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 unit is equal to 512 bit times. By 1999, several vendors supported receiving pause frames, but fewer implemented sending them. Issues One original motivation for the pause frame was to handle network interface controllers (NICs) that did not have enough buffering to handle full-speed reception. This problem is not as common with advances in bus speeds and memory sizes. A more likely scenario is network congestion within a switch. For example, a flow can come into a switch on a higher speed link than the one it goes out, or several flows can come in over two or more links that total more than an output link's bandwidth. These will eventually exhaust any amount of buffering in the switch. However, blocking the sending link will cause all flows over that link to be delayed, even those that are not causing any congestion. This situation is a case of head-of-line blocking (HOL), 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. Subsequent efforts Congestion management Another effort began in March 2004, and in May 2004 it became the IEEE P802.3ar Congestion Management Task Force. 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. The request was withdrawn and the task force was disbanded in 2008. Priority flow control Ethernet flow control disturbs the Ethernet class of service (defined in IEEE 802.1p), as the data of all priorities are stopped to clear the existing buffers which might also consist of low priority data. As a remedy to this problem, Cisco Systems defined their own priority flow control extension to the standard protocol. This mechanism uses 14 bytes of the 42-byte padding in a regular pause frame. The