如果您经常在同一时间下载多个项目，那么您可能已经注意到一个下载连接往往会主导其他连接，直到它完成。为什么？今天的超级用户问答帖子回答了一位好奇的读者的问题。

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，是一个由社区驱动的问答网站分组。

图片由Armando Sotoca（Flickr）提供。

问题

超级用户读者Keltari想知道为什么一些下载（网络）连接主导了其他连接：

I have my suspici*** as to why, but I want someone with a stronger background in networking to explain.

Why is it that when downloading from multiple (different) locati***, certain connecti*** saturate the bandwidth, leaving other connecti*** almost idle until the dominant connection is finished?

为什么一些下载（网络）连接主导了其他连接？

答案

超级用户贡献者cybernard为我们提供了答案：

Generally, the fastest connection wins. However, before Windows 7, networking was a mess and performance was awful all around. Trying to get more than 20mb/s from Windows XP, for example? Good luck with that.

• The standard TCP congestion avoidance algorithm employs an additive increase and multiplicative decrease (AIMD) scheme. When there is no packet loss detected (by means of three duplicate-ACKs or retran**ission timeout), the congestion window (cwnd) is increased by one Maximum Segment Size (MSS) every RTT. Otherwise, if a packet loss is detected, the TCP sender decreases cwnd by half. In a high-speed and long delay network, it requires a very large window, e.g. thousands of packets, to fully utilize the link capacity. Therefore, it will take the standard TCP many RTTs to recover the sending rate upon a single loss event. Moreover, it is well-known now that the average TCP congestion window is inversely proportional to the square root of the packet loss rate.

Source: A Compound TCP Approach for High-speed and Long Distance Networks [Microsoft]

A faster connection has more successful packets, therefore its cwnd/MSS is increased and it gets even more of the total connection.

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