Connecting Stub Networks to Existing Infrastructure
draft-lemon-stub-networks-00

Internet Engineering Task Force                                 T. Lemon
Internet-Draft                                                Apple Inc.
Intended status: Best Current Practice                  22 February 2021
Expires: 26 August 2021

          Connecting Stub Networks to Existing Infrastructure
                      draft-lemon-stub-networks-00

Abstract

   This document describes a set of practices for connecting stub
   networks to adjacent infrastructure networks, as well as to larger
   network fabrics.  This is applicable in cases such as constrained
   (Internet of Things) networks where there is a need to provide
   functional parity of service discovery and reachability between
   devices on the stub network and devices on an adjacent infrastructure
   link (for example, a home network).  The stub networks use case is
   intended to fully address the need to attach a single network link to
   an infrastructure network, where the attached link provides no
   through routing and in cases where integration to the infrastructure
   routing fabric (if any) is not available.

Status of This Memo

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   This Internet-Draft will expire on 26 August 2021.

Copyright Notice

   Copyright (c) 2021 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 (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Glossary  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Support for adjacent infrastructure links . . . . . . . . . .   4
     3.1.  Managing addressability on the adjacent infrastructure
           link  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
       3.1.1.  IP addressability already present on adjacent
               infrastructure link . . . . . . . . . . . . . . . . .   4
       3.1.2.  IP addressability not present on adjacent
               infrastructure link . . . . . . . . . . . . . . . . .   5
       3.1.3.  Handling the presence of multiple stub routers  . . .   6
     3.2.  Managing addressability on the stub network . . . . . . .   6
       3.2.1.  Maintenance across stub router restarts . . . . . . .   7
       3.2.2.  Generating a ULA prefix to provide addressability . .   8
     3.3.  Managing reachability on the adjacent infrastructure
           link  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Managing reachability on the stub network . . . . . . . .   8
     3.5.  Providing discoverability of stub network hosts on the
           adjacent infrastructure link  . . . . . . . . . . . . . .   9
     3.6.  Providing discoverability of adjacent infrastructure hosts
           on the stub network . . . . . . . . . . . . . . . . . . .  11
   4.  Providing reachability to IPv4 services to the stub
           network . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  NAT64 provided by infrastructure  . . . . . . . . . . . .  11
     4.2.  NAT64 provided by stub router(s)  . . . . . . . . . . . .  12
   5.  Handling partitioning events on a stub network  . . . . . . .  13
   6.  Support for non-adjacent links  . . . . . . . . . . . . . . .  13
     6.1.  Acquiring an off-stub-network-routable prefix for the stub
           network . . . . . . . . . . . . . . . . . . . . . . . . .  14
     6.2.  Arranging for routing to a stub network's off-mesh routable
           prefix  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     6.3.  Making service advertisements available on non-adjacent
           infrastructure  . . . . . . . . . . . . . . . . . . . . .  15
     6.4.  Making service advertisements available on the
           internet  . . . . . . . . . . . . . . . . . . . . . . . .  15
     6.5.  Distinction between non-adjacent infrastructure and global
           internet connectivity . . . . . . . . . . . . . . . . . .  15
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .  16
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  16

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1.  Introduction

   This document describes a set of practices for connecting stub
   networks to adjacent infrastructure networks, as well as to larger
   network fabrics.  This is applicable in cases such as constrained
   (Internet of Things) networks where there is a need to provide
   functional parity of service discovery and reachability between
   devices on the stub network and devices on an adjacent infrastructure
   link (for example, a home network).

   The stub networks use case is intended to fully address the need to
   attach a single network link to an infrastructure network, where the
   attached link provides no through routing and in cases where
   integration to the infrastructure routing fabric (if any) is not
   available.

2.  Glossary

   Addressability  The ability to associate each node on a link with its
      own IPv6 address.

   Reachability  Given an IPv6 destination address that is not on-link
      for any link to which a node is attached, the information required
      that allows the node to send packets to a router that can forward
      those packets towards a link where the destination address is on-
      link.

   Infrastructure network  the network infrastructure to which a stub
      router connects.  This network can be a single link, or a network
      of links.  The network may also provide some services, such as a
      DNS resolver, a DHCPv4 server, and a DHCPv6 prefix delegation
      server, for example.

   Infrastructure link  any link in a network infrastructure that is
      managed by a single entity.

   Adjacent infrastructure link (AIL)  an infrastructure link to which a
      stub router is directly connected.

   Non-adjacent infrastructure link (NAIL)  an infrastructure link to
      which a stub router is not directly connected.

   Non-adjacent link (NAL)  any link to which the stub router is not
      directly connected, whether within an infrastructure or elsewhere
      on the Internet.

   Off-Stub-Network-Routable (OSNR) Prefix  a prefix advertised on the

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      stub network that can be used for communication with hosts not on
      the stub network.

3.  Support for adjacent infrastructure links

   We assume that adjacent infrastructure link supports Router and
   Prefix Discovery using router advertisements.  Adjacent
   infrastructure links on networks where this is not supported are out
   of scope for this document.

3.1.  Managing addressability on the adjacent infrastructure link

   In order to provide IPv6 routing to the stub network, IPv6 addressing
   must be available on the adjacent infrastructure link.  In the ideal
   case, such addressing is already present on the link, and need not be
   provided.  In this case, the stub router SHOULD NOT provide
   addressability on the adjacent infrastructure link.

3.1.1.  IP addressability already present on adjacent infrastructure
        link

   IPv6 addressing is considered to be present on the link if a usable
   on-link prefix is advertised on the adjacent infrastructure link.  A
   usable on-link prefix could be a prefix advertised on the link that
   is on-link and allows autonomous configuration.  A prefix is also a
   usable on-link prefix if it is advertised on the link as on-link, and
   if the 'm' bit is set in the Router Advertisement message header
   ([RFC4861], Section 4.2) that contains the Prefix option.  This
   indicates that node addressibility is being managed using DHCPv6.

   A prefix is advertised on the link if, when a Router Solicit message
   ([RFC4861], Section 4.1) is sent, a Router Advertisement message is
   received in response which contains a prefix information option
   ([RFC4861], Section 4.6.2) for that prefix.

   After such an RA message has been received, it can be assumed for
   some period of time thereafter that the prefix is still valid on the
   link.  However, prefix lifetimes and router lifetimes are often quite
   long.  The mere fact that a prefix that has been advertised is still
   within its valid lifetime does not mean that that prefix is still
   being advertised on the link.

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   This is important because when a new host appears on the adjacent
   infrastructure link and sends an initial router solicit, if it does
   not receive a usable on-link prefix, it will not be able to
   communicate.  Consequently, the stub router MUST monitor router
   solicits and advertisements on the link in order to determine whether
   a prefix that has been advertised on the link is still being
   advertised.

   There are several methods that can be used to accomplish this:

   The stub router MUST listen for router advertisements on the adjacent
   infrastructure link, and record the time at which each router
   advertisement was received.  A router advertisement that is more than
   STALE_RA_TIME seconds old MUST be assumed to no longer be advertised
   on the link.  When the last non-stale router advertisement containing
   a usable prefixes on the link is marked stale, the stub router should
   begin router discovery.

   The stub router MUST listen for router solicits on the adjacent
   infrastructure link.  When a router solicit is received, the router
   SHOULD set a timer for VICARIOUS_SOLICIT_TIME seconds.  If, after
   that amount of time, no router advertisements are received that
   contain a usable on-link prefix, the stub router MUST begin router
   discovery.  This is necessary in case the response to the router
   solicit was unicast, since in this case the stub router would not see
   that response.  When the stub router first connects to the adjacent
   infrastructure link, it MUST begin router discovery.

   When router discovery completes, the stub router evaluates whether or
   not a usable on-link prefix has been seen in a non-stale router
   advertisement during router discovery.  If no usable on-link prefix
   has been seen, then the stub router MUST begin to provide a usable
   on-link prefix.

3.1.2.  IP addressability not present on adjacent infrastructure link

   When there is no usable on-link prefix on the adjacent infrastructure
   network, the stub router provides its own on-link prefix.  This
   prefix has a valid and preferred lifetime of
   STUB_PROVIDED_PREFIX_LIFETIME seconds.  This prefix MUST allow for
   autonomous configuration (SLAAC).

   The stub router must advertise this prefix every BEACON_INTERVAL
   seconds.  When the stub router is advertising reachability to the
   stub network, the on-link prefix advertisement and the route
   information advertisement must be contained in the same router
   advertisement.  IP addressability becomes present on adjacent
   infrastructure link

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   When the stub router is advertising an on-link prefix on the AIL, it
   may receive a router advertisement containing a usable on-link prefix
   for the AIL.  In this case, the stub router should begin to deprecate
   the on-link prefix it is advertising on the AIL.  The preferred
   lifetime for this prefix should be set to zero in subsequent
   advertisements.

   The valid lifetime (VALID) is computed based on three values: the
   current time when a router advertisement is being generated (NOW),
   the time at which the new usable on-link prefix advertisement was
   received (DEPRECATE_TIME), and STUB_PROVIDED_PREFIX_LIFETIME.  All of
   these values are in seconds.  VALID is computed as follows:

   VALID = STUB_PROVIDED_PREFIX_LIFETIME - (NOW - DEPRECATE_TIME)

   If VALID is less than BEACON_INTERVAL, the stub router does not
   include the deprecated prefix in the router advertisement.  Note that
   VALID could be less than zero.  Otherwise, the prefix is provided in
   the advertisement, but with a valid lifetime of VALID.

3.1.3.  Handling the presence of multiple stub routers

   When multiple stub routers are connected to the same AIL, and no
   usable on-link prefix is being provided on that link by the
   infrastructure, there will be a competition between routers to
   provide a usable on-link prefix.  In order to avoid duplication, stub
   routers MUST include a random offset in the time interval across
   which router discovery is performed.  This ensures that after a power
   failure, not all stub routers will exit router discovery at the exact
   same time, and so one stub router should advertise a usable on-link
   prefix before the others.  This should prevent the other stub routers
   from advertising additional on-link prefixes.

   There is no particular harm caused by advertising multiple on-link
   prefixes, but it is preferable to minimize this, because each on-link
   prefix consumes space in every on-link host's routing table, and
   consumes time when making source address selection and routing
   decisions.

3.2.  Managing addressability on the stub network

   How addressability is managed on stub networks depends on the nature
   of the stub network.  For some stub networks, the stub router can be
   sure that it is the only router.  For example, a stub router that is
   providing a Wi-Fi network for tethering will advertise its own SSID
   and use its own joining credentials; in this case, it can assume that
   it is the only router for that network, and advertise a default route
   and on-link prefix just like any other router.

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   However, some stub networks are more cooperative in nature, for
   example IP mesh networks.  On such networks, multiple stub routers
   may be present and be providing addressability and reachability.

   In either case, some stub router connected to the stub network MUST
   provide a usable on-link prefix (the OSNR prefix) for the stub
   network.  If the stub network is a multicast-capable medium where
   Router Advertisements are used for router discovery, the same
   mechanism described in section [Support for adjacent infrastructure
   links] is used.

   Stub networks that do not support the use of Router Advertisements
   for router discovery must use some similar mechanism that is
   compatible with that type of network.  Describing the process of
   establishing a common OSNR prefix on such networks is out of scope
   for this document.

3.2.1.  Maintenance across stub router restarts

   Stub routers may restart from time to time; when a restart occurs,
   the stub router may have been advertising state to the network which,
   following the restart, is no longer required.

   For example, suppose there are two stub routers connected to the same
   infrastructure link.  When the first stub router is restarted, the
   second takes over providing an on-link prefix.  Now the first router
   rejoins the link.  It sees that the second stub router's prefix is
   advertised on the infrastructure link, and therefore does not
   advertise its own.

   This behavior can cause problems because the first stub router no
   longer sees the on-link prefix it had been advertising on
   infrastructure as on-link.  Consequently, if it receives a packet to
   forward to such an address, it will forward that packet directly to a
   default router, if one is present; otherwise, it will have no route
   to the destination, and will drop the packet.

   To address this problem, stub routers SHOULD remember the last time a
   prefix was advertised across restarts.  On restart, the router can
   immediately begin deprecating the prefix, and can stop after the
   prefix valid lifetime goes to zero, based on the recorded time that
   the last advertisement was sent.

   When a stub router has only flash memory with limited write lifetime,
   it may be inappropriate to do a write to flash every time a prefix
   beacon happens.  In this case, the router SHOULD record the set of
   prefixes that have been advertised on infrastructure and the maximum
   valid lifetime that was advertised.  On restart, the router should

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   assume that hosts on the infrastructure link have received
   advertisements for any such prefixes, and should immediately
   deprecate them, and continue to do so until the maximum valid
   lifetime has elapsed after restart.

3.2.2.  Generating a ULA prefix to provide addressability

   In order to be able to provide addressability either on the stub
   network or on an adjacent infrastructure network, a stub router must
   allocate its own ULA prefix.  ULA prefixes, described in Unique Local
   IPv6 Unicast Addresses ([RFC4193]) are randomly allocated prefixes.
   A stub router MUST allocate a single ULA prefix for use in providing
   on-link prefixes to the stub network and the infrastructure network,
   as needed.

   The ULA prefix allocated by a stub router SHOULD be maintained across
   reboots, and SHOULD remain stable over time.  For privacy reasons, a
   stub router that roams from network to network may wish to allocate a
   different ULA prefix each time it connects to a different
   infrastructure network.

   If IPv6 prefix delegation is available, which implies that IPv6
   service is also available on the infrastructure link, then the stub
   router MAY use IPv6 prefix delegation to acquire a prefix to
   advertise on the stub network, rather than allocating one out of its
   ULA prefix.

3.3.  Managing reachability on the adjacent infrastructure link

   Stub routers MUST advertise reachability to stub network OSNR
   prefixes on any AIL to which they are connected.

   Each stub network will have some set of prefixes that are advertised
   as on-link for that network.  A stub router connected to that network
   SHOULD advertise reachability to all such links on any AIL to which
   it is attached using router advertisements

3.4.  Managing reachability on the stub network

   The stub router MAY advertise itself as a default router on the stub
   network, if it itself has a default route on the AIL.  In some cases
   it may not be desirable to advertise reachability to the Internet as
   a whole; in this case the stub router need not advertise itself as a
   default router.

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   If the stub router is not advertising itself as a default on the stub
   network, it MUST advertise reachability to any prefixes that are
   being advertised as on-link on AILs to which it is attached.  This is
   true for prefixes it is advertising, and for other prefixes being
   advertised on that link.

   Note that in some stub network configurations, it is possible for
   more than one stub router to be connected to the stub network, and
   each stub router may be connected to a different AIL.  In this case,
   a stub router advertising a default route may receive a packet
   destined for a link that is not an AIL for that router, but is an AIL
   for a different router.  In such a case, if the infrastructure is not
   capable of routing between these two AILs, a packet which could have
   been delivered by another stub router will be lost by the stub router
   that received it.

   Consequently, stub routers SHOULD be configurable to not advertise
   themselves as default routers on the stub network.  Stub routers
   SHOULD be configurable to explicitly advertise AIL prefixes on the
   stub network even if they are advertising as a default router.  Stub
   routers SHOULD be configurable to advertise NAIL prefixes on the stub
   network; such configuration would include a list of NAIL prefixes to
   advertise.  This list may be configured in a management interface or
   as a result of these routes being delivered in a routing protocol or
   through router discovery.  The mechanisms by which such configuration
   can be accomplished are out of scope for this document.

3.5.  Providing discoverability of stub network hosts on the adjacent
      infrastructure link

   In some cases it will be necessary for hosts on the adjacent
   infrastructure link to be able to discover devices on the stub
   network.  In other cases, this will be unnecessary or even
   undesirable.  For example, it may be undesirable for devices on an
   adjacent infrastructure link to be able to discovery devices on a
   WiFi tether, for example provided by a mobile phone.

   One example of a use case for stub networks where such discovery is
   desirable is the constrained network use case.  In this case a low-
   power, low-cost stub network provides connectivity for devices that
   provide services to the infrastructure.  For such networks, it is
   necessary that devices on the infrastructure be able to discover
   devices on the stub network.

   The most basic use case for this is to provide feature parity with
   existing solutions like multicast DNS (mDNS).  For example, a light
   bulb with built-in WiFi connectivity might be discoverable on the
   infrastructure link to which it is connected, using mDNS, but likely

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   is not discoverable on other links.  To provide equivalent
   functionality for an equivalent device on a constrained network that
   is a stub network, the stub network device must be discoverable on
   the infrastructure link (which is an AIL from the perspective of the
   stub network) using mDNS.

   Stub network routers cannot be assumed to be able to integrate into
   the DNS naming hierarchy of the infrastructure network.  Therefore,
   stub networks are forced to rely on ad-hoc service advertisement
   protocols.  Since mDNS is in wide use, this is a suitable protocol
   for use in this context.  This is not to say that mDNS is the only
   such protocol that could be used, but it is the one that we suggest
   implementing.

   In order to provide mDNS discovery for devices on the stub network,
   one of two solutions is likely to be applicable, depending on the
   operational practicalities of the stub network.  For a constrained
   stub network, on which battery operated devices may be attached, mass
   multicast traffic for service discovery is impractical, since every
   device needs to wake up for every service discovery, even if they
   don't offer that service, and since many such devices may be
   operating on battery power.  For such a network, multicast DNS is not
   a good choice.

   For such networks, a unicast service registration protocol such as
   DNS-SD SRP is a good solution.  The stub router can act as an SRP
   server on the stub network, accepting service advertisements from
   stub network devices.  On the adjacent infrastructure network, it can
   advertise those services as multicast DNS Advertising Proxy.

   For other stub networks, for example a WiFi-based Personal Area
   Network provided as part of a tethering function on a mobile device,
   multicast DNS may be the only option.  For WiFi stub networks, there
   is such a large installed base of devices supporting mDNS that
   requiring some other service advertisement solution would be
   problematic simply because it would require new software for that
   entire installed base.  For other networks, particularly constrained
   networks, where devices do not currently support mDNS, no such
   obstacle exists.

   Because the primary use case for discovery of devices on a stub
   network is the use case where the stub network is joining a
   constrained network to an existing infrastructure link, we currently
   only describe a solution (DNS-SD SRP) for that use case.  A solution
   for the use case where the stub router must provide discoverability
   for a stub network where mDNS advertising is preferred is out of
   scope for this document.

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3.6.  Providing discoverability of adjacent infrastructure hosts on the
      stub network

   Hosts on the stub network may need to discover hosts on the adjacent
   infrastructure network.  In the IoT network example we've been using,
   there might be a light switch on the stub network which needs to be
   able to actuate a light bulb connected to the adjacent infrastructure
   network.  In order to know where to send the actuation messages, the
   light switch will need to be able to discover the light bulb's
   address somehow.

   In the case of a WiFi stub network, devices on the stub network will
   need to be able to access the Internet, and may also need to be able
   to access local services on the adjacent infrastructure link.

   In order to address these use cases, the stub network router SHOULD
   provide a DNS-SD Discovery Proxy and a DNS resolver.  Since these two
   functions are combined, if the stub router provides them, it MUST
   offer both services on the standard DNS UDP and TCP ports.

4.  Providing reachability to IPv4 services to the stub network

4.1.  NAT64 provided by infrastructure

   Stub networks are defined to be IPv6-only because it would be
   difficult to implement a stub network using IPv4 technology.
   However, stub network devices may need to be able to communicate with
   IPv4-only services either on the adjacent infrastructure, or on the
   global internet.  Ideally, the infrastructure network fully supports
   IPv6, and all services on the infrastructure network are
   IPv6-capable.  In this case, perhaps the infrastructure network
   provides NAT64 service to IPv4-only hosts on the internet.  In this
   ideal setting, the stub router need do nothing-the infrastructure
   network is doing it all.

   In this situation, if there are multiple stub routers, each connected
   to the same adjacent infrastructure link, there is no need for
   special behavior-each stub router can advertise a default route, and
   any stub router will do to route NAT64 traffix.  If some stub routers
   are connected to different adjacent infrastructure links than others,
   some of which support NAT64 and some of which do not, then the
   default route may not carry traffic to the correct link for NAT64
   service.  In this case, a more specific address to the infrastructure
   NAT64 prefix(es) MUST be advertised by those stub routers that are
   able to discover it.

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4.2.  NAT64 provided by stub router(s)

   Most infrastructure networks at present do not provide NAT64 service.
   It is therefore necessary for stub routers to be able to provide
   NAT64 service if IPv4 hosts are to be reachable from the stub
   network.

   To provide NAT64 service, a stub router must allocate a NAT64 prefix.
   For convenience, the stub network allocates a single prefix out of
   the /48 ULA prefix that it maintains.  Out of the 2^16 possible
   subnets of the /48, the stub router SHOULD use the numerically
   highest /64 prefix.

   If there are multiple stub routers providing connectivity between the
   stub network and infrastructure, each stub network uses its own NAT64
   prefix-there is no common NAT64 prefix.  The reason for this is that
   NAT64 translation is not stateless, and is tied to the stub router's
   IPv4 address.  Therefore each NAT64 egress is not equivalent.

   A stub network that services a WFfi stub network SHOULD provide DNS64
   translation: hosts on the stub network cannot be assumed to be able
   to do DNS64 synthesis in the stub resolver.  In this case the DNS
   resolver on the stub router MUST honor the CD and DO bits if received
   in a request, since this indicates that the stub resolver on the
   requestor intends to do DNSSEC validation.  In this case, the
   resolver on the stub router MUST NOT perform DNS64 synthesis.

   On specific stub networks it may be desirable to require the stub
   network device to perform DNS64 synthesis.  Stub network routers for
   such networks do not need to provide DNS64 synthesis.  Instead, they
   MUST provide an ipv4only.arpa answer that advertises the NAT64 prefix
   for that stub router, and MUST provide an explicit route to that
   NAT64 prefix on the stub network using RA or whatever technology is
   specific to that stub network type.

   In constrained networks it can be very useful if stub network
   resolvers provide the information required to do DNS64 translation in
   the answer to the AAAA query.  If the answer to an AAAA query comes
   back with "no data" (not NXDOMAIN), this suggests that there may be
   an A record.  In this case, the stub network's resolver SHOULD
   attempt to look up an A record on the same name.  If such a record
   exists, the resolver SHOULD return no data in the Answer section of
   the DNS response, and SHOULD provide any CNAME records that were
   involved in returning the "no data" answer to the AAAA query, and
   SHOULD provide any A records that were ultimately returned, in the
   Additional section.  The resolver should also include an
   ipv4only.arpa record in the Additional section.

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5.  Handling partitioning events on a stub network

   If a stub network is constructed using mesh technology, it may become
   partitioned.  In such a situation, it may be one stub router is
   connected to one partition, and another stub router is connected to
   the other partition.  In this situation, in order for all nodes to be
   reachable, it is necessary that each partition of the stub network
   have its own prefix.  When such a partition occurs, the stub routers
   must detect that it has occurred.  If a stub router is currently
   providing a prefix on the stub network, it need take no action.  If a
   stub router had not been providing a prefix on the stub network, and
   now discovers that there is no stub router providing a prefix on the
   network, it MUST begin to provide its own prefix on the stub network.
   It MUST also advertise reachability to that new prefix on its
   adjacent infrastructure link(s).

   When partitions of this type occur, they may also heal.  When a
   partition heals in a situation where two stub routers have both been
   advertising a prefix, it will now appear that there are two prefixes
   on the stub network.  Since partition events may represent a
   recurring situation, stub routers SHOULD wait for at least
   PARTITION_HEAL_WAIT_TIME before deprecating one of these prefixes.

6.  Support for non-adjacent links

   There are two ways that connectivity to non-adjacent links can be
   effected.  The first is that if the infrastructure network as a whole
   has a working IPv4 routing fabric, NAT64 can be used to enable hosts
   on the stub network to establish communications with hosts on non-
   adjacent links, including the Internet.  In some cases, this is all
   that is needed.

   However, if it will be necessary for nodes on non-adjacent networks
   to establish communications with nodes on the stub network, this will
   require a working IPv6 routing fabric connecting the stub network to
   any non-adjacent links from which communications will need to be
   established.

   In order for such routing to work, the stub network will also need to
   acquire a prefix that the infrastructure network is aware of and can
   route to.  The ULA prefix that can work for communicating to adjacent
   infrastructure links will not work for communicating to non-adjacent
   links.

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6.1.  Acquiring an off-stub-network-routable prefix for the stub network

   A prefix may be acquired by using DHCPv6 Prefix Delegation
   ([RFC8415], Section 6.3).  The stub router then advertises this
   prefix as the on-link prefix for the stub network, as before.  It
   also advertises reachability to this prefix using router
   advertisements, as before.

   In the case where there is more than one stub router, it would be
   best if only one stub router requested a delegated prefix.  This can
   be managed through the mechanism described earlier: the stub router
   only acquires a prefix to advertise when it has decided that it needs
   to advertise a prefix, and so in most cases only one stub router at a
   time will request a delegated prefix.

   In order to avoid excessive consumption of delegated prefixes, stub
   routers connected to stub networks that support multiple stub routers
   SHOULD request short lifetimes for delegated prefixes and renew
   frequently.  Stub routers SHOULD request a lifetime of
   PREFIX_DELEGATION_INTERVAL.  Stub routers SHOULD record the time that
   a prefix was acquired in stable storage, and SHOULD release the
   prefix using a "DHCP Release" transaction when shutting down, or when
   it determines that a prefix is no longer needed (See "graceful
   shutdown" in Figure 9 of [RFC8415] for details).  Stub routers SHOULD
   release any remembered still-valid prefix after reboot, if after
   rebooting it is discovered that another prefix is being advertised on
   the stub network.

6.2.  Arranging for routing to a stub network's off-mesh routable prefix

   We can assume that a side effect of the prefix delegation process
   will be to establish routing to the stub router that requested the
   prefix.  This should mean that any node that wishes to establish
   communication with a node on the stub network will be able to do so
   through the delegating router that provides the prefix or, if it is
   attached to an infrastructure link that is adjacent to the stub
   router, through the stub router itself by means of the router
   advertisement it is providing.

   The case of multiple stub routers is more complicated however.  Any
   routing that comes as a side-effect of DHCPv6 Prefix Delegation will
   only route through the stub router that acquired the prefix.  Other
   stub routers can provide reachability on their respective adjacent
   infrastructure links, but reachability across the full routing fabric
   of the infrastructure network will only be possible if there is some
   routing protocol present on the infrastructure network.  Addressing
   this problem is out of scope for this document.

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6.3.  Making service advertisements available on non-adjacent
      infrastructure

   In order for service advertisements to be available on non-adjacent
   infrastructure, the infrastructure must provide SRP service for
   constrained stub networks, and must advertise the availability of
   such service so that stub routers can forward SRP updates to that SRP
   service, rather than providing SRP as a local service.

   For stub networks that use multicast DNS, stub routers must provide a
   discovery proxy service, and most advertise that service to the
   infrastructure.  In turn, the infrastructure must configure that
   service to be discoverable by devices on the infrastructure, as
   described in [RFC8766], Section 6.

6.4.  Making service advertisements available on the internet

   The mechanism described previously for making service advertisements
   available to non-adjacent infrastructure also scales to the internet,
   since it uses DNS.  Indeed, the question an operator should ask
   before enabling such discovery is, do they want their stub network
   devices to be discoverable on the internet.  If it becomes possible
   to configure service advertising automatically, behavior similar to
   that specified in [RFC6092], Section 3.2 and 3.3, would be advised:
   do not automatically advertise stub network devices on the Internet.

6.5.  Distinction between non-adjacent infrastructure and global
      internet connectivity

   Stub routers may be mobile, or fixed.  That is, they may move from
   location to location along with some or all of their connected
   devices, attaching to whatever infrastructure is available.  Or they
   may be fixed devices that are only ever expected to exist in one
   particular location.

   For devices that are intended to be in a fixed location, the
   distinction between infrastructure links and the internet as a whole
   is meaningful; for mobile nodes it most likely is not, unless such a
   node is only going to ever attach to trusted infrastructure as it
   moves from location to location-not a common scenario.

   For fixed links, the infrastructure may be trusted, in which case the
   distinction between infrastructure and internet can be expected to be
   managed by the infrastructure, and therefore only visible to the stub
   router in the sense that some non-adjacent destinations may be
   reachable (infrastructure destinations, for example) while others are
   not.

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   The reason for mentioning this here is to point out that the stub
   router can't be expected to manage this interface: it is up to the
   infrastructure network to do so, either implicitly or explicitly.
   [RFC7084] provides a set of default behaviors for home routers that
   may be adequate for automatically managing this interface, but
   further work in this area may be warranted.

7.  Normative References

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

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

   [RFC6092]  Woodyatt, J., Ed., "Recommended Simple Security
              Capabilities in Customer Premises Equipment (CPE) for
              Providing Residential IPv6 Internet Service", RFC 6092,
              DOI 10.17487/RFC6092, January 2011,
              <https://www.rfc-editor.org/info/rfc6092>.

   [RFC7084]  Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
              Requirements for IPv6 Customer Edge Routers", RFC 7084,
              DOI 10.17487/RFC7084, November 2013,
              <https://www.rfc-editor.org/info/rfc7084>.

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

   [RFC8766]  Cheshire, S., "Discovery Proxy for Multicast DNS-Based
              Service Discovery", RFC 8766, DOI 10.17487/RFC8766, June
              2020, <https://www.rfc-editor.org/info/rfc8766>.

Author's Address

   Ted Lemon
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America

   Email: mellon@fugue.com

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