NEMO Working Group                                            P. Thubert
Internet-Draft                                                     Cisco
Expires: January 16, 2006                                     C. Bontoux
                                                            N. Montavont
                                                             LSIIT - ULP
                                                           July 15, 2005

                       Nested Nemo Tree Discovery

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   The purpose of this paper is to describe a minimum set of features
   that extends the Nemo basic support [4] in order to avoid loops in
   the nested Nemo case.  As a result, Mobile Routers assemble into a
   tree that can be optimized based on various metrics.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3

   2.  Terms and Abbreviations  . . . . . . . . . . . . . . . . . . .  3

   3.  Motivations  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1   Multi-Homed nested mobile network  . . . . . . . . . . . .  4
     3.2   Loops in nested Nemo . . . . . . . . . . . . . . . . . . .  5

   4.  Router Advertisement extensions  . . . . . . . . . . . . . . .  6
     4.1   Router Advertisement message . . . . . . . . . . . . . . .  6
     4.2   Tree Information Option  . . . . . . . . . . . . . . . . .  7

   5.  Tree Discovery . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.1   tree selection . . . . . . . . . . . . . . . . . . . . . . 11
     5.2   Sub-tree mobility  . . . . . . . . . . . . . . . . . . . . 11
     5.3   DRL entries states and stability . . . . . . . . . . . . . 11
       5.3.1   Held-Up  . . . . . . . . . . . . . . . . . . . . . . . 12
       5.3.2   Held-Down  . . . . . . . . . . . . . . . . . . . . . . 13
       5.3.3   Collision  . . . . . . . . . . . . . . . . . . . . . . 13
       5.3.4   Instability  . . . . . . . . . . . . . . . . . . . . . 14
     5.4   Legacy Routers . . . . . . . . . . . . . . . . . . . . . . 14

   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14

   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15

   8.  Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     8.1   Changes from version 00 to 01  . . . . . . . . . . . . . . 16
     8.2   Changes from version 01 to 02  . . . . . . . . . . . . . . 16

   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16

   10.   References . . . . . . . . . . . . . . . . . . . . . . . . . 17
     10.1  Normative Reference  . . . . . . . . . . . . . . . . . . . 17
     10.2  Informative Reference  . . . . . . . . . . . . . . . . . . 17

       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18

       Intellectual Property and Copyright Statements . . . . . . . . 19

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1.  Introduction

   As per Nemo Basic support [4], a Mobile Router autoconfigures a
   single Care of Address (CoA) to register to its Home Agent and
   terminate its Mobile Router-Home Agent tunnel.  That Care of Address
   is the Mobile Router point of attachment to the nested Nemo.

   Consequently, if loops are avoided, the nested Nemo assumes the shape
   of a tree.  The nodes of the tree are Mobile Routers, the root is
   either a fixed or a Mobile Router, called in the latter case the root
   Mobile Router in NEMO terminology [6].  The leaves are mobile or
   fixed hosts, called Local Fixed Nodes, Local Mobile Nodes and
   Visiting Mobile Nodes in the NEMO terminology.

   This paper provides (1) a minimum extension to IPv6 Neighbor
   Discovery Router Advertisements in order to ensure that Mobile
   Routers attaching to one another actually avoid loops and end up
   forming a tree, and (2) the minimum common part of all Mobile Router
   algorithms that is required to ensure that whatever their specific
   decisions, loops between Mobile Routers will be avoided.

   The method is based on an autonomous decision by each Mobile Router
   with no global state convergence such as a MANET proactive routing
   protocol.  In fact, Mobile Routers may make different decisions from
   a same input, based on their own configuration and their own

   In order to build trees of Mobile Routers, we propose an extension to
   the ICMP Router Advertisement (RA) message, the Tree Information
   Option (TIO).  The RA-TIO allows Mobile Routers to advertise the tree
   they belong to, and to select and move to the best location within
   the available trees.  Mobile Routers propagate the TIO down the tree,
   updating some metrics such as the tree depth, leaving alone root
   information such as the tree identifier, and sending the result in
   RAs over the ingress interfaces.

2.  Terms and Abbreviations

   This document assumes that the reader is familiar with Mobile IPv6 as
   defined in [3] and with the concept of Mobile Router defined in the
   Nemo terminology document [6].

   For the needs of this paper, the following new definitions are

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   Nemo clusterhead: The root of a tree of mobile routers.  When the
      tree of Mobile Routers is attached to the infrastructure, the
      fixed Access Router may act as cluster head if it supports the
      Tree Information Option described in this document.  If it does
      not, then the clusterhead coincides with the root Mobile Router in
      NEMO terminology.  A clusterhead is elected even when the tree is
      not attached to the infrastructure.  A stand-alone Mobile Router
      is a clusterhead.

   Floating Tree: A Nested Nemo which clusterhead is a Mobile Router
      that is not attached to an Access Router.

   Grounded Tree: A Nested Nemo whose clusterhead is attached to the
      infrastructure.  In other words, the clusterhead is either a fixed
      router that supports Router Advertisement - Tree Information
      Option or is a Mobile Router which attachment router is a fixed
      router that does not support Router Advertisement - Tree
      Information Option.

   Mobile Access Router: A Mobile Router that provides Access Router
      services to other Mobile Routers.

   Attachment Router: The Router that is selected as Access Router by a
      Mobile Router, making it its parent in the nested NEMO tree.

   Propagation: The action by a Mobile Router that consists in receiving
      a Router Advertisement - Tree Information Option from its
      Attachment Router, recomputing a few specific fields, removing
      unknown suboption, and appending the resulting TIO to RAs sent
      over the ingress interfaces.

3.  Motivations

3.1  Multi-Homed nested mobile network

   A nested mobile network that is made of multiple Mobile Routers
   having a direct connection to the Internet is said to be multi-homed.
   Multihoming in Nemo offers useful properties to Mobile Network Nodes.
   The NEMO multihoming issues [9] draft lists potential multi-homed
   configurations for Nemo and explains the different problems and
   advantages that some configurations may introduce.  Multihoming
   offers three main abilities to the Nemo: it allows route recovery on
   failure, redundancy and load-sharing between Mobile Routers (or
   between interfaces of a given Mobile Router).  However, for the
   moment, there is no requirements nor protocol that would define in
   interaction between several egress interfaces inside a Nemo.

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   In a nested Nemo, the hierarchy of Mobile Routers increases the
   complexity of the route and/or router selection for Mobile Network
   Nodes.  Each level of a Nemo implies the usage of a new tunnel
   between the Mobile Router and its home agent.  Thus if a Mobile
   Network Node connects to a sub-Nemo which is also a sub-Nemo, packets
   from the Mobile Network Node will be encapsulated three times.

   When the Nemo where the MN is connected to is multi-homed, the MN may
   have the choice between several Attachment Router to be its default
   router.  Reference [7] introduces new options in Router Advertisement
   to allow any node on a link to choose between several routers.  This
   option mainly consists of a 2-bits flag that indicates the preference
   of the router (low, medium or high).  Furthermore, the same flag can
   be set in the Route Information option indicating the preference of a
   specific prefix.  Therefore, any node can determine its best default
   router(s) according to a given destination and its best router for
   default, which will be used by default.

   However this preference is only useful in a flat topology; It gives a
   way to the node to choose between different attachment routers
   advertising prefixes on the node link.  But if the node is inside a
   hierarchical topology the node can not learn the depth of each
   attachment router, and might not select the most efficient path.

   One of the usage of the new option introduced in this document is to
   distribute information on the hierarchy of Mobile Routers.  This
   information can be distributed to Attachment Routers, Mobile Routers
   and Mobile Network Nodes as well in order to allow better route
   selection and to increase the knowledge of the Nemo topology on each

3.2  Loops in nested Nemo

   When several Mobile Routers attach to each other to form a nested
   Nemo, loops can be created if they are not explicitly avoided.  In
   the simplest case, when egress and ingress interfaces of an Mobile
   Router are all wireless, a mobile router may be listening to Router
   Advertisement from its own ingress interface, creating a confliction
   problem.  In the general case, arbitrary attachment of Mobile Routers
   will form graphs that are not exempt of loops.  For instance: Assume
   a nested Nemo where Mobile Router1 is connected to the
   infrastructure, and Mobile Router3 is attached to Mobile Router2.
   Say that Mobile Router2 can hear both Mobile Router3 and Mobile
   Router1 over its wireless egress interface.  If Mobile Router2 select
   Mobile Router1, the connectivity to the infrastructure is provided
   for all.  But if Mobile Router2 selects Mobile Router3, Mobile
   Router2 and Mobile Router3 end up forming a loop and are disconnected
   from their Home Agents.

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   With Nemo basic support, a Mobile Router uses a single primary Care
   Of Address to attach to the nested structure.  As a result, if loops
   are avoided, the nested NEMO end up forming a tree.  It is beneficial
   to be able to form that tree in an optimum fashion for a given set of
   metrics such as tree depth.

   The shape of a nested Nemo may change rapidly due to Mobile Routers
   movement.  It is thus impractical to expect each Mobile Router to be
   able to maintain states about the whole tree structure in a link
   state fashion.  On the contrary, it is also beneficial to allow each
   Mobile Router to make its own independent selection based on a
   minimum information about its immediate neighbors, in order to
   reestablish the tree quickly upon erratic movements.

   Each Mobile Router should be able to make its own attachment router
   selection based on its own condition (eg battery level), its own set
   of constraints that may not apply to other Mobile Routers in the
   tree, and in general its own algorithm.  As a result, the
   standardization effort should concentrate on a common minimum set of
   rules that must be common to all Mobile Routers in order to prevent
   routing loops in the nested NEMO while leaving Mobile Routers
   independent in their Attachment Router selection algorithms.

4.  Router Advertisement extensions

   New extensions of Router Advertisement are proposed to distribute the
   knowledge of the Mobile Router hierarchy inside a nested Nemo.  These
   extensions are defined in different options/sub-options: a flag bit
   from the reserved flag field of Router Advertisement message is used
   to indicate whether the sending router is a Mobile Router or not; a
   new option is defined to transport minimum information on the tree to
   avoid loops generation;

4.1  Router Advertisement message

   We propose to use a reserved flag of the Router Advertisement message
   to inform whether the sending router is a Mobile Router or not.

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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |     Type      |     Code      |          Checksum             |
       | Cur Hop Limit |M|O|H|N|Reservd|       Router Lifetime         |
       |                         Reachable Time                        |
       |                          Retrans Timer                        |
       |   Options ...

                      Figure 1: Router Advertisement

   Nemo enabled router (N)

   The Nemo enabled router (N) bit is set when the sending router is a
   Mobile Router.

4.2  Tree Information Option

   The following option regroups the minimum information that allows a
   Mobile Router to discover a tree and select its point of attachment
   while avoiding loop generation.  It can also be used by Mobile
   Network Nodes to select their best default router.  If the default
   router of a non-Mobile Router sends Router Advertisements with a tree
   discovery option, the non-Mobile Router MUST set the N flag of its
   own Router Advertisement to 0 and copy the Tree Discovery Option in
   its own Router Advertisement.

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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |     Type      |    Length     |G|H|       Reserved            |
       |  TreePref.    |  Preference   |    BootTimeRandom             |
       |   TreeDepth   |   Reserved    |         TreeDelay             |
       |                           PathDigest                          |
       |                                                               |
       +                                                               +
       |                            TreeID                             |
       +                                                               +
       |                                                               |
       +                                                               +
       |                                                               |
       |   sub-option(s)...

                     Figure 2: Tree Information Option

   Type 8-bit unsigned integer set to 10 by the clusterhead.  Value is

   Length 8-bit unsigned integer set to 4.  The length of the option
      (including the type and length fields) in units of 8 octets.

   Grounded (G) The Grounded (G) flag is set when the clusterhead is
      attached to a fixed network infrastructure (such as the Internet).

   Home (H) The Home (H) flag is set when the clusterhead is attached to
      its home network.

   Reserved 16-bit unsigned integer set to 0 by the clusterhead.

   TreePreference 8-bit unsigned integer set by the clusterhead to its
      preference and unchanged at propagation.  Default is 0 (lowest

   Preference The administrative preference of that (mobile) Access
      Router.  Default is 0. 255 is the highest possible preference.
      Set by each Mobile Router at propagation time.

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   BootTimeRandom A random value computed at boot time and recomputed in
      case of a duplication with another Attachment Router.  The
      concatenation of the Preference and the BootTimeRandom is a 32-bit
      extended preference that is used to resolve collisions.  It is set
      by each Mobile Router at propagation time.

   TreeDepth 8-bit unsigned integer.  The tree depth of the clusterhead
      is 0 if it is a fixed router and 1 if it is a Mobile Router.  The
      tree Depth of a tree Node is the depth of its attachment router as
      received in a TIO, incremented by one.  All nodes in the tree
      advertise their tree depth in the Tree Information Options that
      they append to the RA messages over their ingress interfaces as
      part of the propagation process.

   TreeDelay 16-bit unsigned integer set by the clusterhead indicating
      the delay before changing the tree configuration, in milliseconds.
      A default value is 128ms.  It is expected to be an order of
      magnitude smaller than the RA-interval so if the clusterhead has a
      sub-second RA-interval, the Tree delay may be shorter than 100ms.
      It is also expected to be an order of magnitude longer than the
      typical propagation delay inside the nested Nemo.

   PathDigest 32-bit unsigned integer CRC, updated by each Mobile
      Router.  This is the result of a CRC-32c computation on a bit
      string obtained by appending the received value and the Mobile
      Router Care of Address. clusterheads use a 'previous value' of
      zeroes to initially set the PathDigest.

   TreeID 128-bit unsigned integer which uniquely identify a tree.  This
      value is set by the clusterhead.  The global IPv6 home address of
      the clusterhead can be used.

   The following values MUST not change during the propagation of the
   TIO down the tree: Type, Length, G, H, TreePreference, TreeDelay and
   TreeID.  All other fields of the TIO are updated at each hop of the

5.  Tree Discovery

   Here follows a set of rules and definitions that MUST be followed by
   all Mobile Routers:

   1.  An Mobile Router that is not attached to an Attachment Router is
       the Nemo clusterhead of its own floating tree.  It's depth is 1.

   2.  An Mobile Router that is attached to an Attachment Router that
       does not support TIO, is the clusterhead of its own grounded
       tree.  It's depth is 1.

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   3.  A router sending a RA without TIO is considered a grounded
       Attachment Router at depth 0.

   4.  The Nemo clusterhead of a tree exposes the tree in the Router
       Advertisement - Tree Information Option and Mobile Routers
       propagate the TIO down the tree with the RAs that they forward
       over their ingress links.

   5.  An Mobile Router that is already part of a tree MAY move at any
       time and with no delay in order to get closer to the clusterhead
       of its current tree - i.e. in order to reduce its own tree depth.
       But an Mobile Router MUST NOT move down the tree that it is
       attached to.  Mobile Routers MUST ignore RAs that are received
       from other routers located deeper or at the same depth within the
       same tree.

   6.  An Mobile Router may move from its current tree into any
       different tree at any time and whatever the depth its reaches in
       the new tree, but it may have to wait for a Tree Hop timer to
       elapse in order to do so.  The Mobile Router will join that other
       tree if it is more preferable for reasons of connectivity,
       configured preference, size, security, bandwidth, tree depth, or
       whatever metrics the Mobile Router cares to use.

   7.  If a Mobile Router has selected a new attachment router but has
       not moved yet (because it is waiting for Tree Hop timer to
       elapse), the Mobile Router is unstable and refrains from sending
       Router Advertisement - Tree Information Options.

   8.  When an Mobile Router joins a tree, moves within its tree, or
       when it receives a modified TIO from its current attachment
       router, the Mobile Router sends an unsolicited Router
       Advertisement message on all its mobile networks (i.e. all its
       ingress interfaces).  The RA contains a TIO that propagates the
       new tree information.  At the same time, the Mobile Router MAY
       send a Binding Update to its home agent or a local proxy of some
       sort, because the tree it is attached to has changed.  If the
       Mobile Router fails to reach its Home Agent, it MAY attempt to
       roll back the movement or to retry the Home Agent discovery

   9.  This allows the new higher parts of the tree to take place first
       eventually dragging their sub-tree with them, and allowing
       stepped sub-tree reconfigurations, limiting relative movements.

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5.1  tree selection

   The tree selection is implementation and algorithm dependent.  In
   order to limit erratic movements, and all metrics being equal, Mobile
   Routers SHOULD stick to their previous selection.  Also, Mobile
   Routers SHOULD provide a mean to filter out candidate Attachment
   Routers whose availability is detected as fluctuating, at least when
   more stable choices are available.  For instance, the Mobile Router
   MAY place the failed Attachment Router in a Hold Down mode that
   ensures that the Attachment Router will not be reused for a given
   period of time.

   The known trees are associated with the Attachment Router that
   advertises them and kept in a list by extending the Default Router
   List.  DRL entries are extended to store the information received
   from the last TIO.  These entries are managed by states and timers
   described in the next section.

   When connection to a fixed network is not possible or preferable for
   security or other reasons, scattered trees should aggregate as much
   as possible into larger trees in order to allow inner connectivity.
   How to balance these trees is implementation dependent, and MAY use a
   specific visitor-counter suboption in the TIO.

5.2  Sub-tree mobility

   It might be perceived as beneficial for a sub-tree to move as a
   whole.  The way it would work is for a Mobile Router to stay root-
   Mobile Router even if itself is attached into a parent tree.  But the
   loop avoidance is based on the knowledge of the tree that the Mobile
   Router visit, preventing a Mobile Router to move down a same tree.
   So without additional support, tree-level loops could form.

   To avoid this, it is possible to add a path vector suboption to the
   TIO that reflects the nesting of trees.  If a root-Mobile Router
   joins a parent tree, then it needs to add its treeID to the path
   vector, but it can not join if the treeID is already listed.

   A specific case is the root-Mobile Router of a tree that attaches to
   a fixed Access Router.  That root-Mobile Router might omit to
   consider a TIO that comes from the new Attachment Router and decide
   to stay root, in order to keep the tree consistency from the nested
   Mobile Routers standpoint.  This does not create loops, even if the
   path vector is not present

5.3  DRL entries states and stability

   Attachment routers in the DRL may or may not be usable for roaming

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   depending on runtime conditions.  The following states are defined:

   Current This Attachment Router is used for roaming

   Candidate This Attachment Router can be used for roaming.

   Held-Up This Attachment Router can not be used till tree hop timer
      elapses.  This does not occur for a fixed Attachment Router that
      does not send a TIO since the tree delay is null in that case.

   Held-Down This Attachment Router can not be used till hold down timer
      elapses.  At the end of the hold-down period, the router is
      removed from the DRL, and will be reinserted if it appears again
      with a RA.

   Collision This Attachment Router can not be used till its next RA.

5.3.1  Held-Up

   This state is managed by the tree Hop timer, it serves 2 purposes:

      Delay the reattachment of a sub-tree that has been forced to
      detach.  This allows to make sure that when a sub-tree has
      detached, the Router Advertisement - Tree Information Option that
      is initiated by the new clusterhead has spread down the sub-tree
      so that two different trees have formed.

      Limit Router Advertisement - Tree Information Option storms when
      two trees collide.  The idea is that between the nodes from tree A
      that wish to move to tree B, those that see the highest place in
      tree B will move first and advertise their new locations before
      other nodes from tree A actually move.

   A new tree is discovered upon a router advertisement message with or
   without a Router Advertisement - Tree Information Option.  The Mobile
   Router joins the tree by selecting the source of the RA message as
   its attachment router (default gateway) and propagating the TIO

   When a new tree is discovered, the candidate Attachment Router that
   advertises the new tree is placed in a held up state for the duration
   of a Tree Hop timer.  If the new Attachment Router is more preferable
   than the current one, the Mobile Router expects to jump and becomes

   A Mobile Router that is unstable may discover other Attachment
   Routers from the same new tree during the instability phase.  It

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   needs to start a new Tree Hop timer for all these.  The first timer
   that elapses for a given new tree clears them all for that tree,
   allowing the Mobile Router to jump to the highest position available
   in the new tree.

   The duration of the Tree Hop timer depends on the tree delay of the
   new tree and on the depth of Attachment Router that triggers it:

   (AR's depth + random) * AR's tree_delay (where 0 <= random < 1).  It
   is randomized in order to limit collisions and synchronizations.

5.3.2  Held-Down

   When a router is 'removed' from the Default Router List, it is
   actually held down for a hold down timer period, in order to prevent
   flapping.  This happens when an Attachment Router disappears (upon
   expiration timer), and when an Attachment Router is tried but can not
   reach the Home Agent (upon expiration of another Attachment Router,
   or upon tree hop for that Attachment Router).

   An Attachment Router that is held down is not considered for the
   purpose of roaming.  When the hold down timer elapses, the Attachment
   Router is removed from the DRL.

5.3.3  Collision

   A race condition occurs if 2 Mobile Routers send Router Advertisement
   - Tree Information Option at the same time and wish to join each
   other.  In order to detect the situation, Mobile Routers time stamp
   the sending of Router Advertisement - Tree Information Option.  Any
   Router Advertisement - Tree Information Option received within a
   short media-dependant period introduces a risk.  To divide the risk,
   A 32bits extended preference is added in the TIO.  The first byte is
   the clusterhead preference, then the router own preference (default
   is 0 for both), the remaining 16 bits is a  boot time computed

   A Mobile Router that decides to join an Attachment Router will do so
   between (Attachment Router depth) and (Attachment Router depth + 1)
   times the Attachment Router tree delay.  But since a Mobile Router is
   unstable as soon as it receives the Router Advertisement - Tree
   Information Option from the preferred Attachment Router, it will
   restrain from sending a Router Advertisement - Tree Information
   Option between the time it receives the RA and the time it actually
   jumps.  So the crossing of RA may only happen during the propagation
   time between the Attachment Router and the Mobile Router, plus some
   internal queuing and processing time within each machine.  It is
   expected that one tree delay normally covers that interval, but

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   ultimately it is up to the implementation and the configuration of
   the Attachment Router to define the duration of risk window.

   There is risk of a collision when a Mobile Router receives an RA, for
   an other mobile router that is more preferable than the current
   Attachment Router, within the risk window.  In the face of a
   potential collision, the Mobile Router with the lowest extended
   preference processes the Router Advertisement - Tree Information
   Option normally, while the router with the highest preference places
   the other in collision state, does not start the tree hop timer, and
   does not become instable.  It is expected that next RAs between the
   two will not cross anyway.

5.3.4  Instability

   A Mobile Router is instable when it is prepared to move shortly to
   another Attachment Router.  This happens typically when the Mobile
   Router has selected a more preferred candidate Attachment Router and
   has to wait for the tree hop timer to elapse before roaming.
   Instability may also occur when the current Attachment Router is lost
   and the next best is still held up.  Instability is resolved when the
   tree hop timer of all the Attachment Router (s) causing instability
   elapse.  Such Attachment Router is changes state to Current or Held-

   Instability is transient (in the order of tree hop timers).  When a
   Mobile Router is unstable, it MUST NOT send RAs with TIO.  This
   avoids loops when Mobile Router A wishes to attach to Mobile Router B
   and Mobile Router B wishes to attach to Mobile Router A. Unless RA
   cross (see Collision section), a Mobile Router receives TIO from
   stable Attachment Routers, which do not plan to attach to itself, so
   the Mobile Router can safely attach to them.

5.4  Legacy Routers

   A legacy router sends its Router Advertisements without a TIO.
   Consequently, a legacy router can be mistaken for a fixed Access
   Router when it is placed within a nested NEMO structure, and defeat
   the loop avoidance mechanism.  Consequently, it is important for the
   administrator to prevent address autoconfiguration by visiting Mobile
   Routers from such a legacy router.

6.  IANA Considerations

   Section 4.2. requires the definition of a new option to Neighbor
   discovery [1] messages, the Router Advertisement - Tree Information
   Option (RA-TIO).  The Router Advertisement - Tree Information Option
   has been assigned the value TBD within the numbering space for IPv6

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   Neighbor Discovery Option Formats.

7.  Security Considerations

   At the current level of this draft, the TIO bears the security level
   of the RA and the link.  Nothing is added to it.  A deeper threat
   analysis would be required to eventually propose additional security.

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8.  Changes

8.1  Changes from version 00 to 01

      Added text on sub-tree mobility from the discussion with Marcelo.

      Added text on nested legacy routers from the discussion with

8.2  Changes from version 01 to 02

      Improved text on instability

      Changed the formula for the 4 bytes number used in collision

9.  Acknowledgments

   The authors wish to thank Marco Molteni and Patrick Wetterwald
   (cisco) for their participation to this design and the review of the
   document, and Massimo Villari (university of Messina), for his early
   work on simulation and research on the subject.  This work is also
   based on prior publications, in particular HMRA [8] by Hosik Cho and
   Eun-Kyoung Paik from Seoul National University and other non IETF
   publications coauthored with Thierry Ernst and Thomas Noel.  Finally,
   thanks to Marcelo Bagnulo Braun for his constructive review.

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10.  References

10.1  Normative Reference

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

   [2]  Thomson, S. and T. Narten, "IPv6 Stateless Address
        Autoconfiguration", RFC 2462, December 1998.

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

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

   [5]  Ernst, T., "Network Mobility Support Goals and Requirements",
        draft-ietf-nemo-requirements-04 (work in progress),
        February 2005.

   [6]  Ernst, T. and H. Lach, "Network Mobility Support Terminology",
        draft-ietf-nemo-terminology-03 (work in progress),
        February 2005.

   [7]  Draves, R. and D. Thaler, "Default Router Preferences and More-
        Specific Routes", draft-ietf-ipv6-router-selection-07 (work in
        progress), January 2005.

10.2  Informative Reference

   [8]  Cho, H., "Hierarchical Mobile Router Advertisement for nested
        mobile networks", draft-cho-nemo-hmra-00 (work in progress),
        January 2004.

   [9]  Ernst, T., "Analysis of Multihoming in Network Mobility
        Support", draft-ietf-nemo-multihoming-issues-02 (work in
        progress), February 2005.

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Authors' Addresses

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

   Phone: +33 4 97 23 26 34

   Caroline Bontoux
   Sophia Antipolis
   Biot  06410


   Nicolas Montavont
   LSIIT - Univerity Louis Pasteur
   Pole API, bureau C444
   Boulevard Sebastien Brant
   Illkirch  67400

   Phone: (33) 3 90 24 45 87

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