How Download accelerator Works?

In this post I will explain the basic principle of Download accelerators (such as DAP, wxDownload Fast, etc) used to download files. 

How it speeds up the downloading. 

First Let me explain the difference between normal downloading of a file and downloading of a file using download accelerator.  If you use regular browser to download a file then it creates only one connection with the server to download the file, whereas if download accelerators is used to download file then it creates multiple connection with the server and downloads a file in chunks and upon completion of download it joins those chunks.  Number of chunks created by download accelerators is depend on its configuration. I used wxDownload Fast and configured it to create 3 chunks of a file. That is, it creates 3 connections with the server to download any file. 

Lets consider an example. 

I used wxDownload Fast as download accelerator and downloaded a file ymsgr8us.exe (Yahoo messenger) which is of size 9.9 MB.
The initial request sent by wxDownload Fast to server was ordinary request to retrieve file, as given below:


Hypertext Transfer Protocol
GET /dl/9073e1f8a8d00eb735874cd9d3b6769c/4b824b4c/30%2Fymsgr8us.exe HTTP/1.1\r\n
Request Method: GET
Request Version: HTTP/1.1
HOST: us.download.soft32.com\r\n
User-Agent: wxDownload Fast\r\n
Range: bytes=0-\r\n

In HTTP response, from Content-Length header field wxDownload Fast learned the actual size of file.  Once the file size is know to wxDownload Fast (in general, to download accelerators), depending on number of connection (chunks) to create it decides how big should be the chunk size. In our case, I configured it to create three chunks, therefore, it divides Content-Length value by 3 to create three chunks.

Then it sends another HTTP request to the server, by creating another connection. The HTTP request made by wxDownload to the server was as follows:

Hypertext Transfer Protocol
GET /download/63-164279-1/ymsgr8us.exe HTTP/1.1\r\n
Request Method: GET
Request URI: /download/63-164279-1/ymsgr8us.exe
Request Version: HTTP/1.1
HOST: www.soft32.com\r\n
User-Agent: wxDownload Fast\r\n
Range: bytes=3474600-\r\n

Note the Range field in HTTP request.  The range header field was
                     Range: bytes=3474600- 


It instructs the server to return a file from 3474600th byte onwards.  Although, download manager requested for entire file in its first connection, it would however terminate that connection as soon as it will receive upto bytes 3474599. Hence it would not wast resources and download duplicate byte streams. 


As now you can image what would be the third HTTP request. It is given below. observe the Range HTTP header field.   

Hypertext Transfer Protocol
GET /download/63-164279-1/ymsgr8us.exe HTTP/1.1\r\n
Request Method: GET
Request URI: /download/63-164279-1/ymsgr8us.exe
Request Version: HTTP/1.1
HOST: www.soft32.com\r\n
User-Agent: wxDownload Fast\r\n
Range: bytes=6949200-\r\n

This is the basic principle followed by download accelerators (such as wxDownload Fast, etc)  to download file quickly than normal browser download.