The Impact of Multihop Wireless Channel on TCP Throughput and Loss

Presented byScott McLaren

Zhenghua Fu, Petros Zerfos, Haiyun Luo, Songwu Lu, Lixia Zhang, Mario Gerla (UCLA), INFOCOM 2003, San Francisco, Mar. 2003.

Overview

Introduction Background Throughput in Multihop Wireless Networks Loss Behavior Improving Performance Conclusions

Introduction

Improve channel utilization by spatial channel reuse

A TCP window size W* exists at which throughput is maximized by achieving best spatial reuse

Increasing the window size past W* will reduce throughput

Standard TCP typically grows its average window much larger than W*

Techniques to improve efficiency

Link-REDTune the wireless link’s drop probability

Adaptive link-layer pacing scheme Increase the spatial reuse of the channel

Allow TCP to operate in the contention avoidance region

802.11

RTS/CTS messagesNodes hearing this handshake defer

transmission until current transmission is finished

Data is dropped if no CTS is received after 7 RTS retries

Data is also dropped if 4 transmissions are sent without an receiving an ACK

Hidden Terminals

A hidden terminal is a node in the receiver’s neighborhood, that can’t detect sender and may disrupt transmissions

Nodes are 200m apart Transmission range is 250m Carrier sensing and interference range is 550m D is a hidden terminal of A B

Cannot hear CTS ( > 250m ) Cannot hear data from A, A is outside of D’s carrier sensing

range D can transmit to E

Causes collision at B, since D is within 550m interference range for B

Contention loss at B

Chain Topology

Best throughput when window size is h/4Assuming ideal MAC protocol and equal packet

sizes Max concurrent senders is h/4, where max

spatial reuse is achieved TCP window size < h/4 under utilization TCP window size > h/4 reduced

throughput

Cross Topology

2 TCP flows Best window W* = 2,

measured window = 12 20% throughput

reduction

Grid Topology

4, 8, and 12 TCP flows

½ of flows in each direction

Measured TCP windows are larger than max achievable throughput

Results

TCP Loss Behavior

Using 8-hop chain, all 165 TCP drops out of 12349 transmissions were due to link drops

TCP Loss Behavior

Corollaries

m – number of backlogged nodes B* – the max number of nodes that can transmit their DATA packets

concurrently without collision C* – denotes the max number of nodes that can initiate RTS messages Corollary 4.1

m < B* Pl ≈ 0

Corollary 4.2 m > B* Pl increases as m increases

Corollary 4.3 m > C* Pl remains constant

Throughput reduction due to Wavg >> W*, Pl > 0, Link contention > 0 reducing spatial reuse

Improving TCP Performance

Distributed Link RED (LRED) Adaptive Pacing

LRED

Easy way is to improve performance by reducing buffer size, but problems with bursty traffic

LRED exploits dropping in 802.11 MAC RED provides a linearly increasing drop curve as

queue exceeds a min size LRED provides a linearly increasing drop curve

as link drop probability exceeds a min size

LRED

Link layer maintains average number of retries

Next packet is dropped/marked with probability based on average number

If average number of retries is small, packets are not dropped/marked

When retries increase, the dropped/marked probability is calculated

Adaptive Pacing

Improve spatial channel reuse by balancing traffic among nodes

Exposed receiver problem Let a node backoff an additional packet

transmission time when necessary

Adaptive Pacing

Enabled from LRED If average retries <

min_th then calculate backoff time as usual

If pacing, backoff time increases by a time equal to the transmission time of the previous packet

Performance

Chain Topology In all cases LRED & Pacing increased TCP

throughput by up to 30%TCP stabilizes at a window size close to the

optimal valueThe longer the chain, the better the

improvement, due to pacing optimizing spatial channel reuse

Chain Topology

Performance

Cross Topology Increased throughput and improves fairness (Jain’s) for

both flows TCP NewReno has large unfairness, due to 802.11

capture characteristic (collision of 2 packets, one weaker than the other. The stronger packet is received)

Performance

Grid TopologyAlso increases throughput and fairness

Conclusions

Only when buffer is small do buffer overflow drops dominate

As buffer increases, link-layer drops dominate

LRED and Adaptive Pacing can be used to fine-tune dropping behaviors and Improve TCP throughput.

Questions