How does class 2 Fibre Channel handle congestion and flow control in a busy environment?

1 Class 2 FC overview

Class 2 FC is a datagram-based service that uses acknowledgements (ACKs) to confirm the receipt of frames. Class 2 FC does not guarantee the order of delivery, nor does it reserve bandwidth or buffer credits for the communication. Instead, it relies on the end devices and the switches to manage the traffic and handle any errors or losses. Class 2 FC is suitable for applications that can cope with variable response times and retransmissions, such as backup, replication, and file transfers.

2 Congestion management

Congestion occurs when the demand for network resources exceeds the supply, resulting in delays, drops, and retries. Class 2 FC uses several mechanisms to manage congestion and avoid performance degradation. One of them is the exchange congestion management (ECM) protocol, which allows the sender and the receiver to negotiate the optimal number of frames to be sent in an exchange. Another one is the buffer-to-buffer (B2B) credit system, which limits the number of frames that a sender can transmit without receiving an ACK from the receiver. The sender must wait until it receives enough credits to send more frames, thus preventing the receiver from being overwhelmed by the incoming traffic.

3 Flow control

Flow control is the process of regulating the rate and volume of data transmission between two devices. Class 2 FC uses two types of flow control: end-to-end and link-level. End-to-end flow control is based on the ECM protocol and the B2B credit system, as described above. Link-level flow control is based on the link control frames (LCFs), which are special frames that can be sent by a switch or an end device to pause or resume the transmission on a link. LCFs can be used to signal congestion, errors, or maintenance events on a link, and to coordinate the recovery of lost or corrupted frames.

4 Congestion avoidance

Congestion avoidance is the process of preventing or reducing congestion before it affects the network performance. Class 2 FC uses several techniques to avoid congestion, such as load balancing, zoning, and quality of service (QoS). Load balancing is the distribution of traffic across multiple paths or links, based on factors such as availability, capacity, and priority. Zoning is the partitioning of a SAN into logical groups of devices, based on their function, location, or access rights. Zoning can isolate traffic and improve security and manageability. QoS is the allocation of network resources according to the importance or urgency of the traffic. QoS can prioritize critical or time-sensitive applications over less important or delay-tolerant ones.

5 Congestion detection

Congestion detection is the process of identifying and measuring the level and impact of congestion in a network. Class 2 FC uses several indicators to detect congestion, such as frame loss, latency, throughput, and buffer occupancy. Frame loss is the percentage of frames that are dropped or discarded due to congestion or errors. Latency is the time it takes for a frame to travel from the sender to the receiver. Throughput is the amount of data that can be transferred in a given time. Buffer occupancy is the percentage of buffer space that is used by the frames in transit. These indicators can be monitored and analyzed by tools such as performance counters, statistics reports, and network analyzers.

6 Congestion mitigation

Congestion mitigation is the process of resolving or alleviating congestion and restoring the network performance. Class 2 FC uses several actions to mitigate congestion, such as retransmission, rerouting, and tuning. Retransmission is the resending of frames that are lost or corrupted due to congestion or errors. Rerouting is the switching of traffic to alternative paths or links, based on the availability and condition of the network. Tuning is the adjustment of network parameters, such as frame size, buffer credits, and QoS settings, to optimize the network efficiency and utilization.