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Versions: (draft-gerla-manet-odmrp) 00 01 02 04

IETF MANET Working Group                                    Sung-Ju Lee
INTERNET-DRAFT                                               William Su
Expiration: December 1999                                   Mario Gerla
                                  University of California, Los Angeles
                                                              June 1999



   On-Demand Multicast Routing Protocol (ODMRP) for Ad Hoc Networks

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


Status of This Memo

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

   On-Demand Multicast Routing Protocol (ODMRP) is a multicast routing
   protocol designed for ad hoc networks with mobile hosts. ODMRP is
   a mesh-based, rather than a conventional tree-based, multicast
   scheme and uses a forwarding group concept (only a subset of nodes
   forwards the multicast packets via scoped flooding). It applies
   on-demand procedures to dynamically build routes and maintain
   multicast group membership. ODMRP is well suited for ad hoc
   wireless networks with mobile hosts where bandwidth is limited,
   topology changes frequently and rapidly, and power is constrained.


Lee, Su, and Gerla                                             [Page 1]


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                                Contents

Status of This Memo                                                   1

Abstract                                                              1

 1. Introduction                                                      3

 2. Terminology                                                       4
     2.1. General Terms . . . . . . . . . . . . . . . . . . . . . .   4
     2.2. Specification Language  . . . . . . . . . . . . . . . . .   4

 3. Protocol Overview                                                 5
     3.1. Group Establishment and Route Construction  . . . . . . .   5
           3.1.1. Mesh Creation . . . . . . . . . . . . . . . . . .   5
           3.1.2. Adapting the Refresh Interval via
                  Mobility Prediction . . . . . . . . . . . . . . .   7
           3.1.3. Soft State  . . . . . . . . . . . . . . . . . . .   7
     3.2. Contents of Tables  . . . . . . . . . . . . . . . . . . .   8
           3.2.1. Routing Table . . . . . . . . . . . . . . . . . .   8
           3.2.2. Forwarding Group Table  . . . . . . . . . . . . .   8
           3.2.3. Message Cache . . . . . . . . . . . . . . . . . .   8
     3.3. Unicast Routing Capability  . . . . . . . . . . . . . . .   8

 4. Packet and Table Formats                                          9
     4.1. Join Data Packet Header  . . . . . . . . . . . . . . . .    9
     4.2. Join Table Packet  . . . . . . . . . . . . . . . . . . .   11

 5. Operation                                                        13
     5.1. Forwarding Group Setup  . . . . . . . . . . . . . . . . .  13
           5.1.1. Originating a Join Data . . . . . . . . . . . . .  13
           5.1.2. Processing a Join Data  . . . . . . . . . . . . .  13
           5.1.3. Processing a Join Data When GPS is Used . . . . .  14
           5.1.4. Originating a Join Table  . . . . . . . . . . . .  15
           5.1.5. Processing a Join Table . . . . . . . . . . . . .  15
           5.1.6. Processing a Join Table When GPS is Used  . . . .  16
           5.1.7. Passive Acknowledgments . . . . . . . . . . . . .  17
     5.2. Handling a Multicast Data Packet  . . . . . . . . . . . .  17

 6. Protocol Applicability                                           18
  6.1. Networking Context . . . . . . . . . . . . . . . . . . . . .  18
  6.2. Protocol Characteristics and Mechanisms  . . . . . . . . . .  18

Acknowledgments                                                      20

References                                                           20

Chair's Address                                                      21

Authors' Addresses                                                   22



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1. Introduction

   This document describes the On-Demand Multicast Routing Protocol
   (ODMRP) developed by the Wireless Adaptive Mobility (WAM) Lab [12]
   at UCLA. ODMRP applies "on-demand" routing techniques to avoid
   channel overhead and improve scalability. It uses the concept of
   "forwarding group,"[3] a set of nodes responsible for forwarding
   multicast data, to build a forwarding mesh for each multicast group.
   By maintaining and using a mesh instead of a tree, the drawbacks of
   multicast trees in mobile wireless networks (e.g., intermittent
   connectivity, traffic concentration, frequent tree reconfiguration,
   non-shortest path in a shared tree, etc.) are avoided. A soft-state
   approach is taken to maintain multicast group members. No explicit
   control message is required to leave the group. We believe the
   reduction of channel/storage overhead and the relaxed connectivity
   make ODMRP more scalable for large networks and more stable for
   mobile wireless networks.

   The following properties of ODMRP highlight its advantages.

   *   Low channel and storage overhead

   *   Usage of stable routes

   *   Robustness to host mobility

   *   Maintenance and exploitation of multiple redundant paths

   *   Scalability to a large number of nodes

   *   Exploitation of the broadcast nature of wireless environments

   *   Adaptivity to node movement patterns

   *   Reconstruction of routes in anticipation of topology changes

















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2. Terminology

2.1. General Terms

   This section defines terminology used in ODMRP.

   node

      A device that implements IP.

   neighbor

      Nodes that are within the radio transmission range.

   forwarding group

       A group of nodes participating in multicast packet forwarding.

   multicast mesh

       The topology defined by the link connection between forwarding
       group members.

   join data

       The special data packet sent by multicast sources to establish
       and update group memberships and routes.

   join table

       The table broadcasted by each multicast receiver and forwarding
       node to establish and update group membership and routes


2.2. 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 [1].













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3. Protocol Overview

3.1. Group Establishment and Route Construction

3.1.1. Mesh Creation

   In ODMRP, group membership and multicast routes are established and
   updated by the source on demand. Similar to on-demand unicast
   routing protocols, a request phase and a reply phase comprise the
   protocol. When a multicast source has packets to send but no route
   and group membership is known, it floods a control packet with data
   payload attached. This packet, called "Join Data" (format shown in
   Section 4.1) is periodically broadcasted to the entire network to
   refresh the membership information and update the routes. When a
   node receives a Join Data packet, it stores the source ID and the
   sequence number to its "Message Cache" to detect duplicates. The
   upstream node ID is inserted or updated as the next node for the
   source node in its "Routing Table." If the Join Data packet is not
   a duplicate and the Time-To-Live value is greater than zero,
   appropriate fields are updated and it is rebroadcasted (operation
   details are explained in Section 5.1.2).

   When a Join Data packet reaches the multicast receiver, it creates
   and broadcasts a "Join Table" to its neighbors. When a node receives
   a Join Table, it checks if the next node ID of one of the entries
   matches its own ID. If it does, the node realizes that it is on the
   path to the source and thus is part of the forwarding group;
   it sets the FG_FLAG. It then broadcasts its own Join Table built
   upon matched entries. The next node ID field is filled in by
   extracting the information from its routing table. This way, the
   Join Table is propagated by each forward group member until it
   reaches the multicast source via the selected path. This process
   constructs (or updates) the routes from sources to receivers and
   builds a mesh of nodes, the forwarding group.


















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   +--+       +--+       +--+
   |S1|-------|I1|-------|R1|
   +--+\      +--+      /+--+  Join Table of Node R1 and Node I1
        \              /        +----------------+  +----------------+
         \            /         |Sender|Next Node|  |Sender|Next Node|
          \          /          |------+---------|  |------+---------|
           \        /           |  S1  |    I1   |  |  S1  |    S1   |
            \      /            |------+---------|  +----------------+
   +--+      \+--+/      +--+   |  S2  |    I2   |
   |S2|-------|I2|-------|R2|   +----------------+
   +--+       +--+       +--+



   Let us consider the above figure as an example of Join Table
   forwarding process. Nodes S1 and S2 are multicast sources, and nodes
   R1 and R2 are multicast receivers. Node R2 sends its Join Table to
   both S1 and S2 via I2, and R1 sends its packet to S1 via I1 and to
   S2 via I2. When receivers send their Join Tables to next hop nodes,
   an intermediate node I1 sets the FG_FLAG and builds its own Join
   Table since there is a next node ID entry in the Join Table received
   from R1 that matches its ID. Note that the Join Table build by I1
   has an entry for sender S1 but not for S2 because the next node ID
   for S2 in the received Join table is not I1. In the meanwhile, node
   I2 sets the FG_FLAG, constructs its own Join Table and sends it to
   its neighbors. Note that I2 broadcasts the join table only once even
   though it receives two Join Tables from the receivers because the
   second table arrival carries no new source information. Channel
   overhead is thus reduced dramatically in cases where numerous
   multicast receivers share the same links to the source.

   After this group establishment and route construction process, a
   source can multicast packets to receivers via selected routes and
   forwarding groups. While outgoing data packets exist, the source
   sends Join Data every REFRESH_INTERVAL. This Join Data and Join
   Table propagation process refreshes forwarding group and routes.
   When receiving the multicast data packet, a node forwards it only
   when it is not a duplicate and the setting of the FG_FLAG for the
   multicast group has not expired. This procedure minimizes the
   traffic overhead and prevents sending packets through stale routes.












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3.1.2 Adapting the Refresh Interval via Mobility Prediction

   ODMRP requires periodic flooding of Join Data} to build and refresh
   routes. Excessive flooding, however, is not desirable in ad hoc
   networks because of bandwidth constraints. Furthermore, flooding
   often causes congestion, contention, and collisions. Finding the
   optimal flooding interval is critical in ODMRP performance. In
   highly mobile networks where nodes are equipped with GPS [9] (e.g.,
   tactical netwoks with tanks, ships, aircrafts, etc.), we can
   efficiently adapt the REFRESH_INTERVAL to mobility patterns and
   speeds by utilizing the location and movement information. Note
   that ODMRP can still operate efficiently in networks where no such
   information is available, but the protocol can be further improved
   if those information can be utilized.

   We use the location and movement information to predict the duration
   of time routes will remain valid (the detail of the process is
   explained in 5.1.3). With the predicted time of route disconnection,
   Join Data are only flooded when route breaks of ongoing data
   sessions are imminent.

   A different route selection method is applied when we use the
   mobility prediction. The idea is inspired by the Associativity-Based
   Routing (ABR) protocol [11] which chooses associatively stable
   routes. In our algorithm, instead of using the minimum delay path,
   we can choose a route that is the most stable (i.e., the one that
   will remain connected for the longest duration of time). To select
   a route, a multicast receiver must wait for an appropriate amount of
   time after receiving the first Join Data so that all possible routes
   and their route qualities will be known. The receiver then chooses
   the most stable route and broadcasts a Join Table. Route breaks will
   occur less often and the number of Join Data propagation will reduce
   because stable routes are used.


3.1.3. Soft State

   In ODMRP, no explicit control packets need to be sent to leave the
   group. If a multicast source wants to leave the group, it simply
   stops sending any Join Data packets since it does not have any
   multicast data to send to the group. If a receiver no longer wants
   to receive from a particular multicast group, it does not send the
   Join Table for that group. Nodes in the forwarding group are demoted
   to non-forwarding nodes if not refreshed (no Join Tables received)
   before they timeout.








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3.2. Contents of Tables

   Nodes running ODMRP are required to maintain the following tables.
   These tables MAY be implemented in any format, but MUST include the
   fields specified in this document.

3.2.1. Routing Table

   A routing table is created on demand and is maintained by each node.
   An entry is inserted or updated when a non-duplicate Join Data is
   received. The node stores the destination (i.e., the source of the
   Join Data) and the next hop to the destination (i.e., the last node
   that propagated the Join Data). The routing table provides the next
   hop information when transmitting Join Tables.

3.2.2. Forwarding Group Table

   When a node is a forwarding group node of the multicast group, it
   maintains the group information in the forwarding group table. The
   multicast group ID and the time when the node was last refreshed are
   recorded.

3.2.3. Message Cache

   The message cache is maintained by each node to detect duplicates.
   When a node receives a new Join Data or data, it stores the source
   address and the sequence number of the packet. Note that entries in
   the message cache need not be maintained permanently. Schemes such
   as LRU (Least Recently Used) or FIFO (First In First Out) can be
   employed to expire and remove old entries and prevent the size of
   the message cache to be extensive.


3.3. Unicast Routing Capability

   One of the major strengths of ODMRP is its unicast routing
   capability. Not only ODMRP can work with any unicast routing
   protocol, it can function as both multicast and unicast. Thus, ODMRP
   can run without any underlying unicast protocol. Other ad hoc
   multicast routing protocols such as AMRoute [2], CAMP [5], RBM [4],
   and LAM [7] must be run on top of a unicast routing protocol. CAMP,
   RBM, and LAM in particular, only work on top of certain underlying
   unicast protocols.










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4. Packet and Table Formats

4.1. Join Data Packet 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Type     |   Reserved    |  Time To Live |   Hop Count   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Multicast Group IP Address                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Sequence Number                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Source IP Address                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   Previous Hop IP Address                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Previous Hop X Coordinate                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Previous Hop Y Coordinate                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Previous Hop Moving Speed   | Previous Hop Moving Direction |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Minimum Link Expiration Time                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type

        01; ODMRP Join Data.

   Reserved

        Sent as 0; ignored on reception.

   Time To Live

        Number of hops this packet can traverse.

   Hop Count

        The number of hops traveled so far by this packet.

   Multicast Group IP Address

        The IP address of the multicast group.







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   Sequence Number

        The sequence number assigned by the source to uniquely
        identify the packet.

   Source IP Address

        The IP address of the node originating the packet.

   Previous Hop IP Address

        The IP address of the last node that has processed this packet.

   Previous Hop X Coordinate (Optional)

        The x-coordinate of the last node that has processed this
        packet. The information can be obtained from the GPS. This
        field is required only when network hosts are GPS equipped.

   Previous Hop Y Coordinate (Optional)

        The y-coordinate of the last node that has processed this
        packet. The information can be obtained from the GPS. This
        field is required only when network hosts are GPS equipped..

   Previous Hop Moving Speed (Optional)

        The mobility speed of the last node that has processed this
        packet. The information can be obtained from the GPS or the
        node's own instruments and sensors (e.g., campus, odometer,
        speed sensors, etc.). This field is required only when network
        hosts are GPS equipped.

   Previous Hop Moving Direction (Optional)

        The moving direction of the last node that has processed this
        packet. The information can be obtained from the GPS or the
        node's own instruments and sensors (e.g., campus, odometer,
        speed sensors, etc.). This field is required only when network
        hosts are GPS equipped.

   Minimum Link Expiration Time (Optional)

        The minimum expiration time among the links taken by this
        packet so far. This field is required only when network hosts
        are GPS equipped.






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4.2. Join Table Packet

     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     |    Count      |R|F|     Reserved              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Multicast Group IP Address                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   Previous Hop IP Address                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Sequence Number                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Sender IP Address [1]                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   Next Hop IP Address [1]                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Route Expiration Time [1]                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :                               |
    |                               :                               |
    |                               :                               |
    |                               :                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Sender IP Address [n]                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   Next Hop IP Address [n]                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Route Expiration Time [n]                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type

        02; ODMRP receiver Join Table.

   Count

        Number of (Sender IP Address, Next Hop IP Address)
        combinations.

   R

        Acknowledgment request flag. This flag is set when active
        acknowledgment packet is requested.

   F

        Forwarding group flag. This flag is set when the packet is
        transmitted by a forwarding group node.


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   Reserved

        Sent as 0; ignored on reception.

   Multicast Group IP address

        The IP address of the multicast group.

   Previous Hop IP Address

        The IP address of the last node that has processed this packet.

   Sequence Number

        The sequence number assigned by the previous hop node to
        uniquely identify the packet.

   Sender IP Address [1..n]

        The IP addresses of the sources of this multicast group.

   Next Hop IP Address [1..n]

        The IP addresses of next nodes that this packet is target to.

   Route Expiration Time [1..n] (Optional)

        The minimum route expiration times of this multicast group.
        This field is required only when network hosts are GPS equipped.























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5. Operation

5.1. Forwarding Group Setup

5.1.1. Originating a Join Data

   When a multicast source has data packets to send but no route is
   known, it originates a "Join Data" packet. The Type field MUST be
   set to 01. TTL MAY be set to TIME_TO_LIVE_VALUE, but SHOULD be
   adjusted based on network size and network diameter. The Sequence
   number MUST be large enough to prevent wraparound ambiguity, and the
   Hop Count is initially set to zero. The source puts its IP address
   in the Source IP Address and Last Hop IP Address field. It appends
   its location, speed, and direction into JOIN DATA.

   When location and movement information is utilized, it sets the
   MIN_LET (Link Expiration Time) field to the MAX_LET_VALUE since the
   source does not have any previous hop node. When the source receives
   Join Tables from multicast receivers, it selects the minimum RET
   (Route Expiration Time) among all the Join Tables received. Then the
   source can build new routes by originating a Join Data before the
   minimum RET approaches (i.e., route breaks of ongoing data sessions
   are imminent).

5.1.2. Processing a Join Data

   When a node receives a Join Data packet:

   1. Check if it is a duplicate by comparing the (Source IP Address,
      Sequence Number) combination with the  entries in message cache.
      If duplicate, then discard the packet. DONE.

   2. If it is not a duplicate, insert an entry into message cache with
      the information of the received packet (i.e., sequence number and
      source IP address) and insert/update the entry for routing table
      (i.e., backward learning).

   3. If the node is a member of the multicast group, it  originates a
      Join Table packet with the RET value enclosed (see Section 5.1.4).

   4. Increase the Hop Count field by 1 and decrease the TTL field by 1.

   5. If the TTL field value is less than or equal to 0, then discard
      the packet. DONE.

   6. If the TTL field value is greater than 0, then set the node's IP
      Address into Last Hop IP Address field and broadcast. DONE.





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5.1.3. Processing a Join Data When GPS is Used

   When a node receives a Join Data packet:

   1. Check if it is a duplicate by comparing the (Source IP Address,
      Sequence Number) combination with the  entries in message cache.
      If duplicate, then discard the packet. DONE.

   2. If it is not a duplicate, insert an entry into message cache with
      the information of the received packet (i.e., sequence number and
      source IP address) and insert/update the entry for routing table
      (i.e., backward learning).

   3. Predict the duration of time the link between the node and the
      upstream node will remain connected using the following equation.

      Assume node i is the upstream node and node j is the current node.
      Let (x_{i}, y_{i}) be the coordinate of node i and (x_{j}, y_{j})
      be that of node j. Also let v_{i} and v_{j} be the speeds, and
      theta_{i} and theta_{j} be the moving directions of nodes i and j,
      respectively. The information of node i (the previous hop node)
      can be obtained from the Join Data and the current node's
      location and mobility information can be provided by the GPS. The
      duration of time that the link between two nodes will stay
      connected, D_{t}, is given by:

           -(a*b + c*d) + sqrt((a^{2} + c^{2})*r^{2} - (a*d - b*c)^{2})
   D_{t} = ------------------------------------------------------------
                             a^{2} + c^{2}

      where
         a = v_{i}*cos(theta_{i}) - v_{j}*cos(theta_{j}),
         b = x_{i} - x_{j},
         c = v_{i}*sin(theta_{i}) - v_{j}*sin(theta_{j}), and
         d = y_{i} - y_{j}.

      Note that when v_{i} = v_{j} and theta_{i} = theta_{j}, D_{t} is
      set to infinity without applying the above equation.

      The minimum between this D_{t} value and the indicated value in
      MIN_LET field of the Join Data is included in the packet. The
      rationale is that as soon as a single link on the path is
      disconnected, the entire path is invalidated. The node also
      overwrites the location and mobility information field written
      by the previous node with its own information.







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   4. If the node is a member of the multicast group, it calculates the
      predicted LET of the last link of the path. The minimum between
      the last link expiration time and the MIN_LET value specified in
      the Join Data is the RET (Route Expiration Time).

      To select a route, a multicast receiver must wait for an
      appropriate amount of time after receiving the first Join Data
      so that all possible routes and their RET will be known. The
      receiver then chooses the most stable route (i.e., the route with
      the largest RET) and originates a Join Table packet with the RET
      value enclosed (see Section 5.1.3.).

   5. Increase the Hop Count field by 1 and decrease the TTL field by 1.

   6. If the TTL field value is less than or equal to 0, then discard
      the packet. DONE.

   7. If the TTL field value is greater than 0, then set the node's IP
      Address into Last Hop IP Address field and broadcast. DONE.


5.1.4. Originating a Join Table

   A multicast receiver transmits a "Join Table" packet after selecting
   the multicast route. Each sender IP address and next hop IP address
   of a multicast group are contained in the Join Table packet. The
   route expiration time is also included if the network hosts operate
   with GPS.


5.1.5. Processing a Join Table

   When a Join Table is received:

   1. The node looks up the Next Hop IP Address field of the received
      Join Table entries. If no entries match the node's IP Address, do
      nothing. DONE.

   2. If one or more entries coincide with the node's IP Address, set
      the FG_FLAG and build its own Join Table. The next hop IP address
       can be obtained from the routing table.

   3. Broadcast the Join Table packet to the neighbor nodes. DONE.










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5.1.6. Processing a Join Table When GPS is Used

   When a Join Table is received:

   1. The node looks up the Next Hop IP Address field of the received
      Join Table entries. If no entries match the node's IP Address, do
      nothing. DONE.

   2. If one or more entries coincide with the node's IP Address, set
      the FG_FLAG and build its own Join Table. If multiple Join Tables
      with different RET values are received (i.e., the node lies in
      paths from the same source to multiple receivers), it selects the
      minimum RET among them and attaches the chosen RET value. Next
      hop IP address can be obtained from the routing table.

   3. Broadcast the Join Table packet to the neighbor nodes.

   4. If the node is a source, it selects the minimum RET among all the
      Join Tables received. Then the source can build new routes by
      flooding a Join Data before the minimum RET approaches (i.e.,
      route breaks of ongoing data sessions are imminent).

      In addition to the estimated RET value, other factors need to be
      considered when choosing the refresh interval of Join Data. If
      the node mobility rate is high and the topology changes
      frequently, routes will expire quickly and often. The source may
      propagate Join Requests excessively and this excessive flooding
      can cause collisions, congestion, and clogs the network with
      control packets. Thus, the MIN_REFRESH_INTERVAL should be
      enforced to avoid control message overflow. On the other hand, if
      nodes are stationary or move slowly and link connectivity remains
      unchanged for a long duration of time, routes will hardly expire
      and the source will rarely send Join Data. A few problems arise
      in this situation. First, if a node in the route suddenly changes
      its movement direction or speed, the predicted RET value becomes
      obsolete and routes will not be reconstructed. Second, when a
      non-member node which is located remotely to multicast members
      wants to join the group, it cannot inform the new membership or
      receive data until a Join Data is received. Hence, the
      MAX_REFRESH_INTERVAL should be set. The selection of the
      MIN_REFRESH_INTERVAL and the MAX_REFRESH_INTERVAL should be
      adaptive to network situations (e.g., traffic type, traffic load,
      mobility pattern, channel capacity, etc.).










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5.1.7. Passive Acknowledgments

      The reliable transmission of Join Tables plays an important role
      in establishing and refreshing multicast routes and forwarding
      groups. Hence, if Join Tables are not properly delivered,
      effective multicast routing cannot be achieved by ODMRP. The
      IEEE 802.11 MAC protocol [6], which is the standard in wireless
      networks, performs reliable transmission by retransmitting the
      packet if no acknowledgment is received. However, if the packet
      is broadcasted, the acknowledgments and retransmissions are not
      sent. In ODMRP, the transmission of Join Tables are mostly
      broadcasted. Thus, the verification of Join Table delivery and
      the retransmissions must be done by the ODMRP layer.

      We adopt a scheme that was used in [8]. When a node transmits a
      Join Table packet to the immediate upstream node of the route,
      the immediate downstream node can hear the transmission if it is
      within the transmitter's radio range. Hence, the packet is used
      as an "passive acknowledgment." We can utilize this passive
      acknowledgments to verify the delivery of Join Tables. Multicast
      sources must send active acknowledgments to the previous hops
      since they do not have any next hops to send Join Tables to
      unless they are forwarding group nodes. When no acknowledgment is
      received within the timeout interval, the node retransmits the
      message. If packet delivery cannot be verified after an
      appropriate number of retransmissions, the node considers the
      route to be invalidated. The node then broadcasts a message to
      its neighbors specifying that the next hop to the source cannot
      be reached. Upon receiving this packet, the neighboring node
      builds and unicasts the Join Table to its next hop if it has a
      route to the multicast source. If no route is known, it simply
      rebroadcasts the packet specifying the next hop is not available.
      In both cases, the node sets its FG_FLAG. The FG_FLAG setting of
      every neighbors may create excessive redundancy, but most of
      these settings will expire because only necessary forwarding
      group nodes will be refreshed in the next Join Table propagation
      phase.


5.2. Handling a Multicast Data Packet

   Multicast sources send the Data whenever they have packets to send.
   Nodes relay data packets only if the packet is not a duplicate and
   the setting of FG_FLAG for the multicast group has not expired.









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6. Protocol Applicability

6.1. Networking Context

   ODMRP is best suited for mobile ad hoc wireless networks.

6.2. Protocol Characteristics and Mechanisms

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

      - No. We assume bidirectional links.

   * 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?)

      - No.

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

      - No.

   * Does the protocol provide support for routing through a multi-
   technology routing fabric? (if so, how?)

      - No.

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

      - No. In this document, we assume each mobile host is combined
        with a router, sharing the same IP address. It is possible,
        however, to extend the protocol to handle multiple hosts per
        router.









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

      - Yes. The message contains host IP address as its identification.

   * 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?)

      - Yes. For example, the source creates and maintains routes and
        multicast group membership only when it has data packets to
        send.

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

      - No.

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

      - Yes. By using the Message Cache, duplicate packets are detected
        and packets can only go through the loop-free route.

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

      - TBD. The work is in progress.

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

      - TBD. The work is in progress.

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

      - This document assumed an arbitrary single channel link-layer
        protocol. The protocol can work with any MAC and link-layer
        technology. It can also support multi-channel link-layer
        technology (e.g., separate channels for data, control packets,
        etc.).






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Acknowledgments

   Authors thank Ching-Chuan Chiang and Guangyu Pei for their initial
   contributions.



References

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

   [2] E. Bommaiah, M. Liu, A. McAuley, and R. Talpade.   AMRoute:
       Adhoc Multicast Routing Protocol.   Internet Draft,
       draft-talpade-manet-amroute-00.txt, Aug. 1998. Work in progress.

   [3] Ching-Chuan Chiang, Mario Gerla, and Lixia Zhang.  Forwarding
       Group Multicast Protocol (FGMP) for Multihop, Mobile Wireless
       Networks.  ACM/Baltzer Cluster Computing, vol. 1, no. 2, 1998.

   [4] M.S. Corson and S.G. Batsell.   A Reservation-Based Multicast
       (RBM) Routing Protocol for Mobile Networks: Initial Route
       Construction Phase.   ACM/Baltzer Wireless Networks, vol. 1,
       no. 4, Dec. 1999, pp. 427-450.

   [5] J.J. Garcia-Luna-Aceves and E.L. Madruga.   A Multicast Routing
       Protocol for Ad-Hoc Networks.   In Proceedings of IEEE
       INFOCOM'99, New York, NY, Mar. 1999, pp. 784-792.

   [6] IEEE Computer Society LAN MAN Standards Committee.   Wireless
       LAN Medium Access Protocol (MAC) and Physical Layer (PHY)
       Specification. IEEE std 802.11-1997. The Institute of Electrical
       and Electronics Engineers, New York, NY, 1997.

   [7] L. Ji and M.S. Corson.   A Lightweight Adaptive Multicast
       Algorithm. In Proceedings of IEEE GLOBECOM'98, Sydney,
       Australia, Nov. 1998, pp. 1036-1042.

   [8] J. Jubin and J.D. Tornow.   The DARPA Packet Radio Network
       Protocols.   Proceedings of the IEEE, vol. 75, no. 1, Jan. 1987,
       pp. 21-32.

   [9] E.D. Kaplan (Editor).   Understanding the GPS: Principles and
       Applications, Artech House, Boston, MA, Feb. 1996.

  [10] S.-J. Lee, M. Gerla, and C.-C. Chiang.   On-Demand Multicast
       Routing Protocol.   In Proceedings of IEEE WCNC'99, New Orleans,
       LA, Sep. 1999 (to appear).




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  [11] C.-K. Toh.  Associativity-Based Routing for Ad-Hoc Mobile
       Networks.   Wireless Personal Communications Journal, Special
       Issue on Mobile Networking and Computing Systems, Kluwer
       Academic Publishers, vol. 4, no. 2, Mar. 1997, pp. 103-139.

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



Chair's Address



   The 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

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


















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Authors' Addresses


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

        Sung-Ju Lee
        3771 Boelter Hall
        Computer Science Department
        University of California
        Los Angeles, CA  90095-1596
        USA

        Phone:  +1 310 206-8589
        Fax:    +1 310 825-7578
        Email:  sjlee@cs.ucla.edu


        William Su
        3771 Boelter Hall
        Computer Science Department
        University of California
        Los Angeles, CA  90095-1596
        USA

        Phone:  +1 310 206-8589
        Fax:    +1 310 825-7578
        Email:  wsu@cs.ucla.edu


        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














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