Chapter 2 - 3. Understanding Congestion in Fibre Channel Fabrics

B2B Credit Counters

Figure 2-3 shows the following counters on an FC port: 图 2-3 显示了 FC 端口上的以下计数器：

1. Rx B2B credits: The number of receive buffers of an FC port. This value does not change while the port is up. FC 端口的接收缓冲区数量。该值在端口正常运行时不会改变。

2. Tx B2B credits: The number of receive buffers of the neighbor FC port as informed during link initialization. This value does not change while the port is up. 在链路初始化过程中告知的邻居 FC 端口的接收缓冲区数量。端口正常运行时，该值不会改变。

3. Remaining-Rx-B2B-credits: The number of free receive buffers on an FC port. Its minimum value can be zero and the maximum value can be Rx B2B credits. FC 端口上可用的接收缓冲区数量。其最小值可以是零，最大值可以是 Rx B2B 信元。

4. Remaining-Tx-B2B-credits: The number of free receive buffers on the neighbor FC port. Its minimum value can be zero and the maximum value can be Tx B2B credits. 邻居 FC 端口上可用的接收缓冲区数量。其最小值可以是零，最大值可以是 Tx B2B 信元。

FC devices that are available in the market may not show all these four counters. Some devices may show only the remaining Rx and Tx B2B credits ((3) and (4)) which are enough because their values are the same as (1) and (2) before traffic starts on a link. The key difference is that (1) and (2) don’t change after a link comes up, whereas (3) and (4) are dynamic and their values change as frames and R_RDYs are sent or received. 市场上的 FC 设备可能不会显示所有这四个计数器。有些设备可能只显示剩余的 Rx 和 Tx B2B 信元（(3) 和 (4)），这就足够了，因为在链路流量开始之前，它们的值与 (1) 和 (2) 相同。主要区别在于，(1) 和 (2) 在链路启动后不会改变，而 (3) 和 (4) 则是动态的，它们的值会随着帧和 R_RDY 的发送或接收而改变。

Some FC devices may choose to show the consumed credits instead of the remaining credits. The difference is that the remaining-Tx-B2B-credit counter on an FC port tr