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IETF MANET Working Group                                    Mario Gerla
INTERNET-DRAFT                                             Xiaoyan Hong
Expiration: November 17, 2001                                     Li Ma
                                  University of California, Los Angeles
                                                            Guangyu Pei
                                                Rockwell Science Center
                                                           May 17, 2001

   Landmark Routing Protocol (LANMAR) for Large Scale Ad Hoc Networks

                  <draft-ietf-manet-lanmar-01.txt>


Status of This Memo

   This document is an Internet-Draft and is NOT offered in accordance
   with Section 10 of RFC2026, and the author does not provide the IETF
   with any rights other than to publish as an Internet-Draft.  This
   document is a submission to the Mobile Ad-hoc Networks (manet)
   Working Group of the Internet Engineering Task Force (IETF).
   Comments should be submitted to the Working Group mailing list at
   "manet@itd.nrl.navy.mil".  Distribution of this memo is unlimited.

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
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   http://www.ietf.org/ietf/1id-abstracts.txt

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Abstract

   The Landmark Routing Protocol (LANMAR) utilizes the concept of
   "landmark" for scalable routing in large, mobile ad hoc networks.
   It relies on the notion of group mobility: i.e., a logical group
   (for example a team of coworkers at a convention) moves in a



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   coordinated fashion.  The existence of such logical group can be
   efficiently reflected in the addressing scheme.  It assumes that
   an IP like address is used consisting of a group ID (or subnet ID)
   and a host ID, i.e. <Group ID, Host ID>.  A landmark is dynamically
   elected in each group.  The route to a landmark is propagated
   throughout the network using a Distance Vector mechanism.
   Separately, each node in the network uses a "scoped" routing
   algorithm (e.g., FSR) to learn about routes within a given (max
   number of hops) scope.  To route a packet to a destination outside
   its scope, a node will direct the packet to the landmark
   corresponding to the group ID of such destination.  Once the packet
   is within the scope of the landmark, it will typically be routed
   directly to the destination.  Remote groups of nodes are
   "summarized" by the corresponding landmarks.  The solution to
   drifters (i.e., nodes outside of the scope of their landmark) is
   also handled by LANMAR.  Landmark dynamic election enables LANMAR
   to cope with mobile environments.  Thus, by using the truncated
   local routing table and the "summarized" landmark distance vector,
   LANMAR dramatically reduces routing table size and update overhead
   in large nets.  LANMAR is well suited to provide an efficient and
   scalable routing solution in large, mobile, ad hoc environments in
   which group behavior applies and high mobility renders traditional
   routing schemes inefficient.


                                Contents

Status of This Memo  . . . . . . . . . . . . . . . . . . . . . . .  1

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1

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

 2. Changes ....................................................... 5

 3. Terminology    . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1. General Terms  . . . . . . . . . . . . . . . . . . . . .  5
     3.2. Specification Language . . . . . . . . . . . . . . . . .  5

 4. Protocol Applicability . . . . . . . . . . . . . . . . . . . .  6
     4.1. Networking Context . . . . . . . . . . . . . . . . . . .  6
     4.2. Protocol Characteristics and Mechanisms  . . . . . . . .  6

 5. Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  8
     5.1. Protocol Descriptions  . . . . . . . . . . . . . . . . .  8
     5.2. Landmark Election  . . . . . . . . . . . . . . . . . . .  9
     5.3. Drifters . . . . . . . . . . . . . . . . . . . . . . . . 10




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 6. Protocol Specifications  . . . . . . . . . . . . . . . . . . . 10
     6.1. Data Structures    . . . . . . . . . . . . . . . . . . . 11
           6.1.1 Landmark Status tuple . . . . . . . . . . . . . . 11
           6.1.2 Landmark Distance Vector  . . . . . . . . . . . . 11
           6.1.3 Drifter List  . . . . . . . . . . . . . . . . . . 11
           6.1.4 Neighbor List . . . . . . . . . . . . . . . . . . 12
     6.2.  LANMAR Update Message Format    . . . . . . . . . . . . 12
           6.2.1 Description of the fields . . . . . . . . . . . . 12
     6.3. Processing Landmark Updates  . . . . . . . . . . . . . . 13
           6.3.1 Originating a Landmark in a Subnet  . . . . . . . 13
           6.3.2 Receiving Landmark Updates  . . . . . . . . . . . 14
           6.3.3 Winner Competition  . . . . . . . . . . . . . . . 14
     6.4. Processing Drifter Updates . . . . . . . . . . . . . . . 14
           6.4.1 Originating a Drifter Entry . . . . . . . . . . . 15
           6.4.2 Receiving Drifter Updates . . . . . . . . . . . . 15
           6.4.3 Removing a Drifter Entry  . . . . . . . . . . . . 15
     6.5. Processing Neighbor List . . . . . . . . . . . . . . . . 15

 7. Data Packet Forwarding . . . . . . . . . . . . . . . . . . . . 15

 8. Discussion about Storage and Processing Overhead for Drifters  16

 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 16

 References  . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

 Chair's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17

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


1. Introduction

   This document describes the Landmark Routing Protocol (LANMAR) [1,2]
   developed by the Wireless Adaptive Mobility (WAM) Laboratory [4] at
   Computer Science Department, University of California, Los Angeles.

   The concept of landmark routing was first introduced in wired area
   networks [6].  The original scheme required predefined multi-level
   hierarchical addressing.  The hierarchical address of each node
   reflects its position within the hierarchy and helps to find a route
   to it.  Each node knows the routes to all the nodes within its
   hierarchical partition.  Moreover, each node knows the routes to
   various "landmarks" at different hierarchical levels.  Packet
   forwarding is consistent with the landmark hierarchy and the path
   is gradually refined from the top level hierarchy to lower levels
   as a packet approaches its destination.




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   LANMAR borrows the concept of landmark and extends it to the
   wireless ad hoc environment.  LANMAR scheme does not require
   predefined hierarchical address, but it uses the notion of landmarks
   to keep track of logical subnets in which the members have a
   commonality of interests and are likely to move as a "group"
   (e.g., brigade in the battlefield, a group of students from same
   class and a team of co-workers at a convention).  Each such
   logical group has an elected landmark.  For each group,
   underlying "scoped" routing algorithm will provide a one-level
   scope.  The routing update packets are restricted only within the
   scope.  Accurate routing information for nodes within scope is
   maintained.  The routing information to remote nodes (nodes outside
   the node's scope) is "summarized" by the corresponding landmarks.
   Thus, the LANMAR scheme largely reduces the routing table size and
   the routing update traffic overhead.  It greatly improves
   scalability.

   In addition, in order to recover from landmark failures,
   a "landmark" node is elected in each subnet.  Landmark election
   provides a flexible way for the LANMAR protocol to cope with a
   dynamic and mobile network.  The protocol also provides a solution
   for drifters (Nodes that are outside the fisheye scopes of the
   landmarks of their logical groups).  Extra storage, processing and
   line overhead will be incurred for landmark election and drifter
   bookkeeping.  However, the design of the algorithms provides
   solutions without compromising scalability.  For example, the
   routing overhead for handling drifters is typically small if the
   fraction of drifting nodes is small.  More analysis is given in
   Section 8.

   The LANMAR runs on top of a proactive routing protocol.  It
   requires that the underlying routing protocol support the scoped
   subnetworking.  Fisheye State Routing Protocol (FSR) [7,8] is
   such a protocol that supports LANMAR.  In FSR, the link state
   protocol is used within the scope.   The fisheye scope
   technique can also be applied to a distance vector type protocol,
   such as DSDV [3], in which the hop distance can be used to
   bind the scope for routing message updating.  The main advantage of
   LANMAR is that the routing table includes only the nodes within the
   scope and the landmark nodes.  This feature greatly improves
   scalability by reducing routing table size and update traffic O/H.

   Thus the Landmark Routing Protocol provides an efficient and
   scalable routing solution for a mobile, ad hoc environment while
   keeping line and storage overhead (O/H) low.  Moreover, the
   election provides a much needed recovery from landmark failures.





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2. Changes

   Major changes from version 00 to version 01:

   -  A destination sequence number for each landmark is used to
      ensure loop-free updates for a particular landmark.

   -  Landmark updates are propagated in separate messages, instead of
      being piggybacked on local routing updates.  This modification
      decouples landmark routing from the underlying proactive routing
      protocol.


3. Terminology

3.1. General Terms

   This section defines terminology used in LANMAR.

      node

         A MANET router that implements Landmark Routing Protocol.

      neighbor

         Nodes that are within the radio transmission range.

      scope

         A zone that is bounded by hop distances.

      host protocol

         A proactive protocol underlies the Landmark Routing Protocol.

      subnet

         Logical groups of nodes that present similar motion behavior.

      group

         This term is used interchangeably with subnet.

3.2. Specification Language

   The keywords "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 [5].



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4. Protocol Applicability

4.1. Networking Context

   LANMAR is best suited for large scale mobile ad hoc wireless
   networks.  The landmark scheme on top of "scoped" routing algorithm
   has large advantages in reducing routing update packet size and
   keeping progressive accurate routes to remote nodes.  It achieves
   high data packet delivery ratio.  Moreover, the fact that the route
   error is blurred by distance obviously reduces the sensitivity to
   network size.

   LANMAR is also suited for high mobility ad hoc wireless networks.
   This is because in a mobility environment, a change on a link far
   away from the source does not necessarily cause a change in the
   routing table at the source and that all the information about
   remote nodes is summarized by landmarks.

4.2. Protocol Characteristics and Mechanisms

   * Does the protocol provide support for unidirectional links?(if so,
   how?)

        No.

   * Does the protocol require the use of tunneling? (if so, how?)
        No.

   * Does the protocol require using some form of source routing? (if
   so, how?)

        No.

   * Does the protocol require the use of periodic messaging? (if so,
   how?)

        Yes.  The LANMAR periodically broadcasts landmark information
        to its neighbors.

   * Does the protocol require the use of reliable or sequenced packet
   delivery? (if so, how?)

        No.  As the packets are sent periodically, they need not be
        sent reliably.

   * Does the protocol provide support for routing through a multi-



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   technology routing fabric? (if so, how?)

        Yes.  It is assumed that each node's network interface is
        assigned a unique IP address.

   * Does the protocol provide support for multiple hosts per router?
   (if so, how?)

        Yes. The router that has multiple hosts can use network ID of
        these hosts as the address to participate LANMAR.

   * Does the protocol support the IP addressing architecture? (if so,
   how?)

        Yes.  Each node is assumed to have a unique IP address (or
        set of unique IP addresses in the case of multiple interfaces).
        The LANMAR references all nodes/interfaces by their IP address.
        This version of the LANMAR also supports IP network addressing
        (network prefixes) for routers that provide access to a
        network of non-router hosts.

   * Does the protocol require link or neighbor status sensing (if so,
   how?)

        No.

   * Does the protocol have dependence on a central entity? (if so,
   how?)

        No.

   * Does the protocol function reactively? (if so, how?)

        No.

   * Does the protocol function proactively? (if so, how?)

        Yes.  The LANMAR proactively maintains landmark information
        at each node.

   * Does the protocol provide loop-free routing? (if so, how?)

        Yes.  For in-scope destinations, the protocol uses routing
        paths learned from the host protocol.  If the host protocol
        provides loop-free routing, e.g., FSR and DSDV, so does LANMAR.
        For out-scope destinations, only routes to landmarks are used.
        Because these routes are DSDV, it is loop free.  When a packet
        approaches the destination, in-scope routes are used again.



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   * Does the protocol provide for sleep period operation?(if so, how?)

        Yes.  However, this requires TDMA MAC layer support.  The
        router can be scheduled to sleep during idle periods.

   * Does the protocol provide some form of security? (if so, how?)

        Yes.  When a node broadcasts routing update message, only
        entries of in-scope nodes and landmarks are included.  This
        will prevent other remote nodes from being heard.

   * Does the protocol provide support for utilizing multi-channel,
     link-layer technologies? (if so, how?)

        Yes.  In fact, the multi-channel can be used to separate
        routing messages from user data packets.


5. Protocol Overview

5.1. Protocol Descriptions

   As mentioned in Section 1, the landmark concept we adopt here uses
   the notion of logical subnets in which the members have a
   commonality of interests and are likely to move as a "group".
   Each logical subnet has one node serving as a "landmark" of that
   subnet.  The protocol requires that the landmark of each subnet have
   the knowledge of all the members in its group.  The LANMAR protocol
   also uses a scope at each node.  The size of the scope is a
   parameter measured in hop distance.  It is chosen in such a way that
   if a node is at the center of a subnet, the scope will cover the
   majority of the subnet members.  If the shape of a subnet is likely
   to be a cycle, the center node's scope will cover all the members of
   the subnet.  If this center node is elected as a landmark, it
   fulfills the requirement of the protocol.  The elected landmark
   uses a destination sequence number to ensure its routing entry
   update is loop-free.  The landmarks are propagated in a
   distance vector mechanism.  Each node maintains a distance vector
   for landmarks of all the subnets.  The size of the landmark distance
   vector equals to the number of logical subnets and thus landmark
   nodes.  If a landmark does not locate at the center, there will be
   some members drifting off its scope.  The landmark will keep a
   distance vector for drifters of its group.  The distance vectors
   for landmarks and drifters are exchanged among neighbors in
   periodical routing update packets.

   The LANMAR relies on an underlying proactive protocol with the



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   ability of providing "scoped" routing.  In the scoped routing, each
   node broadcasts routing information periodically to its immediate
   neighbors.  In each update packet, the node sends routing table
   entries within its scope.  The host routing protocol uses sequence
   numbers for routing entries.  Each node advances its sequence
   number before sending an update packet.  Through the exchange
   process, the table entries with larger sequence numbers replace
   the ones with smaller sequence numbers.

   Let's take, for example, the FSR as our host protocol.  For nodes
   within the scope, the updating is in a certain frequency.  But
   for nodes beyond the scope, the update frequency is reduced to
   zero;  Only the update frequency of the landmark nodes remains
   unaltered.  As a result, each node maintains accurate routing
   information for in-scope nodes and keep routing directions to the
   landmark nodes for out-scope nodes, or say, for remote groups.
   A packet directed to a remote destination initially aims at the
   landmark of that remote group; as it gets closer to the landmark,
   it may eventually switch to the accurate route to the destination
   provided by in-scope nodes of the destination.

5.2. Landmark Election

   Dynamic election/re-election of landmark node is essential for
   LANMAR to work in a wireless mobile environment.  Basically, each
   node tracks other nodes of its group in its scope and computes
   "weight", e.g. the number of the nodes it has found.  At the
   beginning of the LANMAR, no landmark exists.  Protocol LANMAR
   only uses the host protocol functionality.  As host routing
   computation progresses, one of the nodes will learn (from
   the host protocol's routing table) that more than a certain number
   of group members (say, T) are in its scope.  It then proclaims
   itself as a landmark for this group and adds itself to the landmark
   distance vector.  Landmarks broadcast the election weights to
   the neighbors jointly with the landmark update packets.

   When more than one node declares itself as a landmark in the same
   group, as the landmark information floods out, each node will
   perform a winner competition procedure.  Only one landmark for each
   group will survive and it will be elected.  To avoid flapping
   between landmarks (very possible in a mobile situation), we use
   hysteresis in the replacement of an existing landmark.  I.e., the
   old Landmark is replaced by the new one only if its weight is, say
   less than 1/2 of the weight of the current election winner.  Once
   ousted, the old leader needs the full weight superiority to be
   reinstated.

   This procedure is carried out periodically in the background (low



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   overhead, anyway).  At steady state, a landmark propagates its
   presence to all other nodes like a sink in DSDV.  It is extremely
   simple and it converges (by definition).  In a mobile environment,
   an elected landmark may eventually lose its role.  The role shifting
   is a frequent event.  In a transient period, there exist several
   landmarks in a single group.  The transient period may be actually
   the norm at high mobility.  This transient behavior can be
   drastically reduced by using hysteresis.

   When a landmark dies, its neighbors will detect the silence after
   a given timeout.  The neighbors of the same group will then take
   the responsibilities as landmarks and broadcast new landmark
   information.  A new round of landmark election then floods over
   the entire network.

5.3. Drifters

   Typically, all members in a logical subnet are within the scope of
   the landmark, thus the landmark has a route to all members.  It may
   happen, however, that some of the members "drift off" the scope,
   for example, a tank in a battalion may become stranded or lost.
   To keep track of such "drifters", i.e., to make the route to them
   known to the landmark, the following modification to the routing
   table exchange is necessary.  Each node, say i, on the shortest
   path between a landmark L and a drifter l associated with that
   landmark keeps a distance vector entry to l.  Note that if l is
   within the scope of i, this entry is already included in the
   routing table of node i.  When i transmits its distance vector to
   neighbor, say j, then j will retain the entry to member l only if
   d(j,l) is smaller than the scope or d(j,L) is smaller than d(i,L).
   The latter condition occurs if j is on the shortest path from i
   (and therefore from l) to L.  This way, a path is maintained from
   the landmark to each of its members, including drifters.  The
   procedure starts from l, at the time when a node finds it becomes
   a drifter.  It informs the landmark hop by hop about its presence.

   The occurrences of drifters are dynamic in a mobile network.  In
   order to timely remove the staled drifter information, the time
   when a node hears a drifter is recorded.


6. Protocol Specifications

   This section discusses the operation of LANMAR routing protocol.
   The sending and receiving of landmark updates are in the proactive
   nature.  The routing packets are processed separately from
   ordinary data packets.




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6.1. Data Structures

   Each node has a unique "logical" identifier defined by a subnet
   field and a host field.  The host field is unique in the subnet and
   might in fact coincide with the physical address.  The "logical"
   identifier can also be an IP address when the subnet address can
   logically group the nodes.  Moreover, each node keeps a landmark
   status tuple.  As LANMAR runs on top of a host routing protocol,
   it shares the underlying routing table structures.  LANMAR
   maintains a neighbor list and shares it with the host protocol.
   In addition, LANMAR keeps a drifter list and a landmark distance
   vector.

6.1.1. Landmark Status Tuple

   Each node has not only a "logical" identifier, which basically is
   its address, but also a landmark status tuple.  The tuple includes
   a flag which indicates whether the node is a landmark or not, a
   election weight (the number of group members the node detects within
   its scope) and a sequence number.  When a node is elected, the
   status tuple will be copied to its landmark distance vector.  The
   sequence number is advanced. There are three fields for a tuple:
   - Landmark flag
   - Number of group members in its scope
   - Sequence number

6.1.2. Landmark Distance Vector

   Landmark distance vector (LMDV) gives the next hop information to
   all landmarks in the network.  Every subnet has an entry in LMDV.
   The latest route information to the landmark of each subnet is
   learned when a landmark update message is received.  LMDV functions
   as a part of the routing table.  It has the following fields:
   - Landmark status tuple
   - Next hop address
   - Distance

6.1.3. Drifter List

   The drifter list (DFDV) of LANMAR provides the next hop information
   of the drifters known to the current node.  The entries are updated
   with landmark update message.  The latest time a drifter is heard
   is recorded in DFDV.  The DFDV works as a part of routing table.
   It has the following fields:
   -  Destination drifter address
   -  Next hop address
   -  Distance
   -  Last heard time



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6.1.4. Neighbor List

   The neighbor list of LANMAR keeps current neighbor information for
   a node.  The latest time a neighbor is heard is recorded.  The
   neighbor list has the following fields:
   -  Neighbor address
   -  Neighbor landmark flag
   -  Last heard time

6.2. LANMAR Update Message Format

   There is only one message type of LANMAR protocol. The messages are
   periodically exchanged with neighbors.  They update the landmark
   distance vector LMDV and the drifter list DFDV.  The processing of
   the LMDV and DFDV will be describe separately.  The following format
   does not include the node's identifier because it can be obtained
   from IP Header.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Landmark Flag |  N_landmarks  |  N_drifters   |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Landmark Address 1                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Next Hop  Address 1                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Distance 1   |  N_members 1  |       Sequence Number 1       :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                             . . .                             :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Drifter Address 1                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Next Hop  Address 1                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Distance 1   |                   ...                         :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                             . . .                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


6.2.1. Description of the fields

   Landmark Flag

      The landmark flag of the sending node.




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   N_landmarks

      The number of entries of the landmark distance vector.

   N_drifters

      The number of entries of the drifter list.

   Reserved

      The bits are set to '0' and are ignored on reception.

   Landmark Address 1, Next Hop Address 1, Distance 1, N_members 1
   and Sequence Number 1

      The first entry in the landmark distance vector.
      Landmark Address 1, N_members 1 and Sequence Number 1 are the
      status tuple of the destination landmark.
      Next Hop Address 1 and Distance 1 is the next hop and distance
      to the landmark.

      These fields are repeated N_landmarks times for each entry in
      landmark distance vector.

   Drifter Address 1, Next Hop Address 1 and Distance 1

      The first entry in the drifter list.
      Next Hop Address 1 and Distance 1 are the next hop and distance
      to the Drifter Address 1.

      These fields are repeated N_drifters times for each entry in
      the drifter list.

   The length of the message is limited to the maximum IP packet size.
   In that case, multiple packets may be required to broadcast all the
   entries.

6.3. Processing Landmark Updates

   Landmark update information is a part of the LANMAR periodic
   routing update message.  The update information includes sender's
   LMDV.  Landmark update message is used for landmark election and
   building paths to landmarks.

6.3.1. Originating a Landmark in a Subnet

   Every time a node detects a neighbor change, it recalculates the
   number of group members in its scope.  The new number of group



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   members is recorded in its election weight field.  If this
   number is greater than a threshold T, the node qualifies as a
   landmark only when it is the only landmark for the group so far,
   or it wins the election when competing with the existing landmark.
   When it becomes a landmark, it increases its sequence number by 2.
   The old landmark entry is replaced with the new winner.  The
   landmark entry will be broadcast to neighbors with the next update
   packet.  Every time before a landmark sends updates, it increases
   its sequence number and copies it to LMDV.

6.3.2. Receiving Landmark Updates

   When a node receives a landmark update message, it compares its
   LMDV entries with the incoming LMDV updates for each subnet.
   A landmark update corresponding to a new subnet will be copied.
   An update having the same landmark as already given (in node's LMDV)
   will be accepted only if it contains a larger sequence number.
   If an update contains a  different landmark for the same subnet as
   recorded in LMDV, only one landmark will be elected through a
   winner competition algorithm.  LMDV will be updated according to
   the outcomes of the competition.

6.3.3. Winner Competition

   When more than one node declares itself as a landmark in the same
   group, a simple solution is to let the node with the largest
   number of group members win the election and in case of tie,
   lowest ID breaks the tie.  The other competing nodes defer.
   However, this method is likely to cause the oscillation of
   landmark roles between nodes.

   To use hysteresis in replacing an existing landmark, let us assume
   the competing node's number of members is M, the existing
   landmark's number of members is N and a factor value S.  When M is
   greater than N*S, then the competing node replaces the existing
   landmark.  Or, when N reduces to a value smaller than a threshold T,
   then it gives up the landmark role.  A tie occurs when M falls
   within an interval [N*1/S, N*S], then the node with larger member
   number wins the election.  If a tie occurs again with equal member
   number, i.e., M equals to N, it is broken using lowest ID.  A tie
   can always be broken using lowest ID as the address is used as ID
   and it is unique.

6.4. Processing Drifter Updates

   Drifter update information is a part of the LANMAR periodical
   routing update message.  The update information is the drifter list
   (DFDV) of the sender.  The computation of the DFDV at each node



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   includes checking the node itself to see whether it is a drifter
   and recording paths to other drifters.

6.4.1. Originating a Drifter Entry

   By checking the distance to the landmark of its group, each node
   easily knows whether it has become a drifter.  If the distance is
   larger than the scope, the node will put itself into its drifter
   list.  This drifter information will be sent back to the landmark
   hop by hop along the shortest path to it which can be learned from
   the LMDV.  For each drifter, only the node on its shortest path
   to the landmark needs to receive its information, so before the
   entry is broadcast, the next hop to landmark is attached with
   its entry.  The DFDV will be propagated with the next update packet.

6.4.2. Receiving Drifter Updates

   Upon receiving an update packet, the DFDV part is retrieved and
   processed.  If an entry of incoming DFDV indicates that the current
   node is its next hop to the landmark, i.e., the current node is on
   the drifter's shortest path to the landmark, the current node will
   insert or update its drifter list.  The receiving time is stamped
   in the DFDV.  The node sending the update packet is recorded as the
   next hop to the drifter.  The reverse path to the drifter is thus
   built up.  The procedure ends when the landmark receives the
   drifter entry.  The updated DFDV will be propagated with the next
   update packet.

6.4.3. Removing a Drifter Entry

   Each entry in DFDV is time stamped of its last receiving time.
   Every time before the DFDV is sent or routing by DFDV is needed,
   the table is checked for staled entries. If such an entry is found,
   it is removed.

6.5. Processing Neighbor List

   When a node receives either a landmark update or a host protocol
   routing update, the neighbor list is inserted if the update comes
   from a new neighbor, or the corresponding neighbor entry is
   updated.  Current time is recorded with the entry.  The
   neighbor list is also search at this time for possible lost
   neighbors according to the time stamps.  If such a neighbor is
   found, it is removed from the list.

7. Data Packet Forwarding

   Data packets are relayed hop by hop.  The host protocol routing



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   table, drifter list and landmark distance vector are looked up
   sequentially for the destination entry.  If the destination is
   within a node's scope, the entry can be found directly in the
   routing table and the packet is forwarded to the next hop node.
   Otherwise, the drifter list DFDV is searched for the destination.
   If the entry is found, the packet is forwarded using the next hop
   address from DFDV.  If not, the logical subnet field of the
   destination is retrieved and the LMDV entry of the landmark
   corresponding to the destination's logical subnet is searched.
   The data packet is then routed towards the landmark using the next
   hop address from LMDV.  The packet, however, is not necessary to
   pass through the landmark.  Rather, once the packet gets within the
   scope of the destination on its way towards the landmark, it is
   routed to the destination directly.


8. Discussion about Storage and Processing Overhead for Drifters

   The routing storage and processing overhead introduced by the
   distance vector extension to handle drifters is typically small
   if the fraction of drifting nodes is small.  Consider a network
   with N nodes and L landmarks, and assume that a fraction F of the
   members of each logical subnet have drifted.  In the worst case,
   the path length from landmark to drifter is the square root of N
   (assuming a grid topology).  Thus, sqrt(N) is the bound on the
   number of extra routing entries required at the nodes along the
   path to the drifter.  The total number of extra routing entries is
   sqrt(N)*L*(F*N/L) where N/L is the average logical group size.
   Thus, the extra storage per node is F*sqrt(N).  Now, let us assume
   that the number of nodes in the scope = # of landmarks = logical
   group size = sqrt(N).  Then, the basic routing table overhead per
   node (excluding drifters) is 3*sqrt(N).  Thus, the extra overhead
   caused by drifters is F/3.  If 20% of the nodes in a group are
   outside of the landmark scope, i.e., have drifted, the extra
   routing O/H required to keep track of them is only 7%.


Acknowledgments

   This work was supported in part by NSF under contract ANI-9814675,
   by DARPA under contract DAAB07-97-C-D321, and by ONR under
   contract N00014-01-C-0016.


References

   [1] G. Pei, M. Gerla and X. Hong, "LANMAR: Landmark Routing for
   Large Scale Wireless Ad Hoc Networks with Group Mobility",
   Proceedings of IEEE/ACM MobiHOC 2000, Boston, MA, Aug. 2000.


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   [2] M. Gerla, X. Hong, G. Pei, "Landmark Routing for Large Ad Hoc
   Wireless Networks", Proceedings of IEEE GLOBECOM 2000,
   San Francisco, CA, Nov. 2000.

   [3] C.E. Perkins and P. Bhagwat, "Highly Dynamic Destination-
   Sequenced Distance-Vector Routing (DSDV) for Mobile Computers,"
   In Proceedings of ACM SIGCOMM'94, London, UK, Sep. 1994,
   pp. 234-244.

   [4] UCLA Wireless Adaptive Mobility (WAM) Laboratory.
       http://www.cs.ucla.edu/NRL/wireless

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

   [6] P. F. Tsuchiya, "The Landmark Hierarchy: a new hierarchy for
   routing in very large networks", Computer Communication Review,
   vol.18, no.4, Aug. 1988, pp. 35-42.

   [7] G. Pei, M. Gerla, and T.-W. Chen, "Fisheye State Routing:
   A Routing Scheme for Ad Hoc Wireless Networks", Proceedings of
   ICC 2000, New Orleans, LA, Jun. 2000.

   [8] G. Pei, M. Gerla, and T.-W. Chen, "Fisheye State Routing in
   Mobile Ad Hoc Networks", Proceedings of Workshop on Wireless
   Networks and Mobile Computing, Taipei, Taiwan, Apr. 2000.


Chair's Address

   The MANET Working Group can be contacted via its current chairs:

        M. Scott Corson
        Institute for Systems Research
        University of Maryland
        College Park, MD  20742
        USA

        Phone:  +1 301 405-6630
        Email:  corson@isr.umd.edu


        Joseph Macker
        Information Technology Division
        Naval Research Laboratory
        Washington, DC  20375
        USA



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        Phone:  +1 202 767-2001
        Email:  macker@itd.nrl.navy.mil


Authors' Addresses

   Questions about this document can also be directed to the authors:

        Mario Gerla
        3732F Boelter Hall
        Computer Science Department
        University of California
        Los Angeles, CA  90095-1596
        USA

        Phone:  +1 310 825-4367
        Fax:    +1 310 825-7578
        Email:  gerla@cs.ucla.edu


        Xiaoyan Hong
        3803F Boelter Hall
        Computer Science Department
        University of California
        Los Angeles, CA  90095-1596
        USA

        Phone:  +1 310 825-4623
        Fax:    +1 310 825-7578
        Email:  hxy@cs.ucla.edu


        Li Ma
        3803D Boelter Hall
        Computer Science Department
        University of California
        Los Angeles, CA  90095-1596
        USA

        Phone:  +1 310 825-1888
        Fax:    +1 310 825-7578
        Email:  mary@cs.ucla.edu


        Guangyu Pei
        Rockwell Science Center
        1049 Camino Dos Rios



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        P.O. Box 1085
        Thousand Oaks, CA 91358-0085
        USA

        Phone:  +1 805 373-4639
        Fax:    +1 805 373-4383
        Email:  gpei@rsc.rockwell.com






































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