Front-End Redirection
As I mentioned before, Exchange Servers research the Active Directory to find Exchange objects. In a normal situation, the server that actually holds the data instructs the referring server to redirect the client. FE servers are told to respond as if they hold the data. In many cases, it is the BE server that tells the FE server what to do.
Following is a packet trace I did in the lab while a client requested an OWA session from the FE server. In this case, the mailbox was on ServerA. First, the client performs a DNS lookup for SERVER1.BRYANT.COM. The response comes and the client then performs a GET request for an OWA session. I started this whole process by accessing http://server1/exchange from Internet Explorer. (Remember that Server1 is the FE.)
The FE then tries to locate the appropriate BE server. This is done through a series of LDAP and DNS calls, as well as scattered RPC sessions.
Once the FE Server finds the appropriate BE Server, it begins to echo the client requests verbatim to the BE server.
You would expect the BE server to then respond with http://servera/exchange/mailbox information, but instead it tells the Front-End server exactly what to say to the client. The BE tells the FE to respond with http://server1/exchange information. Basically, the BE is doing a lot more than I thought it would.
I have already mentioned WebDAV in this article because this protocol is at the heart of Exchange 2000. WebDAV, an extension to HTTP, is an emerging protocol standard for performing basic file system operations across the Web using Distributed Authoring and Versioning (DAV). Because it is based on HTTP, it is an excellent way to code through firewalls. When used in conjunction with XMLHTTP, WebDAV can also define the XML post data structure. OWA for Exchange 2000 is a prime example of this combination. In fact, it is this combination that we analyzed earlier in this article.
WebDAV and pure HTTP are currently the only (programmers) protocol that FE servers can “proxy” to BE servers. Let me repeat that:
FE servers will only process WebDAV requests destined for an Exchange 2000 Web Storage Sytem. FE servers will not proxy CDO, ADO, MAPI or any other programming protocol other than WebDAV.
To learn more about WebDAV, see:
http://msdn.microsoft.com/workshop/standards/webdav.asp
http://msdn.microsoft.com/library/default.asp?URL=/library/techart/wssdevaccess.htm
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