1. TCP in Mobile Ad-hoc Networks
CSE 6590
11/29/2018 4:34 AM

2. Overview What is TCP? TCP Challenges in MANETs TCP Based Solutions
Split-TCP
ATCP
Recap

3. Transport Layer
In the OSI model, the transport layer is responsible for:
Reliable end-to-end connection
End-to-end delivery
Flow control
Congestion control
In-order packet delivery

4. 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

5. 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

6. 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.
Random backoff timer done to avoid further congestion

7. TCP Congestion Window

8. Why does TCP perform badly in MANETs?
Dynamic network topology
Node mobility
Network partition
Multi-hop paths
Variable path lengths
Longer path = higher failure rate
1) Dynamic network topology means the nodes are mobile, and the structure of the network changes regularly. TCP must account for gaps in transmission where the routing protocol must update the paths before TCP can continue
2) The

9. Why does TCP struggle in MANETs? (2)
Lost packets due to high BER (Bit Error Rate):
BER in wired: 10-8 – 10-10
BER in wireless: 10-3 – 10-5

10. 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

11. Solution Topology

12. 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

13. Solutions for TCP

14. Split-TCP and ATCP

15. TCP Recap Works well in wired
Fails in wireless due to frequent connection breaks:
Mobile nodes being rerouted
Packets lost due to lossy channel
Multi-hop paths more prone to failure
Present solutions tackle subset of problems
Two solutions: Split-TCP and ATCP

16. Split-TCP Overview: Motivation for Split-TCP How does Split-TCP work?
Advantages/Disadvantages
Performance Evaluation:
Throughput vs. TCP
Channel Capture Effect
Recap

17. Split-TCP in Solution Topology

18. 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

19. 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. 1)

20. 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)

21. Split-TCP Segmentation

22. 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

23. 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

24. 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).

25. Channel capture alleviated

26. 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

27. 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%

28. Performance vs. TCP: Throughput Comparison

29. Performance vs. TCP: Channel Capture Effect
Regular TCP Throughput
Split-TCP Throughput

30. Split-TCP Recap 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

31. 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

32. 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
[5] Improving TCP Performance over Wireless Networks; Kenan Xu, Queen’s University 2003