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Versions: 00 01

IPv6 Operations Working Group (v6ops)                            F. Gont
Internet-Draft                                                   G. Gont
Intended status: Informational                              SI6 Networks
Expires: June 14, 2021                                 December 11, 2020


                     IPv6 Addressing Considerations
           draft-gont-v6ops-ipv6-addressing-considerations-00

Abstract

   IPv6 addresses can differ in a number of properties, such as scope,
   stability, and intended usage type.  This document analyzes the
   impact of these properties on aspects such as security, privacy,
   interoperability, and network operations.  Additionally, it
   identifies challenges and gaps that currently prevent systems and
   applications from leveraging the increased flexibility and
   availability of IPv6 addresses.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on June 14, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may not be modified, and derivative works of it may not
   be created, and it may not be published except as an Internet-Draft.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Legacy Specifications and Schemes . . . . . . . . . . . .   4
     3.2.  Unique Local IPv6 Unicast Addresses (ULAs)  . . . . . . .   5
   4.  IPv6 Address Properties . . . . . . . . . . . . . . . . . . .   5
     4.1.  Address Scope Considerations  . . . . . . . . . . . . . .   6
     4.2.  Provider Dependency . . . . . . . . . . . . . . . . . . .   7
     4.3.  Address Reachability  . . . . . . . . . . . . . . . . . .   8
     4.4.  Address Stability Considerations  . . . . . . . . . . . .   9
   5.  IPv6 Address Usage  . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  Default Address Selection . . . . . . . . . . . . . . . .  11
     5.2.  Usage Type Considerations . . . . . . . . . . . . . . . .  12
     5.3.  Current Alternatives for IPv6 Address Usage . . . . . . .  12
       5.3.1.  Incoming communications . . . . . . . . . . . . . . .  12
       5.3.2.  Outgoing communications . . . . . . . . . . . . . . .  14
   6.  Current Issues  . . . . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Sub-optimal IPv6 Address Usage  . . . . . . . . . . . . .  14
       6.1.1.  Correlation of Network Activity . . . . . . . . . . .  14
       6.1.2.  Passive Host Tracking . . . . . . . . . . . . . . . .  15
       6.1.3.  Unintended Service Disclosure . . . . . . . . . . . .  15
       6.1.4.  Availability Outside the Expected Scope . . . . . . .  16
     6.2.  Sub-optimal Address Configuration . . . . . . . . . . . .  16
       6.2.1.  Number of Addresses . . . . . . . . . . . . . . . . .  16
       6.2.2.  SLAAC/DHCPv6 Interaction  . . . . . . . . . . . . . .  17
     6.3.  Operation of Multi-Prefix/Multi-Address/Multi-Router
           Networks  . . . . . . . . . . . . . . . . . . . . . . . .  17
       6.3.1.  Implications of Addresses . . . . . . . . . . . . . .  17
       6.3.2.  Legitimate Network Activity Correlation . . . . . . .  17
       6.3.3.  Routing in Multi-Prefix/Multi-Router Networks . . . .  18
       6.3.4.  Renumbering . . . . . . . . . . . . . . . . . . . . .  18
   7.  Current Gaps that Prevent Leveraging IPv6 Addressing  . . . .  19
     7.1.  Better Address Selection APIs . . . . . . . . . . . . . .  19
     7.2.  Universal Support of Multi-prefix/Multi-router Networks .  20
     7.3.  Profile-based IPv6 Address Configuration  . . . . . . . .  20
     7.4.  Protocol Improvements to Deal with Many Addresses . . . .  21
     7.5.  Support for Firewall Traversal in CE Routers  . . . . . .  21
     7.6.  Advice on IPv6 Address Usage  . . . . . . . . . . . . . .  21
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  22



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   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     11.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   IPv6 addresses can differ in a number of properties, such as address
   scope (e.g. link-local vs. global), stability (e.g. stable addresses
   vs. temporary addresses), and intended usage type (outgoing
   communications vs. incoming communications).  While often overlooked,
   these properties have direct impact on areas such as security,
   privacy, interoperability, and network operations.

   IPv6 hosts typically configure addresses based on local system
   policy, which tends to be static and irrespective of the specific
   network the host attaches to.  For example, most IPv6 host
   implementations configure one link-local address for each network
   interface, and one stable and one (or more) temporary addresses per
   each autoconfiguration prefix advertised via Stateless Address Auto-
   configuration (SLAAC) [RFC4862] for each network interface.
   Additionally, in scenarios where the network provides address
   configuration via both SLAAC and DHCPv6, host typically configure
   addresses using both protocols; as a result, hosts will typically add
   one DHPv6-leased address per local prefix to the set of configured
   addresses.

   Each application on a given system could have its own set of
   requirements or expectations for the properties of the underlying
   IPv6 addresses.  For example, an application meaning to offer a
   public service might expect to employ addresses that are both
   globally-reachable [RFC8190] and stable [RFC7721] [RFC8064], while a
   privacy-sensible client application might prefer short-lived
   temporary addresses [I-D.ietf-6man-rfc4941bis], or might even expect
   to employ single-use ("throw-away") IPv6 addresses when connecting to
   public servers.  However, the subtleties associated with IPv6
   addresses are often ignored or overlooked by application programmers
   and network administrators, and thus hosts could end up making
   suboptimal choices when configuring addresses, while applications
   could fail to signal their requirements and preferences to the
   underlying system.  Additionally, limitations in the current
   Application Programming Interfaces (APIs) along with implementation
   constraints in some network devices can also limit the ability of
   applications to leverage the increased flexibility of IPv6
   addressing.  This not only results in a failure to leverage the
   increased addressing flexibility provided by IPv6 but, at times, also
   in unintended consequences.



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   This document identifies a set of properties that can be associated
   with IPv6 addresses (such as scope and stability), and analyzes the
   impact of these properties on areas ranging from security and privacy
   to network operations, with the goal of providing guidance about IPv6
   address usage, and identifying challenges and gaps that currently
   prevent systems and applications from leveraging the increased
   flexibility and availability of IPv6 addresses.

2.  Terminology

   This document employs the definitions of "public address", "stable
   address", and "temporary address" from Section 2 of [RFC7721].

   This document employs the definition of "globally reachable" from
   Section 2.1 of [RFC8190].

   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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Conventions

3.1.  Legacy Specifications and Schemes

   IPv6 SLAAC has traditionally employed schemes for generating
   Interface Identifiers (IIDs) that have negatively affected the
   security and privacy properties of IPv6 addresses.  For example, IPv6
   SLAAC originally generated stable addresses by embedding the
   underlying link-layer address in the IPv6 Interface Identifier (IID),
   thus negatively affecting the security and privacy properties of IPv6
   addresses [RFC7721] [RFC7707].  Similarly, IPv6 temporary addresses
   [RFC4941] reused the same randomized IID for multiple auto-
   configuration prefixes [RFC4941], thus allowing for network activity
   correlation across different addresses of the same host.

   These schemes have become formally superseded by other schemes, such
   as [RFC7217] and [I-D.ietf-6man-rfc4941bis], that mitigate the issues
   present in the legacy schemes.  Therefore, does not discuss any
   implications arising from legacy IID generation algorithms.

   NOTE:
      The security and privacy implications of such schemes are
      discussed in [RFC7721], [RFC7707], and [RFC7217].






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3.2.  Unique Local IPv6 Unicast Addresses (ULAs)

   Unique Local IPv6 Unicast Addresses (ULAs) are considered to have
   global scope, but non-global reachability.  However, the only reasons
   for which these addresses are considered to have "global scope" are:

   o  given two different networks that employ ULAs, it's unlikely that
      they will employ the same ULA prefix

   o  the ULA address block formally belongs to the Global Unicast
      Address (GUA) address block

   The ULA prefixes of two different networks that e.g. need to be
   merged will be (statistically) different *if* sites generate their
   ULA prefixes following the recommendations in [RFC4193] -- i.e.,
   randomize the ULA prefix.  However, this is not enforceable, and
   there exists anecdotal evidence that some sites employ non-randomized
   ULA prefixes, possibly in the hope of employing prefixes that are
   easier to express via the IPv6 address notation (e.g. fd00::/48).

   The ULA address block has been carved out of the GUA address block,
   without updating the IPv6 Addressing Architecture ([RFC4291]) to
   define the ULA address block as a different address type (e.g.  "IPv6
   private address space").  However, [RFC4193] notes that ULAs are not
   expected to be globally-routable, and the ULA prefix is commonly
   included in "bogon filters" typically enforced by network operators
   and administrators, thus limiting the reachability of these
   addresses.

   Therefore, ULAs are not globally meaningful and thus, for most (if
   not all) practical purposes, ULAs can be considered to have non-
   global scope.  For this reason, ULAs are treated as non-global scope
   addresses, even when from a specifications point of view they have
   global scope.

4.  IPv6 Address Properties

   There are, at least, four properties that can be associated with
   every IPv6 address:

   o  Scope

   o  Reachability

   o  Stability

   o  Provider Dependency




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   The address scope essentially represents the area of the network
   where an address can be expected to uniquely identify a system or set
   of systems, or conceptually, the area of the network where an given
   address is meaningful.  For example, link-local addresses are only
   meaningful within a given network link, and are expected to be unique
   only within such network link.

   Address reachability represents the area of the network, where an
   address can be expected to be used for receiving and transmitting
   packets.  Besides the semantics of specific address types,
   reachability can be affected by network devices: for example,
   Customer Edge Routers (CE Routers) that enforce a filtering policy of
   "only allowing outgoing communications" can render otherwise globally
   reachable addresses as "unreachable from the public Internet, unless
   communication is initiated from the customer's network".

   The stability of an address is associated with the invariance of an
   address over time.  For example, a manually-configured address will
   typically remain stable while the node remains attached to the same
   subnet, while a temporary address will, by definition, change over
   time.  While address stability does depend on the inherent properties
   of a given address (e.g. stable vs. temporary), it also depends on
   other factors, such as provider dependency: if a network employs a
   prefix that is assigned/leased by an upstream provider, then the
   overall stability of the addresses will also depend on the stability
   of the network prefix leased/assigned by the upstream.

   Provider-dependency is typically discussed in the context of Global
   Unicast Addresses, where the address space may be allocated by an
   Internet Service Provider (ISP) (and hence "provider aggregatable")
   or by a Regional Internet Registry (RIR) (and hence "provider
   independent").  However, this document considers "provider
   dependency" in a more general way: "provider aggregatable" address
   space is assigned or leased by the upstream (which may or may not be
   an ISP) from the provider's address space, and thus has a topological
   relationship with the upstream's address space, whereas "provider
   independent" address space is "owned" by the network in question and
   does not necessarily have a topological relationship with the
   upstream.

4.1.  Address Scope Considerations

   The IPv6 address scope [RFC4007] has a direct implication on address
   reachability: the address scope essentially limits reachability.  For
   example, addresses that have a non-global scope are not, in
   principle, globally reachable.

   NOTE:



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      This assumption becomes invalid if technologies such as Network
      Prefix Translation (NPT) [RFC6296] are employed, though.  However,
      strictly speaking, in these scenarios the non-global addresses are
      still not globally reachable, but rather the middle-box acts as an
      interface to the "external realm" via globally-reachable addresses
      (i.e., the middle-box has an interface within the scope of the
      non-global addresses, and globally-reachables address that are
      used to communicate with the "external realm").

   The IPv6 address scope can, in some scenarios, limit the attack
   exposure of a node as a result of the implicit isolation provided by
   a non-global address scopes.  For example, a node that only employs
   link-local addresses will, in principle, only be exposed to attacks
   from other nodes on the same local link.

   The potential protection provided by a non-global-scope addresses
   should not be regarded as a complete security strategy, but rather as
   a form of "prophylactic" security (see
   [I-D.gont-opsawg-firewalls-analysis]).

   We note that non-global scope addresses are normally only of use for
   limited number of applications/protocols that operate on a limited
   scope (e.g., mDNS), or deployments where the intended participants
   have been deployed in a limited area of the network topology (e.g.,
   OpenSSH client and server attached to the same link, both employing
   link-local addresses).

   The address scope can at times be somewhat related with the provider
   dependency property.  For example, link-local addresses are, by
   definition, provider independent.  In the same light, a ULA prefix
   generated by a local router will be, by definition, provider
   independent.  However, a router might also be leased an ULA sub-
   prefix from its upstream, in which case this prefix would be
   "provider dependent".

4.2.  Provider Dependency

   Provider-dependency is typically discussed in the context of Global
   Unicast Addresses, where the address space may be allocated by an
   Internet Service Provider (ISP) (and hence "provider agregatable") or
   by a Regional Internet Registry (RIR) (and hence "provider
   independent").  However, this document considers "provider
   dependency" in a more general way: "provider agregatable" address
   space is assigned or leased by the upstream (which may or may not be
   an ISP) from the provider's address space, and thus has a topological
   relationship with the upstream's address space, whereas "provider
   independent" address space is "owned" by the network in question and




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   does not necessarily have a topological relationship with the
   upstream.

   An implicit consequence of PA address space is that its use is tied
   to the specific provider/upstream that has assigned/leased the
   address space.  This means that multi-homing (employing the address
   space with multiple providers/upstreams) is not possible, and that a
   renumbering event at the upstream could lead to a renumbering event
   of the local network.

   As a result, provider-dependency can affect address stability, with
   PI address space generally having better stability properties.  For
   example, a home network could internally employ both ULAs and GUAs,
   where a ULA prefix is locally generated by the Customer Edge Router
   (CE Router), and global prefix is leased by the ISP via DHCPv6 Prefix
   Delegation (DHCPv6-PD) [RFC8415].  If for some reason there was an
   outage involving the connection with the upstream ISP, the prefix
   lease time would eventually expire, and therefore addresses
   configured for such prefix would need to be invalidated.  Similarly,
   if upon prefix lease expiration the ISP were to lease a new IPv6
   prefix rather than renew the previously employed prefix, the network
   would need to be renumbered.  On the other hand, ULA prefixes locally
   generated and advertised by the CE Router will not require renewal
   from the ISP, since they are locally generated by the CE Router.

   NOTE:
      As noted above, while strictly speaking ULAs belong to the global
      unicast address space, they can be considered non-global addresses
      for all practical purposes.

   Similarly, an organizational network that employs PI address space
   obtained from a RIR would be able to keep the same address space,
   even if the upstream network is renumbered.

4.3.  Address Reachability

   Address reachability represents the area of the network (and the
   associated conditions), where an address can be expected to be used
   for receiving and transmitting packets.  As noted in Section 4.1, the
   address scope has a direct implication on address reachability, since
   address reachability is limited to a subset of the address scope.
   For example, only global-scope addresses can be globally reachable.

   However, besides the reachability semantics of each address type,
   network filtering policies may also affect address reachability.  For
   example, there is widespread deployment of Customer Edge Routers that
   implement a (stateful) filtering policy of "only allowing outgoing
   communications" -- mimicking the filtering policy enforced (as a



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   side-effect) by IPv4 NATs.  In such scenarios, even otherwise
   globally-reachable addresses become unreachable, unless:

   o  communication is initiated from the internal network, or,

   o  the CE Router is manually configured override the default
      filtering policy, or,

   o  a technology to dynamically override the filtering policy (such as
      UPnP [UPnP] or PCP [RFC6887]) is employed.

   Address reachability is what ultimately determines the application
   architecture that may be employed by an IPv6 node.

   NOTE:
      Ironically, an IPv6-only host (with global-scope addresses)
      attached to a home network where the CE Router "only allows
      outgoing communications" and does not implement a protocol such as
      UPnP [UPnP] or PCP [RFC6887], will normally have a harder time
      using peer-to-peer (P2P) applications than an IPv4-only host (with
      a private address) attached to a home network where the CE Router
      employs NAT but implements a protocol such as UPnP or PCP.

   Address reachability has a direct impact on security, since the
   ability to attack a system normally relies on the ability of the
   attacker to reach the system in the first place.  Firewalls
   [I-D.gont-opsawg-firewalls-analysis] are, indeed, devices that are
   specifically devoted to administer address reachability.

4.4.  Address Stability Considerations

   The stability of an address has two associated security/privacy
   implications:

   o  Ability of an attacker to correlate network activity

   o  Exposure to attack

   For obvious reasons, an address that is employed for multiple
   communication instances allows the aforementioned network activities
   to be correlated.  The longer an address is employed (i.e., the more
   stable it is), the longer such correlation will be possible.  In the
   worst-case scenario, a stable address that is employed for multiple
   communication instances over time will allow all such activities to
   be correlated.  On the other hand, if a host were to generate (and
   eventually "throw away") one new address for each communication
   instance (e.g., TCP connection), network activity correlation would
   be mitigated.



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   NOTE:
      The security and privacy implications of predictable addresses are
      discussed in [RFC7721] and [RFC7707].

   Typically, when it comes to attack exposure, the longer an address is
   employed the longer an attacker is exposed to attacks.  While such
   exposure is traditionally associated with the stability of the
   address, the usage type of the address may also have an impact on
   attack exposure (see Section 5.2).

   A popular approach to mitigate network activity correlation is the
   use of "temporary addresses" [RFC4941].  Temporary addresses are
   typically auto-configured and employed along with stable addresses,
   with the temporary addresses employed for outgoing communications,
   and the stable addresses employed for incoming communications.

   NOTE:
      Ongoing work [I-D.ietf-6man-rfc4941bis] aims at updating [RFC4941]
      such that temporary addresses can be employed without the need to
      configure stable addresses.

   We note that the extent to which temporary addresses provide improved
   mitigation of network activity correlation and/or reduced attack
   exposure may be questionable and/or limited in some scenarios.  For
   example, a temporary address that is reachable for, say, a few hours
   has a questionable "reduced exposure" (particularly when automated
   attack tools do not typically require such a long period of time to
   complete their task).  Similarly, if network activity can be
   correlated for the life of such address (e.g., on the order of
   several hours), such period of time might be long enough for the
   attacker to correlate all the network activity he is meaning to
   correlate.  However, they do introduce a limit to the attack window,
   and the amount of time during which address-based network activity
   correlation can be performed.

   In order to better mitigate network activity correlation and/or
   possibly reduce host exposure, an implementation might want to either
   reduce the preferred lifetime of a temporary address, or even better,
   generate one new temporary address for each new transport protocol
   instance.  However, the associated lifetime/stability of an address
   may have a negative impact on the network (please see Section 6.3.

   Additionally, enforcing a maximum lifetime on IPv6 addresses may
   cause long-lived TCP connections to fail.  For example, an address
   becoming "Invalid" (after transitioning through the "Preferred" and
   "Deprecated" states) would cause the TCP connections employing them
   to break, which would in turn cause e.g. long-lived SSH sessions to
   break/fail.



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   In some scenarios, attack exposure may be mitigated by limiting the
   usage of temporary addresses to outgoing connections, and prevent
   such addresses from being used for incoming connections (please see
   Section 5.2).

5.  IPv6 Address Usage

5.1.  Default Address Selection

   Applications use system API's to select the IPv6 addresses that will
   be used for incoming and outgoing connections.  These choices have
   consequences in terms of privacy, security, stability and
   performance.

   Default Address Selection for IPv6 is specified in [RFC6724].  The
   selection starts with a set of potential destination addresses, such
   as returned by getaddrinfo(), and the set of potential source
   addresses currently configured for the selected interfaces.  For each
   potential destination address, the algorithm will select the source
   address that provides the best route to the destination, while
   choosing the appropriate scope and preferring temporary addresses.
   The algorithm will then select the destination address, while giving
   a preference to reachable addresses with the smallest scope.  The
   selection may be affected by system settings.  We note that [RFC6724]
   only applies for outgoing connections, such as those made by clients
   trying to use services offered by other hosts.

   We note that [RFC6724] selects IPv6 addresses from all the currently
   available addresses on the host, and there is currently no way for an
   application to indicate expected or desirable properties for the IPv6
   source addresses employed for such outgoing communications.  For
   example, a privacy-sensitive application might want that each
   outgoing communication instance employs a new, single-use IPv6
   address, or to employ a new reusable address that is not employed or
   reusable by any other application on the host.  Reuse of an IPv6
   address by an application would allow the correlation of all network
   activities corresponding to such application as being performed by
   the same host, while reuse of an IPv6 address by multiple different
   applications would allow the correlation of all such network
   activities as being performed by the host with such IPv6 address.

   When devices provide a service, the common pattern is to just wait
   for connections over all addresses configured on the device.  For
   example, applications using the BSD Sockets API will commonly bind()
   the listening socket to the undefined address.  This long-established
   behavior is appropriate for devices providing public services, but
   can have unexpected results for devices providing semi-private




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   services, such as various forms of peer-to-peer or local-only
   applications.

   This behavior leads to three problems: device tracking, discussed in
   Section 6.1.2; unexpected address discovery, discussed in
   Section 6.1.3; and availability outside the expected scope, discussed
   in Section 6.1.4.  These problems are caused in part by the
   limitations of available address selection API, discussed in
   Section 7.1.

5.2.  Usage Type Considerations

   IPv6 hosts can both stable [RFC8064] and/or temporary [RFC4941]
   addresses, in which case stable addresses are typically employed for
   incoming (server-like) communications, while temporary addresses are
   employed for outgoing (client-like) communications.  That is, the
   stability properties of an address have an implicitly associated
   usage type.

   A node that employs one of its addresses to communicate with an
   external server (i.e., to perform an "outgoing connection") will
   expose that address to the other communicating system.  For example,
   once the external server receives an incoming connection, the
   corresponding server might launch an attack against the
   aforementioned address.  A real-world instance of this type of
   scenario has been documented in [Hein].

   However, we note that employing an IPv6 address for outgoing
   communications need not increase the exposure of local services to
   other parties.  For example, nodes could employ temporary addresses
   only for outgoing communications, and disallow their use for incoming
   communications.  Thus, external nodes that learn about a client's
   addresses could not really leverage such addresses for actively
   contacting clients.

   Section 5 discusses current IPv6 address usage, along with possible
   improvements.

5.3.  Current Alternatives for IPv6 Address Usage

5.3.1.  Incoming communications

   There are a number of ways in which a system or network may affect
   which addresses (and how) may be employed for different services and
   cases.  Namely,

   o  TCP/IP stack address filtering




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   o  Application-based address filtering

   o  Firewall-based address filtering

   Clearly, the most elegant approach for address selection would be for
   applications to be able to specify the properties of the addresses
   they are willing to employ by means of an API, such the TCP/IP stack
   itself could "filter" which addresses are allowed for the given
   service/application.  For example, an application could specify the
   stability and scope properties of the addresses on which incoming
   communications should be accepted, such that the application can be
   relieved from dealing with low-level networking details, portability
   is improved, and duplicate code in applications is avoided.  However,
   constraints in the current APIs (see Section 7.1) limit the ability
   of application programmers for leveraging this technique.
   Alternatively, services could be bound to specific (explicit)
   addresses, rather than to all locally-configured addresses.  However,
   there are a number of short-comings associated with this approach.
   Firstly, an application would need to be able to learn all of its
   addresses and associated properties, something that tends to be non-
   trivial and non-portable, and that also makes applications protocol-
   dependent, unnecessarily.  Secondly, the BSD Sockets API does not
   allow a socket to be bound to a subset of the node's addresses.  That
   is, sockets can be bound to a single address or to all available
   addresses (wildcard), but not to a subset of all the configured
   addresses.

   Another possible approach would be for applications to e.g. bind
   services to all available addresses, and perform the associated
   selection/filtering at the application level.  While possible, this
   would have a number of drawbacks.  Firstly, it would require
   applications to deal with low-level networking details, lead to
   duplicated code in all applications, and also negatively affect
   portability.  Secondly, performing address/selection filtering at the
   application level may not mitigate some possible attacks.  For
   example, port scanning will still be possible, since the
   aforementioned filtering would only be performed once e.g.  UDP
   packets are received or TCP connections are established.

   A client could simply run a host-based firewall that only allows
   incoming connections on the stable addresses.  This would be clearly
   more of an operational approach for achieving the desired
   functionality, and would require good firewall/host integration
   (e.g., the firewall should be able to tell stable vs. temporary
   addresses), would require the client to run additional firewall
   software for this specific purpose, etc.  In other scenarios, a
   network-based firewall could be configured to allow outgoing
   communications from all internal addresses, but only allow incoming



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   communications to stable addresses (i.e., allow such addresses either
   manually, or via a helper protocol such as [UPnP] or PCP [RFC6887]).
   For obvious reasons, this is generally only applicable to networks
   where incoming communications are allowed to a limited number of
   hosts/servers.

5.3.2.  Outgoing communications

   An application might be able to obtain the list of currently-
   configured addresses, and subsequently select an address with desired
   properties, and explicitly "bind" the address to the socket, to
   override the default source address selection.

   However, this approach is problematic for a number of reasons.
   Firstly, there is no portable way of obtaining the list of currently-
   configured addresses on the local node, and even less to check for
   address properties such "valid lifetime".  Secondly, as discussed in
   Section 5.3.1, it would require application programmers to understand
   all the subtleties associated with IPv6 addressing, and would also
   lead to duplicate code on all applications.  Finally, applications
   would be limited to use already-configured addresses and unable to
   trigger the generation of new addresses where desirable (e.g. the
   generation of a new single-use address for this application instance
   or communication instance).

6.  Current Issues

6.1.  Sub-optimal IPv6 Address Usage

6.1.1.  Correlation of Network Activity

   As discussed in [RFC7721], a node that reuses an IPv6 address for
   multiple communication instances will enable the correlation of such
   network activities.  This could be the case when the same IPv6
   address is employed by several instances of the same application
   (e.g., a browser in "privacy" mode and a browser in "normal" mode),
   or when the same IPv6 address is employed by two different
   applications on the same node (e.g., a browser in "privacy" mode, and
   an email client).

   Particularly for privacy-sensitive applications, an application or
   system might want to limit the usage of a given IPv6 address to a
   single communication instance, a single application, a single user on
   the system, etc.  However, given current APIs, this is practically
   impossible.






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6.1.2.  Passive Host Tracking

   The stable addresses recommended in [RFC8064] use stable IIDs defined
   in [RFC7217].  One key part of that algorithm is that if a device
   connects to a given network at different times, it will always
   configure the same IPv6 addresses on that network.  If the device
   hosts a service ready to accept connections on that stable address,
   adversaries can test the presence of the device on the network by
   attempting connections to that stable address.  Stable addresses used
   by listening services will thus enable testing whether a specific
   device is returning to a particular network, which in a number of
   cases might be considered a privacy issue.

6.1.3.  Unintended Service Disclosure

   Systems like DNS-Based Service Discovery [RFC6763] allow clients to
   discover services within a limited scope, that can be defined by a
   domain name.  These services are not advertised outside of that
   scope, and thus do not expect to be discovered by random parties on
   the Internet.  However, such services may be easily discoverable if
   they listen for connections to IPv6 addresses that a client process
   also uses as source address when connecting to remote servers.

   NOTE:
      An example of such service disclosure is described in [Hein].  A
      network manager observed scanning traffic directed at the
      temporary addresses of local devices.  The analysis in [Hein]
      shows that the scanners learned the addresses by observing the
      device contact an NTP service ([RFC5905]).  The remote scanning
      was possible because the local services were accepting connections
      on all configured addresses, including temporary addresses.

   It is obvious from this example that local services are disclosed
   because they are bond to the same IPv6 addresses that are also used
   by clients for outgoing communications with remote systems.  But the
   overlap between "client" and "server" addresses is only one part of
   the problem.  Suppose that a device hosts both a video game and a
   home automation application.  The video game users will be able to
   discover the IPv6 address of the game server.  If the home automation
   server listens to the same IPv6 addresses, it is now exposed to
   connection attempts by all these users.  That, too, increases the
   exposure of the home automation server.

   We note that a host or network that wants to limit access to local
   services should filter incoming connection attempts by affecting
   address reachability (see Section 4.3) via firewalls
   [I-D.gont-opsawg-firewalls-analysis] and/or the use of IPv6 addresses
   of appropriate scope (see Section 4.1).  However, it is also prudent



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   to avoid unintended service disclosure by suboptimal reuse of IPv6
   addresses, as discussed in this section.

6.1.4.  Availability Outside the Expected Scope

   The IPv6 addressing architecture [RFC4291] defines multiple address
   scopes, with devices often configured with globally reachable unicast
   addresses, link local addresses, and Unique Local IPv6 Unicast
   Addresses (ULA) [RFC4193].  Availability outside the expected scope
   happens when a service is expected to be available only in some local
   scope, but inadvertently becomes available from outside of that
   scope.  That could happen, for example, if a service is meant to be
   available only on a given link, but becomes reachable through ULA or
   through globally reachable addresses, or if a service is meant to be
   available only inside some organization's perimeter and becomes
   reachable through globally reachable addresses.  This will commonly
   happen if a service intended for some local scope is programmed to
   bind() to the "unspecified" addresses, which in practice means every
   address configured for the device (please see Section 7.1).

6.2.  Sub-optimal Address Configuration

6.2.1.  Number of Addresses

   Two mechanisms exist for automatic network configuration: SLAAC and
   DHCPv6.  DHCPv6 centralizes network configuration and address
   assignment, and may thus prevent hosts from leveraging the increased
   flexibility and availability of IPv6 addresses.  On the other hand,
   SLAAC may also result in network configuration anarchy, where hosts
   may e.g. configure and use addresses in a way that may render the
   network virtually unusable (please see Section 6.3.1).

   Most of the challenges associated with the use of multiple addresses
   can be addressed by allocating one /64 per host via mechanisms such
   as DHCPv6-PD.  However, support for such mechanisms in host
   implementations and e.g. the LAN-side of CE Routers is not
   widespread, and thus is currently unfeasible.  On the other hand,
   SLAAC lacks the means for the network to convey information about
   e.g., the number of addresses per host the network is able or willing
   to support.

   NOTE:
      Use of a /64 prefix per host could render techniques such as
      temporary addresses [RFC4941] ineffective, since hosts would
      become identified by corresponding /64 prefix.






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6.2.2.  SLAAC/DHCPv6 Interaction

   Many CE Routers offer address configuration via both SLAAC and
   DHCPv6, by including Prefix Information Options (PIOs) in Router
   Advertisement messages, and also setting the "M" such messages.  This
   has a number of implications:

   o  The outcome of the configuration process is non-deterministic,
      difficulting network troubleshooting (see
      [I-D.ietf-v6ops-dhcpv6-slaac-problem]).

   o  Nodes end up configuring more addresses than needed (or even
      used), normally configuring multiple stable addresses for each
      autoconfiguration prefix, with at least one address for each
      configuration mechanism (SLAAC and DHCPv6).

   o  A host can end up employing stable and predictable addresses
      resulting configured via DHCPv6, even when effort has been made to
      mitigate security and privacy issues associated with IPv6
      addresses for the SLAAC-configured addresses (i.e., [RFC7217] and
      [RFC4941]).

6.3.  Operation of Multi-Prefix/Multi-Address/Multi-Router Networks

6.3.1.  Implications of Addresses

   Network deployments are currently recommended to provide multiple
   IPv6 addresses to general-purpose hosts [RFC7934].  However, in some
   scenarios, use of a large number of IPv6 addresses may have negative
   implications on network devices that need to maintain entries for
   each IPv6 address in network data structures (e.g., [RFC7039]).
   Additionally, concurrent active use of multiple IPv6 addresses will
   normally increase neighbour discovery traffic if Neighbour Caches in
   network devices are not large enough to store all addresses on the
   link.  This can impact performance and energy efficiency on networks
   on which multicast is expensive (e.g.
   [I-D.ietf-mboned-ieee802-mcast-problems]).  Finally, network devices
   may interpret the use of a number of addresses above a certain
   threshold as a security event, and block the offending device from
   using the network.

6.3.2.  Legitimate Network Activity Correlation

   The desires of protecting individual privacy versus the desire to
   effectively maintain and debug a network can conflict with each
   other.  For example, 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



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   could become more difficult to determine whether one is seeing
   behavior caused by a single errant machine, or by a number of them.

6.3.3.  Routing in Multi-Prefix/Multi-Router Networks

   If the network is provided with multiple upstreams via different
   routers, each of the upstream will provide its PA address space (see
   Section 4.2) and local hosts will typically configure addresses for
   each of such prefixes.  In this scenarios, packets sourced from a
   given prefix should only employ the local router that announced that
   prefix, since otherwise the packets might be dropped as a result of
   ingress/egress filtering [RFC2827].  Unfortunately, the traditional
   Neighbor Discovery [RFC4861] can advertise routes only with a per-
   destination granularity, irrespective of the source address/prefix.

   [RFC8028] finally addressed the most important challenges associated
   with these scenarios.  However, [RFC8028] is not widely implemented.
   As a result, operating a multi-prefix/multi-router IPv6 network
   represents a major challenge -- if at all possible.

6.3.4.  Renumbering

   The challenges posed by network renumbering have been known for a
   very long time [RFC5887], with renumbering being analyzed with the
   assumption that the network topology remains stable and the network
   is renumbered.

   However, in scenarios where a host is moved to a different network
   without the host detecting the network disattach/re-attach event, or
   where the network a host attaches to is moved to a different point of
   the network topology, the aforementioned host will also perceive a
   renumbering event.  In an era in which moving virtual machines,
   containers, and networks around a network topology is commonplace,
   and where mobile systems changing network connectivity to and from
   e.g.  WiFi and 4G is also commonplace, renumbering events are
   anything but rare.

   One of the challenges represented by network renumbering is how hosts
   can infer that an existing network prefix and associated address(es)
   have become stale and should be removed and replaced by new prefixes
   and addresses.  In scenarios where the network topology does not
   change and the network is renumbered, network elements may be aware
   about the renumbering event and signal this condition to attached
   systems (i.e., signal that existing network configuration information
   should be removed and replaced).  However, in scenarios where it is
   the host, virtual machine or container (or the network they are
   attached to) that move around the network topology, the network will
   not be able to signal the "renumbering event", and the renumbered



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   host, virtual machine, or container might or might not be able to
   detect the event (e.g., via link-down/link-up events).

   Unfortunately, both SLAAC and DHCPv6 assume network configuration
   information to be somewhat stable.  SLAAC has traditionally employed
   long lifetimes for network configuration information, meaning that
   stale information could be employed for an unacceptably long period
   of time.  DCHPv6 has traditionally suffered from the same problem
   but, in addition, there is no widespread support for RECONFIGURE
   messages, so even if the network were in a position to signal a
   renumbering event, in practice hosts would normally have to rely on
   expiration of lease times for stale information to be cleared up.

   Some of these problems have been discussed in detail in
   [I-D.ietf-v6ops-slaac-renum], and there is ongoing work
   [I-D.ietf-6man-slaac-renum] [I-D.ietf-v6ops-cpe-slaac-renum] to
   mitigate their effects.

7.  Current Gaps that Prevent Leveraging IPv6 Addressing

7.1.  Better Address Selection APIs

   Application developers using the BSD Sockets API can "bind()" a
   listening socket to a specific address, and ensure that the
   application is only reachable through that address.  In theory,
   careful selection of the binding address could mitigate the problems
   described in Section 6.1.  Binding services to temporary addresses
   could mitigate the ability of an attacker from testing for the
   presence of the node in the network.  Binding different services to
   different addresses could mitigate unexpected discovery.  Binding
   services to non-global addresses (e.g. link-local addresses or ULAs)
   could mitigate availability outside the expected scope.  However,
   explicitly managing addresses adds significant complexity to the
   application development.  It requires that application developers
   master addressing architecture subtleties, and implement logic that
   reacts adequately to connectivity events and address changes.
   Experience shows that application developers would probably prefer
   some much simpler solution.

   In addition, we note that many application developers use high level
   APIs that listen to TLS, HTTP, or some other application protocol.
   These high level APIs seldomly provide detailed access to specific IP
   addresses, and typically default to listening to all available
   addresses.

   A more advanced API could allow application programmers to select
   desired properties in an address (scope, stability, etc.), such that
   the best-suitable addresses are selected, while relieving the



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   application from low-level IPv6 addressing details.  Such API could
   also trigger the generation of new IPv6 addresses if/when the
   specified properties required so.

7.2.  Universal Support of Multi-prefix/Multi-router Networks

   To put it bluntly, multi-prefix/multi-router networks cannot possibly
   work properly without implementation of [RFC8028].  Unfortunately,
   [RFC8028] is not widely implemented.  On the protocol standardization
   side, the IETF should consider elevating the requirement to support
   RFC8028 in the IPv6 Node Requirements RFC [RFC8504] from "SHOULD" to
   "MUST".

7.3.  Profile-based IPv6 Address Configuration

   Most operating systems configure the same type of addresses
   regardless of the current "operating mode" or "profile" of the device
   (e.g., device connected to an enterprise network vs roaming across
   untrusted networks).  For example, many operating systems configure
   both stable [RFC8064] and temporary [RFC4941] addresses for all
   network interfaces.  However, this "one size fits all" approach tends
   to be sub-optimal or inappropriate for some scenarios.  For example,
   enterprise networks typically prefer usage of only stable addresses,
   thus meaning that a network administrator needs to find the means for
   disabling the generation of temporary addresses on all those systems
   that would otherwise generate them.  On the other hand, some mobile
   devices normally configure both stable and temporary addresses, even
   when their usage type (client-like operation) would allow for the
   more privacy-sensible option of configuring only temporary addresses.

   The lack of better-tuned address configuration policies has helped
   establish the "one size fits all" approach that, as noted, usually
   leads to suboptimal results.  Advice in this area might help achieve
   more optional address configuration policies such that IPv6
   addressing capabilities are fully leveraged.

   NOTE:
      One might envision a document that provides advice regarding the
      address generation for different typical scenarios (e.g., when to
      configure stable-only, temporary-only, or stable+temporary).  In
      the most simple analysis, one might expect nodes in a typical
      enterprise network to employ only stable addresses.  General-
      purpose nodes in a home or "trusted" network might want to employ
      both stable and temporary addresses.  Finally, mobile nodes (e.g.
      when roaming across non-trusted networks) might want to employ
      only temporary addresses).





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7.4.  Protocol Improvements to Deal with Many Addresses

   Possible improvements to IPv6 SLAAC should be evaluated, including:

   o  Enabling IPv6 routers to convey information about network
      constraints such as maximum number of addressees per node

   o  Enabling hosts to register/de-register configured addresses, such
      that e.g. routers need not tie resources to addresses that are no
      longer used

   If a /64 prefix is to be assigned for each host in order to leverage
   IPv6 address availability while mitigating the possible effects on
   network elements of employing large numbers of addresses, widespread
   support for DHCPv6-PD (or some proposed alternative mechanism) should
   be considered.

7.5.  Support for Firewall Traversal in CE Routers

   Customer Edge Routers that implement a default filtering policy of
   "only allowing outgoing communications" need to support helper
   protocols such as [UPnP] or PCP [RFC6887], so that applications can
   punch holes in the CE Router firewall for applications that need to
   receive incoming communications.  Otherwise, P2P applications that
   currently work in IPv4 will not function properly in IPv6-only
   networks.

   Support for these protocols is particularly important for IPv6
   deployments since, as hosts will normally employ "provider
   aggregatable" addresses (see Section 4.2), renumbering events will
   result in host address changes, and thus static firewall rules will
   become invalid/outdated.  Additionally, use of temporary addresses
   [RFC4941] will also lead to changing IPv6 addresses, which will
   require that the associated firewall rules be updated.

7.6.  Advice on IPv6 Address Usage

   An application programmer, left with the question of which are the
   most appropriate addresses for a given usage type and application,
   typically resorts to the Default IPv6 Address Selection for IPv6 (see
   Section 5.1) for outgoing communications, and to accepting incoming
   communications on all available addresses for incoming
   communications.  As discussed throughout this document, this leads to
   sub-optimal results.  Besides, all applications on a node share the
   same pool of configured addresses, and applications are also
   prevented from triggering the generation of new addresses (e.g. to be
   employed for a particular application or communication instance).




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   Guidance in this area is warranted such that applications and systems
   fully-leverage IPv6 addressing.

   NOTE:
      Such guidance would elaborate, among other things, on the usage of
      IPv6 addresses when offering network services and when performing
      client-like communications.  For example, for incoming
      communications, hosts might want to employ only the smallest-scope
      applicable addresses (if available) and, if stable addresses are
      available, they might want to accept incoming connections only on
      such addresses (but *not* on temporary addresses).  For client-
      like communications, hosts might prefer temporary addresses,
      unless the corresponding communication instances are expected to
      be long-lived (e.g., SSH sessions).

8.  IANA Considerations

   There are no IANA registries within this document.  The RFC-Editor
   can remove this section before publication of this document as an
   RFC.

9.  Security Considerations

   The security and privacy implications associated with the
   predictability and lifetime of IPv6 addresses has been analyzed in
   [RFC7217] [RFC7721], and [RFC7707].  This document complements and
   extends the aforementioned analysis by considering other IPv6
   properties such as the address scope and address reachability, and
   the associated trade-offs.

10.  Acknowledgements

   The authors would like to thank (in alphabetical order) Mikael
   Abrahamsson, Fred Baker, Owen DeLong, Francis Dupont, Tatuya Jinmei,
   and Dave Thaler for providing valuable comments on earlier versions
   of this document.

11.  References

11.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.






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   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <https://www.rfc-editor.org/info/rfc2827>.

   [RFC4007]  Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
              B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
              DOI 10.17487/RFC4007, March 2005,
              <https://www.rfc-editor.org/info/rfc4007>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://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,
              <https://www.rfc-editor.org/info/rfc4941>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <https://www.rfc-editor.org/info/rfc5905>.

   [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,
              <https://www.rfc-editor.org/info/rfc6724>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.





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   [RFC6887]  Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
              P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
              DOI 10.17487/RFC6887, April 2013,
              <https://www.rfc-editor.org/info/rfc6887>.

   [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,
              <https://www.rfc-editor.org/info/rfc7217>.

   [RFC7934]  Colitti, L., Cerf, V., Cheshire, S., and D. Schinazi,
              "Host Address Availability Recommendations", BCP 204,
              RFC 7934, DOI 10.17487/RFC7934, July 2016,
              <https://www.rfc-editor.org/info/rfc7934>.

   [RFC8028]  Baker, F. and B. Carpenter, "First-Hop Router Selection by
              Hosts in a Multi-Prefix Network", RFC 8028,
              DOI 10.17487/RFC8028, November 2016,
              <https://www.rfc-editor.org/info/rfc8028>.

   [RFC8064]  Gont, F., Cooper, A., Thaler, D., and W. Liu,
              "Recommendation on Stable IPv6 Interface Identifiers",
              RFC 8064, DOI 10.17487/RFC8064, February 2017,
              <https://www.rfc-editor.org/info/rfc8064>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,
              <https://www.rfc-editor.org/info/rfc8415>.

   [RFC8504]  Chown, T., Loughney, J., and T. Winters, "IPv6 Node
              Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504,
              January 2019, <https://www.rfc-editor.org/info/rfc8504>.

11.2.  Informative References

   [Barnes2012]
              Barnes, R., Altmann, R., and D. Kerr, "Mapping the Great
              Void Smarter scanning for IPv6",  ISMA 2012 AIMS-4 -
              Workshop on Active Internet Measurements, February 2012,
              <https://www.caida.org/workshops/isma/1202/slides/
              aims1202_rbarnes.pdf>.



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   [Hein]     Hein, B., "The Rising Sophistication of Network
              Scanning",  January 2016,
              <http://netpatterns.blogspot.be/2016/01/the-rising-
              sophistication-of-network.html>.

   [I-D.gont-opsawg-firewalls-analysis]
              Gont, F. and F. Baker, "On Firewalls in Network Security",
              draft-gont-opsawg-firewalls-analysis-02 (work in
              progress), February 2016.

   [I-D.ietf-6man-rfc4941bis]
              Gont, F., Krishnan, S., Narten, T., and R. Draves,
              "Temporary Address Extensions for Stateless Address
              Autoconfiguration in IPv6", draft-ietf-6man-rfc4941bis-12
              (work in progress), November 2020.

   [I-D.ietf-6man-slaac-renum]
              Gont, F., Zorz, J., and R. Patterson, "Improving the
              Robustness of Stateless Address Autoconfiguration (SLAAC)
              to Flash Renumbering Events", draft-ietf-6man-slaac-
              renum-01 (work in progress), August 2020.

   [I-D.ietf-mboned-ieee802-mcast-problems]
              Perkins, C., McBride, M., Stanley, D., Kumari, W., and J.
              Zuniga, "Multicast Considerations over IEEE 802 Wireless
              Media", draft-ietf-mboned-ieee802-mcast-problems-12 (work
              in progress), October 2020.

   [I-D.ietf-v6ops-cpe-slaac-renum]
              Gont, F., Zorz, J., Patterson, R., and B. Volz, "Improving
              the Reaction of Customer Edge Routers to Renumbering
              Events", draft-ietf-v6ops-cpe-slaac-renum-05 (work in
              progress), September 2020.

   [I-D.ietf-v6ops-dhcpv6-slaac-problem]
              Liu, B., Jiang, S., Gong, X., Wang, W., and E. Rey,
              "DHCPv6/SLAAC Interaction Problems on Address and DNS
              Configuration", draft-ietf-v6ops-dhcpv6-slaac-problem-07
              (work in progress), August 2016.

   [I-D.ietf-v6ops-slaac-renum]
              Gont, F., Zorz, J., and R. Patterson, "Reaction of
              Stateless Address Autoconfiguration (SLAAC) to Flash-
              Renumbering Events", draft-ietf-v6ops-slaac-renum-05 (work
              in progress), November 2020.






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   [I-D.ietf-v6ops-ula-usage-considerations]
              Liu, B. and S. Jiang, "Considerations For Using Unique
              Local Addresses", draft-ietf-v6ops-ula-usage-
              considerations-02 (work in progress), March 2017.

   [RFC5887]  Carpenter, B., Atkinson, R., and H. Flinck, "Renumbering
              Still Needs Work", RFC 5887, DOI 10.17487/RFC5887, May
              2010, <https://www.rfc-editor.org/info/rfc5887>.

   [RFC6296]  Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
              Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011,
              <https://www.rfc-editor.org/info/rfc6296>.

   [RFC7039]  Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed.,
              "Source Address Validation Improvement (SAVI) Framework",
              RFC 7039, DOI 10.17487/RFC7039, October 2013,
              <https://www.rfc-editor.org/info/rfc7039>.

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

   [RFC7721]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              RFC 7721, DOI 10.17487/RFC7721, March 2016,
              <https://www.rfc-editor.org/info/rfc7721>.

   [RFC8190]  Bonica, R., Cotton, M., Haberman, B., and L. Vegoda,
              "Updates to the Special-Purpose IP Address Registries",
              BCP 153, RFC 8190, DOI 10.17487/RFC8190, June 2017,
              <https://www.rfc-editor.org/info/rfc8190>.

   [UPnP]     UPnP, "UPnP Device Architecture 2.0",  April 17, 2020,
              <https://openconnectivity.org/upnp-specs/UPnP-arch-
              DeviceArchitecture-v2.0-20200417.pdf>.

Authors' Addresses

   Fernando Gont
   SI6 Networks
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Email: fgont@si6networks.com
   URI:   https://www.si6networks.com





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   Guillermo Gont
   SI6 Networks
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Email: ggont@si6networks.com
   URI:   https://www.si6networks.com











































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