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

IETF MANET Working Group                                   Yunjung Yi
INTERNET-DRAFT                                             Sung-Ju Lee
Expiration: Febuary 2003                                   William Su
                                                           Mario Gerla
                                  University of California, Los Angeles
                                                           November 2002



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

                  <draft-ietf-manet-odmrp-04.txt>


Status of This Memo


     This document is an Internet-Draft and is subject to
     all provisions of Section 10 of RFC2026 except that the
     right to produce derivative works is not granted.

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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.







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                                Contents



Status of This Memo                                                   1

Abstract                                                              1

 1. Introduction                                                      4

 2. Terminology                                                       5
     2.1. General Terms . . . . . . . . . . . . . . . . . . . . . .   5
     2.2. Specification Language  . . . . . . . . . . . . . . . . .   5

 3. Protocol Overview                                                 6
     3.1. Multicast Route and Mesh Creation . . . . . . . . . . . .   6
     3.2. Reliability . . . . . . . . . . . . . . . . . . . . . . .   8
     3.3. Soft State  . . . . . . . . . . . . . . . . . . . . . . .  10
     3.4. Selection of Timer Values . . . . . . . . . . . . . . . .  10
     3.5. Unicast Capability  . . . . . . . . . . . . . . . . . . .  10
     3.6. Contents of Tables  . . . . . . . . . . . . . . . . . . .  11
           3.6.1. Routing Table . . . . . . . . . . . . . . . . . .  11
           3.6.2. Forwarding Group Table  . . . . . . . . . . . . .  11
           3.6.3. Message Cache . . . . . . . . . . . . . . . . . .  11
     3.7. Mobility Prediction                                        12
           3.7.1. Adapting the Refresh Interval via
                  Mobility Prediction . . . . . . . . . . . . . . .  12
           3.7.2. Route Selection Criteria  . . . . . . . . . . . .  14
           3.7.3. Alternative Method of Prediction  . . . . . . . .  14
    3.8. Scalability via Efficient Flooding . . . . . . . . . . . .  14
           3.8.1. Passive Clustering  . . . . . . . . . . . . . . .  15

 4. Packet and Table Formats                                         16
     4.1. Join Query Packet Header   . . . . . . . . . . . . . . .   16
     4.2. Join Reply Packet  . . . . . . . . . . . . . . . . . . .   18


 5. Operation                                                        20
     5.1. Forwarding Group Setup  . . . . . . . . . . . . . . . . .  20
           5.1.1. Originating a Join Query  . . . . . . . . . . . .  20
           5.1.2. Processing a Join Query . . . . . . . . . . . . .  20
           5.1.3. Processing a Join Query When GPS is Used  . . . .  21
           5.1.4. Processing a Join Query When PC is Used . . . . .  22
           5.1.5. Originating a Join Reply  . . . . . . . . . . . .  23
           5.1.6. Processing a Join Reply . . . . . . . . . . . . .  23
           5.1.7. Processing a Join Reply When GPS is Used  . . . .  23
           5.1.8. Processing a Join Reply When PC is Used . . . . .  23
     5.2. Handling a Multicast Data Packet  . . . . . . . . . . . .  24




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 6. Protocol Applicability                                           25
  6.1. Networking Context . . . . . . . . . . . . . . . . . . . . .  25
  6.2. Protocol Characteristics and Mechanisms  . . . . . . . . . .  25

Acknowledgments                                                      27

References                                                           27

Chair's Address                                                      28

Authors' Addresses                                                   29










































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1. Introduction

   This document describes the On-Demand Multicast Routing Protocol
   (ODMRP) [14][15] developed by the Wireless Adaptive Mobility (WAM)
   Laboratory [20] at University of California, Los Angeles. ODMRP
   applies "on-demand" routing techniques to avoid channel overhead and
   improve scalability. It uses the concept of "forwarding group," [5]
   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 attractive in mobile wireless networks.

   The following properties of ODMRP highlight its advantages.

   *   Simplicity

   *   Low channel and storage overhead

   *   Usage of up-to-date shortest routes

   *   Reliable construction of routes and forwarding group

   *   Robustness to host mobility

   *   Maintenance and exploitation of multiple redundant paths

   *   Exploitation of the broadcast nature of wireless environments

   *   Unicast routing capability

   *   Scalability using efficient flooding
















Yi, Lee, Su, and Gerla                                             [Page 4]


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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 query

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

   join reply

       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 [4].













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3. Protocol Overview

3.1. Multicast Route and 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 member advertising packet
   with data payload piggybacked. This packet, called "Join Query"
   (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 Query packet, it stores the
   source address and the unique identifier of the packet to its
   "Message Cache" to detect duplicates. The upstream node address is
   inserted or updated as the next node for the source node in its
   "Routing Table." If the Join Query packet is not a duplicate and
   the Time-To-Live value is greater than zero, appropriate fields are
   updated and it is rebroadcast (operation details are illustrated
   in Section 5.1.2).

   When a Join Query packet reaches the multicast receiver, it creates
   and broadcasts a "Join Reply" to its neighbors. When a node receives
   a Join Reply, it checks if the next node address of one of the
   entries matches its own address. 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 (Forwarding Group Flag). It then
   broadcasts its own Join Reply built upon matched entries. The next
   node address field is filled in by extracting the information from
   its routing table. This way, the Join Reply 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 Replies 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 Reply
   forwarding process. Nodes S1 and S2 are multicast sources, and nodes
   R1 and R2 are multicast receivers. Node R2 sends its Join Reply 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 Replies to next hop nodes,
   an intermediate node I1 sets the FG_FLAG and builds its own Join
   Reply since there is a next node ID entry in the Join Reply received
   from R1 that matches its ID. Note that the Join Reply built by I1
   has an entry for sender S1 but not for S2 because the next node
   address for S2 in the received Join Reply is not I1. In the
   meanwhile, node I2 sets the FG_FLAG, constructs its own Join Reply
   and sends it to its neighbors. Note that I2 broadcasts the Join
   Reply  only once even though it receives two Join Replies 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 Query every REFRESH_INTERVAL. This Join Query and Join
   Reply 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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3.2. Reliability

   The reliable transmission of Join Replies plays an important role
   in establishing and refreshing multicast routes and forwarding
   groups. Hence, if Join Replies are not properly delivered,
   effective multicast routing cannot be achieved by ODMRP. The IEEE
   802.11 MAC (Medium Access Control) protocol [8], which is the
   emerging standard in wireless networks, performs reliable
   transmission by retransmitting the packet if no acknowledgment is
   received. However, if the packet is broadcasted, no acknowledgments
   or retransmissions are sent. In ODMRP, the transmission of Join
   Replies are often broadcasted to more than one upstream neighbors
   since we are handling multiple sources (e.g., see the Join Reply
   from node R1 in the example of Section 3.1.). In such cases, the
   hop-by-hop verification of Join Reply delivery and the
   retransmission cannot be handled by the MAC layer. It must be done
   indirectly by ODMRP. Another option for reliable delivery is to
   subdivide the Join Reply into separate sub-tables, one for each
   distinct next node. In the figure of Section 3.1. for example, the
   Join Reply at node R1 is split into two Join Replies, one for
   neighbor I1 and the other for neighbor I2. These Join Replies are
   separately unicasted using a reliable MAC protocol such as IEEE
   802.11 or MACAW [3]. Since the number of neighbors is generally
   limited (typically, about six neighbors in the optimum in a
   multihop network [12]), the scheme still scales well to large number
   of sources. This option can actually be used as backup to the

   passive acknowledgment option as discussed below.

   We adopt a scheme that was used in [10]. When a node transmits a
   Join Reply 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
   acknowledgment to verify the delivery of a Join Reply. Note that
   the source itself must send an active acknowledgment to the
   previous hop since it does not have any next hop to send a Join
   Reply to unless it is also a forwarding group node for other
   sources.














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   Considering the case in figure of Section 3.1. again, we note that
   once the nodes I1 and I2 receive the Join Reply from node R1, they
   will construct and forward their own Join Replies to next hops (in
   this case, sources S1 and S2). In transmitting their Join Replies,
   nodes I1 and I2 may overlap with each other. If I1 and I2} are
   within receiving range, they will recover because of the carrier
   sense feature in CSMA (Carrier Sense Multiple Access) [13]. However,
   if they are out of range, they will be unaware of the "hidden
   terminal" condition of node R1, which cannot hear the (overlapped)
   passive acknowledgments. Thus, a node may not hear the passive
   acknowledgments of its upstream neighbor because of conflicts due
   to the hidden terminal problem. It will also not hear the passive
   acknowledgment if the upstream neighbor has moved away. In either
   case, when no acknowledgment is received within the timeout
   interval, the node retransmits the message. Note that the node may
   get acknowledgments from some, but not all upstream neighbors. As
   an option, the retransmission could be carried out in unicast mode,
   to selected neighbors, with reduced sub-tables. If packet delivery
   cannot be verified after an appropriate number of retransmissions,
   the node considers the route to be invalidated. At this point, the
   most likely cause of route failure is the fact that a node on the
   route has failed or has moved out of range. An alternate route must
   be found "on the spot." The node thus broadcasts a message to its
   neighbors specifying that the next hop to a set of sources cannot
   be reached. Upon receiving this packet, each neighbor builds and
   unicasts the Join Reply to its next hop if it has a route to the
   multicast sources. If no route is known, it simply broadcasts the
   packet specifying the next hop is not available. In both cases, the
   node sets its FG_FLAG. In practical implementations, this
   redundancy is sufficient to establish alternate paths until a more
   efficient route is established during the next refresh phase. The
   FG_FLAG setting of every neighbor may create excessive redundancy,
   but most of these settings will expire because only necessary
   forwarding group nodes will be refreshed in the next Join Reply
   propagation phase.

















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3.3. 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 Query 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 Reply for that group. Nodes in the forwarding group are demoted
   to non-forwarding nodes if not refreshed (no Join Replies received)
   before they timeout.


3.4. Selection of Timer Values

   Timer values for route refresh interval and forwarding group
   timeout interval can have impacts on ODMRP performance. The
   selection of these soft state timers should be adaptive to network
   environment (e.g., traffic type, traffic load, mobility pattern,
   mobility speed, channel capacity, etc.). When small route refresh
   interval values are used, fresh route and membership information
   can be obtained frequently at the expense of producing more packets
   and causing network congestion. On the other hand, when large route
   refresh values are selected, even though less control traffic will
   be generated, nodes may not know up-to-date route and multicast
   membership. Thus in highly mobile networks, using large route
   refresh interval values can yield poor protocol performance. The
   forwarding group timeout interval should also be carefully
   selected. In networks with heavy traffic load, small values should
   be used so that unnecessary nodes can timeout quickly and not
   create excessive redundancy. In situations with high mobility,
   however, large values should be chosen so that more alternative
   paths can be provided. It is important to note that the forwarding
   group timeout value must be larger (e.g., 3 to 5 times) than the
   value of route refresh interval.


3.5. Unicast Capability

   One of the major strengths of ODMRP is its unicast routing
   capability. Not only can ODMRP coexist with any unicast routing
   protocol, it can also operate very efficiently as an unicast
   routing protocol. Thus, a network equipped with ODMRP does not
   require a separate unicast protocol. Other ad hoc multicast routing
   protocols such as AMRoute [5], CAMP [7], RBM [6], and LAM [9]
   must be run on top of a unicast routing protocol. CAMP, RBM, and
   LAM in particular, only work with certain underlying unicast
   protocols.





Yi, Lee, Su, and Gerla                                            [Page 10]


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3.6. 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.

3.6.1. 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 Query is
   received. The node stores the destination (i.e., the source of the
   Join Query) and the next hop to the destination (i.e., the last
   node that propagated the Join Query). The routing table provides
   the next hop information when transmitting Join Replies.

3.6.2. 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.

3.6.3. Message Cache

   The message cache is maintained by each node to detect duplicates.
   When a node receives a new Join Query or data, it stores the source
   address and the unique identifier 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.





















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3.7. Mobility Prediction

3.7.1 Adapting the Refresh Interval via Mobility Prediction

   ODMRP requires periodic flooding of Join Query 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 [11] (e.g.,
   tactical networks with tanks, ships, aircrafts, etc.), we can
   efficiently adapt the REFRESH_INTERVAL to mobility patterns and
   speeds by utilizing the location and movement information. We use
   the location and movement information to predict the duration
   of time routes will remain valid. With the predicted time of route
   disconnection, Join Queries are only flooded when route breaks of
   ongoing data sessions are imminent. 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.

   In our prediction method, we assume a free space propagation model,
   where the received signal strength solely depends on its distance
   to the transmitter. We also assume that all nodes in the network
   have their clock synchronized (e.g., by using the NTP (Network Time
   Protocol) [16] or the GPS clock itself). Therefore, if the motion
   parameters of two neighbors (e.g., speed, direction, radio
   propagation range, etc.) are known, we can determine the duration
   of time these two nodes will remain connected. Assume two nodes i
   and j are within the transmission range r of each other. 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} (0 <= theta_{i}, theta_{j} < 2 * pi) be the
   moving directions of nodes i and j, respectively. Then, 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.


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   To utilize the information obtained from the prediction, extra
   fields must be added into Join Query and Join Reply packets. When a
   source sends Join Query, it appends its location, speed, and
   direction. It sets the MIN_LET (Minimum Link Expiration Time) field
   to the MAX_LET_VALUE since the source does not have any previous
   hop node. The next hop neighbor, upon receiving a Join Query,
   predicts the link expiration time between itself and the previous
   hop using the above equation. The minimum between this value and
   the MIN_LET indicated by the Join Query is included in the packet.
   The rationale is that as soon as a single link on a 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. When a multicast member
   receives the Join Query, 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 Query is the RET
   (Route Expiration Time). This RET value is enclosed in the Join
   Reply and broadcasted. If a forwarding group node receives multiple
   Join Replies with different RET values (i.e., lies in paths from
   the same source to multiple receivers), it selects the minimum RET
   among them and sends its own Join Reply with the chosen RET value
   attached. When the source receives Join Replies, it selects the
   minimum RET among all the Join Replies received. Then the source
   can build new routes by flooding a Join Query before the minimum
   RET approaches (i.e., route breaks).

   In addition to the estimated RET value, other factors need to be
   considered when choosing the refresh interval. If the node mobility
   rate is high and the topology changes frequently, routes will
   expire quickly and often. The source may propagate Join Query
   excessively and this excessive flooding can cause collisions and
   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
   Query. 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 in time. 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
   Query is received. Hence, the MAX_REFRESH_INTERVAL should be set.
   The selection of the MIN_REFRESH_INTERVAL and the
   MAX_REFRESH_INTERVAL values should be adaptive to network
   environments.






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3.7.2. Route Selection Criteria

   In ODMRP, a multicast receiver selects routes based on the minimum
   delay (i.e., routes taken by the first Join Query received. A
   different route selection method is applied when we use the
   mobility prediction. The idea is inspired by the Associativity-Based
   Routing (ABR) protocol [18] 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 Query so that all possible
   routes and their route qualities will be known. The receiver then
   chooses the most stable route and broadcasts a Join Reply. Route
   breaks will occur less often and the number of Join Query
   propagation will reduce because stable routes are used.


3.7.3. Alternative Method of Prediction

   Since GPS may not work properly in certain situations (e.g.,
   indoor, fading, etc.), we may not always be able to accurately
   predict the link expiration time for a particular link.  However,
   there is an alternative method to predict the LET. This method is
   based on a more realistic propagation model and has been proposed
   in [1] and [17]. Basically, transmission power samples are measured
   periodically from packets received from a mobile's neighbor. From

   this information it is possible to compute the rate of change for a
   particular neighbor's transmission power level. Therefore, the time
   when the transmission power level will drop below the acceptable
   value (i.e., hysteresis region) can be predicted. We plan to
   investigate this option in our future work.

3.8. Scalabiity via Efficient Flooding

   Periodic Join Query floods to maintain a multicast mesh structure in
   ODMRP may significantly increase the network congestion and thus
   dramatically degrade network throughput. Especially, excessive
   flooding can become very inefficient as the node geographic density
   (i.e., the number of neighbors within a node's radio reach) grows
   due to redundant, "superfluous" forwarding. In fact, superfluous
   flooding increases the link overhead and wireless medium congestion
   and contention. In a large network, with heavy load, this extra overhead
   can have severe impact on performance and should be eliminated.

   We developed passive clustering [21], a cluster formation protocol
   mechanism designed for mobile ad hoc networks. Passive clustering (PC)
   constructs and maintains clusters using on-going data packets instead of

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   extra explicit control messages. Thus PC does not require excessive extra overhead.
   The cluster platform developed by PC can be used for efficient flooding, where only
   a set of dominant nodes forwards a flood packet rather all nodes participate in flooding.
   With cluster platform, only non-ORDINARY nodes (e.g., cluster heads and
   gateways) are allowed to relay the flood packet. Since the portion of cluster
   heads and gateways decreases as node geographical density increases, PC helps
   the more effectively as the network becomes denser.


3.8.1.  Passive clustering

   Passive Clustering (PC) dynamically partitions the network in clusters
   interconnected by gateway nodes. The resulting clusters have only one
   CLUSTER HEAD (a representative node) on the center of each cluster and
   the transmission range of each cluster head defines the area of the
   cluster.
   Other nodes not cluster head in the cluster are called cluster members.
   A GATEWAY node, one of cluster members, interconnects two adjacent
   clusters.
   If a node is not a cluster head nor gateway, the node is an
   ORDINARY NODE.
   Note that clusters are formed only when on-going traffic exists.
   Initially or without on-going traffic, each node set its state to INITIAL NODE.

   PC is an "on demand" protocol. It constructs and maintains the clus-
   ter architecture only when there are on-going data packets that pig-
   gyback "cluster-related information" such as the state of a node in a
   cluster and an IP address of a node.  Each node collects neighbor
   information through promiscuous packet receptions. Thus, PC does not
   necessitate background overhead of clustering.
   PC has two innovative mechanisms for the cluster formation: the
   "First Declaration Wins" rule and "Gateway Selec-
   tion Heuristic". With the "First Declaration Wins"
   rule, a node that first claims to be a CLUSTER HEAD 'rules' the rest
   of nodes in its clustered area (radio coverage).  There is no waiting
   period (to make sure all the neighbors have been checked).  Also,
   "Gateway Selection Heuristic"  provides a procedure to
   elect the minimal number of gateways required to maintain connectiv-
   ity (including distributed gateway nodes) in a distributed manner [21].


"




















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4. Packet and Table Formats

4.1. Join Query 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 Query.

   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 Reply 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 Join Reply.

   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 Query

   When a multicast source has data packets to send but no route is
   known, it originates a "Join Query" 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 Query if nodes in the
   network are equipped with GPS.

   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 Replies from multicast receivers, it selects the minimum RET
   (Route Expiration Time) among all the Join Replies received. Then the
   source can build new routes by originating a Join Query before the
   minimum RET approaches (i.e., route breaks of ongoing data sessions
   are imminent).

5.1.2. Processing a Join Query

   When a node receives a Join Query packet:

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

   2. If it is not a duplicate, insert an entry into the 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 Reply packet with the RET value enclosed (see Section 5.1.5).

   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 Query When GPS is Used

   When a node receives a Join Query packet:

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

   2. If it is not a duplicate, insert an entry into the 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 equation given in
      Section 3.7.1.

      The minimum between the newly obtained D_{t} value and the
      indicated value in MIN_LET field of the Join Query 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.

   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 Query 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 Query
      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 Reply 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.









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5.1.4. Processing a Join Query When PC is Used

    When a node receives a Join Query packet:

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

   2. If it is not a duplicate, insert an entry into the 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 Reply 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 and the state of node in the
      cluster platform is NOT ORDINARY_NODE, then set the node's IP
      Address into Last Hop IP Address field and broadcast.
      Otherwise, discard the packet. DONE.


































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5.1.5. Originating a Join Reply

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


5.1.6. Processing a Join Reply

   When a Join Reply is received:

   1. The node looks up the Next Hop IP Address field of the received
      Join Reply 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 Reply. The next hop IP address
      can be obtained from the routing table.

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


5.1.7. Processing a Join Reply When GPS is Used

   When a Join Reply is received:

   1. The node looks up the Next Hop IP Address field of the received
      Join Reply 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 Reply. If multiple Join Replies
      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 Reply packet to the neighbor nodes.

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



5.1.8. Processing a Join Reply When PC is Used

   Same to 5.1.6.








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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?)

      - Yes, but only when multicast sources have data packets to send.

   * 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. We also send our gratitude to Sang Ho Bae who
   implemented ODMRP in a real ad hoc network testbed.



References

   [1] P. Agrawal, D.K. Anvekar, and B. Narendran.   Optimal
       Prioritization of Handovers in Mobile Cellular Networks.   In
       Proceedings of IEEE PIMRC'94, The Hague, Netherlands, Sep. 1994,
       pp. 1393-1398.

   [2] R. Bagrodia, R. Meyer, M. Takai, Y. Chen, X. Zeng, J. Martin,
       and H.Y. Song.   PARSEC: A Parallel Simulation Environment for
       Complex Systems.   IEEE Computer, vol. 31, no. 10, Oct. 1998,
       pp.77-85.

   [3] V. Bharghavan, A. Demers, S. Shenker, and L. Zhang.   MACAW: A
       Media Access Protocol for Wireless LANs.   In Proceedings of ACM
       SIGCOMM'94, London, UK, Sep. 1994, pp. 212-225.

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

   [5] 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.

   [5] C.-C. Chiang, M. Gerla, and L. Zhang.  Forwarding Group
       Multicast Protocol (FGMP) for Multihop, Mobile Wireless Networks.
       ACM/Baltzer Cluster Computing, vol. 1, no. 2, 1998.

   [6] 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.

   [7] 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.

   [8] 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.




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   [9] L. Ji and M.S. Corson.   A Lightweight Adaptive Multicast
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  [10] J. Jubin and J.D. Tornow.   The DARPA Packet Radio Network
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  [11] E.D. Kaplan (Editor).   Understanding the GPS: Principles and
       Applications, Artech House, Boston, MA, Feb. 1996.

  [12] L. Kleinrock and J. Silvester.   Optimum Transmission Radii for
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       Proceedings of National Telecommunications Conference,
       Birmingham, AL, Dec. 1978, pp. 4.3.2-4.3.5.

  [13] L. Kleinrock and F.A. Tobagi.   Packet Switching in Radio
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       Throughput-Delay Characteristics.   IEEE Transactions on
       Communications, vol. COM-23, no. 12, Dec. 1975, pp. 1400-1416.

  [14] 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, pp. 1298-1302.

  [15] S.-J. Lee, W. Su, and M. Gerla.   Ad hoc Wireless Multicast with
       Mobility Prediction.   In Proceedings of IEEE ICCCN'99, Boston,
       MA, Oct. 1999, pp. 4-9.

  [16] D.L. Mills.   Internet Time Synchronization: the Network Time
       Protocol.   IEEE Transactions on Communications, vol. 39, no. 10,
       Oct. 1991, pp. 1482-1493.

  [17]  B. Narendran, P. Agrawal, and D.K. Anvekar.   Minimizing
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        In Proceedings of IEEE GLOBECOM'94, San Francisco, CA,
        Dec. 1994, pp. 1679-1685.

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  [19] UCLA Parallel Computing Laboratory and Wireless Adaptive Mobility
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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:


        Yunjung Yi
        3771 Boelter Hall
        Computer Science Department
        University of California
        Los Angeles, CA 90095
        USA

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

        Sung-Ju Lee
        Internet Systems
        Storage Laboratory
        Hwelett-Packard Laboratories
        1501 Page Mill Road, M/S 1138
        Palo Alto, CA  94304
        USA

        Phone:  +1 650 857-3894
        Fax:    +1 650 857-5100
        Email:  sjlee@hpl.hp.com


        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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