TCP/IP over ATM

The Internet Engineering Task Force RFC1577: Classical IP and ARP over ATM standard specifies the mechanism for implementing Internet Protocol (IP) over ATM. Because ATM is connection-oriented technology and IP is a datagram-oriented technology, mapping the IP over ATM is not trivial.

In general, the ATM network is divided into logical IP subnetworks (LISs). Each LIS is comprised of some number of ATM stations. LISs are analogous to traditional LAN segments. LISs are interconnected using routers. A particular adapter (on an ATM station) can be part of multiple LISs. This feature can be very useful for implementing routers.

RFC1577 specifies RFC1483, which specifies logical link control/Sub-Network Access Protocol (LLC/SNAP) encapsulation as the default. In PVC networks for each IP station, all PVCs must be manually defined by configuring VPI:VCI values. If LLC/SNAP encapsulation is not being used, the destination IP address associated with each VPI:VCI must be defined. If LLC/SNAP encapsulation is being used, the IP station can learn the remote IP address by an InARP mechanism.

For SVC networks, RFC1577 specifies an ARP server per LIS. The purpose of the ARP server is to resolve IP addresses into ATM addresses without using broadcasts. Each IP station is configured with the ATM address of the ARP server. IP stations set up SVCs with the ARP server, which in turn, sends InARP requests to the IP stations. Based on InARP reply, an ARP server sets up IP to ATM address maps. IP stations send ARP packets to the ARP server to resolve addresses, which returns ATM addresses. IP stations then set up a SVC to the destination station and data transfer begins. The ARP entries in IP stations and the ARP server age based on a well defined mechanism. For both the PVC and SVC environments, each IP station has at least one virtual circuit per destination address.

The Internet Engineering Task Force RFC2225 adds the support of ATM ARP Request Address list to RFC1577. The ATM ARP Request Address list is a list containing one or more ATM addresses of individual ATM ARP servers located within the LIS. The RFC2225 client eliminates the single point of failure associated with the 1577 clients' ATM ARP services. The 2225 clients have the ability to switch to backup ARP servers when the current ATM ARP server fails.

RS/6000® sets the first entry in the ATM ARP Request Address list as the Primary ATM ARP server and the rest of the entries as Secondary ATM ARP servers.

The client will always try to use the Primary ATM ARP server. If the effort to connect to the Primary ATM ARP server fails, the client tries to connect to the first Secondary server (the position in the ATM ARP Request Address list determines the order of the Secondary ATM ARP server). If the connection to the first Secondary ATM ARP server fails, the client tries to contact the next Secondary ATM ARP server in the list. This process continues until the connection is successful.

If the connection to the Primary ATM ARP server fails, regardless of which Secondary ATM ARP server it is connected to or attempting to connect to, the client continues to retry the Primary ATM ARP server every 15 minutes. If it finally connects to the Primary ATM ARP server, then the connection to the current Secondary ATM ARP server is dropped.

The ATM ARP Request Address list is entered manually either through SMIT or by using the ifconfig command. The ATM ARP Request Address list cannot be configured with the Management Information Base (MIB).