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Linkova 3 Internet-Draft Google 4 Updates: 4861 (if approved) July 25, 2020 5 Intended status: Standards Track 6 Expires: January 26, 2021 8 Gratuitous Neighbor Discovery: Creating Neighbor Cache Entries on First- 9 Hop Routers 10 draft-ietf-6man-grand-01 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 January 26, 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 59 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 60 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 61 2. Proposed Changes to Neighbor Discovery . . . . . . . . . . . 4 62 2.1. Nodes Sending Gratuitous Neighbor Advertisements . . . . 4 63 2.2. Routers Creating Cache Entries Upon Receiving Unsolicited 64 Neighbor Advertisements . . . . . . . . . . . . . . . . . 5 65 3. Avoiding Disruption . . . . . . . . . . . . . . . . . . . . . 5 66 3.1. Neighbor Cache Entry Exists in Any State Other That 67 INCOMPLETE . . . . . . . . . . . . . . . . . . . . . . . 6 68 3.2. Neighbor Cache Entry is in INCOMPLETE state . . . . . . . 6 69 3.3. Neighbor Cache Entry Does Not Exist . . . . . . . . . . . 6 70 3.3.1. The Rightful Owner Is Not Sending Packets From The 71 Address . . . . . . . . . . . . . . . . . . . . . . . 7 72 3.3.2. The Rightful Owner Has Started Sending Packets From 73 The Address . . . . . . . . . . . . . . . . . . . . . 7 74 4. Modifications to RFC-Mandated Behavior . . . . . . . . . . . 9 75 4.1. Modification to RFC4861 Neighbor Discovery for IP version 76 6 (IPv6) . . . . . . . . . . . . . . . . . . . . . . . . 9 77 4.1.1. Modification to the section 7.2.5 . . . . . . . . . . 9 78 4.1.2. Modification to the section 7.2.6 . . . . . . . . . . 9 79 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 80 6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 81 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 82 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 83 8.1. Normative References . . . . . . . . . . . . . . . . . . 11 84 8.2. Informative References . . . . . . . . . . . . . . . . . 12 85 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12 87 1. Introduction 89 The Neighbor Discovery state machine defined in [RFC4861] assumes 90 that communications between IPv6 nodes are in most cases bi- 91 directional and if a node A is trying to communicate to its neighbor, 92 neighbor B, the return traffic flows could be expected. So when the 93 node A starts the address resolution process, the target node would 94 also create an entry for A address in its neighbor cache. That entry 95 will be used for sending the return traffic to A. 97 However when a host sends traffic to off-link destinations a 98 different scenario is observed. After receiving a Router 99 Advertisement the host populates its neighbor cache with the default 100 router IPv6 and link-layer addresses and is able to send traffic to 101 off-link destinations. At the same time the router does not have any 102 cache entries for the host global addresses yet and only starts 103 address resolution upon receiving the first packet of the return 104 traffic flow. While waiting for the resolution to complete routers 105 only keep a very small number of packets in the queue, as recommended 106 in Section 7.2.2 [RFC4861]. All subsequent packets arriving before 107 the resolution process finishes are likely to be dropped. It might 108 cause user-visible packet loss and performance degradation. 110 The detailed problem statement and the various solution approaches 111 could be found in [I-D.ietf-v6ops-nd-cache-init]. This document 112 summarizes the proposed neighbor discovery updates to address the 113 issue. 115 1.1. Requirements Language 117 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 118 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 119 "OPTIONAL" in this document are to be interpreted as described in BCP 120 14 [RFC2119] [RFC8174] when, and only when, they appear in all 121 capitals, as shown here. 123 1.2. Terminology 125 Node: a device that implements IP, [RFC4861]. 127 Host: any node that is not a router, [RFC4861]. 129 ND: Neighbor Discovery, [RFC4861]. 131 SLAAC: IPv6 Stateless Address Autoconfiguration, [RFC4862]. 133 NS: Neighbor Solicitation, [RFC4861]. 135 NA: Neighbor Advertisement, [RFC4861]. 137 RS: Router Solicitation, [RFC4861]. 139 RA: Router Advertisement, [RFC4861]. 141 SLLA: Source link-layer Address, an option in the ND packets 142 containing the link-layer address of the sender of the packet 143 [RFC4861]. 145 TLLA: Target link-layer Address, an option in the ND packets 146 containing the link-layer address of the target [RFC4861]. 148 GUA: Global Unicast Address [RFC4291]. 150 DAD: Duplicate Address Detection, [RFC4862]. 152 Optimistic DAD: a modification of DAD, [RFC4429]. 154 2. Proposed Changes to Neighbor Discovery 156 The following changes are proposed to minimize the delay in creating 157 new entries in a router neighbor cache 159 o A node sends unsolicited NAs upon assigning a new IPv6 address to 160 its interface. 162 o A router creates a new cache entry upon receiving an unsolicited 163 NA from a host. 165 The following sections discuss these changes in more detail. 167 2.1. Nodes Sending Gratuitous Neighbor Advertisements 169 The section 7.2.6 of [RFC4861] discusses using unsolicited Neighbor 170 Advertisement to inform node neighbors of the new link-layer address 171 quickly. The same mechanism could be used to notify the node 172 neighbors about the new network-layer address as well: the node can 173 send gratuitous unsolicited Neighbor Advertisements upon assigning a 174 new IPv6 address to its interface. 176 To minimize the potential disruption in case of duplicate addresses 177 the node should not set the Override flag for a preferred address and 178 must not set the Override flag if the address is in Optimistic 179 [RFC4429] state. 181 As the main purpose of sending unsolicited NAs upon configuring a new 182 address is to proactively create a Neighbor Cache entry on the first- 183 hop routers, the gratuitous NAs are sent to all-routers multicast 184 address (ff02::2). Limiting the recipients to routers only would 185 help reduce the multicast noise level. If the link-layer devices are 186 performing MLD snooping [RFC4541] then those unsolicited NAs will be 187 only sent to onlink routers instead of being flooded to all nodes. 189 It should be noted that the proposed mechanism does not cause any 190 significant increase in the multicast traffic. The additional 191 multicast unsolicited NA would proactively create a STALE cache entry 192 on routers as discussed below. When the router receives the return 193 traffic flows it does not need to send multicast NSes to the 194 solicited node multicast address but would be sending unicast NSes 195 instead. Therefore total amount of multicast traffic should not 196 increase. 198 2.2. Routers Creating Cache Entries Upon Receiving Unsolicited Neighbor 199 Advertisements 201 The section 7.2.5 of [RFC4861] states: "When a valid Neighbor 202 Advertisement is received (either solicited or unsolicited), the 203 Neighbor Cache is searched for the target's entry. If no entry 204 exists, the advertisement SHOULD be silently discarded. There is no 205 need to create an entry if none exists, since the recipient has 206 apparently not initiated any communication with the target". 208 The reasoning behind dropping unsolicited Neighbor Advertisements 209 ("the recipient has apparently not initiated any communication with 210 the target") is valid for onlink host-to-host communication but, as 211 discussed in [I-D.ietf-v6ops-nd-cache-init] it does not really apply 212 for the scenario when the host is announcing its address to routers. 213 Therefore it would be beneficial to allow routers creating new 214 entries upon receiving an unsolicited Neighbor Advertisement. 216 This document updates [RFC4861] so that routers create a new Neighbor 217 Cache entry upon receiving an unsolicited Neighbor Advertisement. 218 The proposed changes do not modify routers behaviour specified in 219 [RFC4861] for the scenario when the corresponding Neighbor Cache 220 entry already exists. 222 3. Avoiding Disruption 224 If hosts following the recommendations in this document are using the 225 DAD mechanism defined in [RFC4862], they would send unsolicited NA as 226 soon as the address changes the state from tentative to preferred 227 (after its uniqueness has been verified). However hosts willing to 228 minimize network stack configuration delays might be using optimistic 229 addresses, which means there is a possibility of the address not 230 being unique on the link. The section 2.2 of [RFC4429] discusses 231 measures to ensure that ND packets from the optimistic address do not 232 override any existing neighbor cache entries as it would cause 233 traffic interruption of the rightful address owner in case of address 234 conflict. As hosts willing to speed up their network stack 235 configuration are most likely to be affected by the problem outlined 236 in this document it seems reasonable for such hosts to advertise 237 their optimistic addresses by sending unsolicited NAs. The main 238 question to consider is the potential risk of overriding the cache 239 entry for the rightful address owner if the optimistic address 240 happens to be duplicated. 242 The following sections are discussing the address collision scenario 243 when a host sends an unsolicited NA for an address in the Optimistic 244 state, while another host has the same address assigned already. 246 3.1. Neighbor Cache Entry Exists in Any State Other That INCOMPLETE 248 If the router Neighbor Cache entry for the target address already 249 exists in any state other than INCOMPLETE, then as per section 7.2.5 250 of [RFC4861] an unsolicited NA with the Override flag cleared would 251 change the entry state from REACHABLE to STALE but would not update 252 the entry in any other way. Therefore even if the host sends an 253 unsolicited NA from the its Optimistic address the router cache entry 254 would not be updated with the new Link-Layer address and no impact to 255 the traffic for the rightful address owner is expected. 257 3.2. Neighbor Cache Entry is in INCOMPLETE state 259 Another corner case is the INCOMPLETE cache entry for the address. 260 If the host sends an unsolicited NA from the Optimistic address it 261 would update the entry with the host link-layer address and set the 262 entry to the STALE state. As the INCOMPLETE entry means that the 263 router has started the ND process for the address and the multicast 264 NS has been sent, the rightful owner is expected to reply with 265 solicited NA with the Override flag set. Upon receiving a solicited 266 NA with the Override flag the cache entry will be updated with the 267 TLLA supplied and (as the NA has the Solicited flag set), the entry 268 state will be set to REACHABLE. It would recover the cache entry and 269 set the link-layer address to the one of the rightful owner. The 270 only potential impact would be for packets arriving to the router 271 after the unsolicited NA from the host but before the rightful owner 272 responded with the solicited NA. Those packets would be sent to the 273 host with the optimistic address instead of its rightful owner. 274 However those packets would have been dropped anyway as until the 275 solicited NA is received the router can not send the traffic. 277 3.3. Neighbor Cache Entry Does Not Exist 279 There are two distinct scenarios which can lead to the situation when 280 the router does not have a NC entry for the IPv6 address: 282 1. The rightful owner of the address has not been using it for 283 communication. 285 2. The rightful owner just started sending packets from that address 286 but the router has not received any return traffic yet. 288 The impact on the rightful owner's traffic flows would be different 289 in those cases. 291 3.3.1. The Rightful Owner Is Not Sending Packets From The Address 293 In this scenario the following events are expected to happen: 295 1. The host configures the address and sets its state to Optimistic. 297 2. The host sends an unsolicited NA with the Override flag set to 298 zero and starts sending traffic from the Optimistic address. 300 3. The router creates a STALE entry for the address and the host 301 link-layer address. 303 4. The host starts DAD and detects the address duplication. 305 5. The router receives the return traffic for the duplicated 306 address. As the NC entry is STALE it sends traffic using that 307 entry, changes it to DELAY and wait up to DELAY_FIRST_PROBE_TIME 308 ([RFC4861]) seconds. 310 6. The router changes the NC entry state to PROBE and sends up to 311 MAX_UNICAST_SOLICIT ([RFC4861]) unicast NSes separated by 312 RetransTimer milliseconds ([RFC4861]) to the host link-layer 313 address. 315 7. As the host has detected the address conflict already it does not 316 respond to the unicast NSes. 318 8. The router sends a multicast NS to the solicited node multicast 319 address, the rightful owner responds and the router NC entry is 320 updated with the rightful owner link-local address. 322 The rightful owner is not experiencing any disruption as it does not 323 send/receive any traffic. If after step 7 the router keeps receiving 324 any return traffic for communication initiated at step 2, those 325 packets would be forwarded to the rightful owner. However the same 326 behaviour would be observed if changes proposed in this document are 327 implemented: if the host starts sending packets from its Optimistic 328 address but then changed the address state to Duplicated, almost all 329 return traffic would be forwarded to the rightful owner of the said 330 address. Therefore it's safe to conclude that the proposed changes 331 do not cause any disruption for the rightful owner. 333 3.3.2. The Rightful Owner Has Started Sending Packets From The Address 335 In this scenario the following events are happening: 337 1. The rightful owner starts sending traffic from the address (e.g. 338 the address has just been configured or has not been recently 339 used). 341 2. The host configures the address and sets its state to Optimistic. 343 3. The host sends an unsolicited NA with the Override flag set to 344 zero and starts sending traffic from the Optimistic address. 346 4. The router creates a STALE entry for the address and the host 347 link-layer address. 349 5. The host starts DAD and detects the address duplication. 351 6. The router receives the return traffic flows for both the 352 rightful owner of the duplicated address and the new host. As 353 the NC entry is STALE it sends traffic using that entry, changes 354 it to DELAY and wait up to DELAY_FIRST_PROBE_TIME ([RFC4861]) 355 seconds. 357 7. The router changes the NC entry state to PROBE and sends up to 358 MAX_UNICAST_SOLICIT ([RFC4861]) unicast NSes separated by 359 RetransTimer milliseconds ([RFC4861]) to the host link-layer 360 address. 362 8. As the host has detected the address conflict already it does not 363 respond to the unicast NSes. 365 9. The router sends a multicast NS to the solicited node multicast 366 address, the rightful owner responds and the router NC entry is 367 updated with the rightful owner link-local address. 369 As a result the traffic for the address rightful owner would be sent 370 to the host with the duplicated address instead. The duration of the 371 disruption can be estimated as DELAY_FIRST_PROBE_TIME*1000 + 372 (MAX_UNICAST_SOLICIT - 1)*RetransTimer milliseconds. As per the 373 constants defined in Section 10 of [RFC4861] this interval is equal 374 to 5*1000 + (3 - 1)*1000 = 7000ms or 7 seconds. 376 However it should be noted that the probability of such scenario is 377 rather low as it would require the following things to happen almost 378 simultaneously (within tens of milliseconds): 380 o One host starts using a new IPv6 address and sending traffic. 382 o Another host configures the same IPv6 address in Optimistic mode 383 before the router receives the return traffic for the first host. 385 4. Modifications to RFC-Mandated Behavior 387 All normative text in this memo is contained in this section. 389 4.1. Modification to RFC4861 Neighbor Discovery for IP version 6 (IPv6) 391 4.1.1. Modification to the section 7.2.5 393 This document proposes the following changes to the section 7.2.5 of 394 [RFC4861]: 396 ------------------------------------------------------------------ 398 OLD TEXT: 400 When a valid Neighbor Advertisement is received (either solicited or 401 unsolicited), the Neighbor Cache is searched for the target's entry. 402 If no entry exists, the advertisement SHOULD be silently discarded. 403 There is no need to create an entry if none exists, since the 404 recipient has apparently not initiated any communication with the 405 target. 407 NEW TEXT: 409 When a valid Neighbor Advertisement is received (either solicited or 410 unsolicited), the Neighbor Cache is searched for the target's entry. 411 If no entry exists, hosts SHOULD silently discard the advertisement. 412 There is no need to create an entry if none exists, since the 413 recipient has apparently not initiated any communication with the 414 target. Routers SHOULD create a new entry for the target address 415 with the link-layer address set to the Target link-layer address 416 option (if supplied). The entry its reachability state MUST also be 417 set to STALE. If the received Neighbor Advertisement does not 418 contain the Target link-layer address option the advertisement SHOULD 419 be silently discarded. 421 ------------------------------------------------------------------ 423 4.1.2. Modification to the section 7.2.6 425 This document proposes the following changes to the section 7.2.6 of 426 [RFC4861]: 428 OLD TEXT: 430 Also, a node belonging to an anycast address MAY multicast 431 unsolicited Neighbor Advertisements for the anycast address when the 432 node's link-layer address changes. 434 NEW TEXT: 436 Also, a node belonging to an anycast address MAY multicast 437 unsolicited Neighbor Advertisements for the anycast address when the 438 node's link-layer address changes. 440 A node may also wish to notify its first-hop routers when it 441 configures a new global IPv6 address so the routers can proactively 442 populate their neighbor caches with the corresponding entries. In 443 such cases a node SHOULD send up to MAX_NEIGHBOR_ADVERTISEMENT 444 Neighbor Advertisement messages. If the address is preferred then 445 the Override flag SHOULD NOT be set. If the address is in the 446 Optimistic state then the Override flag MUST NOT be set. The 447 destination address SHOULD be set to the all-routers multicast 448 address. These advertisements MUST be separated by at least 449 RetransTimer seconds. The first advertisement SHOULD be sent as soon 450 as one of the following events happens: 452 o if Optimistic DAD [RFC4429] is used: a new Optimistic address is 453 assigned to the node interface. 455 o if Optimistic DAD is not used: an address changes the state from 456 tentative to preferred. 458 ------------------------------------------------------------------ 460 5. IANA Considerations 462 This memo asks the IANA for no new parameters. 464 6. Security Considerations 466 One of the potential attack vectors to consider is a cache spoofing 467 when the attacker might try to install a cache entry for the victim's 468 IPv6 address and the attacker's Link-Layer address. However it 469 should be noted that this document does not propose any changes for 470 the scenario when the ND cache for the given IPv6 address already 471 exists. Therefore it is not possible for the attacker to override 472 any existing cache entry. 474 A malicious host could attempt to exhaust the neighbor cache on the 475 router by creating a large number of STALE entries. However this 476 attack vector is not new and this document does not increase the risk 477 of such an attack: the attacker could do it, for example, by sending 478 a NS or RS packet with SLLAO included. All recommendations from 479 [RFC6583] still apply. 481 Announcing a new address to all-routers multicast address may inform 482 an on-link attacker about IPv6 addresses assigned to the host. 483 However hiding information about the specific IPv6 address should not 484 be considered a security measure as such information is usually 485 disclosed via DAD to all nodes anyway. Network administrators can 486 also mitigate this issue by enabling MLD snooping on the link-layer 487 devices to prevent IPv6 link-local multicast packets being flooded to 488 all onlink nodes. If peer-to-peer onlink communications are not 489 desirable for the given network segment they should be prevented by 490 proper layer2 security mechanisms. Therefore the risk of allowing 491 hosts to send unsolicited Neighbor Advertisements to all-routers 492 multicast address is low. 494 It should be noted that the proposed mechanism allows hosts to 495 proactively inform their routers about global IPv6 addresses existing 496 on-link. Routers could use that information to distinguish between 497 used and unused addresses to mitigate ND cache exhaustion DoS attacks 498 described in Section 4.3.2 [RFC3756] and [RFC6583]. 500 7. Acknowledgements 502 Thanks to the following people (in alphabetical order) for their 503 comments, review and feedback: Lorenzo Colitti, Fernando Gont, Tatuya 504 Jinmei, Erik Kline, Warren Kumari, Erik Nordmark, Michael Richardson, 505 Mark Smith, Dave Thaler, Pascal Thubert, Loganaden Velvindron, Eric 506 Vyncke. 508 8. References 510 8.1. Normative References 512 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 513 Requirement Levels", BCP 14, RFC 2119, 514 DOI 10.17487/RFC2119, March 1997, 515 . 517 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 518 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 519 2006, . 521 [RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD) 522 for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006, 523 . 525 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 526 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 527 DOI 10.17487/RFC4861, September 2007, 528 . 530 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 531 Address Autoconfiguration", RFC 4862, 532 DOI 10.17487/RFC4862, September 2007, 533 . 535 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 536 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 537 May 2017, . 539 8.2. Informative References 541 [I-D.ietf-v6ops-nd-cache-init] 542 Linkova, J., "Neighbor Cache Entries on First-Hop Routers: 543 Operational Considerations", draft-ietf-v6ops-nd-cache- 544 init-03 (work in progress), July 2020. 546 [RFC3756] Nikander, P., Ed., Kempf, J., and E. Nordmark, "IPv6 547 Neighbor Discovery (ND) Trust Models and Threats", 548 RFC 3756, DOI 10.17487/RFC3756, May 2004, 549 . 551 [RFC4541] Christensen, M., Kimball, K., and F. Solensky, 552 "Considerations for Internet Group Management Protocol 553 (IGMP) and Multicast Listener Discovery (MLD) Snooping 554 Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006, 555 . 557 [RFC6583] Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational 558 Neighbor Discovery Problems", RFC 6583, 559 DOI 10.17487/RFC6583, March 2012, 560 . 562 Author's Address 564 Jen Linkova 565 Google 566 1 Darling Island Rd 567 Pyrmont, NSW 2009 568 AU 570 Email: furry@google.com