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长肥网络，LFN，Long Fat Network，好名字！
Bandwidth*RTT= maximum amount of data on the network circuit at any given time, i.e., data that has been transmitted but not yet acknowledged.
带宽时延积（bandwidth-delay product） = 最大传输过程中的数据量
A network with a large bandwidth-delay product is commonly known as a long fat network (LFN). As defined in
RFC 1072, a network is considered an LFN if its bandwidth-delay product is significantly larger than 10^5 bits (12,500 bytes).
高速网络比较容易落入LFN范围。对于100G的链接，时延大于1微秒，就可以算LFN了！
Ultra-high speed LAN: 100 Gbit/s, 30 μs RTT
B x D = 3Mbit = 375kB
Ultra-high speed local area networks (LANs) may fall into this category, where protocol tuning is critical for achieving peak throughput, on account of their extremely high bandwidth, even though their delay is not great. While a connection with 1 Gbit/s and a round-trip time below 100 μs is no LFN, a connection with 100 Gbit/s would need to stay below 1 μs RTT to not be considered an LFN.
卫星通信是一个典型的LFN场景，时延很大，吞吐量可能也不小。这对于在传输时，需要停下来等待的协议和需要快速相应的应用，会是个麻烦。
An important example of a system where the bandwidth-delay product is large is that of geostationary satellite connections, where end-to-end delivery time is very high and link throughput may also be high. The high end-to-end delivery time makes life difficult for stop-and-wait protocols and applications that assume rapid end-to-end response.
面对LFN网络，TCP必须要启用window scaling机制，才能够在传输时，充分利用带宽。同时，也存在很多TCP的变体（详情查询wiki），用来应对LFN网络传输场景。
A high bandwidth-delay product is an important problem case in the design of protocols such as Transmission Control Protocol (TCP) in respect of TCP tuning, because the protocol can only achieve optimum throughput if a sender sends a sufficiently large quantity of data before being required to stop and wait until a confirming message is received from the receiver, acknowledging successful receipt of that data. If the quantity of data sent is insufficient compared with the bandwidth-delay product, then the link is not being kept busy and the protocol is operating below peak efficiency for the link. Protocols that hope to succeed in this respect need carefully designed self-monitoring, self-tuning algorithms. The TCP window scale option may be used to solve this problem caused by insufficient window size, which is limited to 65,535 bytes without scaling.
本文链接：https://cs.pynote.net/net/202305251/
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