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

"Inter-Vehicular Communication -- Quo Vadis?". With this question in mind, leading experts in the field of vehicular networking met in Dagstuhl to discuss the current state of the art and, most importantly, the open challenges in R&D from both an scientific and an industry point of view. After more than a decade of research on vehicular networks, the experts very seriously asked the question whether all of the initial research issues had been solved so far.
It turned out that the perspective changed in the last few years, mainly thanks to the ongoing field operational tests in Europe and the U.S. The results point to new research directions and new challenges that need to be solved for a second generation of vehicular networking applications and protocols.
In four working groups, the experts studied these new challenges and derived recommendations that are also very helpful for the respective funding organizations.

https://drops.dagstuhl.de/opus/volltexte/2014/4422/pdf/dagrep_v003_i009_p190_s13392.pdf/

Inter-Vehicular Communication -- Quo Vadis (Dagstuhl Seminar 13392)

Matrix is a new kind of decentralized, topic-based publish-subscribe middleware for communication and data storage that is getting particularly popular as a basis for secure instant messaging. By comparison with traditional decentralized communication systems, Matrix replaces pure message passing with a replicated data structure. This data structure, which we extract and call the Matrix Event Graph (MEG), depicts the causal history of messages. We show that this MEG represents an interesting and important replicated data type for decentralized applications that are based on causal histories of publish-subscribe events: First, we prove that the MEG is a Conflict-Free Replicated Data Type for causal histories and, thus, provides Strong Eventual Consistency (SEC). With SEC being among the best known achievable trade-offs in the scope of the well-known CAP theorem, the MEG provides a powerful consistency guarantee while being available during network partition. Second, we discuss the implications of byzantine attackers on the data type’s properties. We note that the MEG, as it does not strive for consensus or strong consistency, can cope with $n > f$ environments with $n$ participants, of which $f$ are byzantine. Furthermore, we analyze scalability: Using Markov chains, we study the number of forward extremities of the MEG over time and observe an almost optimal evolution. We conjecture that this property is inherent to the underlying spatially inhomogeneous random walk. With the properties shown, a MEG represents a promising element in the set of data structures for decentralized applications, but with distinct trade-offs compared to traditional blockchains and distributed ledger technologies.

/pdf/analysis-of-the-matrix-event-graph-replicated-data-type-5ea2tg9ouv.pdf

Analysis of the Matrix Event Graph Replicated Data Type

We explore challenges of and present a concept for a decentralized payment service which is based on trusted execution environments. The system guarantees that users can always cash out their funds without depending on the cooperation of other network members, hence minimizing the trust required in other network members. We present an overview of the system, motivate key components for a secure architecture and provide a communication protocol. We prove that the payment service users can cash out their funds at any time without any dependence on other network members.

/pdf/banklaves-concept-for-a-trustworthy-decentralized-payment-48cw85qjbi.pdf

Banklaves: Concept for a Trustworthy Decentralized Payment Service for Bitcoin

Secure Data Sharing (SDS) enables users to share data in the cloud in a confidential and integrity-preserving manner. Many recent SDS approaches are based on Attribute-Based Encryption (ABE), leveraging the advantage that ABE allows to address a multitude of users with only one ciphertext. However, ABE approaches often come with the downside that they require a central fully-trusted entity that is able to decrypt any ciphertext in the system. In this paper, we investigate on whether ABE could be used to efficiently implement Decentralized Secure Data Sharing (D-SDS), which explicitly demands that the authorization and access control enforcement is carried out solely by the owner of the data, without the help of a fully-trusted third party. For this purpose, we did a comprehensive analysis of recent ABE approaches with regard to D-SDS requirements. We found one ABE approach to be suitable, and we show different alternatives to employ this ABE approach in a group-based D-SDS scenario. For a realistic estimation of the resource consumption, we give concrete resource consumption values for workloads taken from real-world system traces and exemplary up-to-date mobile devices. Our results indicate that for the most D-SDS operations, the resulting computation times and outgoing network traffic will be acceptable in many use cases. However, the computation times and outgoing traffic for the management of large groups might prevent using mobile devices.

Decentralized Secure Data Sharing with Attribute-Based Encryption: A Resource Consumption Analysis

Currently, the topic of `Bandwidth on Demand' is actively discussed in the US (eg with respect to `network neutrality') not only from a technological point of view, but also from an economic and legal perspective Technically, bandwidth and resulting throughput can be controlled by ISPs on OSI layers 1-3, but additionally `bandwidth on demand' can also be provided by means of the application layer utilizing peer-to-peer (P2P) systems (eg BitTorrent) In this paper we analyze from a European perspective legal status and legal consequences (rights and obligations) of using and operating such overlay networks We first provide a classification of the most relevant European directives, laws, and legal opinions on electronic communication systems Afterwards, we categorize P2P systems and their involved components from the legal point of view Based on this categorization, we present developer/operator guidelines with their corresponding legal consequences for various P2P systems

Hannes Hartenstein

Papers

Inter-Vehicular Communication -- Quo Vadis (Dagstuhl Seminar 13392)

Analysis of the Matrix Event Graph Replicated Data Type

Banklaves: Concept for a Trustworthy Decentralized Payment Service for Bitcoin

Decentralized Secure Data Sharing with Attribute-Based Encryption: A Resource Consumption Analysis

A Techno-Legal Perspective on Peer-to-Peer-Based Bandwidth on Demand Management