We will review some of the major results in random graphs and some of the more challenging open problems. We will cover algorithmic and structural questions. We will touch on newer models, including those related to the WWW.

Random graphs

Intermittently connected mobile networks are sparse wireless networks where most of the time there does not exist a complete path from the source to the destination. These networks fall into the general category of Delay Tolerant Networks. There are many real networks that follow this paradigm, for example, wildlife tracking sensor networks, military networks, inter-planetary networks, etc. In this context, conventional routing schemes would fail.To deal with such networks researchers have suggested to use flooding-based routing schemes. While flooding-based schemes have a high probability of delivery, they waste a lot of energy and suffer from severe contention, which can significantly degrade their performance. Furthermore, proposed efforts to significantly reduce the overhead of flooding-based schemes have often be plagued by large delays. With this in mind, we introduce a new routing scheme, called Spray and Wait, that "sprays" a number of copies into the network, and then "waits" till one of these nodes meets the destination.Using theory and simulations we show that Spray and Wait outperforms all existing schemes with respect to both average message delivery delay and number of transmissions per message delivered; its overall performance is close to the optimal scheme. Furthermore, it is highly scalable retaining good performance under a large range of scenarios, unlike other schemes. Finally, it is simple to implement and to optimize in order to achieve given performance goals in practice.

/pdf/spray-and-wait-an-efficient-routing-scheme-for-50seq3wqos.pdf

Spray and wait: an efficient routing scheme for intermittently connected mobile networks

Wireless vehicular communication has the potential to enable a host of new applications, the most important of which are a class of safety applications that can prevent collisions and save thousands of lives. The automotive industry is working to develop the dedicated short-range communication (DSRC) technology, for use in vehicle-to-vehicle and vehicle-to-roadside communication. The effectiveness of this technology is highly dependent on cooperative standards for interoperability. This paper explains the content and status of the DSRC standards being developed for deployment in the United States. Included in the discussion are the IEEE 802.11p amendment for wireless access in vehicular environments (WAVE), the IEEE 1609.2, 1609.3, and 1609.4 standards for Security, Network Services and Multi-Channel Operation, the SAE J2735 Message Set Dictionary, and the emerging SAE J2945.1 Communication Minimum Performance Requirements standard. The paper shows how these standards fit together to provide a comprehensive solution for DSRC. Most of the key standards are either recently published or expected to be completed in the coming year. A reader will gain a thorough understanding of DSRC technology for vehicular communication, including insights into why specific technical solutions are being adopted, and key challenges remaining for successful DSRC deployment. The U.S. Department of Transportation is planning to decide in 2013 whether to require DSRC equipment in new vehicles.

Dedicated Short-Range Communications (DSRC) Standards in the United States

In this paper, we define and explore proofs of retrievability (PORs). A POR scheme enables an archive or back-up service (prover) to produce a concise proof that a user (verifier) can retrieve a target file F, that is, that the archive retains and reliably transmits file data sufficient for the user to recover F in its entirety.A POR may be viewed as a kind of cryptographic proof of knowledge (POK), but one specially designed to handle a large file (or bitstring) F. We explore POR protocols here in which the communication costs, number of memory accesses for the prover, and storage requirements of the user (verifier) are small parameters essentially independent of the length of F. In addition to proposing new, practical POR constructions, we explore implementation considerations and optimizations that bear on previously explored, related schemes.In a POR, unlike a POK, neither the prover nor the verifier need actually have knowledge of F. PORs give rise to a new and unusual security definition whose formulation is another contribution of our work.We view PORs as an important tool for semi-trusted online archives. Existing cryptographic techniques help users ensure the privacy and integrity of files they retrieve. It is also natural, however, for users to want to verify that archives do not delete or modify files prior to retrieval. The goal of a POR is to accomplish these checks without users having to download the files themselves. A POR can also provide quality-of-service guarantees, i.e., show that a file is retrievable within a certain time bound.

PORs: Proofs of Retrievability for Large Files

Vehicular networks are very likely to be deployed in the coming years and thus become the most relevant form of mobile ad hoc networks. In this paper, we address the security of these networks. We provide a detailed threat analysis and devise an appropriate security architecture. We also describe some major design decisions still to be made, which in some cases have more than mere technical implications. We provide a set of security protocols, we show that they protect privacy and we analyze their robustness and efficiency.

/pdf/securing-vehicular-ad-hoc-networks-4sgegi8hgk.pdf

Securing vehicular ad hoc networks

Permissionless blockchains reach decentralized consensus without requiring pre-established identities or trusted third parties, thus enabling applications such as cryptocurrencies and smart contracts Consensus is agreed on data that is generated by the application and transmitted by the system’s (peer-to-peer) network layer While many attacks on the network layer were discussed so far, there is no systematic approach that brings together known attacks, the requirements, and the design space of the network layer In this paper, we survey attacks on the network layer of permissionless blockchains, and derive five requirements: 1) performance; 2) low cost of participation; 3) anonymity; 4) DoS resistance; and 5) topology hiding Furthermore, we survey the design space of the network layer and qualitatively show the effect of each design decisions on the fulfillment of the requirements Finally, we pick two aspects of the design space, in-band peer discovery and relay delay, and demonstrate possible directions of future research by quantitatively analyzing and optimizing simplified scenarios We show that while most design decisions imply certain tradeoffs, there is a lack of models that analyze and formalize these tradeoffs Such models could aid the design of the network layer of permissionless blockchains One reason for the lack of models is the deliberately limited observability of deployed blockchains We emphasize that simulation based approaches cope with these limitations and are suited for the analysis of the network layer of permissionless blockchains

/pdf/network-layer-aspects-of-permissionless-blockchains-2vqqg3lr5o.pdf

Network Layer Aspects of Permissionless Blockchains

Car-to-car communication by means of wireless technology shows a strong potential to enhance both safety and comfort of road users. The necessity to reach cars beyond the transmission range of the own radio creates the challenge to find algorithms to efficiently forward packets. In these so-called vehicular ad hoc networks, position information has been identified as a crucial component that alleviates some of the limitations of existing topology-based protocols. Thus, we see the use of positional information at the core of a car-to-car communication system that provides advanced applications for active safety, distributed floating car data, as well as user communication and information. A major thrust to leverage a car-to-car communication platform was provided by the FleetNet project, partly funded by the German Ministry of Education and Research BMB+F and led by DaimlerChrysler AG. Based on ad hoc networking principles and the availability of position information, the project developed suitable communication concepts. In addition to simulation studies, a prototype communication and application system was implemented and deployed, embedding safety and convenience applications. With car-based real world field trials successfully conducted, first results and experiences look promising.

FleetNet: Bringing Car-to-Car Communication into the Real World

In wireless one-hop broadcast communications, each node broadcasts messages to inform all neighbors within an intended communication range. Clearly, the packet transmission of the various nodes might interfere with each other due to the overlapping communication ranges. IEEE 802.11 Distributed Coordination Function (DCF) provides some basic mechanisms for coordination of these transmissions. However, due to its distributed nature, DCF suffers from coordination failures, e.g., due to the very well-known hidden terminal problem. When considering realistic radio propagation phenomena to various degrees, these 'incoordination problems' can drastically increase. On the other hand, transceivers with packet capturing capability might be able to deal with some of the uncoordinated packets. In order to understand the effects of different radio propagation environments on packet level incoordination, we provide a detailed simulation study measuring six transmission success/failure categories and five performance metrics. We report quantitative results w.r.t. probability of incoordination and the effects of power-setting and packet retransmission strategies.

IEEE 802.11-based one-hop broadcast communications: understanding transmission success and failure under different radio propagation environments

Communication between vehicles is likely to be one key area where mobile ad-hoc networks will be used in the near future. Currently there are several projects ([1, 3]) that investigate this application area, while car manufacturers and their suppliers aim at the development of products within the next few years. This raises the question what kind of routing algorithm is suited for communication between vehicles. Existing performance studies for various ad-hoc routing protocols do not use movement patterns which resemble the movement of cars and are thus not well suited to answer this question. We have conducted an extensive simulation study based on realistic vehicle movement patterns. Our main aim was to investigate how a topology-based approach compares to a location-based routing scheme when applied to vehicular networks. As representatives we have chosen DSR [4] as a topology-based approach and GPSR [5] complemented with a simple location service as a location-based scheme. For a general discussion of topology-based and location-based ad-hoc routing please refer to [2] and [6]. Vehicular ad-hoc networks have several unique characteristics. The main challenge is the high speed with which nodes move in respect to each other. If oncoming traffic is included in the forwarding of packets, then relative speeds of 250 km/h to 300 km/h are common. This implies a very high rate of topology changes. Also it is not clear if one single partition spans sufficient distance to enable meaningful applications such as emergency warnings or vehicle-to-vehicle entertainment. On the other hand it is safe to assume that battery power is not an issue for vehicular ad-hoc networks. Location-based approaches further benefit from the fact that vehicles are aware of their geographical positions through the use of on-board navigation systems.

Location-based routing for vehicular ad hoc networks

Optimizing vehicular communication strategies is important for an efficient usage of the available wireless bandwidth and also critical for the success of VANETs. In this paper we address the fundamental and practical question whether the load on the wireless channel can be reduced if periodic beacon messages are transmitted over multiple hops with reduced transmit power instead of being transmitted over one hop with high transmit power. In particular, we look at the possible bandwidth savings that can be achieved by piggybacking forwarded messages into the own next beacon transmission. For that matter, we first propose an analytical model to compute a lower bound for the resulting channel load when single- or multi-hop dissemination of beacons is performed. In this model we assume optimal channel conditions and perfect relaying and piggybacking decisions to show that a reduction of the load by multi-hop is possible and closely related to piggybacking. Further, we show that the possible savings depend on the ratio between the size of the header and the payload of a beacon and that a reduction of the load is theoretically possible if the header is larger than the payload - what would be the case in VANETs if security overheads are considered part of the header. We then perform a simulative comparison of single- and multi-hop beaconing to evaluate the impact of effects such as packet collisions and channel fading. We show that the possible savings of multi-hop beaconing are difficult to exploit under non-perfect channel conditions and suboptimal relaying decisions.

Hannes Hartenstein

Papers

Network Layer Aspects of Permissionless Blockchains

FleetNet: Bringing Car-to-Car Communication into the Real World

IEEE 802.11-based one-hop broadcast communications: understanding transmission success and failure under different radio propagation environments

Location-based routing for vehicular ad hoc networks

A comparison of single- and multi-hop beaconing in VANETs