Back to: FC (Fibre Channel) fundamentals
FC (Fibre Channel) Flow Control is a mechanism that ensures data is transmitted efficiently and without loss in a Fibre Channel (FC) network. Since FC is a lossless protocol, it relies on flow control techniques to prevent buffer overflows and ensure smooth data transmission.
Types of Flow Control in Fibre Channel
- Buffer-to-Buffer Credit (B2B Credit)
- Used for flow control between directly connected FC devices (e.g., between a host and a switch or between two switches).
- Works by assigning a certain number of buffer credits (BB_Credits) to each device.
- A device can only send frames if it has available buffer credits.
- Once the receiving device processes and clears frames from its buffer, it sends an acknowledgment (R_RDY) to replenish the credit.
- End-to-End Credit (E2E Credit)
- Used for flow control between the source and destination nodes (e.g., between a server and a storage array).
- Helps manage congestion across an entire FC path.
- Similar to B2B credit but operates on an end-to-end basis rather than hop-by-hop.
- Priority Flow Control (PFC)
- Used in converged networks such as FCoE (Fibre Channel over Ethernet).
- Ensures that Fibre Channel traffic is lossless over an Ethernet network by pausing lower-priority traffic while allowing FC traffic to continue.
Why FC Flow Control is Important
- Prevents Frame Loss: Since FC does not support packet retransmission like TCP/IP, flow control ensures frames are never dropped.
- Optimizes Performance: Efficient credit management allows optimal use of available bandwidth.
- Maintains Low Latency: Helps avoid congestion and delays in the network.
- Ensures Reliable Data Transfer: Critical for high-performance storage applications such as databases and virtualized environments.