 548 node's link-layer address changes. 550 ------------------------------------------------------------------ 552 NEW TEXT: 554 ------------------------------------------------------------------ 556 Also, a node belonging to an anycast address MAY multicast 557 unsolicited Neighbor Advertisements for the anycast address when the 558 node's link-layer address changes. 560 A node may also wish to notify its first-hop routers when it 561 configures a new global IPv6 address so the routers can proactively 562 populate their neighbor caches with the corresponding entries. In 563 such cases a node SHOULD send up to MAX_NEIGHBOR_ADVERTISEMENT 564 Neighbor Advertisement messages. If the address is preferred then 565 the Override flag SHOULD NOT be set. If the address is in the 566 Optimistic state then the Override flag MUST NOT be set. The 567 destination address SHOULD be set to the all-routers multicast 568 address. These advertisements MUST be separated by at least 569 RetransTimer seconds. The first advertisement SHOULD be sent as soon 570 as one of the following events happens: 572 ------------------------------------------------------------------ 574 o if Optimistic DAD [RFC4429] is used: a new Optimistic address is 575 assigned to the node interface. 577 o if Optimistic DAD is not used: an address changes the state from 578 tentative to preferred. 580 7. Solution Limitations 582 The solution described in this document provides some improvement for 583 a node configuring a new IPv6 address and start sending traffic from 584 it. However that approach does not completely eliminate the scenario 585 when a router receives some transit traffic for an address without 586 the corresponding Neighbor Cache entry. For example: 588 o If the host starts using an already configured IPv6 address after 589 a long period of inactivity, the router might not have the NC 590 entry for that address anymore, as old/expired entries are 591 deleted. 593 o Clearing the router Neighbor Cache would trigger the packet loss 594 for all actively used addresses removed from the cache. 596 8. Solutions Considered but Discarded 598 There are other possible approaches to address the problem, for 599 example: 601 o Just do nothing. 603 o Migrating from the "reactive" Neighbor Discovery ([RFC4861]) to 604 the registration-based mechanisms ([RFC8505]). 606 o Creating new entries in routers Neighbor Cache by gleaning from 607 Neighbor Discovery DAD messages. 609 o Initiates bidirectional communication from the host to the router 610 using the host GUA. 612 o Making the probing logic on hosts more robust. 614 o Increasing the buffer size on routers. 616 o Transit dataplane traffic from an unknown address (an address w/o 617 the corresponding neighbor cache entry) triggers an address 618 resolution process on the router. 620 It should be noted that some of those options are already implemented 621 by some vendors. The following sections discuss those approaches and 622 the reasons they were discarded. 624 8.1. Do Nothing 626 One of the possible approaches might be to declare that everything is 627 working as intended and let the upper-layer protocols to deal with 628 packet loss. The obvious drawbacks include: 630 o Unhappy users. 632 o Many support tickets. 634 o More resistance to deploy IPv6 and IPv6-Only networks. 636 8.2. Change to the Registration-Based Neighbor Discovery 638 The most radical approach would be to move away from the reactive ND 639 as defined in [RFC4861] and expand the registration-based ND 640 ([RFC6775], [RFC8505]) used in Low-Power Wireless Personal Area 641 Networks (6LoWPANs) to the rest of IPv6 deployments. This option 642 requires some investigation and discussion. However the 643 implementation complexity and unclear adoption timeline makes this 644 approach less preferable than one proposed in this document. 646 8.3. Host Sending NS to the Router Address from Its GUA 648 The host could force creating a STALE entry for its GUA in the router 649 ND cache by sending the following Neighbor Solicitation message: 651 o The NS source address is the host GUA. 653 o The destination address is the default router IPv6 address. 655 o The Source Link-Layer Address option contains the host link-layer 656 address. 658 o The target address is the host default router address (the default 659 router address the host received in the RA). 661 The main disadvantages of this approach are: 663 o Would not work for Optimistic addresses as section 2.2 of 664 [RFC4429] explicitly prohibits sending Neighbor Solicitations from 665 an Optimistic Address. 667 o If first-hop redundancy is deployed in the network, the NS would 668 reach the active router only, so all backup routers (or all active 669 routers except one) would not get their neighbor cache updated. 671 o Some wireless devices are known to alter ND packets and perform 672 various non-obvious forms of ND proxy actions. In some cases, 673 unsolicited NAs might not even reach the routers. 675 8.4. Host Sending Router Solicitation from its GUA 677 The host could send a router solicitation message to 'all routers' 678 multicast address, using its GUA as a source. If the host link-layer 679 address is included in the Source Link-Layer Address option, the 680 router would create a STALE entry for the host GUA as per the section 681 6.2.6 of [RFC4861]. However, this approach can not be used if the 682 GUA is in optimistic state: section 2.2 of [RFC4429] explicitly 683 prohibits using an Optimistic Address as the source address of a 684 Router Solicitation with a SLLAO as it might disrupt the rightful 685 owner of the address in the case of a collision. So for the 686 optimistic addresses the host can send an RS without SLLAO included. 687 In that case the router may respond with either a multicast or a 688 unicast RA (only the latter would create a cache entry). 690 This approach has the following drawbacks: 692 o If the address is in the Optimistic state the RS can not contain 693 SLLAO. As a result the router would only create a cache entry if 694 solicited RAs are sent as unicast. Routers sending solicited RAs 695 as multicast would not create a new cache entry as they do not 696 need to send a unicast packet back to the host. 698 o There might be a random delay between receiving an RS and sending 699 a unicast RA back (and creating a cache entry) which might 700 undermine the idea of creating the cache entry proactively. 702 o Some wireless devices are known to intercept ND packets and 703 perform various non-obvious forms of ND proxy actions. In some 704 cases the RS might not even reach the routers. 706 8.5. Routers Populating Their Caches by Gleaning From Neighbor 707 Discovery Packets 709 Routers may be able to learn about new addresses by gleaning from the 710 DAD Neighbor Solicitation messages. The router could listen to all 711 solicited node multicast address groups and upon receiving a Neighbor 712 Solicitation from the unspecified address search its Neighbor Cache 713 for the solicitation's Target Address. If no entry exists, the 714 router may create an entry, set its reachability state to 715 'INCOMPLETE' and start the address resolution for that entry. 717 The same solution was proposed in 718 [I-D.halpern-6man-nd-pre-resolve-addr]. Some routing vendors support 719 such optimization already. However, this approach has a number of 720 drawbacks and therefore should not be used as the only solution: 722 o Routers need to receive all multicast Neighbor Discovery packets 723 which might negatively impact the routers CPU. 725 o If the router starts the address resolution as soon as it receives 726 the DAD Neighbor Solicitation the host might be still performing 727 DAD and the target address might be tentative. In that case, the 728 host SHOULD silently ignore the received Neighbor Solicitation 729 from the router as per the Section 5.4.3 of [RFC4862]. As a 730 result the router might not be able to complete the address 731 resolution before the return traffic arrives. 733 8.6. Initiating Hosts-to-Routers Communication 735 The host may force the router to start address resolution by sending 736 a data packet such as ping or traceroute to its default router link- 737 local address, using the GUA as a source address. As the RTT to the 738 default router is lower than RTT to any off-link destinations it's 739 quite likely that the router would start the neighbor discovery 740 process for the host GUA before the first packet of the returning 741 traffic arrives. 743 This approach has the following drawbacks: 745 o Data packets to the router link-local address could be blocked by 746 security policy or control plane protection mechanism. 748 o It introduces an additional overhead for routers control plane (in 749 addition to processing ND packets, the data packet needs to be 750 processed as well). 752 o Unless the data packet is sent to 'all routers' ff02::2 multicast 753 address, if the network provides a first-hop redundancy then only 754 the active router would create a new cache entry. 756 8.7. Making the Probing Logic on Hosts More Robust 758 Theoretically the probing logic on hosts might be modified to deal 759 better with initial packet loss. For example, only one probe can be 760 sent or probes retransmit intervals can be reduced. However, this 761 approach has a number of drawbacks: 763 o It would require updating all possible applications performing 764 probing, while the proposed solution is implemented on operating 765 systems level. 767 o Some implementations need to send multiple probes. Examples are 768 including but not limited to: 770 * Sending AAAA and A records DNS probes in parallel. 772 * Detecting captive portals often require sending multiple 773 packets. 775 o While it would increase the probability of the probing to complete 776 successfully, there are multiple cases when packet loss would 777 still occur: 779 * The probe response consists of multiple packets, so all but the 780 first one are dropped. 782 * There are multiple applications on the same host sending 783 traffic and return packets arrive simultaneously. 785 * There are multiple first-hop routers in the network. The first 786 probe packet creates the NC entry on one of them. The 787 subsequent return traffic flows might cross other routers and 788 still experince the issue. 790 8.8. Increasing the Buffer Size on Routers 792 Increasing the buffer size and buffering more packets would 793 exacerbate issues described in [RFC6583] and make the router more 794 vulnerable to ND-based denial of service attacks. 796 8.9. Transit Dataplane Traffic From a New Address Triggering Address 797 Resolution 799 When a router receives a transit packet, it might check the presence 800 of the neighbor cache entry for the packet source address and if the 801 entry does not, exist start address resolution process. This 802 approach does ensure that a Neighbor Cache entry is proactively 803 created every time a new, previously unseen GUA is used for sending 804 offlink traffic. However this approach has a number of limitations, 805 in particular: 807 o If traffic flows are asymmetrical the return traffic might not 808 transit the same router as the original traffic which triggered 809 the address resolution. So the neighbor cache entry is created on 810 the "wrong" router, not the one which actually needs the neighbor 811 cache entry for the host address. 813 o The functionality needs to be limited to explicitly configured 814 networks/interfaces, as the router needs to distinguish between 815 onlink addresses (ones the router needs to have Neighbor Cache 816 entries for) and the rest of the address space. 818 o Implementing such functionality is much more complicated than all 819 other solutions as it would involve complex data-control planes 820 interaction. 822 9. IANA Considerations 824 This memo asks the IANA for no new parameters. 826 10. Security Considerations 828 One of the potential attack vectors to consider is a cache spoofing 829 when the attacker might try to install a cache entry for the victim's 830 IPv6 address and the attacker's Link-Layer address. However it 831 should be noted that this document does not propose any changes for 832 the scenario when the ND cache for the given IPv6 address already 833 exists. Therefore it is not possible for the attacker to override 834 any existing cache entry. 836 A malicious host could attempt to exhaust the neighbor cache on the 837 router by creating a large number of STALE entries. However this 838 attack vector is not new and this document does not increase the risk 839 of such an attack: the attacker could do it, for example, by sending 840 a NS or RS packet with SLLAO included. All recommendations from 841 [RFC6583] still apply. 843 Announcing a new address to all-routers multicast address may inform 844 an on-link attacker about IPv6 addresses assigned to the host. 845 However hiding information about the specific IPv6 address should not 846 be considered a security measure as such information is usually 847 disclosed via DAD to all nodes anyway. Network administrators can 848 also mitigate this issue by enabling MLD snooping on the link-layer 849 devices to prevent IPv6 link-local multicast packets being flooded to 850 all onlink nodes. If peer-to-peer onlink communications are not 851 desirable for the given network segment they should be prevented by 852 proper layer2 security mechanisms. Therefore the risk of allowing 853 hosts to send unsolicited Neighbor Advertisements to all-routers 854 multicast address is low. 856 It should be noted that the proposed mechanism allows hosts to 857 proactively inform their routers about global IPv6 addresses existing 858 on-link. Routers could use that information to distinguish between 859 used and unused addresses to mitigate ND cache exhaustion DoS attacks 860 described in Section 4.3.2 [RFC3756] and [RFC6583]. 862 11. Acknowledgements 864 Thanks to the following people (in alphabetical order) for their 865 comments, review and feedback: Mikael Abrahamsson, Stewart Bryant, 866 Lorenzo Colitti, Owen DeLong, Igor Gashinsky, Fernando Gont, Tatuya 867 Jinmei, Erik Kline, Warren Kumari, Barry Leiba, Jordi Palet Martinez, 868 Erik Nordmark, Michael Richardson, Mark Smith, Dave Thaler, Pascal 869 Thubert, Loganaden Velvindron, Eric Vyncke. 871 12. References 873 12.1. Normative References 875 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 876 Requirement Levels", BCP 14, RFC 2119, 877 DOI 10.17487/RFC2119, March 1997, 878 . 880 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 881 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 882 2006, . 884 [RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD) 885 for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006, 886 . 888 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 889 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 890 DOI 10.17487/RFC4861, September 2007, 891 . 893 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 894 Address Autoconfiguration", RFC 4862, 895 DOI 10.17487/RFC4862, September 2007, 896 . 898 [RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C. 899 Bormann, "Neighbor Discovery Optimization for IPv6 over 900 Low-Power Wireless Personal Area Networks (6LoWPANs)", 901 RFC 6775, DOI 10.17487/RFC6775, November 2012, 902 . 904 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 905 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 906 May 2017, . 908 [RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: 909 Better Connectivity Using Concurrency", RFC 8305, 910 DOI 10.17487/RFC8305, December 2017, 911 . 913 [RFC8505] Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C. 914 Perkins, "Registration Extensions for IPv6 over Low-Power 915 Wireless Personal Area Network (6LoWPAN) Neighbor 916 Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018, 917 . 919 12.2. Informative References 921 [I-D.halpern-6man-nd-pre-resolve-addr] 922 Chen, I. and J. Halpern, "Triggering ND Address Resolution 923 on Receiving DAD-NS", draft-halpern-6man-nd-pre-resolve- 924 addr-00 (work in progress), January 2014. 926 [RFC3756] Nikander, P., Ed., Kempf, J., and E. Nordmark, "IPv6 927 Neighbor Discovery (ND) Trust Models and Threats", 928 RFC 3756, DOI 10.17487/RFC3756, May 2004, 929 . 931 [RFC4541] Christensen, M., Kimball, K., and F. Solensky, 932 "Considerations for Internet Group Management Protocol 933 (IGMP) and Multicast Listener Discovery (MLD) Snooping 934 Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006, 935 . 937 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy 938 Extensions for Stateless Address Autoconfiguration in 939 IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, 940 . 942 [RFC6583] Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational 943 Neighbor Discovery Problems", RFC 6583, 944 DOI 10.17487/RFC6583, March 2012, 945 . 947 Author's Address 949 Jen Linkova 950 Google 951 1 Darling Island Rd 952 Pyrmont, NSW 2009 953 AU 955 Email: furry@google.com