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rfc:rfc3041

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.

Narten & Draves Standards Track [Page 1] RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001

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

Narten & Draves Standards Track [Page 2] RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001

 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).

Narten & Draves Standards Track [Page 3] RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001

 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

Narten & Draves Standards Track [Page 4] RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001

 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

Narten & Draves Standards Track [Page 5] RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001

 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

Narten & Draves Standards Track [Page 6] RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001

 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

Narten & Draves Standards Track [Page 7] RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001

    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).

Narten & Draves Standards Track [Page 8] RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001

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

Narten & Draves Standards Track [Page 9] RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001

 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.

Narten & Draves Standards Track [Page 10] RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001

 3) When creating a temporary address, the lifetime values are derived
    from the corresponding public address as follows:
  1. Its Valid Lifetime is the lower of the Valid Lifetime of the

public address or TEMP_VALID_LIFETIME.

  1. 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.

Narten & Draves Standards Track [Page 11] RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001

 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

Narten & Draves Standards Track [Page 12] RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001

 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

Narten & Draves Standards Track [Page 13] RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001

 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).

Narten & Draves Standards Track [Page 14] RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001

 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.

Narten & Draves Standards Track [Page 15] 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

Narten & Draves Standards Track [Page 16] RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001

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.

Narten & Draves Standards Track [Page 17]

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