Reliable Broadcast Protocols for Mobile Ad Hoc Networks with Liveness Property

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1 Poznań University of Technology Institute of Computing Science Michał Kalewski Reliable Broadcast Protocols for Mobile Ad Hoc Networks with Liveness Property Doctoral Dissertation Submitted to the Council of the Faculty of Computing Science of Poznań University of Technology Supervisor: Ph. D., Dr. Habil., Professor Jerzy Brzeziński Poznań 2014

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3 Politechnika Poznańska Instytut Informatyki Michał Kalewski Niezawodne protokoły rozgłaszania dla mobilnych sieci ad hoc z właściwością żywotności Rozprawa doktorska Przedłożono Radzie Wydziału Informatyki Politechniki Poznańskiej Promotor: prof. dr hab. inż. Jerzy Brzeziński Poznań 2014

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5 There s a Legion that never was listed, That carries no colours or crest, But, split in a thousand detachments, Is breaking the road for the rest. Rudyard Kipling The Lost Legion (1895) To Patrycja, Wiktoria, and Jakub

6 A dissertation submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Computing Science. Michał Kalewski Distributed Systems Group Institute of Computing Science Poznań University of Technology Typeset by the author in L A TEX. Copyright 2014 by Michał Kalewski This dissertation and associated materials can be downloaded from: Institute of Computing Science Poznań University of Technology Piotrowo 2, Poznań, Poland The research presented in this dissertation was partially supported by the European Union in the scope of the European Regional Development Fund program no. POIG /08. The use in this dissertation of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights.

7 Acknowledgements I would like to first express my special gratitude and thanks to my supervisor, Prof. Jerzy Brzeziński, in recognition and appreciation of his kind support, enduring commitment, and continued guidance. I am sure that everyone reading this thesis will benefit from his constant assistance towards making the reasoning more apparent and unambiguous, and the text more clear and comprehensive. I am also very grateful to all the members of the Distributed Systems Group, whose cooperation, during our seminars and meetings, went way beyond the call of duty. Amongst them, I would like to direct my particular thanks to Dr. Michał Sajkowski for his friendly attitude, continued encouragement, and all his advice. My special thanks go to my favourite distributed system my family, for their continuous support and motivation, for their faith in my abilities and my work. Finally, the most important everyday help, which I received, was from my wife Patrycja, who made my work and writing possible. For your patience, tolerance and understanding, help and support, and countless other big and small things, thank you, Patrycja.

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9 Abstract Mobile ad hoc networks are composed of autonomous and mobile hosts which communicate through wireless links without any additional network infrastructure or central points. Each pair of such hosts, between whom the distance is less than their transmission range, can communicate directly with each other. In mobile ad hoc networks, hosts are free to move and organise themselves arbitrary, so the resulting network topology may change rapidly and can get partitioned and reconnected unpredictably. One of the fundamental communication operations in mobile ad hoc networks is broadcasting a process of sending a message from one host to all hosts in a network. Since broadcast is also a basic communication requirement to construct other and more complex distributed algorithms, it is important for any broadcast protocol to provide some message delivery guarantees, especially if host failures are taken into account. In the case of mobile and dynamic environments, however, this can be hard or even impossible to achieve. On the other hand, if it can be assumed that a group of collaborating hosts in a mobile ad hoc network can be partitioned and that partitions heal eventually, it is then possible to develop broadcast protocols with deterministic guarantees, even if hosts are unreliable. Informally, the assumption that any partition is not allowed to be permanently isolated is called the network liveness property. The broadcast protocols for mobile ad hoc networks with the network liveness property proposed till now guarantee, unconventionally, that only at least an arbitrary majority of operative hosts (i.e. hosts that have not failed) receives each broadcast message. This means that the guarantees of these protocols are different from those of uniform reliable, regular reliable, and best-effort reliable broadcasts. Moreover, in all these protocols, it is assumed that the minimum time of direct connectivity between any neighbouring hosts is much longer than maximum message transmission time. This assumption covers, however, the dependence of the required minimum time of direct communications, and hence of the correctness of these proto-

10 x Abstract cols, on some system parameters. In the context of the above observations, in this dissertation we propose novel reliable broadcast protocols for mobile ad hoc networks with the network liveness property. Towards the end, we first explicitly define a formal model of mobile ad hoc networks and the network liveness property. Additionally, we also present some results on the estimation of select parameters of the property, determined experimentally by simulation of several entity and group mobility models. Next, we scrutinise the broadcast protocols that have so far been proposed for mobile ad hoc networks with the network liveness property, thoroughly analyse their time constraints, and prove that the minimum time of direct connectivity, required by the protocols, depends on the total number of hosts in a network and on the total number of messages that can be disseminated by each host concurrently. Based on this observation, we then propose time-adjusted versions of the protocols by eliminating the dependencies of their time constraints. Since the guarantees of these protocols are unconventionally defined in terms of operative hosts, we thereafter design a set of new uniform reliable and regular reliable broadcast protocols, and discuss the implementation of best-effort reliable broadcast. For the proposed protocols, we show that their time constraints are independent of system parameters, prove analytically their correctness, and evaluate experimentally their performance in simulation tests. Finally, we describe Service-Oriented Ad Hoc System, a system that allows its users to create, publish, and access web services in mobile ad hoc networks, which was created to serve as a proof of the concept of the introduced broadcast solutions.

11 Contents 1 Introduction 1 2 System Model and Foundational Concepts Connectivity graph Processing abstractions Node failures Direct connectivities Network liveness property Mobility models Entity models Group models Distribution of the connectivity time interval Reliable broadcast abstractions Best-effort reliable broadcast Regular reliable broadcast Uniform reliable broadcast Safety and progress Crash-Tolerant Broadcast Protocols Crash-tolerant broadcast Proactive Dissemination Protocol Reactive Dissemination Protocol Proactive Knowledge and Reactive Message Optimised PKRM Analysis of time constraints of the crash-tolerant broadcast protocols Related work Concluding remarks

12 xii Contents 4 Time-Adjusted Crash-Tolerant Broadcast Protocols Time-Adjusted Proactive Dissemination Protocol Time constraints of the TAPDP protocol Correctness of the TAPDP protocol Protocol modifications and improvements Time-Adjusted Reactive Dissemination Protocol Time-Adjusted Proactive Knowledge and Reactive Message Time-Adjusted Uniform Reliable Broadcast Protocols Time-Adjusted Proactive Uniform Reliable Broadcast Protocol Time constraints of the TAPURBP protocol Correctness of the TAPURBP protocol Protocol modifications and improvements Time-Adjusted Reactive Uniform Reliable Broadcast Protocol Time-Adjusted PKRM Uniform Reliable Broadcast Protocol Time-Adjusted Regular Reliable Broadcast Protocols Time-Adjusted Proactive Regular Reliable Broadcast Protocol Correctness of the TAPRRBP protocol Protocol modifications and improvements A note on the implementation of best-effort reliable broadcast Performance Evaluation of the Proposed Protocols The simulator Simulation parameters Performance measures and simulation tests Simulation results T T m and T T pi measures U Rpi and M 0 p i ratios Concluding remarks Service-Oriented Ad Hoc System System overview Replication Modes of communication System architecture Service interface Exemplary applications Conclusions 107 STRESZCZENIE (Extended Abstract in Polish) 113 REFERENCES 131

13 List of Symbols C L The preceding relation according to sequence counters and node identifiers The preceding relation according to Lamport logical clocks and node identifiers e a, b An event named e consisting of two attributes a and b f l m p i α β δ B The assumed maximum number of faulty nodes The concurrent dissemination limit A broadcast message The i-th node The threshold of transmission suppression The time of periodic transmissions The maximum message transmission time between neighbouring nodes The application-specified parameter of direct connectivity time D i I K i (m) KK i (m) KR i (m) L M The delivery vector of node p i The network liveness property connectivity time interval The knowledge vector of node p i for message m The knowledge vector on the propagation knowledge of message m of node p i The knowledge vector on the realisation of message m of node p i The total number of messages that can be disseminated concurrently in an ad hoc network A number of additional messages sent periodically

14 xiv List of Symbols N R i C i C(m) L i L(m) The total number of nodes in an ad hoc network The realisation vector of node p i The sequence counter of node p i A sequence counter value associated with m by its originator Lamport logical clock of node p i Lamport logical clock value associated with m by its originator A i E E The knowledge array of node p i A set of links between neighbouring nodes A product set of V Γ (E ) The set of all subsets (power set) of E E i G O P P Realised i Received i Selected i Unrealised i V The dynamic set of node p i The connectivity graph The set of all operative nodes at some time t A non-empty subset of O A complementary set of P in O The set of realised messages of node p i The set of messages received by node p i The set of messages considered for transmission of node p i The set of unrealised messages of node p i The set of all nodes in an ad hoc network

15 1 Introduction Ad hoc (Latin, for this) [adverb]: for the particular end or case at hand without consideration of wider application; [adjective]: concerned with a particular end or purpose; formed or used for specific or immediate problems or needs; fashioned from whatever is immediately available. Merriam-Webster Dictionary In 1964, the famous British science fiction writer and inventor Arthur C. Clarke was asked to ponder the hardest question: to predict the future. In his answer, aired on the BBC s Horizon programme 1, he foresaw that in the future: We could be in instant contact with each other, wherever we may be, where we can contact our friends anywhere on earth, even if we don t know their actual physical location. It will be possible in that age, perhaps only 50 years from now, for a man to conduct his business from Tahiti or Bali just as well as he could from London. In fact, if it proves worthwhile, almost any executive skill, any administrative skill, even any physical skill, could be made independent of distance. I am perfectly serious when I suggest that one day we may have brain surgeons in Edinburgh operating on patients in New Zealand. Half a century later, after an enormous progress in the field of computing and communication technologies and their applications, this vision may seem to be impressively accurate as it is being realised before our very eyes. The greatest manifestation of all these achievements, which enables us to further develop new communication abilities, is the Internet. Nowadays, the Internet technology is (almost) everywhere, it is (almost) always accessible, and it is always on. In fact, we now have entered an era, in which users seek to be provided with trouble-free Internet access from any device, in any place, and at any time. Consequently, as observed by Kleinrock ([Kle03]), emerging communication technologies are heading to form a new invisible global infrastructure, which is formed by the three areas of: nomadicity, embeddedness, and ubiquity. Following Kleinrock, nomadicity (or nomadic computing and communication) is the system support needed to provide a rich set of computing and communication capabili- 1

16 2 1 Introduction ties and services to mobile users, or nomads, as they move from place to place in a way that is transparent, integrated, convenient, and adaptive. Embeddedness (or embedded technology) is made of small intelligent devices embedded in the physical world and interconnected with each other or connected to the Internet. Whereas ubiquity (or ubiquitous access) means Internet service availability wherever the nomads travel on a global basis. One of the new communication technologies that may support and merge nomadicity and embeddedness, extend ubiquity, and provide communication in places where no infrastructure is available are mobile ad hoc networks. Mobile ad hoc networks Mobile ad hoc networks (abbreviated to MANETs) are composed of autonomous and mobile hosts (or communication devices), which communicate through wireless links without any additional network infrastructure or central points ([Per01, Agg04, MM04, BKL06, TF06, BK07, Bou08, MWM09]). The distance from a transmitting host at which its wireless signal strength remains above the minimal usable level is called transmission (or wireless) range of that host. Therefore, each pair of such devices, between whom the distance is less than their transmission range, is referred to as neighbouring hosts and the hosts are able to communicate directly with each other. In mobile ad hoc networks, hosts are free to move and organise themselves arbitrary, so the resulting network topology may change rapidly and can get partitioned (i.e. disconnected into isolated subnets) and reconnected unpredictably. The absence of any centralised coordination is the reason why network operations in MANETs are performed in a distributed, that is decentralised, manner. Similar descriptions of ad hoc networks are in common use in the literature with only minor differences between them. For instance, Stojmenović and Wu point out in [SW04] that ad hoc networks may also contain static devices (e.g. embedded sensors); Gerla et al., in [GLR05], emphasise that an ad hoc network is often built to support a specific application, and thus, the networking is application-driven ; and Gačnik, characterising MANETs in [Gač04], indicates additionally that hosts in such networks may have limited resources, like battery power supply, and security, because of the wireless communication, which is more vulnerable to security issues than wired connections. At last, the Institute of Electrical and Electronics Engineers (IEEE), in the ANSI/IEEE standard on Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications ([ANS99]), defines MANET as: A network composed solely of stations within mutual communication range of each other via the wireless medium. An ad hoc network is typically created in a spontaneous manner. The principal distinguishing characteristic of an ad hoc network is its limited temporal and spatial extent. These limitations allow the act of creating and dissolving the ad hoc network to be sufficiently straightforward and convenient so as to be achievable by non-technical users of the network facilities; i.e., no specialized technical skills are required and little or no investment of time or additional resources is required beyond the stations that are to participate in the ad hoc network.

17 1 Introduction 3 Figure 1.1: Illustration of the hidden and exposed host problems. At present, there are many different wireless technologies which allow professionals as well as laymen for a rapid and simple establishment of mobile networks. Most of them use the Industrial, Scientific, and Medical (ISM) bands that are free from licensing formalities. Most notably, the technologies include the highly popular IEEE and Bluetooth 3 networks, which both contain direct provisions for ad hoc networking. Furthermore, we currently observe the emergence not only of versatile smartphones and tablets equipped with modern wireless communication interfaces, but also open-source hardware development platforms that support construction of mobile devices interconnected by wireless links, such as Arduino 4, BeagleBoard 5, Raspberry Pi 6, Xbee 7, or ZigBee 8, to name only the most notable ones. All these successful technologies provide an opportunity for ad hoc networking and bring the idea of the invisible global infrastructure closer. The same nature of the wireless communication, which makes mobile networks possible, causes also unique issues like the hidden-host and exposed-host problems, meaning that the popular Carrier Sense Multiple Access (CSMA) scheme ([KT75a]) and its variants such as CSMA with Collision Detection (CSMA/CD), developed for wired networks, cannot be directly used in ad hoc networks at the MAC layer. To explain both problems let us refer to Figure 1.1, where host B is within the transmission range of hosts A and C, but hosts A and C are not in each other s transmission ranges. Let us suppose that host A is transmitting data to host B. Host C, being out of the transmission range of host A, cannot

18 4 1 Introduction detect the carrier, and hence, may start sending data to host B, causing a collision at B. This is referred to as the hidden-host (or hidden-terminal) problem ([KT75b]), because hosts A and C are hidden from each other. Let us now suppose another case where host B is transmitting data to host A. Since host C is within the transmission range of host B, it senses the carrier and defers its own transmissions. However, this is unnecessary because transmissions of host C cannot cause collisions at A. This is referred to as the exposed-host (or exposed-terminal) problem ([PKD01]), as host B being exposed to host C caused the latter to needlessly defer its transmissions. In order to solve the problems of the hidden and exposed host, researchers have come up with many protocols, which are contention based, but involve some forms of dynamic channel reservation prior to data transmission or collision resolution techniques (the protocols are surveyed, among others, in [KRD06]). For instance, the ANSI/IEEE standard specifies two different modes of the MAC protocol: Distributed Coordination Function (DCF) as its fundamental method for use also within ad hoc networks, and Point Coordination Function (PCF) as an optional method for use only within centrally coordinated infrastructure-based networks ([ANS99, Che94]). The DCF mode is a combination of the CSMA and Multiple Access Collision Avoidance (MACA) protocol, which has been proposed by Karn in [Kar90] and modified later by Bharghavan et al. in [BDSZ94]. In the DCF mode, three short signalling packets are used: RT S (or Request-To- Send), CT S (or Clear-To-Send), and ACK (or Acknowledgement). In Figure 1.1, if host A needs to transmit data to host B, it first sends a RT S packet to that host. Such packets, however, are also overheard by all other hosts within the sender s transmission range. When host B receives this packet, it responses with a CT S packet to host A. Again, such packets are also overheard by all other hosts within the sender s transmission range, i.e. by host C in Figure 1.1. Next, when host A receives the CT S packet, it immediately commences transmission of the actual data to host B, and on receiving the data, host B sends an ACK packet to host A, which is overheard by host C. Additionally, the protocol applies the concept of virtual carrier sensing implemented in the form of a Network Allocation Vector (NAV), which is maintained by every host. The vector contains a time value that represents the duration up to which the wireless medium is expected to be occupied by transmissions of other hosts. Since every packet contains the duration information for the remainder of the message, each host overhearing a packet continuously updates its own NAV. The key idea of the protocol is that any neighbouring host that overhears a RT S or CT S packet has to defer its own transmissions for the length of the expected data transmission. So, in the hidden-host scenario as in Figure 1.1, host C will not overhear the RT S packet sent by host A, but it would overhear the CT S and ACK packets sent by host B. Thus, host C will defer its transmissions during the communication between hosts A and B. Similarly, in the exposed-host scenario, host C would overhear the RT S packet sent by host B, but not the CT S sent by host A, and hence, will consider the medium to be free for its own transmissions. Usually, medium access protocols, as these mentioned above, are designed to provide delivery of unicast data, i.e. they involve the procedures necessary to transfer data between two interconnected hosts in the network. But if there is a need to transmit data to more than one receiver in an ad hoc network, then MAC protocols that transfer data packets to all neighbouring hosts may instead be used to improve the overall

19 1 Introduction 5 efficiency of the communication. Several such protocols have so far been proposed, e.g., in [TG01, SL05, KK07, LYXX11], and most of them utilise similar RT S, CT S, and ACK control packets, but let the transmitted data to be received by all hosts neighbouring to the sender. For example, the general idea of the Broadcast Medium Window (BMW) protocol, proposed in [TG01], is to transmit each data packet analogously as it is done in DCF, but to all neighbours in a round robin fashion. Despite the additional handicaps at the MAC layer, and due to their unique features, mobile ad hoc networks have vast practical and commercial potential, which has already been recognised and found many applications that range from military systems to sensor networks. Historically, MANETs have primarily been used for tactical network related applications to improve battlefield communications and survivability ([CCL03]). In military, decentralised and mobile networks, which can be set up anywhere, at any time, and without the complexities of infrastructure assembling, are an operative advantage or even a necessity, and thus the emergence of such systems, which may be exemplified by the BOWMAN family of tactical radios 9 used by British Army, or Radio over Internet Protocol Routed Network (RIPRNet), a United States military tactical network ([TTO10]). Another area of applications that has been receiving a lot of attention are search and rescue missions and emergency services, where MANETs can help overcome network impairment. As firefighter and police squads sometimes have to operate in places where no infrastructure is present, e.g. after natural disasters, and operations still need to be coordinated, then with the use of ad hoc networks, mobile units may build up emergency information structures. Examples of such systems are the crisis communication, security, and critical operations applications developed by the AtHoc company 10, or BRICK, a platform for communicating information during a crisis developed by the Ushahidi company 11. Arguably much less complicated, smaller in size, but perhaps the prototypical application requiring the establishment of an ad hoc network is mobile conferencing, when users gather outside their normal office environment, and where they may lack network infrastructure ([Per01]). Personal Area Networks (PANs) are used in an even smaller range, as they are intended to operate in a personal environment, interconnecting electronic devices for everyday use. The most popular and wide-spread technology for this purpose is Bluetooth, which enables users to interconnect devices into so-called piconets ([Blu13]). Recently, Bluetooth technology has a strong competitor in the form of Wi-Fi Direct 12 (or Wi-Fi Peer-to-Peer) networks, i.e. IEEE ad hoc networks with an enhanced security level and additional features designed for easy and convenient establishment of connections between mobile phones, cameras, printers, gaming consoles, and other devices to transfer content and share applications. The other uses of MANETs include multimedia applications and home networking ([Gač04]), Internet connections sharing in places that lack infrastructure ([BABM05]), wireless sensor networks that consist of spatially distributed autonomous sensors to monitor physical or environmental conditions ([APM05]), and vehicular ad hoc networks that use cars as mobile hosts, and which become so popular that gained widely recognised VANETs acronym ([ZLESB13]). A partial list of wireless network commercial products can be

20 6 1 Introduction found, for instance, in [Agg04]. Given the fact that mobile devices outsold in 2011 personal computers for the first time 13, and that most of them, if not all, are equipped with wireless interfaces, the question that arises is what other useful applications can be created having at our disposal such a large potential that is reaching the kind of audience it used to take technologies decades to reach. Responding to this question, in its most basic terms, should, therefore, be regarded as the responsibility of scientists and researchers. Reliable broadcast Broadcast is the process of sending a message from one host to all hosts in a network, and, to quote Lipman et al. from their chapter on broadcast protocols in [MWM09], it is a fundamental operation for communication in ad hoc networks as it can be used for updating network information or for discovering and maintaining routes to support other multi-hop communication primitives. Since broadcast is also a basic communication requirement to construct other and more complex distributed algorithms, like consensus or coherency protocols ([PSSK10]), it is important for any broadcast protocol to provide some message delivery guarantees. Therefore, as in almost any other network or distributed protocol, a key issue consists here in finding concepts and solutions that are resilient enough to allow reducing, or even eliminating, the underlying uncertainty. This uncertainty may be created by asynchrony, unstable behaviours, low computing capability, or scalability requirements ([BBRTP07]). But in mobile ad hoc networks, this uncertainty is mainly caused by highly dynamic network topologies and limited resources, like power supply, which may lead to host failures and temporal or even permanent disconnections resulting in the same effects as failures a permanently disconnected host cannot take part in the processing just as a faulty one. For that reason, providing strong, i.e. deterministic, delivery guarantees in such changeable environments can be hard or even impossible to achieve ([GL02, LS03]). As a result, looking across all the research, one finds that heuristic broadcast protocols with only probabilistic guarantees have been mostly proposed for use in MANETs. The major techniques in this regard comprise gossip (or epidemic) protocols that disseminate messages in a manner similar to the spread of rumours or a virus in a biological community ([CRB01, LEH03]), protocols based on clustering algorithms that constantly group hosts into interconnected clusters with a selected representative host in each of them ([LG97, LW02]), and protocols that resort to other forms of host arrangements like connected dominating sets, where any receiving host either belongs to the set or is directly connected with some host from the set ([SSZ02, Wu03]), or spanning trees, where receiving hosts are connected without cycles ([BJ02, JM05]). For all such protocols, the flooding protocol, where each host resends each broadcast message only once, is a benchmark for various aspects of efficiency and performance analysis, and a comprehensive survey of these kind of broadcast solutions for mobile ad hoc networks is presented, i.a., in [MWM09]. On the other hand, if it can be assumed that a group of collaborating hosts in an ad hoc network can be partitioned and that partitions heal eventually, it is then possible to develop broadcast protocols with deterministic guarantees, even if hosts are unreliable. 13

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