Security and Privacy Considerations for IPv6 Address Generation Mechanisms
RFC 7721

Internet Engineering Task Force (IETF)                         A. Cooper
Request for Comments: 7721                                         Cisco
Category: Informational                                          F. Gont
ISSN: 2070-1721                                      Huawei Technologies
                                                               D. Thaler
                                                               Microsoft
                                                              March 2016

                Security and Privacy Considerations for
                   IPv6 Address Generation Mechanisms

Abstract

   This document discusses privacy and security considerations for
   several IPv6 address generation mechanisms, both standardized and
   non-standardized.  It evaluates how different mechanisms mitigate
   different threats and the trade-offs that implementors, developers,
   and users face in choosing different addresses or address generation
   mechanisms.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7721.

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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Weaknesses in IEEE-Identifier-Based IIDs  . . . . . . . . . .   5
     3.1.  Correlation of Activities over Time . . . . . . . . . . .   5
     3.2.  Location Tracking . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Address Scanning  . . . . . . . . . . . . . . . . . . . .   7
     3.4.  Device-Specific Vulnerability Exploitation  . . . . . . .   7
   4.  Privacy and Security Properties of Address Generation
       Mechanisms  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  IEEE-Identifier-Based IIDs  . . . . . . . . . . . . . . .  10
     4.2.  Static, Manually Configured IIDs  . . . . . . . . . . . .  10
     4.3.  Constant, Semantically Opaque IIDs  . . . . . . . . . . .  10
     4.4.  Cryptographically Generated IIDs  . . . . . . . . . . . .  10
     4.5.  Stable, Semantically Opaque IIDs  . . . . . . . . . . . .  11
     4.6.  Temporary IIDs  . . . . . . . . . . . . . . . . . . . . .  11
     4.7.  DHCPv6 Generation of IIDs . . . . . . . . . . . . . . . .  12
     4.8.  Transition and Coexistence Technologies . . . . . . . . .  12
   5.  Miscellaneous Issues with IPv6 Addressing . . . . . . . . . .  13
     5.1.  Network Operation . . . . . . . . . . . . . . . . . . . .  13
     5.2.  Compliance  . . . . . . . . . . . . . . . . . . . . . . .  13
     5.3.  Intellectual Property Rights (IPRs) . . . . . . . . . . .  13
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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1.  Introduction

   IPv6 was designed to improve upon IPv4 in many respects, and
   mechanisms for address assignment were one such area for improvement.
   In addition to static address assignment and DHCP, stateless
   autoconfiguration was developed as a less intensive, fate-shared
   means of performing address assignment.  With stateless
   autoconfiguration, routers advertise on-link prefixes and hosts
   generate their own Interface Identifiers (IIDs) to complete their
   addresses.  [RFC7136] clarifies that the IID should be treated as an
   opaque value, while [RFC7421] provides an analysis of the 64-bit
   boundary in IPv6 addressing (e.g., the implications of the IID length
   on security and privacy).  Over the years, many IID generation
   techniques have been defined, both standardized and non-standardized:

   o  Manual configuration [RFC7707]

      *  IPv4 address

      *  Service port

      *  Wordy

      *  Low-byte

   o  Stateless Address Autoconfiguration (SLAAC)

      *  IEEE 802 48-bit Media Access Control (MAC) or IEEE 64-bit
         Extended Unique Identifier (EUI-64) [RFC2464]

      *  Cryptographically generated [RFC3972]

      *  Temporary (also known as "privacy addresses") [RFC4941]

      *  Constant, semantically opaque (also known as "random")
         [Microsoft]

      *  Stable, semantically opaque [RFC7217]

   o  DHCPv6 based [RFC3315]

   o  Specified by transition/co-existence technologies

      *  Derived from an IPv4 address (e.g., [RFC5214], [RFC6052])

      *  Derived from an IPv4 address and port set ID (e.g., [RFC7596],
         [RFC7597], [RFC7599])

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      *  Derived from an IPv4 address and port (e.g., [RFC4380])

   Deriving the IID from a globally unique IEEE identifier [RFC2464]
   [RFC4862] was one of the earliest mechanisms developed (and
   originally specified in [RFC1971] and [RFC1972]).  A number of
   privacy and security issues related to the IIDs derived from IEEE
   identifiers were discovered after their standardization, and many of
   the mechanisms developed later aimed to mitigate some or all of these
   weaknesses.  This document identifies four types of attacks against
   IEEE-identifier-based IIDs and discusses how other existing
   techniques for generating IIDs do or do not mitigate those attacks.

2.  Terminology

   This section clarifies the terminology used throughout this document.

   Public address:
      An address that has been published in a directory or other public
      location, such as the DNS, a SIP proxy [RFC3261], an application-
      specific Distributed Hash Table (DHT), or a publicly available
      URI.  A host's public addresses are intended to be discoverable by
      third parties.

   Stable address:
      An address that does not vary over time within the same IPv6 link.
      Note that [RFC4941] refers to these as "public" addresses, but
      "stable" is used here for reasons explained in Section 4.

   Temporary address:
      An address that varies over time within the same IPv6 link.

   Constant IID:
      An IPv6 interface identifier that is globally stable.  That is,
      the Interface ID will remain constant even if the node moves from
      one IPv6 link to another.

   Stable IID:
      An IPv6 interface identifier that is stable within some specified
      context.  For example, an Interface ID can be globally stable
      (constant) or could be stable per IPv6 link (meaning that the
      Interface ID will remain unchanged as long as the node stays on
      the same IPv6 link but may change when the node moves from one
      IPv6 link to another).

   Temporary IID:
      An IPv6 interface identifier that varies over time.

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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].  These words take their normative meanings only when they
   are presented in ALL UPPERCASE.

3.  Weaknesses in IEEE-Identifier-Based IIDs

   There are a number of privacy and security implications that exist
   for hosts that use IEEE-identifier-based IIDs.  This section
   discusses four generic attack types: correlation of activities over
   time, location tracking, address scanning, and device-specific
   vulnerability exploitation.  The first three of these rely on the
   attacker first gaining knowledge of the IID of the target host.  This
   could be achieved by a number of different entities: the operator of
   a server to which the host connects, such as a web server or a peer-
   to-peer server; an entity that connects to the same IPv6 link as the
   target (such as a conference network or any public network); a
   passive observer of traffic that the host broadcasts; or an entity
   that is on path to the destinations with which the host communicates,
   such as a network operator.

3.1.  Correlation of Activities over Time

   As with other identifiers, an IPv6 address can be used to correlate
   the activities of a host for at least as long as the lifetime of the
   address.  The correlation made possible by IEEE-identifier-based IIDs
   is of particular concern since they last roughly for the lifetime of
   a device's network interface, allowing correlation on the order of
   years.

   As [RFC4941] explains,

      [t]he 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. ... 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.

   IP addresses are just one example of information that can be used to
   correlate activities over time.  DNS names, cookies [RFC6265],
   browser fingerprints [Panopticlick], and application-layer usernames

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   can all be used to link a host's activities together.  Although IEEE-
   identifier-based IIDs are likely to last at least as long or longer
   than these other identifiers, IIDs generated in other ways may have
   shorter or longer lifetimes than these identifiers depending on how
   they are generated.  Therefore, the extent to which a host's
   activities can be correlated depends on whether the host uses
   multiple identifiers together and the lifetimes of all of those
   identifiers.  Frequently refreshing an IPv6 address may not mitigate
   correlation if an attacker has access to other longer-lived
   identifiers for a particular host.  This is an important caveat to
   keep in mind throughout the discussion of correlation in this
   document.  For further discussion of correlation, see Section 5.2.1
   of [RFC6973].

   As noted in [RFC4941], in some cases correlation is just as feasible
   for a host using an IPv4 address as for a host using an IEEE
   identifier to generate its IID in its IPv6 address.  Hosts that use
   static IPv4 addressing or who are consistently allocated the same
   address via DHCPv4 can be tracked as described above.  However, the
   widespread use of both NAT and DHCPv4 implementations that assign the
   same host a different address upon lease expiration mitigates this
   threat in the IPv4 case as compared to the IEEE identifier case in
   IPv6.

3.2.  Location Tracking

   Because the IPv6 address structure is divided between a topological
   portion and an interface identifier portion, an interface identifier
   that remains constant when a host connects to different IPv6 links
   (as an IEEE-identifier-based IID does) provides a way for observers
   to track the movements of that host.  In a passive attack on a mobile
   host, a server that receives connections from the same host over time
   would be able to determine the host's movements as its prefix
   changes.

   Active attacks are also possible.  An attacker that first learns the
   host's interface identifier by being connected to the same IPv6 link,
   running a server that the host connects to, or being on path to the
   host's communications could subsequently probe other networks for the
   presence of the same interface identifier by sending a probe packet
   (e.g., ICMPv6 Echo Request, or any other probe packet).  Even if the
   host does not respond, the first-hop router will usually respond with
   an ICMP Destination Unreachable/Address Unreachable (type 1, code 3)
   when the host is not present and be silent when the host is present.

   Location tracking based on IP address is generally not possible in
   IPv4 since hosts get assigned wholly new addresses when they change
   networks.

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3.3.  Address Scanning

   The structure of IEEE-based identifiers used for address generation
   can be leveraged by an attacker to reduce the target search space
   [RFC7707].  The 24-bit Organizationally Unique Identifier (OUI) of
   MAC addresses, together with the fixed value (0xff, 0xfe) used to
   form a Modified EUI-64 interface identifier, greatly help to reduce
   the search space, making it easier for an attacker to scan for
   individual addresses using widely known popular OUIs.  This erases
   much of the protection against address scanning that the larger IPv6
   address space could provide as compared to IPv4.

3.4.  Device-Specific Vulnerability Exploitation

   IPv6 addresses that embed IEEE identifiers leak information about the
   device (e.g., Network Interface Card vendor, or even Operating System
   and/or software type), which could be leveraged by an attacker with
   knowledge of device- or software-specific vulnerabilities to quickly
   find possible targets.  Attackers can exploit vulnerabilities in
   hosts whose IIDs they have previously obtained or scan an address
   space to find potential targets.

4.  Privacy and Security Properties of Address Generation Mechanisms

   Analysis of the extent to which a particular host is protected
   against the attacks described in Section 3 depends on how each of a
   host's addresses is generated and used.  In some scenarios, a host
   configures a single global address and uses it for all
   communications.  In other scenarios, a host configures multiple
   addresses using different mechanisms and may use any or all of them.

   [RFC3041] (later obsoleted by [RFC4941]) sought to address some of
   the problems described in Section 3 by defining "temporary addresses"
   for outbound connections.  Temporary addresses are meant to
   supplement the other addresses that a device might use, not to
   replace them.  They use IIDs that are randomly generated and change
   daily by default.  The idea was for temporary addresses to be used
   for outgoing connections (e.g., web browsing) while maintaining the
   ability to use a stable address when more address stability is
   desired (e.g., for IPv6 addresses published in the DNS).

   [RFC3484] originally specified that stable addresses be used for
   outbound connections unless an application explicitly prefers
   temporary addresses.  The default preference for stable addresses was
   established to avoid applications potentially failing due to the
   short lifetime of temporary addresses or the possibility of a reverse
   look-up failure or error.  However, [RFC3484] allowed that
   "implementations for which privacy considerations outweigh these

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   application-compatibility concerns MAY reverse the sense of this
   rule" and instead prefer by default temporary addresses rather than
   stable addresses.  Indeed, most implementations (notably including
   Windows) chose to default to temporary addresses for outbound
   connections since privacy was considered more important (and few
   applications supported IPv6 at the time, so application compatibility
   concerns were minimal).  [RFC6724] then obsoleted [RFC3484] and
   changed the default to match what implementations actually did.

   The envisioned relationship in [RFC3484] between stability of an
   address and its use in "public" can be misleading when conducting
   privacy analysis.  The stability of an address and the extent to
   which it is linkable to some other public identifier are independent
   of one another.  For example, there is nothing that prevents a host
   from publishing a temporary address in a public place, such as the
   DNS.  Publishing both a stable address and a temporary address in the
   DNS or elsewhere where they can be linked together by a public
   identifier allows the host's activities when using either address to
   be correlated together.

   Moreover, because temporary addresses were designed to supplement
   other addresses generated by a host, the host may still configure a
   more stable address even if it only ever intentionally uses temporary
   addresses (as source addresses) for communication to off-link
   destinations.  An attacker can probe for the stable address even if
   it is never used as such a source address or advertised outside the
   link (e.g., in DNS or SIP).

   This section compares the privacy and security properties of a
   variety of IID generation mechanisms and their possible usage
   scenarios, including scenarios in which a single mechanism is used to
   generate all of a host's IIDs and those in which temporary addresses
   are used together with addresses generated using a different IID
   generation mechanism.  The analysis of the exposure of each IID type
   to correlation assumes that IPv6 prefixes are shared by a reasonably
   large number of nodes.  As [RFC4941] notes, if a very small number of
   nodes (say, only one) use a particular prefix for an extended period
   of time, the prefix itself can be used to correlate the host's
   activities regardless of how the IID is generated.  For example,
   [RFC3314] recommends that prefixes be uniquely assigned to mobile
   handsets where IPv6 is used within General Packet Radio Service
   (GPRS).  In cases where this advice is followed and prefixes persist
   for extended periods of time (or get reassigned to the same handsets
   whenever those handsets reconnect to the same network router), hosts'
   activities could be correlatable for longer periods than the analysis
   below would suggest.

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   The table below provides a summary of the whole analysis.  A "No"
   entry indicates that the attack is prevented from being carried out
   on the basis of the IID, but the host may still be vulnerable
   depending on how it employs other protocols.

   +--------------+-------------+----------+-------------+-------------+
   | Mechanism(s) | Correlation | Location | Address     | Device      |
   |              |             | tracking | scanning    | exploits    |
   +--------------+-------------+----------+-------------+-------------+
   | IEEE         | For device  | For      | Possible    | Possible    |
   | identifier   | lifetime    | device   |             |             |
   |              |             | lifetime |             |             |
   |              |             |          |             |             |
   | Static       | For address | For      | Depends on  | Depends on  |
   | manual       | lifetime    | address  | generation  | generation  |
   |              |             | lifetime | mechanism   | mechanism   |
   |              |             |          |             |             |
   | Constant,    | For address | For      | No          | No          |
   | semantically | lifetime    | address  |             |             |
   | opaque       |             | lifetime |             |             |
   |              |             |          |             |             |
   | CGA          | For         | No       | No          | No          |
   |              | lifetime of |          |             |             |
   |              | (modifier   |          |             |             |
   |              | block +     |          |             |             |
   |              | public key) |          |             |             |
   |              |             |          |             |             |
   | Stable,      | Within      | No       | No          | No          |
   | semantically | single IPv6 |          |             |             |
   | opaque       | link        |          |             |             |
   |              |             |          |             |             |
   | Temporary    | For temp    | No       | No          | No          |
   |              | address     |          |             |             |
   |              | lifetime    |          |             |             |
   |              |             |          |             |             |
   | DHCPv6       | For lease   | No       | Depends on  | No          |
   |              | lifetime    |          | generation  |             |
   |              |             |          | mechanism   |             |
   +--------------+-------------+----------+-------------+-------------+

   Table 1: Privacy and Security Properties of IID Generation Mechanisms

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4.1.  IEEE-Identifier-Based IIDs

   As discussed in Section 3, addresses that use IIDs based on IEEE
   identifiers are vulnerable to all four attacks.  They allow
   correlation and location tracking for the lifetime of the device
   since IEEE identifiers last that long and their structure makes
   address scanning and device exploits possible.

4.2.  Static, Manually Configured IIDs

   Because static, manually configured IIDs are stable, both correlation
   and location tracking are possible for the life of the address.

   The extent to which location tracking can be successfully performed
   depends, to some extent, on the uniqueness of the employed IID.  For
   example, one would expect "low byte" IIDs to be more widely reused
   than, for example, IIDs where the whole 64 bits follow some pattern
   that is unique to a specific organization.  Widely reused IIDs will
   typically lead to false positives when performing location tracking.

   Whether manually configured addresses are vulnerable to address
   scanning and device exploits depends on the specifics of how the IIDs
   are generated.

4.3.  Constant, Semantically Opaque IIDs

   Although a mechanism to generate a constant, semantically opaque IID
   has not been standardized, it has been in wide use for many years on
   at least one platform (Windows).  Windows uses the random generation
   mechanism described in [RFC4941] in lieu of generating an IEEE-
   identifier-based IID.  This mitigates the device-specific
   exploitation and address-scanning attacks but still allows
   correlation and location tracking because the IID is constant across
   IPv6 links and time.

4.4.  Cryptographically Generated IIDs

   Cryptographically Generated Addresses (CGAs) [RFC3972] bind a hash of
   the host's public key to an IPv6 address in the SEcure Neighbor
   Discovery (SEND) protocol [RFC3971].  CGAs may be regenerated for
   each subnet prefix, but this is not required given that they are
   computationally expensive to generate.  A host using a CGA can be
   correlated for as long as the lifetime of the combination of the
   public key and the chosen modifier block since it is possible to
   rotate modifier blocks without generating new public keys.  Because
   the cryptographic hash of the host's public key uses the subnet
   prefix as an input, even if the host does not generate a new public
   key or modifier block when it moves to a different IPv6 link, its

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   location cannot be tracked via the IID.  CGAs do not allow device-
   specific exploitation or address-scanning attacks.

4.5.  Stable, Semantically Opaque IIDs

   [RFC7217] specifies an algorithm that generates, for each network
   interface, a unique random IID per IPv6 link.  The aforementioned
   algorithm is employed not only for global unicast addresses, but also
   for unique local unicast addresses and link-local unicast addresses
   since these addresses may leak out via application protocols (e.g.,
   IPv6 addresses embedded in email headers).

   A host that stays connected to the same IPv6 link could therefore be
   tracked at length, whereas a mobile host's activities could only be
   correlated for the duration of each network connection.  Location
   tracking is not possible with these addresses.  They also do not
   allow device-specific exploitation or address-scanning attacks.

4.6.  Temporary IIDs

   A host that uses only a temporary address mitigates all four threats.
   Its activities may only be correlated for the lifetime of a single
   temporary address.

   A host that configures both an IEEE-identifier-based IID and
   temporary addresses makes the host vulnerable to the same attacks as
   if temporary addresses were not in use, although the viability of
   some of them depends on how the host uses each address.  An attacker
   can correlate all of the host's activities for which it uses its
   IEEE-identifier-based IID.  Once an attacker has obtained the IEEE-
   identifier-based IID, location tracking becomes possible on other
   IPv6 links even if the host only makes use of temporary addresses on
   those other IPv6 links; the attacker can actively probe the other
   IPv6 links for the presence of the IEEE-identifier-based IID.
   Device-specific vulnerabilities can still be exploited.  Address
   scanning is also still possible because the IEEE-identifier-based
   address can be probed.

   If the host instead generates a constant, semantically opaque IID to
   use in a stable address for server-like connections together with
   temporary addresses for outbound connections (as is the default in
   Windows), it sees some improvements over the previous scenario.  The
   address-scanning attacks and device-specific exploitation attacks are
   no longer possible because the OUI is no longer embedded in any of
   the host's addresses.  However, correlation of some activities across
   time and location tracking are both still possible because the
   semantically opaque IID is constant.  And once an attacker has
   obtained the host's semantically opaque IID, location tracking is

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   possible on any network by probing for that IID, even if the host
   only uses temporary addresses on those networks.  However, if the
   host generates but never uses a constant, semantically opaque IID, it
   mitigates all four threats.

   When used together with temporary addresses, the stable, semantically
   opaque IID generation mechanism [RFC7217] improves upon the previous
   scenario by limiting the potential for correlation to the lifetime of
   the stable address (which may still be lengthy for hosts that are not
   mobile) and by eliminating the possibility for location tracking
   (since a different IID is generated for each subnet prefix).  As in
   the previous scenario, a host that configures but does not use a
   stable, semantically opaque address mitigates all four threats.

4.7.  DHCPv6 Generation of IIDs

   The security and privacy implications of DHCPv6-based addresses will
   typically depend on whether the client requests an IA_NA (Identity
   Association for Non-temporary Addresses) or an IA_TA (Identity
   Association for Temporary Addresses) [RFC3315] and the specific
   DHCPv6 server software being employed.

   DHCPv6 temporary addresses have the same properties as SLAAC
   temporary addresses (see Section 4.6).  On the other hand, the
   properties of DHCPv6 non-temporary addresses typically depend on the
   specific DHCPv6 server software being employed.  Recent releases of
   most popular DHCPv6 server software typically lease random addresses
   with a similar lease time as that of IPv4.  Thus, these addresses can
   be considered to be "stable, semantically opaque".  [DHCPv6-IID]
   specifies an algorithm that can be employed by DHCPv6 servers to
   generate "stable, semantically opaque" addresses.

   On the other hand, some DHCPv6 software leases sequential addresses
   (typically low-byte addresses).  These addresses can be considered to
   be stable addresses.  The drawback of this address generation scheme
   compared to "stable, semantically opaque" addresses is that, since
   they follow specific patterns, they enable IPv6 address scans.

4.8.  Transition and Coexistence Technologies

   Addresses specified based on transition or coexistence technologies
   that embed an IPv4 address within an IPv6 address are not included in
   Table 1 because their privacy and security properties are inherited
   from the embedded address.  For example, Teredo [RFC4380] specifies a
   means to generate an IPv6 address from the underlying IPv4 address
   and port, leaving many other bits set to zero.  This makes it
   relatively easy for an attacker to scan for IPv6 addresses by
   guessing the Teredo client's IPv4 address and port (which for many

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   NATs is not randomized).  For this reason, popular implementations
   (e.g., Windows) began deviating from the standard by including 12
   random bits in place of zero bits.  This modification was later
   standardized in [RFC5991].

   Some other transition technologies (e.g., [RFC5214], [RFC6052])
   specify means to generate an IPv6 address from an underlying IPv4
   address without a port.  Such mechanisms thus make it much easier for
   an attacker to conduct an address scan than for mechanisms that
   require finding a port number as well.

   Finally, still other mechanisms (e.g., [RFC7596], [RFC7597],
   [RFC7599]) are somewhere in between, using an IPv4 address and a port
   set ID (which for many NATs is not randomized).  In general, such
   mechanisms are thus typically as easy to scan as in the Teredo
   example above without the 12-bit mitigation.

5.  Miscellaneous Issues with IPv6 Addressing

5.1.  Network Operation

   It is generally agreed that IPv6 addresses that vary over time in a
   specific IPv6 link tend to increase the complexity of event logging,
   trouble-shooting, enforcement of access controls and quality of
   service, etc.  As a result, some organizations disable the use of
   temporary addresses [RFC4941] even at the expense of reduced privacy
   [Broersma].

5.2.  Compliance

   Some IPv6 compliance testing suites required (and might still
   require) implementations to support IEEE-identifier-based IIDs in
   order to be approved as compliant.  This document recommends that
   compliance testing suites be relaxed to allow other forms of address
   generation that are more amenable to privacy.

5.3.  Intellectual Property Rights (IPRs)

   Some IPv6 addressing techniques might be covered by Intellectual
   Property rights, which might limit their implementation in different
   operating systems.  [CGA-IPR] and [KAME-CGA] discuss the IPRs on
   CGAs.

6.  Security Considerations

   This whole document concerns the privacy and security properties of
   different IPv6 address generation mechanisms.

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7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
              Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998,
              <http://www.rfc-editor.org/info/rfc2464>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <http://www.rfc-editor.org/info/rfc3315>.

   [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
              "SEcure Neighbor Discovery (SEND)", RFC 3971,
              DOI 10.17487/RFC3971, March 2005,
              <http://www.rfc-editor.org/info/rfc3971>.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,
              <http://www.rfc-editor.org/info/rfc3972>.

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              DOI 10.17487/RFC4380, February 2006,
              <http://www.rfc-editor.org/info/rfc4380>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <http://www.rfc-editor.org/info/rfc4862>.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <http://www.rfc-editor.org/info/rfc4941>.

   [RFC5991]  Thaler, D., Krishnan, S., and J. Hoagland, "Teredo
              Security Updates", RFC 5991, DOI 10.17487/RFC5991,
              September 2010, <http://www.rfc-editor.org/info/rfc5991>.

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   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
              <http://www.rfc-editor.org/info/rfc6724>.

   [RFC7136]  Carpenter, B. and S. Jiang, "Significance of IPv6
              Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
              February 2014, <http://www.rfc-editor.org/info/rfc7136>.

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217,
              DOI 10.17487/RFC7217, April 2014,
              <http://www.rfc-editor.org/info/rfc7217>.

7.2.  Informative References

   [Broersma] Broersma, R., "IPv6 Everywhere: Living with a Fully
              IPv6-enabled environment",  Australian IPv6 Summit 2010,
              Melbourne, VIC Australia, October 2010,
              <http://www.ipv6.org.au/10ipv6summit/talks/
              Ron_Broersma.pdf>.

   [CGA-IPR]  IETF, "IPR Details: Microsoft's Statement about IPR
              claimed in RFC 3972", November 2005,
              <https://datatracker.ietf.org/ipr/676/>.

   [DHCPv6-IID]
              Gont, F. and W. Liu, "A Method for Generating Semantically
              Opaque Interface Identifiers with Dynamic Host
              Configuration Protocol for IPv6 (DHCPv6)", Work in
              Progress, draft-ietf-dhc-stable-privacy-addresses-02,
              April 2015.

   [KAME-CGA] The KAME Project, "The KAME IPR policy and concerns of
              some technologies which have IPR claims", November 2005,
              <http://www.kame.net/newsletter/20040525/>.

   [Microsoft]
              Microsoft, "IPv6 interface identifiers", 2013,
              <http://www.microsoft.com/resources/documentation/
              windows/xp/all/proddocs/en-us/
              sag_ip_v6_imp_addr7.mspx?mfr=true>.

   [Panopticlick]
              Electronic Frontier Foundation, "Panopticlick", 2011,
              <http://panopticlick.eff.org>.

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   [RFC1971]  Thomson, S. and T. Narten, "IPv6 Stateless Address
              Autoconfiguration", RFC 1971, DOI 10.17487/RFC1971, August
              1996, <http://www.rfc-editor.org/info/rfc1971>.

   [RFC1972]  Crawford, M., "A Method for the Transmission of IPv6
              Packets over Ethernet Networks", RFC 1972,
              DOI 10.17487/RFC1972, August 1996,
              <http://www.rfc-editor.org/info/rfc1972>.

   [RFC3041]  Narten, T. and R. Draves, "Privacy Extensions for
              Stateless Address Autoconfiguration in IPv6", RFC 3041,
              DOI 10.17487/RFC3041, January 2001,
              <http://www.rfc-editor.org/info/rfc3041>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <http://www.rfc-editor.org/info/rfc3261>.

   [RFC3314]  Wasserman, M., Ed., "Recommendations for IPv6 in Third
              Generation Partnership Project (3GPP) Standards",
              RFC 3314, DOI 10.17487/RFC3314, September 2002,
              <http://www.rfc-editor.org/info/rfc3314>.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484,
              DOI 10.17487/RFC3484, February 2003,
              <http://www.rfc-editor.org/info/rfc3484>.

   [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
              Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
              DOI 10.17487/RFC5214, March 2008,
              <http://www.rfc-editor.org/info/rfc5214>.

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              DOI 10.17487/RFC6052, October 2010,
              <http://www.rfc-editor.org/info/rfc6052>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <http://www.rfc-editor.org/info/rfc6265>.

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   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <http://www.rfc-editor.org/info/rfc6973>.

   [RFC7421]  Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S.,
              Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit
              Boundary in IPv6 Addressing", RFC 7421,
              DOI 10.17487/RFC7421, January 2015,
              <http://www.rfc-editor.org/info/rfc7421>.

   [RFC7596]  Cui, Y., Sun, Q., Boucadair, M., Tsou, T., Lee, Y., and I.
              Farrer, "Lightweight 4over6: An Extension to the Dual-
              Stack Lite Architecture", RFC 7596, DOI 10.17487/RFC7596,
              July 2015, <http://www.rfc-editor.org/info/rfc7596>.

   [RFC7597]  Troan, O., Ed., Dec, W., Li, X., Bao, C., Matsushima, S.,
              Murakami, T., and T. Taylor, Ed., "Mapping of Address and
              Port with Encapsulation (MAP-E)", RFC 7597,
              DOI 10.17487/RFC7597, July 2015,
              <http://www.rfc-editor.org/info/rfc7597>.

   [RFC7599]  Li, X., Bao, C., Dec, W., Ed., Troan, O., Matsushima, S.,
              and T. Murakami, "Mapping of Address and Port using
              Translation (MAP-T)", RFC 7599, DOI 10.17487/RFC7599, July
              2015, <http://www.rfc-editor.org/info/rfc7599>.

   [RFC7707]  Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
              <http://www.rfc-editor.org/info/rfc7707>.

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Acknowledgements

   The authors would like to thank Bernard Aboba, Brian Carpenter, Tim
   Chown, Lorenzo Colitti, Rich Draves, Robert Hinden, Robert Moskowitz,
   Erik Nordmark, Mark Smith, Ole Troan, and James Woodyatt for
   providing valuable comments on earlier draft versions of this
   document.

Authors' Addresses

   Alissa Cooper
   Cisco
   707 Tasman Drive
   Milpitas, CA  95035
   United States

   Phone: +1-408-902-3950
   Email: alcoop@cisco.com
   URI:   https://www.cisco.com/

   Fernando Gont
   Huawei Technologies
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com

   Dave Thaler
   Microsoft
   One Microsoft Way
   Redmond, WA  98052
   United States

   Phone: +1 425 703 8835
   Email: dthaler@microsoft.com

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