RAS Server Architecture
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4/8/2010
In the Windows Embedded CE networking architecture, the Windows Embedded CE-based device functioning as a RAS server communicates directly with PPP. PPP corresponds with the underlying WAN miniport — either PPTP or AsyncMAC — through Transmission Control Protocol/Internet Protocol (TCP/IP). When PPP receives requests to send IP packets from TCP/IP, it passes the packet on to the AsyncMAC miniport. After receiving a packet from PPP, the AsyncMAC miniport performs the asynchronous framing, and then forwards the packet to the TAPI device by calling Microsoft® Win32® serial APIs. When receiving a packet through the network, the AsyncMAC miniport strips the asynchronous framing off the packet, checks the Cyclic Redundancy Check (CRC), and passes the packet to PPP through the NDIS layer.
In a VPN, the PPTP WAN miniport communicates directly with TCP/IP. After receiving a packet that is addressed to the private network from TCP/IP, PPP performs the framing, and then forwards the packet to the PPTP WAN miniport. The PPTP WAN miniport encapsulates the information contained in the packet header, and then reroutes the packet back to TCP/IP. IP then attaches another header that contains the address of the PPTP server to the packet, and passes the packet on to PPP. PPP forwards the packet to either the AsyncMAC WAN miniport or a local area network (LAN) adapter, such as an NE2000 adapter. After receiving the packet over the network, the PPTP server strips the PPTP header off the packet, and then passes the packet to PPP.