# Inter-Data Center RDMA: Challenges, Status, and Future Directions
## 4 Existing Representative Works
### Others  
In addition to the aforementioned optimization schemes for RDMA in inter-DC scenarios, some other optimization schemes targeting non-RDMA inter-DC data transmission have also been proposed. These methods can provide valuable references for the design of future inter-DC RDMA solutions.  
GEMINI [52] is a congestion control algorithm that integrates both ECN and delay signals to handle the distinct characteristics of DCNs and WANs. GEMINI aims to achieve low latency and high throughput by modulating window dynamics and maintaining low buffer occupancy. GEMINI is designed based on the TCP protocol, being developed using the generic congestion control interface of the TCP kernel stack.  
Compared to the reactive congestion control mechanism of GEMINI, FlashPass [53] proposes a proactive congestion control mechanism designed specifically for shallow-buffered WANs, aiming to avoid the high packet loss and degraded throughput in shallow-buffered WANs due to large bandwidth-delay product traffic. FlashPass employs a sender-driven emulation process with send time calibration to avoid data packet crashes and incorporates early data transmission and over-provisioning with selective dropping to efficiently manage credit allocation.  
GTCP [54] introduces a hybrid congestion control mechanism to address the challenges in inter-DC networks by switching between sender-based and receiver-driven modes based  
Figure 7: Design diagram of LoWAR (adapted from [51], Figure 5).  
on congestion detection. When it detects congestion within a DC, the inter-DC flow switches to the receiver-driven mode to avoid the impact on intra-DC flow, and inter-DC flow periodically probes the available bandwidth to switch back to sender-based mode when there is spare bandwidth, avoiding bandwidth wastage. This allows it to balance low latency for intra-DC flows and high throughput for inter-DC flows. Also, for intra-DC flows, GTCP uses a pausing mechanism to prevent queue build-up, ensuring low latency.  
IDCC [55] is a delay-based congestion control scheme that uses delay measurements to handle congestion in both intra-DC and inter-DC networks, which also adopts a hybrid congestion control mechanism. IDCC leverages In-band Network Telemetry (INT) to measure intra-DC queuing delay and uses RTT to monitor WAN delay. By leveraging queue depth information from INT, IDCC dynamically adjusts inter-DC traffic behavior to minimize latency for intra-DC traffic. When INT is unavailable, RTT signals are used to adapt the congestion window, ensuring efficient utilization of WAN bandwidth. To enhance stability and convergence speed, IDCC employs a Proportional Integral Derivative (PID) controller for window adjustment. Furthermore, IDCC incorporates an active control mechanism to optimize the performance of short flows within the DC. It introduces a weight function based on sent bytes, prioritizing short flows during congestion window adjustments. This approach reduces the completion time of short flows while maintaining overall network efficiency.  
Similar to LSCC and BiCC, Annulus [56] employs a separate control loop to achieve better congestion control in inter-DC scenarios. Due to the completely different characteristics and requirements of WAN and DCN traffic. Annulus uses one loop to handle bottlenecks with only one type of traffic (WAN or DCN), while the other loop handles bottlenecks shared between WAN and DCN traffic near the traffic source, using direct feedback from the bottleneck. The near-source control loop relies on direct feedback from switches to quickly adjust the sending rate, reducing the feedback delay for WAN traffic and improving overall performance.  
As illustrated in Table 1, existing optimization approaches for non-RDMA inter-DC data transmission primarily focus on congestion control, encompassing all three categories of congestion control methods: reactive, proactive, and hybrid.