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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: RFC5558 (if approved)                          April 19, 2013
Intended status: Informational
Expires: October 21, 2013


        Boeing's Virtual Enterprise Traversal (VET) Abstraction
                    draft-templin-intarea-vet-39.txt

Abstract

   Enterprise networks connect hosts and routers over various link
   types, and often also connect to the global Internet either directly
   or via a provider network.  Enterprise network nodes require a means
   to automatically provision addresses/prefixes and support
   internetworking operation in a wide variety of use cases including
   Small Office / Home Office (SOHO) networks, Mobile Ad hoc Networks
   (MANETs), ISP networks, multi-organizational corporate networks and
   the interdomain core of the global Internet itself.  This document
   specifies a Virtual Enterprise Traversal (VET) abstraction developed
   by Boeing for autoconfiguration and operation of nodes in dynamic
   enterprise networks.

Status of this Memo

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

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

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

   This Internet-Draft will expire on October 21, 2013.

Copyright Notice

   Copyright (c) 2013 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



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   (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.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Differences with RFC5558 . . . . . . . . . . . . . . . . . . .  6
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Enterprise Network Characteristics . . . . . . . . . . . . . . 12
   5.  Autoconfiguration  . . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  Enterprise Router (ER) Autoconfiguration . . . . . . . . . 14
     5.2.  VET Border Router (VBR) Autoconfiguration  . . . . . . . . 16
       5.2.1.  VET Interface Initialization . . . . . . . . . . . . . 16
       5.2.2.  Potential Router List (PRL) Discovery  . . . . . . . . 17
       5.2.3.  Provider-Aggregated (PA) EID Prefix
               Autoconfiguration  . . . . . . . . . . . . . . . . . . 18
       5.2.4.  Provider-Independent EID Prefix Autoconfiguration  . . 19
     5.3.  VET Border Gateway (VBG) Autoconfiguration . . . . . . . . 19
     5.4.  VET Host Autoconfiguration . . . . . . . . . . . . . . . . 20
   6.  Internetworking Operation  . . . . . . . . . . . . . . . . . . 21
     6.1.  Routing Protocol Participation . . . . . . . . . . . . . . 21
       6.1.1.  PI Prefix Routing Considerations . . . . . . . . . . . 22
       6.1.2.  Client Prefix (CP) Routing Considerations  . . . . . . 22
     6.2.  Default Route Configuration and Selection  . . . . . . . . 22
     6.3.  Address Selection  . . . . . . . . . . . . . . . . . . . . 22
     6.4.  Next Hop Determination . . . . . . . . . . . . . . . . . . 23
     6.5.  VET Interface Encapsulation/Decapsulation  . . . . . . . . 24
       6.5.1.  Inner Network Layer Protocol . . . . . . . . . . . . . 24
       6.5.2.  SEAL Encapsulation . . . . . . . . . . . . . . . . . . 25
       6.5.3.  UDP Encapsulation  . . . . . . . . . . . . . . . . . . 25
       6.5.4.  Outer IP Header Encapsulation  . . . . . . . . . . . . 26
       6.5.5.  Decapsulation and Re-Encapsulation . . . . . . . . . . 26
     6.6.  Neighbor Coordination on VET Interfaces that use SEAL  . . 27
       6.6.1.  Router Discovery . . . . . . . . . . . . . . . . . . . 28
       6.6.2.  Neighbor Unreachability Detection  . . . . . . . . . . 29
       6.6.3.  Redirection  . . . . . . . . . . . . . . . . . . . . . 29
       6.6.4.  Bidirectional Neighbor Synchronization . . . . . . . . 32
     6.7.  Neighbor Coordination on VET Interfaces using IPsec  . . . 33
     6.8.  Mobility and Multihoming Considerations  . . . . . . . . . 33
     6.9.  Multicast  . . . . . . . . . . . . . . . . . . . . . . . . 34
       6.9.1.  Multicast over Non-Multicast Enterprise Networks . . . 34
       6.9.2.  Multicast Over Multicast-Capable Enterprise



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               Networks . . . . . . . . . . . . . . . . . . . . . . . 34
     6.10. Service Discovery  . . . . . . . . . . . . . . . . . . . . 35
     6.11. VET Link Partitioning  . . . . . . . . . . . . . . . . . . 35
     6.12. VBG Prefix State Recovery  . . . . . . . . . . . . . . . . 36
     6.13. Legacy ISATAP Services . . . . . . . . . . . . . . . . . . 36
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 36
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 36
   9.  Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 37
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 37
   11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 38
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 38
     12.2. Informative References . . . . . . . . . . . . . . . . . . 40
   Appendix A.  Duplicate Address Detection (DAD) Considerations  . . 44
   Appendix B.  Anycast Services  . . . . . . . . . . . . . . . . . . 45
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 46



































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1.  Introduction

   Enterprise networks [RFC4852] connect hosts and routers over various
   link types (see [RFC4861], Section 2.2).  The term "enterprise
   network" in this context extends to a wide variety of use cases and
   deployment scenarios.  For example, an "enterprise" can be as small
   as a Small Office / Home Office (SOHO) network, as complex as a
   multi-organizational corporation, or as large as the global Internet
   itself.  Internet Service Provider (ISP) networks are another example
   use case that fits well with the VET enterprise network model.
   Mobile Ad hoc Networks (MANETs) [RFC2501] can also be considered as a
   challenging example of an enterprise network, in that their
   topologies may change dynamically over time and that they may employ
   little/no active management by a centralized network administrative
   authority.  These specialized characteristics for MANETs require
   careful consideration, but the same principles apply equally to other
   enterprise network scenarios.

   In many cases, enterprise networks must present a stable
   manifestation to the outside world (e.g., the Internet Default Free
   Zone) while their internal topologies may be changing dynamically.
   This is often the case when portions of the enterprise network are
   mobile, partitioned for security purposes, employ different IP
   protocol versions, etc. and is most often addressed through
   encapsulation (also known as tunneling).  This document therefore
   focuses on provisions for accommodating dynamic enterprise networks
   while presenting an outward appearance of stability and uniformity.

   This document specifies a Virtual Enterprise Traversal (VET)
   abstraction developed by Boeing for autoconfiguration and
   internetworking operation in dynamic enterprise networks, where
   addresses of different scopes may be assigned on various types of
   interfaces with diverse properties.  Both IPv4 [RFC0791][RFC0792] and
   IPv6 [RFC2460][RFC4443] are discussed within this context (other
   network layer protocols are also considered).  The use of standard
   DHCP [RFC2131] [RFC3315] is assumed unless otherwise specified.















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                             Provider-Edge Interfaces
                                  x   x        x
                                  |   |        |
             +--------------------+---+--------+----------+    E
             |                    |   |        |          |    n
             |    I               |   |  ....  |          |    t
             |    n           +---+---+--------+---+      |    e
             |    t           |   +--------+      /|      |    r
             |    e  I   x----+   |  Host  |   I /*+------+--< p  I
             |    r  n        |   |Function|   n|**|      |    r  n
             |    n  t        |   +--------+   t|**|      |    i  t
             |    a  e   x----+              V e|**+------+--< s  e
             |    l  r      . |              E r|**|  .   |    e  r
             |       f      . |              T f|**|  .   |       f
             |    V  a      . |   +--------+   a|**|  .   |    I  a
             |    i  c      . |   | Router |   c|**|  .   |    n  c
             |    r  e   x----+   |Function|   e \*+------+--< t  e
             |    t  s        |   +--------+      \|      |    e  s
             |    u           +---+---+--------+---+      |    r
             |    a               |   |  ....  |          |    i
             |    l               |   |        |          |    o
             +--------------------+---+--------+----------+    r
                                  |   |        |
                                  x   x        x
                           Enterprise-Edge Interfaces

               Figure 1: Enterprise Router (ER) Architecture

   Figure 1 above depicts the architectural model for an Enterprise
   Router (ER).  As shown in the figure, an ER may have a variety of
   interface types including enterprise-edge, enterprise-interior,
   provider-edge, internal-virtual, as well as VET interfaces used for
   encapsulating inner network layer protocol packets for transmission
   over an underlying IPv4 or IPv6 network.  The different types of
   interfaces are defined, and the autoconfiguration mechanisms used for
   each type are specified.  This architecture applies equally for MANET
   routers, in which enterprise-interior interfaces typically correspond
   to the wireless multihop radio interfaces associated with MANETs.
   Out of scope for this document is the autoconfiguration of provider
   interfaces, which must be coordinated in a manner specific to the
   service provider's network.

   The VET framework builds on a Non-Broadcast Multiple Access (NBMA)
   [RFC2491] virtual interface model in a manner similar to other
   automatic tunneling technologies [RFC2529][RFC5214].  VET interfaces
   support the encapsulation of inner network layer protocol packets
   over IP networks (i.e., either IPv4 or IPv6), and provide an NBMA
   interface abstraction for coordination between tunnel endpoint



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   "neighbors".

   VET and its associated technologies (including the Subnetwork
   Encapsulation and Adaptation Layer (SEAL) [I-D.templin-intarea-seal]
   and Asymmetric Extended Route Optimization (AERO) [RFC6706]) are
   functional building blocks for related architectures known as the
   Interior Routing Overlay Network (IRON) [I-D.templin-ironbis] and
   Routing and Addressing in Networks with Global Enterprise Recursion
   (RANGER) [RFC5720][RFC6139].  Many of the VET principles can be
   traced to the deliberations of the ROAD group in January 1992, and
   also to still earlier initiatives including the Catenet model for
   internetworking [CATENET] [IEN48] [RFC2775] and NIMROD [RFC1753].
   The high-level architectural aspects of the ROAD group deliberations
   are captured in a "New Scheme for Internet Routing and Addressing
   (ENCAPS) for IPNG" [RFC1955].

   VET is related to the present-day activities of the IETF INTAREA,
   AUTOCONF, DHC, IPv6, MANET, RENUM and V6OPS working groups, as well
   as the IRTF RRG working group.


2.  Differences with RFC5558

   This document is based on [RFC5558] but makes significant changes
   over that earlier work.  The most important difference is that this
   document breaks the linkage between VET and earlier NBMA tunneling
   mechanisms such as 6over4 and ISATAP.  The document therefore no
   longer has backwards-compatible dependencies on these technologies.

   The terminology section has seen some new terms added and some
   existing terms renamed and/or clarified.  Important new terms
   including "Client Prefix (CP)" and "VET link" have been added, while
   other terms including VET Border Router and VET Border Gateway have
   been renamed for greater clarity.  RFC2119 terminology has also been
   added.

   "Enterprise Network Characteristics" now also considers cases in
   which an enterprise network may contain many internal partitions,
   which is an area that was left underspecified in RFC5558.  These
   partitions may be necessary for such uses as load balancing,
   organizational separation, etc.  The section now also discusses both
   unidirectional and bidirectional neighbor relationships.

   The "Enterprise Router (ER) Autoconfiguration" section now provides a
   discussion on DHCP relaying considerations, including replay
   detection.  These considerations are important for instances in which
   DHCP relaying may be excessive (e.g., Mobile Ad-Hoc Networks
   (MANETs)).



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   The "VET Border Router Autoconfiguration" section now draws a
   distinction between what is meant by "VET link" and "VET interface",
   and explains the cases in which link local addresses can and cannot
   be used.  Provider Aggregated (PA) prefix autoconfiguration now also
   discusses both stateful and stateless autoconfiguration.  The
   subsection on "ISP-Independent EID Prefix Autoconfiguration" now also
   introduces the capability of registering Client Prefixes (CPs) with
   Virtual Service Providers (VSPs).

   The "VET Border Gateway (VBG) Autoconfiguration" section now explains
   the manner in which VBGs can act as "half gateways" in the IRON
   Client/Server/Relay architecture.  The "VET Host Autoconfiguration"
   section now explains cases in which prefixes may be provided to
   hosts, i.e., if there is assurance that the link will not partition.

   Under "Internetworking Operation", "Routing Protocol Participation"
   now discusses the case of receiving on-demand redirection messages as
   a form of routing.  The section further discusses PI prefix and CP
   prefix routing considerations.  "Default Route Configuration",
   "Address Selection" and "Next-Hop Determination" are newly rewritten
   sections that completely replace significant portions of this major
   section.  "VET Interface Encapsulation/Decapsulation" now gives
   important details on encapsulation procedures and header formats that
   were not present in RFC5558.  The new section on "Neighbor
   Coordination" (including discussions of unidirectional and
   bidirectional neighbor relationships as well as redirection) is also
   key to understanding the new operational model.  The remaining
   sections of "Internetworking Operation" have received minor and/or
   substantial rewrites with most of the specification intact from
   RFC5558.  The document finally adds a new appendix on Anycast
   Services.


3.  Terminology

   The mechanisms within this document build upon the fundamental
   principles of IP encapsulation.  The term "inner" refers to the
   innermost {address, protocol, header, packet, etc.} *before*
   encapsulation, and the term "outer" refers to the outermost {address,
   protocol, header, packet, etc.} *after* encapsulation.  VET also
   accommodates "mid-layer" encapsulations such as SEAL
   [I-D.templin-intarea-seal] and IPsec [RFC4301].

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






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   Virtual Enterprise Traversal (VET)
      an abstraction that uses encapsulation to create virtual overlays
      for transporting inner network layer packets over outer IPv4 and
      IPv6 enterprise networks.

   enterprise network
      the same as defined in [RFC4852].  An enterprise network is
      further understood to refer to a cooperative networked collective
      of devices within a structured IP routing and addressing plan and
      with a commonality of business, social, political, etc.,
      interests.  Minimally, the only commonality of interest in some
      enterprise network scenarios may be the cooperative provisioning
      of connectivity itself.

   subnetwork
      the same as defined in [RFC3819].

   site
      a logical and/or physical grouping of interfaces that connect a
      topological area less than or equal to an enterprise network in
      scope.  From a network organizational standpoint, a site within an
      enterprise network can be considered as an enterprise network unto
      itself.

   Mobile Ad hoc Network (MANET)
      a connected topology of mobile or fixed routers that maintain a
      routing structure among themselves over links that often have
      dynamic connectivity properties.  The characteristics of MANETs
      are described in [RFC2501], Section 3, and a wide variety of
      MANETs share common properties with enterprise networks.

   enterprise/site/MANET
      throughout the remainder of this document, the term "enterprise
      network" is used to collectively refer to any of {enterprise,
      site, MANET}, i.e., the VET mechanisms and operational principles
      can be applied to enterprises, sites, and MANETs of any size or
      shape.

   VET link
      a virtual link that uses automatic tunneling to create an overlay
      network that spans an enterprise network routing region.  VET
      links can be segmented (e.g., by filtering gateways) into multiple
      distinct segments that can be joined together by bridges or IP
      routers the same as for any link.  Bridging would view the
      multiple (bridged) segments as a single VET link, whereas IP
      routing would view the multiple segments as multiple distinct VET
      links.  VET links can further be partitioned into multiple logical
      areas, where each area is identified by a distinct set of border



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      nodes.

      VET links configured over non-multicast enterprise networks
      support only Non-Broadcast, Multiple Access (NBMA) services; VET
      links configured over multicast-capable enterprise networks can
      support both unicast and native multicast services.  All nodes
      connected to the same VET link appear as neighbors from the
      standpoint of the inner network layer.

   Enterprise Router (ER)
      As depicted in Figure 1, an Enterprise Router (ER) is a fixed or
      mobile router that comprises a router function, a host function,
      one or more enterprise-interior interfaces, and zero or more
      internal virtual, enterprise-edge, provider-edge, and VET
      interfaces.  At a minimum, an ER forwards outer IP packets over
      one or more sets of enterprise-interior interfaces, where each set
      connects to a distinct enterprise network.

   VET Border Router (VBR)
      an ER that connects end user networks (EUNs) to VET links and/or
      connects multiple VET links together.  A VBR is a tunnel endpoint
      router, and it configures a separate VET interface for each
      distinct VET link.  All VBRs are also ERs.

   VET Border Gateway (VBG)
      a VBR that connects VET links to provider networks.  A VBG may in
      some circumstances act as a "half-gateway", and forward the
      packets it receives from neighbors on the VET link to another VBG
      on the same VET link.  All VBGs are also VBRs.

   VET host  any node (host or router) that configures a VET interface
      for host-operation only.  Note that a node may configure some of
      its VET interfaces as host interfaces and others as router
      interfaces.

   VET node
      any node (host or router) that configures and uses a VET
      interface.

   enterprise-interior interface
      an ER's attachment to a link within an enterprise network.
      Packets sent over enterprise-interior interfaces may be forwarded
      over multiple additional enterprise-interior interfaces before
      they reach either their final destination or a border router/
      gateway.  Enterprise-interior interfaces connect laterally within
      the IP network hierarchy.





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   enterprise-edge interface
      a VBR's attachment to a link (e.g., an Ethernet, a wireless
      personal area network, etc.) on an arbitrarily complex EUN that
      the VBR connects to a VET link and/or a provider network.
      Enterprise-edge interfaces connect to lower levels within the IP
      network hierarchy.

   provider-edge interface
      a VBR's attachment to the Internet or to a provider network via
      which the Internet can be reached.  Provider-edge interfaces
      connect to higher levels within the IP network hierarchy.

   internal-virtual interface
      an interface that is internal to a VET node and does not in itself
      directly attach to a tangible link, e.g., a loopback interface, a
      tunnel virtual interface, etc.

   VET interface
      a VET node's attachment to a VET link.  VET nodes configure each
      VET interface over a set of underlying enterprise-interior
      interfaces that connect to a routing region spanned by a single
      VET link.  When there are multiple distinct VET links (each with
      their own distinct set of underlying interfaces), the VET node
      configures a separate VET interface for each link.

      The VET interface encapsulates each inner packet in any mid-layer
      headers followed by an outer IP header, then forwards the packet
      on an underlying interface such that the Time to Live (TTL) - Hop
      Limit in the inner header is not decremented as the packet
      traverses the link.  The VET interface therefore presents an
      automatic tunneling abstraction that represents the VET link as a
      single hop to the inner network layer.

   Provider Aggregated (PA) prefix
      a network layer protocol prefix that is delegated to an enterprise
      by a provider network.

   Provider Independent (PI) prefix
      a network layer protocol prefix that is delegated to an enterprise
      by an independent registration authority.  The enterprise then
      becomes solely responsible for representing the PI prefix into the
      global Internet routing system on its own behalf.

   Client Prefix (CP)
      a network layer protocol prefix that is delegated to a VET node by
      a Virtual Service Provider (VSP) that may operate independently of
      the node's provider networks.  The term "Client Prefix (CP)" is
      the same as used in IRON [I-D.templin-ironbis].



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   Routing Locator (RLOC)
      a public-scope or enterprise-local-scope IP address.  Public-scope
      RLOCs are delegated to specific enterprise networks and routable
      within both the enterprise-interior and interdomain routing
      regions.  Enterprise-local-scope RLOCs (e.g., IPv6 Unique Local
      Addresses [RFC4193], IPv4 privacy addresses [RFC1918], etc.) are
      self-generated by individual enterprise networks and routable only
      within the enterprise-interior routing region.

      ERs use RLOCs for operating the enterprise-interior routing
      protocol and for next-hop determination in forwarding packets
      addressed to other RLOCs.  End systems can use RLOCs as addresses
      for end-to-end communications between peers within the same
      enterprise network.  VET interfaces treat RLOCs as *outer* IP
      addresses during encapsulation.

   Endpoint Interface iDentifier (EID)
      a public-scope network layer address that is routable within
      enterprise-edge and/or VET overlay networks.  In a pure mapping
      system, EID prefixes are not routable within the interdomain
      routing system.  In a hybrid routing/mapping system, EID prefixes
      may be represented within the same interdomain routing instances
      that distribute RLOC prefixes.  In either case, EID prefixes are
      separate and distinct from any RLOC prefix space, but they are
      mapped to RLOC addresses to support packet forwarding over VET
      interfaces.

      VBRs participate in any EID-based routing instances and use EID
      addresses for next-hop determination.  End systems can use EIDs as
      addresses for end-to-end communications between peers either
      within the same enterprise network or within different enterprise
      networks.  VET interfaces treat EIDs as *inner* network layer
      addresses during encapsulation.

      Note that an EID can also be used as an *outer* network layer
      address if there are nested encapsulations.  In that case, the EID
      would appear as an RLOC to the innermost encapsulation.

   The following additional acronyms are used throughout the document:

   CGA - Cryptographically Generated Address
   DHCP(v4, v6) - Dynamic Host Configuration Protocol
   ECMP - Equal Cost Multi Path
   ESK - Encrypted Secret Key
   EUN - End User Network
   FIB - Forwarding Information Base
   ICMP - either ICMPv4 or ICMPv6
   ICV - Integrity Check Vector



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   IP - either IPv4 or IPv6
   ISATAP - Intra-Site Automatic Tunnel Addressing Protocol
   MAC - Message Authentication Code
   NBMA - Non-Broadcast, Multiple Access
   ND - Neighbor Discovery
   PIO - Prefix Information Option
   PRL - Potential Router List
   PRLNAME - Identifying name for the PRL
   RIB - Routing Information Base
   RIO - Route Information Option
   SCMP - SEAL Control Message Protocol
   SEAL - Subnetwork Encapsulation and Adaptation Layer
   SLAAC - IPv6 StateLess Address AutoConfiguration
   SNS/SNA - SCMP Neighbor Solicitation/Advertisement
   SPD - SCMP Predirect
   SRD - SCMP Redirect
   SRS/SRA - SCMP Router Solicitation/Advertisement

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].  When used
   in lower case (e.g., must, must not, etc.), these words MUST NOT be
   interpreted as described in [RFC2119], but are rather interpreted as
   they would be in common English.


4.  Enterprise Network Characteristics

   Enterprise networks consist of links that are connected by Enterprise
   Routers (ERs) as depicted in Figure 1.  ERs typically participate in
   a routing protocol over enterprise-interior interfaces to discover
   routes that may include multiple Layer 2 or Layer 3 forwarding hops.
   VET Border Routers (VBRs) are ERs that connect End User Networks
   (EUNs) to VET links that span enterprise networks.  VET Border
   Gateways (VBGs) are VBRs that connect VET links to provider networks.

   Conceptually, an ER embodies both a host function and router
   function, and supports communications according to the weak end-
   system model [RFC1122].  The router function engages in the
   enterprise-interior routing protocol on its enterprise-interior
   interfaces, connects any of the ER's EUNs to its VET links, and may
   also connect the VET links to provider networks (see Figure 1).  The
   host function typically supports network management applications, but
   may also support diverse applications typically associated with
   general-purpose computing platforms.

   An enterprise network may be as simple as a small collection of ERs
   and their attached EUNs; an enterprise network may also contain other



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   enterprise networks and/or be a subnetwork of a larger enterprise
   network.  An enterprise network may further encompass a set of branch
   offices and/or nomadic hosts connected to a home office over one or
   several service providers, e.g., through Virtual Private Network
   (VPN) tunnels.  Finally, an enterprise network may contain many
   internal partitions that are logical or physical groupings of nodes
   for the purpose of load balancing, organizational separation, etc.
   In that case, each internal partition resembles an individual segment
   of a bridged LAN.

   Enterprise networks that comprise link types with sufficiently
   similar properties (e.g., Layer 2 (L2) address formats, maximum
   transmission units (MTUs), etc.) can configure a subnetwork routing
   service such that the network layer sees the underlying network as an
   ordinary shared link the same as for a (bridged) campus LAN (this is
   often the case with large cellular operator networks).  In that case,
   a single network layer hop is sufficient to traverse the underlying
   network.  Enterprise networks that comprise link types with diverse
   properties and/or configure multiple IP subnets must also provide an
   enterprise-interior routing service that operates as an IP layer
   mechanism.  In that case, multiple network layer hops may be
   necessary to traverse the underlying network.

   In addition to other interface types, VET nodes configure VET
   interfaces that view all other nodes on the VET link as neighbors on
   a virtual NBMA link.  VET nodes configure a separate VET interface
   for each distinct VET link to which they connect, and discover
   neighbors on the link that can be used for forwarding packets to off-
   link destinations.  VET interface neighbor relationships may be
   either unidirectional or bidirectional.

   A unidirectional neighbor relationship is typically established and
   maintained as a result of network layer control protocol messaging in
   a manner that parallels IPv6 neighbor discovery [RFC4861].  A
   bidirectional neighbor relationship is typically established and
   maintained as result of a short transaction between the neighbors
   (see Section 6.6.4).

   For each distinct VET link, a trust basis must be established and
   consistently applied.  For example, for VET links configured over
   enterprise networks in which VBRs establish symmetric security
   associations, mechanisms such as IPsec [RFC4301] can be used to
   assure authentication and confidentiality.  In other enterprise
   network scenarios, VET links may require asymmetric securing
   mechanisms such as SEcure Neighbor Discovery (SEND) [RFC3971].  VET
   links configured over still other enterprise networks may find it
   sufficient to employ only the services provided by SEAL
   [I-D.templin-intarea-seal] (including anti-replay, packet header



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   integrity, and message origin authentication) and defer strong
   security services to higher layer functions.

   Finally, for VET links configured over enterprise networks with a
   centralized management structure (e.g., a corporate campus network,
   an ISP network, etc.), a hybrid routing/mapping service can be
   deployed using a synchronized set of VBGs.  In that case, the VBGs
   can provide a mapping service (similar to the "default mapper"
   described in [I-D.jen-apt]) used for short-term packet forwarding
   until route-optimized paths can be established.  For VET links
   configured over enterprise networks with a distributed management
   structure (e.g., disconnected MANETs), interdomain coordination
   between the VET nodes themselves without the assistance of VBGs may
   be required.  Recognizing that various use cases may entail a
   continuum between a fully centralized and fully distributed approach,
   the following sections present the mechanisms of Virtual Enterprise
   Traversal as they apply to a wide variety of scenarios.


5.  Autoconfiguration

   ERs, VBRs, VBGs, and VET hosts configure themselves for operation as
   specified in the following subsections.

5.1.  Enterprise Router (ER) Autoconfiguration

   ERs configure enterprise-interior interfaces and engage in any
   routing protocols over those interfaces.

   When an ER joins an enterprise network, it first configures an IPv6
   link-local address on each enterprise-interior interface that
   requires an IPv6 link-local capability and configures an IPv4 link-
   local address on each enterprise-interior interface that requires an
   IPv4 link-local capability.  IPv6 link-local address generation
   mechanisms include Cryptographically Generated Addresses (CGAs)
   [RFC3972], IPv6 Privacy Addresses [RFC4941], StateLess Address
   AutoConfiguration (SLAAC) using EUI-64 interface identifiers
   [RFC4291] [RFC4862], etc.  The mechanisms specified in [RFC3927]
   provide an IPv4 link-local address generation capability.

   Next, the ER configures one or more RLOCs and engages in any routing
   protocols on its enterprise-interior interfaces.  The ER can
   configure RLOCs via administrative configuration, pseudo-random self-
   generation from a suitably large address pool, SLAAC, DHCP
   autoconfiguration, or through an alternate autoconfiguration
   mechanism.

   Pseudo-random self-generation of IPv6 RLOCs can be from a large



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   public or local-use IPv6 address range (e.g., IPv6 Unique Local
   Addresses [RFC4193]).  Pseudo-random self-generation of IPv4 RLOCs
   can be from a large public or local-use IPv4 address range (e.g.,
   [RFC1918]).  When self-generation is used alone, the ER continuously
   monitors the RLOCs for uniqueness, e.g., by monitoring the
   enterprise-interior routing protocol.  (Note however that anycast
   RLOCs may be assigned to multiple enterprise-interior interfaces;
   hence, monitoring for uniqueness applies only to RLOCs that are
   provisioned as unicast.)

   SLAAC autoconfiguration of RLOCs can be through the receipt of IPv6
   Router Advertisements (RAs) followed by the stateless configuration
   of addresses based on any included Prefix Information Options (PIOs)
   [RFC4861][RFC4862].

   DHCP autoconfiguration of RLOCs uses standard DHCP procedures,
   however ERs acting as DHCP clients SHOULD also use DHCP
   Authentication [RFC3118] [RFC3315].  In typical enterprise network
   scenarios (i.e., those with stable links), it may be sufficient to
   configure one or a few DHCP relays on each link that does not include
   a DHCP server.  In more extreme scenarios (e.g., MANETs that include
   links with dynamic connectivity properties), DHCP operation may
   require any ERs that have already configured RLOCs to act as DHCP
   relays to ensure that client DHCP requests eventually reach a DHCP
   server.  This may result in considerable DHCP message relaying until
   a server is located, but the DHCP Authentication Replay Detection
   option [RFC4030] provides relays with a means for avoiding message
   duplication.

   In all enterprise network scenarios, the amount of DHCP relaying
   required can be significantly reduced if each relay has a way of
   contacting a DHCP server directly.  In particular, if the relay can
   discover the unicast addresses for one or more servers (e.g., by
   discovering the unicast RLOC addresses of VBGs as described in
   Section 5.2.2) it can forward DHCP requests directly to the unicast
   address(es) of the server(s).  If the relay does not know the unicast
   address of a server, it can forward DHCP requests to a site-scoped
   DHCP server multicast address if the enterprise network supports
   site-scoped multicast services.  For DHCPv6, relays can forward
   requests to the site-scoped IPv6 multicast group address
   'All_DHCP_Servers' [RFC3315].  For DHCPv4, relays can forward
   requests to the site-scoped IPv4 multicast group address
   'All_DHCPv4_Servers', which SHOULD be set to a well-known site-scoped
   IPv4 multicast group address for the enterprise network.  DHCPv4
   servers that delegate RLOCs SHOULD therefore join the
   'All_DHCPv4_Servers' multicast group and service any DHCPv4 messages
   received for that group.




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   A combined approach using both DHCP and self-generation is also
   possible when the ER configures both a DHCP client and relay that are
   connected, e.g., via a pair of back-to-back connected Ethernet
   interfaces, a tun/tap interface, a loopback interface, inter-process
   communication, etc.  The ER first self-generates an RLOC taken from a
   temporary addressing range used only for the bootstrapping purpose of
   procuring an actual RLOC taken from a delegated addressing range.
   The ER then engages in the enterprise-interior routing protocol and
   performs a DHCP exchange as above using the temporary RLOC as the
   address of its relay function.  When the DHCP server delegates an
   actual RLOC address/prefix, the ER abandons the temporary RLOC and
   re-engages in the enterprise-interior routing protocol using an RLOC
   taken from the delegation.

   Alternatively (or in addition to the above), the ER can request RLOC
   prefix delegations via an automated prefix delegation exchange over
   an enterprise-interior interface and can assign the prefix(es) on
   enterprise-edge interfaces.  Note that in some cases, the same
   enterprise-edge interfaces may assign both RLOC and EID addresses if
   there is a means for source address selection.  In other cases (e.g.,
   for separation of security domains), RLOCs and EIDs are assigned on
   separate sets of enterprise-edge interfaces.

   In some enterprise network scenarios (e.g., MANETs that include links
   with dynamic connectivity properties), assignment of RLOCs on
   enterprise-interior interfaces as singleton addresses (i.e., as
   addresses with /32 prefix lengths for IPv4, or as addresses with /128
   prefix lengths for IPv6) MAY be necessary to avoid multi-link subnet
   issues [RFC4903].

5.2.  VET Border Router (VBR) Autoconfiguration

   VBRs are ERs that configure and use one or more VET interfaces.  In
   addition to the ER autoconfiguration procedures specified in
   Section 5.1, VBRs perform the following autoconfiguration operations.

5.2.1.  VET Interface Initialization

   VBRs configure a separate VET interface for each VET link, where each
   VET link spans a distinct sets of underlying links belonging to the
   same enterprise network.  All nodes on the VET link appear as single-
   hop neighbors from the standpoint of the inner network layer protocol
   through the use of encapsulation.

   The VBR binds each VET interface to one or more underlying
   interfaces, and uses the underlying interface addresses as RLOCs to
   serve as the outer source addresses for encapsulated packets.  The
   VBR then assigns a link-local address to each VET interface if



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   possible (*).  When IPv6 and IPv4 are used as the inner/outer
   protocols (respectively), the VBR can autoconfigure an IPv6 link-
   local address on the VET interface using a modified EUI-64 interface
   identifier based on an IPv4 RLOC address (see Section 2.2.1 of
   [RFC5342]).  Link-local address configuration for other inner/outer
   protocol combinations is through administrative configuration, random
   self-generation (e.g., [RFC4941], etc.) or through an unspecified
   alternate method.

   (*) In some applications, assignment of link-local addresses on a VET
   interface may be impractical due to an indefinite mapping of the
   inner link-local address to an outer RLOC address.  For example, if
   there are VET link neighbors located behind Network Address
   Translators (NATs) any inner link-local address to outer RLOC address
   mapping may be subject to change due to changes in NAT state.  In
   that case, inner network layer protocol services such as the IPv6
   Neighbor Discovery (ND) protocol [RFC4861] that depend on link-local
   addressing may not be able to function in the normal manner over the
   VET link.

5.2.2.  Potential Router List (PRL) Discovery

   After initializing the VET interface, the VBR next discovers a
   Potential Router List (PRL) for the VET link that includes the RLOC
   addresses of VBGs.  The VBR discovers the PRL through administrative
   configuration, as part of an arrangement with a Virtual Service
   Provider (VSP) (see: Section 5.2.4), through information conveyed in
   the enterprise-interior routing protocol, via a multicast beacon, via
   an anycast VBG discovery message exchange, or through some other
   means specific to the enterprise network.

   If no such enterprise-specific information is available, the VBR can
   instead resolve an identifying name for the PRL ('PRLNAME') formed as
   'hostname.domainname', where 'hostname' is an enterprise-specific
   name string and 'domainname' is an enterprise-specific Domain Name
   System (DNS) suffix [RFC1035].  The VBR can discover 'domainname'
   through the DHCP Domain Name option [RFC2132], administrative
   configuration, etc.  The VBR can discover 'hostname' via link-layer
   information (e.g., an IEEE 802.11 Service Set Identifier (SSID)),
   administrative configuration, etc.

   In the absence of other information, the VBR sets 'hostname' to
   "linkupnetworks" and sets 'domainname' to an enterprise-specific DNS
   suffix, e.g., "example.com".  (VBRs that connect directly to the
   Internet set hostname/domainname to "linkupnetworks.net".)  Isolated
   enterprise networks that do not connect to the outside world may have
   no enterprise-specific DNS suffix, in which case the 'PRLNAME'
   consists only of the 'hostname' component.



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   After discovering 'PRLNAME', the VBR resolves the name into a list of
   RLOC addresses through a name service lookup.  For centrally managed
   enterprise networks, the VBR resolves 'PRLNAME' using an enterprise-
   local name service (e.g., the DNS).  For enterprises with no
   centralized management structure, the VBR resolves 'PRLNAME' using a
   distributed name service query such as Link-Local Multicast Name
   Resolution (LLMNR) [RFC4795] over the VET interface.  In that case,
   all VBGs in the PRL respond to the query, and the VBR accepts the
   union of all responses.

5.2.3.  Provider-Aggregated (PA) EID Prefix Autoconfiguration

   VBRs that connect their enterprise networks to a provider network can
   obtain Provider-Aggregated (PA) EID prefixes.  For IPv4, VBRs acquire
   IPv4 PA EID prefixes through administrative configuration, an
   automated IPv4 prefix delegation exchange, etc.

   For IPv6, VBRs acquire IPv6 PA EID prefixes through administrative
   configuration or through DHCPv6 Prefix Delegation exchanges with a
   VBG acting as a DHCP relay/server.  In particular, the VBR (acting as
   a requesting router) can use DHCPv6 prefix delegation [RFC3633] over
   the VET interface to obtain prefixes from the VBG (acting as a
   delegating router).  The VBR obtains prefixes using either a
   2-message or 4-message DHCPv6 exchange [RFC3315].  When the VBR acts
   as a DHCPv6 client, it maps the IPv6
   "All_DHCP_Relay_Agents_and_Servers" link-scoped multicast address to
   the VBG's outer RLOC address.

   To perform the 2-message exchange, the VBR's DHCPv6 client function
   can send a Solicit message with an IA_PD option either directly or
   via the VBR's own DHCPv6 relay function (see Section 5.1).  The VBR's
   VET interface then forwards the message using VET encapsulation (see
   Section 6.4) to a VBG which either services the request or relays it
   further.  The forwarded Solicit message will elicit a Reply message
   from the server containing prefix delegations.  The VBR can also
   propose a specific prefix to the DHCPv6 server per Section 7 of
   [RFC3633].  The server will check the proposed prefix for consistency
   and uniqueness, then return it in the Reply message if it was able to
   perform the delegation.

   After the VBR receives IPv4 and/or IPv6 prefix delegations, it can
   provision the prefixes on enterprise-edge interfaces as well as on
   other VET interfaces configured over child enterprise networks for
   which it acts as a VBG.  The VBR can also provision the prefixes on
   enterprise-interior interfaces to service directly-attached hosts on
   the enterprise-interior link.

   The prefix delegations remain active as long as the VBR continues to



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   renew them via the delegating VBG before lease lifetimes expire.  The
   lease lifetime also keeps the delegation state active even if
   communications between the VBR and delegating VBG are disrupted for a
   period of time (e.g., due to an enterprise network partition, power
   failure, etc.).  Note however that if the VBR abandons or otherwise
   loses continuity with the prefixes, it may be obliged to perform
   network-wide renumbering if it subsequently receives a new and
   different set of prefixes.

   Prefix delegation for non-IP protocols is out of scope.

5.2.4.  Provider-Independent EID Prefix Autoconfiguration

   VBRs can acquire Provider-Independent (PI) prefixes to facilitate
   multihoming, mobility and traffic engineering without requiring site-
   wide renumbering events due to a change in ISP connections.

   VBRs that connect major enterprise networks (e.g., large
   corporations, academic campuses, ISP networks, etc.) to the global
   Internet can acquire short PI prefixes (e.g., an IPv6 /32, an IPv4
   /16, etc.) through a registration authority such as the Internet
   Assigned Numbers Authority (IANA) or a major regional Internet
   registry.  The VBR then advertises the PI prefixes into the global
   Internet on the behalf of its enterprise network without the
   assistance of an ISP.

   VBRs that connect enterprise networks to a provider network can
   acquire longer Client Prefixes (CPs) (e.g., an IPv6 /56, an IPv4 /24,
   etc.) through arrangements with a Virtual Service Provider (VSP) that
   may or may not be associated with a specific ISP.  The VBR then
   coordinates its CPs with a VSP independently of any of its directly
   attached ISPs.  (In many cases, the "VSP" may in fact be a major
   enterprise network that delegates CPs from its PI prefixes.)

   After a VBR receives prefix delegations, it can sub-delegate portions
   of the prefixes on enterprise-edge interfaces, on child VET
   interfaces for which it is configured as a VBG and on enterprise-
   interior interfaces to service directly-attached hosts on the
   enterprise-interior link.  The VBR can also sub-delegate portions of
   its prefixes to requesting routers connected to child enterprise
   networks.  These requesting routers consider their sub-delegated
   prefixes as PA, and consider the delegating routers as their points
   of connection to a provider network.

5.3.  VET Border Gateway (VBG) Autoconfiguration

   VBGs are VBRs that connect VET links configured over child enterprise
   networks to provider networks via provider-edge interfaces and/or via



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   VET links configured over parent enterprise networks.  A VBG may also
   act as a "half-gateway", in that it may need to forward the packets
   it receives from neighbors on the VET link via another VBG associated
   with the same VET link.  This model is seen in the IRON
   [I-D.templin-ironbis] Client/Server/Relay architecture, in which a
   Server "half-gateway" is a VBG that forwards packets with enterprise-
   external destinations via a Relay "half-gateway" that connects the
   VET link to the provider network.

   VBGs autoconfigure their provider-edge interfaces in a manner that is
   specific to the provider connections, and they autoconfigure their
   VET interfaces that were configured over parent VET links using the
   VBR autoconfiguration procedures specified in Section 5.2.  For each
   of its VET interfaces connected to child VET links, the VBG
   initializes the interface the same as for an ordinary VBR (see
   Section 5.2.1).  It then arranges to add one or more of its RLOCs
   associated with the child VET link to the PRL.

   VBGs configure a DHCP relay/server on VET interfaces connected to
   child VET links that require DHCP services.  VBGs may also engage in
   an unspecified anycast VBG discovery message exchange if they are
   configured to do so.  Finally, VBGs respond to distributed name
   service queries for 'PRLNAME' on VET interfaces connected to VET
   links that span child enterprise networks with a distributed
   management structure.

5.4.  VET Host Autoconfiguration

   Nodes that cannot be attached via a VBR's enterprise-edge interface
   (e.g., nomadic laptops that connect to a home office via a Virtual
   Private Network (VPN)) can instead be configured for operation as a
   simple host on the VET link.  Each VET host performs the same
   enterprise interior interface RLOC configuration procedures as
   specified for ERs in Section 5.1.  The VET host next performs the
   same VET interface initialization and PRL discovery procedures as
   specified for VBRs in Section 5.2, except that it configures its VET
   interfaces as host interfaces (and not router interfaces).  Note also
   that a node may be configured as a host on some VET interfaces and as
   a VBR/VBG on other VET interfaces.

   A VET host may receive non-link-local addresses and/or prefixes to
   assign to the VET interface via administrative configuration, DHCP
   exchanges and/or through SLAAC information conveyed in RAs.  If
   prefixes are provided, however, there must be assurance that either
   1) the VET link will not partition, or 2) that each VET host
   interface connected to the VET link will configure a unique set of
   prefixes.  VET hosts therefore depend on DHCP and/or RA exchanges to
   provide only addresses/prefixes that are appropriate for assignment



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   to the VET interface according to these specific cases, and depend on
   the VBGs within the enterprise keeping track of which addresses/
   prefixes were assigned to which hosts.

   When the VET host solicits a DHCP-assigned EID address/prefix over a
   (non-multicast) VET interface, it maps the DHCP relay/server
   multicast inner destination address to the outer RLOC address of a
   VBG that it has selected as a default router.  The VET host then
   assigns any resulting DHCP-delegated addresses/prefixes to the VET
   interface for use as the source address of inner packets.  The host
   will subsequently send all packets destined to EID correspondents via
   a default router on the VET link, and may discover more-specific
   routes based on any redirection messages it receives.


6.  Internetworking Operation

   Following the autoconfiguration procedures specified in Section 5,
   ERs, VBRs, VBGs, and VET hosts engage in normal internetworking
   operations as discussed in the following sections.

6.1.  Routing Protocol Participation

   ERs engage in any RLOC-based routing protocols over enterprise-
   interior interfaces to exchange routing information for forwarding IP
   packets with RLOC addresses.  VBRs and VBGs can additionally engage
   in any EID-based routing protocols over VET, enterprise-edge and
   provider-edge interfaces to exchange routing information for
   forwarding inner network layer packets with EID addresses.  Note that
   any EID-based routing instances are separate and distinct from any
   RLOC-based routing instances.

   VBR/VBG routing protocol participation on non-multicast VET
   interfaces uses the NBMA interface model, e.g., in the same manner as
   for OSPF over NBMA interfaces [RFC5340].  (VBR/VBG routing protocol
   participation on multicast-capable VET interfaces can alternatively
   use the standard multicast interface model, but this may result in
   excessive multicast control message overhead.)

   VBRs can use the list of VBGs in the PRL (see Section 5.2.1) as an
   initial list of neighbors for EID-based routing protocol
   participation.  VBRs can alternatively use the list of VBGs as
   potential default routers instead of engaging in an EID-based routing
   protocol instance.  In that case, when the VBR forwards a packet via
   a VBG it may receive a redirection message indicating a different VET
   node as a better next hop.





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6.1.1.  PI Prefix Routing Considerations

   VBRs that connect large enterprise networks to the global Internet
   advertise their EID PI prefixes directly into the Internet default-
   free RIB via the Border Gateway Protocol (BGP) [RFC4271] on their own
   behalf the same as for a major service provider network.  VBRs that
   connect large enterprise networks to provider networks can instead
   advertise their EID PI prefixes into their providers' routing
   system(s) if the provider networks are configured to accept them.

6.1.2.  Client Prefix (CP) Routing Considerations

   VBRs that obtain CPs from a VSP can register them with a serving VBG
   in the VSP's network (e.g., through a vendor-specific short TCP
   transaction).  The VSP network then acts as a virtual "home"
   enterprise network that connects its customer enterprise networks to
   the Internet routing system.  The customer enterprise networks in
   turn appear as mobile components of the VSP's network, while the
   customer network uses its ISP connections as transits.  (In many
   cases, the "VSP" may itself be a major enterprise network that
   delegates CPs from its PI prefixes to child enterprise networks.)

6.2.  Default Route Configuration and Selection

   Configuration of default routes in the presence of VET interfaces
   must be carefully coordinated according to the inner and outer
   network protocols.  If the inner and outer protocols are different
   (e.g., IPv6 in IPv4) then default routes of the inner protocol
   version can be configured with next-hops corresponding to default
   routers on a VET interface while default routes of the outer protocol
   version can be configured with next-hops corresponding to default
   routers on an underlying interface.

   If the inner and outer protocols are the same (e.g., IPv4 in IPv4),
   care must be taken in setting the default route to avoid ambiguity.
   For example, if default routes are configured on the VET interface
   then more-specific routes could be configured on underlying
   interfaces to avoid looping.  Alternatively, multiple default routes
   can be configured with some having next-hops corresponding to (EID-
   based) default routers on VET interfaces and others having next-hops
   corresponding to (RLOC-based) default routers on underlying
   interfaces.  In that case, special next-hop determination rules must
   be used (see Section 6.4).

6.3.  Address Selection

   When permitted by policy and supported by enterprise-interior
   routing, VET nodes can avoid encapsulation through communications



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   that directly invoke the outer IP protocol using RLOC addresses
   instead of EID addresses for end-to-end communications.  For example,
   an enterprise network that provides native IPv4 intra-enterprise
   services can provide continued support for native IPv4 communications
   even when encapsulated IPv6 services are available for inter-
   enterprise communications.

   In other enterprise network scenarios, the use of EID-based
   communications (i.e., instead of RLOC-based communications) may be
   necessary and/or beneficial to support address scaling, transparent
   NAT traversal, security domain separation, site multihoming, traffic
   engineering, etc.

   VET nodes can use source address selection rules [RFC6724] (e.g.,
   based on name service information) to determine whether to use EID-
   based or RLOC-based addressing.  The remainder of this section
   discusses internetworking operation for EID-based communications
   using the VET interface abstraction.

6.4.  Next Hop Determination

   VET nodes perform normal next-hop determination via longest prefix
   match, and send packets according to the most-specific matching entry
   in the FIB.  If the FIB entry has multiple next-hop addresses, the
   VET node selects the next-hop with the best metric value.  If
   multiple next hops have the same metric value, the VET node MAY use
   Equal Cost Multi Path (ECMP) to forward different flows via different
   next-hop addresses, where flows are determined, e.g., by computing a
   hash of the inner packet's source address, destination address and
   flow label fields.  Note that it is not important that all VET nodes
   use the same hashing algorithm nor that they perform ECMP at all;
   however, each VET node SHOULD apply ECMP in a consistent fashion.

   If the VET node has multiple default routes of the same inner and
   outer protocol versions, with some corresponding to EID-based default
   routers and others corresponding to RLOC-based default routers, it
   must perform source address based selection of a default route.  In
   particular, if the packet's source address is taken from an EID
   prefix the VET node selects a default route configured over the VET
   interface; otherwise, it selects a default route configured over an
   underlying interface.

   As a last resort when there is no matching entry in the FIB (i.e.,
   not even default), VET nodes can discover neighbors within the
   enterprise network through on-demand name service queries for the
   packet's destination address.  For example, for the IPv6 destination
   address '2001:DB8:1:2::1' and 'PRLNAME' "linkupnetworks.example.com"
   the VET node can perform a name service lookup for the domain name:



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   '1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.2.0.0.0.1.0.0.0.8.b.d.0.1.0.0.2.ip6.
   linkupnetworks.example.com'.

   The name service employs wildcard matching (e.g., per [RFC4592]) to
   determine the most-specific matching entry.  For example, if the
   most-specific prefix that covers the IPv6 destination address is
   '2001:DB8:1::/48' the matching entry is:

   '*.1.0.0.0.8.b.d.0.1.0.0.2.ip6.linkupnetworks.example.com'.

   If the name-service lookup succeeds, it will return RLOC addresses
   (e.g., in DNS A records) that correspond to neighbors to which the
   VET node can forward packets.  Note that this implies that, in
   enterprise networks in which a last resort address resolution service
   is necessary, the enterprise administrator MUST publish name service
   resource records that satisfy the address mapping requirements
   described above.

   Name-service lookups in enterprise networks with a centralized
   management structure use an infrastructure-based service, e.g., an
   enterprise-local DNS.  Name-service lookups in enterprise networks
   with a distributed management structure and/or that lack an
   infrastructure-based name service instead use a distributed name
   service such as LLMNR over the VET interface.  When a distributed
   name service is used, the VBR that performs the lookup sends a
   multicast query and accepts the union of all replies it receives from
   neighbors on the VET interface.  When a VET node receives the query,
   it responds IFF it aggregates an IP prefix that covers the prefix in
   the query.

6.5.  VET Interface Encapsulation/Decapsulation

   VET interfaces encapsulate inner network layer packets in a SEAL
   header followed by an outer transport-layer header such as UDP (if
   necessary) followed by an outer IP header.  Following all
   encapsulations, the VET interface submits the encapsulated packet to
   the outer IP forwarding engine for transmission on an underlying
   interface.  The following sections provide further details on
   encapsulation.

6.5.1.  Inner Network Layer Protocol

   The inner network layer protocol sees the VET interface as an
   ordinary network interface, and views the outer network layer
   protocol as an ordinary L2 transport.  The inner- and outer network
   layer protocol types are mutually independent and can be used in any
   combination.  Inner network layer protocol types include IPv6
   [RFC2460] and IPv4 [RFC0791], but they may also include non-IP



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   protocols such as OSI/CLNP [RFC0994][RFC1070][RFC4548].

6.5.2.  SEAL Encapsulation

   VET interfaces that use SEAL encapsulate the inner packet in a SEAL
   header as specified in [I-D.templin-intarea-seal].  SEAL
   encapsulation must be applied uniformly between all neighbors on the
   VET link.  Note that when a VET node sends a SEAL-encapsulated packet
   to a neighbor that does not use SEAL encapsulation, it may receive an
   ICMP "port unreachable" or "protocol unreachable" message.  If so,
   the VET node SHOULD treat the message as a hint that the prospective
   neighbor is unreachable via the VET link.

   The VET interface sets the 'NEXTHDR' value in the SEAL header to the
   IP protocol number associated with the protocol number of the inner
   network layer.  The VET interface sets the other fields in the SEAL
   header as specified in [I-D.templin-intarea-seal].

6.5.3.  UDP Encapsulation

   Following SEAL encapsulation, VET interfaces that use UDP
   encapsulation add an outer UDP header.  Inclusion of an outer UDP
   header MUST be applied by all neighbors on the VET link.  Note that
   when a VET node sends a UDP-encapsulated packet to a neighbor that
   does not recognize the UDP port number, it may receive an ICMP "port
   unreachable" message.  If so, the VET node SHOULD treat the message
   as a hint that the prospective neighbor is unreachable via the VET
   link.

   VET interfaces use UDP encapsulation on VET links that may traverse
   NATs and/or traffic conditioning network gear (e.g., Equal Cost
   MultiPath (ECMP) routers, Link Aggregation Gateways (LAGs), etc.)
   that only recognize well-known network layer protocols.  When UDP
   encapsulation is used with SEAL, the VET interface encapsulates the
   mid-layer packet in an outer UDP header then sets the UDP port number
   to the port number reserved for SEAL [I-D.templin-intarea-seal].

   The VET interface maintains per-neighbor local and remote UDP port
   numbers.  For bidirectional neighbors, the VET interface sets the
   local UDP port number to the value reserved for SEAL and sets the
   remote UDP port number to the observed UDP source port number in
   packets that it receives from the neighbor.  In cases in which one of
   the bidirectional neighbors is behind a NAT, this implies that the
   one behind the NAT initiates the neighbor relationship.  If both
   neighbors have a way of knowing that there are no NATs in the path,
   then they may select and set port numbers as for unidirectional
   neighbors.




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   For unidirectional neighbors, the VET interface sets the remote UDP
   port number to the value reserved for SEAL, and additionally selects
   a small set of dynamic port number values for use as local UDP port
   numbers.  The VET interface then selects one of this set of local
   port numbers for the UDP source port for each inner packet it sends,
   where the port number can be determined e.g., by a hash calculated
   over the inner network layer addresses and inner transport layer port
   numbers.  The VET interface uses a hash function of its own choosing
   when selecting a dynamic port number value, but it should choose a
   function that provides uniform distribution between the set of
   values, and it should be consistent in the manner in which the hash
   is applied.  This procedure is RECOMMENDED in order to support
   adequate load balancing, e.g., when Link Aggregation based on UDP
   port numbers occurs within the path.

   Finally, the VET interface SHOULD set the UDP checksum field to zero
   regardless of the IP protocol version (see
   [I-D.ietf-6man-udpzero][I-D.ietf-6man-udpchecksums]).

6.5.4.  Outer IP Header Encapsulation

   Following any mid-layer and/or UDP encapsulations, the VET interface
   next adds an outer IP header.  Outer IP header construction is the
   same as specified for ordinary IP encapsulation (e.g.,
   [RFC1070][RFC2003], [RFC2473], [RFC4213], etc.) except that the "TTL/
   Hop Limit", "Type of Service/Traffic Class" and "Congestion
   Experienced" values in the inner network layer header are copied into
   the corresponding fields in the outer IP header.  The VET interface
   also sets the IP protocol number to the appropriate value for the
   first protocol layer within the encapsulation (e.g., UDP, SEAL,
   IPsec, etc.).  When IPv6 is used as the outer IP protocol, the VET
   interface sets the flow label value in the outer IPv6 header the same
   as described in [RFC6438].

6.5.5.  Decapsulation and Re-Encapsulation

   When a VET node receives an encapsulated packet, it retains the outer
   headers, processes the SEAL header (if present) as specified in
   [I-D.templin-intarea-seal], then performs next hop determination on
   the packet's inner destination address.  If the inner packet will be
   forwarded out a different interface than it arrived on, the VET node
   copies the "Congestion Experienced" value in the outer IP header into
   the corresponding field in the inner network layer header.  The VET
   node then forwards the packet to the next inner network layer hop, or
   delivers the packet locally if the inner packet is addressed to
   itself.

   If the inner packet will be forwarded out the same VET interface that



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   it arrived on, however, the VET node copies the "TTL/Hop Limit",
   "Type of Service/Traffic Class" and "Congestion Experienced" values
   in the outer IP header of the received packet into the corresponding
   fields in the outer IP header of the packet to be forwarded (i.e.,
   the values are transferred between outer headers and *not* copied
   from the inner network layer header).  This is true even if the outer
   IP protocol version of the received packet is different than the
   outer IP protocol version of the packet to be forwarded, i.e., the
   same as for bridging dissimilar L2 media segments.  This re-
   encapsulation procedure is necessary to support diagnostic functions
   (e.g., 'traceroute'), and to ensure that the TTL/Hop Limit eventually
   decrements to 0 in case of transient routing loops.

6.6.  Neighbor Coordination on VET Interfaces that use SEAL

   VET interfaces that use SEAL use the SEAL Control Message Protocol
   (SCMP) as specified in Section 4.6 of [I-D.templin-intarea-seal] to
   coordinate reachability, routing information, and mappings between
   the inner and outer network layer protocols.  SCMP parallels the IPv6
   ND [RFC4861] and ICMPv6 [RFC4443] protocols, but operates from within
   the tunnel and supports operation for any combinations of inner and
   outer network layer protocols.

   When a VET interface prepares a neighbor coordination SCMP message,
   the message is formatted the same as described for the corresponding
   IPv6 ND message, except that the message is preceded by a SEAL header
   the same as for SCMP error messages.  The interface sets the SEAL
   header flags, NEXTHDR, LINK_ID, Identification, and Integrity Check
   Vector (ICV) fields the same as for SCMP error messages.

   The VET interface next fills out the SCMP message header fields the
   same as for SCMP error messages, calculates the SCMP message
   Checksum, encapsulates the message in the requisite outer headers,
   then calculates the SEAL header ICV if it is configured to do so and
   places the result in the ICV field.  The VET interface finally sends
   the message to the neighbor, which will verify the ICV and Checksum
   before accepting the message.

   VET and SEAL are specifically designed for encapsulation of inner
   network layer payloads over outer IPv4 and IPv6 networks as a link
   layer.  VET interfaces therefore require a new Source/Target Link-
   Layer Address Option (S/TLLAO) format that encapsulates IPv4
   addresses as shown in Figure 2 and IPv6 addresses as shown in
   Figure 3:







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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Type = 2   |   Length = 1  |          Reserved             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          IPv4 address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 2: SCMP S/TLLAO Option for IPv4 RLOCs

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Type = 2   |   Length = 3  |          Reserved             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Reserved                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               IPv6 address (bytes 0 thru 3)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               IPv6 address (bytes 4 thru 7)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               IPv6 address (bytes 8 thru 11)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               IPv6 address (bytes 12 thru 15)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 3: SCMP S/TLLAO Option for IPv6 RLOCs

   The following subsections discuss VET interface neighbor coordination
   using SCMP.

6.6.1.  Router Discovery

   VET hosts and VBRs can send SCMP Router Solicitation (SRS) messages
   to one or more VBGs in the PRL to receive solicited SCMP Router
   Advertisements (SRAs).

   When a VBG receives an SRS message on a VET interface, it prepares a
   solicited SRA message.  The SRA includes Router Lifetimes, Default
   Router Preferences, PIOs and any other options/parameters that the
   VBG is configured to include.

   The VBG finally includes one or more SLLAOs formatted as specified
   above that encode the IPv6 and/or IPv4 RLOC unicast addresses of its
   own enterprise-interior interfaces or the enterprise-interior
   interfaces of other nearby VBGs.





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6.6.2.  Neighbor Unreachability Detection

   VET nodes perform Neighbor Unreachability Detection (NUD) by
   monitoring hints of forward progress.  The VET node can periodically
   set the 'A' bit in the header of SEAL data packets to elicit SCMP
   responses from the neighbor.  The VET node can also send SCMP
   Neighbor Solicitation (SNS) messages to the neighbor to elicit SCMP
   Neighbor Advertisement (SNA) messages.

   Responsiveness to routing changes is directly related to the delay in
   detecting that a neighbor has gone unreachable.  In order to provide
   responsiveness comparable to dynamic routing protocols, a reasonably
   short neighbor reachable time (e.g., 5sec) SHOULD be used.

   Additionally, a VET node may receive outer IP ICMP "Destination
   Unreachable; net / host unreachable" messages from an ER on the path
   indicating that the path to a neighbor may be failing.  If the node
   receives excessive ICMP unreachable errors through multiple RLOCs
   associated with the same FIB entry, it SHOULD delete the FIB entry
   and allow subsequent packets to flow through a different route (e.g.,
   a default route with a VBG as the next hop).

6.6.3.  Redirection

   The VET node connected to the source EUN (i.e., the source VET node)
   can set R=1 in the SEAL header of a data packet to be forwarded as an
   indication that redirection messages will be accepted from the VET
   node connected to the destination EUN (i.e., the target VET node).
   Each VBG on the VET interface chain to the target preserves the state
   of the R bit when it re-encapsulates and forwards the packet.

   When the VET node that acts as server to the target VET node receives
   the packet, it sends an SCMP "Predirect" (SPD) message forward to the
   target VET node.  The target VET node in turn creates an SCMP
   "Redirect" (SRD) message to send back to the source VET node.  The
   SPD and SRD messages exchanes are coordinated exactly as specified in
   AERO [RFC6706], while the message format is specified as shown in
   Figure 4













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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type (=137)  |   Code (=0)   |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |P|V|                        Reserved                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                        Target Address                         +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                     Destination Address                       +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Authentication Key (optional)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-   ...
   |   Options ...
   +-+-+-+-+-+-+-+-+-+-+-+-

             Figure 4: SCMP Redirect/Predirect Message Format

   In Figure 4, each VET node sets the P bit to 1 for SPD messages and
   sets P to 0 for SRD messages, i.e., exactly as specified in
   [RFC6706].  A VET node that is a target of a Predirect sets the V bit
   to 1 if an Authentication Key is to be included in subsequent
   Redirects and sets V to 0 otherwise.  The Authentication Key includes
   a control octet followed by an Encrypted Secret Key (ESK) as shown in
   Figure 5:


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |F|Key|Algorithm|         Encrypted Secret Key (ESK)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-   ...

                    Figure 5: Authentication Key Format

   The target VET node sets the F and Algorithm fields in the control



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   octet to 0 to indicate "HMAC-SHA1 with 160 bit keys and 80 bit
   Message Authentication Code (MAC)", i.e., exactly as specified in
   [I-D.templin-intarea-seal].  (Other values for the F bit and
   Algorithm IDs are out of scope.)  The target VET node then creates a
   secret authentication key that it will use to validate the SEAL
   header ICV in future packets it will receive from the (redirected)
   source VET node.  The target encrypts the authentication key with an
   encryption key it shares with the previous VET interface hop using an
   agreed cryptographic algorithm (e.g., 3DES [RFC2451]).  It then
   writes the encrypted value in the Encrypted Secret Key (ESK) field.

   Whether or not an Authentication Key is included, the SRD/SPD message
   MUST include a Redirected Header Option (RHO) containing the leading
   portion of the packet that triggered the redirection event in the
   Options field.The target VET node then encapsulates the SRD message
   as specified in [I-D.templin-intarea-seal] and returns the message to
   the previous hop VBG on the chain toward the source.

   When the target returns the SRD message, each intermediate VBG in the
   chain toward the source relays the message by examining the source
   address of the inner packet within the RHO to determine the previous
   hop toward the source.  When an Authentication Key is included, each
   intermediate VBG in the chain decrypts the ESK value in the SRD
   message using its own secret encryption key.  The VBG then re-
   encrypts the ESK value using the encryption key corresponding to the
   previous hop toward the source, then re-encapsulates the SRD message
   and sends it to the previous hop.  This relaying process is otherwise
   the same as for SCMP error message relaying specified in Section 4.6
   of [I-D.templin-intarea-seal].

   When the source VET node receives the SRD message, it discovers both
   the target's delegated prefix and candidate link layer addresses for
   this new (unidirectional) target VET node.  The source VET node then
   installs the prefix included in the Redirect message in a forwarding
   table entry with the target as the next hop.  When an Authentication
   Key is included, the source node also caches the ESK value and uses
   it to calculate the ICVs it will include in the SEAL header of
   subsequent packets it sends to the target.

   The source can subsequently send packets destined to an address
   covered by the destination prefix using SEAL encapsulation via the
   target as the next hop.  The target can then use the ICVs in the SEAL
   data packets for message origin authentication, but it need not also
   check the outer source addresses/port numbers of the packets.
   Therefore, the outer addresses may change over time even if the inner
   source address stays the same.

   Following redirection, if the source is subsequently unable to reach



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   the target via the route-optimized path, it deletes the destination
   prefix forwarding table entry and installs a new forwarding table
   entry for the destination prefix with a default router as the next
   hop.  The source VET node thereafter sets R=0 in the SEAL headers of
   data packets that it sends toward the destination prefix, but it may
   attempt redirection again at a later time by again setting R=1.

   Finally, the source and target VET nodes set an expiration timer on
   the destination forwarding table entry so that stale entries are
   deleted in a timely fashion as specified in AERO [RFC6706].  The
   source MAY further engage the target in a bidirectional neighbor
   synchronization exchange as described in Section 6.6.4 if it is
   configured to do so.

6.6.4.  Bidirectional Neighbor Synchronization

   The tunnel neighbor relationship between a pair of VET interface
   tunnel neighbors can be either unidirectional or bidirectional.  A
   unidirectional relationship (see Section 6.6.3) can be established
   when the source VET node 'A' will tunnel data packets directly to a
   target VET node 'B', but 'B' will not tunnel data packets directly to
   'A'.  A bidirectional relationship is necessary, e.g., when a pair of
   VET nodes require a client/server or peer-to-peer binding.

   In order to establish a bidirectional tunnel neighbor relationship,
   the initiator (call it "A") performs a reliable exchange (e.g., a
   short TCP transaction, a DHCP client/server exchange, etc.) with the
   responder (call it "B").  The Tunnel Setup Protocol (TSP) [RFC5572]
   is an example of a short TCP transaction that can be used, but the
   exact mechanism need not be standardized as long as both the
   initiator and responder observe the same specifications.  Note that a
   short transaction instead of a persistent connection is advised if
   the outer network layer protocol addresses may change, e.g., due to a
   mobility event, due to loss of state in network middleboxes, etc.

   During the transaction, "A" and "B" first authenticate themselves to
   each other, then exchange information regarding the inner network
   layer prefixes that will be used for conveying inner packets that
   will be forwarded over the tunnel.  In this process, the initiator
   and responder register one or more link identifiers (LINK_IDs) with
   one another to provide "handles" for outer IP connection addresses.
   When authentication services are necessary, "A" and "B" then
   establish a shared secret authentication key that will be used for
   ICV generation in future packets as well as a shared secret
   encryption key that will be used in encrypting future key exchanges
   as described in Section 6.6.5.

   Following this bidirectional tunnel neighbor establishment, the



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   neighbors monitor the soft state for liveness, e.g., using Neighbor
   Unreachability Detection hints of forward progress.  When one of the
   neighbors wishes to terminate the relationship, it performs another
   short transaction to request the termination, then both neighbors
   delete their respective tunnel soft state.

   Once a bidirectional neighbor relationship has been established, the
   initiator and responder can further engage in a dynamic routing
   protocol (e.g., OSPF[RFC5340], etc.) to exchange inner network layer
   prefix information if they are configured to do so.

6.7.  Neighbor Coordination on VET Interfaces using IPsec

   VET interfaces that use IPsec encapsulation [RFC4301] use the
   Internet Key Exchange protocol, version 2 (IKEv2) [RFC4306] to manage
   security association setup and maintenance.  IKEv2 provides a logical
   equivalent of the SCMP in terms of VET interface neighbor
   coordinations; for example, IKEv2 also provides mechanisms for
   redirection [RFC5685] and mobility ][RFC4555].

   IPsec additionally provides an extended Identification field and ICV;
   these features allow IPsec to utilize outer IP fragmentation and
   reassembly with less risk of exposure to data corruption due to
   reassembly misassociations.

6.8.  Mobility and Multihoming Considerations

   VBRs that travel between distinct enterprise networks must either
   abandon their PA prefixes that are relative to the "old" network and
   obtain PA prefixes relative to the "new" network, or somehow
   coordinate with a "home" network to retain ownership of the prefixes.
   In the first instance, the VBR would be required to coordinate a
   network renumbering event on its attached networks using the new PA
   prefixes [RFC4192][RFC5887].  In the second instance, an adjunct
   mobility management mechanism is required.

   VBRs can retain their CPs as they travel between distinct network
   points of attachment as long as they continue to refresh their CP-to-
   RLOC address mappings with their serving VBG in a bidirectional
   neighbor exchange (see Section 6.6.4).  (When the VBR moves far from
   its serving VBG, it can also select a new VBG in order to maintain
   optimal routing.)  In this way, VBRs can update their CP-to-RLOC
   mappings in real time and without requiring an adjunct mobility
   management mechanism.

   VBRs that have true PI prefixes can withdraw the prefixes from former
   Internet points of attachment and re-advertise them at new points of
   attachment as they move.  However, this method has been shown to



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   produce excessive routing churn in the global internet BGP tables,
   and should be avoided for any mobility scenarios that may occur along
   short timescales.  The alternative is to employ a system in which the
   true PI prefixes are not injected into the Internet routing system,
   but rather managed through some separate global mapping database.
   This latter method is employed by the LISP proposal [RFC6830].

   The VBGs of a multihomed enterprise network participate in a private
   inner network layer routing protocol instance (e.g., via an interior
   BGP instance) to accommodate network partitions/merges as well as
   intra-enterprise mobility events.

6.9.  Multicast

6.9.1.  Multicast over Non-Multicast Enterprise Networks

   Whether or not the underlying enterprise network supports a native
   multicasting service, the VET node can act as an inner network layer
   IGMP/MLD proxy [RFC4605] on behalf of its attached EUNs and convey
   its multicast group memberships over the VET interface to a VBG
   acting as a multicast router.  The VET node's inner network layer
   multicast transmissions will therefore be encapsulated in outer
   headers with the unicast address of the VBG as the destination.

6.9.2.  Multicast Over Multicast-Capable Enterprise Networks

   In multicast-capable enterprise networks, ERs provide an enterprise-
   wide multicasting service (e.g., Simplified Multicast Forwarding
   (SMF) [RFC6621], Protocol Independent Multicast (PIM) routing,
   Distance Vector Multicast Routing Protocol (DVMRP) routing, etc.)
   over their enterprise-interior interfaces such that outer IP
   multicast messages of site-scope or greater scope will be propagated
   across the enterprise network.  For such deployments, VET nodes can
   optionally provide a native inner multicast/broadcast capability over
   their VET interfaces through mapping of the inner multicast address
   space to the outer multicast address space.  In that case, operation
   of link-or greater-scoped inner multicasting services (e.g., a link-
   scoped neighbor discovery protocol) over the VET interface is
   available, but SHOULD be used sparingly to minimize enterprise-wide
   flooding.

   VET nodes encapsulate inner multicast messages sent over the VET
   interface in any mid-layer headers followed by an outer IP header
   with a site-scoped outer IP multicast address as the destination.
   For the case of IPv6 and IPv4 as the inner/outer protocols
   (respectively), [RFC2529] provides mappings from the IPv6 multicast
   address space to a site-scoped IPv4 multicast address space (for
   other encapsulations, mappings are established through administrative



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   configuration or through an unspecified alternate static mapping).
   Note that VET links will use mid-layer encapsulations as the means
   for distinguishing VET nodes from legacy RFC2529 nodes.

   Multicast mapping for inner multicast groups over outer IP multicast
   groups can be accommodated, e.g., through VET interface snooping of
   inner multicast group membership and routing protocol control
   messages.  To support inner-to-outer multicast address mapping, the
   VET interface acts as a virtual outer IP multicast host connected to
   its underlying interfaces.  When the VET interface detects that an
   inner multicast group joins or leaves, it forwards corresponding
   outer IP multicast group membership reports on an underlying
   interface over which the VET interface is configured.  If the VET
   node is configured as an outer IP multicast router on the underlying
   interfaces, the VET interface forwards locally looped-back group
   membership reports to the outer IP multicast routing process.  If the
   VET node is configured as a simple outer IP multicast host, the VET
   interface instead forwards actual group membership reports (e.g.,
   IGMP messages) directly over an underlying interface.

   Since inner multicast groups are mapped to site-scoped outer IP
   multicast groups, the site administrator MUST ensure that the site-
   scoped outer IP multicast messages received on the underlying
   interfaces for one VET interface do not "leak out" to the underlying
   interfaces of another VET interface.  This is accommodated through
   normal site-scoped outer IP multicast group filtering at enterprise
   network boundaries.

6.10.  Service Discovery

   VET nodes can perform enterprise-wide service discovery using a
   suitable name-to-address resolution service.  Examples of flooding-
   based services include the use of LLMNR [RFC4795] over the VET
   interface or multicast DNS (mDNS) [RFC6762] over an underlying
   interface.  More scalable and efficient service discovery mechanisms
   (e.g., anycast) are for further study.

6.11.  VET Link Partitioning

   A VET link can be partitioned into multiple distinct logical
   groupings.  In that case, each partition configures its own distinct
   'PRLNAME' (e.g., 'linkupnetworks.zone1.example.com',
   'linkupnetworks.zone2.example.com', etc.).

   VBGs that are configured to support partitioning MAY further create
   multiple IP subnets within a partition, e.g., by sending SRAs with
   PIOs containing different IP prefixes to different groups of VET
   hosts.  VBGs can identify subnets, e.g., by examining RLOC prefixes,



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   observing the enterprise-interior interfaces over which SRSs are
   received, etc.

   In the limiting case, VBGs can advertise a unique set of IP prefixes
   to each VET host such that each host belongs to a different subnet
   (or set of subnets) on the VET interface.

6.12.  VBG Prefix State Recovery

   VBGs retain explicit state that tracks the inner network layer
   prefixes delegated to VBRs connected to the VET link, e.g., so that
   packets are delivered to the correct VBRs.  When a VBG loses some or
   all of its state (e.g., due to a power failure), client VBRs MUST
   refresh the VBG's state so that packets can be forwarded over correct
   routes.

6.13.  Legacy ISATAP Services

   VBGs can support legacy ISATAP services according to the
   specifications in [RFC5214].  In particular, VBGs can configure
   legacy ISATAP interfaces and VET interfaces over the same sets of
   underlying interfaces as long as the PRLs and IPv6 prefixes
   associated with the ISATAP/VET interfaces are distinct.


7.  IANA Considerations

   There are no IANA considerations for this document.


8.  Security Considerations

   Security considerations for MANETs are found in [RFC2501].

   The security considerations found in [RFC2529][RFC5214][RFC6324] also
   apply to VET.

   SEND [RFC3971] and/or IPsec [RFC4301] can be used in environments
   where attacks on the neighbor coordination protocol are possible.
   SEAL [I-D.templin-intarea-seal] supports path MTU discovery, and
   provides per-packet authenticating information for message origin
   authentication, anti-replay and message header integrity.

   Rogue neighbor coordination messages with spoofed RLOC source
   addresses can consume network resources and cause VET nodes to
   perform extra work.  Nonetheless, VET nodes SHOULD NOT "blacklist"
   such RLOCs, as that may result in a denial of service to the RLOCs'
   legitimate owners.



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   VBRs and VBGs observe the recommendations for network ingress
   filtering [RFC2827].


9.  Related Work

   Brian Carpenter and Cyndi Jung introduced the concept of intra-site
   automatic tunneling in [RFC2529]; this concept was later called:
   "Virtual Ethernet" and investigated by Quang Nguyen under the
   guidance of Dr. Lixia Zhang.  Subsequent works by these authors and
   their colleagues have motivated a number of foundational concepts on
   which this work is based.

   Telcordia has proposed DHCP-related solutions for MANETs through the
   CECOM MOSAIC program.

   The Naval Research Lab (NRL) Information Technology Division uses
   DHCP in their MANET research testbeds.

   Security concerns pertaining to tunneling mechanisms are discussed in
   [RFC6169].

   Default router and prefix information options for DHCPv6 are
   discussed in [I-D.droms-dhc-dhcpv6-default-router].

   An automated IPv4 prefix delegation mechanism is proposed in
   [RFC6656].

   RLOC prefix delegation for enterprise-edge interfaces is discussed in
   [I-D.clausen-manet-autoconf-recommendations].

   MANET link types are discussed in [I-D.clausen-manet-linktype].

   The LISP proposal [RFC6830] examines encapsulation/decapsulation
   issues and other aspects of tunneling.

   Various proposals within the IETF have suggested similar mechanisms.


10.  Acknowledgements

   The following individuals gave direct and/or indirect input that was
   essential to the work: Jari Arkko, Teco Boot, Emmanuel Bacelli, Fred
   Baker, James Bound, Scott Brim, Brian Carpenter, Thomas Clausen,
   Claudiu Danilov, Chris Dearlove, Remi Despres, Gert Doering, Ralph
   Droms, Washam Fan, Dino Farinacci, Vince Fuller, Thomas Goff, David
   Green, Joel Halpern, Bob Hinden, Sascha Hlusiak, Sapumal Jayatissa,
   Dan Jen, Darrel Lewis, Tony Li, Joe Macker, David Meyer, Gabi



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   Nakibly, Thomas Narten, Pekka Nikander, Dave Oran, Alexandru
   Petrescu, Mark Smith, John Spence, Jinmei Tatuya, Dave Thaler, Mark
   Townsley, Ole Troan, Michaela Vanderveen, Robin Whittle, James
   Woodyatt, Lixia Zhang, and others in the IETF AUTOCONF and MANET
   working groups.  Many others have provided guidance over the course
   of many years.

   Discussions with colleagues following the publication of RFC5558 have
   provided useful insights that have resulted in significant
   improvements to this, the Second Edition of VET.


11.  Contributors

   The following individuals have contributed to this document:

   Eric Fleischman (eric.fleischman@boeing.com)
   Thomas Henderson (thomas.r.henderson@boeing.com)
   Steven Russert (steven.w.russert@boeing.com)
   Seung Yi (seung.yi@boeing.com)

   Ian Chakeres (ian.chakeres@gmail.com) contributed to earlier versions
   of the document.

   Jim Bound's foundational work on enterprise networks provided
   significant guidance for this effort.  We mourn his loss and honor
   his contributions.


12.  References

12.1.  Normative References

   [I-D.templin-intarea-seal]
              Templin, F., "The Subnetwork Encapsulation and Adaptation
              Layer (SEAL)", draft-templin-intarea-seal-53 (work in
              progress), April 2013.

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

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",



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              RFC 2131, March 1997.

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

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

   [RFC3118]  Droms, R. and W. Arbaugh, "Authentication for DHCP
              Messages", RFC 3118, June 2001.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, March 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

   [RFC5342]  Eastlake, D., "IANA Considerations and IETF Protocol Usage
              for IEEE 802 Parameters", BCP 141, RFC 5342,
              September 2008.

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, November 2011.




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   [RFC6706]  Templin, F., "Asymmetric Extended Route Optimization
              (AERO)", RFC 6706, August 2012.

   [RFC6724]  Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, September 2012.

12.2.  Informative References

   [CATENET]  Pouzin, L., "A Proposal for Interconnecting Packet
              Switching Networks", May 1974.

   [I-D.clausen-manet-autoconf-recommendations]
              Clausen, T. and U. Herberg, "MANET Router Configuration
              Recommendations",
              draft-clausen-manet-autoconf-recommendations-00 (work in
              progress), February 2009.

   [I-D.clausen-manet-linktype]
              Clausen, T., "The MANET Link Type",
              draft-clausen-manet-linktype-00 (work in progress),
              October 2008.

   [I-D.droms-dhc-dhcpv6-default-router]
              Droms, R. and T. Narten, "Default Router and Prefix
              Advertisement Options for DHCPv6",
              draft-droms-dhc-dhcpv6-default-router-00 (work in
              progress), March 2009.

   [I-D.ietf-6man-udpchecksums]
              Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets",
              draft-ietf-6man-udpchecksums-08 (work in progress),
              February 2013.

   [I-D.ietf-6man-udpzero]
              Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the use of IPv6 UDP Datagrams with Zero Checksums",
              draft-ietf-6man-udpzero-12 (work in progress),
              February 2013.

   [I-D.ietf-grow-va]
              Francis, P., Xu, X., Ballani, H., Jen, D., Raszuk, R., and
              L. Zhang, "FIB Suppression with Virtual Aggregation",
              draft-ietf-grow-va-06 (work in progress), December 2011.

   [I-D.jen-apt]
              Jen, D., Meisel, M., Massey, D., Wang, L., Zhang, B., and



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              L. Zhang, "APT: A Practical Transit Mapping Service",
              draft-jen-apt-01 (work in progress), November 2007.

   [I-D.templin-ironbis]
              Templin, F., "The Interior Routing Overlay Network
              (IRON)", draft-templin-ironbis-13 (work in progress),
              March 2013.

   [IEN48]    Cerf, V., "The Catenet Model for Internetworking",
              July 1978.

   [RASADV]   Microsoft, "Remote Access Server Advertisement (RASADV)
              Protocol Specification", October 2008.

   [RFC0994]  International Organization for Standardization (ISO) and
              American National Standards Institute (ANSI), "Final text
              of DIS 8473, Protocol for Providing the Connectionless-
              mode Network Service", RFC 994, March 1986.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [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.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC1753]  Chiappa, J., "IPng Technical Requirements Of the Nimrod
              Routing and Addressing Architecture", RFC 1753,
              December 1994.

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

   [RFC1955]  Hinden, R., "New Scheme for Internet Routing and
              Addressing (ENCAPS) for IPNG", RFC 1955, June 1996.

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              October 1996.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, March 1997.

   [RFC2451]  Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher
              Algorithms", RFC 2451, November 1998.



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   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, December 1998.

   [RFC2491]  Armitage, G., Schulter, P., Jork, M., and G. Harter, "IPv6
              over Non-Broadcast Multiple Access (NBMA) networks",
              RFC 2491, January 1999.

   [RFC2501]  Corson, M. and J. Macker, "Mobile Ad hoc Networking
              (MANET): Routing Protocol Performance Issues and
              Evaluation Considerations", RFC 2501, January 1999.

   [RFC2529]  Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
              Domains without Explicit Tunnels", RFC 2529, March 1999.

   [RFC2775]  Carpenter, B., "Internet Transparency", RFC 2775,
              February 2000.

   [RFC3819]  Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, July 2004.

   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927,
              May 2005.

   [RFC3947]  Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
              "Negotiation of NAT-Traversal in the IKE", RFC 3947,
              January 2005.

   [RFC3948]  Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
              Stenberg, "UDP Encapsulation of IPsec ESP Packets",
              RFC 3948, January 2005.

   [RFC4030]  Stapp, M. and T. Lemon, "The Authentication Suboption for
              the Dynamic Host Configuration Protocol (DHCP) Relay Agent
              Option", RFC 4030, March 2005.

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

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.




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

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 2005.

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

   [RFC4555]  Eronen, P., "IKEv2 Mobility and Multihoming Protocol
              (MOBIKE)", RFC 4555, June 2006.

   [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name
              System", RFC 4592, July 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.

   [RFC4795]  Aboba, B., Thaler, D., and L. Esibov, "Link-local
              Multicast Name Resolution (LLMNR)", RFC 4795,
              January 2007.

   [RFC4852]  Bound, J., Pouffary, Y., Klynsma, S., Chown, T., and D.
              Green, "IPv6 Enterprise Network Analysis - IP Layer 3
              Focus", RFC 4852, April 2007.

   [RFC4903]  Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
              June 2007.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, September 2007.

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

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, July 2008.

   [RFC5558]  Templin, F., "Virtual Enterprise Traversal (VET)",
              RFC 5558, February 2010.



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   [RFC5572]  Blanchet, M. and F. Parent, "IPv6 Tunnel Broker with the
              Tunnel Setup Protocol (TSP)", RFC 5572, February 2010.

   [RFC5685]  Devarapalli, V. and K. Weniger, "Redirect Mechanism for
              the Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 5685, November 2009.

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

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

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

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

   [RFC6324]  Nakibly, G. and F. Templin, "Routing Loop Attack Using
              IPv6 Automatic Tunnels: Problem Statement and Proposed
              Mitigations", RFC 6324, August 2011.

   [RFC6621]  Macker, J., "Simplified Multicast Forwarding", RFC 6621,
              May 2012.

   [RFC6656]  Johnson, R., Kinnear, K., and M. Stapp, "Description of
              Cisco Systems' Subnet Allocation Option for DHCPv4",
              RFC 6656, July 2012.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              February 2013.

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              January 2013.


Appendix A.  Duplicate Address Detection (DAD) Considerations

   A priori uniqueness determination (also known as "pre-service DAD")
   for an RLOC assigned on an enterprise-interior interface would
   require either flooding the entire enterprise network or somehow
   discovering a link in the network on which a node that configures a
   duplicate address is attached and performing a localized DAD exchange
   on that link.  But, the control message overhead for such an



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   enterprise-wide DAD would be substantial and prone to false-negatives
   due to packet loss and intermittent connectivity.  An alternative to
   pre-service DAD is to autoconfigure pseudo-random RLOCs on
   enterprise-interior interfaces and employ a passive in-service DAD
   (e.g., one that monitors routing protocol messages for duplicate
   assignments).

   Pseudo-random IPv6 RLOCs can be generated with mechanisms such as
   CGAs, IPv6 privacy addresses, etc. with very small probability of
   collision.  Pseudo-random IPv4 RLOCs can be generated through random
   assignment from a suitably large IPv4 prefix space.

   Consistent operational practices can assure uniqueness for VBG-
   aggregated addresses/prefixes, while statistical properties for
   pseudo-random address self-generation can assure uniqueness for the
   RLOCs assigned on an ER's enterprise-interior interfaces.  Still, an
   RLOC delegation authority should be used when available, while a
   passive in-service DAD mechanism should be used to detect RLOC
   duplications when there is no RLOC delegation authority.


Appendix B.  Anycast Services

   Some of the IPv4 addresses that appear in the Potential Router List
   may be anycast addresses, i.e., they may be configured on the VET
   interfaces of multiple VBRs/VBGs.  In that case, each VET router
   interface that configures the same anycast address must exhibit
   equivalent outward behavior.

   Use of an anycast address as the IP destination address of tunneled
   packets can have subtle interactions with tunnel path MTU and
   neighbor discovery.  For example, if the initial fragments of a
   fragmented tunneled packet with an anycast IP destination address are
   routed to different egress tunnel endpoints than the remaining
   fragments, the multiple endpoints will be left with incomplete
   reassembly buffers.  This issue can be mitigated by ensuring that
   each egress tunnel endpoint implements a proactive reassembly buffer
   garbage collection strategy.  Additionally, ingress tunnel endpoints
   that send packets with an anycast IP destination address must use the
   minimum path MTU for all egress tunnel endpoints that configure the
   same anycast address as the tunnel MTU.  Finally, ingress tunnel
   endpoints SHOULD treat ICMP unreachable messages from a router within
   the tunnel as at most a weak indication of neighbor unreachability,
   since the failures may only be transient and a different path to an
   alternate anycast router quickly selected through reconvergence of
   the underlying routing protocol.

   Use of an anycast address as the IP source address of tunneled



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   packets can lead to more serious issues.  For example, when the IP
   source address of a tunneled packet is anycast, ICMP messages
   produced by routers within the tunnel might be delivered to different
   ingress tunnel endpoints than the ones that produced the packets.  In
   that case, functions such as path MTU discovery and neighbor
   unreachability detection may experience non-deterministic behavior
   that can lead to communications failures.  Additionally, the
   fragments of multiple tunneled packets produced by multiple ingress
   tunnel endpoints may be delivered to the same reassembly buffer at a
   single egress tunnel endpoint.  In that case, data corruption may
   result due to fragment misassociation during reassembly.

   In view of these considerations, VBGs that configure an anycast
   address SHOULD also configure one or more unicast addresses from the
   Potential Router List; they SHOULD further accept tunneled packets
   destined to any of their anycast or unicast addresses, but SHOULD
   send tunneled packets using a unicast address as the source address.


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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