# Inter-Data Center RDMA: Challenges, Status, and Future Directions
## 4 Existing Representative Works
### Comparison
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Scheme**} & \multirow{2}{*}{**Control Type**} & \multirow{2}{*}{**Control Loop**} & **RDMA Specific** & **Specific-Hardware Dependence** & \multirow{2}{*}{**Key Innovation**} \\ \cline{4-6}  & & & & & & PFC-relay mechanism; \\ \hline SWING [19] & Reactive & Single loop & Yes & Yes & single-BDP buffering; \\  & & & & & compatible with existing infrastructure \\ \hline BiFrost [23] & Reactive & Single loop & Yes & Yes & \begin{tabular}{c} Downstream-driven control; \\ virtual incoming packets; \\ one-hop BDP buffering \\ \end{tabular} \\ \hline LSCC [49] & Hybrid & Multi-loop & Yes & Yes & \begin{tabular}{c} Segmented feedback loop; segmented \\ link control \\ \end{tabular} \\ \hline BiCC [50] & Hybrid & Multi-loop & Yes & Yes & \
\begin{tabular}{c} Multi-loop congestion control; \\ two-sided management \\ \end{tabular} \\ \hline GEMINI [52] & Reactive & Single loop & No & No &
\begin{tabular}{c} ECN and delay integration; \\ RTT-adaptive window adjustment \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of existing works in terms of congestion control. Table 3 presents a comparison of existing works in terms of network reliability requirements, where RDMA-specific solutions such as SWING [19], BiFrost [23], LSCC [49], and BiCC [50] are primarily designed for lossless networks. These solutions often incorporate reliability techniques like optimization based on PFC (for SWING, BiFrost, and LSCC) or collaboration with host-driven congestion control (for BiCC), yet they rarely include optimizations for packet retransmission mechanisms in lossy environments. Only one solution, LoWAR [51], considers the RDMA transmission in a lossy WAN environment, which adopts proactive error correction to recover loss packets. In contrast, non-RDMA solutions do not restrict themselves to specific network reliability types. For instance, GEMINI [52] and GTCP [54] work under lossy environments and rely on TCP reliability mechanisms. FlashPass [53] uses tail loss probing with credit scheduled retransmission. However, solutions like IDCC [55] and Annulus [56] have no special optimizations for reliability, basically reusing the native reliability mechanisms of the transmission protocols they employ. Despite these varied approaches, there is a notable lack of discussion on handling packet loss in cross-DC scenarios across all these schemes, leaving a gap in addressing reliability challenges specific to such environments. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Scheme**} & \multicolumn{2}{c}{**Network Reliability**} & \multirow{2}{*}{**RDMA Specific**} & \multirow{2}{*}{**Reliability Technique**} \\  & & **Req.** & & & **RolumnA** \\ \hline SWING [19] & Lossless & Yes & Optimization based on PFC \\ \hline BiFrost [23] & Lossless & Yes & Optimization based on PFC \\ \hline LSCC [49] & Lossless & Yes & Optimization based on PFC \\ \hline BiCC [50] & Lossless & Yes & Collaboration with host-driven CC \\ \hline LoWAR [51] & Lossy & Yes & Proactive error correction \\ \hline GEMINI [52] & Lossy & No & \begin{tabular}{c} Relying on TCP reliability \\ mechanism \\ \end{tabular} \\ \hline FlashPass [53] & Lossy & No & \
\begin{tabular}{c} Tail loss probing with credit \\ scheduled retransmission \\ \end{tabular} \\ \hline GTCP [54] & Lossy & No &
\begin{tabular}{c} Relying on TCP reliability \\ mechanism \\ \end{tabular} \\ \hline IDCC [55] & Lossy & No & No special optimization \\ \hline Annulus [56] & Lossy & No & No special optimization \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of existing works in terms of network reliability requirement. With regard to implementation methods and locations, RDMA-specific solutions exhibit a variety of implementation strategies.