CS 577 / EE 537 Advanced Computer Networks Fall 2006 1 ExOR: Opportunistic Multi-Hop Routing for Wireless Networks Sanjit Biswas and Robert Morris M.I.T.

CS 577 / EE 537 Advanced Computer NetworksFall 2006

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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

Sigcomm 2005, Philadelphia

Presented by

Saurabh Gupta


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Content

Background

ExOR: Overview

ExOR: Design

ExOR: Protocol

Evaluation

Summary


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Wired Networks differ from Wireless Networks

Fixed nodes and fixed topology

Fixed Links between nodes

Independent Links between neighboring nodes

No great variance in link quality


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Challenges for Wireless Routing

Capability constraints- Limited and varying range of transmission- Low Bandwidth Link- Common transmission medium shared by all nodes- Transmit power limitation

Mobility Dynamics- Topology changes frequently- Varying capacity

Transmission loss- Interference- Fading


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Challenges for Wireless Routing (contd..)

Power Conservation

Asymmetry in forward and reverse links

Ease of snooping


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ETX The predicted number of data transmissions required to send a packet

over a link

The ETX of a path is the sum of the ETX values of the links over that path

Examples:

- ETX of a 3-hop route with perfect links is 3

- ETX of a 1-hop route with 50% loss is 2

Expected probability that a transmission is successfully received and acknowledged is df x dr

- df is forward delivery ratio

- dr is reverse delivery ratio

Each attempt to transmit a packet is a Bernoulli trial, so…


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Traditional Routing

Abstract radio to look like a wired link

Identify a route

Packets get forwarded on fixed path

Retried on failures

Looks like a circuit switched network

packet

packet

packet

src

A B

dstC

D


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Cooperative Diversity

Broadcast transmission over Wireless Links involves probabilistic delivery

Sends information through multiple relays, concurrently

Destination chooses best of many relayed signals, or combine information from multiple signals

Requires radios capable of simultaneous, synchronized repeating of signals or additional radio channels for each relay

123456123 63 51 42345612 456 src

A B

dst

C


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Background

ExOR: Overview

ExOR: Design

ExOR: Protocol

Evaluation

Summary


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ExOR (Extremely Opportunistic Routing)

Integrated link/network-layer diversity routing technique

Realizes some of the gains of cooperative diversity on standard radio hardware, such as 802.11

Uses broadcasts for large unicast transfers in multi-hop wireless networks

Uses Delayed Forwarding

- does not make the forwarding decision until after reception

- only the “best” receiver of each packet forwards it

- avoids duplication

Operates on batches of packets, to reduce communication cost of agreement


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packet

packetpacketpacketpacketpacketsrc

A B

dst

C

packetpacketpacket

ExOR: Basic Functionality

Decide which nodes receive broadcasts

Decide who forwards, after reception

Node closest to the destination forwards


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packet

packetpacketpacketpacketpacketsrc

A B

dst

C

packetpacketpacket

ExOR: Basic Functionality(contd…)


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N1 N3 N5 N7N6N2 N4 N8S D

Traditional Path

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

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

It takes advantage of transmissions that reach unexpectedly far, or fall unexpectedly 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

25%

25%

25%

25%

100%

100%

100%

100%

How ExOR might provide more throughput (contd..)


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Background

ExOR: Overview

ExOR: Design

ExOR: Protocol

Evaluation

Summary


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ExOR: Design Challenges

Agreement amongst the nodes on which sub-set of them received each packet

Algorithm to decide, from amongst the receiving nodes, the node “closest” to the ultimate destination that forwards the packet

– requires a metric reflecting the likely cost of moving a packet from any node to the destination

Algorithm to choose only the most useful nodes as participants

Avoid simultaneous transmissions by different nodes to minimize collisions


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Node States for each batch it participates in

Packet Buffer- stores successfully received packets, according to the batch numbers

Local Forwarder List- is the copy of prioritized list of nodes, copied from one of the packets

- for a given batch, all nodes use same forwarder list as specified by the sender

Forwarding Timer- time at which the node predicts that it should forward packets

- timer set far enough ahead to give higher priority nodes enough time to send

- adjusted when packets from other nodes heard


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Node States for each batch it participates in (contd…)

Transmission Tracker- records measured rate of current sending node and expected number of packets left to

send

- used by node to adjust forwarding timer

- adapts to competing traffic

Batch map- indicates, for each packet in a batch, the highest-priority node to have received a copy

of that packet


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ExOR: Packet Format

-HdrLen & PayloadLen indicate size of ExOR header and payload respectively

-PktNum is current packet’s offset in the batch, corresponding to the current batch-map entry

-FragSz is size of currently sending node’s fragment (in packets)

-FragNum is current packet’s offset within the fragment

-FwdListSise is is number of forwarders in list

-ForwarderNum is current sender’s offset within the list

-Forwarder List is copy of sender’s local forwarder list

-Batch Map is copy of sending node’s batch map, where each entry is an index into Forwarder List


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Background

ExOR: Overview

ExOR: Design

ExOR: Protocol

Evaluation

Summary


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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


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Who should participate?

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

- Only considers forward delivery rate

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

- A background process collects ETX information via periodic link-state flooding


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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 of the batch

The highest priority forwarder transmits from its buffer when the batch ends

- These transmissions are called the node’s fragment of 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 durations

if 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 sender’s batch map, containing the sender’s 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 node’s 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 destination’s 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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Gossip Mechanism for Reliable summaries

Repeat summaries in every data packet

Cumulative: what all previous nodes rx’d

N0

N1

N2

N3

1st round Tx: 1, 2, 3, 4, 5, 6, 7, 8

Forwarder list:

N3(dst), N2, N1, N0 (src)

Rx:2,5,8

Rx: 2,4

Rx: 1,2,7,8 Batch map: 03030000

Batch map: 03032002

Batch map: 13032012

Tx: 5,8

Tx: 1,7

2nd round Tx: 3,6

Batch map: 13032012


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Transmission Timeline for an ExOR transfer

N24 not able to listen to N5.

N8 does not send

N17 might have missed some batch-maps


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Background

ExOR: Overview

ExOR: Design

ExOR: Protocol

Evaluation

Summary


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1 kilometer

65 Roofnet node pairs


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Evaluation Details 65 Node pairs 1.0MByte file transfer 1 Mbit/s 802.11 bit rate 1 KByte packets Batch size: 100 packets ExOR Header: 44-114 bytes

Traditional Routing ExOR

802.11 unicast with link-level retransmissions

Hop-by-hop batching

UDP, sending as MAC allows

802.11 broadcasts

100 packet batch size

Reported values are median of nine experimental runs, to reduce effect from other user traffic and other sources


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Throughput (Kbits/sec)

1.0

0.8

0.6

0.4

0.2

00 200 400 600 800C

um

ula

tive F

ract

ion o

f N

ode P

air

s

ExORTraditional

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

Thro

ughput

(Kbit

s/se

c)

0

200

400

600

800

1000 ExORTraditional Routing

1 Traditional Hop

1.14x

2 Traditional Hops1.7x


For single hop pairs ExOR provides the advantage of lower probability of source resending packets, as there’s higher probability of source receiving the destination’s 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


Longer Routes

Thro

ughput

(Kbit

s/se

c)

0

200

400

600

800

1000 ExORTraditional Routing

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 ‘gossip’ 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 which results 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 di®erence 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

Fract

ion o

f Tra

nsm

issi

ons

0

0.1

0.2

0.6 ExORTraditional 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 Routing3 forwarders

4 links

ExOR7 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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Background

ExOR: Overview

ExOR: Design

ExOR: Protocol

Evaluation

Summary


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Summary

Integrated routing and MAC protocol for Multi-Hop Wireless Networks

Uses Delayed forwarding mechanism, whereby the forwarding decision is made only after reception of packets

Takes advantage of the probabilistic nature of wireless broadcast transmissions, and does not hide it

ExOR does not require separate control signals for agreement

Forwarding based on reception of the given data packets and not on signal

strength measurements or previous control/data packets

Less dependent upon channel stability

“Gossip” mechanism reduces the likelihood of duplicate transmissions


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Summary (contd..)

Fewer transmissions of each packet

Utilizes long asymmetric links

Increases total network capacity

Increases individual connection throughput (approx. 2x)

Requires link-state graphs

Introduces overhead in form of “batch info”

Difficult to scale over large, dense networks


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Acknowledgements

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

• Course material on “Net Centric Systems” taught at TECHNISCHE UNIVERSITÄT DARMSTADT

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

• Presentation on “ExOR: Opportunistic Multi-Hop Routing for Wireless Networks”, by Sanjit Biswas and 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 of University of California, Santa Barbara

• Presentation on “ExOR: Opportunistic Multi-Hop Routing for Wireless Networks”, by Yu Sun of University of Texas, Austin

• Presentation on “ExOR: Opportunistic Multi-Hop Routing for Wireless Networks”, by Gaurav Gupta, University of Southern California

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


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Questions?

Thank you!!

CS 577 / EE 537 Advanced Computer Networks Fall 2006 1 ExOR: Opportunistic Multi-Hop Routing for Wireless Networks Sanjit Biswas and Robert Morris M.I.T.

Documents

advanced computer networks

wired networks

wireless networks fixed

multihop wireless networks

computer science

c slide

wireless links

c d slide