Improving TCP Performance over MANETs by Exploiting Cross-Layer

Information Awareness

Xin Yu

NYU

Presented by:

David Choffnes

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Outline

Intro to MANETs and DSR

Problems with TCP over MANETs

EPLN and BEAD

Results

Conclusion

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Mobile Ad-Hoc Networks (MANETs)

Differences from wired networks– Wireless link much less predictable

• Pathloss• Interference

– Mobility leads to rapidly changing topology– Every host is a router

Routing in MANETs– Approaches

• Link state – Requires every node to know about all other nodes– Too much wireless transmission overhead in MANETS

• Distance vector (e.g., DSDV, AODV)• Source routing (e.g., DSR)

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Routing in MANETs (cont)

Key observation– Interference/contention can significantly reduce performance– To reduce overhead, discover routes only when they are

needed; otherwise, aggressively cache overhead/previously heard routes

Ad-hoc On-Demand Distance Vector (AODV)– DV algorithm, floods route request to find path

Dynamic Source Routing (DSR) – Similar to AODV, but returns the entire path– Establishes bi-directional paths– Route failure: send ROUTE ERROR messages, try to use

cached route

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Mobility and TCP

0 38 9

39

131

TCP connection (0,1)Link failure

Link failure

Node 31 drops all in-flight packets to Node 1

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ELFN: A Solution to Mobile TCP

Explicit Link Failure Notification (ELFN)– Sends ICMP message to TCP

• Retransmission timer disabled• Sets thaw timer to 2s• Sends a probe data packet to determine if a new route

was established

Issues– How do we set RTO and cwnd after thaw?

• Small receive window can cause idle state after thaw• Smaller RTO leads to quicker recovery from freeze

– When do we freeze TCP?• ELFN does not distinguish packet loss from link failures• Try to notify TCP of lost data and lost acks

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EPLN and BEAD Overview

EPLN (Early Packet Loss Notification)– Intermediate nodes notify TCP senders about lost data

packets

BEAD (Best-Effort ACK Delivery)– Intermediate nodes retransmit ACKs by extensively using

cached routes when links fail

Rules when dropping packets– First link failure: notify TCP sender– After link failure recovery

• Data: notify intermediate node, which tries to resend and notifies sender

• ACK: notify intermediate node, try to resend using cached route; if no luck, notify TCP receiver

– Similar for notification packet losses

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Examples: EPLN

A B C

G

ED

Data packets (A,E)Link failure

F

Node C uses cachedroute: F-G-E

Link failure

Data packets dropped at Node F

Notification to intermediate node (F,C)

Node C has no other cached route, sends packet loss notification to sender (A)

Notification (C,A)

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Examples: BEAD

A B C ED

Link failure

H I

J K

ACK (E, A)

Node C retransmits using cached route C-I-H-A

Link failureNode I drops ACK

Node I sends notification to intermediate node (C)

Node C sends notification to sender (A)

Node D hears notification, retransmits ACK using cached path D-K-J-B-A

ACK received at A

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Cross Layer Interactions

TCP Sender– Notified about lost packets, not simply broken links

• ICMP message contains seq number, packet status

– Freeze TCP even if packet is salvaged, but retransmit packet only if this is the first packet lost (serves as probe packet)

– Otherwise, wait for ACK to resume TCP– ACK received: restore TCP state to values before

freezing

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Evaluation Setup

Simulator: NS-2

Mobility: RWP (boo!)

Speed: 1± v, where v is the mean speed

Field: 1500x1000m (50 nodes), 2200x600m (100 nodes)– Why?

MAC: 802.11, 2Mbps

Transmission radius: 250m (boo!)

TCP: Reno, 1460B packets, rwnd: 8

App: FTP

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Setting RTO and cwnd

Using “old” values is better than resetting them– Reducing cwnd can cause TCP to enter idle state,

so TCP throughput becomes dependent on RTO– Lower RTO due to “old” value improves

throughput (reduces slow starts)– Improvement not as much for higher traffic load;

due to fresher routes– Can lead to lower performance in heavily

congested scenarios (MAC contention due to increased route discoveries)

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Evaluation: Throughput

27%-210% higher throughput over EPLN

Improvement increases as– Traffic load increases

• No reason given

– Number of nodes increase• More cached routes leads to improved delivery

DSR-Update, a distributed cache update algorithm for DSR, further improves performance

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Evaluation: Number of Slow Starts

Reduces timeouts by as much as 90%

Mostly due to cache update algorithm

Diminishes for larger numbers of nodes, larger traffic load– Probably due to contention

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Packet Overhead

Increases overhead for small #’s of connections and low speed

Reduces overhead otherwise

Distributed DSR route update algorithm generally reduces overhead compared to standard DSR– Fair? How big is DSR’s FIFO cache compared to

the cache for the dist. cache update algo?

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Conclusion

Cross-layer information awareness is key to improving TCP performance

Efficient route updates significantly improve performance

Should we ditch TCP? See ATP.