[core-dev] Bandwidth drop-off in Limewire 4.2.6 (test results)

Hello everyone, I've finished testing Limewire 4.2.6 to 
determine if the bandwidth drop-off that I reported 
earlier is fundamental to the magnet links (my original 
testing method) or is independent of magnet links. In my 
earlier tests with earlier versions of Limewire 4.2.x I used 
magnet links with multiple sources (using the as and xt 
attributes), each of which are my own servers. That is, 
the magnet link sources pointed to my own servers so that 
I could monitor bandwidth during the upload from those 
servers to the client that clicked on the magnet links. 

What I observed was very surprising: with each new source 
the bandwidth for ongoing uploads dropped. For example, 
if I used just one of my servers I got the download at 
full speed, but when two were used both of those sources 
only uploaded at about half-speed -- it was no faster to 
upload from two sources than one because of the upload 
speed drop-off. Things got worse with each new source; 
with a handful of sources or more my own two servers 
dropped off  to just a fraction of the bandwidth they 
had available. As a result, most of the available 
bandwidth on my servers was not used despite the fact 
that the client actually started with them (that is, 
my two servers are the first sources that the client 
starts with and they have ample bandwidth, but as the client 
starts to connect with other clients on the Gnutella 
network the upload/download speed between the client and 
my servers drops off to the point where only a fraction 
of the available bandwidth is used).

With Limewire 4.2.6 I decided to eliminate the magnet links 
from the equation entirely, and provide a testing method 
that is easy enough for anyone to try. If you do the same 
and get the same results we can then assume that there is 
a fundamental upload/download issue with Limewire and that 
magnet links have nothing to do with it. I ran these new 
tests and had the same result as previously; with each new 
source the upload/download speed drops off until eventually 
only a fraction of the available bandwidth on my own servers 
is used. Following are the steps I took for these new tests:

 1) Using a personal file that I know isn't available on the Gnutella 
    network I tested the upload/download speed on my own servers. 
    Because the file isn't available on the larger network I didn't 
    have to worry about the client finding new sources; I got full 
    speed downloads from each of my servers with this test when only
    one of them was used by the client. But, when both were used the
    drop-off occured as previously (each server used only about half
    of their bandwidth when both servers were used, yet transferred at
    full speed when only one for the transfer).
 

 2) Using popular files on the Gnutalla networks (movies, music files, etc.)
    that have a large number of sources) I was able to determine that the 
    download speed drops-off as more sources are found. To observe this I 
    would download the file onto my servers using Limewire, so that my servers 
    had the files *and* retained any alternate sources associated with the files. 
    Then, using my client, I established a direction connection to my server and 
    manually downloaded the media file. This would start the download at full 
    speed (one source - my server), and within a minute or so the client would 
    also start downloading from the alternate sources associated with that file 
    (multiple sources on the Gnutella network). That's when the drop-off started...

As I observed previously when conducting tests with magnet links the upload/
download bandwidth for my own server began to drop-off as the client adds 
more sources; this drop-off continues to the point where only a fraction of 
the available bandwidth my server has is actually used. In other words, 
the exact same behavior occurred when magnet links weren't involved, 
indicating that the problem is isolated withing the core upload/download 
code in Limewire.

Can others replicate the drop-off in bandwidth? The test is rather simple, 
and you can do it as long as you have two computer (one to act as the source, 
the other the client). Merely share a private file that isn't on the larger 
Gnutella network and record how fast the upload/download happens, and then 
do the same for a popular file available on the Gnutella network -- download 
it first to your server, then direct connect to your server from the client 
and manually start the download. The download starts at full speed, and should 
drop-off in speed as the alternate sources are found and added.

Unless my particular test results are unique to me it seems like a big 
problem that limits Limewire's overall effectiveness since it 
loses speed with each new source added to the download (in order to get 
decent speed a lot of sources are needed to compensate for the drop-off).
In my case it's faster to have the download from my own server than to 
swarm it from a few (2-3) sources (in which case a standard Web server 
would be better to transfer the files since users only need a Web 
browser to get the files at the same speed as Limewire offers). If I have
enough sources however, with ample bandwidth available from each, the 
drop-off in bandwidth is overcome. This means that more sources are tapped
just to overcome the problem, which in turn means more traffic on the 
network and more resources consumed than needed.

But if the swarming didn't drop-off in speed it would offer a 
tremendous advantage for such file transfers. We're looking at 
Limewire's potential as a file transfer tool for companies, but 
so far it seems like the bandwidth drop-off makes it on par with 
traditional file sharing. But it could just be my setup, so I wonder:

  Can anyone confirm that this happens to them as well?

Brian

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