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TCP/IP over ATM The protocol for classical IP over ATM (sometimes abbreviated as CLIP/ATM) is a well-established standard spelled out in RFC 1577 and subsequent documents. Windows 2000 provides a full implementation of this standard. The IP over ATM approach provides several attractive advantages over ELAN solutions. The most obvious advantages are its ability to support QoS interfaces, its lower overhead (as it requires no MAC header), and its lack of a frame size limit. All of these features are discussed in the following sections. IP over ATM Architecture IP over ATM is a group of components that do not necessarily reside in one place, and, in this case, the services are not usually on an ATM switch. In some cases, switch vendors provide some IP over ATM support, but not always. (For the purposes of this discussion, it is assumed the IP over ATM server services reside on a Windows 2000 server.) The core components required for IP over ATM are roughly the same as those required for LANE, as both approaches require the mapping of a connectionless medium to a connection-oriented medium, and vice versa. In IP over ATM, these services are provided by an IP ATMARP server for each IP subnet. This server maintains a database of IP and ATM, and provides configuration and broadcast services, as described in the following section. IP over ATM Components IP over ATM is a very small layer between the ATM protocol and the TCP/IP protocol. As with LANE, the client emulates standard IP to the TCP/IP protocol at its top edge while simultaneously issuing native ATM commands to the ATM protocol layers underneath. IP over ATM is often preferred to LANE because it is faster than LANE. One key reason for this performance advantage is that IP over ATM adds almost no additional header information to packets as they are handed down the stack. Once it has established a connection, the IP over ATM client can generally transfer data without modification. As with LANE, IP over ATM is handled by two main components: the IP over ATM server and the IP over ATM client. The IP over ATM server is composed of an ATMARP server and Multicast Address Resolution Service (MARS). The ATMARP server provides services to map network layer IP unicast addresses to ATM addresses, while MARS provides similar services for broadcast and multicast addresses. Both services maintain IP address databases just as LANE services do. The IP over ATM server can reside on more than one computer, but the ATMARP and MARS databases cannot be distributed. You can have one IP over ATM server handle ATMARP traffic, and one handle MARS. If, however, you divided the ATMARP Server between servers, it would effectively create two different IP networks. All IP over ATM clients in the same logical IP subnet (LIS) need to be configured to use the same ATMARP server. Traditional routing methods are used to route between logical IP subnets, even if they are on the same physical network.