Lecture Routing ExOR

7/29/2019 Lecture Routing ExOR

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ExOR: Opportunistic Multi-Hop Routing

for Wireless Networks

Sanjit Biswas and Robert Morris

M.I.T. Computer Science and Artificial Intelligence Laboratory

Presented by Deepak Bastakoty

With slides from :

Sanjit Biswas, Robert Morris, (MIT)

Saurabh Gupta (WPI)

Yao Zhao (Northwestern)


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Multi-Hop Wireless Networks

Dense 802.11b-based mesh, all sorts of loss rates

Goal is efficiency and high-throughput

Gateway

s

2km

2km

Gateway


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packet

packet

packet

The Traditional View

Identify a route, forward over links

Use link level retransmissions

src

A B

dst

C

packet

packet


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How Radios Actually Work

Every packet is broadcast

src

A B

dst

C

1 2 3 4 5 6

12 3 6

3 51

42 3

4 5 6

1 2 4 5 6

No such thing as a link


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packet

packet

packet

packet

packet

packet

ExOR: Exploiting the Insight

src

A B

dst

C

packet

packet

packet

Figure out which nodes rxd broadcast

Node closest to destination forwards


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packet

packet

packet

packet

packet

packet

ExOR: Exploiting the Insight

src

A B

dst

C

packet

packet

packet

Figure out which nodes rxd broadcast

Node closest to destination forwards


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ExORs Assumptions

1. Many receivers hear every broadcast

2. Gradual distance-vs-reception tradeoff

3. Receiver losses are uncorrelated

src

A B

dst

C


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1. Multiple Receivers per

Transmission

Broadcast tests onrooftop network

Source sendspackets at max rate

Receivers recorddelivery ratios

Omni-directionalantennas

Multiple nodes inradio range

1km

S

100%

75%

50%

25%

0%


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2. Gradual Distance vs. Reception

Tradeoff

Wide spread of ranges, delivery ratios

Transmissions may get lucky and travel long distances

Distance (meters)

DeliveryRatio

Same Source


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3. Receiver Losses are Uncorrelated

Two 50% links dont lose the same 50% of packets

Losses not due to common source of interference

Example Broadcast trace:

Receiver 1 (38%):

Receiver 2 (40%):

Receiver 3 (74%):

Receiver 4 (12%):


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Extremely Opportunistic Routing (ExOR)

Design Goals

Ensure only one receiver forwards the packet

The receiver closest to the destination should forward

Lost agreement messages may be common

Lets not get eaten alive by overheads


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N1 N3 N5 N7N6N2 N4 N8S D

Traditional Path

Traditional routing must compromise between hops to choose ones that arelong enough to make good progress but short enough for low loss rate

With ExOR each transmission may have more independent chances of beingreceived and forwarded

It takes advantage of transmissions that reach unexpectedly far, or fallunexpectedly short

How ExOR might provide more throughput


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Traditional routing: 1/0.25 + 1 = 5 transmissions

ExOR: 1/(1 (1 0.25)4) + 1 = 2.5 transmissions

Assumes independent losses

N1

src dst

N2

N3

N4

How ExOR might provide more throughput (contd..)


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How often should ExOR run?

- Per packet is expensive- Use batches

Who should participate in the forwarding?

- Too many participants cause large overhead

When should each participant forward?

- Avoid simultaneous transmission

What should each participant forward?

- Avoid duplicate transmission

How and When does the process complete?

- Identify the convergence of the algorithm

ExOR: Protocol

Jump Ahead


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Who should participate?

The source chooses the participants (forwarder list) using ETX-likemetric

- Only considers forward delivery rate

The source runs a simulation and selects only the nodes whichtransmit at least 10% of the total transmission in a batch

- A background process collects ETX information via periodic link-stateflooding


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When should each participant forward?

Forwarders are prioritized by ETX-like metric to the destination

Receiving nodes buffer successfully received packets till the end ofthe batch

The highest priority forwarder transmits from its buffer when the batch

ends- These transmissions are called the nodes f ragmentof the batch

The remaining forwarders transmit in prioritized order

Question: How does each forwarder know it is its turn to transmit

- Assume other higher priority nodes send for five packet durationsif not hearing anything from them


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What should each participant forward?

Packets it receives yet not received by higher priority forwarders

Each packet includes a copy of the senders batch map, containing the

senders best guess of the highest priority node to have received each

packet in the batch

Question: How does a node know the set of packets received by

higher priority nodes?

- Using batch map


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How and When does the process complete?

If a nodes batch map indicates that over 90% of the batch has been

received by higher priority nodes, the node sends nothing when its

turn comes

When ultimate destinations turn comes to send, it transmits 10

packets including only its batch map and no data

Question: How is the remaining 10% data delivered?

- Using traditional routing


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Who Received the Packet?

Slotting prevents collisions (802.11 ACKs aresynchronous)

Only 2% overhead per candidate, assuming 1500 byteframes

payload ACK

payload ACK1cand1

src dest

cand2 cand3src ACK2 ACK3

src cand1

cand2

cand3

src dest

Standard unicast 802.11 frame with ACK:

ExOR frame with slotted ACKs:


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Slotted ACK Example

Packet to be forwarded by Node C But if ACKs are lost, causes confusion

payloadD C BA

A D

ACK

C

ACK

B

A B C D

X


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Agreeing on the Best Candidate

A: Sends frame with (D, C, B) as candidate set

A B C D

C: Broadcasts ACK C in second slot (not rxd by D)D: Broadcasts ACK D in first slot (not rxd by C, A)

B: Broadcasts ACK D in third slot

Node D is now responsible for forwarding the packet

XX

X


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ExOR: Packet Format

-HdrLen & PayloadLen indicate size of ExOR header and

payload respectively

-PktNum is current packets offset in the batch, corresponding

to the current batch-map entry

-FragSz is size of currently sending nodes fragment (in

packets)

-FragNum is current packets offset within the fragment

-FwdListSise is is number of forwarders in list

-ForwarderNum is current senders offset within the list

-Forwarder List is copy of senders local forwarder list

-Batch Map is copy of sending nodes batch map, where each

entry is an index into Forwarder List


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Transmission Timeline for an ExOR transfer

N24 not able tolisten to N5.

N8 does

not send

N17 might have

missed somebatch-maps


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Preliminary Concept Evaluation

Strengths ExOR is nimble Efficient in total number of packet transmissions

Weaknesses Requires (partial) link-state graph Candidate selection is tricky

Requires changes to MAC


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1 kilometer

65 Roofnet node pairs


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Throughput (Kbits/sec)

1.0

0.8

0.6

0.4

0.2

0

0 200 400 600 800CumulativeFractionofNodeP

airs

ExOR

Traditional

ExOR: 2x Improvement in throughput

Median throughputs: 240 Kbits/sec for ExOR,

121 Kbits/sec for Traditional

Figure 8: The distribution of throughputs of ExOR and traditional routing between the 65 node

pairs. The plots shows the median throughput achieved for each pair over nine experimental

runs.


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25 Highest throughput pairs

Node Pair

Throughput

(Kbits/sec)

0

200

400

600

800

1000 ExOR

TraditionalRouting

1 Traditional Hop

1.14x

2 Traditional Hops

1.7x

3 Traditional Hops

2.3x

For single hop pairs ExOR provides the advantage of lower probability of source

resending packets, as theres higher probability of source receiving the

destinations 10 batch-map packets

Figure 9: The 25 highest throughput pairs, sorted by traditional routing throughput. The bars show each pair's median

throughput, and the error bars show the lowest and highest of the nine experiments.


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25 Lowest throughput pairs

Node Pair

4 Traditional Hops

3.3x

Longer Routes

Throughput(Kbits/s

ec)

0

200

400

600

800

1000 ExOR

TraditionalRouting

Figure 10: The 25 lowest throughput pairs. The bars show each pair's median throughput, and the error bars show the

lowest and the highest of the nine experiments. ExOR outperforms traditional routing by a factor of two or more.

As number of node pairs increases along a route, the likelihood of increased choice

of forwarding nodes and multiple ways to goss ip back batch-maps, increases

With greater routing length ExOR is able to take advantage of asymmetric links also


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Retransmissions affected by selection of hops

Traditional routing has to select the shortest path whichresults in compromise on selecting drop probability, thus

increasing the number of transmissions

ExOR has no limitations on number of nodes, from the forwarder list, that

can forward the packet. Hence it uses both nodes closer to source and

nodes closer to destination, irrespective of their drop probability

Figure 11: The number of transmissions made by each node during a 1000-packet transfer from N5 to N24. The X axis

indicates the sender's ETX metric to N24. The Y axis indicates the number of packet transmissions that node performs.

Bars higher than 1000 indicate nodes that had to re-send packets due to losses.


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ExOR moves packets farther

Figure 12: Distance traveled towards N24 in ETX space by each transmission. The X axis indicates the dierence in ETX

metric between the sending and receiving nodes; the receiver is the next hop for traditional routing, and the highest-priority

receiving node for ExOR. The Y axis indicates the number of transmissions that travel the corresponding distance. Packets

with zero progress are not received by the next hop (for traditional routing) or by any higher-priority node (for ExOR).

Max. distance traveled by

hops in traditional routing

Distance traveled by

transmissions in ExOR

Big chunk of transmission, in

traditional routing, takes place

over shorter distances

Number of packets carried over

individual long distance links is

small

But cumulative

transmission is substantial


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ExOR moves packets farther

Delivery Probability decreases with distance

ExOR average: 422 meters/transmission

Traditional Routing average: 205 meters/tx

FractionofTra

nsmissions

0

0.1

0.2

0.6ExOR

Traditional Routing

0 100 200 300 400 500 600 700 800 900 1000

Distance (meters)

25% of ExOR transmissions

58% of Traditional Routing transmissions


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ExOR uses links in parallel

Traditional Routing

3 forwarders

4 links

ExOR

7 forwarders

18 links


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Batch Size

ExOr header grows with the batch size

Large batches work well for low-throughput pairs due to redundant batch map

transmissions

Small batches work well for high throughput pairs due to lower header

overhead


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Critical Analysis

Static No mobility

Small Scale Tens of nodes

Dense network- Maybe Only Rooftop Networks

File downloading application No voice, maybe not web (No reliable guarantee)

No Cross Traffic


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Static

Knowing the whole topology

In a mobile network, this is expensive

EXT is costly Measure link states of all possible links

Route change

During a batch, route may change


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Small Scale

Knowing the whole topology In a mobile network, this is expensive

EXT is costly Measure link states of all possible links

Large overhead in ExOR packet header All the forwarders are included in ExOR header Long vain waiting of forwarding timer The larger the network, the longer the average distance between

S and D, the more forwarders in the list

Traditional header (24~48 -> 8 if AODV) ExOR header (44~114 for 38-node network)


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Dense Networks

S

A

B

C

D

EX


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Dense Networks


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More Critical AnalysisYao Zhao (Northwestern)

No TCP and hence proxy

Voice Jitter

Web service Is batch good? May introduce large delay

Large file download Best for ExOR


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Cross TrafficYao Zhao (Northwestern)

Forwarding timer

Give higher-priority nodes enough time to send?

Assume 5 packets sent if a node cannot hear another node with

higher priority hard to justify heuristic. Also, forwarders couldbe consistently mutually inaccessible

802.11 use CSMA/CA, competition based MAC

If there is cross traffic, hard to estimate the transmission time ofother nodes


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Unfair Comparison ?Yao Zhao (Northwestern)

Bad Traditional routing (DSR) Dont think about link state changing Long packet header Send the entire file to the next node before the next node

starts sending

Bad MAC Selection Retransmit packet if ACK is lost Why not packet train?

A Paper in 2005 compared to some works before 1999 ?


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Questions?


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Acknowledgements

Many sketches, animated-diagrams, as well as some text have been sourced from the followingmaterials-

Course material on Net Centric Systems taught at TECHNISCHE UNIVERSITT DARMSTADT

Presentation on A High Throughput Route-Metric for Multi-Hop Wireless Routing by Eric Roznerof University of Texas, Austin

Presentation on ExOR: Opportunistic Multi-Hop Routing for Wireless Networks, by Sanjit Biswasand Robert Morris at Siggcomm

ExOR: Opportunistic Multi-Hop Routing for Wireless Networks - Sanjit Biswas and Robert Morris

Presentation on ExOR: Opportunistic Multi-Hop Routing for Wireless Networks, by Avijit ofUniversity of California, Santa Barbara

Presentation on ExOR: Opportunistic Multi-Hop Routing for Wireless Networks, by Yu Sun ofUniversity of Texas, Austin

Presentation on ExOR: Opportunistic Multi-Hop Routing for Wireless Networks, by GauravGupta, University of Southern California

Presentation on ExOR: Opportunistic Multi-Hop Routing for Wireless Networks, by Ao-Jan Su,Northwestern University

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