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rfc:rfc6179

Internet Research Task Force (IRTF) F. Templin, Ed. Request for Comments: 6179 Boeing Research & Technology Category: Experimental March 2011 ISSN: 2070-1721

            The Internet Routing Overlay Network (IRON)

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) 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.  This document is a product of the
 IRTF Routing Research Group.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for examination, experimental implementation, and
 evaluation.
 This document defines an Experimental Protocol for the Internet
 community.  This document is a product of the Internet Research Task
 Force (IRTF).  The IRTF publishes the results of Internet-related
 research and development activities.  These results might not be
 suitable for deployment.  This RFC represents the individual
 opinion(s) of one or more members of the Internet Research Task Force
 (IRTF) Research Group of the Internet Research Task Force (IRTF).
 Documents approved for publication by the IRSG are not a candidate
 for any level of Internet Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6179.

Templin Experimental [Page 1] RFC 6179 IRON March 2011

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
 carefully, as they describe your rights and restrictions with respect
 to this document.

Templin Experimental [Page 2] RFC 6179 IRON March 2011

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 Initialization ............................................13
    5.1. IRON Relay Router Initialization ..........................13
    5.2. IRON Serving Router Initialization ........................14
    5.3. IRON Client Initialization ................................15
 6. IRON Operation .................................................15
    6.1. IRON Client Operation .....................................16
    6.2. IRON Serving Router Operation .............................17
    6.3. IRON Relay Router Operation ...............................18
    6.4. IRON Reference Operating Scenarios ........................18
         6.4.1. Both Hosts within IRON EUNs ........................19
         6.4.2. Mixed IRON and Non-IRON Hosts ......................21
    6.5. Mobility, Multihoming, and Traffic Engineering
         Considerations ............................................24
         6.5.1. Mobility Management ................................24
         6.5.2. Multihoming ........................................25
         6.5.3. Inbound Traffic Engineering ........................25
         6.5.4. Outbound Traffic Engineering .......................25
    6.6. Renumbering Considerations ................................25
    6.7. NAT Traversal Considerations ..............................26
    6.8. Multicast Considerations ..................................26
    6.9. Nested EUN Considerations .................................26
         6.9.1. Host A Sends Packets to Host Z .....................28
         6.9.2. Host Z Sends Packets to Host A .....................28
 7. Implications for the Internet ..................................29
 8. Additional Considerations ......................................30
 9. Related Initiatives ............................................30
 10. Security Considerations .......................................31
 11. Acknowledgements ..............................................31
 12. References ....................................................32
    12.1. Normative References .....................................32
    12.2. Informative References ...................................32
 Appendix A. IRON VPs over Internetworks with Different
             Address Families ......................................35
 Appendix B. Scaling Considerations ................................36

Templin Experimental [Page 3] RFC 6179 IRON March 2011

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)
 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].

Templin Experimental [Page 4] RFC 6179 IRON March 2011

 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
 Network Protocol) [RFC1070], etc.) are also within scope.
 The IRON is a global routing system comprising virtual overlay
 networks managed by Virtual Prefix Companies (VPCs) that own and
 manage Virtual Prefixes (VPs) from which End User Network (EUN)
 prefixes (EPs) are delegated to customer sites.  The 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].  The
 IRON uses 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).  The IRON requires
 deployment of a small number of new BGP core routers and supporting
 servers, as well as IRON-aware routers/servers in customer EUNs.  No
 modifications to hosts, and no modifications to most routers, are
 required.
 Note: This document is offered in compliance with Internet Research
 Task Force (IRTF) document stream procedures [RFC5743]; it is not an
 IETF product and is not a standard.  The views in this document were
 considered controversial by the IRTF Routing Research Group (RRG),
 but the RG reached a consensus that the document should still be
 published.  The document will undergo a period of review within the
 RRG and through selected expert reviewers prior to publication.  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 derived from a Virtual
    Prefix (VP) (e.g., an IPv4 /28, an IPv6 /56, etc.) and delegated
    to an EUN by a Virtual Prefix Company (VPC).
 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.

Templin Experimental [Page 5] RFC 6179 IRON March 2011

 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):
    a composite virtual overlay network that comprises the union of
    all VPC overlay networks configured over a common Internetwork.
    The IRON supports routing through encapsulation of inner packets
    with EPA addresses within outer headers that use locator
    addresses.
 IRON Client Router/Host ("Client"):
    a customer's router or host that logically connects the customer's
    EUNs and their associated EPs to the IRON via an NBMA tunnel
    virtual interface.
 IRON Serving Router ("Server"):
    a VPC's overlay network router that provides forwarding and
    mapping services for the EPs owned by customer Clients.
 IRON Relay Router ("Relay"):
    a VPC's overlay network 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.

Templin Experimental [Page 6] RFC 6179 IRON March 2011

 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 Prefix Company (VPC).
 Virtual Prefix Company (VPC):
    a company that owns and manages a set of VPs from which it
    delegates EPs to EUNs.
 VPC Overlay Network
    a specialized set of routers deployed by a VPC to service customer
    EUNs through a virtual overlay network configured over an
    underlying Internetwork (e.g., the global Internet).

3. The Internet Routing Overlay Network

 The Internet Routing Overlay Network (IRON) is a system of virtual
 overlay networks configured over 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.
 The IRON 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.

Templin Experimental [Page 7] RFC 6179 IRON March 2011

                                       +-------------------------+
                                       |    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.
 The IRON is the union of all virtual overlay networks that are
 configured over a common underlying Internet and are owned and
 managed by Virtual Prefix Companies (VPCs).  Each such virtual
 overlay network comprises a set of IAs distributed throughout the
 Internet to serve highly aggregated Virtual Prefixes (VPs).  VPCs
 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 IRON.
 VPCs 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 broker or through separate entities.  In that case, the VPC
 can open for business and begin serving its customers immediately

Templin Experimental [Page 8] RFC 6179 IRON March 2011

 without the need to coordinate its activities with ISPs or other
 VPCs.  Further details on business considerations are out of scope
 for this document.
 The IRON requires no changes to end systems or to most routers in the
 Internet.  Instead, the IRON comprises IAs that 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 the
 IRON.  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 IRON via tunnels, as shown in Figure 2.  Client routers obtain
 EPs from VPCs 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 EPA and exhibits the outward appearance of a host.
                         .-.
                      ,-(  _)-.
      +--------+   .-(_    (_  )-.
      | Client |--(_     ISP      )
      +---+----+     `-(______)-'
          |   <= T         \     .-.
         .-.       u        \ ,-(  _)-.
      ,-(  _)-.       n     .-(_    (-  )-.
   .-(_    (_  )-.      n  (_   Internet   )
  (_     EUN      )       e   `-(______)-
     `-(______)-'           l          ___
          |                   s =>    (:::)-.
     +----+---+                   .-(::::::::)
     |  Host  |                .-(::::::::::::)-.
     +--------+               (:::: The IRON ::::)
                               `-(::::::::::::)-'
                                  `-(::::::)-'
        Figure 2: IRON Client Router Connecting EUN to the IRON

Templin Experimental [Page 9] RFC 6179 IRON March 2011

3.2. IRON Serving Router

 An IRON serving router (or, simply, "Server") is a VPC's overlay
 network router that provides forwarding and mapping services for the
 EPs owned by customer Client routers.  In typical deployments, a VPC
 will deploy many Servers around the IRON in a globally distributed
 fashion (e.g., as depicted in Figure 3) so that Clients can discover
 those that are nearby.
           +--------+    +--------+
           | Boston |    | Tokyo  |
           | Server |    | Server |
           +--+-----+    ++-------+
   +--------+  \         /
   | Seattle|   \   ___ /
   | Server |    \ (:::)-.       +--------+
   +------+-+  .-(::::::::)------+ Paris  |
           \.-(::::::::::::)-.   | Server |
           (:::: The IRON ::::)  +--------+
            `-(::::::::::::)-'
 +--------+ /  `-(::::::)-'  \     +--------+
 | Moscow +          |        \--- + Sydney |
 | Server |     +----+---+         | Server |
 +--------+     | Cairo  |         +--------+
                | Server |
                +--------+
       Figure 3: IRON Serving Router Global Distribution Example
 Each Server acts as a tunnel-endpoint router that forms a
 bidirectional tunnel-neighbor relationship with each of its Client
 customers.  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 VPC's overlay network
 router that acts as a relay between the IRON and the native Internet.
 Therefore, it also serves as an Autonomous System Border Router
 (ASBR) that is owned and managed by the VPC.
 Each VPC 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 VPC's overlay network Servers, e.g.,
 via tunnels over the IRON, 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.

Templin Experimental [Page 10] RFC 6179 IRON March 2011

                    .-.
                 ,-(  _)-.
              .-(_    (_  )-.
             (_   Internet   )
                `-(______)-'   |  +--------+
                      |        |--| Server |
                 +----+---+    |  +--------+
                 | Relay  |----|  +--------+
                 +--------+    |--| Server |
                     _||       |  +--------+
                    (:::)-.  (Ethernet)
                .-(::::::::)
 +--------+  .-(::::::::::::)-.  +--------+
 | Server |=(:::: The IRON ::::)=| Server |
 +--------+  `-(::::::::::::)-'  +--------+
                `-(::::::)-'
                     ||      (Tunnels)
                 +--------+
                 | Server |
                 +--------+
    Figure 4: IRON Relay Router Connecting IRON to Native Internet

4. IRON Organizational Principles

 The IRON consists of the union of all VPC overlay networks configured
 over a common Internetwork (e.g., the public Internet).  Each such
 overlay network represents a distinct "patch" on the Internet
 "quilt", where the patches are stitched together by tunnels over the
 links, routers, bridges, etc. that connect the underlying
 Internetwork.  When a new VPC overlay network is deployed, it becomes
 yet another patch on the quilt.  The IRON is therefore a composite
 overlay network consisting of multiple individual patches, where each
 patch coordinates its activities independently of all others (with
 the exception that the Servers of each patch must be aware of all VPs
 in the IRON).  In order to ensure mutual cooperation between all VPC
 overlay networks, sufficient address space portions of the inner
 network-layer protocol (e.g., IPv4, IPv6, etc.) should be set aside
 and designated as VP space.
 Each VPC overlay network in the IRON maintains a set of Relays and
 Servers that provide services to their Client customers.  In order to
 ensure adequate customer service levels, the VPC 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.

Templin Experimental [Page 11] RFC 6179 IRON March 2011

                            .-.
                         ,-(  _)-.
                      .-(_    (_  )-.
                     (__ Internet   _)
                        `-(______)-'
        <------------     Relays      ------------>
                  ________________________
                 (::::::::::::::::::::::::)-.
             .-(:::::::::::::::::::::::::::::)
          .-(:::::::::::::::::::::::::::::::::)-.
         (:::::::::::   The IRON  :::::::::::::::)
          `-(:::::::::::::::::::::::::::::::::)-'
             `-(::::::::::::::::::::::::::::)-'
        <------------    Servers      ------------>
        .-.                .-.                     .-.
     ,-(  _)-.          ,-(  _)-.               ,-(  _)-.
  .-(_    (_  )-.    .-(_    (_  )-.         .-(_    (_  )-.
 (__   ISP A    _)  (__   ISP B    _)  ...  (__   ISP x    _)
    `-(______)-'       `-(______)-'            `-(______)-'
         <-----------      NATs        ------------>
         <----------- Clients and EUNs ----------->
            Figure 5: Virtual Overlay Network Organization
 Each Relay in the VPC overlay network connects the overlay directly
 to the underlying IPv4 and IPv6 Internets.  It also advertises the
 VPC overlay network's IPv4 VPs into the IPv4 BGP routing system and
 advertises the overlay network'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 VPC overlay network.
 Each VPC overlay network also manages a set of Servers that connect
 their Clients and associated EUNs to the IRON 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 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 VPC overlay network via a dynamic

Templin Experimental [Page 12] RFC 6179 IRON March 2011

 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 VPC overlay network.
 Customers establish Clients that obtain their basic Internet
 connectivity from ISPs and connect to Servers to attach their EUNs to
 the IRON.  Each EUN can connect to the IRON via one or 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 that serves
 the final destination.  The Client will subsequently receive redirect
 messages informing it of a more direct route through a Server that
 serves the final destination EUN.
 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 initialization entails the startup actions of IAs within the VPC
 overlay network and customer EUNs.  The following sub-sections
 discuss these startup procedures.

5.1. IRON Relay Router Initialization

 Before its first operational use, each Relay in a VPC overlay network
 is provisioned with the list of VPs that it will serve as well as the
 locators for all Servers that belong to the same overlay network.
 The Relay is also provisioned with external BGP interconnections --
 the same as for any BGP router.

Templin Experimental [Page 13] RFC 6179 IRON March 2011

 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 subnet-router anycast addresses
 [RFC3068][RFC2526] for the VPC overlay network.  (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 Server in a VPC overlay
 network is provisioned with the locators for all Relays that
 aggregate the overlay network's VPs.  In order to support route
 optimization, the Server must also be provisioned with the list of
 all VPs in the IRON (i.e., not just the VPs of its own overlay
 network) so that it can discern EPA and non-EPA addresses.
 (Therefore, the Server could be greatly simplified if the list of VPs
 could be covered within a small number of very short prefixes, e.g.,
 one or a few IPv6 ::/20's).  The Server must also discover the VP
 companion prefix relationships discussed in Section 5.1, e.g., via a
 global database such as discussed in Appendix A.
 Upon startup, each Server must connect to all of the Relays within
 its overlay network (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 registers its EP
 prefixes, the Server announces the new EP additions to all Relays;
 when an existing Client unregisters its EP prefixes, the Server
 withdraws its announcements.

Templin Experimental [Page 14] RFC 6179 IRON March 2011

5.3. IRON Client Initialization

 Before its first operational use, each Client must obtain one or more
 EPs from its VPC as well as the companion prefixes associated with
 the VPC overlay network (see Section 5.1).  The Client must also
 obtain a certificate and a public/private key pair from the VPC that
 it can later use to prove ownership of its EPs.  This implies that
 each VPC 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 VPC need not coordinate its public key infrastructure
 with any other organization.
 Upon startup, the Client sends an SCMP Router Solicitation (SRS)
 message to the VPC overlay network subnet-router 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 VPC, 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 VPC 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 VPC overlay network mapping system with a new EP-to-Server
 mapping.

6. IRON Operation

 Following the IRON initialization detailed in Section 5, IAs engage
 in the steady-state process of receiving and forwarding packets.  All
 IAs forward encapsulated packets over the IRON using the mechanisms
 of VET [INTAREA-VET] and SEAL [INTAREA-SEAL], while Relays (and in
 some cases Servers) 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.

Templin Experimental [Page 15] RFC 6179 IRON March 2011

6.1. IRON Client Operation

 After selecting its Server as specified in Section 5.3, the Client
 should register each of its ISP connections with the Server for
 multihoming purposes.  To do so, it sends periodic beacons (e.g., SRS
 messages) to its Server via each of its ISPs to establish additional
 tunnel-neighbor state.  This implies that a single tunnel-neighbor
 identifier (i.e., a "nonce") is used to represent the set of all ISP
 paths between the Client and the Server.  Therefore, the nonce names
 this "bundle" of ISP paths.
 If the Client ceases to receive acknowledgements from its Server via
 a specific ISP connection, it marks the Server as unreachable from
 that address and therefore over that ISP connection.  (The Client
 should also inform its Server of this outage via one of its working
 ISP connections.)  If the Client ceases to receive acknowledgements
 from its 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 propagate the new client/server relationship to the VPC overlay
 network 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 SCMP messages, such as indications of
 PMTU limitations, redirects that point to a better 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 SCMP messages.  When the Client receives an SCMP message,
 it checks the nonce field of the encapsulated packet-in-error to
 verify that the message corresponds to the tunnel-neighbor state for
 its Server and accepts the message if the nonce matches.  (Note
 however that the outer source and destination addresses of the
 packet-in-error may be different than those in the original packet
 due to possible Server and/or Relay address rewritings.)

Templin Experimental [Page 16] RFC 6179 IRON March 2011

6.2. IRON Serving Router Operation

 After the Server is initialized, it responds to SRSs from Clients by
 sending SRAs.  When the Server receives a SEAL-encapsulated packet
 from one of its Client tunnel neighbors, it examines the inner
 destination address.  If the inner destination address is not an EPA,
 the Server decapsulates the packet and forwards it unencapsulated
 into the Internet if it is able to do so without loss due to ingress
 filtering.  Otherwise, the Server re-encapsulates the packet (i.e.,
 it removes the outer header and replaces it with a new outer header
 of the same address family) and sets the outer destination address to
 the locator address of a Relay within its VPC overlay network.  It
 then forwards the re-encapsulated packet to the Relay, which will, in
 turn, decapsulate it and forward it into the Internet.
 If the inner destination address is an EPA, however, the Server
 rewrites the outer source address to one of its own locator addresses
 and rewrites the outer destination address to the subnet-router
 anycast address taken from the companion prefix associated with the
 inner destination address (where the companion prefix of the same
 address family as the outer IP protocol is used).  The Server then
 forwards the revised encapsulated packet into the Internet via a
 default or more specific route, where it will be directed to the
 closest Relay within the destination VPC overlay network.  After
 sending the packet, the Server may then receive an SCMP error or
 redirect message from a Relay/Server within the destination VPC
 overlay network.  In that case, the Server verifies that the nonce in
 the message matches the Client that sent the original inner packet
 and discards the message if the nonce does not match.  Otherwise, the
 Server re-encapsulates the SCMP message in a new outer header that
 uses the source address, destination address, and nonce parameters
 associated with the Client's tunnel-neighbor state; it then forwards
 the message to the Client.  This arrangement is necessary to allow
 SCMP messages to flow through any NATs on the path.
 When a Server ('A') receives a SEAL-encapsulated packet from a Relay
 or from the Internet, if the inner destination address matches an EP
 in its FIB, 'A' re-encapsulates the packet in a new outer header and
 forwards it to a Client ('B'), which, in turn, decapsulates the
 packet and forwards it to the correct end system in the EUN.
 However, if 'B' has left notice with 'A' that it has moved to a new
 Server ('C'), 'A' will instead forward the packet to 'C' and also
 send an SCMP redirect message back to the source of the packet.  In
 this way, 'B' can leave behind forwarding information when changing
 between Servers 'A' and 'C' (e.g., due to mobility events) without
 exposing packets to loss.

Templin Experimental [Page 17] RFC 6179 IRON March 2011

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 VPC overlay network'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 subnet-router 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.
 If the packet is an ordinary SEAL packet (i.e., one that encapsulates
 an inner packet), the Relay sends an SCMP redirect message of the
 same address family back to the source with the locator of the Server
 that serves the EPA destination in the inner packet as the redirected
 target.  The source and destination addresses of the SCMP redirect
 message use the outer destination and source addresses of the
 original packet, respectively.  After sending the redirect message,
 the Relay then rewrites the outer destination address of the SEAL-
 encapsulated packet to the locator of the Server and forwards the
 revised packet to the Server.  Note that in this arrangement, any
 errors that occur on the path between the Relay and the Server will
 be delivered to the original source but with a different destination
 address due to this Relay address rewriting.

6.4. IRON Reference Operating Scenarios

 The IRON supports communications when one or both hosts are located
 within EP-addressed EUNs, regardless of whether the EPs are
 provisioned by the same VPC or by different VPCs.  When both hosts
 are within IRON EUNs, 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.

Templin Experimental [Page 18] RFC 6179 IRON March 2011

6.4.1. Both Hosts within IRON EUNs

 When both hosts are within IRON EUNs, 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 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 two IRON EUNs
 (the same scenario applies whether the two EUNs are within the same
 VPC overlay network or different overlay networks).
               ________________________________________
            .-(                 .-.                    )-.
         .-(                 ,-(  _)-.                    )-.
      .-(          +========+(_    (_  +=====+               )-.
    .(             ||    (_|| Internet ||_) ||                  ).
  .(               ||      ||-(______)-||   vv                    ).
.(        +--------++--+   ||          ||   +------------+          ).
(     +==>| Server(A)  |   vv          ||   | Server(B)  |====+      )
(    //   +---------|\-+   +--++----++--+   +------------+    \\     )
(   //  .-.         | \    |  Relay(B)  |                  .-. \\    )
(  //,-(  _)-.      |  \   +-v----------+               ,-(  _)-\\   )
( .||_    (_  )-.   |   \____|                       .-(_    (_  ||. )
( _||  ISP A    .)  |                               (__   ISP B  ||_))
(  ||-(______)-'    | (redirect)                       `-(______)||  )
(  ||    |          |                                       |    vv  )
 ( +-----+-----+    |                                 +-----+-----+ )
   | Client(A) | <--+                                 | Client(B) |
   +-----+-----+              The IRON                +-----+-----+
         |    (   (Overlaid on the Native Internet)     )   |
        .-.     .-(                                .-)     .-.
     ,-(  _)-.      .-(________________________)-.      ,-(  _)-.
  .-(_    (_  )-.                                    .-(_    (_  )-.
 (_  IRON EUN A  )                                  (_  IRON EUN B  )
    `-(______)-'                                       `-(______)-'
         |                                                  |
     +---+----+                                         +---+----+
     | Host A |                                         | Host B |
     +--------+                                         +--------+
            Figure 6: Initial Packet Flow before Redirects

Templin Experimental [Page 19] RFC 6179 IRON March 2011

 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 subnet-router
 anycast address of the appropriate address family associated with the
 inner destination address.  Server(A) then forwards the revised
 encapsulated packets into the Internet, where routing will direct
 them to Relay(B), which services the VPC overlay network associated
 with host B.
 Relay(B) 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(B) 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 nonce 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(B) 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.

Templin Experimental [Page 20] RFC 6179 IRON March 2011

               ________________________________________
            .-(                 .-.                    )-.
         .-(                 ,-(  _)-.                    )-.
      .-( +=============> .-(_    (_  )-.======+             )-.
    .(   //              (__ Internet   _)    ||                ).
  .(    //                  `-(______)-'      vv                  ).
.(     //                                   +------------+          ).
(     //                                    |  Server(B) |====+      )
(    //                                     +------------+    \\     )
(   //  .-.                                                .-. \\    )
(  //,-(  _)-.                                          ,-(  _)-\\   )
( .||_    (_  )-.                                    .-(_    (_  ||. )
( _||  ISP A    .)                                  (__   ISP B  ||_))
(  ||-(______)-'                                       `-(______)||  )
(  ||    |                                                  |    vv  )
 ( +-----+-----+              The IRON                +-----+-----+ )
   | 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.

Templin Experimental [Page 21] RFC 6179 IRON March 2011

                _________________________________________
             .-(         )-.                             )-.
          .-(      +-------)----+                           )-.
       .-(         |  Relay(A)  |--------------+               )-.
     .(            +------------+               \                ).
   .(     +=======>|  Server(A) |                \                ).
 .(     //         +--------)---+                 \                 ).
 (     //                   )                      \                 )
 (    //      The IRON      )                       \                )
 (   //  .-.                )                        \     .-.       )
 (  //,-(  _)-.             )                         \ ,-(  _)-.    )
 ( .||_    (_  )-.          ) 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

Templin Experimental [Page 22] RFC 6179 IRON March 2011

 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-
 instruction imaginary operation within the gateway.)
 This scenario always involves a Server and Relay owned by the VPC
 that provides service to IRON EUN A. Therefore, it imparts a cost
 that would need to be borne by either the VPC or its customers.

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) |                \                ).
 .(     //         +--------)---+                 \                 ).
 (     //                   )                      \                 )
 (    //      The IRON      )                       \                )
 (   //  .-.                )                        \     .-.       )
 (  //,-(  _)-.             )                         \ ,-(  _)-.    )
 ( .||_    (_  )-.          ) 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.

Templin Experimental [Page 23] RFC 6179 IRON March 2011

 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
 (re-)encapsulates the packets in an outer header that uses the source
 address, destination address, and nonce 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 VPC
 that provides service to IRON EUN A. Therefore, it imparts a cost
 that would need to be borne by either the VPC or its customers.

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.
 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 VPC, 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 moved by

Templin Experimental [Page 24] RFC 6179 IRON March 2011

 providing it with the locator address of the new Server; again, via a
 VPC-specific reliable transaction.  The former Server will then
 garbage-collect the stale FIB entries when their lifetime expires.
 This will allow the former Server to redirect existing correspondents
 to the new Server so that no packets are lost.

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 VPC 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
 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 VPC, it can change between
 ISPs seamlessly and without need to renumber.  If the VPC itself
 applies unreasonable costing structures for use of the EPs, however,
 the customer may be compelled to seek a different VPC and would again
 be required to confront a renumbering scenario.  The IRON approach to
 renumbering avoidance therefore depends on VPCs conducting ethical
 business practices and offering reasonable rates.

Templin Experimental [Page 25] RFC 6179 IRON March 2011

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, namely UDP.
 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 packets.  When there is a NAT in the path, the UDP 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 UDP 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.

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.

Templin Experimental [Page 26] RFC 6179 IRON March 2011

 For example, in the network scenario depicted in Figure 10, 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.
                          .-.
               ISP(D)  ,-(  _)-.
    +-----------+   .-(_    (_  )-.
    | Client(C) |--(_    ISP(D)    )
    +-----+-----+     `-(______)-'
          |   <= T         \     .-.
         .-.       u        \ ,-(  _)-.
      ,-(  _)-.       n     .-(_    (-  )-.
   .-(_    (_  )-.      n  (_   Internet   )
  (_    EUN(C)    )       e   `-(______)-'
     `-(______)-'           l          ___
          | EPA(C)           s =>     (:::)-.
    +-----+-----+                 .-(::::::::)
    | Client(B) |              .-(::::::::::::)-.  +-----------+
    +-----+-----+             (:::: The IRON ::::) |  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 10: 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.

Templin Experimental [Page 27] RFC 6179 IRON March 2011

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
 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

Templin Experimental [Page 28] RFC 6179 IRON March 2011

 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)
 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 deployment of new routers/servers
 throughout the Internet and/or provider networks 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

Templin Experimental [Page 29] RFC 6179 IRON March 2011

 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.
 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]
 [RFC6139], 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].

Templin Experimental [Page 30] RFC 6179 IRON March 2011

 [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
 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] [RFC6139], 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 followed by tunnel-neighbor
 nonces 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).
 Each VPC overlay network 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.
 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.

Templin Experimental [Page 31] RFC 6179 IRON March 2011

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.
 [RFC2526]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
            Addresses", RFC 2526, March 1999.
 [RFC3068]  Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
            RFC 3068, June 2001.

Templin Experimental [Page 32] RFC 6179 IRON March 2011

 [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.

Templin Experimental [Page 33] RFC 6179 IRON March 2011

 [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
            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.

Templin Experimental [Page 34] RFC 6179 IRON March 2011

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

Templin Experimental [Page 35] RFC 6179 IRON March 2011

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 VPC 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 VPC 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 VPC 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 VPC overlay network.
 Sizing the Relay to accommodate state information for all EUNs is
 therefore required during VPC overlay network planning.  In terms of
 state requirements for Servers, each Server maintains tunnel-neighbor
 state for each of the customer EUNs it serves, but it need not keep

Templin Experimental [Page 36] RFC 6179 IRON March 2011

 state for all EUNs in the VPC 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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