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Explicit Congestion Notification (ECN) is an extension to the Internet Protocol and to the Transmission Control Protocol and is defined in RFC 3168 (2001). ECN allows end-to-end notification of network congestion without dropping packets. ECN is an optional feature that may be used between two ECN-enabled endpoints when the underlying network infrastructure also supports it. Conventionally, TCP/IP networks signal congestion by dropping packets. When ECN is successfully negotiated, an ECN-aware router may set a mark in the IP header instead of dropping a packet in order to signal impending congestion. The receiver of the packet echoes the congestion indication to the sender, which reduces its transmission rate as if it detected a dropped packet. Rather than responding properly or ignoring the bits, some outdated or faulty network equipment has historically dropped or mangled packets that have ECN bits set.〔(【引用サイトリンク】title=Measuring Interactions Between Transport Protocols and Middleboxes )〕 , measurements suggest that the fraction of web servers on the public Internet for which setting ECN prevents network connections has been reduced to less than 1%. In June 2015, Apple Inc. announced that it will enable ECN signalling by default on its hundreds of millions of deployed products, in a move that Apple believes will help drive the adoption of ECN in other products.〔(【引用サイトリンク】title=Your App and Next Generation Networks )〕 ==Operation== ECN requires specific support at the Internet layer and the transport layer for the following reasons: * In TCP/IP, routers operate within the Internet layer, while the transmission rate is handled by the endpoints at the transport layer. * Congestion may be handled only by the transmitter, but since it is known to have happened only after a packet was sent, there must be an echo of the congestion indication by the receiver to the transmitter. Without ECN, congestion indication echo is achieved indirectly by the detection of lost packets. With ECN, the congestion is indicated by setting the ECN field within an IP packet to CE and is echoed back by the receiver to the transmitter by setting proper bits in the header of the transport protocol. For example, when using TCP, the congestion indication is echoed back by setting the ECE bit. 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「Explicit Congestion Notification」の詳細全文を読む スポンサード リンク
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