Why CIFS and MAPI Are Poor Network Citizens

Among the applications most likely impaired by file server centralization are Word, PowerPoint, and Excel, which use Microsoft's Common Internet File System (CIFS) file sharing protocol, as well as Outlook which uses Microsoft's Messaging Applications Programming Interface (MAPI) email protocol for Exchange.  CIFS and MAPI-based applications degrade badly as distance to users increases, and here's why.

The problem is that CIFS and MAPI are just plain poor network citizens.  They were designed to run over local area networks, where the performance price for application protocol "chattiness" is negligible.  Unfortunately, when run over a WAN, such chattiness (application turns) exacts a heavy toll. 

CIFS runs slowly over a WAN because when a client requests a server to open, close, or read a file, CIFS breaks it into blocks and transmits the blocks sequentially rather than send the whole file,.  For example, if a client requests a 1Mb file from a remote server, CIFS breaks the file into hundreds of data blocks and sends the blocks one at a time.  The client acknowledges receipt of each block, and only after receiving the acknowledgement, does the server release the next block.  This back and forth chattiness adds unwanted seconds, or even minutes to the simple task of opening the file.  Microsoft's MAPI protocol exhibits similar behavior (or misbehavior) over a WAN.

To place CIFS and MAPI-based application performance into perspective, it is useful to compare performance profiles for CIFS and MAPI tasks with other common application tasks.  The following figure shows application turns and payload per task on logarithmic scales.  As you can see, the number of turns required for CIFS and MAPI-based applications are exponentially larger than for client server, SNA, and even most Web-based applications.

Application Profiles

Because the scales are logarithmic, small distances on the chart reflect huge differences in the user's experience.  With this in mind, you can see that the turns and payload burden for MAPI and CIFS-based applications is exponentially greater than for SNA and client-server applications.

In a future blog we'll tell you how to "fix" CIFS and MAPI's performance problems.

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Copyright © 2008 IDG Communications, Inc.

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