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Augmented SplitAugmented Split--TCP TCP

over Wireless LANsover Wireless LANs

IEEE International Conference on Communications (ICC) 2006, Istanbul, Turkey

H. Jung, N. Choi, Y. Seok, T. Kwon, Y. ChoiSeoul National University, Korea

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ContentsContents

� Observation

� The problem of TCP over wireless LANs

� Augmented Split-TCP (AS-TCP)

� Basic Idea

� Protocol Description

� Simulation Results

� Conclusion

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Observation (1/2)Observation (1/2)

� TCP is not aggressive over 802.11 WLAN

� Self-contention in a single TCP flow

� In congested WLAN, only a small number of mobile hosts contend for a channel [Choi05]

� Power-saving mode (PSM) [Krashinsky02]� TCP ACK signaling is common reason!

� Without addressing these factors, we cannot achieve good TCP performance

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Observation (2/2)Observation (2/2)

� Both TCP and 802.11 DCF support duplicated retransmission mechanism

� TCP retransmits data packets when losses are detected by duplicated ACKs or TCP timeout

� 802.11 DCF retransmits data frames in the absence of MAC ACKs

� BEB in 802.11 DCF gives it a similar role to that of TCP in congestion control

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Basic IdeaBasic Idea

� This functional duplication motivates our work to eliminate TCP ACK signaling in wireless area

� A sender regards MAC ACKs as indicating successful reception of data packet, and TCP ACKs can be eliminated from the wireless link

� To extend this concept to a more general case,we extend the idea of I-TCP

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II--TCP and ASTCP and AS--TCPTCP

2.TCP DATA1.TCP DATA

3.TCP ACK2.TCP ACK

I-TCP :[Bakre95]

1.TCP DATA2.TCP DATA

3.TCP ACK

AS-TCP :2.TCP ACK

FH Router AP MH

FH Router AP MH

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Protocol DescriptionProtocol Description

� TCP-aware sublayer (TAS) is inserted between MAC and IP layer

� TAS at MH can emulate TCP ACK

� TAS at AP can discard TCP ACK

PHY

MAC

TAS

IP

TCP

PHY

MAC

TAS

IP

TCP

AP MH

4. TCP ACK(Emulated)

4. TCP ACK 1. TCP DATA

2. MAC DATA(TCP DATA)

3. MAC ACK

3. TCP DATA

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Simulation OverviewSimulation Overview

� Performance evaluation

� NS2 simulator

� Scenarios varying the IFQLEN at AP, PER, PSM

� compared with normal TCP and I-TCP

� Topology

n FH

Router AP

n MH

… …

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Results (1/4)Results (1/4)

� Aggregate TCP throughput as the number of MH varies

� AS-TCP outperforms the alternatives when the number of flows is small or moderate

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Results (2/4)Results (2/4)

� Fairness index with varying IFQLEN

� The fairness index of AS-TCP is not influenced by packet loss at the AP

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Results (3/4)Results (3/4)

� Aggregate TCP throughput with varying PER

� Higher throughput than alternatives

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Results (4/4)Results (4/4)

� Transfer time in PSM with varying file size

� The delays of AS-TCP are almost constant, regardless of PSM

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ConclusionConclusion

� We have proposed AS-TCP

� eliminates real TCP ACK packets by exploiting the duplicated functionality of the MAC and transport layer

� We compared AS-TCP with normal TCP and I-TCP

� AS-TCP outperforms alternatives

• in terms of aggregate throughput and fairness index, even in lossy environments

• When PSM is enabled

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Thank you! Any questions?