An Adaptive Delayed Acknowledgment Strategy to Improve TCP Performance in Multi-hop Wireless Networks

In multi-hop wireless networks, transmission control protocol (TCP) suffers from performance deterioration due to poor wireless channel characteristics. Earlier studies have shown that the small TCP acknowledgments consume as much wireless resources as the long TCP data packets. Moreover, generating an acknowledgment (ACK) for each incoming data packet reduces the performance of TCP. The main factor affecting TCP performance in multi-hop wireless networks is the contention and collision between ACK and data packets that share the same path. Thus, lowering the number of ACKs using the delayed acknowledgment option defined in IETF RFC 1122 will improve TCP performance. However, large cumulative ACKs will induce packet loss due to retransmission time-out at the sender side of TCP. Motivated by this understanding, we propose a new TCP receiver with an adaptive delayed ACK strategy to improve TCP performance in multi-hop wireless networks. Extensive simulations have been done to prove and evaluate our strategy over different topologies. The simulation results demonstrate that our strategy can improve TCP performance significantly.     

Performance Analysis of TCP with Delayed Acknowledgments in Multi-hop Ad-hoc Networks

The acknowledgment strategy has great potential to increase TCP throughput when it runs over 802.11 MAC protocol. In particular, TCP acknowledgments (ACK) carry out an extensive number of medium accesses as they compete in the same route as data packets for media. In this paper, we first propose a dynamic TCP-MAC interaction strategy which tries to reduce the number of induced ACKs by monitoring the channel condition. To this end, the total collision probability collected along the path from sender to receiver in MAC layer has been used to properly set the number of delayed ACKs (DA) in TCP. Based on the estimated collision probability, TCP sender dynamically adjusts itself to the channel condition by delaying less ACKs in high traffic conditions and more ACKs in low traffic conditions. The simulation results show a throughput improvement up to 15% over the existing method called Dynamic Adaptive Acknowledgment (TCP-DAA) and much more over the regular TCP in different scenarios dealing with a dynamic loss rate. In addition, we show that our proposed strategy does not always benefit from a fixed delay policy along with a fixed congestion window size. In fact, the optimal number of delayed ACKs is based on the path length of a TCP connection and a large delay window may solely improve TCP throughput in short ranges with less number of flows. However, in a longer path congestion window limit provides more throughput gain.     

A Smart TCP Acknowledgment Approach for Multihop Wireless Networks

Reliable data transfer is one of the most difficult tasks to be accomplished in multihop wireless networks. Traditional transport protocols like TCP face severe performance degradation over multihop networks given the noisy nature of wireless media as well as unstable connectivity conditions in place. The success of TCP in wired networks motivates its extension to wireless networks. A crucial challenge faced by TCP over these networks is how to operate smoothly with the 802.11 wireless MAC protocol which also implements a retransmission mechanism at link level in addition to short RTS/CTS control frames for avoiding collisions. These features render TCP acknowledgments (ACK) transmission quite costly. Data and ACK packets cause similar medium access overheads despite the much smaller size of the ACKs. In this paper, we further evaluate our dynamic adaptive strategy for reducing ACK-induced overhead and consequent collisions. Our approach resembles the sender side's congestion control. The receiver is self-adaptive by delaying more ACKs under nonconstrained channels and less otherwise. This improves not only throughput but also power consumption. Simulation evaluations exhibit significant improvement in several scenarios     

Formal specification and verification of safety and performance ofTCP selective acknowledgment

We present a formal specification of the selective acknowledgment (SACK) mechanism that is being proposed as a new standard option for TCP. The formal specification allows one to reason about the SACK protocol; thus, we are able to formally prove that the SACK mechanism does not violate the safety properties (reliable, at most once, and in order message delivery) of the acknowledgment (ACK) mechanism that is currently used with TCP. The new mechanism is being proposed to improve the performance of TCP when multiple packets are lost from one window of data. The proposed mechanism for implementing the SACK option for TCP is sufficiently complicated that it is not obvious that it is indeed safe, so we think it is important to formally verify its safety properties. In addition to safety, we are also able to show that SACK can improve the time it takes for the sender to recover from multiple packet losses. With the additional information available at a SACK sender, the round-trip time that a cumulative ACK sender waits before retransmitting each subsequent packet lost after the very first loss can be saved. We also show that SACK can improve performance even with window sizes as small as four packets and in situations where acknowledgment packets are lost     

Using TCP rate control for queue input‐output rate matching

In explicit TCP rate control, the receiver's advertised window size in acknowledgment (ACK) packets can be modified by intermediate network elements to reflect network congestion conditions. The TCP receiver's advertised window (i.e. the receive buffer of a TCP connection) limits the maximum window and consequently the throughput that can be achieved by the sender. Appropriate reduction of the advertised window can control the number of packets allowed to be sent from a TCP source. This paper evaluates the performance of a TCP rate control scheme in which the receiver's advertised window size in ACK packets are modified in a network node in order to match the generated load to the assigned bandwidth in the node. Using simulation and performance metrics such as the packet loss rates and the cumulative number of TCP timeouts, we examine the service improvement provided by the TCP rate control scheme to the users. The modified advertised windows computed in the network elements and the link utilization are also examined. Copyright © 2002 John Wiley & Sons, Ltd.     

A buffer management algorithm for improving up/down transmission congestion protocol fairness in IEEE 802.11 wireless local area networks

The fair allocation of the resources is an important issue in wireless local area network (WLAN) because all wireless nodes compete for the same wireless radio channel. When uplink and downlink transmission congestion protocol (TCP) flows coexist in WLAN, the network service is biased toward the uplink TCP flows, and the downlink TCP flows tend to starve. In this article, we investigate the special up/down TCP unfairness problem and point out that the direct cause is the uplink acknowledgement (ACK) packets occupy most buffer space of access point. We thus propose a buffer management algorithm to ensure the fairness among uplink and downlink TCP flows. In order to limit the greedy behavior of ACK packets, the proposed algorithm adjusts the maximum size of buffer allocated for the ACK packets. Analysis and simulation results show that the proposed solution not only provides the fairness but also achieves 10–20% lower queue delay and higher network goodput than the other solutions. Copyright © 2012 John Wiley & Sons, Ltd.     

Fair and efficient Transmission Control Protocol access in the IEEE 802.11 infrastructure basic service set

When the stations in an IEEE 802.11 infrastructure basic service set employ Transmission Control Protocol (TCP), this exacerbates per‐flow unfair access problem. We propose a novel analytical model to approximately calculate the maximum per‐flow TCP congestion window limit that prevents packet losses at the access point buffer and therefore provides fair TCP access both in the downlink and uplink. The proposed analysis is unique in considering the effects of varying number of uplink and downlink TCP flows, differing round trip times among TCP connections and the use of delayed TCP acknowledgment (ACK) mechanism. Motivated by the findings of this theoretical analysis and simulations, we design a link layer access control block to be employed only at the access point in order to resolve the unfair access problem. The proposed link layer access control block uses congestion control and ACK filtering approach by prioritizing the access of TCP data packets of downlink flows over TCP ACK packets of uplink flows. Via simulations, we show that the proposed algorithm can provide both short‐term and long‐term fair accesses while improving channel utilization and access delay. Copyright © 2013 John Wiley & Sons, Ltd.     

Explicit TCP window adaptation in a shared memory architecture

A multiple (priority) queueing system allows a network node to manage the queueing of packets in such a way that higher priority packets will always be served first, low priority packets will be discarded when the queue is full, and for same‐priority packets any interference between them will be prevented. This paper describes a TCP window control scheme for a shared memory device that has buffer memory logically organized into multiple queues. To handle changing queue traffic loads, the shared memory device uses a dynamic buffer threshold mechanism to allocate buffer space to the queues. The TCP window control scheme allows the receiver's advertised window size in ACK packets to be modified at the network queue in order to maintain the queue size at a computed dynamic threshold. Copyright © 2003 John Wiley & Sons, Ltd.     

TCP transmission rate control mechanism based on channel utilization and contention ratio in AD hoc networks

In ad hoc networks, both contention and congestion can severely affect the performance of TCP. In our work, we first show that the over-injection of conventional TCP window mechanism results in severe contentions, and medium contentions cause network congestion. Furthermore, introducing two metrics, channel utilization (CU) and contention ratio (CR), we characterize the network status. Then, based on these two metrics, we propose a new TCP transmission rate control mechanism based on Channel utilization and Contention ratio (TCPCC). In this mechanism, each node collects the information about the network busy status and determines the CU and CR accordingly. The CU and CR values fed back through ACK are ultimately determined by the bottleneck node along the flow. The TCP sender controls its transmission rate based on the feedback information. Simulation results show that the proposed TCPCC mechanism significantly outperforms the conventional TCP mechanism and the TCP contention control mechanism in terms of throughput and end-to-end delay.     

Adding network-layer intelligence to mobile receivers for solving spurious TCP timeout during vertical handoff

With the help of mobile IP/IPv6 and soft handoff, ongoing TCP sessions can remain active and handoff packet loss can be avoided. However, TCP still faces several performance degradation issues due to the disparities in bandwidth and propagation delay between different access networks. Particularly, during vertical handoffs, some undesirable phenomena may erroneously trigger TCP congestion-control actions and thus degrade TCP performance. In this article we tackle the spurious timeout problem frequently associated with handovers from fast to slow links. We propose three network-layer schemes: fast ACK, slow ACK, and ACK delaying. These schemes require only minor modifications to the network layer of mobile receivers and no change to the TCP protocol and the TCP sender. The simulation results show that these schemes can effectively improve TCP performance during soft vertical handoffs     

VoIP transmission mechanism based on TCP

As the widespread employment of firewalls on the Internet, user datagram protocol (UDP) based voice over Internet protocol (VoIP) system will be unable to transmit voice data. This paper proposed a novel method to transmit voice data based on transmission control protocol (TCP). The method adopts a disorder TCP transmission strategy, which allows discontinuous data packets in TCP queues read by application layer directly without waiting for the retransmission of lost data packets. A byte stream data boundary identification algorithm based on consistent overhead byte stuffing algorithm is designed to efficiently identify complete voice data packets from disordered TCP packets arrived so as to transmit the data to the audio processing module timely. Then, by implementing the prototype system and testing, we verified that the proposed algorithm can solve the high time delay, jitter and discontinuity problems in standard TCP protocol when transmitting voice data packets, which caused by its error control and retransmission mechanism. We proved that the method proposed in this paper is effective and practical.     

Performance evaluation of TCP algorithms in multi‐hop wireless packet networks

Wireless packet ad hoc networks are characterized by multi‐hop wireless connectivity and limited bandwidth competed among neighboring nodes. In this paper, we investigate and evaluate the performance of several prevalent TCP algorithms in this kind of network over the wireless LAN standard IEEE 802.11 MAC layer. After extensively comparing the existing TCP versions (including Tahoe, Reno, New Reno, Sack and Vegas) in simulations, we show that, in most cases, the Vegas version works best. We reveal the reason why other TCP versions perform worse than Vegas and show a method to avoid this by tuning a TCP parameter— maximum window size. Furthermore, we investigate the performance of these TCP algorithms when they run with the delayed acknowledgment (DA) option defined in IETF RFC 1122, which allows the TCP receiver to transmit an ACK for every two incoming packets. We show that the TCP connection can gain 15 to 32 per cent good‐put improvement by using the DA option. For all the TCP versions investigated in this work, the simulation results show that with the maximum window size set at approximately 4, TCP connections perform best and then all these TCP variants differ little in performance. Copyright © 2001 John Wiley & Sons, Ltd.     

Multi‐channel power‐controlled directional MAC for wireless mesh networks

Wireless Mesh Networks (WMNs) have emerged recently as a technology for providing high‐speed last mile connectivity in next‐generation wireless networks. Several MAC protocols that exploit multiple channels and directional antennas have been proposed in the literature to increase the performance of WMNs. However, while these techniques can improve the wireless medium utilization by reducing radio interference and the impact of the exposed nodes problem, they can also exacerbate the hidden nodes problem. Therefore, efficient MAC protocols need to be carefully designed to fully exploit the features offered by multiple channels and directional antennas. In this paper we propose a novel Multi‐Channel Power‐Controlled Directional MAC protocol (MPCD‐MAC) for nodes equipped with multiple network interfaces and directional antennas. MPCD‐MAC uses the standard RTS‐CTS‐DATA‐ACK exchange procedure. The novel difference is the transmission of the RTS and CTS packets in all directions on a separate control channel, while the DATA and ACK packets are transmitted only directionally on an available data channel at the minimum required power, taking into account the interference generated on already active connections. This solution spreads the information on wireless medium reservation (RTS/CTS) to the largest set of neighbors, while data transfers take place directionally on separate channels to increase spatial reuse. Furthermore, power control is used to limit the interference produced over active nodes. We measure the performance of MPCD‐MAC by simulation of several realistic network scenarios, and we compare it with other approaches proposed in the literature. The results show that our scheme increases considerably both the total traffic accepted by the network and the fairness among competing connections. Copyright © 2010 John Wiley & Sons, Ltd.     

A High-Throughput MAC Protocol for Wireless Ad Hoc Networks

One way to improve the throughput of a wireless ad hoc network at the media access (MAC) layer is to allow as much as possible concurrent transmissions among neighboring nodes. In this paper, we present a novel high-throughput MAC protocol, called Concurrent Transmission MAC(CTMAC), which supports concurrent transmission while allowing the network to have a simple design with a single channel, single transceiver, and single transmission power architecture. CTMAC inserts additional control gap between the transmission of control packets (RTS/CTS) and data packets (DATA/ACK), which allows a series of RTS/CTS exchanges to take place between the nodes in the vicinity of the transmitting or receiving node to schedule possible multiple, concurrent data transmissions. To safeguard the concurrent data transmission, collision avoidance information is included in the control packets and used by the neighboring nodes to determine whether they should begin their transmissions. Also, to isolate the possible interference between DATA packets and ACK packets, a new ACK sequence mechanism is proposed. Simulation results show that a significant gain in throughput can be obtained by the CTMAC protocol compared with the existing work including the IEEE 802.11 MAC protocol.     

一个基于主动网络的大规模可靠组播协议的设计和实现

本文提出一个基于主动网络的大规模可靠组播协议LARMP(Large-scale Active Reliable Multicast Protocol),它较全面地解决了NACK/ACK (Negotiate Acknowledge / Acknowledge Implosion)风暴、选择重发、分布恢复负担、拥塞控制、健壮性这五个Internet上的可靠组播面临的关键问题.LARMP采用一个由四级主动路由器支持的主动组播树结构,利用组播树中的主动路由器实现动态主动组播树的建立和维护、NACK抑制、NACK/ACK聚合、缓存数据报文并为组播树中报文丢失节点恢复报文、及时检测网络拥塞并反馈给发送者以便其调整发送速度等功能.实验测试表明LARMP具有良好性能.     

Improving TCP/IP Performance over Third-Generation Wireless Networks

As third-generation (3G) wireless networks with high data rate get widely deployed, optimizing the transmission control protocol (TCP) performance over these networks would have a broad and significant impact on data application performance. In this paper, we make two main contributions. First, one of the biggest challenges in optimizing the TCP performance over the 3G wireless networks is adapting to the significant delay and rate variations over the wireless channel. We present window regulator algorithms that use the receiver window field in the acknowledgment (ACK) packets to convey the instantaneous wireless channel conditions to the TCP source and an ACK buffer to absorb the channel variations, thereby maximizing long-lived TCP performance. It improves the performance of TCP selective ACK (SACK) by up to 100 percent over a simple drop-tail policy, with small buffer sizes at the congested router. Second, we present a wireless channel and TCP-aware scheduling and buffer sharing algorithm that reduces the latency of short flows while still exploiting user diversity for a wide range of user and traffic mix.     

EPCM – an efficient power controlled MAC protocol for mobile ad hoc network

To reduce interference and to save a significant amount of energy, a control of transmission power is employed in Mobile Ad hoc Network. Many researchers have reported numerous transmission power control schemes to achieve the objective. Some of those techniques use higher transmission power for control packets (Request To Send/Clear To Send) and lesser power for Data and ACK packets. These schemes, though save some amount of energy, achieve least aggregate throughput due to poor spatial reuse and hidden terminal interference. In this paper, an efficient Power Controlled Medium Access Control (EPCM) scheme is evinced, which uses uniform interference aware and minimum transmission power for both Control and Data packet. The performance of EPCM is evaluated and compared with three reported Medium Access Control protocols which are based on transmission power control schemes and is observed that the proposed protocol achieves better throughput and minimal energy consumption while avoiding the hidden terminal problem.     

Start-up transition behaviour of TCP NewReno

NewReno has been proposed as the sender to recover multiple packet losses within a window by responding to a partial ACK. Since a great deal of time is required to recover all losses, the behaviour of the TCP sender during fast recovery greatly affects the overall performance. The dynamics of TCP NewReno variants during fast recovery are analysed     

iMAC: improved Medium Access Control for multi‐channel multi‐hop wireless networks

Trends in wireless networks are increasingly pointing towards a future with multi‐hop networks deployed in multi‐channel environments. In this paper, we present the design for iMAC—a protocol targeted at Medium Access Control in such environments. iMAC uses control packets on a common control channel to facilitate a three‐way handshake between the sender and the receiver for every packet transmission. This handshake enables the sender and the receiver to come to a consensus on a channel to use for data transmission and also signals to neighboring nodes about the contention on that channel. iMAC then uses a mechanism similar to 802.11 for data communication. Our evaluation of iMAC shows that it provides significant gains in throughput in comparison with uninformed channel selection, especially when contention for channel bandwidth is neither too low nor too high; intelligent selection of channels by iMAC is necessary to harness available bandwidth resources in the presence of medium levels of contention. Copyright © 2011 John Wiley & Sons, Ltd.     

TCP拥塞控制机制浅析

TCP是当今网络中主要的传输协议,它采用了慢启动、拥塞避免、快速重传、快速恢复四种算法,能满足IP网络中数据的可靠传输。但是当出现多个数据包丢失时,由于TCP采用了累计确认机制,造成系统吞吐量下降。文章介绍了一种SACK拥塞控制机制,与传统的Tahoe、Reno对比,并通过仿真实验说明了SACK是一种最好的TCP恢复机制。