Improving Throughput of Starved TCP Flow by Sidestepping Bottleneck Nodes Using Concurrent Transmission

Introduction

The traditional transport layer protocol TCP behaves very unpredictably with degraded throughput in multi-hop wireless ad hoc networks, as it was designed to provide reliable end-to-end delivery of data packets over wired networks. In wireless scenarios, the main problem of TCP (Chen, Zhai, Wang & Fang, 2004; Fu, Meng & Lu, 2002) is the performance of congestion control mechanisms in case of losses which are not occurred due to network congestion. TCP is unable to distinguish between non-congestive losses (due to route failures because of mobility, random errors, contentions on the wireless channel and unfairness) and due to network congestion. So, TCP performs poorly in wireless ad hoc networks because it has to face new challenges due to several causes specific to these networks such as lossy channels, hidden and exposed nodes, path asymmetry, network partitions & route failures (in multi-hop environments) and power limitations.

When IEEE 802.11 MAC protocol is used in the multi-hop ad hoc networks, the traditional TCP congestion control mechanism encounters several new problems. In these networks, the medium contention is high which results in severe unfairness and starvation problems for TCP flows. The congestion control, fairness and medium contention are all related and closely coupled issues for TCP end-to-end throughput and the end-to-end delay. Several works have been done in the past related to greedy nature of TCP resulting in high level of congestion and thus degrading the performance of the wireless ad hoc networks. For example, Figure 1 shows a simple 9 static node chain topology with a single TCP flow from node 1 to node 9 with 1000 bytes payload, it is observed that the TCP traffic introduces a lot of collisions and therefore retransmissions of control (RTS/CTS) and Data packets at the MAC layer [3] and there are many TCP packets dropped at a rate of 0.83 to 3.63 packets/sec due to medium contentions without any queue overflow.

Figure 1.

A 9-node topology with a single flow

Again, TCP traffic is unstable in the wireless multi-hop scenario because the round trip time (RTT) oscillates in a larger range and so does the instantaneous throughput. These observations are due to the greedy nature of TCP and the strong dependencies between congestion and the medium contentions. The TCP increases the congestion window size till the first packet loss and when the transmission rate of the sender crosses the channel capacity, the subsequent incoming packets are accumulating at various nodes along the directed route. When all the neighboring nodes have packets in queue to transmit, they try for contending the channel and thus number of collisions increases and so is the channel contention delay, thereby slowing down the packet forwarding rate and frequent congestion occurs in the network. Thus congestion and the collision work jointly and continuous retransmission timeouts and delayed duplicate ACKs occurs giving poor end-to-end throughput and larger delays. Here in this simulation work, we will be using concurrent transmission MAC protocol (CTMAC), which was derived for single-hop independent multiple concurrent transmissions in MANETs for increasing the spatial reuse of the channel within the interfering regions in our proposed static ad hoc networks (SANETs) chain topology scenario because of its simple implementation and better results.

The rest of the paper is organized as follows: In section II, we discuss the related work on solutions to starvation problem and concurrent transmission protocols at the MAC layer level. We present a summary of IEEE 802.11 and working of a concurrent transmission protocol, CTMAC, which we are using in our proposed work, in section III. The motivation and problem formulation is discussed in section IV. In section V, we describe the proposed solution to the congestion and starvation problem. Section VI discusses the simulation results followed by conclusion in section VII.