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Windows will detect if the network hardware has capabilities for offloading of the tasks required to maintain the TCP/IP stack.

• TCP/IP checksum calculation - Each packet sent needs a verification checksum, so the receiver knows that packet wasn't damaged during the transfer. This checksum can be calculated by the NIC.
• TCP/IP segmentation - Also known as TCP Large Send where Windows delivers a large message to the NIC and it is responsible for chopping up the message according to network MTU. This feature is not enabled by default, and Microsoft sees this as an experimental feature.
• Internet Protocol Security (IPSec) Encryption Ciphers and Message Digests - Provides encryption of the packets sent/received at the hardware level.

Checksum offloading should not be activated if one of the following features are turned on:

• Windows Firewall

Note to disable the offloading of processor tasks to the network adapter:

[HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Services \Tcpip \Parameters]
DisableTaskOffload = 1 (Default not set)

More Info MS KB 888750
More Info MS KB 904946
More Info MS KB 936702

More Info WHDC - NDIS 5.0 Overview
More Info WHDC - Windows Network Task Offload