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(See Section 2.2 of https://www.ietf.org/id-info/checklist for how to handle the case when there are no actions for IANA.) == The 'Obsoletes: ' line in the draft header should list only the _numbers_ of the RFCs which will be obsoleted by this document (if approved); it should not include the word 'RFC' in the list. -- The abstract seems to indicate that this document obsoletes RFC4941, but the header doesn't have an 'Obsoletes:' line to match this. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (February 26, 2020) is 1518 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 4941 (Obsoleted by RFC 8981) == Outdated reference: A later version (-15) exists of draft-ietf-mboned-ieee802-mcast-problems-11 Summary: 2 errors (**), 0 flaws (~~), 3 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IPv6 Maintenance (6man) Working Group F. Gont 3 Internet-Draft SI6 Networks / UTN-FRH 4 Obsoletes: rfc4941 (if approved) S. Krishnan 5 Intended status: Standards Track Ericsson Research 6 Expires: August 29, 2020 T. Narten 7 IBM Corporation 8 R. Draves 9 Microsoft Research 10 February 26, 2020 12 Privacy Extensions for Stateless Address Autoconfiguration in IPv6 13 draft-ietf-6man-rfc4941bis-07 15 Abstract 17 Nodes use IPv6 stateless address autoconfiguration to generate 18 addresses using a combination of locally available information and 19 information advertised by routers. Addresses are formed by combining 20 network prefixes with an interface identifier. This document 21 describes an extension that causes nodes to generate global scope 22 addresses with randomized interface identifiers that change over 23 time. Changing global scope addresses over time makes it more 24 difficult for eavesdroppers and other information collectors to 25 identify when different addresses used in different transactions 26 correspond to the same node. This document obsoletes RFC4941. 28 Status of This Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at https://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on August 29, 2020. 45 Copyright Notice 47 Copyright (c) 2020 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (https://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 63 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 64 1.2. Problem Statement . . . . . . . . . . . . . . . . . . . . 4 65 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 66 2.1. Extended Use of the Same Identifier . . . . . . . . . . . 4 67 2.2. Possible Approaches . . . . . . . . . . . . . . . . . . . 6 68 3. Protocol Description . . . . . . . . . . . . . . . . . . . . 6 69 3.1. Design Guidelines . . . . . . . . . . . . . . . . . . . . 6 70 3.2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 7 71 3.3. Generation of Randomized Interface Identifiers . . . . . 8 72 3.3.1. Simple Randomized Interface Identifiers . . . . . . . 8 73 3.3.2. Hash-based Generation of Randomized Interface 74 Identifiers . . . . . . . . . . . . . . . . . . . . . 9 75 3.4. Generating Temporary Addresses . . . . . . . . . . . . . 10 76 3.5. Expiration of Temporary Addresses . . . . . . . . . . . . 12 77 3.6. Regeneration of Temporary Addresses . . . . . . . . . . . 12 78 3.7. Implementation Considerations . . . . . . . . . . . . . . 14 79 3.8. Defined Constants . . . . . . . . . . . . . . . . . . . . 14 80 4. Implications of Changing Interface Identifiers . . . . . . . 15 81 5. Significant Changes from RFC4941 . . . . . . . . . . . . . . 16 82 6. Future Work . . . . . . . . . . . . . . . . . . . . . . . . . 16 83 7. Security Considerations . . . . . . . . . . . . . . . . . . . 17 84 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 85 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 86 9.1. Normative References . . . . . . . . . . . . . . . . . . 18 87 9.2. Informative References . . . . . . . . . . . . . . . . . 19 88 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 90 1. Introduction 92 Stateless address autoconfiguration (SLAAC) [RFC4862] defines how an 93 IPv6 node generates addresses without the need for a Dynamic Host 94 Configuration Protocol for IPv6 (DHCPv6) server. The security and 95 privacy implications of such addresses have been discussed in great 96 detail in [RFC7721],[RFC7217], and RFC7707. This document specifies 97 an extension for SLAAC to generate temporary addresses, such that the 98 aforementioned issues are mitigated. This is a revision of RFC4941, 99 and formally obsoletes RFC4941. Section 5 describes the changes from 100 [RFC4941]. 102 The default address selection for IPv6 has been specified in 103 [RFC6724]. The determination as to whether to use stable versus 104 temporary addresses can in some cases only be made by an application. 105 For example, some applications may always want to use temporary 106 addresses, while others may want to use them only in some 107 circumstances or not at all. An Application Programming Interface 108 (API) such as that specified in [RFC5014] can enable individual 109 applications to indicate a preference for the use of temporary 110 addresses. 112 Section 2 provides background information on the issue. Section 3 113 describes a procedure for generating temporary addresses. Section 4 114 discusses implications of changing interface identifiers (IIDs). 115 Section 5 describes the changes from [RFC4941]. 117 1.1. Terminology 119 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 120 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 121 "OPTIONAL" in this document are to be interpreted as described in BCP 122 14 [RFC2119] [RFC8174] when, and only when, they appear in all 123 capitals, as shown here. 125 The terms "public address", "stable address", "temporary address", 126 "constant IID", "stable IID", and "temporary IID" are to be 127 interpreted as specified in [RFC7721]. 129 The term "global scope addresses" is used in this document to 130 collectively refer to "Global unicast addresses" as defined in 131 [RFC4291] and "Unique local addresses" as defined in [RFC4193], and 132 not to "globally reachable" as defined in [RFC8190]. 134 1.2. Problem Statement 136 Addresses generated using stateless address autoconfiguration 137 [RFC4862] contain an embedded interface identifier, which may remain 138 stable over time. Anytime a fixed identifier is used in multiple 139 contexts, it becomes possible to correlate seemingly unrelated 140 activity using this identifier. 142 The correlation can be performed by 144 o An attacker who is in the path between the node in question and 145 the peer(s) to which it is communicating, and who can view the 146 IPv6 addresses present in the datagrams. 148 o An attacker who can access the communication logs of the peers 149 with which the node has communicated. 151 Since the identifier is embedded within the IPv6 address, it cannot 152 be hidden. This document proposes a solution to this issue by 153 generating interface identifiers that vary over time. 155 Note that an attacker, who is on path, may be able to perform 156 significant correlation on unencrypted packets based on 158 o The payload contents of the packets on the wire 160 o The characteristics of the packets such as packet size and timing 162 Use of temporary addresses will not prevent such payload-based 163 correlation, which can only be addressed by widespread deployment of 164 encryption as advocated in [RFC7624]. Nor will it prevent an on-link 165 observer (e.g. the node's default router) to track all the node's 166 addresses. 168 2. Background 170 This section discusses the problem in more detail, and provides 171 context for evaluating the significance of the concerns in specific 172 environments and makes comparisons with existing practices. 174 2.1. Extended Use of the Same Identifier 176 The use of a non-changing interface identifier to form addresses is a 177 specific instance of the more general case where a constant 178 identifier is reused over an extended period of time and in multiple 179 independent activities. Any time the same identifier is used in 180 multiple contexts, it becomes possible for that identifier to be used 181 to correlate seemingly unrelated activity. For example, a network 182 sniffer placed strategically on a link across which all traffic to/ 183 from a particular host crosses could keep track of which destinations 184 a node communicated with and at what times. Such information can in 185 some cases be used to infer things, such as what hours an employee 186 was active, when someone is at home, etc. Although it might appear 187 that changing an address regularly in such environments would be 188 desirable to lessen privacy concerns, it should be noted that the 189 network prefix portion of an address also serves as a constant 190 identifier. All nodes at, say, a home, would have the same network 191 prefix, which identifies the topological location of those nodes. 192 This has implications for privacy, though not at the same granularity 193 as the concern that this document addresses. Specifically, all nodes 194 within a home could be grouped together for the purposes of 195 collecting information. If the network contains a very small number 196 of nodes, say, just one, changing just the interface identifier will 197 not enhance privacy, since the prefix serves as a constant 198 identifier. 200 One of the requirements for correlating seemingly unrelated 201 activities is the use (and reuse) of an identifier that is 202 recognizable over time within different contexts. IP addresses 203 provide one obvious example, but there are more. Many nodes also 204 have DNS names associated with their addresses, in which case the DNS 205 name serves as a similar identifier. Although the DNS name 206 associated with an address is more work to obtain (it may require a 207 DNS query), the information is often readily available. In such 208 cases, changing the address on a machine over time would do little to 209 address the concerns raised in this document, unless the DNS name is 210 changed as well (see Section 4). 212 Web browsers and servers typically exchange "cookies" with each other 213 [RFC6265]. Cookies allow web servers to correlate a current activity 214 with a previous activity. One common usage is to send back targeted 215 advertising to a user by using the cookie supplied by the browser to 216 identify what earlier queries had been made (e.g., for what type of 217 information). Based on the earlier queries, advertisements can be 218 targeted to match the (assumed) interests of the end-user. 220 The use of a constant identifier within an address is of special 221 concern because addresses are a fundamental requirement of 222 communication and cannot easily be hidden from eavesdroppers and 223 other parties. Even when higher layers encrypt their payloads, 224 addresses in packet headers appear in the clear. Consequently, if a 225 mobile host (e.g., laptop) accessed the network from several 226 different locations, an eavesdropper might be able to track the 227 movement of that mobile host from place to place, even if the upper 228 layer payloads were encrypted. 230 The security and privacy implications of IPv6 addresses are discussed 231 in detail in [RFC7721], [RFC7707], and [RFC7217]. 233 Using temporary addresses alone is not sufficient to prevent all 234 forms of tracking. It is however clear that temporary addresses are 235 useful to improve user privacy. 237 2.2. Possible Approaches 239 One approach, compatible with the stateless address autoconfiguration 240 architecture, would be to change the interface identifier portion of 241 an address over time. Changing the interface identifier can make it 242 more difficult to look at the IP addresses in independent 243 transactions and identify which ones actually correspond to the same 244 node, both in the case where the routing prefix portion of an address 245 changes and when it does not. 247 Many machines function as both clients and servers. In such cases, 248 the machine would need a DNS name for its use as a server. Whether 249 the address stays fixed or changes has little privacy implication 250 since the DNS name remains constant and serves as a constant 251 identifier. When acting as a client (e.g., initiating 252 communication), however, such a machine may want to vary the 253 addresses it uses. In such environments, one may need multiple 254 addresses: a stable address registered in the DNS, that is used to 255 accept incoming connection requests from other machines, and a 256 temporary address used to shield the identity of the client when it 257 initiates communication. 259 On the other hand, a machine that functions only as a client may want 260 to employ only temporary addresses for public communication. 262 To make it difficult to make educated guesses as to whether two 263 different interface identifiers belong to the same node, the 264 algorithm for generating alternate identifiers must include input 265 that has an unpredictable component from the perspective of the 266 outside entities that are collecting information. 268 3. Protocol Description 270 The following subsections define the procedures for the generation of 271 IPv6 temporary addresses. 273 3.1. Design Guidelines 275 Temporary addresses observe the following properties: 277 1. Temporary addresses are typically employed for initiating 278 outgoing sessions. 280 2. Temporary addresses are used for a short period of time 281 (typically hours to days) and are subsequently deprecated. 282 Deprecated addresses can continue to be used for established 283 connections, but are not used to initiate new connections. 285 3. New temporary addresses are generated periodically to replace 286 temporary addresses that expire. 288 4. Temporary addresses must have a limited lifetime (limited "valid 289 lifetime" and "preferred lifetime" from [RFC4862]), that should 290 be statistically different for different addresses. The lifetime 291 of an address should be further reduced when privacy-meaningful 292 events (such as a node attaching to a different network, or the 293 regeneration of a new randomized MAC address) takes place. 295 5. By default, one address is generated for each prefix advertised 296 by stateless address autoconfiguration. The resulting Interface 297 Identifiers must be statistically different when addresses are 298 configured for different prefixes. That is, when temporary 299 addresses are generated for different autoconfiguration prefixes 300 for the same network interface, the resulting Interface 301 Identifiers must be statistically different. This means that, 302 given two addresses that employ different prefixes, it must be 303 difficult for an outside entity to tell whether the addresses 304 correspond to the same network interface or even whether they 305 have been generated by the same host. 307 6. It must be difficult for an outside entity to predict the 308 Interface Identifiers that will be employed for temporary 309 addresses, even with knowledge of the algorithm/method employed 310 to generate them and/or knowledge of the Interface Identifiers 311 previously employed for other temporary addresses. These 312 Interface Identifiers must be semantically opaque [RFC7136] and 313 must not follow any specific patterns. 315 3.2. Assumptions 317 The following algorithm assumes that for a given temporary address, 318 an implementation can determine the prefix from which it was 319 generated. When a temporary address is deprecated, a new temporary 320 address is generated. The specific valid and preferred lifetimes for 321 the new address are dependent on the corresponding lifetime values 322 set for the prefix from which it was generated. 324 Finally, this document assumes that when a node initiates outgoing 325 communication, temporary addresses can be given preference over 326 stable addresses (if available), when the device is configured to do 327 so. [RFC6724] mandates implementations to provide a mechanism, which 328 allows an application to configure its preference for temporary 329 addresses over stable addresses. It also allows for an 330 implementation to prefer temporary addresses by default, so that the 331 connections initiated by the node can use temporary addresses without 332 requiring application-specific enablement. This document also 333 assumes that an API will exist that allows individual applications to 334 indicate whether they prefer to use temporary or stable addresses and 335 override the system defaults (see e.g. [RFC5014]). 337 3.3. Generation of Randomized Interface Identifiers 339 The following subsections specify example algorithms for generating 340 temporary interface identifiers that follow the guidelines in 341 Section 3.1 of this document. The algorithm specified in 342 Section 3.3.1 benefits from a Pseudo-Random Number Generator (PRNG) 343 available on the system. The algorithm specified in Section 3.3.2 344 allows for code reuse by nodes that implement [RFC7217]. 346 3.3.1. Simple Randomized Interface Identifiers 348 One approach is to select a pseudorandom number of the appropriate 349 length. A node employing this algorithm should generate IIDs as 350 follows: 352 1. Obtain a random number (see [RFC4086] for randomness requirements 353 for security). 355 2. The Interface Identifier is obtained by taking as many bits from 356 the random number obtained in the previous step as necessary. 357 Note: there are no special bits in an Interface Identifier 358 [RFC7136]. 360 We note that [RFC4291] requires that the Interface IDs of all 361 unicast addresses (except those that start with the binary 362 value 000) be 64 bits long. However, the method discussed in 363 this document could be employed for generating Interface IDs 364 of any arbitrary length, albeit at the expense of reduced 365 entropy (when employing Interface IDs smaller than 64 bits). 366 The privacy implications of the IID length are discussed in 367 [RFC7421]. 369 3. The resulting Interface Identifier SHOULD be compared against the 370 reserved IPv6 Interface Identifiers [RFC5453] [IANA-RESERVED-IID] 371 and against those Interface Identifiers already employed in an 372 address of the same network interface and the same network 373 prefix. In the event that an unacceptable identifier has been 374 generated, a new interface identifier should be generated, by 375 repeating the algorithm from the first step. 377 3.3.2. Hash-based Generation of Randomized Interface Identifiers 379 The algorithm in [RFC7217] can be augmented for the generation of 380 temporary addresses. The benefit of this would be that a node could 381 employ a single algorithm for generating stable and temporary 382 addresses, by employing appropriate parameters. 384 Nodes would employ the following algorithm for generating the 385 temporary IID: 387 1. Compute a random identifier with the expression: 389 RID = F(Prefix, Net_Iface, Network_ID, Time, DAD_Counter, 390 secret_key) 392 Where: 394 RID: 395 Random Identifier 397 F(): 398 A pseudorandom function (PRF) that MUST NOT be computable from 399 the outside (without knowledge of the secret key). F() MUST 400 also be difficult to reverse, such that it resists attempts to 401 obtain the secret_key, even when given samples of the output 402 of F() and knowledge or control of the other input parameters. 403 F() SHOULD produce an output of at least 64 bits. F() could 404 be implemented as a cryptographic hash of the concatenation of 405 each of the function parameters. SHA-256 [FIPS-SHS] is one 406 possible option for F(). Note: MD5 [RFC1321] is considered 407 unacceptable for F() [RFC6151]. 409 Prefix: 410 The prefix to be used for SLAAC, as learned from an ICMPv6 411 Router Advertisement message. 413 Net_Iface: 414 The MAC address corresponding to the underlying network 415 interface card, in the case the link uses IEEE802 link-layer 416 identifiers. Employing the MAC address for this parameter 417 (over the other suggested options in RFC7217) means that the 418 re-generation of a randomized MAC address will result in a 419 different temporary address. 421 Network_ID: 422 Some network-specific data that identifies the subnet to which 423 this interface is attached -- for example, the IEEE 802.11 424 Service Set Identifier (SSID) corresponding to the network to 425 which this interface is associated. Additionally, Simple DNA 426 [RFC6059] describes ideas that could be leveraged to generate 427 a Network_ID parameter. This parameter is SHOULD be employed 428 if some form of "Network_ID" is available. 430 Time: 431 An implementation-dependent representation of time. One 432 possible example is the representation in UNIX-like systems 433 [OPEN-GROUP], that measure time in terms of the number of 434 seconds elapsed since the Epoch (00:00:00 Coordinated 435 Universal Time (UTC), 1 January 1970). The addition of the 436 "Time" argument results in (statistically) different interface 437 identifiers over time. 439 DAD_Counter: 440 A counter that is employed to resolve Duplicate Address 441 Detection (DAD) conflicts. 443 secret_key: 444 A secret key that is not known by the attacker. The secret 445 key SHOULD be of at least 128 bits. It MUST be initialized to 446 a pseudo-random number (see [RFC4086] for randomness 447 requirements for security) when the operating system is 448 "bootstrapped". 450 2. The Interface Identifier is finally obtained by taking as many 451 bits from the RID value (computed in the previous step) as 452 necessary, starting from the least significant bit. The 453 resulting Interface Identifier SHOULD be compared against the 454 reserved IPv6 Interface Identifiers [RFC5453] [IANA-RESERVED-IID] 455 and against those Interface Identifiers already employed in an 456 address of the same network interface and the same network 457 prefix. In the event that an unacceptable identifier has been 458 generated, the value DAD_Counter should be incremented by 1, and 459 the algorithm should be restarted from the first step. 461 3.4. Generating Temporary Addresses 463 [RFC4862] describes the steps for generating a link-local address 464 when an interface becomes enabled as well as the steps for generating 465 addresses for other scopes. This document extends [RFC4862] as 466 follows. When processing a Router Advertisement with a Prefix 467 Information option carrying a prefix for the purposes of address 468 autoconfiguration (i.e., the A bit is set), the node MUST perform the 469 following steps: 471 1. Process the Prefix Information Option as defined in [RFC4862], 472 adjusting the lifetimes of existing temporary addresses. If a 473 received option may extend the lifetimes of temporary addresses, 474 with the overall constraint that no temporary addresses should 475 ever remain "valid" or "preferred" for a time longer than 476 (TEMP_VALID_LIFETIME) or (TEMP_PREFERRED_LIFETIME - 477 DESYNC_FACTOR) respectively. The configuration variables 478 TEMP_VALID_LIFETIME and TEMP_PREFERRED_LIFETIME correspond to 479 approximate target lifetimes for temporary addresses. 481 2. One way an implementation can satisfy the above constraints is to 482 associate with each temporary address a creation time (called 483 CREATION_TIME) that indicates the time at which the address was 484 created. When updating the preferred lifetime of an existing 485 temporary address, it would be set to expire at whichever time is 486 earlier: the time indicated by the received lifetime or 487 (CREATION_TIME + TEMP_PREFERRED_LIFETIME - DESYNC_FACTOR). A 488 similar approach can be used with the valid lifetime. 490 3. If the node has not configured any temporary address for the 491 corresponding prefix, the node SHOULD create a new temporary 492 address for such prefix. 494 Note: 495 For example, a host might implement prefix-specific policies 496 such as not configuring temporary addresses for the Unique 497 Local IPv6 Unicast Addresses (ULA) [RFC4193] prefix. 499 4. When creating a temporary address, the lifetime values MUST be 500 derived from the corresponding prefix as follows: 502 * Its Valid Lifetime is the lower of the Valid Lifetime of the 503 prefix and TEMP_VALID_LIFETIME 505 * Its Preferred Lifetime is the lower of the Preferred Lifetime 506 of the prefix and TEMP_PREFERRED_LIFETIME - DESYNC_FACTOR. 508 5. A temporary address is created only if this calculated Preferred 509 Lifetime is greater than REGEN_ADVANCE time units. In 510 particular, an implementation MUST NOT create a temporary address 511 with a zero Preferred Lifetime. 513 6. New temporary addresses MUST be created by appending a randomized 514 interface identifier (generates as described in Section 3.3 of 515 this document) to the prefix that was received. 517 7. The node MUST perform duplicate address detection (DAD) on the 518 generated temporary address. If DAD indicates the address is 519 already in use, the node MUST generate a new randomized interface 520 identifier, and repeat the previous steps as appropriate up to 521 TEMP_IDGEN_RETRIES times. If after TEMP_IDGEN_RETRIES 522 consecutive attempts no non-unique address was generated, the 523 node MUST log a system error and MUST NOT attempt to generate 524 temporary addresses for that interface. This allows hosts to 525 recover from occasional DAD failures, or otherwise log the 526 recurrent address collisions. 528 3.5. Expiration of Temporary Addresses 530 When a temporary address becomes deprecated, a new one MUST be 531 generated. This is done by repeating the actions described in 532 Section 3.4, starting at step 4). Note that, except for the 533 transient period when a temporary address is being regenerated, in 534 normal operation at most one temporary address per prefix should be 535 in a non-deprecated state at any given time on a given interface. 536 Note that if a temporary address becomes deprecated as result of 537 processing a Prefix Information Option with a zero Preferred 538 Lifetime, then a new temporary address MUST NOT be generated. To 539 ensure that a preferred temporary address is always available, a new 540 temporary address SHOULD be regenerated slightly before its 541 predecessor is deprecated. This is to allow sufficient time to avoid 542 race conditions in the case where generating a new temporary address 543 is not instantaneous, such as when duplicate address detection must 544 be run. The node SHOULD start the address regeneration process 545 REGEN_ADVANCE time units before a temporary address would actually be 546 deprecated. 548 As an optional optimization, an implementation MAY remove a 549 deprecated temporary address that is not in use by applications or 550 upper layers as detailed in Section 6. 552 3.6. Regeneration of Temporary Addresses 554 The frequency at which temporary addresses change depends on how a 555 device is being used (e.g., how frequently it initiates new 556 communication) and the concerns of the end user. The most egregious 557 privacy concerns appear to involve addresses used for long periods of 558 time (weeks to months to years). The more frequently an address 559 changes, the less feasible collecting or coordinating information 560 keyed on interface identifiers becomes. Moreover, the cost of 561 collecting information and attempting to correlate it based on 562 interface identifiers will only be justified if enough addresses 563 contain non-changing identifiers to make it worthwhile. Thus, having 564 large numbers of clients change their address on a daily or weekly 565 basis is likely to be sufficient to alleviate most privacy concerns. 567 There are also client costs associated with having a large number of 568 addresses associated with a node (e.g., in doing address lookups, the 569 need to join many multicast groups, etc.). Thus, changing addresses 570 frequently (e.g., every few minutes) may have performance 571 implications. 573 Nodes following this specification SHOULD generate new temporary 574 addresses on a periodic basis. This can be achieved by generating a 575 new temporary address at least once every (TEMP_PREFERRED_LIFETIME - 576 REGEN_ADVANCE - DESYNC_FACTOR) time units. As described above, 577 generating a new temporary address REGEN_ADVANCE time units before a 578 temporary address becomes deprecated produces addresses with a 579 preferred lifetime no larger than TEMP_PREFERRED_LIFETIME. The value 580 DESYNC_FACTOR is a random value (different for each client) that 581 ensures that clients don't synchronize with each other and generate 582 new addresses at exactly the same time. When the preferred lifetime 583 expires, a new temporary address MUST be generated using the new 584 randomized interface identifier. 586 Because the precise frequency at which it is appropriate to generate 587 new addresses varies from one environment to another, implementations 588 SHOULD provide end users with the ability to change the frequency at 589 which addresses are regenerated. The default value is given in 590 TEMP_PREFERRED_LIFETIME and is one day. In addition, the exact time 591 at which to invalidate a temporary address depends on how 592 applications are used by end users. Thus, the suggested default 593 value of two days (TEMP_VALID_LIFETIME) may not be appropriate in all 594 environments. Implementations SHOULD provide end users with the 595 ability to override both of these default values. 597 Finally, when an interface connects to a new (different) link, a new 598 set of temporary addresses MUST be generated immediately for use on 599 the new link. If a device moves from one link to another, generating 600 a new set of temporary addresses ensures that the device uses 601 different randomized interface identifiers for the temporary 602 addresses associated with the two links, making it more difficult to 603 correlate addresses from the two different links as being from the 604 same node. The node MAY follow any process available to it, to 605 determine that the link change has occurred. One such process is 606 described by "Simple Procedures for Detecting Network Attachment in 607 IPv6" [RFC6059]. Detecting link changes would prevent link down/up 608 events from causing temporary addresses to be (unnecessarily) 609 regenerated. 611 3.7. Implementation Considerations 613 Devices implementing this specification MUST provide a way for the 614 end user to explicitly enable or disable the use of temporary 615 addresses. In addition, a site might wish to disable the use of 616 temporary addresses in order to simplify network debugging and 617 operations. Consequently, implementations SHOULD provide a way for 618 trusted system administrators to enable or disable the use of 619 temporary addresses. 621 Additionally, sites might wish to selectively enable or disable the 622 use of temporary addresses for some prefixes. For example, a site 623 might wish to disable temporary address generation for "Unique local" 624 [RFC4193] prefixes while still generating temporary addresses for all 625 other global prefixes. Another site might wish to enable temporary 626 address generation only for the prefixes 2001:db8:1::/48 and 627 2001:db8:2::/48 while disabling it for all other prefixes. To 628 support this behavior, implementations SHOULD provide a way to enable 629 and disable generation of temporary addresses for specific prefix 630 subranges. This per-prefix setting SHOULD override the global 631 settings on the node with respect to the specified prefix subranges. 632 Note that the per-prefix setting can be applied at any granularity, 633 and not necessarily on a per subnet basis. 635 Use of the extensions defined in this document may complicate 636 debugging and other operational troubleshooting activities. 637 Consequently, it may be site policy that temporary addresses should 638 not be used. Consequently, implementations MUST provide a method for 639 the end user or trusted administrator to override the use of 640 temporary addresses. 642 3.8. Defined Constants 644 Constants defined in this document include: 646 TEMP_VALID_LIFETIME -- Default value: 2 days. Users should be able 647 to override the default value. 649 TEMP_PREFERRED_LIFETIME -- Default value: 1 day. Users should be 650 able to override the default value. 652 REGEN_ADVANCE -- 5 seconds 654 MAX_DESYNC_FACTOR -- 10 minutes. Upper bound on DESYNC_FACTOR. 656 DESYNC_FACTOR -- A random value within the range 0 - 657 MAX_DESYNC_FACTOR. It is computed once at system start (rather than 658 each time it is used) and must never be greater than 659 (TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE). 661 TEMP_IDGEN_RETRIES -- Default value: 3 663 4. Implications of Changing Interface Identifiers 665 The desires of protecting individual privacy versus the desire to 666 effectively maintain and debug a network can conflict with each 667 other. Having clients use addresses that change over time will make 668 it more difficult to track down and isolate operational problems. 669 For example, when looking at packet traces, it could become more 670 difficult to determine whether one is seeing behavior caused by a 671 single errant machine, or by a number of them. 673 Network deployments are currently recommended to provide multiple 674 IPv6 addresses from each prefix to general-purpose hosts [RFC7934]. 675 However, in some scenarios, use of a large number of IPv6 addresses 676 may have negative implications on network devices that need to 677 maintain entries for each IPv6 address in some data structures (e.g., 678 [RFC7039]). Additionally, concurrent active use of multiple IPv6 679 addresses will increase neighbour discovery traffic if Neighbour 680 Caches in network devices are not large enough to store all addresses 681 on the link. This can impact performance and energy efficiency on 682 networks on which multicast is expensive (e.g. 683 [I-D.ietf-mboned-ieee802-mcast-problems]). 685 The use of temporary addresses may cause unexpected difficulties with 686 some applications. For example, some servers refuse to accept 687 communications from clients for which they cannot map the IP address 688 into a DNS name. That is, they perform a DNS PTR query to determine 689 the DNS name, and may then also perform an AAAA query on the returned 690 name to verify that the returned DNS name maps back into the address 691 being used. Consequently, clients not properly registered in the DNS 692 may be unable to access some services. As noted earlier, however, a 693 node's DNS name (if non-changing) serves as a constant identifier. 694 The wide deployment of the extension described in this document could 695 challenge the practice of inverse-DNS-based "authentication," which 696 has little validity, though it is widely implemented. In order to 697 meet server challenges, nodes could register temporary addresses in 698 the DNS using random names (for example, a string version of the 699 random address itself). 701 In addition, some applications may not behave robustly if temporary 702 addresses are used and an address expires before the application has 703 terminated, or if it opens multiple sessions, but expects them to all 704 use the same addresses. 706 5. Significant Changes from RFC4941 708 This section summarizes the changes in this document relative to RFC 709 4941 that an implementer of RFC 4941 should be aware of. 711 Broadly speaking, this document introduces the following changes: 713 o Addresses a number of flaws in the algorithm for generating 714 temporary addresses: The aforementioned flaws include the use of 715 MD5 for computing the temporary IIDs, and reusing the same IID for 716 multiple prefixes (see [RAID2015] and [RFC7721] for further 717 details). 719 o Allows hosts to employ only temporary addresses: 720 [RFC4941] assumed that temporary addresses were configured in 721 addition to stable addresses. This document does not imply or 722 require the configuration of stable addresses, and thus 723 implementations can now configure both stable and temporary 724 addresses, or temporary addresses only. 726 o Recommends that temporary addresses be enabled by default: 727 Enabling temporary addresses by default is in line with BCP188 728 ([RFC7258]), and also with BCP204 ([RFC7934]). 730 o Reduces the default Valid Lifetime for temporary addresses: 731 The default Valid Lifetime for temporary addresses has been 732 reduced from 1 week to 2 days, decreasing the typical number of 733 concurrent temporary addresses from 7 to 2. This reduces the 734 possible stress on network elements (see Section 4 for further 735 details). 737 o Addresses all errata submitted for [RFC4941]. 739 6. Future Work 741 An implementation might want to keep track of which addresses are 742 being used by upper layers so as to be able to remove a deprecated 743 temporary address from internal data structures once no upper layer 744 protocols are using it (but not before). This is in contrast to 745 current approaches where addresses are removed from an interface when 746 they become invalid [RFC4862], independent of whether or not upper 747 layer protocols are still using them. For TCP connections, such 748 information is available in control blocks. For UDP-based 749 applications, it may be the case that only the applications have 750 knowledge about what addresses are actually in use. Consequently, an 751 implementation generally will need to use heuristics in deciding when 752 an address is no longer in use. 754 7. Security Considerations 756 If a very small number of nodes (say, only one) use a given prefix 757 for extended periods of time, just changing the interface identifier 758 part of the address may not be sufficient to address-based network 759 activity correlation, since the prefix acts as a constant identifier. 760 The procedures described in this document are most effective when the 761 prefix is reasonably non static or is used by a fairly large number 762 of nodes. 764 While this document discusses ways of obscuring a user's IP address, 765 the method described is believed to be ineffective against 766 sophisticated forms of traffic analysis. To increase effectiveness, 767 one may need to consider the use of more advanced techniques, such as 768 Onion Routing [ONION]. 770 Ingress filtering has been and is being deployed as a means of 771 preventing the use of spoofed source addresses in Distributed Denial 772 of Service (DDoS) attacks. In a network with a large number of 773 nodes, new temporary addresses are created at a fairly high rate. 774 This might make it difficult for ingress filtering mechanisms to 775 distinguish between legitimately changing temporary addresses and 776 spoofed source addresses, which are "in-prefix" (using a 777 topologically correct prefix and non-existent interface ID). This 778 can be addressed by using access control mechanisms on a per-address 779 basis on the network egress point. 781 8. Acknowledgments 783 The authors would like to thank (in alphabetical order) Fred Baker, 784 Brian Carpenter, Tim Chown, Lorenzo Colitti, David Farmer, Tom 785 Herbert, Bob Hinden, Christian Huitema, Erik Kline, Gyan Mishra, Dave 786 Plonka, Michael Richardson, Mark Smith, Pascal Thubert, Ole Troan, 787 Johanna Ullrich, and Timothy Winters, for providing valuable comments 788 on earlier versions of this document. 790 This document incorporates errata submitted for [RFC4941] by Jiri 791 Bohac and Alfred Hoenes. 793 This document is based on [RFC4941] (a revision of RFC3041). Suresh 794 Krishnan was the sole author of RFC4941. He would like to 795 acknowledge the contributions of the IPv6 working group and, in 796 particular, Jari Arkko, Pekka Nikander, Pekka Savola, Francis Dupont, 797 Brian Haberman, Tatuya Jinmei, and Margaret Wasserman for their 798 detailed comments. 800 Rich Draves and Thomas Narten were the authors of RFC 3041. They 801 would like to acknowledge the contributions of the IPv6 working group 802 and, in particular, Ran Atkinson, Matt Crawford, Steve Deering, 803 Allison Mankin, and Peter Bieringer. 805 9. References 807 9.1. Normative References 809 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 810 Requirement Levels", BCP 14, RFC 2119, 811 DOI 10.17487/RFC2119, March 1997, 812 . 814 [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, 815 "Randomness Requirements for Security", BCP 106, RFC 4086, 816 DOI 10.17487/RFC4086, June 2005, 817 . 819 [RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast 820 Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005, 821 . 823 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 824 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 825 2006, . 827 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 828 Address Autoconfiguration", RFC 4862, 829 DOI 10.17487/RFC4862, September 2007, 830 . 832 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy 833 Extensions for Stateless Address Autoconfiguration in 834 IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, 835 . 837 [RFC5453] Krishnan, S., "Reserved IPv6 Interface Identifiers", 838 RFC 5453, DOI 10.17487/RFC5453, February 2009, 839 . 841 [RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown, 842 "Default Address Selection for Internet Protocol Version 6 843 (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012, 844 . 846 [RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6 847 Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136, 848 February 2014, . 850 [RFC7217] Gont, F., "A Method for Generating Semantically Opaque 851 Interface Identifiers with IPv6 Stateless Address 852 Autoconfiguration (SLAAC)", RFC 7217, 853 DOI 10.17487/RFC7217, April 2014, 854 . 856 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 857 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 858 May 2017, . 860 [RFC8190] Bonica, R., Cotton, M., Haberman, B., and L. Vegoda, 861 "Updates to the Special-Purpose IP Address Registries", 862 BCP 153, RFC 8190, DOI 10.17487/RFC8190, June 2017, 863 . 865 9.2. Informative References 867 [FIPS-SHS] 868 NIST, "Secure Hash Standard (SHS)", FIPS 869 Publication 180-4, August 2015, 870 . 873 [I-D.ietf-mboned-ieee802-mcast-problems] 874 Perkins, C., McBride, M., Stanley, D., Kumari, W., and J. 875 Zuniga, "Multicast Considerations over IEEE 802 Wireless 876 Media", draft-ietf-mboned-ieee802-mcast-problems-11 (work 877 in progress), December 2019. 879 [IANA-RESERVED-IID] 880 IANA, "Reserved IPv6 Interface Identifiers", 881 . 883 [ONION] Reed, MGR., Syverson, PFS., and DMG. Goldschlag, "Proxies 884 for Anonymous Routing", Proceedings of the 12th Annual 885 Computer Security Applications Conference, San Diego, CA, 886 December 1996. 888 [OPEN-GROUP] 889 The Open Group, "The Open Group Base Specifications Issue 890 7 / IEEE Std 1003.1-2008, 2016 Edition", 891 Section 4.16 Seconds Since the Epoch, 2016, 892 . 895 [RAID2015] 896 Ullrich, J. and E. Weippl, "Privacy is Not an Option: 897 Attacking the IPv6 Privacy Extension", International 898 Symposium on Recent Advances in Intrusion Detection 899 (RAID), 2015, . 902 [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, 903 DOI 10.17487/RFC1321, April 1992, 904 . 906 [RFC5014] Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6 907 Socket API for Source Address Selection", RFC 5014, 908 DOI 10.17487/RFC5014, September 2007, 909 . 911 [RFC6059] Krishnan, S. and G. Daley, "Simple Procedures for 912 Detecting Network Attachment in IPv6", RFC 6059, 913 DOI 10.17487/RFC6059, November 2010, 914 . 916 [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations 917 for the MD5 Message-Digest and the HMAC-MD5 Algorithms", 918 RFC 6151, DOI 10.17487/RFC6151, March 2011, 919 . 921 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, 922 DOI 10.17487/RFC6265, April 2011, 923 . 925 [RFC7039] Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed., 926 "Source Address Validation Improvement (SAVI) Framework", 927 RFC 7039, DOI 10.17487/RFC7039, October 2013, 928 . 930 [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an 931 Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 932 2014, . 934 [RFC7421] Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S., 935 Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit 936 Boundary in IPv6 Addressing", RFC 7421, 937 DOI 10.17487/RFC7421, January 2015, 938 . 940 [RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T., 941 Trammell, B., Huitema, C., and D. Borkmann, 942 "Confidentiality in the Face of Pervasive Surveillance: A 943 Threat Model and Problem Statement", RFC 7624, 944 DOI 10.17487/RFC7624, August 2015, 945 . 947 [RFC7707] Gont, F. and T. Chown, "Network Reconnaissance in IPv6 948 Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016, 949 . 951 [RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy 952 Considerations for IPv6 Address Generation Mechanisms", 953 RFC 7721, DOI 10.17487/RFC7721, March 2016, 954 . 956 [RFC7934] Colitti, L., Cerf, V., Cheshire, S., and D. Schinazi, 957 "Host Address Availability Recommendations", BCP 204, 958 RFC 7934, DOI 10.17487/RFC7934, July 2016, 959 . 961 Authors' Addresses 963 Fernando Gont 964 SI6 Networks / UTN-FRH 965 Evaristo Carriego 2644 966 Haedo, Provincia de Buenos Aires 1706 967 Argentina 969 Phone: +54 11 4650 8472 970 Email: fgont@si6networks.com 971 URI: https://www.si6networks.com 973 Suresh Krishnan 974 Ericsson Research 975 8400 Decarie Blvd. 976 Town of Mount Royal, QC 977 Canada 979 Email: suresh.krishnan@ericsson.com 980 Thomas Narten 981 IBM Corporation 982 P.O. Box 12195 983 Research Triangle Park, NC 984 USA 986 Email: narten@us.ibm.com 988 Richard Draves 989 Microsoft Research 990 One Microsoft Way 991 Redmond, WA 992 USA 994 Email: richdr@microsoft.com