Head-of-line blocking (HOL) is a phenomenon that appears in buffered telecommunication network switches. A switch is usually made of buffered input ports, a switch fabric and buffered output ports. Because of the FIFO nature of the input buffers and switch design, the switch fabric can only switch the packets at the head of the buffer per cycle. HOL arises when packets arriving at different input ports are destined for the same output port. If the HOL packet of a certain buffer at the input cannot be switched to an output port because of contention, the rest of the packets in that buffer are blocked by that Head-of-Line packet, even if there is no contention at the destination output ports for those packets. The phenomenon may have severe performance-degrading effects in input-buffered systems.

Effect on switch throughput

This phenomenon limits the throughput of switches to 58,6%^[1]

Overcoming HOL

One way this drawback is overcome is by using Virtual Output Queues.

Packet reordering

HOL can significantly increase packet reordering - Jon C. R. Bennett , Craig Partridge , Nicholas Shectman, Packet reordering is not pathological network behavior, IEEE/ACM Transactions on Networking (TON), v.7 n.6, p.789-798, Dec. 1999. In the paper, it gives detail diagram and description how HOL occurs and results.

References