# Inter-Data Center RDMA: Challenges, Status, and Future Directions
###### Abstract  
Remote Direct Memory Access (RDMA) has been widely implemented in data centers (DCs) due to its high-bandwidth, low-latency, and low-overhead characteristics. In recent years, as various applications relying on inter-DC interconnection have continuously emerged, the demand for deploying RDMA across DCs has been on the rise. Numerous studies have focused on intra-DC RDMA. However, research on inter-DC RDMA is relatively scarce, yet it is showing a growing tendency. Inspired by this trend, this article identifies and discusses specific challenges in inter-DC RDMA deployment, such as congestion control and load balancing. Subsequently, it surveys the recent progress in enhancing the applicability of inter-DC RDMA. Finally, it presents future research directions and opportunities. As the first review article focusing on inter-DC RDMA, this article aims to provide valuable insights and guidance for future research in this emerging field. By systematically reviewing the current state of inter-DC RDMA, we hope to establish a foundation that will inspire and direct subsequent studies. c 2025 17 242 17 2025  
such as land, energy, and connectivity [5; 6; 7], cloud service providers (CSPs) typically deploy multiple DCs in different regions to balance user demand and operational costs [8]. Meanwhile, applications relying on inter-DC networks are continuously evolving. Many data-intensive and computing-intensive applications, such as data analysis [9; 10], machine learning [11; 12], graph processing [13; 14], and high-performance computing (HPC) [15; 16], involve substantial amounts of data distributed across multiple DCs. Specifically, with the rapid development of Artificial Intelligence (AI) applications, the explosive growth in parameters of large language models (LLMs) has led to a significant increase in the computing power required for LLM pre-training; consequently, leveraging the computing power of multi-regional DCs to provide bigger computing capabilities to support ultra-scale computing requirements has become a recent research focus [17; 18]. To meet the demands for high-throughput, low-latency, and lossless data transmission across DCs, applying RDMA to inter-DC scenarios is a natural choice. This approach extends the performance and cost-efficiency advantages observed in intra-DC applications to inter-DC environments. However, when RDMA is deployed across DCs, a series of new challenges are presented, such as congestion control, buffer pressure, load balance, and so on. For instance, regarding congestion control, the existing RDMA congestion control algorithms are mainly designed for the intra-DC network, and it is proved that the performance of these algorithms is severely impaired in inter-DC networks. The long-haul, high-bandwidth link introduces a large round-trip time (RTT) and bandwidth delay product (BDP) [19], which delays the network signals and results in a large queueing latency. To address these challenges, various methods have been proposed by researchers, focusing on long-range congestion control or packet loss control to optimize the performance of RDMA in specific inter-DC application scenarios. However, these methods still exhibit certain limitations. Therefore, further research is required to explore additional optimizations to enhance the effectiveness of RDMA in inter-DC applications. As illustrated in Figure 1, the evolution of RDMA has progressed through three key stages: proposal, maturation, and modernization. Its application scope has also broadened, extending from intra-DC to inter-DC use cases. This article will align its structure with this developmental trajectory. We first introduce the background of RDMA, including its working principles and related technologies applied in DCN. Next, the requirements and challenges of inter-DC RDMA are analyzed. Subsequently, a survey of the existing inter-DC RDMA solutions and related research is conducted. Following this, several future research directions for inter-DC RDMA are examined.