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Download Accelerators 101
Posted By: Brian <>Date: 2/6/03 9:42 p.m.

In Response To: Re: this is for louis (DeAdLy_cOoKiE)

Since it has come up a bunch of times in the past few weeks, I figured I'd give everyone the lowdown on what download accelerators are and why they're bad.

When your web browser tries to download something from a site (like, it opens up a connection to the site's HTTP server and downloads the file through that (single) connection. This is how things are supposed to work.

One of the swell features of modern HTTP servers is the ability to request certain "ranges", or parts, of a file. The idea behind this is that, if you get disconnected while you're downloading a file, you can resume the download later and not have to start over.

Download accelerators capitalize on the idea of resumable downloads by downloading multiple ranges at the same time, hoping against hope that this will somehow make things go faster. Once in a blue moon, when the site that the file is being downloaded from is artificially capping download speeds, a download accelerator will actually work. It will work by bypassing limits which were probably put in place for a very good reason, but it'll work.

To download multiple ranges at the same time, the download accelerator has to open multiple connections to the HTTP server. This is bad, because HTTP servers can only handle a finite number of simultaneous connections, and so when one person is opening up four or eight or twenty connections, he's filling up spaces that could otherwise be used to allow that many other people to connect to the server. Get enough people doing this, and soon no one will be able to connect to the server.

So what Mythica (and many other sites that serve mostly large files) does to prevent this is to limit the number of simultaneous connections to the HTTP server from any given IP address. In Mythica's case, a single IP address is allowed to open up three connections simultaneously. This works quite well to protect the HTTP server from the abuse that download accelerators throw at it.

Now enter "Download Accelerator Plus", probably the most popular download accelerator around. When it starts a download it does its thing, flooding the server with connection attempts and trying to download certain pieces of the file. After three connections, Mythica's HTTP server sends it a "503 Service Unavailable" message. This is supposed to mean that the client (i.e. DAP) should stop trying to connect to the server for some period of time. And so DAP _does_ stop trying to connect. But what it also does is to close the existing, perfectly legitimate connections to the HTTP server. Why it does this, I'm not entirely sure. But the net effect is that it's more-or-less impossible to download anything from mythica using DAP.

At any rate, the solution is -- you guessed it -- to stop using Download Accelerator Plus. Lots of folks evidently use DAP solely because it can resume downloads (the ability to resume downloads is a good, useful feature which is beneficial to both the server and the client), and they can't get that functionality in any useful form from their web browsers. But they're not out of luck -- there are lots of other more reasonable, more cluefully designed programs which can resume downloads. GetRight (, for one.

To sum it all up: Download accelerators are bad. Don't use them.

Message Index


this is for louisSniPeR 2/6/03 6:42 p.m.
     Re: this is for louisSniPeR 2/6/03 6:47 p.m.
           Re: this is for louisDeAdLy_cOoKiE 2/6/03 6:51 p.m.
                 Re: this is for louisLouis Wu 2/6/03 6:52 p.m.
                 Re: this is for louisBrian 2/6/03 7:26 p.m.
                 Download Accelerators 101Brian 2/6/03 9:42 p.m.
                       Re: Download Accelerators 101Louis Wu 2/7/03 8:09 a.m.
     Louis has email.Louis Wu 2/6/03 6:51 p.m.
           Re: Louis has email.Dan Chosich 2/6/03 7:31 p.m.
                 Re: Louis has email.oddworld18 2/6/03 8:53 p.m.
           Re: Louis has email.P_40E 2/6/03 9:39 p.m.
                 Re: Louis has email.Brian 2/6/03 9:47 p.m.
                       Average speed question then....skavenger_s7 2/7/03 8:36 a.m.
                             Re: Average speed question then....Louis Wu 2/7/03 9:41 a.m.
                       Re: Louis has email.P_40E 2/7/03 2:56 p.m.
                             Re: Louis has email.Brian 2/7/03 3:37 p.m.
                                   Re: Louis has email.Louis Wu 2/7/03 3:45 p.m.
                                         Re: Louis has email.Warbow 2/7/03 3:48 p.m.
                             Re: Louis has email.Louis Wu 2/7/03 3:40 p.m.
                                   Re: Louis has email.P_40E 2/7/03 4:36 p.m.
                                         Re: Louis has email.Brian 2/7/03 5:03 p.m.

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