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Scalable Networking Pack and the ISA Firewall

February 18, 2009

If you’ve spent any time at all looking at alerts on a Microsoft ISA firewall, you have no doubt seen the following alert:

“The Windows Server 2003 Scalable Network Pack, which is included in Windows Server 2003 Service Pack 2, is enabled. Some ISA Server features will not work properly if a network adapter installed on an ISA Server computer supports and uses the Scalable Network Pack features. For more information, see the Microsoft Knowledge Base article 948496. If you do not have a network adapter that supports the Scalable Network Pack features, you can disable the Windows Server 2003 Scalable Network Pack Enabled alert.”

What does this mean? What is the Scalable Network Pack? Why are we being alerted about it? How did it get installed? Well, let me answer some of those questions for you!

The Microsoft Windows Server 2003 Scalable Networking Pack (SNP) provides support for network acceleration and offloading technologies available in today’s advanced network interface adapters to increase performance and scalability. It was made available as an update to Microsoft Windows Server 2003 SP1, and was later included in Windows Server 2003 SP2. The Scalable Networking Pack consists of the following three new features:

TCP Chimney Offload – Provides for automated, stateful offload of TCP processing to a network adapter that includes a TCP offload engine (TOE). For certain types of network communication (typically large file transfers) TCP Chimney Offload reduces CPU overhead by offloading network packet processing tasks such as packet segmentation and reassembly to the network adapter.

Receiver-side Scaling – Enables the processing of inbound networking traffic to be shared across multiple CPUs. Applications that rely heavy on inbound network communication that run on a multiprocessor system can benefit from Receiver-side Scaling.

NetDMA – Enables memory management efficiencies through direct memory access (DMA), provided the server supports this.

All of these features are wonderful, and have the ability to dramatically increase throughput and substantially reduce processor utilization for network intensive applications. At first you might think that these features would be beneficial to the ISA firewall, but unfortunately, they are not. TCP offloading is designed primarily for large file transfers, and provides little benefit for short-lived conversations that are typical of network traffic handled by the ISA firewall. There are also some incompatibilities as well. For instance, Receiver-side Scaling is incompatible with both NAT and NLB, both commonly configured on an ISA firewall.

It is for these reasons that you will see this alert generated by the ISA firewall, and as a best practice you should disable these features to prevent unexpected behavior. Microsoft Knowledge Base article 948496 includes an update to turn off the default SNP features, as well as instructions on how to disable these features by editing the registry.