# Reliable User Datagram Protocol as a Solution to Latencies in Network Games
## 2 Related Research
### Reliable User Datagram Protocol (RulDP) for Network Games  
Communication protocols such as TCP and UDP are adopted in the transport layer by most network game companies due to their adaptability and universality in the design process [25; 26]. However, although UDP is preferred more as its structure is relatively simple and offers a higher processing capability, its reliability is low as it does not check whether the packets have been delivered to the destination or not, and just keeps on transmitting the packets regardless. An additional problem is that sometimes packets will not be delivered sequentially. That is, ones that have been sent earlier than others could arrive at the destination faster. Figure 2 shows the R UDP for network games.  
On the other hand, TCP is relatively complex in structure and checks the packet delivery situation for re-transmission purposes. As it continuously checks delivery acknowledgement signals, its communication process becomes slower than that of UDP but offers higher reliability. In this regard, TCP is not suitable for network games but is adequate for routine internet operations such as mail transmission or web browsing.  
Considering these factors, R UDP has become the first choice for network game companies since 2015. Smartphone-based game companies are also adopting R UDP as their standard communication protocol in the ROK. R UDP guarantees higher reliability by specifying the number of packets successfully delivered to the destination in the subsequent packet to be sent, and omitted packets will be re-transmitted accordingly [27; 28].  
Next, this paper will describe examples of performance by UDP and R UDP in an actual game play. The main techniques used in a fighting game called 'The Virtual Fighter 5 (VF5), Live Arena' are 'Perfect Synchronization' and 'Key-input synchronous communications', both of which assume  
Figure 2: The reliable user datagram protocol (R UDP) for network games.  
that the players' screens are consistent with each other and that each key-input-data is transmitted to the opponent and synchronized by each frame. Synchronization can be achieved with simple key inputs if the game network is highly reliable or under the LAN environment, like the heavy fighting games where battles are engaged in every floor unit. Here data is transmitted through the UDP-based communication. It is important to check delays prior to initiating the games. Figure 3 shows a delay of approximately 40 ms until the data reaches the Two Frame (2F).  
Based on this calculation, the Third Frame (3F) was set as a weight where synchronization will be achieved. In this structure, synchronization will be carried out by each frame. However, UDP does not always guarantee a complete key data delivery when a packet(s) is lost during communication. In such a case, the game will stop and wait for the next packet which will have the record of past data worth 10F. With this packet, the previous packet that has been processed will be dealt with. With the data worth 10F, one or two continuous packet losses can be recovered [25].  
One-on-one fighting games like Virtual Fighter 5 will not often require a wide range of bandwidths but the multiple player games will have some problems [26]. That is, network games that invite multiple-fighters (e.g., 2 vs. 2, 3 vs. 3, or 4 vs. 4) will reduce the frequency of packet transmissions in order to enable much complex computing processes in large-scale games developed due to the evolution of smartphone games and interface upgrades, meaning that the adoption of RUDP should be considered.