Abstract not found

There are many contexts in distributed wireless networks where there is a critical threshold, corresponding to a minimum amount of the communication effort or power expenditure by individual nodes, above which a desirable global property exists with high probability. When this individual node effort is below the threshold the desired global property exists with a low probability. This "phase transition" is typically seen to become sharper as the number of nodes in the network increases. We discuss in this paper some examples of properties that exhibit such critical behavior: node reachability with probabilistic flooding, ad-hoc network connectivity, and sensor network coordination. We discuss the connections between these phenomena and the phase transitions that have been shown to arise in random graphs. We argue that a good understanding of these phase transition phenomena can provide useful design principles for engineering distributed wireless networks.

Rumor mongering (also known as gossip) is an epidemiological protocol that implements broadcasting with a reliability that can be very high. Rumor mongering is attractive because it is generic, scalable, adapts well to failures and recoveries, and has a reliability that gracefully degrades with the number of failures in a run. However, rumor mongering uses random selection for communications. We study the impact of using random selection in this paper. We present a protocol that superficially resembles rumor mongering but is deterministic. We show that this new protocol has most of the same attractions as rumor mongering. The one attraction that rumor mongering has---namely graceful degradation---comes at a high cost in terms of the number of messages sent. We compare the two approaches both at an abstract level and in terms of how they perform in an Ethernet and small wide area network of Ethernets.

Distributed content-based publish-subscribe middleware provides the necessary decoupling, flexibility, expressiveness, and scalability required by modern distributed applications. Unfortunately, this kind of middleware usually does not provide reliability guarantees, as this problem has been thus far largely disregarded by the research community and solutions developed in other contexts are not immediately applicable.

Mainstream approaches to content-based distributed publish-subscribe typically route events deterministically based on information collected from subscribers, and do so by relying on a tree-shaped overlay network. While this solution achieves scalability in fixed, large-scale settings, it is less appealing in scenarios characterized by high dynamicity, e.g., mobile ad hoc networks or peer-to-peer systems. At the other extreme, researchers in the related fields of multicast and group communication have successfully exploited probabilistic techniques that provide increased fault tolerance, resilience to changes, and yet are scalable. In this paper, we propose a novel approach where event routing relies on deterministic decisions driven by a limited view on the subscription information and, when this is not sufficient, resorts to probabilistic decisions performed by selecting links at random. Simulations show that the particular mix of deterministic and probabilistic decisions we put forth in this work is very effective at providing high event delivery and low overhead in highly dynamic scenarios, without sacrificing scalability. 1.

Gossip-based multicast can be an effective tool for providing highly reliable and scalable message dissemination. Previous work has shown it to be useful in a variety of group communication settings when processes all belong to a single process group. In this paper, we consider the problem of gossiping within overlapping process subgroups. If each subgroup independently runs the standard gossip protocol, then the total gossip overhead could be high for a process that is a member of many subgroups. We present anovel gossip protocol that allows individual subgroup members to trade-off update quality for gossip overhead, enabling processes to belong to several subgroups while maintaining a low total gossip overhead. Our results include a mathematical model for message dissemination under this modified gossip protocol, and an algorithm that computes gossip parameters such that all processes within a subgroup achieve their desired update quality. Preliminary results are promising.

We present experimental and analytical results showing "zero-one" phase transitions for network connectivity, multi-path reliability, neighbor count, Hamiltonian cycle formation, multiple-clique formation, and probabilistic flooding. These transitions are characterized by critical density thresholds such that a global property exists with negligible probability on one side of the threshold, and exists with high probability on the other. We discuss the connections between these phase transitions and some known results on random graphs, and indicate their significance for the design of resource-efficient wireless networks.

Multicast avoids sending repeated packets over the same network links and thus offers the promise of supporting multimedia streaming over wide-area networks. Previously, two opposite multicast schemes – forward-path forwarding and reverse-path forwarding – have been proposed on top of structured peer-to-peer (p2p) overlay networks. This paper presents Borg, a new scalable application-level multicast system built on top of p2p overlay networks. Borg is a hybrid protocol that exploits the asymmetry in p2p routing and leverages the reverse-path multicast scheme for its low link stress on the physical networks. Borg has been implemented on top of Pastry, a generic, structured p2p routing substrate. Simulation results based on a realistic network topology model shows that Borg induces significantly lower routing delay penalty than both forward-path and reversepath multicast schemes while retaining the low link stress of the reverse-path multicast scheme. 1.

Tree-based reliable multicast protocols provide scalability by distributing error-recovery tasks among several repair nodes. These repair nodes integrate the status information of several receiver nodes and perform local error recovery for these nodes using the data stored in their buffers. We propose a buffer management scheme that uses both positive and negative acknowledgments to manage these buffers in an efficient manner. Under our scheme, receiver nodes send negative acknowledgments to repair nodes to request packet retransmissions. At infrequent intervals, they also send positive acknowledgments to indi-cate which packets can be safely discarded from the buffer of their repair node. Our scheme reduces delay in error recovery, because the packets requested from the repair nodes are always available in their buffers. It achieves this goal without increasing the server workload because (a) each receiver node only sends infrequent positive acknowledgments and (b) their sending times are randomized among all the receiver nodes. We also show how our scheme can be extended to provide full flow control and eliminate buffer overflows. I.

There are many contexts in distributed wireless networks where there is a critical threshold, corresponding to a minimum amount of the communication effort or power expenditure by individual nodes, above which a desirable global property exists with high probability. When this individual node effort is below the threshold the desired global property exists with a low probability. This "phase transition" is typically seen to become sharper as the number of nodes in the network increases. We discuss in this paper some examples of properties that exhibit such critical behavior: node reachability with probabilistic flooding, ad-hoc network connectivity, and sensor network coordination. We discuss the connections between these phenomena and the phase transitions that have been shown to arise in random graphs. We argue that a good understanding of these phase transition phenomena can provide useful design principles for engineering distributed wireless networks.