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Improving TCP Performance over Mobile Ad Hoc Networksby Exploiting Cross-Layer Information Awareness

CS495 – Spring 2005Northwestern University

Sausan Yazji

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Overview

Packet losses effects TCP performance in a negative way. We present two mechanisms to enhance TCP performance:

Early Packet Loss Notification (EPLN) Best Effort ACK Delivery (BEAD)

Extensive use of cached routes No route discoveries at intermediate node. Significant improvement in TCP performance.

Cross-layer information awareness is a key in order to make TCP efficient in the presence of mobility.

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Previous Work

Route failures due to mobility Congestion control => throughput reduction Explicit Link Failure Notification, ELFN, TCP freezes its

retransmission timer and periodically sends a probing packet until it receives an ACK.

Dynamic Source Routing protocol, DSR, reduces route failures by making the network layer more mobility-aware.

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Goals

Focus on the network layer, transport layer, and the interactions between these two layers.

What should be the appropriate responses of TCP to frequent route failures and packet losses?

How can TCP be made efficient through approaches at the network layer and cross-layer?

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Our Algorithms

Early Packet Loss Notification, EPLN

Best-Effort ACK Delivery, BEAD

In order to: make routing protocols aware of lost data packets and ACKs help reduce TCP timeouts for mobility-induced losses

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EPLN

When a node detects a link failure:if data packets have not been salvaged

the node sends a notification to the TCP senderif data packets were salvaged by an intermediate node

the node sends a notification to the intermediate nodeWhen TCP sender’s routing protocol receives a notification:

notifies TCP about all lost packets TCP disables its retransmission timerrecords these lost packetsretransmits the lost packet with the lowest sequence

numberWhen an ACK arrives,

TCP restores its retransmission timer retransmits the remaining lost packets

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BEAD

When a node detects a link failure:if it cannot salvage ACKs it sends a notification about the lost ACK to the TCP receiver if it can salvage ACKs it sends a notification to the intermediate node

Upon receiving the notification

a node attempts to retransmit an ACK with the highest sequence number to the TCP sender using a cached route

If the intermediate node cannot retransmit the ACKit sends a notification to the TCP receiverTCP receiver’s routing protocol retransmits the ACK with the highest sequence number

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Simulation Environment

Ns-2 network simulator with Monarch Project’s wireless and mobile extensions

The network interface is modeled after the Lucent’s Wave LAN The network interface uses IEEE 802.11 DCF MAC protocol. The mobility model is random waypoint model in a rectangular

field. The two field configurations we used were 1500m × 1000m field

with 50 nodes and 2200m × 600m field with 100 nodes. We used TCP-Reno with the packet size of 1460 bytes. The maximum size of both congestion window and receiver’s

advertised window is 8. We used pause time 0 s for all simulations. FTP is the application that we used over TCP. The node speed was randomly chosen from 10±1 m/s.

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Example

0 38 9 31 1

39

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Summery

1. If data packets or ACKs are dropped and this is the first time they encounter a link failure, then the current node sends a notification using the route obtained by reversing the source route.

2. If data packets are dropped after being salvaged by an intermediate node, then the current node notifies the intermediate node about lost packets. The intermediate node sends a notification to the TCP sender if it has a cached route.

3. If ACKs are dropped after being salvaged by an intermediate node, then the current node notifies the intermediate node about lost ACKs. That node first attempts to retransmit an ACK with the highest sequence number among lost ACKs using a cached route; if it cannot, it sends a notification about lost ACKs to the TCP receiver.

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Summery - Continued

4. When forwarding a notification about lost ACKs, a node attempts to retransmit an ACK with the highest sequence number among lost ACKs to the TCP sender using a cached route. If it can do so, it marks the notification to indicate that an ACK has been retransmitted. If none of the intermediate nodes is able to retransmit an ACK, the routing protocol at the TCP receiver retransmits an ACK if it has a cached route.

5. If a notification packet is dropped due to a link failure, the node detecting the link failure notifies the node that is the source of the notification. That source node will send another notification to the TCP sender or the TCP receiver using a cached route. Thus, the network layer

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Packet Loss Notifications Drop List: to record dropped packets:

Source address, Source port, Destination address, Destination port, Packet type (data or ACK), TCP sequence number, and the source route used in routing the packet

Conn info to record connection information: Source address, Source port, Destination address, Destination port, Packet type (data or ACK), and the TCP sequence numbers of lost packets

ROUTE ERROR includes an optional field called conn list

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EPLN Example

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BEAD Example

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PERFORMANCE EVALUATION

Two sets of experiments:

Evaluated the effects of the two choices for setting RTO and cwnd on TCP performance:

TCP-ELFN with default RTO = 6s and cwnd=2, TCP-ELFN with old RTO and cwnd, TCP-ELFN w default RTO/cwnd, no tapping TCP-ELFN w old RTO/cwnd, no tapping

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PERFORMANCE EVALUATION

Evaluated the effectiveness of EPLN and BEAD under two caching strategies for DSR, default DSR & cache update DSR :

TCP-ELFN with default RTO= 6s and cwnd 2, and DSR TCP-ELFN with old RTO/cwnd and DSR with EPLN & BEAD TCP-ELFN with old RTO/cwnd and DSR with EPLN, BEAD,

and DSR-Update. And we used the same three set of tests with no tapping

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PERFORMANCE EVALUATION

1, 5, and 10 TCP connections Did not use higher traffic=>no congestion effect Node mean speed is 5 m/s, 10 m/s, 15 m/s, & 20 m/s Node pause time was 0 s for all scenarios. Each simulation ran for 900 s. Each data point represents an average of 10 runs The probing interval of ELFN was 2 s

Three metrics: TCP Throughput Average Number of Slow-starts Packet Overhead

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Results-1

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Results-2

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Results-3

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Results-4

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Results-5

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Conclusion

•Cross-layer information awareness is key to making TCP efficient in the presence of mobility. It is necessary for the network layer to notify TCP senders about lost packets and to retransmit ACKs for lost ACKs, so that TCP reacts quickly to frequent packet losses and is unaware of lost ACKs.

•It is important to make route caches adapt fast to topology changes, because the validity of cached routes affects not only TCP performance but also the effectiveness of the mechanisms used to improve TCP performance, whether at the network layer or cross-layer.

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Problems

Increase of TCP throughput doesn’t mean indicate improve in the performance

The model to evaluate the ELFN is different than the model to evaluate EPLD & BEAD

Complicated analogy