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Refining Tcp´S Rtt Dependent Mechanisms With An Advent Of Link Retransmission Delay Measurement In Wireless Lan

3133 words - 13 pages

Network utilization by legacy Transmission Control Protocol (TCP) is determined by its Round Trip Time (RTT) dependent mechanisms for flow control and loss recovery. The RTT estimation does not always reflect actual network conditions especially in case of Wireless Local Area Network (WLAN). It can in turn influence the mechanisms falsely. This article proposes a Cross-Layer mechanism between Medium Access Control (MAC) and TCP for correcting RTT estimation at sender and subsequent TCP actions for better performance in WLAN. Authors have come with a novel proposal with two refinements. The first refinement incorporates mechanism for correcting RTT estimation in presence of Automatic Repeat ...view middle of the document...

Both of the above incidences are greatly dependent on RTT, estimated by TCP sender on end-to-end basis [5][6]. Therefore, efficiency claimed by these schemes is largely depending upon their accuracy for RTT estimation. It is believed that the RTT estimation at TCP sender mainly includes packet queuing delay, transmission time, processing delays and propagation delay that increases with path lengths [7] and hence correctly infers congestion in the end-to-end network path. This belief may go wrong when TCP runs over a wireless network like WLAN.

In wireless network, the quality of the radio link can fluctuate greatly in time due to channel fading, interference and mobility [8][9]. Moreover wireless channel is a shared medium. The above characteristics of wireless networks attribute to the delay variability at TCP. Firstly, with the support for mobility and reconfigurable architecture, network topology may not remain same throughout the TCP connection and hence may incur delay variability in lieu of route failure and re-establishment [10]. Second, random movement of a mobile user varies its distance from a Base Station (BS), resulting in transitory delay variations. Here, BS indicates the network element which is responsible for reliable transmission over a wireless link, for example, Radio Network Controller (RNC) in UMTS and Access Point (AP) in IEEE 802.11 WLAN [11]. Third, link-layer errors and contention along the path that leads to a longer waiting time and many link-level retransmission attempts before the packets in the queue are transmitted [11]. These delay variations are, in general, weakly correlated with network congestion [12]. In absence of the support for discrimination, these delay variations inappropriately reflect in RTT estimation and are difficult for TCP to handle gracefully. In fact, all major TCP proposals implemented in Internet, fail in achieving the goal of high bandwidth utilization with fast convergence (required especially with wireless losses) when end-to-end network path comprises of wireless link [13].

With incorrect RTT estimation, TCP spuriously retransmits segments along with avoidable shrinkage of cwnd, loses its self-clocking and hence underutilizes the network [14][15]. Therefore, it is crucial to have either fair RTT estimation that reflects actual level of congestion in the network or a rational congestion response of TCP to treat the above issues. Many studies treat effects of route failures, contention and packet reordering on TCP in wireless networks [10][16][17][18]. However, a very few [19][20] treat for sacrifice in TCP performance in presence of delay due to link ARQs, though they are very common in recent wireless architectures. Link ARQs protect TCP from transmission losses but in return they increase the end-to-end RTT estimate at TCP sender [21]. Since, the end-to-end TCP throughput is limited by the augmentation in cwnd per RTT [22], the inappropriate rise in RTT may pull down TCP´s throughput....

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