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

Why is it that when downloading from multiple different locations, certain connections saturate the bandwidth, leaving other connections almost idle until the dominant connection is finished?

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up vote 24 down vote accepted

Generally, the fastest connection wins. However, before Windows 7 networking was a mess and performance was awful all around. Try to get more than 20mb/s from XP 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 retransmission 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.

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

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That was in line with what I was thinking, albeit more technical. – Keltari Feb 18 at 3:18
20 millibits per second is pretty low, even for Windows XP. :P – CodesInChaos Feb 18 at 12:22
@codesinchaos where's a pedant badge when you need one... – Ashley Steel Feb 23 at 19:41
@CodesInChaos I think there isn't really an ambiguity, since you can't split a bit, but Mb/s would probably read nicer ;-) A bigger issue is when people confuse "Megabit" with "Megabyte".. – comfreak Feb 24 at 17:01

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