Buffering and Ability to Absorb Congestion
The ability to absorb congestion increases as the number of buffers (credits) increases. When more buffers are available, a traffic burst consumes many buffers, but may still leave free buffers. When buffers are free, a port in a lossless network does not invoke hop-by-hop flow control to pace ingress traffic, and hence, there is no congestion spreading. 吸收拥塞的能力随着缓冲区（信元）数量的增加而提高。当可用缓冲区较多时，流量突发会消耗很多缓冲区，但仍可能留下空闲缓冲区。当缓冲区空闲时，无损网络中的端口不会调用逐跳流量控制来加快入口流量，因此不会出现拥塞扩散。
To understand this further, consider the example from Figure 2-9. ISL port on Switch-1 has 500 remaining-Rx-B2B-credits. This port is not returning any credits because of downstream congestion caused by the Host. But Switch-2 does not stop transmission until those 500 credits are used. Recall that one credit is for one frame. If the port is operating at 32GFC, a full-sized FC frame of 2148 bytes takes approximately 600 nanoseconds for transmission. Therefore, consuming 500 credits by 500 frames takes approximately 300 microseconds (600 x 500). In other words, congestion up to 300 microseconds can be absorbed by 500 credits. If Switch-1 had only 50 credits, it could absorb congestion only up to 30 microseconds (time to transmit 50 full-sized frames). If the traffic has a microburst pattern that causes congestion for small durations or if the host becomes only momentarily busy (such as 200 microseconds), 50 credits will spread this congestion to Switch-2 forcing it to stop transmission until a credit is returned, whereas 500 credits will absorb the microburst on Switch-1, and Switch-2 will not stop transmission. 要进一步理解这一点，请参考图 2-9 中的示例。交换机-1 上的 ISL 端口有 500 个剩余 Rx-B2B 信元。由于主机造成的下游拥塞，该端口没有返回任何信元。但 Switch-2 在使用完这 500