Ethernet Flow Control
The purpose of Ethernet Flow Control (which is defined in IEE 802.3x) is to allow an Ethernet receiving station to tell a sending station to slow down transmission so that, typically, its buffers won't overflow. It does this by sending a PAUSE frame to multi-cast address 01-80-C2-00-00-01. This address is reserved for use by PAUSE frames and comes from a range of multicast addresses which are treated specially by Ethernet stations - they are not flooded, but rather are delivered to the Ethernet station itself.
The PAUSE frame uses MAC Control opcode 0X0001, and has two bytes which indicate how long the sender should pause sending. Each unit of 'pause' represents 512 'bit times' (ie the time it takes to send 1 bit). Therefore, for a 1 Gbps Ethernet port (with a 'bit time' of 1ns) a single PAUSE frame can stall a sender for between 0 and 65us. Since a second PAUSE frame supersedes the first, it is possible to cancel a previously sent PAUSE frame by sending another PAUSE frame with wait=0.
-- Main.TobyRodwell - Source: Wikipedia 16 Nov 2007