Internet Bandwidth Throughput

Unanswered Question
Nov 16th, 2008

I have 10Mbps of IP transit (Internet) through my provider, my PRTG graph shows utilization of 10Mbps but the through put is low, if I try to download from FTP servers that hang off the provider I get less than 690KByte/sec. I don't know what to do, are there any setting I can do enhance my throughput or my download speed.

I have this problem too.
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scottmac Sun, 11/16/2008 - 20:53

One system is usually challenged to consume the entire pipe's bandwidth. Often as not, it's constrained by the TCP session and the link's latency (the rules say it can't send anymore until it get an "ACK" ... latency delays the "ACK" so it slows things down).

If you're using straight FTP, that is part of the problem. Download Filezilla (free) client and configure it for multiple threads/sessions.

Then aim it at a directory of stuff to download ... it will download one file per thread and do a better job of using the bandwidth ... though I doubt it will use the entire pipe ... just more of it.

It's likely that to use the entire 10M, you'll need several clients each running multiple sessions full tilt.

There are a number of tweaks that can be done, depending on {every component, every link, every server, every host}. Can't really comment without details.

Good Luck


Joseph W. Doherty Mon, 11/17/2008 - 04:18

There are many things that can account for low throughput.

TCP's (used by FTP) flow rate is very dependent on the receiver's TCP receive window size. Ideally, the receive window should be large enough to cover BDP (bandwidth delay product). For 10 Mbps and 100 ms latency, the BDP is 125 KB. TCP is unable to have a use a receive window larger than 64 KB unless both sides support extended window sizing. Without it, your maximum thoughput for a single flow has just been halved. Further, many TCP stacks don't default to either an extended window or even 64 KB, the actual receive window might be much smaller. For instance, XP hosts connected 100 Mbps Ethernet, set their default to about 16 KB, I believe. (NB: Vista's TCP NextGen stack will default to much larger default receive window, i.e. using it vs. XP you might see a considerable jump in thoughput.)

TCP flow rate is very sensitive to dropped packets.

On the Internet, although you have a 10 Mbps connection, your source may not, or it could intentionally limit the sending bandwidth per flow so that one high speed receiver doesn't monopolize the source's bandwidth at the expense of other receivers.

tcordier Mon, 11/17/2008 - 07:52

If you are downloading at 690KB/s, you are in fact using more than 5Mbps of your line for a single download. That is quite a decent download speed, given the constraints described by Joseph. What is the round-trip delay when you ping the FTP server?

- Thomas


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