IPv6 Maintenance (6man) Working Group                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Obsoletes: rfc4941 (if approved)                             S. Krishnan
Intended status: Standards Track                       Ericsson Research
Expires: September 12, 2019 January 9, 2020                                       T. Narten
                                                         IBM Corporation
                                                               R. Draves
                                                      Microsoft Research
                                                          March 11,
                                                            July 8, 2019

   Privacy Extensions for Stateless Address Autoconfiguration in IPv6


   Nodes use IPv6 stateless address autoconfiguration to generate
   addresses using a combination of locally available information and
   information advertised by routers.  Addresses are formed by combining
   network prefixes with an interface identifier.  This document
   describes an extension that causes nodes to generate global scope
   addresses from interface identifiers that change over time.  Changing
   the interface identifier (and the global scope addresses generated
   from it) over time makes it more difficult for eavesdroppers and
   other information collectors to identify when different addresses
   used in different transactions actually correspond to the same node.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 12, 2019. January 9, 2020.

Copyright Notice

   Copyright (c) 2019 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Problem Statement . . . . . . . . . . . . . . . . . . . .   3
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Extended Use of the Same Identifier . . . . . . . . . . .   4
     2.2.  Possible Approaches . . . . . . . . . . . . . . . . . . .   5
   3.  Protocol Description  . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Assumptions . . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Generation of Randomized Interface Identifiers  . . . . .   7
       3.2.1.  Simple Randomized Interface Identifiers . . . . . . .   8
       3.2.2.  Hash-based Generation of Randomized Interface
               Identifiers . . . . . . . . . . . . . . . . . . . . .   8
     3.3.  Generating Temporary Addresses  . . . . . . . . . . . . .  10
     3.4.  Expiration of Temporary Addresses . . . . . . . . . . . .  11
     3.5.  Regeneration of Randomized Interface Identifiers  . . . .  12
     3.6.  Deployment Considerations . . . . . . . . . . . . . . . .  13
   4.  Implications of Changing Interface Identifiers  . . . . . . .  14
   5.  Defined Constants . . . . . . . . . . . . . . . . . . . . . .  14
   6.  Future Work . . . . . . . . . . . . . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   8.  Significant Changes from RFC4941  . . . . . . . . . . . . . .  16
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   Stateless address autoconfiguration [RFC4862] defines how an IPv6
   node generates addresses without the need for a Dynamic Host
   Configuration Protocol for IPv6 (DHCPv6) server.  The security and
   privacy implications of such addresses have been discussed in great
   detail in [RFC7721],[RFC7217], and RFC7707.  This document specifies
   an extension for SLAAC to generate temporary addresses, such that the
   aforementioned issues are mitigated.

   The default address selection for IPv6 has been specified in
   [RFC6724].  We note that the determination as to whether to use
   stable versus temporary addresses can in some cases 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 API such as that specified in
   [RFC5014] can enable individual applications to indicate with
   sufficient granularity their needs with regards to the use of
   temporary addresses.

   Section 2 provides background information on the issue.  Section 3
   describes a procedure for generating temporary interface identifiers
   and global scope addresses.  Section 4 discusses implications of
   changing interface identifiers.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   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].

1.2.  Problem Statement

   Addresses generated using stateless address autoconfiguration
   [RFC4862] contain an embedded interface identifier, which remains
   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
   o  An attacker who is in the path between the node in question and
      the peer(s) to which it is communicating, and who can view the
      IPv6 addresses present in the datagrams.

   o  An attacker who can access the communication logs of the peers
      with which the node has communicated.

   Since the identifier is embedded within the IPv6 address, which is a
   fundamental requirement of communication, it cannot be easily 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

   o  The payload contents of the packets on the wire

   o  The characteristics of the packets such as packet size and timing

   Use of temporary addresses will not prevent such payload-based

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 interface identifier 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.  Any time 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 across which all traffic to/
   from a particular host crosses could keep track of which destinations
   a node communicated with and at what times.  Such information can in
   some cases 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 nodes at, say, a home, would have the same network
   prefix, which identifies the topological location of those nodes.
   This has implications for privacy, though not at the same granularity
   as the concern that this document addresses.  Specifically, all nodes
   within a home could be grouped together for the purposes of
   collecting information.  If the network contains a very small number
   of nodes, say, just one, changing just the interface identifier will
   not enhance privacy at all, since the prefix serves as a constant

   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.  Many nodes 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 machine over time would do little to
   address the concerns raised in this document, unless the DNS name is
   changed as well (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.

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

2.2.  Possible Approaches

   One way to avoid having a stable non-changing address is to use
   DHCPv6 [RFC3315] for obtaining addresses.  Section 12 of [RFC3315]
   discusses the use of DHCPv6 for the assignment and management of
   "temporary addresses", which are never renewed and provide the same
   property of temporary addresses described in this document with
   regards to the privacy concern.

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

   Many machines function as both clients and servers.  In such cases,
   the machine would need a DNS name for its use as a server.  Whether
   the address stays fixed or changes has little privacy implication
   since the DNS name remains constant and serves as a constant
   identifier.  When acting as a client (e.g., initiating
   communication), however, such a machine may want to vary the
   addresses it uses.  In such environments, one may need multiple
   addresses: a stable address registered in the DNS, that is used to
   accept incoming connection requests from other machines, and a
   temporary address used to shield the identity of the client when it
   initiates communication.  These two cases are roughly analogous to
   telephone numbers and caller ID, where a user may list their
   telephone number in the public phone book, but disable the display of
   its number via caller ID when initiating calls.

   On the other hand, a machine 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 interface identifiers belong to the same node, 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.

   [I-D.gont-6man-non-stable-iids] specifies requirements for temporary
   addresses.  This document specifies a number of algorithms for
   generating temporary addresses that comply with the aforementioned

3.  Protocol Description

   The goal of this section is to define procedures that:

   1.  Do not result in any changes to the basic behavior of addresses
       generated via stateless address autoconfiguration [RFC4862].

   2.  Create temporary addresses based on an unpredictable interface
       identifier for the purpose of initiating outgoing sessions.
       These temporary addresses would be used for a short period of
       time (hours to days) and would then be deprecated.  Deprecated
       addresses can continue to be used for already established
       connections, but are not used to initiate new connections.  New
       temporary addresses are generated periodically to replace
       temporary addresses that expire, with the exact time between
       address generation a matter of local policy.

   3.  Produce a sequence of temporary global scope addresses from a
       sequence of interface identifiers that appear to be random in the
       sense that it is difficult for an outside observer to predict a
       future address (or identifier) based on a current one and it is
       difficult to determine previous addresses (or identifiers)
       knowing only the present one.

   4.  By default, generate one address for each prefix advertised for
       stateless address autoconfiguration.  The interface identifier
       generated for each of those prefixes should be (statistically)
       different.  That is, a new interface identifier should be
       computed for each temporary address that is to be generated.

3.1.  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 node initiates outgoing
   communication, temporary addresses can be given preference over
   stable addresses (if available), when the device is configured to do
   so.  [RFC6724] mandates implementations to provide a mechanism, which
   allows an application to configure its preference for temporary
   addresses over stable addresses.  It also allows for an
   implementation to prefer temporary addresses by default, so that the
   connections initiated by the node 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.

3.2.  Generation of Randomized Interface Identifiers

   The following subsections specify some possible algorithms for
   generating temporary interface identifiers that comply with the
   requirements in [I-D.gont-6man-non-stable-iids].  The algorithm
   specified in Section 3.2.1 benefits from a Peseudo-Random Pseudo-Random Number
   Generator (PRNG) available on the system.  On the other hand, the
   algorithm specified in Section 3.2.2 allows for code reuse by nodes
   that implement [RFC7217].

3.2.1.  Simple Randomized Interface Identifiers

   One possible approach would be to select a pseudorandom number of the
   appropriate length.  A node employing this algorithm should generate
   IIDs as follows:

   1.  Obtain a random number (see [RFC4086] for randomness requirements
       for security)

   2.  The Interface Identifier is obtained by taking as many bits from
       the aforementioned random number (obtained in the previous step)
       as necessary.

          We note that [RFC4291] requires that the Interface IDs of all
          unicast addresses (except those that start with the binary
          value 000) be 64 bits long.  However, the method discussed in
          this document could be employed for generating Interface IDs
          of any arbitrary length, albeit at the expense of reduced
          entropy (when employing Interface IDs smaller than 64 bits).

   3.  The resulting Interface Identifier SHOULD be compared against the
       reserved IPv6 Interface Identifiers [RFC5453] [IANA-RESERVED-IID]
       and against those Interface Identifiers 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 interface identifier should be generated, by
       repeating the algorithm from the first step.

3.2.2.  Hash-based Generation of Randomized Interface Identifiers

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

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

   1.  Compute a random identifier with the expression:

       RID = F(Prefix, MAC_Address, Network_ID, Time, DAD_Counter,


          Random Identifier

          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 64 bits.  F() could
          be implemented as a cryptographic hash of the concatenation of
          each of the function parameters.  SHA-1 [FIPS-SHS] and SHA-256
          are two possible options for F().  Note: MD5 [RFC1321] is
          considered unacceptable for F() [RFC6151].

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

          The MAC address corresponding to the underlying network
          interface card.  Employing the MAC address in this expression
          (in replacement of the Net_Iface parameter of the expression
          in RFC7217) means that the re-generation of a randomized MAC
          address will result in a different temporary address.

          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 DNA
          [RFC6059] describes ideas that could be leveraged to generate
          a Network_ID parameter.  This parameter is SHOULD be employed
          if some form of "Network_ID" is available.

          An implementation-dependent representation of time.  One
          possible example is the representation in UNIX-like systems
          [OPEN-GROUP], that measure time in terms of the number of
          seconds elapsed since the Epoch (00:00:00 Coordinated
          Universal Time (UTC), 1 January 1970).

          A counter that is employed to resolve Duplicate Address
          Detection (DAD) conflicts.


          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 pseudo-random number (see [RFC4086] for randomness
          requirements for security) when the operating system is

   2.  The Interface Identifier 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.  The
       resulting Interface Identifier SHOULD be compared against the
       reserved IPv6 Interface Identifiers [RFC5453] [IANA-RESERVED-IID]
       and against those Interface Identifiers 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 value DAD_Counter should be incremented by 1, and
       the algorithm should be restarted from the first step.

3.3.  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 global scope prefix for the purposes of
   address autoconfiguration (i.e., the A bit is set), the node MUST
   perform the following steps:

   1.  Process the Prefix Information Option as defined in [RFC4862],
       either creating a new stable address or
       adjusting the lifetimes of existing addresses, both stable and temporary. temporary addresses.  If a
       received option will may extend the lifetime of a stable address, the lifetimes of temporary addresses should be extended, subject to addresses,
       with the overall constraint that no temporary addresses should
       ever remain "valid" or "preferred" for a time longer than
       DESYNC_FACTOR) respectively.  The configuration variables
       approximate target lifetimes for temporary addresses.

   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
       similar approach can be used with the valid lifetime.

   3.  When a new stable address is created as described in [RFC4862],
       or if  If the node has not configured any temporary address for the
       corresponding prefix, the node SHOULD create a new temporary
       address for such prefix.

   4.  When creating a temporary address, the 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

   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 the
       interface's current a randomized
       interface identifier (generates as described in Section 3.2 of
       this document) to the prefix that was received.

   7.  The node MUST perform duplicate address detection (DAD) on the
       generated temporary address.  If DAD indicates the address is
       already in use, the node MUST generate a new randomized interface
       identifier, and repeat the previous steps as appropriate up to
       consecutive attempts no non-unique address was generated, the
       node MUST log a system error and MUST NOT attempt to generate
       temporary addresses for that interface.  Note that DAD MUST be
       performed on every unicast address generated from this randomized
       interface identifier.

3.4.  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 3.3, starting at step 4).  Note that, except for the
   transient period when a temporary address is being regenerated, in
   normal operation at most one temporary address per prefix should be
   in a non-deprecated 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.  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 duplicate address detection must
   be run.  The node SHOULD start the address regeneration process
   REGEN_ADVANCE time units before a temporary address would actually be

   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.

3.5.  Regeneration of Randomized Interface Identifiers

   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 (weeks to months to years).  The more frequently an address
   changes, the less feasible collecting or coordinating information
   keyed on interface identifiers becomes.  Moreover, the cost of
   collecting information and attempting to correlate it based on
   interface identifiers 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 node (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

   Nodes following this specification SHOULD generate new temporary
   addresses on a periodic basis.  This can be achieved automatically by
   generating a new randomized interface identifier at least once every
   As described above, generating a new temporary address REGEN_ADVANCE
   time units before a temporary address becomes deprecated produces
   addresses with a preferred lifetime no larger than
   TEMP_PREFERRED_LIFETIME.  The value DESYNC_FACTOR is a random value
   (different for each client) that ensures that clients don't
   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 new randomized interface

   Because the precise 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 one week (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, a new
   set of temporary addresses MUST be generated immediately.  If a
   device moves from one ethernet to another, generating a new set of
   temporary addresses ensures that the device uses different randomized
   interface identifiers 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 node.  The node MAY follow
   any process available to it, to determine that the link change has
   occurred.  One such process is described by "Simple Procedures for
   Detecting Network Attachment [RFC4135]. in IPv6" [RFC6059].  Detecting link
   changes would prevent link down/up events from causing temporary
   addresses to be (unnecessarily) regenerated.

3.6.  Deployment 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 "Unique local"
   [RFC4193] prefixes while still generating temporary addresses for all
   other global prefixes.  Another site might wish to enable temporary
   address generation only for the prefixes 2001::/16 and 2002::/16
   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 node 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.

   The use of temporary addresses may cause unexpected difficulties with
   some applications.  As described below, some servers refuse to accept
   communications from clients for which they cannot map the IP address
   into a DNS name.  In addition, some applications may not behave
   robustly if temporary addresses are used and an address expires
   before the application has terminated, or if it opens multiple
   sessions, but expects them to all use the same addresses.

   If a very small number of nodes (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 ensure privacy, since
   the prefix acts as a constant identifier.  The procedures described
   in this document are most effective when the prefix is reasonably non
   static or is used by a fairly large number of nodes.

4.  Implications of Changing Interface Identifiers

   The desires of protecting individual privacy versus the desire to
   effectively maintain and debug a network can conflict with each
   other.  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 machine, or by a number of them.

   Some servers refuse to grant access to clients for which no DNS name
   exists.  That is, they perform a DNS PTR query to determine the DNS
   name, and may then also perform an AAAA query on the returned name to
   verify that the returned DNS name maps back into the address being
   used.  Consequently, clients not properly registered in the DNS may
   be unable to access some services.  As noted earlier, however, a
   node's DNS name (if non-changing) serves as a constant identifier.
   The wide deployment of the extension described in this document could
   challenge the practice of inverse-DNS-based "authentication," which
   has little validity, though it is widely implemented.  In order to
   meet server challenges, nodes could register temporary addresses in
   the DNS using random names (for example, a string version of the
   random address itself).

   Use of the extensions defined in this document may complicate
   debugging and other operational troubleshooting activities.
   Consequently, it may be site policy that temporary addresses should
   not be used.  Consequently, implementations MUST provide a method for
   the end user or trusted administrator to override the use of
   temporary addresses.

5.  Defined Constants

   Constants defined in this document include:

   TEMP_VALID_LIFETIME -- Default value: 1 week.  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.

   REGEN_ADVANCE -- 5 seconds

   MAX_DESYNC_FACTOR -- 10 minutes.  Upper bound on DESYNC_FACTOR.

   DESYNC_FACTOR -- A random value within the range 0 -
   MAX_DESYNC_FACTOR.  It is computed once at system start (rather than
   each time it is used) and must never be greater than

   TEMP_IDGEN_RETRIES -- Default value: 3

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.

   Recommendations on DNS practices to avoid the problem described in
   Section 4 when reverse DNS lookups fail may be needed.  [RFC4472]
   contains a more detailed discussion of the DNS-related issues.

   While this document discusses ways of obscuring a user's IP address,
   the method described is believed to be ineffective against
   sophisticated forms of traffic analysis.  To increase effectiveness,
   one may need to consider use of more advanced techniques, such as
   Onion Routing [ONION].

7.  Security Considerations

   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
   nodes, new temporary addresses are created at a fairly high rate.
   This might make it difficult for ingress filtering mechanisms to
   distinguish between legitimately changing temporary addresses and
   spoofed source addresses, which are "in-prefix" (using a
   topologically correct prefix and non-existent interface ID).  This
   can be addressed by using access control mechanisms on a per-address
   basis on the network egress point.

8.  Significant Changes from RFC4941

   This section summarizes the changes in this document relative to RFC
   4941 that an implementer of RFC 4941 should be aware of.

   1.  Discussion of IEEE-based IIDs has been removed, since the current
       recommendation ([RFC8064]) is to employ [RFC7217]).

   2.  The document employs the terminology from [RFC7721].

   3.  Sections 2.2 and 2.3 of [RFC4941] have been removed since the
       topic has been discussed in more detail in e.g.  [RFC7721].

   4.  The algorithm to generate randomized interface identifiers was
       replaced by two possible alternative algorithms.

   5.  Generation of stable addresses is not implied or required by this

   6.  Temporary addresses are *not* disabled by default.

   7.  Section 3.2.1 and 3.2.2 from [RFC4941] were replaced with
       alternative algorithms.

   8.  Section 3.2.3 from [RFC4941] was removed, based on the
       explanation of that very section of RFC4941.

   9.  All the verified errata for [RFC4941] has been incorporated.

9.  Acknowledgments

   The authors would like to thank (in alphabetical order) Brian
   Carpenter, Tim Chown, Lorenzo Colitti, David Farmer, Tom Herbert, Bob
   Hinden, Christian Huitema, Dave Plonka, Michael Richardson, Mark
   Smith, and Johanna Ullrich for providing valuable comments on earlier
   versions of this document.

   This document is based on [RFC4941] (a revision of RFC3041).  Suresh
   Krishnan was the sole author of RFC4941.  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.

10.  References

10.1.  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,

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,

   [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>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,

   [RFC5453]  Krishnan, S., "Reserved IPv6 Interface Identifiers",
              RFC 5453, DOI 10.17487/RFC5453, February 2009,

   [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,

   [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>.

   [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,

   [RFC8064]  Gont, F., Cooper, A., Thaler, D., and W. Liu,
              "Recommendation on Stable IPv6 Interface Identifiers",
              RFC 8064, DOI 10.17487/RFC8064, February 2017,

10.2.  Informative References

              NIST, "Secure Hash Standard (SHS)", FIPS
              Publication 180-4, March 2012,

              Gont, F., Huitema, C., Krishnan, S., Gont, G., and M.
              Corbo, "Recommendation on Temporary IPv6 Interface
              Identifiers", draft-gont-6man-non-stable-iids-04 (work in
              progress), March 2018.

              IANA, "Reserved IPv6 Interface Identifiers",

   [ONION]    Reed, MGR., Syverson, PFS., and DMG. Goldschlag, "Proxies
              for Anonymous Routing",  Proceedings of the 12th Annual
              Computer Security Applications Conference, San Diego, CA,
              December 1996.

              The Open Group, "The Open Group Base Specifications Issue
              7 / IEEE Std 1003.1-2008, 2016 Edition",
              Section 4.16 Seconds Since the Epoch, 2016,

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <https://www.rfc-editor.org/info/rfc3315>.

   [RFC4135]  Choi, JH. and G. Daley, "Goals of Detecting Network
              Attachment in IPv6", RFC 4135, DOI 10.17487/RFC4135,
              August 2005, <https://www.rfc-editor.org/info/rfc4135>.

   [RFC4472]  Durand, A., Ihren, J., and P. Savola, "Operational
              Considerations and Issues with IPv6 DNS", RFC 4472,
              DOI 10.17487/RFC4472, April 2006,

   [RFC5014]  Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6
              Socket API for Source Address Selection", RFC 5014,
              DOI 10.17487/RFC5014, September 2007,

   [RFC6059]  Krishnan, S. and G. Daley, "Simple Procedures for
              Detecting Network Attachment in IPv6", RFC 6059,
              DOI 10.17487/RFC6059, November 2010,

   [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,

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,

   [RFC7707]  Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,

   [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,

Authors' Addresses
   Fernando Gont
   SI6 Networks / UTN-FRH
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com

   Suresh Krishnan
   Ericsson Research
   8400 Decarie Blvd.
   Town of Mount Royal, QC

   Email: suresh.krishnan@ericsson.com

   Thomas Narten
   IBM Corporation
   P.O. Box 12195
   Research Triangle Park, NC

   Email: narten@us.ibm.com

   Richard Draves
   Microsoft Research
   One Microsoft Way
   Redmond, WA

   Email: richdr@microsoft.com