ATM ARP Address Resolution Tables

Depending on the aarconfig file, the Classical IP software runs as either a server or a client. As a server, the Classical IP software handles ATM ARP requests originating from its clients. An ATM server has to be configured for each subnet. The ATM ARP server code conforms to RFC 1577: clients send ATM ARP requests to the server to resolve a destination IP address to an ATM address. The server then replies to ATM ARP requests by sending an ATM ARP response. If the server does not have the IP-to-ATM address entry, then it replies with NAK.

All the IP-to-ATM address entries specified in the /etc/opt/SUNWconn/atm/aarconfig file are entered into a kernel resident table by the ATM ARP setup program, aarsetup. Additional entries in the kernel table are added dynamically using the inverse ARP process. When a client connects to the server, the server sends an inverse ARP request back to the client to obtain the client's IP address. When a response is received, an entry is created for that client. The Classical IP software also responds to client ARP requests. The software looks up a kernel IP-to-ATM address entry and responds to an ATM ARP request with either an ATM ARP reply or ATM ARP NAK (if there is no entry in the table). Note that an ATM ARP client uses the virtual channel (VC) specified in the /etc/opt/SUNWconn/atm/aarconfig file to communicate with the server; or, if an ATM address is specified, it establishes a switched virtual circuit (SVC) connection to communicate with the server.

While dynamic entries in the ARP server's table make network administration less complex, they also create a security problem. Any host can register with the ARP server and therefore gain access to the subnet. To resolve this issue, you can provide a list of hosts or networks with a entries in the server's /etc/opt/SUNWconn/atm/aarconfig file. If no a entries appear, any host can connect to the server. If any a entries exist, only those hosts whose addresses match those specified will be allowed to connect.

Although the a entry requires a complete ATM address, you can reference multiple addresses in a single entry using the provided wildcards. See "Using Variables in the  /etc/opt/SUNWconn/atm/aarconfig File" for more information about this feature.

The advantage of having an ATM ARP server in the subnet is that it represents a known source for all address resolutions. It is the only host that a client must know about to have IP addresses resolved to ATM connections, and it allows for access control in the ATM network.

When the /etc/opt/SUNWconn/atm/aarconfig file has been modified on a system, it is necessary to rerun aarsetup.

For better caching, all clients have the option of adding to their configuration file the IP-to-ATM address information for other clients. This can benefit clients that communicate frequently because it eliminates having to go through the ATM ARP server for IP-to-ATM address resolution.

If a host has multiple SunATM cards, the host can be a server for one IP subnet and a client for another. This is handled transparently by aarsetup.