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Network Working Group                                          T. NartenRequest for Comments: 3041                                           IBMCategory: Standards Track                                      R. Draves                                                      Microsoft Research                                                            January 2001Privacy Extensions for Stateless Address Autoconfiguration in IPv6Status 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 2001Table of Contents1.  Introduction.............................................22.  Background...............................................32.1.  Extended Use of the Same Identifier.................32.2.  Address Usage in IPv4 Today.........................42.3.  The Concern With IPv6 Addresses.....................52.4.  Possible Approaches.................................63.  Protocol Description.....................................73.1.  Assumptions.........................................83.2.  Generation Of Randomized Interface Identifiers......93.3.  Generating Temporary Addresses......................103.4.  Expiration of Temporary Addresses...................113.5.  Regeneration of Randomized Interface Identifiers....124.  Implications of Changing Interface Identifiers...........135.  Defined Constants........................................146.  Future Work..............................................147.  Security Considerations..................................158.  Acknowledgments..........................................159.  References...............................................1510. Authors' Addresses.......................................1611. Full Copyright Statement.................................171.  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 theNarten & 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 (seeSection 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 privacyNarten & 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 hasNarten & 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, registeredNarten & 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 aNarten & 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 20013.2.  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 randomizedNarten & 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:      -  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      inSection 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 inSection 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'tNarten & 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 mayNarten & 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 secondsMAX_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 inSection3.4).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",RFC2131, 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",RFC1321, 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.comNarten & Draves             Standards Track                    [Page 16]

RFC 3041      Extensions to IPv6 Address Autoconfiguration  January 200111.  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]