Temporary Address Extensions for Stateless Address Autoconfiguration in IPv6
RFC 8981

Document Type RFC - Proposed Standard (February 2021; No errata)
Obsoletes RFC 4941
Authors Fernando Gont  , Suresh Krishnan  , Thomas Narten  , Richard Draves 
Last updated 2021-02-28
Replaces draft-fgont-6man-rfc4941bis
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Internet Engineering Task Force (IETF)                           F. Gont
Request for Comments: 8981                                  SI6 Networks
Obsoletes: 4941                                              S. Krishnan
Category: Standards Track                                         Kaloom
ISSN: 2070-1721                                                T. Narten
                                                                        
                                                               R. Draves
                                                      Microsoft Research
                                                           February 2021

Temporary Address Extensions for Stateless Address Autoconfiguration in
                                  IPv6

Abstract

   This document describes an extension to IPv6 Stateless Address
   Autoconfiguration that causes hosts to generate temporary addresses
   with randomized interface identifiers for each prefix advertised with
   autoconfiguration enabled.  Changing addresses over time limits the
   window of time during which eavesdroppers and other information
   collectors may trivially perform address-based network-activity
   correlation when the same address is employed for multiple
   transactions by the same host.  Additionally, it reduces the window
   of exposure of a host as being accessible via an address that becomes
   revealed as a result of active communication.  This document
   obsoletes RFC 4941.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8981.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
     1.2.  Problem Statement
   2.  Background
     2.1.  Extended Use of the Same Identifier
     2.2.  Possible Approaches
   3.  Protocol Description
     3.1.  Design Guidelines
     3.2.  Assumptions
     3.3.  Generation of Randomized IIDs
       3.3.1.  Simple Randomized IIDs
       3.3.2.  Generation of IIDs with Pseudorandom Functions
     3.4.  Generating Temporary Addresses
     3.5.  Expiration of Temporary Addresses
     3.6.  Regeneration of Temporary Addresses
     3.7.  Implementation Considerations
     3.8.  Defined Protocol Parameters and Configuration Variables
   4.  Implications of Changing IIDs
   5.  Significant Changes from RFC 4941
   6.  Future Work
   7.  IANA Considerations
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   [RFC4862] specifies Stateless Address Autoconfiguration (SLAAC) for
   IPv6, which typically results in hosts configuring one or more
   "stable" IPv6 addresses composed of a network prefix advertised by a
   local router and a locally generated interface identifier (IID).  The
   security and privacy implications of such addresses have been
   discussed in detail in [RFC7721], [RFC7217], and [RFC7707].  This
   document specifies an extension to SLAAC for generating temporary
   addresses that can help mitigate some of the aforementioned issues.
   This document is a revision of RFC 4941 and formally obsoletes it.
   Section 5 describes the changes from [RFC4941].

   The default address selection for IPv6 has been specified in
   [RFC6724].  In some cases, the determination as to whether to use
   stable versus temporary addresses can only be made by an application.
   For example, some applications may always want to use temporary
   addresses, while others may want to use them only in some
   circumstances or not at all.  An Application Programming Interface
   (API) such as that specified in [RFC5014] can enable individual
   applications to indicate a preference for the use of temporary
   addresses.

   Section 2 provides background information.  Section 3 describes a
   procedure for generating temporary addresses.  Section 4 discusses
   implications of changing IIDs.  Section 5 describes the changes from
   [RFC4941].

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The terms "public address", "stable address", "temporary address",
   "constant IID", "stable IID", and "temporary IID" are to be
   interpreted as specified in [RFC7721].

   The term "global-scope addresses" is used in this document to
   collectively refer to "Global unicast addresses" as defined in
   [RFC4291] and "Unique local addresses" as defined in [RFC4193], and
   not to "globally reachable addresses" as defined in [RFC8190].

1.2.  Problem Statement

   Addresses generated using SLAAC [RFC4862] contain an embedded
   interface identifier, which may remain stable over time.  Anytime a
   fixed identifier is used in multiple contexts, it becomes possible to
   correlate seemingly unrelated activity using this identifier.

   The correlation can be performed by:

   *  An attacker who is in the path between the host in question and
      the peer(s) to which it is communicating, who can view the IPv6
      addresses present in the datagrams.

   *  An attacker who can access the communication logs of the peers
      with which the host has communicated.

   Since the identifier is embedded within the IPv6 address, it cannot
   be hidden.  This document proposes a solution to this issue by
   generating interface identifiers that vary over time.

   Note that an attacker, who is on path, may be able to perform
   significant correlation based on:

   *  The payload contents of unencrypted packets on the wire.

   *  The characteristics of the packets, such as packet size and
      timing.

   Use of temporary addresses will not prevent such correlation, nor
   will it prevent an on-link observer (e.g., the host's default router)
   from tracking all the host's addresses.

2.  Background

   This section discusses the problem in more detail, provides context
   for evaluating the significance of the concerns in specific
   environments, and makes comparisons with existing practices.

2.1.  Extended Use of the Same Identifier

   The use of a non-changing IID to form addresses is a specific
   instance of the more general case where a constant identifier is
   reused over an extended period of time and in multiple independent
   activities.  Anytime the same identifier is used in multiple
   contexts, it becomes possible for that identifier to be used to
   correlate seemingly unrelated activity.  For example, a network
   sniffer placed strategically on a link traversed by all traffic to/
   from a particular host could keep track of which destinations a host
   communicated with and at what times.  In some cases, such information
   can be used to infer things, such as what hours an employee was
   active, when someone is at home, etc.  Although it might appear that
   changing an address regularly in such environments would be desirable
   to lessen privacy concerns, it should be noted that the network-
   prefix portion of an address also serves as a constant identifier.
   All hosts at, say, a home would have the same network prefix, which
   identifies the topological location of those hosts.  This has
   implications for privacy, though not at the same granularity as the
   concern that this document addresses.  Specifically, all hosts within
   a home could be grouped together for the purposes of collecting
   information.  If the network contains a very small number of hosts --
   say, just one -- changing just the IID will not enhance privacy,
   since the prefix serves as a constant identifier.

   One of the requirements for correlating seemingly unrelated
   activities is the use (and reuse) of an identifier that is
   recognizable over time within different contexts.  IP addresses
   provide one obvious example, but there are more.  For example:

   *  Many hosts also have DNS names associated with their addresses, in
      which case, the DNS name serves as a similar identifier.  Although
      the DNS name associated with an address is more work to obtain (it
      may require a DNS query), the information is often readily
      available.  In such cases, changing the address on a host over
      time would do little to address the concerns raised in this
      document, unless the DNS name is also changed at the same time
      (see Section 4).

   *  Web browsers and servers typically exchange "cookies" with each
      other [RFC6265].  Cookies allow web servers to correlate a current
      activity with a previous activity.  One common usage is to send
      back targeted advertising to a user by using the cookie supplied
      by the browser to identify what earlier queries had been made
      (e.g., for what type of information).  Based on the earlier
      queries, advertisements can be targeted to match the (assumed)
      interests of the end user.

   The use of a constant identifier within an address is of special
   concern, because addresses are a fundamental requirement of
   communication and cannot easily be hidden from eavesdroppers and
   other parties.  Even when higher layers encrypt their payloads,
   addresses in packet headers appear in the clear.  Consequently, if a
   mobile host (e.g., laptop) accessed the network from several
   different locations, an eavesdropper might be able to track the
   movement of that mobile host from place to place, even if the upper-
   layer payloads were encrypted.

   Changing addresses over time limits the time window over which
   eavesdroppers and other information collectors may trivially
   correlate network activity when the same address is employed for
   multiple transactions by the same host.  Additionally, it reduces the
   window of exposure during which a host is accessible via an address
   that becomes revealed as a result of active communication.

   The security and privacy implications of IPv6 addresses are discussed
   in detail in [RFC7721], [RFC7707], and [RFC7217].

2.2.  Possible Approaches

   One approach, compatible with the SLAAC architecture, would be to
   change the IID portion of an address over time.  Changing the IID can
   make it more difficult to look at the IP addresses in independent
   transactions and identify which ones actually correspond to the same
   host, both in the case where the routing-prefix portion of an address
   changes and when it does not.

   Many hosts function as both clients and servers.  In such cases, the
   host would need a name (e.g., a DNS domain name) for its use as a
   server.  Whether the address stays fixed or changes has little impact
   on privacy, since the name remains constant and serves as a constant
   identifier.  However, when acting as a client (e.g., initiating
   communication), such a host may want to vary the addresses it uses.
   In such environments, one may need multiple addresses: a stable
   address associated with the name, which is used to accept incoming
   connection requests from other hosts, and a temporary address used to
   shield the identity of the client when it initiates communication.

   On the other hand, a host that functions only as a client may want to
   employ only temporary addresses for public communication.

   To make it difficult to make educated guesses as to whether two
   different IIDs belong to the same host, the algorithm for generating
   alternate identifiers must include input that has an unpredictable
   component from the perspective of the outside entities that are
   collecting information.

3.  Protocol Description

   The following subsections define the procedures for the generation of
   IPv6 temporary addresses.

3.1.  Design Guidelines

   Temporary addresses observe the following properties:

   1.  Temporary addresses are typically employed for initiating
       outgoing sessions.

   2.  Temporary addresses are used for a short period of time
       (typically hours to days) and are subsequently deprecated.
       Deprecated addresses can continue to be used for established
       connections but are not used to initiate new connections.

   3.  New temporary addresses are generated over time to replace
       temporary addresses that expire (i.e., become deprecated and
       eventually invalidated).

   4.  Temporary addresses must have a limited lifetime (limited "valid
       lifetime" and "preferred lifetime" from [RFC4862]).  The lifetime
       of an address should be further reduced when privacy-meaningful
       events (such as a host attaching to a different network, or the
       regeneration of a new randomized Media Access Control (MAC)
       address) take place.  The lifetime of temporary addresses must be
       statistically different for different addresses, such that it is
       hard to predict or infer when a new temporary address is
       generated or correlate a newly generated address with an existing
       one.

   5.  By default, one address is generated for each prefix advertised
       by SLAAC.  The resulting interface identifiers must be
       statistically different when addresses are configured for
       different prefixes or different network interfaces.  This means
       that, given two addresses, it must be difficult for an outside
       entity to infer whether the addresses correspond to the same host
       or network interface.

   6.  It must be difficult for an outside entity to predict the
       interface identifiers that will be employed for temporary
       addresses, even with knowledge of the algorithm/method employed
       to generate them and/or knowledge of the IIDs previously employed
       for other temporary addresses.  These IIDs must be semantically
       opaque [RFC7136] and must not follow any specific patterns.

3.2.  Assumptions

   The following algorithm assumes that, for a given temporary address,
   an implementation can determine the prefix from which it was
   generated.  When a temporary address is deprecated, a new temporary
   address is generated.  The specific valid and preferred lifetimes for
   the new address are dependent on the corresponding lifetime values
   set for the prefix from which it was generated.

   Finally, this document assumes that, when a host initiates outgoing
   communications, temporary addresses can be given preference over
   stable addresses (if available), when the device is configured to do
   so.  [RFC6724] mandates that implementations provide a mechanism that
   allows an application to configure its preference for temporary
   addresses over stable addresses.  It also allows an implementation to
   prefer temporary addresses by default, so that the connections
   initiated by the host can use temporary addresses without requiring
   application-specific enablement.  This document also assumes that an
   API will exist that allows individual applications to indicate
   whether they prefer to use temporary or stable addresses and override
   the system defaults (see, for example, [RFC5014]).

3.3.  Generation of Randomized IIDs

   The following subsections specify example algorithms for generating
   temporary IIDs that follow the guidelines in Section 3.1 of this
   document.  The algorithm specified in Section 3.3.1 assumes a
   pseudorandom number generator (PRNG) is available on the system.  The
   algorithm specified in Section 3.3.2 allows for code reuse by hosts
   that implement [RFC7217].

3.3.1.  Simple Randomized IIDs

   One approach is to select a pseudorandom number of the appropriate
   length.  A host employing this algorithm should generate IIDs as
   follows:

   1.  Obtain a random number from a PRNG that can produce random
       numbers of at least as many bits as required for the IID (please
       see the next step).  [RFC4086] specifies randomness requirements
       for security.

   2.  The IID is obtained by taking as many bits from the random number
       obtained in the previous step as necessary.  See [RFC7136] for
       the necessary number of bits (i.e., the length of the IID).  See
       also [RFC7421] for a discussion of the privacy implications of
       the IID length.  Note: there are no special bits in an IID
       [RFC7136].

   3.  The resulting IID MUST be compared against the reserved IPv6 IIDs
       [RFC5453] [IANA-RESERVED-IID] and against those IIDs already
       employed in an address of the same network interface and the same
       network prefix.  In the event that an unacceptable identifier has
       been generated, a new IID should be generated by repeating the
       algorithm from the first step.

3.3.2.  Generation of IIDs with Pseudorandom Functions

   The algorithm in [RFC7217] can be augmented for the generation of
   temporary addresses.  The benefit of this is that a host could employ
   a single algorithm for generating stable and temporary addresses by
   employing appropriate parameters.

   Hosts would employ the following algorithm for generating the
   temporary IID:

   1.  Compute a random identifier with the expression:

       RID = F(Prefix, Net_Iface, Network_ID, Time, DAD_Counter,
       secret_key)

       Where:

       RID:
          Random Identifier

       F():
          A pseudorandom function (PRF) that MUST NOT be computable from
          the outside (without knowledge of the secret key).  F() MUST
          also be difficult to reverse, such that it resists attempts to
          obtain the secret_key, even when given samples of the output
          of F() and knowledge or control of the other input parameters.
          F() SHOULD produce an output of at least as many bits as
          required for the IID.  BLAKE3 (256-bit key, arbitrary-length
          output) [BLAKE3] is one possible option for F().
          Alternatively, F() could be implemented with a keyed-hash
          message authentication code (HMAC) [RFC2104].  HMAC-SHA-256
          [FIPS-SHS] is one possible option for such an implementation
          alternative.  Note: use of HMAC-MD5 [RFC1321] is considered
          unacceptable for F() [RFC6151].

       Prefix:
          The prefix to be used for SLAAC, as learned from an ICMPv6
          Router Advertisement message.

       Net_Iface:
          The MAC address corresponding to the underlying network-
          interface card, in the case the link uses IEEE 802 link-layer
          identifiers.  Employing the MAC address for this parameter
          (over the other suggested options in [RFC7217]) means that the
          regeneration of a randomized MAC address will result in a
          different temporary address.

       Network_ID:
          Some network-specific data that identifies the subnet to which
          this interface is attached -- for example, the IEEE 802.11
          Service Set Identifier (SSID) corresponding to the network to
          which this interface is associated.  Additionally, "Simple
          Procedures for Detecting Network Attachment in IPv6" ("Simple
          DNA") [RFC6059] describes ideas that could be leveraged to
          generate a Network_ID parameter.  This parameter SHOULD be
          employed if some form of "Network_ID" is available.

       Time:
          An implementation-dependent representation of time.  One
          possible example is the representation in UNIX-like systems
          [OPEN-GROUP], which measure time in terms of the number of
          seconds elapsed since the Epoch (00:00:00 Coordinated
          Universal Time (UTC), 1 January 1970).  The addition of the
          "Time" argument results in (statistically) different IIDs over
          time.

       DAD_Counter:
          A counter that is employed to resolve the conflict where an
          unacceptable identifier has been generated.  This can be
          result of Duplicate Address Detection (DAD), or step 3 below.

       secret_key:
          A secret key that is not known by the attacker.  The secret
          key SHOULD be of at least 128 bits.  It MUST be initialized to
          a pseudorandom number (see [RFC4086] for randomness
          requirements for security) when the operating system is
          "bootstrapped".  The secret_key MUST NOT be employed for any
          other purpose than the one discussed in this section.  For
          example, implementations MUST NOT employ the same secret_key
          for the generation of stable addresses [RFC7217] and the
          generation of temporary addresses via this algorithm.

   2.  The IID is finally obtained by taking as many bits from the RID
       value (computed in the previous step) as necessary, starting from
       the least significant bit.  See [RFC7136] for the necessary
       number of bits (i.e., the length of the IID).  See also [RFC7421]
       for a discussion of the privacy implications of the IID length.
       Note: there are no special bits in an IID [RFC7136].

   3.  The resulting IID MUST be compared against the reserved IPv6 IIDs
       [RFC5453] [IANA-RESERVED-IID] and against those IIDs already
       employed in an address of the same network interface and the same
       network prefix.  In the event that an unacceptable identifier has
       been generated, the DAD_Counter should be incremented by 1, and
       the algorithm should be restarted from the first step.

3.4.  Generating Temporary Addresses

   [RFC4862] describes the steps for generating a link-local address
   when an interface becomes enabled, as well as the steps for
   generating addresses for other scopes.  This document extends
   [RFC4862] as follows.  When processing a Router Advertisement with a
   Prefix Information option carrying a prefix for the purposes of
   address autoconfiguration (i.e., the A bit is set), the host MUST
   perform the following steps:

   1.  Process the Prefix Information option as specified in [RFC4862],
       adjusting the lifetimes of existing temporary addresses, with the
       overall constraint that no temporary addresses should ever remain
       "valid" or "preferred" for a time longer than
       (TEMP_VALID_LIFETIME) or (TEMP_PREFERRED_LIFETIME -
       DESYNC_FACTOR), respectively.  The configuration variables
       TEMP_VALID_LIFETIME and TEMP_PREFERRED_LIFETIME correspond to the
       maximum valid lifetime and the maximum preferred lifetime of
       temporary addresses, respectively.

       Note:
          DESYNC_FACTOR is the value computed when the address was
          created (see step 4 below).

   2.  One way an implementation can satisfy the above constraints is to
       associate with each temporary address a creation time (called
       CREATION_TIME) that indicates the time at which the address was
       created.  When updating the preferred lifetime of an existing
       temporary address, it would be set to expire at whichever time is
       earlier: the time indicated by the received lifetime or
       (CREATION_TIME + TEMP_PREFERRED_LIFETIME - DESYNC_FACTOR).  A
       similar approach can be used with the valid lifetime.

       Note:
          DESYNC_FACTOR is the value computed when the address was
          created (see step 4 below).

   3.  If the host has not configured any temporary address for the
       corresponding prefix, the host SHOULD create a new temporary
       address for such prefix.

       Note:
          For example, a host might implement prefix-specific policies
          such as not configuring temporary addresses for the Unique
          Local IPv6 Unicast Addresses (ULAs) [RFC4193] prefix.

   4.  When creating a temporary address, DESYNC_FACTOR MUST be computed
       and associated with the newly created address, and the address
       lifetime values MUST be derived from the corresponding prefix as
       follows:

       *  Its valid lifetime is the lower of the Valid Lifetime of the
          prefix and TEMP_VALID_LIFETIME.

       *  Its preferred lifetime is the lower of the Preferred Lifetime
          of the prefix and TEMP_PREFERRED_LIFETIME - DESYNC_FACTOR.

   5.  A temporary address is created only if this calculated preferred
       lifetime is greater than REGEN_ADVANCE time units.  In
       particular, an implementation MUST NOT create a temporary address
       with a zero preferred lifetime.

   6.  New temporary addresses MUST be created by appending a randomized
       IID to the prefix that was received.  Section 3.3 of this
       document specifies some sample algorithms for generating the
       randomized IID.

   7.  The host MUST perform DAD on the generated temporary address.  If
       DAD indicates the address is already in use, the host MUST
       generate a new randomized IID and repeat the previous steps as
       appropriate (starting from step 4), up to TEMP_IDGEN_RETRIES
       times.  If, after TEMP_IDGEN_RETRIES consecutive attempts, the
       host is unable to generate a unique temporary address, the host
       MUST log a system error and SHOULD NOT attempt to generate a
       temporary address for the given prefix for the duration of the
       host's attachment to the network via this interface.  This allows
       hosts to recover from occasional DAD failures or otherwise log
       the recurrent address collisions.

3.5.  Expiration of Temporary Addresses

   When a temporary address becomes deprecated, a new one MUST be
   generated.  This is done by repeating the actions described in
   Section 3.4, starting at step 4).  Note that, in normal operation,
   except for the transient period when a temporary address is being
   regenerated, at most one temporary address per prefix should be in a
   nondeprecated state at any given time on a given interface.  Note
   that if a temporary address becomes deprecated as result of
   processing a Prefix Information option with a zero preferred
   lifetime, then a new temporary address MUST NOT be generated (in
   response to the same Prefix Information option).  To ensure that a
   preferred temporary address is always available, a new temporary
   address SHOULD be regenerated slightly before its predecessor is
   deprecated.  This is to allow sufficient time to avoid race
   conditions in the case where generating a new temporary address is
   not instantaneous, such as when DAD must be performed.  The host
   SHOULD start the process of address regeneration REGEN_ADVANCE time
   units before a temporary address is deprecated.

   As an optional optimization, an implementation MAY remove a
   deprecated temporary address that is not in use by applications or
   upper layers, as detailed in Section 6.

3.6.  Regeneration of Temporary Addresses

   The frequency at which temporary addresses change depends on how a
   device is being used (e.g., how frequently it initiates new
   communication) and the concerns of the end user.  The most egregious
   privacy concerns appear to involve addresses used for long periods of
   time (from weeks to years).  The more frequently an address changes,
   the less feasible collecting or coordinating information keyed on
   IIDs becomes.  Moreover, the cost of collecting information and
   attempting to correlate it based on IIDs will only be justified if
   enough addresses contain non-changing identifiers to make it
   worthwhile.  Thus, having large numbers of clients change their
   address on a daily or weekly basis is likely to be sufficient to
   alleviate most privacy concerns.

   There are also client costs associated with having a large number of
   addresses associated with a host (e.g., in doing address lookups, the
   need to join many multicast groups, etc.).  Thus, changing addresses
   frequently (e.g., every few minutes) may have performance
   implications.

   Hosts following this specification SHOULD generate new temporary
   addresses over time.  This can be achieved by generating a new
   temporary address REGEN_ADVANCE time units before a temporary address
   becomes deprecated.  As described above, this produces addresses with
   a preferred lifetime no larger than TEMP_PREFERRED_LIFETIME.  The
   value DESYNC_FACTOR is a random value computed when a temporary
   address is generated; it ensures that clients do not generate new
   addresses at a fixed frequency and that clients do not synchronize
   with each other and generate new addresses at exactly the same time.
   When the preferred lifetime expires, a new temporary address MUST be
   generated using the algorithm specified in Section 3.4 (starting at
   step 4).

   Because the frequency at which it is appropriate to generate new
   addresses varies from one environment to another, implementations
   SHOULD provide end users with the ability to change the frequency at
   which addresses are regenerated.  The default value is given in
   TEMP_PREFERRED_LIFETIME and is one day.  In addition, the exact time
   at which to invalidate a temporary address depends on how
   applications are used by end users.  Thus, the suggested default
   value of two days (TEMP_VALID_LIFETIME) may not be appropriate in all
   environments.  Implementations SHOULD provide end users with the
   ability to override both of these default values.

   Finally, when an interface connects to a new (different) link,
   existing temporary addresses for the corresponding interface MUST be
   removed, and new temporary addresses MUST be generated for use on the
   new link, using the algorithm in Section 3.4.  If a device moves from
   one link to another, generating new temporary addresses ensures that
   the device uses different randomized IIDs for the temporary addresses
   associated with the two links, making it more difficult to correlate
   addresses from the two different links as being from the same host.
   The host MAY follow any process available to it to determine that the
   link change has occurred.  One such process is described by "Simple
   DNA" [RFC6059].  Detecting link changes would prevent link down/up
   events from causing temporary addresses to be (unnecessarily)
   regenerated.

3.7.  Implementation Considerations

   Devices implementing this specification MUST provide a way for the
   end user to explicitly enable or disable the use of temporary
   addresses.  In addition, a site might wish to disable the use of
   temporary addresses in order to simplify network debugging and
   operations.  Consequently, implementations SHOULD provide a way for
   trusted system administrators to enable or disable the use of
   temporary addresses.

   Additionally, sites might wish to selectively enable or disable the
   use of temporary addresses for some prefixes.  For example, a site
   might wish to disable temporary-address generation for ULA [RFC4193]
   prefixes while still generating temporary addresses for all other
   prefixes advertised via PIOs for address configuration.  Another site
   might wish to enable temporary-address generation only for the
   prefixes 2001:db8:1::/48 and 2001:db8:2::/48 while disabling it for
   all other prefixes.  To support this behavior, implementations SHOULD
   provide a way to enable and disable generation of temporary addresses
   for specific prefix subranges.  This per-prefix setting SHOULD
   override the global settings on the host with respect to the
   specified prefix subranges.  Note that the per-prefix setting can be
   applied at any granularity, and not necessarily on a per-subnet
   basis.

3.8.  Defined Protocol Parameters and Configuration Variables

   Protocol parameters and configuration variables defined in this
   document include:

   TEMP_VALID_LIFETIME
      Default value: 2 days.  Users should be able to override the
      default value.

   TEMP_PREFERRED_LIFETIME
      Default value: 1 day.  Users should be able to override the
      default value.  Note: The TEMP_PREFERRED_LIFETIME value MUST be
      smaller than the TEMP_VALID_LIFETIME value, to avoid the
      pathological case where an address is employed for new
      communications but becomes invalid in less than 1 second,
      disrupting those communications.

   REGEN_ADVANCE
      2 + (TEMP_IDGEN_RETRIES * DupAddrDetectTransmits * RetransTimer /
      1000)

      |  Rationale: This parameter is specified as a function of other
      |  protocol parameters, to account for the time possibly spent in
      |  DAD in the worst-case scenario of TEMP_IDGEN_RETRIES.  This
      |  prevents the pathological case where the generation of a new
      |  temporary address is not started with enough anticipation, such
      |  that a new preferred address is generated before the currently
      |  preferred temporary address becomes deprecated.
      |  
      |  RetransTimer is specified in [RFC4861], while
      |  DupAddrDetectTransmits is specified in [RFC4862].  Since
      |  RetransTimer is specified in units of milliseconds, this
      |  expression employs the constant "1000", such that REGEN_ADVANCE
      |  is expressed in seconds.

   MAX_DESYNC_FACTOR
      0.4 * TEMP_PREFERRED_LIFETIME.  Upper bound on DESYNC_FACTOR.

      |  Rationale: Setting MAX_DESYNC_FACTOR to 0.4
      |  TEMP_PREFERRED_LIFETIME results in addresses that have
      |  statistically different lifetimes, and a maximum of three
      |  concurrent temporary addresses when the default values
      |  specified in this section are employed.

   DESYNC_FACTOR
      A random value within the range 0 - MAX_DESYNC_FACTOR.  It is
      computed each time a temporary address is generated, and is
      associated with the corresponding address.  It MUST be smaller
      than (TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE).

   TEMP_IDGEN_RETRIES
      Default value: 3

4.  Implications of Changing IIDs

   The desire to protect individual privacy can conflict with the desire
   to effectively maintain and debug a network.  Having clients use
   addresses that change over time will make it more difficult to track
   down and isolate operational problems.  For example, when looking at
   packet traces, it could become more difficult to determine whether
   one is seeing behavior caused by a single errant host or a number of
   them.

   It is currently recommended that network deployments provide multiple
   IPv6 addresses from each prefix to general-purpose hosts [RFC7934].
   However, in some scenarios, use of a large number of IPv6 addresses
   may have negative implications on network devices that need to
   maintain entries for each IPv6 address in some data structures (e.g.,
   SAVI [RFC7039]).  For example, concurrent active use of multiple IPv6
   addresses will increase Neighbor Discovery traffic if Neighbor Caches
   in network devices are not large enough to store all addresses on the
   link.  This can impact performance and energy efficiency on networks
   on which multicast is expensive (see e.g., [MCAST-PROBLEMS]).
   Additionally, some network-security devices might incorrectly infer
   IPv6 address forging if temporary addresses are regenerated at a high
   rate.

   The use of temporary addresses may cause unexpected difficulties with
   some applications.  For example, some servers refuse to accept
   communications from clients for which they cannot map the IP address
   into a DNS name.  That is, they perform a DNS PTR query to determine
   the DNS name corresponding to an IPv6 address, and may then also
   perform a AAAA query on the returned name to verify it maps back into
   the same address.  Consequently, clients not properly registered in
   the DNS may be unable to access some services.  However, a host's DNS
   name (if non-changing) would serve as a constant identifier.  The
   wide deployment of the extension described in this document could
   challenge the practice of inverse-DNS-based "validation", which has
   little validity, though it is widely implemented.  In order to meet
   server challenges, hosts could register temporary addresses in the
   DNS using random names (for example, a string version of the random
   address itself), albeit at the expense of increased complexity.

   In addition, some applications may not behave robustly if an address
   becomes invalid while it is still in use by the application or if the
   application opens multiple sessions and expects them to all use the
   same address.

   [RFC4941] employed a randomized temporary IID for generating a set of
   temporary addresses, such that temporary addresses configured at a
   given time for multiple SLAAC prefixes would employ the same IID.
   Sharing the same IID among multiple addresses allowed a host to join
   only one solicited-node multicast group per temporary address set.

   This document requires that the IIDs of all temporary addresses on a
   host are statistically different from each other.  This means that
   when a network employs multiple prefixes, each temporary address of a
   set will result in a different solicited-node multicast address, and,
   thus, the number of multicast groups that a host must join becomes a
   function of the number of SLAAC prefixes employed for generating
   temporary addresses.

   Thus, a network that employs multiple prefixes may require hosts to
   join more multicast groups than in the case of implementations of RFC
   4941.  If the number of multicast groups were large enough, a host
   might need to resort to setting the network interface card to
   promiscuous mode.  This could cause the host to process more packets
   than strictly necessary and might have a negative impact on battery
   life and system performance in general.

   We note that since this document reduces the default
   TEMP_VALID_LIFETIME from 7 days (in [RFC4941]) to 2 days, the number
   of concurrent temporary addresses per SLAAC prefix will be smaller
   than for RFC 4941 implementations; thus, the number of multicast
   groups for a network that employs, say, between 1 and 3 prefixes,
   will be similar to the number of such groups for RFC 4941
   implementations.

   Implementations concerned with the maximum number of multicast groups
   that would be required to join as a result of configured addresses,
   or the overall number of configured addresses, should consider
   enforcing implementation-specific limits on, e.g., the maximum number
   of configured addresses, the maximum number of SLAAC prefixes that
   are employed for autoconfiguration, and/or the maximum ratio for
   TEMP_VALID_LIFETIME/TEMP_PREFERRED_LIFETIME (which ultimately
   controls the approximate number of concurrent temporary addresses per
   SLAAC prefix).  Many of these configuration limits are readily
   available in SLAAC and RFC 4941 implementations.  We note that these
   configurable limits are meant to prevent pathological behaviors (as
   opposed to simply limiting the usage of IPv6 addresses), since IPv6
   implementations are expected to leverage the usage of multiple
   addresses [RFC7934].

5.  Significant Changes from RFC 4941

   This section summarizes the substantive changes in this document
   relative to RFC 4941.

   Broadly speaking, this document introduces the following changes:

   *  Addresses a number of flaws in the algorithm for generating
      temporary addresses.  The aforementioned flaws include the use of
      MD5 for computing the temporary IIDs, and reusing the same IID for
      multiple prefixes (see [RAID2015] and [RFC7721] for further
      details).

   *  Allows hosts to employ only temporary addresses.  [RFC4941]
      assumed that temporary addresses were configured in addition to
      stable addresses.  This document does not imply or require the
      configuration of stable addresses; thus, implementations can now
      configure both stable and temporary addresses or temporary
      addresses only.

   *  Removes the recommendation that temporary addresses be disabled by
      default.  This is in line with BCP 188 ([RFC7258]) and also with
      BCP 204 ([RFC7934]).

   *  Reduces the default maximum valid lifetime for temporary addresses
      (TEMP_VALID_LIFETIME).  TEMP_VALID_LIFETIME has been reduced from
      1 week to 2 days, decreasing the typical number of concurrent
      temporary addresses from 7 to 3.  This reduces the possible stress
      on network elements (see Section 4 for further details).

   *  DESYNC_FACTOR is computed each time a temporary address is
      generated and is associated with the corresponding temporary
      address, such that each temporary address has a statistically
      different preferred lifetime, and thus temporary addresses are not
      generated at any specific frequency.

   *  Changes the requirement to not try to regenerate temporary
      addresses upon TEMP_IDGEN_RETRIES consecutive DAD failures from
      "MUST NOT" to "SHOULD NOT".

   *  The discussion about the security and privacy implications of
      different address generation techniques has been replaced with
      references to recent work in this area ([RFC7707], [RFC7721], and
      [RFC7217]).

   *  This document incorporates errata submitted (at the time of
      writing) for [RFC4941] by Jiri Bohac and Alfred Hoenes.

6.  Future Work

   An implementation might want to keep track of which addresses are
   being used by upper layers so as to be able to remove a deprecated
   temporary address from internal data structures once no upper-layer
   protocols are using it (but not before).  This is in contrast to
   current approaches, where addresses are removed from an interface
   when they become invalid [RFC4862], independent of whether or not
   upper-layer protocols are still using them.  For TCP connections,
   such information is available in control blocks.  For UDP-based
   applications, it may be the case that only the applications have
   knowledge about what addresses are actually in use.  Consequently, an
   implementation generally will need to use heuristics in deciding when
   an address is no longer in use.

7.  IANA Considerations

   This document has no IANA actions.

8.  Security Considerations

   If a very small number of hosts (say, only one) use a given prefix
   for extended periods of time, just changing the interface-identifier
   part of the address may not be sufficient to mitigate address-based
   network-activity correlation, since the prefix acts as a constant
   identifier.  The procedures described in this document are most
   effective when the prefix is reasonably nonstatic or used by a fairly
   large number of hosts.  Additionally, if a temporary address is used
   in a session where the user authenticates, any notion of "privacy"
   for that address is compromised for the party or parties that receive
   the authentication information.

   While this document discusses ways to limit the lifetime of interface
   identifiers to reduce the ability of attackers to perform address-
   based network-activity correlation, the method described is believed
   to be ineffective against sophisticated forms of traffic analysis.
   To increase effectiveness, one may need to consider the use of more
   advanced techniques, such as onion routing [ONION].

   Ingress filtering has been and is being deployed as a means of
   preventing the use of spoofed source addresses in Distributed Denial
   of Service (DDoS) attacks.  In a network with a large number of
   hosts, new temporary addresses are created at a fairly high rate.
   This might make it difficult for ingress-/egress-filtering mechanisms
   to distinguish between legitimately changing temporary addresses and
   spoofed source addresses, which are "in-prefix" (using a
   topologically correct prefix and nonexistent interface identifier).
   This can be addressed by using access-control mechanisms on a per-
   address basis on the network ingress point -- though, as noted in
   Section 4, there are corresponding costs for doing so.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [RFC5453]  Krishnan, S., "Reserved IPv6 Interface Identifiers",
              RFC 5453, DOI 10.17487/RFC5453, February 2009,
              <https://www.rfc-editor.org/info/rfc5453>.

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
              <https://www.rfc-editor.org/info/rfc6724>.

   [RFC7136]  Carpenter, B. and S. Jiang, "Significance of IPv6
              Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
              February 2014, <https://www.rfc-editor.org/info/rfc7136>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [BLAKE3]   O'Connor, J., Aumasson, J. P., Neves, S., and Z. Wilcox-
              O'Hearn, "BLAKE3: one function, fast everywhere", 2020,
              <https://blake3.io/>.

   [FIPS-SHS] NIST, "Secure Hash Standard (SHS)", FIPS PUB 180-4,
              DOI 10.6028/NIST.FIPS.180-4, August 2015,
              <https://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.180-4.pdf>.

   [IANA-RESERVED-IID]
              IANA, "Reserved IPv6 Interface Identifiers",
              <https://www.iana.org/assignments/ipv6-interface-ids>.

   [MCAST-PROBLEMS]
              Perkins, C. E., McBride, M., Stanley, D., Kumari, W., and
              J. C. Zuniga, "Multicast Considerations over IEEE 802
              Wireless Media", Work in Progress, Internet-Draft, draft-
              ietf-mboned-ieee802-mcast-problems-13, 4 February 2021,
              <https://tools.ietf.org/html/draft-ietf-mboned-ieee802-
              mcast-problems-13>.

   [ONION]    Reed, M.G., Syverson, P.F., and D.M. Goldschlag, "Proxies
              for Anonymous Routing", Proceedings of the 12th Annual
              Computer Security Applications Conference,
              DOI 10.1109/CSAC.1996.569678, December 1996,
              <https://doi.org/10.1109/CSAC.1996.569678>.

   [OPEN-GROUP]
              The Open Group, "The Open Group Base Specifications Issue
              7", Section 4.16 Seconds Since the Epoch, IEEE Std 1003.1,
              2016,
              <http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
              contents.html>.

   [RAID2015] Ullrich, J. and E.R. Weippl, "Privacy is Not an Option:
              Attacking the IPv6 Privacy Extension",  International
              Symposium on Recent Advances in Intrusion Detection
              (RAID), 2015, <https://publications.sba-
              research.org/publications/Ullrich2015Privacy.pdf>.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <https://www.rfc-editor.org/info/rfc1321>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <https://www.rfc-editor.org/info/rfc4941>.

   [RFC5014]  Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6
              Socket API for Source Address Selection", RFC 5014,
              DOI 10.17487/RFC5014, September 2007,
              <https://www.rfc-editor.org/info/rfc5014>.

   [RFC6059]  Krishnan, S. and G. Daley, "Simple Procedures for
              Detecting Network Attachment in IPv6", RFC 6059,
              DOI 10.17487/RFC6059, November 2010,
              <https://www.rfc-editor.org/info/rfc6059>.

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,
              <https://www.rfc-editor.org/info/rfc6151>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.

   [RFC7039]  Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed.,
              "Source Address Validation Improvement (SAVI) Framework",
              RFC 7039, DOI 10.17487/RFC7039, October 2013,
              <https://www.rfc-editor.org/info/rfc7039>.

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217,
              DOI 10.17487/RFC7217, April 2014,
              <https://www.rfc-editor.org/info/rfc7217>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

   [RFC7421]  Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S.,
              Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit
              Boundary in IPv6 Addressing", RFC 7421,
              DOI 10.17487/RFC7421, January 2015,
              <https://www.rfc-editor.org/info/rfc7421>.

   [RFC7707]  Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
              <https://www.rfc-editor.org/info/rfc7707>.

   [RFC7721]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              RFC 7721, DOI 10.17487/RFC7721, March 2016,
              <https://www.rfc-editor.org/info/rfc7721>.

   [RFC7934]  Colitti, L., Cerf, V., Cheshire, S., and D. Schinazi,
              "Host Address Availability Recommendations", BCP 204,
              RFC 7934, DOI 10.17487/RFC7934, July 2016,
              <https://www.rfc-editor.org/info/rfc7934>.

   [RFC8190]  Bonica, R., Cotton, M., Haberman, B., and L. Vegoda,
              "Updates to the Special-Purpose IP Address Registries",
              BCP 153, RFC 8190, DOI 10.17487/RFC8190, June 2017,
              <https://www.rfc-editor.org/info/rfc8190>.

Acknowledgments

   Fernando Gont was the sole author of this document (a revision of RFC
   4941).  He would like to thank (in alphabetical order) Fred Baker,
   Brian Carpenter, Tim Chown, Lorenzo Colitti, Roman Danyliw, David
   Farmer, Tom Herbert, Bob Hinden, Christian Huitema, Benjamin Kaduk,
   Erik Kline, Gyan Mishra, Dave Plonka, Alvaro Retana, Michael
   Richardson, Mark Smith, Dave Thaler, Pascal Thubert, Ole Troan,
   Johanna Ullrich, Eric Vyncke, Timothy Winters, and Christopher Wood
   for providing valuable comments on earlier draft versions of this
   document.

   This document incorporates errata submitted for RFC 4941 by Jiri
   Bohac and Alfred Hoenes (at the time of writing).

   Suresh Krishnan was the sole author of RFC 4941 (a revision of RFC
   3041).  He would like to acknowledge the contributions of the IPv6
   Working Group and, in particular, Jari Arkko, Pekka Nikander, Pekka
   Savola, Francis Dupont, Brian Haberman, Tatuya Jinmei, and Margaret
   Wasserman for their detailed comments.

   Rich Draves and Thomas Narten were the authors of RFC 3041.  They
   would like to acknowledge the contributions of the IPv6 Working Group
   and, in particular, Ran Atkinson, Matt Crawford, Steve Deering,
   Allison Mankin, and Peter Bieringer.

Authors' Addresses

   Fernando Gont
   SI6 Networks
   Segurola y Habana 4310, 7mo Piso
   Villa Devoto
   Ciudad Autonoma de Buenos Aires
   Argentina

   Email: fgont@si6networks.com
   URI:   https://www.si6networks.com

   Suresh Krishnan
   Kaloom

   Email: suresh@kaloom.com

   Thomas Narten

   Email: narten@cs.duke.edu

   Richard Draves
   Microsoft Research
   One Microsoft Way
   Redmond, WA
   United States of America

   Email: richdr@microsoft.com