path: root/doc/rfc/rfc7368.txt

Internet Engineering Task Force (IETF)                     T. Chown, Ed.
Request for Comments: 7368                     University of Southampton
Category: Informational                                         J. Arkko
ISSN: 2070-1721                                                 Ericsson
                                                               A. Brandt
                                                           Sigma Designs
                                                                O. Troan
                                                     Cisco Systems, Inc.
                                                                 J. Weil
                                                       Time Warner Cable
                                                            October 2014


              IPv6 Home Networking Architecture Principles

Abstract

   This text describes evolving networking technology within residential
   home networks with increasing numbers of devices and a trend towards
   increased internal routing.  The goal of this document is to define a
   general architecture for IPv6-based home networking, describing the
   associated principles, considerations, and requirements.  The text
   briefly highlights specific implications of the introduction of IPv6
   for home networking, discusses the elements of the architecture, and
   suggests how standard IPv6 mechanisms and addressing can be employed
   in home networking.  The architecture describes the need for specific
   protocol extensions for certain additional functionality.  It is
   assumed that the IPv6 home network is not actively managed and runs
   as an IPv6-only or dual-stack network.  There are no recommendations
   in this text for the IPv4 part of the network.

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/rfc7368.





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Copyright Notice

   Copyright (c) 2014 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  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology and Abbreviations . . . . . . . . . . . . . .   5
   2.  Effects of IPv6 on Home Networking  . . . . . . . . . . . . .   6
     2.1.  Multiple Subnets and Routers  . . . . . . . . . . . . . .   7
     2.2.  Global Addressability and Elimination of NAT  . . . . . .   8
     2.3.  Multi-Addressing of Devices . . . . . . . . . . . . . . .   8
     2.4.  Unique Local Addresses (ULAs) . . . . . . . . . . . . . .   9
     2.5.  Avoiding Manual Configuration of IP Addresses . . . . . .  10
     2.6.  IPv6-Only Operation . . . . . . . . . . . . . . . . . . .  11
   3.  Homenet Architecture Principles . . . . . . . . . . . . . . .  11
     3.1.  General Principles  . . . . . . . . . . . . . . . . . . .  12
       3.1.1.  Reuse Existing Protocols  . . . . . . . . . . . . . .  12
       3.1.2.  Minimise Changes to Hosts and Routers . . . . . . . .  13
     3.2.  Homenet Topology  . . . . . . . . . . . . . . . . . . . .  13
       3.2.1.  Supporting Arbitrary Topologies . . . . . . . . . . .  13
       3.2.2.  Network Topology Models . . . . . . . . . . . . . . .  14
       3.2.3.  Dual-Stack Topologies . . . . . . . . . . . . . . . .  18
       3.2.4.  Multihoming . . . . . . . . . . . . . . . . . . . . .  19
       3.2.5.  Mobility Support  . . . . . . . . . . . . . . . . . .  20
     3.3.  A Self-Organising Network . . . . . . . . . . . . . . . .  21
       3.3.1.  Differentiating Neighbouring Homenets . . . . . . . .  21
       3.3.2.  Largest Practical Subnets . . . . . . . . . . . . . .  21
       3.3.3.  Handling Varying Link Technologies  . . . . . . . . .  22
       3.3.4.  Homenet Realms and Borders  . . . . . . . . . . . . .  22
       3.3.5.  Configuration Information from the ISP  . . . . . . .  23
     3.4.  Homenet Addressing  . . . . . . . . . . . . . . . . . . .  24
       3.4.1.  Use of ISP-Delegated IPv6 Prefixes  . . . . . . . . .  24
       3.4.2.  Stable Internal IP Addresses  . . . . . . . . . . . .  26
       3.4.3.  Internal Prefix Delegation  . . . . . . . . . . . . .  27
       3.4.4.  Coordination of Configuration Information . . . . . .  28
       3.4.5.  Privacy . . . . . . . . . . . . . . . . . . . . . . .  28



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     3.5.  Routing Functionality . . . . . . . . . . . . . . . . . .  28
       3.5.1.  Unicast Routing within the Homenet  . . . . . . . . .  30
       3.5.2.  Unicast Routing at the Homenet Border . . . . . . . .  31
       3.5.3.  Multicast Support . . . . . . . . . . . . . . . . . .  31
     3.6.  Security  . . . . . . . . . . . . . . . . . . . . . . . .  32
       3.6.1.  Addressability vs. Reachability . . . . . . . . . . .  32
       3.6.2.  Filtering at Borders  . . . . . . . . . . . . . . . .  33
       3.6.3.  Partial Effectiveness of NAT and Firewalls  . . . . .  34
       3.6.4.  Exfiltration Concerns . . . . . . . . . . . . . . . .  34
       3.6.5.  Device Capabilities . . . . . . . . . . . . . . . . .  34
       3.6.6.  ULAs as a Hint of Connection Origin . . . . . . . . .  35
     3.7.  Naming and Service Discovery  . . . . . . . . . . . . . .  35
       3.7.1.  Discovering Services  . . . . . . . . . . . . . . . .  35
       3.7.2.  Assigning Names to Devices  . . . . . . . . . . . . .  36
       3.7.3.  The Homenet Name Service  . . . . . . . . . . . . . .  37
       3.7.4.  Name Spaces . . . . . . . . . . . . . . . . . . . . .  38
       3.7.5.  Independent Operation . . . . . . . . . . . . . . . .  40
       3.7.6.  Considerations for LLNs . . . . . . . . . . . . . . .  40
       3.7.7.  DNS Resolver Discovery  . . . . . . . . . . . . . . .  41
       3.7.8.  Devices Roaming to/from the Homenet . . . . . . . . .  41
     3.8.  Other Considerations  . . . . . . . . . . . . . . . . . .  41
       3.8.1.  Quality of Service  . . . . . . . . . . . . . . . . .  41
       3.8.2.  Operations and Management . . . . . . . . . . . . . .  42
     3.9.  Implementing the Architecture on IPv6 . . . . . . . . . .  43
   4.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  44
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  44
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  44
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  44
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  44
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  48
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  49




















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1.  Introduction

   This document focuses on evolving networking technology within
   residential home networks with increasing numbers of devices and a
   trend towards increased internal routing, as well as the associated
   challenges with their deployment and operation.  There is a growing
   trend in home networking for the proliferation of networking
   technology through an increasingly broad range of devices and media.
   This evolution in scale and diversity sets requirements on IETF
   protocols.  Some of these requirements relate to the introduction of
   IPv6, while others relate to the introduction of specialised networks
   for home automation and sensors.

   While at the time of writing some complex home network topologies
   exist, most are relatively simple single subnet networks and
   ostensibly operate using just IPv4.  While there may be IPv6 traffic
   within the network, e.g., for service discovery, the homenet is
   provisioned by the ISP as an IPv4 network.  Such networks also
   typically employ solutions that should be avoided, such as private
   [RFC1918] addressing with (cascaded) Network Address Translation
   (NAT) [RFC3022], or they may require expert assistance to set up.

   In contrast, emerging IPv6-capable home networks are very likely to
   have multiple internal subnets, e.g., to facilitate private and guest
   networks, heterogeneous link layers, and smart grid components, and
   have enough address space available to allow every device to have a
   globally unique address.  This implies that internal routing
   functionality is required, and that the homenet's ISP delegates a
   large enough address block, to allow assignment of a prefix to each
   subnet in the home network.

   It is not practical to expect home users to configure their networks.
   Thus, the assumption of this document is that the homenet is as far
   as possible self-organising and self-configuring, i.e., it should
   function without proactive management by the residential user.

   The architectural constructs in this document are focused on the
   problems to be solved when introducing IPv6, with an eye towards a
   better result than what we have today with IPv4, as well as aiming
   for a more consistent solution that addresses as many of the
   identified requirements as possible.  This document aims to provide
   the basis and guiding principles for how standard IPv6 mechanisms and
   addressing [RFC2460] [RFC4291] can be employed in home networking,
   while coexisting with existing IPv4 mechanisms.  In emerging dual-
   stack home networks, it is vital that introducing IPv6 does not
   adversely affect IPv4 operation.  We assume that the IPv4 network
   architecture in home networks is what it is and cannot be modified by
   new recommendations.  This document does not discuss how IPv4 home



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   networks provision or deliver support for multiple subnets.  It
   should not be assumed that any future new functionality created with
   IPv6 in mind will be backward compatible to include IPv4 support.
   Further, future deployments, or specific subnets within an otherwise
   dual-stack home network, may be IPv6-only, in which case
   considerations for IPv4 impact would not apply.

   This document proposes a baseline homenet architecture, using
   protocols and implementations that are as far as possible proven and
   robust.  The scope of the document is primarily the network-layer
   technologies that provide the basic functionality to enable
   addressing, connectivity, routing, naming, and service discovery.
   While it may, for example, state that homenet components must be
   simple to deploy and use, it does not discuss specific user
   interfaces, nor does it discuss specific physical, wireless, or data-
   link-layer considerations.  Likewise, we also do not specify the
   whole design of a homenet router from top to bottom; rather, we focus
   on the Layer 3 aspects.  This means that Layer 2 is largely out of
   scope, we're assuming a data-link layer that supports IPv6 is
   present, and we react accordingly.  Any IPv6-over-Foo definitions
   occur elsewhere.

   [RFC7084], which has obsoleted [RFC6204], defines basic requirements
   for Customer Edge (CE) routers.  The update includes the definition
   of requirements for specific transition tools on the CE router,
   specifically Dual-Stack Lite (DS-Lite) [RFC6333] and IPv6 Rapid
   Deployment on IPv4 Infrastructures (6rd) [RFC5969].  Such detailed
   specification of CE router devices is considered out of scope of this
   architecture document, and we assume that any required update of the
   CE router device specification as a result of adopting this
   architecture will be handled as separate and specific updates to
   these existing documents.  Further, the scope of this text is the
   internal homenet, and thus specific features on the WAN side of the
   CE router are out of scope for this text.

1.1.  Terminology and Abbreviations

   In this section, we define terminology and abbreviations used
   throughout the text.

   o  Border: A point, typically resident on a router, between two
      networks, e.g., between the main internal homenet and a guest
      network.  This defines a point(s) at which filtering and
      forwarding policies for different types of traffic may be applied.

   o  CE router: Customer Edge router.  A border router intended for use
      in a homenet.  A CE router connects the homenet to a service
      provider network.



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   o  FQDN: Fully Qualified Domain Name.  A globally unique name.

   o  Guest network: A part of the home network intended for use by
      visitors or guests to the home(net).  Devices on the guest network
      may typically not see or be able to use all services in the
      home(net).

   o  Homenet: A home network, comprising host and router equipment,
      with one or more CE routers providing connectivity to a service
      provider network(s).

   o  ISP: Internet Service Provider.  An entity that provides access to
      the Internet.  In this document, a service provider specifically
      offers Internet access using IPv6 and may also offer IPv4 Internet
      access.  The service provider can provide such access over a
      variety of different transport methods such as DSL, cable,
      wireless, and others.

   o  LLN: Low-power and Lossy Network.

   o  LQDN: Locally Qualified Domain Name.  A name local to the homenet.

   o  NAT: Network Address Translation.  Typically referring to IPv4
      Network Address Port Translation (NAPT) [RFC3022].

   o  NPTv6: IPv6-to-IPv6 Network Prefix Translation [RFC6296].

   o  PCP: Port Control Protocol [RFC6887].

   o  Realm: A network delimited by a defined border.  A guest network
      within a homenet may form one realm.

   o  'Simple Security': Defined in [RFC4864] and expanded further in
      [RFC6092]; describes recommended perimeter security capabilities
      for IPv6 networks.

   o  ULA: IPv6 Unique Local Address [RFC4193].

   o  VM: Virtual Machine.

2.  Effects of IPv6 on Home Networking

   While IPv6 resembles IPv4 in many ways, there are some notable
   differences in the way it may typically be deployed.  It changes
   address allocation principles, making multi-addressing the norm, and
   through the vastly increased address space, it allows globally unique
   IP addresses to be used for all devices in a home network.  This
   section presents an overview of some of the key implications of the



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   introduction of IPv6 for home networking that are simultaneously both
   promising and problematic.

2.1.  Multiple Subnets and Routers

   While simple Layer 3 topologies involving as few subnets as possible
   are preferred in home networks, the incorporation of dedicated
   (routed) subnets remains necessary for a variety of reasons.  For
   instance, an increasingly common feature in modern home routers is
   the ability to support both guest and private network subnets.
   Likewise, there may be a need to separate home automation or
   corporate extension LANs (whereby a home worker can have their
   corporate network extended into the home using a virtual private
   network, commonly presented as one port on an Ethernet device) from
   the main Internet access network, or different subnets may in general
   be associated with parts of the homenet that have different routing
   and security policies.  Further, link-layer networking technology is
   poised to become more heterogeneous as networks begin to employ both
   traditional Ethernet technology and link layers designed for Low-
   power and Lossy Networks (LLNs), such as those used for certain types
   of sensor devices.  Constraining the flow of certain traffic from
   Ethernet links to links of much lower capacity thus becomes an
   important topic.

   The introduction of IPv6 for home networking makes it possible for
   every home network to be delegated enough address space from its ISP
   to provision globally unique prefixes for each such subnet in the
   home.  While the number of addresses in a standard /64 IPv6 prefix is
   practically unlimited, the number of prefixes available for
   assignment to the home network is not.  As a result, the growth
   inhibitor for the home network shifts from the number of addresses to
   the number of prefixes offered by the provider; this topic is
   discussed in BCP 157 [RFC6177], which recommends that "end sites
   always be able to obtain a reasonable amount of address space for
   their actual and planned usage."

   The addition of routing between subnets raises a number of issues.
   One is a method by which prefixes can be efficiently allocated to
   each subnet, without user intervention.  Another issue is how to
   extend mechanisms such as zero-configuration service discovery that
   currently only operate within a single subnet using link-local
   traffic.  In a typical IPv4 home network, there is only one subnet,
   so such mechanisms would normally operate as expected.  For multi-
   subnet IPv6 home networks, there are two broad choices to enable such
   protocols to work across the scope of the entire homenet: extend
   existing protocols to work across that scope or introduce proxies for
   existing link-layer protocols.  This topic is discussed in
   Section 3.7.



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2.2.  Global Addressability and Elimination of NAT

   The possibility for direct end-to-end communication on the Internet
   to be restored by the introduction of IPv6 is, on the one hand, an
   incredible opportunity for innovation and simpler network operation,
   but on the other hand, it is also a concern as it potentially exposes
   nodes in the internal networks to receipt of unwanted and possibly
   malicious traffic from the Internet.

   With devices and applications able to talk directly to each other
   when they have globally unique addresses, there may be an expectation
   of improved host security to compensate for this.  It should be noted
   that many devices may (for example) ship with default settings that
   make them readily vulnerable to compromise by external attackers if
   globally accessible, or they may simply not be robust by design
   because it was assumed that either such devices would only be used on
   private networks or the devices don't have the computing power to
   apply the necessary security methods.  In addition, the upgrade cycle
   for devices (or their firmware) may be slow and/or lack auto-update
   mechanisms.

   It is thus important to distinguish between addressability and
   reachability.  While IPv6 offers global addressability through the
   use of globally unique addresses in the home, whether devices are
   globally reachable or not would depend on any firewall or filtering
   configuration, and not, as is commonly the case with IPv4, the
   presence or use of NAT.  In this respect, IPv6 networks may or may
   not have filters applied at their borders to control such traffic,
   i.e., at the homenet CE router.  [RFC4864] and [RFC6092] discuss such
   filtering and the merits of 'default allow' against 'default deny'
   policies for external traffic initiated into a homenet.  This topic
   is discussed further in Section 3.6.1.

2.3.  Multi-Addressing of Devices

   In an IPv6 network, devices will often acquire multiple addresses,
   typically at least a link-local address and one or more globally
   unique addresses (GUAs).  Where a homenet is multihomed, a device
   would typically receive a GUA from within the delegated prefix from
   each upstream ISP.  Devices may also have an IPv4 address if the
   network is dual stack, an IPv6 Unique Local Address (ULA) [RFC4193]
   (see below), and one or more IPv6 privacy addresses [RFC4941].

   It should thus be considered the norm for devices on IPv6 home
   networks to be multi-addressed and to need to make appropriate
   address selection decisions for the candidate source and destination
   address pairs for any given connection.  In multihoming scenarios,
   nodes will be configured with one address from each upstream ISP



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   prefix.  In such cases, the presence of upstream ingress filtering as
   described in BCP 38 [RFC2827] requires such multi-addressed nodes to
   select the correct source address to be used for the corresponding
   uplink.  Default address selection for IPv6 [RFC6724] provides a
   solution for this, but a challenge here is that the node may not have
   the information it needs to make that decision based on addresses
   alone.  We discuss this challenge in Section 3.2.4.

2.4.  Unique Local Addresses (ULAs)

   [RFC4193] defines ULAs for IPv6 that may be used to address devices
   within the scope of a single site.  Support for ULAs for IPv6 CE
   routers is described in [RFC7084].  A home network running IPv6
   should deploy ULAs alongside its globally unique prefix(es) to allow
   stable communication between devices (on different subnets) within
   the homenet where that externally allocated globally unique prefix
   may change over time, e.g., due to renumbering within the
   subscriber's ISP, or where external connectivity may be temporarily
   unavailable.  A homenet using provider-assigned global addresses is
   exposed to its ISP renumbering the network to a much larger degree
   than before whereas, for IPv4, NAT isolated the user against ISP
   renumbering to some extent.

   While setting up a network, there may be a period where it has no
   external connectivity, in which case ULAs would be required for
   inter-subnet communication.  In the case where home automation
   networks are being set up in a new home/deployment (as early as
   during construction of the home), such networks will likely need to
   use their own /48 ULA prefix.  Depending upon circumstances beyond
   the control of the owner of the homenet, it may be impossible to
   renumber the ULA used by the home automation network so routing
   between ULA /48s may be required.  Also, some devices, particularly
   constrained devices, may have only a ULA (in addition to a link-
   local), while others may have both a GUA and a ULA.

   Note that unlike private IPv4 space as described in RFC 1918, the use
   of ULAs does not imply use of an IPv6 equivalent of a traditional
   IPv4 NAT [RFC3022] or of NPTv6 prefix-based NAT [RFC6296].  When an
   IPv6 node in a homenet has both a ULA and a globally unique IPv6
   address, it should only use its ULA address internally and use its
   additional globally unique IPv6 address as a source address for
   external communications.  This should be the natural behaviour given
   support for default address selection for IPv6 [RFC6724].  By using
   such globally unique addresses between hosts and devices in remote
   networks, the architectural cost and complexity, particularly to
   applications, of NAT or NPTv6 translation are avoided.  As such,
   neither IPv6 NAT nor NPTv6 is recommended for use in the homenet
   architecture.  Further, the homenet border router(s) should filter



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   packets with ULA source/destination addresses as discussed in
   Section 3.4.2.

   Devices in a homenet may be given only a ULA as a means to restrict
   reachability from outside the homenet.  ULAs can be used by default
   for devices that, without additional configuration (e.g., via a web
   interface), would only offer services to the internal network.  For
   example, a printer might only accept incoming connections on a ULA
   until configured to be globally reachable, at which point it acquires
   a global IPv6 address and may be advertised via a global name space.

   Where both a ULA and a global prefix are in use, the ULA source
   address is used to communicate with ULA destination addresses when
   appropriate, i.e., when the ULA source and destination lie within the
   /48 ULA prefix(es) known to be used within the same homenet.  In
   cases where multiple /48 ULA prefixes are in use within a single
   homenet (perhaps because multiple homenet routers each independently
   auto-generate a /48 ULA prefix and then share prefix/routing
   information), utilising a ULA source address and a ULA destination
   address from two disjoint internal ULA prefixes is preferable to
   using GUAs.

   While a homenet should operate correctly with two or more /48 ULAs
   enabled, a mechanism for the creation and use of a single /48 ULA
   prefix is desirable for addressing consistency and policy
   enforcement.

   A counter argument to using ULAs is that it is undesirable to
   aggressively deprecate global prefixes for temporary loss of
   connectivity, so for a host to lose its global address, there would
   have to be a connection breakage longer than the lease period, and
   even then, deprecating prefixes when there is no connectivity may not
   be advisable.  However, it is assumed in this architecture that
   homenets should support and use ULAs.

2.5.  Avoiding Manual Configuration of IP Addresses

   Some IPv4 home networking devices expose IPv4 addresses to users,
   e.g., the IPv4 address of a home IPv4 CE router that may be
   configured via a web interface.  In potentially complex future IPv6
   homenets, users should not be expected to enter IPv6 literal
   addresses in devices or applications, given their much greater length
   and the apparent randomness of such addresses to a typical home user.
   Thus, even for the simplest of functions, simple naming and the
   associated (minimal, and ideally zero configuration) discovery of
   services are imperative for the easy deployment and use of homenet
   devices and applications.




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2.6.  IPv6-Only Operation

   It is likely that IPv6-only networking will be deployed first in new
   home network deployments, often referred to as 'greenfield'
   scenarios, where there is no existing IPv4 capability, or perhaps as
   one element of an otherwise dual-stack network.  Running IPv6-only
   adds additional requirements, e.g., for devices to get configuration
   information via IPv6 transport (not relying on an IPv4 protocol such
   as IPv4 DHCP) and for devices to be able to initiate communications
   to external devices that are IPv4-only.

   Some specific transition technologies that may be deployed by the
   homenet's ISP are discussed in [RFC7084].  In addition, certain other
   functions may be desirable on the CE router, e.g., to access content
   in the IPv4 Internet, NAT64 [RFC6144] and DNS64 [RFC6145] may be
   applicable.

   The widespread availability of robust solutions to these types of
   requirements will help accelerate the uptake of IPv6-only homenets.
   The specifics of these are, however, beyond the scope of this
   document, especially those functions that reside on the CE router.

3.  Homenet Architecture Principles

   The aim of this text is to outline how to construct advanced IPv6-
   based home networks involving multiple routers and subnets using
   standard IPv6 addressing and protocols [RFC2460] [RFC4291] as the
   basis.  As described in Section 3.1, solutions should as far as
   possible reuse existing protocols and minimise changes to hosts and
   routers, but some new protocols or extensions are likely to be
   required.  In this section, we present the elements of the proposed
   home networking architecture with discussion of the associated design
   principles.

   In general, home network equipment needs to be able to operate in
   networks with a range of different properties and topologies, where
   home users may plug components together in arbitrary ways and expect
   the resulting network to operate.  Significant manual configuration
   is rarely, if at all, possible or even desirable given the knowledge
   level of typical home users.  Thus, the network should, as far as
   possible, be self-configuring, though configuration by advanced users
   should not be precluded.









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   The homenet needs to be able to handle or provision at least the
   following:

   o  Routing

   o  Prefix configuration for routers

   o  Name resolution

   o  Service discovery

   o  Network security

   The remainder of this document describes the principles by which the
   homenet architecture may deliver these properties.

3.1.  General Principles

   There is little that the Internet standards community can do about
   the physical topologies or the need for some networks to be separated
   at the network layer for policy or link-layer compatibility reasons.
   However, there is a lot of flexibility in using IP addressing and
   internetworking mechanisms.  This text discusses how such flexibility
   should be used to provide the best user experience and ensure that
   the network can evolve with new applications in the future.  The
   principles described in this text should be followed when designing
   homenet protocol solutions.

3.1.1.  Reuse Existing Protocols

   Existing protocols will be used to meet the requirements of home
   networks.  Where necessary, extensions will be made to those
   protocols.  When no existing protocol is found to be suitable, a new
   or emerging protocol may be used.  Therefore, it is important that no
   design or architectural decisions be made that would preclude the use
   of new or emerging protocols.

   A generally conservative approach, giving weight to running (and
   available) code, is preferable.  Where new protocols are required,
   evidence of commitment to implementation by appropriate vendors or
   development communities is highly desirable.  Protocols used should
   be backward compatible and forward compatible where changes are made.









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3.1.2.  Minimise Changes to Hosts and Routers

   In order to maximise the deployability of new homenets, any
   requirement for changes to hosts and routers should be minimised
   where possible; however, solutions that, for example, incrementally
   improve capability via host or router changes may be acceptable.
   There may be cases where changes are unavoidable, e.g., to allow a
   given homenet routing protocol to be self-configuring or to support
   routing based on source addresses in addition to destination
   addresses (to improve multihoming support, as discussed in
   Section 3.2.4).

3.2.  Homenet Topology

   This section considers homenet topologies and the principles that may
   be applied in designing an architecture to support as wide a range of
   such topologies as possible.

3.2.1.  Supporting Arbitrary Topologies

   There should ideally be no built-in assumptions about the topology in
   home networks, as users are capable of connecting their devices in
   'ingenious' ways.  Thus, arbitrary topologies and arbitrary routing
   will need to be supported, or at least the failure mode for when the
   user makes a mistake should be as robust as possible, e.g.,
   deactivating a certain part of the infrastructure to allow the rest
   to operate.  In such cases, the user should ideally have some useful
   indication of the failure mode encountered.

   There should be no topology scenarios that cause a loss of
   connectivity, except when the user creates a physical island within
   the topology.  Some potentially pathological cases that can be
   created include bridging ports of a router together; however, this
   case can be detected and dealt with by the router.  Loops within a
   routed topology are in a sense good in that they offer redundancy.
   Topologies that include potential bridging loops can be dangerous but
   are also detectable when a switch learns the Media Access Control
   (MAC) address of one of its interfaces on another or runs a spanning
   tree or link-state protocol.  It is only topologies with such
   potential loops using simple repeaters that are truly pathological.

   The topology of the homenet may change over time, due to the addition
   or removal of equipment but also due to temporary failures or
   connectivity problems.  In some cases, this may lead to, for example,
   a multihomed homenet being split into two isolated homenets or, after
   such a fault is remedied, two isolated parts reconfiguring back to a
   single network.




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3.2.2.  Network Topology Models

   As hinted above, while the architecture may focus on likely common
   topologies, it should not preclude any arbitrary topology from being
   constructed.

   At the time of writing, most IPv4 home network models tend to be
   relatively simple, typically a single NAT router to the ISP and a
   single internal subnet but, as discussed earlier, evolution in
   network architectures is driving more complex topologies, such as the
   separation of guest and private networks.  There may also be some
   cascaded IPv4 NAT scenarios, which we mention in the next section.
   For IPv6 homenets, the network architectures described in [RFC7084]
   should, as a minimum, be supported.

   There are a number of properties or attributes of a home network that
   we can use to describe its topology and operation.  The following
   properties apply to any IPv6 home network:

   o  Presence of internal routers.  The homenet may have one or more
      internal routers or may only provide subnetting from interfaces on
      the CE router.

   o  Presence of isolated internal subnets.  There may be isolated
      internal subnets, with no direct connectivity between them within
      the homenet (with each having its own external connectivity).
      Isolation may be physical or implemented via IEEE 802.1q VLANs.
      The latter is, however, not something a typical user would be
      expected to configure.

   o  Demarcation of the CE router.  The CE router(s) may or may not be
      managed by the ISP.  If the demarcation point is such that the
      customer can provide or manage the CE router, its configuration
      must be simple.  Both models must be supported.

   Various forms of multihoming are likely to become more prevalent with
   IPv6 home networks, where the homenet may have two or more external
   ISP connections, as discussed further below.  Thus, the following
   properties should also be considered for such networks:

   o  Number of upstream providers.  The majority of home networks today
      consist of a single upstream ISP, but it may become more common in
      the future for there to be multiple ISPs, whether for resilience
      or provision of additional services.  Each would offer its own
      prefix.  Some may or may not provide a default route to the public
      Internet.





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   o  Number of CE routers.  The homenet may have a single CE router,
      which might be used for one or more providers, or multiple CE
      routers.  The presence of multiple CE routers adds additional
      complexity for multihoming scenarios and protocols like PCP that
      may need to manage connection-oriented state mappings on the same
      CE router as used for subsequent traffic flows.

   In the following sections, we give some examples of the types of
   homenet topologies we may see in the future.  This is not intended to
   be an exhaustive or complete list but rather an indicative one to
   facilitate the discussion in this text.

3.2.2.1.  A: Single ISP, Single CE Router, and Internal Routers

   Figure 1 shows a home network with multiple local area networks.
   These may be needed for reasons relating to different link-layer
   technologies in use or for policy reasons, e.g., classic Ethernet in
   one subnet and an LLN link-layer technology in another.  In this
   example, there is no single router that a priori understands the
   entire topology.  The topology itself may also be complex, and it may
   not be possible to assume a pure tree form, for instance (because
   home users may plug routers together to form arbitrary topologies,
   including those with potential loops in them).




























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                     +-------+-------+                     \
                     |   Service     |                      \
                     |   Provider    |                       | Service
                     |    Router     |                       | Provider
                     +-------+-------+                       | Network
                             |                              /
                             | Customer                    /
                             | Internet Connection
                             |
                      +------+--------+                    \
                      |     IPv6      |                     \
                      | Customer Edge |                      \
                      |    Router     |                      |
                      +----+-+---+----+                      |
          Network A        | |   |      Network B(E)         |
    ----+-------------+----+ |   +---+-------------+------+  |
        |             |      |       |             |      |  |
   +----+-----+ +-----+----+ |  +----+-----+ +-----+----+ |  |
   |IPv6 Host | |IPv6 Host | |  | IPv6 Host| |IPv6 Host | |  |
   |    H1    | |    H2    | |  |    H3    | |    H4    | |  |
   +----------+ +----------+ |  +----------+ +----------+ |  |
                             |        |             |     |  |
                      Link F |     ---+------+------+-----+  |
                             |               | Network E(B)  |
                      +------+--------+      |               | End-User
                      |     IPv6      |      |               | Networks
                      |   Interior    +------+               |
                      |    Router     |                      |
                      +---+-------+-+-+                      |
          Network C       |       |   Network D              |
    ----+-------------+---+       +---+-------------+---     |
        |             |               |             |        |
   +----+-----+ +-----+----+     +----+-----+ +-----+----+   |
   |IPv6 Host | |IPv6 Host |     | IPv6 Host| |IPv6 Host |   |
   |   H5     | |   H6     |     |    H7    | |    H8    |   /
   +----------+ +----------+     +----------+ +----------+  /

                                 Figure 1

   In this diagram, there is one CE router.  It has a single uplink
   interface.  It has three additional interfaces connected to Network
   A, Link F, and Network B.  The IPv6 Internal Router (IR) has four
   interfaces connected to Link F, Network C, Network D, and Network E.
   Network B and Network E have been bridged, likely inadvertently.
   This could be as a result of connecting a wire between a switch for
   Network B and a switch for Network E.





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   Any of logical Networks A through F might be wired or wireless.
   Where multiple hosts are shown, this might be through one or more
   physical ports on the CE router or IPv6 (IR), wireless networks, or
   through one or more Ethernet switches that are Layer 2 only.

3.2.2.2.  B: Two ISPs, Two CE Routers, and Shared Subnet

           +-------+-------+     +-------+-------+         \
           |   Service     |     |   Service     |          \
           |  Provider A   |     |  Provider B   |           | Service
           |    Router     |     |    Router     |           | Provider
           +------+--------+     +-------+-------+           | Network
                  |                      |                   /
                  |      Customer        |                  /
                  | Internet Connections |                 /
                  |                      |
           +------+--------+     +-------+-------+         \
           |     IPv6      |     |    IPv6       |          \
           | Customer Edge |     | Customer Edge |           \
           |   Router 1    |     |   Router 2    |           /
           +------+--------+     +-------+-------+          /
                  |                      |                 /
                  |                      |                | End-User
     ---+---------+---+---------------+--+----------+---  | Network(s)
        |             |               |             |      \
   +----+-----+ +-----+----+     +----+-----+ +-----+----+  \
   |IPv6 Host | |IPv6 Host |     | IPv6 Host| |IPv6 Host |  /
   |   H1     | |   H2     |     |    H3    | |    H4    | /
   +----------+ +----------+     +----------+ +----------+

                                 Figure 2

   Figure 2 illustrates a multihomed homenet model, where the customer
   has connectivity via CE router 1 to ISP A and via CE router 2 to ISP
   B.  This example shows one shared subnet where IPv6 nodes would
   potentially be multihomed and receive multiple IPv6 global prefixes,
   one per ISP.  This model may also be combined with that shown in
   Figure 1 to create a more complex scenario with multiple internal
   routers.  Or, the above shared subnet may be split in two, such that
   each CE router serves a separate isolated subnet, which is a scenario
   seen with some IPv4 networks today.










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3.2.2.3.  C: Two ISPs, One CE Router, and Shared Subnet

           +-------+-------+    +-------+-------+          \
           |   Service     |    |   Service     |           \
           |  Provider A   |    |  Provider B   |            | Service
           |    Router     |    |    Router     |            | Provider
           +-------+-------+    +------+--------+            | Network
                   |                   |                     /
                   |     Customer      |                    /
                   |     Internet      |                   /
                   |    Connections    |
                 +-----------+-----------+                 \
                 |         IPv6          |                  \
                 |     Customer Edge     |                   \
                 |        Router         |                   /
                 +-----------+-----------+                  /
                             |                             /
                             |                            | End-User
     ---+------------+-------+--------+-------------+---  | Network(s)
        |            |                |             |      \
   +----+-----+ +----+-----+     +----+-----+ +-----+----+  \
   |IPv6 Host | |IPv6 Host |     | IPv6 Host| |IPv6 Host |  /
   |   H1     | |   H2     |     |    H3    | |   H4     | /
   +----------+ +----------+     +----------+ +----------+

                                 Figure 3

   Figure 3 illustrates a model where a home network may have multiple
   connections to multiple providers or multiple logical connections to
   the same provider, with shared internal subnets.

3.2.3.  Dual-Stack Topologies

   For the immediate future, it is expected that most homenet
   deployments will be dual-stack IPv4/IPv6.  In such networks, it is
   important not to introduce new IPv6 capabilities that would cause a
   failure if used alongside IPv4+NAT, given that such dual-stack
   homenets will be commonplace for some time.  That said, it is
   desirable that IPv6 works better than IPv4 in as many scenarios as
   possible.  Further, the homenet architecture must operate in the
   absence of IPv4.

   A general recommendation is to follow the same topology for IPv6 as
   is used for IPv4 but not to use NAT.  Thus, there should be routed
   IPv6 where an IPv4 NAT is used, and where there is no NAT, routing or
   bridging may be used.  Routing may have advantages when compared to
   bridging together high- and lower-speed shared media, and in




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   addition, bridging may not be suitable for some networks, such as ad
   hoc mobile networks.

   In some cases, IPv4 home networks may feature cascaded NATs.  End
   users are frequently unaware that they have created such networks, as
   'home routers' and 'home switches' are frequently confused.  In
   addition, there are cases where NAT routers are included within
   Virtual Machine Hypervisors or where Internet connection-sharing
   services have been enabled.  This document applies equally to such
   hidden NAT 'routers'.  IPv6-routed versions of such cases will be
   required.  We should thus also note that routers in the homenet may
   not be separate physical devices; they may be embedded within other
   devices.

3.2.4.  Multihoming

   A homenet may be multihomed to multiple providers, as the network
   models above illustrate.  This may take a form where there are either
   multiple isolated networks within the home or a more integrated
   network where the connectivity selection needs to be dynamic.
   Current practice is typically of the former kind, but the latter is
   expected to become more commonplace.

   In the general homenet architecture, multihomed hosts should be
   multi-addressed with a global IPv6 address from the global prefix
   delegated from each ISP they communicate with or through.  When such
   multi-addressing is in use, hosts need some way to pick source and
   destination address pairs for connections.  A host may choose a
   source address to use by various methods, most commonly [RFC6724].
   Applications may of course do different things, and this should not
   be precluded.

   For the single CE Router Network Model C illustrated above,
   multihoming may be offered by source-based routing at the CE router.
   With multiple exit routers, as in CE Router Network Model B, the
   complexity rises.  Given a packet with a source address on the home
   network, the packet must be routed to the proper egress to avoid
   ingress filtering as described in BCP 38 if exiting through the wrong
   ISP.  It is highly desirable that the packet is routed in the most
   efficient manner to the correct exit, though as a minimum requirement
   the packet should not be dropped.

   The homenet architecture should support both the above models, i.e.,
   one or more CE routers.  However, the general multihoming problem is
   broad, and solutions suggested to date within the IETF have included
   complex architectures for monitoring connectivity, traffic
   engineering, identifier-locator separation, connection survivability
   across multihoming events, and so on.  It is thus important that the



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   homenet architecture should as far as possible minimise the
   complexity of any multihoming support.

   An example of such a 'simpler' approach has been documented in
   [RFC7157].  Alternatively, a flooding/routing protocol could
   potentially be used to pass information through the homenet, such
   that internal routers and ultimately end hosts could learn per-prefix
   configuration information, allowing better address selection
   decisions to be made.  However, this would imply router and, most
   likely, host changes.  Another avenue is to introduce support
   throughout the homenet for routing that is based on the source as
   well as the destination address of each packet.  While greatly
   improving the 'intelligence' of routing decisions within the homenet,
   such an approach would require relatively significant router changes
   but avoid host changes.

   As explained previously, while NPTv6 has been proposed for providing
   multihoming support in networks, its use is not recommended in the
   homenet architecture.

   It should be noted that some multihoming scenarios may see one
   upstream being a "walled garden" and thus only appropriate for
   connectivity to the services of that provider; an example may be a
   VPN service that only routes back to the enterprise business network
   of a user in the homenet.  As per Section 4.2.1 of [RFC3002], we do
   not specifically target walled-garden multihoming as a goal of this
   document.

   The homenet architecture should also not preclude use of host or
   application-oriented tools, e.g., Shim6 [RFC5533], Multipath TCP
   (MPTCP) [RFC6824], or Happy Eyeballs [RFC6555].  In general, any
   incremental improvements obtained by host changes should give benefit
   for the hosts introducing them but should not be required.

3.2.5.  Mobility Support

   Devices may be mobile within the homenet.  While resident on the same
   subnet, their address will remain persistent, but should devices move
   to a different (wireless) subnet, they will acquire a new address in
   that subnet.  It is desirable that the homenet supports internal
   device mobility.  To do so, the homenet may either extend the reach
   of specific wireless subnets to enable wireless roaming across the
   home (availability of a specific subnet across the home) or support
   mobility protocols to facilitate such roaming where multiple subnets
   are used.






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3.3.  A Self-Organising Network

   The home network infrastructure should be naturally self-organising
   and self-configuring under different circumstances relating to the
   connectivity status to the Internet, number of devices, and physical
   topology.  At the same time, it should be possible for advanced users
   to manually adjust (override) the current configuration.

   While a goal of the homenet architecture is for the network to be as
   self-organising as possible, there may be instances where some manual
   configuration is required, e.g., the entry of a cryptographic key to
   apply wireless security or to configure a shared routing secret.  The
   latter may be relevant when considering how to bootstrap a routing
   configuration.  It is highly desirable that the number of such
   configurations is minimised.

3.3.1.  Differentiating Neighbouring Homenets

   It is important that self-configuration with 'unintended' devices be
   avoided.  There should be a way for a user to administratively assert
   in a simple way whether or not a device belongs to a given homenet.
   The goal is to allow the establishment of borders, particularly
   between two adjacent homenets, and to avoid unauthorised devices from
   participating in the homenet.  Such an authorisation capability may
   need to operate through multiple hops in the homenet.

   The homenet should thus support a way for a homenet owner to claim
   ownership of their devices in a reasonably secure way.  This could be
   achieved by a pairing mechanism by, for example, pressing buttons
   simultaneously on an authenticated and a new homenet device or by an
   enrollment process as part of an autonomic networking environment.

   While there may be scenarios where one homenet may wish to
   intentionally gain access through another, e.g., to share external
   connectivity costs, such scenarios are not discussed in this
   document.

3.3.2.  Largest Practical Subnets

   Today's IPv4 home networks generally have a single subnet, and early
   dual-stack deployments have a single congruent IPv6 subnet, possibly
   with some bridging functionality.  More recently, some vendors have
   started to introduce 'home' and 'guest' functions, which in IPv6
   would be implemented as two subnets.

   Future home networks are highly likely to have one or more internal
   routers and thus need multiple subnets for the reasons described
   earlier.  As part of the self-organisation of the network, the



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   homenet should subdivide itself into the largest practical subnets
   that can be constructed within the constraints of link-layer
   mechanisms, bridging, physical connectivity, and policy, and where
   applicable, performance or other criteria.  In such subdivisions, the
   logical topology may not necessarily match the physical topology.
   This text does not, however, make recommendations on how such
   subdivision should occur.  It is expected that subsequent documents
   will address this problem.

   While it may be desirable to maximise the chance of link-local
   protocols operating across a homenet by maximising the size of a
   subnet, multi-subnet home networks are inevitable, so their support
   must be included.

3.3.3.  Handling Varying Link Technologies

   Homenets tend to grow organically over many years, and a homenet will
   typically be built over link-layer technologies from different
   generations.  Current homenets typically use links ranging from 1
   Mbit/s up to 1 Gbit/s -- a throughput discrepancy of three orders of
   magnitude.  We expect this discrepancy to widen further as both high-
   speed and low-power technologies are deployed.

   Homenet protocols should be designed to deal well with
   interconnecting links of very different throughputs.  In particular,
   flows local to a link should not be flooded throughout the homenet,
   even when sent over multicast, and, whenever possible, the homenet
   protocols should be able to choose the faster links and avoid the
   slower ones.

   Links (particularly wireless links) may also have limited numbers of
   transmit opportunities (txops), and there is a clear trend driven by
   both power and downward compatibility constraints toward aggregation
   of packets into these limited txops while increasing throughput.
   Transmit opportunities may be a system's scarcest resource and,
   therefore, also strongly limit actual throughput available.

3.3.4.  Homenet Realms and Borders

   The homenet will need to be aware of the extent of its own 'site',
   which will, for example, define the borders for ULA and site scope
   multicast traffic and may require specific security policies to be
   applied.  The homenet will have one or more such borders with
   external connectivity providers.

   A homenet will most likely also have internal borders between
   internal realms, e.g., a guest realm or a corporate network extension
   realm.  It is desirable that appropriate borders can be configured to



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   determine, for example, the scope of where network prefixes, routing
   information, network traffic, service discovery, and naming may be
   shared.  The default mode internally should be to share everything.

   It is expected that a realm would span at least an entire subnet, and
   thus the borders lie at routers that receive delegated prefixes
   within the homenet.  It is also desirable, for a richer security
   model, that hosts are able to make communication decisions based on
   available realm and associated prefix information in the same way
   that routers at realm borders can.

   A simple homenet model may just consider three types of realms and
   the borders between them, namely the internal homenet, the ISP, and a
   guest network.  In this case, the borders will include the border
   from the homenet to the ISP, the border from the guest network to the
   ISP, and the border from the homenet to the guest network.
   Regardless, it should be possible for additional types of realms and
   borders to be defined, e.g., for some specific LLN-based network,
   such as Smart Grid, and for these to be detected automatically and
   for an appropriate default policy to be applied as to what type of
   traffic/data can flow across such borders.

   It is desirable to classify the external border of the home network
   as a unique logical interface separating the home network from a
   service provider network(s).  This border interface may be a single
   physical interface to a single service provider, multiple Layer 2
   sub-interfaces to a single service provider, or multiple connections
   to a single or multiple providers.  This border makes it possible to
   describe edge operations and interface requirements across multiple
   functional areas including security, routing, service discovery, and
   router discovery.

   It should be possible for the homenet user to override any
   automatically determined borders and the default policies applied
   between them, the exception being that it may not be possible to
   override policies defined by the ISP at the external border.

3.3.5.  Configuration Information from the ISP

   In certain cases, it may be useful for the homenet to get certain
   configuration information from its ISP.  For example, the homenet
   DHCP server may request and forward some options that it gets from
   its upstream DHCP server, though the specifics of the options may
   vary across deployments.  There is potential complexity here, of
   course, should the homenet be multihomed.






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3.4.  Homenet Addressing

   The IPv6 addressing scheme used within a homenet must conform to the
   IPv6 addressing architecture [RFC4291].  In this section, we discuss
   how the homenet needs to adapt to the prefixes made available to it
   by its upstream ISP, such that internal subnets, hosts, and devices
   can obtain and configure the necessary addressing information to
   operate.

3.4.1.  Use of ISP-Delegated IPv6 Prefixes

   Discussion of IPv6 prefix allocation policies is included in
   [RFC6177].  In practice, a homenet may receive an arbitrary length
   IPv6 prefix from its provider, e.g., /60, /56, or /48.  The offered
   prefix may be stable or change from time to time; it is generally
   expected that ISPs will offer relatively stable prefixes to their
   residential customers.  Regardless, the home network needs to be
   adaptable as far as possible to ISP prefix allocation policies and
   assume nothing about the stability of the prefix received from an ISP
   or the length of the prefix that may be offered.

   However, if, for example, only a /64 is offered by the ISP, the
   homenet may be severely constrained or even unable to function.  BCP
   157 [RFC6177] states the following:

      A key principle for address management is that end sites always be
      able to obtain a reasonable amount of address space for their
      actual and planned usage, and over time ranges specified in years
      rather than just months.  In practice, that means at least one
      /64, and in most cases significantly more.  One particular
      situation that must be avoided is having an end site feel
      compelled to use IPv6-to-IPv6 Network Address Translation or other
      burdensome address conservation techniques because it could not
      get sufficient address space.

   This architecture document assumes that the guidance in the quoted
   text is being followed by ISPs.

   There are many problems that would arise from a homenet not being
   offered a sufficient prefix size for its needs.  Rather than attempt
   to contrive a method for a homenet to operate in a constrained manner
   when faced with insufficient prefixes, such as the use of subnet
   prefixes longer than /64 (which would break stateless address
   autoconfiguration [RFC4862]), the use of NPTv6, or falling back to
   bridging across potentially very different media, it is recommended
   that the receiving router instead enters an error state and issues
   appropriate warnings.  Some consideration may need to be given to how




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   such a warning or error state should best be presented to a typical
   home user.

   Thus, a homenet CE router should request, for example, via DHCP
   Prefix Delegation (DHCP PD) [RFC3633], that it would like a /48
   prefix from its ISP, i.e., it asks the ISP for the maximum size
   prefix it might expect to be offered, even if in practice it may only
   be offered a /56 or /60.  For a typical IPv6 homenet, it is not
   recommended that an ISP offers less than a /60 prefix, and it is
   highly preferable that the ISP offers at least a /56.  It is expected
   that the allocated prefix to the homenet from any single ISP is a
   contiguous, aggregated one.  While it may be possible for a homenet
   CE router to issue multiple prefix requests to attempt to obtain
   multiple delegations, such behaviour is out of scope of this
   document.

   The norm for residential customers of large ISPs may be similar to
   their single IPv4 address provision; by default it is likely to
   remain persistent for some time, but changes in the ISP's own
   provisioning systems may lead to the customer's IP (and in the IPv6
   case their prefix pool) changing.  It is not expected that ISPs will
   generally support Provider Independent (PI) addressing for
   residential homenets.

   When an ISP does need to restructure, and in doing so renumber its
   customer homenets, 'flash' renumbering is likely to be imposed.  This
   implies a need for the homenet to be able to handle a sudden
   renumbering event that, unlike the process described in [RFC4192],
   would be a 'flag day' event, which means that a graceful renumbering
   process moving through a state with two active prefixes in use would
   not be possible.  While renumbering can be viewed as an extended
   version of an initial numbering process, the difference between flash
   renumbering and an initial 'cold start' is the need to provide
   service continuity.

   There may be cases where local law means some ISPs are required to
   change IPv6 prefixes (current IPv4 addresses) for privacy reasons for
   their customers.  In such cases, it may be possible to avoid an
   instant 'flash' renumbering and plan a non-flag day renumbering as
   per RFC 4192.  Similarly, if an ISP has a planned renumbering
   process, it may be able to adjust lease timers, etc., appropriately.

   The customer may of course also choose to move to a new ISP and thus
   begin using a new prefix.  In such cases, the customer should expect
   a discontinuity, and not only may the prefix change, but potentially
   also the prefix length if the new ISP offers a different default size
   prefix.  The homenet may also be forced to renumber itself if
   significant internal 'replumbing' is undertaken by the user.



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   Regardless, it's desirable that homenet protocols support rapid
   renumbering and that operational processes don't add unnecessary
   complexity for the renumbering process.  Further, the introduction of
   any new homenet protocols should not make any form of renumbering any
   more complex than it already is.

   Finally, the internal operation of the home network should also not
   depend on the availability of the ISP network at any given time,
   other than, of course, for connectivity to services or systems off
   the home network.  This reinforces the use of ULAs for stable
   internal communication and the need for a naming and service
   discovery mechanism that can operate independently within the
   homenet.

3.4.2.  Stable Internal IP Addresses

   The network should by default attempt to provide IP-layer
   connectivity between all internal parts of the homenet as well as to
   and from the external Internet, subject to the filtering policies or
   other policy constraints discussed later in the security section.

   ULAs should be used within the scope of a homenet to support stable
   routing and connectivity between subnets and hosts regardless of
   whether a globally unique ISP-provided prefix is available.  In the
   case of a prolonged external connectivity outage, ULAs allow internal
   operations across routed subnets to continue.  ULA addresses also
   allow constrained devices to create permanent relationships between
   IPv6 addresses, e.g., from a wall controller to a lamp, where
   symbolic host names would require additional non-volatile memory, and
   updating global prefixes in sleeping devices might also be
   problematic.

   As discussed previously, it would be expected that ULAs would
   normally be used alongside one or more global prefixes in a homenet,
   such that hosts become multi-addressed with both globally unique and
   ULA prefixes.  ULAs should be used for all devices, not just those
   intended to only have internal connectivity.  Default address
   selection would then enable ULAs to be preferred for internal
   communications between devices that are using ULA prefixes generated
   within the same homenet.

   In cases where ULA prefixes are in use within a homenet but there is
   no external IPv6 connectivity (and thus no GUAs in use),
   recommendations ULA-5, L-3, and L-4 in RFC 7084 should be followed to
   ensure correct operation, in particular where the homenet may be dual
   stack with IPv4 external connectivity.  The use of the Route
   Information Option described in [RFC4191] provides a mechanism to
   advertise such more-specific ULA routes.



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   The use of ULAs should be restricted to the homenet scope through
   filtering at the border(s) of the homenet, as mandated by RFC 7084
   requirement S-2.

   Note that in some cases, it is possible that multiple /48 ULA
   prefixes may be in use within the same homenet, e.g., when the
   network is being deployed, perhaps also without external
   connectivity.  In cases where multiple ULA /48s are in use, hosts
   need to know that each /48 is local to the homenet, e.g., by
   inclusion in their local address selection policy table.

3.4.3.  Internal Prefix Delegation

   As mentioned above, there are various sources of prefixes.  From the
   homenet perspective, a single global prefix from each ISP should be
   received on the border CE router [RFC3633].  Where multiple CE
   routers exist with multiple ISP prefix pools, it is expected that
   routers within the homenet would assign themselves prefixes from each
   ISP they communicate with/through.  As discussed above, a ULA prefix
   should be provisioned for stable internal communications or for use
   on constrained/LLN networks.

   The delegation or availability of a prefix pool to the homenet should
   allow subsequent internal autonomous assignment of prefixes for use
   within the homenet.  Such internal assignment should not assume a
   flat or hierarchical model, nor should it make an assumption about
   whether the assignment of internal prefixes is distributed or
   centralised.  The assignment mechanism should provide reasonable
   efficiency, so that typical home network prefix allocation sizes can
   accommodate all the necessary /64 allocations in most cases, and not
   waste prefixes.  Further, duplicate assignment of multiple /64s to
   the same network should be avoided, and the network should behave as
   gracefully as possible in the event of prefix exhaustion (though the
   options in such cases may be limited).

   Where the home network has multiple CE routers and these are
   delegated prefix pools from their attached ISPs, the internal prefix
   assignment would be expected to be served by each CE router for each
   prefix associated with it.  Where ULAs are used, it is preferable
   that only one /48 ULA covers the whole homenet, from which /64s can
   be assigned to the subnets.  In cases where two /48 ULAs are
   generated within a homenet, the network should still continue to
   function, meaning that hosts will need to determine that each ULA is
   local to the homenet.

   Prefix assignment within the homenet should result in each link being
   assigned a stable prefix that is persistent across reboots, power
   outages, and similar short-term outages.  The availability of



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   persistent prefixes should not depend on the router boot order.  The
   addition of a new routing device should not affect existing
   persistent prefixes, but persistence may not be expected in the face
   of significant 'replumbing' of the homenet.  However, assigned ULA
   prefixes within the homenet should remain persistent through an ISP-
   driven renumbering event.

   Provisioning such persistent prefixes may imply the need for stable
   storage on routing devices and also a method for a home user to
   'reset' the stored prefix should a significant reconfiguration be
   required (though ideally the home user should not be involved at
   all).

   This document makes no specific recommendation towards solutions but
   notes that it is very likely that all routing devices participating
   in a homenet must use the same internal prefix delegation method.
   This implies that only one delegation method should be in use.

3.4.4.  Coordination of Configuration Information

   The network elements will need to be integrated in a way that takes
   account of the various lifetimes on timers that are used on different
   elements, e.g., DHCPv6 PD, router, valid prefix, and preferred prefix
   timers.

3.4.5.  Privacy

   If ISPs offer relatively stable IPv6 prefixes to customers, the
   network prefix part of addresses associated with the homenet may not
   change over a reasonably long period of time.

   The exposure of which traffic is sourced from the same homenet is
   thus similar to IPv4; the single IPv4 global address seen through use
   of IPv4 NAT gives the same hint as the global IPv6 prefix seen for
   IPv6 traffic.

   While IPv4 NAT may obfuscate to an external observer which internal
   devices traffic is sourced from, IPv6, even with use of privacy
   addresses [RFC4941], adds additional exposure of which traffic is
   sourced from the same internal device through use of the same IPv6
   source address for a period of time.

3.5.  Routing Functionality

   Routing functionality is required when there are multiple routers
   deployed within the internal home network.  This functionality could
   be as simple as the current 'default route is up' model of IPv4 NAT,




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   or more likely, it would involve running an appropriate routing
   protocol.

   A mechanism is required to discover which router(s) in the homenet is
   providing the CE router function.  Borders may include but are not
   limited to the interface to the upstream ISP, a gateway device to a
   separate home network such as an LLN network, or a gateway to a guest
   or private corporate extension network.  In some cases, there may be
   no border present, which may, for example, occur before an upstream
   connection has been established.

   The routing environment should be self-configuring, as discussed
   previously.  The homenet self-configuration process and the routing
   protocol must interact in a predictable manner, especially during
   startup and reconvergence.  The border discovery functionality and
   other self-configuration functionality may be integrated into the
   routing protocol itself but may also be imported via a separate
   discovery mechanism.

   It is preferable that configuration information is distributed and
   synchronised within the homenet by a separate configuration protocol.

   The homenet routing protocol should be based on a previously deployed
   protocol that has been shown to be reliable and robust.  This does
   not preclude the selection of a newer protocol for which a high-
   quality open source implementation becomes available.  The resulting
   code must support lightweight implementations and be suitable for
   incorporation into consumer devices, where both fixed and temporary
   storage and processing power are at a premium.

   At most, one unicast and one multicast routing protocol should be in
   use at a given time in a given homenet.  In some simple topologies,
   no routing protocol may be needed.  If more than one routing protocol
   is supported by routers in a given homenet, then a mechanism is
   required to ensure that all routers in that homenet use the same
   protocol.

   The homenet architecture is IPv6-only.  In practice, dual-stack
   homenets are still likely for the foreseeable future, as described in
   Section 3.2.3.  Whilst support for IPv4 and other address families
   may therefore be beneficial, it is not an explicit requirement to
   carry the routing information in the same routing protocol.

   Multiple types of physical interfaces must be accounted for in the
   homenet routing topology.  Technologies such as Ethernet, Wi-Fi,
   Multimedia over Coax Alliance (MoCA), etc., must be capable of
   coexisting in the same environment and should be treated as part of
   any routed deployment.  The inclusion of physical-layer



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   characteristics in path computation should be considered for
   optimising communication in the homenet.

3.5.1.  Unicast Routing within the Homenet

   The role of the unicast routing protocol is to provide good enough
   end-to-end connectivity often enough, where good/often enough is
   defined by user expectations.

   Due to the use of a variety of diverse underlying link technologies,
   path selection in a homenet may benefit from being more refined than
   minimising hop count.  It may also be beneficial for traffic to use
   multiple paths to a given destination within the homenet where
   available rather than just a single best path.

   Minimising convergence time should be a goal in any routed
   environment.  It is reasonable to assume that convergence time should
   not be significantly longer than network outages users are accustomed
   to should their CE router reboot.

   The homenet architecture is agnostic as to the choice of underlying
   routing technology, e.g., link state versus Bellman-Ford.

   The routing protocol should support the generic use of multiple
   customer Internet connections and the concurrent use of multiple
   delegated prefixes.  A routing protocol that can make routing
   decisions based on source and destination addresses is thus highly
   desirable, to avoid problems with upstream ISP ingress filtering as
   described in BCP 38.  Multihoming support may also include load
   balancing to multiple providers and failover from a primary to a
   backup link when available.  The protocol should not require upstream
   ISP connectivity to be established to continue routing within the
   homenet.

   The homenet architecture is agnostic on a minimum hop count that has
   to be supported by the routing protocol.  The architecture should,
   however, be scalable to other scenarios where homenet technology may
   be deployed, which may include small office and small enterprise
   sites.  To allow for such cases, it would be desirable that the
   architecture is scalable to higher hop counts and to larger numbers
   of routers than would be typical in a true home network.

   At the time of writing, link-layer networking technology is poised to
   become more heterogeneous, as networks begin to employ both
   traditional Ethernet technology and link layers designed for LLNs,
   such as those used for certain types of sensor devices.





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   Ideally, LLN or other logically separate networks should be able to
   exchange routes such that IP traffic may be forwarded among the
   networks via gateway routers that interoperate with both the homenet
   and any LLNs.  Current home deployments use largely different
   mechanisms in sensor and basic Internet connectivity networks.  IPv6
   virtual machine (VM) solutions may also add additional routing
   requirements.

   In this homenet architecture, LLNs and other specialised networks are
   considered stub areas of the homenet and are thus not expected to act
   as a transit for traffic between more traditional media.

3.5.2.  Unicast Routing at the Homenet Border

   The current practice defined in [RFC7084] would suggest that routing
   between the homenet CE router and the service provider router follow
   the WAN-side requirements model in [RFC7084], Section 4 (WAN-side
   requirements), at least in initial deployments.  However,
   consideration of whether a routing protocol is used between the
   homenet CE router and the service provider router is out of scope of
   this document.

3.5.3.  Multicast Support

   It is desirable that, subject to the capacities of devices on certain
   media types, multicast routing is supported across the homenet,
   including source-specific multicast (SSM) [RFC4607].

   [RFC4291] requires that any boundary of scope 4 or higher (i.e.,
   admin-local or higher) be administratively configured.  Thus, the
   boundary at the homenet-ISP border must be administratively
   configured, though that may be triggered by an administrative
   function such as DHCP PD.  Other multicast forwarding policy borders
   may also exist within the homenet, e.g., to/from a guest subnet,
   whilst the use of certain link media types may also affect where
   specific multicast traffic is forwarded or routed.

   There may be different drivers for multicast to be supported across
   the homenet -- for example,

   o  for homenet-wide service discovery, should a multicast service
      discovery protocol of scope greater than link-local be defined

   o  for multicast-based streaming or file-sharing applications

   Where multicast is routed across a homenet, an appropriate multicast
   routing protocol is required, one that as per the unicast routing
   protocol should be self-configuring.  As hinted above, it must be



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   possible to scope or filter multicast traffic to avoid it being
   flooded to network media where devices cannot reasonably support it.

   A homenet may not only use multicast internally, it may also be a
   consumer or provider of external multicast traffic, where the
   homenet's ISP supports such multicast operation.  This may be
   valuable, for example, where live video applications are being
   sourced to/from the homenet.

   The multicast environment should support the ability for applications
   to pick a unique multicast group to use.

3.6.  Security

   The security of an IPv6 homenet is an important consideration.  The
   most notable difference to the IPv4 operational model is the removal
   of NAT, the introduction of global addressability of devices, and
   thus a need to consider whether devices should have global
   reachability.  Regardless, hosts need to be able to operate securely,
   end to end where required, and also be robust against malicious
   traffic directed towards them.  However, there are other challenges
   introduced, e.g., default filtering policies at the borders between
   various homenet realms.

3.6.1.  Addressability vs. Reachability

   An IPv6-based home network architecture should embrace the
   transparent end-to-end communications model as described in
   [RFC2775].  Each device should be globally addressable, and those
   addresses must not be altered in transit.  However, security
   perimeters can be applied to restrict end-to-end communications, and
   thus while a host may be globally addressable, it may not be globally
   reachable.

   [RFC4864] describes a 'Simple Security' model for IPv6 networks,
   whereby stateful perimeter filtering can be applied to control the
   reachability of devices in a homenet.  RFC 4864 states in Section 4.2
   that "the use of firewalls...is recommended for those that want
   boundary protection in addition to host defences."  It should be
   noted that a 'default deny' filtering approach would effectively
   replace the need for IPv4 NAT traversal protocols with a need to use
   a signalling protocol to request a firewall hole be opened, e.g., a
   protocol such as PCP [RFC6887].  In networks with multiple CE
   routers, the signalling would need to handle the cases of flows that
   may use one or more exit routers.  CE routers would need to be able
   to advertise their existence for such protocols.





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   [RFC6092] expands on RFC 4864, giving a more detailed discussion of
   IPv6 perimeter security recommendations, without mandating a 'default
   deny' approach.  Indeed, RFC 6092 does not enforce a particular mode
   of operation, instead stating that CE routers must provide an easily
   selected configuration option that permits a 'transparent' mode, thus
   ensuring a 'default allow' model is available.

   The topic of whether future home networks as described in this
   document should have a 'default deny' or 'default allow' position has
   been discussed at length in various IETF meetings without any
   consensus being reached on which approach is more appropriate.
   Further, the choice of which default to apply may be situational, and
   thus this text makes no recommendation on the default setting beyond
   what is written on this topic in RFC 6092.  We note in Section 3.6.3
   below that the implicit firewall function of an IPv4 NAT is
   commonplace today, and thus future CE routers targeted at home
   networks should continue to support the option of running in 'default
   deny mode', whether or not that is the default setting.

3.6.2.  Filtering at Borders

   It is desirable that there are mechanisms to detect different types
   of borders within the homenet, as discussed previously, and further
   mechanisms to then apply different types of filtering policies at
   those borders, e.g., whether naming and service discovery should pass
   a given border.  Any such policies should be able to be easily
   applied by typical home users, e.g., to give a user in a guest
   network access to media services in the home or access to a printer.
   Simple mechanisms to apply policy changes, or associations between
   devices, will be required.

   There are cases where full internal connectivity may not be
   desirable, e.g., in certain utility networking scenarios, or where
   filtering is required for policy reasons against a guest network
   subnet(s).  As a result, some scenarios/models may involve running an
   isolated subnet(s) with their own CE routers.  In such cases,
   connectivity would only be expected within each isolated network
   (though traffic may potentially pass between them via external
   providers).

   LLNs provide another example of where there may be secure perimeters
   inside the homenet.  Constrained LLN nodes may implement network key
   security but may depend on access policies enforced by the LLN border
   router.

   Considerations for differentiating neighbouring homenets are
   discussed in Section 3.3.1.




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3.6.3.  Partial Effectiveness of NAT and Firewalls

   Security by way of obscurity (address translation) or through
   firewalls (filtering) is at best only partially effective.  The very
   poor security track record of home computers, home networking, and
   business PC computers and networking is testimony to this.  A
   security compromise behind the firewall of any device exposes all
   others, making an entire network that relies on obscurity or a
   firewall as vulnerable as the most insecure device on the private
   side of the network.

   However, given current evidence of home network products with very
   poor default device security, putting a firewall in place does
   provide some level of protection.  The use of firewalls today,
   whether a good practice or not, is common practice, and the
   capability to afford protection via a 'default deny' setting, even if
   marginally effective, should not be lost.  Thus, while it is highly
   desirable that all hosts in a homenet be adequately protected by
   built-in security functions, it should also be assumed that all CE
   routers will continue to support appropriate perimeter defence
   functions, as per [RFC7084].

3.6.4.  Exfiltration Concerns

   As homenets become more complex, with more devices, and with service
   discovery potentially enabled across the whole home, there are
   potential concerns over the leakage of information should devices use
   discovery protocols to gather information and report it to equipment
   vendors or application service providers.

   While it is not clear how such exfiltration could be easily avoided,
   the threat should be recognised, be it from a new piece of hardware
   or some 'app' installed on a personal device.

3.6.5.  Device Capabilities

   In terms of the devices, homenet hosts should implement their own
   security policies in accordance to their computing capabilities.
   They should have the means to request transparent communications that
   can be initiated to them through security filters in the homenet, for
   either all ports or specific services.  Users should have simple
   methods to associate devices to services that they wish to operate
   transparently through (CE router) borders.








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3.6.6.  ULAs as a Hint of Connection Origin

   As noted in Section 3.6, if appropriate filtering is in place on the
   CE router(s), as mandated by requirement S-2 in RFC 7084, a ULA
   source address may be taken as an indication of locally sourced
   traffic.  This indication could then be used with security settings
   to designate between which nodes a particular application is allowed
   to communicate, provided ULA address space is filtered appropriately
   at the boundary of the realm.

3.7.  Naming and Service Discovery

   The homenet requires devices to be able to determine and use unique
   names by which they can be accessed on the network and that are not
   used by other devices on the network.  Users and devices will need to
   be able to discover devices and services available on the network,
   e.g., media servers, printers, displays, or specific home automation
   devices.  Thus, naming and service discovery must be supported in the
   homenet, and given the nature of typical home network users, the
   service(s) providing this function must as far as possible support
   unmanaged operation.

   The naming system will be required to work internally or externally,
   whether the user is within or outside of the homenet, i.e., the user
   should be able to refer to devices by name, and potentially connect
   to them, wherever they may be.  The most natural way to think about
   such naming and service discovery is to enable it to work across the
   entire homenet residence (site), disregarding technical borders such
   as subnets but respecting policy borders such as those between guest
   and other internal network realms.  Remote access may be desired by
   the homenet residents while travelling but also potentially by
   manufacturers or other 'benevolent' third parties.

3.7.1.  Discovering Services

   Users will typically perform service discovery through graphical user
   interfaces (GUIs) that allow them to browse services on their network
   in an appropriate and intuitive way.  Devices may also need to
   discover other devices, without any user intervention or choice.
   Either way, such interfaces are beyond the scope of this document,
   but the interface should have an appropriate application programming
   interface (API) for the discovery to be performed.

   Such interfaces may also typically hide the local domain name element
   from users, especially where only one name space is available.
   However, as we discuss below, in some cases the ability to discover
   available domains may be useful.




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   We note that current zero-configuration service discovery protocols
   are generally aimed at single subnets.  There is thus a choice to
   make for multi-subnet homenets as to whether such protocols should be
   proxied or extended to operate across a whole homenet.  In this
   context, that may mean bridging a link-local method, taking care to
   avoid packets entering looping paths, or extending the scope of
   multicast traffic used for the purpose.  It may mean that some proxy
   or hybrid service is utilised, perhaps co-resident on the CE router.
   Or, it may be that a new approach is preferable, e.g., flooding
   information around the homenet as attributes within the routing
   protocol (which could allow per-prefix configuration).  However, we
   should prefer approaches that are backward compatible and allow
   current implementations to continue to be used.  Note that this
   document does not mandate a particular solution; rather, it expresses
   the principles that should be used for a homenet naming and service
   discovery environment.

   One of the primary challenges facing service discovery today is lack
   of interoperability due to the ever increasing number of service
   discovery protocols available.  While it is conceivable for consumer
   devices to support multiple discovery protocols, this is clearly not
   the most efficient use of network and computational resources.  One
   goal of the homenet architecture should be a path to service
   discovery protocol interoperability through either a standards-based
   translation scheme, hooks into current protocols to allow some form
   of communication among discovery protocols, extensions to support a
   central service repository in the homenet, or simply convergence
   towards a unified protocol suite.

3.7.2.  Assigning Names to Devices

   Given the large number of devices that may be networked in the
   future, devices should have a means to generate their own unique
   names within a homenet and to detect clashes should they arise, e.g.,
   where a second device of the same type/vendor as an existing device
   with the same default name is deployed or where a new subnet is added
   to the homenet that already has a device of the same name.  It is
   expected that a device should have a fixed name while within the
   scope of the homenet.

   Users will also want simple ways to (re)name devices, again most
   likely through an appropriate and intuitive interface that is beyond
   the scope of this document.  Note that the name a user assigns to a
   device may be a label that is stored on the device as an attribute of
   the device, and it may be distinct from the name used in a name
   service, e.g., 'Study Laser Printer' as opposed to
   printer2.<somedomain>.




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3.7.3.  The Homenet Name Service

   The homenet name service should support both lookups and discovery.
   A lookup would operate via a direct query to a known service, while
   discovery may use multicast messages or a service where applications
   register in order to be found.

   It is highly desirable that the homenet name service must at the very
   least coexist with the Internet name service.  There should also be a
   bias towards proven, existing solutions.  The strong implication is
   thus that the homenet service is DNS based, or DNS compatible.  There
   are naming protocols that are designed to be configured and operate
   Internet-wide, like unicast-based DNS, but also protocols that are
   designed for zero-configuration local environments, like Multicast
   DNS (mDNS) [RFC6762].

   When DNS is used as the homenet name service, it typically includes
   both a resolving service and an authoritative service.  The
   authoritative service hosts the homenet-related zone.  One approach
   when provisioning such a name service, which is designed to
   facilitate name resolution from the global Internet, is to run an
   authoritative name service on the CE router and a secondary
   authoritative name service provided by the ISP or perhaps an external
   third party.

   Where zero-configuration name services are used, it is desirable that
   these can also coexist with the Internet name service.  In
   particular, where the homenet is using a global name space, it is
   desirable that devices have the ability, where desired, to add
   entries to that name space.  There should also be a mechanism for
   such entries to be removed or expired from the global name space.

   To protect against attacks such as cache poisoning, where an attacker
   is able to insert a bogus DNS entry in the local cache, it is
   desirable to support appropriate name service security methods,
   including DNS Security Extensions (DNSSEC) [RFC4033], on both the
   authoritative server and the resolver sides.  Where DNS is used, the
   homenet router or naming service must not prevent DNSSEC from
   operating.

   While this document does not specify hardware requirements, it is
   worth noting briefly here that, e.g., in support of DNSSEC,
   appropriate homenet devices should have good random number generation
   capability, and future homenet specifications should indicate where
   high-quality random number generators, i.e., with decent entropy, are
   needed.





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   Finally, the impact of a change in the CE router must be considered.
   It would be desirable to retain any relevant state (configuration)
   that was held in the old CE router.  This might imply that state
   information should be distributed in the homenet, to be recoverable
   by/to the new CE router, or to the homenet's ISP or a third-party
   externally provided service by some means.

3.7.4.  Name Spaces

   If access to homenet devices is required remotely from anywhere on
   the Internet, then at least one globally unique name space is
   required, though the use of multiple name spaces should not be
   precluded.  One approach is that the name space(s) used for the
   homenet would be served authoritatively by the homenet, most likely
   by a server resident on the CE router.  Such name spaces may be
   acquired by the user or provided/generated by their ISP or an
   alternative externally provided service.  It is likely that the
   default case is that a homenet will use a global domain provided by
   the ISP, but advanced users wishing to use a name space that is
   independent of their provider in the longer term should be able to
   acquire and use their own domain name.  For users wanting to use
   their own independent domain names, such services are already
   available.

   Devices may also be assigned different names in different name
   spaces, e.g., by third parties who may manage systems or devices in
   the homenet on behalf of the resident(s).  Remote management of the
   homenet is out of scope of this document.

   If, however, a global name space is not available, the homenet will
   need to pick and use a local name space, which would only have
   meaning within the local homenet (i.e., it would not be used for
   remote access to the homenet).  The .local name space currently has a
   special meaning for certain existing protocols that have link-local
   scope and is thus not appropriate for multi-subnet home networks.  A
   different name space is thus required for the homenet.

   One approach for picking a local name space is to use an Ambiguous
   Local Qualified Domain Name (ALQDN) space, such as .sitelocal (or an
   appropriate name reserved for the purpose).  While this is a simple
   approach, there is the potential in principle for devices that are
   bookmarked somehow by name by an application in one homenet to be
   confused with a device with the same name in another homenet.  In
   practice, however, the underlying service discovery protocols should
   be capable of handling moving to a network where a new device is
   using the same name as a device used previously in another homenet.





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   An alternative approach for a local name space would be to use a
   Unique Locally Qualified Domain Name (ULQDN) space such as
   .<UniqueString>.sitelocal.  The <UniqueString> could be generated in
   a variety of ways, one potentially being based on the local /48 ULA
   prefix being used across the homenet.  Such a <UniqueString> should
   survive a cold restart, i.e., be consistent after a network power-
   down, or if a value is not set on startup, the CE router or device
   running the name service should generate a default value.  It would
   be desirable for the homenet user to be able to override the
   <UniqueString> with a value of their choice, but that would increase
   the likelihood of a name conflict.  Any generated <UniqueString>
   should not be predictable; thus, adding a salt/hash function would be
   desirable.

   In the (likely) event that the homenet is accessible from outside the
   homenet (using the global name space), it is vital that the homenet
   name space follow the rules and conventions of the global name space.
   In this mode of operation, names in the homenet (including those
   automatically generated by devices) must be usable as labels in the
   global name space.  [RFC5890] describes considerations for
   Internationalizing Domain Names in Applications (IDNA).

   Also, with the introduction of new 'dotless' top-level domains, there
   is also potential for ambiguity between, for example, a local host
   called 'computer' and (if it is registered) a .computer Generic Top
   Level Domain (gTLD).  Thus, qualified names should always be used,
   whether these are exposed to the user or not.  The IAB has issued a
   statement that explains why dotless domains should be considered
   harmful [IABdotless].

   There may be use cases where different name spaces may be desired for
   either different realms in the homenet or segmentation of a single
   name space within the homenet.  Thus, hierarchical name space
   management is likely to be required.  There should also be nothing to
   prevent an individual device(s) from being independently registered
   in external name spaces.

   It may be the case that if there are two or more CE routers serving
   the home network, if each has a name space delegated from a different
   ISP, there is the potential for devices in the home to have multiple
   fully qualified names under multiple domains.

   Where a user is in a remote network wishing to access devices in
   their home network, there may be a requirement to consider the domain
   search order presented where multiple associated name spaces exist.
   This also implies that a domain discovery function is desirable.





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   It may be the case that not all devices in the homenet are made
   available by name via an Internet name space, and that a 'split view'
   (as described in [RFC6950], Section 4) is preferred for certain
   devices, whereby devices inside the homenet see different DNS
   responses to those outside.

   Finally, this document makes no assumption about the presence or
   omission of a reverse lookup service.  There is an argument that it
   may be useful for presenting logging information to users with
   meaningful device names rather than literal addresses.  There are
   also some services, most notably email mail exchangers, where some
   operators have chosen to require a valid reverse lookup before
   accepting connections.

3.7.5.  Independent Operation

   Name resolution and service discovery for reachable devices must
   continue to function if the local network is disconnected from the
   global Internet, e.g., a local media server should still be available
   even if the Internet link is down for an extended period.  This
   implies that the local network should also be able to perform a
   complete restart in the absence of external connectivity and have
   local naming and service discovery operate correctly.

   As described above, the approach of a local authoritative name
   service with a cache would allow local operation for sustained ISP
   outages.

   Having an independent local trust anchor is desirable, to support
   secure exchanges should external connectivity be unavailable.

   A change in ISP should not affect local naming and service discovery.
   However, if the homenet uses a global name space provided by the ISP,
   then this will obviously have an impact if the user changes their
   network provider.

3.7.6.  Considerations for LLNs

   In some parts of the homenet, in particular LLNs or any devices where
   battery power is used, devices may be sleeping, in which case a proxy
   for such nodes may be required that could respond (for example) to
   multicast service discovery requests.  Those same devices or parts of
   the network may have less capacity for multicast traffic that may be
   flooded from other parts of the network.  In general, message
   utilisation should be efficient considering the network technologies
   and constrained devices that the service may need to operate over.





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   There are efforts underway to determine naming and discovery
   solutions for use by the Constrained Application Protocol (CoAP)
   [RFC7252] in LLN networks.  These are outside the scope of this
   document.

3.7.7.  DNS Resolver Discovery

   Automatic discovery of a name service to allow client devices in the
   homenet to resolve external domains on the Internet is required, and
   such discovery must support clients that may be a number of router
   hops away from the name service.  Similarly, it may be desirable to
   convey any DNS domain search list that may be in effect for the
   homenet.

3.7.8.  Devices Roaming to/from the Homenet

   It is likely that some devices that have registered names within the
   homenet Internet name space and that are mobile will attach to the
   Internet at other locations and acquire an IP address at those
   locations.  Devices may move between different homenets.  In such
   cases, it is desirable that devices may be accessed by the same name
   as is used in their home network.

   Solutions to this problem are not discussed in this document.  They
   may include the use of Mobile IPv6 or Dynamic DNS -- either of which
   would put additional requirements on the homenet -- or establishment
   of a (VPN) tunnel to a server in the home network.

3.8.  Other Considerations

   This section discusses two other considerations for home networking
   that the architecture should not preclude but that this text is
   neutral towards.

3.8.1.  Quality of Service

   Support for Quality of Service (QoS) in a multi-service homenet may
   be a requirement, e.g., for a critical system (perhaps health care
   related) or for differentiation between different types of traffic
   (file sharing, cloud storage, live streaming, Voice over IP (VoIP),
   etc).  Different link media types may have different such properties
   or capabilities.

   However, homenet scenarios should require no new QoS protocols.  A
   Diffserv [RFC2475] approach with a small number of predefined traffic
   classes may generally be sufficient, though at present there is
   little experience of QoS deployment in home networks.  It is likely
   that QoS, or traffic prioritisation, methods will be required at the



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   CE router and potentially around boundaries between different link
   media types (where, for example, some traffic may simply not be
   appropriate for some media and need to be dropped to avoid
   overloading the constrained media).

   There may also be complementary mechanisms that could be beneficial
   to application performance and behaviour in the homenet domain, such
   as ensuring proper buffering algorithms are used as described in
   [Gettys11].

3.8.2.  Operations and Management

   In this section, we briefly review some initial considerations for
   operations and management in the type of homenet described in this
   document.  It is expected that a separate document will define an
   appropriate operations and management framework for such homenets.

   As described in this document, the homenet should have the general
   goal of being self-organising and self-configuring from the network-
   layer perspective, e.g., prefixes should be able to be assigned to
   router interfaces.  Further, applications running on devices should
   be able to use zero-configuration service discovery protocols to
   discover services of interest to the home user.  In contrast, a home
   user would not be expected, for example, to have to assign prefixes
   to links or manage the DNS entries for the home network.  Such expert
   operation should not be precluded, but it is not the norm.

   The user may still be required to, or wish to, perform some
   configuration of the network and the devices on it.  Examples might
   include entering a security key to enable access to their wireless
   network or choosing to give a 'friendly name' to a device presented
   to them through service discovery.  Configuration of link- and
   application-layer services is out of scope of this architectural
   principles document but is likely to be required in an operational
   homenet.

   While not being expected to actively configure the networking
   elements of their homenet, users may be interested in being able to
   view the status of their networks and the devices connected to it, in
   which case appropriate network monitoring protocols will be required
   to allow them to view their network, and its status, e.g., via a web
   interface or equivalent.  While the user may not understand how the
   network operates, it is reasonable to assume they are interested in
   understanding what faults or problems may exist on it.  Such
   monitoring may extend to other devices on the network, e.g., storage
   devices or web cameras, but such devices are beyond the scope of this
   document.




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   It may also be the case that an ISP, or a third party, might wish to
   offer a remote management service for the homenet on behalf of the
   user, or to be able to assist the user in the event of some problem
   they are experiencing, in which case appropriate management and
   monitoring protocols would be required.

   Specifying the required protocols to facilitate homenet management
   and monitoring is out of scope of this document.  As stated above, it
   is expected that a separate document will be produced to describe the
   operations and management framework for the types of home networks
   presented in this document.

   As a final point, we note that it is desirable that all network
   management and monitoring functions should be available over IPv6
   transport, even where the homenet is dual stack.

3.9.  Implementing the Architecture on IPv6

   This architecture text encourages reuse of existing protocols.  Thus,
   the necessary mechanisms are largely already part of the IPv6
   protocol set and common implementations, though there are some
   exceptions.

   For automatic routing, it is expected that solutions can be found
   based on existing protocols.  Some relatively smaller updates are
   likely to be required, e.g., a new mechanism may be needed in order
   to turn a selected protocol on by default, or a mechanism may be
   required to automatically assign prefixes to links within the
   homenet.

   Some functionality, if required by the architecture, may need more
   significant changes or require development of new protocols, e.g.,
   support for multihoming with multiple exit routers would likely
   require extensions to support source and destination address-based
   routing within the homenet.

   Some protocol changes are, however, required in the architecture,
   e.g., for name resolution and service discovery, extensions to
   existing zero-configuration link-local name resolution protocols are
   needed to enable them to work across subnets, within the scope of the
   home network site.

   Some of the hardest problems in developing solutions for home
   networking IPv6 architectures include discovering the right borders
   where the 'home' domain ends and the service provider domain begins,
   deciding whether some of the necessary discovery mechanism extensions
   should affect only the network infrastructure or also hosts, and the




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   ability to turn on routing, prefix delegation, and other functions in
   a backwards-compatible manner.

4.  Conclusions

   This text defines principles and requirements for a homenet
   architecture.  The principles and requirements documented here should
   be observed by any future texts describing homenet protocols for
   routing, prefix management, security, naming, or service discovery.

5.  Security Considerations

   Security considerations for the homenet architecture are discussed in
   Section 3.6 above.

6.  References

6.1.  Normative References

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998,
              <http://www.rfc-editor.org/info/rfc2460>.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003, <http://www.rfc-editor.org/info/rfc3633>.

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

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

6.2.  Informative References

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets", BCP
              5, RFC 1918, February 1996,
              <http://www.rfc-editor.org/info/rfc1918>.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, December 1998,
              <http://www.rfc-editor.org/info/rfc2475>.





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   [RFC2775]  Carpenter, B., "Internet Transparency", RFC 2775, February
              2000, <http://www.rfc-editor.org/info/rfc2775>.

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

   [RFC3002]  Mitzel, D., "Overview of 2000 IAB Wireless Internetworking
              Workshop", RFC 3002, December 2000,
              <http://www.rfc-editor.org/info/rfc3002>.

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022, January
              2001, <http://www.rfc-editor.org/info/rfc3022>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, March 2005,
              <http://www.rfc-editor.org/info/rfc4033>.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, November 2005,
              <http://www.rfc-editor.org/info/rfc4191>.

   [RFC4192]  Baker, F., Lear, E., and R. Droms, "Procedures for
              Renumbering an IPv6 Network without a Flag Day", RFC 4192,
              September 2005, <http://www.rfc-editor.org/info/rfc4192>.

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, August 2006,
              <http://www.rfc-editor.org/info/rfc4607>.

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

   [RFC4864]  Van de Velde, G., Hain, T., Droms, R., Carpenter, B., and
              E. Klein, "Local Network Protection for IPv6", RFC 4864,
              May 2007, <http://www.rfc-editor.org/info/rfc4864>.

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






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   [RFC5533]  Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
              Shim Protocol for IPv6", RFC 5533, June 2009,
              <http://www.rfc-editor.org/info/rfc5533>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010,
              <http://www.rfc-editor.org/info/rfc5890>.

   [RFC5969]  Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd) -- Protocol Specification", RFC
              5969, August 2010,
              <http://www.rfc-editor.org/info/rfc5969>.

   [RFC6092]  Woodyatt, J., "Recommended Simple Security Capabilities in
              Customer Premises Equipment (CPE) for Providing
              Residential IPv6 Internet Service", RFC 6092, January
              2011, <http://www.rfc-editor.org/info/rfc6092>.

   [RFC6144]  Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
              IPv4/IPv6 Translation", RFC 6144, April 2011,
              <http://www.rfc-editor.org/info/rfc6144>.

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, April 2011,
              <http://www.rfc-editor.org/info/rfc6145>.

   [RFC6177]  Narten, T., Huston, G., and L. Roberts, "IPv6 Address
              Assignment to End Sites", BCP 157, RFC 6177, March 2011,
              <http://www.rfc-editor.org/info/rfc6177>.

   [RFC6204]  Singh, H., Beebee, W., Donley, C., Stark, B., and O.
              Troan, "Basic Requirements for IPv6 Customer Edge
              Routers", RFC 6204, April 2011,
              <http://www.rfc-editor.org/info/rfc6204>.

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

   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, August 2011,
              <http://www.rfc-editor.org/info/rfc6333>.

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, April 2012,
              <http://www.rfc-editor.org/info/rfc6555>.



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

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              February 2013, <http://www.rfc-editor.org/info/rfc6762>.

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, January 2013,
              <http://www.rfc-editor.org/info/rfc6824>.

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

   [RFC6950]  Peterson, J., Kolkman, O., Tschofenig, H., and B. Aboba,
              "Architectural Considerations on Application Features in
              the DNS", RFC 6950, October 2013,
              <http://www.rfc-editor.org/info/rfc6950>.

   [RFC7084]  Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
              Requirements for IPv6 Customer Edge Routers", RFC 7084,
              November 2013, <http://www.rfc-editor.org/info/rfc7084>.

   [RFC7157]  Troan, O., Miles, D., Matsushima, S., Okimoto, T., and D.
              Wing, "IPv6 Multihoming without Network Address
              Translation", RFC 7157, March 2014,
              <http://www.rfc-editor.org/info/rfc7157>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252, June 2014,
              <http://www.rfc-editor.org/info/rfc7252>.

   [IABdotless]
              IAB, "IAB Statement: Dotless Domains Considered Harmful",
              February 2013, <http://www.iab.org/documents/
              correspondence-reports-documents/2013-2/
              iab-statement-dotless-domains-considered-harmful>.

   [Gettys11]
              Gettys, J., "Bufferbloat: Dark Buffers in the Internet",
              March 2011,
              <http://www.ietf.org/proceedings/80/slides/tsvarea-1.pdf>.






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Acknowledgments

   The authors would like to thank Mikael Abrahamsson, Aamer Akhter,
   Mark Andrews, Dmitry Anipko, Ran Atkinson, Fred Baker, Ray Bellis,
   Teco Boot, John Brzozowski, Cameron Byrne, Brian Carpenter, Stuart
   Cheshire, Julius Chroboczek, Lorenzo Colitti, Robert Cragie, Elwyn
   Davies, Ralph Droms, Lars Eggert, Jim Gettys, Olafur Gudmundsson,
   Wassim Haddad, Joel M. Halpern, David Harrington, Lee Howard, Ray
   Hunter, Joel Jaeggli, Heather Kirksey, Ted Lemon, Acee Lindem, Kerry
   Lynn, Daniel Migault, Erik Nordmark, Michael Richardson, Mattia
   Rossi, Barbara Stark, Sander Steffann, Markus Stenberg, Don Sturek,
   Andrew Sullivan, Dave Taht, Dave Thaler, Michael Thomas, Mark
   Townsley, JP Vasseur, Curtis Villamizar, Russ White, Dan Wing, and
   James Woodyatt for their comments and contributions within homenet WG
   meetings and on the WG mailing list.  An acknowledgment generally
   means that a person's text made it into the document or was helpful
   in clarifying or reinforcing an aspect of the document.  It does not
   imply that each contributor agrees with every point in the document.

































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Authors' Addresses

   Tim Chown (editor)
   University of Southampton
   Highfield
   Southampton, Hampshire  SO17 1BJ
   United Kingdom

   EMail: tjc@ecs.soton.ac.uk


   Jari Arkko
   Ericsson
   Jorvas  02420
   Finland

   EMail: jari.arkko@piuha.net


   Anders Brandt
   Sigma Designs
   Emdrupvej 26A, 1
   Copenhagen  DK-2100
   Denmark

   EMail: anders_brandt@sigmadesigns.com


   Ole Troan
   Cisco Systems, Inc.
   Philip Pedersensvei 1
   Lysaker,  N-1325
   Norway

   EMail: ot@cisco.com


   Jason Weil
   Time Warner Cable
   13820 Sunrise Valley Drive
   Herndon, VA  20171
   United States

   EMail: jason.weil@twcable.com







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