Buffer Overrun Situation

What happens if a Fibre Channel port has zero remaining-Rx-B2B-credits, and it is still sent a frame by its directly connected neighbor? This condition is considered a severe error. The port takes two actions. First, it drops the incoming frame due to buffer overrun. Second, it initiates Link Reset Protocol, which results in a bidirectional traffic halt on the link until the Link Reset Protocol completes. For more details refer to the sections on Link Initialization Counters and Credit Loss Recovery Using Link Reset Protocol. 如果光纤通道端口的剩余 Rx-B2B 信元为零，但其直接连接的邻居仍向其发送帧，会发生什么情况？这种情况被视为严重错误。端口会采取两种措施。首先，由于缓冲区超限而丢弃接收到的帧。其次，启动链路重置协议，导致链路上的双向通信停止，直到链路重置协议完成。详情请参阅 "链路初始化计数器 "和 "使用链路重置协议恢复信用损失 "章节。

Frame Rate Equalization using B2B Flow Control

The illustration in Figure 2-9 assumes that the destination (Host) stops processing the frames completely. However, a more realistic scenario is when the receiver processes the frames slowly. In other words, the receiver is unable to process the frames as fast as the source (Target) is sending them. For example, Target sends 800 frames per second, but the Host can accept only 200 frames per second. Hence, the Host port starts pacing the rate of R_RDY. Because one R_RDY returns one credit which is used to send only one frame, sending 200 R_RDYs per second results in a frame rate of 200 per second. 图 2-9 中的插图假定目的地（主机）完全停止处理帧。然而，更现实的情况是接收方缓慢处理帧。换句话说，接收方处理帧的速度无法达到源（目标）发送帧的速度。例如，Target 每秒发送 800 个帧，但主机每秒只能接受 200 个帧。因此，主机端口开始调整 R_RDY 的速率。由于一个 R_RDY 返回一个信元，而信元只能用于发送一个帧，因此每秒发送 200 个 R_RDY 会导致帧速率为每秒 200 帧。

Overall, the B2B flow control mechanism achieves the following two results: 总的来说，B2B 流量控制机制实现了以下两个结果：

 It equalizes the rate of ingress traffic with the rate of egress traffic. 它能使入口流量速率与出口流量速率相等。

 Because of this ingress/egress rate equalization, there are no frame drops, therefore achieving a lossless fabric. 由于这种入口