path: root/doc/rfc/rfc4887.txt

Network Working Group                                         P. Thubert
Request for Comments: 4887                                 Cisco Systems
Category: Informational                                      R. Wakikawa
                                                Keio University and WIDE
                                                          V. Devarapalli
                                                         Azaire Networks
                                                               July 2007


                  Network Mobility Home Network Models

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This paper documents some of the usage patterns and the associated
   issues when deploying a Home Network for Network Mobility (NEMO)-
   enabled Mobile Routers, conforming to the NEMO Basic Support.  The
   aim here is specifically to provide some examples of organization of
   the Home Network, as they were discussed in NEMO-related mailing
   lists.






















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology and Concepts . . . . . . . . . . . . . . . . . . .  4
   3.  General Expectations . . . . . . . . . . . . . . . . . . . . .  4
   4.  MIP Home Network . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  NEMO Extended Home Network . . . . . . . . . . . . . . . . . .  5
     5.1.  Configuration  . . . . . . . . . . . . . . . . . . . . . .  5
     5.2.  Returning Home . . . . . . . . . . . . . . . . . . . . . .  6
     5.3.  Home Address from MNP  . . . . . . . . . . . . . . . . . .  7
     5.4.  Deployment Caveats . . . . . . . . . . . . . . . . . . . .  8
       5.4.1.  Mobile Router Side . . . . . . . . . . . . . . . . . .  8
     5.5.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  8
   6.  NEMO Aggregated Home Network . . . . . . . . . . . . . . . . .  8
     6.1.  Configuration  . . . . . . . . . . . . . . . . . . . . . .  8
     6.2.  Returning Home . . . . . . . . . . . . . . . . . . . . . .  9
       6.2.1.  Returning Home with the Egress Interface . . . . . . . 10
       6.2.2.  Returning Home with the Ingress Interface  . . . . . . 10
     6.3.  Applicability  . . . . . . . . . . . . . . . . . . . . . . 11
     6.4.  Deployment Caveats . . . . . . . . . . . . . . . . . . . . 11
       6.4.1.  Home Agent Side  . . . . . . . . . . . . . . . . . . . 11
       6.4.2.  Mobile Router Side . . . . . . . . . . . . . . . . . . 11
   7.  NEMO Virtual Home Network  . . . . . . . . . . . . . . . . . . 12
     7.1.  Configuration  . . . . . . . . . . . . . . . . . . . . . . 12
     7.2.  Applicability  . . . . . . . . . . . . . . . . . . . . . . 14
   8.  NEMO Mobile Home Network . . . . . . . . . . . . . . . . . . . 14
     8.1.  Configuration  . . . . . . . . . . . . . . . . . . . . . . 14
     8.2.  Applicability  . . . . . . . . . . . . . . . . . . . . . . 17
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     11.2. Informative References . . . . . . . . . . . . . . . . . . 18


















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1.  Introduction

   This document assumes that the reader is familiar with IPv6 Mobility
   as defined by Mobile IPv6 and the Network Mobility (NEMO) Basic
   Support.  In order to read this document properly, it is important to
   realize that in NEMO, the Home Network can encompass much more than
   the Home Link, as it spans the Home Link and all the Links that the
   Mobile Routers (MRs) carry with them.  Exactly how the two concepts
   relate in a given deployment depends on the organization of the Home
   Network, as described below.

   Five different organizations of the Home Network including a
   hierarchical construction are documented:

   MIPv6 Home Network:  A short reminder of what the Home Network is
      with Mobile IP, in order to help the reader figure out the
      evolution toward NEMO.

   NEMO Extended Home Network:  In this arrangement, the Home Network is
      only one subnet of a larger aggregation that encompasses the
      Mobile Networks, called Extended Home Network.  When at home, a
      Mobile Router performs normal routing between the Home Link and
      the Mobile Networks.  More in Section 5.

   NEMO Aggregated Home Network:  In this arrangement, the Home Network
      actually overlaps with the Mobile Networks.  When at home, a
      Mobile Router acts as a bridge between the Home Link and the
      Mobile Networks.  More in Section 6.

   Virtual Home Network:  In this arrangement, there is no physical Home
      Link at all for the Mobile Routers to come back home to.  More in
      Section 7.

   NEMO Mobile Home Network:  In this arrangement, there is a bitwise
      hierarchy of Home Networks.  A global Home Network is advertised
      to the infrastructure by a head Home Agent (HA) and further
      subnetted into Mobile Networks.  Each subnet is owned by a Mobile
      Router that registers it in a NEMO fashion while acting as a Home
      Agent for that network.  More in Section 8.

   In all cases, the Home Agents collectively advertise only the
   aggregation of the Mobile Networks.  The subnetting is kept within
   the Home Agents and the Mobile Routers, as opposed to advertised by
   means of routing protocols to other parties.

   The examples provided here aim at illustrating the NEMO Basic Support
   [5] but do not aim at limiting its scope of application; additional
   cases may be added in the future.



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2.  Terminology and Concepts

   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 RFC 2119 [2].

   Most of the mobility-related terms used in this document are defined
   in the Mobility Related Terminology document [3] and in the Mobile
   IPv6 (MIP6) specification [4].

   In addition, some terms were created or extended for NEMO.  These
   specific terms are defined in the Mobile Network Terminology document
   [6]:

      Home Link

      Home Network

      Home Address

      MRHA Tunnel

      Mobile Aggregated Prefix

      Aggregated Home Network

      Extended Home Network

      Virtual Home Network

      Mobile Home Network

3.  General Expectations

   With Mobile IPv6, the Home Network is generally a physical network
   interconnecting the Home Agents and the Mobile Nodes that are at
   home.  NEMO extends the concept of home so that it is not only a flat
   subnet composed of Home Addresses but an aggregation that is itself
   subnetted in Mobile and Home Networks.  This aggregation is still
   referred to as home.

   As an example, consider the case where the aggregation has a global
   routing prefix of m = 48 bits (A:B:C::/48), with a subnet ID size of
   n = 16 bits (n + m = 64):







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   When a Mobile Router, MR1, uses the Mobile Network Prefix (MNP) A:B:
   C:1::/64 with the NEMO Basic Support, MR1 may register using a Home
   Address from the Home network (i.e., A:B:C:0::1) or a Home Address
   from one of its MNPs (i.e., A:B:C:1::1) depending on the deployment.

   In a given deployment, one subnet may be reserved for the Home Link
   (A:B:C:0::/64) while the others are attributed to Mobile Routers as
   Mobile Networks (as A:B:C:1::/64 for MR1).  Another approach could be
   to configure the aggregation of Mobile Networks as the subnet on the
   Home Link, and let the Mobile Routers manage the overlapping
   networks.  Finally, the aggregation could be configured on a virtual
   network, with no physical Home Link at all, in which case home means
   topologically and administratively close to the Home Agent that
   advertises the virtual network.

   The following sections provide additional information on these forms
   of Home Network.

4.  MIP Home Network

   In the Mobile IPv6 (MIP6) specification [4], Mobile Nodes are at home
   when they are connected to their Home Link, where they recognize
   their Home Prefix in Router Advertisement messages.  Also, a binding
   is checked using Duplicate Address Detection (DAD) on the Home Link,
   and Home Agents discover each other by means of Neighbor Discovery
   (ND) extensions over that link.

   The Home Prefix that is advertized on the Home Link is a final
   prefix, as opposed to an aggregation, and it may be used by hosts on
   the Home Link for autoconfiguration purposes.

   As we see, the concept of a Home Network for Mobile IPv6 is really a
   prefix on a link, served by one or more Home Agents as opposed to a
   routed mesh.  We will see in the next sections that NEMO needs
   additional prefixes for use by the Mobile Networks.  For that reason,
   NEMO extends the concept of Home Network into a more complex,
   aggregated structure.

5.  NEMO Extended Home Network

5.1.  Configuration

   One simple way of extending the MIP Home Network is to use additional
   prefixes, contiguous to the Home Link Prefix inherited from MIPv6, as
   Mobile Network Prefixes.  As this model trivially extends the MIP
   Home Network, the resulting aggregation is called a NEMO Extended
   Home Network.  It is depicted in Figure 1.




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                        |
              route     v  /48                        A:B:C::/48

                        HA
                        | /64         Home Link: A:B:C:0::/64
             --+-----+--+- . -+- . -+--
               |     |        |     |
               MR1   MR2      MRi   MRN
               |     |        |     |
            ------  ------  ------ ------
              /64   /64      /64   /64   MNP:  A:B:C:i::/64


                             Extended Home Network
           <----------------------------------------------------------->

             Home Net      Mobile Net    Mobile Net   ...   Mobile Net
           <------------><------------><------------> ... <------------>

                      Figure 1: Extended Home Network

   In that arrangement:

   o  There is one physical Home Network and multiple Mobile Networks

   o  The Home Prefix and the MNPs are tailored to allow for IPv6
      Stateless Address Autoconfiguration with typical interface
      identifier length for the type of interface (for example, can be
      /64).

   o  The prefix length of the Extended Home Network is shorter than
      that of the Home Network and the MNPs, since it is an aggregation
      (for example, can be /48).

   o  Since the Extended Home Network operations inherit trivially from
      MIPv6, it can be seen as natural that the Mobile Routers be
      assigned their Home Addresses from the prefix on the Home Link.
      In that case, a Home Agent can perform DAD on the Home Link as
      prescribed by Mobile IPv6 for the Mobile Router Home Addresses
      (MRHAs).

5.2.  Returning Home

   In the Extended Home Network model, the Home Network is configured on
   a physical interface of the Home Agent, the Home Link.

   A Mobile Router returns home by connecting directly to the Home Link,
   and dropping the MRHA tunnel.



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   When at home, the Mobile Router ensures the connectivity of the
   Mobile Network using standard router operations.

   In implicit mode, the Home Agent has the necessary information to
   continue routing to the MNPs in the absence of registration, assuming
   that the Mobile Router is at home, and the participation of the
   Mobile Router to the home Interior Gateway Protocol (IGP) is not
   required.

   But in explicit mode, or if the Mobile Router uses an IGP over the
   MRHA tunnel, then it needs to resume its IGP operations on the Home
   Link in order to advertise its Mobile Networks to the HA, unless some
   other means such as static routes are deployed to cover the case.

   Alternative procedures for ensuring the connectivity of the Mobile
   Networks when at home are described in Section 7.

5.3.  Home Address from MNP

   We saw that a natural extension of the MIP procedure is to derive the
   Home Address of a Mobile Router from the prefix on the Home Link.
   Alternatively, NEMO basic support allows that a Mobile Router forms
   its Home Address from one of its Mobile Network Prefixes.

   In that case, the Home Address does not match the Home Link Prefix,
   and there is a need to configure the Home Agent in a specific mode
   with the support for the Extended Home Network and the range of the
   Mobile Network Prefixes.  Based on that new configuration, the Home
   Agent can accept a Home Address that is not from the Home Link, and
   it will know that it should not perform any DAD.

   Also, if the Mobile Router uses a Home Address that is derived from
   its MNP, some specific support is required on the Mobile Router as
   well.  In order to determine that it is at home, the Mobile Router
   recognizes the well-known prefix of its Home Agent as opposed to
   matching the prefix on the Home Link with that of its Home Address.

   When connecting to the Home Link, the Mobile Router also need to
   autoconfigure an address on the Egress interface as opposed to
   assigning its home Address to the interface.

   For all these reasons, this submode of Extended Home Network is not a
   trivial extension of the MIPv6 Home Model, and it might not be
   compatible with all implementations.







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5.4.  Deployment Caveats

5.4.1.  Mobile Router Side

   In explicit mode, the routing to the MNP via the Mobile Router must
   be restored when the Mobile Router is at home.  This is normally
   performed by the Mobile Router by means of the existing IGP.  In that
   case, a specific support is required on the Mobile Router to control
   the routing protocol operation, enabling the participation in the IGP
   if and only if the Mobile Router is at home.

   The NEMO Basic Support does not mandate a specific routing protocol
   though the support for some well-known routing protocols can be
   expected from many implementations.  An implementation might provide
   an automatic toggle to start/stop routing on an egress interface when
   the mobile router comes back/leaves home.  When such a toggle is
   unavailable, then a specific interface should be reserved to attach
   to home with the appropriate settings for security and routing.

5.5.  Applicability

   The Extended Home Network keeps the MIP6 concept of a Home Network
   for both Mobile Nodes and Mobile Routers to take their Home Address
   from.  Since there is no overlap between the prefixes that are
   assigned to MNPs and prefix(es) that are dedicated to the Home Link,
   it is possible for MNs and Mobile Routers to coexist with that model.

   Also, when the Home Address is derived from the prefix on the Home
   Link, the Home Agent behavior on the link trivially extends that of
   MIP and the support for that configuration should be available with
   all implementations.

   There are a number of issues with returning home when a Mobile Router
   configures its Home Address from the MNP as described in Section 5.3.
   Therefore, we do not recommend this mechanism if the Mobile Routers
   attach to the Home Network.

6.  NEMO Aggregated Home Network

6.1.  Configuration

   One other approach is to consider that the aggregation of all the
   MNPs is used plainly as the Home Link Prefix.  In this model, the
   Home Network is referred to as a NEMO Aggregated Home Network.  This
   means that the Mobile Aggregated Prefix is configured on the Home
   Link and advertised by the Home Agent as a subnet, as depicted in
   Figure 2.




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                    HA
                     | /56                       Aggreg /56
          --+-----+--+- . -+- . -+--
            |     |        |     |
           MR1   MR2      MRi   MRN
            |     |        |     |
        ------  ------  ------ ------
            /64   /64     /64   /64         Aggreg|i /64  0 < i <= N


                  Aggregated Home Network == Home Network
        <----------------------------------------------------------->

         Mobile Net    Mobile Net    Mobile Net    ...   Mobile Net
        <------------><------------><------------> ... <------------>

                     Figure 2: Aggregated Home Network

   In that model, it seems natural to subnet the whole range of
   addresses into Mobile Network prefixes, as opposed to reserving one
   prefix for the Home Link, which would boil down to the Extended Home
   Network model.  If the prefix on the Home Link is really an
   aggregation and not a final prefix, it should not be allowed for
   autoconfiguration or Home Address allocation.

   Note that in that case, it makes sense for a Mobile Router to
   register using a Home Address from one of its own MNPs.  Taking the
   Home Address from its own range guarantees the uniqueness of the
   suffix.  That uniqueness can be checked by the Mobile Router on its
   Ingress network (see [3]) using DAD.

6.2.  Returning Home

   The Aggregated Home Prefix is configured on a physical interface of
   the Home Agent, the Home Link.  As a consequence, the Home Agent has
   a connected route to the Aggregated Home Network over the Home Link.

   A Mobile Router returns home by connecting directly to the Home Link,
   and dropping the MRHA tunnel.  The Mobile Router recognizes its Home
   Link by a prefix match with its Home Agent.

   When the Mobile Router forms its Home Address out of one of its MNPs,
   since the Home Network prefix is an aggregation that encompasses all
   the MNPs, the Home Address actually matches both prefixes.  To
   properly identify the Home Network as it returns home, the MR must
   expect a shorter prefix length than that of the MNP from which the
   Home Address was formed.




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6.2.1.  Returning Home with the Egress Interface

   A Mobile Router coming home via its Egress interface sees overlapping
   prefixes between the Ingress and the Egress interfaces and some
   specific support may be needed:

   When a Mobile Router connects to the Home Link using its Egress
   Interface, it might set up a bridge between its Ingress interface(s)
   and the Home Link, if the interfaces are compatible.

   Alternatively, the Mobile Router might perform ND proxying for all
   addresses in its MNPs, between the Egress interface and the related
   Ingress interface, as described in [8].  Since the prefixes on the
   Egress and Ingress interfaces are overlapping, routing is disallowed.

   The Mobile Router does not need to join the local IGP when returning
   home, even if it is using the explicit Prefix Mode.  When the Mobile
   Router is not registered, the Home Agent simply expects that all
   Mobile Network Nodes (MNNs) will be reachable over the Home Link.

                    HA
                     |
           -------+--+--- /56
                  |
           Egress |
                 MR at home
                  |
                --+---  /64

               Figure 3: Bridging between Egress and Ingress

6.2.2.  Returning Home with the Ingress Interface

   Alternatively, if the Mobile Router has a single Ingress interface,
   the Mobile Router may use the NEMO-Link to connect to the Home Link,
   merging the two links in a single consistent network.

                    HA
                    |
           -------+-+---- /56
                  |
               ---+-- /64
                  |
                 MR at home
           Egress |

            Figure 4: Merging the Home and the Mobile Networks




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   This fits the connected route model, since the Aggregated Home
   Network is truly located on that network.  Note that in that case, it
   makes sense for a Mobile Router to register using a Home Address from
   one of its own MNPs.

6.3.  Applicability

   With this model, there is no specific space for independent nodes, as
   any address in the aggregation belongs to a MNP, and thus to a Mobile
   Router.  This configuration excludes the cohabitation with MIP6 MNs
   on the Home Link.

6.4.  Deployment Caveats

6.4.1.  Home Agent Side

   A node on the Home Link receiving a Router Advertisement that
   includes the Aggregated Home Network prefix might use that prefix for
   Address Autoconfiguration.  Such a node would also install a
   connected route to the Aggregated Home Network over the Home Link.

   As a result, unless the node has a better (longest match) route to a
   given Mobile Network Prefix, it would look up all MNNs on that MNP
   using Neighbor Discovery over its interface to the Home Link, and
   fail.

   Thus, on the Home Link, the Home Agent must intercept all the packets
   for ALL the Mobile Network Nodes on the registered prefixes; that is,
   for ALL nodes attached to Mobile Routers that are away from home.
   This should be a layer 2 operation, rather than layer 3.  The Home
   Agent might, for example, perform some form of ND proxying for all
   addresses in all registered Mobile Network Prefixes.

   The Home Agent must also protect the MNP space from autoconfiguration
   by uncontrolled visitors at Neighbor Discovery level.

   There is a need to provide a specific configuration on the Home Agent
   to specify that it operates in Aggregated Mode.  If a Home Agent
   implementation is simply derived from that of MIP, then the
   capability to perform the required proxying might not exist, and the
   Aggregated Mode will not operate properly for nodes on the Home Link.

6.4.2.  Mobile Router Side

   If the Mobile Router returns home by Egress, a specific support is
   required to control the bridging operation depending on whether or
   not a Mobile Router is at home.  This support might not be present in
   all implementations.



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   The NEMO Basic Support does not mention a specific behavior for
   bridging though bridging capabilities can be expected from many
   implementations.  An implementation might provide an automatic toggle
   to start/stop bridging on an Egress interface when the Mobile Router
   comes back/leaves home.  When such a toggle is unavailable, then a
   specific interface should be reserved to attach to home with the
   appropriate settings for security and bridging.

   Also, note that NEMO authorizes multiple registrations for a same MNP
   by different Mobile Routers.  This is a case of multihoming, and it
   normally means that the Mobile Routers are interconnected by the
   Ingress network that bears the common MNP.  But there is no provision
   in NEMO Basic Support to test that this condition is met at binding
   time and maintained over time.

   It is thus possible for 2 different Mobile Routers to register the
   same prefix with different Home Addresses, and this will cause an
   undetected problem if the corresponding Ingress interfaces are not
   connected.

   When the Home Address of a Mobile Router is derived from its MNP,
   there is thus an additional risk of an undetected misconfiguration if
   the Home Address is autoconfigured from the Ingress interface as
   opposed to statically assigning an address and MNP.

   A Mobile Router that is at home must own an address from the
   aggregation on its Egress interface and an address from its MNP -- a
   subnet of that aggregation -- on its Ingress interface.  A pure
   router will reject that configuration, and the Mobile Router needs to
   act as a bridge to use it.  In order to deploy the Aggregated Home
   Network model, one must check whether that support is available in
   the Mobile Routers if returning home is required.

7.  NEMO Virtual Home Network

7.1.  Configuration

   The Home Link can be configured on the Home Agent on a virtual link,
   in which case there is no physical Home Link for Mobile Routers to
   return home to, or for Home Agents to discover each other and perform
   the ND-level interactions on, as described in Mobile IPv6 [4].










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                    /48                       e.g.: A:B:C::/48
                    HA
                    | /64                         A:B:C::/64
         --+-----+--+- . -+- . -+--
           |     |        |     |
           MR1   MR2      MRi   MRN
           /64   /64      /64   /64            A:B:C:i::/64  0 < i <= N

                      Figure 5: Virtual Home Network

   The Extended Home Network and the Aggregated Home Network models can
   be adapted for virtual links.

   As in the case of a physical link, the Home Address of a Mobile
   Router can be constructed based on a dedicated subnet of the Home
   Prefix or one of the Mobile Router MNPs.

   Note that since the Home Address is never checked for DAD, it makes
   the configuration easier to take it from the MNP as opposed to a
   specific subnet.

   There are certain advantages to making the Home Link a virtual link:

      A virtual link may not experience any disruption related to
      physical maintenance or to hardware problems, so it is more
      available than a physical link.  The high availability of the Home
      Link is critical for the mobility service.

      The Home Agent does not have to defend the Mobile Router's Home
      Address through Proxy Neighbor Discovery.  The Home Agent does not
      also have to perform Duplicate Address Detection (DAD) for the
      Mobile Router's Home Address when it receives a Binding Update
      from the Mobile Router.

      The Mobile Router does not have to implement the Returning Home
      procedure (Section 11.5.4 of Mobile IPv6 [4]).

   There are also some drawbacks to the Virtual Home Link approach:

      RFC 3775 [4] and RFC 3963 [5] do not provide the specific support
      for a Mobile Node to emulate returning home on a Virtual Home
      Network.  In particular, in the case of NEMO, the routing
      information from the Mobile Router being injected on the IGP might
      adversely affect IPv6 route aggregation on the Home Network.

      There can be only one Home Agent since Mobile IPv6 relies on
      Neighbor Discovery on the Home Link for other Home Agent discovery
      and for Duplicate Address Detection.



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      The Home Agent must maintain a Binding Cache entry for a Mobile
      Router and forwarding state for its Mobile Network even when the
      Mobile Router is directly connected to it.  All traffic to and
      from the Mobile Network is sent through the bi-directional tunnel
      regardless of the Mobile Router location.  This results in a
      tunneling overhead even though the Mobile Router is connected to
      the Home Network.

   Suggestions on how to perform an equivalent of returning home on a
   Virtual Home Network have been proposed, but this topic is outside of
   the scope of this document.

7.2.  Applicability

   NEMO operations rely on ND extensions over the Home Link for the Home
   Agent to Home Agent communication.

   Making the Home Link virtual bars the deployment of multiple Home
   Agents, which may be desirable for reasons of load balancing.  Please
   refer to the NEMO multihoming issues [9] for more on this.

   Yet, for a deployment where a single Home Agent is enough, making the
   Home Link virtual reduces the vulnerability to some attacks and to
   some hardware failures, while making the Home Agent operation faster.

   Note that NEMO basic does not mandate the support of Virtual Home
   Networks.

8.  NEMO Mobile Home Network

8.1.  Configuration

   In this arrangement, there is a bitwise hierarchy of Home Networks.
   A global Home Network is advertised to the infrastructure by a head
   Home Agent(s) and further subnetted into Mobile Networks.  As a
   result, only the Home Agent(s) responsible for the most global
   (shortest prefix) aggregation receive all the packets for all the
   MNPs, which are leaves in the hierarchy tree.

   Each subnet is owned by a Mobile Router that registers it in a NEMO
   fashion while acting as a Home Agent for that network.  This Mobile
   Router is at home at the upper level of hierarchy.  This
   configuration is referred to as Mobile Home.

   An example of this is the Cab Co configuration.  Cab Co is a taxi
   company that uses a /32 prefix for its Home Network, this prefix
   being advertised by the company headquarters (HQ).  Regional offices
   are deployed around the country.  Even though these regional offices



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   are relatively stable in terms of location and prefix requirement --
   say, this changes every few years -- making them mobile allows a
   simpler management when a move has to take place, or should the ISP
   service change.

   To illustrate this configuration, we make up the prefixes to reflect
   their role, like CAB:C0::/32 for the Home Network:

         global Home Network   CAB:C0::/32  advertised by HQ
    <------------------------------------------------------------------>

      HQ Extended Home Net              Mobile Home for SFO office
          (casa)
      CAB:C0:CA5A::/48                          CAB:C0:5F0::/48
    <----------------------------> ... <------------------------------->
                                                       |
      Home for offices        HQ                       |
     CAB:C0:CA5A:CA5A::/64    MN                       |
    <----------------------><---->                     |
     CAB:C0:CA5A:CA5A::CA5A                            |
     CAB:C0:CA5A:CA5A::CA5B                            |
     are HAs on link with for each office a route like |
                                                       |
     CAB:C0:CA5A:CA5A::5F0    <---------------------- via
       is the Home addr
       of SFO office

                  Figure 6:  CAB Company HQ Configuration

   Finally, each regional office owns a number of taxis, each one
   equipped with a mobile router and an associated /64 prefix.

   For each Office, say San Francisco (SFO) as an example:


















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        Mobile Home Network CAB:C0:5F0::/48  owned by SFO office
    <------------------------------------------------------------------>

      SFO Home Network             Mobile Networks for taxis
        for taxis        <---------------------...--------------------->
     CAB:C0:5F0:5F0::/64  CAB:C0:5F0:CAB1::/64     CAB:C0:5F0:....::/64
    <-------------------><-------------------> ... <------------------->
     CAB:C0:5F0:5F0::5F0           |
     is HA on link with for        |
     each taxi a route like        |
                                   |
     CAB:C0:5F0:5F0::CAB1 <------ via
       is the Home Address
       of CAB 1

               Figure 7:  CAB Company regional configuration

   Note that this is a hierarchy in terms of MR-HA relationship, which
   may not be reflected in the physical arrangement of nodes at a given
   point of time.  For instance, in the Cab Co case, some SFO cabs might
   attach to any hot spot or Cab Co office in a different city, and the
   SFO office might be at home if it is co-located with the
   headquarters.  But note that SFO should never attach to one of its
   own cabs.  This would create a stalemate situation, as documented in
   the NEMO Route Optimization (RO) problem statement [7].

   But it is also possible to reflect the organizational hierarchy in a
   moving cloud of Mobile Routers.  If a Mobile Home Agent acts as
   root-MR for a nested configuration of its own Mobile Routers, then
   the communication between Mobile Routers is confined within the
   nested structure.

   This can be illustrated in the case of a fleet at sea.  Assume that
   SFO is a communication ship of a fleet, using a satellite link to
   join the infrastructure, and that the cabs are Mobile Routers
   installed on smaller ships, equipped with low-range radios.

   If SFO is also the root-MR of a nested structure of its own cabs, the
   communication between cabs is relayed by SFO and does not require the
   satellite link.  As for traffic to the outside of the nested NEMO,
   SFO recursively terminates the nested tunnels from its cabs and
   reencapsulates all the packets between the nested cloud and
   correspondents in the infrastructure in a single tunnel to CA5A.  As
   a result, the unwanted effect of nesting of tunnels is avoided over
   the Internet part of the packet path.






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8.2.  Applicability

   This complex topology applies to a large distributed fleet, mostly if
   there is a single interchange point with the Internet (e.g., a
   Network Address Transition (NAT) or a SOCKS [1] server farm) where
   the super Home Agent could be located.

   One specific benefit is that when 2 Mobile Routers travel together
   with a common Home Agent, the traffic between the 2 is not
   necessarily routed via the infrastructure, but can stay confined
   within the mobile cloud, the Mobile Home Agent acting as a rendezvous
   point between the Mobile Routers.  This applies particularly well for
   a fleet at sea when the long-haul access may be as expensive as a
   satellite link.

9.  Security Considerations

   This document only explains how a Home Network can be deployed to
   support Mobile Routers and does not introduce any additional security
   concerns.  Please see RFC 3963 [5] for security considerations for
   the NEMO Basic Support protocol.

10.  Acknowledgements

   The authors wish to thank Erik Nordmark, Jari Arkko, Henrik
   Levkowetz, Scott Hollenbeck, Ted Hardie, David Kessens, Pekka Savola,
   Kent Leung, Thierry Ernst, TJ Kniveton, Patrick Wetterwald, Alexandru
   Petrescu, and David Binet for their contributions.

11.  References

11.1.  Normative References

   [1]  Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and L.
        Jones, "SOCKS Protocol Version 5", RFC 1928, March 1996.

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

   [3]  Manner, J. and M. Kojo, "Mobility Related Terminology",
        RFC 3753, June 2004.

   [4]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
        IPv6", RFC 3775, June 2004.

   [5]  Devarapalli, V., Wakikawa, R., Petrescu, A., and P. Thubert,
        "Network Mobility (NEMO) Basic Support Protocol", RFC 3963,
        January 2005.



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   [6]  Ernst, T. and H. Lach, "Network Mobility Support Terminology",
        July 2007.

11.2.  Informative References

   [7]  Ng, C., Thubert, P., Watari, M., and F. Zhao, "Network Mobility
        Route Optimization Problem Statement", RFC 4888, July 2007.

   [8]  Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
        Proxies (ND Proxy)", RFC 4389, April 2006.

   [9]  Ng, C., "Analysis of Multihoming in Network Mobility Support",
        Work in Progress, February 2007.

Authors' Addresses

   Pascal Thubert
   Cisco Systems
   Village d'Entreprises Green Side
   400, Avenue de Roumanille
   Batiment T3, Biot - Sophia Antipolis  06410
   FRANCE

   Phone: +33 4 97 23 26 34
   EMail: pthubert@cisco.com


   Ryuji Wakikawa
   Keio University and WIDE
   5322 Endo Fujisawa Kanagawa
   252-8520
   JAPAN

   EMail: ryuji@sfc.wide.ad.jp


   Vijay Devarapalli
   Azaire Networks
   3121 Jay Street
   Santa Clara, CA  94054
   USA

   EMail: vijay.devarapalli@azairenet.com








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