# Reliable User Datagram Protocol as a Solution to Latencies in Network Games
## 2 Related Research
### Communication Types of Network Games  
Today, TCP and UDP are widely used transmission protocols for both wired and wireless networks. Even though they both guarantee sequential data delivery and provide a congestion control function [25; 26; 27], UDP displays better packet delivery performance (real-time delivery). Based on reliable data protocol (RDP) [28; 29; 30; 31; 32], reliable UDP was proposed in the 1999 Internet Draft [3] as a transmission protocol that could offer reliability similar to TCP while preserving the low-delay characteristics of UDP [3]. Table 1 shows these communication types.  
Figure 3: Example of Virtual Fighter 5 Using RUDP.  
The main purpose of the simulations performed for the RUDP-based system in this study is to supplement the \"unreliable\" nature of UDP, which has been considered as its major problem, and thereby produce reliable data like TCP produces, by employing the better nature of UDP--not just imitating TCP's characteristics.  
This study focuses on an RUDP-based communication system which has been proposed and supported by many researchers for its efficiency but has been difficult to validate as there has been no testbed large enough to conduct a direct experiment. So far the performance data of such a system can be collected from the small-scale testbeds only, but it is obvious that constructing an effective and adaptive analytical model based on these testbeds will not be possible due to the fact that some essential data collected from them would not reflect all the necessary elements required to generate a practical model.  
The OPNET simulation tool was used to construct the RUDP-based communication system model proposed in this research, in which an advanced photonics technology had been adopted to establish a communication link with the metro networks.  
Since UDP is basically a message-oriented and connectionless protocol, the network stream sent is not defined and relevant data will follow the pass through the router(s) operating under the less crowded network environment to process messages so that there is no process like the TCP's congestion control that controls the network flows [33; 34; 35; 36; 37]. If desired data is sent with RUDP, its transmission reliability can be guaranteed as much as TCP can offer as they will be retransmitted repetitively when transmission fails due to the poor network environment. However, here, a notable difference with TCP is that RUDP can solely use the network as it does not require any congestion control. The contribution of UDP in network games is great as it exhibits a reaction speed close to the physical network latency.  
However, as the transmitted data is less reliable, UDP is often used in a limited way. UDP will be able to effectively achieve TCP-level reliability through establishing better reliability by compensating for the weakness of UDP more actively.  
Such an implementation of RUDP has been attempted many times by famous overseas games and network engines and the author expects that it would become an important axis in technological development, along with the activation of MMORPG. RUDP is in fact being used by Korean game companies but they are not disclosing the details for security reasons. Although the following is not an unfamiliar subject, the sections below describe design and implementation phases of RUDP that focus on the utilization of UDP.