Every so often Microsoft puts out what are called "out-of-band" (OOB) releases for Windows and its associated products. Such releases are add-ons that allow you to add major functionality from a future iteration of a given product -- such as the next Service Pack revision -- without having to wait for that iteration to be formally released.
One of the most significant OOB releases for Windows Server 2003 (and 64-bit Windows XP) is an add-on called the Microsoft Windows Server 2003 Scalable Networking Pack. This add-on enables several new functions in the Windows network stack; these functions allow you to take advantage of abilities in new network hardware that are not enabled or exploited by default.
The Networking Pack adds the following functions:
- TCP Chimney. This allows TCP/IP processing to be offloaded to any network adapter that supports it (similar to how a graphics processor handles 3D geometry and polygons for games in lieu of the CPU doing that work). Likewise, this allows the network adapter to do some of the work in processing TCP connections that would normally be done by the CPU.
- RSS. In this case, RSS means Receive-Side Scaling, and is not to be confused with the Really Simple Syndication protocol. This allows the load from a single network adapter to be balanced equally across multiple CPUs.
- NetDMA. Direct Memory Access (DMA) technology is used for hard drives, and allows the disk controller (or other hardware device) to copy data directly into memory instead of requiring the CPU to do the same, which is slower. NetDMA allows network hardware to do the same thing, provided it supports it: The TCP stack can copy data directly into memory without CPU intervention.
However, there are several limitations to the way the Networking Pack operates. First of all, TCP Chimneying can only support a finite number of connections at a time on a given network adapter. If you go over that limit for a given adapter, any connections over that limit will be handled instead by the CPU directly. The exact number of connections varies from manufacturer to manufacturer.
A second limitation is that NetDMA will not work on adapters that support TCP chimneying, and vice versa. A third limitation is that third-party firewalls will not be able to gain access to the network stack through undocumented APIs if chimneying is enabled.
By default all of the above functions are disabled, even when the networking pack is installed, and will need to be enabled manually by editing the Registry. The link mentioned above contains the pack for download and also explains how to enable everything.
About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter. Check it out for the latest advice and musings on the world of Windows network administrators.
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