A remote access protocol manages the connection between a remote computer and a remote access server. These are the primary remote access protocols that are in use today:
Serial Line Internet Protocol (SLIP)
In 1984, students at the University of California, Berkeley, developed SLIP for UNIX as a way to transmit TCP/IP over serial connections (such as modem connections over POTS). SLIP operates at both the Physical and Data Link layers of the OSI model. Today, SLIP is found in many network operating systems in addition to UNIX. It is being used less frequently with each passing year, though, because it lacks features when compared with other protocols. Although a low overhead is associated with using SLIP and you can use it to transport TCP/IP over serial connections, it does no error checking or packet addressing and can be used only on serial connections. SLIP is used today primarily to connect a workstation to the Internet or to another network running TCP/IP.
Setting up SLIP for a remote connection requires a SLIP account on the host machine and usually a batch file or a script on the workstation. When SLIP is used to log in to a remote machine, a terminal mode must be configured after login to the remote site so that the script can enter each parameter. If you don’t use a script, you will have to establish the connection and then open a terminal window to log in to the remote access server manually.
Point-to-Point Protocol (PPP) and PPPoE (Point-to-Point Protocol over Ethernet)
PPP is used to implement TCP/IP; it is the protocol that establishes a connection over point-to point links (for example, dial-up and dedicated leased lines). It is most commonly used for remote connections to ISPs and LANs.
PPP uses the Link Control Protocol (LCP) to communicate between PPP client and host. LCP tests the link between client and PPP host and specifies PPP client configuration. Through LCP, PPP also supports authentication negotiation, as well as negotiation of encryption and compression between client and server, using compression control protocols (CCPs) and encryption control protocols (ECPs). PPP can support several network protocols through the use of protocol specific network control protocols (NCPs), and because it features error checking and can run over many types of physical media, PPP has almost completely replaced SLIP. In addition, PPP can automatically configure TCP/IP and other protocol parameters through the use of the IP control protocol (IPCP) NCP. On the downside, high overhead is associated with using PPP, and it is not compatible with some older configurations.
From the technician’s standpoint, PPP is easy to configure. Once you connect to a router using PPP, the router assigns all other TCP/IP parameters. This is typically done with the Dynamic Host Configuration Protocol (DHCP). DHCP is the protocol within the TCP/IP protocol stack that is used to assign TCP/IP addressing information, including host IP address, subnet mask, and DNS configuration. This information can be assigned over a LAN connection or a dial-up connection. When you connect to an ISP, you are most likely getting your IP address from a DHCP server.
To configure a Windows 2000 Professional client to dial up a remote access server and connect using PPP, follow these steps:
Point-to-Point Tunneling Protocol (PPTP)
PPTP is the Microsoft-created protocol based on PPP. It is used to create virtual connections across the Internet using TCP/IP and PPP so that two networks can use the Internet as their WAN link and yet retain private network security. PPTP is both simple and secure.
To use PPTP, you set up a PPP session between the client and server, typically over the Internet. Once the session is established, you create a second dial-up session that dials through the existing PPP session using PPTP. The PPTP session tunnels through the existing PPP connection, creating a secure session. In this way, you can use the Internet to create a secure session between the client and the server. Also called a virtual private network (VPN), this type of connection is very inexpensive when compared with a direct connection.
PPTP is a good idea for network administrators who want to connect several LANs but don’t want to pay for dedicated leased lines. But, as with any network technology, there can be disadvantages:
You can implement PPTP in two ways. First, you can set up a server to act as the gateway to the Internet and the one that does all the tunneling. The workstations will run normally without any additional configuration. You would normally use this method to connect entire networks. Notice how the TCP/IP packets are tunneled through an intermediate TCP/IP network (in this case, the Internet).
A PPTP implementation connecting two LAN's over the internet
The second way to use PPTP is to configure a single, remote workstation to connect to a corporate network over the Internet. The workstation is configured to connect to the Internet via an ISP, and the VPN client is configured with the address of the VPN remote access server PPTP is often used to connect remote workstations to corporate LANs when a workstation must communicate with a corporate network over a dial-up PPP link through an ISP and the link must be secure.
A workstation is connected to a corporate LAN over the internet using PPTP
To configure a Windows 2000 Professional client to create a VPN connection using PPTP over a PPP connection to a remote access server, follow these steps:
You can now double-click the VPN connection you made in the Network and Dial-up Connections window, choose to connect to the PPP connection through which you wish to tunnel (which may involve dialing up a remote access server, requiring you to first enter your username and password), and click Dial to establish the connection.
Windows Remote Access Services (RAS)
Both Windows NT and Windows 2000 include technology to allow users to dial up a server and connect to not only that server, but also to that server’s host network. This technology is known as RAS. RAS is used in smaller networks where a dedicated dial-up router is not practical or possible. In a RAS setup, you can basically connect a modem to a Windows NT or Windows 2000 server and, by way of the RAS, configure that modem as dial-out only, dial-up only, or a combination.
It is important to note that RAS, without help, provides access to only the LAN to remote users; it does not allow LAN users to use the modem to, say, dial their AOL account. For that, they would need Microsoft’s Shared Modem Services, which comes with the Small Business Server edition of Windows NT. Windows 2000, however, comes with the ability to share outbound connections. This is set up with Windows 2000’s RRAS utility.
Remote Desktop Protocol (RDP)
The Remote Desktop Protocol (RDP) is very similar to the Independent Computing Architecture (ICA) protocol used by Citrix products. As a matter of fact, RDP is used to access Windows Terminal Services, a close relative of the Citrix WinFrame product line. RDP performs the same basic functions as ICA, but it does it with a lot less functionality. RDP provides remote access for Windows clients only, whereas ICA provides it for multiple platforms, including DOS, Linux, Macintosh and many others. ICA is also a much more full-featured platform, including support for automatic client updates, publishing an application to a web browser, and much more.
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