# Congestion Control Mechanism Based on Backpressure Feedback in Data Center Networks
## 2 Motivation and Related Work
### Congestion Control in Data Centers  
The complex network environment in data centers has posed a substantial challenge for traffic management. Researchers constantly introduce new techniques, resulting in the proposals of various congestion control schemes. In the concrete implementations, these schemes rely on end-to-end feedback loops. The sender adjusts the sending rate based on the congestion feedback signals. Regardless of the type of congestion signal (e.g., ECN, RTT, INT), the sender must wait at least 1-RTT to receive the feedback [2; 14].  
DCTCP [15] is the first to design a dedicated congestion control protocol for data centers, which uses the ECN as a feedback signal and greatly reduces the transmission delay transmission latency. Based on DCTCP, D2 TCP [16] introduces a flow deadline factor to prevent urgent flows from missing the final deadline. In contrast, ICTCP [17] implements a congestion avoidance mechanism on the receiver side, unlike DCTCP and D2 TCP. The available remaining bandwidth is used to dynamically resize the receive window in real time to regulate TCP throughput and avoid network congestion. DCQCN [18] is an improved scheme based on DCTCP and QCN [19]. It detects congestion based on buffer size. The sender reduces the sending rate based on the congestion information fed back from the receiver. DCQCN is primarily designed as a rate-based congestion control scheme for initiating remote direct memory access (RDMA) communication in data centers. There are other options for detecting congestion through ECN marking, such as ECN* [20], L2 DCT [21], CEDM [22], and so on. The key to these methods is the selection of ECN markings threshold [18; 23].  
Timely [24] used the RTT as a congestion signal to adjust the sending rate of senders. This approach relies on the end-to-end RTT measurement and rate control and is mainly used to enable RDMA in DCN. Swift [25] introduced a congestion control system that classifies delay into endpoint delay and structural target delay. It employs different target delays for different flows to enhance fairness. The requirement for hardware to accurately measure the RTT is high in schemes where the delay is used as a congestion signal.  
Poseidon [26] leveraged INT to solve multi-hop and reverse path congestion problems. In addition, there exist congestion control methods based on different information. For instance, FlexPass [27] relies on credit-based congestion control protocols. This type of method usually requires the sender to send additional information or to redefine the process of sending the packet, ultimately resulting in excessive bandwidth usage.  
The main concern in an end-to-end congestion control scheme is the selection of congestion signals. While a great deal of work has already been accomplished in the field of congestion control, there are still significant challenges ahead. On the one hand, the capacities of data center switches and link speeds continue to increase, requiring DCN to be able to accomplish more and more small flows quickly. On the other hand, the buffer capacity of the switch does not expand with the switch capacity. It makes it easier for buffers to reach the limits of high-bandwidth links. These make the design of end-to-end congestion control schemes more difficult. It is necessary to balance the transmission of various types of traffic while ensuring low buffer occupancy of switches.