The HTTP/1.1 protocol include support for range-specific requests, which allow a client to optionally request aparticular range of bytes rather than request the entire file. This functionality is most commonly used by download manager programs, which allow users to pause and resume downloads. In a nutshell, a download manager will start by asking for the entire contents of a file (the default behavior). If the user pauses the download or if the download manager is shut down, the last downloaded byte is remembers. Later, the download manager can resume the download by sending a request to the server for the file starting from the last downloaded byte.
The following diagram illustrates the range-specific request workflow just described when downloading a large file named DancingHampsters.zip. The diagram shows what happens when the download is paused (or interrupted) after having downloaded the first 500,000 bytes, and how using range-specific requests the client can resume the download starting from a specified location in the file (rather than having to re-download the file in its entirety).
While IIS natively handles range-specific requests, ASP.NET does not. If you are serving binary content from an ASP.NET HTTP Handler and if you want (or need) to support range-specific requests then you'll need toaddsuch functionality yourself. I bumped into this requirement when working on a project that serves videos to Apple's handheld devices - the iPhone and iPod Touch. The iPhone and iPod Touch request video files using range-specific requests. Therefore, if you are serving these videos straight from the file system via IIS then everything will work as expected, but if you serve the video from an HTTP Handler in order to implement authorization rules or if the files are dynamically generated then you'll need to write code in your HTTP Handler that will handle the range-specific requests sent by the Apple devices.
In my research I stumbled upon an article by Alexander Schaaf titled Tracking and Resuming Large File Downloads in ASP.NET, which presented Visual Basic code showing how to implement range-specific requests in an ASP.NET 1.x application. I took this code, refactored it,ported it to C#,and utilized a number of language enhancements added since the .NET 1.x days. This code, along with a discussion on how range-specific requests work and a look at how to use the code, is now available on DotNetSlackers.
Read more at:Range-Specific Requests in ASP.NET.
Happy Programming!