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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Informational                          October 14, 2011
Expires: April 16, 2012


              The Internet Routing Overlay Network (IRON)
                      draft-templin-ironbis-04.txt

Abstract

   Since the Internet must continue to support escalating growth due to
   increasing demand, it is clear that current routing architectures and
   operational practices must be updated.  This document proposes an
   Internet Routing Overlay Network (IRON) architecture that supports
   sustainable growth while requiring no changes to end systems and no
   changes to the existing routing system.  In addition to routing
   scaling, IRON further addresses other important issues including
   mobility management, mobile networks, multihoming, traffic
   engineering and NAT traversal.  While business considerations are an
   important determining factor for widespread adoption, they are out of
   scope for 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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on April 16, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of



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   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.













































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  The Internet Routing Overlay Network . . . . . . . . . . . . .  7
     3.1.  IRON Client  . . . . . . . . . . . . . . . . . . . . . . .  9
     3.2.  IRON Serving Router  . . . . . . . . . . . . . . . . . . . 10
     3.3.  IRON Relay Router  . . . . . . . . . . . . . . . . . . . . 10
   4.  IRON Organizational Principles . . . . . . . . . . . . . . . . 11
   5.  IRON Control Plane Operation . . . . . . . . . . . . . . . . . 13
     5.1.  IRON Client Operation  . . . . . . . . . . . . . . . . . . 13
     5.2.  IRON Server Operation  . . . . . . . . . . . . . . . . . . 14
     5.3.  IRON Relay Operation . . . . . . . . . . . . . . . . . . . 14
   6.  IRON Forwarding Plane Operation  . . . . . . . . . . . . . . . 15
     6.1.  IRON Client Operation  . . . . . . . . . . . . . . . . . . 15
     6.2.  IRON Server Operation  . . . . . . . . . . . . . . . . . . 16
     6.3.  IRON Relay Operation . . . . . . . . . . . . . . . . . . . 17
   7.  IRON Reference Operating Scenarios . . . . . . . . . . . . . . 18
     7.1.  Both Hosts within Same IRON Instance . . . . . . . . . . . 18
       7.1.1.  EUNs Served by Same Server . . . . . . . . . . . . . . 18
       7.1.2.  EUNs Served by Different Servers . . . . . . . . . . . 20
     7.2.  Mixed IRON and Non-IRON Hosts  . . . . . . . . . . . . . . 22
       7.2.1.  From IRON Host A to Non-IRON Host B  . . . . . . . . . 22
       7.2.2.  From Non-IRON Host B to IRON Host A  . . . . . . . . . 24
     7.3.  Hosts within Different IRON Instances  . . . . . . . . . . 25
   8.  Mobility, Multiple Interfaces, Multihoming, and Traffic
       Engineering  . . . . . . . . . . . . . . . . . . . . . . . . . 25
     8.1.  Mobility Management and Mobile Networks  . . . . . . . . . 26
     8.2.  Multiple Interfaces and Multihoming  . . . . . . . . . . . 26
     8.3.  Traffic Engineering  . . . . . . . . . . . . . . . . . . . 27
   9.  Renumbering Considerations . . . . . . . . . . . . . . . . . . 27
   10. NAT Traversal Considerations . . . . . . . . . . . . . . . . . 27
   11. Multicast Considerations . . . . . . . . . . . . . . . . . . . 28
   12. Nested EUN Considerations  . . . . . . . . . . . . . . . . . . 28
     12.1. Host A Sends Packets to Host Z . . . . . . . . . . . . . . 29
     12.2. Host Z Sends Packets to Host A . . . . . . . . . . . . . . 30
   13. Implications for the Internet  . . . . . . . . . . . . . . . . 31
   14. Additional Considerations  . . . . . . . . . . . . . . . . . . 32
   15. Related Initiatives  . . . . . . . . . . . . . . . . . . . . . 32
   16. Security Considerations  . . . . . . . . . . . . . . . . . . . 33
   17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34
   18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
     18.1. Normative References . . . . . . . . . . . . . . . . . . . 34
     18.2. Informative References . . . . . . . . . . . . . . . . . . 34
   Appendix A.  IRON Operation over Internetworks with Different
                Address Families  . . . . . . . . . . . . . . . . . . 37
   Appendix B.  Scaling Considerations  . . . . . . . . . . . . . . . 38
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 39



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1.  Introduction

   Growth in the number of entries instantiated in the Internet routing
   system has led to concerns regarding unsustainable routing scaling
   [RFC4984][RADIR].  Operational practices such as the increased use of
   multihoming with Provider-Independent (PI) addressing are resulting
   in more and more de-aggregated prefixes being injected into the
   routing system from more and more end user networks.  Furthermore,
   depletion of the public IPv4 address space has raised concerns for
   both increased de-aggregation (leading to yet further routing system
   entries) and an impending address space run-out scenario.  At the
   same time, the IPv6 routing system is beginning to see growth
   [BGPMON] which must be managed in order to avoid the same routing
   scaling issues the IPv4 Internet now faces.  Since the Internet must
   continue to scale to accommodate increasing demand, it is clear that
   new methodologies and operational practices are needed.

   Several related works have investigated routing scaling issues.
   Virtual Aggregation (VA) [GROW-VA] and Aggregation in Increasing
   Scopes (AIS) [EVOLUTION] are global routing proposals that introduce
   routing overlays with Virtual Prefixes (VPs) to reduce the number of
   entries required in each router's Forwarding Information Base (FIB)
   and Routing Information Base (RIB).  Routing and Addressing in
   Networks with Global Enterprise Recursion (RANGER) [RFC5720] examines
   recursive arrangements of enterprise networks that can apply to a
   very broad set of use-case scenarios [RFC6139].  IRON specifically
   adopts the RANGER Non-Broadcast, Multiple Access (NBMA) tunnel
   virtual-interface model, and uses Virtual Enterprise Traversal (VET)
   [INTAREA-VET] the Subnetwork Adaptation and Encapsulation Layer
   (SEAL) [INTAREA-SEAL] and Asymmetric Extended Route Optimization
   [AERO] as its functional building blocks.

   This document proposes an Internet Routing Overlay Network (IRON)
   architecture with goals of supporting scalable routing and addressing
   while requiring no changes to the Internet's Border Gateway Protocol
   (BGP) interdomain routing system [RFC4271].  IRON observes the
   Internet Protocol standards [RFC0791][RFC2460], while other network-
   layer protocols that can be encapsulated within IP packets (e.g.,
   OSI/CLNP (Connectionless Network Protocol) [RFC1070], etc.) are also
   within scope.

   IRON borrows concepts from VA and AIS, and further borrows concepts
   from the Internet Vastly Improved Plumbing (Ivip) [IVIP-ARCH]
   architecture proposal along with its associated Translating Tunnel
   Router (TTR) mobility extensions [TTRMOB].  Indeed, the TTR model to
   a great degree inspired the IRON mobility architecture design
   discussed in this document.  The Network Address Translator (NAT)
   traversal techniques adapted for IRON were inspired by the Simple



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   Address Mapping for Premises Legacy Equipment (SAMPLE) proposal
   [SAMPLE].

   IRON is a global virtual routing system comprising Virtual Service
   Provider (VSP) overlay networks that service Aggregated Prefixes
   (APs) from which more-specific Client Prefixes (CPs) are delegated.
   IRON is motivated by a growing end user demand for mobility
   management, mobile networks, multihoming and traffic engineering
   while using stable addressing to minimize dependence on network
   renumbering [RFC4192][RFC5887].  IRON VSP overlay network instances
   use the existing IPv4 and IPv6 Internets as virtual NBMA links for
   tunneling inner network layer packets within outer network layer
   headers (see Section 3).  Each IRON instance requires deployment of a
   small number of relays and servers in the Internet, as well as client
   devices that connect End User Networks (EUNs).  No modifications to
   hosts, and no modifications to existing routers, are required.  The
   following sections discuss details of the IRON architecture.


2.  Terminology

   This document makes use of the following terms:

   Aggregated Prefix (AP):
      a short network-layer prefix (e.g., an IPv4 /16, an IPv6 /20, an
      OSI Network Service Access Protocol (NSAP) prefix, etc.) that is
      owned and managed by a Virtual Service Provider (VSP).  The term
      "Aggregated Prefix (AP)" used in this document is the equivalent
      to the term "Virtual Prefix (VP)" used in Virtual Aggregation (VA)
      [GROW-VA].

   Client Prefix (CP):
      a more-specific network-layer prefix (e.g., an IPv4 /28, an IPv6
      /56, etc.) derived from an AP and delegated to a client end user
      network.

   Client Prefix Address (CPA):
      a network-layer address belonging to a CP and assigned to an
      interface in an End User Network (EUN).

   End User Network (EUN):
      an edge network that connects an end user's devices (e.g.,
      computers, routers, printers, etc.) to the Internet.  IRON EUNs
      are mobile networks, and can change their ISP attachments without
      having to renumber.






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   Internet Routing Overlay Network (IRON):
      the union of all VSP overlay network instances.  Each such IRON
      instance supports routing within the overlay through encapsulation
      of inner packets within outer headers.  Each IRON instance appears
      as a virtual enterprise network, and connects to the global
      Internet the same as for any Autonomous System (AS).

   IRON Client Router/Host ("Client"):
      a customer device that logically connects EUNs to an IRON instance
      via an NBMA tunnel virtual interface.  The device is normally a
      router, but may instead be a host if the "EUN" is a singleton end
      system.

   IRON Serving Router ("Server"):
      a VSP's IRON instance router that provides forwarding and mapping
      services for Clients.

   IRON Relay Router ("Relay"):
      a VSP's router that acts as a relay between the IRON instance and
      the (native) Internet.

   IRON Agent (IA):
      generically refers to any of an IRON Client/Server/Relay.

   IRON Instance:
      a set of IRON Agents deployed by a VSP to service EUNs through
      automatic tunneling over the Internet.

   Internet Service Provider (ISP):
      a service provider that connects EUNs to the Internet.  In other
      words, an ISP is responsible for providing EUNs with data link
      services for basic Internet connectivity.

   Locator:
      an IP address assigned to the interface of a router or end system
      connected to a public or private network over which tunnels are
      formed.  Locators taken from public IP prefixes are routable on a
      global basis, while locators taken from private IP prefixes
      [RFC1918] are made public via Network Address Translation (NAT).

   Routing and Addressing in Networks with Global Enterprise Recursion
   (RANGER):
      an architectural examination of virtual overlay networks applied
      to enterprise network scenarios, with implications for a wider
      variety of use cases.






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   Subnetwork Encapsulation and Adaptation Layer (SEAL):
      an encapsulation sublayer that provides extended identification
      fields and control messages to ensure deterministic network-layer
      feedback.

   Virtual Enterprise Traversal (VET):
      a method for discovering border routers and forming dynamic
      tunnel-neighbor relationships over enterprise networks (or sites)
      with varying properties.

   Virtual Service Provider (VSP):
      a company that owns and manages a set of APs from which it
      delegates CPs to EUNs.

   VSP Overlay Network:
      the same as defined above for IRON Instance.


3.  The Internet Routing Overlay Network

   The Internet Routing Overlay Network (IRON) is the union of all
   Virtual Service Provider (VSP) overlay networks (also known as "IRON
   instances").  IRON provides a number of important services to End
   User Networks (EUNs) that are not well supported in the current
   Internet architecture, including routing scaling, mobility
   management, mobile networks, multihoming, traffic engineering and NAT
   traversal.  While the principles presented in this document are
   discussed within the context of the public global Internet, they can
   also be applied to any other form of autonomous internetwork (e.g.,
   corporate enterprise networks, civil aviation networks, tactical
   military networks, etc.).  Hence, the terms "Internet" and
   "internetwork" are used interchangeably within this document.

   Each IRON instance consists of IRON Agents (IAs) that automatically
   tunnel the packets of end-to-end communication sessions within
   encapsulating headers used for Internet routing.  IAs use the Virtual
   Enterprise Traversal (VET) [INTAREA-VET] virtual NBMA link model in
   conjunction with the Subnetwork Encapsulation and Adaptation Layer
   (SEAL) [INTAREA-SEAL] to encapsulate inner network-layer packets
   within outer network layer headers, as shown in Figure 1.











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                                         +-------------------------+
                                         |    Outer headers with   |
                                         ~     locator addresses   ~
                                         |     (IPv4 or IPv6)      |
                                         +-------------------------+
                                         |       SEAL Header       |
       +-------------------------+       +-------------------------+
       |   Inner Packet Header   |  -->  |   Inner Packet Header   |
       ~    with CPA addresses   ~  -->  ~    with CPA addresses   ~
       | (IPv4, IPv6, OSI, etc.) |  -->  | (IPv4, IPv6, OSI, etc.) |
       +-------------------------+       +-------------------------+
       |                         |  -->  |                         |
       ~    Inner Packet Body    ~  -->  ~    Inner Packet Body    ~
       |                         |  -->  |                         |
       +-------------------------+       +-------------------------+

          Inner packet before                Outer packet after
           encapsulation                       encapsulation

     Figure 1: Encapsulation of Inner Packets within Outer IP Headers

   VET specifies automatic tunneling and tunnel neighbor coordination
   mechanisms, where IAs appear as neighbors on an NBMA tunnel virtual
   link.  SEAL specifies the format and usage of the SEAL header used
   for encapsulation.  Additionally, Asymmetric Extended Route
   Optimization (AERO) [AERO] specifies the method for reducing routing
   path stretch.  Together, these documents specify elements of a SEAL
   Control Message Protocol (SCMP) used to deterministically exchange
   and authenticate neighbor discovery messages, route redirections,
   indications of Path Maximum Transmission Unit (PMTU) limitations,
   destination unreachables, etc.

   Each IRON instance comprises a set of IAs distributed throughout the
   Internet to provide internetworking services for a set of Aggregated
   Prefixes (APs).  VSPs delegate sub-prefixes from their APs, which
   they provide to end users as Client Prefixes (CPs).  In turn, end
   users assign CPs to Client IAs which connect their End User Networks
   (EUNs) to the VSP IRON instance.

   VSPs may have no affiliation with the ISP networks from which end
   users obtain their basic Internet connectivity.  In that case, the
   VSP can service its end users without the need to coordinate its
   activities with ISPs or other VSPs.  Further details on VSP business
   considerations are out of scope for this document.

   IRON requires no changes to end systems or to existing routers.
   Instead, IAs are deployed either as new platforms or as modifications
   to existing platforms.  IAs may be deployed incrementally without



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   disturbing the existing Internet routing system, and act as waypoints
   (or "cairns") for navigating VSP overly networks.  The functional
   roles for IAs are described in the following sections.

3.1.  IRON Client

   An IRON Client (or, simply, "Client") is a router or host that
   logically connects EUNs to the VSP's IRON instance via tunnels, as
   shown in Figure 2.  Clients obtain CPs from their VSPs and use them
   to number subnets and interfaces within the EUNs.

   Each Client connects to one or more Servers in the IRON instance
   which serve as default routers.  The Servers in turn consider this
   class of Clients as "connected" Clients.  Clients also dynamically
   discover destination-specific Servers through the receipt of redirect
   messages.  These destination-specific Servers in turn consider this
   class of Clients as "foreign" Clients.

   A Client can be deployed on the same physical platform that also
   connects EUNs to the end user's ISPs, but it may also be deployed as
   a separate router within the EUN.  (This model applies even if the
   EUN connects to the ISP via a Network Address Translator (NAT) -- see
   Section 6.7).  Finally, a Client may also be a simple end system that
   connects a singleton EUN and exhibits the outward appearance of a
   host.
                           .-.
                        ,-(  _)-.
        +--------+   .-(_    (_  )-.
        | Client |--(_     ISP      )
        +---+----+     `-(______)-'
            |   <= T         \     .-.
           .-.       u        \ ,-(  _)-.
        ,-(  _)-.       n     .-(_    (-  )-.
     .-(_    (_  )-.      n  (_   Internet   )
    (_     EUN      )       e   `-(______)-
       `-(______)-'           l          ___
            |                   s =>    (:::)-.
       +----+---+                   .-(::::::::)
       |  Host  |                .-(::: IRON :::)-.
       +--------+               (:::: Instance ::::)
                                 `-(::::::::::::)-'
                                    `-(::::::)-'

           Figure 2: IRON Client Connecting EUN to IRON Instance







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3.2.  IRON Serving Router

   An IRON serving router (or, simply, "Server") is a VSP's router that
   provides forwarding and mapping services within the IRON instance for
   the CPs that have been delegated to end user Clients.  In typical
   deployments, a VSP will deploy many Servers for the IRON instance in
   a globally distributed fashion (e.g., as depicted in Figure 3) around
   the Internet so that Clients can discover those that are nearby.

             +--------+    +--------+
             | Boston |    | Tokyo  |
             | Server |    | Server |
             +--+-----+    ++-------+
     +--------+  \         /
     | Seattle|   \   ___ /
     | Server |    \ (:::)-.       +--------+
     +------+-+  .-(::::::::)------+ Paris  |
             \.-(::: IRON :::)-.   | Server |
             (:::: Instance ::::)  +--------+
              `-(::::::::::::)-'
   +--------+ /  `-(::::::)-'  \     +--------+
   | Moscow +          |        \--- + Sydney |
   | Server |     +----+---+         | Server |
   +--------+     | Cairo  |         +--------+
                  | Server |
                  +--------+

             Figure 3: IRON Server Global Distribution Example

   Each Server acts as a tunnel-endpoint router.  The Server forms
   bidirectional tunnel-neighbor relationships with each of its
   connected Clients, and also serves as the unidirectional tunnel-
   neighbor egress for dynamically discovered foreign Clients.  Each
   Server also forms bidirectional tunnel-neighbor relationships with a
   set of Relays that can forward packets from the IRON instance out to
   the native Internet and vice versa, as discussed in the next section.

3.3.  IRON Relay Router

   An IRON Relay Router (or, simply, "Relay") is a router that connects
   the VSP's IRON instance to the Internet as an Autonomous System (AS).
   The Relay therefore also serves as an Autonomous System Border Router
   (ASBR) that is owned and managed by the VSP.

   Each VSP configures one or more Relays that advertise the VSP's APs
   into the IPv4 and/or IPv6 global Internet routing systems.  Each
   Relay associates with the VSP's IRON instance Servers, e.g., via
   tunnel virtual links over the IRON instance, via a physical



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   interconnect such as an Ethernet cable, etc.  The Relay role is
   depicted in Figure 4.


                      .-.
                   ,-(  _)-.
                .-(_    (_  )-.
               (_   Internet   )
                  `-(______)-'   |  +--------+
                        |        |--| Server |
                   +----+---+    |  +--------+
                   | Relay  |----|  +--------+
                   +--------+    |--| Server |
                       _||       |  +--------+
                      (:::)-.  (Physical Interconnects)
                  .-(::::::::)
   +--------+  .-(::: IRON :::)-.  +--------+
   | Server |=(:::: Instance ::::)=| Server |
   +--------+  `-(::::::::::::)-'  +--------+
                  `-(::::::)-'
                       ||      (Tunnels)
                   +--------+
                   | Server |
                   +--------+

      Figure 4: IRON Relay Router Connecting IRON Instance to Native
                                 Internet


4.  IRON Organizational Principles

   The IRON consists of the union of all VSP overlay networks configured
   over the Internet.  Each such IRON instance represents a distinct
   "patch" on the underlying Internet "quilt", where the patches are
   stitched together by standard Internet routing.  When a new IRON
   instance is deployed, it becomes yet another patch on the quilt and
   coordinates its internal routing system independently of all other
   patches.

   Each IRON instance connects to the Internet as an AS in the Internet
   routing system using a public BGP Autonomous System Number (ASN).
   The IRON instance maintains a set of Relays that serve as ASBRs as
   well as a set of Servers that provide routing and addressing services
   to Clients.  Figure 5 depicts the logical arrangement of Relays,
   Servers, and Clients in an IRON instance.






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                              .-.
                           ,-(  _)-.
                        .-(_    (_  )-.
                       (__ Internet   _)
                          `-(______)-'

          <------------     Relays      ------------>
                    ________________________
                   (::::::::::::::::::::::::)-.
               .-(:::::::::::::::::::::::::::::)
            .-(:::::::::::::::::::::::::::::::::)-.
           (::::::::::: IRON Instance :::::::::::::)
            `-(:::::::::::::::::::::::::::::::::)-'
               `-(::::::::::::::::::::::::::::)-'

          <------------    Servers      ------------>
          .-.                .-.                     .-.
       ,-(  _)-.          ,-(  _)-.               ,-(  _)-.
    .-(_    (_  )-.    .-(_    (_  )-.         .-(_    (_  )-.
   (__   ISP A    _)  (__   ISP B    _)  ...  (__   ISP x    _)
      `-(______)-'       `-(______)-'            `-(______)-'
           <-----------      NATs        ------------>

           <----------- Clients and EUNs ----------->

                        Figure 5: IRON Organization

   Each Relay connects the IRON instance directly to the underlying IPv4
   and/or IPv6 Internets via external BGP (eBGP) peerings with
   neighboring ASes.  It also advertises the VSP's IPv4 APs into the
   IPv4 Internet routing system and advertises the VSP's IPv6 APs into
   the IPv6 Internet routing system.  Relays will therefore receive
   packets with CPA destination addresses sent by end systems in the
   Internet and forward them to a Server that connects the Client to
   which the corresponding CP has been delegated.  Finally, the IRON
   instance Relays maintain synchronization by running interior BGP
   (iBGP) between themselves the same as for ordinary ASBRs.

   In a simple VSP overlay network arrangement, each Server can be
   configured as an ASBR for a stub AS using a private ASN [RFC1930] to
   peer with each IRON instance Relay the same as for an ordinary eBGP
   neighbor.  (The Server and Relay functions can instead be deployed
   together on the same physical platform as a unified gateway.)  Each
   Server maintains a working set of connected Clients for which it
   caches CP-to-Client mappings in its forwarding table.  Each Server
   also, in turn, propagates the list of CPs in its working set to its
   neighboring Relays via eBGP.  Therefore, each Server only needs to
   track the CPs for its current working set of Clients, while each



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   Relay will maintain a full CP-to-Server forwarding table that
   represents reachability information for all CPs in the IRON instance.

   Each Client obtains its basic connectivity from ISPs, and connects to
   Servers to attach its EUNs to the IRON instance.  Each EUN can
   further connect to the IRON instance via multiple Clients as long as
   the Clients coordinate with one another, e.g., to mitigate EUN
   partitions.  Unlike Relays and Servers, Clients may use private
   addresses behind one or several layers of NATs.  Each Client
   initially discovers a list of nearby Servers then forms a
   bidirectional tunnel-neighbor relationship with one or more Servers
   through an initial exchange followed by periodic keepalives.

   After a Client connects to Servers, it forwards initial outbound
   packets from its EUNs by tunneling them to a Server, which may, in
   turn, forward them to a nearby Relay within the IRON instance.  The
   Client may subsequently receive redirect messages informing it of a
   more direct route through a different Server within the IRON instance
   that serves the final destination EUN.  This foreign Server in turn
   provides the Client with a unidirectional tunnel-neighbor egress for
   route optimization purposes,.

   IRON can also be used to support APs of network-layer address
   families that cannot be routed natively in the underlying
   Internetwork (e.g., OSI/CLNP over the public Internet, IPv6 over
   IPv4-only Internetworks, IPv4 over IPv6-only Internetworks, etc.).
   Further details for the support of IRON APs of one address family
   over Internetworks based on different address families are discussed
   in Appendix A.


5.  IRON Control Plane Operation

   Each IRON instance supports routing through the control plane startup
   and runtime dynamic routing operation of IAs.  The following sub-
   sections discuss control plane considerations for initializing and
   maintaining the IRON instance routing system.

5.1.  IRON Client Operation

   Each Client obtains one or more CPs in a secured exchange with the
   VSP as part of the initial end user registration.  Upon startup, the
   Client discovers a list of nearby VSP Servers via, e.g., a location
   broker, a well known website, a static map, etc.

   After the Client obtains a list of nearby Servers, it initiates short
   transactions to connect to one or more Servers, e.g., via secured TCP
   connections.  During the transaction, each Server provides the Client



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   with a tunnel-neighbor identifier ("NBR_ID") and a Shared Secret that
   the Client will use to sign and authenticate control messages.  The
   protocol details of the transaction are specific to the VSP, and
   hence out of scope for this document.

   After the Client connects to Servers, it configures default routes
   that list the Servers as next hops on the tunnel virtual interface.
   The Client may subsequently discover more-specific routes through
   receipt of redirect messages.

5.2.  IRON Server Operation

   In a simple VSP overlay network arrangement, each IRON Server is
   provisioned with the locators for Relays within the IRON instance.
   The Server is further configured as an ASBR for a stub AS and uses
   eBGP with a private ASN to peer with each Relay.

   Upon startup, the Server reports the list of CPs it is currently
   serving to the overlay network Relays.  The Server then actively
   listens for Clients that register their CPs as part of their
   connection establishment procedure.  When a new Client connects, the
   Server announces the new CP routes to its neighboring Relays; when an
   existing Client disconnects, the Server withdraws its CP
   announcements.  This process can often be accommodated through
   standard router configurations, e.g., on routers that can announce
   and withdraw prefixes based on kernel route additions and deletions.

5.3.  IRON Relay Operation

   Each IRON Relay is provisioned with the list of APs that it will
   serve, as well as the locators for Servers within the IRON instance.
   The Relay is also provisioned with eBGP peerings with neighboring
   ASes in the Internet -- the same as for any ASBR.

   In a simple VSP overlay network arrangement, each Relay connects to
   each Server via IRON instance-internal eBGP peerings for the purpose
   of discovering CP-to-Server mappings, and connects to all other
   Relays using iBGP either in a full mesh or using route reflectors.
   (The Relay only uses iBGP to announce those prefixes it has learned
   from AS peerings external to the IRON instance, however, since all
   Relays will already discover all CPs in the IRON instance via their
   eBGP peerings with Servers.)  The Relay then engages in eBGP routing
   exchanges with peer ASes in the IPv4 and/or IPv6 Internets the same
   as for any ASBR.

   After this initial synchronization procedure, the Relay advertises
   the APs to its eBGP peers in the Internet.  In particular, the Relay
   advertises the IPv6 APs into the IPv6 Internet routing system and



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   advertises the IPv4 APs into the IPv4 Internet routing system, but it
   does not advertise the full list of the IRON overlay's CPs to any of
   its eBGP peers.  The Relay further advertises "default" via eBGP to
   its associated Servers, then engages in ordinary packet-forwarding
   operations.


6.  IRON Forwarding Plane Operation

   Following control plane initialization, IAs engage in the cooperative
   process of receiving and forwarding packets.  IAs forward
   encapsulated packets over the IRON instance using the mechanisms of
   VET [INTAREA-VET], AERO [AERO] and SEAL [INTAREA-SEAL], while Relays
   additionally forward packets to and from the native IPv6 and/or IPv4
   Internets.  IAs also use SCMP to coordinate with other IAs, including
   the process of sending and receiving redirect messages, error
   messages, etc.  Each IA operates as specified in the following sub-
   sections.

6.1.  IRON Client Operation

   After connecting to Servers as specified in Section 5.1, the Client
   registers one or more active ISP connections with each Server.
   Thereafter, it sends periodic beacons (e.g., cryptographically signed
   SRS messages) to the Server via each ISP connection to maintain
   tunnel-neighbor address mapping state.  The beacons should be sent at
   no more than 60 second intervals (subject to a small random delay) so
   that state in NATs on the path as well as on the Server itself is
   refreshed regularly.  Although the Client may connect via multiple
   ISPs, a single NBR_ID is used to represent the set of all ISP paths
   the Client has registered with this Server.  The NBR_ID therefore
   names this "bundle" of ISP connections.

   If the Client ceases to receive acknowledgements from a Server via a
   specific ISP connection, it marks the Server as unreachable from that
   ISP.  (The Client should also inform the Server of this outage via
   one of its working ISP connections.)  If the Client ceases to receive
   acknowledgements from the Server via multiple ISP connections, it
   disconnects from this server and connects to a new nearby Server.
   The act of disconnecting from old servers and connecting to new
   servers will soon propagate the appropriate routing information among
   the IRON instance's Relays.

   When an end system in an EUN sends a flow of packets to a
   correspondent in a different network, the packets are forwarded
   through the EUN via normal routing until they reach the Client, which
   then tunnels the initial packets to a Server as its default router.
   In particular, the Client encapsulates each packet in an outer header



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   with its locator as the source address and the locator of the Server
   as the destination address.

   The Client uses the mechanisms specified in VET, AERO and SEAL to
   encapsulate each packet to be forwarded.  The Client further accepts
   SCMP protocol messages from its Servers, including neighbor
   coordination exchanges, indications of PMTU limitations, redirects
   and other control messages.  When the Client is redirected to a
   foreign Server that serves a destination CP, it sends future packets
   toward that destination CP directly to the foreign Server instead of
   via one of its connected Servers.

   Note that Client-to-Client tunneling is not accommodated, since this
   could result in unpredictable behavior when one or both Clients are
   located behind a NAT, or when one or both Clients are mobile.
   Therefore, Client-to-Client mobility binding updates are not required
   in the IRON model.

6.2.  IRON Server Operation

   After the Server associates with nearby Relays, it accepts Client
   connections and authenticates the SRS messages it receives from its
   already-connected Clients.  The Server discards any SRS messages that
   failed authentication, and responds to authentic SRS messages by
   returning signed SRAs.

   When the Server receives a SEAL-encapsulated data packet from one of
   its connected Clients, it uses normal longest-prefix-match rules to
   locate a forwarding table entry that matches the packet's inner
   destination address.  If the matching forwarding table entry is more-
   specific than default, the next hop is another of the Server's
   connected Clients; otherwise, the next-hop is a Relay which serves as
   a default router.  The Server then re-encapsulates the packet (i.e.,
   it removes the outer header and replaces it with a new outer header
   of the same address family), sets the outer destination address to
   the locator address of the next hop and tunnels the packet to the
   next hop.

   When the Server receives a SEAL-encapsulated data packet from a
   foreign Client, it accepts the packet only if there is a matching
   ingress filter table entry; otherwise, it silently drops the packet.
   The Server then locates a forwarding table entry that matches the
   packet's inner destination address.  If there is no matching
   forwarding table entry more-specific than default (i.e., the
   destination does not correspond to a connected Client), the Server
   silently drops the packet.  Otherwise, the Server re-encapsulates the
   packet and forwards it to the correct connected Client.  If the
   Client is in the process of disconnecting (e.g., due to mobility),



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   the Server also returns a redirect message listing a NULL next hop to
   inform the foreign Client that the connected Client has moved.

   When the Server receives a SEAL-encapsulated data packet from a
   Relay, it again locates a forwarding table entry that matches the
   packet's inner destination.  If there is no matching forwarding table
   entry more-specific than default, the Server drops the packet and
   sends a destination unreachable message.  Otherwise, the Server re-
   encapsulates the packet and forwards it to the correct connected
   Client.

   Note that Server-to-Server tunneling is not accommodated, since this
   could result in sustained routing loops in which Server A has a route
   to Server B, and Server B has a route to Server A. This implies that
   a Server must never accept and process a redirect message, but must
   instead relay the redirect message to the appropriate Client.

   The permissible data flow paths for tunneled packets that flow
   through a Server are therefore:

   o  From a connected Client to another connected Client (i.e., a
      hairpin route)

   o  From a connected Client to a default Relay router

   o  From a default Relay router to a connected Client

   o  From a foreign Client to a connected Client

   These data flow paths are shown diagrammatically in Section 7.

6.3.  IRON Relay Operation

   After each Relay has synchronized its APs (see Section 5.3) it
   advertises them in the IPv4 and/or IPv6 Internet routing systems.
   These APs will be represented as ordinary routing information in the
   interdomain routing system, and any packets originating from the IPv4
   or IPv6 Internet destined to an address covered by one of the APs
   will be forwarded to one of the VSP's Relays.

   When a Relay receives a packet from the Internet destined to a CPA
   covered by one of its APs, it behaves as an ordinary IP router.  In
   particular, the Relay looks in its forwarding table to discover a
   locator of a Server that serves the CP covering the destination
   address.  The Relay then simply forwards the packet to the Server,
   e.g., via SEAL encapsulation over a tunnel virtual link, via a
   physical interconnect, etc.




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   When a Relay receives a packet from a Server destined to a CPA
   serviced by a different Server, the Relay forwards the packet toward
   the correct Server and initiates an AERO redirection procedure, which
   both establishes the necessary ingress filtering state in the target
   Server and conveys a better next hop to the source Client.


7.  IRON Reference Operating Scenarios

   IRON supports communications when one or both hosts are located
   within CP-addressed EUNs.  The following sections discuss the
   reference operating scenarios.

7.1.  Both Hosts within Same IRON Instance

   When both hosts are within EUNs served by the same IRON instance, it
   is sufficient to consider the scenario in a unidirectional fashion,
   i.e., by tracing packet flows only in the forward direction from
   source host to destination host.  The reverse direction can be
   considered separately and incurs the same considerations as for the
   forward direction.  The simplest case occurs when the EUNs that
   service the source and destination hosts are connected to the same
   server, while the general case occurs when the EUNs are connected to
   different Servers.  The two cases are discussed in the following
   sections.

7.1.1.  EUNs Served by Same Server

   In this scenario, the packet flow from the source host is forwarded
   through the EUN to the source's Client.  The Client then tunnels the
   packets to the Server, which simply re-encapsulates and forwards the
   tunneled packets to the destination's Client.  The destination's
   Client then removes the packets from the tunnel and forwards them
   over the EUN to the destination.  Figure 6 depicts the sustained flow
   of packets from Host A to Host B within EUNs serviced by the same
   Server via a "hairpinned" route:















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                 ________________________________________
              .-(                                         )-.
           .-(                                              )-.
        .-(                                                    )-.
      .(                                                          ).
    .(                                                              ).
  .(                         +------------+                           ).
  (     +===================>| Server(S)  |=====================+      )
  (    //                    +------------+                     \\     )
  (   //  .-.                                                .-. \\    )
  (  //,-(  _)-.                                          ,-(  _)-\\   )
  ( .||_    (_  )-.                                    .-(_    (_  ||. )
  ((_||  ISP A    .)                                  (__   ISP B  ||_))
  (  ||-(______)-'                                       `-(______)||  )
  (  ||    |                                                  |    vv  )
   ( +-----+-----+                                      +-----+-----+ )
     | Client(A) |                                      | Client(B) |
     +-----+-----+           VSP IRON Instance          +-----+-----+
     ^     |    (   (Overlaid on the Native Internet)     )   |     |
     |    .-.     .-(                                .-)     .-.    |
     | ,-(  _)-.      .-(________________________)-.      ,-(  _)-. |
    .|(_    (_  )-.                                    .-(_    (_  )|
   (_|   EUN A     )                                  (_    EUN B   |)
     |`-(______)-'                                       `-(______)-|
     |     |               Legend:                            |     |
     | +---+----+            <---> == Native             +----+---+ |
     +-| Host A |            <===> == Tunnel             | Host B |<+
       +--------+                                        +--------+

           Figure 6: Sustained Packet Flow via Hairpinned Route

   With reference to Figure 6, Host A sends packets destined to Host B
   via its network interface connected to EUN A.  Routing within EUN A
   will direct the packets to Client(A) as a default router for the EUN,
   which then encapsulates them in outer IP/SEAL/* headers with its
   locator address as the outer source address, the locator address of
   Server(S) as the outer destination address, and the NBR_ID parameters
   associated with its tunnel-neighbor state as the identity.  Client(A)
   then simply forwards the encapsulated packets into the ISP network
   connection that provided its locator.  The ISP will forward the
   encapsulated packets into the Internet without filtering since the
   (outer) source address is topologically correct.  Once the packets
   have been forwarded into the Internet, routing will direct them to
   Server(S).

   Server(S) will receive the encapsulated packets from Client(A) then
   check its forwarding table to discover an entry that covers
   destination address B with Client(B) as the next hop.  Server(S) then



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   re-encapsulates the packets in a new outer header that uses the
   source address, destination address, and NBR_ID parameters associated
   with the tunnel-neighbor state for Client(B).  Server(S) then
   forwards these re-encapsulated packets into the Internet, where
   routing will direct them to Client(B).  Client(B) will, in turn,
   decapsulate the packets and forward the inner packets to Host B via
   EUN B.

7.1.2.  EUNs Served by Different Servers

   In this scenario, the initial packets of a flow produced by a source
   host within an EUN connected to the IRON instance by a Client must
   flow through both the Server of the source host and a nearby Relay,
   but route optimization can eliminate these elements from the path for
   subsequent packets in the flow.  Figure 7 shows the flow of initial
   packets from Host A to Host B within EUNs of the same IRON instance:

                 ________________________________________
              .-(                                         )-.
           .-(               +------------+                 )-.
        .-(          +======>|  Relay(R)  |=======+            )-.
      .(             ||      +*-----------+      ||               ).
    .(               ||     *                    vv                 ).
  .(        +--------++--+*                   +--++--------+          ).
  (     +==>| Server(A) *|                    | Server(B)  |====+      )
  (    //   +----------*-+                    +------------+    \\     )
  (   //  .-.         *                                      .-. \\    )
  (  //,-(  _)-.      *                                   ,-(  _)-\\   )
  ( .||_    (_  )-.   *                                .-(_    (_  ||. )
  ((_||  ISP A    .)  *                               (__   ISP B  ||_))
  (  ||-(______)-'    *                                  `-(______)||  )
  (  ||    |          *                                       |    vv  )
   ( +-----+-----+   *                                  +-----+-----+ )
     | Client(A) |<*                                    | Client(B) |
     +-----+-----+           VSP IRON Instance          +-----+-----+
     ^     |    (   (Overlaid on the Native Internet)     )   |     |
     |    .-.     .-(                                .-)     .-.    |
     | ,-(  _)-.      .-(________________________)-.      ,-(  _)-. |
    .|(_    (_  )-.                                    .-(_    (_  )|
   (_|   EUN A     )                                  (_    EUN B   |)
     |`-(______)-'                                       `-(______)-|
     |     |               Legend:                            |     |
     | +---+----+            <---> == Native             +----+---+ |
     +-| Host A |            <===> == Tunnel             | Host B |<+
       +--------+            ****> == Redirect           +--------+

              Figure 7: Initial Packet Flow Before Redirects




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   With reference to Figure 7, Host A sends packets destined to Host B
   via its network interface connected to EUN A.  Routing within EUN A
   will direct the packets to Client(A) as a default router for the EUN,
   which then encapsulates them in outer IP/SEAL/* headers that use the
   source address, destination address, and NBR_ID parameters associated
   with the tunnel-neighbor state for Server(A).  Client(A) then
   forwards the encapsulated packets into the ISP network connection
   that provided its locator, which will forward the encapsulated
   packets into the Internet where routing will direct them to
   Server(A).

   Server(A) receives the encapsulated packets from Client(A) and
   consults its forwarding table to determine that the most-specific
   matching route is "default" with Relay(R) as the next hop.  Server(A)
   then re-encapsulates the packets in outer headers that use the source
   address, destination address, and NBR_ID parameters associated with
   Relay (R), and forwards them into the Internet where routing will
   direct them to Relay(R).  (Note that the Server could instead forward
   the packets directly to the Relay without encapsulation when the
   Relay is connected via a physical interconnect.)

   Relay(R) receives the forwarded packets from Server(A) then checks
   its forwarding table to discover a CP entry that covers inner
   destination address B with Server(B) as the next hop.  Relay(R) then
   returns redirect messages to Server(A), which forwards (or,
   "proxies") the redirects to Client(A).  Relay(R) finally re-
   encapsulates the packets in outer headers that use the source
   address, destination address, and NBR_ID parameters associated with
   Server(B), then forwards them into the Internet where routing will
   direct them to Server(B).  (Note again that the Relay could instead
   forward the packets directly to the Server via a physical
   interconnect.)

   Server(B) receives the forwarded packets from Relay(R) then checks
   its forwarding table to discover a CP entry that covers destination
   address B with Client(B) as the next hop.  Server(B) then re-
   encapsulates the packets in outer headers that use the source
   address, destination address, and NBR_ID parameters associated with
   Client(B), then forwards them into the Internet where routing will
   direct them to Client(B).  Client(B) will, in turn, decapsulate the
   packets and forward the inner packets to Host B via EUN B.

   After the initial flow of packets, Server(A) will have received one
   or more redirect messages from Relay(R) listing Server(B) as a better
   next hop.  Server(A) will, in turn, proxy the redirects to Client(A),
   which will establish unidirectional tunnel-neighbor state listing
   Server(B) as the next hop toward the CP that covers Host B. Client(A)
   thereafter forwards its encapsulated packets directly to the locator



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   address of Server(B) without involving either Server(A) or Relay(B),
   as shown in Figure 8.

                 ________________________________________
              .-(                                         )-.
           .-(                                              )-.
        .-(                                                    )-.
      .(                                                          ).
    .(                                                              ).
  .(                                          +------------+          ).
  (     +====================================>|  Server(B) |====+      )
  (    //                                     +------------+    \\     )
  (   //  .-.                                                .-. \\    )
  (  //,-(  _)-.                                          ,-(  _)-\\   )
  ( .||_    (_  )-.                                    .-(_    (_  ||. )
  ((_||  ISP A    .)                                  (__   ISP B  ||_))
  (  ||-(______)-'                                       `-(______)||  )
  (  ||    |                                                  |    vv  )
   ( +-----+-----+                                      +-----+-----+ )
     | Client(A) |                                      | Client(B) |
     +-----+-----+             IRON Instance            +-----+-----+
     ^     |    (   (Overlaid on the Native Internet)     )   |     |
     |    .-.     .-(                                .-)     .-.    |
     | ,-(  _)-.      .-(________________________)-.      ,-(  _)-. |
    .|(_    (_  )-.                                    .-(_    (_  )|
   (_|   EUN A     )                                  (_    EUN B   |)
     |`-(______)-'                                       `-(______)-|
     |     |               Legend:                            |     |
     | +---+----+            <---> == Native             +----+---+ |
     +-| Host A |            <===> == Tunnel             | Host B |<+
       +--------+                                        +--------+

              Figure 8: Sustained Packet Flow After Redirects

7.2.  Mixed IRON and Non-IRON Hosts

   The cases in which one host is within an IRON EUN and the other is in
   a non-IRON EUN (i.e., one that connects to the native Internet
   instead of the IRON) are described in the following sub-sections.

7.2.1.  From IRON Host A to Non-IRON Host B

   Figure 9 depicts the IRON reference operating scenario for packets
   flowing from Host A in an IRON EUN to Host B in a non-IRON EUN.







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                  _________________________________________
               .-(         )-.                             )-.
            .-(      +-------)----+                           )-.
         .-(         |  Relay(A)  |--------------------------+   )-.
       .(            +------------+                           \     ).
     .(     +=======>|  Server(A) |                            \     ).
   .(     //         +--------)---+                             \     ).
   (     //                   )                                  \     )
   (    //         IRON       )                                   \    )
   (   //  .-.   Instance     )                              .-.   \   )
   (  //,-(  _)-.             )                           ,-(  _)-. \  )
   ( .||_    (_  )-.          ) The Native Internet    .- _    (_  )-| )
   ( _||  ISP A     )         )                       (_    ISP B    |))
   (  ||-(______)-'           )                         `-(______)-' | )
   (  ||    |             )-.                                  |     v )
    ( +-----+ ----+    )-.                               +-----+-----+ )
      | Client(A) |)-.                                   | Router(B) |
      +-----+-----+                                      +-----+-----+
      ^     |    (                                         )   |     |
      |    .-.     .-(                                .-)     .-.    |
      | ,-(  _)-.      .-(________________________)-.      ,-(  _)-. |
     .|(_    (_  )-.                                    .-(_    (_  )|
    (_|   EUN A     )                                 (      EUN B   |)
      |`-(______)-'                                       `-(______)-|
      |     |               Legend:                            |     |
      | +---+----+            <---> == Native             +----+---+ |
      +-| Host A |            <===> == Tunnel             | Host B |<+
        +--------+                                        +--------+

               Figure 9: From IRON Host A to Non-IRON Host B

   In this scenario, Host A sends packets destined to Host B via its
   network interface connected to IRON EUN A.  Routing within EUN A will
   direct the packets to Client(A) as a default router for the EUN,
   which then encapsulates them and forwards them into the Internet
   routing system where they will be directed to Server(A).

   Server(A) receives the encapsulated packets from Client(A) then
   forwards them to Relay(A), which simply forwards the unencapsulated
   packets into the Internet.  Once the packets are released into the
   Internet, routing will direct them to the final destination B. (Note
   that for simplicity Server(A) and Relay(A) are depicted in Figure 9
   as two concatenated "half-routers", and the forwarding between the
   two halves is via encapsulation, via a physical interconnect, via a
   shared memory operation when the two halves are within the same
   physical platform, etc.)





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7.2.2.  From Non-IRON Host B to IRON Host A

   Figure 10 depicts the IRON reference operating scenario for packets
   flowing from Host B in an Non-IRON EUN to Host A in an IRON EUN.

                  _________________________________________
               .-(         )-.                             )-.
            .-(      +-------)----+                           )-.
         .-(         |  Relay(A)  |<-------------------------+   )-.
       .(            +------------+                           \     ).
     .(     +========|  Server(A) |                            \     ).
   .(     //         +--------)---+                             \     ).
   (     //                   )                                  \     )
   (    //         IRON       )                                   \    )
   (   //  .-.   Instance     )                              .-.   \   )
   (  //,-(  _)-.             )                           ,-(  _)-. \  )
   ( .||_    (_  )-.          ) The Native Internet    .- _    (_  )-| )
   ( _||  ISP A     )         )                       (_    ISP B    |))
   (  ||-(______)-'           )                         `-(______)-' | )
   (  vv    |             )-.                                  |     | )
    ( +-----+ ----+    )-.                               +-----+-----+ )
      | Client(A) |)-.                                   | Router(B) |
      +-----+-----+                                      +-----+-----+
      |     |    (                                         )   |     |
      |    .-.     .-(                                .-)     .-.    |
      | ,-(  _)-.      .-(________________________)-.      ,-(  _)-. |
     .|(_    (_  )-.                                    .-(_    (_  )|
    (_|   EUN A     )                                 (      EUN B   |)
      |`-(______)-'                                       `-(______)-|
      |     |               Legend:                            |     |
      | +---+----+            <---> == Native             +----+---+ |
      +>| Host A |            <===> == Tunnel             | Host B |-+
        +--------+                                        +--------+

              Figure 10: From Non-IRON Host B to IRON Host A

   In this scenario, Host B sends packets destined to Host A via its
   network interface connected to non-IRON EUN B. Internet routing will
   direct the packets to Relay(A), which then forwards them to
   Server(A).

   Server(A) will then check its forwarding table to discover an entry
   that covers destination address A with Client(A) as the next hop.
   Server(A) then (re-)encapsulates the packets and forwards them into
   the Internet, where routing will direct them to Client(A).  Client(A)
   will, in turn, decapsulate the packets and forward the inner packets
   to Host A via its network interface connected to IRON EUN A.




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7.3.  Hosts within Different IRON Instances

   Figure 11 depicts the IRON reference operating scenario for packets
   flowing between Host A in an IRON instance A and Host B in a
   different IRON instance B. In that case, forwarding between hosts A
   and B always involves the Servers and Relays of both IRON instances,
   i.e., the scenario is no different than if one of the hosts was
   serviced by an IRON EUN and the other was serviced by a non-IRON EUN.
                  _________________________________________
               .-(         )-.                  .-(        )-.
            .-(      +-------)----+       +---(--------+      )-.
         .-(         |  Relay(A)  | <---> |  Relay(B)  |        )-.
       .(            +------------+       +------------+          ).
     .(     +=======>|  Server(A) |       |  Server(B) |<======+    ).
   .(     //         +--------)---+       +---(--------+        \\   ).
   (     //                   )               (                  \\   )
   (    //          IRON      )               (     IRON          \\   )
   (   //  .-.   Instance A   )               (  Instance B  .-.   \\  )
   (  //,-(  _)-.             )               (           ,-(  _).  || )
   ( .||_    (_  )-.          )               (        .-'_    (_  )|| )
   ( _||  ISP A     )         )               (       (_    ISP B   ||))
   (  ||-(______)-'           )               (          '-(______)-|| )
   (  vv    |             )-.                   .-(            |    vv )
    ( +-----+ ----+    )-.                         .-(   +-----+-----+ )
      | Client(A) |)-.                                .-(| Client(B) |
      +-----+-----+           The Native Internet        +-----+-----+
      ^     |    (                                         )   |     ^
      |    .-.     .-(                                .-)     .-.    |
      | ,-(  _)-.      .-(________________________)-.      ,-(  _)-. |
     .|(_    (_  )-.                                    .-(_    (_  )|
    (_|   EUN A     )                                  (_    EUN B   |)
      |`-(______)-'                                       `-(______)-|
      |     |               Legend:                            |     |
      | +---+----+            <---> == Native             +----+---+ |
      +>| Host A |            <===> == Tunnel             | Host B |<+
        +--------+                                        +--------+

             Figure 11: Hosts within Different IRON Instances


8.  Mobility, Multiple Interfaces, Multihoming, and Traffic Engineering

   While IRON Servers and Relays are typically arranged as fixed
   infrastructure, Clients may need to move between different network
   points of attachment, connect to multiple ISPs, or explicitly manage
   their traffic flows.  The following sections discuss mobility,
   multihoming, and traffic engineering considerations for IRON Clients.




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8.1.  Mobility Management and Mobile Networks

   When a Client changes its network point of attachment (e.g., due to a
   mobility event), it configures one or more new locators.  If the
   Client has not moved far away from its previous network point of
   attachment, it simply informs its Server of any locator changes.
   This operation is performance sensitive and should be conducted
   immediately to avoid packet loss.  This aspect of mobility can be
   classified as a "localized mobility event".

   If the Client has moved far away from its previous network point of
   attachment, however, it re-issues the Server discovery procedure
   described in Section 5.3.  If the Client's current Server is no
   longer close by, the Client may wish to move to a new Server in order
   to reduce routing stretch.  This operation is not performance
   critical, and therefore can be conducted over a matter of seconds/
   minutes instead of milliseconds/microseconds.  This aspect of
   mobility can be classified as a "global mobility event".

   To move to a new Server, the Client first engages in the CP
   registration process with the new Server, as described in Section
   5.3.  The Client then informs its former Server that it has departed;
   again, via a VSP-specific secured reliable transport connection.  The
   former Server will then withdraw its CP advertisements from the IRON
   instance routing system and retain the (stale) forwarding table
   entries until their lifetime expires.  In the interim, the former
   Server continues to deliver packets to the Client's last-known
   locator addresses for the short term while informing any
   unidirectional tunnel-neighbors that the Client has moved.

   Note that the Client may be either a mobile host or a mobile router.
   In the case of a mobile router, the Client's EUN becomes a mobile
   network, and can continue to use the Client's CPs without renumbering
   even as it moves between different network attachment points.

8.2.  Multiple Interfaces and Multihoming

   A Client may register multiple ISP connections with each Server such
   that multiple interfaces are naturally supported.  This feature
   results in the Client "harnessing" its multiple ISP connections into
   a "bundle" that is represented as a single entity at the network
   layer, and therefore allows for ISP independence at the link-layer.

   A Client may further register with multiple Servers for fault
   tolerance and reduced routing stretch.  In that case, the Client
   should register its full bundle of ISP connections with each of its
   Servers unless it has a way of carefully coordinating its ISP-to-
   Server mappings.



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   Client registration with multiple Servers results in "pseudo-
   multihoming", in which the multiple homes are within the same VSP
   IRON instance and hence share fate with the health of the IRON
   instance itself.

8.3.  Traffic Engineering

   A Client can dynamically adjust its ISP-to-Server mappings in order
   to influence inbound traffic flows.  It can also change between
   Servers when multiple Servers are available, but should strive for
   stability in its Server selection in order to limit VSP network
   routing churn.

   A Client can select outgoing ISPs, e.g., based on current Quality-of-
   Service (QoS) considerations such as minimizing delay or variance.


9.  Renumbering Considerations

   As new link-layer technologies and/or service models emerge, end
   users will be motivated to select their basic Internet connectivity
   solutions through healthy competition between ISPs.  If an end user's
   network-layer addresses are tied to a specific ISP, however, they may
   be forced to undergo a painstaking renumbering process if they wish
   to change to a different ISP [RFC4192][RFC5887].

   When an end user Client obtains CPs from a VSP, it can change between
   ISPs seamlessly and without need to renumber the CPs.  IRON therefore
   provides ISP independence at the link layer.  If the VSP itself
   applies unreasonable costing structures or policies for use of the
   CPs, however, the end user may be compelled to seek a different VSP
   and would again be required to engage in a network layer renumbering
   event.


10.  NAT Traversal Considerations

   The Internet today consists of a global public IPv4 routing and
   addressing system with non-IRON EUNs that use either public or
   private IPv4 addressing.  The latter class of EUNs connect to the
   public Internet via Network Address Translators (NATs).  When an IRON
   Client is located behind a NAT, it selects Servers using the same
   procedures as for Clients with public addresses and can then send SRS
   messages to Servers in order to get SRA messages in return.  The only
   requirement is that the Client must configure its encapsulation
   format to use a transport protocol that supports NAT traversal, e.g.,
   UDP, TCP, TLS/SSL, etc.




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   Since the Server maintains state about its connected Clients, it can
   discover locator information for each Client by examining the
   transport port number and IP address in the outer headers of the
   Client's encapsulated packets.  When there is a NAT in the path, the
   transport port number and IP address in each encapsulated packet will
   correspond to state in the NAT box and might not correspond to the
   actual values assigned to the Client.  The Server can then
   encapsulate packets destined to hosts in the Client's EUN within
   outer headers that use this IP address and transport port number.
   The NAT box will receive the packets, translate the values in the
   outer headers, then forward the packets to the Client.  In this
   sense, the Server's "locator" for the Client consists of the
   concatenation of the IP address and transport port number.

   In order to keep NAT and Server connection state alive, the Client
   sends periodic beacons to the server, e.g., by sending an SRS message
   to elicit an SRA message from the Server.  IRON does not otherwise
   introduce any new issues to complications raised for NAT traversal or
   for applications embedding address referrals in their payload.


11.  Multicast Considerations

   IRON Servers and Relays are topologically positioned to provide
   Internet Group Management Protocol (IGMP) / Multicast Listener
   Discovery (MLD) proxying for their Clients [RFC4605].  Further
   multicast considerations for IRON (e.g., interactions with multicast
   routing protocols, traffic scaling, etc.) are out of scope and will
   be discussed in a future document.


12.  Nested EUN Considerations

   Each Client configures a locator that may be taken from an ordinary
   non-CPA address assigned by an ISP or from a CPA address taken from a
   CP assigned to another Client.  In that case, the Client is said to
   be "nested" within the EUN of another Client, and recursive nestings
   of multiple layers of encapsulations may be necessary.

   For example, in the network scenario depicted in Figure 12, Client(A)
   configures a locator CPA(B) taken from the CP assigned to EUN(B).
   Client(B) in turn configures a locator CPA(C) taken from the CP
   assigned to EUN(C).  Finally, Client(C) configures a locator ISP(D)
   taken from a non-CPA address delegated by an ordinary ISP(D).

   Using this example, the "nested-IRON" case must be examined in which
   a Host A, which configures the address CPA(A) within EUN(A),
   exchanges packets with Host Z located elsewhere in a different IRON



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   instance EUN(Z).

                            .-.
                 ISP(D)  ,-(  _)-.
      +-----------+   .-(_    (_  )-.
      | Client(C) |--(_    ISP(D)    )
      +-----+-----+     `-(______)-'
            |   <= T         \     .-.
           .-.       u        \ ,-(  _)-.
        ,-(  _)-.       n     .-(_    (-  )-.
     .-(_    (_  )-.      n  (_   Internet   )
    (_    EUN(C)    )       e   `-(______)-'
       `-(______)-'           l          ___
            | CPA(C)           s =>     (:::)-.
      +-----+-----+                 .-(::::::::)
      | Client(B) |              .-(: Multiple :)-.    +-----------+
      +-----+-----+             (:::::: IRON ::::::)   |  Relay(Z) |
            |                    `-(: Instances:)-'    +-----------+
           .-.                      `-(::::::)-'       +-----------+
        ,-(  _)-.                                      | Server(Z) |
     .-(_    (_  )-.            +---------------+      +-----------+
    (_    EUN(B)    )           |Relay/Server(C)|      +-----------+
       `-(______)-'             +---------------+      | Client(Z) |
            | CPA(B)            +---------------+      +-----------+
      +-----+-----+             |Relay/Server(B)|          |
      | Client(A) |             +---------------+         .-.
      +-----------+             +---------------+      ,-(  _)-.
            |                   |Relay/Server(A)|   .-(_    (_  )-.
           .-.                  +---------------+  (_    EUN(Z)    )
        ,-(  _)-.  CPA(A)                             `-(______)-'
     .-(_    (_  )-.    +--------+                     +--------+
    (_    EUN(A)    )---| Host A |                     | Host Z |
       `-(______)-'     +--------+                     +--------+

                       Figure 12: Nested EUN Example

   The two cases of Host A sending packets to Host Z, and Host Z sending
   packets to Host A, must be considered separately, as described below.

12.1.  Host A Sends Packets to Host Z

   Host A first forwards a packet with source address CPA(A) and
   destination address Z into EUN(A).  Routing within EUN(A) will direct
   the packet to Client(A), which encapsulates it in an outer header
   with CPA(B) as the outer source address and Server(A) as the outer
   destination address then forwards the once-encapsulated packet into
   EUN(B).




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   Routing within EUN(B) will direct the packet to Client(B), which
   encapsulates it in an outer header with CPA(C) as the outer source
   address and Server(B) as the outer destination address then forwards
   the twice-encapsulated packet into EUN(C).  Routing within EUN(C)
   will direct the packet to Client(C), which encapsulates it in an
   outer header with ISP(D) as the outer source address and Server(C) as
   the outer destination address.  Client(C) then sends this triple-
   encapsulated packet into the ISP(D) network, where it will be routed
   via the Internet to Server(C).

   When Server(C) receives the triple-encapsulated packet, it forwards
   it to Relay(C) which removes the outer layer of encapsulation and
   forwards the resulting twice-encapsulated packet into the Internet to
   Server(B).  Next, Server(B) forwards the packet to Relay(B) which
   removes the outer layer of encapsulation and forwards the resulting
   once-encapsulated packet into the Internet to Server(A).  Next,
   Server(A) forwards the packet to Relay(A), which decapsulates it and
   forwards the resulting inner packet via the Internet to Relay(Z).
   Relay(Z), in turn, forwards the packet to Server(Z), which
   encapsulates and forwards the packet to Client(Z), which decapsulates
   it and forwards the inner packet to Host Z.

12.2.  Host Z Sends Packets to Host A

   When Host Z sends a packet to Host A, forwarding in EUN(Z) will
   direct it to Client(Z), which encapsulates and forwards the packet to
   Server(Z).  Server(Z) will forward the packet to Relay(Z), which will
   then decapsulate and forward the inner packet into the Internet.
   Internet routing will convey the packet to Relay(A) as the next-hop
   towards CPA(A), which then forwards it to Server(A).

   Server (A) encapsulates the packet and forwards it to Relay(B) as the
   next-hop towards CPA(B) (i.e., the locator for CPA(A)).  Relay(B)
   then forwards the packet to Server(B), which encapsulates it a second
   time and forwards it to Relay(C) as the next-hop towards CPA(C)
   (i.e., the locator for CPA(B)).  Relay(C) then forwards the packet to
   Server(C), which encapsulates it a third time and forwards it to
   Client(C).

   Client(C) then decapsulates the packet and forwards the resulting
   twice-encapsulated packet via EUN(C) to Client(B).  Client(B) in turn
   decapsulates the packet and forwards the resulting once-encapsulated
   packet via EUN(B) to Client(A).  Client(A) finally decapsulates and
   forwards the inner packet to Host A.







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13.  Implications for the Internet

   The IRON architecture envisions a hybrid routing/mapping system that
   benefits from both the shortest-path routing afforded by pure dynamic
   routing systems and the routing-scaling suppression afforded by pure
   mapping systems.  Therefore, IRON targets the elusive "sweet spot"
   that pure routing and pure mapping systems alone cannot satisfy.

   The IRON system requires a VSP deployment of new routers/servers
   throughout the Internet to maintain well-balanced virtual overlay
   networks.  These routers/servers can be deployed incrementally
   without disruption to existing Internet infrastructure as long as
   they are appropriately managed to provide acceptable service levels
   to end users.

   End-to-end traffic that traverses an IRON instance may experience
   delay variance between the initial packets and subsequent packets of
   a flow.  This is due to the IRON system allowing a longer path
   stretch for initial packets followed by timely route optimizations to
   utilize better next hop routers/servers for subsequent packets.

   IRON instances work seamlessly with existing and emerging services
   within the native Internet.  In particular, end users serviced by an
   IRON instance will receive the same service enjoyed by end users
   serviced by non-IRON service providers.  Internet services already
   deployed within the native Internet also need not make any changes to
   accommodate IRON end users.

   The IRON system operates between IAs within the Internet and EUNs.
   Within these networks, the underlying paths traversed by the virtual
   overlay networks may comprise links that accommodate varying MTUs.
   While the IRON system imposes an additional per-packet overhead that
   may cause the size of packets to become slightly larger than the
   underlying path can accommodate, IAs have a method for naturally
   detecting and tuning out instances of path MTU underruns.  In some
   cases, these MTU underruns may need to be reported back to the
   original hosts; however, the system will also allow for MTUs much
   larger than those typically available in current Internet paths to be
   discovered and utilized as more links with larger MTUs are deployed.

   Finally, and perhaps most importantly, the IRON system provides in-
   built mobility management, mobile networks, multihoming and traffic
   engineering capabilities that allow end user devices and networks to
   move about freely while both imparting minimal oscillations in the
   routing system and maintaining generally shortest-path routes.  This
   mobility management is afforded through the very nature of the IRON
   service model, and therefore requires no adjunct mechanisms.  The
   mobility management and multihoming capabilities are further



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   supported by forward-path reachability detection that provides "hints
   of forward progress" in the same spirit as for IPv6 Neighbor
   Discovery (ND).


14.  Additional Considerations

   Considerations for the scalability of Internet Routing due to
   multihoming, traffic engineering, and provider-independent addressing
   are discussed in [RADIR].  Other scaling considerations specific to
   IRON are discussed in Appendix B.

   Route optimization considerations for mobile networks are found in
   [RFC5522].

   In order to ensure acceptable end user service levels, the VSP should
   conduct a traffic scaling analysis and distribute sufficient Relays
   and Servers for the IRON instance globally throughout the Internet.


15.  Related Initiatives

   IRON builds upon the concepts of the RANGER architecture [RFC5720] ,
   and therefore inherits the same set of related initiatives.  The
   Internet Research Task Force (IRTF) Routing Research Group (RRG)
   mentions IRON in its recommendation for a routing architecture
   [RFC6115].

   Virtual Aggregation (VA) [GROW-VA] and Aggregation in Increasing
   Scopes (AIS) [EVOLUTION] provide the basis for the Virtual Prefix
   concepts.

   Internet Vastly Improved Plumbing (Ivip) [IVIP-ARCH] has contributed
   valuable insights, including the use of real-time mapping.  The use
   of Servers as mobility anchor points is directly influenced by Ivip's
   associated TTR mobility extensions [TTRMOB].

   [RO-CR] discusses a route optimization approach using a Correspondent
   Router (CR) model.  The IRON Server construct is similar to the CR
   concept described in this work; however, the manner in which Clients
   coordinate with Servers is different and based on the NBMA virtual
   link model [RFC5214].

   Numerous publications have proposed NAT traversal techniques.  The
   NAT traversal techniques adapted for IRON were inspired by the Simple
   Address Mapping for Premises Legacy Equipment (SAMPLE) proposal
   [SAMPLE].




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   The IRON Client-Server relationship is managed in essentially the
   same way as for the Tunnel Broker model [RFC3053].  Numerous existing
   tunnel broker provider networks (e.g., Hurricane Electric, SixXS,
   freenet6, etc.) provide existence proofs that IRON-like overlay
   network services can be deployed and managed on a global basis
   [BROKER].


16.  Security Considerations

   Security considerations that apply to tunneling in general are
   discussed in [RFC6169].  Additional considerations that apply also to
   IRON are discussed in RANGER [RFC5720] , VET [INTAREA-VET] and SEAL
   [INTAREA-SEAL].

   The IRON system further depends on mutual authentication of IRON
   Clients to Servers and Servers to Relays.  This is accomplished
   through initial authentication exchanges that establish tunnel-
   neighbor NBR_ID values that can be used to defeat off-path attacks.
   As for all Internet communications, the IRON system also depends on
   Relays acting with integrity and not injecting false advertisements
   into the Internet routing system (e.g., to mount traffic siphoning
   attacks).

   IRON Servers must ensure that any changes in a Client's locator
   addresses are communicated only through an authenticated exchange
   that is not subject to replay.  For this reason, Clients periodically
   send digitally-signed SRS messages to the Server.  If the Client's
   locator address stays the same, the Server can accept the SRS message
   without verifying the signature as long as the NBR_ID of the SRS
   matches the Client.  If the Client's locator address changes, the
   Server must verify the SRS message's signature before accepting the
   message.  Once the message has been authenticated, the Server updates
   the Client's locator address to the new address.

   Each IRON instance requires a means for assuring the integrity of the
   interior routing system so that all Relays and Servers in the overlay
   have a consistent view of CP<->Server bindings.  Also, Denial-of-
   Service (DoS) attacks on IRON Relays and Servers can occur when
   packets with spoofed source addresses arrive at high data rates.
   However, this issue is no different than for any border router in the
   public Internet today.

   Middleboxes can interfere with tunneled packets within an IRON
   instance in various ways.  For example, a middlebox may alter a
   packet's contents, change a packet's locator addresses, inject
   spurious packets, replay old packets, etc.  These issues are no
   different than for middlebox interactions with ordinary Internet



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   communications.  If man-in-the-middle attacks are a matter for
   concern in certain deployments, however, IRON Agents can use IPsec to
   protect the authenticity, integrity and (if necessary) privacy of
   their tunneled packets.


17.  Acknowledgements

   The ideas behind this work have benefited greatly from discussions
   with colleagues; some of which appear on the RRG and other IRTF/IETF
   mailing lists.  Robin Whittle and Steve Russert co-authored the TTR
   mobility architecture, which strongly influenced IRON.  Eric
   Fleischman pointed out the opportunity to leverage anycast for
   discovering topologically close Servers.  Thomas Henderson
   recommended a quantitative analysis of scaling properties.

   The following individuals provided essential review input: Jari
   Arkko, Mohamed Boucadair, Stewart Bryant, John Buford, Ralph Droms,
   Wesley Eddy, Adrian Farrel, Dae Young Kim, and Robin Whittle.

   Further discussions with colleagues following the publication of
   RFC6179 have provided useful insights that have resulted in
   significant improvments to this, the Second Editon of IRON.


18.  References

18.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

18.2.  Informative References

   [AERO]     Templin, F., Ed., "Asymmetric Extended Route Optimization
              (AERO)", Work in Progress, June 2011.

   [BGPMON]   net, B., "BGPmon.net - Monitoring Your Prefixes,
              http://bgpmon.net/stat.php", June 2010.

   [BROKER]   Wikipedia, W., "List of IPv6 Tunnel Brokers,
              http://en.wikipedia.org/wiki/List_of_IPv6_tunnel_brokers",
              August 2011.

   [EVOLUTION]



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              Zhang, B., Zhang, L., and L. Wang, "Evolution Towards
              Global Routing Scalability", Work in Progress,
              October 2009.

   [GROW-VA]  Francis, P., Xu, X., Ballani, H., Jen, D., Raszuk, R., and
              L. Zhang, "FIB Suppression with Virtual Aggregation", Work
              in Progress, February 2011.

   [INTAREA-SEAL]
              Templin, F., Ed., "The Subnetwork Encapsulation and
              Adaptation Layer (SEAL)", Work in Progress, February 2011.

   [INTAREA-VET]
              Templin, F., Ed., "Virtual Enterprise Traversal (VET)",
              Work in Progress, January 2011.

   [IVIP-ARCH]
              Whittle, R., "Ivip (Internet Vastly Improved Plumbing)
              Architecture", Work in Progress, March 2010.

   [RADIR]    Narten, T., "On the Scalability of Internet Routing", Work
              in Progress, February 2010.

   [RFC1070]  Hagens, R., Hall, N., and M. Rose, "Use of the Internet as
              a subnetwork for experimentation with the OSI network
              layer", RFC 1070, February 1989.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC1930]  Hawkinson, J. and T. Bates, "Guidelines for creation,
              selection, and registration of an Autonomous System (AS)",
              BCP 6, RFC 1930, March 1996.

   [RFC3053]  Durand, A., Fasano, P., Guardini, I., and D. Lento, "IPv6
              Tunnel Broker", RFC 3053, January 2001.

   [RFC4192]  Baker, F., Lear, E., and R. Droms, "Procedures for
              Renumbering an IPv6 Network without a Flag Day", RFC 4192,
              September 2005.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC4548]  Gray, E., Rutemiller, J., and G. Swallow, "Internet Code
              Point (ICP) Assignments for NSAP Addresses", RFC 4548,
              May 2006.



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   [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, August 2006.

   [RFC4984]  Meyer, D., Zhang, L., and K. Fall, "Report from the IAB
              Workshop on Routing and Addressing", RFC 4984,
              September 2007.

   [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
              Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
              March 2008.

   [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, October 2009.

   [RFC5720]  Templin, F., "Routing and Addressing in Networks with
              Global Enterprise Recursion (RANGER)", RFC 5720,
              February 2010.

   [RFC5743]  Falk, A., "Definition of an Internet Research Task Force
              (IRTF) Document Stream", RFC 5743, December 2009.

   [RFC5887]  Carpenter, B., Atkinson, R., and H. Flinck, "Renumbering
              Still Needs Work", RFC 5887, May 2010.

   [RFC6115]  Li, T., "Recommendation for a Routing Architecture",
              RFC 6115, February 2011.

   [RFC6139]  Russert, S., Fleischman, E., and F. Templin, "Routing and
              Addressing in Networks with Global Enterprise Recursion
              (RANGER) Scenarios", RFC 6139, February 2011.

   [RFC6169]  Krishnan, S., Thaler, D., and J. Hoagland, "Security
              Concerns with IP Tunneling", RFC 6169, April 2011.

   [RO-CR]    Bernardos, C., Calderon, M., and I. Soto, "Correspondent
              Router based Route Optimisation for NEMO (CRON)", Work
              in Progress, July 2008.

   [SAMPLE]   Carpenter, B. and S. Jiang, "Legacy NAT Traversal for
              IPv6: Simple Address Mapping for Premises Legacy Equipment
              (SAMPLE)", Work in Progress, June 2010.

   [TTRMOB]   Whittle, R. and S. Russert, "TTR Mobility Extensions for
              Core-Edge Separation Solutions to the Internet's Routing



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              Scaling Problem,
              http://www.firstpr.com.au/ip/ivip/TTR-Mobility.pdf",
              August 2008.

Appendix A.  IRON Operation over Internetworks with Different Address
             Families

   The IRON architecture leverages the routing system by providing
   generally shortest-path routing for packets with CPA addresses from
   APs that match the address family of the underlying Internetwork.
   When the APs are of an address family that is not routable within the
   underlying Internetwork, however, (e.g., when OSI/NSAP [RFC4548] APs
   are used over an IPv4 Internetwork) a global AP mapping database is
   required.  The mapping database allows the Relays of the local IRON
   instance to map APs belonging to other IRON instances to companion
   prefixes taken from address families that are routable within the
   Internetwork.  For example, an IPv6 AP (e.g., 2001:DB8::/32) could be
   paired with a companion IPv4 prefix (e.g., 192.0.2.0/24) so that
   encapsulated IPv6 packets can be forwarded over IPv4-only
   Internetworks.

   In that case, every AP must be represented in a globally distributed
   Master AP database (MAP) that maintains AP-to-companion prefix
   mappings for all APs in the IRON.  The MAP is maintained by a
   globally managed assigned numbers authority in the same manner as the
   Internet Assigned Numbers Authority (IANA) currently maintains the
   master list of all top-level IPv4 and IPv6 delegations.  The database
   can be replicated across multiple servers for load balancing, much in
   the same way that FTP mirror sites are used to manage software
   distributions.

   Upon startup, each Relay advertises an IPv4 companion prefix (e.g.,
   192.0.2.0/24) into the IPv4 internetwork routing system and/or an
   IPv6 companion prefix (e.g., 2001:DB8::/64) into the IPv6
   internetwork routing system for the IRON instance that it serves.
   The Relay then configures the host number '1' in the IPv4 companion
   prefix (e.g., as 192.0.2.1) and the interface identifier '0' in the
   IPv6 companion prefix (e.g., as 2001:DB8::0), and assigns the
   resulting addresses as "Relay anycast" addresses for the IRON
   instance.

   The Relay then discovers the full set of APs for all other IRON
   instances by reading the MAP.  The Relay reads the MAP from a nearby
   server and periodically checks the server for deltas since the
   database was last read.  After reading the MAP, the Relay has a full
   list of AP-to-companion prefix mappings.  The Relay can then forward
   packets toward CPAs belonging to other IRON instances by
   encapsulating them in an outer header of the companion prefix address



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   family and using the Relay anycast address as the outer destination
   address.

   Possible encapsulations in this model include IPv6-in-IPv4, IPv4-in-
   IPv6, OSI/CLNP-in-IPv6, OSI/CLNP-in-IPv4, etc.

Appendix B.  Scaling Considerations

   Scaling aspects of the IRON architecture have strong implications for
   its applicability in practical deployments.  Scaling must be
   considered along multiple vectors, including Interdomain core routing
   scaling, scaling to accommodate large numbers of EUNs, traffic
   scaling, state requirements, etc.

   In terms of routing scaling, each VSP will advertise one or more APs
   into the global Internet routing system from which CPs are delegated
   to end users.  Routing scaling will therefore be minimized when each
   AP covers many CPs.  For example, the IPv6 prefix 2001:DB8::/32
   contains 2^24 ::/56 CP prefixes for assignment to EUNs; therefore,
   the VSP could accommodate 2^32 ::/56 CPs with only 2^8 ::/32 APs
   advertised in the interdomain routing core.  (When even longer CP
   prefixes are used, e.g., /64s assigned to individual handsets in a
   cellular provider network, many more EUNs can be represented within
   only a single AP.)

   In terms of traffic scaling for Relays, each Relay represents an ASBR
   of a "shell" enterprise network that simply directs arriving traffic
   packets with CPA destination addresses towards Servers that service
   the corresponding Clients.  Moreover, the Relay sheds traffic
   destined to CPAs through redirection, which removes it from the path
   for the majority of traffic packets between Clients within the same
   IRON instance.  On the other hand, each Relay must handle all traffic
   packets forwarded between the CPs it manages and the rest of the
   Internet.  The scaling concerns for this latter class of traffic are
   no different than for ASBR routers that connect large enterprise
   networks to the Internet.  In terms of traffic scaling for Servers,
   each Server services a set of CPs.  The Server services all traffic
   packets destined to its own CPs but only services the initial packets
   of flows initiated from its own CPs and destined to other CPs.
   Therefore, traffic scaling for CPA-addressed traffic is an asymmetric
   consideration and is proportional to the number of CPs each Server
   serves.

   In terms of state requirements for Relays, each Relay maintains a
   list of Servers in the IRON instance as well as forwarding table
   entries for the CPs that each Server handles.  This Relay state is
   therefore dominated by the total number of CPs handled by the Relay's
   associated Servers.  Keeping in mind that current day core router



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   technologies are only capable of handling fast-path FIB cache sizes
   of O(1M) entries, a large-scale deployment may require that the total
   CP database for the VSP overlay be spread between the FIBs of a mesh
   of Relays rather than fully-resident in the FIB of each Relay.  In
   that case, the techniques of Virtual Aggregation (VA) may be useful
   in bridging together the mesh of Relays.  Alternatively, each Relay
   could elect to keep some or all CP prefixes out of the FIB and
   maintain them only in a slow-path forwarding table.  In that case,
   considerably more CP entries could be kept in each Relay at the cost
   of incurring slow-path processing for the initial packets of a flow.

   In terms of state requirements for Servers, each Server maintains
   state only for the CPs it serves, and not for the CPs handled by
   other Servers in the IRON instance.  Finally, neither Relays nor
   Servers need keep state for final destinations of outbound traffic.

   Clients source and sink all traffic packets originating from or
   destined to the CP.  Therefore, traffic scaling considerations for
   Clients are the same as for any site border router.  Clients also
   retain unidirectional tunnel-neighbor state for the Servers for final
   destinations of outbound traffic flows.  This can be managed as soft
   state, since stale entries purged from the cache will be refreshed
   when new traffic packets are sent.

Author's Address

   Fred L. Templin (editor)
   Boeing Research & Technology
   P.O. Box 3707 MC 7L-49
   Seattle, WA  98124
   USA

   EMail: fltemplin@acm.org


















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