Ethernet flow control

A sending station (computer or network switch) may be transmitting data faster than the other end of the link can accept it.

[1] An overwhelmed network node can send a pause frame, which halts the transmission of the sender for a specified period of time.

This situation is a case of head-of-line (HOL) blocking, and can happen more often in core network switches due to the large numbers of flows generally being aggregated.

Many switches use a technique called virtual output queues to eliminate the HOL blocking internally, so will never send pause frames.

In May 2006, the objectives of the task force were revised to specify a mechanism to limit the transmitted data rate at about 1% granularity.

As a remedy to this problem, Cisco Systems defined their own priority flow control extension to the standard protocol.

[6] The extension was subsequently standardized by the Priority-based Flow Control (PFC) project authorized on March 27, 2008, as IEEE 802.1Qbb.

[8] The effort was part of the data center bridging task group, which developed Fibre Channel over Ethernet.