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A Distributed Multiplayer Game Server System

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A Distributed Multiplayer Game Server System A Distributed Multiplayer Game Server System Eric Cronin Burton Filstrup Anthony Kurc Electrical Engineering and Computer Science Department University of Michigan Ann Arbor, MI 48109-2122 g fecronin,bfilstru,tkurc @eecs.umich.edu May 4, 2001 2 Chapter 1 Introduction 1.1 Background client-server architectures or, less frequently, on top of peer-to-peer architectures. Client-server archi- Online multiplayer games have improved dramati- tectures are simple to implement and allow gam- cally in the past few years. The newest online virtual ing companies to retain control over the game state. worlds such as Everquest [5] feature fantastic art- Peer-to-peer architectures deliver lower latencies work, realistic graphics, imaginative gameplay and and eliminate bottlenecks at the server. Our pro- sophisticated artificial intelligence. Before the next posed Mirrored-Server architecture requires a com- generation of multiplayer games can complete the plex consistency protocol, but it achieves low latency transition to lifelike virtual worlds, they must be able and allows administrative control over the game to support real-time interactions. The main obsta- state. cle to real-time interactions is the Internet’s inability In order to make our mirrored server architecture to provide low-latency guarantees. In this paper, we attractive, we have developed a consistency proto- present a system for enabling real-time multiplayer col that can be used to port a client-server game to games. a Mirrored-Server architecture with minimal modifi- In particular, we focus on real-time multiplayer cations. The two key pieces of the consistency proto- games that have strong consistency requirements. col are a synchronization mechanism called trailing That is, all players must share a common view of state synchronization and CRIMP, a reliable multi- a complex virtual world. The combination of low la- cast protocol. tency and absolute consistency is difficult to achieve Previous synchronization mechanisms such as because messages may be delayed indefinitely in the bucket synchronization and Time Warp are not well network. Our system realizes these goals using three suited to the demands of a real-time multiplayer building blocks: a Mirrored-Server architecture, a game. Trailing state synchronization is designed trailing state synchronization protocol, and a low- specifically for real-time multiplayer games, and latency reliable multicast protocol. We have imple- achieves better responsiveness and scalability while mented and integrated these three components into a maintaining near-perfect consistency. working prototype and performed some preliminary The consistency protocol assumes a low-latency, experiments on our system. many-to-many reliable multicast protocol. Previous Commercial games are either designed on top of reliable multicast protocols focus more on scalability 3 of state rather than overall packet latency. CRIMP is We outline Quake’s architecture, including its mas- a custom-built, application-specific multicast proto- ter server. We describe Quake’s interest management col that provides the infrastructure requirements of techniques and then delve into details about the mes- our architecture in a low latency manner. saging protocols. Two bootstrapping services will be included in the final system, although they have not yet been imple- 1.2.1 Introduction mented. The first is a master server that allows users to locate candidate game servers. The second is a Quake is a 3-D first player shooter, by id Soft- server selection service named qm-find that can be ware, the creators of a revolutionary and spectac- used to locate the closest game server to a particular ularly successful line of games including Wolfen- client. stein 3D, Doom, Quake and Quake III. Besides 3- The most significant result of this project is a D graphics, their major innovation was the online fully operational, distributed Quake system, com- multiplayer game. College students found that they plete with a working synchronization mechanism could put the newly installed dorm LAN’s to use and a functional multicast protocol. In addition, we by engaging in mortal combat with their neighbors. have established the infrastructure to test and evalu- Multiplayer Quake is a simple game. Your avatar ate our system under different network conditions. has a gun and so does everyone else. You’re going to The experimental evaluation of trailing state syn- die in the next minute or so, but before that happens, chronization is in its early phases, and our prelim- your goal is to finish off as many of your opponents inary results do not adequately explore the mecha- as possible. All interactions take place in real-time, nism’s potential to respond to different network con- and more than a 100 ms lay can be a crippling hand- ditions. In future work, we would like to concen- icap. trate on understanding the effects of different con- John Carmack, the mastermind behind Quake, re- figurations on trailing state synchronization’s perfor- leased the source code in 1999. Two open source mance. projects arose to improve Quake, QuakeForge and QuakeWorld. The two subsequently merged their sources into Quakeforge, which has developed sev- 1.2 Quake eral new versions of the original game. Develop- ment in Quakeforge is sporadic, with a handful of We chose Quake as our proof-of-concept game be- people doing most of the work. Their major contri- cause it is very sensitive to latency and provides a butions have been porting Quake to new platforms, virtual world environment. Quake is a commercial supporting more sound and graphics hardware, and game that was not designed for a distributed archi- enhancing the graphics. Quakeforge has also built in tecture. As such, Quake forces us to address the some simple cheat detection algorithm and a denial- needs of a real game, as opposed to a game that fits of-service attack prevention mechanism. neatly into our architecture. Another reason for using A significant amount of information about Quake Quake is that it is one of the few commercial games has been compiled by the Quake Documentation for which the source code has been released. Project. Two pieces of this project describe the net- In this section, we introduce Quake and describe work code: The Unofficial Quake Specification and the features of Quake that are relevant to our report. The Unofficial DEM Format Description. For the 4 most part, these documents focus on the details of server also informs clients of world state: gravity, the message formats. Neither is a comprehensive lighting and map information. Aside from its role in look at the networking issues in Quake. Other Quake gameplay, the server is a session manager that starts resources can be found at the Quake Developer’s and ends games, accepts client connections and per- Pages. forms numerous other administrative tasks. The purpose of this document is to give a high- The client sends simple avatar control commands level overview of the technologies Quake uses to en- (movements, tilts, buttons) to the server. It uses able multiplayer games. The first section describes its cache of the game state plus any updates from the Quake Client-Server architecture. Next, three op- the server to render a 3-D representation of the vir- timizations are discussed: dead reckoning, area of in- tual world. The graphics rendering is performed terest filtering and packet compression. The Quake- on the client, as is fine-grained collision detection. forge modifications to prevent cheating and denial- The client has been the main area of interest in the of-service attacks are discussed. Finally, the last sec- game developer community because it is where all tion presents the network protocols. the ’cool graphics’ are done. However, from an ar- chitectural or networking point of view, the Quake 1.2.2 Architecture client is relatively simple compared to the server. Quake uses a centralized Client-Server architecture. 1.2.3 Master Server In this architecture, the server is responsible for computing all game state and distributing updates Quake supports a master server that keeps track of about that game state to the client. The client. The active game servers on the Internet. Although the client acts as a viewport into the server’s state and master server is not packaged with Quake, GameSpy passes primitive commands such as button presses provides a full-featured master server service. For and movement commands to the server. Because this a Quake server to be listed in GameSpy’s database, architecture allows most of the game state computa- the administrator must manually add GameSpy’s IP tion to be performed on the server, there is no consis- address to the master server list. The server then pe- tency problem. However, the gameplay is very sen- riodically sends heartbeats containing the number of sitive to latencies between the client and the server, connected client to the master server. The server also as users must wait at least a round trip time for key informs the master server when it is shutting down. presses to be reflected in reality. The server is responsible for receiving input com- 1.2.4 Area of Interest Filtering mands from client, updating the game state, and sending out updates to the clients. The control loop One of the advantages of having a server architecture for this procedure is discussed in more detail in Ap- is that the server can dynamically filter information pendix A. The server’s biggest job is keeping track before sending it to the clients. The Quake server of entities. An entity is almost any dynamic object: only sends a client information for entities that are avatars, monsters, rockets and backpacks. The server within that player’s potential field of sight. Simi- must decide when a player is hit, use physics models larly, sounds are filtered to those that are within that to calculate the position of free-falling objects, turn player’s earshot.
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