Multicast in Mobile Ad-Hoc Networks

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Multicast in Mobile Ad-Hoc Networks

• Routing and Reliability

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Outline of the Talk

• Characteristics of Ad-Hoc Networks
• Issues in Multicast Routing
• AODV
• Tree-based Multicast Routing
• MCEDAR
• Reliability Issues
• Our Work in Progress

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Characteristics of Ad-Hoc Networks

• All mobile platforms(nodes) are capable of motion
• All the nodes have routing functionality. There
  is no need for centralized infrastructure for
  communication.
• Each node is equipped with wireless transmitters
  / receivers
• The node may be directly connected to a fixed
  network on a foreign subnet, or be connected via
  a wireless link, dial-up line, etc.

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Salient features of MANETS

• Dynamic Topologies Nodes move arbitrarily, and
  links can be uni as well as bidirectional.
• Bandwidth Constrained links Significant lower
  capacity of wireless links. Congestion is the
  norm rather than the exception.
• Energy Constrained Operation All the nodes rely
  on some exhaustible source for their energy.
• Limited Physical Security More prone to
  spoofing, DoS attacks, eavesdropping, etc.

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A Future Internetwork
Base Station
Fixed Network
Ad Hoc Network Switch
Mobile Host
Wired Link
Wireless Link
Ad Hoc Network
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An Ad Hoc Network Our View

• A Graph with n nodes
• A node can move in any direction with any speed
• Connectivity is defined on the basis of power
  considerations and land features, implies
  frequently changing connectivity, neighborhood of
  the nodes in the graph.

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Issues in Multicast Routing

• Information stored We want to store as less
  state as possible in the hosts.
• Messages Exchanged Because the networks are
  bandwidth constrained, we would like as less
  exchange of state as possible between the nodes.
• Active Adaptability We would like the nodes to
  adapt themselves to mobility, power
  considerations, environment, etc.
• Local Effect of Link Breakages

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Multicast Routing Algorithms

• Some of them
• AODV (Ad Hoc On Demand Distance Vector) Routing
• Tree based Algorithms
• MCEDAR (Multicast Core-Extraction Distributed Ad
  Hoc Routing)

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AODV

• Initially developed as a reactive, unicast
  routing protocol.
• Elegantly adapts to a Multicast routing protocol.
• Based on building a Multicast Routing Tree on
  demand.
• The current version assumes bidirectional links.

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AODV Unicast Route Discovery
Source

• Source broadcasts route request. (RREQ)
• Node can reply to RREQ
• If it is destination
• It has a fresh route to destination
• Nodes create reverse route entry
• Record Source IP address/Broadcast ID to prevent
  multiple processing

Destination
Route Propagation
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AODV Forward Path Setup
Source

• Destination or Intermediate Node with route to
  destination unicasts RREP back to source.
• Nodes along path create forward route to
  destination
• Source begins sending data when it receives the
  first RREP.

Destination
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AODV Local Connectivity Management

• Nodes must periodically hear from their active
  neighbors to know that they are still within
  range
• Every time it hears the broadcast, it updates the
  lifetime
• If it does not broadcast within hello_interval,
  it broadcasts a hello packet.
• Failure to hear from neighbor within
• (1 allowed_hello_loss)hello_lifetime
• indicates loss of link.

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AODV Multicast Overview

• Utilizes same RREQ/RREP message cycle
• Shared tree composed of group members and
  connecting nodes is formed
• Dynamic Group Membership
• Group Leader
• Maintains and distributes group sequence number
• Is not a central point of failure.
• Multicast Group Members are also routers of the
  Multicast Tree.

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AODV Multicast Routing Tables

• Multicast Group IP Address
• Multicast Group Leader IP Address
• Multicast Group Sequence Number
• Hop Count to Multicast Leader
• Next Hops
• Lifetime

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AODV Multicast Route Discovery

• Source node sends RREQ
• Sets J flag if joining
• If no reply recd try rebroadcast RREQ
  rreq_retries additional times
• If still no reply, then become the group leader
• Nodes receiving RREQ set up reverse route entries.

Source
Multicast Group Members
Multicast Tree Members
Non-Members
Multicast Group Leader
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AODV Route Reply Generation

• Only members of multicast tree can respond to
  join request
• Any node with route to multicast tree can reply
  to non-join request
• RREP generated and unicast back to the source
• RREP has address of group member and distance
  from closest tree member
• Nodes forwarding RREP update RT and MRT entries.

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AODV Route Activation

• Source waits rte_disc_tmo
• Notes route with largest seq and smallest hopcnt
  to nearest tree member
• After rte_disc_tmo, unicast MACT (Multicast
  Activation) to selected next hop.
• Node receiving MACT enables MRT entry for source
• Unicasts own MACT if not member of tree.

Multicast Tree
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AODV Leaving the Group

• Node may revoke its member status at any time
• Unicast MACT with P(prune) flag set to next hop
• If node is a leaf and not a group member, prunes
  self

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AODV Link Breakages

• Node downstream of the break initiates repair
• Broadcast RREQ with Multicast Group hop count
  field and small TTL
• Accept RREPs as before

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AODV Reconnecting Partitioned Trees

• New partition detected by differing Group Leader
  Information
• Any member whose Group Leader has lower IP
  address initiates repair
• Unicasts RREQ with R(Repair) flag set to the
  other Group Leader
• The other Group Leader does not give permission
  to any other node to initiate repair unless this
  fails.

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Group Hello Messages

• First member of group becomes the Group Leader
• Maintains, disseminates the Group Sequence Number
• Broadcasts Group Hello every group_hello_interval
  seconds
• Multicast Group IP address
• Multicast Group leader IP address
• Current Group Sequence Number
• Hopcount
• Used by multicast tree members to update current
  distance to Group Leader

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AODV Simulation

• Used Glomosim
• Each node chooses destination, speed
• Carrier Sensing performed before every
  transmission
• Simulated length of time 300 seconds
• Data Rate 1 Mbit/sec
• Data packet size 64 bytes
• Transmission Radius 10 m

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AODV Performance

• 50m x 50m Multicast slightly reduced Goodput
  Ratio
• 85m x 85m Multicast has high rate of group merges
  and partitions.

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Multicast Routing Algorithms

• Some of them
• AODV (Ad Hoc On Demand Distance Vector) Routing
• Tree based Routing Algorithms
• MCEDAR (Multicast Core-Extraction Distributed Ad
  Hoc Routing)

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Per-Source Multicast

• A Proactive Protocol
• Extension of DVMRP for fixed networks
• DVMRP
• Each sender selectively floods multicast
  packets to all nodes within a specified range
• They use reverse shortest path forwarding scheme
• Periodically non-member leaf nodes and nodes
  without any downstream members send prune
  messages
• They become alive again after a timeout.

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Per Source Multicast

• Problems of DVMRP in Ad-Hoc Networks
• Leaf Node Detection
• Flooding for Grafting/Pruning
• Reverse Path Forwarding does not work due to
  mobility.
• Scalability ??? Very poor!!!

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Shared Tree Multicast

• Another Proactive Protocol
• Based on the concept of a rendezvous point (RP)
• Sender Messages send multicast packets to the RP.
• Join requests are also sent to the RP
• Multicast packets are forwarded to receiver
  members along the multicast forwarding tree,
  either in the unicast mode or multicast mode.

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Multicast Routing Algorithms

• Some of them
• AODV (Ad Hoc On Demand Distance Vector) Routing
• Tree based Routing Algorithms
• MCEDAR (Multicast Core-Extraction Distributed Ad
  Hoc Routing)

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Ad-hoc routing using CEDAR

• Core subset of nodes in network involved in
  route computation and management, with tunnels
  between them.
• Core Broadcast an efficient broadcast mechanism
  among core nodes using O(V) messages
• Increase/Decrease waves the state propagation
  mechanism in CEDAR
• Route Computation approximation to shortest
  widest path.

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CEDAR components in MCEDAR

• Core
• Only core nodes become part of the multicast mesh
• Core Broadcast
• for joining the multicast mesh
• for data forwarding on the mesh

M
M
M
Multicast Mesh (subgraph of Core)
Ad-hoc network
Core Graph
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MCEDAR Characteristics

• Robustness of a mesh
• Efficiency of a tree based forwarding protocol.
• Involves only a subset of nodes (core nodes) in
  multicast route management
• Independent of the underlying unicast routing
  protocol.

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MCEDAR - Two aspects

• Route Management
• the multicast infrastructure
• joins
• leaves
• Data forwarding

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MCEDAR The Multicast Infrastructure

• A mesh of core nodes
• A non-core node requests its dominator (a core
  node in its one hop neighborhood)to become a
  member on its behalf.
• Senders and receivers are not distinguished
• Has a robustness factor of R

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MCEDAR Joining a Group

• Joining core nodes send Join Request using Core
  broadcast
• Members with a lesser JoinID reply with Join-ACK
• On the reverse (Join-ACK) path, each node accepts
  upto R acks.
• Upto R paths to the mesh.
• Each member has a JoinID and non members have a
  joinID of -INF
• Members (including intermediate core nodes), keep
  track of parents and children

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MCEDAR Joins (contd.)

• Mesh is essentially a DAG where the JoinIDs
  increase as we go down the DAG
• On accepting a Join-ACK
• JoinID lt- MAX(JoinID, ID in ACK 1)
• MAX allows a node to distinguish between set of
  ancestors and descendants

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Illustration (R2)
1
3
2
4
3
Core node Multicast member Multicast mesh link
New member
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MCEDAR Leaving a Group

• A node can leave if
• A member becomes a non-core node, OR
• It has no members attached to it AND it does not
  have any children
• Send a leave message to each of its parents
• Set JoinID to -INF

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MCEDAR Data Forwarding

• Forward data on all mesh links except on the link
  from which it came from
• Core broadcast mechanism used for data forwarding
  on the multicast mesh
• Use overheard RTS/CTS packets to optimize data
  forwarding

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MCEDAR Link Failures/Partitions

• A member does a new join only if it loses
  connection with all parents
• Only members of lesser JoinID respond
• avoids joining back with the descendants
• if no response for time Tpartition then a
  partition is assumed.

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Conclusions

• MCEDAR
• Provides robustness of a mesh based mechanism.
• Provides efficiency of a tree based forwarding
  protocol.
• Involves only few nodes in multicast route
  management.
• No results available yet, so cannot predict
  performance.

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Multicast Routing Our Views

• The Tree based Algorithms are
• Too costly w.r.t. messages exchanged
• Shared Tree depends on the correct functioning of
  a single node
• Both these algorithms are not at all scalable
• Hence neither algorithm is useful.

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Multicast Routing Our Views

• AODV has
• Less overhead because it is a reactive protocol
• Not as good as it can be, because again most of
  the traffic is directed towards the Multicast
  Group Leader
• Another improvement could be to incorporate a
  mesh-like routing infrastructure
• The results of AODV do not give any result on
  scalability.

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Multicast Routing Our Views

• MCEDAR, we believe
• Is good in that it has distributed computation.
• But again, your performance depends on the
  performance of your core nodes, is that
  acceptable??
• Shouldnt power awareness be a feature of routing
  protocols too??
• Is it necessary to have some central control for
  good performance??

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Reliability

• Different Aspects
• QoS guarantees
• Eventual Delivery
• Consistency Properties
• All group members deliver all the messages with a
  high probability

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Reliability Previous Work

• Pagani et al. in 1997
• Reliable Multicast
• Validity and Agreement at least once delivery
• Integrity Message m is delivered only if m has
  been multicast by a group member
• Termination Integrity, validity and termination
  are guaranteed for m within a finite time

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Reliability Pagani et al

• These guarantees hold only as long as there
  is
• Eventual Subsidence For each m, eventually no
  more messages are generated regarding m
• Liveness Each mobile is connected for at least
  a given time to its clusterhead
• Clusterhead Stability A node chosen as the
  clusterhead remains as one for at least a given
  duration.

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Reliability Pagani et al

• Drawbacks
• No performance results were given
• Is dependent on the underlying multicast protocol
• Based on ack-mechanism, so scalability is an
  issue, since much more failures
• The conditions are difficult to maintain in the
  mobile environment
• Can we really provide strong guarantees ??

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Reliability Previous work

• Viswanath et al, 1999
• Reliability
• Robustness and efficiency specifically for high
  speed ad hoc networks
• No preset speed constraints
• No direction constraints
• Environment has high mobility and frequent outages

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Reliability Viswanath et al

• Adaptive Flooding as their technique
• Routes stored as states become stale soon
• So resort to techniques where minimum state
  stored in the routers
• Simulation Environment
• 50 nodes places in a 1000m x 1000m field
• Each node sends 25 packets/sec
• Packet Loss ratio of unique packets not sent to
  packets sent
• Overhead Number of duplicate packets received

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Reliability Viswanath et al
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Reliability Viswanath et al
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Reliability Our views

• Flooding is valid only for very high speed AHNs
• Paganis work requires too many restrictions to
  hold
• Can we have probabilistic guarantees of delivery
  ??

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Reliability Our Work in Progress

• We are designing a gossip protocol on top of AODV
• Our protocol does not add any significant
  overhead to AODV, in messages and even the
  algorithm.
• How will this effect performance and
  reliability??? Simulations going on!!

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Future Work

• Develop Power Aware Algorithms..
• Have a theoretical model for our environment, and
  prove its properties
• How do these algorithms perform in reality??
• In what environment will these mobiles operate??
  Are the current algorithms suited for it??

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Questions ??