Microsoft KB Archive/229704

= IIS 4.0 Supports HTTP 1.1 File Download Resume Capability =

Article ID: 229704

Article Last Modified on 11/21/2006

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APPLIES TO


 * Microsoft Internet Information Server 4.0
 * Microsoft Internet Information Services 5.0

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This article was previously published under Q229704



We strongly recommend that all users upgrade to Microsoft Internet Information Services (IIS) version 6.0 running on Microsoft Windows Server 2003. IIS 6.0 significantly increases Web infrastructure security. For more information about IIS security-related topics, visit the following Microsoft Web site:

http://www.microsoft.com/technet/security/prodtech/IIS.mspx



SUMMARY
When you host Web content on a computer running IIS, it is possible for Web browsers that support HTTP 1.1 protocol to resume interrupted file downloads where they left off, without having to re-download the entire file again. This functionality is provided in HTTP 1.1 protocol, as noted in the HTTP 1.1 RFC.



MORE INFORMATION
Specifically, the Range Retrieval Request field allows for the following functionality:

14.36.2 Range Retrieval Requests

HTTP retrieval requests using conditional or unconditional GET methods may request one or more sub-ranges of the entity, instead of the entire entity, using the Range request header, which applies to the entity returned as the result of the request:

Range = "Range" ":" ranges-specifier

A server MAY ignore the Range header. However, HTTP/1.1 origin servers and intermediate caches SHOULD support byte ranges when possible, since Range supports efficient recovery from partially failed transfers, and supports efficient partial retrieval of large entities.

Because IIS 4.0 and Internet Explorer versions 4.0 and 5.0 support HTTP 1.1, a client can re-initiate a broken or interrupted download and continue downloading the file at the point where the download was interrupted. This can be extremely helpful from both a customer satisfaction and network bandwidth management viewpoint for companies that host large files for general download on their Web site. However, there are some caveats, such as the following:
 * The local cache settings on the browser must be set large enough, so that the received portion of the downloaded file fits in the browser cache. If an empty browser cache has a size limit of 10 MB configured and a 15-MB file is being downloaded, the resume feature will not work if more than 10 MB of the file was downloaded before the download was cancelled. Internet Explorer will immediately remove this partially downloaded file from its cache to keep it within the size limit specified. This prevents the resume feature from working properly. To resolve this issue, set the cache size to a value that is large enough to accommodate large files being downloaded.
 * If the Web site you are browsing is actually a Web farm located behind some sort of load balancing device, such as a Cisco LocalDirector, the load balancing device will have to be configured to allow a user to remain connected to the specific Web server in the farm that the file was originally being downloaded from. If a user starts and stops the download on one server, and then gets routed to a different identical server in the farm to resume the download, the resume feature will fail. The file resume request must be sent to the same server that the file was originally being downloaded from.
 * The default LocalDirector settings assume that it will be used for standard Web server functions. As a result, the configuration is geared toward the small quick sessions typical for Web page service; it not geared towards the larger and longer sessions that are typical for large product downloads. Most notably, the sticky value controls the window in which a user may reconnect to the site and have a guarantee of hitting the same server behind the LocalDirector proxy. By default, this is set to 0 (minutes), which means that a user will never be guaranteed to hit the same server and as such, resumable downloads will not function. By increasing the value modestly (for example, to 10 minutes), resumable downloads can then be supported.

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