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Introduction
Receiver Credit-Based Congestion Control (RCCC) is a cornerstone of the Ultra Ethernet transport architecture, specifically designed to eliminate incast congestion. Incast occurs at the last-hop switch when the aggregate data rate from multiple senders exceeds the egress interface capacity of the target’s link. This mismatch leads to rapid buffer exhaustion on the outgoing interface, resulting in packet drops and severe performance degradation.
The RCCC Mechanism
Figure 8-1 illustrates the operational flow of the RCCC algorithm. In a standard scenario without credit limits, source Rank 0 and Rank 1 might attempt to transmit at their full 100G line rates simultaneously. If the backbone fabric consists of 400G inter-switch links, the core utilization remains a comfortable 50% (200G total traffic). However, because the target host link is only 100G, the last-hop switch (Leaf 1B-1) becomes an immediate bottleneck. The switch is forced to queue packets that cannot be forwarded at the 100G egress rate, eventually triggering incast congestion and buffer overflows.
While "incast" occurs at the egress interface and can resemble head-of-line blocking, it is fundamentally a "fan-in" problem where multiple sources converge on a single receiver. Under RCCC, standard Explicit Congestion Notification (ECN) on the last-hop switch's egress interface is Continue reading
