TCP in Mobile Ad-hoc
Networks ─ Split TCP
CSE 6590
4/8/2015 2:10 AM
1
Overview



What is TCP?
TCP challenges in MANETs
TCP-based solutions


Split-TCP
ATCP
2
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
3
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
4
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.
5
TCP Congestion Window
6
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
7
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
8
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
9
Solution Topology
10
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
11
Solutions for TCP
12
Split-TCP and ATCP
13
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
14
Split-TCP Overview




Motivation for Split-TCP
How does Split-TCP work?
Advantages/Disadvantages
Performance Evaluation:



Throughput vs. TCP
Channel Capture Effect
Summary
15
Split-TCP in Solution Topology
16
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
17
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.
18
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)
19
Split-TCP Segmentation
20
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
21
Split-TCP in a MANET (2)

Steps:





Node 1 initiates TCP
session
Nodes 4 and 13 are
chosen as proxies ondemand
Upon trx, 4 buffers
packets
If a packet lost at 15,
request made to 13 to
retransmit
1 unaware of link
failure at 15
22
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).
23
Channel capture alleviated
24
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
25
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%
26
Performance vs. TCP:
Throughput Comparison
27
Performance vs. TCP:
Channel Capture Effect
Regular TCP Throughput
Split-TCP Throughput
28
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
29
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
30
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.
31