Pause frame

An overwhelmed network node can send a pause frame, which halts the transmission of the sender for a specified period of time. A media access control (MAC) frame (EtherType 0x8808) is used to carry the pause command, with the Control opcode set to 0x0001 ([hexadecimal][https://en.wikipedia.org/wiki/Hexadecimal "Hexadecimal")]1 Only stations configured for full-duplex operation may send pause frames. When a station wishes to pause the other end of a link, it sends a pause frame to either the unique 48-bit destination address of this link or to the 48-bit reserved multicast address of 01-80-C2-00-00-01.2. The use of a well-known address makes it unnecessary for a station to discover and store the address of the station at the other end of the link.

Another advantage of using this multicast address arises from the use of flow control between network switches. The particular multicast address used is selected from a range of address which have been reserved by the IEEE 802.1D standard which specifies the operation of switches used for bridging. Normally, a frame with a multicast destination sent to a switch will be forwarded out to all other ports of the switch. However, this range of multicast address is special and will not be forwarded by an 802.1D-compliant switch. Instead, frames sent to this range are understood to be frames meant to be acted upon only within the switch.

A pause frame includes the period of pause time being requested, in the form of a two-byte (16-bit), unsigned integer (0 through 65535). This number is the requested duration of the pause. The pause time is measured in units of pause quanta, where each quanta is equal to 512 bit times.