The core protocol stack components are those shown between the NDIS and TDI interfaces in Figure 1. They are implemented in the Windows Server 2003 Tcpip.sys driver. The TCP/IP stack is accessible through the TDI interface and the NDIS interface. The Winsock2 interface also provides some support for direct access to the protocol stack.
Address Resolution Protocol (ARP)
ARP performs IP address-to-MAC address resolution for outgoing packets. As each outgoing IP datagram is encapsulated in a frame, source and destination MAC addresses must be added. Determining the destination MAC address for each frame is the responsibility of ARP.
ARP compares the next-hop IP address on every outbound IP datagram to the ARP cache for the NIC over which the frame will be sent. If there is a matching entry, the MAC address is retrieved from the cache. If not, ARP broadcasts an ARP Request frame on the local subnet, requesting that the owner of the IP address in question reply with its MAC address. If the packet is going through a router, the next-hop address is that of a neighboring router and ARP resolves the MAC address for that next-hop router, rather than the final destination host. When an ARP reply is received, the ARP cache is updated with the new information, and it is used to address the packet at the link layer.
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