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RFC 4888 - Network Mobility Route Optimization Problem Statement


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Network Working Group                                              C. Ng
Request for Comments: 4888                      Panasonic Singapore Labs
Category: Informational                                       P. Thubert
                                                           Cisco Systems
                                                               M. Watari
                                                           KDDI R&D Labs
                                                                 F. Zhao
                                                                UC Davis
                                                               July 2007

         Network Mobility Route Optimization Problem Statement

Status of This Memo

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

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   With current Network Mobility (NEMO) Basic Support, all
   communications to and from Mobile Network Nodes must go through the
   bi-directional tunnel established between the Mobile Router and Home
   Agent when the mobile network is away.  This sub-optimal routing
   results in various inefficiencies associated with packet delivery,
   such as increased delay and bottleneck links leading to traffic
   congestion, which can ultimately disrupt all communications to and
   from the Mobile Network Nodes.  Additionally, with nesting of Mobile
   Networks, these inefficiencies get compounded, and stalemate
   conditions may occur in specific dispositions.  This document
   investigates such problems and provides the motivation behind Route
   Optimization (RO) for NEMO.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  NEMO Route Optimization Problem Statement  . . . . . . . . . .  3
     2.1.  Sub-Optimality with NEMO Basic Support . . . . . . . . . .  4
     2.2.  Bottleneck in the Home Network . . . . . . . . . . . . . .  6
     2.3.  Amplified Sub-Optimality in Nested Mobile Networks . . . .  6
     2.4.  Sub-Optimality with Combined Mobile IPv6 Route
           Optimization . . . . . . . . . . . . . . . . . . . . . . .  8
     2.5.  Security Policy Prohibiting Traffic from Visiting Nodes  .  9
     2.6.  Instability of Communications within a Nested Mobile
           Network  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     2.7.  Stalemate with a Home Agent Nested in a Mobile Network . . 10
   3.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   5.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.1.  Normative Reference  . . . . . . . . . . . . . . . . . . . 12
     6.2.  Informative Reference  . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Various Configurations Involving Nested Mobile
                Networks  . . . . . . . . . . . . . . . . . . . . . . 13
     A.1.  CN Located in the Fixed Infrastructure . . . . . . . . . . 13
       A.1.1.  Case A: LFN and Standard IPv6 CN . . . . . . . . . . . 14
       A.1.2.  Case B: VMN and MIPv6 CN . . . . . . . . . . . . . . . 14
       A.1.3.  Case C: VMN and Standard IPv6 CN . . . . . . . . . . . 14
     A.2.  CN Located in Distinct Nested NEMOs  . . . . . . . . . . . 15
       A.2.1.  Case D: LFN and Standard IPv6 CN . . . . . . . . . . . 16
       A.2.2.  Case E: VMN and MIPv6 CN . . . . . . . . . . . . . . . 16
       A.2.3.  Case F: VMN and Standard IPv6 CN . . . . . . . . . . . 16
     A.3.  MNN and CN Located in the Same Nested NEMO . . . . . . . . 17
       A.3.1.  Case G: LFN and Standard IPv6 CN . . . . . . . . . . . 18
       A.3.2.  Case H: VMN and MIPv6 CN . . . . . . . . . . . . . . . 18
       A.3.3.  Case I: VMN and Standard IPv6 CN . . . . . . . . . . . 19
     A.4.  CN Located Behind the Same Nested MR . . . . . . . . . . . 19
       A.4.1.  Case J: LFN and Standard IPv6 CN . . . . . . . . . . . 20
       A.4.2.  Case K: VMN and MIPv6 CN . . . . . . . . . . . . . . . 20
       A.4.3.  Case L: VMN and Standard IPv6 CN . . . . . . . . . . . 21
   Appendix B.  Example of How a Stalemate Situation Can Occur  . . . 22

1.  Introduction

   With current Network Mobility (NEMO) Basic Support [1], all
   communications to and from nodes in a mobile network must go through
   the bi-directional tunnel established between the Mobile Router and
   its Home Agent (also known as the MRHA tunnel) when the mobile
   network is away.  Although such an arrangement allows Mobile Network
   Nodes to reach and be reached by any node on the Internet,
   limitations associated to the base protocol degrade overall
   performance of the network and, ultimately, can prevent all
   communications to and from the Mobile Network Nodes.

   Some of these concerns already exist with Mobile IPv6 [4] and were
   addressed by the mechanism known as Route Optimization, which is part
   of the base protocol.  With Mobile IPv6, Route Optimization mostly
   improves the end-to-end path between the Mobile Node and
   Correspondent Node, with an additional benefit of reducing the load
   of the Home Network, thus its name.

   NEMO Basic Support presents a number of additional issues, making the
   problem more complex, so it was decided to address Route Optimization
   separately.  In that case, the expected benefits are more dramatic,
   and a Route Optimization mechanism could enable connectivity that
   would be broken otherwise.  In that sense, Route Optimization is even
   more important to NEMO Basic Support than it is to Mobile IPv6.

   This document explores limitations inherent in NEMO Basic Support,
   and their effects on communications between a Mobile Network Node and
   its corresponding peer.  This is detailed in Section 2.  It is
   expected that readers are familiar with general terminologies related
   to mobility in [4][2], NEMO-related terms defined in [3], and NEMO
   goals and requirements [5].

2.  NEMO Route Optimization Problem Statement

   Given the NEMO Basic Support protocol, all data packets to and from
   Mobile Network Nodes must go through the Home Agent, even though a
   shorter path may exist between the Mobile Network Node and its
   Correspondent Node.  In addition, with the nesting of Mobile Routers,
   these data packets must go through multiple Home Agents and several
   levels of encapsulation, which may be avoided.  This results in
   various inefficiencies and problems with packet delivery, which can
   ultimately disrupt all communications to and from the Mobile Network
   Nodes.

   In the following sub-sections, we will describe the effects of a
   pinball route with NEMO Basic Support, how it may cause a bottleneck
   to be formed in the Home Network, and how these get amplified with

   nesting of mobile networks.  Closely related to nesting, we will also
   look into the sub-optimality even when Mobile IPv6 Route Optimization
   is used over NEMO Basic Support.  This is followed by a description
   of security policy in the Home Network that may forbid transit
   traffic from Visiting Mobile Nodes in mobile networks.  In addition,
   we will explore the impact of the MRHA tunnel on communications
   between two Mobile Network Nodes on different links of the same
   mobile network.  We will also provide additional motivations for
   Route Optimization by considering the potential stalemate situation
   when a Home Agent is part of a mobile network.

2.1.  Sub-Optimality with NEMO Basic Support

   With NEMO Basic Support, all packets sent between a Mobile Network
   Node and its Correspondent Node are forwarded through the MRHA
   tunnel, resulting in a pinball route between the two nodes.  This has
   the following sub-optimal effects:

   o  Longer Route Leading to Increased Delay and Additional
      Infrastructure Load

      Because a packet must transit from a mobile network to the Home
      Agent then to the Correspondent Node, the transit time of the
      packet is usually longer than if the packet were to go straight
      from the mobile network to the Correspondent Node.  When the
      Correspondent Node (or the mobile network) resides near the Home
      Agent, the increase in packet delay can be very small.  However,
      when the mobile network and the Correspondent Node are relatively
      near to one another but far away from the Home Agent on the
      Internet, the increase in delay is very large.  Applications such
      as real-time multimedia streaming may not be able to tolerate such
      increase in packet delay.  In general, the increase in delay may
      also impact the performance of transport protocols such as TCP,
      since the sending rate of TCP is partly determined by the round-
      trip time (RTT) perceived by the communication peers.

      Moreover, by using a longer route, the total resource utilization
      for the traffic would be much higher than if the packets were to
      follow a direct path between the Mobile Network Node and
      Correspondent Node.  This would result in additional load in the
      infrastructure.

   o  Increased Packet Overhead

      The encapsulation of packets in the MRHA tunnel results in
      increased packet size due to the addition of an outer header.
      This reduces the bandwidth efficiency, as an IPv6 header can be

      quite substantial relative to the payload for applications such as
      voice samples.  For instance, given a voice application using an 8
      kbps algorithm (e.g., G.729) and taking a voice sample every 20 ms
      (as in RFC 1889 [6]), the packet transmission rate will be 50
      packets per second.  Each additional IPv6 header is an extra 320
      bits per packet (i.e., 16 kbps), which is twice the actual
      payload!

   o  Increased Processing Delay

      The encapsulation of packets in the MRHA tunnel also results in
      increased processing delay at the points of encapsulation and
      decapsulation.  Such increased processing may include encryption/
      decryption, topological correctness verifications, MTU
      computation, fragmentation, and reassembly.

   o  Increased Chances of Packet Fragmentation

      The augmentation in packet size due to packet encapsulation may
      increase the chances of the packet being fragmented along the MRHA
      tunnel.  This can occur if there is no prior path MTU discovery
      conducted, or if the MTU discovery mechanism did not take into
      account the encapsulation of packets.  Packet fragmentation will
      result in a further increase in packet delays and further
      reduction of bandwidth efficiency.

   o  Increased Susceptibility to Link Failure

      Under the assumption that each link has the same probability of
      link failure, a longer routing path would be more susceptible to
      link failure.  Thus, packets routed through the MRHA tunnel may be
      subjected to a higher probability of being lost or delayed due to
      link failure, compared to packets that traverse directly between
      the Mobile Network Node and its Correspondent Node.

2.2.  Bottleneck in the Home Network

   Apart from the increase in packet delay and infrastructure load,
   forwarding packets through the Home Agent may also lead to either the
   Home Agent or the Home Link becoming a bottleneck for the aggregated
   traffic from/to all the Mobile Network Nodes.  A congestion at home
   would lead to additional packet delay, or even packet loss.  In
   addition, Home Agent operations such as security check, packet
   interception, and tunneling might not be as optimized in the Home
   Agent software as plain packet forwarding.  This could further limit
   the Home Agent capacity for data traffic.  Furthermore, with all
   traffic having to pass through the Home Link, the Home Link becomes a
   single point of failure for the mobile network.

   Data packets that are delayed or discarded due to congestion at the
   Home Network would cause additional performance degradation to
   applications.  Signaling packets, such as Binding Update messages,
   that are delayed or discarded due to congestion at the Home Network
   may affect the establishment or update of bi-directional tunnels,
   causing disruption of all traffic flow through these tunnels.

   A NEMO Route Optimization mechanism that allows the Mobile Network
   Nodes to communicate with their Correspondent Nodes via a path that
   is different from the MRHA tunneling and thereby avoiding the Home
   Agent may alleviate or even prevent the congestion at the Home Agent
   or Home Link.

2.3.  Amplified Sub-Optimality in Nested Mobile Networks

   By allowing other mobile nodes to join a mobile network, and in
   particular mobile routers, it is possible to form arbitrary levels of
   nesting of mobile networks.  With such nesting, the use of NEMO Basic
   Support further amplifies the sub-optimality of routing.  We call
   this the amplification effect of nesting, where the undesirable
   effects of a pinball route with NEMO Basic Support are amplified with
   each level of nesting of mobile networks.  This is best illustrated
   by an example shown in Figure 1.

               +--------+  +--------+  +--------+  +--------+
               | MR2_HA |  | MR3_HA |  | MR4_HA |  | MR5_HA |
               +------+-+  +---+----+  +---+----+  +-+------+
                       \       |           |        /
        +--------+    +------------------------------+
        | MR1_HA |----|         Internet             |-----CN1
        +--------+    +------------------------------+
                                    |
                                +---+---+
                      root-MR   |  MR1  |
                                +-------+
                                 |     |
                          +-------+   +-------+
                 sub-MR   |  MR2  |   |  MR4  |
                          +---+---+   +---+---+
                              |           |
                          +---+---+   +---+---+
                 sub-MR   |  MR3  |   |  MR5  |
                          +---+---+   +---+---+
                              |           |
                          ----+----   ----+----
                             MNN         CN2

              Figure 1: An Example of a Nested Mobile Network

   Using NEMO Basic Support, the flow of packets between a Mobile
   Network Node, MNN, and a Correspondent Node, CN1, would need to go
   through three separate tunnels, illustrated in Figure 2 below.

                                ----------.
                      ---------/         /----------.
              -------/        |         |          /-------
    MNN -----( -  - | -  -  - | -  -  - | -  -  - |  -  - (------ CN1
           MR3-------\        |         |          \-------MR3_HA
                    MR2--------\         \----------MR2_HA
                              MR1---------MR1_HA

                Figure 2: Nesting of Bi-Directional Tunnels

   This leads to the following problems:

   o  Pinball Route

      Both inbound and outbound packets will flow via the Home Agents of
      all the Mobile Routers on their paths within the mobile network,
      with increased latency, less resilience, and more bandwidth usage.
      Appendix A illustrates in detail the packets' routes under
      different nesting configurations of the Mobile Network Nodes.

   o  Increased Packet Size

      An extra IPv6 header is added per level of nesting to all the
      packets.  The header compression suggested in [7] cannot be
      applied because both the source and destination (the intermediate
      Mobile Router and its Home Agent) are different hop to hop.

   Nesting also amplifies the probability of congestion at the Home
   Networks of the upstream Mobile Routers.  In addition, the Home Link
   of each upstream Mobile Router will also be a single point of failure
   for the nested Mobile Router.

2.4.  Sub-Optimality with Combined Mobile IPv6 Route Optimization

   When a Mobile IPv6 host joins a mobile network, it becomes a Visiting
   Mobile Node of the mobile network.  Packets sent to and from the
   Visiting Mobile Node will have to be routed not only via the Home
   Agent of the Visiting Mobile Node, but also via the Home Agent of the
   Mobile Router in the mobile network.  This suffers the same
   amplification effect of nested mobile network mentioned in
   Section 2.3.

   In addition, although Mobile IPv6 [4] allows a mobile host to perform
   Route Optimization with its Correspondent Node in order to avoid
   tunneling with its Home Agent, the "optimized" route is no longer
   optimized when the mobile host is attached to a mobile network.  This
   is because the route between the mobile host and its Correspondent
   Node is subjected to the sub-optimality introduced by the MRHA
   tunnel.  Interested readers may refer to Appendix A for examples of
   how the routes will appear with nesting of Mobile IPv6 hosts in
   mobile networks.

   The readers should also note that the same sub-optimality would apply
   when the mobile host is outside the mobile network and its
   Correspondent Node is in the mobile network.

2.5.  Security Policy Prohibiting Traffic from Visiting Nodes

   NEMO Basic Support requires all traffic from visitors to be tunneled
   to the Mobile Router's Home Agent.  This might represent a breach in
   the security of the Home Network (some specific attacks against the
   Mobile Router's binding by rogue visitors have been documented in
   [8][9]).  Administrators might thus fear that malicious packets will
   be routed into the Home Network via the bi-directional tunnel.  As a
   consequence, it can be expected that in many deployment scenarios,
   policies will be put in place to prevent unauthorized Visiting Mobile
   Nodes from attaching to the Mobile Router.

   However, there are deployment scenarios where allowing unauthorized
   Visiting Mobile Nodes is actually desirable.  For instance, when
   Mobile Routers attach to other Mobile Routers and form a nested NEMO,
   they depend on each other to reach the Internet.  When Mobile Routers
   have no prior knowledge of one another (no security association,
   Authentication, Authorization, and Accounting (AAA), Public-Key
   Infrastructure (PKI), etc.), it could still be acceptable to forward
   packets, provided that the packets are not tunneled back to the Home
   Networks.

   A Route Optimization mechanism that allows traffic from Mobile
   Network Nodes to bypass the bi-directional tunnel between a Mobile
   Router and its Home Agent would be a necessary first step towards a
   Tit for Tat model, where MRs would benefit from a reciprocal
   altruism, based on anonymity and innocuousness, to extend the
   Internet infrastructure dynamically.

2.6.  Instability of Communications within a Nested Mobile Network

   Within a nested mobile network, two Mobile Network Nodes may
   communicate with each other.  Let us consider the previous example
   illustrated in Figure 1 where MNN and CN2 are sharing a communication
   session.  With NEMO Basic Support, a packet sent from MNN to CN2 will
   need to be forwarded to the Home Agent of each Mobile Router before
   reaching CN2, whereas, a packet following the direct path between
   them need not even leave the mobile network.  Readers are referred to
   Appendix A.3 for detailed illustration of the resulting routing
   paths.

   Apart from the consequences of increased packet delay and packet
   size, which are discussed in previous sub-sections, there are two
   additional effects that are undesirable:

   o  when the nested mobile network is disconnected from the Internet
      (e.g., MR1 loses its egress connectivity), MNN and CN2 can no

      longer communicate with each other, even though the direct path
      from MNN to CN2 is unaffected;

   o  the egress link(s) of the root Mobile Router (i.e., MR1) becomes a
      bottleneck for all the traffic that is coming in and out of the
      nested mobile network.

   A Route Optimization mechanism could allow traffic between two Mobile
   Network Nodes nested within the same mobile network to follow a
   direct path between them, without being routed out of the mobile
   network.  This may also off-load the processing burden of the
   upstream Mobile Routers when the direct path between the two Mobile
   Network Nodes does not traverse these Mobile Routers.

2.7.  Stalemate with a Home Agent Nested in a Mobile Network

   Several configurations for the Home Network are described in [10].
   In particular, there is a mobile home scenario where a (parent)
   Mobile Router is also a Home Agent for its mobile network.  In other
   words, the mobile network is itself an aggregation of Mobile Network
   Prefixes assigned to (children) Mobile Routers.

   A stalemate situation exists in the case where the parent Mobile
   Router visits one of its children.  The child Mobile Router cannot
   find its Home Agent in the Internet and thus cannot establish its
   MRHA tunnel and forward the visitor's traffic.  The traffic from the
   parent is thus blocked from reaching the Internet, and it will never
   bind to its own (grandparent) Home Agent.  Appendix B gives a
   detailed illustration of how such a situation can occur.

   Then again, a Route Optimization mechanism that bypasses the nested
   tunnel might enable the parent traffic to reach the Internet and let
   it bind.  At that point, the child Mobile Router would be able to
   reach its parent and bind in turn.  Additional nested Route
   Optimization solutions might also enable the child to locate its Home
   Agent in the nested structure and bind regardless of whether or not
   the Internet is reachable.

3.  Conclusion

   With current NEMO Basic Support, all communications to and from
   Mobile Network Nodes must go through the MRHA tunnel when the mobile
   network is away.  This results in various inefficiencies associated
   with packet delivery.  This document investigates such inefficiencies
   and provides the motivation behind Route Optimization for NEMO.

   We have described the sub-optimal effects of pinball routes with NEMO
   Basic Support, how they may cause a bottleneck to be formed in the
   Home Network, and how they get amplified with nesting of mobile
   networks.  These effects will also be seen even when Mobile IPv6
   Route Optimization is used over NEMO Basic Support.  In addition,
   other issues concerning the nesting of mobile networks that might
   provide additional motivation for a NEMO Route Optimization mechanism
   were also explored, such as the prohibition of forwarding traffic
   from a Visiting Mobile Node through an MRHA tunnel due to security
   concerns, the impact of the MRHA tunnel on communications between two
   Mobile Network Nodes on different links of the same mobile network,
   and the possibility of a stalemate situation when Home Agents are
   nested within a mobile network.

4.  Security Considerations

   This document highlights some limitations of NEMO Basic Support.  In
   particular, some security concerns could prevent interesting
   applications of the protocol, as detailed in Section 2.5.

   Route Optimization for RFC 3963 [1] might introduce new threats, just
   as it might alleviate existing ones.  This aspect will certainly be a
   key criterion in the evaluation of the proposed solutions.

5.  Acknowledgments

   The authors wish to thank the co-authors of previous versions from
   which this document is derived: Marco Molteni, Paik Eun-Kyoung,
   Hiroyuki Ohnishi, Thierry Ernst, Felix Wu, and Souhwan Jung.  Early
   work by Masafumi Watari on the extracted appendix was written while
   still at Keio University.  In addition, sincere appreciation is also
   extended to Jari Arkko, Carlos Bernardos, Greg Daley, T.J. Kniveton,
   Henrik Levkowetz, Erik Nordmark, Alexandru Petrescu, Hesham Soliman,
   Ryuji Wakikawa, and Patrick Wetterwald for their various
   contributions.

6.  References

6.1.  Normative Reference

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

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

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

6.2.  Informative Reference

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

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

   [6]   Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications",
         RFC 1889, January 1996.

   [7]   Deering, S. and B. Zill, "Redundant Address Deletion when
         Encapsulating IPv6 in IPv6", Work in Progress, November 2001.

   [8]   Petrescu, A., Olivereau, A., Janneteau, C., and H-Y. Lach,
         "Threats for Basic Network Mobility Support (NEMO threats)",
         Work in Progress, January 2004.

   [9]   Jung, S., Zhao, F., Wu, S., Kim, H-G., and S-W. Sohn, "Threat
         Analysis on NEMO Basic Operations", Work in Progress,
         July 2004.

   [10]  Thubert, P., Wakikawa, R., and V. Devarapalli, "Network
         Mobility Home Network Models", RFC RFC4887, July 2007.

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

Appendix A.  Various Configurations Involving Nested Mobile Networks

   In the following sections, we try to describe different communication
   models that involve a nested mobile network and to clarify the issues
   for each case.  We illustrate the path followed by packets if we
   assume nodes only use Mobile IPv6 and NEMO Basic Support mechanisms.
   Different cases are considered where a Correspondent Node is located
   in the fixed infrastructure, in a distinct nested mobile network as
   the Mobile Network Node, or in the same nested mobile network as the
   Mobile Network Node.  Additionally, cases where Correspondent Nodes
   and Mobile Network Nodes are either standard IPv6 nodes or Mobile
   IPv6 nodes are considered.  As defined in [3], standard IPv6 nodes
   are nodes with no mobility functions whatsoever, i.e., they are not
   Mobile IPv6 or NEMO enabled.  This means that they cannot move around
   keeping open connections and that they cannot process Binding Updates
   sent by peers.

A.1.  CN Located in the Fixed Infrastructure

   The most typical configuration is the case where a Mobile Network
   Node communicates with a Correspondent Node attached in the fixed
   infrastructure.  Figure 3 below shows an example of such topology.

                    +--------+  +--------+  +--------+
                    | MR1_HA |  | MR2_HA |  | MR3_HA |
                    +---+----+  +---+----+  +---+----+
                        |           |           |
                       +-------------------------+
                       |        Internet         |----+ CN
                       +-------------------------+
                               |               |
                           +---+---+        +--+-----+
                 root-MR   |  MR1  |        | VMN_HA |
                           +---+---+        +--------+
                               |
                           +---+---+
                  sub-MR   |  MR2  |
                           +---+---+
                               |
                           +---+---+
                  sub-MR   |  MR3  |
                           +---+---+
                               |
                           ----+----
                              MNN

                Figure 3: CN Located at the Infrastructure

A.1.1.  Case A: LFN and Standard IPv6 CN

   The simplest case is where both MNN and CN are fixed nodes with no
   mobility functions.  That is, MNN is a Local Fixed Node, and CN is a
   standard IPv6 node.  Packets are encapsulated between each Mobile
   Router and its respective Home Agent (HA).  As shown in Figure 4, in
   such a case, the path between the two nodes would go through:

        1       2       3       4          3          2          1
   MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA --- CN
   LFN                                                         IPv6 Node

             The digits represent the number of IPv6 headers.

               Figure 4: MNN and CN Are Standard IPv6 Nodes

A.1.2.  Case B: VMN and MIPv6 CN

   In this second case, both end nodes are Mobile IPv6-enabled mobile
   nodes, that is, MNN is a Visiting Mobile Node.  Mobile IPv6 Route
   Optimization may thus be initiated between the two and packets would
   not go through the Home Agent of the Visiting Mobile Node or the Home
   Agent of the Correspondent Node (not shown in the figure).  However,
   packets will still be tunneled between each Mobile Router and its
   respective Home Agent, in both directions.  As shown in Figure 5, the
   path between MNN and CN would go through:

        1       2       3       4          3          2          1
   MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA --- CN
   VMN                                                             MIPv6

                Figure 5: MNN and CN Are MIPv6 Mobile Nodes

A.1.3.  Case C: VMN and Standard IPv6 CN

   When the communication involves a Mobile IPv6 node either as a
   Visiting Mobile Node or as a Correspondent Node, Mobile IPv6 Route
   Optimization cannot be performed because the standard IPv6
   Correspondent Node cannot process Mobile IPv6 signaling.  Therefore,
   MNN would establish a bi-directional tunnel with its HA, which causes
   the flow to go out the nested NEMO.  Packets between MNN and CN would
   thus go through MNN's own Home Agent (VMN_HA).  The path would
   therefore be as shown in Figure 6:

               2       3       4       5          4
          MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA
          VMN                                           |
                                                        | 3
                                       1          2     |
                                   CN --- VMN_HA --- MR3_HA
                                IPv6 Node

   Figure 6: MNN is an MIPv6 Mobile Node and CN is a Standard IPv6 Node

   Providing Route Optimization involving a Mobile IPv6 node may require
   optimization among the Mobile Routers and the Mobile IPv6 node.

A.2.  CN Located in Distinct Nested NEMOs

   The Correspondent Node may be located in another nested mobile
   network, different from the one MNN is attached to, as shown in
   Figure 7.  We define such configuration as "distinct nested mobile
   networks".

              +--------+  +--------+  +--------+  +--------+
              | MR2_HA |  | MR3_HA |  | MR4_HA |  | MR5_HA |
              +------+-+  +---+----+  +---+----+  +-+------+
                      \       |           |        /
         +--------+    +-------------------------+    +--------+
         | MR1_HA |----|        Internet         |----| VMN_HA |
         +--------+    +-------------------------+    +--------+
                          |                   |
                      +---+---+           +---+---+
            root-MR   |  MR1  |           |  MR4  |
                      +---+---+           +---+---+
                          |                   |
                      +---+---+           +---+---+
             sub-MR   |  MR2  |           |  MR5  |
                      +---+---+           +---+---+
                          |                   |
                      +---+---+           ----+----
             sub-MR   |  MR3  |              CN
                      +---+---+
                          |
                      ----+----
                         MNN

           Figure 7: MNN and CN Located in Distinct Nested NEMOs

A.2.1.  Case D: LFN and Standard IPv6 CN

   Similar to Case A, we start off with the case where both end nodes do
   not have any mobility functions.  Packets are encapsulated at every
   Mobile Router on the way out of the nested mobile network,
   decapsulated by the Home Agents, and then encapsulated again on their
   way down the nested mobile network.

            1       2       3       4          3          2
       MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA
       LFN                                                      |
                                                                | 1
                               1       2       3          2     |
                           CN --- MR5 --- MR4 --- MR4_HA --- MR5_HA
                        IPv6 Node

               Figure 8: MNN and CN Are Standard IPv6 Nodes

A.2.2.  Case E: VMN and MIPv6 CN

   Similar to Case B, when both end nodes are Mobile IPv6 nodes, the two
   nodes may initiate Mobile IPv6 Route Optimization.  Again, packets
   will not go through the Home Agent of the MNN or the Home Agent of
   the Mobile IPv6 Correspondent Node (not shown in the figure).
   However, packets will still be tunneled for each Mobile Router to its
   Home Agent and vice versa.  Therefore, the path between MNN and CN
   would go through:

            1       2       3       4          3          2
       MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA
       VMN                                                      |
                                                                | 1
                               1       2       3          2     |
                           CN --- MR5 --- MR4 --- MR4_HA --- MR5_HA
                       MIPv6 Node

                Figure 9: MNN and CN Are MIPv6 Mobile Nodes

A.2.3.  Case F: VMN and Standard IPv6 CN

   Similar to Case C, when the communication involves a Mobile IPv6 node
   either as a Visiting Mobile Node or as a Correspondent Node, MIPv6
   Route Optimization cannot be performed because the standard IPv6
   Correspondent Node cannot process Mobile IPv6 signaling.  MNN would

   therefore establish a bi-directional tunnel with its Home Agent.
   Packets between MNN and CN would thus go through MNN's own Home Agent
   as shown in Figure 10:

            2       3       4       5          4          3
       MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA
       VMN                                                      |
                                                                | 2
                   1       2       3           2          1     |
               CN --- MR5 --- MR4 --- MR4_HA  --- MR5_HA --- VMN_HA
            IPv6 Node

   Figure 10: MNN is an MIPv6 Mobile Node and CN is a Standard IPv6 Node

A.3.  MNN and CN Located in the Same Nested NEMO

   Figure 11 below shows the case where the two communicating nodes are
   connected behind different Mobile Routers that are connected in the
   same nested mobile network, and thus behind the same root Mobile
   Router.  Route Optimization can avoid packets being tunneled outside
   the nested mobile network.

              +--------+  +--------+  +--------+  +--------+
              | MR2_HA |  | MR3_HA |  | MR4_HA |  | MR5_HA |
              +------+-+  +---+----+  +---+----+  +-+------+
                      \       |           |        /
         +--------+    +-------------------------+    +--------+
         | MR1_HA |----|        Internet         |----| VMN_HA |
         +--------+    +-------------------------+    +--------+
                                    |
                                +---+---+
                      root-MR   |  MR1  |
                                +-------+
                                 |     |
                          +-------+   +-------+
                 sub-MR   |  MR2  |   |  MR4  |
                          +---+---+   +---+---+
                              |           |
                          +---+---+   +---+---+
                 sub-MR   |  MR3  |   |  MR5  |
                          +---+---+   +---+---+
                              |           |
                          ----+----   ----+----
                             MNN          CN

           Figure 11: MNN and CN Located in the Same Nested NEMO

A.3.1.  Case G: LFN and Standard IPv6 CN

   Again, we start off with the case where both end nodes do not have
   any mobility functions.  Packets are encapsulated at every Mobile
   Router on the way out of the nested mobile network via the root
   Mobile Router, decapsulated and encapsulated by the Home Agents, and
   then make their way back to the nested mobile network through the
   same root Mobile Router.  Therefore, the path between MNN and CN
   would go through:

            1       2       3       4          3          2
       MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA
       LFN                                                      |
                                                                | 1
            1       2       3       4          3          2     |
        CN --- MR5 --- MR4 --- MR1 --- MR1_HA --- MR4_HA --- MR5_HA
     IPv6 Node

               Figure 12: MNN and CN Are Standard IPv6 nodes

A.3.2.  Case H: VMN and MIPv6 CN

   Similar to Case B and Case E, when both end nodes are Mobile IPv6
   nodes, the two nodes may initiate Mobile IPv6 Route Optimization,
   which will avoid the packets going through the Home Agent of MNN or
   the Home Agent of the Mobile IPv6 CN (not shown in the figure).
   However, packets will still be tunneled between each Mobile Router
   and its respective Home Agent in both directions.  Therefore, the
   path would be the same as with Case G and go through:

             1       2       3       4          3          2
        MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA
        LFN                                                      |
                                                                 | 1
             1       2       3       4          3          2     |
         CN --- MR5 --- MR4 --- MR1 --- MR1_HA --- MR4_HA --- MR5_HA
     MIPv6 Node

               Figure 13: MNN and CN Are MIPv6 Mobile Nodes

A.3.3.  Case I: VMN and Standard IPv6 CN

   As for Case C and Case F, when the communication involves a Mobile
   IPv6 node either as a Visiting Mobile Node or as a Correspondent
   Node, Mobile IPv6 Route Optimization cannot be performed.  Therefore,
   MNN will establish a bi-directional tunnel with its Home Agent.
   Packets between MNN and CN would thus go through MNN's own Home
   Agent.  The path would therefore be as shown in Figure 14:

            2       3       4       5          4          3
       MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA
       VMN                                                      |
                                                                | 2
                                                                |
                                                             VMN_HA
                                                                |
                                                                | 1
             1       2       3       4          3          2    |
         CN --- MR5 --- MR4 --- MR1 --- MR1_HA --- MR4_HA --- MR5_HA
      IPv6 Node

   Figure 14: MNN is an MIPv6 Mobile Node and CN is a Standard IPv6 Node

A.4.  CN Located Behind the Same Nested MR

   Figure 15 below shows the case where the two communicating nodes are
   connected behind the same nested Mobile Router.  The optimization is
   required when the communication involves MIPv6-enabled nodes.

              +--------+  +--------+  +--------+  +--------+
              | MR2_HA |  | MR3_HA |  | MR4_HA |  | MR5_HA |
              +------+-+  +---+----+  +---+----+  +-+------+
                      \       |           |        /
         +--------+    +-------------------------+    +--------+
         | MR1_HA |----|        Internet         |----| VMN_HA |
         +--------+    +-------------------------+    +--------+
                                    |
                                +---+---+
                      root-MR   |  MR1  |
                                +---+---+
                                    |
                                +-------+
                       sub-MR   |  MR2  |
                                +---+---+
                                    |
                                +---+---+
                       sub-MR   |  MR3  |
                                +---+---+
                                    |
                                -+--+--+-
                                MNN    CN

          Figure 15: MNN and CN Located Behind the Same Nested MR

A.4.1.  Case J: LFN and Standard IPv6 CN

   If both end nodes are Local Fixed Nodes, no special function is
   necessary for optimization of their communications.  The path between
   the two nodes would go through:

                                  1
                             MNN --- CN
                             LFN   IPv6 Node

               Figure 16: MNN and CN Are Standard IPv6 Nodes

A.4.2.  Case K: VMN and MIPv6 CN

   Similar to Case H, when both end nodes are Mobile IPv6 nodes, the two
   nodes may initiate Mobile IPv6 Route Optimization.  Although few
   packets would go out the nested mobile network for the Return
   Routability initialization, however, unlike Case B and Case E,
   packets will not get tunneled outside the nested mobile network.
   Therefore, packets between MNN and CN would eventually go through:

                                  1
                             MNN --- CN
                             VMN   MIPv6 Node

               Figure 17: MNN and CN are MIPv6 Mobile Nodes

   If the root Mobile Router is disconnected while the nodes exchange
   keys for the Return Routability procedure, they may not communicate
   even though they are connected on the same link.

A.4.3.  Case L: VMN and Standard IPv6 CN

   When the communication involves a Mobile IPv6 node either as a
   Visiting Mobile Network Node or as a Correspondent Node, Mobile IPv6
   Route Optimization cannot be performed.  Therefore, even though the
   two nodes are on the same link, MNN will establish a bi-directional
   tunnel with its Home Agent, which causes the flow to go out the
   nested mobile network.  The path between MNN and CN would require
   another Home Agent (VMN_HA) to go through for this Mobile IPv6 node:

            2       3       4       5          4          3
       MNN --- MR3 --- MR2 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA
       VMN                                                      |
                                                                | 2
                                                                |
                                                             VMN_HA
                                                                |
                                                                | 1
             1       2       3       4          3          2    |
         CN --- MR5 --- MR4 --- MR1 --- MR1_HA --- MR2_HA --- MR3_HA
      IPv6 Node

   Figure 18: MNN is an MIPv6 Mobile Node and CN is a Standard IPv6 Node

   However, MNN may also decide to use its Care-of Address (CoA) as the
   source address of the packets, thus avoiding the tunneling with the
   MNN's Home Agent.  This is particularly useful for a short-term
   communications that may easily be retried if it fails.  Default
   Address Selection [11] provides some mechanisms for controlling the
   choice of the source address.

Appendix B.  Example of How a Stalemate Situation Can Occur

   Section 2.7 describes the occurrence of a stalemate situation where a
   Home Agent of a Mobile Router is nested behind the Mobile Router.
   Here, we illustrate a simple example where such a situation can
   occur.

   Consider a mobility configuration depicted in Figure 19 below.  MR1
   is served by HA1/BR and MR2 is served by HA2.  The 'BR' designation
   indicates that HA1 is a border router.  Both MR1 and MR2 are at home
   in the initial step.  HA2 is placed inside the first mobile network,
   thus representing a "mobile" Home Agent.

                                                     /-----CN
                                         +----------+
        home link 1         +--------+   |          |
      ----+-----------------| HA1/BR |---| Internet |
          |                 +--------+   |          |
          |                              +----------+
       +--+--+  +-----+
       | MR1 |  | HA2 |
       +--+--+  +--+--+
          |        |
         -+--------+-- mobile net 1 / home link 2
          |
       +--+--+  +--+--+
       | MR2 |  | LFN |
       +--+--+  +--+--+
           |        |
          -+--------+- mobile net 2

                       Figure 19: Initial Deployment

   In Figure 19 above, communications between CN and LFN follow a direct
   path as long as both MR1 and MR2 are positioned at home.  No
   encapsulation intervenes.

   In the next step, consider that the MR2's mobile network leaves home
   and visits a foreign network, under Access Router (AR) like in
   Figure 20 below.

                                               /-----CN
                                   +----------+
        home link 1   +--------+   |          |
        --+-----------| HA1/BR |---| Internet |
          |           +--------+   |          |
       +--+--+  +-----+            +----------+
       | MR1 |  | HA2 |                        \
       +--+--+  +--+--+                        +-----+
          |        |                           | AR  |
         -+--------+- mobile net 1             +--+--+
                      home link 2                 |
                                               +--+--+  +-----+
                                               | MR2 |  | LFN |
                                               +--+--+  +--+--+
                                                  |        |
                                    mobile net 2 -+--------+-

                  Figure 20: Mobile Network 2 Leaves Home

   Once MR2 acquires a Care-of Address under AR, the tunnel setup
   procedure occurs between MR2 and HA2.  MR2 sends a Binding Update to
   HA2 and HA2 replies with a Binding Acknowledgement to MR2.  The bi-
   directional tunnel has MR2 and HA2 as tunnel endpoints.  After the
   tunnel MR2HA2 has been set up, the path taken by a packet from CN
   towards LFN can be summarized as:

       CN->BR->MR1->HA2=>MR1=>BR=>AR=>MR2->LFN.

   Non-encapsulated packets are marked "->" while encapsulated packets
   are marked "=>".

   Consider next the attachment of the first mobile network under the
   second mobile network, like in Figure 21 below.

   After this movement, MR1 acquires a Care-of Address valid in the
   second mobile network.  Subsequently, it sends a Binding Update (BU)
   message addressed to HA1.  This Binding Update is encapsulated by MR2
   and sent towards HA2, which is expected to be placed in mobile net 1
   and expected to be at home.  Once HA1/BR receives this encapsulated
   BU, it tries to deliver to MR1.  Since MR1 is not at home, and a
   tunnel has not yet been set up between MR1 and HA1, HA1 is not able
   to route this packet and drops it.  Thus, the tunnel establishment
   procedure between MR1 and HA1 is not possible, because the tunnel
   between MR2 and HA2 had been previously torn down (when the mobile
   net 1 moved from home).  The communications between CN and LFN stops,
   even though both mobile networks are connected to the Internet.

                                      /-----CN
                          +----------+
             +--------+   |          |
             | HA1/BR |---| Internet |
             +--------+   |          |
                          +----------+
                                      \
                                      +-----+
                                      | AR  |
                                      +--+--+
                                         |
                                      +--+--+  +-----+
                                      | MR2 |  | LFN |
                                      +--+--+  +--+--+
                                         |        |
                           mobile net 2 -+--------+-
                                         |
                                      +--+--+  +-----+
                                      | MR1 |  | HA2 |
                                      +--+--+  +--+--+
                                         |        |
                           mobile net 1 -+--------+-

                   Figure 21: Stalemate Situation Occurs

   If both tunnels between MR1 and HA1, and between MR2 and HA2, were up
   simultaneously, they would have "crossed over" each other.  If the
   tunnels MR1-HA1 and MR2-HA2 were drawn in Figure 21, it could be
   noticed that the path of the tunnel MR1-HA1 includes only one
   endpoint of the tunnel MR2-HA2 (the MR2 endpoint).  Two MR-HA tunnels
   are crossing over each other if the IP path between two endpoints of
   one tunnel includes one and only one endpoint of the other tunnel
   (assuming that both tunnels are up).  When both endpoints of one
   tunnel are included in the path of the other tunnel, then tunnels are
   simply encapsulating each other.

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

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

   EMail: pthubert@cisco.com

   Masafumi Watari
   KDDI R&D Laboratories Inc.
   2-1-15 Ohara
   Fujimino, Saitama  356-8502
   JAPAN

   EMail: watari@kddilabs.jp

   Fan Zhao
   UC Davis
   One Shields Avenue
   Davis, CA  95616
   US

   Phone: +1 530 752 3128
   EMail: fanzhao@ucdavis.edu

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