    Network Working Group                                          T. Narten
    Request for Comments: 3041                                           IBM
    Category: Standards Track                                      R. Draves
                                                          Microsoft Research
                                                                January 2001

       Privacy Extensions for Stateless Address Autoconfiguration in IPv6

    Status of this Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2001).  All Rights Reserved.

    Abstract

       Nodes use IPv6 stateless address autoconfiguration to generate
       addresses without the necessity of a Dynamic Host Configuration
       Protocol (DHCP) server.  Addresses are formed by combining network
       prefixes with an interface identifier.  On interfaces that contain
       embedded IEEE Identifiers, the interface identifier is typically
       derived from it.  On other interface types, the interface identifier
       is generated through other means, for example, via random number
       generation.  This document describes an extension to IPv6 stateless
       address autoconfiguration for interfaces whose interface identifier
       is derived from an IEEE identifier.  Use of the extension causes
       nodes to generate global-scope addresses from interface identifiers
       that change over time, even in cases where the interface contains an
       embedded IEEE identifier.  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.

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    Table of Contents

       1.  Introduction.............................................    2
       2.  Background...............................................    3
          2.1.  Extended Use of the Same Identifier.................    3
          2.2.  Address Usage in IPv4 Today.........................    4
          2.3.  The Concern With IPv6 Addresses.....................    5
          2.4.  Possible Approaches.................................    6
       3.  Protocol Description.....................................    7
          3.1.  Assumptions.........................................    8
          3.2.  Generation Of Randomized Interface Identifiers......    9
          3.3.  Generating Temporary Addresses......................   10
          3.4.  Expiration of Temporary Addresses...................   11
          3.5.  Regeneration of Randomized Interface Identifiers....   12
       4.  Implications of Changing Interface Identifiers...........   13
       5.  Defined Constants........................................   14
       6.  Future Work..............................................   14
       7.  Security Considerations..................................   15
       8.  Acknowledgments..........................................   15
       9.  References...............................................   15
       10. Authors' Addresses.......................................   16
       11. Full Copyright Statement.................................   17

    1.  Introduction

       Stateless address autoconfiguration [ADDRCONF] defines how an IPv6
       node generates addresses without the need for a DHCP server.  Some
       types of network interfaces come with an embedded IEEE Identifier
       (i.e., a link-layer MAC address), and in those cases stateless
       address autoconfiguration uses the IEEE identifier to generate a 64-
       bit interface identifier [ADDRARCH].  By design, the interface
       identifier is likely to be globally unique when generated in this
       fashion.  The interface identifier is in turn appended to a prefix to
       form a 128-bit IPv6 address.

       All nodes combine interface identifiers (whether derived from an IEEE
       identifier or generated through some other technique) with the
       reserved link-local prefix to generate link-local addresses for their
       attached interfaces.  Additional addresses, including site-local and
       global-scope addresses, are then created by combining prefixes
       advertised in Router Advertisements via Neighbor Discovery
       [DISCOVERY] with the interface identifier.

       Not all nodes and interfaces contain IEEE identifiers.  In such
       cases, an interface identifier is generated through some other means
       (e.g., at random), and the resultant interface identifier is not
       globally unique and may also change over time.  The focus of this
       document is on addresses derived from IEEE identifiers, as the

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       concern being addressed exists only in those cases where the
       interface identifier is globally unique and non-changing.  The rest
       of this document assumes that IEEE identifiers are being used, but
       the techniques described may also apply to interfaces with other
       types of globally unique and/or persistent identifiers.

       This document discusses concerns associated with the embedding of
       non-changing interface identifiers within IPv6 addresses and
       describes extensions to stateless address autoconfiguration that can
       help mitigate those concerns for individual users and in environments
       where such concerns are significant.  Section 2 provides background
       information on the issue.  Section 3 describes a procedure for
       generating alternate interface identifiers and global-scope
       addresses.  Section 4 discusses implications of changing interface
       identifiers.

    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.  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 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.

       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).

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       Web browsers and servers typically exchange "cookies" with each other
       [COOKIES].  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 [SERIALNUM].

    2.2.  Address Usage in IPv4 Today

       Addresses used in today's Internet are often non-changing in practice
       for extended periods of time, especially in non-home environments
       (e.g., corporations, campuses, etc.).  In such sites, addresses are
       assigned statically and typically change infrequently.  Over the last
       few years, sites have begun moving away from static allocation to
       dynamic allocation via DHCP [DHCP].  In theory, the address a client
       gets via DHCP can change over time, but in practice servers often
       return the same address to the same client (unless addresses are in
       such short supply that they are reused immediately by a different
       node when they become free).  Thus, even within sites using DHCP,
       clients frequently end up using the same address for weeks to months
       at a time.

       For home users accessing the Internet over dialup lines, the
       situation is generally different.  Such users do not have permanent
       connections and are often assigned temporary addresses each time they
       connect to their ISP (e.g., AOL).  Consequently, the addresses they
       use change frequently over time and are shared among a number of
       different users.  Thus, an address does not reliably identify a
       particular device over time spans of more than a few minutes.

       A more interesting case concerns always-on connections (e.g., cable
       modems, ISDN, DSL, etc.) that result in a home site using the same
       address for extended periods of time.  This is a scenario that is
       just starting to become common in IPv4 and promises to become more of
       a concern as always-on internet connectivity becomes widely
       available.  Although it might appear that changing an address
       regularly in such environments would be desirable to lessen privacy

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       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 would be
       grouped together for the purposes of collecting information.  This
       issue is difficult to address, because the routing prefix part of an
       address contains topology information and cannot contain arbitrary
       values.

       Finally, it should be noted that nodes that need a (non-changing) DNS
       name generally have static addresses assigned to them to simplify the
       configuration of DNS servers.  Although Dynamic DNS [DDNS] can be
       used to update the DNS dynamically, it is not yet widely deployed.
       In addition, changing an address but keeping the same DNS name does
       not really address the underlying concern, since the DNS name becomes
       a non-changing identifier.  Servers generally require a DNS name (so
       clients can connect to them), and clients often do as well (e.g.,
       some servers refuse to speak to a client whose address cannot be
       mapped into a DNS name that also maps back into the same address).
       Section 4 describes one approach to this issue.

    2.3.  The Concern With IPv6 Addresses

       The division of IPv6 addresses into distinct topology and interface
       identifier portions raises an issue new to IPv6 in that a fixed
       portion of an IPv6 address (i.e., the interface identifier) can
       contain an identifier that remains constant even when the topology
       portion of an address changes (e.g., as the result of connecting to a
       different part of the Internet).  In IPv4, when an address changes,
       the entire address (including the local part of the address) usually
       changes.  It is this new issue that this document addresses.

       If addresses are generated from an interface identifier, a home
       user's address could contain an interface identifier that remains the
       same from one dialup session to the next, even if the rest of the
       address changes.  The way PPP is used today, however, PPP servers
       typically unilaterally inform the client what address they are to use
       (i.e., the client doesn't generate one on its own).  This practice,
       if continued in IPv6, would avoid the concerns that are the focus of
       this document.

       A more troubling case concerns mobile devices (e.g., laptops, PDAs,
       etc.) that move topologically within the Internet.  Whenever they
       move (in the absence of technology such as mobile IP [MOBILEIP]),
       they form new addresses for their current topological point of
       attachment.  This is typified today by the "road warrior" who has

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       Internet connectivity both at home and at the office.  While the
       node's address changes as it moves, however, the interface identifier
       contained within the address remains the same (when derived from an
       IEEE Identifier).  In such cases, the interface identifier can be
       used to track the movement and usage of a particular machine
       [SERIALNUM].  For example, a server that logs usage information
       together with a source addresses, is also recording the interface
       identifier since it is embedded within an address.  Consequently, any
       data-mining technique that correlates activity based on addresses
       could easily be extended to do the same using the interface
       identifier.  This is of particular concern with the expected
       proliferation of next-generation network-connected devices (e.g.,
       PDAs, cell phones, etc.) in which large numbers of devices are in
       practice associated with individual users (i.e., not shared).  Thus,
       the interface identifier embedded within an address could be used to
       track activities of an individual, even as they move topologically
       within the internet.

       In summary, IPv6 addresses on a given interface generated via
       Stateless Autoconfiguration contain the same interface identifier,
       regardless of where within the Internet the device connects.  This
       facilitates the tracking of individual devices (and thus potentially
       users).  The purpose of this document is to define mechanisms that
       eliminate this issue, in those situations where it is a concern.

    2.4.  Possible Approaches

       One way to avoid some of the problems discussed above is to use DHCP
       for obtaining addresses.  With DHCP, the DHCP server could arrange to
       hand out addresses that change over time.

       Another approach, compatible with the stateless address
       autoconfiguration architecture, would be to change the interface id
       portion of an address over time and generate new addresses from the
       interface identifier for some address scopes.  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 "public" (i.e., non-secret) server address, registered

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       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.

       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.  Picking
       identifiers from a pseudo-random sequence suffices, so long as the
       specific sequence cannot be determined by an outsider examining
       information that is readily available or easily determinable (e.g.,
       by examining packet contents).  This document proposes the generation
       of a pseudo-random sequence of interface identifiers via an MD5 hash.
       Periodically, the next interface identifier in the sequence is
       generated, a new set of temporary addresses is created, and the
       previous temporary addresses are deprecated to discourage their
       further use.  The precise pseudo-random sequence depends on both a
       random component and the globally unique interface identifier (when
       available), to increase the likelihood that different nodes generate
       different sequences.

    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 [ADDRCONF].

       2) Create additional global-scope addresses based on a random
          interface identifier for use with global scope addresses.  Such
          addresses would be used to initiate outgoing sessions.  These
          "random" or temporary addresses would be used for a short period
          of time (hours to days) and would then be deprecated.  Deprecated
          address 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

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          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) Generate a set of addresses from the same (randomized) interface
          identifier, one address for each prefix for which a global address
          has been generated via stateless address autoconfiguration.  Using
          the same interface identifier to generate a set of temporary
          addresses reduces the number of IP multicast groups a host must
          join.  Nodes join the solicited-node multicast address for each
          unicast address they support, and solicited-node addresses are
          dependent only on the low-order bits of the corresponding address.
          This decision was made to address the concern that a node that
          joins a large number of multicast groups may be required to put
          its interface into promiscuous mode, resulting in possible reduced
          performance.

    3.1.  Assumptions

       The following algorithm assumes that each interface maintains an
       associated randomized interface identifier.  When temporary addresses
       are generated, the current value of the associated randomized
       interface identifier is used.  The actual value of the identifier
       changes over time as described below, but the same identifier can be
       used to generate more than one temporary address.

       The algorithm also assumes that for a given temporary address, an
       implementation can determine the corresponding public address 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 in the public address.

       Finally, this document assumes that when a node initiates outgoing
       communication, temporary addresses can be given preference over
       public addresses.  This can mean that all connections initiated by
       the node use temporary addresses by default, or that applications
       individually indicate whether they prefer to use temporary or public
       addresses.  Giving preference to temporary address is consistent with
       on-going work that addresses the topic of source-address selection in
       the more general case [ADDR_SELECT].  An implementation may make it a
       policy that it does not select a public address in the event that no
       temporary address is available (e.g., if generation of a useable
       temporary address fails).

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    3.2.  Generation Of Randomized Interface Identifiers.

       We describe two approaches for the maintenance of the randomized
       interface identifier.  The first assumes the presence of stable
       storage that can be used to record state history for use as input
       into the next iteration of the algorithm across system restarts.  A
       second approach addresses the case where stable storage is
       unavailable and there is a need to generate randomized interface
       identifiers without previous state.

    3.2.1.  When Stable Storage Is Present

       The following algorithm assumes the presence of a 64-bit "history
       value" that is used as input in generating a randomized interface
       identifier.  The very first time the system boots (i.e., out-of-the-
       box), a random value should be generated using techniques that help
       ensure the initial value is hard to guess [RANDOM].  Whenever a new
       interface identifier is generated, a value generated by the
       computation is saved in the history value for the next iteration of
       the algorithm.

       A randomized interface identifier is created as follows:

       1) Take the history value from the previous iteration of this
          algorithm (or a random value if there is no previous value) and
          append to it the interface identifier generated as described in
          [ADDRARCH].
       2) Compute the MD5 message digest [MD5] over the quantity created in
          the previous step.
       3) Take the left-most 64-bits of the MD5 digest and set bit 6 (the
          left-most bit is numbered 0) to zero.  This creates an interface
          identifier with the universal/local bit indicating local
          significance only.  Save the generated identifier as the
          associated randomized interface identifier.
       4) Take the rightmost 64-bits of the MD5 digest computed in step 2)
          and save them in stable storage as the history value to be used in
          the next iteration of the algorithm.

       MD5 was chosen for convenience, and because its particular properties
       were adequate to produce the desired level of randomization.  IPv6
       nodes are already required to implement MD5 as part of IPsec [IPSEC],
       thus the code will already be present on IPv6 machines.

       In theory, generating successive randomized interface identifiers
       using a history scheme as above has no advantages over generating
       them at random.  In practice, however, generating truly random
       numbers can be tricky.  Use of a history value is intended to avoid
       the particular scenario where two nodes generate the same randomized

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       interface identifier, both detect the situation via DAD, but then
       proceed to generate identical randomized interface identifiers via
       the same (flawed) random number generation algorithm.  The above
       algorithm avoids this problem by having the interface identifier
       (which will often be globally unique) used in the calculation that
       generates subsequent randomized interface identifiers.  Thus, if two
       nodes happen to generate the same randomized interface identifier,
       they should generate different ones on the followup attempt.

    3.2.2.  In The Absence of Stable Storage

       In the absence of stable storage, no history value will be available
       across system restarts to generate a pseudo-random sequence of
       interface identifiers.  Consequently, the initial history value used
       above will need to be generated at random.  A number of techniques
       might be appropriate.  Consult [RANDOM] for suggestions on good
       sources for obtaining random numbers.  Note that even though machines
       may not have stable storage for storing a history value, they will in
       many cases have configuration information that differs from one
       machine to another (e.g., user identity, security keys, serial
       numbers, etc.).  One approach to generating a random initial history
       value in such cases is to use the configuration information to
       generate some data bits (which may remain constant for the life of
       the machine, but will vary from one machine to another), append some
       random data and compute the MD5 digest as before.

    3.3.  Generating Temporary Addresses

       [ADDRCONF] 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 [ADDRCONF] 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), perform the
       following steps:

       1) Process the Prefix Information Option as defined in [ADDRCONF],
          either creating a public address or adjusting the lifetimes of
          existing addresses, both public and temporary.  When adjusting the
          lifetimes of an existing temporary address, only lower the
          lifetimes.  Implementations must not increase the lifetimes of an
          existing temporary address when processing a Prefix Information
          Option.
       2) When a new public address is created as described in [ADDRCONF]
          (because the prefix advertised does not match the prefix of any
          address already assigned to the interface, and the Valid Lifetime
          in the option is not zero), also create a new temporary address.

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       3) When creating a temporary address, the lifetime values are derived
          from the corresponding public address as follows:

          -  Its Valid Lifetime is the lower of the Valid Lifetime of the
             public address or TEMP_VALID_LIFETIME.
          -  Its Preferred Lifetime is the lower of the Preferred Lifetime
             of the public address or TEMP_PREFERRED_LIFETIME -
             DESYNC_FACTOR.

          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.
       4) New temporary addresses are created by appending the interface's
          current randomized interface identifier to the prefix that was
          used to generate the corresponding public address.  If by chance
          the new temporary address is the same as an address already
          assigned to the interface, generate a new randomized interface
          identifier and repeat this step.
       5) Perform duplicate address detection (DAD) on the generated
          temporary address.  If DAD indicates the address is already in
          use, generate a new randomized interface identifier as described
          in Section 3.2 above, and repeat the previous steps as appropriate
          up to 5 times.  If after 5 consecutive attempts no non-unique
          address was generated, log a system error and give up attempting
          to generate temporary addresses for that interface.

          Note: because multiple temporary addresses are generated from the
          same associated randomized interface identifier, there is little
          benefit in running DAD on every temporary address.  This document
          recommends that DAD be run on the first address generated from a
          given randomized identifier, but that DAD be skipped on all
          subsequent addresses generated from the same randomized interface
          identifier.

    3.4.  Expiration of Temporary Addresses

       When a temporary address becomes deprecated, a new one should be
       generated.  This is done by repeating the actions described in
       Section 3.3, starting at step 3).  Note that, except for the
       transient period when a temporary address is being regenerated, in
       normal operation at most one temporary address corresponding to a
       public address should be in a non-deprecated state at any given time.
       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.  The
       Prefix Information Option will also deprecate the corresponding
       public address.

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       To insure 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.  It is recommended that an implementation start the address
       regeneration process REGEN_ADVANCE time units before a temporary
       address would actually be deprecated.

       As an optional optimization, an implementation may wish to remove a
       deprecated temporary address that is not in use by applications or
       upper-layers.  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, one may need to use heuristics in
       deciding when an address is no longer in use (e.g., the default
       TEMP_VALID_LIFETIME suggested above).

    3.5.  Regeneration of Randomized Interface Identifiers

       The frequency at which temporary addresses should 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
       implications.

       This document recommends that implementations generate new temporary
       addresses on a periodic basis.  This can be achieved automatically by
       generating a new randomized interface identifier at least once every
       (TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE - DESYNC_FACTOR) time units.
       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

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       synchronize with each other and generate new addresses at exactly the
       same time.  When the preferred lifetime expires, a new temporary
       address is generated using the new randomized interface identifier.

       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 default value given 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 link, a new randomized
       interface identifier should be generated immediately together with a
       new set of temporary addresses.  If a device moves from one ethernet
       to another, generating a new set of temporary addresses from a
       different randomized interface identifier 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.

    4.  Implications of Changing Interface Identifiers

       The IPv6 addressing architecture goes to some lengths to ensure that
       interface identifiers are likely to be globally unique where easy to
       do so.  During the IPng discussions of the GSE proposal [GSE], it was
       felt that keeping interface identifiers globally unique in practice
       might prove useful to future transport protocols.  Usage of the
       algorithms in this document may complicate providing such a future
       flexibility.

       The desires of protecting individual privacy vs. 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 A 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

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       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.  Implementations may provide a method for a 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_VALID_LIFETIME - REGEN_ADVANCE).

    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 [ADDRCONF], 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 (e.g., as is suggested in Section
       3.4).

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       The determination as to whether to use public vs. 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.
       Suitable API extensions will likely need to be developed to enable
       individual applications to indicate with sufficient granularity their
       needs with regards to the use of temporary addresses.

    7.  Security Considerations

       The motivation for this document stems from privacy concerns for
       individuals.  This document does not appear to add any security
       issues beyond those already associated with stateless address
       autoconfiguration [ADDRCONF].

    8.  Acknowledgments

       The authors would like to acknowledge the contributions of the IPNGWG
       working group and, in particular, Matt Crawford, Steve Deering and
       Allison Mankin for their detailed comments.

    9.  References

       [ADDRARCH]    Hinden, R. and S. Deering, "IP Version 6 Addressing
                     Architecture", RFC 2373, July 1998.

       [ADDRCONF]    Thomson, S. and T. Narten, "IPv6 Address
                     Autoconfiguration", RFC 2462, December 1998.

       [ADDR_SELECT] Draves, R. "Default Address Selection for IPv6", Work
                     in Progress.

       [COOKIES]     Kristol, D. and L. Montulli, "HTTP State Management
                     Mechanism", RFC 2965, October 2000.

       [DHCP]        Droms, R., "Dynamic Host Configuration Protocol", RFC
                     2131, March 1997.

       [DDNS]        Vixie, R., Thomson, S., Rekhter, Y. and J. Bound,
                     "Dynamic Updates in the Domain Name System (DNS
                     UPDATE)", RFC 2136, April 1997.

       [DISCOVERY]   Narten, T., Nordmark, E. and W. Simpson, "Neighbor
                     Discovery for IP Version 6 (IPv6)", RFC 2461, December
                     1998.

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    RFC 3041      Extensions to IPv6 Address Autoconfiguration  January 2001

       [GSE]         Crawford, et al., "Separating Identifiers and Locators
                     in Addresses: An Analysis of the GSE Proposal for
                     IPv6", Work in Progress.

       [IPSEC]       Kent, S., Atkinson, R., "Security Architecture for the
                     Internet Protocol", RFC 2401, November 1998.

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

       [MOBILEIP]    Perkins, C., "IP Mobility Support", RFC 2002, October
                     1996.

       [RANDOM]      Eastlake 3rd, D., Crocker S. and J. Schiller,
                     "Randomness Recommendations for Security", RFC 1750,
                     December 1994.

       [SERIALNUM]   Moore, K., "Privacy Considerations for the Use of
                     Hardware Serial Numbers in End-to-End Network
                     Protocols", Work in Progress.

    10. Authors' Addresses

       Thomas Narten
       IBM Corporation
       P.O. Box 12195
       Research Triangle Park, NC 27709-2195
       USA

       Phone: +1 919 254 7798
       EMail: narten@raleigh.ibm.com

       Richard Draves
       Microsoft Research
       One Microsoft Way
       Redmond, WA 98052

       Phone: +1 425 936 2268
       EMail: richdr@microsoft.com

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    11.  Full Copyright Statement

       Copyright (C) The Internet Society (2001).  All Rights Reserved.

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
       BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
       HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
       MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

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