rfc3041.txt

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   TEMP_VALID_LIFETIME suggested above).

3.5.  Regeneration of Randomized Interface Identifiers

   The frequency at which temporary addresses should change depends on
   how a device is being used (e.g., how frequently it initiates new
   communication) and the concerns of the end user.  The most egregious
   privacy concerns appear to involve addresses used for long periods of
   time (weeks to months to years).  The more frequently an address
   changes, the less feasible collecting or coordinating information
   keyed on interface identifiers becomes.  Moreover, the cost of
   collecting information and attempting to correlate it based on
   interface identifiers will only be justified if enough addresses
   contain non-changing identifiers to make it worthwhile.  Thus, having
   large numbers of clients change their address on a daily or weekly
   basis is likely to be sufficient to alleviate most privacy concerns.

   There are also client costs associated with having a large number of
   addresses associated with a node (e.g., in doing address lookups, the
   need to join many multicast groups, etc.).  Thus, changing addresses
   frequently (e.g., every few minutes) may have performance
   implications.

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



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

   Because the precise frequency at which it is appropriate to generate
   new addresses varies from one environment to another, implementations
   should provide end users with the ability to change the frequency at
   which addresses are regenerated.  The default value is given in
   TEMP_PREFERRED_LIFETIME and is one day.  In addition, the exact time
   at which to invalidate a temporary address depends on how
   applications are used by end users.  Thus the default value given of
   one week (TEMP_VALID_LIFETIME) may not be appropriate in all
   environments.  Implementations should provide end users with the
   ability to override both of these default values.

   Finally, when an interface connects to a new link, a new randomized
   interface identifier should be generated immediately together with a
   new set of temporary addresses.  If a device moves from one ethernet
   to another, generating a new set of temporary addresses from a
   different randomized interface identifier ensures that the device
   uses different randomized interface identifiers for the temporary
   addresses associated with the two links, making it more difficult to
   correlate addresses from the two different links as being from the
   same node.

4.  Implications of Changing Interface Identifiers

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

   The desires of protecting individual privacy vs. the desire to
   effectively maintain and debug a network can conflict with each
   other.  Having clients use addresses that change over time will make
   it more difficult to track down and isolate operational problems.
   For example, when looking at packet traces, it could become more
   difficult to determine whether one is seeing behavior caused by a
   single errant machine, or by a number of them.

   Some servers refuse to grant access to clients for which no DNS name
   exists.  That is, they perform a DNS PTR query to determine the DNS
   name, and may then also perform an A query on the returned name to
   verify that the returned DNS name maps back into the address being
   used.  Consequently, clients not properly registered in the DNS may



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   be unable to access some services.  As noted earlier, however, a
   node's DNS name (if non-changing) serves as a constant identifier.
   The wide deployment of the extension described in this document could
   challenge the practice of inverse-DNS-based "authentication," which
   has little validity, though it is widely implemented.  In order to
   meet server challenges, nodes could register temporary addresses in
   the DNS using random names (for example a string version of the
   random address itself).

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

5.  Defined Constants

   Constants defined in this document include:

TEMP_VALID_LIFETIME -- Default value: 1 week.  Users should be able
          to override the default value.
TEMP_PREFERRED_LIFETIME -- Default value: 1 day.  Users should be
          able to override the default value.
REGEN_ADVANCE -- 5 seconds
MAX_DESYNC_FACTOR -- 10 minutes.  Upper bound on DESYNC_FACTOR.
DESYNC_FACTOR -- A random value within the range 0 - MAX_DESYNC_FACTOR.
          It is computed once at system start (rather than each time
          it is used) and must never be greater than
          (TEMP_VALID_LIFETIME - REGEN_ADVANCE).

6.  Future Work

   An implementation might want to keep track of which addresses are
   being used by upper layers so as to be able to remove a deprecated
   temporary address from internal data structures once no upper layer
   protocols are using it (but not before).  This is in contrast to
   current approaches where addresses are removed from an interface when
   they become invalid [ADDRCONF], independent of whether or not upper
   layer protocols are still using them.  For TCP connections, such
   information is available in control blocks.  For UDP-based
   applications, it may be the case that only the applications have
   knowledge about what addresses are actually in use.  Consequently, an
   implementation generally will need to use heuristics in deciding when
   an address is no longer in use (e.g., as is suggested in Section
   3.4).






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   The determination as to whether to use public vs. temporary addresses
   can in some cases only be made by an application.  For example, some
   applications may always want to use temporary addresses, while others
   may want to use them only in some circumstances or not at all.
   Suitable API extensions will likely need to be developed to enable
   individual applications to indicate with sufficient granularity their
   needs with regards to the use of temporary addresses.

7.  Security Considerations

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

8.  Acknowledgments

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

9.  References

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

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

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

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

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

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

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






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   [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: [phone]
   EMail: [e-mail]


   Richard Draves
   Microsoft Research
   One Microsoft Way
   Redmond, WA 98052

   Phone: [phone]
   EMail: [e-mail]











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11.  Full Copyright Statement

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

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

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

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

Acknowledgement

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



















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