ExOR to Change Ttl

8/12/2019 ExOR to Change Ttl

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ExOR:Opportunistic Multi-Hop

Routing For Wireless Networks

Sanjit Biswas and Robert Morris

MIT CSAILhttp://pdos.csail.mit.edu/roofnet


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What is Ex Opportunistic Routing ?

A Link/Network Layer diversity routing

technique that uses standard radio hardware

Achieves substantial increase in throughput

for large unicast transfers in mesh network.

Thoughts :

Does opportunistic mean Greedy ?

Do we need Dijkstras algorithm?

How is it different?


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Traditional Routing

S

A B

C

D

Packets get forwarded on fixed path

Retried on failures

Looks like a circuit switched network

Packet

Packet Packet


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Cooperative Diversity /

Probabilistic Broadcast

S

A B

C

D

Packets are broadcasted

Selects each hop, after the transmission for that hop.

Unlike Coop Diversity only a single node forwards each packet


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Why ExOR promises high throughput?

N1 N3 N5 N7N6N2 N4 N8S D

Gradual falloff of probability with distance (80%, 40%, 20%..)

Lucky longer path can reduce transmission count

Shorter path ensures some forward progress

Traditional Path


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Why ExOR promises high throughput?

S

S

S

D

S

S

25%

25%

25%

25%

100%

100%

100%

100%

Reception at different node is independent, no interference

Traditional Routing: 1/ 0.25 + 1 = 5tx

ExOR: 1/ (1-(1-0.25) ) + 1 = 2.5tx4


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Design ChallengesOrder in Anarchy

1) The nodes must agree on which subset of

them received each packetProtocol ?

2) A metric to measure the probable cost of

moving packet from any node to destination

3) Choosing most useful participants

4) Avoid simultaneous transmission to

minimize collisions.


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Agreement using Gossip and Batch

S

N2

N3

N4

N1N5

N6

D

N7N8

Batch

F F

F

F

F

A complete schedule, undelivered packet are retried in subsequent one

A subset within a transmission batch is called Fragment (F)

After each batch destination sends packet just containing batch map

Okay, where is the agreement ?

1st round2nd round

3rd round


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GossipProtocolVery similar to P2P

Only one kind of packet/message.

Batch Map (Summary) - Map of each packet

against the received highest priority node.

Forwarder ListA list of nodes ordered in priorityi.e. proximity towards the destination.

Knowledge is shared by means of Gossip.

Each data packet contains summary.

Summary is updated, retrofitted and

transmitted by each node.


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Cost Metric and Selection of Participants

ETX (Expected Transmission Count) metric

based on Forward Delivery Probability

Periodically link state flooding

Path is weighted shortest path (Dijkstra)

The source runs ExOR simulation and

selects only the nodes which transmit at least

10% of total transmissions in a batch. Need to know about entire Mesh before

hand? Is that okay?


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Forwarding Timer and

Transmission Tracker

S

N2

N3

N4

N1N5

N6

D

N7N8

Batch

F F

F

F

F

1st round2nd round

3rd round

Header contains information to predict source transmission rate

Transmission schedule allows high priority node to send first

Uses EMWA to set Forwarding Timer


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Batch? Whats going on with TCP

Window?


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25 Highest Throughput Pairs

ACK might get dropped even for single hop.


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25 Lowest Throughput Pairs

Asymmetric long links affect ACK handling


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Transmission Range

ExOR requires less packet transmissions to travel far.


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Going Forward

Leverage 802.11 bit rate for throughput

improvement.

Cooperation between simultaneous flows.

Transport level protocol development ?

Security ?

More organic improvements based on

cooperative diversity theory ?

Documents

ExOR to Change Ttl