Sameh El-Ansary

Efficient broadcast in structured P2P networks

In this position paper, we present an efficient algorithm for performing a broadcast operation with minimal cost in structured DHT-based P2P networks. In a system of N nodes, a broadcast message originating at an arbitrary node reaches all other nodes after exactly N − 1 messages. We emphasize the perception of a class of DHT systems as a form of distributed k-ary search and we take advantage of that perception in constructing a spanning tree that is utilized for efficient broadcasting. We consider broadcasting as a basic service that adds to existing DHTs the ability to search using arbitrary queries as well as dissiminate/collect global information.

DKS(N, k, f): a family of low communication, scalable and fault-tolerant infrastructures for P2P applications

In this paper, we present DKS(N, k, f), a family of infrastructures for building Peer-To-Peer applications. Each instance of DKS(N, k, f) is a fully decentralized overlay network characterized by three parameters: N the maximum number of nodes that can be in the network; k the search arity within the network and f the degree of fault-tolerance. Once these parameters are instantiated, the resulting network has several desirable properties. The first property, which is the main contribution of this paper, is that there is no separate procedure for maintaining routing tables; instead, any out-of-date or erroneous routing entry is eventually corrected on-the-fly thereby, eliminating unnecessary bandwidth consumption. The second property is that each lookup request is resolved in at most log/sub k/(N) overlay hops under normal operations. Third, each node maintains only (k-1) log/sub k/(N) + 1 addresses of other nodes for routing purposes. Fourth, new nodes can join and existing nodes can leave at will with a negligible disturbance to the ability to resolve lookups in logk(N) hops in average. Fifth, any pair key/value that is inserted into the system is guaranteed to be located even in the presence of concurrent joins. Sixth, even if f consecutive nodes fail simultaneously, correct lookup is still guaranteed.

NATCracker: NAT Combinations Matter

In this paper, we report our experience in working with Network Address Translators (NATs). Traditionally, there were only 4 types of NATs. For each type, the (im)possibility of traversal is well-known. Recently, the NAT community has provided a deeper dissection of NAT behaviors resulting into at least 27 types and documented the (im)possibility of traversal for some types. There are, however, two fundamental issues that were not previously tackled by the community. First, given the more elaborate set of behaviors, it is incorrect to reason about traversing a single NAT, instead combinations must be considered and we have not found any study that comprehensively states, for every possible combination, whether direct connectivity with no relay is feasible. Such a statement is the first outcome of the paper. Second, there is a serious need for some kind of formalism to reason about NATs which is a second outcome of this paper. The results were obtained using our own scheme which is an augmentation of currently-known traversal methods. The scheme is validated by reasoning using our formalism, simulation and implementation in a real P2P network.

An analytical study of a structured overlay in the presence of dynamic membership

In this paper, we present an analytical study of dynamic membership (aka churn) in structured peer-to-peer networks. We use a fluid model approach to describe steady-state or transient phenomena and apply it to the Chord system. For any rate of churn and stabilization rates and any system size, we accurately account for the functional form of the probability of network disconnection as well as the fraction of failed or incorrect successor and finger pointers. We show how we can use these quantities to predict both the performance and consistency of lookups under churn. All theoretical predictions match simulation results. The analysis includes both features that are generic to structured overlays deploying a ring as well as Chord-specific details and opens the door to a systematic comparative analysis of, at least, ring-based structured overlay systems under churn.

Multicast in DKS (N,k,f )overlay networks

In [1] a family of DHT-based infrastructures, termed DKS (N, k, f), with a number of desirable properties is presented. In the current paper, we show how multicast is achieved in DKS (N, k, f) overlay networks. Each multicast group is represented by an instance of DKS (N, k, f), which is created and maintained exactly as the underlying overlay network. Multicast messages are efficiently disseminated thanks to a correcting broadcast algorithm that allow each multicast message to be delivered exactly once to all application layer proccesses despite the presence of erroneous routing information.

On the feasibility of centrally-coordinated Peer-to-Peer live streaming

In this paper we present an exploration of central coordination as a way of managing P2P live streaming overlays. The main point is to show the elements needed to construct a system with that approach. A key element in the feasibility of this approach is a near real-time optimization engine for peer selection. Peer organization in a way that enables high bandwidth utilization plus optimized peer selection based on multiple utility factors make it possible to achieve large source bandwidth savings and provide high quality of user experience. The benefits of our approach are also seen most when NAT constraints come into play.

Comparing Maintenance Strategies for Overlays

In this paper, we present an analytical tool for understanding the performance of structured overlay networks under churn based on the master-equation approach of physics. We motivate and derive an equation for the average number of hops taken by lookups during churn, for the chord network. We analyse this equation in detail to understand the behaviour with and without churn. We then use this understanding to predict how lookups will scale for varying peer population as well as varying the sizes of the routing tables. We also consider a change in the maintenance algorithm of the overlay, from periodic stabilisation to a reactive one which corrects fingers only when a change is detected. We generalise our earlier analysis to understand how the reactive strategy compares with the periodic one.

SmoothCache: HTTP-Live streaming goes peer-to-peer

In this paper, we present SmoothCache, a peer-to-peer live video streaming (P2PLS) system. The novelty of SmoothCache is threefold: i ) It is the first P2PLS system that is built to support the relatively-new approach of using HTTP as the transport protocol for live content, ii ) The system supports both single and multi-bitrate streaming modes of operation, and iii ) In Smoothcache, we make use of recent advances in application-layer dynamic congestion control to manage priorities of transfers according to their urgency. We start by explaining why the HTTP live streaming semantics render many of the existing assumptions used in P2PLS protocols obsolete. Afterwards, we present our design starting with a baseline P2P caching model. We, then, show a number of optimizations related to aspects such as neighborhood management, uploader selection and proactive caching. Finally, we present our evaluation conducted on a real yet instrumented test network. Our results show that we can achieve substantial traffic savings on the source of the stream without major degradation in user experience.

Peer2View: A peer-to-peer HTTP-live streaming platform

Peer2View is a commercial peer-to-peer live video streaming (P2PLS) system. The novelty of Peer2View is threefold: i) It is the first P2PLS platform to support HTTP as transport protocol for live content, ii) The system supports both single and multi-bitrate streaming modes of operation, and iii) It makes use of an application-layer dynamic congestion control to manage priorities of transfers. Peer2View goals are to achieve substantial savings towards the source of the stream while providing the same quality of user experience of a CDN.

SmoothCache 2.0: CDN-quality adaptive HTTP live streaming on peer-to-peer overlays

In recent years, adaptive HTTP streaming protocols have become the de-facto standard in the industry for the distribution of live and video-on-demand content over the Internet. This paper presents SmoothCache 2.0, a distributed cache platform for adaptive HTTP live streaming content based on peer-to-peer (P2P) overlays. The contribution of this work is twofold. From a systems perspective, to the best of our knowledge, it is the only P2P platform which supports recent live streaming protocols based on HTTP as a transport and the concept of adaptive bitrate switching. From an algorithmic perspective, the system describes a novel set of overlay construction and prefetching techniques that realize: i) substantial savings in terms of the bandwidth load on the source of the stream, and ii) CDN-quality user experience in terms of playback latency and the watched bitrate. In order to support our claims, we conduct a methodical evaluation on thousands of real consumer machines.