How do Download Managers accelerate file downloads?
Although broadband Internet access is getting more and more
common, people's demand for faster file downloads has never
stopped. It is this desire for speed that gave birth to Download
Managers, which accelerate file downloads for people like
you.
Plenty of Download Managers are out there on the market for
you to choose from. Just to name a few, there are GetRight,
Download Accelerator, FlashGet...etc. They
may use different strategies to speed up file downloads, but
their basic principle is the same. In this article, we are going
to discuss how they work.
Before we delve into the technical details, let's have some
water fun first. How would you fill up 10 buckets with water
within the shortest time? One of the many ways of doing it is
to line them all up in front of a faucet, turn on the faucet
at full blast and then fill up the buckets one by one. Another
way of doing it might be to put them under 10 different faucets
and fill them up at the same time. Which way would you choose?
I would go for the second strategy.
When it comes to downloading files, the situation is essentially
the same. Say, you are going to download a file whose size is
100 MB (Mega Bytes). If you use a regular browser to download
the file, it will take 100 seconds to download the file if the
server delivers a maximum download bandwidth of 1MB per second
per connection. However, if you can somehow
break up the file into 2 halves and download them simultaneously,
you can cut the download time to 50 seconds. That is a 100%
increase in speed! By doing the math, you know that the more
pieces you break up the file into, the more time you will save.
Does it sound like a crazy idea?
That is not a crazy idea at all. In fact, that is exactly how
most (if not all) Download Managers accelerate file downloads.
Let's take a look at a real-life example:
I used a Download Manager to download the file "gaim-0.79.exe"
(size: 6.2MB) from one of the sourceforge servers (in this case,
it is belnet.dl.sourceforge.net). The very
first thing it did was to make an ordinary HTTP request to retrieve
the file as shown below:
There was nothing special in the HTTP request made by the Download
Manager. From the Content-Length header of
the HTTP response, the Download Manager learned that the file
size was 6,284,421 bytes:
Once the Download Manager knew the file size, it calculated
how many pieces it wanted to break the file into and how big
each piece should be. Then, it opened up another connection
(c.f. turning on another faucet) to the sourceforge server to
download the "second piece":
This time, the HTTP request made by the Download Manager was
a bit unusual. There was a Range header:
It instructed the sourceforge server to return the file from
the 1,413,878 th byte onwards. You may ask: The Download
Manager was already downloading the entire file via its first
connection, would doing this actually increase the download
time? That was a good question. Although the Download Manager
requested the entire file in its first connection, it would
however terminate the connection as soon as it obtained the
first 1,413,877 bytes so that it wouldn't waste any time and
resources downloading the same portion of data twice.
As you can imagine, the Download Manager would make several
HTTP requests to download different portions of the same file
simultaneously. The following screenshot shows you what it did
to obtain the "third piece":
Being a smart dude that you are, you must have spotted that
the Range header was different from last time:
This process continued until the Download Manager finished breaking
up the file into its desired number of pieces. When the Download
Manager obtained all the pieces, it would assemble them into
one single file that you had requested -- i.e. "gaim-0.79.exe"
in this case. Now that you know how Download Managers work,
do you agree that accelerating file downloads isn't actually
that mysterious.
That's all. I hope you enjoyed the discussion. I found out
the above with HttpRevealer. You
can explore the web yourself too! [See
more info]
Steven Chau
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