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rfc:rfc4941

Network Working Group T. Narten Request for Comments: 4941 IBM Corporation Obsoletes: 3041 R. Draves Category: Standards Track Microsoft Research

                                                           S. Krishnan
                                                     Ericsson Research
                                                        September 2007
 Privacy Extensions for Stateless Address Autoconfiguration in IPv6

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 Nodes use IPv6 stateless address autoconfiguration to generate
 addresses using a combination of locally available information and
 information advertised by routers.  Addresses are formed by combining
 network prefixes with an interface identifier.  On an interface that
 contains an embedded IEEE Identifier, 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, et al. Standards Track [Page 1] RFC 4941 Privacy Extensions to Autoconf September 2007

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.  Conventions Used in This Document  . . . . . . . . . . . .  4
   1.2.  Problem Statement  . . . . . . . . . . . . . . . . . . . .  4
 2.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.1.  Extended Use of the Same Identifier  . . . . . . . . . . .  5
   2.2.  Address Usage in IPv4 Today  . . . . . . . . . . . . . . .  6
   2.3.  The Concern with IPv6 Addresses  . . . . . . . . . . . . .  7
   2.4.  Possible Approaches  . . . . . . . . . . . . . . . . . . .  8
 3.  Protocol Description . . . . . . . . . . . . . . . . . . . . .  9
   3.1.  Assumptions  . . . . . . . . . . . . . . . . . . . . . . . 10
   3.2.  Generation of Randomized Interface Identifiers . . . . . . 10
     3.2.1.  When Stable Storage Is Present . . . . . . . . . . . . 11
     3.2.2.  In The Absence of Stable Storage . . . . . . . . . . . 12
     3.2.3.  Alternate Approaches . . . . . . . . . . . . . . . . . 12
   3.3.  Generating Temporary Addresses . . . . . . . . . . . . . . 13
   3.4.  Expiration of Temporary Addresses  . . . . . . . . . . . . 14
   3.5.  Regeneration of Randomized Interface Identifiers . . . . . 15
   3.6.  Deployment Considerations  . . . . . . . . . . . . . . . . 16
 4.  Implications of Changing Interface Identifiers . . . . . . . . 17
 5.  Defined Constants  . . . . . . . . . . . . . . . . . . . . . . 18
 6.  Future Work  . . . . . . . . . . . . . . . . . . . . . . . . . 18
 7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
 8.  Significant Changes from RFC 3041  . . . . . . . . . . . . . . 19
 9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 20
 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
   10.2. Informative References . . . . . . . . . . . . . . . . . . 20

Narten, et al. Standards Track [Page 2] RFC 4941 Privacy Extensions to Autoconf September 2007

1. Introduction

 Stateless address autoconfiguration [ADDRCONF] defines how an IPv6
 node generates addresses without the need for a Dynamic Host
 Configuration Protocol for IPv6 (DHCPv6) 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.  Note that an IPv6 identifier does not
 necessarily have to be 64 bits in length, but the algorithm specified
 in this document is targeted towards 64-bit interface identifiers.
 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 can then be 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 may not be
 globally unique and may also change over time.  The focus of this
 document is on addresses derived from IEEE identifiers because
 tracking of individual devices, the concern being addressed here, is
 possible 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.  The term "global scope addresses" is used in this
 document to collectively refer to "Global unicast addresses" as
 defined in [ADDRARCH] and "Unique local addresses" as defined in
 [ULA].

Narten, et al. Standards Track [Page 3] RFC 4941 Privacy Extensions to Autoconf September 2007

1.1. Conventions Used in This Document

 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 [RFC2119].

1.2. Problem Statement

 Addresses generated using stateless address autoconfiguration
 [ADDRCONF] contain an embedded interface identifier, which remains
 constant over time.  Anytime a fixed identifier is used in multiple
 contexts, it becomes possible to correlate seemingly unrelated
 activity using this identifier.
 The correlation can be performed by
 o  An attacker who is in the path between the node in question and
    the peer(s) to which it is communicating, and who can view the
    IPv6 addresses present in the datagrams.
 o  An attacker who can access the communication logs of the peers
    with which the node has communicated.
 Since the identifier is embedded within the IPv6 address, which is a
 fundamental requirement of communication, it cannot be easily hidden.
 This document proposes a solution to this issue by generating
 interface identifiers that vary over time.
 Note that an attacker, who is on path, may be able to perform
 significant correlation based on
 o  The payload contents of the packets on the wire
 o  The characteristics of the packets such as packet size and timing
 Use of temporary addresses will not prevent such payload-based
 correlation.

Narten, et al. Standards Track [Page 4] RFC 4941 Privacy Extensions to Autoconf September 2007

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.  Any time 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.  Although it might appear
 that changing an address regularly in such environments would be
 desirable to lessen privacy concerns, it should be noted that the
 network prefix portion of an address also serves as a constant
 identifier.  All nodes at, say, a home, would have the same network
 prefix, which identifies the topological location of those nodes.
 This has implications for privacy, though not at the same granularity
 as the concern that this document addresses.  Specifically, all nodes
 within a home could be grouped together for the purposes of
 collecting information.  If the network contains a very small number
 of nodes, say, just one, changing just the interface identifier will
 not enhance privacy at all, since the prefix serves as a constant
 identifier.
 One of the requirements for correlating seemingly unrelated
 activities is the use (and reuse) of an identifier that is
 recognizable over time within different contexts.  IP addresses
 provide one obvious example, but there are more.  Many nodes also
 have DNS names associated with their addresses, in which case the DNS
 name serves as a similar identifier.  Although the DNS name
 associated with an address is more work to obtain (it may require a
 DNS query), the information is often readily available.  In such
 cases, changing the address on a machine over time would do little to
 address the concerns raised in this document, unless the DNS name is
 changed as well (see Section 4).
 Web browsers and servers typically exchange "cookies" with each other
 [COOKIES].  Cookies allow Web servers to correlate a current activity
 with a previous activity.  One common usage is to send back targeted
 advertising to a user by using the cookie supplied by the browser to

Narten, et al. Standards Track [Page 5] RFC 4941 Privacy Extensions to Autoconf September 2007

 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.

2.2. Address Usage in IPv4 Today

 Addresses used in today's Internet are often non-changing in practice
 for extended periods of time.  In an increasing number of 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 dial-up 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.  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.
 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 may not always be available
 depending on the administrative policy.  In addition, changing an

Narten, et al. Standards Track [Page 6] RFC 4941 Privacy Extensions to Autoconf September 2007

 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 dial-up 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, 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, 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.  For
 example, a server that logs usage information together with 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.

Narten, et al. Standards Track [Page 7] RFC 4941 Privacy Extensions to Autoconf September 2007

 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 having a static non-changing address is to use
 DHCPv6 [DHCPV6] for obtaining addresses.  Section 12 of [DHCPV6]
 discusses the use of DHCPv6 for the assignment and management of
 "temporary addresses", which are never renewed and provide the same
 property of temporary addresses described in this document with
 regards to the privacy concern.
 Another approach, compatible with the stateless address
 autoconfiguration architecture, would be to change the interface
 identifier portion of an address over time and generate new addresses
 from the interface identifier for some address scopes.  Changing the
 interface identifier can make it more difficult to look at the IP
 addresses in independent transactions and identify which ones
 actually correspond to the same node, both in the case where the
 routing prefix portion of an address changes and when it does not.
 Many machines function as both clients and servers.  In such cases,
 the machine would need a DNS name for its use as a server.  Whether
 the address stays fixed or changes has little privacy implication
 since the DNS name remains constant and serves as a constant
 identifier.  When acting as a client (e.g., initiating
 communication), however, such a machine may want to vary the
 addresses it uses.  In such environments, one may need multiple
 addresses: a "public" (i.e., non-secret) server address, registered
 in the DNS, that is used to accept incoming connection requests from
 other machines, and a "temporary" address used to shield the identity
 of the client when it initiates communication.  These two cases are
 roughly analogous to telephone numbers and caller ID, where a user
 may list their telephone number in the public phone book, but disable
 the display of its number via caller ID when initiating calls.
 To make it difficult to make educated guesses as to whether two
 different interface identifiers belong to the same node, the
 algorithm for generating alternate identifiers must include input
 that has an unpredictable component from the perspective of the
 outside entities that are collecting information.  Picking
 identifiers from a pseudo-random sequence suffices, so long as the
 specific sequence cannot be determined by an outsider examining

Narten, et al. Standards Track [Page 8] RFC 4941 Privacy Extensions to Autoconf September 2007

 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 addresses based on a random interface
     identifier for the purpose of initiating outgoing sessions.
     These "random" or temporary addresses would be used for a short
     period of time (hours to days) and would then be deprecated.
     Deprecated address can continue to be used for already
     established connections, but are not used to initiate new
     connections.  New temporary addresses are generated periodically
     to replace temporary addresses that expire, with the exact time
     between address generation a matter of local policy.
 3.  Produce a sequence of temporary global scope addresses from a
     sequence of interface identifiers that appear to be random in the
     sense that it is difficult for an outside observer to predict a
     future address (or identifier) based on a current one, and it is
     difficult to determine previous addresses (or identifiers)
     knowing only the present one.
 4.  By default, 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 default behavior 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.

Narten, et al. Standards Track [Page 9] RFC 4941 Privacy Extensions to Autoconf September 2007

     A node highly concerned about privacy MAY use different interface
     identifiers on different prefixes, resulting in a set of global
     addresses that cannot be easily tied to each other.  For example
     a node MAY create different interface identifiers I1, I2, and I3
     for use with different prefixes P1, P2, and P3 on the same
     interface.

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.  While the same identifier can be used
 to create more than one temporary address, the value SHOULD change
 over time as described in Section 3.5.
 The algorithm also assumes that, for a given temporary address, an
 implementation can determine the prefix 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 set for
 the prefix from which it was generated.
 Finally, this document assumes that when a node initiates outgoing
 communication, temporary addresses can be given preference over
 public addresses when the device is configured to do so.
 [ADDR_SELECT] mandates implementations to provide a mechanism, which
 allows an application to configure its preference for temporary
 addresses over public addresses.  It also allows for an
 implementation to prefer temporary addresses by default, so that the
 connections initiated by the node can use temporary addresses without
 requiring application-specific enablement.  This document also
 assumes that an API will exist that allows individual applications to
 indicate whether they prefer to use temporary or public addresses and
 override the system defaults.

3.2. Generation of Randomized Interface Identifiers

 We describe two approaches for the generation and 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.

Narten, et al. Standards Track [Page 10] RFC 4941 Privacy Extensions to Autoconf September 2007

 The random interface identifier generation algorithm, as described in
 this document, uses MD5 as the hash algorithm.  The node MAY use
 another algorithm instead of MD5 to produce the random interface
 identifier.

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 leftmost 64-bits of the MD5 digest and set bit 6 (the
     leftmost bit is numbered 0) to zero.  This creates an interface
     identifier with the universal/local bit indicating local
     significance only.
 4.  Compare the generated identifier against a list of reserved
     interface identifiers and to those already assigned to an address
     on the local device.  In the event that an unacceptable
     identifier has been generated, the node MUST restart the process
     at step 1 above, using the rightmost 64 bits of the MD5 digest
     obtained in step 2 in place of the history value in step 1.
 5.  Save the generated identifier as the associated randomized
     interface identifier.
 6.  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.

Narten, et al. Standards Track [Page 11] RFC 4941 Privacy Extensions to Autoconf September 2007

 MD5 was chosen for convenience, and because its particular properties
 were adequate to produce the desired level of randomization.  The
 node MAY use another algorithm instead of MD5 to produce the random
 interface identifier
 In theory, generating successive randomized interface identifiers
 using a history scheme as above has no advantages over generating
 them at random.  In practice, however, generating truly random
 numbers can be tricky.  Use of a history value is intended to avoid
 the particular scenario where two nodes generate the same randomized
 interface identifier, both detect the situation via DAD, but then
 proceed to generate identical randomized interface identifiers via
 the same (flawed) random number generation algorithm.  The above
 algorithm avoids this problem by having the interface identifier
 (which will often be globally unique) used in the calculation that
 generates subsequent randomized interface identifiers.  Thus, if two
 nodes happen to generate the same randomized interface identifier,
 they should generate different ones on the follow-up 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 SHOULD 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.

3.2.3. Alternate Approaches

 Note that there are other approaches to generate random interface
 identifiers, albeit with different goals and applicability.  One such
 approach is Cryptographically Generated Addresses (CGAs) [CGA], which
 generate a random interface identifier based on the public key of the
 node.  The goal of CGAs is to prove ownership of an address and to
 prevent spoofing and stealing of existing IPv6 addresses.  They are
 used for securing neighbor discovery using [SEND].  The CGA random
 interface identifier generation algorithm may not be suitable for
 privacy addresses because of the following properties:

Narten, et al. Standards Track [Page 12] RFC 4941 Privacy Extensions to Autoconf September 2007

 o  It requires the node to have a public key.  This means that the
    node can still be identified by its public key.
 o  The random interface identifier process is computationally
    intensive and hence discourages frequent regeneration.

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), the node MUST
 perform the following steps:
 1.  Process the Prefix Information Option as defined in [ADDRCONF],
     either creating a new public address or adjusting the lifetimes
     of existing addresses, both public and temporary.  If a received
     option will extend the lifetime of a public address, the
     lifetimes of temporary addresses should be extended, subject to
     the overall constraint that no temporary addresses should ever
     remain "valid" or "preferred" for a time longer than
     (TEMP_VALID_LIFETIME) or (TEMP_PREFERRED_LIFETIME -
     DESYNC_FACTOR), respectively.  The configuration variables
     TEMP_VALID_LIFETIME and TEMP_PREFERRED_LIFETIME correspond to
     approximate target lifetimes for temporary addresses.
 2.  One way an implementation can satisfy the above constraints is to
     associate with each temporary address a creation time (called
     CREATION_TIME) that indicates the time at which the address was
     created.  When updating the preferred lifetime of an existing
     temporary address, it would be set to expire at whichever time is
     earlier: the time indicated by the received lifetime or
     (CREATION_TIME + TEMP_PREFERRED_LIFETIME - DESYNC_FACTOR).  A
     similar approach can be used with the valid lifetime.
 3.  When a new public address is created as described in [ADDRCONF],
     the node SHOULD also create a new temporary address.
 4.  When creating a temporary address, the lifetime values MUST be
     derived from the corresponding prefix as follows:
  • Its Valid Lifetime is the lower of the Valid Lifetime of the

public address or TEMP_VALID_LIFETIME.

Narten, et al. Standards Track [Page 13] RFC 4941 Privacy Extensions to Autoconf September 2007

  • Its Preferred Lifetime is the lower of the Preferred Lifetime

of the public address or TEMP_PREFERRED_LIFETIME -

        DESYNC_FACTOR.
 5.  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.
 6.  New temporary addresses MUST be created by appending the
     interface's current randomized interface identifier to the prefix
     that was received.
 7.  The node MUST perform duplicate address detection (DAD) on the
     generated temporary address.  If DAD indicates the address is
     already in use, the node MUST generate a new randomized interface
     identifier as described in Section 3.2 above, and repeat the
     previous steps as appropriate up to TEMP_IDGEN_RETRIES times.  If
     after TEMP_IDGEN_RETRIES consecutive attempts no non-unique
     address was generated, the node MUST log a system error and MUST
     NOT attempt to generate temporary addresses for that interface.
     Note that DAD MUST be performed on every unicast address
     generated from this randomized interface identifier.

3.4. Expiration of Temporary Addresses

 When a temporary address becomes deprecated, a new one MUST be
 generated.  This is done by repeating the actions described in
 Section 3.3, starting at step 3).  Note that, except for the
 transient period when a temporary address is being regenerated, in
 normal operation at most one temporary address per prefix should be
 in a non-deprecated state at any given time on a given interface.
 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.  To
 ensure 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.  The node SHOULD start the address regeneration process
 REGEN_ADVANCE time units before a temporary address would actually be
 deprecated.
 As an optional optimization, an implementation MAY remove a
 deprecated temporary address that is not in use by applications or
 upper layers as detailed in Section 6.

Narten, et al. Standards Track [Page 14] RFC 4941 Privacy Extensions to Autoconf September 2007

3.5. Regeneration of Randomized Interface Identifiers

 The frequency at which temporary addresses changes 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.
 Nodes following this specification SHOULD generate new temporary
 addresses on a periodic basis.  This can be achieved automatically by
 generating a new randomized interface identifier at least once every
 (TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE - DESYNC_FACTOR) time units.
 As described above, generating a new temporary address REGEN_ADVANCE
 time units before a temporary address becomes deprecated produces
 addresses with a preferred lifetime no larger than
 TEMP_PREFERRED_LIFETIME.  The value DESYNC_FACTOR is a random value
 (different for each client) that ensures that clients don't
 synchronize with each other and generate new addresses at exactly the
 same time.  When the preferred lifetime expires, a new temporary
 address MUST be 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 suggested default
 value 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.

Narten, et al. Standards Track [Page 15] RFC 4941 Privacy Extensions to Autoconf September 2007

 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.  The node MAY follow any process available to it, to
 determine that the link change has occurred.  One such process is
 described by Detecting Network Attachment [DNA].

3.6. Deployment Considerations

 Devices implementing this specification MUST provide a way for the
 end user to explicitly enable or disable the use of temporary
 addresses.  In addition, a site might wish to disable the use of
 temporary addresses in order to simplify network debugging and
 operations.  Consequently, implementations SHOULD provide a way for
 trusted system administrators to enable or disable the use of
 temporary addresses.
 Additionally, sites might wish to selectively enable or disable the
 use of temporary addresses for some prefixes.  For example, a site
 might wish to disable temporary address generation for "Unique local"
 [ULA] prefixes while still generating temporary addresses for all
 other global prefixes.  Another site might wish to enable temporary
 address generation only for the prefixes 2001::/16 and 2002::/16,
 while disabling it for all other prefixes.  To support this behavior,
 implementations SHOULD provide a way to enable and disable generation
 of temporary addresses for specific prefix subranges.  This per-
 prefix setting SHOULD override the global settings on the node with
 respect to the specified prefix subranges.  Note that the pre-prefix
 setting can be applied at any granularity, and not necessarily on a
 per-subnet basis.
 The use of temporary addresses may cause unexpected difficulties with
 some applications.  As described below, some servers refuse to accept
 communications from clients for which they cannot map the IP address
 into a DNS name.  In addition, some applications may not behave
 robustly if temporary addresses are used and an address expires
 before the application has terminated, or if it opens multiple
 sessions, but expects them to all use the same addresses.
 Consequently, the use of temporary addresses SHOULD be disabled by
 default in order to minimize potential disruptions.  Individual
 applications, which have specific knowledge about the normal duration
 of connections, MAY override this as appropriate.

Narten, et al. Standards Track [Page 16] RFC 4941 Privacy Extensions to Autoconf September 2007

 If a very small number of nodes (say, only one) use a given prefix
 for extended periods of time, just changing the interface identifier
 part of the address may not be sufficient to ensure privacy, since
 the prefix acts as a constant identifier.  The procedures described
 in this document are most effective when the prefix is reasonably non
 static or is used by a fairly large number of nodes.

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.  The widespread use of temporary addresses may result in a
 significant fraction of Internet traffic not using addresses in which
 the interface identifier portion is globally unique.  Consequently,
 usage of the algorithms in this document may complicate providing
 such a future flexibility, if global uniqueness is necessary.
 The desires of protecting individual privacy versus 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 AAAA query on the returned name to
 verify that the returned DNS name maps back into the address being
 used.  Consequently, clients not properly registered in the DNS may
 be unable to access some services.  As noted earlier, however, a
 node's DNS name (if non-changing) serves as a constant identifier.
 The wide deployment of the extension described in this document could
 challenge the practice of inverse-DNS-based "authentication," which
 has little validity, though it is widely implemented.  In order to
 meet server challenges, nodes could register temporary addresses in
 the DNS using random names (for example, a string version of the
 random address itself).
 Use of the extensions defined in this document may complicate
 debugging and other operational troubleshooting activities.
 Consequently, it may be site policy that temporary addresses should
 not be used.  Consequently, implementations MUST provide a method for
 the end user or trusted administrator to override the use of
 temporary addresses.

Narten, et al. Standards Track [Page 17] RFC 4941 Privacy Extensions to Autoconf September 2007

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).
 TEMP_IDGEN_RETRIES -- Default value: 3

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.
 The determination as to whether to use public versus 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.  Recommendations on DNS practices to avoid the
 problem described in Section 4 when reverse DNS lookups fail may be
 needed.  [DNSOP] contains a more detailed discussion of the DNS-
 related issues.

Narten, et al. Standards Track [Page 18] RFC 4941 Privacy Extensions to Autoconf September 2007

 While this document discusses ways of obscuring a user's permanent IP
 address, the method described is believed to be ineffective against
 sophisticated forms of traffic analysis.  To increase effectiveness,
 one may need to consider use of more advanced techniques, such as
 Onion Routing [ONION].

7. Security Considerations

 Ingress filtering has been and is being deployed as a means of
 preventing the use of spoofed source addresses in Distributed Denial
 of Service (DDoS) attacks.  In a network with a large number of
 nodes, new temporary addresses are created at a fairly high rate.
 This might make it difficult for ingress filtering mechanisms to
 distinguish between legitimately changing temporary addresses and
 spoofed source addresses, which are "in-prefix" (using a
 topologically correct prefix and non-existent interface ID).  This
 can be addressed by using access control mechanisms on a per-address
 basis on the network egress point.

8. Significant Changes from RFC 3041

 This section summarizes the changes in this document relative to RFC
 3041 that an implementer of RFC 3041 should be aware of.
 1.  Excluded certain interface identifiers from the range of
     acceptable interface identifiers.  Interface IDs such as those
     for reserved anycast addresses [RFC2526], etc.
 2.  Added a configuration knob that provides the end user with a way
     to enable or disable the use of temporary addresses on a per-
     prefix basis.
 3.  Added a check for denial of service attacks using low valid
     lifetimes in router advertisements.
 4.  DAD is now run on all temporary addresses, not just the first one
     generated from an interface identifier.
 5.  Changed the default setting for usage of temporary addresses to
     be disabled.
 6.  The node is now allowed to generate different interface
     identifiers for different prefixes, if it so desires.
 7.  The algorithm used for generating random interface identifiers is
     no longer restricted to just MD5.

Narten, et al. Standards Track [Page 19] RFC 4941 Privacy Extensions to Autoconf September 2007

 8.  Reduced default number of retries to 3 and added a configuration
     variable.
 9.  Router advertisement (RA) processing algorithm is no longer
     included in the document, and is replaced by a reference to
     [ADDRCONF].

9. Acknowledgments

 Rich Draves and Thomas Narten were the authors of RFC 3041.  They
 would like to acknowledge the contributions of the ipv6 working group
 and, in particular, Ran Atkinson, Matt Crawford, Steve Deering,
 Allison Mankin, and Peter Bieringer.
 Suresh Krishnan was the sole author of this version of the document.
 He would like to acknowledge the contributions of the ipv6 working
 group and, in particular, Jari Arkko, Pekka Nikander, Pekka Savola,
 Francis Dupont, Brian Haberman, Tatuya Jinmei, and Margaret Wasserman
 for their detailed comments.

10. References

10.1. Normative References

 [ADDRARCH]     Hinden, R. and S. Deering, "IP Version 6 Addressing
                Architecture", RFC 4291, February 2006.
 [ADDRCONF]     Thomson, S., Narten, T., and T. Jinmei, "IPv6
                Stateless Address Autoconfiguration", RFC 4862,
                September 2007.
 [DISCOVERY]    Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
                "Neighbor Discovery for IP version 6 (IPv6)",
                RFC 4861, September 2007.
 [MD5]          Rivest, R., "The MD5 Message-Digest Algorithm",
                RFC 1321, April 1992.
 [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", RFC 2119, March 1997.

10.2. Informative References

 [ADDR_SELECT]  Draves, R., "Default Address Selection for Internet
                Protocol version 6 (IPv6)", RFC 3484, February 2003.
 [CGA]          Aura, T., "Cryptographically Generated Addresses
                (CGA)", RFC 3972, March 2005.

Narten, et al. Standards Track [Page 20] RFC 4941 Privacy Extensions to Autoconf September 2007

 [COOKIES]      Kristol, D. and L. Montulli, "HTTP State Management
                Mechanism", RFC 2965, October 2000.
 [DDNS]         Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
                "Dynamic Updates in the Domain Name System (DNS
                UPDATE)", RFC 2136, April 1997.
 [DHCP]         Droms, R., "Dynamic Host Configuration Protocol",
                RFC 2131, March 1997.
 [DHCPV6]       Droms, R., Bound, J., Volz, B., Lemon, T., Perkins,
                C., and M. Carney, "Dynamic Host Configuration
                Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.
 [DNA]          Choi, JH. and G. Daley, "Goals of Detecting Network
                Attachment in IPv6", RFC 4135, August 2005.
 [DNSOP]        Durand, A., Ihren, J., and P. Savola, "Operational
                Considerations and Issues with IPv6 DNS", RFC 4472,
                April 2006.
 [ONION]        Reed, MGR., Syverson, PFS., and DMG. Goldschlag,
                "Proxies for Anonymous Routing",  Proceedings of the
                12th Annual Computer Security Applications Conference,
                San Diego, CA, December 1996.
 [RANDOM]       Eastlake, D., Schiller, J., and S. Crocker,
                "Randomness Requirements for Security", BCP 106,
                RFC 4086, June 2005.
 [RFC2526]      Johnson, D. and S. Deering, "Reserved IPv6 Subnet
                Anycast Addresses", RFC 2526, March 1999.
 [SEND]         Arkko, J., Kempf, J., Zill, B., and P. Nikander,
                "SEcure Neighbor Discovery (SEND)", RFC 3971,
                March 2005.
 [ULA]          Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
                Addresses", RFC 4193, October 2005.

Narten, et al. Standards Track [Page 21] RFC 4941 Privacy Extensions to Autoconf September 2007

Authors' Addresses

 Thomas Narten
 IBM Corporation
 P.O. Box 12195
 Research Triangle Park, NC
 USA
 EMail: narten@us.ibm.com
 Richard Draves
 Microsoft Research
 One Microsoft Way
 Redmond, WA
 USA
 EMail: richdr@microsoft.com
 Suresh Krishnan
 Ericsson Research
 8400 Decarie Blvd.
 Town of Mount Royal, QC
 Canada
 EMail: suresh.krishnan@ericsson.com

Narten, et al. Standards Track [Page 22] RFC 4941 Privacy Extensions to Autoconf September 2007

Full Copyright Statement

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 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
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Narten, et al. Standards Track [Page 23]

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