04/19/23 19:27 1

TCP in Mobile Ad-hoc Networks ─ Split TCP

CSE 6590

2

Overview

What is TCP? TCP challenges in MANETs TCP-based solutions

Split-TCP ATCP

3

TCP: A Brief Review

TCP: Transmission Control Protocol Specified in 1974 (TCP Tahoe) Data stream TCP packets Reliable end-to-end connection In-order packet delivery Flow and congestion control

4

How does TCP work? Establishes an end-to-end connection:

Acknowledgement based packet delivery Assigns a congestion window Cw:

Initial value of Cw = 1 (packet) If trx successful, congestion window

doubled. Continues until Cmax is reached After Cw ≥ Cmax, Cw = Cw + 1 If timeout before ACK, TCP assumes

congestion

5

How does TCP work? (2) TCP response to congestion is drastic:

A random backoff timer disables all transmissions for duration of timer

Cw is set to 1 Cmax is set to Cmax / 2

Congestion window can become quite small for successive packet losses.

Throughput falls dramatically as a result.

6

TCP Congestion Window

7

Why does TCP perform badly in MANETs?

1. Dynamic network topology Node mobility Network partition

2. Multi-hop paths Variable path lengths Longer path = higher failure rate

8

Why does TCP struggle in MANETs? (2)

3. Lost packets due to high BER (Bit Error Rate):

BER in wired: 10-8 – 10-10

BER in wireless: 10-3 – 10-5

9

Solutions for TCP in MANETs

Various solutions present Most solutions generally tackle a

subset of the problem Often, fixing one part of TCP

breaks another part Competing interests exist in the

standards laid out by OSI

10

Solution Topology

11

Why focus on TCP-based solutions? We want to choose solutions which

maintain close connection to TCP Upper layers in the OSI model

affected by choice of transport layer protocol

Modifications may affect interactions with the Internet

Alternative methods only useful for isolated networks

12

Solutions for TCP

13

Split-TCP and ATCP

14

TCP Summary Works well in wired Fails in wireless networks due to

frequent connection breaks: Mobile nodes move Packets lost due to lossy channels Multi-hop paths more prone to failure

Present solutions tackle subset of problems

Two solutions: Split-TCP and ATCP

15

Split-TCP Overview

Motivation for Split-TCP How does Split-TCP work? Advantages/Disadvantages Performance Evaluation:

Throughput vs. TCP Channel Capture Effect

Summary

16

Split-TCP in Solution Topology

17

Motivation for Split-TCP

Issues addressed by Split-TCP: Throughput degradation with

increasing path length Channel capture effect (802.11) Mobility issues with regular TCP

18

Channel Capture Effect

Definition: “The most data-intense connection

dominates the multiple-access wireless channel” [1]

Higher SNR Early start Example: 2 simultaneous heavy-load

TCP flows located close to each other.

19

How does Split-TCP work?

Connection between sender and receiver broken into segments

A proxy controls each segment Regular TCP is used within

segments Global end-to-end connection with

periodic ACKs (for multiple packets)

20

Split-TCP Segmentation

21

Split-TCP in a MANET: Proxy Functionality

Proxies: Intercept and buffer TCP packets Transmit packet, wait for LACK Send local ACK (LACK) to previous

proxy Packets cleared upon reception of LACK Increase fairness by maintaining equal

connection length

22

Split-TCP in a MANET (2) Steps:

Node 1 initiates TCP session

Nodes 4 and 13 are chosen as proxies on-demand

Upon trx, 4 buffers packets

If a packet lost at 15, request made to 13 to retransmit

1 unaware of link failure at 15

23

Split-TCP in a MANET (3)

Sender is unaware of transient link failure. Congestion window not reduced.

Packet retransmissions only incorporate part of a path bandwidth usage is reduced.

Channel capture effect is alleviated (see next slide).

24

Channel capture alleviated

25

Is Split-TCP successful? Pros:

Increased throughput Increased fairness Restricted channel capture effect

Cons: Modified end-to-end connection Proxy movement/failure adversely affects

protocol performance Congestion at proxy nodes if another fails

26

Performance Evaluation Test bench Specifics:

ns-2 Simulator 50 mobile nodes initially equidistant 1 km2 Area Nodes maintain constant velocity:

Arbitrary direction Random changes at periodic intervals

Optimal segment length: 3 ≤ n ≤ 5 nodes Measured improvement: Throughput

increases by 5% to 30%

27

Performance vs. TCP:Throughput Comparison

28

Performance vs. TCP:Channel Capture Effect

Regular TCP Throughput

Split-TCP Throughput

29

Split-TCP: Summary

Break link into segments with proxies

Use proxies to buffer packets at segments

Employ TCP locally in segments Reduce bandwidth consumption

and channel capture effect

30

Issues Not Addressed

Does not maintain end-to-end semantics Periodic ACK failures means major

retransmissions Packet loss due to high BER Out-of-order packets Proxy link failure affects

performance

31

References [1] Split-TCP for Mobile Ad Hoc Networks;

Kopparty et al. [2] ATCP: TCP for Mobile Ad Hoc Networks; Jian

Liu, Suresh Singh, IEEE Journal, 2001. [3] A Feedback-Based Scheme for Improving

TCP Performance in Ad Hoc Wireless Networks; Kartik Chandran et al.

[4] Ad Hoc Wireless Networks: Architectures and Protocols; C. Siva Ram Murthy and B. S. Manoj; section 9.5.7.

[5] Improving TCP Performance over Wireless Networks; Kenan Xu, Queen’s University 2003.