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Automatic Extended Route Optimization (AERO)
draft-templin-6man-aero-24

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Author Fred Templin
Last updated 2021-08-12 (Latest revision 2021-08-10)
Replaces draft-templin-intarea-6706bis
Replaced by draft-templin-intarea-aero
RFC stream Internet Engineering Task Force (IETF)
Formats
Additional resources
Stream WG state (None)
Document shepherd Eliot Lear
IESG IESG state I-D Exists
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD (None)
Send notices to rfc-ise@rfc-editor.org
draft-templin-6man-aero-24
#x27;s delegated MNP regardless of their
   OMNI link point of origin.  These checks are necessary to prevent
   Clients from either accidentally or intentionally establishing
   endless loops that could congest Proxy/Servers and/or ANET/INET
   links.

   Note: Proxy/Servers forward secure control plane carrier packets via
   the SRT secured spanning tree and forward other carrier packets via
   the unsecured spanning tree.  When a Proxy/Server receives a carrier
   packet from the secured spanning tree, it considers the message as
   authentic without having to verify upper layer authentication
   signatures.  When a Proxy/Server receives a carrier packet from the
   unsecured spanning tree, it verifies any upper layer authentication
   signatures and/or forwards the unsecured message toward the
   destination which must apply data origin authentication.

   Note: If the Proxy/Server has multiple original IP packets to send to
   the same neighbor, it can concatenate them in a single OAL super-
   packet [I-D.templin-6man-omni].

3.10.3.  Bridge Forwarding Algorithm

   Bridges forward spanning tree carrier packets while decrementing the
   OAL header Hop Count but not the original IP header Hop Count/TTL.
   Bridges convey carrier packets that encapsulate critical IPv6 ND
   control messages or routing protocol control messages via the secured
   spanning tree, and may convey other carrier packets via the unsecured
   spanning tree or via more direct paths according to MFIB information.
   When the Bridge receives a carrier packet, it removes the outer *NET
   header and searches for an MFIB entry that matches an MFVI or an IP
   forwarding table entry that matches the OAL destination address.

   Bridges process carrier packets with OAL destinations that do not
   match their ADM-ULA or the SRT Subnet Router Anycast address in the
   same manner as for traditional IP forwarding within the OAL, i.e.,
   nodes use IP forwarding to forward packets not explicitly addressed
   to themselves.  Bridges process carrier packets with their ADM-ULA or
   the SRT Subnet Router Anycast address as the destination by first
   examining the packet for a CRH-32 header or a compressed OAL header.
   In that case, the Bridge examines the next MFVI in the carrier packet
   to locate the MFV entry in the MFIB for next hop forwarding (i.e.,
   without examining IP addresses).  When the Bridge forwards the
   carrier packet, it changes the destination address according to the

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   MFVI value for the next hop found either in the CRH-32 header or in
   the node's own MFIB.

   Bridges forward carrier packets received from a first segment via the
   SRT secured spanning tree to the next segment also via the secured
   spanning tree.  Bridges forward carrier packets received from a first
   segment via the unsecured spanning tree to the next segment also via
   the unsecured spanning tree.  Bridges use a single IPv6 routing table
   that always determines the same next hop for a given OAL destination,
   where the secured/unsecured spanning tree is determined through the
   selection of the underlying interface to be used for transmission
   (i.e., a secured tunnel or an open INET interface).

3.11.  OMNI Interface Error Handling

   When an AERO node admits an original IP packet into the OMNI
   interface, it may receive link-layer or network-layer error
   indications.  The AERO node may also receive OMNI link error
   indications in OAL-encapsulated uNA messages that include
   authentication signatures.

   A link-layer error indication is an ICMP error message generated by a
   router in the INET on the path to the neighbor or by the neighbor
   itself.  The message includes an IP header with the address of the
   node that generated the error as the source address and with the
   link-layer address of the AERO node as the destination address.

   The IP header is followed by an ICMP header that includes an error
   Type, Code and Checksum.  Valid type values include "Destination
   Unreachable", "Time Exceeded" and "Parameter Problem"
   [RFC0792][RFC4443].  (OMNI interfaces ignore link-layer IPv4
   "Fragmentation Needed" and IPv6 "Packet Too Big" messages for carrier
   packets that are no larger than the minimum/path MPS as discussed in
   Section 3.9, however these messages may provide useful hints of probe
   failures during path MPS probing.)

   The ICMP header is followed by the leading portion of the carrier
   packet that generated the error, also known as the "packet-in-error".
   For ICMPv6, [RFC4443] specifies that the packet-in-error includes:
   "As much of invoking packet as possible without the ICMPv6 packet
   exceeding the minimum IPv6 MTU" (i.e., no more than 1280 bytes).  For
   ICMPv4, [RFC0792] specifies that the packet-in-error includes:
   "Internet Header + 64 bits of Original Data Datagram", however
   [RFC1812] Section 4.3.2.3 updates this specification by stating: "the
   ICMP datagram SHOULD contain as much of the original datagram as
   possible without the length of the ICMP datagram exceeding 576
   bytes".

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   The link-layer error message format is shown in Figure 4:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~                               ~
        |    IP Header of link layer    |
        |         error message         |
        ~                               ~
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |          ICMP Header          |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
        ~                               ~   P
        |  carrier packet *NET and OAL  |   a
        |     encapsulation headers     |   c
        ~                               ~   k
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   e
        ~                               ~   t
        |  original IP packet headers   |
        |    (first-fragment only)      |   i
        ~                               ~   n
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~                               ~   e
        |    Portion of the body of     |   r
        |    the original IP packet     |   r
        |       (all fragments)         |   o
        ~                               ~   r
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---

         Figure 4: OMNI Interface Link-Layer Error Message Format

   The AERO node rules for processing these link-layer error messages
   are as follows:

   o  When an AERO node receives a link-layer Parameter Problem message,
      it processes the message the same as described as for ordinary
      ICMP errors in the normative references [RFC0792][RFC4443].

   o  When an AERO node receives persistent link-layer Time Exceeded
      messages, the IP ID field may be wrapping before earlier fragments
      awaiting reassembly have been processed.  In that case, the node
      should begin including integrity checks and/or institute rate
      limits for subsequent packets.

   o  When an AERO node receives persistent link-layer Destination
      Unreachable messages in response to carrier packets that it sends
      to one of its neighbor correspondents, the node should process the
      message as an indication that a path may be failing, and
      optionally initiate NUD over that path.  If it receives
      Destination Unreachable messages over multiple paths, the node

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      should allow future carrier packets destined to the correspondent
      to flow through a default route and re-initiate route
      optimization.

   o  When an AERO Client receives persistent link-layer Destination
      Unreachable messages in response to carrier packets that it sends
      to one of its neighbor Proxy/Servers, the Client should mark the
      path as unusable and use another path.  If it receives Destination
      Unreachable messages on many or all paths, the Client should
      associate with a new Proxy/Server and release its association with
      the old Proxy/Server as specified in Section 3.16.5.

   o  When an AERO Proxy/Server receives persistent link-layer
      Destination Unreachable messages in response to carrier packets
      that it sends to one of its neighbor Clients, the Proxy/Server
      should mark the underlying path as unusable and use another
      underlying path.

   o  When an AERO Proxy/Server receives link-layer Destination
      Unreachable messages in response to a carrier packet that it sends
      to one of its permanent neighbors, it treats the messages as an
      indication that the path to the neighbor may be failing.  However,
      the dynamic routing protocol should soon reconverge and correct
      the temporary outage.

   When an AERO Bridge receives a carrier packet for which the network-
   layer destination address is covered by an MSP, the Bridge drops the
   packet if there is no more-specific routing information for the
   destination and returns an OMNI interface Destination Unreachable
   message subject to rate limiting.

   When an AERO node receives a carrier packet for which reassembly is
   currently congested, it returns an OMNI interface Packet Too Big
   (PTB) message as discussed in [I-D.templin-6man-omni] (note that the
   PTB messages could indicate either "hard" or "soft" errors).

   AERO nodes include ICMPv6 error messages intended for the OAL source
   as sub-options in the OMNI option of secured uNA messages.  When the
   OAL source receives the uNA message, it can extract the ICMPv6 error
   message enclosed in the OMNI option and either process it locally or
   translate it into a network-layer error to return to the original
   source.

3.12.  AERO Router Discovery, Prefix Delegation and Autoconfiguration

   AERO Router Discovery, Prefix Delegation and Autoconfiguration are
   coordinated as discussed in the following Sections.

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3.12.1.  AERO Service Model

   Each AERO Proxy/Server on the OMNI link is configured to facilitate
   Client prefix delegation/registration requests.  Each Proxy/Server is
   provisioned with a database of MNP-to-Client ID mappings for all
   Clients enrolled in the AERO service, as well as any information
   necessary to authenticate each Client.  The Client database is
   maintained by a central administrative authority for the OMNI link
   and securely distributed to all Proxy/Servers, e.g., via the
   Lightweight Directory Access Protocol (LDAP) [RFC4511], via static
   configuration, etc.  Clients receive the same service regardless of
   the Proxy/Servers they select.

   Clients associate each of their underlying interfaces with a FHS
   Proxy/Server.  Each FHS Proxy/Server may locally service one or more
   of the Client's underlying interfaces, and the Client selects one
   among them to serve as the Hub Proxy/Server.  The Hub Proxy/Server is
   responsible for short-term packet forwarding, for acting as a
   mobility anchor point and for acting as an ROR for NS(AR) messages
   directed to the Client.  All of the Client's other FHS Proxy/Servers
   forward proxyed copies of RS/RA messages between the Hub Proxy/Server
   and Client without assuming the Hub role functions themselves.

   Each Client associates with a single Hub Proxy/Server at a time,
   while all FHS Proxy/Servers are candidates for providing the Hub role
   for other Clients.  An FHS Proxy/Server assumes the Hub role when it
   receives an RS message with its own ADM-LLA or link-scoped All-
   Routers multicast as the destination.  An FHS Proxy/Server assumes
   the proxy role when it receives an RS message with the ADM-LLA of
   another Proxy/Server as the destination.  (An FHS Proxy/Server can
   also assume the proxy role when it receives an RS message addressed
   to link-scoped All-Routers multicast if it can determine the ADM-LLA
   of another Proxy/Server to serve as a Hub.)

   AERO Clients and Proxy/Servers use IPv6 ND messages to maintain
   neighbor cache entries.  AERO Proxy/Servers configure their OMNI
   interfaces as advertising NBMA interfaces, and therefore send unicast
   RA messages with a short Router Lifetime value (e.g., ReachableTime
   seconds) in response to a Client's RS message.  Thereafter, Clients
   send additional RS messages to keep Proxy/Server state alive.

   AERO Clients and Hub Proxy/Servers include prefix delegation and/or
   registration parameters in RS/RA messages (see
   [I-D.templin-6man-omni]).  The IPv6 ND messages are exchanged between
   the Client and Hub Proxy/Server (via any FHS Proxy/Servers acting as
   proxies) according to the prefix management schedule required by the
   service.  If the Client knows its MNP in advance, it can employ
   prefix registration by including its MNP-LLA as the source address of

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   an RS message and with an OMNI option with valid prefix registration
   information for the MNP.  If the Hub Proxy/Server accepts the
   Client's MNP assertion, it injects the MNP into the routing system
   and establishes the necessary neighbor cache state.  If the Client
   does not have a pre-assigned MNP, it can instead employ prefix
   delegation by including the unspecified address (::) as the source
   address of an RS message and with an OMNI option with prefix
   delegation parameters to request an MNP.

   The following sections specify the Client and Proxy/Server behavior.

3.12.2.  AERO Client Behavior

   AERO Clients discover the addresses of candidate FHS Proxy/Servers by
   resolving the Potential Router List (PRL) in a similar manner as
   described in [RFC5214].  Discovery methods include static
   configuration (e.g., a flat-file map of Proxy/Server addresses and
   locations), or through an automated means such as Domain Name System
   (DNS) name resolution [RFC1035].  Alternatively, the Client can
   discover Proxy/Server addresses through a layer 2 data link login
   exchange, or through a unicast RA response to a multicast/anycast RS
   as described below.  In the absence of other information, the Client
   can resolve the DNS Fully-Qualified Domain Name (FQDN)
   "linkupnetworks.[domainname]" where "linkupnetworks" is a constant
   text string and "[domainname]" is a DNS suffix for the OMNI link
   (e.g., "example.com").

   To associate with a Hub Proxy/Server over a first underlying
   interface, the Client acts as a requesting router to request MNPs by
   preparing an RS message with prefix management parameters.  If the
   Client already knows the Proxy/Server's ADM-LLA, it includes the LLA
   as the network-layer destination address; otherwise, the Client
   includes link-scoped All-Routers multicast as the network-layer
   destination.  The Client can use its MNP-LLA as the network-layer
   source address and include an OMNI option with prefix registration
   information.  If the Client does not yet have an MNP-LLA, it instead
   sets the network-layer source address to unspecified (::) and
   includes prefix delegation parameters in the OMNI option (see:
   [I-D.templin-6man-omni]).

   The Client next includes an authentication sub-option if necessary
   and Multilink Forwarding Parameters corresponding to the underlying
   interface over which it will send the RS message.  Next, the Client
   submits the RS for OAL encapsulation and fragmentation if necessary
   with its own MNP-ULA and the Proxy/Server's ADM-ULA or an OMNI IPv6
   anycast address as the OAL addresses while selecting an
   Identification value and invoking window synchronization as specified
   in [I-D.templin-6man-omni].

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   The Client then sends the RS (either directly via Direct interfaces,
   via a VPN for VPNed interfaces, via an access router for ANET
   interfaces or via INET encapsulation for INET interfaces) then waits
   up to RetransTimer milliseconds for an RA message reply (see
   Section 3.12.3) (retrying up to MAX_RTR_SOLICITATIONS).  If the
   Client receives no RAs, or if it receives an RA with Router Lifetime
   set to 0, the Client SHOULD abandon attempts through the first
   candidate Hub Proxy/Server and try another FHS Proxy/Server.
   Otherwise, the Client processes the prefix information found in the
   RA message.

   When the Client processes an RA, it first performs OAL reassembly and
   decapsulation if necessary then creates a NCE with the Hub Proxy/
   Server's ADM-LLA as the network-layer address and the Hub Proxy/
   Server's encapsulation and/or link-layer addresses as the link-layer
   address.  The Client then caches the Multilink Forwarding Parameters
   information.  The Client next records the RA Router Lifetime field
   value in the NCE as the time for which the Hub Proxy/Server has
   committed to maintaining the MNP in the routing system via this
   underlying interface, and caches the other RA configuration
   information including Cur Hop Limit, M and O flags, Reachable Time
   and Retrans Timer.  The Client then autoconfigures MNP-LLAs for any
   delegated MNPs and assigns them to the OMNI interface.  The Client
   also caches any MSPs included in Route Information Options (RIOs)
   [RFC4191] as MSPs to associate with the OMNI link, and assigns the
   MTU value in the MTU option to the underlying interface.

   The Client then registers its additional underlying interfaces with
   FHS Proxy/Servers for those interfaces discovered by sending RS
   messages via each additional interface but with the ADM-LLA of the
   Hub Proxy/Server as the destination.  The additional FHS Proxy/
   Servers will assume the proxy role and forward proxyed copies of the
   RS/RA exchanges between the Client and Hub Proxy/Server.  The Client
   finally sub-delegates the MNPs to its attached EUNs and/or the
   Client's own internal virtual interfaces as described in
   [I-D.templin-v6ops-pdhost] to support the Client's downstream
   attached "Internet of Things (IoT)".  The Client then sends
   additional RS messages over each underlying interface before the
   Router Lifetime received for that interface expires.

   After the Client registers its underlying interfaces, it may wish to
   change one or more registrations, e.g., if an interface changes
   address or becomes unavailable, if traffic selectors change, etc.  To
   do so, the Client prepares an RS message to send over any available
   underlying interface as above.  The RS includes an OMNI option with
   prefix registration/delegation information and with Multilink
   Forwarding Parameters specific to the selected underlying interface.
   When the Client receives the Hub Proxy/Server's RA response, it has

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   assurance that the Proxy/Server has been updated with the new
   information.

   If the Client wishes to discontinue use of a Hub Proxy/Server it
   issues an RS message over any underlying interface with an OMNI
   option with a prefix release indication.  When the Hub Proxy/Server
   processes the message, it releases the MNP, sets the NCE state for
   the Client to DEPARTED and returns an RA reply with Router Lifetime
   set to 0.  After a short delay (e.g., 2 seconds), the Hub Proxy/
   Server withdraws the MNP from the routing system.

3.12.3.  AERO Proxy/Server Behavior

   AERO Proxy/Servers act as both IP routers and IPv6 ND proxies, and
   support a prefix delegation/registration service for Clients.  Proxy/
   Servers arrange to add their ADM-LLAs to the PRL maintained in a
   static map of Proxy/Server addresses for the link, the DNS resource
   records for the FQDN "linkupnetworks.[domainname]", etc.  before
   entering service.  The PRL should be arranged such that Clients can
   discover the addresses of Proxy/Servers that are geographically and/
   or topologically "close" to their underlying network connections.

   When an FHS Proxy/Server receives a prospective Client's RS message,
   it SHOULD return an immediate RA reply with Router Lifetime set to 0
   if it is currently too busy or otherwise unable to service the
   Client.  Otherwise, the Proxy/Server performs OAL reassembly if
   necessary, then decapsulates and authenticates the RS message.  If
   the RS message destination is link-scoped All-Routers multicast or
   the Proxy/Server's own ADM-LLA, the Proxy/Server assumes the Hub
   role.  If the RS message destination is the ADM-LLA of another node,
   the Proxy/Server assumes the proxy role and forwards the RS to the
   Hub Proxy/server via the secured spanning tree.  (An FHS Proxy/Server
   can also assume the proxy role when it receives an RS message
   addressed to link-scoped All-Routers multicast if it can determine
   the ADM-LLA of another Proxy/Server to serve as a Hub.)

   The Hub Proxy/Server then determines the correct MNPs to provide to
   the Client by processing the MNP-LLA prefix parameters and/or the
   DHCPv6 OMNI sub-option.  When the Hub Proxy/Server returns the MNPs,
   it also creates a forwarding table entry for the MNP-ULA
   corresponding to each MNP resulting in a BGP update (see:
   Section 3.2.3).  For IPv6, the Hub Proxy/Server creates an IPv6
   forwarding table entry for each MNP.  For IPv4, the Hub Proxy/Server
   creates an IPv6 forwarding table entry with the IPv4-compatibility
   MNP-ULA prefix corresponding to the IPv4 address.

   The Hub Proxy/Server next creates a NCE for the Client using the base
   MNP-LLA as the network-layer address.  Next, the Hub Proxy/Server

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   updates the NCE by recording the information in the Multilink
   Forwarding Parameters sub-option in the RS OMNI option.  The Hub
   Proxy/Server also records the actual OAL/*NET addresses and RS
   message window synchronization parameters (if any) in the NCE.

   Next, the Hub Proxy/Server prepares an RA message using its ADM-LLA
   as the network-layer source address and the network-layer source
   address of the RS message as the network-layer destination address.
   The Hub Proxy/Server sets the Router Lifetime to the time for which
   it will maintain both this underlying interface individually and the
   NCE as a whole.  The Hub Proxy/Server also sets Cur Hop Limit, M and
   O flags, Reachable Time and Retrans Timer to values appropriate for
   the OMNI link.  The Hub Proxy/Server includes the MNPs, any other
   prefix management parameters and an OMNI option with a Multilink
   Forwarding Parameters sub-option with FHS addressing information
   filled out.  The Hub Proxy/Server then includes one or more RIOs that
   encode the MSPs for the OMNI link, plus an MTU option (see
   Section 3.9).  The Hub Proxy/Server finally forwards the message to
   the Client using OAL encapsulation/fragmentation if necessary while
   including an acknowledgement if the RS invoked window
   synchronization.

   After the initial RS/RA exchange, the Hub Proxy/Server maintains a
   ReachableTime timer for each of the Client's underlying interfaces
   individually (and for the Client's NCE collectively) set to expire
   after ReachableTime seconds.  If the Client (or an FHS Proxy/Server)
   issues additional RS messages, the Hub Proxy/Server sends an RA
   response and resets ReachableTime.  If the Hub Proxy/Server receives
   an IPv6 ND message with a prefix release indication it sets the
   Client's NCE to the DEPARTED state and withdraws the MNP from the
   routing system after a short delay (e.g., 2 seconds).  If
   ReachableTime expires before a new RS is received on an individual
   underlying interface, the Hub Proxy/Server marks the interface as
   DOWN.  If ReachableTime expires before any new RS is received on any
   individual underlying interface, the Hub Proxy/Server sets the NCE
   state to STALE and sets a 10 second timer.  If the Hub Proxy/Server
   has not received a new RS or uNA message with a prefix release
   indication before the 10 second timer expires, it deletes the NCE and
   withdraws the MNP from the routing system.

   The Hub Proxy/Server processes any IPv6 ND messages pertaining to the
   Client and returns an NA/RA reply in response to solicitations.  The
   Hub Proxy/Server may also issue unsolicited RA messages, e.g., with
   reconfigure parameters to cause the Client to renegotiate its prefix
   delegation/registrations, with Router Lifetime set to 0 if it can no
   longer service this Client, etc.  Finally, If the NCE is in the
   DEPARTED state, the Hub Proxy/Server deletes the entry after
   DepartTime expires.

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   The Hub Proxy/Server may also receive carrier packets via the secured
   spanning tree that contain initial data packets sent while route
   optimization is in progress.  The Hub Proxy/Server reassembles, then
   re-encapsulates/re-fragments and forwards the packets to the target
   Client.  Although these fragments will have traversed the secured
   spanning tree, the security only assures correct reassembly and does
   not assure message content security.

   Note: Clients SHOULD notify former Hub Proxy/Servers of their
   departures, but Hub Proxy/Servers are responsible for expiring
   neighbor cache entries and withdrawing routes even if no departure
   notification is received (e.g., if the Client leaves the network
   unexpectedly).  Hub Proxy/Servers SHOULD therefore set Router
   Lifetime to ReachableTime seconds in solicited RA messages to
   minimize persistent stale cache information in the absence of Client
   departure notifications.  A short Router Lifetime also ensures that
   proactive RS/RA messaging between Clients and FHS Proxy/Servers will
   keep any NAT state alive (see above).

   Note: All Proxy/Servers on an OMNI link MUST advertise consistent
   values in the RA Cur Hop Limit, M and O flags, Reachable Time and
   Retrans Timer fields the same as for any link, since unpredictable
   behavior could result if different Proxy/Servers on the same link
   advertised different values.

3.12.3.1.  DHCPv6-Based Prefix Registration

   When a Client is not pre-provisioned with an MNP-LLA, it will need
   for the Hub Proxy/Server to select one or more MNPs on its behalf and
   set up the correct state in the AERO routing service.  (A Client with
   a pre-provisioned MNP may also request the Hub Proxy/Server to select
   additional MNPs.)  The DHCPv6 service [RFC8415] is used to support
   this requirement.

   When a Client needs to have the Hub Proxy/Server select MNPs, it
   sends an RS message with source address set to the unspecified
   address (::) and with an OMNI option that includes a DHCPv6 message
   sub-option with DHCPv6 Prefix Delegation (DHCPv6-PD) parameters.
   When the Hub Proxy/Server receives the RS message, it extracts the
   DHCPv6-PD message from the OMNI option.

   The Hub Proxy/Server then acts as a "Proxy DHCPv6 Client" in a
   message exchange with the locally-resident DHCPv6 server, which
   delegates MNPs and returns a DHCPv6-PD Reply message.  (If the Hub
   Proxy/Server wishes to defer creation of MN state until the DHCPv6-PD
   Reply is received, it can instead act as a Lightweight DHCPv6 Relay
   Agent per [RFC6221] by encapsulating the DHCPv6-PD message in a

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   Relay-forward/reply exchange with Relay Message and Interface ID
   options.)

   When the Hub Proxy/Server receives the DHCPv6-PD Reply, it adds a
   route to the routing system and creates an MNP-LLA based on the
   delegated MNP.  The Hub Proxy/Server then sends an RA back to the
   Client with the (newly-created) MNP-LLA as the destination address
   and with the DHCPv6-PD Reply message coded in the OMNI option.  When
   the Client receives the RA, it creates a default route, assigns the
   Subnet Router Anycast address and sets its MNP-LLA based on the
   delegated MNP.

   Note: See [I-D.templin-6man-omni] for an MNP delegation alternative
   that avoids including a DHCPv6 message sub-option in the RS.  Namely,
   when the Client requests a single MNP it can set the RS source to the
   unspecified address (::) and include a Node Identification sub-option
   and Preflen in the OMNI option (but with no DHCPv6 message sub-
   option).  When the Hub Proxy/Server receives the RS message, it
   forwards a self-generated DHCPv6 Solicit message to the DHCPv6 server
   on behalf of the Client.  When the Hub Proxy/Server receives the
   DHCPv6 Reply, it prepares an RA message with an OMNI option with
   Preflen information (but with no DHCPv6 message sub-option), then
   places the (newly-created) MNP-LLA in the RA destination address and
   returns the message to the Client.

3.13.  AERO Proxy/Server Coordination

   OMNI link Clients register with FHS Proxy/Servers for each underlying
   interface.  Each of the Client's FHS Proxy/Servers must inform a
   single Hub Proxy/Server of all of the Client's additional underlying
   interfaces.  For Clients on Direct and VPNed underlying interfaces,
   the FHS Proxy/Server for each interface is directly connected, for
   Clients on ANET underlying interfaces the FHS Proxy/Server is located
   on the ANET/INET boundary, and for Clients on INET underlying
   interfaces the FHS Proxy/Server is located somewhere in the connected
   Internetwork.  When FHS Proxy/Server "A" processes a Client
   registration, it must either assume the Hub role or forward a proxyed
   registration to another FHS Proxy/Server acting as the Hub.  Proxy/
   Servers satisfy these requirements as follows:

   o  when Proxy/Server "A" receives a Client RS message, it first
      verifies that the OAL Identification is within the window for the
      NCE that matches the MNP-ULA for this Client neighbor and
      authenticates the message.  (If no NCE was found, Proxy/Server "A
      instead creates one in the STALE state and returns an RA message
      with an authentication signature if necessary and any window
      synchronization parameters.)  Proxy/Server "A" then examines the
      network-layer destination address.  If the destination address is

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      the ADM-LLA of a different Proxy/Server "B", Proxy/Server "A"
      prepares a separate proxyed version of the RS message with an OAL
      header with source set to its own ADM-ULA and destination set to
      Proxy/Server B's ADM-ULA.  Proxy/Server "A" also writes its own
      information over the Multilink Forwarding Parameters sub-option
      supplied by the Client then sets the S/T-omIndex to the value for
      this Client underlying interface, then forwards the message into
      the OMNI link secured spanning tree.

   o  when Proxy/Server "B" receives the RS, it assume the Hub role and
      creates or updates a NCE for the Client with FHS Proxy/Server
      "A"'s Multilink Forwarding Parameters as the link-layer address
      information for this S/T-omIndex and caches any window
      synchronization parameters supplied by the Client.  Hub Proxy/
      Server "B" then prepares an RA message with source set to its own
      LLA and destination set to the Client's MNP-LLA, and with any
      window synchronization acknowledgements.  Hub Proxy/Server "B"
      then encapsulates the RA in an OAL header with source set to its
      own ADM-ULA and destination set to the ADM-ULA of FHS Proxy/Server
      "A, performs fragmentation if necessary, then sends the resulting
      carrier packets into the secured spanning tree.

   o  when Proxy/Server "A" reassembles the RA, it locates the Client
      NCE based on the RA destination LLA.  Proxy/Server "A" then re-
      encapsulates the RA message with OAL source set to its own ADM-ULA
      and OAL destination set to the MNP-ULA of the Client, includes an
      authentication signature if necessary, and echoes the Multilink
      Forwarding Parameters sub-option.  Proxy/Server "A" sets the P
      flag in the RA flags field to indicate that the message has passed
      through a proxy [RFC4389], then fragments the RA if necessary and
      returns the fragments to the Client.

   o  The Client repeats this process over each of its additional
      underlying interfaces while treating each FHS Proxy/Server "C",
      "D", "E", etc. as a proxy to facilitate RS/RA exchanges between
      the Hub and the Client.

   After the initial RS/RA exchanges each FHS Proxy/Server forwards any
   of the Client's carrier packets with OAL destinations for which there
   is no matching NCE to a Bridge using OAL encapsulation with its own
   ADM-ULA as the source and with destination determined by the Client.
   The Proxy/Server instead forwards any carrier packets destined to a
   neighbor cache target directly to the target according to the OAL/
   link-layer information - the process of establishing neighbor cache
   entries is specified in Section 3.14.

   While the Client is still associated with each FHS Proxy/Server "A",
   "A" can send NS, RS and/or unsolicited NA messages to update the

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   neighbor cache entries of other AERO nodes on behalf of the Client
   and/or to convey Multilink Forwarding Parameter updates.  This allows
   for higher-frequency Proxy-initiated RS/RA messaging over well-
   connected INET infrastructure supplemented by lower-frequency Client-
   initiated RS/RA messaging over constrained ANET data links.

   If the Hub Proxy/Server "A" ceases to send solicited RAs, Proxy/
   Servers "B", "C", "D" send unsolicited RAs over the Client's
   underlying interface with destination set to (link-local) All-Nodes
   multicast and with Router Lifetime set to zero to inform Clients that
   the Hub Proxy/Server has failed.  Although Proxy/Servers "B", "C" and
   "D" can engage in IPv6 ND exchanges on behalf of the Client, the
   Client can also send IPv6 ND messages on its own behalf, e.g., if it
   is in a better position to convey state changes.  The IPv6 ND
   messages sent by the Client include the Client's MNP-LLA as the
   source in order to differentiate them from the IPv6 ND messages sent
   by Proxy/Server "A".

   If the Client becomes unreachable over all underlying interface it
   serves, the Hub Proxy/Server sets the NCE state to DEPARTED and
   retains the entry for DepartTime seconds.  While the state is
   DEPARTED, the Hub Proxy/Server forwards any carrier packets destined
   to the Client to a Bridge via OAL encapsulation.  When DepartTime
   expires, the Hub Proxy/Server deletes the NCE and discards any
   further carrier packets destined to the former Client.

   In some ANETs that employ a Proxy/Server, the Client's MNP can be
   injected into the ANET routing system.  In that case, the Client can
   send original IP packets without invoking the OAL so that the ANET
   routing system transports the original IP packets to the Proxy.  This
   can be very beneficial, e.g., if the Client connects to the ANET via
   low-end data links such as some aviation wireless links.

   If the ANET first-hop access router is on the same underlying link as
   the Client and recognizes the AERO/OMNI protocol, the Client can
   avoid OAL encapsulation for both its control and data messages.  When
   the Client connects to the link, it can send an unencapsulated RS
   message with source address set to its own MNP-LLA (or to a Temporary
   LLA), and with destination address set to the ADM-LLA of the Client's
   selected Proxy/Server or to link-scoped All-Routers multicast.  The
   Client includes an OMNI option formatted as specified in
   [I-D.templin-6man-omni].  The Client then sends the unencapsulated RS
   message, which will be intercepted by the AERO-Aware access router.

   The ANET access router then performs OAL encapsulation on the RS
   message and forwards it to a Proxy/Server at the ANET/INET boundary.
   When the access router and Proxy/Server are one and the same node,
   the Proxy/Server would share and underlying link with the Client but

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   its message exchanges with outside correspondents would need to pass
   through a security gateway at the ANET/INET border.  The method for
   deploying access routers and Proxys (i.e. as a single node or
   multiple nodes) is an ANET-local administrative consideration.

   Note: When a Proxy/Server alters the IPv6 ND message contents before
   forwarding (e.g., such as altering the OMNI option contents), the
   IPv6 ND message checksum and/or authentication signature are
   invalidated.  If the Proxy/Server forwards the message over the
   secured spanning tree, however, it need not re-calculate the
   checksum/signature since they will not be examined by the next hop.

   Note: When a Proxy/Server receives a secured Client NS message, it
   performs the same proxying procedures as for described for RS
   messages above.  The proxying procedures for NS/NA message exchanges
   is specified in Section 3.14.

3.13.1.  Detecting and Responding to Proxy/Server Failures

   In environments where fast recovery from Proxy/Server failure is
   required, FHS Proxy/Servers SHOULD use proactive Neighbor
   Unreachability Detection (NUD) to track Hub Proxy/Server reachability
   in a similar fashion as for Bidirectional Forwarding Detection (BFD)
   [RFC5880].  Each FHS Proxy/Server can then quickly detect and react
   to failures so that cached information is re-established through
   alternate paths.  The NS/NA(NUD) control messaging is carried only
   over well-connected ground domain networks (i.e., and not low-end
   aeronautical radio links) and can therefore be tuned for rapid
   response.

   FHS Proxy/Servers perform continuous NS/NA(NUD) exchanges with the
   Hub Proxy/Server in rapid succession, e.g., one exchange per second.
   The FHS Proxy/Server sends the NS(NUD) message via the spanning tree
   with its own ADM-LLA as the source and the ADM-LLA of the Hub Proxy/
   Server as the destination, and the Hub Proxy/Server responds with an
   NA(NUD).  When the FHS Proxy/Server is also sending RS messages to a
   Hub Proxy/Server on behalf of Clients, the resulting RA responses can
   be considered as equivalent hints of forward progress.  This means
   that the FHS Proxy/Server need not also send a periodic NS(NUD) if it
   has already sent an RS within the same period.  If the Hub Proxy/
   Server fails (i.e., if the FHS Proxy/Server ceases to receive
   advertisements), the FHS Proxy/Server can quickly inform Clients by
   sending unsolicited RA messages

   The FHS Proxy/Server sends unsolicited RA messages with source
   address set to the Hub Proxy/Server's address, destination address
   set to (link-local) All-Nodes multicast, and Router Lifetime set to
   0.  The FHS Proxy/Server SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA

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   messages separated by small delays [RFC4861].  Any Clients that had
   been using the failed Hub Proxy/Server will receive the RA messages
   and select one of its other FHS Proxy/Servers to assume the Hub role
   (i.e., by sending an RS with destination set to the ADM-LLA of the
   new Hub).

3.14.  AERO Route Optimization

   AERO nodes invoke route optimization when they need to forward
   packets to new target destinations.  Route optimization is based on
   IPv6 ND Address Resolution messaging between a Route Optimization
   Source (ROS) and the target Client's current Hub Proxy/Server acting
   as a Route Optimization Responder (ROR).  Route optimization is
   initiated by the first eligible ROS closest to the source as follows:

   o  For Clients on VPNed and Direct interfaces, the Client's FHS
      Proxy/Server is the ROS.

   o  For Clients on ANET interfaces, either the Client or the FHS
      Proxy/Server may be the ROS.

   o  For Clients on INET interfaces, the Client itself is the ROS.

   o  For correspondent nodes on INET/EUN interfaces serviced by a
      Relay, the Relay is the ROS.

   The route optimization procedure is conducted between the ROS and the
   LHS Hub Proxy/Server/Relay for the target selected by routing as the
   ROR.  In this arrangement, the ROS is always the Client or Proxy/
   Server (or Relay) nearest the source over the selected source
   underlying interface, while the ROR is always the target's current
   Hub Proxy/Server.

   The AERO routing system directs a route optimization request sent by
   the ROS to the ROR, which returns a route optimization reply which
   must include information that is current, consistent and authentic.
   The ROS is responsible for periodically refreshing the route
   optimization, and the ROR is responsible for quickly informing the
   ROS of any changes.

   The procedures are specified in the following sections.

3.14.1.  Route Optimization Initiation

   When an original IP packet from a source node destined to a target
   node arrives, the ROS checks for a NCE with an MNP-LLA that matches
   the target destination.  If there is a NCE in the REACHABLE state,
   the ROS invokes the OAL and forwards the resulting carrier packets

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   according to the cached state then returns from processing.
   Otherwise, if there is no NCE the ROS creates one in the INCOMPLETE
   state.

   The ROS next invokes the OAL and forwards the resulting carrier
   packets into the secured spanning tree, then sends an NS message for
   Address Resolution (NS(AR)) to receive a solicited NA(AR) message
   from the ROR.  While route optimization is in progress, the ROS may
   forward additional original IP packets into the secured spanning tree
   but if so must impose rate limiting to minimize secured spanning tree
   traffic as well as ROR reassembly.

   The NS(AR) message must be sent securely, and includes:

   o  the LLA of the ROS as the source address.

   o  the MNP-LLA corresponding to the original IP packet's destination
      as the Target Address, e.g., for 2001:db8:1:2::10:2000 the Target
      Address is fe80::2001:db8:1:2.

   o  the Solicited-Node multicast address [RFC4291] formed from the
      lower 24 bits of the original IP packet's destination as the
      destination address, e.g., for 2001:db8:1:2::10:2000 the NS(AR)
      destination address is ff02:0:0:0:0:1:ff10:2000.

   The NS(AR) message also includes an OMNI option with an
   authentication sub-option if necessary and with Preflen set to the
   prefix length associated with the NS(AR) source.  The ROS then
   selects an Identification value and submits the NS(AR) message for
   OAL encapsulation with OAL source set to its own ULA and OAL
   destination set to the ULA corresponding to the target.  (The ROS
   does not include any window synchronization parameters, since it will
   not exchange other packet types with the ROR.)  The ROS then sends
   the resulting carrier packet into the SRT secured spanning tree
   without decrementing the network-layer TTL/Hop Limit field.

   When the ROS is an INET Client, it must instead forward the resulting
   carrier packet to the ADM-ULA of one of its current Proxy/Servers.
   The Proxy/Server then verifies the NS(AR) authentication signature,
   then re-encapsulates with the OAL source set to its own ADM-ULA and
   OAL destination set to the ULA corresponding to the target and
   forwards the resulting carrier packets into the secured spanning tree
   on behalf of the Client.

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3.14.2.  Relaying the NS(AR) *NET Packet(s)

   When the Bridge receives the carrier packet containing the RS from
   the ROS, it discards the *NET headers and determines the next hop by
   consulting its standard IPv6 forwarding table for the OAL header
   destination address.  The Bridge then decrements the OAL header Hop-
   Limit, then re-encapsulates and forwards the carrier packet(s) via
   the secured spanning tree the same as for any IPv6 router, where it
   may traverse multiple OMNI link segments.  The final-hop Bridge will
   deliver the carrier packet via the secured spanning tree to the ROR
   for the target.

3.14.3.  Processing the NS(AR) and Sending the NA(AR)

   When the ROR for the target receives the secured carrier packet, it
   examines the NS(AR) target to determine whether it has a matching NCE
   and/or non-MNP route.  If there is no match, the ROR drops the
   message.  Otherwise, the ROR continues processing as follows:

   o  if the NS(AR) target matches a Client NCE in the DEPARTED state,
      the ROR re-encapsulates while setting the OAL source to the ULA of
      the ROS and OAL destination address to the ADM-ULA of the Client's
      new Proxy/Server.  The ROR then forwards the resulting carrier
      packet over the secured spanning tree then returns from
      processing.

   o  If the NS(AR) target matches the MNP-LLA of a Client NCE in the
      REACHABLE state, the ROR notes whether the NS (AR) arrived from
      the secured spanning tree then provides route optimization
      information on behalf of the Client.  If the message arrived via
      the secured spanning tree the ROR need not perform further
      authentication; otherwise, it must verify the message
      authentication signature before accepting.

   o  If the NS(AR) target matches one of its non-MNP routes, the ROR
      serves as both a Relay and a route optimization target, since the
      Relay forwards IP packets toward the (fixed network) target at the
      network layer.

   The ROR next checks the target NCE for a Report List entry that
   matches the NS(AR) source LLA/ULA of the ROS.  If there is a Report
   List entry, the ROR refreshes ReportTime for this ROR; otherwise, the
   ROR creates a new entry for the ROS and records both the LLA and ULA.

   The ROR then prepares a (solicited) NA(AR) message to return to the
   ROS with the source address set to its own ADM-LLA, the destination
   address set to the NS(AR) LLA source address and the Target Address
   set to the target Client's MNP-LLA.  The ROR includes an OMNI option

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   with Preflen set to the prefix length associated with the NA(AR)
   source address, with S/T-omIndex set to the value that appeared in
   the NS(AR) and with Interface Attributes sub-options for all of the
   target's underlying interfaces with current information for each
   interface.

   For each Interface Attributes sub-option, the ROR sets the L2ADDR
   according to its own INET address for VPNed, Direct, ANET and NATed
   Client interfaces, or to the Client's INET address for native Client
   interfaces.  The ROR then includes the lower 32 bits of its ADM-ULA
   as the LHS, encodes the ADM-ULA SRT prefix length in the SRT field
   and sets FMT as specified in Section 3.3.

   The ROR then sets the NA(AR) message R flag to 1 (as a router) and S
   flag to 1 (as a response to a solicitation) and sets the O flag to 0
   (as a proxy).  The ROR finally submits the NA(AR) for OAL
   encapsulation with source set to its own ULA and destination set to
   the same ULA that appeared in the NS(AR) OAL source, then performs
   OAL encapsulation using the same Identification value that appeared
   in the NS(AR) and finally forwards the resulting (*NET-encapsulated)
   carrier packet via the secured spanning tree without decrementing the
   network-layer TTL/Hop Limit field.

3.14.4.  Relaying the NA(AR)

   When the Bridge receives NA(AR) carrier packet from the ROR, it
   discards the *NET header and determines the next hop by consulting
   its standard IPv6 forwarding table for the OAL header destination
   address.  The Bridge then decrements the OAL header Hop-Limit, re-
   encapsulates the carrier packet and forwards it via the SRT secured
   spanning tree, where it may traverse multiple OMNI link segments.
   The final-hop Bridge will deliver the carrier packet via the secured
   spanning tree to a Proxy/Server for the ROS.

3.14.5.  Processing the NA(AR)

   When the ROS receives the NA(AR) message, it first searches for a NCE
   that matches the NA(AR) target address.  The ROS then processes the
   message the same as for standard IPv6 Address Resolution [RFC4861].
   In the process, it caches all OMNI option information in the target
   NCE (including all Interface Attributes), and caches the NA(AR)
   ADM-{LLA,ULA} source addresses as the addresses of the ROR.  If the
   ROS receives additional NA(AR) or uNA messages for this target Client
   with the same ADM-LLA source address but a different ADM-ULA source
   address, it configures the ADM-LLA corresponding to the new ADM-ULA,
   then caches the new ADM-{LLA,ULA} and deprecates the former
   ADM-{LLA,ULA}.

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   When the ROS is a Client, the SRT secured spanning tree will first
   deliver the solicited NA(AR) message to the local Proxy/Server, which
   re-encapsulates and forwards the message to the Client.  If the
   Client is on a well-managed ANET, physical security and protected
   spectrum ensures security for the unmodified NA(AR); if the Client is
   on the open INET the Proxy/Server must instead include an
   authentication signature (while adjusting the OMNI option size, if
   necessary).  The Proxy/Server uses its own ADM-ULA as the OAL source
   and the MNP-ULA of the Client as the OAL destination.

3.14.6.  Forwarding Packets to Route Optimized Targets

   After the ROS receives the route optimization NA(AR) and updates the
   target NCE, it sends additional NS(AR) messages to the ADM-ULA of the
   ROR to refresh the NCE ReachableTime before expiration while it still
   has sustained interest in this target.  While the NCE remains
   REACHABLE, the ROS can forward packets along paths that use best
   underlying interface pairs based on local preferences and target
   Interface Attributes.  The ROS selects target underlying interfaces
   according to traffic selectors and/or any other traffic
   discriminators, but must first establish window synchronization state
   for each target if necessary.

   The ROS initiates window synchronization through a secured uncast NS/
   NA(WIN) exchange as specified in Section 3.2.7.  The NS/NA(WIN)
   exchange is conducted over a first underlying interface pair and
   registers only those interfaces.  If the ROS and target have
   additional underlying interface pairs serviced by the same source/
   destination LLAs, they may register new interfaces by sending
   additional NS/NA(WIN) messages but need not include window
   synchronization parameters.  If the ROS and target have additional
   underlying interface pairs services by different source/destination
   LLAs, they must include window synchronization parameters when they
   send NS/NA(WIN) messages to establish NCE state for the new source/
   destination LLAs.

   After window synchronization state has been established, the ROS and
   target Client can begin forwarding carrier packets while performing
   additional NS/NA(WIN) exchanges as above to update window state,
   register new interfaces and/or test reachability.  The ROS sends
   carrier packets to the FHS Bridge discovered through the NS/NA(WIN)
   exchange which verifies the Identification is in window for the
   target Client.  The FHS Bridge then forwards the carrier packets over
   the unsecured spanning tree to the LHS Bridge, which forwards them
   via LHS encapsulation to the LHS Proxy/Server or directly to the
   target Client itself.  The target Client in turn sends packets to the
   ROS in the reverse direction while forwarding through the Bridges to
   minimize Proxy/Server load whenever possible.

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   While the ROS continues to actively forward packets to the target
   Client, it is responsible for updating window synchronization state
   and per-interface reachability before expiration.  Window
   synchronization state is shared by all underlying interfaces in the
   ROS' NCE that use the same destination LLA so that a single NS/
   NA(WIN) exchange applies for all interfaces regardless of the
   (single) interface used to conduct the exchange.  However, the window
   synchronization exchange only confirms target Client reachability
   over the specific interface used to conduct the exchange.
   Reachability for other underlying interfaces that share the same
   window synchronization state must be determined individually using
   NS/NA(NUD) messages which need not be secured as long as they use in-
   window Identifications and do not update other state information.

3.15.  Neighbor Unreachability Detection (NUD)

   AERO nodes perform Neighbor Unreachability Detection (NUD) per
   [RFC4861] either reactively in response to persistent link-layer
   errors (see Section 3.11) or proactively to confirm reachability.
   The NUD algorithm is based on periodic control message exchanges and
   may further be seeded by IPv6 ND hints of forward progress, but care
   must be taken to avoid inferring reachability based on spoofed
   information.  For example, IPv6 ND message exchanges that include
   authentication codes and/or in-window Identifications may be
   considered as acceptable hints of forward progress, while spurious
   random carrier packets should be ignored.

   AERO nodes can perform NS/NA(NUD) exchanges over the OMNI link
   secured spanning tree (i.e. the same as described above for NS/
   NA(WIN)) to test reachability without risk of DoS attacks from nodes
   pretending to be a neighbor.  These NS/NA(NUD) messages use the
   unicast LLAs and ULAs of the parties involved in the NUD test.  When
   only reachability information is required without updating any other
   NCE state, AERO nodes can instead perform NS/NA(NUD) exchanges
   directly between neighbors without employing the secured spanning
   tree as long as they include in-window Identifications and either an
   authentication signature or checksum.

   When an ROR directs an ROS to a target neighbor with one or more
   link-layer addresses, the ROS probes each unsecured target underlying
   interface either proactively or on-demand of carrier packets directed
   to the path by multilink forwarding to maintain the interface's state
   as reachable.  Probing is performed through NS(NUD) messages over the
   SRT secured or unsecured spanning tree, or through NS(NUD) messages
   sent directly to an underlying interface of the target itself.  While
   testing a target underlying interface, the ROS can optionally
   continue to forward carrier packets via alternate interfaces and/or

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   maintain a small queue of carrier packets until target reachability
   is confirmed.

   NS(NUD) messages are encapsulated, fragmented and transmitted as
   carrier packets the same as for ordinary original IP data packets,
   however the encapsulated destinations are the LLA of the ROS and
   either the ADM-LLA of the LHS Proxy/Server or the MNP-LLA of the
   target itself.  The ROS encapsulates the NS(NUD) message the same as
   described in Section 3.2.7 and sets the NS(NUD) OMNI header S/
   T-omIndex to identify the underlying interface used for forwarding
   (or to 0 if any underlying interface can be used).  The ROS then
   fragments the OAL packet and forwards the resulting carrier packets
   into the unsecured spanning tree or via direct encapsulation for
   local segment targets.

   When the target receives the NS(NUD) carrier packets, it verifies
   that it has a NCE for this ROS and that the Identification is in-
   window, then submits the carrier packets for reassembly.  The node
   then verifies the authentication signature or checksum, then searches
   for Interface Attributes in its NCE for the ROS that match the
   NS(NUD) S/T-omIndex for the NA(NUD) reply.  The node then prepares
   the NA(NUD) with the source and destination LLAs reversed,
   encapsulates and sets the OAL source and destination, sets the
   NA(NUD) S/T-omIndex to the index of the underlying interface the
   NS(NUD) arrived on and sets the Target Address to the same value
   included in the NS(NUD).  The target next sets the R flag to 1, the S
   flag to 1 and the O flag to 1, then selects an in-window
   Identification for the ROS and performs fragmentation.  The node then
   forwards the carrier packets into the unsecured spanning tree,
   directly to the ROS if it is in the local segment or directly to a
   Bridge in the local segment.

   When the ROS receives the NA(NUD), it marks the target underlying
   interface tested as "reachable".  Note that underlying interface
   states are maintained independently of the overall NCE REACHABLE
   state, and that a single NCE may have multiple target underlying
   interfaces in various states "reachable" and otherwise while the NCE
   state as a whole remains REACHABLE.

   Note also that the exchange of NS/NA(NUD) messages has the useful
   side-benefit of opening holes in NATs that may be useful for NAT
   traversal.  For example, a Client that discovers the address of a
   Bridge on the local SRT segment during an NS/NA(WIN) exchange with a
   peer that established MFIB state can send an NS(NUD) message directly
   to the INET address of the Bridge while including an authentication
   signature.  The NS(NUD) will open a hole in any NATs on the path from
   the Client to the Bridge, and the Bridge can verify the
   authentication signature before returning a direct NA(NUD) to the

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   Client's NATed L2ADDR while also including an authentication
   signature.  Future carrier packets exchanged between the Client and
   peer can then be forwarded directly via the Bridge while bypassing
   the Client's FHS Proxy/Server.

3.16.  Mobility Management and Quality of Service (QoS)

   AERO is a Distributed Mobility Management (DMM) service.  Each Proxy/
   Server is responsible for only a subset of the Clients on the OMNI
   link, as opposed to a Centralized Mobility Management (CMM) service
   where there is a single network mobility collective entity for all
   Clients.  Clients coordinate with their associated FHS and Hub Proxy/
   Servers via RS/RA exchanges to maintain the DMM profile, and the AERO
   routing system tracks all current Client/Proxy/Server peering
   relationships.

   Hub Proxy/Servers provide ROR, default routing and mobility anchor
   point services for their dependent Clients, while FHS Proxy/Servers
   provide a proxy conduit between the Client and the Hub. Clients are
   responsible for maintaining neighbor relationships with their Proxy/
   Servers through periodic RS/RA exchanges, which also serves to
   confirm neighbor reachability.  When a Client's underlying interface
   attributes change, the Client is responsible for updating the Hub
   Proxy/Server with this new information while using the FHS Proxy/
   Server as a first-hop conduit.  The FHS Proxy/Server can also act as
   a proxy to perform some IPv6 ND exchanges on the Client's behalf
   without consuming bandwidth on the Client underlying interface.

   Mobility management considerations are specified in the following
   sections.

3.16.1.  Mobility Update Messaging

   RORs accommodate Client mobility and/or multilink change events by
   sending secured uNA messages to each ROS in the target Client's
   Report List.  When an ROR sends a uNA message, it sets the IPv6
   source address to the its own ADM-LLA, sets the destination address
   to the ROS LLA (i.e., an MNP-LLA if the ROS is a Client and an ADM-
   LLA if the ROS is a Proxy/Server) and sets the Target Address to the
   Client's MNP-LLA.  The ROR also includes an OMNI option with Preflen
   set to the prefix length associated with the Client's MNP-LLA, with
   Interface Attributes for the target Client's underlying interfaces
   and with the OMNI header S/T-omIndex set to 0.  The ROR then sets the
   uNA R flag to 1, S flag to 0 and O flag to 1, then encapsulates the
   message in an OAL header with source set to its own ADM-ULA and
   destination set to the ROS ULA (i.e., the ADM-ULA of the ROS Proxy/
   Server) and sends the message into the secured spanning tree.

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   As discussed in Section 7.2.6 of [RFC4861], the transmission and
   reception of uNA messages is unreliable but provides a useful
   optimization.  In well-connected Internetworks with robust data links
   uNA messages will be delivered with high probability, but in any case
   the ROR can optionally send up to MAX_NEIGHBOR_ADVERTISEMENT uNAs to
   each ROS to increase the likelihood that at least one will be
   received.  Alternatively, the ROR can set the PNG flag in the uNA
   OMNI option header to request a solicited NA acknowledgement as
   specified in [I-D.templin-6man-omni].

   When the ROS Proxy/Server receives a uNA message prepared as above,
   it ignores the message if the OAL destination is not its own ADM-ULA.
   If the uNA destination was its own ADM-LLA, the ROS Proxy/Server uses
   the included OMNI option information to update its NCE for the target
   but does not reset ReachableTime since the receipt of an unsolicited
   NA message from the ROR does not provide confirmation that any
   forward paths to the target Client are working.  If the destination
   was the MNP-LLA of the ROS Client, the Proxy/Server instead re-
   encapsulates with the OAL source set to its own ADM-ULA, OAL
   destination set to the MNP-ULA of the ROS Client with an
   authentication signature if necessary, and with an in-window
   Identification for this Client.  Finally, if the uNA message PNG flag
   was set, the ROS returns a solicited NA acknowledgement as specified
   in [I-D.templin-6man-omni].

   In addition to sending uNA messages to the current set of ROSs for
   the target Client, the ROR also sends uNAs to the former Proxy/Server
   associated with the underlying interface for which the link-layer
   address has changed.  These uNA messages update former Proxy/Servers
   that cannot easily detect (e.g., without active probing) when a
   formerly-active Client has departed.  When the ROR sends the uNA, it
   sets the source address to its ADM-LLA, sets the destination address
   to the former Proxy/Server's ADM-LLA, and sets the Target Address to
   the Client's MNP-LLA.  The ROR also includes an OMNI option with
   Preflen set to the prefix length associated with the Client's MNP-
   LLA, with Interface Attributes for the changed underlying interface,
   and with the OMNI header S/T-omIndex set to 0.  The ROR then sets the
   uNA R flag to 1, S flag to 0 and O flag to 1, then encapsulates the
   message in an OAL header with source set to its own ADM-ULA and
   destination set to the ADM-ULA of the former Proxy/Server and sends
   the message into the secured spanning tree.

3.16.2.  Announcing Link-Layer Address and/or QoS Preference Changes

   When a Client needs to change its underlying Interface Attributes
   (e.g., due to a mobility event), the Client sends an RS message to
   its Hub Proxy/Server (i.e., the ROR) via a first-hop FHS Proxy/
   Server, if necessary.  The RS includes an OMNI option with a

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   Multilink Forwarding Parameters sub-option with the new link quality
   and address information.  Note that the first FHS Proxy/Server may
   change due to the underlying interface change; any stale state in
   former FHS Proxy/Servers will simply expire after ReachableTime
   expires with no effect on the Hub Proxy/Server.

   Up to MAX_RTR_SOLICITATIONS RS messages MAY be sent in parallel with
   sending carrier packets containing user data in case one or more RAs
   are lost.  If all RAs are lost, the Client SHOULD re-associate with a
   new Proxy/Server.

   When the Proxy/Server receives the Client's changes, it sends uNA
   messages to all nodes in the Report List the same as described in the
   previous section.

3.16.3.  Bringing New Links Into Service

   When a Client needs to bring new underlying interfaces into service
   (e.g., when it activates a new data link), it sends an RS message to
   the Hub Proxy/Server via a FHS Proxy/Server for the underlying
   interface (if necessary) with an OMNI option that includes Multilink
   Forwarding Parameters with appropriate link quality values and with
   link-layer address information for the new link.

3.16.4.  Deactivating Existing Links

   When a Client needs to deactivate an existing underlying interface,
   it sends an RS message to an FHS Proxy/Server with an OMNI option
   with appropriate Multilink Forwarding Parameter values for the
   deactivated link - in particular, the link quality value 0 assures
   that neighbors will cease to use the link.

   If the Client needs to send RS messages over an underlying interface
   other than the one being deactivated, it MUST include Interface
   Attributes with appropriate link quality values for any underlying
   interfaces being deactivated.

   Note that when a Client deactivates an underlying interface,
   neighbors that have received the RS/uNA messages need not purge all
   references for the underlying interface from their neighbor cache
   entries.  The Client may reactivate or reuse the underlying interface
   and/or its omIndex at a later point in time, when it will send new RS
   messages to an FHS Proxy/Server with fresh interface parameters to
   update any neighbors.

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3.16.5.  Moving Between Proxy/Servers

   The Client performs the procedures specified in Section 3.12.2 when
   it first associates with a new Hub Proxy/Server or renews its
   association with an existing Hub Proxy/Server.

   When an FHS Proxy/Server receives the Client's RS message destined to
   a new Hub Proxy/Server, it forwards the RS and also sends uNA
   messages to inform the old Hub Proxy/Server that the Client has
   DEPARTED.  The FHS Proxy/Server sets the uNA source to the ADM-LLA of
   the new Hub Proxy/Server, sets the destination to the ADM-LLA of the
   old Hub Proxy/Server, sets the OAL source to its own ADM-ULA and sets
   the OAL destination to the ADM-ULA of the old Hub Proxy/Server.  The
   FHS Proxy/Server then submits the uNA for OAL encapsulation and
   fragmentation, then forwards the resulting carrier packets into the
   secured spanning tree.

   When the old Hub Proxy/Server receives the uNA, it changes the
   Client's NCE state to DEPARTED, sets the interface attributes
   information for the Client to point to the new Hub Proxy/Server, and
   resets DepartTime.  After a short delay (e.g., 2 seconds) the old Hub
   Proxy/Server withdraws the Client's MNP from the routing system.
   After DepartTime expires, the old Hub Proxy/Server deletes the
   Client's NCE.

   The old Hub Proxy/Server also iteratively sends uNA messages to each
   ROS in the Client's Report List with its own ADM-LLA as the source
   and the LLA of the ROS as the destination.  The old Proxy/Server then
   encapsulates the uNA with OAL source address set to the ADM-ULA of
   the new Hub Proxy/Server and OAL destination address set to the ADM-
   ULA of the ROS Proxy/Server and sends the carrier packets over the
   secured spanning tree.  When the ROS Proxy/Server receives the uNA,
   it forwards the message to the ROS Client if the destination is an
   MNP-LLA.  The ROS then examines the uNA Target Address to locate the
   target Client's NCE and the ADM-LLA source address to identify the
   old Hub Proxy/Server.  The ROS then caches the ULA source address as
   the ADM-{LLA/ULA} for the new Hub Proxy/Server for this target NCE
   and marks the entry as STALE.  While in the STALE state, the ROS
   sends new NS(AR) messages using its own ULA as the OAL source and the
   ADM-ULA of the new Hub Proxy/Server as the OAL destination address.
   The new Hub Proxy/Server will then process the NS(AR) and return an
   NA(AR) response.

   Clients SHOULD NOT move rapidly between Hub Proxy/Servers in order to
   avoid causing excessive oscillations in the AERO routing system.
   Examples of when a Client might wish to change to a different Hub
   Proxy/Server include a Hub Proxy/Server that has gone unreachable,

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   topological movements of significant distance, movement to a new
   geographic region, movement to a new OMNI link segment, etc.

3.17.  Multicast

   Clients provide an IGMP (IPv4) [RFC2236] or MLD (IPv6) [RFC3810]
   proxy service for its EUNs and/or hosted applications [RFC4605] and
   act as a Protocol Independent Multicast - Sparse-Mode (PIM-SM, or
   simply "PIM") Designated Router (DR) [RFC7761] on the OMNI link.
   Proxy/Servers act as OMNI link PIM routers for Clients on ANET, VPNed
   or Direct interfaces, and Relays also act as OMNI link PIM routers on
   behalf of nodes on other links/networks.

   Clients on VPNed, Direct or ANET underlying interfaces for which the
   ANET has deployed native multicast services forward IGMP/MLD messages
   into the ANET.  The IGMP/MLD messages may be further forwarded by a
   first-hop ANET access router acting as an IGMP/MLD-snooping switch
   [RFC4541], then ultimately delivered to an ANET Proxy/Server.  The
   Proxy/Server then acts as an ROS to send NS(AR) messages to an ROR.
   Clients on INET and ANET underlying interfaces without native
   multicast services instead send NS(AR) messages as an ROS to cause
   their Proxy/Server forward the message to an ROR.  When the ROR
   receives an NA(AR) response, it initiates PIM protocol messaging
   according to the Source-Specific Multicast (SSM) and Any-Source
   Multicast (ASM) operational modes as discussed in the following
   sections.

3.17.1.  Source-Specific Multicast (SSM)

   When an ROS "X" (i.e., either a Client or Proxy Server) acting as PIM
   router receives a Join/Prune message from a node on its downstream
   interfaces containing one or more ((S)ource, (G)roup) pairs, it
   updates its Multicast Routing Information Base (MRIB) accordingly.
   For each S belonging to a prefix reachable via X's non-OMNI
   interfaces, X then forwards the (S, G) Join/Prune to any PIM routers
   on those interfaces per [RFC7761].

   For each S belonging to a prefix reachable via X's OMNI interface, X
   sends an NS(AR) message (see: Section 3.14) using its own LLA as the
   source address, the solicited node multicast address corresponding to
   S as the destination and the LLA of S as the target address.  X then
   encapsulates the NS(AR) in an OAL header with source address set to
   its own ULA and destination address set to the ULA for S, then
   forwards the message into the secured spanning tree which delivers it
   to ROR "Y" that services S.  The resulting NA(AR) will return an OMNI
   option with Interface Attributes for any underlying interfaces that
   are currently servicing S.

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   When X processes the NA(AR) it selects one or more underlying
   interfaces for S and performs an NS/NA(WIN) exchange over the secured
   spanning tree while including a PIM Join/Prune message for each
   multicast group of interest in the OMNI option.  If S is located
   behind any Proxys "Z"*, each Z* then updates its MRIB accordingly and
   maintains the LLA of X as the next hop in the reverse path.  Since
   Bridges forward messages not addressed to themselves without
   examining them, this means that the (reverse) multicast tree path is
   simply from each Z* (and/or S) to X with no other multicast-aware
   routers in the path.

   Following the initial combined Join/Prune and NS/NA(WIN) messaging, X
   maintains a NCE for each S the same as if X was sending unicast data
   traffic to S.  In particular, X performs additional NS/NA(WIN)
   exchanges to keep the NCE alive for up to t_periodic seconds
   [RFC7761].  If no new Joins are received within t_periodic seconds, X
   allows the NCE to expire.  Finally, if X receives any additional
   Join/Prune messages for (S,G) it forwards the messages over the
   secured spanning tree.

   Client C that holds an MNP for source S may later depart from a first
   Proxy/Server Z1 and/or connect via a new Proxy/Server Z2.  In that
   case, Y sends a uNA message to X the same as specified for unicast
   mobility in Section 3.16.  When X receives the uNA message, it
   updates its NCE for the LLA for source S and sends new Join messages
   in NS/NA(WIN) exchanges addressed to the new target Client underlying
   interface connection for S.  There is no requirement to send any
   Prune messages to old Proxy/Server Z1 since source S will no longer
   source any multicast data traffic via Z1.  Instead, the multicast
   state for (S,G) in Proxy/Server Z1 will soon expire since no new
   Joins will arrive.

3.17.2.  Any-Source Multicast (ASM)

   When an ROS X acting as a PIM router receives Join/Prune messages
   from a node on its downstream interfaces containing one or more (*,G)
   pairs, it updates its Multicast Routing Information Base (MRIB)
   accordingly.  X first performs an NS/NA(AR) exchange to receive route
   optimization information for Rendezvous Point (RP) R for each G.  X
   then includes a copy of each Join/Prune message in the OMNI option of
   an NS(WIN) message with its own LLA as the source address and the LLA
   for R as the destination address, then encapsulates the NS(WIN)
   message in an OAL header with its own ULA as the source and the ADM-
   ULA of R's Proxy/Server as the destination then sends the message
   into the secured spanning tree.

   For each source S that sends multicast traffic to group G via R,
   Client S* that aggregates S (or its Proxy/Server) encapsulates the

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   original IP packets in PIM Register messages, includes the PIM
   Register messages in the OMNI options of uNA messages, performs OAL
   encapsulation and fragmentation then forwards the resulting carrier
   packets with Identification values within the receive window for
   Client R* that aggregates R.  Client R* may then elect to send a PIM
   Join to S* in the OMNI option of a uNA over the secured spanning
   tree.  This will result in an (S,G) tree rooted at S* with R as the
   next hop so that R will begin to receive two copies of the original
   IP packet; one native copy from the (S, G) tree and a second copy
   from the pre-existing (*, G) tree that still uses uNA PIM Register
   encapsulation.  R can then issue a uNA PIM Register-stop message over
   the secured spanning tree to suppress the Register-encapsulated
   stream.  At some later time, if Client S* moves to a new Proxy/
   Server, it resumes sending original IP packets via uNA PIM Register
   encapsulation via the new Proxy/Server.

   At the same time, as multicast listeners discover individual S's for
   a given G, they can initiate an (S,G) Join for each S under the same
   procedures discussed in Section 3.17.1.  Once the (S,G) tree is
   established, the listeners can send (S, G) Prune messages to R so
   that multicast original IP packets for group G sourced by S will only
   be delivered via the (S, G) tree and not from the (*, G) tree rooted
   at R.  All mobility considerations discussed for SSM apply.

3.17.3.  Bi-Directional PIM (BIDIR-PIM)

   Bi-Directional PIM (BIDIR-PIM) [RFC5015] provides an alternate
   approach to ASM that treats the Rendezvous Point (RP) as a Designated
   Forwarder (DF).  Further considerations for BIDIR-PIM are out of
   scope.

3.18.  Operation over Multiple OMNI Links

   An AERO Client can connect to multiple OMNI links the same as for any
   data link service.  In that case, the Client maintains a distinct
   OMNI interface for each link, e.g., 'omni0' for the first link,
   'omni1' for the second, 'omni2' for the third, etc.  Each OMNI link
   would include its own distinct set of Bridges and Proxy/Servers,
   thereby providing redundancy in case of failures.

   Each OMNI link could utilize the same or different ANET connections.
   The links can be distinguished at the link-layer via the SRT prefix
   in a similar fashion as for Virtual Local Area Network (VLAN) tagging
   (e.g., IEEE 802.1Q) and/or through assignment of distinct sets of
   MSPs on each link.  This gives rise to the opportunity for supporting
   multiple redundant networked paths (see: Section 3.2.5).

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   The Client's IP layer can select the outgoing OMNI interface
   appropriate for a given traffic profile while (in the reverse
   direction) correspondent nodes must have some way of steering their
   original IP packets destined to a target via the correct OMNI link.

   In a first alternative, if each OMNI link services different MSPs the
   Client can receive a distinct MNP from each of the links.  IP routing
   will therefore assure that the correct OMNI link is used for both
   outbound and inbound traffic.  This can be accomplished using
   existing technologies and approaches, and without requiring any
   special supporting code in correspondent nodes or Bridges.

   In a second alternative, if each OMNI link services the same MSP(s)
   then each link could assign a distinct "OMNI link Anycast" address
   that is configured by all Bridges on the link.  Correspondent nodes
   can then perform Segment Routing to select the correct SRT, which
   will then direct the original IP packet over multiple hops to the
   target.

3.19.  DNS Considerations

   AERO Client MNs and INET correspondent nodes consult the Domain Name
   System (DNS) the same as for any Internetworking node.  When
   correspondent nodes and Client MNs use different IP protocol versions
   (e.g., IPv4 correspondents and IPv6 MNs), the INET DNS must maintain
   A records for IPv4 address mappings to MNs which must then be
   populated in Relay NAT64 mapping caches.  In that way, an IPv4
   correspondent node can send original IPv4 packets to the IPv4 address
   mapping of the target MN, and the Relay will translate the IPv4
   header and destination address into an IPv6 header and IPv6
   destination address of the MN.

   When an AERO Client registers with an AERO Proxy/Server, the Proxy/
   Server can return the address(es) of DNS servers in RDNSS options
   [RFC6106].  The DNS server provides the IP addresses of other MNs and
   correspondent nodes in AAAA records for IPv6 or A records for IPv4.

3.20.  Transition/Coexistence Considerations

   OAL encapsulation ensures that dissimilar INET partitions can be
   joined into a single unified OMNI link, even though the partitions
   themselves may have differing protocol versions and/or incompatible
   addressing plans.  However, a commonality can be achieved by
   incrementally distributing globally routable (i.e., native) IP
   prefixes to eventually reach all nodes (both mobile and fixed) in all
   OMNI link segments.  This can be accomplished by incrementally
   deploying AERO Bridges on each INET partition, with each Bridge

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   distributing its MNPs and/or discovering non-MNP IP GUA prefixes on
   its INET links.

   This gives rise to the opportunity to eventually distribute native IP
   addresses to all nodes, and to present a unified OMNI link view even
   if the INET partitions remain in their current protocol and
   addressing plans.  In that way, the OMNI link can serve the dual
   purpose of providing a mobility/multilink service and a transition/
   coexistence service.  Or, if an INET partition is transitioned to a
   native IP protocol version and addressing scheme that is compatible
   with the OMNI link MNP-based addressing scheme, the partition and
   OMNI link can be joined by Bridges.

   Relays that connect INETs/EUNs with dissimilar IP protocol versions
   may need to employ a network address and protocol translation
   function such as NAT64 [RFC6146].

3.21.  Detecting and Reacting to Proxy/Server and Bridge Failures

   In environments where rapid failure recovery is required, Proxy/
   Servers and Bridges SHOULD use Bidirectional Forwarding Detection
   (BFD) [RFC5880].  Nodes that use BFD can quickly detect and react to
   failures so that cached information is re-established through
   alternate nodes.  BFD control messaging is carried only over well-
   connected ground domain networks (i.e., and not low-end radio links)
   and can therefore be tuned for rapid response.

   Proxy/Servers and Bridges maintain BFD sessions in parallel with
   their BGP peerings.  If a Proxy/Server or Bridge fails, BGP peers
   will quickly re-establish routes through alternate paths the same as
   for common BGP deployments.  Similarly, Proxys maintain BFD sessions
   with their associated Bridges even though they do not establish BGP
   peerings with them.

3.22.  AERO Clients on the Open Internet

   AERO Clients that connect to the open Internet via INET interfaces
   can establish a VPN or direct link to securely connect to a FHS/Hub
   Proxy/Server in a "tethered" arrangement with all of the Client's
   traffic transiting the Proxy/Server which acts as a router.
   Alternatively, the Client can associate with an INET FHS/Hub Proxy/
   Server using UDP/IP encapsulation and control message securing
   services as discussed in the following sections.

   When a Client's OMNI interface enables an INET underlying interface,
   it first examines the INET address.  For IPv4, the Client assumes it
   is on the open Internet if the INET address is not a special-use IPv4
   address per [RFC3330].  Similarly for IPv6, the Client assumes it is

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   on the open Internet if the INET address is a Global Unicast Address
   (GUA) [RFC4291].  Otherwise, the Client should assume it is behind
   one or several NATs.

   The Client then prepares an RS message with IPv6 source address set
   to its MNP-LLA, with IPv6 destination set to link-scoped All-Routers
   multicast and with an OMNI option with underlying interface
   attributes.  If the Client believes that it is on the open Internet,
   it SHOULD include its IP address and UDP port number in the Multilink
   Forwarding Parameters sub-option corresponding to the underlying
   interface.  If the underlying address is IPv4, the Client includes
   the Port Number and IPv4 address written in obfuscated form [RFC4380]
   as discussed in Section 3.3.  If the underlying interface address is
   IPv6, the Client instead includes the Port Number and IPv6 address in
   obfuscated form.  The Client finally includes an authentication
   signature per [I-D.templin-6man-omni] to provide message
   authentication, selects an Identification value and window
   synchronization parameters, and submits the RS for OAL encapsulation.
   The Client then encapsulates the OAL atomic fragment in UDP/IP
   headers to form a carrier packet, sets the UDP/IP source to its INET
   address and UDP port, sets the UDP/IP destination to the FHS Proxy/
   Server's INET address and the AERO service port number (8060), then
   sends the carrier packet to the Proxy/Server.

   When the FHS Proxy/Server receives the RS, it discards the OAL
   encapsulation, authenticates the RS message, and examines the
   destination address.  If the destination is the ADM-LLA of another
   Proxy/Server, the FHS Proxy/Server assumes the proxy role and
   forwards the message into the secured spanning tree.  If the
   destination is its own ADM-LLA, the FHS Proxy/Server instead assumes
   the Hub role, creates a NCE and registers the Client's MNP, window
   synchronization state and INET interface information according to the
   OMNI option parameters.  (If the destination is link-scoped All-
   Routers multicast, the FHS Proxy/Server can assume either the proxy
   or Hub role.)

   If the Multilink Forwarding Paramters sub-option includes a non-zero
   L2ADDR, the Hub Proxy/Server compares the encapsulation IP address
   and UDP port number with the (unobfuscated) values.  If the values
   are the same, the Hub Proxy/Server caches the Client's information as
   an "INET" address meaning that the Client is likely to accept direct
   messages without requiring NAT traversal exchanges.  If the values
   are different (or, if the OMNI option did not include an L2ADDR) the
   Hub Proxy/Server instead caches the Client's information as a
   "mapped" address meaning that NAT traversal exchanges may be
   necessary.

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   The Hub Proxy/Server then prepares an RA message with IPv6 source and
   destination set corresponding to the addresses in the RS, and with an
   OMNI option with an Origin Indication sub-option per
   [I-D.templin-6man-omni] with the mapped and obfuscated Port Number
   and IP address observed in the encapsulation headers.  The Proxy/
   Server also includes a Multilink Forwarding Parameters sub-option, an
   authentication signature sub-option per [I-D.templin-6man-omni] and/
   or a symmetric window synchronization/acknowledgement if necessary.
   The Hub Proxy/Server then performs OAL encapsulation then
   encapsulates the carrier packet in UDP/IP headers with addresses set
   per the L2ADDR information in the NCE for the Client.

   When the Client receives the RA, it authenticates the message then
   process the window synchronization/acknowledgement and compares the
   mapped Port Number and IP address from the Multilink Forwarding
   Parameters sub-option with its own address.  If the addresses are the
   same, the Client assumes the open Internet / Cone NAT principle; if
   the addresses are different, the Client instead assumes that further
   qualification procedures are necessary to detect the type of NAT and
   performs NAT traversal on-demand according to standard procedures
   [RFC6081][RFC4380].  The Client also caches the RA rest of the
   Multilink Forwarding Parameters information to discover the FHS
   Proxy/Server's local spanning tree segment.  The Client finally
   arranges to return an explicit/implicit acknowledgement, and sends
   periodic RS messages to receive fresh RA messages before the Router
   Lifetime received on each INET interface expires.

   When the Client sends messages to target IP addresses, it also
   invokes route optimization per Section 3.14.  For route optimized
   targets in the same OMNI link segment, if the target's L2ADDR is on
   the open INET, the Client forwards carrier packets directly to the
   target INET address.  If the target is behind a NAT, the Client first
   establishes NAT state for the L2ADDR using the "direct bubble" and
   NS/NA(NUD) mechanisms discussed in Section 3.10.1.  The Client
   continues to send carrier packets via the local Bridge discovered
   during window synchronization until NAT state is populated, then
   begins forwarding carrier packets via the direct path through the NAT
   to the target.  For targets in different OMNI link segments, the
   Client forwards carrier packets to the local Bridge.

   The Client can send original IP packets to route-optimized neighbors
   in the same OMNI link segment no larger than the minimum/path MPS in
   one piece and with OAL encapsulation as atomic fragments.  For larger
   original IP packets, the Client applies OAL encapsulation then
   fragments if necessary according to Section 3.9, with OAL header with
   source set to its own MNP-ULA and destination set to the MNP-ULA of
   the target, and with an in-window Identification value.  The Client

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   then encapsulates each resulting carrier packet in UDP/IP *NET
   headers and sends them to the neighbor.

   INET Clients exchange NS/NA(WIN) messages to associate with a new
   peer as discussed in Section 3.2.7.  The exchange establishes MFIB
   state in the Client, peer and all OMNI intermediate nodes in the
   path.  After MFIB state is established, INET Clients and peers can
   exchange carrier packets with compressed headers that include an MFVI
   which is updated on a hop-by-hop basis, while employing "shortcuts"
   to skip any unnecessary hops.

   Note: The NAT traversal procedures specified in this document are
   applicable for Cone, Address-Restricted and Port-Restricted NATs
   only.  While future updates to this document may specify procedures
   for other NAT variations (e.g., hairpinning and various forms of
   Symmetric NATs), it should be noted that continuous communications
   are always possible through Proxy/Server forwarding even for these
   other NAT variations.

3.23.  Time-Varying MNPs

   In some use cases, it is desirable, beneficial and efficient for the
   Client to receive a constant MNP that travels with the Client
   wherever it moves.  For example, this would allow air traffic
   controllers to easily track aircraft, etc.  In other cases, however
   (e.g., intelligent transportation systems), the MN may be willing to
   sacrifice a modicum of efficiency in order to have time-varying MNPs
   that can be changed every so often to defeat adversarial tracking.

   The DHCPv6 service offers a way for Clients that desire time-varying
   MNPs to obtain short-lived prefixes (e.g., on the order of a small
   number of minutes).  In that case, the identity of the Client would
   not be bound to the MNP but rather to a Node Identification value
   (see: [I-D.templin-6man-omni]) to be used as the Client ID seed for
   MNP prefix delegation.  The Client would then be obligated to
   renumber its internal networks whenever its MNP (and therefore also
   its MNP-LLA) changes.  This should not present a challenge for
   Clients with automated network renumbering services, however presents
   limits for the durations of ongoing sessions that would prefer to use
   a constant address.

4.  Implementation Status

   An early AERO implementation based on OpenVPN (https://openvpn.net/)
   was announced on the v6ops mailing list on January 10, 2018 and an
   initial public release of the AERO proof-of-concept source code was
   announced on the intarea mailing list on August 21, 2015.

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   AERO Release-3.2 was tagged on March 30, 2021, and is undergoing
   internal testing.  Additional internal releases expected within the
   coming months, with first public release expected end of 1H2021.

   Many AERO/OMNI functions are implemented and undergoing final
   integration.  OAL fragmentation/reassembly buffer management code has
   been cleared for public release and will be presented at the June
   2021 ICAO mobility subgroup meeting.

5.  IANA Considerations

   The IANA has assigned the UDP port number "8060" for an earlier
   experimental first version of AERO [RFC6706].  This document together
   with [I-D.templin-6man-omni] reclaims UDP port number "8060" as the
   service port for UDP/IP encapsulation.  This document makes no
   request of IANA, since [I-D.templin-6man-omni] already provides
   instructions.  (Note: although [RFC6706] was not widely implemented
   or deployed, it need not be obsoleted since its messages use the
   invalid ICMPv6 message type number '0' which implementations of this
   specification can easily distinguish and ignore.)

   No further IANA actions are required.

6.  Security Considerations

   AERO Bridges configure secured tunnels with AERO Proxy/Servers and
   Relays within their local OMNI link segments.  Applicable secured
   tunnel alternatives include IPsec [RFC4301], TLS/SSL [RFC8446], DTLS
   [RFC6347], WireGuard [WG], etc.  The AERO Bridges of all OMNI link
   segments in turn configure secured tunnels for their neighboring AERO
   Bridges in a secured spanning tree topology.  Therefore, control
   messages exchanged between any pair of OMNI link neighbors over the
   secured spanning tree are already protected.

   To prevent spoofing vectors, Proxy/Servers MUST discard without
   responding to any unsecured NS(AR) messages.  Also, Proxy/Servers
   MUST discard without forwarding any original IP packets received from
   one of their own Clients (whether directly or following OAL
   reassembly) with a source address that does not match the Client's
   MNP and/or a destination address that does match the Client's MNP.
   Finally, Proxy/Servers MUST discard without forwarding any carrier
   packets with an OAL source and destination that both match the same
   MNP.

   For INET partitions that require strong security in the data plane,
   two options for securing communications include 1) disable route
   optimization so that all traffic is conveyed over secured tunnels, or
   2) enable on-demand secure tunnel creation between Client neighbors.

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   Option 1) would result in longer routes than necessary and impose
   traffic concentration on critical infrastructure elements.  Option 2)
   could be coordinated between Clients using NS/NA messages with OMNI
   Host Identity Protocol (HIP) "Initiator/Responder" message sub-
   options [RFC7401][I-D.templin-6man-omni] to create a secured tunnel
   on-demand.

   AERO Clients that connect to secured ANETs need not apply security to
   their IPv6 ND messages, since the messages will be authenticated and
   forwarded by a perimeter Proxy/Server that applies security on its
   INET-facing interface as part of the spanning tree (see above).  AERO
   Clients connected to the open INET can use network and/or transport
   layer security services such as VPNs or can by some other means
   establish a direct link to a Proxy/Server.  When a VPN or direct link
   may be impractical, however, INET Clients and Proxy/Servers SHOULD
   include and verify authentication signatures for their IPv6 ND
   messages as specified in [I-D.templin-6man-omni].

   Application endpoints SHOULD use transport-layer (or higher-layer)
   security services such as TLS/SSL, DTLS or SSH [RFC4251] to assure
   the same level of protection as for critical secured Internet
   services.  AERO Clients that require host-based VPN services SHOULD
   use network and/or transport layer security services such as IPsec,
   TLS/SSL, DTLS, etc.  AERO Proxys and Proxy/Servers can also provide a
   network-based VPN service on behalf of the Client, e.g., if the
   Client is located within a secured enclave and cannot establish a VPN
   on its own behalf.

   AERO Proxy/Servers and Bridges present targets for traffic
   amplification Denial of Service (DoS) attacks.  This concern is no
   different than for widely-deployed VPN security gateways in the
   Internet, where attackers could send spoofed packets to the gateways
   at high data rates.  This can be mitigated through the AERO/OMNI data
   origin authentication procedures, as well as connecting Proxy/Servers
   and Bridges over dedicated links with no connections to the Internet
   and/or when connections to the Internet are only permitted through
   well-managed firewalls.  Traffic amplification DoS attacks can also
   target an AERO Client's low data rate links.  This is a concern not
   only for Clients located on the open Internet but also for Clients in
   secured enclaves.  AERO Proxy/Servers and Proxys can institute rate
   limits that protect Clients from receiving packet floods that could
   DoS low data rate links.

   AERO Relays must implement ingress filtering to avoid a spoofing
   attack in which spurious messages with ULA addresses are injected
   into an OMNI link from an outside attacker.  AERO Clients MUST ensure
   that their connectivity is not used by unauthorized nodes on their
   EUNs to gain access to a protected network, i.e., AERO Clients that

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   act as routers MUST NOT provide routing services for unauthorized
   nodes.  (This concern is no different than for ordinary hosts that
   receive an IP address delegation but then "share" the address with
   other nodes via some form of Internet connection sharing such as
   tethering.)

   The PRL MUST be well-managed and secured from unauthorized tampering,
   even though the list contains only public information.  The PRL can
   be conveyed to the Client in a similar fashion as in [RFC5214] (e.g.,
   through layer 2 data link login messaging, secure upload of a static
   file, DNS lookups, etc.).

   The AERO service for open INET Clients depends on a public key
   distribution service in which Client public keys and identities are
   maintained in a shared database accessible to all open INET Proxy/
   Servers.  Similarly, each Client must be able to determine the public
   key of each Proxy/Server, e.g. by consulting an online database.
   When AERO nodes register their public keys indexed by a unique Host
   Identity Tag (HIT) [RFC7401] in a distributed database such as the
   DNS, and use the HIT as an identity for applying IPv6 ND message
   authentication signatures, a means for determining public key
   attestation is available.

   Security considerations for IPv6 fragmentation and reassembly are
   discussed in [I-D.templin-6man-omni].  In environments where spoofing
   is considered a threat, OMNI nodes SHOULD employ Identification
   window synchronization and OAL destinations SHOULD configure an (end-
   system-based) firewall.

   SRH authentication facilities are specified in [RFC8754].  Security
   considerations for accepting link-layer ICMP messages and reflected
   packets are discussed throughout the document.

7.  Acknowledgements

   Discussions in the IETF, aviation standards communities and private
   exchanges helped shape some of the concepts in this work.
   Individuals who contributed insights include Mikael Abrahamsson, Mark
   Andrews, Fred Baker, Bob Braden, Stewart Bryant, Scott Burleigh,
   Brian Carpenter, Wojciech Dec, Pavel Drasil, Ralph Droms, Adrian
   Farrel, Nick Green, Sri Gundavelli, Brian Haberman, Bernhard Haindl,
   Joel Halpern, Tom Herbert, Bob Hinden, Sascha Hlusiak, Lee Howard,
   Christian Huitema, Zdenek Jaron, Andre Kostur, Hubert Kuenig, Ted
   Lemon, Andy Malis, Satoru Matsushima, Tomek Mrugalski, Thomas Narten,
   Madhu Niraula, Alexandru Petrescu, Behcet Saikaya, Michal Skorepa,
   Dave Thaler, Joe Touch, Bernie Volz, Ryuji Wakikawa, Tony Whyman,
   Lloyd Wood and James Woodyatt.  Members of the IESG also provided
   valuable input during their review process that greatly improved the

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   document.  Special thanks go to Stewart Bryant, Joel Halpern and
   Brian Haberman for their shepherding guidance during the publication
   of the AERO first edition.

   This work has further been encouraged and supported by Boeing
   colleagues including Kyle Bae, M.  Wayne Benson, Dave Bernhardt, Cam
   Brodie, John Bush, Balaguruna Chidambaram, Irene Chin, Bruce Cornish,
   Claudiu Danilov, Don Dillenburg, Joe Dudkowski, Wen Fang, Samad
   Farooqui, Anthony Gregory, Jeff Holland, Seth Jahne, Brian Jaury,
   Greg Kimberly, Ed King, Madhuri Madhava Badgandi, Laurel Matthew,
   Gene MacLean III, Kyle Mikos, Rob Muszkiewicz, Sean O'Sullivan, Vijay
   Rajagopalan, Greg Saccone, Rod Santiago, Kent Shuey, Brian Skeen,
   Mike Slane, Carrie Spiker, Katie Tran, Brendan Williams, Amelia
   Wilson, Julie Wulff, Yueli Yang, Eric Yeh and other members of the
   Boeing mobility, networking and autonomy teams.  Kyle Bae, Wayne
   Benson, Madhuri Madhava Badgandi, Vijayasarathy Rajagopalan, Katie
   Tran and Eric Yeh are especially acknowledged for their work on the
   AERO implementation.  Chuck Klabunde is honored and remembered for
   his early leadership, and we mourn his untimely loss.

   This work was inspired by the support and encouragement of countless
   outstanding colleagues, managers and program directors over the span
   of many decades.  Beginning in the late 1980s,' the Digital Equipment
   Corporation (DEC) Ultrix Engineering and DECnet Architects groups
   identified early issues with fragmentation and bridging links with
   diverse MTUs.  In the early 1990s, engagements at DEC Project Sequoia
   at UC Berkeley and the DEC Western Research Lab in Palo Alto included
   investigations into large-scale networked filesystems, ATM vs
   Internet and network security proxies.  In the mid-1990s to early
   2000s employment at the NASA Ames Research Center (Sterling Software)
   and SRI International supported early investigations of IPv6, ONR UAV
   Communications and the IETF.  An employment at Nokia where important
   IETF documents were published gave way to a present-day engagement
   with The Boeing Company.  The work matured at Boeing through major
   programs including Future Combat Systems, Advanced Airplane Program,
   DTN for the International Space Station, Mobility Vision Lab, CAST,
   Caravan, Airplane Internet of Things, the NASA UAS/CNS program, the
   FAA/ICAO ATN/IPS program and many others.  An attempt to name all who
   gave support and encouragement would double the current document size
   and result in many unintentional omissions - but to all a humble
   thanks.

   Earlier works on NBMA tunneling approaches are found in
   [RFC2529][RFC5214][RFC5569].

   Many of the constructs presented in this second edition of AERO are
   based on the author's earlier works, including:

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   o  The Internet Routing Overlay Network (IRON)
      [RFC6179][I-D.templin-ironbis]

   o  Virtual Enterprise Traversal (VET)
      [RFC5558][I-D.templin-intarea-vet]

   o  The Subnetwork Encapsulation and Adaptation Layer (SEAL)
      [RFC5320][I-D.templin-intarea-seal]

   o  AERO, First Edition [RFC6706]

   Note that these works cite numerous earlier efforts that are not also
   cited here due to space limitations.  The authors of those earlier
   works are acknowledged for their insights.

   This work is aligned with the NASA Safe Autonomous Systems Operation
   (SASO) program under NASA contract number NNA16BD84C.

   This work is aligned with the FAA as per the SE2025 contract number
   DTFAWA-15-D-00030.

   This work is aligned with the Boeing Commercial Airplanes (BCA)
   Internet of Things (IoT) and autonomy programs.

   This work is aligned with the Boeing Information Technology (BIT)
   MobileNet program.

8.  References

8.1.  Normative References

   [I-D.templin-6man-omni]
              Templin, F. L. and T. Whyman, "Transmission of IP Packets
              over Overlay Multilink Network (OMNI) Interfaces", draft-
              templin-6man-omni-35 (work in progress), August 2021.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/info/rfc791>.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <https://www.rfc-editor.org/info/rfc792>.

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

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   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
              December 1998, <https://www.rfc-editor.org/info/rfc2473>.

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

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

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
              November 2005, <https://www.rfc-editor.org/info/rfc4191>.

   [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>.

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

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

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

   [RFC6081]  Thaler, D., "Teredo Extensions", RFC 6081,
              DOI 10.17487/RFC6081, January 2011,
              <https://www.rfc-editor.org/info/rfc6081>.

   [RFC7401]  Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
              Henderson, "Host Identity Protocol Version 2 (HIPv2)",
              RFC 7401, DOI 10.17487/RFC7401, April 2015,
              <https://www.rfc-editor.org/info/rfc7401>.

   [RFC7739]  Gont, F., "Security Implications of Predictable Fragment
              Identification Values", RFC 7739, DOI 10.17487/RFC7739,
              February 2016, <https://www.rfc-editor.org/info/rfc7739>.

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

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [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>.

8.2.  Informative References

   [BGP]      Huston, G., "BGP in 2015, http://potaroo.net", January
              2016.

   [I-D.bonica-6man-comp-rtg-hdr]
              Bonica, R., Kamite, Y., Alston, A., Henriques, D., and L.
              Jalil, "The IPv6 Compact Routing Header (CRH)", draft-
              bonica-6man-comp-rtg-hdr-26 (work in progress), May 2021.

   [I-D.bonica-6man-crh-helper-opt]
              Li, X., Bao, C., Ruan, E., and R. Bonica, "Compressed
              Routing Header (CRH) Helper Option", draft-bonica-6man-
              crh-helper-opt-03 (work in progress), April 2021.

   [I-D.ietf-intarea-frag-fragile]
              Bonica, R., Baker, F., Huston, G., Hinden, R. M., Troan,
              O., and F. Gont, "IP Fragmentation Considered Fragile",
              draft-ietf-intarea-frag-fragile-17 (work in progress),
              September 2019.

   [I-D.ietf-intarea-tunnels]
              Touch, J. and M. Townsley, "IP Tunnels in the Internet
              Architecture", draft-ietf-intarea-tunnels-10 (work in
              progress), September 2019.

   [I-D.ietf-ipwave-vehicular-networking]
              (editor), J. (. J., "IPv6 Wireless Access in Vehicular
              Environments (IPWAVE): Problem Statement and Use Cases",
              draft-ietf-ipwave-vehicular-networking-20 (work in
              progress), March 2021.

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   [I-D.ietf-rtgwg-atn-bgp]
              Templin, F. L., Saccone, G., Dawra, G., Lindem, A., and V.
              Moreno, "A Simple BGP-based Mobile Routing System for the
              Aeronautical Telecommunications Network", draft-ietf-
              rtgwg-atn-bgp-11 (work in progress), July 2021.

   [I-D.templin-6man-dhcpv6-ndopt]
              Templin, F. L., "A Unified Stateful/Stateless
              Configuration Service for IPv6", draft-templin-6man-
              dhcpv6-ndopt-11 (work in progress), January 2021.

   [I-D.templin-intarea-seal]
              Templin, F. L., "The Subnetwork Encapsulation and
              Adaptation Layer (SEAL)", draft-templin-intarea-seal-68
              (work in progress), January 2014.

   [I-D.templin-intarea-vet]
              Templin, F. L., "Virtual Enterprise Traversal (VET)",
              draft-templin-intarea-vet-40 (work in progress), May 2013.

   [I-D.templin-ipwave-uam-its]
              Templin, F. L., "Urban Air Mobility Implications for
              Intelligent Transportation Systems", draft-templin-ipwave-
              uam-its-04 (work in progress), January 2021.

   [I-D.templin-ironbis]
              Templin, F. L., "The Interior Routing Overlay Network
              (IRON)", draft-templin-ironbis-16 (work in progress),
              March 2014.

   [I-D.templin-v6ops-pdhost]
              Templin, F. L., "IPv6 Prefix Delegation and Multi-
              Addressing Models", draft-templin-v6ops-pdhost-27 (work in
              progress), January 2021.

   [OVPN]     OpenVPN, O., "http://openvpn.net", October 2016.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,
              <https://www.rfc-editor.org/info/rfc1812>.

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              DOI 10.17487/RFC2003, October 1996,
              <https://www.rfc-editor.org/info/rfc2003>.

Templin                 Expires February 13, 2022              [Page 89]
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   [RFC2004]  Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
              DOI 10.17487/RFC2004, October 1996,
              <https://www.rfc-editor.org/info/rfc2004>.

   [RFC2236]  Fenner, W., "Internet Group Management Protocol, Version
              2", RFC 2236, DOI 10.17487/RFC2236, November 1997,
              <https://www.rfc-editor.org/info/rfc2236>.

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

   [RFC2529]  Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
              Domains without Explicit Tunnels", RFC 2529,
              DOI 10.17487/RFC2529, March 1999,
              <https://www.rfc-editor.org/info/rfc2529>.

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, DOI 10.17487/RFC2983, October 2000,
              <https://www.rfc-editor.org/info/rfc2983>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC3330]  IANA, "Special-Use IPv4 Addresses", RFC 3330,
              DOI 10.17487/RFC3330, September 2002,
              <https://www.rfc-editor.org/info/rfc3330>.

   [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              DOI 10.17487/RFC3810, June 2004,
              <https://www.rfc-editor.org/info/rfc3810>.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,
              <https://www.rfc-editor.org/info/rfc4122>.

   [RFC4251]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
              January 2006, <https://www.rfc-editor.org/info/rfc4251>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

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

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [RFC4389]  Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
              Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April
              2006, <https://www.rfc-editor.org/info/rfc4389>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC4511]  Sermersheim, J., Ed., "Lightweight Directory Access
              Protocol (LDAP): The Protocol", RFC 4511,
              DOI 10.17487/RFC4511, June 2006,
              <https://www.rfc-editor.org/info/rfc4511>.

   [RFC4541]  Christensen, M., Kimball, K., and F. Solensky,
              "Considerations for Internet Group Management Protocol
              (IGMP) and Multicast Listener Discovery (MLD) Snooping
              Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006,
              <https://www.rfc-editor.org/info/rfc4541>.

   [RFC4605]  Fenner, B., He, H., Haberman, B., and H. Sandick,
              "Internet Group Management Protocol (IGMP) / Multicast
              Listener Discovery (MLD)-Based Multicast Forwarding
              ("IGMP/MLD Proxying")", RFC 4605, DOI 10.17487/RFC4605,
              August 2006, <https://www.rfc-editor.org/info/rfc4605>.

   [RFC4982]  Bagnulo, M. and J. Arkko, "Support for Multiple Hash
              Algorithms in Cryptographically Generated Addresses
              (CGAs)", RFC 4982, DOI 10.17487/RFC4982, July 2007,
              <https://www.rfc-editor.org/info/rfc4982>.

   [RFC5015]  Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
              "Bidirectional Protocol Independent Multicast (BIDIR-
              PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,
              <https://www.rfc-editor.org/info/rfc5015>.

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

   [RFC5320]  Templin, F., Ed., "The Subnetwork Encapsulation and
              Adaptation Layer (SEAL)", RFC 5320, DOI 10.17487/RFC5320,
              February 2010, <https://www.rfc-editor.org/info/rfc5320>.

   [RFC5522]  Eddy, W., Ivancic, W., and T. Davis, "Network Mobility
              Route Optimization Requirements for Operational Use in
              Aeronautics and Space Exploration Mobile Networks",
              RFC 5522, DOI 10.17487/RFC5522, October 2009,
              <https://www.rfc-editor.org/info/rfc5522>.

   [RFC5558]  Templin, F., Ed., "Virtual Enterprise Traversal (VET)",
              RFC 5558, DOI 10.17487/RFC5558, February 2010,
              <https://www.rfc-editor.org/info/rfc5558>.

   [RFC5569]  Despres, R., "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd)", RFC 5569, DOI 10.17487/RFC5569,
              January 2010, <https://www.rfc-editor.org/info/rfc5569>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <https://www.rfc-editor.org/info/rfc5880>.

   [RFC6106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Options for DNS Configuration",
              RFC 6106, DOI 10.17487/RFC6106, November 2010,
              <https://www.rfc-editor.org/info/rfc6106>.

   [RFC6139]  Russert, S., Ed., Fleischman, E., Ed., and F. Templin,
              Ed., "Routing and Addressing in Networks with Global
              Enterprise Recursion (RANGER) Scenarios", RFC 6139,
              DOI 10.17487/RFC6139, February 2011,
              <https://www.rfc-editor.org/info/rfc6139>.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <https://www.rfc-editor.org/info/rfc6146>.

   [RFC6179]  Templin, F., Ed., "The Internet Routing Overlay Network
              (IRON)", RFC 6179, DOI 10.17487/RFC6179, March 2011,
              <https://www.rfc-editor.org/info/rfc6179>.

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   [RFC6221]  Miles, D., Ed., Ooghe, S., Dec, W., Krishnan, S., and A.
              Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221,
              DOI 10.17487/RFC6221, May 2011,
              <https://www.rfc-editor.org/info/rfc6221>.

   [RFC6273]  Kukec, A., Krishnan, S., and S. Jiang, "The Secure
              Neighbor Discovery (SEND) Hash Threat Analysis", RFC 6273,
              DOI 10.17487/RFC6273, June 2011,
              <https://www.rfc-editor.org/info/rfc6273>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6355]  Narten, T. and J. Johnson, "Definition of the UUID-Based
              DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355,
              DOI 10.17487/RFC6355, August 2011,
              <https://www.rfc-editor.org/info/rfc6355>.

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
              <https://www.rfc-editor.org/info/rfc6438>.

   [RFC6706]  Templin, F., Ed., "Asymmetric Extended Route Optimization
              (AERO)", RFC 6706, DOI 10.17487/RFC6706, August 2012,
              <https://www.rfc-editor.org/info/rfc6706>.

   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935,
              DOI 10.17487/RFC6935, April 2013,
              <https://www.rfc-editor.org/info/rfc6935>.

   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, DOI 10.17487/RFC6936, April 2013,
              <https://www.rfc-editor.org/info/rfc6936>.

   [RFC7333]  Chan, H., Ed., Liu, D., Seite, P., Yokota, H., and J.
              Korhonen, "Requirements for Distributed Mobility
              Management", RFC 7333, DOI 10.17487/RFC7333, August 2014,
              <https://www.rfc-editor.org/info/rfc7333>.

   [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
              Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
              Multicast - Sparse Mode (PIM-SM): Protocol Specification
              (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
              2016, <https://www.rfc-editor.org/info/rfc7761>.

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   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

   [WG]       Wireguard, "WireGuard, https://www.wireguard.com", August
              2020.

Appendix A.  Non-Normative Considerations

   AERO can be applied to a multitude of Internetworking scenarios, with
   each having its own adaptations.  The following considerations are
   provided as non-normative guidance:

A.1.  Implementation Strategies for Route Optimization

   Route optimization as discussed in Section 3.14 results in the route
   optimization source (ROS) creating a NCE for the target neighbor.
   The NCE state is set to REACHABLE for at most ReachableTime seconds.
   In order to refresh the NCE lifetime before the ReachableTime timer
   expires, the specification requires implementations to issue a new
   NS/NA(AR) exchange to reset ReachableTime while data packets are
   still flowing.  However, the decision of when to initiate a new NS/
   NA(AR) exchange and to perpetuate the process is left as an
   implementation detail.

   One possible strategy may be to monitor the NCE watching for data
   packets for (ReachableTime - 5) seconds.  If any data packets have
   been sent to the neighbor within this timeframe, then send an NS(AR)
   to receive a new NA(AR).  If no data packets have been sent, wait for
   5 additional seconds and send an immediate NS(AR) if any data packets
   are sent within this "expiration pending" 5 second window.  If no
   additional data packets are sent within the 5 second window, reset
   the NCE state to STALE.

   The monitoring of the neighbor data packet traffic therefore becomes
   an ongoing process during the NCE lifetime.  If the NCE expires,
   future data packets will trigger a new NS/NA(AR) exchange while the

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   packets themselves are delivered over a longer path until route
   optimization state is re-established.

A.2.  Implicit Mobility Management

   OMNI interface neighbors MAY provide a configuration option that
   allows them to perform implicit mobility management in which no IPv6
   ND messaging is used.  In that case, the Client only transmits
   packets over a single interface at a time, and the neighbor always
   observes packets arriving from the Client from the same link-layer
   source address.

   If the Client's underlying interface address changes (either due to a
   readdressing of the original interface or switching to a new
   interface) the neighbor immediately updates the NCE for the Client
   and begins accepting and sending packets according to the Client's
   new address.  This implicit mobility method applies to use cases such
   as cellphones with both WiFi and Cellular interfaces where only one
   of the interfaces is active at a given time, and the Client
   automatically switches over to the backup interface if the primary
   interface fails.

A.3.  Direct Underlying Interfaces

   When a Client's OMNI interface is configured over a Direct interface,
   the neighbor at the other end of the Direct link can receive packets
   without any encapsulation.  In that case, the Client sends packets
   over the Direct link according to traffic selectors.  If the Direct
   interface is selected, then the Client's IP packets are transmitted
   directly to the peer without going through an ANET/INET.  If other
   interfaces are selected, then the Client's IP packets are transmitted
   via a different interface, which may result in the inclusion of
   Proxy/Servers and Bridges in the communications path.  Direct
   interfaces must be tested periodically for reachability, e.g., via
   NUD.

A.4.  AERO Critical Infrastructure Considerations

   AERO Bridges can be either Commercial off-the Shelf (COTS) standard
   IP routers or virtual machines in the cloud.  Bridges must be
   provisioned, supported and managed by the INET administrative
   authority, and connected to the Bridges of other INETs via inter-
   domain peerings.  Cost for purchasing, configuring and managing
   Bridges is nominal even for very large OMNI links.

   AERO INET Proxy/Servers can be standard dedicated server platforms,
   but most often will be deployed as virtual machines in the cloud.
   The only requirements for INET Proxy/Servers are that they can run

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   the AERO/OMNI code and have at least one network interface connection
   to the INET.  INET Proxy/Servers must be provisioned, supported and
   managed by the INET administrative authority.  Cost for purchasing,
   configuring and managing cloud Proxy/Servers is nominal especially
   for virtual machines.

   AERO ANET Proxy/Servers are most often standard dedicated server
   platforms with one underlying interface connected to the ANET and a
   second interface connected to an INET.  As with INET Proxy/Servers,
   the only requirements are that they can run the AERO/OMNI code and
   have at least one interface connection to the INET.  ANET Proxy/
   Servers must be provisioned, supported and managed by the ANET
   administrative authority.  Cost for purchasing, configuring and
   managing Proxys is nominal, and borne by the ANET administrative
   authority.

   AERO Relays are simply Proxy/Servers connected to INETs and/or EUNs
   that provide forwarding services for non-MNP destinations.  The Relay
   connects to the OMNI link and engages in eBGP peering with one or
   more Bridges as a stub AS.  The Relay then injects its MNPs and/or
   non-MNP prefixes into the BGP routing system, and provisions the
   prefixes to its downstream-attached networks.  The Relay can perform
   ROS/ROR services the same as for any Proxy/Server, and can route
   between the MNP and non-MNP address spaces.

A.5.  AERO Server Failure Implications

   AERO Proxy/Servers may appear as a single point of failure in the
   architecture, but such is not the case since all Proxy/Servers on the
   link provide identical services and loss of a Proxy/Server does not
   imply immediate and/or comprehensive communication failures.  Proxy/
   Server failure is quickly detected and conveyed by Bidirectional
   Forward Detection (BFD) and/or proactive NUD allowing Clients to
   migrate to new Proxy/Servers.

   If a Proxy/Server fails, ongoing packet forwarding to Clients will
   continue by virtue of the neighbor cache entries that have already
   been established in route optimization sources (ROSs).  If a Client
   also experiences mobility events at roughly the same time the Proxy/
   Server fails, uNA messages may be lost but neighbor cache entries in
   the DEPARTED state will ensure that packet forwarding to the Client's
   new locations will continue for up to DepartTime seconds.

   If a Client is left without a Proxy/Server for a considerable length
   of time (e.g., greater than ReachableTime seconds) then existing
   neighbor cache entries will eventually expire and both ongoing and
   new communications will fail.  The original source will continue to

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   retransmit until the Client has established a new Proxy/Server
   relationship, after which time continuous communications will resume.

   Therefore, providing many Proxy/Servers on the link with high
   availability profiles provides resilience against loss of individual
   Proxy/Servers and assurance that Clients can establish new Proxy/
   Server relationships quickly in event of a Proxy/Server failure.

A.6.  AERO Client / Server Architecture

   The AERO architectural model is client / server in the control plane,
   with route optimization in the data plane.  The same as for common
   Internet services, the AERO Client discovers the addresses of AERO
   Proxy/Servers and connects to one or more of them.  The AERO service
   is analogous to common Internet services such as google.com,
   yahoo.com, cnn.com, etc.  However, there is only one AERO service for
   the link and all Proxy/Servers provide identical services.

   Common Internet services provide differing strategies for advertising
   server addresses to clients.  The strategy is conveyed through the
   DNS resource records returned in response to name resolution queries.
   As of January 2020 Internet-based 'nslookup' services were used to
   determine the following:

   o  When a client resolves the domainname "google.com", the DNS always
      returns one A record (i.e., an IPv4 address) and one AAAA record
      (i.e., an IPv6 address).  The client receives the same addresses
      each time it resolves the domainname via the same DNS resolver,
      but may receive different addresses when it resolves the
      domainname via different DNS resolvers.  But, in each case,
      exactly one A and one AAAA record are returned.

   o  When a client resolves the domainname "ietf.org", the DNS always
      returns one A record and one AAAA record with the same addresses
      regardless of which DNS resolver is used.

   o  When a client resolves the domainname "yahoo.com", the DNS always
      returns a list of 4 A records and 4 AAAA records.  Each time the
      client resolves the domainname via the same DNS resolver, the same
      list of addresses are returned but in randomized order (i.e.,
      consistent with a DNS round-robin strategy).  But, interestingly,
      the same addresses are returned (albeit in randomized order) when
      the domainname is resolved via different DNS resolvers.

   o  When a client resolves the domainname "amazon.com", the DNS always
      returns a list of 3 A records and no AAAA records.  As with
      "yahoo.com", the same three A records are returned from any
      worldwide Internet connection point in randomized order.

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   The above example strategies show differing approaches to Internet
   resilience and service distribution offered by major Internet
   services.  The Google approach exposes only a single IPv4 and a
   single IPv6 address to clients.  Clients can then select whichever IP
   protocol version offers the best response, but will always use the
   same IP address according to the current Internet connection point.
   This means that the IP address offered by the network must lead to a
   highly-available server and/or service distribution point.  In other
   words, resilience is predicated on high availability within the
   network and with no client-initiated failovers expected (i.e., it is
   all-or-nothing from the client's perspective).  However, Google does
   provide for worldwide distributed service distribution by virtue of
   the fact that each Internet connection point responds with a
   different IPv6 and IPv4 address.  The IETF approach is like google
   (all-or-nothing from the client's perspective), but provides only a
   single IPv4 or IPv6 address on a worldwide basis.  This means that
   the addresses must be made highly-available at the network level with
   no client failover possibility, and if there is any worldwide service
   distribution it would need to be conducted by a network element that
   is reached via the IP address acting as a service distribution point.

   In contrast to the Google and IETF philosophies, Yahoo and Amazon
   both provide clients with a (short) list of IP addresses with Yahoo
   providing both IP protocol versions and Amazon as IPv4-only.  The
   order of the list is randomized with each name service query
   response, with the effect of round-robin load balancing for service
   distribution.  With a short list of addresses, there is still
   expectation that the network will implement high availability for
   each address but in case any single address fails the client can
   switch over to using a different address.  The balance then becomes
   one of function in the network vs function in the end system.

   The same implications observed for common highly-available services
   in the Internet apply also to the AERO client/server architecture.
   When an AERO Client connects to one or more ANETs, it discovers one
   or more AERO Proxy/Server addresses through the mechanisms discussed
   in earlier sections.  Each Proxy/Server address presumably leads to a
   fault-tolerant clustering arrangement such as supported by Linux-HA,
   Extended Virtual Synchrony or Paxos.  Such an arrangement has
   precedence in common Internet service deployments in lightweight
   virtual machines without requiring expensive hardware deployment.
   Similarly, common Internet service deployments set service IP
   addresses on service distribution points that may relay requests to
   many different servers.

   For AERO, the expectation is that a combination of the Google/IETF
   and Yahoo/Amazon philosophies would be employed.  The AERO Client
   connects to different ANET access points and can receive 1-2 Proxy/

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   Server ADM-LLAs at each point.  It then selects one AERO Proxy/Server
   address, and engages in RS/RA exchanges with the same Proxy/Server
   from all ANET connections.  The Client remains with this Proxy/Server
   unless or until the Proxy/Server fails, in which case it can switch
   over to an alternate Proxy/Server.  The Client can likewise switch
   over to a different Proxy/Server at any time if there is some reason
   for it to do so.  So, the AERO expectation is for a balance of
   function in the network and end system, with fault tolerance and
   resilience at both levels.

Appendix B.  Change Log

   << RFC Editor - remove prior to publication >>

   Changes from draft-templin-6man-aero-21 to draft-templin-6man-aero-
   22:

   o  That's it, folks.

   Changes from draft-templin-6man-aero-20 to draft-templin-6man-aero-
   21:

   o  Major updates to Hub-and-Spokes Proxy/Server coordination.

   Changes from draft-templin-6man-aero-19 to draft-templin-6man-aero-
   20:

   o  Major updates especially in Section 3.2.7.

   Changes from draft-templin-6man-aero-18 to draft-templin-6man-aero-
   19:

   o  Major revision update for review.

   Changes from draft-templin-6man-aero-17 to draft-templin-6man-aero-
   18:

   o  Interim version with extensive new text - cleanup planned for next
      release.

   Changes from draft-templin-6man-aero-16 to draft-templin-6man-aero-
   17:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

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   Changes from draft-templin-6man-aero-15 to draft-templin-6man-aero-
   16:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-14 to draft-templin-6man-aero-
   15:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-13 to draft-templin-6man-aero-
   14:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-12 to draft-templin-6man-aero-
   13:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-11 to draft-templin-6man-aero-
   12:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-10 to draft-templin-6man-aero-
   11:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-09 to draft-templin-6man-aero-
   10:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

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   Changes from draft-templin-6man-aero-08 to draft-templin-6man-aero-
   09:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-07 to draft-templin-6man-aero-
   08:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-06 to draft-templin-6man-aero-
   07:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-05 to draft-templin-6man-aero-
   06:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval.

   Changes from draft-templin-6man-aero-04 to draft-templin-6man-aero-
   05:

   o  Changed to use traffic selectors instead of the former multilink
      selection strategy.

   Changes from draft-templin-6man-aero-03 to draft-templin-6man-aero-
   04:

   o  Removed documents from "Obsoletes" list.

   o  Introduced the concept of "secured" and "unsecured" spanning tree.

   o  Additional security considerations.

   o  Additional route optimization considerations.

   Changes from draft-templin-6man-aero-02 to draft-templin-6man-aero-
   03:

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   o  Support for extended route optimization from ROR to target over
      target's underlying interfaces.

   Changes from draft-templin-6man-aero-01 to draft-templin-6man-aero-
   02:

   o  Changed reference citations to "draft-templin-6man-omni".

   o  Several important updates to IPv6 ND cache states and route
      optimization message addressing.

   o  Included introductory description of the "6M's".

   o  Updated Multicast specification.

   Changes from draft-templin-6man-aero-00 to draft-templin-6man-aero-
   01:

   o  Changed category to "Informational".

   o  Updated implementation status.

   Changes from earlier versions to draft-templin-6man-aero-00:

   o  Established working baseline reference.

Author's Address

   Fred L. Templin (editor)
   Boeing Research & Technology
   P.O. Box 3707
   Seattle, WA  98124
   USA

   Email: fltemplin@acm.org

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