How HTTP verbs can 'hang' Outlook Web Access

Hypertext Transfer Protocol (HTTP) uses a series of commands, called "verbs," to describe what action is taking place between the client and server. Two of the most common HTTP verbs are GET and POST. The former is used for retrieving a Web document and the latter is used for sending form information to a server.

One of the newer HTTP verb sets is implemented in a standard called Distributed Authoring Version, or HTTP-DAV, which has been specified in RFC 2518.

Microsoft Exchange Server uses HTTP-DAV to send and receive requests called Distributed Authoring and Versioning Searching and Locating (DASL), which help run Outlook Web Access (OWA). Internet Explorer versions 5.0 and up use HTTP-DAV, but other browsers -- such as earlier versions of Netscape -- do not.

Proxy servers or firewalls that closely screen HTTP traffic may unintentionally block legitimate OWA traffic if they don't allow the use of HTTP-DAV verbs. Under these conditions, the infamous "Loading …" screen will appear indefinitely when users try to access OWA.

NOTE: This issue only occurs with browsers that explicitly support HTTP-DAV; browsers that don't support HTTP-DAV will revert to a lower level of functionality.

The two most basic ways to work around this problem:

• Update the firewall or proxy server to support HTTP-DAV verbs. This isn't always possible or practical, but it is the best first approach, since it may solve a number of other problems as well.

• Set up Secure Sockets Layer (SSL) for OWA. The firewall or proxy server will not be able to screen encrypted traffic, so it will pass through as-is. This is useful if you can't upgrade the firewall or proxy server; it also adds another level of security to OWA.

About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter.

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