Privacy Extensions for Stateless Address Autoconfiguration in IPv6
draft-ietf-ipngwg-addrconf-privacy-03

Please post. Tnx.

INTERNET-DRAFT                                             Thomas Narten
<draft-ietf-ipngwg-addrconf-privacy-03.txt>                          IBM
                                                          Richard Draves
                                                      Microsoft Research
                                                      September 19, 2000

    Privacy Extensions for Stateless Address Autoconfiguration in IPv6

                <draft-ietf-ipngwg-addrconf-privacy-03.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

   Nodes use IPv6 stateless address autoconfiguration to generate
   addresses without the necessity of a 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

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   different transactions actually correspond to the same node.

   Contents

   Status of this Memo..........................................    1

   1.  Introduction.............................................    2

   2.  Background...............................................    3
      2.1.  Extended Use of the Same Identifier.................    3
      2.2.  Not a New Issue.....................................    4
      2.3.  Possible Approaches.................................    6

   3.  Protocol Description.....................................    7
      3.1.  Assumptions.........................................    8
      3.2.  Generation Of Randomized Interface Identifiers......    9
      3.3.  Generating Anonymous Addresses......................   10
      3.4.  Expiration of Anonymous Addresses...................   11
      3.5.  Regeneration of Randomized Interface Identifiers....   12

   4.  Implications of Changing Interface Identifiers...........   13

   5.  Defined Constants........................................   14

   6.  Open Issues and Future Work..............................   14

   7.  Security Considerations..................................   14

   8.  Acknowledgments..........................................   14

   9.  References...............................................   15

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 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

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   attached interfaces. Additional addresses, including site-local and
   global-scope addresses, are then created by combining prefixes
   advertised in Router Advertisements via Neighbor Discovery
   [DISCOVERY] with the interface identifier.

   Not all nodes and interfaces contain IEEE identifiers. In such cases,
   an interface identifier is generated through some other means (e.g.,
   at random), and the resultant interface identifier is not globally
   unique and may also change over time. The focus of this document is
   on addresses derived from IEEE identifiers, as the concern being
   addressed exists only in those cases where the interface identifier
   is globally unique and non-changing. The rest of this document
   assumes that IEEE identifiers are being used, but the techniques
   described may also apply to interfaces with other types of globally
   unique and 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 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.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [KEYWORDS].

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

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   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 concern
   raised in this document, as the DNS name would become the correlating
   identifier.

   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.  Not a New Issue

   Although the topic of this document may at first appear to be an
   issue new to IPv6, similar issues exist in today's Internet already.
   That is, addresses used in today's Internet are often non-changing in
   practice for extended periods of time. In many sites, addresses are
   assigned statically; such addresses typically change infrequently.
   However, many sites are 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 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, although many sites use DHCP, clients end up using the
   same address for months at a time.

   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 widely deployed today. 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

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   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).

   Many network services require that the client authenticate itself to
   the server before gaining access to a resource. The authentication
   step binds the activity (e.g., TCP connection) to a specific entity
   (e.g., an end user). In such cases, a server already has the ability
   to track usage by an individual, independent of the address they
   happen to use. Indeed, such tracking is an important part of
   accounting.

   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 non-changing interface identifiers in IPv6 has
   implications in two quite different contexts: stationary devices
   (i.e., those that generally do not move physically such as desktop
   PCs), and mobile devices (i.e., those that move frequently, including
   laptops, cell phones, etc.).

   In today's internet, many home users do not have permanent
   connections and indeed are assigned temporary addresses each time
   they connect to their ISP. Consequently, the addresses they use
   change frequently over time and are shared among a number of
   different users. If addresses are generated from an interface
   identifier, however, a home user's address could contain an interface
   identifier that remains the same from one dialup session to the next.
   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 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.
   The technique described later in the document attempts to address
   this concern by changing the interface identifier portion of an
   address. However, it should be noted that in the case of always-on
   connections, the network prefix portion of an address is in effect a

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   constant identifier. All nodes at (say) a home, would have the same
   network prefix. This has implications for privacy, though not at the
   same granularity (i.e., all nodes within a home would be lumped
   together for the purposes of collecting information). This issue is
   also non-trivial to address, because the routing prefix part of an
   address contains topology information and cannot contain arbitrary
   values.

   Another case concerns mobile devices (e.g., laptops, PDAs, etc.) that
   move topologically within the Internet. Whenever they move (in the
   absence of technology such as mobile IP [MOBILEIP]), they form new
   addresses for their current topological point of attachment. This is
   typified today by the "road warrior" who has Internet connectivity
   both at home and at the office. While the node's address changes as
   it moves, however, the interface identifier contained within the
   address remains the same (when derived from an IEEE Identifier). In
   such cases, the interface identifier could (in theory) 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.

2.3.  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

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   the address stays fixed or changes has little privacy implication
   since the DNS name remains constant and serves as a constant
   identifier. When acting as a client (e.g., initiating communication),
   however, such a machine may want to vary the addresses it uses. In
   such environments, one may need multiple addresses: a "public" (i.e.,
   non-secret) server address, registered in the DNS, that is used to
   accept incoming connection requests from other machines, and
   (possibly) an "anonymous" 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 just the
   identifiers that appear in addresses or are otherwise readily
   available (e.g., a node's link-layer address). 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 anonymous addresses is created,
   and the previous anonymous 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) Define new procedures that 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 anonymous 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 anonymous addresses are generated periodically to
      replace anonymous addresses that expire, with the exact time
      between address generation a matter of local policy.

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   3) Produce a sequence of anonymous global-scope addresses from a
      sequence of interface identifiers that appear to be random in the
      sense that it is difficult for an outside observer to predict a
      future address (or identifier) based on a current one and it is
      difficult to determine previous addresses (or identifiers) knowing
      only the present one.

   4) Generate a set of addresses from the same (randomized) interface
      identifier, one address for each prefix for which a global address
      has been generated via stateless address autoconfiguration. Using
      the same interface identifier to generate a set of anonymous
      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 anonymous 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 anonymous address.

   The algorithm also assumes that for a given anonymous address, one
   can determine the corresponding public address. When an anonymous
   address is deprecated, a new anonymous 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, anonymous addresses can be given preference over other
   public addresses. This can mean that all outgoing connections use
   anonymous addresses by default, or that applications individually
   indicate whether they prefer to use anonymous or public addresses.
   Giving preference to anonymous address is consistent with on-going
   work that addresses the topic of source address-selection in the more
   general case [ADDR_SELECT].

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3.2.  Generation Of Randomized Interface Identifiers.

   We describe two approaches for the maintenance of the randomized
   interface identifier. The first assumes the presence of stable
   storage that can be used to record state history for use as input
   into the next iteration of the algorithm across system restarts. A
   second approach addresses the case where stable storage is
   unavailable and a randomized interface identifier may need to be
   generated at random.

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

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

3.2.2.  In The Absence of Stable Storage

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

3.3.  Generating Anonymous 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 anonymous.  When adjusting the
      lifetimes of an existing anonymous address, only lower the
      lifetimes. Implementations MUST NOT increase the lifetimes of an
      existing anonymous 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

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      address already assigned to the interface, and the Valid Lifetime
      in the option is not zero), also create a new anonymous address.
   3) When creating an anonymous 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 ANON_VALID_LIFETIME.
       - Its Preferred Lifetime is the lower of the Preferred Lifetime
         of the public address or ANON_PREFERRED_LIFETIME.

      An anonymous address is created only if this calculated Preferred
      Lifetime is greater than REGEN_ADVANCE time units. In particular,
      an implementation MUST NOT create an anonymous address with a zero
      Preferred Lifetime.
   4) New anonymous 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 anonymous 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
      anonymous 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 anonymous addresses for that interface.

      Note: because multiple anonymous addresses are generated from the
      same associated randomized interface identifier, there is little
      benefit in running DAD on every anonymous 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 Anonymous Addresses

   When an anonymous 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 an anonymous address is being regenerated, in normal operation
   at most one anonymous address corresponding to a public address
   should be in a non-deprecated state at any given time. Note that if
   an anonymous address becomes deprecated as result of processing a
   Prefix Information Option with a zero Preferred Lifetime, then a new
   anonymous address MUST NOT be generated.  The Prefix Information

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   Option will also deprecate the corresponding public address.

   To insure that a preferred anonymous address is always available, a
   new anonymous 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 anonymous 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 an anonymous
   address would actually be deprecated.

   As an optional optimization, an implementation may wish to remove a
   deprecated anonymous 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
   ANON_VALID_LIFETIME suggested above).

3.5.  Regeneration of Randomized Interface Identifiers

   The frequency at which anonymous 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 anonymous
   addresses on a periodic basis. This can be achieved automatically by
   generating a new randomized interface identifier at least once every
   (ANON_PREFERRED_LIFETIME - REGEN_ADVANCE) time units. As described
   above, generating a new anonymous address REGEN_ADVANCE time units
   before an anonymous address becomes deprecated produces addresses

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   with a preferred lifetime no larger than ANON_PREFERRED_LIFETIME.
   When the preferred lifetime expires, a new anonymous 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
   ANON_PREFERRED_LIFETIME and is one day. In addition, the exact time
   at which to invalidate an anonymous address depends on how
   applications are used by end users. Thus the default value given of
   one week (ANON_PREFERRED_LIFETIME) may not be appropriate in all
   environments. Implementations should provide end users with the
   ability to override both of these default values.

4.  Implications of Changing Interface Identifiers

   The IPv6 addressing architecture goes to great lengths to ensure that
   interface identifiers are globally unique. 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 would eliminate that future flexibility.

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

   Some servers refuse to grant access to clients for which no DNS name
   exists. That is, they perform a DNS PTR query to determine the DNS
   name, and may then also perform an A query on the returned name to
   verify that the returned DNS name maps back into the address being
   used. Consequently, clients not properly registered in the DNS may be
   unable to access some services. As noted earlier, however, a node's
   DNS name (if non-changing) serves as a constant identifier. If the
   extension described in this document becomes widely deployed, servers
   will likely need to change their behavior to not require every
   address be in the DNS.  Another alternative is to register anonymous
   addresses in DNS using random names (for example a string version of
   the address itself).

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5.  Defined Constants

   Constants defined in this document include:

   ANON_VALID_LIFETIME -- Default value: 1 week. Users should be able to
             override the default value.
   ANON_PREFERRED_LIFETIME -- Default value: 1 day. Users should be able
             to override the default value.
   REGEN_ADVANCE -- 5 seconds

6.  Open Issues and 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
   anonymous 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, it
   may need to use heuristics in deciding when an address is no longer
   in use (e.g., as is suggested in Section 3.4).

   Use of the extensions defined in this document is likely to make
   debugging and other operational troubleshooting activities more
   difficult. Consequently, it may be site policy that anonymous
   addresses should not be used. Implementations MAY provide a method
   for a trusted administrator to override the use of anonymous
   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 and Steve Deering for
   their detailed comments.

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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", draft-
           ietf-ipngwg-default-addr-select-00.txt.

   [COOKIES] Kristol, D., Montulli, L., "HTTP State Management
           Mechanism", draft-ietf-http-state-man-mec-12.txt.

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

   [DDNS] Vixie et. al., "Dynamic Updates in the Domain Name System (DNS
           UPDATE)", RFC 2136, April 1997.

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

   [GSE] Crawford et. al., "Separating Identifiers and Locators in
           Addresses: An Analysis of the GSE Proposal for IPv6 ", draft-
           ietf-ipngwg-esd-analysis-04.txt.

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

   [KEYWORDS] Bradner,S. "Key words for use in RFCs to Indicate
           Requirement Levels" RFC 2119, March 1997.

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

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

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

   [SERIALNUM] Moore, K., "Privacy Considerations for the Use of
           Hardware Serial Numbers in End-to-End Network Protocols",
           draft-iesg-serno-privacy-00.txt.

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10.
   Authors' Addresses

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

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

   Richard Draves
   Microsoft Research
   One Microsoft Way
   Redmond, WA 98052

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

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