## 2 Background and Motivation  
### Comparison of RDMA Technology and TCP  
Derived from previous studies [9; 10; 11; 32; 33; 34], one of the primary reasons affecting the TCP transmission rate is that data are copied back and forth between the kernel and user space, significantly increasing CPU usage. Therefore, the critical problem in improving the performance is eliminating cumbersome data in the host CPU. Copy and move operations reduce CPU usage in the communication process, and RDMA uses these two methods to optimize communication performance. It is a technology that can directly access the memory of a remote host. There are two types of operations in RDMA: RDMA Read and RDMA Write. Whether RDMA Read or RDMA Write, the requester only needs to obtain the memory address of the accessed remote node in advance, register the target memory, obtain the memory key of the target memory, and then perform kernel bypass data access. In addition to the studies mentioned above, there are other studies [13; 14; 15; 16; 17; 18] that have also compared TCP with RDMA technology. It can also be seen from the research conclusions of these documents that the RDMA protocol significantly reduces the number of memory copies due to its zero-copy and kernel bypass characteristics. Compared with the TCP, it has obvious advantages and can achieve higher performance. For example, a 40 Gbps TCP stream consumes the CPU resources of mainstream servers. RDMA addresses the technical pain points of traditional TCP/IP communication: In the 40 Gbps scenario, the CPU usage is reduced from 100% to 5%, and the network latency is reduced from _ms_ level to less than 10 _us_[19]. Analyzing the TCP architecture, when an application wants to use TCP/IP, it must use the programming interface socket provided by the protocol to realize network communication between applications or processes. Commonly used TCP/IP protocols for network communication programs use sockets for programming. Whether you write a client-side or server-side program, the system creates a socket handle for each TCP connection. This results in each transmission of traffic, through the OS and protocol stack management, so whether it is synchronous communication or asynchronous communication socket, there will be a phenomenon of high CPU consumption. In the RDMA model, the core problem is how to realize the simplest, more efficient, and direct communication between applications. RDMA provides peer-to-peer communication based on message queues. As long as it is registered in advance, each application can directly obtain its messages from the remote end. Figure 1 shows the architecture of the RDMA technology and its comparison with the TCP/IP architecture. The thin blue line in the figure represents the data flow in the TCP, and the thick red line represents the data flow in the RDMA communication. It is clear from the figure that RDMA saves a lot of data copying trouble compared to TCP, and the data are directly transmitted from the application buffer to the application buffer of another node through the host adapter HCA without passing through the kernel space of both parties. RDMA uses core bypass and zero-copy technology to provide low-latency features while reducing CPU usage, breaking through the memory bandwidth bottleneck, and providing high bandwidth utilization. This benefits from how the RDMA message service is presented to the application and the underlying technology used to send and deliver these messages [31]. Compared with a TCP/IP network, RDMA technology also eliminates the retransmission delay caused by data packet loss. There is no need to confirm and resend the lost data packet as in the TCP/IP network, saving this part of the time overhead, so that the overall performance is improved in this respect, and the transmission efficiency is also improved.