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

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.

   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

   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).

   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

   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

   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

   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

      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).

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

   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.

   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.

   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

   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

   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).

   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.

   [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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