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Versions: 00 01 02 03 04 05 06 07 rfc4980                               
NEMO Working Group                                                 C. Ng
Internet-Draft                                  Panasonic Singapore Labs
Expires: August 25, 2005                                         E. Paik
                                                                      KT
                                                                T. Ernst
                                                 WIDE at Keio University
                                                       February 21, 2005


          Analysis of Multihoming in Network Mobility Support
                 draft-ietf-nemo-multihoming-issues-02

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  By submitting this Internet-Draft, each
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   This Internet-Draft will expire on August 25, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document is an analysis of multihoming in the context of network
   mobility (NEMO).  As there are many situations in which mobile
   networks may be multihomed, a taxonomy is proposed to classify the



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   possible configurations.  We also describe possible deployment
   scenarios and we attempt to identify issues that arise when mobile
   networks are multihomed while mobility supports is taken care by NEMO
   Basic Support.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4

   2.  Classification . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1   (1,1,1): Single MR, Single HA, Single MNP  . . . . . . . .  7
     2.2   (1,1,n): Single MR, Single HA, Multiple MNPs . . . . . . .  7
     2.3   (1,n,1): Single MR, Multiple HAs, Single MNP . . . . . . .  8
     2.4   (1,n,n): Single MR, Multiple HAs, Multiple MNPs  . . . . .  8
     2.5   (n,1,1): Multiple MRs, Single HA, Single MNP . . . . . . .  9
     2.6   (n,1,n): Multiple MRs, Single HA, Multiple MNPs  . . . . .  9
     2.7   (n,n,1): Multiple MRs, Multiple HAs, Single MNP  . . . . . 10
     2.8   (n,n,n): Multiple MRs, Multiple HAs, Multiple MNPs . . . . 10

   3.  Deployment Scenarios and Prerequisites . . . . . . . . . . . . 12
     3.1   Deployment Scenarios . . . . . . . . . . . . . . . . . . . 12
     3.2   Prerequisites  . . . . . . . . . . . . . . . . . . . . . . 13

   4.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . . 15
     4.1   Path Survival  . . . . . . . . . . . . . . . . . . . . . . 15
     4.2   Path Selection . . . . . . . . . . . . . . . . . . . . . . 15
     4.3   Ingress Filtering  . . . . . . . . . . . . . . . . . . . . 16
     4.4   Failure Detection  . . . . . . . . . . . . . . . . . . . . 18
     4.5   Media Detection  . . . . . . . . . . . . . . . . . . . . . 19
     4.6   HA Synchronization . . . . . . . . . . . . . . . . . . . . 20
     4.7   MR Synchronization . . . . . . . . . . . . . . . . . . . . 20
     4.8   Prefix Delegation  . . . . . . . . . . . . . . . . . . . . 21
     4.9   Multiple Bindings/Registrations  . . . . . . . . . . . . . 21
     4.10  Source Address Selection . . . . . . . . . . . . . . . . . 21
     4.11  Impact on the Routing Infrastructure . . . . . . . . . . . 22
     4.12  Nested Mobile Networks . . . . . . . . . . . . . . . . . . 22
     4.13  Split Mobile Networks  . . . . . . . . . . . . . . . . . . 22

   5.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 23

   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 24

   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 24

       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 26

   A.  Alternative Classifications Approach . . . . . . . . . . . . . 27
     A.1   Ownership-Oriented Approach  . . . . . . . . . . . . . . . 27



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       A.1.1   ISP Model  . . . . . . . . . . . . . . . . . . . . . . 27
       A.1.2   Subscriber/Provider Model  . . . . . . . . . . . . . . 28
     A.2   Problem-Oriented Approach  . . . . . . . . . . . . . . . . 30

   B.  Nested Tunneling for Fault Tolerance . . . . . . . . . . . . . 31
     B.1   Detecting Presence of Alternate Routes . . . . . . . . . . 31
     B.2   Re-Establishment of Bi-Directional Tunnels . . . . . . . . 31
       B.2.1   Using Alternate Egress Interface . . . . . . . . . . . 32
       B.2.2   Using Alternate Mobile Router  . . . . . . . . . . . . 32
     B.3   To Avoid Tunneling Loop  . . . . . . . . . . . . . . . . . 33

   C.  Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 34

       Intellectual Property and Copyright Statements . . . . . . . . 36





































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1.  Introduction

   The design goals and objectives of Network Mobility Support (NEMO)
   are identified in [1] while the terminology is being described in [2]
   and [3].  NEMO Basic Support [4] is the solution proposed by the NEMO
   Working Group to provide continuous Internet connectivity to nodes
   located in a mobile network.  This solutions basically solves the
   problem by setting up bi-directional tunnels between the mobile
   routers (MRs) connecting the mobile network to the Internet and their
   respective Home Agents (HAs), much how this is done in Mobile IPv6
   [5], the solution for host mobility.  NEMO Basic Support is
   transparent to nodes located behind the mobile router (i.e.  the
   mobile network nodes, or MNNs) and as such doesn't require MNNs to
   take any action in the mobility management.

   However, mobile networks are typically connected by means of wireless
   and thus less reliable links; there could also be many nodes behind
   the MR.  A loss of connectivity or a failure to connect to the
   Internet has thus a more significant impact than for a single node.
   Real-life scenarios as illustrated in [6] demonstrate that providing
   a permanent access to mobile networks such as vehicles typically
   require the use of several interfaces and technologies since the
   mobile network may be moving in distant geographical locations where
   different access technologies are provided and governed by distinct
   access control policies.

   As specified by R.12 in section 5 of [1], the NEMO WG must ensure
   that NEMO Basic Support does not prevent mobile networks to be
   multihomed, i.e.  when there is more than one point of attachment
   between the mobile network and the Internet (see definitions in
   [3])..  This arises either when a MR has multiple egress interfaces.
   Using NEMO Basic Support, this translates into having multiple
   bi-directional tunnels between the mobile network and its HA(s), and
   may result into multiple MNPs being advertised to the MNNs.  However,
   NEMO Basic Support does not specify any particular mechanism to
   manage multiple bi-directional tunnels.  The objective of this memo
   is thus three-folds:

   o  to identify which multihoming configurations are useful,

   o  to identify issues that may prevent useful multihomed
      configurations to be supported under the operation of NEMO Basic
      Support.  It doesn't mean that those not supported will not work
      with NEMO Basic Support, just that it is up to the implementors to
      make it work (hopefully issues discussed in this memo will be
      helpful to these implementors).

   For doing so, a taxonomy to classify the possible multihomed



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   configurations is described in Section 2.  Deployment scenarios,
   their benefits, and requirements to meet these benefits are
   illustrated in Section 3.  Following this, we study the general
   issues in Section 4, and we conclude with an evaluation of NEMO Basic
   Support for multihomed configurations.  Alternative classifications
   are outlined in the Appendix.

   In order to understand this memo, the reader is expected to be
   familiar with the above cited documents, i.e.  with the NEMO
   terminology as defined in [2] (section 3) and [3], Goals and Benefits
   of Multihoming [6], Goals and Requirements of Network Mobility
   Support [1], and the NEMO Basic Support specification [4].  Goals and
   benefits for multihoming as discussed in [6] are applicable to fixed
   nodes, mobile nodes, fixed networks and mobile networks.

   This document considers multihoming only from the IPv6 point of view.



































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2.  Classification

   Various discussions on the topic of multihoming issues in NEMO have
   been carried out on the mailing list and at IETF meetings.  As there
   are several configurations in which mobile networks are multihomed,
   there is a need to classify them into a clearly defined taxonomy.
   This can be done in various ways.  Three approaches have been
   proposed on the NEMO mailing list.  These are, namely, (i) the
   Configuration-Oriented Approach, (ii) the Ownership-Oriented
   Approach, and (iii) the Problem-Oriented Approach.  As the WG
   consensus seems to have converged to the Configuration-Oriented
   Approach, we only describe this approach here.  The other two
   approaches are outlined in Appendix A.1 and Appendix A.2.

   Multihomed configurations can be classified depending on how many
   mobile routers are present, how many egress interfaces, Care-of
   Address (CoA) and Home Addresses (HoA) the mobile routers have, how
   many prefixes (MNPs) are advertised to the mobile network nodes, etc.
   For doing so, we use three key parameters differentiating different
   multihomed configurations.  With these parameters, we can refer to
   each configuration by the 3-tuple (x,y,z), where 'x', 'y', 'z' are
   defined as follows:

   o  'x' indicates the number of MRs where:

      x=1 implies a mobile network has only a single MR, presumably
         multihomed.

      x=N implies a mobile network has more than one MR.

   o  'y' indicates the number of HAs associated with the entire mobile
      network, where:

      y=1 implies that a single HA is assigned to the mobile network.

      y=N implies that multiple HAs (possibly in different
         administrative domains) are assigned to the mobile network.

   o  'z' indicates the number of MNPs announced to MNNs, where:

      z=1 implies that a single MNP is advertised to the MNNs.

      z=N implies that multiple MNPs are advertised to the MNNs.

   It can be seen that the above three parameters are fairly orthogonal
   to one another.  Thus different values of 'x', 'y' and 'z' give rise
   to different combinations of the 3-tuple (x,y,z).  As described in
   the sub-sections below, a total of 8 possible configurations can be



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   identified.

   One thing the reader has to keep in mind is that in each of the
   following 8 cases, the MR may be multihomed if either (i) multiple
   prefixes are advertised (on the home link, or on the visited link),
   or (ii) the MR is equipped with multiple interfaces.  In such a case,
   the MR would have a combination of Home Address / Care-of Address
   pairs.  Issues pertaining to a multihomed MR are also addressed in
   the companion document [7].

   A simplified analysis of multihoming configuration in NEMO Basic
   Support using the same classification can be found in [8].


2.1  (1,1,1): Single MR, Single HA, Single MNP

   The (1,1,1) mobile network has only one MR advertising a single MNP.
   In addition, the MR is associated with only one HA at any one time.
   A bi-directional tunnel may be established between each pair of Home
   Address / Care-of address if the MR is itself multihomed.

   The MR may be multihomed and MNNs are (usually) not multihomed since
   they would configure a single address on the single MNP announced on
   the link they are attached to.

                                   _____
                   _    p      _  |     |
                  |_|-|<-_  |-|_|-|     |-|        _
                   _  |-|_|=|     |_____| |  _  |-|_|
                  |_|-|     |             |-|_|-|
                                                |
                  MNNs   MR   AR  Internet   AR    HA

                  Figure 1: (1,1,1): 1 MR, 1 HA, 1 MNP


2.2  (1,1,n): Single MR, Single HA, Multiple MNPs

   The (1,1,n) mobile network has only one MR, which is associated to
   only one HA at any one time.  However, two or more MNPs are
   advertised to the mobile network nodes.

   The MR may be itself multihomed, and MNNs are multihomed if several
   MNPs are advertised on the link they are attached to.  If that
   conditions holds, MNNs would configure an address with each MNP
   announced on the link.





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                                   _____
                   _   p1,p2   _  |     |
                  |_|-|<-_  |-|_|-|     |-|        _
                   _  |-|_|=|     |_____| |  _  |-|_|
                  |_|-|     |             |-|_|-|
                                                |
                  MNNs   MR   AR  Internet   AR    HA

              Figure 2: (1,1,n): 1 MR, 1 HA, multiple MNPs


2.3  (1,n,1): Single MR, Multiple HAs, Single MNP

   The (1,n,1) mobile network has only one MR advertising a single MNP.
   The MR, however, is associated to multiple HAs, possibly one HA per
   Home Address, or one HA per egress interface.

   The MR may be multihomed whereas MNNs are (usually) not multihomed
   since they would configure a single address on the single MNP
   announced on the link they are attached to.

                                          AR    HA2
                                           _  |
                                        |-|_|-|  _
                                 _____  |     |-|_|
                 _    p      _  |     |-|
                |_|-|<-_  |-|_|-|     |
                 _  |-|_|=|     |_____|-|        _
                |_|-|     |             |  _  |-|_|
                                        |-|_|-|
                                              |
                MNNs  MR    AR  Internet  AR    HA1

              Figure 3: (1,n,1): 1 MR, multiple HAs, 1 MNP


2.4  (1,n,n): Single MR, Multiple HAs, Multiple MNPs

   The (1,n,n) mobile network has only one MR.  However, the MR is
   associated with multiple HAs, possibly one per Home Address (or one
   HA per egress interface), and the MR advertises more than one MNP on
   its ingress interfaces.  No assumption is made on whether or not the
   HAs belongs to the same administrative domain.

   The MR may be multihomed, and the MNNs are generally multihomed since
   they would configure an address on each MNP announced on the link
   they are attached to.




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                                         AR    HA2
                                          _  |  _
                                _____  |-|_|-|-|_|
                _   p1,p2   _  |     |-|     |
               |_|-|<-_  |-|_|-|     |          _
                _  |-|_|=|     |_____|-|  _  |-|_|
               |_|-|     |             |-|_|-|
                                       |     |
               MNNs  MR    AR  Internet  AR    HA1

          Figure 4: (1,n,n): 1 MR, multiple HAs, multiple MNPs


2.5  (n,1,1): Multiple MRs, Single HA, Single MNP

   The (n,1,1) mobile network has more than one MR advertising global
   routes.  These MRs, however, advertise the same MNP and are
   associated with the same HA.

   Each MR may be itself multihomed, whereas MNNs are (usually) not
   multihomed since they would configure a single address on the single
   MNP announced on the link they are attached to.

                        MR2
                      p<-_  |
                   _  |-|_|-|  _____
                  |_|-|     |-|     |
                   _  |       |     |-|        _
                  |_|-|  _  |-|_____| |  _  |-|_|
                      |-|_|-|         |-|_|-|
                      p<-   |               |
                  MNNs  MR1   Internet   AR    HA

              Figure 5: (n,1,1): Multiple MRs, 1 HA, 1 MNP


2.6  (n,1,n): Multiple MRs, Single HA, Multiple MNPs

   The (n,1,n) mobile network has more than one MR; multiple global
   routes and different MNPs are advertised by the MRs.

   Each MR may be itself multihomed, and MNNs are generally multihomed
   since they would configure an address on each MNP announced on the
   link they are attached to.







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                        MR2
                     p2<-_  |
                   _  |-|_|-|  _____
                  |_|-|     |-|     |
                   _  |       |     |-|        _
                  |_|-|  _  |-|_____| |  _  |-|_|
                      |-|_|-|         |-|_|-|
                     p1<-   |               |
                  MNNs  MR1   Internet   AR    HA

          Figure 6: (n,1,n): Multiple MRs, 1 HA, multiple MNPs


2.7  (n,n,1): Multiple MRs, Multiple HAs, Single MNP

   The (n,n,1) mobile network has more than one MR advertising multiple
   global routes.  The mobile network is associated with multiple HAs at
   any one time.  No assumptions are made on whether or not the HAs
   belongs to the same administrative domain.  However, the MRs
   advertises the same MNP.

   Each MR may be itself multihomed whereas MNNs are (usually) not
   multihomed since they would configure a single address on the single
   MNP announced on the link they are attached to.

                        MR2             AR    HA2
                        p                _  |
                       <-_  |         |-|_|-|  _
                   _  |-|_|-|  _____  |     |-|_|
                  |_|-|     |-|     |-|
                   _  |       |     |
                  |_|-|  _  |-|_____|-|        _
                      |-|_|-|         |  _  |-|_|
                       <-   |         |-|_|-|
                        p                   |
                  MNNs  MR1   Internet  AR    HA1

          Figure 7: (n,n,1): Multiple MRs, Multiple HAs, 1 MNP


2.8  (n,n,n): Multiple MRs, Multiple HAs, Multiple MNPs

   The (n,n,n) mobile network has multiple MRs advertising different
   global routes and different MNPs.  The mobile network is associated
   with more than one HA at any one time.  No assumptions is made on
   whether or not the HA belongs to the same administrative domain.

   Each MR may be itself multihomed and MNNs are generally multihomed



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   since they would configure an address on each MNP announced on the
   link they are attached to

                        MR2             AR    HA2
                        p2               _  |
                       <-_  |         |-|_|-|  _
                   _  |-|_|-|  _____  |     |-|_|
                  |_|-|     |-|     |-|
                   _  |       |     |
                  |_|-|  _  |-|_____|-|        _
                      |-|_|-|         |  _  |-|_|
                       <-   |         |-|_|-|
                        p1                  |
                  MNNs  MR1   Internet  AR    HA1

             Figure 8: (n,n,n): Multiple MRs, HAs, and MNPs



































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3.  Deployment Scenarios and Prerequisites

   The following generic goals and benefits of multihoming are discussed
   in a companion document [6]:

   1.  Permanent and Ubiquitous Access

   2.  Redundancy/Fault-Recovery

   3.  Load Sharing

   4.  Load Balancing

   5.  Preference Settings

   These benefits are now illustrated from a NEMO perspective with a
   typical instance scenario for each case in the taxonomy.  We then
   discuss the prerequisites to fulfill these.

3.1  Deployment Scenarios

   x=1: Multihomed mobile network with a single MR

      o  Example: an MR with dual/multiple access interfaces (e.g.
         802.11 and GPRS capabilities).  This is a S/P-(1,1,*) if both
         accesses are subscribed to the same ISP.  If the two accesses
         are offered by independent ISPs, this is a S/mP-(1,n,n) [for
         the meaning of this abbreviation, see Appendix A.1].

         Benefits: Ubiquity, Redundancy/Fault-Recovery, Load Sharing,
         Preference Settings


   x=N:  Multihomed mobile networks with multiple MRs

      o  Example 1: a train with one MR in each car, all served by the
         same HA, thus a (n,1,*).  Alternatively, the train company
         might be forced to use different ISPs when the train go to
         different locations, thus it is a (n,n,n).

         Benefits: Load Sharing, Redundancy/Fault-Recovery, Ubiquity

      o  Example 2: W-PAN with a GPRS_enabled phone and a WiFi-enabled
         PDA.  This is a S/mP-(n,n,n) if the two access technologies are
         subscribed separately.

         Benefits: Ubiquity, Redundancy/Fault-Recovery, Preference
         Settings



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   y=1:  Multihomed mobile networks with a single HA

      o  Most single ISP cases in above examples.


   y=N:  Multihomed mobile networks with multiple HAs

      o  Most multiple ISP cases in above examples.

      o  Example: a transatlantic flight with a HA in each continent.
         This is a (1,n,1) network if there is only one MR.

         Benefits: Ubiquity, Preferences (reduced delay, shortest path)


   z=1:  Multihomed mobile networks with a single MNP

      o  Most single ISP cases in above examples.


   z=N:  Multihomed mobile networks with multiple MNPs

      o  Most multiple ISP cases in above examples.

      o  Example: a car with a prefix taken from home (personal traffic
         transit on this prefix and is paid by the owner) and one that
         belongs to the car manufacturer (maintenance traffic is paid by
         the car-manufacturer).  This will typically be a (1,1,n) or a
         (1,n,n,).

         Benefits: Preference Settings


3.2  Prerequisites

   In this section, we try to define the requirements in order to
   fulfill the expected multihoming benefits as detailed in [6].

   At least one bi-directional tunnel must be available at any point in
   time between the mobile network and the fixed network to meet all
   expectations.  But for most goals to be effective, multiple tunnels
   must be maintained simultaneously:

   o  Permanent and Ubiquitous Access:

      At least one bi-directional tunnel must be available at any point
      in time.



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   o  Redundancy and Fault-Recovery:

      Both the inbound and outbound traffic must be transmitted or
      diverted over another bi-directional tunnel once a bi-directional
      tunnel is broken or disrupted.

   o  Load Sharing and Load Balancing:

      Multiple tunnels must be maintained simultaneously.

   o  Preference Settings:

      Implicitly, multiple tunnels must be maintained simultaneously if
      preferences are set for deciding which of the available
      bi-directional tunnels should be used.  A mechanism must be
      provided to the user/application about the availability of
      multiple bi-direction tunnels, and perhaps also to set the
      preference.  The preference may reside in the mobile router or
      mobile network nodes (using [9] for instance).
































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4.  Problem Statement

   In order to meet the multihoming benefits, multiple tunnels may be
   maintained simultaneously (e.g.  load balancing, load sharing) or not
   (e.g.  redundancy) between the mobile network and the fixed network.
   In some cases, it may be necessary to divert packets from a (perhaps
   failed) bi-directional tunnel to an alternative (perhaps newly
   established) bi-directional tunnel (i.e.  for matters of fault
   recovery, preferences), or to split traffic between multiple tunnel
   (load sharing, load balancing).

   For doing so, the issues discussed below must be addressed,
   preferably dynamically.  For each issue, we also investigate
   potential ways to solve the problem and recommend which IETF WG(s)
   should look into these issues.

4.1  Path Survival

   Internet connectivity is guaranteed for all MNNs as long as at least
   one bi-directional tunnel is maintained between the mobile network
   and the fixed Internet.  When an alternative tunnel must be found to
   substitute for the failed one, the loss of one tunnel to the Internet
   may result in broken sessions.  In this case, new transport sessions
   will have to be established over the alternate tunnel if no mechanism
   is provided to make this change transparent at layers above layer 3.

   The tunnel can be changed transparently to the MNNs if mechanisms
   such as MMI [10] are brought to the MR.

   For instance, in the (1,1,1) case specifically, packets are always
   transmitted to/from the same MR's ingress interface, i.e.
   independently of MR's links connectivity status.

   This is a general problem faced by any node with multiple egress
   paths.  It is recommended that this issue be considered by other WGs
   (such as IPv6, Multi6), and NEMO WG to contribute any NEMO specific
   requirements.

4.2  Path Selection

   When multiple bi-directional tunnels are available and possibly used
   simultaneously, the mode of operation may be either primary-secondary
   (one tunnel is precedent over the others and used as the default
   tunnel, while the other serves as a back-up) or peer-to-peer (no
   tunnel has precedence over one another, they are used with the same
   priority).  This questions which bi-directional tunnel would be
   selected, and based on which parameter (e.g.  type of flow that goes
   into/out of the mobile network).



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   A dynamic path selection mechanism is thus needed so that this path
   could be selected by:

   o  The HA: it should be able to select the path based on some
      information recorded in the binding cache.

   o  The MR: it should be able to select the path based on router
      advertisements received on both its egress interfaces or on its
      ingress interfaces for the (n,*,*) case.

   o  The MNN: it should be able to select the path based on "Default
      Router Selection" (see [Section 6.3.6. Default Router Selection]
      [11]) in the (n,*,*) case or based on "Source Address Selection"
      in the (*,*,n) cases (see Section 4.10 of the present memo).

   o  The user or the application: e.g.  in case where a user wants to
      select a particular access technology among the available
      technologies for reasons of cost or data rate.

   o  A combination of any of the above: a hybrid mechanism should be
      also available, e.g.  one in which the HA, the MR, and/or the MNNs
      are coordinated to select the path.

   This is a general problem faced by any node with multiple egress
   paths.  It is recommended that this issue be considered by other WGs
   (such as IPv6, Multi6), and NEMO WG to contribute any NEMO specific
   requirements.

4.3  Ingress Filtering

   Ingress filtering mechanisms may drop the outgoing packets when
   multiple bi-directional tunnels end up at different HAs.  This could
   particularly occur if different MNPs are handled by different home
   agents.  If packet with a source address configured from a specific
   MNP is tunnelled to a home agent that does not handle that specific
   MNP the packet may be discarded due to ingress filtering (either by
   the home agent or by a border gateway in the home network).

   As an example of how this could happen, consider the deployment
   scenario illustrated in Figure 9.  In Figure 9, the mobile network
   has two mobile routers MR1 and MR2, with home agents HA1 and HA2
   respectively.  Two bi-directional tunnels are established are
   established between the two pairs.  Each mobile router advertises a
   different MNP (P1 and P2).  MNP P1 is registered to HA1, and MNP P2
   is registered to HA2.  Thus, MNNs should be free to auto-configure
   their addresses on any of P1 or P2.  Ingress filtering could thus
   happen in two cases:




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   o  If the two tunnels are available, MNN cannot forward packet with
      source address equals P1.MNN to MR2.  This would cause ingress
      filtering at HA2 to occur (or even at MR2).  This is contrary to
      normal Neighbor Discovery [11] practice that an IPv6 node is free
      to choose any router as its default router regardless of the
      prefix it chooses to use.  A simple solution is to require all
      MNNs to set their default router to the MR that advertises the MNP
      the MNNs configured their addresses from.  If such requirement is
      not placed on mobile network nodes, then a multihoming solution
      for mobile networks must address this problem.  For a possible
      approach, see [12].  However, this is not enough to maintain
      connectivity if a tunnel fails (see Section 4.1 for a discussion
      of this issue).

   o  If the tunnel to HA1 is broken, packets would be sent through the
      tunnel to HA1 are diverted through the tunnel to HA2.  If HA2 (or
      some border gateway in the domain of HA2) performs ingress
      filtering, packets with source address configured from MNP P1 may
      be discarded.  It should be noted that this problem may be faced
      by any (*,n,n) mobile network, even if MR1 and MR2 are in fact the
      same entity in Figure 9.

   To avoid ingress filtering mechanisms dropping packets in such
   situations, MR(s) can stop advertising P1.  This would prevent MNNs
   from using the address auto-configured on this prefix.  However, such
   a method suffers from the following two limitations:

   o  Switching addresses is time consuming since nodes have to wait for
      addresses to get deprecated [9].

   o  Switching addresses force transport sessions without multihoming
      capabilities (such as TCP) to terminate, and be re-established
      using the alternative source address.  Transport sessions with
      multihoming capabilities (such as SCTP) may be able to continue
      without disruption (see also Section 4.1)

   Although one way to avoid the long wait for address deprecation by
   sending a router advertisement with zero Lifetime, the
   termination/disruption of transport sessions may render this solution
   unattractive.  It is possible to overcome these limitations by using
   nested tunnels.  Appendix B describes one such approach.










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               Prefix: P1 +-----+  +----+  +----------+   +-----+
                       +--| MR1 |--| AR |--|          |---| HA1 |
                       |  +-----+  +----+  |          |   +-----+
       IP:    +-----+  |                   |          | Prefix: P1
    P1.MNN or | MNN |--+                   | Internet |
      P2.MNN  +-----+  |                   |          | Prefix: P2
                       |  +-----+  +----+  |          |   +-----+
                       +--| MR2 |--| AR |--|          |---| HA2 |
               Prefix: P2 +-----+  +----+  +----------+   +-----+

                  Figure 9: An (n,n,n) mobile network

   It is recommended that the NEMO specific issue of ingress filtering
   be tackled by the NEMO WG, either through the standardization of the
   technique described in Appendix B, or by specifying a statement to
   define the mobile router behavior with respect to fault recovery in
   an (*,n,n) mobile network.











4.4  Failure Detection

   It is expected for faults to occur more readily at the edge of the
   network (i.e.  the mobile nodes), due to the use of wireless
   connections.  Efficient fault detection mechanisms are necessary to
   recover in timely fashion.  In order for fault recovery to work, the
   MRs and HAs must first possess a means to detect failures:

   o  On the MR's side, the MR can also rely on router advertisements
      from access routers, or other layer-2 trigger mechanisms to detect
      faults, e.g.  [13] or [14].  (For a related issue, see
      Section 4.5.)

   o  On the HA's side, it is more difficult for HAs to detect tunnel
      failures.  For an ISP deployment model, the HAs and MRs can use
      proprietary methods (such as constant transmission of heartbeat
      signals) to detect failures and check tunnel liveness.  In the S/P
      model (see Appendix A.2), a lack of standardized "tunnel liveness"
      protocol means that it is harder to detect failures.




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      A possible method is for the MRs to send binding updates more
      regularly with shorter Lifetime values.  Similarly, HAs can return
      binding acknowledgment messages with smaller Lifetime values, thus
      forcing the MRs to send binding updates more frequently.  These
      binding updates can be used to emulate "tunnel heartbeats".  This
      however may lead to more traffic and processing overhead, since
      binding updates sent to HAs must be protected (and possibly
      encrypted) with security associations.

   There are other failure modes to consider as well, such as failure at
   home agent, failure at access routers, or in general failure of a
   link or node along the path from the mobile router to the home agent.
   By the nature of the routing infrastructure, failure of intermediate
   nodes or links are recovered by the the routing infrastructure by
   choosing a different route.  For those failures that can't be
   receovered (such a failure of the access router), a heartbeat
   protocol or the use of small-lifetime binding updates described above
   can also be used to detect tunnel failures.

   This is a general problem faced by all nodes communicating with a
   peer.  It is recommended that NEMO WG adopts any failure detection
   mechansim standardized in the IETF, and, should the need arise,adapts
   such mechanism specifically for detecting tunnel failures.











4.5  Media Detection

   In order to achieve benefits such as ubiquity or fault recovery, it
   is necessary for mobile router to detect the availability of network
   media.  This may be achieved using layer 2 triggers [13], or other
   mechanism developed/recommended by the Detecting Network Attachment
   (DNA) Working Group.

   This is related to Section 4.4, since the ability to detect media
   availability would often implies the ability to detect media
   in-availability.






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4.6  HA Synchronization

   In the (*,n,1) mobile networks, a single MNP would be registered at
   different HAs.  This gives rise to the following issues:

   o  Only one HA may actively advertise a route to the MNP.

   o  Multiple HAs at different domains may advertise a route to the
      same MNP

   This may pose a problem in the routing infrastructure as a whole.
   The implications of this aspect needs further exploration.  Certain
   level of HA co-ordination may be required.  A possible approach is to
   adopt a HA synchronization mechanism such as that described in [15]
   and [16].  Such synchronization might also be necessary in a (*,n,*)
   mobile network, when a MR sends binding update messages to only one
   HA (instead of all HAs).  In such cases, the binding update
   information might have to be synchronized betweeen HAs.  The mode of
   synchoronization may be either primary-secondary or peer-to-peer.
   See also Section 4.7.

   This problem is general in the sense that Mobile IPv6 will have to
   consider similar issues.  It is recommended that the issue be looked
   at in one of the Mobile IP WG, and NEMO WG to contribute any NEMO
   specific requirements.

4.7  MR Synchronization

   In the (n,*,*) mobile network, different MRs may need to be
   synchronized in order to take common decisions.  The mode of
   synchoronization may be either primary-secondary or peer-to-peer.
   This may include:

   o  In the (n,*,1) case, advertising the same MNP (see also "prefix
      delegation" in Section 4.8).

   o  In the (n,*,n) case, a MR relaying the advertisement of the MNP
      from another failed MR.

   o  In the (n,*,*) cases, relaying between MRs everything that needs
      to be relayed, such as data packets, creating a tunnel from the
      ingress interface, etc.

   This problem is general in the sense that a multi-router site would
   require the same level of router synchronization as well.  It is
   recommended that the issue be looked at in IPv6 or Multi6 WG, and
   NEMO WG to contribute any NEMO specific requirements.




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4.8  Prefix Delegation

   In the (*,*,1) mobile network, the same MNP must be advertised to the
   MNNs through different paths.  This questions how to perform prefix
   delegation:

   o  For the (*,n,1) mobile network, how multiple HAs would delegate
      the same MNP to the mobile network.  For doing so, the HAs may be
      somehow configured to advertise the same MNP.  (see also "HA
      Synchronization" in Section 4.6).

   o  For the (n,*,n) mobile network, how multiple mobile routers would
      be synchronized to advertise the same MNP down the NEMO-link.  For
      doing so, the MRs may be somehow configured to advertise the same
      MNP (see also "MR Synchronization" in Section 4.7).

   This could be configured manually, or dynamically.  Alternatively,
   prefix delegation mechanisms [17][18] could be used to ensure all
   routers advertise the same MNP.

   Prefix delegation is currently being explored in the NEMO WG.  Should
   the WG decides to standardize a prefix delegation mechanism, the WG
   should also consider its application to a multihomed mobile network.

4.9  Multiple Bindings/Registrations

   When a MR is configured with multiple Care-of Addresses, it is often
   necessary for it to bind these Care-of Addresses to the same MNP.

   This is a generic issue, since Mobile IPv6 nodes face a similar
   problem if they wish to bind multiple Care-of Addresses to the same
   Home Address".  This is better discussed in [7].  It is sufficient to
   note that solutions like [19] can solve this.

4.10  Source Address Selection

   In the (*,*,n) mobile networks, MNNs would be configured with
   multiple addresses.  Source address selection mechanisms are needed
   to decide which address to choose from.

   It may be desirable for MNN to be able to acquire "preference"
   information on each MNP from the MRs.  This allows default address
   selection mechanism such as that specified in [9] to be used.
   Further exploration on setting such "preference" information in
   Router Advertisement based on performance of the bi-directional
   tunnel might prove to be useful.

   This is a general issue faced by any node when offered multiple



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   prefixes.  It is recommended that the issue be examined by other WG
   (such as IPv6).

4.11  Impact on the Routing Infrastructure

   In the (1,n,1) case with HAs located in distinct ISPs/ASs, multiple
   routes directed to the mobile network may be advertised in the
   Internet.  Although this may provide shorter paths, it would add a
   burden to routing tables as multiple routes to the same prefix are
   injected into the routing infrastructure.

   Such issues are investigated in the MULTI6 working group at the IETF,
   and the NEMO WG should adopt solutions designed there whenever
   appropriate.

4.12  Nested Mobile Networks

   When a multihomed mobile network is nested within another mobile
   network, it can result in very complex topologies.  For instance, a
   nested mobile network may be attached two different root-MRs, thus
   the aggregated network no longer forms a simple tree structure.  As
   such, a solution to prevent an infinite loop of multiple mobile
   routers must be provided.

   This problem is specific to NEMO WG.  It is recommended that the NEMO
   WG standardizes a solution to solve this problem (such as the use of
   a tree-spanning algorithm).

4.13  Split Mobile Networks

   When a (n,*,1) network splits, (i.e.  the two MRs split themselves
   up), the only available MNP will then be registered by two different
   MRs on different links.  This cannot be allowed, as the HA has no way
   to know which node with an address configured from that MNP is
   attached to which MR.  Some mechanism must be present for the MNP to
   either be forcibly removed from one (or all) MRs, or the implementors
   must not allow a (n,*,1) network to split.

   A possible approach to solving this problem is described in [20].

   This problem is specific to NEMO WG.  It is recommended that the NEMO
   WG standardizes a solution to solve this problem, or specifies that
   the split of a (n,*,1) network cannot be allowed without a router
   renumbering.







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5.  Conclusion

   This document is an analysis of multihoming in the context of network
   mobility.  The purpose of this memo is to investigate issues related
   to such a bi-directional tunneling mechanism when mobile networks are
   multihomed.

   Several issues that might impact the deployment of NEMO with
   multihoming capabilities were identified in Section 4. They include
   :

   1.  Path Survival

   2.  Path Availability

   3.  Ingress Filtering

   4.  Failure Detection

   5.  Media Detection

   6.  HA Synchronization

   7.  MR Synchronization

   8.  Prefix Delegation

   9.  Multiple Binding/Registrations

   10.  Source Address Selection

   11.  Imapct on the Routing Infrastructure

   12.  Nested Mobile Networks

   13.  Split Mobile Networks.

   This study is a work in progress and need to be improved by a
   thorough study of each individual issues.  Particularly, this memo
   should be completed by a thorough threat analysis of multihoming
   configurations of mobile network.  We will add security threat issues
   here as and when they are encountered, such as those described in
   [21].  We also encourage interested people to contribute to this
   part.







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6.  Acknowledgments

   The authors would like to thank people who have given valuable
   comments on various multihoming issues on the mailing list, and also
   those who have suggested directions in the 56th - 61st IETF Meetings.
   Sincere gratitude is also extended to Marcelo Bagnulo Braun for his
   extensive review and comments on the -00 version of this draft.  The
   initial evaluation of NEMO Basic Support is a contribution from
   Julien Charbon.


7.  References

   [1]   Ernst, T., "Network Mobility Support Goals and Requirements",
         Internet-Draft draft-ietf-nemo-requirements-04, February 2005.

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

   [3]   Ernst, T. and H. Lach, "Network Mobility Support Terminology",
         Internet-Draft draft-ietf-nemo-terminology-03, February 2005.

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

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

   [6]   Ernst, T., Montavont, N. and R. Wakikawa, "Goals and Benefits
         of Multihoming",
         Internet-Draft draft-ernst-generic-goals-and-benefits-01,
         February 2005.

   [7]   Montavont, N., Wakikawa, R. and T. Ernst, "Analysis of
         Multihoming in Mobile IPv6",
         Internet-Draft draft-montavont-mobileip-multihoming-pb-statement-03
         , January 2005.

   [8]   Ernst, T. and J. Charbon, "Multihoming with NEMO Basic
         Support", Proceedings First International Conference on Mobile
         Computing and Ubiquitous Networking (ICMU), January 2004.

   [9]   Draves, R., "Default Address Selection for Internet Protocol
         version 6 (IPv6)", RFC 3484, February 2003.

   [10]  Montavont, N., Noel, T. and M. Kassi-Lahlou, "MIPv6 for
         Multiple Interfaces",



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         Internet-Draft draft-montavont-mobileip-mmi-00, July 2002.

   [11]  Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
         for IP Version 6 (IPv6)", RFC 2461, December 1998.

   [12]  Draves, R. and D. Thaler, "Default Router Preferences and
         More-Specific Routes",
         Internet-Draft draft-ietf-ipv6-router-selection-06, October
         2004.

   [13]  Yegin, A., "Link-layer Hints for Detecting Network
         Attachments", Internet-Draft draft-yegin-dna-l2-hints-01,
         February 2004.

   [14]  Yegin, A., "Supporting Optimized Handover for IP Mobility
         -Requirements for Underlying  Systems",
         Internet-Draft draft-manyfolks-l2-mobilereq-02, July 2002.

   [15]  Wakikawa, R., Devarapalli, V. and P. Thubert, "Inter Home
         Agents Protocol (HAHA)",
         Internet-Draft draft-wakikawa-mip6-nemo-haha-01, February 2004.

   [16]  Koh, B., Ng, C. and J. Hirano, "Dynamic Inter Home Agent
         Protocol", Internet-Draft draft-koh-mip6-nemo-dhap-00, July
         2004.

   [17]  Miyakawa, S. and R. Droms, "Requirements for IPv6 Prefix
         Delegation", RFC 3769, June 2004.

   [18]  Droms, R. and P. Thubert, "DHCPv6 Prefix Delegation for NEMO",
         Internet-Draft draft-droms-nemo-dhcpv6-pd-01, February 2004.

   [19]  Wakikawa, R., Uehara, K., Ernst, T. and K. Nagami, "Multiple
         Care-of Addresses Registration",
         Internet-Draft draft-wakikawa-mobileip-multiplecoa-04, February
         2005.

   [20]  Kumazawa, M., "Token based Duplicate Network Detection for
         split mobile network (Token  based DND)",
         Internet-Draft draft-kumazawa-nemo-tbdnd-01, October 2004.

   [21]  Choi, S., "Threat for Multi-homed Mobile Networks",
         Internet-Draft draft-cho-nemo-threat-multihoming-00, February
         2004.







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Authors' Addresses

   Chan-Wah Ng
   Panasonic Singapore Laboratories Pte Ltd
   Blk 1022 Tai Seng Ave #06-3530
   Tai Seng Industrial Estate
   Singapore  534415
   SG

   Phone: +65 65505420
   Email: cwng@psl.com.sg


   Eun Kyoung Paik
   KT
   Portable Internet Team, Convergence Lab., KT
   17 Woomyeon-dong, Seocho-gu
   Seoul  137-792
   Korea

   Phone: +82-2-526-5233
   Fax:   +82-2-526-5200
   Email: euna@kt.co.kr
   URI:   http://mmlab.snu.ac.kr/~eun/


   Thierry Ernst
   WIDE at Keio University
   Jun Murai Lab., Keio University.
   K-square Town Campus, 1488-8 Ogura, Saiwa-Ku
   Kawasaki, Kanagawa  212-0054
   Japan

   Phone: +81-44-580-1600
   Fax:   +81-44-580-1437
   Email: ernst@sfc.wide.ad.jp
   URI:   http://www.sfc.wide.ad.jp/~ernst/














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Appendix A.  Alternative Classifications Approach

A.1  Ownership-Oriented Approach

   An alternative approach to classifying multihomed mobile network is
   proposed by Erik Nordmark (Sun Microsystems) by breaking the
   classification of multihomed network based on ownership.  This is
   more of a tree-like top-down classification.  Starting from the
   control and ownership of the HA(s) and MR(s), there are two different
   possibilities: either (i) the HA(s) and MR(s) are controlled by a
   single entity, or (ii) the HA(s) and MR(s) are controlled by separate
   entities.  We called the first possibility the 'ISP Model', and the
   second the 'Subscriber/Provider Model'.

A.1.1  ISP Model

   The case of the HA(s) and MR(s) are controlled by the same entity can
   be best illustrated as an Internet Service Provider (ISP) installing
   mobile routers on trains, ships or planes.  It is up to the ISP to
   deploy a certain configuration of mobile network; all 8
   configurations as described in the Configuration-Oriented Approach
   are possible.  In the remaining portion of this document, when
   specifically referring to a mobile network configuration that is
   controlled by a single entity, we will add an 'ISP' prefix: for
   example: ISP-(1,1,1) or ISP-(1,N,N).

   When the HA(s) and MR(s) are controlled by a single entity (such as
   an ISP), the ISP can decide whether it wants to assign one or
   multiple MNPs to the mobile network just like it can make the same
   decision for any other link in its network (wired or otherwise).  In
   any case, the ISP will make the routing between the mobile networks
   and its core routers (such as the HAs) work.  This include not
   introducing any aggregation between the HAs which will filter out
   routing announcements for the MNP(es).

   To such ends, the ISP has various means and mechanisms.  For one, the
   ISP can run its Interior Gateway Protocol (IGP) over bi-directional
   tunnels between the MR(s) and HA(s).  Alternatively, static routes
   may be used with the tunnels.  When static routes are used, a
   mechanism  to test "tunnel liveness" might be necessary to avoid
   maintaining stale routes.  Such "tunnel liveness" may be tested by
   sending heartbeats signals from MR(s) to the HA(s).  A possibility is
   to simulate heartbeats using Binding Updates messages by controlling
   the "Lifetime" field of the Binding Acknowledgment message to force
   the MR to send Binding Update messages at regular interval.  However,
   a more appropriate tool might be the Binding Refresh Request message,
   though conformance to the Binding Refresh Request message may be less
   strictly enforced in implementations since it serves a somewhat



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   secondary role when compared to Binding Update messages.

A.1.2  Subscriber/Provider Model

   The case of the HA(s) and MR(s) are controlled by the separate
   entities can be best illustrated with a subscriber/provider model,
   where the MRs belongs to a single subscriber and subscribes to one or
   more ISPs for HA services.  There is two sub-categories in this case:
   when the subscriber subscribes to a single ISP, and when the
   subscriber subscribes to multiple ISPs.  In the remaining portion of
   this document, when specifically referring to a mobile network
   configuration that is in the subscriber/provider model where the
   subscriber subscribes to only one ISP, we will add an 'S/P' prefix:
   for example: S/P-(1,1,1) or S/P-(1,n,n).  When specifically referring
   to a mobile network configuration that is in the subscriber/provider
   model where the subscriber subscribes to multiple ISPs, we will add
   an 'S/mP' prefix: for example: S/mP-(1,1,1) or S/mP-(1,n,n).

   Not all 8 configurations are likely to be deployed for the S/P and
   S/mP models.  For instance, it is unlikely to foresee a S/mP-(*,1,1)
   mobile network where there is only a single HA.  For the S/P model,
   the following configurations are likely to be deployed:

   o  S/P-(1,1,1): Single Provider, Single MR, Single HA, Single MNP

   o  S/P-(1,1,n): Single Provider, Single MR, Single HA, Multiple MNPs

   o  S/P-(1,n,1): Single Provider, Single MR, Multiple HAs, Single MNP

   o  S/P-(1,n,n): Single Provider, Single MR, Multiple HAs, Multiple
      MNPs

   o  S/P-(n,n,1): Single Provider, Multiple MRs, Single HA, Single MNP

   o  S/P-(n,1,n): Single Provider, Multiple MRs, Single HA, Multiple
      MNPs

   o  S/P-(n,n,1): Single Provider, Multiple MRs, Multiple HAs, Single
      MNP

   o  S/P-(n,n,n): Single Provider, Multiple MRs, Multiple HAs, Multiple
      MNPs


   For the S/mP model, the following configurations are likely to be
   deployed:

   o  S/mP-(1,n,1): Multiple Providers, Single MR, Multiple HAs, Single



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      MNP

   o  S/mP-(1,n,n): Multiple Providers, Single MR, Multiple HAs,
      Multiple MNPs

   o  S/mP-(n,n,n): Multiple Providers, Multiple MRs, Multiple HAs,
      Multiple MNPs

   When the HA(s) and MR(s) are controlled by different entities, it is
   more likely the scenario where the MR is controlled by one entity
   (i.e.  the subscriber), and the MR is establishing multiple
   bi-directional tunnels to one or more HA(s) provided by one or more
   ISP(s).  In such case, it is unlikely for the ISP to run IGP over the
   bi-directional tunnel, since ISP would most certainly wish to retain
   full control of its routing domain.




































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A.2  Problem-Oriented Approach

   A third approach is proposed by Pascal Thubert (Cisco System).  This
   focused on the problems of multihomed mobile networks rather than the
   configuration or ownership.  With this approach, there is a set of 4
   categories based on two orthogonal parameters: the number of HAs, and
   the number of MNPs advertised.  Since the two parameters are
   orthogonal, the categories are not mutually exclusive.  The four
   categories are:

   o  Tarzan: Single HA for Different Care-of Addresses of Same MNP

      This is the case where one mobile router registers different
      Care-of Addresses to the same home agent for the same subnet
      prefix.  This is equivalent to the case of y=1, i.e.  the (1,1,*)
      mobile network.

   o  JetSet: Multiple HAs for Different Care-of Addresses of Same MNP

      This is the case where the mobile router registers different
      Care-of Addresses to different home agents for the same subnet
      prefix.  This is equivalent to the case of y=n, i.e.  the (1,n,*)
      mobile network.

   o  Shinkansen: Single MNP Advertised by MR(s)

      This is the case where one MNP is announced by different MRs.
      This is equivalent to the case of x=n and z=1, i.e.  the (n,*,1)
      mobile network.

   o  DoubleBed: Multiple MNPs Advertised by MR(s)

      This is the case where more than one MNPs are announced by the
      different MRs.  This is equivalent to the case of x=n and z=n,
      i.e.  the (n,*,n) mobile network.
















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Appendix B.  Nested Tunneling for Fault Tolerance

   In order to utilize the additional robustness provided by
   multihoming, MRs that employ bi-directional tunneling with their HAs
   should dynamically change their tunnel exit points depending on the
   link status.  For instance, if a MR detects that one of its egress
   interface is down, it should detect if any other alternate route to
   the global Internet exists.  This alternate route may be provided by
   any other MRs connected to one of its ingress interfaces that has an
   independent route to the global Internet, or by another active egress
   interface the MR itself possess.  If such an alternate route exists,
   the MR should re-establish the bi-directional tunnel using this
   alternate route.

   In the remaining part of this section, we will attempt to investigate
   methods of performing such re-establishment of bi-directional
   tunnels.  It is not the objective of this memo to specify a new
   protocol specifically tailored to provide this form of re-
   establishments.  Instead, we will limit ourselves to currently
   available mechanisms specified in Mobile IPv6 [5] and Neighbor
   Discovery in IPv6 [11].

B.1  Detecting Presence of Alternate Routes

   To actively utilize the robustness provided by multihoming, a MR must
   first be capable of detecting alternate routes.  This can be manually
   configured into the MR by the administrators if the configuration of
   the mobile network is relatively static.  It is however highly
   desirable for MRs to be able to discover alternate routes
   automatically for greater flexibility.

   The case where a MR possesses multiple egress interface (bound to the
   same HA or otherwise) should be trivial, since the MR should be able
   to "realize" it has multiple routes to the global Internet.

   In the case where multiple MRs are on the mobile network, each MR has
   to detect the presence of other MR.  A MR can do so by listening for
   Router Advertisement message on its *ingress* interfaces.  When a MR
   receives a Router Advertisement message with a non-zero Router
   Lifetime field from one of its ingress interfaces, it knows that
   another MR which can provide an alternate route to the global
   Internet is present in the mobile network.

B.2  Re-Establishment of Bi-Directional Tunnels

   When a MR detects that the link by which its current bi-directional
   tunnel with its HA is using is down,  it needs to re-establish the
   bi-directional tunnel using an alternate route detected.  We consider



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   two separate cases here: firstly, the alternate route is provided by
   another egress interface that belongs to the MR; secondly, the
   alternate route is provided by another MR connected to the mobile
   network.  We refer to the former case as an alternate route provided
   by an alternate egress interface, and the latter case as an alternate
   route provided by an alternate MR.

B.2.1  Using Alternate Egress Interface

   When an egress interface of a MR loses the connection to the global
   Internet, the MR can make use of its alternate egress interface
   should it possess multiple egress interfaces.  The most direct way to
   do so is for the mobile router to send a binding update to the home
   agent of the failed interface using the Care-of Address assigned to
   the alternate interface in order to re-establish the bi-directional
   tunneling using the Care-of Address on the alternate egress
   interface.  After a successful binding update, the MR encapsulates
   outgoing packets through the bi-directional tunnel using the
   alternate egress interface.

   The idea is to use the global address (most likely a Care-of Address)
   assigned to an alternate egress interface as the new (back-up)
   Care-of Address of the mobile router to re-establish the
   bi-directional tunneling with its home agent.

B.2.2  Using Alternate Mobile Router

   When the MR loses a connection to the global Internet, the MR can
   utilize a route provided by an alternate MR (if one exists) to
   re-establish the bi-directional tunnel with its HA.  First, the MR
   has to obtain a Care-of Address from the alternate MR (i.e.  attaches
   itself to the alternate MR).  Next, it sends binding update to its HA
   using the Care-of Address obtained from the alternate MR.  From then
   on, the MR can encapsulates outgoing packets through the
   bi-directional tunnel using via the alternate MR.

   The idea is to obtain a Care-of Address from the alternate MR and use
   this as the new (back-up) Care-of Address of the MR to re-establish
   the bi-directional tunneling with its HA.

   Note that every packet sent between MNNs and their correspondent
   nodes will experience two levels of encapsulation.  First level of
   tunneling occurs between a MR which the MNN uses as its default
   router and the MR's HA.  The second level of tunneling occurs between
   the alternate MR and its HA.






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B.3  To Avoid Tunneling Loop

   The method of re-establishing the bi-directional tunnel as described
   in Appendix B.2 may lead to infinite loops of tunneling.  This
   happens when two MRs on a mobile network lose connection to the
   global Internet at the same time and each MR tries to re-establish
   bi-directional tunnel using the other MR.  We refer to this
   phenomenon as tunneling loop.

   One approach to avoid tunneling loop is for a MR that has lost
   connection to the global Internet to insert an option into the Router
   Advertisement message it broadcasts periodically.  This option serves
   to notify other MRs on the link that the sender no longer provides
   global connection.  Note that setting a zero Router Lifetime field
   will not work well since it will cause MNNs that are attached to the
   MR to stop using the MR as their default router too  (in which case,
   things are back to square one).


































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Appendix C.  Change Log

   o  This draft is an update of draft-ng-nemo-multihoming-issues-03.txt
      which is itself a merge of 3 previous drafts
      draft-ng-nemo-multihoming-issues-02.txt,
      draft-eun-nemo-multihoming-problem-statement-00.txt, and
      draft-charbon-nemo-multihoming-evaluation-00.txt

   o  Changes from draft-ietf-nemo-multihoming-issues-01 to -02:

      *  Added recommendations/suggestion of which WG each issue should
         be addressed as pointed out in 61st IETF.

      *  Minor updates on references.

   o  Changes from draft-ietf-nemo-multihoming-issues-00 to -01:

      *  Replaced NEMO-Prefix with MNP as decided by the WG at 60th IETF

      *  Addressed Issue #1 in Section 1: Added a note to remind readers
         that IPv6 is implicitly assumed

      *  Addressed Issue #3 in Section 2.3: Removed text on assumption

      *  Addressed Issue #6 in Section 3.1: Added benefits

      *  Addressed Issue #7 in Section 3.2: Modified text

      *  Addressed Issue #9 in Section 4.3: Modified text

      *  Addressed Issue #10 in Section 4.4: Added paragraph on other
         failure modes

      *  Addressed Issue #10: New Section 4.5 on media detection

      *  Addressed Issue #11 in Section 4.11: modified text

   o  Changes from draft-ng-multihoming-issues-03 to
      draft-ietf-nemo-multihoming-issues-00:

      *  Expanded "Problem Statement" (Section 4)

      *  Merged "Evaluation" Section into "Problem Statement"
         (Section 4)

      *  Cleaned up description in "Classification" (Section 2), and
         clearly indicate in each classification, what are the
         multihomed entities



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      *  Re-organized "Deployment Scenarios and Prerequisites"
         (Section 3), and created the "Prerequisites" sub-section.

   o  Changes from draft-ng-multihoming-issues-02 to
      draft-ng-multihoming-issues-03:

      *  Merged with draft-eun-nemo-multihoming-problem-statement (see
         "Problem Statement" (Section 4))

      *  Included conclusions from
         draft-charbon-nemo-multihoming-evaluation-00

      *  Re-organized some part of "Benefits/Issues of Multhoming in
         NEMO" to "Problem Statement" (Section 4)

      *  Removed lots of text to be in sync with [6].

      *  Title changed from "Multihoming Issues in NEMO Basic Support"
         to "Analysis of Multihoming in NEMO"

      *  Changed (w,x,y) to (x,y,z) in taxonomy.

      *  Moved alternative approaches of classification to Appendix

      *  Creation of this Change-Log itself ;-)


























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