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42 Networking and Online Games: Understanding and Engineering Multiplayer Internet Games IP Transport [packet delivery, TCP, UDP, multicast, QoS, ….] Services [POP3, SMTP, HTTP, DNS, DHCP, Routing protocols, ….] Applications [Half-Life, Everquest, Mozilla, IM,….] Figure 4.1 End-user applications and services utilise packet transport services provided by the IP network ethernet Local Area Networks (LANs), 802.11/WiFi networks, cable modems and so on). IP provides a single, global addressing scheme across all the underlying technologies. 4.1 IP Networks as seen from the Edge From an end user s perspective, there is a reasonable analogy between the postal service and an IP network. With traditional postal service we place letters into envelopes, address them to the final destination, and place them into a local post box. After that, we simply trust the postal service to transport our envelope to its destination in some reasonable time. We neither know nor care how the envelope gets to the destination, the delivery time is measured in days, and we accept that envelopes are sometimes lost. And we implement our own end-to-end strategies to confirm delivery (such as a phone call to the recipient some days later, or reposting a copy of the original letter every few days until the recipient responds). The postal service is a network, and its edges are the post-boxes and letter-boxes where we drop off and pick up our mail. A simple way to view an IP network is as an opaque cloud with devices attached around the edges. These edge devices may be any piece of hardware (or software) that transmits or receives digital information. The primary goal of an IP network is to provide connectivity, that is, deliver data (in packets) from sources (who transmit packets) to destinations (who receive packets). Devices at the edges of IP networks typically act as both sources and receivers. IP edge devices (or endpoints) are identified with IP addresses, and endpoints are required to look after their own needs for reliable delivery of data. And finally, the IP network is presumed to be totally agnostic towards the actual contents of the packets each endpoint is sending. Traditionally, IP networks provide few guarantees of timeliness or certainty in packet delivery usually referred to as best effort service (although it might be more aptly considered a no guarantees services). Milliseconds, hundreds of milliseconds, or seconds may elapse between the time a source injects (transmits) a packet into the network cloud, and the destination edge receives that same packet. Sometimes packets simply get lost inside the network and never arrive at all.

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Basic Internet Architecture The time it takes for a packet to reach its destination is often referred to as latency. Short-term variation in this latency from one packet to the next is referred to as jitter. Packet loss is often described in terms of a packet loss rate the probability of an IP packet being lost across a certain part of the IP network. Despite the simplicity of best effort service, IP networks can be quite complex internally. In all but the most trivial networks, there will be multiple internal paths a packet may take between any given source and destination. The network must continually make internal choices as to which path is most optimal at any given time. This is the task of routing protocols, which we will discuss later in the chapter. 4.1.1 Endpoints and Addressing IPv4 endpoints are identified with numerical 32-bit (4 byte) values, conventionally written in dotted-quad form four decimal numbers (representing each of the four bytes making up the IP address) separated by periods. For example, the 32-bit binary address 1000 0000 0101 0000 1100 0101 0000 0111 is written as 128.80.195.7 in dotted-quad form. In theory, this allows for 232 IPv4 addresses. Later in this chapter we will discuss why significantly fewer than 232 IPv4 addresses are available in practice. Endpoints are also often referred to as hosts, although a host (whether a Personal Computer (PC), game console with network port, or any device capable of attaching to a IP network) may have multiple interfaces to an IP network. A host with multiple interfaces (commonly referred to as a multihomed host) will have unique IP addresses on each of its interfaces to the IP network. Servers (e.g. web sites or game servers) and clients (e.g. someone running a browser on their home computer) are both hosts on the IP network. Figure 4.2 shows a simple case where an IP network has three hosts attached, with IP addresses 136.80.1.2, 21.80.1.32, and 142.8.20.8. When host 136.80.1.2 wants to send an IP packet to another host (for example, host 142.8.20.8), it specifies this destination address 142.8.20.8 in the packet and passes the packet to the IP network. The IP network itself worries about how to locate and reach 142.8.20.8. As a consequence of IP networking s hierarchical routing scheme (described later in this chapter), an IP address is tied closely to where the host is attached in the network. 136.80.1.2 IP Network 142.8.20.8 21.80.1.32 Destination 142.8.20.8 Source 136.80.1.2 Payload Destination 142.8.20.8 Source 136.80.1.2 Payload Figure 4.2 An IP Network opaque cloud with attached devices looks after getting packets to their destinations

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40 Networking and Online Games: Understanding and Engineering Multiplayer Internet Games [CFG+03] A. David Cheok, S. Wan Fong, K. Hwee Goh, X. Yang, W. Liu and F. Farzbiz, Human Pacman: A Sensing-based Mobile Entertainment System with Ubiquitous Computing and Tangible Interaction , Proceedings of ACM Network and System Support for Games Workshop (NetGames), Redwood City, CA, May 2003. [CNET05] CNET Networks, Inc., Playstation3 Xbox 360: Tech Head to Head , 05/16/2005 [Online] http:// hardware.gamespot.com/Story-ST-x-1985-x-x-x [CSD+92] C. Cruz-Neira, D.J. Sandin, T.A. DeFanti, R. Kenyon and J. Hart, The CAVE: audio visual experience automatic virtual environment , Communications of the ACM, Vol. 35, No. 6, pp. 65 72, 1992. [FS98] E. Freecon and M. Stenius, DIVE: A scalable network architecture for distributed virtual environments , Distributed Systems Engineering, Vol. 5, No. 3, pp. 91 100, 1998. [Ger02] B. Geryk, A History of Real-Time Strategy Games, Part I: 1989-1998 , Gamespot, 2002, [Online]http://www.gamespot.com/gamespot/features/all/real time. [Ken03] S.L. Kent, Making an MMOG for the Masses , GameSpy.com, October 10, 2003. [Kus03] D. Kushner, Masters of Doom, Random House, 2003, ISBN: 0-375-50524-5. [MADDEN05] Wikipedia, Madden NFL , (as of 20 January 2006) [http://en.wikipedia.org/w/index.php?title= Madden NFL&oldid=36020223 [MS94] M.J. Massimino and T.B. Sheridan, Teleoperator Performance with Varying Force and Visual Feedback , Journal of Human Factors, Mark Vol. 36, No. 1, pp. 145 157, 1994. [MSA+94] W. Dean McCarty, S. Sheasby, P. Amburn, M.R. Stytz and C. Switzer, A Virtual Cockpit for a Distributed Interactive Simulation , IEEE Computer Graphics and Applications, Vol. 14, No. 1, pp. 49 54, January 1994. [MSTM04] K. Mansley, D. Scott, A. Tse and A. Madhavapeddy, Feedback, Latency, Accuracy: Exploiting Tradeoffs in Location-Aware Gaming , Proceedings of ACM Network and System Support for Games Workshop (NetGames), Portand, OG, 2004. [NC04] J. Nichols and M. Claypool, The Effects of Latency on Online Madden NFL Football , In Proceedings of the 14th ACM International Workshop on Network and Operating Systems Support for Digital Audio and Video (NOSSDAV), Kinsale, County Cork, Ireland, June 16 18, 2004. [Ney97] D.L. Neyland, Virtual Combat: A Guide to Distributed Interactive Simulation , Stackpole Books, Mechanicsburg, PA, 1997. [PCS+06] L.S. Ping, A.D. Cheok, J.T. Kheng Soon, G.P. Lyn Debra, C.W. Jie, W. Chuang and F. Farzbiz, A Mobile Pet Wearable Computer and Mixed Reality System for Human Poultry Interaction through the Internet , Springer Journal of Personal and Ubiquitous Computing, ISSN: 1617-4909, (Paper) 1617-4917, (Online)03 November, 2005. [PT02] W. Piekarski and B. Thomas, ARQuake: the outdoor augmented reality gaming system, Communications of the ACM, Vol. 45, No. 1, pp 36 38, 2002. [QWORLD] Quake World.net, http://www.quakeworld.net/, Accessed 2006. [SG01] S. Shirmohammadi and N.D. Georganas, An End-to-end Communication Architecture for Collaborative Virtual Environments , Computer Networks, Vol. 35, No. 2 3, pp. 351 367, 2001. [SKH02] J. Smed, T. Kaukoranta and H. Hakonen, Aspects of networking in multiplayer computer games, The Electronic Library, Vol. 20, No. 2, pp. 87 97, 2002. [Tut04] W. Tuttle, The Future of Live? , GameSpy, October 24, 2004, [Online] http://xbox.gamespy.com/ articles/559/559846p1.html. [Tys06] J. Tyson, How Video Game Systems Work , HowStuffWorks, Inc, [Online] http://computer. howstuffworks.com/video-game5.htm [Woo05] B.S. Woodcock. An Analysis of MMOG Subscription Growth , MMOGCHART.COM 12.02929 2004. January 1, 2005. Online: http://www.mmogchart.com/.

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38 Networking and Online Games: Understanding and Engineering Multiplayer Internet Games atr eoa nt h mlc (a) Bat sensors SPIRIT Game server Bat feedback Phone Interface Environmental Display ClickslocationBeeps Events Events Events Video Audio Bluetooth Daid Horn (b) Figure 3.26 The BATS game, with a mobile phone providing the interface (a), and the game architecture (b)

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Recent Online and Multiplayer Games AR provides a means of combining computer gameplay, physical exercise and social interaction. Beyond entertainment, more serious uses include the potential for military or disaster response training, allowing physical training exercises with computer augmenting physical reality with virtual conflicts. 3.5.4 Distributed Virtual Environments Most computer games are in the virtual reality cluster, and within this cluster, virtual reality has produced three branches of research that have some relevance to computer games [SKH02]: military simulations, Computer Supported Cooperative Work (CSCW), and virtual reality. (a) Military simulations have proceeded through Distributed Interactive Simulation (DIS ), an IEEE standard for communication designed to allow networked simulators to interact through simulation using compliant architecture, modelling, protocols, standards and databases [Ney97]. A DIS system must be flexible and powerful enough to support up to hundreds or even thousands of simulators. The Virtual Cockpit, a tactics trainer for pilots, is an example of a typical vehicle that participates in DIS [MSA+94]. The Virtual Cockpit is a low-cost, manned flight simulator of an F-15E jet fighter built by the Air Force Institute of Technology. A soldier flies the Virtual Cockpit using the hands-on throttle and stick, while the interior and out-the-window views are viewed within a head-mounted display. (b) Computer Supported Cooperative Work (CSCW) focuses on using computers to support the collaboration of users. An example of a CSCW application would be a shared whiteboard, where participants can read, edit, and annotate a shared document using a variety of media including text, pictures, video and voice. Efforts to combine CSCW with virtual reality has produced collaborative virtual environments (CVEs) [BGRP01] with network architectures to support their online use [SG01]. Successful CVE applications allow users to remotely control avatars to operate on a shared media, allowing 3-D editing, product development and even game design. (c) Distributed Virtual Environments (DVEs) are technology-oriented environments for users to immerse themselves in the virtual world. An example of a DVE is a Cave wherein users immerse themselves in an artificial 3-D world using various combinations of VR goggles, VR gloves and wands and various 3-D audio components [http://www.evl.uic.edu/pape/CAVE/, CSD+92]. DVEs concentrate on the player representation and the technology for interacting with the world. Typically, they only support participation by a few, physically local users, although architectures are being developed to allow larger-scale interaction by more participants [BWA96, FS98]. References [BGR+98] S. Benford, C. Greenhalgh, G. Reynard, C. Brown and B. Koleva, Understanding and constructing shared spaces with mixed-reality boundaries , ACM Transactions on Computer-Human Interaction (TOCHI), Vol. 5, No. 3, pp. 185 223, 1998. [BGRP01] S. Benford, C. Greenhalgh, T. Rodden and J. Pycock, Collaborative virtual environments , Communications of the ACM, Vol. 44, No. 7, pp. 79 85, 2001. [BWA96] J.W. Barrus, R.C. Waters and D.B. Anderson, Locales: supporting large multiuser virtual environments , IEEE Computer Graphics and Applications, Vol. 16, No. 6, 1996.

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Recent Online and Multiplayer Games (a) (b) Figure 3.24 ARQuake, with user wearing head-mounted display and holding a gun (a) and game scene depiction (b). Reproduced by permission of Wayne Piekarski. Health Karma (a) (b) Figure 3.25 Real Tournament, showing super-soaker gun with attached IPAQ computer (a), and game scene depiction (b) Another AR, location-action game uses mobile phones, Bluetooth network technology and high-precision location-aware hardware, called BATS [MSTM04]. The BATS mobile device has enough precision to infer user gestures such as picking virtual objects up or shooting (Figure 3.26). The mobile phone provides map information with the location of game objects to augment the reality of the physical walls and doors. A centralised computer that controls the game play also allows observers to monitor game play or even serve as commanders surveying the game battlefield.

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Recent Online and Multiplayer Games Figure 3.21 Picture of an arcade version of Dance, Dance Revolution. Reproduced by permission of Konami. display, mobile computer, head tracker, and GPS system to provide inputs to the game computer, allowing the player to walk around the real, physical world and fight against virtual Quake monsters. A game called Real Tournament5 is also an AR shooting game. However, instead of a head-mounted display, Real Tournament (Figure 3.25) attaches an IPAQ Pocket PC with a 802.11 WLAN network card to a super-soaker squirt gun. A microcontroller resolves communication between the trigger, the GPS and an electronic compass and sends the information to the IPAQ. The IPAQ also provides team communication by a push-to-talk audio feature over the wireless network, allowing players to synchronise their actions. The network includes a wireless network around parts of a University campus, based on IEEE 802.11b and GPRS. The IPv6-enabled IPAQs are mobile, allowing players to transparently roam between the wireless networks without losing connectivity or disrupting the game state. 5 The name is a play of words from Epic s Unreal Tournament.

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36 Networking and Online Games: Understanding and Engineering Multiplayer Internet Games Figure 3.22 Screenshot of Human Pacman showing first person view of game from head-mounted display. Reproduced by permission of Adrian Cheok. Figure 3.23 Information flow in Human Pacman. Reproduced by permission of Adrian Cheok.

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Recent Online and Multiplayer Games Figure 3.20 Real people playing Rugby. Do not underestimate the immersiveness of Physical Reality!. Reproduced by permission of Tiffany Wolf. with the game objects being tangible and concrete. The variety of such physical reality games range from sports, such as cricket and rugby (Figure 3.20), to board games such as chess and backgammon. While games in this cluster are generally not computer supported, there can be computers enhancing aspects of the game. For example, sensors and computer equipment allow accurate calling of the boundary lines in professional tennis. In addition, asynchronous communication by computers can allow games to be played without participants being in the same physical location. For example, chess can be played by two people who are geographically separated and use email or even VoIP to communicate their moves to their opponent. 3.5.2 Telepresence Telepresence uses computers and networks to provide a feeling of immersiveness for a user that is physically separated from the environment. Sensors gathering information on the local environment transmit information to a physically separated user, providing feedback that allows some recreation of the local world from the remote location. With telepresence, managing latency is critical for providing a feeling of immersiveness. In the local world, an action has an immediate consequence, whereas in telepresence, an action done remotely is delayed by the time it takes to travel from the remote user to the local environment, plus any additional processing time. Force feedback, where tension or pressure occurs from physically manipulating the local environment, is also often necessary to make the remote user immersed in the local world and can often help overcome deficiencies in the display from a low resolution or frame rate [MS94]. Examples of telepresence include undersea work to repair pipelines and cabling system. Deep water diving is risky and costly, so telepresence systems can prove cheaper as well as

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34 Networking and Online Games: Understanding and Engineering Multiplayer Internet Games posing fewer risks to humans. Similarly, other hazardous environments such as explosive disposal, rescue work and even mining can benefit from telepresence systems. Another example of telepresence is computer-assisted surgery, which is gaining in popularity because it allows specialists to provide medical care to a geographically broader audience, as well as providing opportunities for computers to make surgeons more precise. When there are concerns about hazardous environments, this brings telepresence to the front line of battlefields, allowing surgeons to aid soldiers during combat. Telepresence in education can allow people to experience environmentally fragile locations, such as the coral reefs or Egyptian tombs. Space exploration can also benefit from telepresence, as in operating a rover on Mars from Earth [http://marsrovers.jpl.nasa.gov/ home/]. Telepresence systems could even be incorporated in entertainment systems beyond games, by allowing the experience of a roller coaster or sky-dive without the associated risks. Telepresence has even been applied to human-animal communication. A system has been developed that allows humans to remotely touch poultry by having the animal wear a lightweight, tactile pet jacket. The human interacts with a physical representation of the animal (a chicken-robot) and the feedback from the robot is passed through the Internet to the real chicken [PCS+06]. 3.5.3 Augmented Reality With Augmented Reality (AR), the users can still interact with the local, physical environment but artificial, computer-generated objects and attributes are used to enhance the local reality. A popular game that is a step toward AR is the arcade hit Dance, Dance Revolution (DDR), initially released by Konami in 1998 with numerous variations available in the arcade, home PCs and game consoles. In DDR, players move their feet to a set pattern, stepping on arrows on the floor to the rhythm of the song. Multiplayer versions allow two players to play cooperatively or in competition. In some sense, DDR as in Figure 3.21 provides a novel interface to a computer game, but in another sense, it augments the reality of dancing to music by providing visual feedback on the dancing itself as gameplay. As an example that augments reality even more, consider the original Pacman game that featured the well-known, cookie-shaped disk that game players manoeuvred through maze munching dots. An AR version of the game called Human Pacman depicted in Figure 3.22 [CFG+03] has game players assume the role of the gobbling disk where the physical world is augmented with virtual dots the players see by means of a wearable computer with a head-mounted display. Sensors on the display indicate the direction the player is looking and the GPS provides positional information to generate virtual dots at the appropriate physical locations. Multiplayer aspects are incorporated by having some players assume the role of the ghosts that chase Pacman. Bluetooth technology provides networking to enable the Pacman player to pickup power pills to then chase the ghosts. Human Pacman goes beyond basic Pacman gameplay of one yellow disc against all the ghosts by enabling an additional player to see an overview of the game field on a networked computer, then relaying information about the location of ghosts and power pills to the human Pacman (Figure 3.23). FPSs have also entered the AR game space, notably with ARQuake an AR version of the popular Quake game [PT02]. ARQuake players as in Figure 3.24 use a head-mounted

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