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Versions: 00 01 02 03                                                   
Internet Draft                                         A. Petrescu, ed.
Document: draft-petrescu-nemo-mrha-03.txt           M. Catalina-Gallego
Expires: April 2004                                        C. Janneteau
                                                             H.-Y. Lach
                                                           A. Olivereau
                                                               Motorola

                                                           October 2003


           Issues in Designing Mobile IPv6 Network Mobility
              with the MR-HA Bidirectional Tunnel (MRHA)
                 <draft-petrescu-nemo-mrha-03.txt>


Status of this Nemo

   This document is an Internet-Draft and is in full conformance
   with all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

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

   The list of current Internet-Drafts can be accessed at
        http://www.ietf.org/ietf/1id-abstracts.txt
   The list of Internet-Draft Shadow Directories can be accessed at
        http://www.ietf.org/shadow.html.

Abstract

   This document describes several issues relevant to the design of a
   network mobility support solution that relies on the bi-directional
   tunnel between Mobile Router and Home Agent, with Mobile IP.
   Examples of issues are: conflicting Mobile IP and RIPng/OSPF
   requirements on link-local addresses, HA/BR co-location,
   disconnected operation and "cross-over" tunnels.















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Table of Contents

   Status of this Nemo................................................i
   Abstract...........................................................i
   Conventions Used in this Document.................................ii
   1. Introduction....................................................1
   2. Definitions.....................................................1
   3. NEMO "Basic" preliminary descriptions...........................1
   4. Issues..........................................................2
     4.1 Implementation-independent specification terms...............2
     4.2 Allow for deployment flexibility.............................3
     4.3 Dynamic routing protocol and the HA..........................3
     4.4 Link-local addresses.........................................3
     4.5 Mobile Router as a Mobile Host...............................4
     4.6 Neighbour Discovery for MR's egress interface................4
     4.7 Separation of routing and mobility for MR....................5
     4.8 Prefix-based routing and host-based routing exceptions.......5
     4.9 IPv4 Issues..................................................5
     4.10 "Cross-over" tunnels........................................6
   5. Security Considerations.........................................6
     5.1 A tool: HA ingress filtering.................................6
   Acknowledgements...................................................7
   References.........................................................7
   Changes............................................................9
   A. Motivation for Full Addresses in Binding Updates................9
     A.1 Description of a Home Network................................9
     A.2 Scenarios...................................................11
       A.2.1 Manual Mobile Networks..................................11
       A.2.2 Scenarios with Co-located HA and BR.....................12
       A.2.3 Scenarios with HA and BR Separated......................16
     A.3 MR Redirects to BR..........................................21
     A.4 Informing the HA about the Route to MR......................21
       A.4.1 ICMP Redirect from BR to HA.............................22
       A.4.2 Static Route Method.....................................22
       A.4.3 Dynamic Route Method....................................24
   B. Examples and Other Issues......................................24
     B.1 Example of issue for Mobile Router as Mobile Host...........24
     B.2 Multicast Subscriptions of the MR...........................25
     B.3 Examples of issues for Neighbour Discovery for MR...........25
     B.4 Router Renumbering..........................................26
     B.5 Example of disconnected operation issue.....................26
     B.6 Example for the "cross-over" tunnels issue..................26
     B.7 Example of use of HA ingress filtering......................29
   C. A Digression...................................................29
   Intellectual Property Rights Considerations.......................29
   Chairs' Addresses.................................................30
   Authors' Addresses................................................30
   Copyright Notice..................................................31

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC-2119 [1].


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1. Introduction

   This document describes several issues relevant to the design of a
   network mobility support solution that relies on the bi-directional
   tunnel between Mobile Router and Home Agent, with Mobile IP.
   Examples of issues are: conflicting Mobile IP and RIPng/OSPF
   requirements on link-local addresses, HA/BR co-location and
   "cross-over" tunnels.

   The Mobile Router is using Binding Updates, Binding
   Acknowledgments, Binding Requests and Binding Errors with the Home
   Agent to maintain the MRHA bidirectional tunnel.

   The document is organized as follows: the next section presents a
   short perspective on three preliminary proposals for a NEMO "Basic"
   type of solution; the following section lists the issues that
   appear in this type of protocols.  Two additional sections, or
   appendices, give more detail of issues by way of motivations,
   examples and other related issues.

2. Definitions

   Complete NEMO terminology can be found in [9].

   MH: a mobile host, which is a mobile node (MN) as defined by Mobile
       IPv6, except all router parts.  In Mobile IPv6 terminology, MN
       can be either a host or a router.  An MH can only be a host.

   MR_HoA: mobile router's Home Address, or the home address of the
           mobile (egress) interface of the mobile router.

   MNP: mobile network prefix, or the prefix of the link of the mobile
        network that will move away.  Note that in the most general
        case a single MR may route multiple prefixes, in which case
        there would be multiples MNPs per one mobile network.

   FN: fixed node on the home link.  It doesn't stand for fixed
       network.

3. NEMO "Basic" preliminary descriptions

   An exhaustive description of the proposals to support mobile
   networks or mobile routers with Mobile IP bi-directional tunnel can
   hardly fit in the usual space reserved for an Internet Draft, which
   is traditionally a short document.  We retain three main
   descriptions: Cisco Mobile IPv4 for Mobile Routers [4], MRTP [13]
   and the "Basic" approach [22].

   MRTP is a method of enabling mobile routers by using dynamic tunnel
   registrations between the AR's point of attachment and its HA.
   This tunnel allows the HA to tunnel all traffic for the mobile
   network prefix to the MR, and also lets the MR forward all mobile
   network traffic back to the home network, where it is topologically
   correct, and can avoid ingress routing in the visited network.
   MRTP does not suffer from the authorization problem of how to show

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   that the MR owns the routing authority for the Mobile Network.

   The approach relies on the bidirectional tunnel between MR and HA.
   The solution proposed is valid for Mobile IPv6 as for Mobile IPv4.
   The MR and HA behaviours still represent a sensitive departure from
   the Mobile IPv6 protocol in that MR informs its HA directly about
   the tunnel interface and dynamically triggers additions of routing
   table entries in the HA's routing table for the MR's tunnel.  In
   addition, the most recent version of the draft proposes usage of
   the PSBUs in order to inform the HA about the prefix of the mobile
   network (thus a combination with the PSBU approach).  Moreover, the
   considerations about dynamic routing in this draft refer only to
   how dynamic routing would work with a MR, but not about the
   necessity of running a routing protocol between HA and MR.

   In the Mobile IPv4 case, the network mobility support with the MRHA
   tunnel has been reported at least by various teams at Cisco [4] and
   NASA [14].

   The Basic protocol proposed in [22] takes a different tack at
   assigning the home addresses: it assigns it to the MR's ingress
   interface, instead of the egress interface.  In addition, it
   proposes a two step approach for the search algorithm in the HA's
   binding cache: the first step is a search based on a key that is a
   full /128 address, while the second step is a search based on
   longest-prefix match.  A new aspect is that this proposals relies
   also on a (yet to be developped) prefix delegation scheme where the
   HA assigns the mobile network prefix to MR, in a dynamic manner.

   For a more detailed analysis on the first two approaches (MRTP and
   Cisco Mobile Routers) see sections A.4.2 and A.4.3.

4. Issues

   The following is a list of issues that we believe might be relevant
   when designing a Basic type of solution by the NEMO WG.  Some of
   the issues are exemplified in the Appendices.

4.1 Implementation-independent specification terms

   The specification of the basic network mobility support should be
   expressed with implementation-independent terms. In other words,
   clear distinction should be made between the specification of the
   protocol and a description of a possible implementation of this
   protocol. Especially, since it is to be based on Mobile IP(v6), the
   basic NEMO support specification should not make any assumption on
   how Mobile IP(v6) is implemented but instead re-use (and possibly
   extend) data structures from the Mobile IP(v6) specification
   (e.g. Binding Cache), and eventually introduce new ones if
   needed. Below are two examples of how attention should be payed in
   the specification of the protocol.

   The bi-directional approach requires MR's HA to configure a
   "forwarding information" for the mobile network prefix towards the
   mobile router.  Since the Mobile IP(v6) specification introduces a

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   dedicated structure, so-called Binding Cache, to store
   mobility-related "forwading information" on the HA, the
   specification of basic NEMO support should re-use/extend the
   Binding Cache to include the new mobility-related "forwarding
   information" for a mobile network.  Even though a Binding Cache may
   be implemented as an extension of a routing table, the
   specification should relate to the Binding Cache and not the
   routing table.  For instance, the specification should relate to
   the "forwading information" to be configured on MR's HA for the
   mobile network prefix in terms of a prefix entry in the Binding
   Cache instead of an entry in the routing table of MR's HA.
   Especially, Mobile IP(v6) specification does not specify any
   routing table for a HA.

   Similarly, the specification should not assume that a tunnel,
   e.g. the MRHA bi-directionnal tunnel, is visible as a virtual
   network interface on the MR or HA. This is an
   implementation-related consideration that may not be true for all
   IP(v6)/MobileIP(v6) stacks.

   Such considerations will allow to clearly draw the line between the
   specification and a description of a possible implementation, and
   as a result ease any future implementation of the basic NEMO
   support as an extention of an existing Mobile IPv6 implementation.

4.2 Allow for deployment flexibility

   The basic NEMO support specification should not assume MR's HA to
   be co-located with the Border Router (BR) of MR's home network. The
   HA should be allowed to be a one-interface machine, separated from
   BR, that does only NEMO HA functionalities (as a Mobile IP(v6) HA
   can be). This way, HA can be deployed in a home domain without the
   need to upgrade deployed BRs offering an easy deployment path.

4.3 Dynamic routing protocol and the HA

   Considering the case of a HA deployed as a one-interface machine
   not co-located with BR, the basic NEMO support specification should
   not mandate the HA to run a routing protocol, even in situation
   when MR runs a routing protocol. On the other hand, such HA should
   allow MR and BR to continue running the dynamic routing protocol as
   if MR was at home. Suffices it for the HA to: (1) join the
   corresponding multicast address, intercept all packets addressed to
   the link-local address of MR and encapsulate towards current MR CoA
   and (2) relay, or forward, towards BR all dynamic routing message
   exchanges coming from MR.

4.4 Link-local addresses

   According to section 10.4.2 of Mobile IPv6 spec [12] the HA will
   not allow re-direction of traffic of a Home Address towards a CoA,
   when that Home Address is link-local.  Two relevant paragraphs:

     "However, packets addressed to the mobile node's link-local
      address MUST NOT be tunneled to the mobile node."

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     "Multicast packets addressed to a multicast address with
      link-local scope [], to which the mobile node is subscribed,
      MUST NOT be tunneled to the mobile node;"

   which exposes, of course, the very nature of link-local addresses:
   they are local, not going anywhere.

   On another hand, OSPF for IPv6 [5] requires that:

     "On all OSPF interfaces except virtual links, OSPF packets are
      sent using the interface's associated link-local unicast address
      as source."

   Moreover, RIPng [16] requires that: (1) next hop addresses in
   routing tables managed by RIPng be link-local and (2) a large part
   of RIPng messages be originated and adressed to link-local
   addresses:

     "An address specified as a next hop must be a link-local
      address."

   or

     "Response Messages: [...] the source of the datagram must be a
      link-local address."

   or

     "Generating Response messages: [...]  The IPv6 source address
      must be a link-local address of the possible addresses of the
      sending router's interface, except when replying to a unicast
      Request Message from a port other than the RIPng port."

   Overall, keeping in mind that Mobile IPv6 is not dealing with
   link-local home addresses and that routing protocols and forwarding
   process make substantial use of link-local addresses, the issue is
   clearly how to make the routing protocols work together with Mobile
   IPv6. Basic NEMO support specification should enable redirection of
   traffic destined to MR's link-local addresses.

4.5 Mobile Router as a Mobile Host

   There are several scenarios that involve an MR that needs to act as
   a MH too, that is, send normal BUs and use existing Mobile IPv6.
   Applications running on the MR should take advantage of MR's
   session continuity and universal reachability at its home address.
   For more example issues see section B.

4.6 Neighbour Discovery for MR's egress interface

   Neighbour Discover on the MR's egress interface is particularly
   delicate in that Neighbour Discovery should act differently when MR
   is at home and when MR is in a foreign network.  A simple example
   is that when MR is at home, it has little reason to listen to RAs.
   However, when MR is in a foreign network, receiving RAs is very

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   important in order to have a good working of Mobile IPv6.  For more
   example issues see section B.

4.7 Separation of routing and mobility for MR

   The necessity of the distinction between mobility vs. routing
   exchanges holds true irrespective to whether dynamic or static
   routing is used.  If static routing is used, then BR has routes
   towards the mobile network through the MR, and MR has routes
   towards the Internet through the BR.  If dynamic routing is used,
   then the MR and BR dynamically exchange routing information that is
   manually configured in the routing configuration files of MR and of
   BR, as well as routing information that is delivered by other
   routers external to the home network (be it beyond the BR or beyond
   the MR).  The entities concerned with routing in the home network
   are only BR and MR.  This behaviour should continue when network
   mobility is introduced, presumably by deploying an HA (but not
   touching the BR).  MR and HA should exchange only the information
   related to mobility but not the information related to routing.

4.8 Prefix-based routing and host-based routing exceptions

   Prefix-based hierarchical routing (where the mobile network link
   has a prefix that is a subset of the home-network link) is the
   preferred type of routing for IPv6.  Practically though, it is
   possible for the BR to have a routing table entry containing the
   prefix of the mobile network, as well as a host-based entry that
   points to a certain LFN also in the mobile network.  Those two
   entries might or might not have the same common sub-prefix.  With a
   MR at home, being a normal router, BR will know how to forward to
   all hosts behind the MR as well as only to the specific LFN of the
   host-based route.  This behaviour should be maintained when the MR
   is no longer at home and when it has a bidirectional tunnel MRHA.

4.9 IPv4 Issues

   The mechanisms and issues described in this draft for IPv6 mobile
   networks can be applied for IPv4 network mobility as well.  RFC
   3344 [21] provides important intuititve support for IPv4 network
   mobility through the 'R' bit in Registration Requests/Replies.
   Some solutions have already been successfully tested in [4] and
   [14].  The support provided in RFC 3344 [21] as well as those
   solutions keep the HA co-located with the BR.  In a general case in
   which the BR and HA are kept on separate machines (scenarios 9 to
   16 in section A.2.3) the same issues as in IPv6 apply to the IPv4
   case.

   Additionally, in Mobile IPv4 there is a distinction between the MN
   and FA functionality, and it is possible to have the FA separated
   from the MN, whereas in IPv6 MN and FA are always co-located.  This
   gets us to the following additional cases:

     -When the MR is in a visited network it can send BU's using a
      co-located care-of address or a Foreing Agent care-of address
      if an FA is available.  In the latter case, two reverse

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      tunneling modes are possible: direct delivery style and
      encapsulated delivery style [17].

     -The MR may be itself a FA for Leaf Mobile Nodes (LMNs), or the
      mobile network may contain a FA for LMNs.

4.10 "Cross-over" tunnels

   Two MR-HA tunnels are "crossing over" each other when the path
   between one tunnel's endpoints includes only one of the other
   tunnel's endpoints.

   Support of nested mobile networks is possible only when the path
   from MR2 to MR1's HA does not go through MR1 (path considered when
   both mobile routers are at home and no tunnels are in place).

   An example of the dynamics of two MR-HA crossing tunnels is given
   in section B.6.

5. Security Considerations

   A detailed threat analysis is to be performed for a NEMO "Basic"
   type of solution.  But that's what the Charter says anyways.

   One issue is related to when the MR runs a dynamic routing
   protocol.  In that case, MR is able to inform the routers in the
   home domain about new routes (or "inject" routes in the home
   domain).  Considering that MR might be a small device, not locked
   in a highly secured room, not a tamper-proof device, potentially
   being stolen, then it is clear that the ability to introduce routes
   in the home domain, and worse, propagating upper to backbones, is
   inducing serious risks.

   It is possible that HA and MR belong to a same administrative
   domain.  It is also possible that HA and MR belong to different
   administrative domains.  In the latter case, there might be
   important security risks, and routing instability risks.  For one,
   if MR advertises a prefix towards HA, HA accepts it and propagates
   it upper stream then an important instability in the Internet at
   large might appear (because MR's prefix is administratively
   assigned somewhere else, in MR's administrative domain).  Second,
   if HA advertises prefixes towards the mobile network, then routing
   instability in the mobile network might appear.  Because of those
   two reasons, it might be recommended for MR and HA to forbid the
   exchange of any routing information (other than Mobile IPv6) when
   MR and HA belong to different administrative domains.

5.1 A tool: HA ingress filtering

   Home Agents supporting mobile networks are normally able to perform
   ingress filtering, so that only topologically correct packets leave
   the HA.  See section B.7 on how HA could do ingress filtering.




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Acknowledgements

   Authors of this document acknowledge the following WG members and
   non-members for their remarks, improvements to this draft and
   fruitful discussions:

   Tim Leinumeller for many insightful remarks and implementation
   aspects.

   Mattias Petterson.

   Vijay Devarapalli.

   TJ Kniveton.

   Pekka Pääkkönen.

   Mooi Choo Chuah.

   Erik Nordmark.

   Takeshi Tanaka.

References

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

   [2]  Arkko, Jari, Devarapalli, Vijay, and Dupont, Francis, "Using
        IPsec to Protect Mobile IPv6 Signaling between Mobile Nodes
        and Home Agents", draft-ietf-mobileip-mipv6-ha-ipsec-03.txt,
        IETF Internet Draft, February 2003. (Work in Progress).

   [3]  Baker, F. and Atkinson, R., "RIP-2 MD5 Authentication", RFC
        2082, January 1997.

   [4]  Cisco authors, "Cisco Mobile Networks", whitepaper browsed
        March 3rd, 2003 at
        http://www.cisco.com/univercd/cc/td/doc/product/software/
        ios122/122newft/122t/122t4/ftmbrout.pdf

   [5]  Coltun, R., Ferguson, D. and Moy, J., "OSPF for IPv6", RFC
        2740, December 1999.

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

   [7]  Crawford, M., "Router Renumbering for IPv6", RFC 2894, August
        2000.

   [8]  Ernst, Thierry, Olivereau, Alexis, Bellier, Ludovic,
        Castelluccia, Claude and Lach, Hong-Yon, "Mobile Networks
        Support in Mobile IPv6",
        draft-ernst-mobileip-v6-network-03.txt, IETF Internet Draft,
        March 2002. (Work in Progress).

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   [9]  Ernst, Thierry and Lach, Hong-Yon, "Network Mobility Support
        Terminology", draft-ernst-nemo-terminology-01.txt, IETF
        Internet Draft, November 2002. (Work in Progress).

   [10] Harkins, D., Mankin, A., Narten, T., Nikander, P., Nordmark,
        E., Patil, B. and Roberts, P., "Threat Models introduced by
        Mobile IPv6 and Requirements for Security",
        draft-ietf-mobileip-mipv6-scrty-reqts-02.txt, IETF Internet
        Draft, November 2001. (Work in Progress).

   [11] Hedrick, C., "Routing Information Protocol", RFC 1058, June
        1998.

   [12] Johnson, David B., Perkins, Charles E. and Arkko, Jari,
        "Mobility Support in IPv6", draft-ietf-mobileip-ipv6-20.txt,
        IETF Internet Draft, January 2003. (Work in Progress).

   [13] Kniveton, Timothy J., Malinen, Jari T. and Devarapalli, Vijay,
        "Mobile Router Tunneling Protocol",
        draft-kniveton-mobrtr-03.txt, IETF Internet Draft, November
        2003. (Work in Progress).

   [14] Leung, K. and Shell, D. and Ivancic, W. D. and Stewart,
        D. H. and Bell, T. L. and Kachmar, B. A., "Application of
        Mobile-IP to Space and Aeronautical Networks", IEEE Proceedngs
        of the Aerospace Conference, 2001.

   [15] Malkin, G., "RIP Version 2, Carrying Additional Information",
        RFC 1723, November 1994.

   [16] Malkin, G., "RIPng for IPv6", RFC 2080, January 1997.

   [17] Montenegro, G., ed., "Reverse Tunneling for Mobile IP,
        revised", RFC 3024, January 2001.

   [18] Moy, J., "OSPF Version 2", RFC 2328, April 1998.

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

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

   [21] Perkins, C., ed., "IP Mobility Support for IPv4", RFC 3344,
        August 2002.

   [22] Wakikawa, R., Uehara, K., Mitsuya, K. and Ernst, T., "Basic
        Network Mobility Support", draft-wakikawa-nemo-basic-00.txt,
        IETF Internet Draft, February 2003. (Work in Progress).

   [23] Devarapalli, V., Wakikawa, R., Petrescu, A. and Thubert, P.,
        "Nemo Basic Support Protocol" (work in progress). Internet
        Draft, IETF. draft-ietf-nemo-basic-support-01.txt.  September
        2003.


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Changes

   October 2002:  revision 00 submitted.
   November 2002: revision 01:
     -added discussion on multicast addresses with link-local scope.
     -added Chairs' addresses.
     -modified the abstract to better express the fact that /128s are
      probably sufficient.
     -added section on v4 issues, and Mobile IPv4 issues.
     -added an empty IPR section.
   March 2003: revision 02:
     -major overhaul from revision 01: shorter, focused on main
      issues, integrated some ml discussions, moved large "Motivation"
      parts to appendices.
     -added MH definition and used MH instead of MN when MR acts as an
      MH.
     -added more detailed acknowledgements.
     -added "cross-over" tunnels discussion.
     -added HA ingress filtering.
   October 2003: revision 03:
     -extended the security section with cases when MR and HA not in
      the same admin domain.
     -added reference to NEMO Basic Support draft.
     -refined the sections B.5 on disconnected operation and B.6 on
      cross-over tunnels.

Appendix A: Motivation for Full Addresses in Binding Updates

   An initial remark is that traffic coming from outside the home
   link, or from other hosts on the home link, and directed to hosts
   in the mobile network (or behind the mobile router) only need to go
   through the L2 address of the mobile router (corresponding to its
   L3 address).  With Proxy ND [19], it is the HA that pretends to own
   MR's L3 address by advertising new associations of the MR's L3
   address to the its own L2 address, thus intercepting MR's home
   traffic and forwarding it to the current CoA of the MR.

   With this in mind, it can be stated that when the MR is in a
   foreign network, traffic coming from hosts in the mobile network
   and towards anywhere to the Internet, is first forwarded by the MR
   through the reverse tunnel MRHA to the HA.  Then HA decapsulates
   and forwards to the address specified in the inner packet.

A.1 Description of a Home Network

   When designing a NEMO solution with the MRHA tunnel, the first
   steps are to carefully consider the actual behaviour of the home
   network when the mobile network is at home, employing normal
   routing.  Then this behaviour should be maintained as much as
   possible when the MR is not at home (e.g. MR should be able to send
   redirects through the MRHA tunnel); reciprocically, the normal
   behaviour of an FR at home should change when that FR is an MR and
   is at home (e.g. when MR at home, the MRHA tunnel should be torn
   down).  When the MR is in a foreign network, its presence at home
   is simulated by the HA (as in Mobile IPv6 for hosts).

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   Let us consider a simple case of a home network that supports
   movement of one of its links.  The home network is made up of a
   home link and a mobile network link, separated by the Mobile
   Router.  The home network is connected to the Internet via the
   Border Router, as presented in the figure:
                             ----
                            | FN |
                             ----
                               |               -------
                 home link -------------------| HA/BR |---> Internet
                               |               -------
                             ----    -----
                            | MR |  | LFN |
                             ----    -----
                               |       |
                   mobile link ---------

   Current specification for Mobile IPv6 implies that the HA can be
   either co-located with the BR, or it can act as a separate
   one-interface machine (this is advantageous for deploying Mobile
   IPv6 without changing BRs).  For mobile networks, the latter mode
   can be pictured like this:
                             ----    ----
                            | FN |  | HA |
                             ----    ----
                               |       |       ----
                 home link -------------------| BR |------> Internet
                               |               ----
                             ----    -----
                            | MR |  | LFN |
                             ----    -----
                               |       |
           mobile network link ---------

   It is assumed that routes outside the home link are managed by BR
   and MR, either in a static manner (operator fills in routing
   tables) or dynamic manner (application software partially manages
   routing tables).  Remark that even when the dynamic style is used,
   it is still true that operator fills initial routing configuration
   files, where she/he puts the image of the network as being what the
   operator believes it to be.  The dynamic behaviour of routing
   protocols intervenes when certain links come down or up due to
   failures, the operator view is no longer true, and the routers
   manage to find alternative paths.  Also, the dynamic behaviour
   helps obtaining shortest paths over large networks, relying on
   several local operator's views of smaller sized networks.  Addition
   of mobility should not change this.

   If static routing is used instead of dynamic routing, then static
   routes are added manually both on MR and on the BR.  When
   considering adding *static* routes in a *dynamic* manner for
   prefixes shorter than /128 by Mobile IP, authors of this document
   realize (in truth, hopefully) that Mobile IP starts using semantics
   that are traditionally belonging to routing protocols.


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A.2 Scenarios

   For the sake of completetess, we first describe a simple "manual"
   scenario for mobile networks based on the MRHA tunnel, that exposes
   relative simplicity, that uses static routing and doesn't use
   Mobile IP.

   Then, adding the Mobile IP behaviour, we present detailed scenarios
   of communication between an FN on the home link and an LFN on the
   mobile network link and a CN on the Internet, when the mobile
   network is at home and away from home in a visited network, and
   when the HA is co-located with the BR and separated from the BR.
   All in all, 16 simple scenarios are presented.

   The scenarios where HA is co-located with BR (1 up to 8) expose
   that there is no need for MR to communicate prefixes to its HA via
   BUs.  In a normal routing function, when the MR is at home, it
   exchanges routing information with the BR (co-located with the HA)
   and thus those prefixes are communicated by e.g. RIP or OSPF.  When
   the MR is not at home, this behaviour continues, but through the
   MRHA tunnel.

   The scenarios where HA and BR are separated (9 up to 16) expose
   that HA needs an entry in its routing table in order to be capable
   of forwarding packets to the MR (when it is not at home).

   An additional scenario is then presented where MR at home is using
   ICMP Redirect, a functionality that might be needed even when the
   MR is not at home.


A.2.1 Manual Mobile Networks

   Authors of this draft have experimented with "manual" mobile
   networks in IPv4, where the addition of routes and tunnels on the
   MR and on the BR are done manually, by operators talking on the
   phone.

   A home network was used that contains about 10 routers and about 12
   subnets.  That home network is connected to the Internet with a BR.
   All routers have static routes among them.

   Then, one slice of that home network (the mobile network)
   containing one "MR", one normal router and 6 subnets, was
   disconnected from home, and moved across the Atlantic.  Once the
   "MR" was connected on the other side, it was manually configured
   with a new IPv4 address, mask and default route.  Then a tunnel
   interface and a route were manually set up on the MR, a tunnel
   interface and a route were manually set up on the BR.  All other
   routes on all other routers where not touched.  Mobile IP software
   was not used.

   The entire network (the home and the mobile network) looked, and
   acted, as if the mobile slice were at home.  During this, several
   applications were tested between hosts in the mobile network, hosts

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   in the home network and hosts on the Internet (incidentally, one of
   the applications was relying on Mobile IPv4 for hosts, but in no
   relation with the mobile network moving).

   Again, this "manual" mobile networks scenario was not using any
   dynamic routing protocol, and the tunnel was not supporting any
   form of broadcast of multicast.

A.2.2 Scenarios with Co-located HA and BR

   1. FN sends packet to LFN, mobile network home, HA/BR colocated
                             ----
                            | FN |
                             ----
                               |               -------
                 home link -------------------| HA/BR |---> Internet
                               |               -------
                             ----    -----
                            | MR |  | LFN |
                             ----    -----
                               |       |
                   mobile link ---------

   -FN scans its routing table for LFN's address, and finds default
    route towards BR.
   -FN sends NS for L2 address of BR.
   -BR replies NA.
   -FN sends packet to BR.
   -HA scans its BC to find out whether MR is at home; BR scans its
    routing table for LFN's address, and finds route through MR;
   -BR sends NS for MR.
   -MR replies NA with its L2 address.
   -BR forwards packet to MR and sends ICMP Redirect to FN such that
    subsequent packets from FN to LFN go straight through MR and not
    through BR.
   -MR forwards packet to FN.

   The sensitive issue exposed here is the way in which initially the
   packet travels from FN to BR to MR, the dynamic addition of an
   entry in the routing table of the FN (even if FN doesn't run a
   routing protocol) and that subsequent packets will not go through
   BR, but from FN to MR to LFN.














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   2. FN sends packet to LFN, mobile network visits, HA/BR colocated
     ----                                   /
    | FN |                                 /
     ----                       ----------/
       |            -------    |          |
   ----------------| HA/BR |---| Internet |
         home link  -------    |          |
                                ----------\
                                           \
                                            \   ----  Visited link
                                             --| AR |------
                                                ----     |
                                                         |
                                                       ----    -----
                                                      | MR |  | LFN |
                                                       ----    -----
                                                         |       |
                                                         ---------
                                                         mobile net

   -FN scans its routing table for LFN's address, and finds default
    route towards BR.
   -FN sends NS for L2 address of BR.
   -BR replies NA.
   -FN sends packet to BR.
   -BR scans its routing table for LFN's address, and finds route
    through MR;
   -BR (being an HA) scans its BC and its routing table and finds it
    needs to encapsulate this packet towards MR's CoA.
   -BR encapsulates through the MRHA tunnel to MR's CoA.
   -MR decapsulates and forwards to LFN.

   3. LFN sends packet to FN, mobile network home, HA/BR colocated
                             ----
                            | FN |
                             ----
                               |               -------
                 home link -------------------| HA/BR |---> Internet
                               |               -------
                             ----    -----
                            | MR |  | LFN |
                             ----    -----
                               |       |
                   mobile link ---------

   -LFN scans its routing table for FN's address, and finds default
    route towards MR.
   -LFN sends NS for L2 address of MR.
   -MR replies NA.
   -LFN sends packet to MR.
   -MR scans its routing table for LFN's address, and finds route
    'on-link';
   -MR sends NS for FN.
   -FN replies NA with its L2 address.
   -MR forwards packet to FN.

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   4. LFN sends packet to FN, mobile network visits, HA/BR colocated
     ----                                   /
    | FN |                                 /
     ----                       ----------/
       |            -------    |          |
   ----------------| HA/BR |---| Internet |
         home link  -------    |          |
                                ----------\
                                           \
                                            \   ----  Visited link
                                             --| AR |------
                                                ----     |
                                                         |
                                                       ----    -----
                                                      | MR |  | LFN |
                                                       ----    -----
                                                         |       |
                                                         ---------
                                                         mobile net

   -LFN scans its routing table for FN's address, and finds default
    route towards MR.
   -LFN sends NS for L2 address of MR.
   -MR replies NA.
   -LFN sends packet to MR.
   -MR encapsulates this packet through the MRHA tunnel and sends to
    HA.
   -HA receives this packet and decapsulates.
   -HA scans its routing table for FN's address, and finds route
    'on-link';
   -HA sends NS for FN.
   -FN replies NA with its L2 address.
   -HA forwards packet to FN (on behalf of the MR).

   5. CN sends packet to LFN, mobile network home, HA/BR co-located
                                                ----  CN link
                                             --| BR1|------
                                            /   ----     |
                                           /             |
                                ----------/            ----
                    -------    |          |           | CN |
   ----------------| HA/BR |---| Internet |            ----
       | home link  -------    |          |
     ----    -----              ----------\
    | MR |  | LFN |                        \
     ----    -----                          \
       |       |
       ---------
     mobile net link

   -BR receives packet from CN towards LFN.
   -HA scans its BC to see whether MR is at home; BR scans its routing
    table and finds dest through MR.
   -BR sends NS for L2 address of MR and MR replies NA.
   -BR forwards packet to MR.

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   -MR forwards packet to LFN.

   6. CN sends packet to LFN, mobile network visits, HA/BR colocated

                                                 ----  CN link
                                              --| BR1|------
                                             /   ----     |
                                            /             |
                                 ----------/            ----
                     -------    |          |           | CN |
                 ---| HA/BR |---| Internet |            ----
                     -------    |          |
                                 ----------\
                                            \
                                             \   ----  Visited link
                                              --| AR |------
                                                 ----     |
                                                          |
                                                        ----    -----
                                                       | MR |  | LFN |
                                                        ----    -----
                                                          |       |
                                                          ---------
                                                          mobile net
   -BR receives packet from CN towards LFN.
   -BR scans its routing table and finds dest through MR.
   -BR scans its routing table and its BC and realizes it needs to
    send this through the MRHA tunnel.
   -BR sends the packet through the MRHA tunnel to MR.
   -MR decapsulates and forwards to LFN.

   (this is sometimes referred to as triangular routing, since the
    packet from CN to LFN travels artificially through BR)


   7. LFN sends packet to CN, mobile network home, HA/BR colocated


                                                ----  CN link
                                             --| BR1|------
                                            /   ----     |
                                           /             |
                                ----------/            ----
                    -------    |          |           | CN |
   ----------------| HA/BR |---| Internet |            ----
       | home link  -------    |          |
     ----    -----              ----------\
    | MR |  | LFN |                        \
     ----    -----                          \
       |       |
       ---------
     mobile net link

   -MR receives packet from LFN towards CN.
   -MR scans its routing table to and finds dest through BR.

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   -BR forwards packet to Internet towards CN.
   -BR1 forwards packet to CN.

   8. LFN sends packet to CN, mobile network visits, HA/BR colocated
                                                 ----  CN link
                                              --| BR1|------
                                             /   ----     |
                                            /             |
                                 ----------/            ----
                     -------    |          |           | CN |
                 ---| HA/BR |---| Internet |            ----
                     -------    |          |
                                 ----------\
                                            \
                                             \   ----  Visited link
                                              --| AR |------
                                                 ----     |
                                                          |
                                                        ----    -----
                                                       | MR |  | LFN |
                                                        ----    -----
                                                          |       |
                                                          ---------
                                                          mobile net
   -MR receives packet from LFN towards CN.
   -MR scans its tables and finds it needs to send it through the MRHA
    tunnel.
   -BR receives this packet, decapsulates and forwards to Internet.
   -BR1 forwards this packet to CN.

   (this is sometimes referred to as triangular routing, since the
    packet from LFN to CN travels artificially through BR)

A.2.3 Scenarios with HA and BR Separated

   9. FN sends packet to LFN, mobile network home, HA separated BR
                             ----    ----
                            | FN |  | HA |
                             ----    ----
                               |       |       ----
                 home link -------------------| BR |------> Internet
                               |               ----
                             ----    -----
                            | MR |  | LFN |
                             ----    -----
                               |       |
           mobile network link ---------


   -FN scans its routing table for LFN's address, and finds default
    route towards BR.
   -FN sends NS for L2 address of BR.
   -BR replies NA.
   -FN sends packet to BR.


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   -BR scans its routing table for LFN's address, and finds route
    through MR;
   -BR sends NS for MR.
   -MR replies NA with its L2 address.
   -BR forwards packet to MR and sends ICMP Redirect to FN such that
    subsequent packets from FN to LFN go straight through MR and not
    through BR.
   -MR forwards packet to FN.


   10. FN sends packet to LFN, mobile network visits, HA separated BR


     ----    ----                           /
    | FN |  | HA |                         /
     ----    ----               ----------/
       |       |       ----    |          |
   -------------------| BR |---| Internet |
         home link     ----    |          |
                                ----------\
                                           \
                                            \   ----  Visited link
                                             --| AR |------
                                                ----     |
                                                         |
                                                       ----    -----
                                                      | MR |  | LFN |
                                                       ----    -----
                                                         |       |
                                                         ---------
                                                         mobile net


   -FN scans its routing table for LFN's address, and finds default
    route towards BR.
   -FN sends NS for L2 address of BR.
   -BR replies NA.
   -FN sends packet to BR.
   -BR scans its routing table for LFN's address, and finds route
    through MR;
   -BR sends NS for MR.
   -HA replies NA with its L2 address (on behalf of MR).
   -BR forwards packet to HA and sends ICMP Redirect to FN such that
    subsequent packets from FN to LFN go straight through MR and not
    through BR.  BR also sends ICMP Redirect to HA, such that HA knows
    a route through MR.  The logic of this last ICMP Redirect is
    described in section 6.1.
   -HA scans its routing table for LFN's address, and finds through MR;
   -HA scans binding cache and finds 'through MRHA tunnel';
   -HA encapsulates and sends packet to MR.
   -MR decapsulates and forwards to LFN.





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   The problem in the above case is how to inform the HA about the
   route towards MR.  When MR at home, and HA being a host, normally
   HA doesn't have a route towards MR.

   11. LFN sends packet to FN, mobile network home, HA separated BR
                             ----    ----
                            | FN |  | HA |
                             ----    ----
                               |       |       ----
                 home link -------------------| BR |------> Internet
                               |               ----
                             ----    -----
                            | MR |  | LFN |
                             ----    -----
                               |       |
           mobile network link ---------

   -LFN scans its routing table for FN's address, and finds default
    route towards MR.
   -LFN sends NS for L2 address of MR.
   -MR replies NA.
   -LFN sends packet to MR.
   -MR scans its routing table for LFN's address, and finds route
    'on-link';
   -MR sends NS for FN.
   -FN replies NA with its L2 address.
   -MR forwards packet to FN.

   12. LFN sends packet to FN, mobile network visits, HA separated BR
     ----    ----                           /
    | FN |  | HA |                         /
     ----    ----               ----------/
       |       |       ----    |          |
   -------------------| BR |---| Internet |
         home link     ----    |          |
                                ----------\
                                           \
                                            \   ----  Visited link
                                             --| AR |------
                                                ----     |
                                                         |
                                                       ----    -----
                                                      | MR |  | LFN |
                                                       ----    -----
                                                         |       |
                                                         ---------
                                                         mobile net









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   -LFN scans its routing table for FN's address, and finds default
    route towards MR.
   -LFN sends NS for L2 address of MR. MR replies NA.
   -LFN sends packet to MR.
   -MR encapsulates this packet through the MRHA tunnel and sends to
    HA.
   -HA receives this packet and decapsulates.
   -HA scans its routing table for FN's address, and finds route
    'on-link';
   -HA sends NS for FN.  FN replies NA with its L2 address.
   -HA forwards packet to FN (on behalf of the MR).

   13. CN sends packet to LFN, mobile network home, HA separated BR

                                                ----  CN link
                                             --| BR1|------
             ----                           /   ----     |
            | HA |                         /             |
             ----               ----------/            ----
               |       ----    |          |           | CN |
     -----------------| BR |---| Internet |            ----
       | home link     ----    |          |
     ----    -----              ----------\
    | MR |  | LFN |                        \
     ----    -----                          \
       |       |
       ---------
     mobile net link

   -BR receives packet from CN towards LFN.
   -BR scans its routing table to and finds dest through MR.
   -BR sends NS for L2 address of MR.
   -MR replies NA.
   -BR forwards packet to MR.
   -MR forwards packet to LFN.





















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   14. CN sends packet to LFN, mobile network visits, HA separated BR

                                                 ----  CN link
                                              --| BR1|------
              ----                           /   ----     |
             | HA |                         /             |
              ----               ----------/            ----
                |       ----    |          |           | CN |
              ---------| BR |---| Internet |            ----
                        ----    |          |
                                 ----------\
                                            \
                                             \   ----  Visited link
                                              --| AR |------
                                                 ----     |
                                                          |
                                                        ----    -----
                                                       | MR |  | LFN |
                                                        ----    -----
                                                          |       |
                                                          ---------
                                                          mobile net
   -BR receives packet from CN towards LFN.
   -BR scans its routing table to and finds dest through MR.
   -BR sends NS for L2 address of MR.  HA replies NA on behalf of MR.
   -BR sends Redirect to HA informing it about a route towards MR.
   -Simultaneously with previous packet, BR forwards packet to HA.
   -HA scans its routing table and finds an entry to MR (added as a
    result to ICMP redirect), it also has a BC entry for MR, so it
    sends the packet through the MRHA tunnel.

   The problem in the above case is how to inform the HA about the
   route towards MR.  When MR at home, and HA being a host, normally
   HA doesn't have a route towards MR.

   15. LFN sends packet to CN, mobile network home, HA separated BR

                                                ----  CN link
                                             --| BR1|------
             ----                           /   ----     |
            | HA |                         /             |
             ----               ----------/            ----
               |       ----    |          |           | CN |
   -------------------| BR |---| Internet |            ----
       | home link     ----    |          |
     ----    -----              ----------\
    | MR |  | LFN |                        \
     ----    -----                          \
       |       |
       ---------
     mobile net link

   -MR receives packet from LFN towards CN.
   -MR scans its routing table and finds dest through BR.
   -BR sends packet to CN

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   16. LFN sends packet to CN, mobile network visits, HA separated BR
                                                 ----  CN link
                                              --| BR1|------
              ----                           /   ----     |
             | HA |                         /             |
              ----               ----------/            ----
                |       ----    |          |           | CN |
             ----------| BR |---| Internet |            ----
                        ----    |          |
                                 ----------\
                                            \
                                             \   ----  Visited link
                                              --| AR |------
                                                 ----     |
                                                          |
                                                        ----    -----
                                                       | MR |  | LFN |
                                                        ----    -----
                                                          |       |
                                                          ---------
                                                          mobile net
   -MR receives packet from LFN towards CN.
   -MR encapsulates this packet through the MRHA tunnel.
   -HA receives this packet, decapsulates and sends to CN.

A.3 MR Redirects to BR

   Also, consider the scenario where the FN has a default route
   towards the MR instead of the BR, and sending packets to a CN on
   the Internet.  This might very well happen when the MR is at home
   and sending RAs, in addition to the RAs sent by the BR.  FN might
   configure a default route through the MR instead of the BR.  If MR
   is at home, MR will redirect the FN towards the BR.  So, even if
   this looks like a wrong configuration on the FN (its default route
   should point to BR and not MR), packets will still travel correctly
   when MR is at home.  This should be maintained when the MR is not
   at home.  There are two possibilities: either the HA (replacing the
   MR) redirects the FN towards the BR, or it is the MR itself that
   sends the respective ICMP redirect message to the FN (through the
   MRHA tunnel).  The first case supposes that HA maintains a routing
   table, which contains routes towards the mobile network.  This is
   less desirable if the HA is not co-located with BR, and where we
   prefer not to have routing interactions with the HA.  The latter
   case is more plausible, keeping the default routing behaviour to
   the MR.

A.4 Informing the HA about the Route to MR

   In certain scenarios presented previously, with the HA dissociated
   from the BR and the MR in the visited network, there is a need for
   the HA to maintain in its routing table an entry towards the MR.  A
   scenario where packets from CN towards LFN are looping between BR
   and HA has been described in detail in section 3.2.4 of [8].
   Several solutions exist to avoid this looping, described below.


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A.4.1 ICMP Redirect from BR to HA

   One alternative for avoiding the loop problem is by using ICMP
   Redirects [19] sent by BR to HA in order to communicate to HA the
   route it misses towards the MR.  ICMP Redirects are deployed and
   used in existing networks.  The classic behaviour of ICMP Redirects
   is presented in scenario 1.  Scenarios 10 and 14 with
   MR-not-at-home and BR dissociated from HA, present the fact that
   classic ICMP Redirects are not triggered normally and thus
   modifications are needed.

   In addition to the normal behaviour with ICMP Redirects, described
   in [19], it could be modified according to the following.  The
   decision by BR to send ICMP Redirect towards HA can be taken in at
   least three ways:

     -allow a number of iterations of a packet looping between HA and
      BR and after this fixed number decide to send the Redirect to HA
      such as the looping stops.  This modifies the normal behaviour
      of BR.

     -another possibility is for BR to react at the moment it receives
      the proxy NA from HA (on behalf of the MR), compare to the
      current entry it has in the Neighbour Cache for MR, and then
      decide that, because MR has moved away, send Redirect to HA to
      inform HA about the route to MR.  This is the route (or set of
      routes) normally maintained by the BR with the MR, doesn't
      contain any form of the new position (CoA) of the MR.  This
      route, or set of routes (in which case a set of Redirects are
      sent), is copied from BR's routing table.  All routes that have
      destination the MR's home address need to be communicated to HA
      with ICMP Redirects.  This modifies the normal behaviour of BR.

     -yet another possibility is to consider modifications on HA (from
      vanilla Mobile IPv6), but don't touch BR, such that HA generates
      a new packet, thus obtaining a classic ICMP Redirect from BR.

      When the HA receives a packet that is not for itself, it
      encapsulates it with an IP-in-IP tunnel, having the src address
      its own address and the destination address copied from the dst
      address of the original packet.  Then try to route this packet
      and find the default route towards BR.  Then BR sends a normal
      ICMP Redirect informing HA there is a better route for this
      packet towards MR.  Thus HA acquires the MR route dynamically.
      The packet will be passed on by BR to HA again, and further
      details are needed here.  Remark that this is equivalent to one
      iteration of the loop (a particular case of the fixed iterations
      loop mentioned previously).

A.4.2 Static Route Method

   This is proposed by [4] and [13], where a route is statically
   introduced in the HA upon receiption of a Binding Update from
   MR. This route for MR's prefix may point towards MR's home address


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   (next hop), towards a specific tunnel to MR's home address(output
   interface), or towards a specific tunnel to MR's care-of address
   (output interface).

   The first approach proposed in [4] suggests to configure a new
   static tunnel on the MR's HA towards MR_HoA.  This static tunnel,
   that we call here MR_HoA_tunnel, is to be used as output interface
   of a new static entry added in the routing table of HA for MR's
   prefix: MR prefix -> MR_HoA_tunnel.  Upon reception of a data
   packet from CN addressed to a LFN, MR's HA will consult its routing
   table and find a match for that packet for this static route since
   LFN address matches MR's prefix.  As a results it will encapsulate
   the packet with an additional header that will have MR's HA as
   source address and MR_HoA as destination address.  In order to
   forward this packet, now addressed to MR's Home Address, the MR
   will first consult its binding cache and discover MR's Care-of
   address.  It will thus send the packet through the MRHA tunnel
   towards MR's current location.  It is worth mentionning that this
   approach introduces a double encapsulation of an incoming packet to
   be forwarded to the MR: the first is due to the MR_HoA_tunnel, the
   second to the MRHA tunnel.

   The second approach proposed in [13] suggests a similar method but
   avoids the overhead introduced by the two tunnels.  It consists in
   configuring a static route in MR's HA routing table for MR's prefix
   towards MR's Home Address: MR prefix -> MR_HoA.  Upon reception of
   a data packet from CN addressed to a LFN, MR's HA will consult its
   routing table and, again, find a match for that packet for this
   static route since LFN address matches MR's prefix. This indicates
   the MR's HA that the packet should be routed towards MR_HoA.  From
   its binding cache it discovers MR's CoA and as a consequence
   forwards the incoming packet from the CN directly through the MRHA
   tunnel.  This approach reduces the overhead of the MR_HoA_tunnel but
   requires a suitable coordination of the routing table and binding
   cache on the HA.

   A third possible approach is similar to the previous one but
   directly uses the MR's care-of address as the tunnel termination
   point instead of MR's home address. As such the new static entry
   added in the routing table of HA for MR's prefix is then MR prefix
   -> MRHA_tunnel.

   Analyzed from the perspective where HA is separated from BR, and
   where MR doesn't normally maintain routes with HA, then this
   addition might seem superfluous.  Consider a situation where MR and
   BR maintain routing information and where that manual route is
   added on HA.  When the MR is not at home, consider that
   administrator decides to add a new fixed subnet at home, with its
   own router neighbouring with BR on the home link.  Consider the new
   subnet's prefix being a longer prefix derived from the prefix
   assigned to the MR's subnet.  This is perfectly feasible by
   changing configurations on the MR and BR.  That can work perfectly
   even if MR is not at home.  But if HA doesn't participate in this
   exchange (which is the case if HA separated from BR) then the
   manual route added previously in the HA is no longer valid.  Thus,
   a potential issue.
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   Using PSBUs as proposed in [8] and [13] has many side-effects not
   clearly considered.  When the mobile network is assigned several
   prefixes instead of one, then it is not clear whether several BUs
   are being sent or only one with several prefixes inside.  Remark
   that in the vanilla Mobile IPv6 case, only one CoA can be sent with
   a BU (the alternative CoA is only an alternative not a substitute).

A.4.3 Dynamic Route Method

   It is possible for the HA, being either separated or co-located
   with the BR, to run a specific routing protocol, participating in
   the routing interactions between BR and all other neighbouring
   routers on the home link.  Thus, the HA always has the necessary
   route it needs to join the MR's network.

   If the HA is a one-interface machine, and separated from the BR, it
   seems that it maintains information that is not always necessary to
   its well working as a HA.  For example, it will maintain routes to
   all neighbouring routers, be it fixed or mobile.  The routes to the
   fixed neighbouring routers are not necessary for its working as a
   host, since it suffices to only have a default route towards a BR,
   that will normally dynamically Redirect it towards the other fixed
   routers.  Moreover, if HA runs a dynamic routing protocol, its
   route updates will never be taken into account by other routers,
   since they will always be one hop further than the routes already
   known by them.  Thus it might be possible to have the HA as a
   silent routing, only receiving route updates from the neighbouring
   routers, but never sending route updates, since it does not have a
   network behind it (it is a "host") whose reachability it needs to
   advertise.

   RIP [11] supports having a silent host that only listens to update
   messages, but does not advertise new routes.  With OSPF [18] the
   "listening only" requirement is complicated by the fact that the HA
   would needs to participate in OSPF's HELLO protocol.

   The advantage of using this solution is that it does not require
   additional changes to Mobile IPv6, and PSBUs are not needed.  The
   disadvantage is that if the MR does not run a routing protocol then
   we still need some way of telling the HA the routes to the MNPs.

Appendix B: Examples and Other Issues

B.1 Example of issue for Mobile Router as Mobile Host

   If the MR is at home and it has an address configured on the moving
   interface other than a link-local address, then the MR can act as
   an MH too, and send normal Mobile IPv6 BUs, binding that Home
   Address to a newly configured CoA; thus allowing the MR to be an MH
   for itself only, ignoring the LFNs.  If the MR at home doesn't have
   other addresses than link-local on the mobile interface then the MR
   can not send normal Mobile IPv6 BUs and can not be an MH.  It can
   however be an MR for the hosts on the mobile network.



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B.2 Multicast Subscriptions of the MR

   When the MR is at home, it normally joins certain multicast groups
   related to routing (e.g. all-routers multicast group with site
   scope).  This is assumed by dynamic routing protocols, or by
   renumbering mechanisms.  When the MR is no longer at home, its
   multicast subscription should continue as if it were at home.  This
   can be achieved by "home subscription" techniques considered in
   relation with Mobile IPv6.

B.3 Examples of issues for Neighbour Discovery for MR

   When MR is at home and sends RA towards the home link, it should
   not advertise itself as being capable of being a default router
   (Router Lifetime should be 0).

   When the MR is visiting, it should not respond to RSs sent on the
   visited link and it should not send RAs on the visited link.

   When the MR is at home, it doesn't normally use any information
   received from RAs sent by a neighbouring router, i.e. the BR.  It
   has a link-local address and if it has a larger scope address
   configured on an interface, then that is normally done manually.
   Actually, routers are usually prohibited from using information
   received in RAs more than for logging and synchronization purposes.
   When the MR is in a foreign network, it needs a way to configure a
   Care-of Address.  In the hosts case this is done by stateless or
   stateful autoconf.  In the MR case, the stateful is possible, while
   the stateless is normally prohibited.  A good way for address
   autococnfiguration for the MR should be identified, be it DHCP, or
   modified RAs, or modified router's behaviour to accept RAs.

   Assume the MR is at home and a non-link-local (site- or global)
   home address is configured on the interface connecting to the home
   link (supposedly the same interface that will change CoAs when
   visiting).  The MR-at-home will do periodic NAs for this home
   address; this behaviour should stop when MR is visiting.  This
   modified behaviour is already taken into consideration by Mobile
   IPv6 MN.  In the particular MR case, most ND operations of MR are
   delegated to the HA, and such most entries of Neighbour Cache,
   Destination Cache that are related to the home link will disappear.
   New entries that are relevant in the foreign network will populate
   those tables.  When coming back home, all ND entries should be
   replaced back with the entries related to the home network.

   Another specific case in point is the default route.  As already
   presented with the router behaviour with respect to RAs, a default
   route is not normally configured by MR from a received RA.  When
   the MR is in a foreign network, it should have a default route that
   points to its BR (but through the MRHA tunnel) and another
   non-tunnelled default route towards the current AR.  Moreover, all
   MR's routing table entries that pointed to BR when the MR was at
   home, should still continue to point to BR (through the MRHA
   tunnel).  The same is true for all routing table entries of the BR.


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B.4 Router Renumbering

   Router Renumbering for IPv6 [7] is a technique where routers of a
   home network are instructed to change the prefixes they advertise.
   In the context here, it should be possible for the MR to be
   re-numbered when it is at home as well as when it is visiting.

   The renumbering mechanisms provided by Mobile IPv6 (mobile prefix
   solicitations and advertisements) are not relevant for changing the
   prefixes advertised by the MR towards the mobile network; but these
   mechanisms should still be used for MR when MR is acting as an MH.
   In order for router renumbering to work for MR when acting as MR,
   the MR should at least be able to maintain its multicast
   subscription to all-routers group valid at home.

B.5 Example of disconnected operation issue

   With the NEMO basic support protocol as described in [23], it is
   possible to accomodate a wide range of simple and nested mobile
   networks.  However, the communication between two entities inside a
   nested aggregation of two mobile networks is not possible if the
   aggregation is not connected to the Internet.  Consider two mobile
   networks, each MR having its own HA in different domains.  The
   first MR attaches to an AR and the second MR attaches under the
   first mobile network.  In this case, the communication between two
   LFNs situated one on the first mobile net and the second on the
   second mobile net is encapsulated by both HA's.  If the first MR
   looses connectivity to AR ("disconnects"), the LFN's are unable to
   communicate, even though the two mobile networks are attached to
   each other.

B.6 Example for the "cross-over" tunnels issue

  With the NEMO basic support protocol as described in [23], it is
  possible to accomodate a wide range of simple and nested mobile
  networks.  However, certain cases of nested network mobility are not
  possible with this protocol, as exemplified by the "cross-over"
  tunnels issue described in this section.

  Consider a mobility configuration depicted below.  MR1 is served by
  HA1/BR and MR2 is served by HA2.  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.













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                                                  /-----CN
                                       ----------/
     home link1            -------    |          |
   -----------------------| HA1/BR|---| Internet |
       |                   -------    |          |
       |                               ----------
     -----    -----
    | MR1 |  | HA2 |
     -----    -----
       |        |
      -------------mobile network 1 / home link2
       |
     -----    -----
    | MR2 |  | LFN |
     -----    -----
        |        |
       ------------mobile network 2

  In the picture above, communication between CN and LFN follows a
  direct path as lons 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 the
  figure below:

                                            /-----CN
                                 ----------/
     home link 1     -------    |          |
     ---------------| HA1/BR|---| Internet |
       |             -------    |          |
     -----    -----              ----------\
    | MR1 |  | HA2 |                        \
     -----    -----                          -----
       |        |                           | AR  |
      ------------mobile net 1               -----
                  home link2                   |
                                             -----    -----
                                            | MR2 |  | LFN |
                                             -----    -----
                                               |        |
                                  mobile net 2------------

  Once MR2 acquires a CoA under AR, the tunnel setup procedure occurs
  between MR2 and HA2.  MR2 sends BU to HA2 and HA2 replies BAck 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 "=>".






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  Consider next the attachment of the first mobile network under the
  second mobile network, like in the figure below:

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

  After this movement, MR1 acquires a CoA valid in the second mobile
  network.  Subsequently, it sends a BU message addressed to HA1.
  This BU 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, due to the fact that the tunnel between MR2 and HA2
  has been previously torn down (when the mobile net 1 has moved from
  home).  The communication between CN and LFN stops, even though both
  mobile networks are connected to the Internet.

  If both the 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 the above picture,
  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.







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B.7 Example of use of HA ingress filtering

   HA should verify that packets it receives from the MRHA tunnel have
   a source address that matches what's in HA's routing table. HA
   should have a route for the mobile prefix pointing into the MRHA
   tunnel, and the LFN should have use a source address derived from
   that prefix when sending its packets. Other packets will be
   dropped.

Appendix C: A Digression

   Two types of approaches have been distinguished in designing a
   network mobility support with Mobile IPv6 and the bidirectional
   tunnel.

   Clean-slate Mobile IP-centric approach

   In this approach, it is assumed that a home network is in fact a
   new 1-link network.  This home network connects to the Internet
   with one or more BRs.  The BRs have HA functionality with Mobile IP
   for hosts.  There are no other routers or hosts in the home network
   than the BRs and the MRs.  MRs are seldom at home.  MRs and BRs
   would presumably have little need to run a dynamic routing
   protocol.  Most, if not all, routing information exhanges happen
   with Mobile IP BUs.

   Nodes in the mobile networks communicate with CNs.  Those CNs are
   anywhere in the Internet, but not in the home network (there's no
   node in the home network than BRs and/or other MRs).

   Evolutionary approach

   In this type of approach, it is assumed that a home network is
   already deployed.  The home network has several routers that run
   dynamic routing protocols (non-Mobile IP) to maintain connectivity
   between various endpoints.  The home network is connected to the
   Internet with one or more BRs.

   From this, it is possible to "mobilize" some slices (or chunks of
   this network), maintaining session continuity and reachability at a
   permanent home address for fixed nodes of that slice.  Consider
   that the slice that needs to be physically disconnected from the
   home network uses a router (called "MR") that connects the slice to
   the home network.  A minimal deployment effort could be the
   following: (1) modify software on MR and (2) place a new box with
   new software on the link where MR was connecting the slice to the
   home network (this entity called "HA").  MR and the slice move away
   and HA stays in place.

Intellectual Property Rights Considerations

   Consult Motorola on IPR (authors believe no IPR here, but depends
   who asks; the complete and authoritative answers can be found from
   IPD or Public Relations of Motorola, corelated with IPD of ECRL).


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Chairs' Addresses

  Thierry Ernst,                      Timothy J. Kniveton
  French National Institute for       Communication Systems Lab
  Research in Computer Science and    Nokia Research Center
  Control                             313 Fairchild Drive
  Visiting Researcher at WIDE         Mountain View, California 94043
  Project                             USA
  Jun Murai lab. Faculty of           Phone: +1 650 625-2025
  Environmental Information,          EMail: timothy.kniveton@nokia.com
  Keio University.                    Fax:  +1 650 625-2502
  5322 Endo, Fujisawa-shi, Kanagawa
  252-8520, Japan.
  Phone : +81-466-49-1100
  Fax   : +81-466-49-1395
  E-mail: ernst@sfc.wide.ad.jp
  Web:
  http://www.sfc.wide.ad.jp/~ernst/

Authors' Addresses

  Alexandru Petrescu                  Miguel Catalina-Gallego
  Motorola Labs                       Motorola Labs
  Parc les Algorithmes St Aubin       Parc les Algorithmes St Aubin
  Gif-sur-Yvette 91193                Gif-sur-Yvette 91193
  France                              France
  Phone:  +33 1 69354827              Phone:  +33 1 69352541
  Alexandru.Petrescu@motorola.com     Miguel.Catalina@motorola.com

  Christophe Janneteau                Hong-Yon Lach
  Motorola Labs                       Motorola Labs
  Parc les Algorithmes St Aubin       Parc les Algorithmes St Aubin
  Gif-sur-Yvette 91193                Gif-sur-Yvette 91193
  France                              France
  Phone:  +33 1 69352548              Phone:  +33 1 69352536
  Christophe.Janneteau@motorola.com   Hong-Yon.Lach@motorola.com

  Alexis Olivereau
  Motorola Labs
  Parc les Algorithmes St Aubin
  Gif-sur-Yvette 91193
  France
  Phone:  +33 1 69352516
  Alexis@motorola.com












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