TCP plays a very vital role in today’s internet communication. The increase in user terminals and other applica- tions requires networks to perform smoothly, and this network requires transport layer to provide reliable data transfer. Therefore congestion is a big challenge in today’s increasing internet traffic. However, we cannot de- termine the optimal TCP performance due to the following main problems:

1) Contention between sharing terminals.

2) Hidden terminal problems.

3) Packet loss at the MAC layer.

4) Path disconnection in case of mobility.

5) Reordering.

6) Exponential retransmission back-off at TCP layer.

To address these challenges, a simple yet effective scheme is proposed in this paper. As its name implies, the scheme contains relevant mechanisms for avoiding congestion intelligently, detecting congestion timely and eliminating congestion reactively. Control operations are performed by choosing a new congestion window on the measured channel bandwidth and RTT. While previous work was performed by selecting number of hops between sender and receiver [1] -[5] , which raise many issues that we will discuss in later sections. Rest of the paper is categorized as; Section 2 will focus some previous work done in this area and some drawbacks at some level, Section 3 will describe our proposed mechanism, Section 4 will based on simulation results, conclusion and future work.

The application requirements and the limitations of congestion control circumscribe a framework for potential improvements. We identify five distinct cases of interest, which have been discussed in the recent literature.

1) Additive Increase leads naturally to congestion, which, in turn, degrades throughput for two reasons: a) Routers need time to recover even from a transitory congestive collapse and sources need time to detect and re- transmit missing packets [6] ; b) Congestion control is triggered upon congestion; the window is adjusted back- wards and the timeout is extended, which in turn degrades the protocol’s capability to detect and exploit error free conditions and bandwidth availability, respectively [6] [7] .

2) Additive Increase is not efficient when the network dynamics encompass rapid changes of bandwidth availability. For example, when short flows that cause congestion complete their task, bandwidth becomes available. Similarly, when a handoff is completed in a cellular network, the entire channel’s bandwidth becomes available. A more rapid response is then clearly indicated [8] .

3) Multiplicative decreases causes transmission gaps that hurt the performance of real-time applications that experience jitter and degraded good put. Furthermore, multiplicative decrease with a factor of ½ or a window adjustment to 2 packets characterizes a rather conservative strategy [9] .

4) Error detection lacks an appropriate classification module that would permit a responsive strategy, oriented by the nature of potential errors. That is, when errors appear to be transient due to short-lived flows or random network interference, congestion control mechanisms (i.e. timeout extension and multiplicative window adjust- ment) are triggered unduly. The insufficient error detection/classification may also lead to unfair bandwidth al- location in mixed networks. By default, flows that experience network errors do not balance their bandwidth loss with a more aggressive recovery although such behavior could be justified: flows that experienced no losses have occupied extra bandwidth at the router temporarily, when the network errors forced some senders to back off. This situation is discussed as an open issue in [10] ; we demonstrate the validity of this argument, based on experimental results, in Section 5.

5) Source-based decisions on the transmission rate, based on the pace of the acknowledgements, necessarily incorporate the potentially asymmetric characteristics (e.g. ack delays and/or losses) of the reverse path [11] . Hence, the sender’s transmission rate does not always reflect the capacity of the forward path. This situation has a direct impact on efficiency since available bandwidth remains unexploited. Several proposals have been pre- sented to tackle the problems of TCP over networks. Most of these proposals rely on some form of local re- transmission at the wired boarder, and do not deal (either directly or indirectly) with real-time application con- straints, see [12] for a detailed description). Some recent protocols restrict the modifications at the transport lev- el. TCP-Freeze [13] distinguishes handoffs from congestion through the use of the Advertised Window. WTCP [14] implements a rate-based congestion control replacing entirely the ACK-clocking mechanism. TCP-Probing [15] grafts a probing cycle and an Immediate Recovery Strategy into standard TCP, in order to control effec- tively the throughput/overhead tradeoff. Although TCP-Probing deals effectively with both throughput and energy performance in heterogeneous networks, due to its probing mechanism, it may not satisfy the requirements of current service.

It is possible to have node- and link-level congestion occurring at the same time in networks. Both of them have direct impacts on network performance and application objective. Thus, the onset of congestion must be predicted in advance or detected in time so that it can be relieved thereafter.

To verify our proposed mechanism we use NS-2 (2.34) simulator, model used in our experiment is consists of 50 nodes in a chain wise topology. We set link bandwidth to 1Gbps in order to analyze maximum cwnd size. Simulated scheme is investigated by increasing number of nodes, and we compared the performance results with TNR.

If we analyze cwnd with saw-tooth effect in Figure 1, it can be seen clearly that the proposed mechanism is more effective and consistent then TNR, as number of users increases the cwnd loses its stability as well as per- formance, but our suggested mechanism has consistent effect in terms of stability as well as performance, which increase system overall performance.

Packet loss in networks affects the overall performance of the system. Packet loss is due to signal degradation, congestion in channel and many other common factors. After sending data in bulk we observe that proposed method has very few ratio of packet loss (0.2%) as compared to TNR. It is also clear that cwnd in this case is bigger then the proposed but on the other hand decrease in cwnd size is also clear which may lead to higher de- lay and decreasing the network throughput. Therefore proposed mechanism has better results than the TNR (Figure 2).

We have modified the default behavior of TCP duplicate acknowledgement from two to three Acks, because it has been observe that 87% of the packets loss after two duplicate Acks, so why not modify this to two instead of three. Which decrease delay and increase throughput as shown in Figure 3.

In this work, we propose a scheme for congestion control, congestion avoidance by selecting appropriate cwnd size which detects and alleviates congestion in networks. The main objective is to provide high transmission quality for the data traffic under conditions of congestion. The scheme comprises three main mechanisms. Firstly, it attempts to suppress the source traffic from event area by carefully selecting a set of representative nodes to be data sources. Secondly, the onset of congestion is indicated in a timely way by jointly checking buffer occupancy and link utilization. Lastly, the network attempts to alleviate congestion in the traffic hotspot by either resource control or traffic control, which is dependent on the specific congestion condition. The ns-2 based simulation has confirmed the advantages of proposed scheme and demonstrated significant performance improvements over existing schemes.

In the future, we hope to extend the proposed scheme to take into account some dynamic behaviors of the network, such as node mobility and link failure. We have also planed to extensively investigate performance on sensor based networks.

This research work was partially supported by NSF of china Grant No.61003247. The assistance of Prof. Xiao- feng Liao in this work is also gratefully acknowledged. The authors also would like to thanks the anonymous reviewers and the editors for insightful comments and suggestions.