A survey of TCP over ad hoc networks
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paper_title: TCP-BuS: Improving TCP performance in wireless Ad Hoc networks

paper_content:

Reliable data transmission over wireless multi-hop networks, called ad hoc networks, has proven to be non-trivial. TCP (Transmission Control Protocol), a widely used end-to-end reliable transport protocol designed for wired networks, is not entirely suitable for wireless ad hoc networks due to the inappropriateness of TCP congestion control schemes. Specifically, the TCP sender concludes that there is network congestion upon detecting packet losses or at time-outs. However, in wireless ad hoc networks, links are broken as a result of node mobility and hence some time is needed to perform route reconfiguration. During this time, packets could be lost or held back. Hence, the TCP sender could mistake this event as congestion, which is untrue. A route disconnection should be handled differently from network congestion. In this paper, we propose a new mechanism that improves TCP performance in a wireless ad hoc network where each node can buffer ongoing packets during a route disconnection and re-establishment. In addition to distinguishing network congestion from route disconnection due to node mobility, we also incorporate new measures to deal with reliable transmission of important control messages and exploitation of TCP fast recovery procedures. Our simulation compares the proposed TCP-BuS approach with general TCP and TCP-Feedback. Results reveal that TCP-BuS outperforms other approaches in terms of communication throughput under the presence of mobility.
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paper_title: CLAMP: differentiated capacity allocation in access networks

paper_content:

The paper presents a solution for providing differentiated capacity allocation in an access network. The system is based on CLAMP (curtailing the large TCP advertised window to maximize performance), an algorithm that can differentiate between flows sharing the same FIFO queue. The system is suitable for access networks, such as those based on DSL and HFC modems and wireless LAN access points. The deployment of CLAMP is completely contained within the access network; no changes to the remainder of the network are required. CLAMP provides the opportunity to enforce local policies on TCP flows that originate from sources distributed globally. The performance of CLAMP is verified by both simulation and analysis.
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paper_title: TCP extensions for space communications

paper_content:

The space communication environment and mobile and wireless communication environments show many similarities when observed from the perspective of a transport protocol. Both types of environments exhibit loss caused by data corruption and link outage, in addition to congestion-related loss. The constraints imposed by the two environments are also similar—power, weight, and physical volume of equipment are scarce resources. Finally, it is not uncommon for communication channel data rates to be severely limited and highly asymmetric. We are working on solutions to these types of problems for space communication environments, and we believe that these solutions may be applicable to the mobile and wireless community. As part of our work, we have defined and implemented the Space Communications Protocol Standards-Transport Protocol (SCPS-TP), a set of extensions to TCP that address the problems that we have identified. The results of our performance tests, both in the laboratory and on actual satellites, indicate that the SCPS-TP extensions yield significant improvements in throughput over unmodified TCP on error-prone links. Additionally, the SCPS modifications significantly improve performance over links with highly asymmetric data rates.
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paper_title: Improving reliable transport and handoff performance in cellular wireless networks

paper_content:

TCP is a reliable transport protocol tuned to perform well in traditional networks where congestion is the primary cause of packet loss. However, networks with wireless links and mobile hosts incur significant losses due to bit-errors and hand-offs. This environment violates many of the assumptions made by TCP, causing degraded end-to-end performance. In this paper, we describe the additions and modifications to the standard Internet protocol stack (TCP/IP) to improve end-to-end reliable transport performance in mobile environments. The protocol changes are made to network-layer software at the base station and mobile host, and preserve the end-to-end semantics of TCP. One part of the modifications, called the snoop module, caches packets at the base station and performs local retransmissions across the wireless link to alleviate the problems caused by high bit-error rates. The second part is a routing protocol that enables low-latency handoff to occur with negligible data loss. We have implemented this new protocol stack on a wireless testbed. Our experiments show that this system is significantly more robust at dealing with unreliable wireless links than normal TCP; we have achieved throughput speedups of up to 20 times over regular TCP and handoff latencies over 10 times shorter than other mobile routing protocols.
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paper_title: Analysis of TCP performance over mobile ad hoc networks

paper_content:

Mobile ad hoc networks have attracted attention lately as a means of providing continuous network connectivity to mobile computing devices regardless of physical location. Recent research has focused primarily on the routing protocols needed in such an environment. In this paper, we investigate the effects that link breakage due to mobility has on TCP performance. Through simulation, we show that TCP throughput drops significantly when nodes move, due to TCP's inability to recognize the difference between link failure and congestion. We also analyze specific examples, such as a situation where throughput is zero for a particular connection. We introduce a new metric, expected throughput, for the comparison of throughput in multi-hop networks, and then use this metric to show how the use of explicit link failure notification (ELFN) techniques can significantly improve TCP performance.
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paper_title: Performance evaluation of TCP algorithms in multi‐hop wireless packet networks

paper_content:

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.
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paper_title: Improving TCP performance over mobile ad-hoc networks with out-of-order detection and response

paper_content:

In a Mobile Ad Hoc Network (MANET), temporary link failures and route changes happen frequently. With the assumption that all packet losses are due to congestion, TCP performs poorly in such environment. While there has been some research on improving TCP performance over MANET, most of them require feedback from the network or the lower layer. In this research, we explore a new approach to improve TCP performance by detecting and responding to out-of-order packet delivery events, which are the results of frequent route changes. In our simulation study, this approach had achieved on average 50% performance improvement, without requiring feedback from the network or the lower layer.
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paper_title: A feedback based scheme for improving TCP performance in ad-hoc wireless networks

paper_content:

Ad-hoc networks are completely wireless networks of mobile hosts, in which the topology rapidly changes due to the movement of mobile hosts. This frequent topology may lead to sudden packet losses and delays. Transport protocols like TCP have been built mainly for reliable, fixed networks. Hence, when used in ad-hoc networks, TCP misinterprets this loss as congestion and invokes congestion control. This leads to unnecessary retransmissions and loss of throughput. To overcome this problem, a feedback scheme is proposed, so that the source can distinguish between route failure and network congestion. When a route is disrupted, the source is sent a route failure notification (RFN) packet, allowing it to freeze its timers and stop sending packets. When the route is re-established, the source is informed through a route re-establishment notification (RRN) packet, upon which it resumes by unfreezing timers and continuing packet transmissions. The simulated performance of TCP on ad-hoc networks with and without feedback is compared and reported. It is observed that in the event of route failures, as the route re-establishment time increases, the use of feedback provides significant gains in throughput as well as savings in unnecessary packet transmissions. Several further enhancements and directions for future work are also sketched.
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paper_title: Novel delayed ACK techniques for improving TCP performance in multihop wireless networks

paper_content:

We study in this paper TCP performance over a static multihop network that uses IEEE 802.11 protocol for access. For such networks it has been shown in [6] that TCP performance is mainly determined by the hidden terminal effects (and not by drop probabilities at buffers) which limits the number of packets that can be transmitted simultaneously in the network. We propose new approaches for improving the performance based on thinning the ACK streams that competes over the same radio resources as the TCP packets. In particular, we propose a new delayed ACK scheme in which the delay coefficient varies with the sequence number of the TCP packet. Through simulations we show that the ACK thinning allows to increase TCP throughput substantially more than previous improvement methods.
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paper_title: Transport Protocols for Internet-Compatible Satellite Networks

paper_content:

We address the question of how well end-to-end transport connections perform in a satellite environment composed of one or more satellites in geostationary orbit (GEO) or low-altitude Earth orbit (LEO), in which the connection may traverse a portion of the wired Internet. We first summarize the various ways in which latency and asymmetry can impair the performance of the Internet's transmission control protocol (TCP), and discuss extensions to standard TCP that alleviate some of these performance problems. Through analysis, simulation, and experiments, we quantify the performance of state-of-the-art TCP implementations in a satellite environment. A key part of the experimental method is the use of traffic models empirically derived from Internet traffic traces. We identify those TCP implementations that can be expected to perform reasonably well, and those that can suffer serious performance degradation. An important result is that, even with the best satellite-optimized TCP implementations, moderate levels of congestion in the wide-area Internet can seriously degrade performance for satellite connections. For scenarios in which TCP performance is poor, we investigate the potential improvement of using a satellite gateway, proxy, or Web cache to "split" transport connections in a manner transparent to end users. Finally, we describe a new transport protocol for use internally within a satellite network or as part of a split connection. This protocol, which we call the satellite transport protocol (STP), is optimized for challenging network impairments such as high latency, asymmetry, and high error rates. Among its chief benefits are up to an order of magnitude reduction in the bandwidth used in the reverse path, as compared to standard TCP, when conducting large file transfers. This is a particularly important attribute for the kind of asymmetric connectivity likely to dominate satellite-based Internet access.
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paper_title: Implementation and performance evaluation of Indirect TCP

paper_content:

With the advent of small portable computers and the technological advances in wireless communications, mobile wireless computing is likely to become very popular in the near future. Wireless links are slower and less reliable compared to wired links and are prone to loss of signal due to noise and fading. Furthermore, host mobility can give rise to periods of disconnection from the fixed network. The use of existing network protocols, which were developed mainly for the high bandwidth and faster wired links, with mobile computers thus gives rise to unique performance problems arising from host mobility and due to the characteristics of wireless medium. Indirect protocols can isolate mobility and wireless related problems using mobility support routers (MSRs) as intermediaries, which also provide backward compatibility with fixed network protocols. We present the implementation and performance evaluation of I-TCP, which is an indirect transport layer protocol for mobile wireless environments. Throughput comparison with regular (BSD) TCP shows that I-TCP performs significantly better in a wide range of conditions related to wireless losses and host mobility. We also describe the implementation and performance of I-TCP handoffs.
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paper_title: M-TCP: TCP for mobile cellular networks

paper_content:

Transport connections set up over wireless links are frequently plagued by problems such as - high bit error rate (BER), frequent disconnections of the mobile user, and low wireless bandwidth that may change dynamically. In this paper, we study the effects of frequent disconnections and low variable bandwidth on TCP throughput and propose a protocol that addresses this problem. We discuss the implementation (in NetBSD) of our protocol called M-TCP and compare its performance against other mobile TCP implementations. We show that M-TCP has two significant advantages over other solutions: (1) it maintains end-to-end TCP semantics and, (2) it delivers excellent performance for environments where the mobile encounters periods of disconnection.
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paper_title: Highly dynamic Destination-Sequenced Distance-Vector routing (DSDV) for mobile computers

paper_content:

An ad-hoc network is the cooperative engagement of a collection of Mobile Hosts without the required intervention of any centralized Access Point. In this paper we present an innovative design for the operation of such ad-hoc networks. The basic idea of the design is to operate each Mobile Host as a specialized router, which periodically advertises its view of the interconnection topology with other Mobile Hosts within the network. This amounts to a new sort of routing protocol. We have investigated modifications to the basic Bellman-Ford routing mechanisms, as specified by RIP [5], to make it suitable for a dynamic and self-starting network mechanism as is required by users wishing to utilize ad hoc networks. Our modifications address some of the previous objections to the use of Bellman-Ford, related to the poor looping properties of such algorithms in the face of broken links and the resulting time dependent nature of the interconnection topology describing the links between the Mobile Hosts. Finally, we describe the ways in which the basic network-layer routing can be modified to provide MAC-layer support for ad-hoc networks.
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paper_title: Implementation experience with MANET routing protocols

paper_content:

This paper outlines our experience with the implementation and deployment of two MANET routing protocols on a five node, four hop, network. The work was prompted by the lack of published results concerning the issues associated with the implementation of MANET routing protocols on actual wireless networks, as opposed to results of simulation experiments. We examined implementations of two distance vector MANET routing protocols and found a number of problems with both protocols during the course of our experiments. The most significant was that neither protocol could provide a stable route over any multi-hop network connection. The route discovery process of both protocols is fooled by the transient availability of network links to nodes that were more than one hop away. Packets transmitted over a fading channel cause the routing protocol to conclude incorrectly that there is a new one hop neighbor that could provide a lower metric (hop count) route to even more distant nodes. This can occur even when nodes are stationary, mobility resulted in even less route stability. We implemented a simple signal strength based neighbor selection procedure to test our assertion that fading channels and unreliable network links were the cause of the failure of the routing protocols. The result was that neighbor discovery and the filtering for neighbors with which nodes could communicate reliably enables the creation of reliable multihop routes. Based on our experiences, we outline several recommendations for future work in MANET research.
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paper_title: Effectiveness of RTS / CTS Handshake in IEEE 802 . 11 based Ad Hoc Networks

paper_content:

Abstract IEEE 802.11 MAC mainly relies on two techniques to combat interference: physical carrier sensing and RTS/CTS handshake (also known as “virtual carrier sensing”). Ideally, the RTS/CTS handshake can eliminate most interference. However, the effectiveness of RTS/CTS handshake is based on the assumption that hidden nodes are within transmission range of receivers. In this paper, we prove using analytic models that in ad hoc networks, such an assumption cannot hold due to the fact that power needed for interrupting a packet reception is much lower than that of delivering a packet successfully. Thus, the “virtual carrier sensing” implemented by RTS/CTS handshake cannot prevent all interference as we expect in theory. Physical carrier sensing can complement this in some degree. However, since interference happens at receivers, while physical carrier sensing is detecting transmitters (the same problem causing the hidden terminal situation), physical carrier sensing cannot help much, unless a very large carrier sensing range is adopted, which is limited by the antenna sensitivity. In this paper, we investigate how effective is the RTS/CTS handshake in terms of reducing interference. We show that in some situations, the interference range is much larger than transmission range, where RTS/CTS cannot function well. Two independent solutions are proposed in this paper. One is a simple enhancement to the IEEE 802.11 MAC protocol. The other is to utilize directional antennas. Simulation results verify that the proposed schemes indeed can help IEEE 802.11 resolve most interference caused by large interference range.
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paper_title: MACAW: a media access protocol for wireless LAN's

paper_content:

In recent years, a wide variety of mobile computing devices has emerged, including portables, palmtops, and personal digital assistants. Providing adequate network connectivity for these devices will require a new generation of wireless LAN technology. In this paper we study media access protocols for a single channel wireless LAN being developed at Xerox Corporation's Palo Alto Research Center. We start with the MACA media access protocol first proposed by Karn [9] and later refined by Biba [3] which uses an RTS-CTS-DATA packet exchange and binary exponential back-off. Using packet-level simulations, we examine various performance and design issues in such protocols. Our analysis leads to a new protocol, MACAW, which uses an RTS-CTS-DS-DATA-ACK message exchange and includes a significantly different backoff algorithm.
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paper_title: TCP extensions for space communications

paper_content:

The space communication environment and mobile and wireless communication environments show many similarities when observed from the perspective of a transport protocol. Both types of environments exhibit loss caused by data corruption and link outage, in addition to congestion-related loss. The constraints imposed by the two environments are also similar—power, weight, and physical volume of equipment are scarce resources. Finally, it is not uncommon for communication channel data rates to be severely limited and highly asymmetric. We are working on solutions to these types of problems for space communication environments, and we believe that these solutions may be applicable to the mobile and wireless community. As part of our work, we have defined and implemented the Space Communications Protocol Standards-Transport Protocol (SCPS-TP), a set of extensions to TCP that address the problems that we have identified. The results of our performance tests, both in the laboratory and on actual satellites, indicate that the SCPS-TP extensions yield significant improvements in throughput over unmodified TCP on error-prone links. Additionally, the SCPS modifications significantly improve performance over links with highly asymmetric data rates.
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paper_title: Novel delayed ACK techniques for improving TCP performance in multihop wireless networks

paper_content:

We study in this paper TCP performance over a static multihop network that uses IEEE 802.11 protocol for access. For such networks it has been shown in [6] that TCP performance is mainly determined by the hidden terminal effects (and not by drop probabilities at buffers) which limits the number of packets that can be transmitted simultaneously in the network. We propose new approaches for improving the performance based on thinning the ACK streams that competes over the same radio resources as the TCP packets. In particular, we propose a new delayed ACK scheme in which the delay coefficient varies with the sequence number of the TCP packet. Through simulations we show that the ACK thinning allows to increase TCP throughput substantially more than previous improvement methods.
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paper_title: WaveLAN®-II: A high-performance wireless LAN for the unlicensed band

paper_content:

In July 1997 the Institute of Electrical and Electronics Engineers (IEEE) completed standard 802.11 for wireless local area networks (LANs). WaveLAN®-II, to be released early in 1998, offers compatibility with the IEEE 802.11 standard for operation in the 2.4-GHz band. It is the successor to WaveLAN-I, which has been in the market since 1991. As a next-generation wireless LAN product, WaveLAN-II has many enhancements to improve performance in various areas. An IEEE 802.11 direct sequence spread spectrum (DSSS) product, WaveLAN-II supports the basic bit rates of 1 and 2 Mb/s, but it can also provide enhanced bit rates as high as 10 Mb/s. This paper discusses various aspects of the system design of WaveLAN-II and characteristics of its antenna, radio-frequency (RF) front-end, digital signal processor (DSP) transceiver chip, and medium access controller (MAC) chip.
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paper_title: Computing TCP's Retransmission Timer

paper_content:

This document defines the standard algorithm that Transmission Control Protocol (TCP) senders are required to use to compute and manage their retransmission timer. It expands on the discussion in section 4.2.3.1 of RFC 1122 and upgrades the requirement of supporting the algorithm from a SHOULD to a MUST.
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paper_title: Alleviating Effects of Mobility on TCP Performance in Ad Hoc Networks Using Signal Strength Based Link Management

paper_content:

Mobility in ad hoc networks causes link failures, which in turn result in packet losses. TCP attributes these losses to congestion. This results in frequent TCP retransmission timeouts and degradation in TCP performance even at light loads. We propose mechanisms that are based on signal strength measurements to alleviate such packet losses due to mobility at light loads. Our key ideas are (a) if the signal strength measurements indicate that a link failure is most likely due to a neighbor moving out of range, in reaction, facilitate the use of temporary high power to re-establish the link and, (b) if the signal strength measurements indicate that a link is likely to fail, initiate a route re-discovery proactively before the link actually fails. We make changes at the MAC and the routing layers to predict link failures and estimate if a link failure is due to mobility. We also propose a simple mechanism that can help alleviate false link failures that occur due to congestion when the IEEE 802.11 MAC protocol is used. We compare the above proactive and reactive schemes and also demonstrate the benefits of using them together. We show that, in high mobility, the performance of a TCP session can increase by as much as 45 percent when our methods are incorporated.
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paper_title: A review of routing protocols for mobile ad hoc networks

paper_content:

Abstract The 1990s have seen a rapid growth of research interests in mobile ad hoc networking. The infrastructureless and the dynamic nature of these networks demands new set of networking strategies to be implemented in order to provide efficient end-to-end communication. This, along with the diverse application of these networks in many different scenarios such as battlefield and disaster recovery, have seen MANETs being researched by many different organisations and institutes. MANETs employ the traditional TCP/IP structure to provide end-to-end communication between nodes. However, due to their mobility and the limited resource in wireless networks, each layer in the TCP/IP model require redefinition or modifications to function efficiently in MANETs. One interesting research area in MANET is routing. Routing in the MANETs is a challenging task and has received a tremendous amount of attention from researches. This has led to development of many different routing protocols for MANETs, and each author of each proposed protocol argues that the strategy proposed provides an improvement over a number of different strategies considered in the literature for a given network scenario. Therefore, it is quite difficult to determine which protocols may perform best under a number of different network scenarios, such as increasing node density and traffic. In this paper, we provide an overview of a wide range of routing protocols proposed in the literature. We also provide a performance comparison of all routing protocols and suggest which protocols may perform best in large networks.
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paper_title: TCP performance over mobile ad hoc networks: a quantitative study

paper_content:

In this paper, we study the performance of the transmission control protocol (TCP) over mobile ad-hoc networks. We present a comprehensive set of simulation results and identify the key factors that impact TCP's performance over ad-hoc networks. We use a variety of parameters including link failure detection latency, route computation latency, packet level route unavailability index, and flow level route unavailability index to capture the impact of mobility. We relate the impact of mobility on the different parameters to TCP's performance by studying the throughput, loss-rate and retransmission timeout values at the TCP layer. We conclude from our results that existing approaches to improve TCP performance over mobile ad-hoc networks have identified and hence focused only on a subset of the affecting factors. In the process, we identify a comprehensive set of factors influencing TCP performance. Finally, using the insights gained through the performance evaluations, we propose a framework called Atra consisting of three simple and easily implementable mechanisms at the MAC and routing layers to improve TCP's performance over ad-hoc networks. We demonstrate that Atra improves on the throughput performance of a default protocol stack by 50%-100%.
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paper_title: Analysis of TCP performance over mobile ad hoc networks

paper_content:

Mobile ad hoc networks have attracted attention lately as a means of providing continuous network connectivity to mobile computing devices regardless of physical location. Recent research has focused primarily on the routing protocols needed in such an environment. In this paper, we investigate the effects that link breakage due to mobility has on TCP performance. Through simulation, we show that TCP throughput drops significantly when nodes move, due to TCP's inability to recognize the difference between link failure and congestion. We also analyze specific examples, such as a situation where throughput is zero for a particular connection. We introduce a new metric, expected throughput, for the comparison of throughput in multi-hop networks, and then use this metric to show how the use of explicit link failure notification (ELFN) techniques can significantly improve TCP performance.
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paper_title: Mobile Ad Hoc Networking: Imperatives and Challenges

paper_content:

A mobile ad hoc network (MANET), sometimes called a mobile mesh network, is a self- configuring network of mobile devices connected by wireless links. The Ad hoc networks are a new wireless networking paradigm for mobile hosts. Unlike traditional mobile wireless networks, ad hoc networks do not rely on any fixed infrastructure. Instead, hosts rely on each other to keep the network connected. It represent complex distributed systems that comprise wireless mobile nodes that can freely and dynamically self-organize into arbitrary and temporary, ''ad-hoc'' network topologies, allowing people and devices to seamlessly internetwork in areas with no pre-existing communication infrastructure. Ad hoc networking concept is not a new one, having been around in various forms for over 20 years. Traditionally, tactical networks have been the only communication networking application that followed the adhoc paradigm. Recently, the introduction of new technologies such as the Bluetooth, IEEE 802.11 and Hyperlan are helping enable eventual commercial MANET deployments outside the military domain. These recent evolutions have been generating a renewed and growing interest in the research and development of MANET.So,we would like to propose the challenges and imperativels that the MANET is facing.
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paper_title: TCP performance over multipath routing in mobile ad hoc networks

paper_content:

In this paper, we investigate TCP performance over a multipath routing protocol. Multipath routing can improve the path availability in mobile environment. Thus, it has a great potential to improve TCP performance in ad hoc networks under mobility. Previous research on multipath routing mostly used UDP traffic for performance evaluation. When TCP is used, we find that most times, using multiple paths simultaneously may actually degrade TCP performance. This is partly due to frequent out-of-order packet delivery via different paths. We then test another multipath routing strategy called backup path routing. Under the backup path routing scheme, TCP is able to gain improvements against mobility. We then further study related issues to backup path routing, which can affect TCP performance. Some important discoveries are reported in the paper and simulation results show that by careful selection of the multipath routing strategies, we can improve TCP performance by more than 30% even under very high mobility.
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paper_title: An adaptive distance vector routing algorithm for mobile, ad hoc networks

paper_content:

We present a new routing algorithm called adaptive distance vector (ADV) for mobile, ad hoc networks (MANETs). ADV is a distance vector routing algorithm that exhibits some on-demand characteristics by varying the frequency and the size of the routing updates in response to the network load and mobility conditions. Using simulations we show that ADV outperforms AODV and DSR especially in high mobility cases by giving significantly higher (50% or more) peak throughputs and lower packet delays. Furthermore, ADV uses fewer routing and control overhead packets than that of AODV and DSR, especially at moderate to high loads. Our results indicate the benefits of combining both proactive and on-demand routing techniques in designing suitable routing protocols for MANETs.
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paper_title: Split multipath routing with maximally disjoint paths in ad hoc networks

paper_content:

In recent years, routing has been the most focused area in ad hoc networks research. On-demand routing in particular, is widely developed in bandwidth constrained mobile wireless ad hoc networks because of its effectiveness and efficiency. Most proposed on-demand routing protocols however, build and rely on a single route for each data session. Whenever there is a link disconnection on the active route, the routing protocol must perform a route recovery process. In QoS routing for wired networks, multiple path routing is popularly used. Multiple routes are however, constructed using link-state or distance vector algorithms which are not well-suited for ad hoc networks. We propose an on-demand routing scheme called split multipath routing (SMR) that establishes and utilizes multiple routes of maximally disjoint paths. Providing multiple routes helps minimizing route recovery process and control message overhead. Our protocol uses a per-packet allocation scheme to distribute data packets into multiple paths of active sessions. This traffic distribution efficiently utilizes available network resources and prevents nodes of the route from being congested in heavily loaded traffic situations. We evaluate the performance of our scheme using extensive simulation.
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paper_title: A comparison of TCP performance over three routing protocols for mobile ad hoc networks

paper_content:

We examine the performance of the TCP protocol for bulk-data transfers in mobile ad hoc networks (MANETs). We vary the number of TCP connections and compare the performances of three recently proposed on-demand (AODV and DSR) and adaptive proactive (ADV) routing algorithms. It has been shown in the literature that the congestion control mechanism of TCP reacts adversely to packet losses due to temporarily broken routes in wireless networks. So, we prospose a simple heuristic, called fixed RTO, to distinguish between route loss and network congestion and thereby improve the performance of the routing algorithms. Using the ns-2 simulator, we evaluate the performances of the three routing algorithms with the standard TCP Reno protocol and Reno with fixed RTO. Our results indicate that the proactive ADV algorithm performs well under a variety of conditions and that the fixed RTO technique improves the performances of the two on-demand algorithms significantly
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paper_title: Fair sharing of mac under tcp in wireless ad hoc networks

paper_content:

In this study we investigate the performance of TCP and MAC layer in a wireless multi-hop network. Using simulation, we provide new insights into the interactions between TCP and various MAC layer protocols, including CSMA, FAMA and 802.11. These MAC protocols were chosen because they provide an evolution of wireless medium access schemes, starting with carrie sing (CSMA), then evolving to the utilization of RTS/CTS control frames (FAMA) and finally progressing to collision avoidance and acknowledgements (802.11). We examine these interactions in various network topologies and in a mobile environment where node movements are unpredictable. In particular, we address the issue of fair sharing of MAC with multiple TCP flows.
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paper_title: On setting TCP's congestion window limit in mobile ad hoc networks

paper_content:

Improving TCP performance has long been the focus of many research efforts in mobile ad hoc networks (MANET). In this paper, we address one aspect of this endeavor: how to properly set TCP's congestion window limit (CWL) to achieve optimal performance. Past research has shown that using a small CWL improves TCP performance in certain scenarios [M. Gerla et al., Feb. 1999], [Z. Fu et al., Apr. 2003], however, no comprehensive study has been given. To this end, we turn the problem of setting TCP's optimal CWL into identifying the bandwidth-delay product (BDP) of a path in MANET. We first show and prove that, independent of the MAC layer protocol being used, the BDP of a path in MANET cannot exceed the round-trip hop-count (RTHC) of the path. We further refine this upper bound based on the IEEE 802.11 MAC layer protocol, and show that in a chain topology, a tighter upper bound exists, which is approximately 1/5 of the RTHC of the path. Based on this tighter bound, we propose an adaptive CWL setting strategy to dynamically adjust TCP's CWL according to the current RTHC of its path. Using ns-2 simulations, we show that our simple strategy improves TCP performance by 8% to 16% in a dynamic MANET environment.
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paper_title: TCP behavior across multihop wireless networks and the wired internet

paper_content:

Emerging wireless ad hoc networks find their most important applications in untethered, mobile, multihop scenarios where there is no wired infrastructure. Yet, when the wired infrastructure (say, the Internet) is within reach, opportunistic connections to Internet sites may be established across the multihop network to transfer files and update databases. These file transfers use TCP for reliability and congestion control. However, recent experiments with ad hoc, multihop 802.11 networks have exposed serious instabilities when TCP connections span both wired and wireless domains. In particular, some TCP connections capture the wireless channel and drive the throughput on other connections virtually to zero. This is most surprising in view of the fact that connections between 802.11 (single hop) wireless LAN stations and the Internet are well behaved. In fact they are routinely used in most Campuses, Businesses and Research Labs. This paper is an experimental study of the unstable behavior of TCP across 802.11 ad hoc networks and the wired Internet. We investigate the fairness issues of multiple TCP flows as well as the coexistence of TCP flows and video streams in the wired/wireless scenario. Detailed analysis of the measurement results is also presented. The paper will prove very valuable to future commercial and military ad hoc networks.
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paper_title: IEEE 802.11 ad hoc networks: performance measurements

paper_content:

In this paper we investigate the performance of IEEE 802.11 ad hoc networks by means of an experimental study. Measurements on IEEE 802.11 ad hoc networks confirm previous simulative results (e.g., TCP connections may actually experience significant throughput unfairness). The analysis of IEEE 802.11b reveals several aspects that are usually neglected in simulative studies. Firstly, since different transmission rates are used for control and data frames, different transmission ranges and carrier-sensing ranges may exist at the same time in the network. In addition, the transmission ranges are in practice much shorter than usually assumed in simulative analysis, and are not constant but highly variable, even in the same session.
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paper_title: Performance evaluation of TCP algorithms in multi‐hop wireless packet networks

paper_content:

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.
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paper_title: Enhancing TCP fairness in ad hoc wireless networks using neighborhood RED

paper_content:

Significant TCP unfairness in ad hoc wireless networks has been reported during the past several years. This unfairness results from the nature of the shared wireless medium and location dependency. If we view a node and its interfering nodes to form a "neighborhood", the aggregate of local queues at these nodes represents the distributed queue for this neighborhood. However, this queue is not a FIFO queue. Flows sharing the queue have different, dynamically changing priorities determined by the topology and traffic patterns. Thus, they get different feedback in terms of packet loss rate and packet delay when congestion occurs. In wired networks, the Randomly Early Detection (RED) scheme was found to improve TCP fairness. In this paper, we show that the RED scheme does not work when running on individual queues in wireless nodes. We then propose a Neighborhood RED (NRED) scheme, which extends the RED concept to the distributed neighborhood queue. Simulation studies confirm that the NRED scheme can improve TCP unfairness substantially in ad hoc networks. Moreover, the NRED scheme acts at the network level, without MAC protocol modifications. This considerably simplifies its deployment.
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paper_title: Novel delayed ACK techniques for improving TCP performance in multihop wireless networks

paper_content:

We study in this paper TCP performance over a static multihop network that uses IEEE 802.11 protocol for access. For such networks it has been shown in [6] that TCP performance is mainly determined by the hidden terminal effects (and not by drop probabilities at buffers) which limits the number of packets that can be transmitted simultaneously in the network. We propose new approaches for improving the performance based on thinning the ACK streams that competes over the same radio resources as the TCP packets. In particular, we propose a new delayed ACK scheme in which the delay coefficient varies with the sequence number of the TCP packet. Through simulations we show that the ACK thinning allows to increase TCP throughput substantially more than previous improvement methods.
---
paper_title: TCP performance in wireless multi-hop networks

paper_content:

We investigate the interaction between TCP and MAC layer in a wireless multi-hop network. Using simulation, we provide new insight into two critical problems of TCP over wireless multi-hop. The first is the conflict between TCP data packets and TCP ACKs, which causes performance to degrade for window sizes greater than 1 packet. The second is the interaction between TCP and MAC layer backoff timers which causes severe unfairness and capture conditions. We show that these effects are particularly pronounced in two families of MAC protocols that have been extensively used in ad-hoc network simulation and implementations, namely CSMA and FAMA (a descendent of MACA). We then demonstrate that these problems are in part overcome by using MACAW, a MAC layer protocol which extends MACA by adding link level ACKs and a less aggressive backoff policy. We argue that link level protection, backoff policy and selective queue scheduling are critical elements for efficient and fair operation of ad-hoc networks under TCP. These conclusions are supported by extensive simulation and measurement experiments.
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paper_title: Does the IEEE 802.11 MAC protocol work well in multihop wireless ad hoc networks

paper_content:

The IEEE 802.11 MAC protocol is the standard for wireless LANs; it is widely used in testbeds and simulations for wireless multihop ad hoc networks. However, this protocol was not designed for multihop networks. Although it can support some ad hoc network architecture, it is not intended to support the wireless mobile ad hoc network, in which multihop connectivity is one of the most prominent features. In this article we focus on the following question: can the IEEE 802.11 MAC protocol function well in multihop networks? By presenting several serious problems encountered in an IEEE 802.11-based multihop network and revealing the in-depth cause of these problems, we conclude that the current version of this wireless LAN protocol does not function well in multihop ad hoc networks. We thus doubt whether the WaveLAN-based system is workable as a mobile ad hoc testbed.
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paper_title: Analysis of TCP performance over mobile ad hoc networks

paper_content:

Mobile ad hoc networks have attracted attention lately as a means of providing continuous network connectivity to mobile computing devices regardless of physical location. Recent research has focused primarily on the routing protocols needed in such an environment. In this paper, we investigate the effects that link breakage due to mobility has on TCP performance. Through simulation, we show that TCP throughput drops significantly when nodes move, due to TCP's inability to recognize the difference between link failure and congestion. We also analyze specific examples, such as a situation where throughput is zero for a particular connection. We introduce a new metric, expected throughput, for the comparison of throughput in multi-hop networks, and then use this metric to show how the use of explicit link failure notification (ELFN) techniques can significantly improve TCP performance.
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paper_title: TCP-BuS: Improving TCP performance in wireless Ad Hoc networks

paper_content:

Reliable data transmission over wireless multi-hop networks, called ad hoc networks, has proven to be non-trivial. TCP (Transmission Control Protocol), a widely used end-to-end reliable transport protocol designed for wired networks, is not entirely suitable for wireless ad hoc networks due to the inappropriateness of TCP congestion control schemes. Specifically, the TCP sender concludes that there is network congestion upon detecting packet losses or at time-outs. However, in wireless ad hoc networks, links are broken as a result of node mobility and hence some time is needed to perform route reconfiguration. During this time, packets could be lost or held back. Hence, the TCP sender could mistake this event as congestion, which is untrue. A route disconnection should be handled differently from network congestion. In this paper, we propose a new mechanism that improves TCP performance in a wireless ad hoc network where each node can buffer ongoing packets during a route disconnection and re-establishment. In addition to distinguishing network congestion from route disconnection due to node mobility, we also incorporate new measures to deal with reliable transmission of important control messages and exploitation of TCP fast recovery procedures. Our simulation compares the proposed TCP-BuS approach with general TCP and TCP-Feedback. Results reveal that TCP-BuS outperforms other approaches in terms of communication throughput under the presence of mobility.
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paper_title: TCP performance over mobile ad hoc networks: a quantitative study

paper_content:

In this paper, we study the performance of the transmission control protocol (TCP) over mobile ad-hoc networks. We present a comprehensive set of simulation results and identify the key factors that impact TCP's performance over ad-hoc networks. We use a variety of parameters including link failure detection latency, route computation latency, packet level route unavailability index, and flow level route unavailability index to capture the impact of mobility. We relate the impact of mobility on the different parameters to TCP's performance by studying the throughput, loss-rate and retransmission timeout values at the TCP layer. We conclude from our results that existing approaches to improve TCP performance over mobile ad-hoc networks have identified and hence focused only on a subset of the affecting factors. In the process, we identify a comprehensive set of factors influencing TCP performance. Finally, using the insights gained through the performance evaluations, we propose a framework called Atra consisting of three simple and easily implementable mechanisms at the MAC and routing layers to improve TCP's performance over ad-hoc networks. We demonstrate that Atra improves on the throughput performance of a default protocol stack by 50%-100%.
---
paper_title: Analysis of TCP performance over mobile ad hoc networks

paper_content:

Mobile ad hoc networks have attracted attention lately as a means of providing continuous network connectivity to mobile computing devices regardless of physical location. Recent research has focused primarily on the routing protocols needed in such an environment. In this paper, we investigate the effects that link breakage due to mobility has on TCP performance. Through simulation, we show that TCP throughput drops significantly when nodes move, due to TCP's inability to recognize the difference between link failure and congestion. We also analyze specific examples, such as a situation where throughput is zero for a particular connection. We introduce a new metric, expected throughput, for the comparison of throughput in multi-hop networks, and then use this metric to show how the use of explicit link failure notification (ELFN) techniques can significantly improve TCP performance.
---
paper_title: The Addition of Explicit Congestion Notification (ECN) to IP

paper_content:

This memo specifies the incorporation of ECN (Explicit Congestion Notification) to TCP and IP, including ECN's use of two bits in the IP header.
---
paper_title: Associativity-Based Routing For Ad-Hoc Mobile Networks

paper_content:

This paper presents a new, simple and bandwidth-efficient distributed routing protocol to support mobile computing in a conference size ad-hoc mobile network environment. Unlike the conventional approaches such as link-state and distance-vector distributed routing algorithms, our protocol does not attempt to consistently maintain routing information in every node. In an ad-hoc mobile network where mobile hosts (MHs) are acting as routers and where routes are made inconsistent by MHs‘ movement, we employ an associativity-based routing scheme where a route is selected based on nodes having associativity states that imply periods of stability. In this manner, the routes selected are likely to be long-lived and hence there is no need to restart frequently, resulting in higher attainable throughput. Route requests are broadcast on a per need basis. The association property also allows the integration of ad-hoc routing into a BS-oriented Wireless LAN (WLAN) environment, providing the fault tolerance in times of base stations (BSs) failures. To discover shorter routes and to shorten the route recovery time when the association property is violated, the localised-query and quick-abort mechanisms are respectively incorporated into the protocol. To further increase cell capacity and lower transmission power requirements, a dynamic cell size adjustment scheme is introduced. The protocol is free from loops, deadlock and packet duplicates and has scalable memory requirements. Simulation results obtained reveal that shorter and better routes can be discovered during route re-constructions.
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paper_title: Optimized Link State Routing Protocol (OLSR)

paper_content:

This document describes the Optimized Link State Routing (OLSR) protocol for mobile ad hoc networks. The protocol is an optimization of the classical link state algorithm tailored to the requirements of a mobile wireless LAN. The key concept used in the protocol is that of multipoint relays (MPRs). MPRs are selected nodes which forward broadcast messages during the flooding process. This technique substantially reduces the message overhead as compared to a classical flooding mechanism, where every node retransmits each message when it receives the first copy of the message. In OLSR, link state information is generated only by nodes elected as MPRs. Thus, a second optimization is achieved by minimizing the number of control messages flooded in the network. As a third optimization, an MPR node may chose to report only links between itself and its MPR selectors. Hence, as contrary to the classic link state algorithm, partial link state information is distributed in the network. This information is then used for route calculation. OLSR provides optimal routes (in terms of number of hops). The protocol is particularly suitable for large and dense networks as the technique of MPRs works well in this context.
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paper_title: Improving TCP performance over mobile ad-hoc networks with out-of-order detection and response

paper_content:

In a Mobile Ad Hoc Network (MANET), temporary link failures and route changes happen frequently. With the assumption that all packet losses are due to congestion, TCP performs poorly in such environment. While there has been some research on improving TCP performance over MANET, most of them require feedback from the network or the lower layer. In this research, we explore a new approach to improve TCP performance by detecting and responding to out-of-order packet delivery events, which are the results of frequent route changes. In our simulation study, this approach had achieved on average 50% performance improvement, without requiring feedback from the network or the lower layer.
---
paper_title: Novel delayed ACK techniques for improving TCP performance in multihop wireless networks

paper_content:

We study in this paper TCP performance over a static multihop network that uses IEEE 802.11 protocol for access. For such networks it has been shown in [6] that TCP performance is mainly determined by the hidden terminal effects (and not by drop probabilities at buffers) which limits the number of packets that can be transmitted simultaneously in the network. We propose new approaches for improving the performance based on thinning the ACK streams that competes over the same radio resources as the TCP packets. In particular, we propose a new delayed ACK scheme in which the delay coefficient varies with the sequence number of the TCP packet. Through simulations we show that the ACK thinning allows to increase TCP throughput substantially more than previous improvement methods.
---
paper_title: A comparison of TCP performance over three routing protocols for mobile ad hoc networks

paper_content:

We examine the performance of the TCP protocol for bulk-data transfers in mobile ad hoc networks (MANETs). We vary the number of TCP connections and compare the performances of three recently proposed on-demand (AODV and DSR) and adaptive proactive (ADV) routing algorithms. It has been shown in the literature that the congestion control mechanism of TCP reacts adversely to packet losses due to temporarily broken routes in wireless networks. So, we prospose a simple heuristic, called fixed RTO, to distinguish between route loss and network congestion and thereby improve the performance of the routing algorithms. Using the ns-2 simulator, we evaluate the performances of the three routing algorithms with the standard TCP Reno protocol and Reno with fixed RTO. Our results indicate that the proactive ADV algorithm performs well under a variety of conditions and that the fixed RTO technique improves the performances of the two on-demand algorithms significantly
---
paper_title: Split TCP for mobile ad hoc networks

paper_content:

The fairness and throughput of TCP suffer when it is used in mobile ad hoc networks. This is because TCP wrongly attributes packet losses due to link failures (a consequence of mobility) to congestion. The resulting overall degradation of throughput especially affects connections with a large number of hops, where link failures are more likely; thus, short connections enjoy an unfair advantage. Furthermore, if the IEEE 802.11 MAC protocol is used, the problems are exacerbated due to the protocol-induced capture effect, leading to greater unfairness and a further throughput degradation. We develop a scheme, called split TCP, which separates the TCP functions of congestion control and reliable packet delivery. For any TCP connection, certain nodes along the route take up the role of being proxies for that connection. The proxies buffer packets upon receipt and administer rate control. The buffering enables dropped packets to be recovered from the most recent proxy. The rate control helps in controlling congestion on inter-proxy segments. Thus, we emulate shorter TCP connections and can thereby achieve better parallelism in the network. Simulations show that the use of proxies improves the total throughput by as much as 30% in typical scenarios and reduces unfairness significantly. In terms of an unfairness metric that we introduce, the unfairness decreases from 0.8 to 0.2 (1.0 being the maximum unfairness). We conclude that incorporating TCP proxies is beneficial in terms of improving TCP performance in ad hoc networks.
---
paper_title: Preemptive routing in ad hoc networks

paper_content:

Routing in ad hoc networks is a challenging problem because nodes are mobile and links are continuously being created and broken. Existing on-demand ad hoc routing algorithms initiate route discovery only after a path breaks, incurring a significant cost in detecting the disconnection and establishing a new route. In this work, we investigate adding proactive route selection and maintenance to on-demand ad hoc routing algorithms. More specifically, when a path is likely to be broken, a warning is sent to the source indicating the likelihood of a disconnection. The source can then initiate path discovery early, potentially avoiding the disconnection altogether. A path is considered likely to break when the received packet power becomes close to the minimum detectable power (other approaches are possible). Care must be taken to avoid initiating false route warnings due to fluctuations in received power caused by fading, multipath effects and similar random transient phenomena. Experiments demonstrate that adding proactive route selection and maintenance to DSR and AODV (on-demand ad hoc routing protocols) significantly reduces the number of broken paths, with a small increase in protocol overhead. Packet latency and jitter go down in most cases. Because preemptive routing reduces the number of broken paths, it also has a secondary effect on TCP performance--unnecessary congestion handling measures are avoided. This is observed for TCP traffic under different traffic patterns (telnet, ftp and http). Additionally, we outline some problems in TCP performance in ad hoc environments.
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paper_title: Alleviating Effects of Mobility on TCP Performance in Ad Hoc Networks Using Signal Strength Based Link Management

paper_content:

Mobility in ad hoc networks causes link failures, which in turn result in packet losses. TCP attributes these losses to congestion. This results in frequent TCP retransmission timeouts and degradation in TCP performance even at light loads. We propose mechanisms that are based on signal strength measurements to alleviate such packet losses due to mobility at light loads. Our key ideas are (a) if the signal strength measurements indicate that a link failure is most likely due to a neighbor moving out of range, in reaction, facilitate the use of temporary high power to re-establish the link and, (b) if the signal strength measurements indicate that a link is likely to fail, initiate a route re-discovery proactively before the link actually fails. We make changes at the MAC and the routing layers to predict link failures and estimate if a link failure is due to mobility. We also propose a simple mechanism that can help alleviate false link failures that occur due to congestion when the IEEE 802.11 MAC protocol is used. We compare the above proactive and reactive schemes and also demonstrate the benefits of using them together. We show that, in high mobility, the performance of a TCP session can increase by as much as 45 percent when our methods are incorporated.
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paper_title: TCP performance over multipath routing in mobile ad hoc networks

paper_content:

In this paper, we investigate TCP performance over a multipath routing protocol. Multipath routing can improve the path availability in mobile environment. Thus, it has a great potential to improve TCP performance in ad hoc networks under mobility. Previous research on multipath routing mostly used UDP traffic for performance evaluation. When TCP is used, we find that most times, using multiple paths simultaneously may actually degrade TCP performance. This is partly due to frequent out-of-order packet delivery via different paths. We then test another multipath routing strategy called backup path routing. Under the backup path routing scheme, TCP is able to gain improvements against mobility. We then further study related issues to backup path routing, which can affect TCP performance. Some important discoveries are reported in the paper and simulation results show that by careful selection of the multipath routing strategies, we can improve TCP performance by more than 30% even under very high mobility.
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paper_title: COPAS: dynamic contention-balancing to enhance the performance of TCP over multi-hop wireless networks

paper_content:

Most studies on TCP over multi-hop wireless ad hoc networks have only addressed the issue of performance degradation due to temporarily broken routes, which results in TCP inability to distinguish between losses due to link failures or congestion. This problem tends to become more serious as network mobility increases. We tackle the equally important capture problem to which there has been little or no solution, and is present mostly in static and low mobility multihop wireless networks. This is a result of the interplay between the MAC layer and TCP backoff policies, which causes nodes to unfairly capture the wireless shared medium, hence preventing neighboring nodes to access the channel. This has been shown to have major negative effects on TCP performance comparable to the impact of mobility. We propose a novel algorithm, called COPAS (COntention-based PAth Selection), which incorporates two mechanisms to enhance TCP performance by avoiding capture conditions. First, it uses disjoint forward (sender to receiver for TCP data) and reverse (receiver to sender for TCP ACKs) paths in order to minimize the conflicts of TCP data and ACK packets. Second, COPAS employs a dynamic contention-balancing scheme where it continuously monitors and changes forward and reverse paths according to the level of MAC layer contention, hence minimizing the likelihood of capture. Through extensive simulation, COPAS is shown to improve TCP throughput by up to 90% while keeping routing overhead low.
---
paper_title: The Addition of Explicit Congestion Notification (ECN) to IP

paper_content:

This memo specifies the incorporation of ECN (Explicit Congestion Notification) to TCP and IP, including ECN's use of two bits in the IP header.
---
paper_title: Enhancing TCP fairness in ad hoc wireless networks using neighborhood RED

paper_content:

Significant TCP unfairness in ad hoc wireless networks has been reported during the past several years. This unfairness results from the nature of the shared wireless medium and location dependency. If we view a node and its interfering nodes to form a "neighborhood", the aggregate of local queues at these nodes represents the distributed queue for this neighborhood. However, this queue is not a FIFO queue. Flows sharing the queue have different, dynamically changing priorities determined by the topology and traffic patterns. Thus, they get different feedback in terms of packet loss rate and packet delay when congestion occurs. In wired networks, the Randomly Early Detection (RED) scheme was found to improve TCP fairness. In this paper, we show that the RED scheme does not work when running on individual queues in wireless nodes. We then propose a Neighborhood RED (NRED) scheme, which extends the RED concept to the distributed neighborhood queue. Simulation studies confirm that the NRED scheme can improve TCP unfairness substantially in ad hoc networks. Moreover, the NRED scheme acts at the network level, without MAC protocol modifications. This considerably simplifies its deployment.
---
paper_title: Improving fairness among TCP flows crossing wireless ad hoc and wired networks

paper_content:

In scenarios where wireless ad hoc networks are deployed, sometimes it would be desirable that ad hoc nodes can communicate with servers in wired networks to upload or download data. In these cases TCP connections will span both wireless ad hoc and wired domains. However, TCP often faces severe unfairness in this type of connection scenario, which forces some TCP flows to completely stop transferring any data despite all links being in good states. In this paper, we propose a simple scheduling scheme, which helps competing TCP connections to achieve fairness without much throughput loss. Simulation results show that our scheme successfully eliminates the extreme unfairness existing in above-mentioned scenarios.
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paper_title: Congestion avoidance and control

paper_content:

In October of '86, the Internet had the first of what became a series of 'congestion collapses'. During this period, the data throughput from LBL to UC Berkeley (sites separated by 400 yards and three IMP hops) dropped from 32 Kbps to 40 bps. Mike Karels 1 and I were fascinated by this sudden factor-of-thousand drop in bandwidth and embarked on an investigation of why things had gotten so bad. We wondered, in particular, if the 4.3BSD (Berkeley UNIX) TCP was mis-behaving or if it could be tuned to work better under abysmal network conditions. The answer to both of these questions was “yes”. Since that time, we have put seven new algorithms into the 4BSD TCP: round-trip-time variance estimation exponential retransmit timer backoff slow-start more aggressive receiver ack policy dynamic window sizing on congestion Karn's clamped retransmit backoff fast retransmit Our measurements and the reports of beta testers suggest that the final product is fairly good at dealing with congested conditions on the Internet. This paper is a brief description of ( i ) - ( v ) and the rationale behind them. ( vi ) is an algorithm recently developed by Phil Karn of Bell Communications Research, described in [KP87]. ( viii ) is described in a soon-to-be-published RFC. Algorithms ( i ) - ( v ) spring from one observation: The flow on a TCP connection (or ISO TP-4 or Xerox NS SPP connection) should obey a 'conservation of packets' principle. And, if this principle were obeyed, congestion collapse would become the exception rather than the rule. Thus congestion control involves finding places that violate conservation and fixing them. By 'conservation of packets' I mean that for a connection 'in equilibrium', i.e., running stably with a full window of data in transit, the packet flow is what a physicist would call 'conservative': A new packet isn't put into the network until an old packet leaves. The physics of flow predicts that systems with this property should be robust in the face of congestion. Observation of the Internet suggests that it was not particularly robust. Why the discrepancy? There are only three ways for packet conservation to fail: The connection doesn't get to equilibrium, or A sender injects a new packet before an old packet has exited, or The equilibrium can't be reached because of resource limits along the path. In the following sections, we treat each of these in turn.
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paper_title: Simulation-based comparisons of Tahoe, Reno and SACK TCP

paper_content:

This paper uses simulations to explore the benefits of adding selective acknowledgments (SACK) and selective repeat to TCP. We compare Tahoe and Reno TCP, the two most common reference implementations for TCP, with two modified versions of Reno TCP. The first version is New-Reno TCP, a modified version of TCP without SACK that avoids some of Reno TCP's performance problems when multiple packets are dropped from a window of data. The second version is SACK TCP, a conservative extension of Reno TCP modified to use the SACK option being proposed in the Internet Engineering Task Force (IETF). We describe the congestion control algorithms in our simulated implementation of SACK TCP and show that while selective acknowledgments are not required to solve Reno TCP's performance problems when multiple packets are dropped, the absence of selective acknowledgments does impose limits to TCP's ultimate performance. In particular, we show that without selective acknowledgments, TCP implementations are constrained to either retransmit at most one dropped packet per round-trip time, or to retransmit packets that might have already been successfully delivered.
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paper_title: Improving the start-up behavior of a congestion control scheme for TCP

paper_content:

Based on experiments conducted in a network simulator and over real networks, this paper proposes changes to the congestion control scheme in current TCP implementations to improve its behavior during the start-up period of a TCP connection.The scheme, which includes Slow-start, Fast Retransmit, and Fast Recovery algorithms, uses acknowledgments from a receiver to dynamically calculate reasonable operating values for a sender's TCP parameters governing when and how much a sender can pump into the network. During the start-up period, because a TCP sender starts with default parameters, it often ends up sending too many packets and too fast, leading to multiple losses of packets from the same window. This paper shows that recovery from losses during this start-up period is often unnecessarily time-consuming.In particular, using the current Fast Retransmit algorithm, when multiple packets in the same window are lost, only one of the packet losses may be recovered by each Fast Retransmit; the rest are often recovered by Slow-start after a usually lengthy retransmission timeout. Thus, this paper proposes changes to the Fast Retransmit algorithm so that it can quickly recover from multiple packet losses without waiting unnecessarily for the timeout. These changes, tested in the simulator and on the real networks, show significant performance improvements, especially for short TCP transfers. The paper also proposes other changes to help minimize the number of packets lost during the start-up period.
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Title: A Survey of TCP over Ad Hoc Networks

Section 1: TCP's Challenges in Ad Hoc Networks
Description 1: Discuss the various challenges TCP faces in ad hoc networks, including lossy channels, hidden and exposed stations, path asymmetry, network partition, routing failures, and power constraints.

Section 2: TCP Performance Over Ad Hoc Networks
Description 2: Report on the studies of TCP performance over mobile ad hoc networks (MANETs) and static ad hoc networks (SANETs), summarizing major problems.

Section 3: Proposals to Improve TCP Performance in Ad Hoc Networks
Description 3: Present various proposals made in literature to improve TCP performance in ad hoc networks, grouped into four sets based on the identified problems.

Section 4: Proposals to Distinguish Between Losses Due to Route Failures and Congestion
Description 4: Discuss proposals to address TCP's inability to distinguish between losses due to route failures and those due to network congestion, including cross layer and TCP layer proposals.

Section 5: Proposals to Reduce Route Failures
Description 5: Explore proposals that aim to reduce frequent route failures in MANETs, including TCP and network cross layer, network and physical cross layer, and network layer proposals.

Section 6: Proposals to Reduce Wireless Channel Contention
Description 6: Review proposals addressing wireless channel contention in SANETs, categorized into TCP layer, network layer, and link layer proposals.

Section 7: Proposals to Improve TCP Fairness
Description 7: Examine proposals aimed at improving TCP fairness in SANETs, classified under link layer proposals.

Section 8: General Comparison and Discussion
Description 8: Compare different proposals aimed at improving TCP performance in MANETs, considering complexity, network type, TCP connection load and type, and evaluation basis.

Section 9: Conclusion
Description 9: Summarize the state-of-the-art TCP over ad hoc networks, highlighting major challenges and proposed solutions, and propose future research directions to further address TCP issues in dynamic ad hoc environments.