Large Uploads in IIS 6

IIS 6 presents two obstacles to very large file transfers: inefficient memory utilization and a 4-GB request size limit. To overcome these problems in IIS6, FileUpEE provides the HttpModule and supports chunked transfer-encoding.

The HttpModule

FileUpEE's HttpModule intercepts selected requests to overcome ASP.NET's inefficient memory utilization and to allow accurate progress indication. The HttpModule intercepts an upload request, reads all the data, and writes it to a temp file chunk by chunk, before sending it to ASP.NET.

For performance reasons, the HttpModule will not intercept all ASP.NET requests. It will intercept ASP.NET requests for files with the extension .uplx. Files with the extension .aspx will not be processed by the filter, and will be sent directly to ASP.NET. This prevents burdening the server with unnecessary use of the HttpModule. So, to use the HttpModule, name your ASP.NET scripts FileName.uplx.

With the FileUpEE registry key FilterAspDotNetFlagExtension, you can change the file extension used to signal the HttpModule in ASP.NET (.uplx) to a different extension of up to seven characters. By using a file extension other than .uplx you can hide your web server implementation (Windows/IIS) from potential hackers.

If you install the ASP.NET version of FileUpEE, the automatic installation package will install the HttpModule for FileUpEE's samples, and create the script mappings for the .uplx extension. See Installing the HttpModule for instructions on installing the HttpModule at the application level or in the Global Assembly Cache.

Chunked Transfer-Encoding

IIS 6 gets the size of a request from the request's content-length header, and does not accept requests larger than 4-GB. By default, IIS 6 accepts requests of no more than 1-MB. You can increase this value by resetting the IIS 6 properties MaxRequestEntityAllowed or AspMaxEntityBodyAllowed, but the maximum value is 4-GB.

To overcome the request size limits imposed by IIS 6, FileUpEE supports chunked transfer-encoding. A chunked file transfer is sent as a series of chunks, each preceded by its own size indicator. A chunked transfer request does not include a content-length header for the whole request. Since IIS 6 rejects a request because of the size of the request's content-length header, it finds no reason to reject a request without a content-length header. With chunked-transfer encoding, no matter how large the upload, it will not be rejected by IIS 6.

To prevent misuse of chunked uploading, FileUpEE allows you to control the maximum allowable file transfer through the MaxKBytes and MaxKBytesToCancel properties.

Browsers do not support chunked transfer-encoding. To send a chunked upload from client to web server you must use SoftArtisans XFileEE. Chunked transfer-encoding is not supported by Microsoft ISA Server and Microsoft Proxy Server. If your requests must go through ISA Server or Proxy Server, you cannot use chunked-transfer encoding.

To send a chunked upload from web server to file server, on the web server set FileUpEE.TransferEncoding to saChunked. Set TransferEncoding after setting TransferStage, and before ProcessRequest.

Set oFileUpEE = Server.CreateObject("SoftArtisans.FileUpEE")
oFileUpEE.TransferStage = saWebServer
oFileUpEE.TransferEncoding(saFileServer) = saChunked
...
oFileUpEE.ProcessRequest Request, False, False

The following 3-tier upload samples send the upload from web server to file server as a chunked request:

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