We describe PeerReview, a system that provides accountability in distributed systems. PeerReview ensures that Byzantine faults whose effects are observed by a correct node are eventually detected and irrefutably linked to a faulty node. At the same time, PeerReview ensures that a correct node can always defend itself against false accusations. These guarantees are particularly important for systems that span multiple administrative domains, which may not trust each other.PeerReview works by maintaining a secure record of the messages sent and received by each node. The record isused to automatically detect when a node's behavior deviates from that of a given reference implementation, thus exposing faulty nodes. PeerReview is widely applicable: it only requires that a correct node's actions are deterministic, that nodes can sign messages, and that each node is periodically checked by a correct node. We demonstrate that PeerReview is practical by applying it to three different types of distributed systems: a network filesystem, a peer-to-peer system, and an overlay multicast system.

/pdf/peerreview-practical-accountability-for-distributed-systems-47i8xmr8tp.pdf

PeerReview: practical accountability for distributed systems

As mobile ad hoc networks (MANETs) become ever more popular, it also becomes more and more interesting to build distributed network applications that one is accustomed to from the Internet on top of MANETs. In the Internet, Distributed Hash Tables (DHTs) have recently proven themselves an efficient building block for such distributed applications. However, DHTs are ill-suited for direct deployment in MANETs as they are largely oblivious of the physical routing. Therefore, we propose MADPastry, a DHT substrate explicitly designed for the use in MANETs. MADPastry considers physical locality and integrates the functionality of a DHT and an ad hoc routing protocol at the network layer to provide an efficient indirect routing primitive in MANETs. To answer the fundamental question whether the extra overhead of maintaining a DHT in MANETs is really worth the effort or, instead, one would be better off broadcasting the actual lookups in the first place, we compare MADPastry's performance against an unstructured Gnutella-style broadcast agent and a DHT substrate without locality awareness.

/pdf/madpastry-a-dht-substrate-for-practicably-sized-manets-15yzl9g5ht.pdf

MADPastry: A DHT Substrate for Practicably Sized MANETs

This dissertation presents algorithms for data structures called distributed hash tables (DHT) or structured overlay networks, which are used to build scalable self-managing distributed systems. The provided algorithms guarantee lookup consistency in the presence of dynamism: they guarantee consistent lookup results in the presence of nodes joining and leaving. Similarly, the algorithms guarantee that routing never fails while nodes join and leave. Previous algorithms for lookup consistency either suffer from starvation, do not work in the presence of failures, or lack proof of correctness.


Several group communication algorithms for structured overlay networks are presented. We provide an overlay broadcast algorithm, which unlike previous algorithms avoids redundant messages, reaching all nodes in O(log n) time, while using O(n) messages, where n is the number of nodes in the system. The broadcast algorithm is used to build overlay multicast.


We introduce bulk operation, which enables a node to efficiently make multiple lookups or send a message to all nodes in a specified set of identifiers. The algorithm ensures that all specified nodes are reached in O(log n) time, sending maximum O(log n) messages per node, regardless of the input size of the bulk operation. Moreover, the algorithm avoids sending redundant messages. Previous approaches required multiple lookups, which consume more messages and can render the initiator a bottleneck. Our algorithms are used in DHT-based storage systems, where nodes can do thousands of lookups to fetch large files. We use the bulk operation algorithm to construct a pseudo-reliable broadcast algorithm. Bulk operations can also be used to implement efficient range queries.


Finally, we describe a novel way to place replicas in a DHT, called symmetric replication, that enables parallel recursive lookups. Parallel lookups are known to reduce latencies. However, costly iterative lookups have previously been used to do parallel lookups. Moreover, joins or leaves only require exchanging O(1) messages, while other schemes require at least log(f) messages for a replication degree of f.


The algorithms have been implemented in a middleware called the Distributed k-ary System (DKS), which is briefly described.

/pdf/distributed-k-ary-system-algorithms-for-distributed-hash-4pdubi1rag.pdf

Distributed k-ary System: Algorithms for Distributed Hash Tables

Peer-to-peer (p2p) technology can potentially be used to build highly reliable applications without a single point of failure. However, most of the existing applications, such as file sharing or web caching, have only moderate reliability demands. Without a challenging proving ground, it remains unclear whether the full potential of p2p systems can be realized.To provide such a proving ground, we have designed, deployed and operated a p2p-based email system. We chose email because users depend on it for their daily work and therefore place high demands on the availability and reliability of the service, as well as the durability, integrity, authenticity and privacy of their email. Our system, ePOST, has been actively used by a small group of participants for over two years.In this paper, we report the problems and pitfalls we encountered in this process. We were able to address some of them by applying known principles of system design, while others turned out to be novel and fundamental, requiring us to devise new solutions. Our findings can be used to guide the design of future reliable p2p systems and provide interesting new directions for future research.

/pdf/experiences-in-building-and-operating-epost-a-reliable-peer-2nhl1qqv4q.pdf

Experiences in building and operating ePOST, a reliable peer-to-peer application

Designing structured peer-to-peer overlays as a platform for distributed network applications in mobile ad hoc networks

POST: A Decentralized Platform for Reliable Collaborative Applications

POST: A decentralized platform for reliable collaborative applications

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POST: A decentralized platform for reliable collaborative applications