An Expedite State Dissemination Mechanism for MMOGs

Dewan Tanvir Ahmed and Shervin Shirmohammadi Distributed and Collaborative Virtual Environments Research Laboratory (DISCOVER Lab)

School of Information Technology and Engineering University of Ottawa, Canada

{dahmed, shervin}@discover.uottawa.ca

Abstract

Massively Multiuser Online Game, i.e. MMOG, is

an active research especially to integrate players to perform routing task with the intention of keeping the game deployment cost to a minimum. This paper proposes a novel state dissemination mechanism for MMOG in the context of hybrid P2P architecture by exploiting the idle period of the participating players. The proposed approach performs such task well ahead of other traditional approaches with the identical environmental setup. It expedites the task of state sharing even with heterogeneous environment. The performance of this technique is compared with the most popular multi-tree dissemination method. The measurement validates its superiority. 1. Introduction

MMOG is a large distributed application with the involvement of potentially thousands highly active players that requires processing of events synchronously. The nature of such application needs sharing of large state information for consistent game world. The centralized approach that most of the commercial companies follow has clear advantage at least in terms of management and central power. But the deployment cost has a significant impact on users. The subscription fee is really high. Thus, the benefit of providing MMOG service over peer-to-peer system follows the necessity.

The feasibility of carrying MMOG over peer-to-peer architecture is now investigating at different angles. Despite the development of some solutions to define appropriate peer-to-peer architectures and distributed communication protocols to carry time dependant state information, still many issues are remain unexplored like ensuring robustness and relaxing time constraint at every possible aspect. In this paper, we shall explore a speedup state distribution approach for MMOG in the peer-to-peer domain. In

earlier works, it has already been mentioned that pure theoretical peer-to-peer approach is technically impossible to provide such game services; hence we propose hybrid architecture that integrates servers and participating players in collaborative manner [1][2].

The consistency preservation of virtual world for gaming applications like First Person Shooters (FPS) has high importance. But the Internet is not well suited in this aspect because of security and timing issues. In this paper, we are going to propose a speedup state dissemination mechanism that addressed second one. The significance will be presented in the measurement section of the paper.

The organization of paper is as follows. Related work is given in Section 2, while in Section 3; a brief MMOG architecture is presented. We elaborate the new concept in Section 4. The measurement and analysis of the proposed expedite state dissemination mechanism is presented in Section 5. Finally, we conclude the paper in Section 6.

2. Literature review

Internet games are very popular to the game communities where a player can play against any player in the world. There are many types of online games each one is quite different from others in terms of complexity, timing requirements and constraints. Other aspect of such games is the number of players involved in the game. For example, the Internet version of Microsoft Hearts or online poker deals with a few players as compared to Second life or Sony’s EverQuest. The second class of games has more complexity and stringent timing constraint and thus requires special care. This class of games is commonly known as Massively Multiplayer Online Games (MMOG) or role playing games. In MMOG, an attractive and challenging virtual setting is given to the players where each player has an objective to improve his skill and/or to enhance his resource.

The commercial MMOGs are centralized. It is effective in terms of controlling the game logic as well

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as less volatile to player dynamics. But it is expensive to deploy and also has single point of failure problem. Recently researchers are looking for hybrid approach as an alternative solution to reduce such huge deployment cost by integrating the participating players to perform some of the state relaying task [3][4][5]. The MMOG covers many aspects like zoning, peer dynamics, inter zone communication, intra zone communication, seamless zone crossing etc. According to our extensive literature review, none of them are complete and there are many ways to enhance the architecture.

MOPAR is an online peer-to-peer game architecture [5]. It integrates both structured and unstructured peer-to-peer system where a master node is chosen in each zone and becomes the root of all other players in the zone. The dependent nodes are called slaves. Each master node supports all of the slaves within its zone. Although the architecture is P2P in the sense that master node is also connected other master nodes and manages inter-zonal communication, the network architecture within a zone looks like the client-server architecture. So, it faces single point of failure problem. One of the main drawbacks of MOPAR is unexploited slaves’ bandwidth as slaves are only connected to the master node, not among themselves. HDSP [6] and DS-ALM [7] are some other approaches those can be classified into hybrid P2P domain. In this paper, we are going to present a novel state dissemination mechanism that would be more effective to use player’s unused resources compared to the above mentioned methods.

3. The overlay architecture

A brief MMOG architecture is given in this section to maintain the continuity of the paper. In MMOG, the game world is partitioned into small manageable areas or zones for proper state sharing. Typically players within the same vicinity form a zone. But there are some other definitions in this regard. Hexagonal shape has a couple of advantages over other shapes as it keeps a regular shape to enable zones to stick together without having any gap in between, which could be the case if we choose circles as zone shape. Such issues are comprehensively covered in [3].

Each zone has a master that supports the interactions of the players within a zone. In most cases, this communication pattern resembles to the multicast structure. It would be perfect if we can use IP multicast. The limitations of IP multicast have been discussed over the last decade. Scalability and deployability are two big problems that make IP multicast unpopular [8]. We are not iterating those

issues in this paper. To overcome the functionality limitations of the IP multicast, application layer multicasting (ALM) has been chosen as an alternative solution. The advantages of such approach were presented and explored extensively in [9]. A set of master nodes manages MMOG operations and provides overlay services with a lively participation of the players. In that sense, the architecture is hybrid as it combines both centralized and distributed features.

The following assumptions are taken to demonstrate our expedite state dissemination mechanism in the context of hybrid MMOG architecture: (a) each player has a unique identification number, (b) each player is unit distance apart from each other to give mathematical equations (but in measurement section we discard this assumption); (c) every player knows the identity of at least one master. These are necessary and reasonable assumptions, which will be used in the design of the system. In this paper, we keep our interest solely to the state dissemination mechanism in a zone. Some other MMOG issues can be found in [1][4] [5][7] [10]. 4. Proposed state dissemination mechanism

The fundamental difference between a game application and a streaming system is the pattern and the timing constraints of transmitted data. In MMOG, the data is represented by states that carry useful information to describe a player in the virtual world and sharing of such states with other interested players are very important. Moreover, an event generated by a player may produce more events because of the reactive actions of the involved players. Thus, the traffic pattern is completely different that a streaming system has. In MMOG, the traffic pattern is not continuous. In most cases, message is short but recurrent and requires sharing it with other interested players in the system synchronously. The limit of this constraint varies from action to action, typically in 100 ms to 1000 ms range [11].

In order to have better state management mechanism, the hybrid MMOG system is decomposed into several static or dynamic zones where each zone is coordinated by a central server. Based on the Area of Interest Management (AoIM) model such as geographical or behavioral, continuous or discrete view, each zone requires different inter zonal and intra zonal communication policies. These policies are very important to satisfy timing requirements as well as to provide efficient MMOG service like seamless zone crossing. In our earlier works, we have given a couple of algorithms that addressed the above mentioned issues [1][4][7]. In addition, each communication

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approach follows either mesh-first or tree-first technique to disseminate state information. In this paper, we present an efficient and elegant state dissemination method that would be a booster for scalability.

Figure 1. State information distribution

through binary tree

Figure 2. State information distribution

through speedup mechanism

Consider a system with N players where each player is evenly apart from each other. Assume that each player can simultaneously relay message to two other players with the bandwidth it has. The best possible solution to distribute the state information is through a binary tree. According the Figure 1, say N equals to 126, 6 time slots are required to complete the task. The number of players those have the state information at a particular time slot is shown in Table 1 (second column). One of the weaknesses of such approach is unexploited bandwidth of the players in the following time slots if the players are idle. Consider Figure 1 again, at time slot 4, 8 players forward state information to 16 other players. But there are 7 other players (1+2+4) who are capable of to do the same task. In this case, they are idle (off course idleness depends on application). We should use the resources that these players have. This may lead to a complex protocol but the scarcity of bandwidth insists to go for such a solution. The slot by slot state forwarding is given in Figure 2. The key concept is to define a routing table that presents the speedup routing mechanism.

Table 1. Number of new players received state information at a particular time slot

Time slot Binary tree Speedup 1 2 2 2 4 6 3 8 14 4 16 30 5 32 62 6 64 126

4.1. Homogeneous case

The homogeneous case is covered in [12]. As the

heterogeneous case formulation depends on homogeneous derivations, we briefly cover relevant points. Let each player has a degree of m. From simple mathematical derivations, it is clear that traditional tree like approach takes Nlogm time slots.

Let p(i) denotes number of players have the state information at the beginning of the time slot i. So, according to the expedite method explained earlier, it has the form like 1111 −+=−×+= i)m()i(p)m()i(p . So the time required to distribute the message is Nlogm 1+ . 4.2. Heterogeneous case

It is more complex to model a system with

heterogeneous features. In this case, we have to consider a case where players have different degrees. We will solve it through the same concept that we have used in the homogenous case. Let us form groups those have same degree. Say, there are g group with different degrees gm,...,m,m 21 and each group has different

gN,...,N,N 21 players respectively, where∑ = NN j . The order of state dissemination has an impact on

performance. For better functioning, it is intuitive to forward state information to the players having higher degrees. Let gm,...,m,m 21 is ordered in the descending order of degree. So, the time required to share state information with N1 players of the Group 1 each having m1 degree is 111

Nlogm + . So, the next task is to distribute the state information to N2 players of the Group 2 each having m2 degree. At the stage of the processing, there are N1 source. The time required to do each parallel task is ( ) 1212

N/Nlogm + by ignoring extra degree (m1-m2) of each N1 players. The time requirement would be ( )( ) 21313

NNNlogm ++ in the next step. So, the total time to complete state dissemination task will be,

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( ) ( )( )

∑+

++++

−

=+

+

++

1

11

2131

12111

3

21

g

jjgm

m

mm

NNlog...

NNNlogN/NlogNlog

g

From the equation, it is clear that each subtask is performed in speedup fashion even in heterogeneous case. Off course this requires special scheduling mechanism to get the maximum benefit from it. In this paper, we shall explore the effectiveness of such special routing policy. But proper routing definition is kept aside for our future research. 5. Measurement and performance analysis

A fast state dissemination mechanism is presented in the context of MMOG in the above sections. We have verified the presented theory thorough the measurement of timing attribute like the maximum time required to flood the state information to the all players in a zone. In the following figures, we compare our expedite approach, i.e. speedup, against tree like structure that most systems follow. In measurement, both approaches (speedup and general) had identical settings in terms of degree, end-to-end delay, ordering of nodes (higher bandwidth to lower bandwidth), etc, until otherwise stated.

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Figure 4. Comparing theories by measurement

In Figure 4, we have 512 players; each of them is 100ms away from each other (this scenario is not real, the purpose is to verify the theory with the numerical values obtained through simulation) and has a degree of 2. The maximum time attribute clearly shows that speedup approach accomplishes the task well ahead of the traditional approach (Figure 4). The average time found in Figure 4 also manifests that speedup approach

satisfies more players than general approach at a particular instant of time.

In Figure 5, we have tried to be as close as possible to the real sceneries. The end-to-end delay between any two players was between 30ms to 100ms. The degree of each player is not identical: 20% players have degree 4, 30% players have degree 3 and the remaining 50% players have degree 2. The key objective of such measurement is to check the significance of the proposed concept in heterogeneous case. Figure 5 votes for the speedup approach in terms of maximum time required for state dissemination for different number of players.

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Figure 5. Measurement of required maximum

time to flood state information

6. Conclusion and future work

The presented expedite state dissemination mechanism clearly shows its significance in terms of utilizing players’ unexploited bandwidth. Most overlay methods use either tree-first or mesh-first approach to define suitable routing paths for the target application. At the end when data or state distribution comes to action, all of them use tree like structure for loop free routing. So from the source to a leaf, packet follows multi-application-hop (different from network hop) path. In the traditional case, a player relays a packet to its children and protocol becomes happy in terms of responsibility. But, after performing the relaying task, player becomes idle for a while until the generation of the next event. It would be more effective in our case as packets are very short and isolated. Here, we are trying to use those idle periods and theoretically shows its influence on the performance. The smart part of this approach comes from the minimum time slots required to distribute state information to all players compared to other currently practiced approaches with identical settings. We also demonstrated the significance through measurement with a couple of assumption.

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Despite we have addressed issues like heterogeneous environment there are some other points to be taken care of. These include delay variance as well as proper algorithmic routing table definition. Those issues shall be addressed in our future research. 7. References [1] D. T. Ahmed and S. Shirmohammadi, “A framework for provisioning overlay network based multimedia distribution services”, in proceedings of IEEE ICME, Beijing, China, 2007. [2] D. T. Ahmed, S. Shirmohammadi, and J. C. de. Oliveira, “A novel method for supporting massively multi-user virtual environments”, in IEEE International Workshop on Haptic Audio Visual Environments and their Applications (HAVE), Ottawa, 2006, pages 72–77. [3] D. T. Ahmed, S. Shirmohammadi, and I. Kazem, “Zone based messaging in collaborative virtual environments”, in IEEE International Workshop on Haptic Audio Visual Environments and their Applications (HAVE), Ottawa, 2006, pages 165– 170. [4] A. E. Rhalibi, M. Merabti, and Y. Shen, “AOIM in peer-to-peer multiplayer online games”, in ACE ’06: Proceedings of the ACM SIGCHI international conference on advances in computer entertainment technology, 2006, page 71. [5] A. P. Yu and S. T. Vuong, “MOPAR: a mobile peer-to-peer overlay architecture for interest management of massively multiplayer online games”, in NOSSDAV ’05: Proceedings of the international workshop on Network and operating systems support for digital audio and video, 2005, pages 99–104. [6] S. Shirmohammadi, A. Diabi, and P. Lacombe, “A peer-to-peer communication architecture for networked games”, in Proceedings of the Future Play, USA, 2005. [7] D. T. Ahmed, S. Shirmohammadi, and A. E. Saddik, “A dominating set based peer-to-peer protocol for real-time multisource collaboration”, in IEEE Workshop on Collaborative P2P Information Systems, Paris, France, 2007. [8] S. E. Deering, “Multicast routing in internetworks and extended LANs”, in SIGCOMM ’88: Symposium proceedings on Communications architectures and protocols, 1988, pages 55–64. [9] M. Hosseini, D. T. Ahmed, S. Shirmohammadi, and N. D. Georganas, “A survey of application-layer multicast protocols”, in IEEE Communication Surveys and Tutorials, 9(3), 2007. [10] D. T. Ahmed, S. Shirmohammadi, and J. Oliveira, “Improving gaming experience in zonal MMOGs”, in

MULTIMEDIA ’07: Proceedings of the 15th international conference on Multimedia, 2007, pages 581–584. [11] M. Claypool and K. Claypool, “Latency and player actions in online games”, Communication ACM, 2006, 49(11):40–45. [12] D. T. Ahmed and S. Shirmohammadi, “Model and Measurement of State Dissemination in MMOGs”, in IEEE International Instrumentation and Measurement Technology Conference (I2MTC), Victoria, BC, Canada, May 2008.

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