Networking: The Need for WAN Optimization

By Jim Metzler

September 15, 2008 —  (Page 1 of 3)

A lot has been written about WAN optimization over the past three years, as companies have discovered significant performance problems when branch office workers try to access applications that are housed in centralized data centers. A host of factors has come into play to complicate the task of ensuring acceptable application performance in the branch. Two of the primary organizational factors are the movement to consolidate IT resources into a smaller number of locations and the simultaneous movement to distribute employees geographically.

Many companies either already have consolidated or are in the process of consolidating servers out of branch offices and into centralized data centers. This consolidation typically reduces cost and enables IT organizations to have better control over the company’s data. Because of server consolidation, chatty protocols such as CIFS (Common Internet File System), Exchange and NFS (Network File System), which were designed to run over the LAN, are now running over the WAN.

A chatty protocol is one that that requires hundreds of round trips to complete a single transaction. To understand the challenges this presents, assume that it takes 300 round trips to complete a transaction, that the LAN round trip delay time is 1ms and that the WAN round trip time is 100ms. The transaction takes a fraction of a second (roughly 300ms) to complete when accessing a server on the LAN. This is such a small increment of time that no user would notice it. The same transaction, however, requires a minimum of 30 seconds to complete when accessing a server over the WAN. This is so large that everybody notices it.

To put chatty protocols in context, CIFS works by decomposing all files into smaller blocks before transmitting them. Assume a client is attempting to open up a 20MB file on a remote server. CIFS would decompose that file into hundreds or possibly thousands of small data blocks. The server would send each of those data blocks to the client, where it would be verified, with an acknowledgement sent back to the server. The server would have to wait for the acknowledgement before sending the next data block. As a result, it could take tens of seconds before the user would be able to open the file.

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