Duncan McCaffery. Personal homepage URL:

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1 Name: Institution: PhD thesis submission date: Duncan McCaffery Lancaster University, UK Not yet determined Personal homepage URL:

2 Peer to Peer: Our Mobile Real-time Saviour? Duncan McCaffery, Joe Finney DMRG, Computing Department, Lancaster University Lancaster, LA1 4YR, UK {mccaffed, Abstract Developments in wireless networking will soon enable high bandwidth, low-latency, wireless wide area networks. In addition the rollout of mobile IPv6 across these wireless technologies will greatly challenge traditionally held ideas regarding support for mobile applications. This paper describes the development of a novel application which has led to the proposal of a new approach to support interactive mobile applications based on peer to peer networks. This approach offers support for real-time interaction between heterogeneous mobile devices and considers a dynamic peer-to-peer topology with shared state and consistency mechanisms. Introduction In the near future wireless networks will appear with greater ubiquity, from public access networks offering connectivity in shopping centres, towns centres and even beaches, to telecoms operated 3G and 4G networks. It is envisioned that a wealth of novel applications could be deployed on these networks through the use of their multimedia capabilities. One of the most prevalent driving forces behind the deployment of these networks is the predicted success of wireless computer games in which some industry analysts [1] believe that the medium will generate $17.5 billion in annual revenue worldwide by One of the greatest concerns with current approaches to mobile applications design within industry is that of making applications which may be deployed across a range of devices that are highly heterogeneous in their hardware and operating systems. In addition to this at the network level there will be a future requirement for these devices to be able to communicate with each other between the heterogeneous set of networks that they might be connected to. Mobile IPv6 is seen as a unifying protocol for these networks since it will not only allow devices to interoperate over different networks but also allows seamless roaming between networks and adds support for devices with multiple network interfaces. For example, this would allow a device which has lost IEEE coverage to continue communications over its 3G network interface since it maintains the same globally unique IPv6 address. When high speed, wireless, wide area networks and Mobile IPv6 are combined, application designers no longer face the sparse network environments traditionally associated with mobile computing. Devices will move from a world were they are intermittently connected, to a richer one in which they will have always on connections and not struggle to find a network connection. They will design for an environment where they often have the choice of a number of wireless networks. The Mobile IPv6 Testbed project[2] at Lancaster University has deployed an experimental metropolitan area mobile IPv6 network in order to provide real world trials through the development of novel applications and services. A large amount of this work involves analysis and testing in order to determine the best way to deploy these networks and services in order to ensure that they interoperate with existing technologies and function adequately. Following on from this they will then be evaluated by members of the

3 public ranging from university students to citizens and visitors to the area. One such application is known as Real Tournament. Real Tournament The goal of Real Tournament (RT) [3] was the development of a non-trivial application which made great demands on both a mobile device (a Compaq ipaq 3970) and a Mobile IPv6 enabled network (running over IEEE b). In terms of the network it was envisaged that such an application would encourage frequent handoffs between IPv6 networks in order to evaluate mobility while at the same time utilising different forms of network traffic. We decided that an application which most suited these requirements would be a mobile game, since many games have a real-time low latency requirement. We also felt that building such an application would enable us to determine new areas of research within mobile computing. RT is novel as a game in that it is played using a custom built augmented reality handset (Figure 1) that maintains a mapping between a player s physical world and the game arena as presented on the handset display (Figure 2). This mapping is accomplished through the use of a built-in GPS device for location information, a digital compass for orientation, and a microswitch behind the handset s trigger which are all multiplexed through the ipaq s serial port. Figure 1 - The Real Tournament handset Figure 2 - Screenshots of the user interface The game is played at a local park that has at its centre a large building from which four wireless access points face North, South, East and West with a separate IPv6 network prefix allocated to each cell. The task of each player is to capture as many monsters in the arena as possible by physically moving around the park using their handsets to track monsters in realtime. As a player navigates the arena their state information is sent over the network to other players within the same game. If players are allocated teams they may also make use of voice over mobile IPv6 which generates additional bursts of real-time network traffic.

4 Identified Issues While developing the game we discovered a number of issues with the traditional approaches to providing both support for mobile devices and real-time gaming. The issues of immediate concern were related to the initial design choice of developing the application using a centralised network architecture. The requirements for our architecture were that we could provide low latency interaction while maintaining a high level of localised consistency. It is widely accepted that to achieve this, most fixed network real-time games such as Unreal Tournament[4] and Half Life[5] make use of client server architectures because they afford the following advantages: The Server can aggregate events from multiple clients into one packet therefore reducing the amount of redundant data such as packet headers etc The server knows the location of each player within a gaming arena and can generate an area of interest for a player and only send events which are relevant to that particular players machine. For example only state information regarding players and projectiles within a users immediate area are sent. Games with larger numbers of players or being played over long distances can ensure low latency by using a mirrored server [6] or proxy based [7] approach. These approaches usually make use of a private multicast network which is used to maintain state between a number of game servers each located close to a subset of the players. However, when applied to a wireless environment a centralised approach suffers because users are mobile and servers are not. The reason for this is made up of these issues: Centralised approaches rely on servers being fixed at a well known address, with this in mind and mobile games being potentially playable in any location, where do we locate your servers and mirrors to minimise latency for all players? What happens if the most suitable location for these servers changes, how can we deal with it? In addition to this, players may switch networks a number of times perhaps radically changing their topological closeness to the nearest game server. A private multicast network may be a political and logistical nightmare to set up, especially over separately administrated mobile networks. Centralised approaches offer little in the way of support for dynamic changing topologies since mobile IPv6 nodes generate signalling information everytime they switch to a new network. Servers could potentially be flooded just by signalling information. We therefore believe that the way in which some services are offered must change radically to reflect the fact that network nodes are mobile.

5 A Peer to Peer Solution We propose a generic architecture based around a hierarchy of peer groups which are responsible for forming a dynamic topology and for the distribution of consistent realtime state. In order for this model to work it is based on the assumption that highly real-time, events (such as firing and player/monster movement) are exchanged within the bounds of a peer group while events which are less real-time but have a more global affect (such as player kills and scores) may be propagated outside of a peer group. It should be noted that we are considering consistency mechanisms to aid in minimising bandwidth while maximising consistency such as Timewarp[8], local lag[9] and dead reckoning[10] algorithms but these are outside of the scope of this paper. We believe that an approach such as this will offer the following advantages: Low latency interaction as highly consistent highly real-time events are passed directly only between nodes of immediate interest. This has as a side effect the benefit of providing area of interest filtering since only important global events are forwarded outside of a peer group Scalability since the network is dynamic and self regulating thus load balancing itself. Since nodes regulate event propagation, determine the size of peer groupings and to some degree network latency, there is also an opportunity to deploy interactive applications which do not require specialised services to have been deployed in strategic points of the core network. This helps avoid prohibitive costs in terms of money and administration time. The use of replicated state which provides increased reliability since state is distributed on multiple client devices and support for late joiners since any mobile node may update another user s state. Furthermore, since events are passed directly between peers and processing takes place on the device, latency in the system is further reduced. We believe that such a model would be highly suited to mobile applications, particularly those that are location sensitive since time could be saved in locating the nearest peer group. In addition to this mapping peer groupings to locations might be useful in the following application instances: Tourist Guides Partitioning the network into a hierarchy so that events of immediate interest relate to a person s location ensures that the user has the most focused information available to them at all times. In addition to this users would be well informed about other locations through global events being passed from other locations which were allowed out of other local peer groupings. Car Safety Systems When travelling at speed on a motorway drivers are immediately interested about what is happening to cars that are close by, however they like to be aware of the state of the road ahead. Once again real-time critical information could be passed locally between a cars peer group and global events that did not have such stringent real time requirements (such as an oil spillage 10 miles away) could be passed about the hierarchy between peer groups.

6 Summary and Future Work This paper has identified the need for new architectures to support next generation mobile networks and services, particularly for those of a real time nature. It has also suggested that a peer to peer architecture might provide more than just support for file sharing applications. We believe that a number of further benefits may exist through a hierarchical peer to peer approach and we intend to discover and evaluate them through deployment on the Lancaster Mobile IPv6 Testbed. Although there may be gains to be made from the use of a peer to peer solution it is not yet apparent whether the approach will be successful, particularly when taking into account both the signalling for Mobile IPv6 and the peer to peer hierarchy. In addition it is still unclear how a user might be able to design programs to easily make use of this architecture. We hope to tackle many of these problems in our future research and testing. References [1] Leavitt, N. Will Wireless Gaming Be a Winner?", IEEE Computer, Volume 36, No 1, pp 24-27, January [2] The Mobile IPv6 Testbed Project. Available via the Internet at [3] Real Tournament. Available via the Internet at [4] Unreal Tournament. Available via the Internet at [5] Half Life. Available via the Internet at [6] Cronin, E. Filstrup, B. Kurc, A. R. and Jamin, S. An Efficient Synchronization Mechanism for Mirrored Game Architectures. Proc. NetGames, Braunschweig, Germany, [7] Bauer, D., Rooney, S. Scotton, P. Network infrastructure for massively distributed games, Proc. NetGames,Braunschweig, Germany, [8] Ferscha, A. Lüthi, J. Estimating Rollback Overhead for Optimism Control in Time Warp. Proc. 28th Annual Simulation Symposium, 1995, Phoenix, Arizona, USA. [9] Mauve, M., Vogel, J., Hilt V. and Effelsberg, W. Local-lag and Timewarp: Providing Consistency for Replicated Continuous Applications, IEEE Transactions on Multimedia, 2002 [10] Mauve, M, How to Keep a Dead Man from Shooting. Proc. Seventh International Workshop on Interactive Distributed Multimedia Systems and Telecommunication Services (IDMS) 2000, Enschede, the Netherlands.