Vehicle Networks

Receiver-based Forwarding & Opportunistic Forwarding

Univ.-Prof. Dr. Thomas Strang, Dipl.-Inform. Matthias Röckl

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Outline

Receiver-based Forwarding:GossipingContention-based ForwardingDestination Attractor & Directed Transmission

Opportunistic ForwardingDelay Tolerant Networks

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Introduction

Problem: In all routing protocols of the last lecture the sender decides on which node becomes the next-forwarder. But how will the sender know which node will be the best forwarder?

Is the forwarder connected to a suitable next hop or is it a dead end?Is the forwarder driving towards the destination or in another direction?What is the link stability of the forwarder?

Idea: Let the receivers decide whether it is worth to forward the message (receiver-based) instead of a next-forwarder selection by the sender (sender-based)

S DSource Destination

Transit node

SenderReceiver

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Receiver-based ForwardingGossiping

Gossiping: Probabilistic version of floodingReceiver becomes the next forwarder, i.e. it re-broadcasts the received packet, with probability pExamples:

Gossiping with p=1 is equivalent to simple floodingGossiping with a decreasing value for p lets the packet die out with an increasing number of hops

Falko Dressler (2008): Self-Organization in Autonomous Sensor/Actuator Networks

S

Haas (2003): Gossip-based Ad hoc Routing

Single-connected path

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Receiver-based Forwarding Optimized gossiping

Problem: How to dimension p?p too small: message can get lost (die out too early)p too large: high redundancy

Adjust p according to:number of hops already traversed,number of nodes in the surrounding,distance to destination,number of copies already received,distance from sender,relevance of the information in current context, etc.

A common heuristic shared by all nodes is required

Contention-based Forwarding

Destination-Attractor,Directed Transmission

Hop-limited Flooding

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Receiver-based Forwarding Contention-based Forwarding (CBF)

CBF is gossiping with greedy selection of p based on the node’s distance to the destination as heuristicTimer-based contention: Contention Window CW in CSMA/CA backoff algorithm is chosen according to distance to destination

If a node’s timer expires, it becomes the next forwarder, i.e. re-broadcasts the packetIn order to avoid duplication of packets, node that wins the contention suppresses other potential forwarders If there is no greedy next-hop, i.e. initiator does not hear any node forwarding the packet, Perimeter Routing (see GPSR) is used as recovery strategyNo periodic beaconing required

significantly reduces control overhead

Füßler (2007): Position-Based Packet Forwarding for Vehicular Ad-Hoc Networks

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S = SenderD = DestinationF = cand. Forwarder

Pr < 0: No progress towards destination (node 1/2)Pr = 0: Node is located on dashed line (node 9)Pr = 1: Only for node 8

Pr));0max(1(T WindowContention −⋅=CW

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Receiver-based Forwarding CBR: Suppression schemes

Basis suppression:If another node hears the forwarding, it skips its timer and does not forward the packet (suppression)Disadvantage: Not all nodes in the greedy area can hear each other (e.g. in the figure at right if node 1 forwards the packet first, nodes 2 and 3 cannot hear the packet forwarding and will create duplicates)

Area-based suppression:First, only nodes located within the Reuleaux triangle (= area (1)) are allowed to become the next forwarderSecond, nodes within the whole greedy area (= area (1),(2) & (3)) are allowed to become the next forwarder Disadvantage: Nodes with nearly the same distance (e.g. located on the dashed line) might still create packet duplicates

Active selection:The sender only sends an Request-To-Forward (RTF)packet instead of the data packetThe potential next forwarder replies with a Clear-To-Forward (CTF) packetIf the sender receives multiple CTF packets, it selects one as next forwarder and sends a unicast packet to this node

Füßler (2007): Position-Based Packet Forwarding for Vehicular Ad-Hoc Networks

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Receiver-based ForwardingDuplication

Especially in safety-critical VANET applications, e.g.Traffic Jam Warning,Wrong-way Driver Warning,Black Spot Warning (ice, obstacles, road-works, aquaplaning, etc.),

receiving duplicates is better than not to receive the packet at allTypes:

Spatial duplication: Same packet in different spatial areas (backup paths)Temporal duplication: Same packet forwarded at different points in time (consecutive re-broadcasts)

In order to increase reliability of reception, duplication of packets is beneficial but increases network load

tradeoff between reception reliability and network utilization

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Receiver-based ForwardingDestination Attractor & Directed Transmission

Whereas CBF tries to minimize spatial redundancy, i.e. only a single node becomes forwarder, by setting p = 0 (suppression) in case another node becomes the next forwarder, alternative approaches exploit the possibility of temporal and spatial duplication, e.g.:

Destination Attractor

Directed Transmission

Barrett et al. (2003): Parametric Probabilistic Sensor Network Routing

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Receiver-based ForwardingDestination Attractor

Increase probability PRiof re-broadcast for

node Ri at hop i, if position is closer to destination, and decrease probability, if getting further away (k as scaling factor)

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Barrett et al. (2003): Parametric Probabilistic Sensor Network Routing

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Receiver-based ForwardingDirected Transmission

Increase probability PRiof re-broadcast for

node Ri at hop i, if position is closer to shortest path between source and destination, and decrease probability, if forwarder position is further away from shortest path

Barrett et al. (2003): Parametric Probabilistic Sensor Network Routing

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Opportunistic Forwarding Introduction

What has to be done in the following situation?No connected network to forward the packet towards its destination

Idea: Exploit mobility to forward packets across disconnected networksNodes can store packets until they reach another node to forward the packet to (store-and-forward / store-carry-forward)

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Opportunistic Forwarding Delay Tolerant Networks

Classical routing protocols will fail in partitioned networks with no continuous end-to-end connectivity, i.e. no connectivity between source and destination at all timesSynonyms: Partitioned Networks, Delay Tolerant Networks (DTN), Disruption Tolerant Networks (DTN), Intermitted Connected Networks (ICN)The main idea in DTNs is to exploit mobility to deliver packets across partitioned networks:

(Store) If no suitable forwarder is available, nodes store the packet,(Carry) carry the packet for some time, and(Forward) forward the packet, when another node is more suitable as next forwarder

Used in sparse and partitioned mobile ad-hoc networks, particularly important in deployment phase of V2V communications with low penetration rates

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Opportunistic Forwarding Opportunistic forwarder selection

Packet forwarders and their connectivity are dynamic a node has to take the opportunity when it is available

(opportunistic forwarding)Optimal node for store-and-forward has to be chosen, e.g. based on:

positionposition and driving direction,Position, driving direction and road map,route information of navigation system, traffic volume on the route, etc.

Switch between different routing modes:

Adapted from Cheng et al. (2008): GeoDTN+Nav: A Hybrid Geographic and DTN Routing with Navigation Assistance in Urban Vehicular Networks

Increasing am

ount of info required

Greedy Recovery OpportunisticForwarding

LocalMaximum

No suitablenext-hop

Availability of a greedy next-hop

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Opportunistic Forwarding Forwarding Heuristics

Exemplary heuristics:

Epidemic Routing: Packet is forwardedwhenever another vehicle is in vicinity

Packet is forwarded to vehicles that drivetowards the destination (e.g. MoVe [1])

Packet is forwarded to the vehicle with the closest distance between its Nearest Point NP and the destination carries the packet

Packet is forwarded to the vehicle with the minimum METD [2], Minimum Estimated Time of Delivery (METD)= Time to reach NP + time to forwardpacket from NP to D

etc.

[1] Leontiadis et al. (2007): GeOpps: Geographical Opportunistic Routing for Vehicular Networks[2] LeBrun et al. (2005): Knowledge-Based Opportunistic Forwarding in Vehicular Wireless Ad Hoc Networks

Nearest Point NPa, NPb, NPc of vehicle a (red), vehicle b (black) and vehicle c (green) to destination D

Forwarding decision

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Questions?