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RFC 7368

 
 
 

IPv6 Home Networking Architecture Principles

Part 2 of 3, p. 11 to 32
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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.



(page 32 continued on part 3)

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