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INFORMATIONAL

Network Working Group                                              C. Ng
Request for Comments: 4980                      Panasonic Singapore Labs
Category: Informational                                         T. Ernst
                                                                   INRIA
                                                                 E. Paik
                                                                      KT
                                                              M. Bagnulo
                                                                    UC3M
                                                            October 2007


          Analysis of Multihoming in Network Mobility Support

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.

Abstract

   This document is an analysis of multihoming in the context of network
   mobility (NEMO) in IPv6.  As there are many situations in which
   mobile networks may be multihomed, a taxonomy is proposed to classify
   the possible configurations.  The possible deployment scenarios of
   multihomed mobile networks are described together with the associated
   issues when network mobility is supported by RFC 3963 (NEMO Basic
   Support).  Recommendations are offered on how to address these
   issues.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Classification . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  (1,1,1): Single MR, Single HA, Single MNP  . . . . . . . .  6
     2.2.  (1,1,n): Single MR, Single HA, Multiple MNPs . . . . . . .  6
     2.3.  (1,n,1): Single MR, Multiple HAs, Single MNP . . . . . . .  7
     2.4.  (1,n,n): Single MR, Multiple HAs, Multiple MNPs  . . . . .  8
     2.5.  (n,1,1): Multiple MRs, Single HA, Single MNP . . . . . . .  8
     2.6.  (n,1,n): Multiple MRs, Single HA, Multiple MNPs  . . . . .  9
     2.7.  (n,n,1): Multiple MRs, Multiple HAs, Single MNP  . . . . .  9
     2.8.  (n,n,n): Multiple MRs, Multiple HAs, Multiple MNPs . . . . 10









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   3.  Deployment Scenarios and Prerequisites . . . . . . . . . . . . 11
     3.1.  Deployment Scenarios . . . . . . . . . . . . . . . . . . . 11
     3.2.  Prerequisites  . . . . . . . . . . . . . . . . . . . . . . 13
   4.  Multihoming Issues . . . . . . . . . . . . . . . . . . . . . . 14
     4.1.  Fault Tolerance  . . . . . . . . . . . . . . . . . . . . . 14
       4.1.1.  Failure Detection  . . . . . . . . . . . . . . . . . . 15
       4.1.2.  Path Exploration . . . . . . . . . . . . . . . . . . . 16
       4.1.3.  Path Selection . . . . . . . . . . . . . . . . . . . . 17
       4.1.4.  Re-Homing  . . . . . . . . . . . . . . . . . . . . . . 19
     4.2.  Ingress Filtering  . . . . . . . . . . . . . . . . . . . . 19
     4.3.  HA Synchronization . . . . . . . . . . . . . . . . . . . . 21
     4.4.  MR Synchronization . . . . . . . . . . . . . . . . . . . . 22
     4.5.  Prefix Delegation  . . . . . . . . . . . . . . . . . . . . 23
     4.6.  Multiple Bindings/Registrations  . . . . . . . . . . . . . 23
     4.7.  Source Address Selection . . . . . . . . . . . . . . . . . 23
     4.8.  Loop Prevention in Nested Mobile Networks  . . . . . . . . 24
     4.9.  Prefix Ownership . . . . . . . . . . . . . . . . . . . . . 24
     4.10. Preference Settings  . . . . . . . . . . . . . . . . . . . 25
   5.  Recommendations to the Working Group . . . . . . . . . . . . . 26
   6.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 28
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 29
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 29
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 29
   Appendix A.  Alternative Classifications Approach  . . . . . . . . 32
     A.1.  Ownership-Oriented Approach  . . . . . . . . . . . . . . . 32
       A.1.1.  ISP Model  . . . . . . . . . . . . . . . . . . . . . . 32
       A.1.2.  Subscriber/Provider Model  . . . . . . . . . . . . . . 33
     A.2.  Problem-Oriented Approach  . . . . . . . . . . . . . . . . 34
   Appendix B.  Nested Tunneling for Fault Tolerance  . . . . . . . . 35
     B.1.  Detecting Presence of Alternate Routes . . . . . . . . . . 35
     B.2.  Re-Establishment of Bi-Directional Tunnels . . . . . . . . 36
       B.2.1.  Using Alternate Egress Interface . . . . . . . . . . . 36
       B.2.2.  Using Alternate Mobile Router  . . . . . . . . . . . . 36
     B.3.  To Avoid Tunneling Loop  . . . . . . . . . . . . . . . . . 37
     B.4.  Points of Considerations . . . . . . . . . . . . . . . . . 37














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1.  Introduction

   The design goals and objectives of Network Mobility (NEMO) support in
   IPv6 are identified in [1], while the terminology is described in [2]
   and [3].  NEMO Basic Support (RFC 3963) [4] is the solution proposed
   by the NEMO Working Group to provide continuous Internet connectivity
   to nodes located in an IPv6 mobile network, e.g., like in an in-
   vehicle embedded IP network.  The NEMO Basic Support solution does so
   by setting up bi-directional tunnels between the mobile routers (MRs)
   connecting the mobile network (NEMO) to the Internet and their
   respective home agents (HAs), much like how this is done in Mobile
   IPv6 [5], the solution for host mobility support.  NEMO Basic Support
   is transparent to nodes located behind the MR (i.e., the mobile
   network nodes, or MNNs), and as such, does not 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 mobile
   node.  Scenarios illustrated in [6] demonstrate that providing a
   permanent access to mobile networks typically require the use of
   several interfaces and technologies.  For example, this is
   particularly useful for Intelligent Transport Systems (ITS)
   applications since vehicles are moving across distant geographical
   locations.  Access would be provided through different access
   technologies (e.g., Wimax, Wifi, 3G) and through different access
   operators.

   As specified in Section 5 of the NEMO Basic Support Requirements [1]
   (R.12), 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:

   o  when an MR has multiple egress interfaces, or

   o  the mobile network has multiple MRs, or

   o  the mobile network is associated with multiple HAs, or

   o  multiple global prefixes are available in the mobile network.

   Using NEMO Basic Support, this would translate into having multiple
   bi-directional tunnels between the MR(s) and the corresponding HA(s),
   and may result in multiple Mobile Network Prefixes (MNPs) available





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   to the MNNs.  However, NEMO Basic Support does not specify any
   particular mechanism to manage multiple bi-directional tunnels.  The
   objectives of this memo are thus multifold:

   o  to determine all the potential multihomed configurations for a
      NEMO, and then to identify which of these may be useful in a real-
      life scenario;

   o  to capture issues that may prevent some multihomed configurations
      to be supported under the operation of NEMO Basic Support.  It
      does not necessarily mean that the ones not supported will not
      work with NEMO Basic Support, as it may be up to the implementors
      to make it work (hopefully this memo will be helpful to these
      implementors);

   o  to decide which issues are worth solving and to determine which WG
      is the most appropriate to address these;

   o  to identify potential solutions to the previously identified
      issues.

   In order to reach these objectives, a taxonomy for classifying the
   possible multihomed configurations is described in Section 2.
   Deployment scenarios, their benefits, and requirements to meet these
   benefits are illustrated in Section 3.  Following this, the related
   issues are studied in Section 4.  The issues are then summarized in a
   matrix for each of the deployment scenario, and recommendations are
   made on which of the issues should be worked on and where in
   Section 5.  This memo concludes with an evaluation of NEMO Basic
   Support for multihomed configurations.  Alternative classifications
   are outlined in the Appendix.

   The readers should note that this document considers multihoming only
   from the point of view of an IPv6 environment.  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], Motivations and Scenarios for Multihoming
   [6], Goals and Requirements of Network Mobility Support [1], and the
   NEMO Basic Support specification [4].  Goals and benefits of
   multihoming as discussed in [6], are applicable to fixed nodes,
   mobile nodes, fixed networks, and mobile networks.

2.  Classification

   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.  A Configuration-
   Oriented taxonomy is described in this section.  Two other



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   taxonomies, namely, the Ownership-Oriented Approach and the Problem-
   Oriented Approach, are outlined in Appendix A.1 and Appendix A.2.

   Multihomed configurations can be classified depending on how many MRs
   are present, how many egress interfaces, Care-of Address (CoA), and
   Home Addresses (HoA) the MRs have, how many prefixes (MNPs) are
   available to the mobile network nodes, etc.  We use three key
   parameters to differentiate the multihomed configurations.  Using
   these parameters, each configuration is referred 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 that a mobile network has only a single MR,
         presumably multihomed.

      x=n  implies that 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 are assigned to the mobile network.

   o  'z' indicates the number of MNPs available within the NEMO, where:

      z=1  implies that a single MNP is available in the NEMO.

      z=N  implies that multiple MNPs are available in the NEMO.

   It can be seen that the above three parameters are fairly orthogonal
   with one another.  Thus, different values of 'x', 'y', and 'z' result
   in different combinations of the 3-tuple (x,y,z).

   As will be described in the sub-sections below, a total of 8 possible
   configurations can be 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 available (on the home
   link, or on the foreign link), or (ii) the MR is equipped with
   multiple interfaces.  In such a case, the MR would have multiple
   (HoA,CoA) pairs.  Issues pertaining to a multihomed MR are also
   addressed in [7].  In addition, the readers should also keep in mind
   that when "MNP(s) is/are available in the NEMO", the MNP(s) may
   either be explicitly announced by the MR via router advertisement, or
   made available through Dynamic Host Configuration Protocol (DHCP)
   [8].




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

   The (1,1,1) configuration has only one MR, it is associated with a
   single HA, and a single MNP is available in the NEMO.  The MR and the
   AR are connected to the Internet via a single Access Router (AR).  To
   fall into a multihomed configuration, at least one of the following
   conditions must hold:

   o  The MR has multiple interfaces and thus it has multiple CoAs;

   o  Multiple global prefixes are available on the foreign link, and
      thus it has multiple CoAs; or

   o  Multiple global prefixes are available on the home link, and thus
      the MR has more than one path to reach the HA.

   Note that the case where multiple prefixes are available on the
   foreign link does not have any bearing on the MNPs.  MNPs are
   independent of prefixes available on the link where the MR is
   attached to, thus prefixes available on the foreign link are not
   announced on the NEMO link.  For the case where multiple prefixes are
   available on the home link, these are only announced on the NEMO link
   if the MR is configured to do so.  In the present (1,1,1)
   configuration, only one MNP is announced.

   A bi-directional tunnel would then be established between each
   (HoA,CoA) pair.

   Regarding MNNs, they are (usually) not multihomed since they would
   configure a single global address from the single MNP available 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) configuration has only one MR, it is associated with a
   single HA, and two or more MNPs are available in the NEMO.





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   The MR may itself be multihomed, as detailed in Section 2.1.  In such
   a case, a bi-directional tunnel would be established between each
   (HoA,CoA) pair.

   Regarding MNNs, they are multihomed because several MNPs are
   available on the link they are attached to.  The MNNs would then
   configure a global address from each MNP available on the link.

                                   _____
                   _   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) configuration has only one MR and a single MNP is
   available in the NEMO.  The MR, however, is associated with multiple
   HAs.

   The NEMO is multihomed since it has multiple HAs, but in addition,
   the conditions detailed in Section 2.1 may also hold for the MR.  A
   bi-directional tunnel would then be established between each
   (HoA,CoA) pair.

   Regarding MNNs, they are (usually) not multihomed since they would
   configure a single global address from the single MNP available 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





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

   The (1,n,n) configuration has only one MR.  However, the MR is
   associated with multiple HAs and more than one MNP is available in
   the NEMO.

   The MR is multihomed since it has multiple HAs, but in addition, the
   conditions detailed in Section 2.1 may also hold.  A bi-directional
   tunnel would then be established between each (HoA,CoA) pair.

   Regarding MNNs, they are multihomed because several MNPs are
   available on the link they are attached to.  The MNNs would then
   configure a global address with each MNP available on the link.

                                         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) configuration has more than one MR advertising global
   routes.  However, the MR(s) are associated with a single HA, and
   there is a single MNP available in the NEMO.

   The NEMO is multihomed since it has multiple MRs, but in addition the
   conditions detailed in Section 2.1 may also hold for each MR.  A bi-
   directional tunnel would then be established between each (HoA,CoA)
   pair.

   Regarding MNNs, they are (usually) not multihomed since they would
   configure a single global address from the single MNP available on
   the link they are attached to.











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                        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) configuration has more than one MR; multiple global
   routes are advertised by the MRs and multiple MNPs are available
   within the NEMO.

   The NEMO is multihomed since it has multiple MRs, but in addition,
   the conditions detailed in Section 2.1 may also hold for each MR.  A
   bi-directional tunnel would then be established between each
   (HoA,CoA) pair.

   Regarding MNNs, they are multihomed because several MNPs are
   available on the link they are attached to.  The MNNs would then
   configure a global address with each MNP available on the link.

                        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) configuration has more than one MR advertising multiple
   global routes.  The mobile network is simultaneously associated with
   multiple HAs and a single MNP is available in the NEMO.







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   The NEMO is multihomed since it has multiple MRs and HAs, but in
   addition, the conditions detailed in Section 2.1 may also hold for
   each MR.  A bi-directional tunnel would then be established between
   each (HoA,CoA) pair.

   Regarding MNNs, they are (usually) not multihomed since they would
   configure a single global address from the single MNP available 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) configuration has multiple MRs advertising different
   global routes.  The mobile network is simultaneously associated with
   more than one HA and multiple MNPs are available in the NEMO.

   The NEMO is multihomed since it has multiple MRs and HAs, but in
   addition, the conditions detailed in Section 2.1 may also hold for
   each MR.  A bi-directional tunnel would then be established between
   each (HoA,CoA) pair.

   Regarding MNNs, they are multihomed because several MNPs are
   available on the link they are attached to.  The MNNs would then
   configure a global address with each MNP available on the link.














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

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

3.  Deployment Scenarios and Prerequisites

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

   1.  Permanent and Ubiquitous Access

   2.  Reliability

   3.  Load Sharing

   4.  Load Balancing/Flow Distribution

   5.  Preference Settings

   6.  Aggregate Bandwidth

   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 networks with a single MR

      o Example 1:

         MR with dual/multiple access interfaces (e.g., 802.11 and GPRS
         capabilities).  This is a (1,1,*) if a single HA is used for
         both.  If two independent HAs are used, this is a (1,n,n)
         configuration.

         Benefits: Ubiquitous Access, Reliability, Load Sharing,
         Preference Settings, Aggregate Bandwidth.



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   x=n: Multihomed mobile networks with multiple MRs

      o Example 1:

         Train with one MR in each car, all served by the same HA, thus
         a (n,1,*) configuration.  Alternatively, the train company
         might use different HAs, in different countries, thus a (n,n,n)
         configuration.

         Benefits: Ubiquitous Access, Reliability, Load Sharing,
         Aggregate Bandwidth.

      o Example 2:

         Wireless personal area network with a GPRS-enabled phone and a
         WiFi-enabled PDA.  This is a (n,n,n) configuration if different
         HAs are also used.

         Benefits: Ubiquitous Access, Reliability, Preference Settings,
         Aggregate Bandwidth.

   y=1: Multihomed mobile networks with a single HA

      o Example:

         Most single HA cases in above examples.

   y=n: Multihomed mobile networks with multiple HAs

      o Example 1:

         Most multiple HAs cases in above examples.

      o Example 2:

         Transatlantic flight with a HA in each continent.  This is a
         (1,n,1) configuration if there is only one MR.

         Benefits: Ubiquitous Access, Reliability, Preference Settings
         (reduced delay, shortest path).

   z=1: Multihomed mobile networks with a single MNP

      o Example:

         Most single HA cases in above examples.





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   z=n: Multihomed mobile networks with multiple MNPs

      o Example 1:

         Most multiple HAs cases in above examples.

      o Example 2:

         Car with a prefix taken from home (personal traffic is
         transmitted using 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,) configuration.

         Benefits: Preference Settings

3.2.  Prerequisites

   In this section, requirements are stated in order to comply with the
   expected benefits of multihoming 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.

   o  Reliability:

      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.  It should be noted that the
      provision of fault tolerance capabilities does not necessarily
      require the existence of multiple bi-directional tunnels
      simultaneously.

   o  Load Sharing and Load Balancing:

      Multiple tunnels must be maintained simultaneously.








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   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.  To allow user/application to
      set the preference, a mechanism should be provided to the user/
      application for the notification of the availability of multiple
      bi-directional tunnels, and perhaps also to set preferences.  A
      similar mechanism should also be provided to network
      administrators to manage preferences.

   o  Aggregate Bandwidth:

      Multiple tunnels must be maintained simultaneously in order to
      increase the total aggregated bandwidth available to the mobile
      network.

4.  Multihoming Issues

   As discussed in the previous section, multiple bi-directional tunnels
   need to be maintained either sequentially (e.g., for fault tolerance)
   or simultaneously (e.g., for load sharing).

   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 tunnels
   (load sharing, load balancing).

   So, depending on the configuration under consideration, the issues
   discussed below may need to be addressed sometimes dynamically.  For
   each issue, potential ways to solve the problem are investigated.

4.1.  Fault Tolerance

   One of the goals of multihoming is the provision of fault tolerance
   capabilities.  In order to provide such features, a set of tasks need
   to be performed, including: failure detection, alternative available
   path exploration, path selection, and re-homing of established
   communications.  These are also discussed in [9] by the Shim6 WG.  In
   the following sub-sections, we look at these issues in the specific
   context of NEMO, rather than the general Shim6 perspective in [9].
   In addition, in some scenarios, it may also be required to provide
   the mechanisms for coordination between different HAs (see
   Section 4.3) and also the coordination between different MRs (see
   Section 4.4).





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4.1.1.  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.

   Depending on the NEMO configuration considered, the failure
   protection domain greatly varies.  In some configurations, the
   protection domain provided by NEMO multihoming is limited to the
   links between the MR(s) and the HA(s).  In other configurations, the
   protection domain allows to recover from failures in other parts of
   the path, so an end-to-end failure detection mechanism is required.

   The failure detection capabilities required for each configuration
   are detailed below:

   o  For the (1,1,*) cases, multiple paths are available between a
      single MR and a single HA.  All the traffic to and from the NEMO
      flows through the MR and HA.  Failure detection mechanisms need
      only to be executed between these two devices.  This is a NEMO-/
      MIPv6-specific issue.

   o  For the (n,1,*) cases, there is a single HA, so all the traffic to
      and from the NEMO will flow through it.  The failure detection
      mechanisms need to be able to detect failure in the path between
      the used MR and the only HA.  Hence, the failure detection
      mechanism needs only to involve the HA and the MRs.  This is a
      NEMO/MIPv6 specific issue.

   o  For the (n,n,*) cases, there are multiple paths between the
      different HAs and the different MRs.  Moreover, the HAs may be
      located in different networks, and have different Internet access
      links.  This implies that changing the HA used may not only allow
      recovering from failures in the link between the HA and the MR,
      but also from other failure modes, affecting other parts of the
      path.  In this case, an end-to-end failure detection mechanism
      would provide additional protection.  However, a higher number of
      failures is likely to occur in the link between the HA and the MR,
      so it may be reasonable to provide optimized failure detection
      mechanisms for this failure mode.  The (n,n,n) case is hybrid,
      since selecting a different prefix results in a change of path.
      For this case, the Shim6 protocols (such as those discussed in
      [9]) may be useful.

   Most of the above cases involve the detection of tunnel failures
   between HA(s) and MR(s).  This is no different from the case of
   failure detection between a mobile host and its HA(s).  As such, a



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   solution for MIPv6 should apply to NEMO as well.  For case (n,*,*),
   an MR synchronization solution (see Section 4.4) should be able to
   complement a MIPv6 failure detection solution to achieve the desired
   functionality for NEMO.

   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 rely on router advertisements from
      access routers, or other layer-2 trigger mechanisms to detect
      faults, e.g., [10] and [11].

   o  On the HA's side, it is more difficult 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 subscriber
      model (see Appendix A.2: S/P model), a lack of standardized
      "tunnel liveness" protocol means that it is harder to detect
      failures.

   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.

4.1.2.  Path Exploration

   Once a failure in the currently used path is detected, alternative
   paths have to be explored in order to identify an available one.
   This process is closely related to failure detection in the sense
   that paths being explored need to be alternative paths to the one
   that has failed.  There are, however, subtle but significant
   differences between path exploration and failure detection.  Failure
   detection occurs on the currently used path while path exploration
   occurs on the alternative paths (not on the one currently being used
   for exchanging packets).  Although both path exploration and failure
   detection are likely to rely on a reachability or keepalive test
   exchange, failure detection also relies on other information, such as
   upper layer information (e.g., positive or negative feedback from
   TCP), lower layer information (e.g., an interface is down), and
   network layer information (e.g., as an address being deprecated or
   ICMP error message).





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   Basically, the same cases as in the analysis of the failure detection
   (Section 4.1.1) issue are identified:

   o  For the (1,1,*) cases, multiple paths are available between a
      single MR and a single HA.  The existing paths between the HA and
      the MR have to be explored to identify an available one.  The
      mechanism involves only the HA and the MR.  This is a NEMO-/
      MIPv6-specific issue.

   o  For the (n,1,*) cases, there is a single HA, so all the traffic to
      and from the NEMO will flow through it.  The available alternative
      paths are the different ones between the different MRs and the HA.
      The path-exploration mechanism only involves the HA and the MRs.
      This is a NEMO/MIPv6 specific issue.

   o  For the (n,n,*) cases, there are multiple paths between the
      different HAs and the different MRs.  In this case, alternative
      paths may be routed completely independent from one another.  An
      end-to-end path-exploration mechanism would be able to discover if
      any of the end-to-end paths is available.  The (n,n,1) case,
      however, seems to be pretty NEMO specific, because of the absence
      of multiple prefixes.  The (n,n,n) case is hybrid, since selecting
      a different prefix results in a change of path.  For this case,
      the Shim6 protocols (such as those discussed in [9]) may be
      useful.

   Most of the above cases involve the path exploration of tunnels
   between HA(s) and MR(s).  This is no different from the case of path
   exploration between a mobile host and its HA(s).  As such, a solution
   for MIPv6 should apply to NEMO as well.  For case (n,*,*), an MR
   synchronization solution (see Section 4.4) should be able to
   complement an MIPv6 path-exploration solution to achieve the desired
   functionality for NEMO.

   In order to perform path exploration, it is sometimes also necessary
   for the MR to detect the availability of network media.  This may be
   achieved using layer 2 triggers [10], or other mechanism developed/
   recommended by the Detecting Network Attachment (DNA) Working Group
   [11].  This is related to Section 4.1.1, since the ability to detect
   media availability would often imply the ability to detect media
   unavailability.

4.1.3.  Path Selection

   A path-selection mechanism is required to select among the multiple
   available paths.  Depending on the NEMO multihoming configuration
   involved, the differences between the paths may affect only the part
   between the HA and the MR, or they may affect the full end-to-end



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   path.  In addition, depending on the configuration, path selection
   may be performed by the HA(s), the MR(s), or the hosts themselves
   through address selection, as will be described in detail next.

   The multiple available paths may differ on the tunnel between the MR
   and the HA used to carry traffic to/from the NEMO.  In this case,
   path selection is performed by the MR and the intra-NEMO routing
   system for traffic flowing from the NEMO, and path selection is
   performed by the HA and intra-Home Network routing system for traffic
   flowing to the NEMO.

   Alternatively, the multiple paths available may differ in more than
   just the tunnel between the MR and the HA, since the usage of
   different prefixes may result in using different providers; hence, in
   completely different paths between the involved endpoints.  In this
   case, besides the mechanisms presented in the previous case,
   additional mechanisms for the end-to-end path selection may be
   needed.  This mechanism may be closely related to source address
   selection mechanisms within the hosts, since selecting a given
   address implies selecting a given prefix, which is associated with a
   given ISP serving one of the home networks.

   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]
      [12]) in the (n,*,*) case or based on "Source Address Selection"
      in the (*,*,n) cases (see Section 4.7 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, e.g., 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.

   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



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   tunnel, while the other serves as a backup) or peer-to-peer (no
   tunnel has precedence over one another, they are used with the same
   priority).  This questions which of the bi-directional tunnels would
   be selected, and based on which of the parameters (e.g., type of flow
   that goes into/out of the mobile network).

   The mechanisms for the selection among the different tunnels between
   the MR(s) and the HA(s) seem to be quite NEMO/MIPv6 specific.

   For (1,*,*) cases, they are no different from the case of path
   selection between a mobile host and its HA(s).  As such, a solution
   for MIPv6 should apply to NEMO as well.  For the (n,*,*) cases, an MR
   synchronization solution (see Section 4.4) should be able to
   complement an MIPv6 path-selection solution to achieve the desired
   functionality for NEMO.

   The mechanisms for selecting among different end-to-end paths based
   on address selection are similar to the ones used in other
   multihoming scenarios, as those considered by Shim6 (e.g., [13]).

4.1.4.  Re-Homing

   After an outage has been detected and an available alternative path
   has been identified, a re-homing event takes place, diverting the
   existing communications from one path to the other.  Similar to the
   previous items involved in this process, the re-homing procedure
   heavily varies depending on the NEMO multihoming configuration.

   o  For the (*,*,1) configurations, the re-homing procedure involves
      only the MR(s) and the HA(s).  The re-homing procedure may involve
      the exchange of additional BU messages.  These mechanisms are
      shared between NEMO Basic Support and MIPv6.

   o  For the (*,*,n) cases, in addition to the previous mechanisms,
      end-to-end mechanisms may be required.  Such mechanisms may
      involve some form of end-to-end signaling or may simply rely on
      using different addresses for the communication.  The involved
      mechanisms may be similar to those required for re-homing Shim6
      communications (e.g., [13]).

4.2.  Ingress Filtering

   Ingress filtering mechanisms [14][15] 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
   HAs.  If a packet with a source address configured from a specific





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   MNP is tunneled to a HA that does not handle that specific MNP, the
   packet may be discarded either by the HA or by a border router in the
   home network.

   The ingress filtering compatibility issue is heavily dependent on the
   particular NEMO multihoming configuration:

   o  For the (*,*,1) cases, there is not such an issue, since there is
      a single MNP.

   o  For the (1,1,*) and (n,1,1) cases, there is not such a problem,
      since there is a single HA, accepting all the MNPs.

   o  For the (n,1,n) case, though ingress filtering would not occur at
      the HA, it may occur at the MRs, when each MR is handling
      different MNPs.

   o  (*,n,n) are the cases where the ingress filtering presents some
      difficulties.  In the (1,n,n) case, the problem is simplified
      because all the traffic to and from the NEMO is routed through a
      single MR.  Such configuration allows the MR to properly route
      packets respecting the constraints imposed by ingress filtering.
      In this case, the single MR may face ingress filtering problems
      that a multihomed mobile node may face, as documented in [7].  The
      more complex case is the (n,n,n) case.  A simplified case occurs
      when all the prefixes are accepted by all the HAs, so that no
      problems occur with the ingress filtering.  However, this cannot
      be always assumed, resulting in the problem described below.

   As an example of how this could happen, consider the deployment
   scenario illustrated 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 between the two pairs.  Each
   MR advertises a different MNP (P1 and P2 respectively).  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:

   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 [12] practice that an IPv6 node is free
      to choose any router as its default router regardless of the
      prefix it chooses to use.







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   o  If the tunnel to HA1 is broken, packets that would normally be
      sent through the tunnel to HA1 should be diverted through the
      tunnel to HA2.  If HA2 (or some border router in HA2's domain)
      performs ingress filtering, packets with source address configured
      from MNP P1 may be discarded.

               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

   Possible solutions to the ingress filtering incompatibility problem
   may be based on the following approaches:

   o  Some form of source address-dependent routing, whether host-based
      and/or router-based where the prefix contained in the source
      address of the packet is considered when deciding which exit
      router to use when forwarding the packet.

   o  The usage of nested tunnels for (*,n,n) cases.  Appendix B
      describes one such approach.

   o  Deprecating those prefixes associated to non-available exit
      routers.

   The ingress filtering incompatibilities problems that appear in some
   NEMO multihoming configurations are similar to those considered in
   Shim6 (e.g., see [16]).

4.3.  HA Synchronization

   In the (*,n,*) configuration, a single MNP would be registered at
   different HAs.  This gives rise to the following cases:

   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.






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   This may pose a problem in the routing infrastructure as a whole if
   the HAs are located in different administrative domains.  The
   implications of this aspect needs further exploration.  A certain
   level of HA coordination may be required.  A possible approach is to
   adopt an HA synchronization mechanism such as that described in [17]
   and [18].  Such synchronization might also be necessary in a (*,n,*)
   configuration, when an 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 between HAs.  The mode of
   synchronization may be either primary-secondary or peer-to-peer.  In
   addition, when a MNP is delegated to the MR (see Section 4.5), some
   level of coordination between the HAs may also be necessary.

   This issue is a general mobility issue that will also have to be
   dealt with by Mobile IPv6 (see Section 6.2.3 in [7]) as well as NEMO
   Basic Support.

4.4.  MR Synchronization

   In the (n,*,*) configurations, there are common decisions that may
   require synchronization among different MRs [19], such as:

   o  advertising the same MNP in the (n,*,1) configurations (see also
      "prefix delegation" in Section 4.5);

   o  one MR relaying the advertisement of the MNP from another failed
      MR in the (n,*,n) configuration; and

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

   However, there is no known standardized protocol for this kind of
   router-to-router synchronization.  Without such synchronization, it
   may not be possible for a (n,*,*) configuration to achieve various
   multihoming goals, such as fault tolerance.

   Such a synchronization mechanism can be primary-secondary (i.e., a
   master MR, with the other MRs as backup) or peer-to-peer (i.e., there
   is no clear administrative hierarchy between the MRs).  The need for
   such mechanism is general in the sense that a multi-router site in
   the fixed network would require the same level of router
   synchronization.

   Thus, this issue is not specific to NEMO Basic Support, though there
   is a more pressing need to develop an MR-to-MR synchronization scheme
   for proving fault tolerances and failure recovery in NEMO
   configurations due to the higher possibility of links failure.



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   In conclusion, it is recommended to investigate a generic solution to
   this issue although the solution would first have to be developed for
   NEMO deployments.

4.5.  Prefix Delegation

   In the (*,*,1) configurations, the same MNP must be advertised to the
   MNNs through different paths.  There is, however, no synchronization
   mechanism available to achieve this.  Without a synchronization
   mechanism, MR may end up announcing incompatible MNPs.  Particularly,

   o  for the (*,n,1) cases, how can multiple HAs 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 4.3).

   o  for the (n,*,1) cases, how can multiple MRs 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 4.4).

   Prefix delegation mechanisms [20][21][22] could be used to ensure all
   routers advertise the same MNP.  Their applicability to a multihomed
   mobile network should be considered.

4.6.  Multiple Bindings/Registrations

   When an MR is configured with multiple CoAs, it is often necessary
   for it to bind these CoAs to the same MNP.

   This is a generic mobility issue, since Mobile IPv6 nodes face a
   similar problem.  This issue is discussed in [7].  It is sufficient
   to note that solutions like [23] can solve this for both Mobile IPv6
   and NEMO Basic Support.  This issue is being dealt with in the
   Monami6 WG.

4.7.  Source Address Selection

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

   However, currently available source address selection mechanisms do
   not allow MNNs to acquire sufficient information to select their
   source addresses intelligently (such as based on the traffic
   condition associated with the home network of each MNP).  It may be
   desirable for MNNs to be able to acquire "preference" information on
   each MNP from the MRs.  This would allow default address selection



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   mechanisms, such as those specified in [24], 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.  Note that source address selection may be
   closely related to path selection procedures (see Section 4.1.3) and
   re-homing techniques (see Section 4.1.4).

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

4.8.  Loop Prevention in 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 to two different root-MRs, thus
   the aggregated network no longer forms a simple tree structure.  In
   such a situation, infinite loop within the mobile network may occur.

   This problem is specific to NEMO Basic Support.  However, at the time
   of writing, more research is recommended to assess the probability of
   loops occurring in a multihomed mobile network.  For related work,
   see [25] for a mechanism to avoid loops in nested NEMO.

4.9.  Prefix Ownership

   When a (n,*,1) network splits, (i.e., the two MRs split themselves
   up), MRs on distinct links may try to register the only available
   MNP.  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 [26].

   This problem is specific to NEMO Basic Support.  However, it is
   unclear whether there is a sufficient deployment scenario for this
   problem to occur.

   It is recommended that the NEMO WG standardize a solution to solve
   this problem if there is sufficient vendor/operator interest, or
   specify that the split of a (n,*,1) network cannot be allowed without
   router renumbering.








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4.10.  Preference Settings

   When a mobile network is multihomed, the MNNs may be able to benefit
   from this configuration, such as to choose among the available paths
   based on cost, transmission delays, bandwidth, etc.  However, in some
   cases, such a choice is not made available to the MNNs.
   Particularly:

   o  In the (*,*,n) configuration, the MNNs can influence the path by
      source address selection (see Section 4.1.3 and Section 4.7).

   o  In the (n,*,*) configuration, the MNNs can influence the path by
      default router selection (see Section 4.1.3).

   o  In the (1,n,1) configuration, the MNNs cannot influence the path
      selection.

   One aspect of preference setting is that the preference of the MNN
   (e.g., application or transport layer configuration) may not be the
   same as the preference used by MR.  Thus, forwarding choices made by
   the MR may not be the best for a particular flow, and may even be
   detrimental to some transport control loops (i.e., the flow control
   algorithm for TCP may be messed up when MR unexpectedly performs load
   balancing on a TCP flow).  A mechanism that allows the MNN to
   indicate its preference for a given traffic might be helpful here.

   Another aspect of preference setting is that the MNN may not even be
   aware of the existence of multiple forwarding paths, e.g., the
   (1,n,1) configuration.  A mechanism for the MR to advertise the
   availability of multiple tunneling paths would allow the MNN to take
   advantage of this, coupled with the previously mentioned mechanism
   that allows the MNN to indicate its preference for a given traffic.

   This problem is general in the sense that IPv6 nodes may wish to
   influence the routing decision done by the upstream routers.  Such a
   mechanism is currently being explored by various WGs, such as the
   NSIS and IPFIX WGs.  It is also possible that the Shim6 layer in the
   MNNs may possess such a capability.  It is recommended for vendors or
   operators to investigate into the solutions developed by these WGs
   when providing multihoming capabilities to mobile networks.

   In addition, the Monami6 WG is currently developing a flow filtering
   solution for mobile nodes to indicate how flows should be forwarded
   by a filtering agent [27] (such as HA and mobile anchor points).  It
   is recommended that the Monami6 WG consider the issues described here
   so that flow filtering can be performed by the MNN to indicate how
   flows should be forwarded by the MR.




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5.  Recommendations to the Working Group

   Several issues that might impact the deployment of NEMO with
   multihoming capabilities were identified in Section 4.  These are
   shown in the matrix below, for each of the eight multihoming
   configurations, together with indications from which WG(s) a solution
   to each issue is most likely to be found.

    +=================================================================+
    |                       # of MRs: | 1 | 1 | 1 | 1 | n | n | n | n |
    |                       # of HAs: | 1 | 1 | n | n | 1 | 1 | n | n |
    |                  # of Prefixes: | 1 | n | 1 | n | 1 | n | 1 | n |
    +=================================================================+
    | Fault Tolerance                 | * | * | * | * | * | * | * | * |
    +---------------------------------+---+---+---+---+---+---+---+---+
    | Failure Detection               |N/M|N/M|N/M|N/M|N/M|N/M| N | S |
    +---------------------------------+---+---+---+---+---+---+---+---+
    | Path Exploration                |N/M|N/M|N/M|N/M|N/M|N/M| N | S |
    +---------------------------------+---+---+---+---+---+---+---+---+
    | Path Selection                  | N |S/M| M |S/M| N |S/N| N |S/N|
    +---------------------------------+---+---+---+---+---+---+---+---+
    | Re-Homing                       |N/M| S |N/M| S |N/M| S |N/M| S |
    +---------------------------------+---+---+---+---+---+---+---+---+
    | Ingress Filtering               | . | . | . | t | . | . | . | N |
    +---------------------------------+---+---+---+---+---+---+---+---+
    | HA Synchronization              | . | . |N/M|N/M| . | . |N/M|N/M|
    +---------------------------------+---+---+---+---+---+---+---+---+
    | MR Synchronization              | . | . | . | . | G | G | G | G |
    +---------------------------------+---+---+---+---+---+---+---+---+
    | Prefix Delegation               | . | . | N | N | N | N | N | N |
    +---------------------------------+---+---+---+---+---+---+---+---+
    | Multiple Binding/Registrations  | M | M | M | M | M | M | M | M |
    +---------------------------------+---+---+---+---+---+---+---+---+
    | Source Address Selection        | . | G | . | G | . | G | . | G |
    +---------------------------------+---+---+---+---+---+---+---+---+
    | Loop Prevention in Nested NEMO  | N | N | N | N | N | N | N | N |
    +---------------------------------+---+---+---+---+---+---+---+---+
    | Prefix Ownership                | . | . | . | . | N | . | N | . |
    +---------------------------------+---+---+---+---+---+---+---+---+
    | Preference Settings             | G | G | G | G | G | G | G | G |
    +=================================================================+
    N - NEMO Specific       M - MIPv6 Specific      G - Generic IPv6
    S - SHIM6 WG            D - DNA WG
    . - Not an Issue        t - trivial
    * - Fault Tolerance is a combination of Failure Detection, Path
        Exploration, Path Selection, and Re-Homing

               Figure 10: Matrix of NEMO Multihoming Issues



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   The above matrix serves to identify which issues are NEMO-specific,
   and which are not.  The readers are reminded that this matrix is a
   simplification of Section 4 as subtle details are not represented in
   Figure 10.

   As can be seen from Figure 10, the following are some concerns that
   are specific to NEMO: Failure Detection, Path Exploration, Path
   Selection, Re-Homing, Ingress Filtering, HA Synchronization, Prefix
   Delegation, Loop Prevention in Nested NEMO, and Prefix Ownership.
   Based on the authors' best knowledge of the possible deployments of
   NEMO, it is recommended that:

   o  A solution for Failure Detection, Path Exploration, Path
      Selection, and Re-Homing be solicited from other WGs.

      Although Path Selection is reflected in Figure 10 as NEMO-
      Specific, the technical consideration of the problem is believed
      to be quite similar to the selection of multiple paths in mobile
      nodes.  As such, we would recommend vendors to solicit a solution
      for these issues from other WGs in the IETF; for instance, the
      Monami6 or Shim6 WG.

   o  Ingress Filtering on the (n,n,n) configuration can be solved by
      the NEMO WG.

      This problem is clearly defined, and can be solved by the WG.
      Deployment of the (n,n,n) configuration can be envisioned on
      vehicles for mass transportation (such as buses, trains) where
      different service providers may install their own MRs on the
      vehicle/vessel.

      It should be noted that the Shim6 WG may be developing a mechanism
      for overcoming ingress filtering in a more general sense.  We thus
      recommend that the NEMO WG concentrate only on the (n,n,n)
      configuration should the WG decide to work on this issue.

   o  A solution for HA Synchronization can be looked at in a mobility-
      specific WG, taking into consideration both mobile hosts operating
      Mobile IPv6 and MRs operating NEMO Basic Support.

   o  A solution for Multiple Bindings/Registrations is presently being
      developed by the Monami6 WG.

   o  Prefix Delegation should be reviewed and checked by the NEMO WG.

      The proposed solutions [22] and [21] providing prefix delegation
      functionality to NEMO Basic Support should be reviewed in order to




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      make sure concerns, as discussed in Section 4.5, are properly
      handled.

   o  Loop Prevention in Nested NEMO should be investigated.

      Further research is recommended to assess the risk of having a
      loop in the nesting of multihomed mobile networks.

   o  Prefix Ownership should be considered by the vendors and
      operators.

      The problem of Prefix Ownership only occurs when a mobile network
      with multiple MRs and a single MNP can arbitrarily join and split.
      Vendors and operators of mobile networks are encouraged to input
      their views on the applicability of deploying such kind of mobile
      networks.

6.  Conclusion

   This memo presented an analysis of multihoming in the context of
   network mobility under the operation of NEMO Basic Support (RFC
   3963).  The purpose was to investigate issues related to such a bi-
   directional tunneling mechanism where mobile networks are multihomed
   and multiple bi-directional tunnels are established between Home
   Agent and Mobile Router pairs.  For doing so, mobile networks were
   classified into a taxonomy comprising eight possible multihomed
   configurations.  Issues were explained one by one and then summarized
   into a table showing the multihomed configurations where they apply,
   suggesting the most relevant IETF working group where they could be
   solved.  This analysis will be helpful to extend the existing
   standards to support multihoming and to implementors of NEMO Basic
   Support and multihoming-related mechanisms.

7.  Security Considerations

   This is an informational document where the multihoming
   configurations under the operation of NEMO Basic Support are
   analyzed.  Security considerations of these multihoming
   configurations, should they be different from those that concern NEMO
   Basic Support, must be considered by forthcoming solutions.  For
   instance, an attacker could try to use the multihomed device as a
   means to access another network that would not be normally reachable
   through the Internet.  Even when forwarding to another network is
   turned off by configuration, an attacker could compromise a system to
   enable it.






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8.  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.
   The initial evaluation of NEMO Basic Support on multihoming
   configurations is a contribution from Julien Charbon.

9.  References

9.1.  Normative References

   [1]   Ernst, T., "Network Mobility Support Goals and Requirements",
         RFC 4886, July 2007.

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

   [3]   Ernst, T. and H-Y. Lach, "Network Mobility Support
         Terminology", RFC 4885, July 2007.

   [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.

9.2.  Informative References

   [6]   Ernst, T., Montavont, N., Wakikawa, R., Ng, C., and K.
         Kuladinithi, "Motivations and Scenarios for Using Multiple
         Interfaces and Global Addresses", Work in Progress,
         October 2006.

   [7]   Montavont, N., Wakikawa, R., Ernst, T., Ng, C., and K.
         Kuladinithi, "Analysis of Multihoming in Mobile IPv6", Work
         in Progress, February 2006.

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

   [9]   Arkko, J. and I. Beijnum, "Failure Detection and Locator Pair
         Exploration Protocol for IPv6  Multihoming", Work in Progress,
         December 2006.





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   [10]  Krishnan, S., Montavont, N., Yegin, A., Veerepalli, S., and A.
         Yegin, "Link-layer Event Notifications for Detecting Network
         Attachments", Work in Progress, November 2006.

   [11]  Narayanan, S., "Detecting Network Attachment in IPv6 Networks
         (DNAv6)", Work in Progress, October 2006.

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

   [13]  Nordmark, E. and M. Bagnulo, "Level 3 multihoming shim
         protocol", Work in Progress, November 2006.

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

   [15]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
         Networks", BCP 84, RFC 3704, March 2004.

   [16]  Huitema, C. and M. Marcelo, "Ingress filtering compatibility
         for IPv6 multihomed sites", Work in Progress, October 2006.

   [17]  Wakikawa, R., Devarapalli, V., and P. Thubert, "Inter Home
         Agents Protocol (HAHA)", Work in Progress, February 2004.

   [18]  Koh, B., Ng, C., and J. Hirano, "Dynamic Inter Home Agent
         Protocol", Work in Progress, July 2004.

   [19]  Tsukada, M., "Analysis of Multiple Mobile Routers Cooperation",
         Work in Progress, October 2005.

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

   [21]  Droms, R. and P. Thubert, "DHCPv6 Prefix Delegation for NEMO",
         Work in Progress, September 2006.

   [22]  Thubert, P. and TJ. Kniveton, "Mobile Network Prefix
         Delegation", Work in Progress, November 2006.

   [23]  Wakikawa, R., "Multiple Care-of Addresses Registration", Work
         in Progress, June 2006.

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





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   [25]  Thubert, P., Bontous, C., and N. Nicolas, "Nested Nemo Tree
         Discovery", Work in Progress, November 2006.

   [26]  Kumazawa, M., "Token based Duplicate Network Detection for
         split mobile network (Token based DND)", Work in Progress,
         July 2005.

   [27]  Soliman, H., "Flow Bindings in Mobile IPv6 and NEMO Basic
         Support", Work in Progress, March 2007.










































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Appendix A.  Alternative Classifications Approach

A.1.  Ownership-Oriented Approach

   An alternative approach to classifying a multihomed mobile network
   was 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
   MRs 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 includes not
   introducing any aggregation between the HAs, which will filter out
   routing announcements for the MNP(s).

   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 intervals.
   However, a more appropriate tool might be the Binding Refresh Request





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   message, though conformance to the Binding Refresh Request message
   may be less strictly enforced in implementations since it serves a
   somewhat secondary role when compared to Binding Update messages.

A.1.2.  Subscriber/Provider Model

   The case of the HA(s) and MR(s) 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







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   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
      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 that 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
   cases, it is unlikely that the ISP will run IGP over the bi-
   directional tunnel, since the ISP will most certainly wish to retain
   full control of its routing domain.

A.2.  Problem-Oriented Approach

   A third approach was proposed by Pascal Thubert (Cisco Systems).
   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 CoAs of Same MNP

      This is the case where one MR registers different CoAs to the same
      HA 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 CoAs of Same MNP

      This is the case where the MR registers different CoAs to
      different HAs 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.




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   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.

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 an MR detects that one of its egress
   interface is down, it should detect if alternate routes 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 possesses.  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 Appendix, we will attempt to
   investigate methods of performing such re-establishment of bi-
   directional tunnels.  This method of tunnel re-establishment is
   particularly useful for the (*,n,n) NEMO configuration.  The method
   described is by no means complete and merely serves as a suggestion
   on how to approach the problem.  It is also not the objective 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 [12].

B.1.  Detecting Presence of Alternate Routes

   To actively utilize the robustness provided by multihoming, an 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 an 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.  An MR can do so by listening for
   Router Advertisement message on its *ingress* interfaces.  When an MR
   receives a Router Advertisement message with a non-zero Router



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   Lifetime field from one of its ingress interfaces, it knows that
   another MR that can provide an alternate route to the global Internet
   is present in the mobile network.

B.2.  Re-Establishment of Bi-Directional Tunnels

   When an 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
   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 an 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 MR to send a binding update to the HA of the failed
   interface using the CoA assigned to the alternate interface in order
   to re-establish the bi-directional tunneling using the CoA 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 CoA) assigned to
   an alternate egress interface as the new (back-up) CoA of the MR to
   re-establish the bi-directional tunneling with its HA.

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 CoA from the alternate MR (i.e., attach itself to the
   alternate MR).  Next, it sends binding update to its HA using the CoA
   obtained from the alternate MR.  From then on, the MR can encapsulate
   outgoing packets through the bi-directional tunnel via the alternate
   MR.

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





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   Note that every packet sent between MNNs and their correspondent
   nodes will experience two levels of encapsulation.  The first level
   of tunneling occurs between an MR that 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.

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 an 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).

B.4.  Points of Considerations

   This method of using tunnel re-establishments is by no means a
   complete solution.  There are still points to consider in order to
   develop it into a fully functional solution.  For instance, in
   Appendix B.1, it was suggested that MR detects the presence of other
   MRs using Router Advertisements.  However, Router Advertisements are
   link scoped, so when there is more than one link, some information
   may be lost.  For instance, suppose a case where there are three MRs
   and three different prefixes and each MR is in a different link with
   regular routers in between.  Suppose now that only a single MR is
   working; how do the other MRs identify which prefix they have to use
   to configure the new CoA?  In this case, there are three prefixes
   being announced, and an MR whose link has failed knows that its
   prefix is not to be used, but it does not have enough information to
   decide which one of the other two prefixes to use to configure the
   new CoA.  In such cases, a mechanism is needed to allow an MR to
   withdraw its own prefix when it discovers that its link is no longer
   working.








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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: chanwah.ng@sg.panasonic.com


   Thierry Ernst
   INRIA
   INRIA Rocquencourt
   Domaine de Voluceau B.P. 105
   Le Chesnay  78153
   France

   Phone: +33-1-39-63-59-30
   Fax:   +33-1-39-63-54-91
   EMail: thierry.ernst@inria.fr
   URI:   http://www.nautilus6.org/~thierry


   Eun Kyoung Paik
   KT
   KT Research Center
   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/


   Marcelo Bagnulo
   Universidad Carlos III de Madrid
   Av. Universidad, 30
   Leganes, Madrid  28911
   Spain

   Phone: +34 91624 8837
   EMail: marcelo@it.uc3m.es




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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
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Ng, et al.                   Informational                     [Page 39]


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