@inproceedings {Simula.ND.219,
	title = {On the Influence of Latency Estimation on Dynamic Group Communication Using Overlays},
	journal = {Proceedings of Multimedia Computing and Networking (MMCN)},
	volume = {7253},
	year = {2009},
	publisher = {SPIE},
	type = {Conference},
	abstract = {Distributed interactive applications tend to have stringent latency requirements and some may have high bandwidth demands. Many of them have also very dynamic user groups for which all-to-all communication is needed. In online multiplayer games, for example, such groups are determined through region-of-interest management in the application. We have investigated a variety of group management approaches for overlay networks in earlier work and shown that several useful tree heuristics exist. However, these heuristics require full knowledge of all overlay link latencies. Since this is not scalable, we investigate the effects that latency estimation techniques have on the quality of overlay tree constructions. We do this by evaluating one example of our group management approaches in Planetlab and examining how latency estimation techniques influence their quality. Specifically, we investigate how two well-known latency estimation techniques, Vivaldi and Netvigator, affect the quality of tree building.},
	isbn = {9780819475039},
	doi = {10.1117/12.815561},
	author = {Vik, Knut-Helge and Griwodz, Carsten and Halvorsen, P{\r a}l},
	editor = {Ketan Mayer-Pate {Rejaie}}
}
@article {Simula.ND.327,
	title = {The Partial Migration of Game State and Dynamic Server Selection to Reduce Latency},
	journal = {Multimedia Tools and Applications},
	volume = {45},
	number = {1-3},
	year = {2009},
	note = {Special Issue on Massively Multiuser Online Gaming Systems and Applications},
	pages = {83-107},
	abstract = {Massively multi-player online games (MMOGs) have stringent latency requirements and must support large numbers of concurrent players. To handle these conflicting requirements, it is common to divide the virtual environment into virtual regions. As MMOGs are world-spanning games, it is plausible to disperse these regions on geographically distributed servers. Core selection can then be applied to locate an optimal server for placing a region, based on player latencies. Functionality for migrating objects supports this objective, with a distributed name server ensuring that references to the moved objects are maintained. As a result we anticipate a decrease in the aggregate latency for the affected players. The core selection relies on a set of servers and measurements of the interacting players latencies. Measuring these latencies by actively probing the network is not scalable for a large number of players. We therefore explore the use of latency estimation techniques to gather this information.},
	doi = {10.1007/s11042-009-0287-7},
	author = {Beskow, Paul and Vik, Knut-Helge and Halvorsen, P{\r a}l and Griwodz, Carsten}
}
@inproceedings {Simula.ND.221,
	title = {Constructing Low-Latency Overlay Networks: Tree Vs. Mesh Algorithms},
	journal = {Proceedings of IEEE Conference on Local Computer Networks (LCN)},
	year = {2008},
	month = {October},
	pages = {36-43},
	publisher = {IEEE},
	type = {Conference},
	abstract = {Distributed interactive applications may have stringent latency requirements and dynamic user groups. These applications may benefit from a group communication system, and to improve the system support for such applications, we investigate graph algorithms that construct low-latency overlay networks for application-layer multicast. In particular, we focus on reducing the diameter and the pair-wise latencies in the overlay. The overlay construction time is also considered, as it is often time-dependent in our dynamic target applications. Here, we have implemented and experimentally analyzed spanning-tree heuristics and mesh construction heuristics, and compared their performance and applicability to distributed interactive applications. We found that trees are faster to construct and save considerable amounts of resources in the network. Meshes, on the other hand, yield lower pair-wise latencies and increases the fault tolerance, but at the expense of increased resource consumption.},
	isbn = {978-1-4244-2412-2},
	doi = {10.1109/LCN.2008.4664149},
	author = {Vik, Knut-Helge and Griwodz, Carsten and Halvorsen, P{\r a}l},
	editor = {Elmallah, Ehab and Younis, Mohamed and Chun Tung {Chou}}
}
@article {Simula.ND.102,
	title = {Evaluating Steiner Tree Heuristics and Diameter Variations for Application Layer Multicast},
	journal = {Computer Networks},
	volume = {52},
	number = {15},
	year = {2008},
	note = {Special issue on Complex Computer and Communication Networks, Elsevier},
	month = {October},
	pages = {2872-2893},
	abstract = {Latency reduction in distributed interactive applications has been studied intensively. Such applications may have stringent latency requirements and dynamic user groups. We focus on application-layer multicast with a centralized approach to the group management. The groups are organized in overlay networks that are created using graph algorithms that create a tree structure for the group. A tree has no cycles and uses a small routing table, as opposed to a connected overlay mesh. We investigate a group of spanning tree problems that are referred to as Steiner tree problems, and we have a particular focus on reducing the diameter of a tree, which is the maximum pairwise latency in a tree. In addition, we focus on reducing the time it takes to execute membership changes. In that context, we use core-selection heuristics to find well-placed client nodes, and edge-pruning algorithms to reduce the number of edges in an otherwise fully meshed overlay. Our edge-pruning algorithms strongly connect well-placed client nodes to the remaining group members, to create new and pruned group graphs. Consequently, when a tree algorithm is applied to a pruned group graph, it is manipulated into creating trees with a smaller diameter.We devised new Steiner-tree heuristics that reduced the diameter, and also proposed new edge-pruning algorithms to make the tree construction faster. These heuristics and algorithms were implemented and analyzed experimentally along with several spanning-tree and core-selection heuristics found in the literature. We found that a full-mesh of shortest paths makes it difficult for Steiner-tree heuristics to find better trees than spanning tree algorithms. The network seen from the application layer is in fact a full mesh of shortest paths. In addition, we found that faster Steiner-tree heuristics that do not explicitly optimize the diameter are able to compete with slower heuristics that do optimize it.},
	doi = {10.1016/j.comnet.2008.06.003},
	author = {Vik, Knut-Helge and Halvorsen, P{\r a}l and Griwodz, Carsten}
}
@phdthesis {Simula.simula.20,
	title = {Group Communication Techniques in Overlay Networks},
	year = {2008},
	month = {December},
	school = {University of Oslo},
	type = {phd},
	address = {Unipub, Kristian Ottosens hus, Pb. 33 Blindern, 0313 Oslo},
	abstract = {One type of Internet services that have recently gained much attention are services that enable people around the world to communicate in real-time. Such services of real-time interaction are offered by applications most commonly referred to as distributed interactive applications. Concrete examples of distributed interactive applications are multiplayer online games, audio/video conferencing, and many virtual-reality applications linked to education, entertainment, military, etc. A time-dependent requirement generally applies to all distributed interactive applications that aim to support real-time interaction, and is usually in terms of a few hundred milliseconds. The latency requirements are manifested in terms of event-distribution, group membership management, group dynamics, etc., far exceeding the requirements of many other applications. One general focal point in this thesis is to enable scalable group communication for managing dynamic groups of clients that interact in real-time. By doing this, we want to enable people around the world to dynamically join networks of participants and interact with them in real-time. The main contributions of the thesis are a number of investigations of a wide variety of group communication techniques. The results from the investigations form a foundation to identify the techniques that are particularly suitable for distributed interactive applications. We investigated membership management techniques, and evaluated both centralized and distributed approaches through empirical and experimental studies on PlanetLab. We proposed 3 membership management techniques and found that a centralized membership management approach is particularly fast and consistent when there are multiple dynamic groups. We also aimed to identify well-placed nodes in the application network that yield low pair-wise latencies to groups of clients. These may, for example, be used for membership managing tasks. We evaluated 5 core-node selection algorithms through group communication simulations and experiments on PlanetLab. From these evaluations we found that there exist core-node selection algorithms that are able to find sufficiently well-placed nodes. We considered overlay network multicast as the better option to distribute time-dependent events in groups, and found that centralized graph algorithms are suited to meet the latency requirements put on the overlay constructions and reconfigurations. We evaluated a variety of centralized overlay construction algorithms that aim to build low-latency overlay networks. Through rather comprehensive analyses we identified suitable algorithms in the investigations. Finally, we investigated whether it is possible to obtain accurate all-to-all path latencies to be used by the centralized graph algorithms. For this, we evaluated 2 latency estimation techniques and measured their accuracy by comparing the estimates to all-to-all ping measurements. For the evaluation, we implemented a real-world system and performed group communication experiments on PlanetLab. We found that when latency estimates are used by core-node selection algorithms and overlay construction algorithms, they are sufficiently accurate such that the graph algorithms still find solutions that are close to the real-world.},
	author = {Vik, Knut-Helge}
}
@inproceedings {Simula.ND.225,
	title = {Latency Reduction by Dynamic Core Selection and Partial Migration of Game State},
	journal = {Proceedings of Workshop on Network and Systems Support for Games (NetGames)},
	year = {2008},
	month = {October},
	pages = {79-84},
	publisher = {ACM},
	type = {Workshop},
	abstract = {Massively multi-player online games (MMOGs) require low latency while supporting a large number of concurrent players, often sharing one worldwide instance of the game. As these are conflicting requirements, a common way of distributing load is by dividing the virtual environment into virtual regions. As MMOGs are world-spanning games, it is plausible to disperse these regions on geographically distributed servers. As such, we propose the use of core selection for finding an optimal server for placing a region, and support for migrating the game state to that server. The first goal relies on a set of servers and measurement of the interacting players latencies. In locating an optimal server, we anticipate a decrease in the overall latency for the majority of players. This reduction occurs by migrating the region to a server closer in proximity to the majority of players in that virtual region, thereby lowering the response time of any interaction.},
	isbn = {978-1-60558-132-3},
	author = {Beskow, Paul and Vik, Knut-Helge and Halvorsen, P{\r a}l and Griwodz, Carsten},
	editor = {Claypool, Mark}
}
@inproceedings {Simula.ND.73,
	title = {Multicast Tree Diameter for Dynamic Distributed Interactive Applications},
	journal = {IEEE International Conference on Computer Communications - IEEE INFOCOM},
	year = {2008},
	month = {April},
	pages = {1597-1605},
	publisher = {IEEE},
	type = {Annual},
	edition = {27th},
	abstract = {Considerable attention has been given to latency reduction in distributed interactive applications. Such applications may have stringent latency requirements and dynamic user groups. In this paper, our focus has been on using application layer multicast with a centralized approach to the group management. The groups are organized in overlay networks that are created using graph algorithms. We investigate many spanning tree problems with particular focus on reducing the diameter of a tree, i.e., the maximum pairwise latency between users. In addition, we focus on reducing the time it takes to execute membership changes. In that context, we use core selection heuristics to find well located group nodes, and edge pruning algorithms to reduce the number of edges in an otherwise fully meshed overlay. Our edge pruning algorithms strongly connect well located group nodes to the remaining group members, to create new and pruned group graphs. Such that, when a tree algorithm is given a pruned group graph, it is manipulated into creating trees with lower diameter. In the investigation presented here, we have implemented and experimentally analyzed spanning tree heuristics, core selection heuristics and edge pruning algorithms. We find that faster heuristics that do not explicitly optimize the diameter are able to compete with slower heuristics that optimize the diameter.},
	doi = {10.1109/INFOCOM.2008.220},
	author = {Vik, Knut-Helge and Halvorsen, P{\r a}l and Griwodz, Carsten},
	editor = {Merill, Douglas and Saha, Debanjan and Jennifer C. {Hou} and Kalyanaraman, Shivkumar and Sivalingam, Krishna}
}
@inproceedings {Simula.ND.33,
	title = {Dynamic Group Membership Management for Distributed Interactive Applications},
	journal = {32nd IEEE Conference on Local Computer Networks (LCN)},
	year = {2007},
	month = {October},
	pages = {141-148},
	publisher = {IEEE},
	type = {Conference},
	abstract = {Distributed interactive applications have become increasingly popular, making it important to address their communication needs, where, one of the needs is group communication. In this paper, we consider the applications in which it is at any given time possible to divide its users into groups. The group membership changes over time and the group division is unrelated to the physical proximity. As a way of enabling the group communication in distributed interactive applications we choose application layer multicast. We use simulation to evaluate several dynamic algorithms for managing overlay multicast trees. They are compared with respect to four metrics that can be relevant for a distributed interactive application. These are total tree cost, diameter, reconfiguration time and stability. We demonstrate algorithms that perform well for these metrics although they do not consider all users during reconfiguration.},
	isbn = {978-0-7695-3000-0},
	author = {Vik, Knut-Helge and Griwodz, Carsten and Halvorsen, P{\r a}l},
	editor = {Christensen, Ken and Frank, Matthias}
}
@inproceedings {Vik.2006.1,
	title = {Applicability of Group Communication for Increased Scalability in MMOGs},
	journal = {Network \& System Support for Games (NetGames 2006)},
	year = {2006},
	month = {October},
	pages = {1-12},
	publisher = {ACM Press},
	type = {Annual},
	edition = {Fifth},
	abstract = {Massive multiplayer online games (MMOGs) are today the driving factor for the development of distributed interactive applications, and they are increasing in size and complexity. Even a small MMOG supports thousands of players, the biggest support hundreds of thousands of concurrent players. Since they are typically built as strict client-server systems, they suffer from the inherent scalability problem of the architecture. Computing power and bandwidth limitations close to the server limit the possible number of players. Also, the latency of communication between players through the server will be higher than using direct communication. In the paper, we address these issues and investigate improvement options. A typical MMOG consists of a virtual world with a concept of time and space that is similar to the real world. In it, players are represented by avatars. Only subsets of these avatars interact with each other at any given time. This allows us to divide them into groups, and communication among group members becomes a multi-party communication problem. Thus, to reduce resource consumption, we compare the performance of several algorithms for group communication with the current central server approach. We use overlay multicast as the means of providing group communication, and research algorithms for creating shortest path trees, spanning trees, delay-bounded spanning trees and, more specific, applying Steiner tree heuristics. Our experimental results indicate that different approaches are useful to reduce resource consumption while achieving a good perceived quality under varying conditions, such as frequent changes in group membership and the demand for low latency.},
	isbn = {1-59593-589-4},
	author = {Vik, Knut-Helge and Griwodz, Carsten and Halvorsen, P{\r a}l},
	editor = {Adrian David {Cheok} and Ishibashi, Yutaka and Natkin, Stephane and Yasumoto, Keiichi}
}
@inproceedings {Griwodz.2006.2,
	title = {Multicast Tree Reconfiguration in Distributed Interactive Applications},
	journal = {Networking Issues in Multimedia Entertainment (NIME 2006)},
	year = {2006},
	month = {Januar},
	pages = {1219-1223},
	publisher = {IEEE},
	type = {Workshop},
	abstract = {Communication in highly interactive distributed applications, such as massive multiplayer online games, can often be performed efficiently using multicast, i.e., application level multicast. However, in applications with a very dynamic group management, the multicast tree will have frequent changes, and in applications that have stringent latency requirement, this operation needs to be fast. Current multicast approaches either have no notion of reconfiguration, they do not care about tree reconstruction latency or wrongly assume that this is a fast, atomic operation. In this paper, we have focused on dynamic reconfiguration and have tested different ways for a node to join a tree. Our results show that this is an important issue for the class of highly interactive distributed applications.},
	isbn = {1-4244-0086-4},
	author = {Griwodz, Carsten and Vik, Knut-Helge and Halvorsen, P{\r a}l},
	editor = {Roccetti, Marco and Pau, Giovanni and El-Rhalibi, Abdennour}
}
@inproceedings {vikACMM05,
	title = {Game State and Event Distribution Using Proxy Technology and Application Layer Multicast},
	journal = {Proceedings of the ACM International Multimedia Conference (ACM MM)},
	year = {2005},
	pages = {1041-1042},
	publisher = {ACM Press},
	type = {Annual},
	isbn = {1-59593-044-2},
	author = {Vik, Knut-Helge},
	editor = {Tat-Seng Chua Hongj {Zhang}}
}
@mastersthesis {Vik.2004.1,
	title = {Quality of Service Aware Source Initiated Ad-Hoc Routing},
	year = {2004},
	month = {May},
	publisher = {Electrical Engineering and Computer Science Department, Washington State University, WA, USA},
	type = {masters},
	abstract = {Mobil Ad-hoc networks (MANETs) are used in a variety of situations ranging from conference meetings to military operations. Among the important challenges facing MANETs is the development of suitable routing protocols. A Routing Protocol must be aware of the diversity in the network if the network is going to perform at its best. The routing protocol should be e\pmcient and able to adapt to rapidly changing topologies. A MANET is applicable to any situation requiring network communication in particular short-lived highly mobile networks. The routing protocol needs mechanisms that handle the mobility such that the communication disruption time is min- imized. Speci{\textasciimacron}cally we desire a routing protocol with quality control mechanisms for every important stage in the lifetime of a route. This thesis proposes a Quality of Service (QoS) aware source initiated ad-hoc routing protocol (QuaSAR) that adds quality control to all the phases of an on-demand routing protocol. QuaSAR gathers statistical information from the network during route discovery, more speci{\textasciimacron}cally battery power, signal strength, bandwidth and latency. In particular we use the signal strength to {\textasciimacron}nd stronger connected routes. The metrics are associated to the individual route and later used when a route to the destination is picked. QuaSAR includes proactive route maintenance features in addition to the reactive maintenance. The proactive mechanism of QuaSAR makes a Mobile Node (MN) on an active route aware of route critical incidents in progress and noti{\textasciimacron}es the appropriate party of the development. Performance experiments were conducted to test the performance of QuaSAR using ns- 2 Network Simulator. Results were compared with Dynamic Source Routing (DSR). The following mobility models were used in the experiments: Random Waypoint, Gauss Markov, Manhattan Grid and Reference Point Group Model. We have demonstrated that using signal strength and proactive mechanisms as a means of routing will increase the throughput and the packet delivery ratio. The quality control mechanisms of QuaSAR enhance the performance experienced by MNs by giving it more stable paths thus minimizing the communication disruption time.},
	author = {Vik, Knut-Helge}
}
@inproceedings {Vik.2004.2,
	title = {Quality of Service-Aware Source-Initiated Ad-Hoc Routing},
	journal = {IEEE Sensor and Ad-hoc Networks Conference (SECON)},
	year = {2004},
	month = {October},
	pages = {108-117},
	publisher = {IEEE},
	type = {Conference},
	edition = {First},
	abstract = {Desirable features of routing protocols for Mobile Ad Hoc Networks (MANETs) include ability to adapt to changing network conditions due to mobility and provide quality control mechanisms during the life time of a route. Current routing protocols that provide Quality of service (QoS) for MANETs have proposed routing based on a single QoS metric. This paper proposes a QoS aware source initiated ad-hoc routing protocol (QuaSAR) that adds quality control to all the phases of an on-demand routing protocol. QuaSAR gathers information about battery power, signal strength, bandwidth and latency during route discovery and uses in route choosing. Additionally, our approach has proactive route maintenance features in addition to the reactive maintenance. We conducted simulation experiments using ns-2 network simulator and compared our results with Dynamic Source Routing (DSR). Our performance results demonstrate that our technique has increased throughput and packet delivery ratio.},
	isbn = {0-7803-8796-1},
	author = {Vik, Knut-Helge and Medidi, Sirisha},
	editor = {IEEE, }
}
