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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Informational                           August 03, 2011
Expires: February 4, 2012


              The Internet Routing Overlay Network (IRON)
                      draft-templin-ironbis-00.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.  IRON further addresses other
   important issues including routing scaling, mobility management,
   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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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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 February 4, 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
   publication of this document.  Please review these documents



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   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  . . . . . . . . . . . . . . . . . . .  9
     3.3.  IRON Relay Router  . . . . . . . . . . . . . . . . . . . . 10
   4.  IRON Organizational Principles . . . . . . . . . . . . . . . . 11
   5.  IRON Initialization  . . . . . . . . . . . . . . . . . . . . . 13
     5.1.  IRON Relay Router Initialization . . . . . . . . . . . . . 13
     5.2.  IRON Serving Router Initialization . . . . . . . . . . . . 14
     5.3.  IRON Client Initialization . . . . . . . . . . . . . . . . 14
   6.  IRON Operation . . . . . . . . . . . . . . . . . . . . . . . . 15
     6.1.  IRON Client Operation  . . . . . . . . . . . . . . . . . . 15
     6.2.  IRON Serving Router Operation  . . . . . . . . . . . . . . 16
     6.3.  IRON Relay Router Operation  . . . . . . . . . . . . . . . 17
     6.4.  IRON Reference Operating Scenarios . . . . . . . . . . . . 18
       6.4.1.  Both Hosts within Same IRON Instance . . . . . . . . . 18
       6.4.2.  Mixed IRON and Non-IRON Hosts  . . . . . . . . . . . . 21
       6.4.3.  Hosts within Different IRON Instances  . . . . . . . . 24
     6.5.  Mobility, Multihoming, and Traffic Engineering
           Considerations . . . . . . . . . . . . . . . . . . . . . . 25
       6.5.1.  Mobility Management  . . . . . . . . . . . . . . . . . 25
       6.5.2.  Multihoming  . . . . . . . . . . . . . . . . . . . . . 26
       6.5.3.  Inbound Traffic Engineering  . . . . . . . . . . . . . 26
       6.5.4.  Outbound Traffic Engineering . . . . . . . . . . . . . 26
     6.6.  Renumbering Considerations . . . . . . . . . . . . . . . . 26
     6.7.  NAT Traversal Considerations . . . . . . . . . . . . . . . 27
     6.8.  Multicast Considerations . . . . . . . . . . . . . . . . . 27
     6.9.  Nested EUN Considerations  . . . . . . . . . . . . . . . . 28
       6.9.1.  Host A Sends Packets to Host Z . . . . . . . . . . . . 29
       6.9.2.  Host Z Sends Packets to Host A . . . . . . . . . . . . 30
   7.  Implications for the Internet  . . . . . . . . . . . . . . . . 31
   8.  Additional Considerations  . . . . . . . . . . . . . . . . . . 32
   9.  Related Initiatives  . . . . . . . . . . . . . . . . . . . . . 32
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 33
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 33
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 34
     12.2. Informative References . . . . . . . . . . . . . . . . . . 34
   Appendix A.  IRON VPs over Internetworks with Different
                Address Families  . . . . . . . . . . . . . . . . . . 36
   Appendix B.  Scaling Considerations  . . . . . . . . . . . . . . . 37
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 38






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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
   [RADIR].  Operational practices such as the increased use of
   multihoming with Provider-Independent (PI) addressing are resulting
   in more and more fine-grained 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 address space fragmentation (leading to yet further
   routing table 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 routing 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] and the Subnetwork Adaptation and Encapsulation Layer
   (SEAL) [INTAREA-SEAL] as its functional building blocks.

   This document proposes an Internet Routing Overlay Network (IRON)
   architecture with goals of supporting sustainable growth while
   requiring no changes to the existing routing system.  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 Address
   Mapping for Premises Legacy Equipment (SAMPLE) proposal [SAMPLE].

   IRON supports scalable addressing without changing the current BGP
   [RFC4271] routing system.  IRON observes the Internet Protocol
   standards [RFC0791][RFC2460].  Other network-layer protocols that can
   be encapsulated within IP packets (e.g., OSI/CLNP (Connectionless



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   Network Protocol) [RFC1070], etc.) are also within scope.

   IRON is a global routing system comprising Virtual Service Provider
   (VSP) overlay networks that service Virtual Prefixes (VPs) from which
   End User Network (EUN) prefixes (EPs) are delegated to customer
   sites.  IRON is motivated by a growing customer demand for
   multihoming, mobility management, 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 global Internet routing systems as virtual
   NBMA links for tunneling inner network protocol packets within outer
   IPv4 or IPv6 headers (see Section 3).  Each IRON instance requires
   deployment of a small number of new BGP core routers and supporting
   servers, as well as IRON-aware clients in customer EUNs.  No
   modifications to hosts, and no modifications to most routers, are
   required.  The following sections discuss details of the IRON
   architecture.


2.  Terminology

   This document makes use of the following terms:

   End User Network (EUN):
      an edge network that connects an organization's devices (e.g.,
      computers, routers, printers, etc.) to the Internet.

   End User Network Prefix (EP):
      a more specific inner network-layer prefix (e.g., an IPv4 /28, an
      IPv6 /56, etc.) derived from an aggregated Virtual Prefix (VP)and
      delegated to an EUN by a Virtual Service Provider (VSP).

   End User Network Prefix Address (EPA):
      a network-layer address belonging to an EP and assigned to the
      interface of an end system in an EUN.

   Forwarding Information Base (FIB):
      a data structure containing network prefixes to next-hop mappings;
      usually maintained in a router's fast-path processing lookup
      tables.

   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 with EPA addresses within outer headers that use
      locator addresses.  Each IRON instance connects to the global
      Internet the same as for any autonomous system.




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   IRON Client Router/Host ("Client"):
      a customer's router or host that logically connects the customer's
      EUNs and their associated EPs to an IRON instance via an NBMA
      tunnel virtual interface.

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

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

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

   Internet Service Provider (ISP):
      a service provider that connects customer EUNs to the underlying
      Internetwork.  In other words, an ISP is responsible for providing
      basic Internet connectivity for customer EUNs.

   Locator
      an IP address assigned to the interface of a router or end system
      within a public or private network.  Locators taken from public IP
      prefixes are routable on a global basis, while locators taken from
      private IP prefixes 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.

   Subnetwork Encapsulation and Adaptation Layer (SEAL):
      an encapsulation sublayer that provides extended packet
      identification and a Control Message Protocol 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 Prefix (VP):
      a prefix block (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).



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

   VSP Overlay Network:
      a specialized set of routers deployed by a VSP to service customer
      EUNs through an IRON instance configured over an underlying
      Internetwork (e.g., the global Internet).


3.  The Internet Routing Overlay Network

   The Internet Routing Overlay Network (IRON) is a union of Virtual
   Service Provider (VSP) overlay network instances connected to a
   common Internetwork.  While the principles presented in this document
   are discussed within the context of the public global Internet, they
   can also be applied to any autonomous Internetwork.  The rest of this
   document therefore refers to the terms "Internet" and "Internetwork"
   interchangeably except in cases where specific distinctions must be
   made.

   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 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 EP addresses    ~  -->  ~    with EP 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 the automatic tunneling mechanisms used for
   encapsulation, while SEAL specifies the format and usage of the SEAL
   header as well as a set of control messages.  Most notably, IAs use
   the SEAL Control Message Protocol (SCMP) to deterministically
   exchange and authenticate control messages such as route
   redirections, indications of Path Maximum Transmission Unit (PMTU)
   limitations, destination unreachables, etc.  IAs appear as neighbors
   on an NBMA virtual link, and form bidirectional and/or unidirectional
   tunnel-neighbor relationships.

   Each IRON instance comprises a set of IAs distributed throughout the
   Internet to serve highly aggregated Virtual Prefixes (VPs).  VSPs
   delegate sub-prefixes from their VPs, which they lease to customers
   as End User Network Prefixes (EPs).  In turn, the customers assign
   the EPs to their customer edge IAs, which connect their End User
   Networks (EUNs) to the VSP IRON instance.

   VSPs may have no affiliation with the ISP networks from which
   customers obtain their basic Internet connectivity.  Therefore, a
   customer could procure its summary network services either through a
   common provider or through separate entities.  In that case, the VSP
   can open for business and begin serving its customers immediately
   without the need to coordinate its activities with ISPs or other
   VSPs.  Further details on business considerations are out of scope
   for this document.

   IRON requires no changes to end systems or to most routers in the



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   Internet.  Instead, IAs are deployed either as new platforms or as
   modifications to existing platforms.  IAs may be deployed
   incrementally without 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 customer's router or host
   that logically connects the customer's EUNs and their associated EPs
   to the VSP IRON instance via tunnels, as shown in Figure 2.  Client
   routers obtain EPs from VSPs and use them to number subnets and
   interfaces within their EUNs.  A Client can be deployed on the same
   physical platform that also connects the customer's EUNs to its ISPs,
   but it may also be a separate router or even a standalone server
   system located 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 Router Connecting EUN to IRON Instance

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 EPs owned by customer Client routers.  In typical deployments, a
   VSP will deploy many Servers around the IRON instance in a globally
   distributed fashion (e.g., as depicted in Figure 3) so that Clients



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   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 Serving Router Global Distribution Example

   Each Server acts as a tunnel-endpoint router that forms bidirectional
   tunnel-neighbor relationships with each of its Client customers and
   also servers as the tunnel egress of dynamically discovered
   unidirectional tunnel-neighbors.  Each Server also associates with a
   set of Relays that can forward packets from the IRON 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 VSP's overlay network
   router that acts as a relay between the IRON instance and the native
   Internet.  Therefore, it 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 company's
   VPs into the IPv4 and IPv6 global Internet BGP routing systems.  Each
   Relay associates with all of the VSP's overlay network Servers, e.g.,
   via tunnels over the IRON instance, via a direct interconnect such as
   an Ethernet cable, etc.  The Relay role (as well as its relationship
   with overlay network Servers) is depicted in Figure 4.









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                      .-.
                   ,-(  _)-.
                .-(_    (_  )-.
               (_   Internet   )
                  `-(______)-'   |  +--------+
                        |        |--| Server |
                   +----+---+    |  +--------+
                   | Relay  |----|  +--------+
                   +--------+    |--| Server |
                       _||       |  +--------+
                      (:::)-.  (Ethernet)
                  .-(::::::::)
   +--------+  .-(::: 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 a common Internetwork (e.g., the public Internet).  Each such
   instance represents a distinct "patch" on the 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, where each patch coordinates its activities independently of
   all others.

   Each VSP IRON instance maintains a set of Relays and Servers that
   provide services to Client customers.  In order to ensure adequate
   customer service levels, the VSP should conduct a traffic scaling
   analysis and distribute sufficient Relays and Servers for the overlay
   network globally throughout the Internet.  Figure 5 depicts the
   logical arrangement of Relays, Servers, and Clients in an IRON
   virtual overlay network.









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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 IPv6 Internets.  It also advertises the VSP's IPv4 VPs into the
   IPv4 BGP routing system and advertises the VSP's IPv6 VPs into the
   IPv6 BGP routing system.  Relays will therefore receive packets with
   EPA destination addresses sent by end systems in the Internet and
   direct them toward EPA-addressed end systems connected to the VSP's
   IRON instance.

   Each VSP also manages a set of Servers that connect their Clients and
   associated EUNs to the IRON instance and to the IPv6 and IPv4
   Internets via their associations with Relays.  IRON Servers therefore
   need not be BGP routers themselves; they can be simple commodity
   hardware platforms.  Moreover, the Server and Relay functions can be
   deployed together on the same physical platform as a unified gateway,
   or they may be deployed on separate platforms (e.g., for load
   balancing purposes).

   Each Server maintains a working set of bidirectional tunnel-neighbor
   Clients for which it caches EP-to-Client mappings in its Forwarding
   Information Base (FIB).  Each Server also, in turn, propagates the
   list of EPs in its working set to each of the Relays in the IRON



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   instance via a dynamic routing protocol (e.g., an overlay network
   internal BGP instance that carries only the EP-to-Server mappings and
   does not interact with the external BGP routing system).  Therefore,
   each Server only needs to track the EPs for its current working set
   of Clients, while each Relay will maintain a full EP-to-Server
   mapping table that represents reachability information for all EPs in
   the VSP overlay network.

   Customers establish Clients that obtain their basic Internet
   connectivity from ISPs and connect to Servers to attach their 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 through an anycast discovery process (described below).  It
   then selects one of these nearby Servers and forms a bidirectional
   tunnel-neighbor relationship with the server through an initial
   exchange followed by periodic keepalives.

   After the Client selects a Server, it forwards initial outbound
   packets from its EUNs by tunneling them to the Server, which, in
   turn, forwards them to the nearest 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 Server in turn
   provides a unidirectional tunnel-neighbor egress for route
   optimization purposes,.

   The IRON can also be used to support VPs 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 VPs of one address family
   over Internetworks based on other address families are discussed in
   Appendix A.


5.  IRON Initialization

   IRON instance initialization entails the startup actions of IAs and
   customer EUNs.  The following sub-sections discuss these startup
   procedures.

5.1.  IRON Relay Router Initialization

   Before its first operational use, each IRON Relay is provisioned with
   the list of VPs that it will serve as well as the locators for all



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   Servers within the IRON instance.  The Relay is also provisioned with
   external BGP interconnections -- the same as for any BGP router.

   Upon startup, the Relay engages in BGP routing exchanges with its
   peers in the IPv4 and IPv6 Internets the same as for any BGP router.
   It then connects to all of the Servers in the overlay network (e.g.,
   via a TCP connection over a bidirectional tunnel, via an Internal BGP
   (IBGP) route reflector, etc.) for the purpose of discovering EP-to-
   Server mappings.  After the Relay has fully populated its EP-to-
   Server mapping information database, it is said to be "synchronized"
   with regard to its VPs.

   After this initial synchronization procedure, the Relay then
   advertises the overlay network's VPs externally.  In particular, the
   Relay advertises the IPv6 VPs into the IPv6 BGP routing system and
   advertises the IPv4 VPs into the IPv4 BGP routing system.  The Relay
   additionally advertises an IPv4 /24 companion prefix (e.g.,
   192.0.2.0/24) into the IPv4 routing system and an IPv6 ::/64
   companion prefix (e.g., 2001:DB8::/64) into the IPv6 routing system
   (note that these may also be sub-prefixes taken from a VP).  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 it assigns the
   resulting addresses as "Relay anycast" addresses for the IRON
   instance.  (See Appendix A for more information on the discovery and
   use of companion prefixes.)  The Relay then engages in ordinary
   packet-forwarding operations.

5.2.  IRON Serving Router Initialization

   Before its first operational use, each IRON Server is provisioned
   with the locators for all Relays within the IRON instance.  Upon
   startup, each Server must connect to all of the Relays within the
   IRON instance (e.g., via a TCP connection, via an IBGP route
   reflector, etc.) for the purpose of reporting its EP-to-Server
   mappings.  The Server then actively listens for Client customers that
   register their EP prefixes as part of establishing a bidirectional
   tunnel-neighbor relationship.  When a new Client connects, the Server
   announces the new EP additions to all Relays; when an existing Client
   disconnects, the Server withdraws its announcements.

5.3.  IRON Client Initialization

   Before its first operational use, each Client must obtain one or more
   EPs from its VSP as well as the companion prefixes associated with
   the VSP's IRON instance (see Section 5.1).  The Client must also
   obtain a certificate and a public/private key pair from the VSP that
   it can later use to prove ownership of its EPs.  This implies that



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   each VSP must run its own public key infrastructure to be used only
   for the purpose of verifying its customers' claimed right to use an
   EP.  Hence, the VSP need not coordinate its public key infrastructure
   with any other organization.

   Upon startup, the Client sends an SCMP Router Solicitation (SRS)
   message to the VSP overlay network Relay anycast address to discover
   the nearest Relay.  The Relay will return an SCMP Router
   Advertisement (SRA) message that lists the locator addresses of one
   or more nearby Servers.  (This list is analogous to the Intra-Site
   Automatic Tunnel Addressing Protocol (ISATAP) Potential Router List
   (PRL) [RFC5214].)

   After the Client receives an SRA message from the nearby Relay
   listing the locator addresses of nearby Servers, it initiates a short
   transaction with one of the Servers carried by a reliable transport
   protocol such as TCP in order to establish a bidirectional tunnel-
   neighbor relationship.  The protocol details of the transaction are
   specific to the VSP, and hence out of scope for this document.

   Note that it is essential that the Client select one and only one
   Server.  This is to allow the VSP overlay network mapping system to
   have one and only one active EP-to-Server mapping at any point in
   time, which shares fate with the Server itself.  If this Server
   fails, the Client can select a new one that will automatically update
   the VSP overlay network mapping system with a new EP-to-Server
   mapping.


6.  IRON Operation

   Following the initialization operations detailed in Section 5, IAs
   engage in the steady-state process of receiving and forwarding
   packets.  All IAs forward encapsulated packets over the IRON instance
   using the mechanisms of VET [INTAREA-VET] and SEAL [INTAREA-SEAL],
   while Relays additionally forward packets to and from the native IPv6
   and IPv4 Internets.  IAs also use SCMP to coordinate with other IAs,
   including the process of sending and receiving redirect messages,
   error messages, etc.  (Note however that an IA must not send an SCMP
   message in response to an SCMP error message.)  Each IA operates as
   specified in the following sub-sections.

6.1.  IRON Client Operation

   After selecting its Server as specified in Section 5.3, the Client
   registers each of its active ISP connections with its IRON instance
   Server.  To do so, it sends periodic beacons (e.g., cryptographically
   signed SRS messages) to the Server via each active ISP to maintain



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   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
   updated.  Although the client may connect via multiple ISPs, a single
   tunnel-neighbor identifier ("NBR_ID") is used to represent the set of
   all ISP paths between the Client and the Server.  The NBR_ID
   therefore names this "bundle" of tunnel-neighbor ISP connections.

   If the Client ceases to receive acknowledgements from its 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 marks the Server as unusable and quickly attempts to
   register with a new Server.  The act of registering with a new Server
   will automatically purge the stale mapping state associated with the
   old Server, since dynamic routing will soon propagate the new Client/
   Server relationship among the IRON instance's Relay Routers.

   When an end system in an EUN sends a flow of packets to a
   correspondent, the packets are forwarded through the EUN via normal
   routing until they reach the Client, which then tunnels the initial
   packets to its Server as the next hop.  In particular, the Client
   encapsulates each packet in an outer header with its locator as the
   source address and the locator of its Server as the destination
   address.  Note that after sending the initial packets of a flow, the
   Client may receive important control messages, such as indications of
   PMTU limitations, redirect messages that indicate a better tunnel-
   neighbor next hop, etc.

   The Client uses the mechanisms specified in VET and SEAL to
   encapsulate each forwarded packet.  The Client further uses the SCMP
   protocol to coordinate with Servers, including accepting redirects
   and other control messages.  When the Client receives an SCMP
   message, it checks the NBR_ID field of the encapsulated packet-in-
   error to verify that the message corresponds to the tunnel-neighbor
   state for its Server.

6.2.  IRON Serving Router Operation

   After the Server is initialized, it responds to SRS messages from
   tunnel-neighbor Clients by sending SRAs.

   When the Server receives a SEAL-encapsulated data packet from one of
   its bidirectional tunnel-neighbor Clients, it uses normal longest-
   prefix-match rules to locate a FIB entry that matches the packet's
   inner destination address.  If the matching FIB entry is more-
   specific than default, the next hop is another tunnel-neighbor



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   Client; 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 either
   a Relay or from a unidirectional tunnel-neighbor Client, it again
   locates a FIB entry that matches the packet's inner destination
   address.  If the matching FIB entry is more-specific than default,
   the server re-encapsulates the packet and forwards it to the correct
   bidirectional tunnel-neighbor Client.  If the Client has recently
   moved to a different Server, however, the Server also returns an SCMP
   redirect message listing a NULL next hop which informs the source
   that the Client has moved.

   After forwarding the packet into the tunnel, the Server may receive
   SCMP error or redirect messages.  The Server then re-encapsulates the
   SCMP message and forwards it to the source of the original packet.

6.3.  IRON Relay Router Operation

   After each Relay has synchronized its VPs (see Section 5.1) it
   advertises the full set of the company's VPs and companion prefixes
   into the IPv4 and IPv6 Internet BGP routing systems.  These prefixes
   will be represented as ordinary routing information in the BGP, and
   any packets originating from the IPv4 or IPv6 Internet destined to an
   address covered by one of the prefixes will be forwarded to one of
   the VSP's Relays.

   When a Relay receives a packet from the Internet destined to an EPA
   covered by one of its VPs, it behaves as an ordinary IP router.  In
   particular, the Relay looks in its FIB to discover a locator of the
   Server that serves the EP covering the destination address.  The
   Relay then simply encapsulates the packet with its own locator as the
   outer source address and the locator of the Server as the outer
   destination address and forwards the packet to the Server.

   When a Relay receives a packet from the Internet destined to one of
   its Relay anycast addresses, it discards the packet if it is not SEAL
   encapsulated.  If the packet is an SCMP SRS message, the Relay
   instead sends an SRA message back to the source listing the locator
   addresses of nearby Servers then discards the message.  The Relay
   otherwise discards all other SCMP messages.







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6.4.  IRON Reference Operating Scenarios

   IRON supports communications when one or both hosts are located
   within EP-addressed EUNs.  When both hosts are within the EUNs of the
   same VSP IRON instance, route redirections that eliminate unnecessary
   Servers and Relays from the path are possible.  When only one host is
   within an IRON EUN, however, route optimization cannot be used.  The
   following sections discuss the two scenarios.

6.4.1.  Both Hosts within Same IRON Instance

   When both hosts are within EUNs served by the same VSP 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.

   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 Relay of the
   destination host, but route optimization can eliminate these elements
   from the path for subsequent packets in the flow.  Figure 6 shows the
   flow of initial packets from host A to host B within EUNs of the same
   IRON instance:


























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

              Figure 6: Initial Packet Flow before Redirects

   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 uses VET and SEAL to encapsulate them in outer headers
   with its locator address as the outer source address and the locator
   address of Server(A) as the outer destination address.  Client(A)
   then simply forwards the encapsulated packets into its 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(A).

   Server(A) receives the encapsulated packets from Client(A) then
   rewrites the outer source address to one of its own locator addresses
   and rewrites the outer destination address to the address of a nearby
   IRON instance Relay(R).  Server(A) then forwards the revised



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   encapsulated packets into the Internet, where routing will direct
   them to Relay(R).

   Relay(R) will intercept the encapsulated packets from Server(A) then
   check its FIB to discover an entry that covers inner destination
   address B with Server(B) as the next hop.  Relay(B) then returns SCMP
   redirect messages to Server(A) (*), rewrites the outer destination
   address of the encapsulated packets to the locator address of
   Server(B), and forwards these revised packets to Server(B).

   Server(B) will receive the encapsulated packets from Relay(R) then
   check its FIB to discover an entry that covers destination address B
   with Client(B) as the next hop.  Server(B) then 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(B) 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.

   (*) Note that after the initial flow of packets, Server(A) will have
   received one or more SCMP redirect messages from Relay(R) listing
   Server(B) as a better next hop.  Server(A) will, in turn, forward the
   redirects to Client(A), which will establish unidirectional tunnel-
   neighbor state and thereafter forward its encapsulated packets
   directly to the locator address of Server(B) without involving either
   Server(A) or Relay(B), as shown in Figure 7.
























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

              Figure 7: Sustained Packet Flow after Redirects

6.4.2.  Mixed IRON and Non-IRON Hosts

   When 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), the IA elements involved depend on the packet-flow directions.
   The cases are described in the following sub-sections.

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

   Figure 8 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 |
      +-----+-----+                                      +-----+-----+
            |  (                                            )  |
           .-.   .-(____________________________________)-.   .-.
        ,-(  _)-.                                          ,-(  _)-.
     .-(_    (_  )-.                                    .-(_    (_  )-.
    (_  IRON EUN A  )                                  (_non-IRON EUN B)
       `-(______)-'                                       `-(______)-'
            |                                                  |
        +---+----+                                         +---+----+
        | Host A |                                         | Host B |
        +--------+                                         +--------+

               Figure 8: 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 uses VET and SEAL to encapsulate them in outer headers
   with its locator address as the outer source address and the locator
   address of Server(A) as the outer destination address.  The ISP will
   pass the packets without filtering since the (outer) source address
   is topologically correct.  Once the packets have been released into
   the native Internet, routing will direct them to Server(A).

   Server(A) receives the encapsulated packets from Client(A) then re-
   encapsulates and forwards them to Relay(A), which simply decapsulates
   them and 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 Server(A) and Relay(A) are
   depicted in Figure 8 as two halves of a unified gateway.  In that
   case, the "forwarding" between Server(A) and Relay(A) is a zero-



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   instruction imaginary operation within the gateway.)

   This scenario always involves a Server and Relay owned by the VSP
   that provides service to IRON EUN A.

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

   Figure 9 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 |
      +-----+-----+                                      +-----+-----+
            |  (                                            )  |
           .-.   .-(____________________________________)-.   .-.
        ,-(  _)-.                                          ,-(  _)-.
     .-(_    (_  )-.                                    .-(_    (_  )-.
    (_  IRON EUN A  )                                  (_non-IRON EUN B)
       `-(______)-'                                       `-(_______)-'
            |                                                  |
        +---+----+                                         +---+----+
        | Host A |                                         | Host B |
        +--------+                                         +--------+

               Figure 9: 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. Routing will direct
   the packets to Relay(A), which then forwards them to Server(A) using
   encapsulation if necessary.

   Server(A) will then check its FIB to discover an entry that covers
   destination address A with Client(A) as the next hop.  Server(A) then



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   (re-)encapsulates the packets in an outer header that uses the source
   address, destination address, and NBR_ID parameters associated with
   the tunnel-neighbor state for Client(A).  Next, Server(A) forwards
   these (re-)encapsulated packets 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.

   This scenario always involves a Server and Relay owned by the VSP
   that provides service to IRON EUN A.

6.4.3.  Hosts within Different IRON Instances

   Figure 10 depicts the IRON reference operating scenario for packets
   flowing from host A in an IRON instance B to 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
   and IRON EUN and the other was serviced by a non-IRON EUN.
































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                  _________________________________________
               .-(         )-.                             )-.
            .-(      +-------)----+       +---(--------+      )-.
         .-(         |  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        +-----+-----+
            |  (                                            )  |
           .-.   .-(____________________________________)-.   .-.
        ,-(  _)-.                                          ,-(  _)-.
     .-(_    (_  )-.                                    .-(_    (_  )-.
    (_  IRON EUN A  )                                  (_  IRON EUN B  )
       `-(______)-'                                       `-(______)-'
            |                                                  |
        +---+----+                                         +---+----+
        | Host A |                                         | Host B |
        +--------+                                         +--------+

             Figure 10: Hosts within Different IRON Instances

6.5.  Mobility, Multihoming, and Traffic Engineering Considerations

   While IRON Servers and Relays can be considered 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 client
   routers.

6.5.1.  Mobility Management

   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 additions or
   deletions.  This operation is performance sensitive and should be
   conducted immediately to avoid packet loss.



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   If the Client has moved far away from its previous network point of
   attachment, however, it re-issues the anycast discovery procedure
   described in Section 6.1 to discover whether its candidate set of
   Servers has changed.  If the Client's current Server is also included
   in the new list received from the VSP, this provides indication that
   the Client has not moved far enough to warrant changing to a new
   Server.  Otherwise, 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.

   To move to a new Server, the Client first engages in the EP
   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 reliable transaction.  The former Server
   will then retain the (stale) FIB entries until their lifetime
   expires, allowing it to continue delivering packets to the Client for
   the short term while informing existing correspondents that the
   Client has moved.

6.5.2.  Multihoming

   A Client may register multiple locators with its Server.  It can
   assign metrics with its registrations to inform the Server of
   preferred locators, and it can select outgoing locators according to
   its local preferences.  Therefore, multihoming is naturally
   supported.

6.5.3.  Inbound Traffic Engineering

   A Client can dynamically adjust the priorities of its prefix
   registrations with its Server 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.

6.5.4.  Outbound Traffic Engineering

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

6.6.  Renumbering Considerations

   As new link-layer technologies and/or service models emerge,
   customers will be motivated to select their service providers through
   healthy competition between ISPs.  If a customer's EUN addresses are
   tied to a specific ISP, however, the customer may be forced to



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   undergo a painstaking EUN renumbering process if it wishes to change
   to a different ISP [RFC4192][RFC5887].

   When a customer obtains EP prefixes from a VSP, it can change between
   ISPs seamlessly and without need to renumber.  If the VSP itself
   applies unreasonable costing structures for use of the EPs, however,
   the customer may be compelled to seek a different VSP and would again
   be required to confront a renumbering scenario.

6.7.  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 a
   Client is located behind a NAT, it selects Servers using the same
   procedures as for Clients with public addresses, e.g., it can send
   SRS messages to Servers in order to get SRA messages in return.  The
   only requirement is that the Client must configure its SEAL
   encapsulation to use a transport protocol that supports NAT
   traversal, e.g., UDP, TCP, SSL, etc.

   Since the Server maintains state about its Client customers, it can
   discover locator information for each Client by examining the UDP
   port number and IP address in the outer headers of the Client's
   encapsulated SRS 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 UDP 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.

   IRON does not introduce any new issues to complications raised for
   NAT traversal or for applications embedding address referrals in
   their payload.

6.8.  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.) will be discussed in a
   separate document.



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6.9.  Nested EUN Considerations

   Each Client configures a locator that may be taken from an ordinary
   non-EPA address assigned by an ISP or from an EPA address taken from
   an EP 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 11, Client(A)
   configures a locator EPA(B) taken from the EP assigned to EUN(B).
   Client(B) in turn configures a locator EPA(C) taken from the EP
   assigned to EUN(C).  Finally, Client(C) configures a locator ISP(D)
   taken from a non-EPA 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 EPA(A) within EUN(A), exchanges
   packets with host Z located elsewhere in the Internet.



































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

                       Figure 11: 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.

6.9.1.  Host A Sends Packets to Host Z

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



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   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
   into the Internet to Server(C).

   When Server(C) receives the triple-encapsulated packet, it removes
   the outer layer of encapsulation and forwards the resulting twice-
   encapsulated packet into the Internet to Server(B).  Next, Server(B)
   removes the outer layer of encapsulation and forwards the resulting
   once-encapsulated packet into the Internet to Server(A).  Next,
   Server(A) checks the address type of the inner address 'Z'.  If Z is
   a non-EPA address, Server(A) simply decapsulates the packet and
   forwards it into the Internet.  Otherwise, Server(A) rewrites the
   outer source and destination addresses of the once-encapsulated
   packet and forwards it to Relay(Z).  Relay(Z), in turn, rewrites the
   outer destination address of the packet to the locator for Server(Z),
   then forwards the packet and sends a redirect to Server(A) (which
   forwards the redirect to Client(A)).  Server(Z) then re-encapsulates
   the packet and forwards it to Client(Z), which decapsulates it and
   forwards the inner packet to host Z.  Subsequent packets from
   Client(A) will then use Server(Z) as the next hop toward host Z,
   which eliminates Server(A) and Relay(Z) from the path.

6.9.2.  Host Z Sends Packets to Host A

   Whether or not host Z configures an EPA address, its packets destined
   to host A will eventually reach Server(A).  Server(A) will have a
   mapping that lists Client(A) as the next hop toward EPA(A).
   Server(A) will then encapsulate the packet with EPA(B) as the outer
   destination address and forward the packet into the Internet.
   Internet routing will convey this once-encapsulated packet to
   Server(B), which will have a mapping that lists Client(B) as the next
   hop toward EPA(B).  Server(B) will then encapsulate the packet with
   EPA(C) as the outer destination address and forward the packet into
   the Internet.  Internet routing will then convey this twice-
   encapsulated packet to Server(C), which will have a mapping that
   lists Client(C) as the next hop toward EPA(C).  Server(C) will then
   encapsulate the packet with ISP(D) as the outer destination address
   and forward the packet into the Internet.  Internet routing will then
   convey this triple-encapsulated packet to Client(C).

   When the triple-encapsulated packet arrives at Client(C), it strips
   the outer layer of encapsulation and forwards the twice-encapsulated
   packet to EPA(C), which is the locator address of Client(B).  When
   Client(B) receives the twice-encapsulated packet, it strips the outer
   layer of encapsulation and forwards the once-encapsulated packet to
   EPA(B), which is the locator address of Client(A).  When Client(A)



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   receives the once-encapsulated packet, it strips the outer layer of
   encapsulation and forwards the unencapsulated packet to EPA(A), which
   is the host address of host A.


7.  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 and
   appropriately managed to provide acceptable service levels to
   customers.

   End-to-end traffic that traverses an IRON virtual overlay network 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 virtual overlay networks also work seamlessly with existing and
   emerging services within the native Internet.  In particular,
   customers serviced by IRON virtual overlay networks will receive the
   same service enjoyed by customers serviced by non-IRON service
   providers.  Internet services already deployed within the native
   Internet also need not make any changes to accommodate IRON virtual
   overlay network customers.

   The IRON system operates between routers within provider networks and
   end user networks.  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, IRON
   routers have a method for naturally detecting and tuning out all
   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.




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   Finally, and perhaps most importantly, the IRON system provides an
   in-built mobility management and multihoming capability that allows
   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 customer/provider relationship,
   and therefore requires no adjunct mechanisms.  The mobility
   management and multihoming capabilities are further supported by
   forward-path reachability detection that provides "hints of forward
   progress" in the same spirit as for IPv6 Neighbor Discovery (ND).


8.  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].


9.  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 customer
   EUNs coordinate with Servers is different and based on the
   redirection model associated with NBMA links.

   Numerous publications have proposed NAT traversal techniques.  The
   NAT traversal techniques adapted for IRON were inspired by the Simple



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


10.  Security Considerations

   Security considerations that apply to tunneling in general are
   discussed in [V6OPS-TUN-SEC].  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 detect 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 BGP (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 Client<->Server bindings.  Finally, 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.


11.  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.



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   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.


12.  References

12.1.  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.

12.2.  Informative References

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

   [EVOLUTION]
              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.




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   [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.

   [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.

   [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.

   [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



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

   [V6OPS-TUN-SEC]
              Krishnan, S., Thaler, D., and J. Hoagland, "Security
              Concerns With IP Tunneling", Work in Progress,
              October 2010.

Appendix A.  IRON VPs over Internetworks with Different Address Families

   The IRON architecture leverages the routing system by providing
   generally shortest-path routing for packets with EPA addresses from
   VPs that match the address family of the underlying Internetwork.
   When the VPs are of an address family that is not routable within the
   underlying Internetwork, however, (e.g., when OSI/NSAP [RFC4548] VPs
   are used within an IPv4 Internetwork) a global mapping database is
   required to allow Servers to map VPs to companion prefixes taken from
   address families that are routable within the Internetwork.  For
   example, an IPv6 VP (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.

   Every VP in the IRON must therefore be represented in a globally
   distributed Master VP database (MVPd) that maintains VP-to-companion
   prefix mappings for all VPs in the IRON.  The MVPd 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 Server discovers the full set of VPs for the IRON
   by reading the MVPd.  The Server reads the MVPd from a nearby server
   and periodically checks the server for deltas since the database was
   last read.  After reading the MVPd, the Server has a full list of VP-
   to-companion prefix mappings.

   The Server can then forward packets toward EPAs covered by a VP by
   encapsulating them in an outer header of the VP's companion prefix
   address family and using any address taken from the companion prefix
   as the outer destination address.  The companion prefix therefore
   serves as an anycast prefix.



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   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 customer EUNs,
   traffic scaling, state requirements, etc.

   In terms of routing scaling, each VSP will advertise one or more VPs
   into the global Internet routing system from which EPs are delegated
   to customer EUNs.  Routing scaling will therefore be minimized when
   each VP covers many EPs.  For example, the IPv6 prefix 2001:DB8::/32
   contains 2^24 ::/56 EP prefixes for assignment to EUNs; therefore,
   the IRON could accommodate 2^32 ::/56 EPs with only 2^8 ::/32 VPs
   advertised in the interdomain routing core.  (When even longer EP
   prefixes are used, e.g., /64s assigned to individual handsets in a
   cellular provider network, considerable numbers of EUNs can be
   represented within only a single VP.)  Each VP also has an associated
   anycast companion prefix; hence, there will be one anycast prefix
   advertised into the global routing system for each VP.

   In terms of traffic scaling for Relays, each Relay represents an ASBR
   of a "shell" enterprise network that simply directs arriving traffic
   packets with EPA destination addresses towards Servers that service
   customer EUNs.  Moreover, the Relay sheds traffic destined to EPAs
   through redirection, which removes it from the path for the vast
   majority of traffic packets.  On the other hand, each Relay must
   handle all traffic packets forwarded between its customer EUNs and
   the non-IRON 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 the VSP overlay network's
   customer EUNs.  The Server services all traffic packets destined to
   its EUNs but only services the initial packets of flows initiated
   from the EUNs and destined to EPAs.  Therefore, traffic scaling for
   EPA-addressed traffic is an asymmetric consideration and is
   proportional to the number of EUNs each Server serves.

   In terms of state requirements for Relays, each Relay maintains a
   list of all Servers in the VSP overlay network as well as FIB entries
   for all customer EUNs that each Server serves.  This state is
   therefore dominated by the number of EUNs in the VSP overlay network.
   Sizing the Relay to accommodate state information for all EUNs is
   therefore required during VSP overlay network planning.  In terms of
   state requirements for Servers, each Server maintains tunnel-neighbor



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   state for each of the customer EUNs it serves, but it need not keep
   state for all EUNs in the VSP overlay network.  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 customer EUN.  Therefore, traffic scaling
   considerations for Clients are the same as for any site border
   router.  Clients also retain 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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