Ethernet Flow Control
NAS Recovery - Ethernet Flow Control
Ethernet flow control is a mechanism by which transmission of data on an Ethernet network is temporarily stopped. It is a method by which a network device tells its immediate neighbor that it is overloaded with data, in situations when the device is receiving data faster than it can afford to process. Ethernet flow control is intended for preventing loss in switches.
Ethernet flow control allows an overloaded device to send a special Ethernet frame called a PAUSE frame. The frame asks the device on the other end to stop sending data temporarily. If the receiving end succeeds in catching the pause frame, the sending device gets enough time to clear the stack of unprocessed data.
PAUSE is a flow control mechanism used on full duplex Ethernet link segments. When a station wishes to send a PAUSE command, it sends the MAC Control frames to the 48-bit destination multicast MAC address of 01-80-C2-00-00-01 to carry the PAUSE commands. This address is specifically reserved for use in PAUSE frames. Only stations configured for full-duplex operation are able to send PAUSE frames.
A PAUSE frame consists of the requested duration of the pause. It is a two byte unsigned integer (0 through 65535). The pause time is measured in units of quanta. Each quantum is equal to 512 bit times.
In half-duplex environments, flow control is provided by a ‘back pressure’ mechanism. In this, the network device sends a back pressure to the sending network interface card (NIC) on ports that are moving too fast and blocks transmission for a stipulated period of time.
