Название-Сеть в Играх 2006год_Язык-Eng

14 Networking and Online Games: Understanding and Engineering Multiplayer Internet Games

Doom peers communicated by using Ethernet broadcasts for all of its traffic. This had the side effect that when a player shot a bullet, the Ethernet packet the Doom peer sent was not only received by all other Doom nodes, but also all other computers on the same LAN were interrupted. The other computers not playing Doom would ignore the broadcast packet, but their processing was still interrupted so they can receive the packet, transfer it to main memory and then have the operating system determine that they do not need it. Normally, LAN traffic is addressed directly to a machine and it is either not received by other machines or it is discarded by the network card before interrupting the processor.

The significant processor time wasted by the computers not participating in a Doom game, but still handling the broadcast packets, was significant, especially for the slower machines of the day, and could even cause them to drop keyboard keystrokes. This was a serious problem for network managers, prompting companies such as Intel and many colleges and universities across the United States to implement specific anti-Doom policies in an attempt to reduce congestion on the local computer networks.

Doom was immensely popular. While the total game sales of 1.5 million copies is not enormous compared with modern blockbuster titles, the shareware version better reflects Doom’s popularity. The shareware version of Doom was estimated to have been downloaded and played by 15 to 20 million people [MOD], and installed on more computers than Microsoft’s Windows NT and IBMs OS/2 combined. The popularity of multiplayer Doom, particularly the death-match mode, influenced the genre of nearly all FPS games to follow, both in terms of game play and in terms of networking code.

In 1994, id Software produced Doom 2, an impressive sequel to Doom. Doom 2 sold over two million copies, making it the highest-selling game by id at that time. Doom 2 could support eight players and, more importantly, Doom’s initial use of broadcast packets was removed, and this change brought with it a marked change in the acceptability of networked games on LANs and wide area links.

References

[DOOMENGINE] http://doom.wikicities.com/wiki/Doom networking engine, Accessed 2006

[PONG] The Pong Story. The Site of the First Video Game. [Online] http://www.pong-story.com/intro.htm [MOD] David Kushner. Masters of Doom, Random House, 2003. ISBN 1588362892.

[PLATO] Dear, B., PLATO People – A History Book Research Project, http://www.platopeople.com/ [RFC854] Postel, J. and Reynolds, J., “Telnet Protocol Specification”, RFC 854. (May 1983)

3

Recent Online and Multiplayer

Games

In this chapter, we will deal with the following:

(a)Introduce game communication architectures and their communication models.

(b)Briefly describe the developments in online game play for First Person Shooter (FPS) games, Massively Multiplayer Online games, Real-Time Strategy (RTS) games, and Sports games.

(c)Briefly describe the evolution of game platforms to support online play, including Personal Computers (PCs), Game Consoles and Handheld Game Consoles.

(d)Put games into the broader context of other immersive environments and distributed simulation, including augmented reality (AR), telepresence and virtual reality.

3.1Communication Architectures

The evolution of online games must consider several different architectures for arranging the communication between game nodes. The different alternatives are depicted in Figure 3.1. The circles represent different processes on remote computers with the links denoting processes that exchange messages.

In the earliest days of multiplayer games, there was no networking between players. Multiplayer functionality was achieved by having both players interacting with the same computer. Players could manipulate their avatars on a shared, common screen or the screen could be physically ‘split’ by partitioning part of the video screen for each player. Many console games that allow multiple players still use the single screen, multiplayer architecture.

In a peer-to-peer architecture, each client process is a peer in that no process has more control over the game than the others. There are no mediator nodes to control game state or route game messages. Peer-to-peer architectures are popular in multiplayer games played on a Local Area Network (LAN) because of the broadcast support of many LANS (e.g. wired or wireless Ethernet) and generally small number of players that participate in a single game. While peer-to-peer architectures can be applied to Wide

Networking and Online Games: Understanding and Engineering Multiplayer Internet Games

Grenville Armitage, Mark Claypool, Philip Branch 2006 John Wiley & Sons, Ltd

16 Networking and Online Games: Understanding and Engineering Multiplayer Internet Games

Area Networks (WANs) (i.e. the Internet), they do not scale well without an additional hierarchical structure.

In a client–server architecture, one process plays the role of the server, communicating with each client and mediating the game state. The clients do not communicate directly with each other but rather have the server route messages to the appropriate clients. The server is the critical part in the communication link; if a client cannot communicate with the server the game cannot be played and if the server cannot keep up with the communication and computation required, the gameplay can degrade for all clients. The client–server architecture is the most popular architecture used in commercial online games as well as in the classic MultiUser Dungeon (MUD) games (see Chapter 2 for a description of a MUD).

In the peer-to-peer, client–server hybrid architecture, the server process mediates game states on the basis of information sent by clients as in the traditional client–server architecture, but the clients are also able to communicate with other clients as in the traditional peer-to-peer architecture. The communication amongst the client peers is generally for game information that is not essential for achieving consistent views of the game state

Figure 3.4 Screenshot of Doom 2. Reproduced by permission of Id Software, Inc.

Note: Tunnelling is an often-applied technology where a network packet from one kind of network protocol is encapsulated, headers and all, into a data packet in another, lower network protocol. One computer then encapsulates the higher protocol packet, sends (tunnels) it over the lower protocol to a destination where another computer will unpack the higher protocol packet and transmit it normally (see Chapter 4 for more details on IP tunnelling).

Multiplayer network gameplay was significantly improved in 1996 with the release of id Software’s Quake (Figure 3.5). Quake featured a method that allowed players to compete against each other over the Internet without the need for tunnelling. Before Quake, players needed to coordinate times and places (Internet addresses) to meet online in a game. Quake addressed these problems with the inclusion of servers that stayed-up for repeated rounds, hosting death-match after death-match, so that players from all over the world could connect to these servers at any time of the day or night, and always be able to find a game. The Quake servers acted as persistent game hosts. Players would connect via their Quake clients, with the player’s input sent to the server, which would keep track of the state of the game world. Information about the world would be periodically transmitted back to each of the clients, updating their view of the world to match the one currently running on the host machine.

This paradigm created new network problems that had not been faced by game developers before. In particular, the client–server architecture had worked well on a LAN with its high-bandwidth, low-latency connections that were capable of quickly sending and receiving many transmissions for each player. Unfortunately, most WAN (Internet) connections were not capable of LAN transmission speeds, with most players at the time connecting to the Internet through relatively low-speed dial-up modems.

20 Networking and Online Games: Understanding and Engineering Multiplayer Internet Games

Figure 3.5 Screenshot of Quake. Reproduced by permission of Id Software, Inc.

In 1996, standard-model modems transmitted at 14,400 bps and it took the packets a long time to travel from a client over the modem and across the Internet to a server and back (in games, this time is often referred to as the ping time). The time it took to send a command to the server (such as the firing of a gun) and have it result in a change to the game world (such as hitting an opponent) directly affected the realism, immersiveness and overall playability of the game. Players became interested in minimising their ping times. Lower ping times lead to smoother and more immersive gameplay, and, for some games, a higher score (see Chapter 5 for more information on the effects of ping times, i.e. latency, on gameplay).

Note: The ping time measured by computer games is somewhat different than the ping time measured by network tools such as the network ping tool found on many systems. In particular, computer game ping time measures the time to send a User Datagram Protocol (UDP) packet from the client game process to the server game process and back again. Network ping times, on the other hand, measure the time to send an Internet Control Message Protocol (ICMP) packet from the client operating system to another host and back again. Game ping times are usually slightly higher than network ping times since they include additional overhead from processing by the server and client applications.

id Software responded by releasing QuakeWorld in 1996, a free add-on to Quake that included rewritten network code and a number of game updates [QWORLD]. QuakeWorld was a specific version of Quake optimized for multiplayer Quake play over a modem connection to the Internet. QuakeWorld allowed players to adjust network parameters to minimise the effect of their slow Internet connection on gameplay. In particular,

Figure 3.8 Active MMORPG subscriptions for games with 120, 000 + players (excluding City of Heroes, EverQuest II, Dark Age of Camelot, Runescape, Ragnarok Online, Star Wars Galaxies which are all smaller) [Woo05]

Figure 3.9 Screenshot of Dune II. Reproduced by permission of Electronic Arts.

hit, Warcraft set the stage for Blizzard’s Warcraft II in 1995, one of the biggest successes the RTS genre has known. WarCraft II allowed up to eight people to play simultaneously on a LAN using the IPX protocol. The v1.2 patch for Warcraft II included optimization for network play, but was not, in fact, playable on the Internet since it still used IPX instead of IP. In the light of this, Blizzard released a special executable to facilitate multiplayer Warcraft II over the Kali network. Warcraft II showed that the Internet could support superb multiplayer RTS gameplay that appeared surprisingly resistant to the effects of latency and bit rate limitations, even on slow modem connections. Because of this, Warcraft II