Last time, we discussed why so many companies either already have, or are currently looking at, the idea of consolidating servers out of branch offices and into centralized data centers. We also mentioned that there is a well-known problem associated with server consolidation: the Common Internet File System (CIFS) protocol. Many Microsoft applications, including Outlook, Word and PowerPoint, rely on CIFS, but the protocol was designed to run over a LAN. Over a WAN, CIFS performs very badly resulting in highly disgruntled users.

In order to understand why CIFS runs badly over the WAN it is helpful to understand a bit about the protocol. CIFS works by sending packets from the client to the server in order to request some kind of service, such as opening, closing or reading a file. However, CIFS does not send the entire file from the server back to the client, instead it decomposes the file into smaller blocks prior to transmission. 

Assume that a client was attempting to open up a 3M-byte file on a remote server. CIFS would decompose that file into tens, or possibly hundreds, of small data blocks. The server sends each of these data blocks one by one to the client where it is verified and an acknowledgement is sent back to the server. The server must wait for an acknowledgement prior to sending the next data block.  When run over a WAN, it can take tens of milliseconds to transmit a single block of data because of the latency that is inherent in the WAN - and that is not present in the LAN. As a result, it can take several seconds for the soon-to-be-disgruntled user to be able to open up the file. 

Subsequent WAN newsletters will discuss techniques to overcome the issues of running CIFS over the WAN. In the meantime, we would like to hear from you.  Have you experienced bad response time as a result of consolidating your servers into a centralized data center? If so, what actions have you taken to improve that response time and how much of an impact did those actions have on response time?

* Note from Steve: There is a Juniper presentation at the Webtorials site that does a nice job of discussing applications acceleration for CIFS (the presentation includes animations). The presentation is near the bottom of the page.

Steve Taylor is president of Distributed Networking Associates and publisher/editor-in-chief of Webtorials. Jim Metzler is vice president of Ashton, Metzler & Associates.