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I have a couple machines experiencing an awful download speed on an awesome Internet connection, and for the life of me I can't come up with a rational explanation as to why.

Speed test to the local ISP shows >10Mbps downspeed. The line is supposed to be symmetrical but test shows ~7Mbps upspeed. Speed test to a server further away shows < 8Mbps. (One server I tested showed under 1Mbps.) And shows 11ms ping, 1ms jitter, 0% packet loss. Internal pings are 1ms or 2ms.

Actual downloads however are terribly slow. The speeds vary between website. Drivers from Dell (over FTP) at 40Kb/s, sometimes 10Kb/s. Windows 8 Preview from Microsoft 100Kb/s, VLC from SourceForge at 60Kb/s.

Firewall blocks all ICMP. (This is clearly the problem, but this question is about to be closed so I won't get a chance to confirm what specifically the problem is in this case.)

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closed as not a real question by Sirex, MaQleod, Diogo, Joe Taylor, Renan Aug 15 '12 at 16:03

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

can you provide some evidence ? pings, trace routes to different places etc. Not a lot to go on. – Sirex Aug 14 '12 at 20:45
Your ISP hates you ;) – Wayne Werner Aug 14 '12 at 20:46
Are these D/L's done through torrents? Is there a chance your ISP has throttled the ports that the clients use? – Everett Aug 14 '12 at 20:48
@Everett Download is straight from Dell. Also tried downloading Win8 preview from Microsoft, also very slow (100kbps). No torrents. – Tanner Faulkner Aug 14 '12 at 20:49
What does say about your connection? – Darth Android Aug 14 '12 at 20:56
up vote 5 down vote accepted

Blocking ICMP will cause horrible TCP performance problems because ICMP is used to perform path MTU discovery. Remove the ICMP block and test again. I bet you'll find the problem goes away.

There are also other possible ways the firewall could be causing this. So I'd suggest shutting off the firewall and re-testing. If that solves the problem, enable the firewall but disable ICMP filtering and test again.

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good shout. May well cause this. – Sirex Aug 14 '12 at 21:41
Didn't have a chance to shut off the firewall entirely, but I did play around with serving files to one of the problem machines with a server using a lower MTU. This maxed out the connection on the server easily. – Tanner Faulkner Aug 15 '12 at 14:01
I would suggest going over all the settings on the firewall. If it was last worked on by someone who thought it was perfectly fine to block all ICMP (or managed to break path MTU some other way), I wouldn't trust any of its configuration. – David Schwartz Aug 15 '12 at 21:32

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