23-07-2012, 04:13 PM
[b]Mobile Transport Layer[/b]
Mobile Transport Layer 1217.docx (Size: 145.71 KB / Downloads: 38)
Abstract
The transport layer is a true end-to-end layer in the sense that two transport entities located at each end of a virtual connection directly communicate with each other without intervention from any intermediate layer or agent.
Introduction
TCP(transport control layer protocol) is one of the core protocols of the Internet protocolsuite, often simply referred to as TCP/IP. TCP is reliable, guarantees in-order delivery of dataand incorporates congestion control and flow control mechanisms.TCP supports many of the Internet's mostpopular application protocols and resultingapplications, including the World Wide Web,e-mail, File Transfer Protocol and Secure Shell.In the Internet protocol suite, TCP is theintermediate layer between the Internet layerand application layer.
TCP fast retransmit/fast recovery
TCP sends an acknowledgement only after receiving a packet
If a sender receives several acknowledgements for the same packet, this is due to a gap in received packets at the receiver.
Sender can retransmit missing packets (fast retransmit)
Also, the receiver got all packets up to the gap and is actually receiving packets
Therefore, packet loss is not due to congestion, continue with current congestion window (fast recovery)
In the following simplied analysis, we do consider neither fast retransmit nor fast recovery.
Transmission/time-out freezing
Mobile hosts can be disconnected for a longer time
no packet exchange possible, e.g., in a tunnel, disconnection due to overloaded cells or multiplex with higher priority traffic
TCP disconnects after time-out completely
TCP freezing
MAC layer is often able to detect interruption in advance
MAC can inform TCP layer of upcoming loss of connection
TCP stops sending, but does now not assume a congested link
MAC layer signals again if reconnected
scheme is independent of data
TCP on mobile host has to be changed, mechanism depends on
MAC layer.
Conclusion
Mobile ad-hoc networks (MANETs) are dynamic in nature. Links between nodes may constantly change due to mobility. The performance of the transmission control protocol (TCP) degrades in such an environment mainly due to its inability to differentiate between congestion and link failures which are frequent in MANETs. Several TCP variants were proposed to adapt TCP to a mobile environment. In addition, other reliable transport protocols were especially designed for MANETs as an alternative to these TCP variants. Several network settings might require different transport protocols to coexist and interoperate. In this paper we present a model that can be used to achieve interoperability between reliable transport protocols. The proposed model consists of forming an intermediate easy-to-implement thin layer between the network layer and the transport layer in the TCP/IP stack.