15-11-2012, 05:01 PM
Transport Layer and Security Protocols for Ad Hoc Wireless Networks
1Transport Layer.ppt (Size: 5.93 MB / Downloads: 193)
Introduction
The objectives of a transport layer protocol include setting up of:
End-to-end connection
End-to-end delivery of data packets
Flow control
Congestion control
Transport layer protocols
User datagram protocol (UDP): unreliable and connection-less transport layer protocols
Transmission control protocol (TCP): reliable, byte-stream-based, and connection-oriented transport layer protocols
These traditional wired transport layer protocols are not suitable for ad hoc wireless networks.
Issues
Issues while designing a transport layer protocol for ad hoc wireless networks:
Induced traffic refers to the traffic at any given link due to the relay traffic through neighboring links.
Induced throughput unfairness refers to the throughput unfairness at the transport layer due to the throughput/delay unfairness existing at the lower layers such as the network and MAC layers.
Separation of congestion control, reliability, and flow control could improve the performance of the transport layer.
Power and bandwidth constraints affects the performance of a transport layer protocol.
Misinterpretation of congestion occurs in ad hoc wireless networks.
Completely decoupled transport layer needs to adapt to the changing network environment.
Dynamic topology affects the performance of a transport layer.
Design Goal
The protocol should maximize the throughput per connection.
It should provide throughout fairness across contending flows.
It should minimize connection setup and connection maintenance overheads.
The protocol should have mechanisms for congestion control and flow control in the network.
It should be able to provide both reliable and unreliable connections.
The protocol should be able to adapt to the dynamics of the network.
One of the important resources must be used efficiently.
The protocol should be aware of resource constraints.
The protocol should make use of information from the lower layer.
It should have a well-defined cross-layer interaction framework.
The protocol should maintain end-to-end semantics.
TCP over Ad Hoc Wireless Networks
TCP taking 90% of the traffic is predominant in the Internet.
This chapter focuses on TCP extension in ad hoc wireless networks.
Transport protocol should be independent of the network layer technology, e.g., no matter fiber or radio is used
But TCP is optimized for wired network Congestion control
TCP assumes timeout is due to congestion
Wireless links are not reliable, packet loss may be as high as 20%
In wired network, packet loss is due to congestion slow down
In wireless network, due to wireless links try harder
Why does TCP not perform well in Ad Hoc Wireless Networks
Misinterpretation of packet loss
Frequent path breaks
Effect of path length
Misinterpretation of congestion window
Asymmetric link behavior
Uni-directional path: TCP ACK requires RTS-CTS-Data-ACK exchange
Multipath routing
Network partitioning and remerging
The use of sliding-window-based transmission
TCP Over Ad Hoc Wireless Network
Feedback-based TCP (TCP Feedback – TCP-F)
Requires the support of a reliable link layer and a routing protocol that can provide feedback to the TCP sender about the path breaks.
The routing protocol is expected to repair the broken path within a reasonable time period.
Advantages: Simple, permits the TCP congestion control mechanism to respond to congestion
Disadvantages:
If a route to the sender is not available at the failure point (FP), then additional control packets may need to be generated for routing the route failure notification (RFN) packet.
Requires modification to the existing TCP.
The congestion window after a new route is obtained may not reflect the achievable transmission rate acceptable to the network and the TCP-F receiver.
Other Transport Layer Protocols
Ad hoc transport protocol (ATP)
specifically designed for ad hoc wireless networks and is not a variant of TCP and differ from TCP in the following ways:
Coordination among multiple layers
Rate based transmissions
Decoupling congestion control and reliability
Assisted congestion control
ATP uses information from lower layers for
Estimation of the initial transmission rate
Detection, avoidance, and control of congestion
Detection of path breaks
Advantages: improved performance, decoupling of the congestion control and reliability mechanisms, and avoidance of congestion window fluctuations
Disadvantages
The lack of interoperability with TCP
Fine-grained per-flow timer may cause the scalable problem
Security in Ad Hoc Wireless Networks
A security protocol should meet following requirements
Data confidentiality/secrecy is concerned with ensuring that data is not exposed to unauthorized users.
Data integrity means that unauthorized users should not be able to modify any data without the owner's permission.
System availability means that nobody can disturb the system to have it unusable.
Authentication is concerned with verifying the identity of a user.
Non-repudiation means that the sender cannot deny having sent a message and the recipient cannot deny have received the message.