[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-templin-aerolink) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc5320, rfc5558, rfc5720,                  February 27, 2019
           rfc6179, rfc6706 (if
           approved)
Intended status: Standards Track
Expires: August 31, 2019


             Asymmetric Extended Route Optimization (AERO)
                  draft-templin-intarea-6706bis-06.txt

Abstract

   This document specifies the operation of IP over tunnel virtual links
   using Asymmetric Extended Route Optimization (AERO).  Nodes attached
   to AERO links can exchange packets via trusted intermediate routers
   that provide forwarding services to reach off-link destinations and
   route optimization services for improved performance.  AERO provides
   an IPv6 link-local address format that supports operation of the IPv6
   Neighbor Discovery (ND) protocol and links ND to IP forwarding.
   Dynamic link selection, mobility management, quality of service (QoS)
   signaling and route optimization are naturally supported through
   dynamic neighbor cache updates, while IPv6 Prefix Delegation (PD) is
   supported by network services such as the Dynamic Host Configuration
   Protocol for IPv6 (DHCPv6).  AERO is a widely-applicable tunneling
   solution especially well-suited to aviation services, mobile Virtual
   Private Networks (VPNs) and other applications as described in this
   document.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 31, 2019.





Templin                  Expires August 31, 2019                [Page 1]


Internet-Draft                    AERO                     February 2019


Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Asymmetric Extended Route Optimization (AERO) . . . . . . . .   7
     3.1.  AERO Link Reference Model . . . . . . . . . . . . . . . .   8
     3.2.  AERO Node Types . . . . . . . . . . . . . . . . . . . . .   9
     3.3.  AERO Routing System . . . . . . . . . . . . . . . . . . .  10
     3.4.  AERO Addresses  . . . . . . . . . . . . . . . . . . . . .  12
     3.5.  AERO Interface Characteristics  . . . . . . . . . . . . .  14
     3.6.  AERO Interface Initialization . . . . . . . . . . . . . .  18
       3.6.1.  AERO Relay Behavior . . . . . . . . . . . . . . . . .  18
       3.6.2.  AERO Server Behavior  . . . . . . . . . . . . . . . .  18
       3.6.3.  AERO Proxy Behavior . . . . . . . . . . . . . . . . .  18
       3.6.4.  AERO Client Behavior  . . . . . . . . . . . . . . . .  19
     3.7.  AERO Interface Neighbor Cache Maintenance . . . . . . . .  19
     3.8.  AERO Interface Forwarding Algorithm . . . . . . . . . . .  21
       3.8.1.  Client Forwarding Algorithm . . . . . . . . . . . . .  22
       3.8.2.  Proxy Forwarding Algorithm  . . . . . . . . . . . . .  22
       3.8.3.  Server Forwarding Algorithm . . . . . . . . . . . . .  23
       3.8.4.  Relay Forwarding Algorithm  . . . . . . . . . . . . .  23
       3.8.5.  Processing Return Packets . . . . . . . . . . . . . .  24
     3.9.  AERO Interface Encapsulation and Re-encapsulation . . . .  24
     3.10. AERO Interface Decapsulation  . . . . . . . . . . . . . .  26
     3.11. AERO Interface Data Origin Authentication . . . . . . . .  26
     3.12. AERO Interface Packet Size Issues . . . . . . . . . . . .  26
     3.13. AERO Interface Error Handling . . . . . . . . . . . . . .  28
     3.14. AERO Router Discovery, Prefix Delegation and
           Autoconfiguration . . . . . . . . . . . . . . . . . . . .  32
       3.14.1.  AERO ND/PD Service Model . . . . . . . . . . . . . .  32
       3.14.2.  AERO Client Behavior . . . . . . . . . . . . . . . .  33
       3.14.3.  AERO Server Behavior . . . . . . . . . . . . . . . .  35
     3.15. The AERO Proxy  . . . . . . . . . . . . . . . . . . . . .  37



Templin                  Expires August 31, 2019                [Page 2]


Internet-Draft                    AERO                     February 2019


     3.16. AERO Route Optimization . . . . . . . . . . . . . . . . .  38
     3.17. Neighbor Unreachability Detection (NUD) . . . . . . . . .  41
     3.18. Mobility Management and Quality of Service (QoS)  . . . .  41
       3.18.1.  Mobility Update Messaging  . . . . . . . . . . . . .  42
       3.18.2.  Forwarding Packets on Behalf of Departed Clients . .  43
       3.18.3.  Announcing Link-Layer Address and/or QoS Preference
                Changes  . . . . . . . . . . . . . . . . . . . . . .  43
       3.18.4.  Bringing New Links Into Service  . . . . . . . . . .  43
       3.18.5.  Removing Existing Links from Service . . . . . . . .  44
       3.18.6.  Implicit Mobility Management . . . . . . . . . . . .  44
       3.18.7.  Moving to a New Server . . . . . . . . . . . . . . .  44
     3.19. Multicast Considerations  . . . . . . . . . . . . . . . .  45
   4.  Direct Underlying Interfaces  . . . . . . . . . . . . . . . .  45
   5.  Operation on AERO Links with /64 ASPs . . . . . . . . . . . .  45
   6.  AERO Adaptations for SEcure Neighbor Discovery (SEND) . . . .  46
   7.  AERO Critical Infrastructure Considerations . . . . . . . . .  47
   8.  Implementation Status . . . . . . . . . . . . . . . . . . . .  47
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  47
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  48
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  49
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  50
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  50
     12.2.  Informative References . . . . . . . . . . . . . . . . .  52
   Appendix A.  AERO Alternate Encapsulations  . . . . . . . . . . .  57
   Appendix B.  S/TLLAO Extensions for Special-Purpose Links . . . .  59
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  61
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  63

1.  Introduction

   This document specifies the operation of IP over tunnel virtual links
   using Asymmetric Extended Route Optimization (AERO).  The AERO link
   can be used for tunneling between neighboring nodes over either IPv6
   or IPv4 networks, i.e., AERO views the IPv6 and IPv4 networks as
   equivalent links for tunneling.  Nodes attached to AERO links can
   exchange packets via trusted intermediate routers that provide
   forwarding services to reach off-link destinations and route
   optimization services for improved performance [RFC5522].

   AERO provides an IPv6 link-local address format that supports
   operation of the IPv6 Neighbor Discovery (ND) [RFC4861] protocol and
   links ND to IP forwarding.  Dynamic link selection, mobility
   management, quality of service (QoS) signaling and route optimization
   are naturally supported through dynamic neighbor cache updates, while
   IPv6 Prefix Delegation (PD) is supported by network services such as
   the Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC8415].





Templin                  Expires August 31, 2019                [Page 3]


Internet-Draft                    AERO                     February 2019


   A node's AERO interface can be configured over multiple underlying
   interfaces.  From the standpoint of ND, AERO interface neighbors
   therefore may appear to have multiple link-layer addresses (i.e., the
   IP addresses assigned to underlying interfaces).  Each link-layer
   address is subject to change due to mobility and/or QoS fluctuations,
   and link-layer address changes are signaled by ND messaging the same
   as for any IPv6 link.

   AERO is applicable to a wide variety of use cases.  For example, it
   can be used to coordinate the Virtual Private Network (VPN) links of
   mobile nodes (e.g., cellphones, tablets, laptop computers, etc.) that
   connect into a home enterprise network via public access networks
   using services such as OpenVPN [OVPN].  AERO is also applicable to
   aviation services for both manned and unmanned aircraft where the
   aircraft is treated as a mobile node that can connect an Internet of
   Things (IoT).  Other applicable use cases are also in scope.

   The following numbered sections present the AERO specification.  The
   appendices at the end of the document are non-normative.

2.  Terminology

   The terminology in the normative references applies; the following
   terms are defined within the scope of this document:

   IPv6 Neighbor Discovery (ND)
      an IPv6 control message service for coordinating neighbor
      relationships between nodes connected to a common link.  The ND
      service used by AERO is specified in [RFC4861].

   IPv6 Prefix Delegation (PD)
      a networking service for delegating IPv6 prefixes to nodes on the
      link.  The nominal PD service is DHCPv6 [RFC8415], however
      alternate services (e.g., based on ND messaging) are also in scope
      [I-D.templin-v6ops-pdhost][I-D.templin-6man-dhcpv6-ndopt].

   (native) Internetwork
      a connected IP network topology over which the AERO link virtual
      overlay is configured and native peer-to-peer communications are
      supported.  Example Internetworks include the global public
      Internet, private enterprise networks, aviation networks, etc.

   AERO link
      a Non-Broadcast, Multiple Access (NBMA) tunnel virtual overlay
      configured over an underlying Internetwork.  Nodes on the AERO
      link appear as single-hop neighbors from the perspective of the
      virtual overlay even though they may be separated by many
      underlying Internetwork hops.  The AERO mechanisms can also



Templin                  Expires August 31, 2019                [Page 4]


Internet-Draft                    AERO                     February 2019


      operate over native link types (e.g., Ethernet, WiFi etc.) when
      tunneling is not needed.

   AERO interface
      a node's attachment to an AERO link.  Since the addresses assigned
      to an AERO interface are managed for uniqueness, AERO interfaces
      do not require Duplicate Address Detection (DAD) and therefore set
      the administrative variable 'DupAddrDetectTransmits' to zero
      [RFC4862].

   AERO address
      an IPv6 link-local address constructed as specified in
      Section 3.4.

   AERO node
      a node that is connected to an AERO link.

   AERO Client ("Client")
      a node that requests PDs from one or more AERO Servers.  Following
      PD, the Client assigns a Client AERO address to the AERO interface
      for use in ND exchanges with other AERO nodes.  A node that acts
      as an AERO Client on one AERO interface can also act as an AERO
      Server on a different AERO interface.

   AERO Server ("Server")
      a node that configures an AERO interface to provide default
      forwarding services and a Mobility Anchor Point (MAP) for AERO
      Clients.  The Server assigns an administratively-provisioned AERO
      address to the AERO interface to support the operation of the ND/
      PD services.  An AERO Server can also act as an AERO Relay.

   AERO Relay ("Relay")
      an IP router that can relay IP packets between AERO Servers and/or
      forward IP packets between the AERO link and the native
      Internetwork.  AERO Relays are standard IP routers that do not
      require any AERO-specific functions.

   AERO Proxy ("Proxy")
      a node that provides proxying services, e.g., when the Client is
      located in a secured internal enclave and the Server is located in
      the external Internetwork.  The AERO Proxy is a conduit between
      the secured enclave and the external Internetwork in the same
      manner as for common web proxies, and behaves in a similar fashion
      as for ND proxies [RFC4389].

   ingress tunnel endpoint (ITE)
      an AERO interface endpoint that injects encapsulated packets into
      an AERO link.



Templin                  Expires August 31, 2019                [Page 5]


Internet-Draft                    AERO                     February 2019


   egress tunnel endpoint (ETE)
      an AERO interface endpoint that receives encapsulated packets from
      an AERO link.

   underlying network
      the same as defined for Internetwork.

   underlying link
      a link that connects an AERO node to the underlying network.

   underlying interface
      an AERO node's interface point of attachment to an underlying
      link.

   link-layer address
      an IP address assigned to an AERO node's underlying interface.
      When UDP encapsulation is used, the UDP port number is also
      considered as part of the link-layer address.  Packets transmitted
      over an AERO interface use link-layer addresses as encapsulation
      header source and destination addresses.  Destination link-layer
      addresses can be either "reachable" or "unreachable" based on
      dynamically-changing network conditions.

   network layer address
      the source or destination address of an encapsulated IP packet.

   end user network (EUN)
      an internal virtual or external edge IP network that an AERO
      Client connects to the rest of the network via the AERO interface.
      The Client sees each EUN as a "downstream" network and sees the
      AERO interface as its point of attachment to the "upstream"
      network.

   AERO Service Prefix (ASP)
      an IP prefix associated with the AERO link and from which more-
      specific AERO Client Prefixes (ACPs) are derived.

   AERO Client Prefix (ACP)
      an IP prefix derived from an ASP and delegated to a Client, where
      the ACP prefix length must be no shorter than the ASP prefix
      length.

   base AERO address
      the lowest-numbered AERO address from the first ACP delegated to
      the Client (see Section 3.4).

   secured enclave




Templin                  Expires August 31, 2019                [Page 6]


Internet-Draft                    AERO                     February 2019


      a private access network (e.g., a corporate enterprise network,
      radio access network, cellular service provider network, etc.)
      with secured links and perimeters.  Link-layer security services
      such as IEEE 802.1X and physical-layer security such as campus
      wired LANs prevent unauthorized access from within the enclave,
      while border network-layer security services such as firewalls and
      proxies prevent unauthorized access from the external
      Internetwork.

   Potential Router List (PRL)
      a geographically and/or topologically referenced list of IP
      addresses of Servers for the AERO link.

   Mobility Anchor Point (MAP)
      an AERO Server that is currently tracking and reporting the
      mobility events of its associated Clients.

   Distributed Mobility Management (DMM)
      an overlay routing service coordinated by Servers and Relays that
      tracks all MAP-to-Client associations.

   Throughout the document, the simple terms "Client", "Server", "Relay"
   and "Proxy" refer to "AERO Client", "AERO Server", "AERO Relay" and
   "AERO Proxy", respectively.  Capitalization is used to distinguish
   these terms from DHCPv6 client/server/relay [RFC8415].

   The terminology of DHCPv6 [RFC8415] and IPv6 ND [RFC4861] (including
   the names of node variables, messages and protocol constants) is used
   throughout this document.  Also, the term "IP" is used to generically
   refer to either Internet Protocol version, i.e., IPv4 [RFC0791] or
   IPv6 [RFC8200].

   The terms Mobility Anchor Point (MAP) and Distributed Mobility
   Management (DMM) are used in the same sense as standard
   Internetworking terminology.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].  Lower case
   uses of these words are not to be interpreted as carrying RFC2119
   significance.

3.  Asymmetric Extended Route Optimization (AERO)

   The following sections specify the operation of IP over Asymmetric
   Extended Route Optimization (AERO) links:





Templin                  Expires August 31, 2019                [Page 7]


Internet-Draft                    AERO                     February 2019


3.1.  AERO Link Reference Model

                              .-(::::::::)
                           .-(::::::::::::)-.
                          (:: Internetwork ::)
                           `-(::::::::::::)-'
                              `-(::::::)-'
                                   |
       +--------------+   +--------+-------+   +--------------+
       |AERO Server S1|   | AERO Relay R1  |   |AERO Server S2|
       |  Nbr: C1, R1 |   |   Nbr: S1, S2  |   |  Nbr: C2, R1 |
       |  default->R1 |   |(X1->S1; X2->S2)|   |  default->R1 |
       |    X1->C1    |   |      ASP A1    |   |    X2->C2    |
       +-------+------+   +--------+-------+   +------+-------+
               |    AERO Link      |                  |
       X---+---+-------------------+-+----------------+---+---X
           |                         |                    |
     +-----+--------+     +----------+------+    +--------+-----+
     |AERO Client C1|     |  AERO Proxy P1  |    |AERO Client C2|
     |    Nbr: S1   |     |(Proxy Nbr Cache)|    |   Nbr: S2    |
     | default->S1  |     +--------+--------+    | default->S2  |
     |    ACP X1    |              |             |    ACP X2    |
     +------+-------+     .--------+------.      +-----+--------+
            |           (- Proxyed Clients -)          |
           .-.            `---------------'           .-.
        ,-(  _)-.                                  ,-(  _)-.
     .-(_  IP   )-.   +-------+     +-------+    .-(_  IP   )-.
   (__    EUN      )--|Host H1|     |Host H2|--(__    EUN      )
      `-(______)-'    +-------+     +-------+     `-(______)-'

                    Figure 1: AERO Link Reference Model

   Figure 1 presents the AERO link reference model.  In this model:

   o  AERO Relay R1 aggregates AERO Service Prefix (ASP) A1, acts as a
      default router for its associated Servers (S1 and S2), and
      connects the AERO link to the rest of the Internetwork.

   o  AERO Servers S1 and S2 associate with Relay R1 and also act as
      default routers for their associated Clients C1 and C2.

   o  AERO Clients C1 and C2 associate with Servers S1 and S2,
      respectively.  They receive AERO Client Prefix (ACP) delegations
      X1 and X2, and also act as default routers for their associated
      physical or internal virtual EUNs.  Simple hosts H1 and H2 attach
      to the EUNs served by Clients C1 and C2, respectively.





Templin                  Expires August 31, 2019                [Page 8]


Internet-Draft                    AERO                     February 2019


   o  AERO Proxy P1 provides proxy services for AERO Clients in secured
      enclaves that cannot associate directly with other AERO link
      neighbors.

   Each node on the AERO link maintains an AERO interface neighbor cache
   and an IP forwarding table the same as for any link.  Although the
   figure shows a limited deployment, in common operational practice
   there may be many additional Relays, Servers, Clients and Proxies.

3.2.  AERO Node Types

   AERO Relays are standard IP routers that provide default forwarding
   services for AERO Servers.  Each Relay also peers with Servers and
   other Relays in a dynamic routing protocol instance to provide a
   Distributed Mobility Management (DMM) service for the list of active
   ACPs (see Section 3.3).  Relays forward packets between neighbors
   connected to the same AERO link and also forward packets between the
   AERO link and the native Internetwork.  Relays present the AERO link
   to the native Internetwork as a set of one or more AERO Service
   Prefixes (ASPs) and serve as a gateway between the AERO link and the
   Internetwork.  Relays maintain tunnels with neighboring Servers, and
   maintain an IP forwarding table entry for each AERO Client Prefix
   (ACP).

   AERO Servers provide default forwarding and Mobility Anchor Point
   (MAP) services for AERO Clients.  Each Server also peers with Relays
   in a dynamic routing protocol instance to advertise its list of
   associated ACPs (see Section 3.3).  Servers facilitate PD exchanges
   with Clients, where each delegated prefix becomes an ACP taken from
   an ASP.  Servers forward packets between AERO interface neighbors,
   and maintain AERO interface neighbor cache entries for Relays.  They
   also maintain both neighbor cache entries and IP forwarding table
   entries for each of their associated Clients, and track each Client's
   mobility profiles.

   AERO Clients act as requesting routers to receive ACPs through PD
   exchanges with AERO Servers over the AERO link.  Each Client can
   associate with a single Server or with multiple Servers, e.g., for
   fault tolerance, load balancing, etc.  Each IPv6 Client receives at
   least a /64 IPv6 ACP, and may receive even shorter prefixes.
   Similarly, each IPv4 Client receives at least a /32 IPv4 ACP (i.e., a
   singleton IPv4 address), and may receive even shorter prefixes.
   Clients maintain an AERO interface neighbor cache entry for each of
   their associated Servers as well as for each of their correspondent
   Clients.

   AERO Proxies provide a transparent conduit for AERO Clients connected
   to secured enclaves to associate with AERO Servers.  The Client sends



Templin                  Expires August 31, 2019                [Page 9]


Internet-Draft                    AERO                     February 2019


   all of its control plane messages to the Server's link-layer address
   and the Proxy intercepts them before they leave the secured enclave.
   The Proxy forwards the Client's control and data plane messages to
   and from the Client's current Server(s).  The Proxy may also discover
   a more direct route toward a target destination via AERO route
   optimization, in which case future outbound data packets would be
   forwarded via the more direct route.  The Proxy function is specified
   in Section 3.15.

   AERO Relays, Servers and Proxies are critical infrastructure elements
   in fixed (i.e., non-mobile) deployments.  Relays, Servers and Proxies
   must use public link-layer addresses that do not change and can be
   reached from any correspondent in the underlying Internetwork (i.e.,
   in the same fashion as for popular Internet services).  AERO Clients
   may be mobile, and may not have any public link-layer addresses,
   e.g., if they are located behind NATs or Proxies.

3.3.  AERO Routing System

   The AERO routing system comprises a private instance of the Border
   Gateway Protocol (BGP) [RFC4271] that is coordinated between Relays
   and Servers and does not interact with either the public Internet BGP
   routing system or the native Internetwork routing system.  Relays
   advertise only a small and unchanging set of ASPs to the native
   Internetwork routing system instead of the full dynamically changing
   set of ACPs.

   In a reference deployment, each Server is configured as an Autonomous
   System Border Router (ASBR) for a stub Autonomous System (AS) using
   an AS Number (ASN) that is unique within the BGP instance, and each
   Server further uses eBGP to peer with one or more Relays but does not
   peer with other Servers.  All Relays are members of the same hub AS
   using a common ASN, and use iBGP to maintain a consistent view of all
   active ACPs currently in service.

   Each Server maintains a working set of associated ACPs, and
   dynamically announces new ACPs and withdraws departed ACPs in its
   eBGP updates to Relays.  Clients are expected to remain associated
   with their current Servers for extended timeframes, however Servers
   SHOULD selectively suppress updates for impatient Clients that
   repeatedly associate and disassociate with them in order to dampen
   routing churn.

   Each Relay configures a black-hole route for each of its ASPs.  By
   black-holing the ASPs, the Relay will maintain forwarding table
   entries only for the ACPs that are currently active, and packets
   destined to all other ACPs will correctly incur Destination
   Unreachable messages due to the black hole route.  Relays do not send



Templin                  Expires August 31, 2019               [Page 10]


Internet-Draft                    AERO                     February 2019


   eBGP updates for ACPs to Servers, but instead only originate a
   default route.  In this way, Servers have only partial topology
   knowledge (i.e., they know only about the ACPs of their directly
   associated Clients) and they forward all other packets to Relays
   which have full topology knowledge.

   Scaling properties of the AERO routing system are limited by the
   number of BGP routes that can be carried by Relays.  As of 2015, the
   global public Internet BGP routing system manages more than 500K
   routes with linear growth and no signs of router resource exhaustion
   [BGP].  More recent network emulation studies have also shown that a
   single Relay can accommodate at least 1M dynamically changing BGP
   routes even on a lightweight virtual machine, i.e., and without
   requiring high-end dedicated router hardware.

   Therefore, assuming each Relay can carry 1M or more routes, this
   means that at least 1M Clients can be serviced by a single set of
   Relays.  A means of increasing scaling would be to assign a different
   set of Relays for each set of ASPs.  In that case, each Server still
   peers with one or more Relays, but the Server institutes route
   filters so that it only sends BGP updates to the specific set of
   Relays that aggregate the ASP.  For example, if the ASP for the AERO
   link is 2001:db8::/32, a first set of Relays could service the ASP
   segment 2001:db8::/40, a second set of Relays could service
   2001:db8:0100::/40, a third set could service 2001:db8:0200::/40,
   etc.

   Assuming up to 1K sets of Relays, the AERO routing system can then
   accommodate 1B or more ACPs with no additional overhead for Servers
   and Relays (for example, it should be possible to service 1B /64 ACPs
   taken from a /34 ASP and even more for shorter prefixes).  In this
   way, each set of Relays services a specific set of ASPs that they
   advertise to the native Internetwork routing system, and each Server
   configures ASP-specific routes that list the correct set of Relays as
   next hops.  This arrangement also allows for natural incremental
   deployment, and can support small scale initial deployments followed
   by dynamic deployment of additional Clients, Servers and Relays
   without disturbing the already-deployed base.

   In an alternate routing arrangement, each set of Relays could
   advertise an aggregated ASP for the link into the native Internetwork
   routing system even though each Relay services only smaller segments
   of the ASP.  In that case, a Relay upon receiving a packet with a
   destination address covered by the ASP segment of another Relay can
   simply tunnel the packet to the other Relay.  The tradeoff then is
   the penalty for Relay-to-Relay tunneling compared with reduced
   routing information in the native routing system.




Templin                  Expires August 31, 2019               [Page 11]


Internet-Draft                    AERO                     February 2019


   A full discussion of the BGP-based routing system used by AERO is
   found in [I-D.ietf-rtgwg-atn-bgp].  The system provides for
   Distributed Mobility Management (DMM) per the distributed mobility
   anchoring architecture [I-D.ietf-dmm-distributed-mobility-anchoring].

3.4.  AERO Addresses

   A Client's AERO address is an IPv6 link-local address with an
   interface identifier based on the Client's delegated ACP.  Relay,
   Server and Proxy AERO addresses are assigned from the range fe80::/96
   and include an administratively-provisioned value in the lower 32
   bits.

   For IPv6, Client AERO addresses begin with the prefix fe80::/64 and
   include in the interface identifier (i.e., the lower 64 bits) a
   64-bit prefix taken from one of the Client's IPv6 ACPs.  For example,
   if the AERO Client receives the IPv6 ACP:

      2001:db8:1000:2000::/56

   it constructs its corresponding AERO addresses as:

      fe80::2001:db8:1000:2000

      fe80::2001:db8:1000:2001

      fe80::2001:db8:1000:2002

      ... etc. ...

      fe80::2001:db8:1000:20ff

   For IPv4, Client AERO addresses are based on an IPv4-mapped IPv6
   address formed from an IPv4 ACP and with a Prefix Length of 96 plus
   the ACP prefix length.  For example, for the IPv4 ACP 192.0.2.32/28
   the IPv4-mapped IPv6 ACP is:

      0:0:0:0:0:FFFF:192.0.2.16/124

   The Client then constructs its AERO addresses with the prefix
   fe80::/64 and with the lower 64 bits of the IPv4-mapped IPv6 address
   in the interface identifier as:

      fe80::FFFF:192.0.2.16

      fe80::FFFF:192.0.2.17

      fe80::FFFF:192.0.2.18



Templin                  Expires August 31, 2019               [Page 12]


Internet-Draft                    AERO                     February 2019


      ... etc. ...

      fe80:FFFF:192.0.2.31

   Relay, Server and Proxy AERO addresses are allocated from the range
   fe80::/96, and MUST be managed for uniqueness by the administrative
   authority for the link.  For interfaces that assign static IPv4
   addresses, the lower 32 bits of the AERO address includes the IPv4
   address, e.g., for the IPv4 address 192.0.2.1 the corresponding AERO
   address is fe80::192.0.2.1.  For other interfaces, the lower 32 bits
   of the AERO address includes a unique integer value, e.g., fe80::1,
   fe80::2, fe80::3, etc.  The address fe80:: is reserved as the IPv6
   link-local Subnet Router Anycast address [RFC4291], and the address
   fe80::ffff:ffff is reserved as the unspecified AERO address; hence,
   these values are not available for administrative assignment.  (Note
   that a special link-local-format unspecified address is defined for
   AERO to satisfy PD services that require a link-local source
   address.)

   When the Server delegates ACPs to the Client, the lowest-numbered
   AERO address from the first ACP delegation serves as the "base" AERO
   address (for example, for the ACP 2001:db8:1000:2000::/56 the base
   AERO address is fe80::2001:db8:1000:2000).  The Client then assigns
   the base AERO address to the AERO interface and uses it for the
   purpose of maintaining the neighbor cache entry.  The Server likewise
   uses the AERO address as its index into the neighbor cache for this
   Client.

   If the Client has multiple AERO addresses (i.e., when there are
   multiple ACPs and/or ACPs with prefix lengths shorter than /64), the
   Client originates ND messages using the base AERO address as the
   source address and accepts and responds to ND messages destined to
   any of its AERO addresses as equivalent to the base AERO address.  In
   this way, the Client maintains a single neighbor cache entry that may
   be indexed by multiple AERO addresses.

   AERO addresses that embed an IPv6 prefix can be statelessly
   transformed into an IPv6 Subnet Router Anycast address and vice-
   versa.  For example, for the AERO address fe80::2001:db8:2000:3000
   the corresponding Subnet Router Anycast address is
   2001:db8:2000:3000::.  In the same way, for the IPv6 Subnet Router
   Anycast address 2001:db8:1:2:: the corresponding AERO address is
   fe80::2001:db8:1:2.  In other words, the low-order 64 bits of an AERO
   address can be used as the high-order 64 bits of a Subnet Router
   Anycast address, and vice-versa.

   AERO links additionally require a reserved IPv6 prefix to support
   encapsulated forwarding of IPv6 ND messages between Servers on the



Templin                  Expires August 31, 2019               [Page 13]


Internet-Draft                    AERO                     February 2019


   link.  Although any non-link-local IPv6 prefix could be reserved for
   this purpose, a Unique Local Address (ULA) prefix [RFC4389] would be
   desireable since it is not routable outside of the AERO link.  For
   example, if the reserved (ULA) prefix is fd00:db8::/64 the AERO
   Server Subnet Router Anycast Address is fd00:db8::.

   A full discussion of the AERO addressing service is found in
   [I-D.templin-6man-aeroaddr].

3.5.  AERO Interface Characteristics

   AERO interfaces use encapsulation (see: Section 3.9) to exchange
   packets with neighbors attached to the AERO link.

   AERO interfaces maintain a neighbor cache for tracking per-neighbor
   state the same as for any interface.  AERO interfaces use ND messages
   including Router Solicitation (RS), Router Advertisement (RA),
   Neighbor Solicitation (NS), Neighbor Advertisement (NA) and Redirect
   for neighbor cache management.

   AERO interface ND messages include one or more Source/Target Link-
   Layer Address Options (S/TLLAOs) formatted as shown in Figure 2:





























Templin                  Expires August 31, 2019               [Page 14]


Internet-Draft                    AERO                     February 2019


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Type     |   Length = 5  | Prefix Length |X|N| Reserved  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Interface ID         |        UDP Port Number        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                          IP Address                           +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P00|P01|P02|P03|P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P16|P17|P18|P19|P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P32|P33|P34|P35|P36|P37|P38|P39|P40|P41|P42|P43|P44|P45|P46|P47|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 2: AERO Source/Target Link-Layer Address Option (S/TLLAO)
                                  Format

   In this format:

   o  Type is set to '1' for SLLAO or '2' for TLLAO.

   o  Length is set to the constant value '5' (i.e., 5 units of 8
      octets).

   o  Prefix Length is set to the ACP prefix length if the ND message
      source address is a Client AERO address, or 128 if the source is
      an administratively-provisioned AERO address.  For example, if the
      source address is fe80::2001:db8:1:0 and the ACP is
      2001:db8:1:0::/56, Prefix Length is set to the value 56.
      Conversely, if the source address is fe80::1, Prefix Length is set
      to 128.  If the ND message contains multiple S/TLLAOs, only the
      Prefix Length value in the first S/TLLAO is consulted and the
      values in other S/TLLAOs are ignored.

   o  The 'X' bit is set to '1' in the SLLAO of RS/RA messages by the
      Proxy when there is a Proxy in the path; otherwise, set to '0'.
      If the ND message contains multiple SLLAOs, only the 'X' value in




Templin                  Expires August 31, 2019               [Page 15]


Internet-Draft                    AERO                     February 2019


      the first SLLAO is consulted and the values in other SLLAOs are
      ignored.

   o  The 'N' bit is set to '1' in the SLLAO of RA messages by the
      Server if there is a NAT in the path; otherwise, set to '0'.  If
      the ND message contains multiple SLLAOs, only the 'N' value in the
      first SLLAO is consulted and the values in other SLLAOs are
      ignored.

   o  Reserved is set to the value '0' on transmission and ignored on
      receipt.

   o  Interface ID is set to a 16-bit integer value corresponding to an
      underlying interface of the AERO node.  Once the node has assigned
      an Interface ID to an underlying interface, the assignment must
      remain unchanged until the node fully detaches from the AERO link.
      The value '255' is reserved as the AERO Server interface ID, i.e.,
      Servers MUST use Interface ID '255', and Clients MUST number their
      Interface IDs with values in the range of 0-254.

   o  UDP Port Number and IP Address are set to the addresses used by
      the AERO node when it sends encapsulated packets over the
      specified underlying interface (or to '0' when the addresses are
      left unspecified).  When UDP is not used as part of the
      encapsulation, UDP Port Number is set to '0'.  When the
      encapsulation IP address family is IPv4, IP Address is formed as
      an IPv4-mapped IPv6 address as specified in Section 3.4.

   o  P(i) is a set of Preferences that correspond to the 64
      Differentiated Service Code Point (DSCP) values [RFC2474].  Each
      P(i) is set to the value '0' ("disabled"), '1' ("low"), '2'
      ("medium") or '3' ("high") to indicate a QoS preference level for
      packet forwarding purposes.

   AERO interfaces may be configured over multiple underlying interface
   connections to underlying links.  For example, common mobile handheld
   devices have both wireless local area network ("WLAN") and cellular
   wireless links.  These links are typically used "one at a time" with
   low-cost WLAN preferred and highly-available cellular wireless as a
   standby.  In a more complex example, aircraft frequently have many
   wireless data link types (e.g. satellite-based, cellular,
   terrestrial, air-to-air directional, etc.) with diverse performance
   and cost properties.

   A Client's underlying interfaces are classified as follows:






Templin                  Expires August 31, 2019               [Page 16]


Internet-Draft                    AERO                     February 2019


   o  Native interfaces connect to the open Internetwork, and have a
      global IP address that is reachable from any open Internetwork
      correspondent.

   o  NATed interfaces connect to a closed network that is separated
      from the open Internetwork by a Network Address Translator (NAT).
      The NAT does not participate in any AERO control message
      signaling, but the AERO Server can issue control messages on
      behalf of the Client.

   o  VPNed interfaces use security encapsulation over the Internetwork
      to a Virtual Private Network (VPN) gateway that also acts as an
      AERO Server.  As with NATed links, the AERO Server can issue
      control messages on behalf of the Client.

   o  Proxyed interfaces connect to a closed network that is separated
      from the open Internetwork by an AERO Proxy.  Unlike NATed and
      VPNed interfaces, the AERO Proxy can also issue control messages
      on behalf of the Client.

   o  Direct interfaces connect the Client directly to a neighbor
      without crossing any networked paths.  An example is a line-of-
      sight link between a remote pilot and an unmanned aircraft.

   If a Client's multiple underlying interfaces are used "one at a time"
   (i.e., all other interfaces are in standby mode while one interface
   is active), then ND messages include only a single S/TLLAO with
   Interface ID set to a constant value.  In that case, the Client would
   appear to have a single underlying interface but with a dynamically
   changing link-layer address.

   If the Client has multiple active underlying interfaces, then from
   the perspective of ND it would appear to have multiple link-layer
   addresses.  In that case, ND messages MAY include multiple S/TLLAOs
   -- each with an Interface ID that corresponds to a specific
   underlying interface of the AERO node.

   When the Client includes an S/TLLAO for an underlying interface for
   which it is aware that there is a NAT or Proxy on the path to the
   Server, or when a node includes an S/TLLAO solely for the purpose of
   announcing new QoS preferences, the node sets both UDP Port Number
   and IP Address to 0 to indicate that the addresses are unspecified at
   the network layer and must instead be derived from the link-layer
   encapsulation headers.

   When an ND message includes multiple S/TLLAOs, the first S/TLLAO MUST
   correspond to the AERO node's underlying interface used to transmit
   the message.



Templin                  Expires August 31, 2019               [Page 17]


Internet-Draft                    AERO                     February 2019


3.6.  AERO Interface Initialization

3.6.1.  AERO Relay Behavior

   When a Relay enables an AERO interface, it first assigns an
   administratively-provisioned AERO address fe80::ID to the interface.
   Each fe80::ID address MUST be unique among all AERO nodes on the
   link.  The Relay then engages in a dynamic routing protocol session
   with one or more Servers and all other Relays on the link (see:
   Section 3.3), and advertises its assigned ASPs into the native
   Internetwork.  Each Relay subsequently maintains an IP forwarding
   table entry for each active ACP covered by its ASP(s).

3.6.2.  AERO Server Behavior

   When a Server enables an AERO interface, it assigns an
   administratively-provisioned AERO address fe80::ID the same as for
   Relays.  The Server further configures a service to facilitate ND/PD
   exchanges with AERO Clients.  The Server maintains neighbor cache
   entries for one or more Relays on the link, and manages per-Client
   neighbor cache entries and IP forwarding table entries based on
   control message exchanges.  The Server also engages in a dynamic
   routing protocol with its neighboring Relays (see: Section 3.3).

   When the Server receives an NS/RS message on the AERO interface it
   authenticates the message and returns an NA/RA message.  (When the
   Server receives an unsolicited NA message, it likewise authenticates
   the message and processes it locally.)  The Server further provides a
   simple link-layer conduit between AERO interface neighbors.  In
   particular, when a packet sent by a source Client arrives on the
   Server's AERO interface and is destined to another AERO node, the
   Server forwards the packet from within the AERO interface at the link
   layer without ever disturbing the network layer.

3.6.3.  AERO Proxy Behavior

   When a Proxy enables an AERO interface, it assigns an
   administratively-provisioned address fe80::ID the same as for Relays
   and Servers.  The Proxy further maintains per-Client proxy neighbor
   cache entries based on control message exchanges.  Proxies forward
   packets between their associated Clients and each Client's associated
   Servers.

   When the Proxy receives an RS message from a Client in the secured
   enclave, it creates an incomplete proxy neighbor cache entry and
   sends a proxyed RS message to a Server selected by the Client while
   using its own link-layer address as the source address.  When the
   Server returns an RA message, the Proxy completes the proxy neighbor



Templin                  Expires August 31, 2019               [Page 18]


Internet-Draft                    AERO                     February 2019


   cache entry based on autoconfiguration information in the RA and
   sends a proxyed RA to the Client while using its own link-layer
   address as the source address.  The Client, Server and Proxy will
   then have the necessary state for managing the proxy neighbor
   association.

3.6.4.  AERO Client Behavior

   When a Client enables an AERO interface, it sends RS messages with
   ND/PD parameters over an underlying interface to one or more AERO
   Servers, which return RA messages with corresponding PD parameters.
   (The RS/RA messages may pass through a Proxy on the path in the case
   of a Client's Proxyed interface.)  See
   [I-D.templin-6man-dhcpv6-ndopt] for the types of ND/PD parameters
   that can be included in the RS/RA message exchanges.

   After the initial ND/PD message exchange, the Client assigns AERO
   addresses to the AERO interface based on the delegated prefix(es).
   The Client can then register additional underlying interfaces with
   the Server by sending a simple RS message (i.e., one with no PD
   parameters) over each underlying interface using its base AERO
   address as the source network layer address.  The Server will update
   its neighbor cache entry for the Client and return a simple RA
   message.

   The Client maintains a neighbor cache entry for each of its Servers
   and each of its active correspondent Clients.  When the Client
   receives ND messages on the AERO interface it updates or creates
   neighbor cache entries, including link-layer address and QoS
   preferences.

3.7.  AERO Interface Neighbor Cache Maintenance

   Each AERO interface maintains a conceptual neighbor cache that
   includes an entry for each neighbor it communicates with on the AERO
   link, the same as for any IPv6 interface [RFC4861].  AERO interface
   neighbor cache entries are said to be one of "permanent", "static",
   "proxy" or "dynamic".

   Permanent neighbor cache entries are created through explicit
   administrative action; they have no timeout values and remain in
   place until explicitly deleted.  AERO Relays maintain permanent
   neighbor cache entries for their associated Relays and Servers on the
   link, and AERO Servers maintain permanent neighbor cache entries for
   their associated Relays.  Each entry maintains the mapping between
   the neighbor's fe80::ID network-layer address and corresponding link-
   layer address.




Templin                  Expires August 31, 2019               [Page 19]


Internet-Draft                    AERO                     February 2019


   Static neighbor cache entries are created and maintained through ND/
   PD exchanges as specified in Section 3.14, and remain in place for
   durations bounded by ND/PD lifetimes.  AERO Servers maintain static
   neighbor cache entries for each of their associated Clients, and AERO
   Clients maintain static neighbor cache entries for each of their
   associated Servers.

   Proxy neighbor cache entries are created and maintained by AERO
   Proxies when they process Client/Server ND/PD exchanges, and remain
   in place for durations bounded by ND/PD lifetimes.  AERO Proxies
   maintain proxy neighbor cache entries for each of their associated
   Clients.

   Dynamic neighbor cache entries are created or updated based on
   receipt of route optimization messages as specified in Section 3.16,
   and are garbage-collected when keepalive timers expire.  AERO route
   optimization sources maintain dynamic neighbor cache entries for each
   of their active target Clients with lifetimes based on ND messaging
   constants.

   When a target AERO Server (acting as a Mobility Anchor Point (MAP))
   receives a valid NS message used for route optimization, it searches
   for a static neighbor cache entry for the target Client.  The Server
   then returns an NA message, and adds the link-layer address of the
   source to a "Report List" associated with the Client's static
   neighbor cache entry.  The Server then sets a "ReportTime" variable
   for the Report list entry to REPORTTIME seconds.  The Server resets
   ReportTime when it receives a new NS message, and otherwise
   decrements ReportTime while no NS messages have been received.  It is
   RECOMMENDED that REPORTTIME be set to the default constant value 40
   seconds to allow a 10 second window so that the AERO route
   optimization procedure can converge before ReportTime decrements
   below REACHABLETIME (see below).

   When the route optimization source receives a valid NA message
   response to its NS message, it creates or updates a dynamic neighbor
   cache entry for the target network-layer and link-layer addresses.
   The node then (re)sets "ReachableTime" for the neighbor cache entry
   to REACHABLETIME seconds and uses this value to determine whether
   packets can be forwarded directly to the target, i.e., instead of via
   a default route.  The node otherwise decrements ReachableTime while
   no further solicited NA messages arrive.  It is RECOMMENDED that
   REACHABLETIME be set to the default constant value 30 seconds as
   specified in [RFC4861].

   The route optimization source also uses the value MAX_UNICAST_SOLICIT
   to limit the number of NS keepalives sent when a correspondent may
   have gone unreachable, the value MAX_RTR_SOLICITATIONS to limit the



Templin                  Expires August 31, 2019               [Page 20]


Internet-Draft                    AERO                     February 2019


   number of RS messages sent without receiving an RA and the value
   MAX_NEIGHBOR_ADVERTISEMENT to limit the number of unsolicited NAs
   that can be sent based on a single event.  It is RECOMMENDED that
   MAX_UNICAST_SOLICIT, MAX_RTR_SOLICITATIONS and
   MAX_NEIGHBOR_ADVERTISEMENT be set to 3 the same as specified in
   [RFC4861].

   Different values for REPORTTIME, REACHABLETIME, MAX_UNICAST_SOLICIT,
   MAX_RTR_SOLCITATIONS and MAX_NEIGHBOR_ADVERTISEMENT MAY be
   administratively set; however, if different values are chosen, all
   nodes on the link MUST consistently configure the same values.  Most
   importantly, REPORTTIME SHOULD be set to a value that is sufficiently
   longer than REACHABLETIME to allow the AERO route optimization
   procedure to converge.

   When there may be a NAT or Proxy between the Client and the Server,
   or if the path from the Client to the Server should be tested for
   reachability, either the Client or the Proxy can send periodic RS
   messages to the Server without PD parameters to receive RA replies.
   The RS/RA messaging will keep NAT/Proxy state alive and test Server
   reachability without disturbing the PD service.

3.8.  AERO Interface Forwarding Algorithm

   IP packets enter a node's AERO interface either from the network
   layer (i.e., from a local application or the IP forwarding system) or
   from the link layer (i.e., from the AERO tunnel virtual link).
   Packets that enter the AERO interface from the network layer are
   encapsulated and forwarded into the AERO link, i.e., they are
   tunneled to an AERO interface neighbor.  Packets that enter the AERO
   interface from the link layer are either re-admitted into the AERO
   link or forwarded to the network layer where they are subject to
   either local delivery or IP forwarding.  In all cases, the AERO
   interface itself MUST NOT decrement the network layer TTL/Hop-count
   since its forwarding actions occur below the network layer.

   AERO interfaces may have multiple underlying interfaces and/or
   neighbor cache entries for neighbors with multiple Interface ID
   registrations (see Section 3.5).  The AERO node uses each packet's
   DSCP value to select an outgoing underlying interface based on the
   node's own QoS preferences, and also to select a destination link-
   layer address based on the neighbor's underlying interface with the
   highest preference.  AERO implementations SHOULD allow for QoS
   preference values to be modified at runtime through network
   management.

   If multiple outgoing interfaces and/or neighbor interfaces have a
   preference of "high", the AERO node sends one copy of the packet via



Templin                  Expires August 31, 2019               [Page 21]


Internet-Draft                    AERO                     February 2019


   each of the (outgoing / neighbor) interface pairs; otherwise, the
   node sends a single copy of the packet via the interface with the
   highest preference.  AERO nodes keep track of which underlying
   interfaces are currently "reachable" or "unreachable", and only use
   "reachable" interfaces for forwarding purposes.

   The following sections discuss the AERO interface forwarding
   algorithms for Clients, Proxies, Servers and Relays.  In the
   following discussion, a packet's destination address is said to
   "match" if it is a non-link-local address with a prefix covered by an
   ASP/ACP, or if it is an AERO address that embeds an ACP, or if it is
   the same as an administratively-provisioned AERO address.

3.8.1.  Client Forwarding Algorithm

   When an IP packet enters a Client's AERO interface from the network
   layer the Client searches for a dynamic neighbor cache entry that
   matches the destination.  If there is a match, the Client uses one or
   more "reachable" link-layer addresses in the entry as the link-layer
   addresses for encapsulation and admits the packet into the AERO link.
   Otherwise, the Client uses the link-layer address in a static
   neighbor cache entry for a Server as the encapsulation address
   (noting that there may be a Proxy on the path to the real Server).

   When an IP packet enters a Client's AERO interface from the link-
   layer, if the destination matches one of the Client's ACPs or link-
   local addresses the Client decapsulates the packet and delivers it to
   the network layer.  Otherwise, the Client drops the packet and MAY
   return a network-layer ICMP Destination Unreachable message subject
   to rate limiting (see: Section 3.13).

3.8.2.  Proxy Forwarding Algorithm

   When the Proxy receives a packet from a Client within the secured
   enclave, the Proxy searches for a dynamic neighbor cache entry that
   matches the destination.  If there is a match, the Proxy uses one or
   more "reachable" link-layer addresses in the entry as the link-layer
   addresses for encapsulation and admits the packet into the AERO link.
   Otherwise, the Proxy uses the link-layer address for one of the
   Client's Servers as the encapsulation address.

   When the Proxy receives a packet from an AERO interface neighbor, it
   searches for a proxy neighbor cache entry for a Client within the
   secured enclave that matches the destination.  If there is a match,
   the Proxy forwards the packet to the Client.  Otherwise, the Proxy
   returns the packet to the neighbor, i.e., by reversing the source and
   destination link-layer addresses and re-admitting the packet into the
   AERO link.



Templin                  Expires August 31, 2019               [Page 22]


Internet-Draft                    AERO                     February 2019


3.8.3.  Server Forwarding Algorithm

   When an IP packet enters a Server's AERO interface from the network
   layer, the Server searches for a static neighbor cache entry for a
   Client that matches the destination.  If there is a match, the Server
   uses one or more link-layer addresses in the entry as the link-layer
   addresses for encapsulation and admits the packet into the AERO link.
   Otherwise, the Server uses the link-layer address in a permanent
   neighbor cache entry for a Relay (selected through longest-prefix
   match) as the link-layer address for encapsulation.

   When an IP packet enters a Server's AERO interface from the link
   layer, the Server processes the packet according to the network-layer
   destination address as follows:

   o  if the destination matches one of the Server's own addresses the
      Server decapsulates the packet and forwards it to the network
      layer for local delivery.

   o  else, if the destination matches a static neighbor cache entry for
      a local Client the Server first determines whether the neighbor is
      the same as the one it received the packet from.  If so, the
      Server drops the packet silently to avoid looping; otherwise, the
      Server uses the neighbor's link-layer address(es) as the
      destination for encapsulation and re-admits the packet into the
      AERO link.

   o  else, if the destination matches a dynamic neighbor cache entry
      for a target Client, the Server forwards the packet according to
      the interface ID settings in the dynamic neighbor cache entry.

   o  else, the Server uses the link-layer address in a neighbor cache
      entry for a Relay (selected through longest-prefix match) as the
      link-layer address for encapsulation.

3.8.4.  Relay Forwarding Algorithm

   Relays forward packets the same as any IP router.  When the Relay
   receives an encapsulated packet from a Server via the AERO link, it
   removes the encapsulation header and searches for a forwarding table
   entry that matches the destination address in the next IP header.
   When the Relay receives an unencapsulated packet from a node outside
   the AERO link, it performs the same forwarding table lookup.  The
   Relay then processes the packet as follows:

   o  if the destination does not match an ASP, or if the destination
      matches one of the Relay's own addresses, the Relay submits the
      packet for either IP forwarding or local delivery.



Templin                  Expires August 31, 2019               [Page 23]


Internet-Draft                    AERO                     February 2019


   o  else, if the destination matches an ACP entry in the IP forwarding
      table the Relay first determines whether the neighbor is the same
      as the one it received the packet from.  If so the Relay MUST drop
      the packet silently to avoid looping; otherwise, the Relay
      encapsulates and forwards the packet using the neighbor's link-
      layer address as the destination for encapsulation.

   o  else, the Relay drops the packet and returns an ICMP Destination
      Unreachable message subject to rate limiting (see: Section 3.13).

   As for any IP router, the Relay decrements the TTL/Hop Count when it
   forwards the packet.

3.8.5.  Processing Return Packets

   When an AERO Server receives a return packet from an AERO Proxy (see
   Section 3.8.2), it proceeds according to the AERO link trust basis.
   Namely, the return packets have the same trust profile as for link-
   layer Destination Unreachable messages.  If the Server has sufficient
   trust basis to accept link-layer Destination Unreachable messages, it
   can then process the return packet by searching for a dynamic
   neighbor cache entry that matches the destination.  If there is a
   match, the Server marks the corresponding link-layer address as
   "unreachable", selects the next-highest priority "reachable" link-
   layer address in the entry as the link-layer address for
   encapsulation then (re)admits the packet into the AERO link.  If
   there are no "reachable" link-layer addresses, the Server instead
   sets ReachableTime in the dynamic neighbor cache entry to 0.
   Otherwise, the Server SHOULD drop the packet and treat it as an
   indication that a path may be failing, and MAY use Neighbor
   Unreachability Detection (NUD) (see: Section 3.13) to test the path
   for reachability.

   When an AERO Relay receives a return packet from an AERO Server, it
   searches its forwarding table for an entry that matches the inner
   destination address.  If there is a forwarding table entry that lists
   a different Server as the next hop, the Relay forwards the packet to
   the different Server; otherwise, the Relay drops the packet.

   See Section 3.18 for further discussion on the nature of return
   packets.

3.9.  AERO Interface Encapsulation and Re-encapsulation

   AERO interfaces encapsulate IP packets according to whether they are
   entering the AERO interface from the network layer or if they are
   being re-admitted into the same AERO link they arrived on.  This
   latter form of encapsulation is known as "re-encapsulation".



Templin                  Expires August 31, 2019               [Page 24]


Internet-Draft                    AERO                     February 2019


   The AERO interface encapsulates packets per the Generic UDP
   Encapsulation (GUE) procedures in
   [I-D.ietf-intarea-gue][I-D.ietf-intarea-gue-extensions], or through
   an alternate encapsulation format (e.g., see: Appendix A, [RFC2784],
   [RFC8086], [RFC4301], etc.).  For packets entering the AERO interface
   from the network layer, the AERO interface copies the "TTL/Hop
   Limit", "Type of Service/Traffic Class" [RFC2983], "Flow
   Label"[RFC6438] (for IPv6) and "Congestion Experienced" [RFC3168]
   values in the packet's IP header into the corresponding fields in the
   encapsulation IP header.  For packets undergoing re-encapsulation,
   the AERO interface instead copies these values from the original
   encapsulation IP header into the new encapsulation header, i.e., the
   values are transferred between encapsulation headers and *not* copied
   from the encapsulated packet's network-layer header.  (Note
   especially that by copying the TTL/Hop Limit between encapsulation
   headers the value will eventually decrement to 0 if there is a
   (temporary) routing loop.)  For IPv4 encapsulation/re-encapsulation,
   the AERO interface sets the DF bit as discussed in Section 3.12.

   When GUE encapsulation is used, the AERO interface next sets the UDP
   source port to a constant value that it will use in each successive
   packet it sends, and sets the UDP length field to the length of the
   encapsulated packet plus 8 bytes for the UDP header itself plus the
   length of the GUE header (or 0 if GUE direct IP encapsulation is
   used).  For packets sent to a Server or Relay, the AERO interface
   sets the UDP destination port to 8060, i.e., the IANA-registered port
   number for AERO.  For packets sent to a Client, the AERO interface
   sets the UDP destination port to the port value stored in the
   neighbor cache entry for this Client.  The AERO interface then either
   includes or omits the UDP checksum according to the GUE
   specification.

   Clients normally use the IP address of the underlying interface as
   the encapsulation source address.  If the underlying interface does
   not have an IP address, however, the Client uses an IP address taken
   from an ACP as the encapsulation source address (assuming the node
   has some way of injecting the ACP into the underlying network routing
   system).  For IPv6 addresses, the Client normally uses the ACP Subnet
   Router Anycast address [RFC4291].

   When GUE encapsulation is not available, encapsulation between
   Servers and Relays can use standard mechanisms such as Generic
   Routing Encapsulation (GRE) [RFC2784], GRE-in-UDP [RFC8086] and IPSec
   [RFC4301] so that Relays can be standard IP routers with no AERO-
   specific mechanisms.






Templin                  Expires August 31, 2019               [Page 25]


Internet-Draft                    AERO                     February 2019


3.10.  AERO Interface Decapsulation

   AERO interfaces decapsulate packets destined either to the AERO node
   itself or to a destination reached via an interface other than the
   AERO interface the packet was received on.  Decapsulation is per the
   procedures specified for the appropriate encapsulation format.

3.11.  AERO Interface Data Origin Authentication

   AERO nodes employ simple data origin authentication procedures for
   encapsulated packets they receive from other nodes on the AERO link.
   In particular:

   o  AERO Relays and Servers accept encapsulated packets with a link-
      layer source address that matches a permanent neighbor cache
      entry.

   o  AERO Servers accept authentic encapsulated ND messages from
      Clients (either directly or via a Proxy), and create or update a
      static neighbor cache entry for the Client based on the specific
      message type.

   o  AERO Clients and Servers accept encapsulated packets if there is a
      static neighbor cache entry with a link-layer address that matches
      the packet's link-layer source address.

   o  AERO Proxies accept encapsulated packets if there is a proxy
      neighbor cache entry that matches the packet's network-layer
      address.

   Each packet should include a signature that the recipient can use to
   authenticate the message origin, e.g., as for common VPN systems such
   as OpenVPN [OVPN].  In some environments, however, it may be
   sufficient to require signatures only for ND control plane messages
   (see: Section 10) and omit signatures for data plane messages.

3.12.  AERO Interface Packet Size Issues

   The AERO interface is the node's attachment to the AERO link.  The
   AERO interface acts as a tunnel ingress when it sends a packet to an
   AERO link neighbor and as a tunnel egress when it receives a packet
   from an AERO link neighbor.  AERO interfaces observe the packet
   sizing considerations for tunnels discussed in
   [I-D.ietf-intarea-tunnels] and as specified below.

   The Internet Protocol expects that IP packets will either be
   delivered to the destination or a suitable Packet Too Big (PTB)
   message returned to support the process known as IP Path MTU



Templin                  Expires August 31, 2019               [Page 26]


Internet-Draft                    AERO                     February 2019


   Discovery (PMTUD) [RFC1191][RFC1981].  However, PTB messages may be
   crafted for malicious purposes such as denial of service, or lost in
   the network [RFC2923].  This can be especially problematic for
   tunnels, where a condition known as a PMTUD "black hole" can result.
   For these reasons, AERO interfaces employ operational procedures that
   avoid interactions with PMTUD, including the use of fragmentation
   when necessary.

   AERO interfaces observe two different types of fragmentation.  Source
   fragmentation occurs when the AERO interface (acting as a tunnel
   ingress) fragments the encapsulated packet into multiple fragments
   before admitting each fragment into the tunnel.  Network
   fragmentation occurs when an encapsulated packet admitted into the
   tunnel by the ingress is fragmented by an IPv4 router on the path to
   the egress.  Note that a packet that incurs source fragmentation may
   also incur network fragmentation.

   IPv6 specifies a minimum link Maximum Transmission Unit (MTU) of 1280
   bytes [RFC8200].  Although IPv4 specifies a smaller minimum link MTU
   of 68 bytes [RFC0791], AERO interfaces also observe the IPv6 minimum
   for IPv4 even if encapsulated packets may incur network
   fragmentation.

   IPv6 specifies a minimum Maximum Reassembly Unit (MRU) of 1500 bytes
   [RFC8200], while the minimum MRU for IPv4 is only 576 bytes [RFC1122]
   (note that common IPv6 over IPv4 tunnels already assume a larger MRU
   than the IPv4 minimum).

   AERO interfaces therefore configure an MTU that MUST NOT be smaller
   than 1280 bytes, MUST NOT be larger than the minimum MRU among all
   nodes on the AERO link minus the encapsulation overhead ("ENCAPS"),
   and SHOULD NOT be smaller than 1500 bytes.  AERO interfaces also
   configure a Maximum Segment Unit (MSU) as the maximum-sized
   encapsulated packet that the ingress can inject into the tunnel
   without source fragmentation.  The MSU value MUST NOT be larger than
   (MTU+ENCAPS) and MUST NOT be larger than 1280 bytes unless there is
   operational assurance that a larger size can traverse the link along
   all paths.

   All AERO nodes MUST configure the same MTU/MSU values for reasons
   cited in [RFC3819][RFC4861]; in particular, multicast support
   requires a common MTU value among all nodes on the link.  All AERO
   nodes MUST configure an MRU large enough to reassemble packets up to
   (MTU+ENCAPS) bytes in length; nodes that cannot configure a large-
   enough MRU MUST NOT enable an AERO interface.

   The network layer proceeds as follow when it presents an IP packet to
   the AERO interface.  For each IPv4 packet that is larger than the



Templin                  Expires August 31, 2019               [Page 27]


Internet-Draft                    AERO                     February 2019


   AERO interface MTU and with the DF bit set to 0, the network layer
   uses IPv4 fragmentation to break the packet into a minimum number of
   non-overlapping fragments where the first fragment is no larger than
   the MTU and the remaining fragments are no larger than the first.
   For all other IP packets, if the packet is larger than the AERO
   interface MTU, the network layer drops the packet and returns a PTB
   message to the original source.  Otherwise, the network layer admits
   each IP packet or fragment into the AERO interface.

   For each IP packet admitted into the AERO interface, the interface
   (acting as a tunnel ingress) encapsulates the packet.  If the
   encapsulated packet is larger than the AERO interface MSU the ingress
   source-fragments the encapsulated packet into a minimum number of
   non-overlapping fragments where the first fragment is no larger than
   the MSU and the remaining fragments are no larger than the first.
   The ingress then admits each encapsulated packet or fragment into the
   tunnel, and for IPv4 sets the DF bit to 0 in the IP encapsulation
   header in case any network fragmentation is necessary.  The
   encapsulated packets will be delivered to the egress, which
   reassembles them into a whole packet if necessary.

   Several factors must be considered when fragmentation is needed.  For
   AERO links over IPv4, the IP ID field is only 16 bits in length,
   meaning that fragmentation at high data rates could result in data
   corruption due to reassembly misassociations [RFC6864][RFC4963].  For
   AERO links over both IPv4 and IPv6, studies have also shown that IP
   fragments are dropped unconditionally over some network paths [I-
   D.taylor-v6ops-fragdrop].  In environments where IP fragmentation
   issues could result in operational problems, the ingress SHOULD
   employ intermediate-layer source fragmentation (see: [RFC2764] and
   [I-D.ietf-intarea-gue-extensions]) before appending the outer
   encapsulation headers to each fragment.  Since the encapsulation
   fragment header reduces the room available for packet data, but the
   original source has no way to control its insertion, the ingress MUST
   include the fragment header length in the ENCAPS length even for
   packets in which the header is absent.

3.13.  AERO Interface Error Handling

   When an AERO node admits encapsulated packets into the AERO
   interface, it may receive link-layer or network-layer error
   indications.

   A link-layer error indication is an ICMP error message generated by a
   router in the underlying network 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




Templin                  Expires August 31, 2019               [Page 28]


Internet-Draft                    AERO                     February 2019


   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].  (AERO interfaces ignore all link-layer IPv4
   "Fragmentation Needed" and IPv6 "Packet Too Big" messages since they
   only emit packets that are guaranteed to be no larger than the IP
   minimum link MTU as discussed in Section 3.12.)

   The ICMP header is followed by the leading portion of the 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".

   The link-layer error message format is shown in Figure 3 (where, "L2"
   and "L3" refer to link-layer and network-layer, respectively):


























Templin                  Expires August 31, 2019               [Page 29]


Internet-Draft                    AERO                     February 2019


        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~                               ~
        |        L2 IP Header of        |
        |         error message         |
        ~                               ~
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |         L2 ICMP Header        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
        ~                               ~   P
        |   IP and other encapsulation  |   a
        | headers of original L3 packet |   c
        ~                               ~   k
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   e
        ~                               ~   t
        |        IP header of           |
        |      original L3 packet       |   i
        ~                               ~   n
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~                               ~   e
        |    Upper layer headers and    |   r
        |    leading portion of body    |   r
        |   of the original L3 packet   |   o
        ~                               ~   r
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---

         Figure 3: AERO 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 encapsulated packets that it
      sends to one of its dynamic neighbor correspondents, the node
      SHOULD process the message as an indication that a path may be
      failing, and MAY initiate NUD over that path.  If it receives
      Destination Unreachable messages on many or all paths, the node
      SHOULD set ReachableTime for the corresponding dynamic neighbor




Templin                  Expires August 31, 2019               [Page 30]


Internet-Draft                    AERO                     February 2019


      cache entry to 0 and allow future packets destined to the
      correspondent to flow through a default route.

   o  When an AERO Client receives persistent link-layer Destination
      Unreachable messages in response to encapsulated packets that it
      sends to one of its static neighbor 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 Server and release its association
      with the old Server as specified in Section 3.18.7.

   o  When an AERO Server receives persistent link-layer Destination
      Unreachable messages in response to encapsulated packets that it
      sends to one of its static neighbor Clients, the Server SHOULD
      mark the underlying path as unusable and use another underlying
      path.  If it receives Destination Unreachable messages on multiple
      paths, the Server should take no further actions unless it
      receives an explicit ND/PD release message or if the PD lifetime
      expires.  In that case, the Server MUST release the Client's
      delegated ACP, withdraw the ACP from the AERO routing system and
      delete the neighbor cache entry.

   o  When an AERO Relay or Server receives link-layer Destination
      Unreachable messages in response to an encapsulated 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 Relay receives a packet for which the network-layer
   destination address is covered by an ASP, if there is no more-
   specific routing information for the destination the Relay drops the
   packet and returns a network-layer Destination Unreachable message
   subject to rate limiting.  The Relay writes the network-layer source
   address of the original packet as the destination address and uses
   one of its non link-local addresses as the source address of the
   message.

   When an AERO node receives an encapsulated packet for which the
   reassembly buffer it too small, it drops the packet and returns a
   network-layer Packet Too Big (PTB) message.  The node first writes
   the MRU value into the PTB message MTU field, writes the network-
   layer source address of the original packet as the destination
   address and writes one of its non link-local addresses as the source
   address.






Templin                  Expires August 31, 2019               [Page 31]


Internet-Draft                    AERO                     February 2019


3.14.  AERO Router Discovery, Prefix Delegation and Autoconfiguration

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

3.14.1.  AERO ND/PD Service Model

   Each AERO Server configures a PD service to facilitate Client
   requests.  Each Server is provisioned with a database of ACP-to-
   Client ID mappings for all Clients enrolled in the AERO system, as
   well as any information necessary to authenticate each Client.  The
   Client database is maintained by a central administrative authority
   for the AERO link and securely distributed to all Servers, e.g., via
   the Lightweight Directory Access Protocol (LDAP) [RFC4511], via
   static configuration, etc.  Therefore, no Server-to-Server PD state
   synchronization is necessary, and Clients can optionally hold
   separate PDs for the same ACPs from multiple Servers.  In this way,
   Clients can associate with multiple Servers, and can receive new PDs
   from new Servers before releasing PDs received from existing Servers.
   This provides the Client with a natural fault-tolerance and/or load
   balancing profile.

   AERO Clients and Servers use ND messages to maintain neighbor cache
   entries.  AERO Servers configure their AERO interfaces as advertising
   interfaces, and therefore send unicast RA messages with configuration
   information in response to a Client's RS message.  Thereafter,
   Clients send additional RS messages to the Server's unicast address
   to refresh prefix and/or router lifetimes.

   AERO Clients and Servers include PD parameters in the RS/RA messages
   they exchange (see: [I-D.templin-6man-dhcpv6-ndopt]).  The unified
   ND/PD messages are exchanged between Client and Server according to
   the prefix management schedule required by the PD service.  If the
   Client knows its ACP in advance, it can include its AERO address as
   the source address of an RS message and with an SLLAO with a valid
   Prefix Length for the ACP.  If the Server (and Proxy) accept the
   Client's ACP assertion, they inject the prefix into the routing
   system and establish the necessary neighbor cache state.  If the
   Client does not know its ACP in advance, or if it wishes to engage in
   a formal PD exchange, it can use a service such as DHCPv6.

   On Some AERO links, PD arrangements may be through some out-of-band
   service such as network management, static configuration, etc.  In
   those cases, AERO nodes can use simple RS/RA message exchanges with
   no explicit PD options.  Instead, the RS/RA messages use AERO
   addresses as a means of representing the delegated prefixes, e.g., if
   a message includes a source address of "fe80::2001:db8:1:2" then the
   recipient can infer that the sender holds the prefix delegation



Templin                  Expires August 31, 2019               [Page 32]


Internet-Draft                    AERO                     February 2019


   "2001:db8:1:2::/N" (where 'N' is the Prefix Length included in the
   first S/TLLAO in the message).

   The following sections specify the Client and Server behavior.

3.14.2.  AERO Client Behavior

   AERO Clients discover the link-layer addresses of AERO Servers in the
   Potential Router List (PRL) via static configuration (e.g., from a
   flat-file map of Server addresses and locations), or through an
   automated means such as Domain Name System (DNS) name resolution
   [RFC1035].  In the absence of other information, the Client resolves
   the DNS Fully-Qualified Domain Name (FQDN)
   "linkupnetworks.[domainname]" where "linkupnetworks" is a constant
   text string and "[domainname]" is a DNS suffix for the Client's
   underlying interface (e.g., "example.com").  After discovering the
   link-layer addresses, the Client associates with one or more of the
   corresponding Servers.

   To associate with a Server, the Client acts as a requesting router to
   request ACPs through an ND/PD message exchange.  The Client sends an
   RS message with PD parameters and with all-routers multicast as the
   IPv6 destination address, the address of the Client's underlying
   interface as the link-layer source address and the link-layer address
   of the Server as the link-layer destination address.  If the Client
   already knows its own AERO address, it uses the AERO address as the
   IPv6 source address; otherwise, it uses the unspecified AERO address
   as the source address.  If the Client's underlying interface connects
   to a subnetwork that supports ACP injection, the Client can use the
   ACP's Subnet Router Anycast address as the link-layer source address.

   The Client next includes one or more SLLAOs in the RS message
   formatted as described in Section 3.5 to register its link-layer
   address(es) with the Server.  The first SLLAO MUST correspond to the
   underlying interface over which the Client will send the RS message.
   The Client MAY include additional SLLAOs specific to other underlying
   interfaces, but if so it sets their UDP Port Number and IP Address
   fields to 0.  The Client can instead register additional link-layer
   addresses with the Server by sending additional RS messages including
   SLLAOs via other underlying interfaces after the initial RS/RA
   exchange.

   The Client then sends the RS message to the AERO Server and waits for
   an RA message reply (see Section 3.14.3) while retrying
   MAX_RTR_SOLICITATIONS times until an RA is received.  If the Client
   receives no RAs, or if it receives an RA with Router Lifetime set to
   0 and/or with no ACP PD parameters, the Client SHOULD discontinue
   autoconfiguration attempts through this Server and try another



Templin                  Expires August 31, 2019               [Page 33]


Internet-Draft                    AERO                     February 2019


   Server.  Otherwise, the Client processes the ACPs found in the RA
   message.

   Next, the Client creates a static neighbor cache entry with the
   Server's link-local address as the network-layer address and the
   address in the first SLLAO as the link-layer address.  The Client
   then autoconfigures AERO addresses for each of the delegated ACPs and
   assigns them to the AERO interface.

   The Client next examines the X and N bits in the first SLLAO of the
   RA message.  If the X bit value is '1' the Client infers that there
   is a Proxy on the path via the interface over which it sent the RS
   message, and if the N bit value is '1' the Client infers that there
   is a NAT on the path.  If N is '1', the Client sets UDP Port Number
   and IP Address to 0 in the first S/TLLAO of any subsequent ND
   messages it sends to the Server over that link.

   The Client also caches any ASPs included in Route Information Options
   (RIOs) [RFC4191] as ASPs to associate with the AERO link, and assigns
   the MTU/MSU values in the MTU options to its AERO interface while
   configuring an appropriate MRU.  This configuration information
   applies to the AERO link as a whole, and all AERO nodes will receive
   the same values.

   Following autoconfiguration, the Client sub-delegates the ACPs 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
   subsequently maintains its ACP delegations through each of its
   Servers by sending RS messages with PD parameters to receive
   corresponding RA messages.

   After the Client registers its Interface IDs and their associated
   UDP/IP addresses and 'P(i)' values, it may wish to change one or more
   Interface ID registrations, e.g., if an underlying interface changes
   address or becomes unavailable, if QoS preferences change, etc.  To
   do so, the Client prepares an RS message to send over any available
   underlying interface.  The RS MUST include a SLLAO specific to the
   selected available underlying interface as the first SLLAO and MAY
   include any additional SLLAOs specific to other underlying
   interfaces.  The Client includes fresh 'P(i)' values in each SLLAO to
   update the Server's neighbor cache entry.  If the Client wishes to
   update 'P(i)' values without updating the link-layer address, it sets
   the UDP Port Number and IP Address fields to 0.  If the Client wishes
   to disable the interface, it sets all 'P(i)' values to '0'
   ("disabled").  When the Client receives the Server's RS response, it
   has assurance that the Server has been updated with the new
   information.



Templin                  Expires August 31, 2019               [Page 34]


Internet-Draft                    AERO                     February 2019


   If the Client wishes to discontinue use of a Server it issues an RS
   message with PD parameters that will cause the Server to release the
   Client.  When the Server processes the message, it releases the ACP,
   deletes its neighbor cache entry for the Client, withdraws the IP
   route from the routing system and returns an RA reply containing any
   necessary PD parameters.

3.14.3.  AERO Server Behavior

   AERO Servers act as IPv6 routers and support a PD service for
   Clients.  AERO Servers arrange to add their encapsulation layer IP
   addresses (i.e., their link-layer addresses) to a static map of
   Server addresses for the link and/or the DNS resource records for the
   FQDN "linkupnetworks.[domainname]" before entering service.  The list
   of Server addresses should be geographically and/or topologically
   referenced, and forms the Potential Router List (PRL) for the AERO
   link.

   When an AERO Server receives a prospective Client's RS message with
   PD parameters on its AERO interface, and the Server is too busy, it
   SHOULD return an immediate RA reply with no ACPs and with Router
   Lifetime set to 0.  Otherwise, the Server authenticates the RS
   message and processes the PD parameters.  The Server first determines
   the correct ACPs to delegate to the Client by searching the Client
   database.  When the Server delegates the ACPs, it also creates an IP
   forwarding table entry for each ACP so that the AERO BGP-based
   routing system will propagate the ACPs to the Relays that aggregate
   the corresponding ASP (see: Section 3.3).

   Next, the Server prepares an RA message that includes the delegated
   ACPs, any other PD parameters and an SLLAO with the Server's link-
   layer address and with Interface ID set to 0.  The Server then
   returns the RA message using its link-local address as the network-
   layer source address, the network-layer source address of the RS
   message as the network-layer destination address, the Server's link-
   layer address as the source link-layer address, and the source link-
   layer address of the RS message as the destination link-layer
   address.  The Server next sets the N flag to 1 if the source link-
   layer address in the RS message was different than the address in the
   first SLLAO to indicate that there is a NAT on the path.  The Server
   then includes one or more RIOs that encode the ASPs for the AERO
   link.  The Server also includes two MTU options - the first MTU
   option includes the MTU for the link and the second MTU option
   includes the MSU for the link (see Section 3.12).  The Server finally
   sends the RA message to the Client.

   The Server next creates a static neighbor cache entry for the Client
   using the base AERO address as the network-layer address and with



Templin                  Expires August 31, 2019               [Page 35]


Internet-Draft                    AERO                     February 2019


   lifetime set to no more than the smallest PD lifetime.  Next, the
   Server updates the neighbor cache entry link-layer address(es) by
   recording the information in each SLLAO in the RS indexed by the
   Interface ID and including the UDP port number, IP address and P(i)
   values.  For the first SLLAO in the list, however, the Server records
   the actual encapsulation source UDP and IP addresses instead of those
   that appear in the SLLAO in case there was a NAT in the path.  The
   Server also records the value of the X bit to indicate whether there
   is a Proxy on the path.

   After the initial RS/RA exchange, the AERO Server maintains the
   neighbor cache entry for the Client until the PD lifetimes expire.
   If the Client (or Proxy) issues additional RS messages with PD
   renewal parameters, the Server extends the PD lifetimes.  If the
   Client (or Proxy) issues an RS with PD release parameters, or if the
   Client (or Proxy) does not issue a renewal before the lifetime
   expires, the Server deletes the neighbor cache entry for the Client
   and withdraws the IP routes from the AERO routing system.  The Server
   processes these and any other Client PD messages, and returns an RA
   reply.  The Server may also issue an unsolicited RA message with PD
   reconfigure parameters to inform the Client that it needs to
   renegotiate its PDs.

3.14.3.1.  Lightweight DHCPv6 Relay Agent (LDRA)

   When DHCPv6 is used as the ND/PD service back end, AERO Clients and
   Servers are always on the same link (i.e., the AERO link) from the
   perspective of DHCPv6.  However, in some implementations the DHCPv6
   server and ND function may be located in separate modules.  In that
   case, the Server's AERO interface module can act as a Lightweight
   DHCPv6 Relay Agent (LDRA)[RFC6221] to relay PD messages to and from
   the DHCPv6 server module.

   When the LDRA receives an authentic RS message, it extracts the PD
   message parameters and uses them to fabricate an IPv6/UDP/DHCPv6
   message.  It sets the IPv6 source address to the source address of
   the RS message, sets the IPv6 destination address to
   'All_DHCP_Relay_Agents_and_Servers' and sets the UDP fields to values
   that will be understood by the DHCPv6 server.

   The LDRA then wraps the message in a DHCPv6 'Relay-Forward' message
   header and includes an 'Interface-Id' option that includes enough
   information to allow the LDRA to forward the resulting Reply message
   back to the Client (e.g., the Client's link-layer addresses, a
   security association identifier, etc.).  The LDRA also wraps the
   information in all of the SLLAOs from the RS message into the
   Interface-Id option, then forwards the message to the DHCPv6 server.




Templin                  Expires August 31, 2019               [Page 36]


Internet-Draft                    AERO                     February 2019


   When the DHCPv6 server prepares a Reply message, it wraps the message
   in a 'Relay-Reply' message and echoes the Interface-Id option.  The
   DHCPv6 server then delivers the Relay-Reply message to the LDRA,
   which discards the Relay-Reply wrapper and IPv6/UDP headers, then
   uses the DHCPv6 message to fabricate an RA response to the Client.
   The Server uses the information in the Interface-Id option to prepare
   the RA message and to cache the link-layer addresses taken from the
   SLLAOs echoed in the Interface-Id option.

3.15.  The AERO Proxy

   In some environments, Clients may be located in secured subnetwork
   enclaves that do not allow direct communications from the Client to a
   Server in the outside Internetwork.  In that case, the secured
   enclave can employ an AERO Proxy.

   The Proxy is located at the secured enclave perimeter and listens for
   encapsulated RS messages originating from or RA messages destined to
   Clients located within the enclave.  The Proxy acts on these control
   messages as follows:

   o  when the Proxy receives an RS message from a Client within the
      secured enclave, it first authenticates the message then creates a
      proxy neighbor cache entry for the Client in the INCOMPLETE State
      and caches the Client and Server link-layer addresses along with
      any identifying information including Transaction IDs, Client
      Identifiers, Nonce values, etc.  The Proxy then re-encapsulates
      the RS message using its own external address as the source link-
      layer address, sets the X flag in the first SLLAO to '1', and
      forwards the message to the Server.

   o  when the Server receives the RS message, it authenticates the
      message then creates a static neighbor cache entry for the Client
      with the Proxy's address as the link-layer address.  The Server
      then sends an RA message back to the Proxy.

   o  when the Proxy receives the RA message, it matches the message
      with the RS that created the (INCOMPLETE) proxy neighbor cache
      entry.  The Proxy then caches the route information in the message
      as a mapping from the Client's ACPs to the Client's address within
      the secured enclave, and sets the neighbor cache entry state to
      REACHABLE.  The Proxy then re-encapsulates the RA message using
      its own internal address as the source link-layer address, sets
      the X flag in the first SLLAO to '1', and forwards the message to
      the Client.

   After the initial RS/RA exchange, the Proxy forwards data packets
   between the Client and Server with the Server acting as the Client's



Templin                  Expires August 31, 2019               [Page 37]


Internet-Draft                    AERO                     February 2019


   default router.  The Proxy can send RS messages to the Client's
   Server(s) to update Server neighbor cache entries on behalf of the
   Client, e.g., to refresh neighbor cache entry lifetimes and/or to
   convey QoS updates.  The Proxy also forwards any Client control and
   data messages to the Client's primary Server.

   In some subnetworks that employ a Proxy, the Client's ACP can be
   injected into the underlying network routing system.  In that case,
   the Client can send data messages without encapsulation so that the
   native underlying network routing system transports the
   unencapsulated packets to the Proxy.  This can be very beneficial,
   e.g., if the Client connects to the network via low-end data links
   such as some aviation wireless links.  In that case, however, the
   Client's control messages are still sent encapsulated so as to supply
   the Proxy with the address of the Server and to transport IPv6 ND
   messages without decrementing the hop-count.  In summary, the
   interface becomes one where control messages are encapsulated while
   data messages are either unencapsulated or encapsulated according to
   the specific use case.  This encapsulation avoidance represents a
   form of "header compression", meaning that the MTU should be sized
   based on the size of full encapsulated messages even if most messages
   are sent unencapsulated.

3.16.  AERO Route Optimization

   While data packets are flowing from a source Client to a target
   Client that are both holders of ACPs belonging to the same AERO link,
   route optimization SHOULD be used to avoid sending traffic through
   sub-optimal routes that consume expensive resources.  Route
   optimization is conducted on a per-interface basis based on the
   source Client's available underlying interfaces, and may need to
   involve Proxies and Servers in the process.

   Route optimization is initiated by the first eligible AERO node
   closest to the source Client (i.e., the route optimization source) as
   follows:

   o  For VPNed, NATed and Direct underlying interfaces, the Server is
      the route optimization source.

   o  For Proxyed underlying interfaces, the Proxy is the route
      optimization source.

   o  For native underlying interfaces, the Client itself is the route
      optimization source.

   While the source Client sends data packets toward a target Client,
   the route optimization source also sends an NS message to receive a



Templin                  Expires August 31, 2019               [Page 38]


Internet-Draft                    AERO                     February 2019


   solicited NA message from a target Server acting as a Mobility Anchor
   Point (MAP) for the target Client.  The route optimization process
   parallels IPv6 ND Address Resolution.

   The NS includes the AERO address of the route optimization source as
   the network-layer source address, the AERO address corresponding to
   the data packet's destination address as the network-layer
   destination address, and the route optimization source address as the
   link-layer source address.  For Clients and Proxies as the route
   optimization source, the address of the Client's Server is used as
   the link-layer destination address.  For Servers as the route
   optimization source, the address of a Relay is used as the link-layer
   destination address.  The NS message also includes a single SLLAO
   with the route optimization source address in the UDP Port Number and
   IP address fields.  Finally, the NS message includes a Timestamp and
   Nonce option that can be used to match against the corresponding
   solicited NA.

   When the source Server receives or originates the NS message, it
   inserts an additional mid-layer IP encapsulation header between the
   NS message link-layer and network-layer headers.  This mid-layer IP
   header uses the AERO Server Subnet Router Anycast address as the
   source address and the Subnet Router Anycast address corresponding to
   the target AERO address as the destination address.  The source
   Server then changes the link-layer source address to its own address
   and the link-layer destination address to the address of a Relay.
   The Server finally forwards the message to the Relay without
   decrementing the network-layer TTL/Hop Limit field.

   When the Relay receives the double-encapsulated NS message from the
   source Server, it discards the link-layer header(s) and determines
   that the target Server is the next hop toward the target Client by
   consulting its standard IP forwarding table for the Client Subnet
   Router Anycast destination address.  The Relay then encapsulates and
   forwards the message to the target Server the same as for any IP
   router.

   When the target Server receives the double-encapsulated NS message
   from the Relay, it removes the link-layer and mid-layer headers and
   examines the network-layer destination address to determine whether
   the target Client is one of its static neighbors.  If the target
   Client is not a static neighbor, the target Server discards the NS
   and prepares an NA message with no TLLAOs to send back to the route
   optimization source.  The target Server then encapsulates the NA
   message and sets the link-layer source address to its own address and
   sets the link-layer destination address to the address found in the
   NS SLLAO.  The target Server finally includes the Nonce value




Templin                  Expires August 31, 2019               [Page 39]


Internet-Draft                    AERO                     February 2019


   received in the NS plus the current Timestamp, then sends the NA
   message to the route optimization source.

   Otherwise, the target Server adds the link-layer address found in the
   NS SLLAO to the "Report" list for the target Client's neighbor cache
   entry with timer set to ReportTime seconds, but it does not create or
   update a neighbor cache entry.  The target Server then prepares an NA
   message to send back to the route optimization source.  The NA
   message includes a first TLLAO with the target Server's address in
   the IP address and UDP Port Number, with Interface ID set to '255',
   with all P(i) values set to "low" and with "Prefix Length" set to the
   prefix length of the target Client's ACP.  The NA message then
   includes additional TLLAOs for all of the target Client's underlying
   interfaces.  For NATed, VPNed and Direct interfaces, the TLLAO
   addresses are the address of the Server.  For Proxyed interfaces, the
   TLLAO addresses are the addresses of the Proxies, and for native
   interfaces the TLLAO addresses are the addresses of the Client.  The
   target Server then encapsulates the NA message and sets the link-
   layer source address to its own address and sets the link-layer
   destination address to the address found in the NS SLLAO.  The target
   Server finally includes the Nonce value received in the NS plus the
   current Timestamp, then sends the NA message to the route
   optimization source.

   When the route optimization source receives the NA message, it
   verifies the Nonce and Timestamp.  If there are no TLLAOs, the route
   optimization source discards the message; otherwise, it creates a
   dynamic neighbor cache entry for the target Client and caches all
   address, Interface ID, P(i) and Prefix Length information found in
   the NA TLLAOs.  The route optimization source also sets the neighbor
   cache entry state to REACHABLE and sets the timeout value to
   ReachableTime.  Future data packets that flow through the route
   optimization source will then go directly to the target Client
   instead of traveling through a dogleg route involving unnecessary
   Servers and/or Relays.  The route optimization further is shared by
   all sources that send packets to the target Client, i.e., and not
   just the original source Client.

   While new data packets destined to the target are flowing through the
   route optimization source, it sends additional NS messages to the
   target Server before ReachableTime expires to receive a fresh NA
   message.  The route optimization source then updates the dynamic
   neighbor cache entry to refresh ReachableTime, while the target
   Server updates the target Client's static neighbor cache entry to
   refresh ReportTime.  While no data packets are flowing, the route
   optimization source instead allows the dynamic neighbor cache entry
   to expire.  Following expiration, future data packets flowing through
   the route optimization source will again trigger a new route



Templin                  Expires August 31, 2019               [Page 40]


Internet-Draft                    AERO                     February 2019


   optimization exchange while initial data packets travel over a
   suboptimal route via Servers and/or Relays.

   In this arrangement, the route optimization source holds a dynamic
   neighbor cache entry for the target, but the target does not hold a
   dynamic neighbor cache entry for the route optimization source.  The
   route optimization neighbor relationship is therefore asymmetric and
   unidirectional.  If the target Client also has packets to send back
   to the source Client, then a separate route optimization procedure is
   required in the reverse direction.  But, there is no requirement that
   the forward and reverse paths be symmetric.

3.17.  Neighbor Unreachability Detection (NUD)

   AERO nodes perform Neighbor Unreachability Detection (NUD) by sending
   NS messages to elicit solicited NA messages from the target node the
   same as described in [RFC4861].  NUD is performed either reactively
   in response to persistent link-layer errors (see Section 3.13) or
   proactively to confirm reachability in the forward direction.

   When an AERO node sends an NS/NA message, it uses one of its link-
   local addresses as the IPv6 source address and a link-local address
   of the neighbor as the IPv6 destination address.  When route
   optimization directs a source AERO node to a target AERO node, the
   source node SHOULD proactively test the direct path by sending an
   initial NS message to elicit a solicited NA response.  While testing
   the path, the source node can optionally continue sending packets via
   its default router, maintain a small queue of packets until target
   reachability is confirmed, or (optimistically) allow packets to flow
   directly to the target.

   Note that AERO nodes may have multiple underlying interface paths
   toward the target neighbor.  In that case, NUD SHOULD be performed
   over each underlying interface individually and the node should only
   consider the neighbor unreachable if NUD fails over multiple
   underlying interface paths.

   Underlying interface paths that pass NUD tests are marked as
   "reachable", while those that do not are marked as "unreachable".
   These markings inform the AERO interface forwarding algorithm
   specified in Section 3.8.

3.18.  Mobility Management and Quality of Service (QoS)

   AERO is an example of a Distributed Mobility Management (DMM)
   service.  Each Server is responsible for only a subset of the Clients
   on the AERO link, as opposed to a Centralized Mobility Management
   (CMM) service where there is a single network mobility service for



Templin                  Expires August 31, 2019               [Page 41]


Internet-Draft                    AERO                     February 2019


   all Clients.  Clients coordinate with their associated Servers via
   RS/RA exchanges to maintain the DMM profile, and the AERO routing
   system tracks all current Client/Server peering relationships.

   Servers provide a Mobility Anchor Point (MAP) for their dependent
   Clients.  Clients are responsible for maintaining neighbor
   relationships with their Servers through periodic RS/RA exchanges,
   which also serves to confirm Client reachability.  When a Client's
   underlying interface address and/or QoS information changes, the
   Client is responsible for updating the Server with this new
   information.  (Note that for Proxyed interfaces, however, the Proxy
   can perform the RS/RA exchanges on the Client's behalf.)

   Mobility management considerations are specified in the following
   sections.

3.18.1.  Mobility Update Messaging

   AERO Servers accommodate mobility and/or QoS change events by sending
   unsolicited NA messages to all route optimization sources for the
   Client.  When a Server sends an unsolicited NA message, it sets the
   IPv6 source address to the Client's AERO address, sets the IPv6
   destination address to all-nodes multicast, sets the link-layer
   source address to its own address and sets the link-layer destination
   address to either a multicast address or the unicast link-layer
   address of a route optimization source in the Client's Report list.
   If there are multiple route optimization sources, the node sends
   identical unicast copies of the unsolicited NA to each source.

   As for the hot-swap of interface cards discusssed in Section 7.2.6 of
   [RFC4861], the transmission and reception of unsolicited NA messages
   is unreliable but provided as a useful optimization.  The Server can
   send up to MAX_NEIGHBOR_ADVERTISEMENT unsolicited NAs to each route
   optimization source, but in the normal case sends only one.

   When a route optimization source receives an unsolicited NA message,
   it ignores the message if there is no existing neighbor cache entry
   for the Client.  Otherwise, it uses the included TLLAOs to update the
   address and QoS information in the neighbor cache entry, but does not
   reset ReachableTime since the receipt of an unsolicited NA message
   from the target Server does not provide confirmation that any forward
   paths to the target Client are working.

   If unsolicited NA messages are lost, the route optimization source
   may be left with stale address and/or QoS information for the Client
   for up to ReachableTime seconds.  During this time, the route
   optimization source can continue sending packets to the target Client
   according to its current neighbor cache information but may receive



Templin                  Expires August 31, 2019               [Page 42]


Internet-Draft                    AERO                     February 2019


   persistent Destination Unreachable messages and/or unsolicited NA
   messages with no TLLAOs as discussed in Section 3.18.2.  In that
   case, the route optimization source SHOULD re-initiate route
   optimization immediately instead of waiting for ReachableTime to
   expire.

3.18.2.  Forwarding Packets on Behalf of Departed Clients

   When a Server receives packets with destination addresses that do not
   match one of its static neighbor cache Clients, it forwards the
   packets to a Relay which delivers them to the target Client's current
   location.  If the source is not one of its static neighbor Clients,
   the Server also returns an unsolicited NA message to the source with
   no TLLAOs - the sender will then realize that it needs to delete its
   neighbor cache entry that associated the target with this Server and
   re-initiate route optimization.

   When a Proxy receives packets with destination addresses that do not
   match of its proxy neighbor cache Clients, if forwards the packets to
   a Server which delivers them to the target Client's current location.
   If the source is not one of its proxy neighbor Clients, the Proxy
   also returns an unsolicited NA message to the source with no TLLAOs
   the same as described for Servers above.

   When a Client receives packets with destination addresses that do not
   match one of its ACPs, it drops the packets and returns an
   unsolicited NA message to the source with no TLLAOs.

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

   When a Client needs to change its link-layer addresses and/or QoS
   preferences (e.g., due to a mobility event), either the Client or
   Proxy sends RS messages to its Servers using the new link-layer
   address as the source address and with TLLAOs that include the new
   Client UDP Port Number, IP Address and P(i) values.  If the RS
   messages are sent solely for the purpose of updating QoS preferences
   without updating the link-layer address, the UDP Port Number and IP
   Address are set to 0.

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

3.18.4.  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 RS messages to




Templin                  Expires August 31, 2019               [Page 43]


Internet-Draft                    AERO                     February 2019


   its Servers using the new link-layer address as the source address
   and with TLLAOs that include the new Client link-layer information.

3.18.5.  Removing Existing Links from Service

   When a Client needs to remove existing underlying interfaces from
   service (e.g., when it de-activates an existing data link), it sends
   RS messages to its Servers with SLLAOs with all P(i) values set to 0.

   If the Client needs to send RS messages over an underlying interface
   other than the one being removed from service, it MUST include a
   current SLLAO for the sending interface as the first SLLAO and
   include SLLAOs for any underlying interface being removed from
   service as additional TLLAOs.

3.18.6.  Implicit Mobility Management

   AERO interface neighbors MAY provide a configuration option that
   allows them to perform implicit mobility management in which no 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 neighbor cache entry
   for the Client and begins accepting and sending packets to the
   Client's new link-layer 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.

3.18.7.  Moving to a New Server

   When a Client associates with a new Server, it performs the Client
   procedures specified in Section 3.14.2.  The Client then sends RS
   messages with PD release parameters to the old Server to release
   itself from that Server's domain.  If the Client does not receive an
   RA reply after MAX_RTR_SOLICITATIONS attempts, the old Server may
   have failed and the Client should discontinue its release attempts.
   When the old Server processes the PD release, it sends unsolicited NA
   messages with no TLLAOs to all route optimization sources in the
   Client's Report list.

   Clients SHOULD NOT move rapidly between Servers in order to avoid
   causing excessive oscillations in the AERO routing system.  Such



Templin                  Expires August 31, 2019               [Page 44]


Internet-Draft                    AERO                     February 2019


   oscillations could result in intermittent reachability for the Client
   itself, while causing no harm to the network.  Examples of when a
   Client might wish to change to a different Server include a Server
   that has gone unreachable, topological movements of significant
   distance, movement to a new geographic region, etc.

3.19.  Multicast Considerations

   When the underlying network does not support multicast, AERO Clients
   map link-scoped multicast addresses to the link-layer address of a
   Server, which acts as a multicast forwarding agent.  The AERO Client
   also serves as an IGMP/MLD Proxy for its EUNs and/or hosted
   applications per [RFC4605] while using the link-layer address of the
   Server as the link-layer address for all multicast packets.

   When the underlying network supports multicast, AERO nodes use the
   multicast address mapping specification found in [RFC2529] for IPv4
   underlying networks and use a TBD site-scoped multicast mapping for
   IPv6 underlying networks.  In that case, border routers must ensure
   that the encapsulated site-scoped multicast packets do not leak
   outside of the site spanned by the AERO link.

4.  Direct Underlying Interfaces

   When a Client's AERO interface is configured over a Direct underlying
   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 the QoS preferences
   associated with its underling interfaces.  If the Direct underlying
   interface has the highest QoS preference, then the Client's IP
   packets are transmitted directly to the peer without going through an
   underlying network.  If other underlying interfaces have higher QoS
   preferences, then the Client's IP packets are transmitted via a
   different underlying interface, which may result in the inclusion of
   Proxies, Servers and Relays in the communications path.  Direct
   underlying interfaces must be tested periodically for reachability,
   e.g., via NUD.

5.  Operation on AERO Links with /64 ASPs

   IPv6 AERO links typically have ASPs that cover many candidate ACPs of
   length /64 or shorter.  However, in some cases it may be desirable to
   use AERO over links that have only a /64 ASP.  This can be
   accommodated by treating all Clients on the AERO link as simple hosts
   that receive /128 prefix delegations.

   In that case, the Client sends an RS message to the Server the same
   as for ordinary AERO links.  The Server responds with an RA message



Templin                  Expires August 31, 2019               [Page 45]


Internet-Draft                    AERO                     February 2019


   that includes one or more /128 prefixes (i.e., singleton addresses)
   that include the /64 ASP prefix along with an interface identifier
   portion to be assigned to the Client.  The Client and Server then
   configure their AERO addresses based on the interface identifier
   portions of the /128s (i.e., the lower 64 bits) and not based on the
   /64 prefix (i.e., the upper 64 bits).

   For example, if the ASP for the host-only IPv6 AERO link is
   2001:db8:1000:2000::/64, each Client will receive one or more /128
   IPv6 prefix delegations such as 2001:db8:1000:2000::1/128,
   2001:db8:1000:2000::2/128, etc.  When the Client receives the prefix
   delegations, it assigns the AERO addresses fe80::1, fe80::2, etc. to
   the AERO interface, and assigns the global IPv6 addresses (i.e., the
   /128s) to either the AERO interface or an internal virtual interface
   such as a loopback.  In this arrangement, the Client conducts route
   optimization in the same sense as discussed in Section 3.16.

   This specification has applicability for nodes that act as a Client
   on an "upstream" AERO link, but also act as a Server on "downstream"
   AERO links.  More specifically, if the node acts as a Client to
   receive a /64 prefix from the upstream AERO link it can then act as a
   Server to provision /128s to Clients on downstream AERO links.

6.  AERO Adaptations for SEcure Neighbor Discovery (SEND)

   SEcure Neighbor Discovery (SEND) [RFC3971] and Cryptographically
   Generated Addresses (CGAs) [RFC3972] were designed to secure IPv6 ND
   messaging in environments where symmetric network and/or transport-
   layer security services are impractical (see: Section 10).  AERO
   nodes that use SEND/CGA employ the following adaptations.

   When a source AERO node prepares a SEND-protected ND message, it uses
   a link-local CGA as the IPv6 source address and writes the prefix
   embedded in its AERO address (i.e., instead of fe80::/64) in the CGA
   parameters Subnet Prefix field.  When the neighbor receives the ND
   message, it first verifies the message checksum and SEND/CGA
   parameters while using the link-local prefix fe80::/64 (i.e., instead
   of the value in the Subnet Prefix field) to match against the IPv6
   source address of the ND message.

   The neighbor then derives the AERO address of the source by using the
   value in the Subnet Prefix field as the interface identifier of an
   AERO address.  For example, if the Subnet Prefix field contains
   2001:db8:1:2, the neighbor constructs the AERO address as
   fe80::2001:db8:1:2.  The neighbor then caches the AERO address in the
   neighbor cache entry it creates for the source, and uses the AERO
   address as the IPv6 destination address of any ND message replies.




Templin                  Expires August 31, 2019               [Page 46]


Internet-Draft                    AERO                     February 2019


7.  AERO Critical Infrastructure Considerations

   AERO Relays are low-end to midrange Commercial off-the Shelf (COTS)
   standard IP routers with no AERO code.  Relays must be provisioned,
   supported and managed by the AERO Link Service Provider.  Cost for
   purchasing, configuring and managing Relays is nominal even for very
   large AERO links.

   AERO Servers can be standard dedicated server platforms, but most
   often will be deployed as virtual machines in the cloud.  The only
   requirements for Servers are that they can run the AERO user-level
   code and have at least one network interface with a public IP
   address.  As with Relays, Servers must be provisioned, supported and
   managed by the AERO Link Service Provider.  Cost for purchasing,
   configuring and managing Servers is nominal especially for virtual
   Servers hosted in the cloud.

   AERO Proxies are most often standard dedicated server platforms with
   one network interface connected to the secured enclave and a second
   interface connected to the public Internetwork.  As with Servers, the
   only requirements are that they can run the AERO user-level code and
   have at least one interface with a public IP address.  Proxies must
   be provisioned, supported and managed by the administrative authority
   for the secured enclave.  Cost for purchasing, configuring and
   managing Proxies is nominal, and borne by the secured enclave
   administrative authority.

8.  Implementation Status

   An AERO implementation based on OpenVPN (https://openvpn.net/) was
   announced on the v6ops mailing list on January 10, 2018.  The latest
   version is available at: http://linkupnetworks.net/aero/AERO-OpenVPN-
   2.0.tgz.

   An initial public release of the AERO proof-of-concept source code
   was announced on the intarea mailing list on August 21, 2015.  The
   latest version is available at: http://linkupnetworks.net/aero/aero-
   4.0.0.tgz.

   A survey of public domain and commercial SEND implementations is
   available at https://www.ietf.org/mail-archive/web/its/current/
   msg02758.html.

9.  IANA Considerations

   The IANA has assigned a 4-octet Private Enterprise Number "45282" for
   AERO in the "enterprise-numbers" registry.




Templin                  Expires August 31, 2019               [Page 47]


Internet-Draft                    AERO                     February 2019


   The IANA has assigned the UDP port number "8060" for an earlier
   experimental version of AERO [RFC6706].  This document obsoletes
   [RFC6706] and claims the UDP port number "8060" for all future use.

   No further IANA actions are required.

10.  Security Considerations

   AERO link security considerations include considerations for both the
   data plane and the control plane.

   Data plane security considerations are the same as for ordinary
   Internet communications.  Application endpoints in AERO Clients and
   their EUNs SHOULD use application-layer security services such as
   TLS/SSL [RFC5246], DTLS [RFC6347] or SSH [RFC4251] to assure the same
   level of protection as for critical secured Internet services such as
   online banking.  AERO Clients that require VPN access to enterprise
   networks SHOULD use symmetric network and/or transport layer security
   services such as TLS/SSL, DTLS, IPsec [RFC4301], etc.

   Control plane security considerations are the same as for standard
   IPv6 Neighbor Discovery [RFC4861], except that the PRL also improves
   security by providing AERO Clients with a list of trusted Servers.
   As fixed infrastructure elements, AERO Proxies and Servers SHOULD
   pre-configure security associations for one another (e.g., using pre-
   placed keys) and use symmetric network and/or transport layer
   security services such as IPsec, TLS/SSL or DTLS to secure ND
   messages.  AERO Clients that connect to secured enclaves need not
   apply security to their ND messages, since the messages will be
   intercepted by an enclave perimeter Proxy.  AERO Clients located
   outside of secured enclaves SHOULD use symmetric network and/or
   transport layer security to secure their ND exchanges with Servers,
   but when there are many prospective neighbors with dynamically
   changing connectivity an asymmetric security service such as SEND may
   be needed (see: Section 6).

   AERO Servers and Relays 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 by connecting Relays and Servers 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 Servers and Proxies can institute rate limits that




Templin                  Expires August 31, 2019               [Page 48]


Internet-Draft                    AERO                     February 2019


   protect Clients from receiving packet floods that could DoS low data
   rate links.

   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 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.)

   Although public domain and commercial SEND implementations exist,
   concerns regarding the strength of the cryptographic hash algorithm
   have been documented [RFC6273] [RFC4982].

   The PRL MUST be well-managed and secured from unauthorized tampering,
   even though the list includes only public information.

   Security considerations for accepting link-layer ICMP messages and
   reflected packets are discussed throughout the document.

11.  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, Brian Carpenter,
   Wojciech Dec, Ralph Droms, Adrian Farrel, Nick Green, Sri Gundavelli,
   Brian Haberman, Bernhard Haindl, Joel Halpern, Tom Herbert, Sascha
   Hlusiak, Lee Howard, Andre Kostur, Hubert Kuenig, Ted Lemon, Andy
   Malis, Satoru Matsushima, Tomek Mrugalski, Madhu Niraula, Alexandru
   Petrescu, Behcet Saikaya, Michal Skorepa, 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 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, Balaguruna Chidambaram, Irene Chin, Bruce Cornish, Claudiu
   Danilov, Wen Fang, Anthony Gregory, Jeff Holland, Seth Jahne, Ed
   King, Gene MacLean III, Rob Muszkiewicz, Sean O'Sullivan, Greg
   Saccone, Kent Shuey, Brian Skeen, Mike Slane, Carrie Spiker, Brendan
   Williams, Julie Wulff, Yueli Yang, Eric Yeh and other members of the
   BR&T and BIT mobile networking teams.  Kyle Bae, Wayne Benson and
   Eric Yeh are especially acknowledged for implementing the AERO
   functions as extensions to the public domain OpenVPN distribution.



Templin                  Expires August 31, 2019               [Page 49]


Internet-Draft                    AERO                     February 2019


   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:

   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 Information Technology (BIT)
   MobileNet program.

   This work is aligned with the Boeing autonomy program.

12.  References

12.1.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.

   [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>.




Templin                  Expires August 31, 2019               [Page 50]


Internet-Draft                    AERO                     February 2019


   [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>.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [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>.

   [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>.

   [RFC5175]  Haberman, B., Ed. and R. Hinden, "IPv6 Router
              Advertisement Flags Option", RFC 5175,
              DOI 10.17487/RFC5175, March 2008,
              <https://www.rfc-editor.org/info/rfc5175>.

   [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>.




Templin                  Expires August 31, 2019               [Page 51]


Internet-Draft                    AERO                     February 2019


   [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>.

12.2.  Informative References

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

   [I-D.ietf-dmm-distributed-mobility-anchoring]
              Chan, A., Wei, X., Lee, J., Jeon, S., and C. Bernardos,
              "Distributed Mobility Anchoring", draft-ietf-dmm-
              distributed-mobility-anchoring-12 (work in progress),
              January 2019.

   [I-D.ietf-intarea-gue]
              Herbert, T., Yong, L., and O. Zia, "Generic UDP
              Encapsulation", draft-ietf-intarea-gue-06 (work in
              progress), August 2018.

   [I-D.ietf-intarea-gue-extensions]
              Herbert, T., Yong, L., and F. Templin, "Extensions for
              Generic UDP Encapsulation", draft-ietf-intarea-gue-
              extensions-05 (work in progress), August 2018.

   [I-D.ietf-intarea-tunnels]
              Touch, J. and M. Townsley, "IP Tunnels in the Internet
              Architecture", draft-ietf-intarea-tunnels-09 (work in
              progress), July 2018.

   [I-D.ietf-rtgwg-atn-bgp]
              Templin, F., 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-01 (work in progress), January 2019.

   [I-D.templin-6man-aeroaddr]
              Templin, F., "The AERO Address", draft-templin-6man-
              aeroaddr-04 (work in progress), December 2018.

   [I-D.templin-6man-dhcpv6-ndopt]
              Templin, F., "A Unified Stateful/Stateless Configuration
              Service for IPv6", draft-templin-6man-dhcpv6-ndopt-07
              (work in progress), December 2018.





Templin                  Expires August 31, 2019               [Page 52]


Internet-Draft                    AERO                     February 2019


   [I-D.templin-6man-rio-redirect]
              Templin, F. and j. woodyatt, "Route Information Options in
              IPv6 Neighbor Discovery", draft-templin-6man-rio-
              redirect-07 (work in progress), December 2018.

   [I-D.templin-intarea-grefrag]
              Templin, F., "GRE Tunnel Level Fragmentation", draft-
              templin-intarea-grefrag-04 (work in progress), July 2016.

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

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

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

   [I-D.templin-v6ops-pdhost]
              Templin, F., "IPv6 Prefix Delegation and Multi-Addressing
              Models", draft-templin-v6ops-pdhost-23 (work in progress),
              December 2018.

   [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>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <https://www.rfc-editor.org/info/rfc1191>.

   [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>.





Templin                  Expires August 31, 2019               [Page 53]


Internet-Draft                    AERO                     February 2019


   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
              1996, <https://www.rfc-editor.org/info/rfc1981>.

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

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [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>.

   [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>.

   [RFC2764]  Gleeson, B., Lin, A., Heinanen, J., Armitage, G., and A.
              Malis, "A Framework for IP Based Virtual Private
              Networks", RFC 2764, DOI 10.17487/RFC2764, February 2000,
              <https://www.rfc-editor.org/info/rfc2764>.

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              DOI 10.17487/RFC2784, March 2000,
              <https://www.rfc-editor.org/info/rfc2784>.

   [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",
              RFC 2890, DOI 10.17487/RFC2890, September 2000,
              <https://www.rfc-editor.org/info/rfc2890>.

   [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery",
              RFC 2923, DOI 10.17487/RFC2923, September 2000,
              <https://www.rfc-editor.org/info/rfc2923>.

   [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>.




Templin                  Expires August 31, 2019               [Page 54]


Internet-Draft                    AERO                     February 2019


   [RFC3819]  Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, DOI 10.17487/RFC3819, July 2004,
              <https://www.rfc-editor.org/info/rfc3819>.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213,
              DOI 10.17487/RFC4213, October 2005,
              <https://www.rfc-editor.org/info/rfc4213>.

   [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>.

   [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>.

   [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>.



Templin                  Expires August 31, 2019               [Page 55]


Internet-Draft                    AERO                     February 2019


   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963,
              DOI 10.17487/RFC4963, July 2007,
              <https://www.rfc-editor.org/info/rfc4963>.

   [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>.

   [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>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [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>.

   [RFC5720]  Templin, F., "Routing and Addressing in Networks with
              Global Enterprise Recursion (RANGER)", RFC 5720,
              DOI 10.17487/RFC5720, February 2010,
              <https://www.rfc-editor.org/info/rfc5720>.

   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 5996, DOI 10.17487/RFC5996, September 2010,
              <https://www.rfc-editor.org/info/rfc5996>.




Templin                  Expires August 31, 2019               [Page 56]


Internet-Draft                    AERO                     February 2019


   [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>.

   [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>.

   [RFC6422]  Lemon, T. and Q. Wu, "Relay-Supplied DHCP Options",
              RFC 6422, DOI 10.17487/RFC6422, December 2011,
              <https://www.rfc-editor.org/info/rfc6422>.

   [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>.

   [RFC6864]  Touch, J., "Updated Specification of the IPv4 ID Field",
              RFC 6864, DOI 10.17487/RFC6864, February 2013,
              <https://www.rfc-editor.org/info/rfc6864>.

   [RFC8086]  Yong, L., Ed., Crabbe, E., Xu, X., and T. Herbert, "GRE-
              in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086,
              March 2017, <https://www.rfc-editor.org/info/rfc8086>.

   [TUNTAP]   Wikipedia, W., "http://en.wikipedia.org/wiki/TUN/TAP",
              October 2014.

Appendix A.  AERO Alternate Encapsulations

   When GUE encapsulation is not needed, AERO can use common
   encapsulations such as IP-in-IP [RFC2003][RFC2473][RFC4213], Generic
   Routing Encapsulation (GRE) [RFC2784][RFC2890] and others.  The
   encapsulation is therefore only differentiated from non-AERO tunnels



Templin                  Expires August 31, 2019               [Page 57]


Internet-Draft                    AERO                     February 2019


   through the application of AERO control messaging and not through,
   e.g., a well-known UDP port number.

   As for GUE encapsulation, alternate AERO encapsulation formats may
   require encapsulation layer fragmentation.  For simple IP-in-IP
   encapsulation, an IPv6 fragment header is inserted directly between
   the inner and outer IP headers when needed, i.e., even if the outer
   header is IPv4.  The IPv6 Fragment Header is identified to the outer
   IP layer by its IP protocol number, and the Next Header field in the
   IPv6 Fragment Header identifies the inner IP header version.  For GRE
   encapsulation, a GRE fragment header is inserted within the GRE
   header [I-D.templin-intarea-grefrag].

   Figure 4 shows the AERO IP-in-IP encapsulation format before any
   fragmentation is applied:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |     Outer IPv4 Header     |      |    Outer IPv6 Header      |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |IPv6 Frag Header (optional)|      |IPv6 Frag Header (optional)|
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |      Inner IP Header      |      |       Inner IP Header     |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                           |      |                           |
        ~                           ~      ~                           ~
        ~    Inner Packet Body      ~      ~     Inner Packet Body     ~
        ~                           ~      ~                           ~
        |                           |      |                           |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Minimal Encapsulation in IPv4      Minimal Encapsulation in IPv6


           Figure 4: Minimal Encapsulation Format using IP-in-IP

   Figure 5 shows the AERO GRE encapsulation format before any
   fragmentation is applied:














Templin                  Expires August 31, 2019               [Page 58]


Internet-Draft                    AERO                     February 2019


        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |        Outer IP Header        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |          GRE Header           |
        | (with checksum, key, etc..)   |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        | GRE Fragment Header (optional)|
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |        Inner IP Header        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                               |
        ~                               ~
        ~      Inner Packet Body        ~
        ~                               ~
        |                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                 Figure 5: Minimal Encapsulation Using GRE

   Alternate encapsulation may be preferred in environments where GUE
   encapsulation would add unnecessary overhead.  For example, certain
   low-bandwidth wireless data links may benefit from a reduced
   encapsulation overhead.

   GUE encapsulation can traverse network paths that are inaccessible to
   non-UDP encapsulations, e.g., for crossing Network Address
   Translators (NATs).  More and more, network middleboxes are also
   being configured to discard packets that include anything other than
   a well-known IP protocol such as UDP and TCP.  It may therefore be
   necessary to determine the potential for middlebox filtering before
   enabling alternate encapsulation in a given environment.

   In addition to IP-in-IP, GRE and GUE, AERO can also use security
   encapsulations such as IPsec, TLS/SSL, DTLS, etc.  In that case, AERO
   control messaging and route determination occur before security
   encapsulation is applied for outgoing packets and after security
   decapsulation is applied for incoming packets.

   AERO is especially well suited for use with VPN system encapsulations
   such as OpenVPN [OVPN].

Appendix B.  S/TLLAO Extensions for Special-Purpose Links

   The AERO S/TLLAO format specified in Section 3.5 includes a Length
   value of 5 (i.e., 5 units of 8 octets).  However, special-purpose
   links may extend the basic format to include additional fields and a
   Length value larger than 5.



Templin                  Expires August 31, 2019               [Page 59]


Internet-Draft                    AERO                     February 2019


   For example, adaptation of AERO to the Aeronautical
   Telecommunications Network with Internet Protocol Services (ATN/IPS)
   includes link selection preferences based on transport port numbers
   in addition to the existing DSCP-based preferences.  ATN/IPS nodes
   maintain a map of transport port numbers to 64 possible preference
   fields, e.g., TCP port 22 maps to preference field 8, TCP port 443
   maps to preference field 20, UDP port 8060 maps to preference field
   34, etc.  The extended S/TLLAO format for ATN/IPS is shown in
   Figure 6, where the Length value is 7 and the 'Q(i)' fields provide
   link preferences for the corresponding transport port number.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Type     |   Length = 7  | Prefix Length |   Reserved    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Interface ID         |        UDP Port Number        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                          IP Address                           +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P00|P01|P02|P03|P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P16|P17|P18|P19|P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P32|P33|P34|P35|P36|P37|P38|P39|P40|P41|P42|P43|P44|P45|P46|P47|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Q00|Q01|Q02|Q03|Q04|Q05|Q06|Q07|Q08|Q09|Q10|Q11|Q12|Q13|Q14|Q15|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Q16|Q17|Q18|Q19|Q20|Q21|Q22|Q23|Q24|Q25|Q26|Q27|Q28|Q29|Q30|Q31|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Q32|Q33|Q34|Q35|Q36|Q37|Q38|Q39|Q40|Q41|Q42|Q43|Q44|Q45|Q46|Q47|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Q48|Q49|Q50|Q51|Q52|Q53|Q54|Q55|Q56|Q57|Q58|Q59|Q60|Q61|Q62|Q63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 6: ATN/IPS Extended S/TLLAO Format







Templin                  Expires August 31, 2019               [Page 60]


Internet-Draft                    AERO                     February 2019


Appendix C.  Change Log

   << RFC Editor - remove prior to publication >>

   Changes from draft-templin-intarea-6706bis-05 to draft-templin-
   intrea-6706bis-06:

   o  Major re-work and simplification of Route Optimization function

   o  Added Distributed Mobility Management (DMM) and Mobility Anchor
      Point (MAP) terminology

   o  New section on "AERO Critical Infrastructure Element
      Considerations" demonstrating low overall cost for the service

   o  minor text revisions and deletions

   o  removed extraneous appendices

   Changes from draft-templin-intarea-6706bis-04 to draft-templin-
   intrea-6706bis-05:

   o  New Appendix E on S/TLLAO Extensions for special-purpose links.
      Discussed ATN/IPS as example.

   o  New sentence in introduction to declare appendices as non-
      normative.

   Changes from draft-templin-intarea-6706bis-03 to draft-templin-
   intrea-6706bis-04:

   o  Added definitions for Potential Router List (PRL) and secure
      enclave

   o  Included text on mapping transport layer port numbers to network
      layer DSCP values

   o  Added reference to DTLS and DMM Distributed Mobility Anchoring
      working group document

   o  Reworked Security Considerations

   o  Updated references.

   Changes from draft-templin-intarea-6706bis-02 to draft-templin-
   intrea-6706bis-03:

   o  Added new section on SEND.



Templin                  Expires August 31, 2019               [Page 61]


Internet-Draft                    AERO                     February 2019


   o  Clarifications on "AERO Address" section.

   o  Updated references and added new reference for RFC8086.

   o  Security considerations updates.

   o  General text clarifications and cleanup.

   Changes from draft-templin-intarea-6706bis-01 to draft-templin-
   intrea-6706bis-02:

   o  Note on encapsulation avoidance in Section 4.

   Changes from draft-templin-intarea-6706bis-00 to draft-templin-
   intrea-6706bis-01:

   o  Remove DHCPv6 Server Release procedures that leveraged the old way
      Relays used to "route" between Server link-local addresses

   o  Remove all text relating to Relays needing to do any AERO-specific
      operations

   o  Proxy sends RS and receives RA from Server using SEND.  Use CGAs
      as source addresses, and destination address of RA reply is to the
      AERO address corresponding to the Client's ACP.

   o  Proxy uses SEND to protect RS and authenticate RA (Client does not
      use SEND, but rather relies on subnetwork security.  When the
      Proxy receives an RS from the Client, it creates a new RS using
      its own addresses as the source and uses SEND with CGAs to send a
      new RS to the Server.

   o  Emphasize distributed mobility management

   o  AERO address-based RS injection of ACP into underlying routing
      system.

   Changes from draft-templin-aerolink-82 to draft-templin-intarea-
   6706bis-00:

   o  Document use of NUD (NS/NA) for reliable link-layer address
      updates as an alternative to unreliable unsolicited NA.
      Consistent with Section 7.2.6 of RFC4861.

   o  Server adds additional layer of encapsulation between outer and
      inner headers of NS/NA messages for transmission through Relays
      that act as vanilla IPv6 routers.  The messages include the AERO
      Server Subnet Router Anycast address as the source and the Subnet



Templin                  Expires August 31, 2019               [Page 62]


Internet-Draft                    AERO                     February 2019


      Router Anycast address corresponding to the Client's ACP as the
      destination.

   o  Clients use Subnet Router Anycast address as the encapsulation
      source address when the access network does not provide a
      topologically-fixed address.

Author's Address

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

   Email: fltemplin@acm.org



































Templin                  Expires August 31, 2019               [Page 63]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/