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Head-of-line blocking (HOL blocking) in computer networking is a performance-limiting phenomenon that occurs when a line of packets is held-up by the first packet, for example in input buffered network switches, out-of-order delivery, and multiple requests in HTTP pipelining. ==Switches== A switch may be composed of buffered input ports, a switch fabric and buffered output ports. If first-in first-out (FIFO) input buffers are used, only the oldest packet is available for forwarding. More recent arrivals cannot be forwarded if the oldest packet cannot be forwarded because its destination output is busy. The output may be busy if: * There is output contention (see diagram) * Or most commonly when the output buffer is full - congestion (for example the combined rate of multiple inputs exceeds the output rate) Without HOL blocking, the new arrivals could potentially be forwarded around the stuck packet to their respective destinations. The phenomenon can have severe performance-degrading effects in input-buffered systems. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Head-of-line blocking」の詳細全文を読む スポンサード リンク
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