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

The Lightning Network is a payment channel network built on top of the cryptocurrency Bitcoin. It allows Bitcoin to scale by performing transactions off-chain to reduce load on the blockchain. Malicious payment channel participants can try to commit fraud by closing channels with outdated balances. The Lightning Network allows resolving this dispute on the blockchain. However, this mechanism forces the channels’ participants to watch the blockchain in regular intervals. It has been proposed to offload this monitoring duty to a third party, called a watchtower. However, existing approaches for watchtowers do not scale as they have storage requirements linear in the number of updates in a channel. In this work, we propose TEE Guard, a new architecture for watchtowers that leverages the features of Trusted Execution Environments to build watchtowers that require only constant memory and are thus able to scale. We show that TEE Guard is deployable because it can run with the existing Bitcoin and Lightning Network protocols. We also show that it is economically viable for a third party to provide watchtower services. As a watchtower needs to be trusted to be watching the blockchain, we also introduce a mechanism that allows customers to verify that a watchtower has been running continuously.

/pdf/tee-based-distributed-watchtowers-for-fraud-protection-in-4ur0jkzm7j.pdf

TEE-Based Distributed Watchtowers for Fraud Protection in the Lightning Network

In distributed IT systems, replication of information is commonly used to strengthen the fault tolerance on a technical level or the autonomy of an organization on a business level. In particular, information related to the identity of a user, which is used to authorize service access, is often replicated for these reasons. To ensure correct authorization decisions, replicas have to be kept consistent. However, an appropriate definition of “consistency” is required that takes into account the need for the following aspects: (i) semantic and causal relations between identity information, and (ii) temporal aspects with respect to an acceptable duration of the dissemination of occurring attribute changes. Both identity-information specifics and temporal aspects are not addressed sufficiently by existing consistency models. In this paper we introduce a consistency model for identity information in distributed systems named ID-consistency. ID-consistency is based on a formalization of identity information and considers semantic and causal relations as well as a so-called inconsistency window that denotes the time period between a change to information and the moment when the change is fully disseminated. Therefore, the model reveals the fundamental structure of an IdM system and helps in the design and analysis of corresponding dissemination middleware in distributed systems. We exemplarily show how to make use of the concept of ID-consistency to analyze and improve a real-world IdM system using CardSpace for demonstration purposes.

A Consistency Model for Identity Information in Distributed Systems

Vehicle-to-vehicle (V2V) and vehicle-to-roadside (V2R) communications is required for numerous applications that aim at improving traffic safety and traffic efficiency. As recent studies have shown, communications in this context is significantly influenced by radio propagation characteristics of the environment and the signal processing algorithms that are executed on the physical layer of the communications stack. Whereas a shadowing of the transmitted signal, e.g. due to buildings, determines the ability to communicate “around corners”, channel estimation, channel equalizing, and advanced coding schemes determine whether a receiver can decode a received signal successfully or not. Consequently, a proper assessment and evaluation of V2V and V2R communications, especially when traffic safety applications are considered, requires an accurate simulation of the wireless channel as well as the physical layer of the protocol stack. To enable a proper assessment, we integrated a physical layer simulator into the popular NS-3 network simulator, validated our implementation against commercial off the shelf transceiver chipsets, and employed ray tracing as a method to accurately simulate the radio propagation characteristics of the Karlsruhe Oststadt. Since the simulation of signal processing details and ray tracing are computationally expensive modeling methods, we based our work on the HP XC4000 to speedup the computation of both aspects.

Accurate Simulation of Wireless Vehicular Networks Based on Ray Tracing and Physical Layer Simulation

Distributed Hash Tables (DHTs) prove valuable for distributed architectures by providing distributed information lookup, routing, and data storage while promising good scalability and robustness at the same time. From this point of view, a DHT could be seen as a basic service that can be used to build distributed applications. Whereas today no widely deployed and publicly accessible basic DHT service exists and thus DHT- based applications have to deploy their very own DHT networks, DHTs consisting of millions of peers are formed by file sharing clients. Although the interfaces of typical DHTs used for file sharing are too narrow, a basic DHT service could probably be created by bundling a suitable client implementation with file sharing software. In this paper, we evaluate whether a basic DHT service could suit the needs of DHT- based applications in terms of stability, number of participating peers, the peers'' session lengths, geographical distribution and peer connectivity when deployed similar to DHTs driven by file sharing. As these metrics mostly depend on end user behavior rather than on the DHT protocol, we report on measurement results gathered from monitoring of the BitTorrent Mainline client''s DHT over six months. We analyze which metrics would fit a basic DHT service and which could prove problematic. Furthermore, we discuss resulting technical requirements for an appropriate DHT protocol.

Towards a Basic DHT Service: Analyzing Network Characteristics of a Widely Deployed DHT

Inter-vehicle communication promises to prevent accidents by enabling applications such as cross-traffic assistance. This application requires information from vehicles in Non-Line-of-Sight (NLOS) areas due to buildings at intersection corners. The periodic Cooperative Awareness Messages (CAM) are foreseen to be sent via 5.9GHz IEEE 802.11p. While reception rate measurements and validated propagation models for vehicular 5.9GHz NLOS conditions recently emerged, NLOS reception rates under competition on the channel are an open issue and require an in-depth evaluation to judge application feasibility. In this paper, we investigate NLOS reception on a 5.9GHz inter-vehicle communication channel by network simulation in ns-2. The simulator has been enhanced to support a state of the art vehicular NLOS propagation model. A traffic flow simulation is used to derive realistic simulation scenarios, reflecting cross-traffic assistance critical traffic situations. Results of the network simulations under different environment conditions show that reception rates in NLOS areas are on acceptable levels under competition on the channel by various transmitters.

Hannes Hartenstein

Papers

TEE-Based Distributed Watchtowers for Fraud Protection in the Lightning Network

A Consistency Model for Identity Information in Distributed Systems

Accurate Simulation of Wireless Vehicular Networks Based on Ray Tracing and Physical Layer Simulation

Towards a Basic DHT Service: Analyzing Network Characteristics of a Widely Deployed DHT

5.9GHz IEEE 802.11p inter-vehicle communication: Non-Line-of-Sight reception under competition