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Platform Notes: The SunATM Driver Software

LAN Emulation

As described in previous sections, Classical IP provides its own (IP-to-ATM) address resolution mechanism that corresponds to and replaces ARP, thus allowing IP-based applications to run transparently over ATM. A shortcoming of Classical IP, and a primary reason it must replace the traditional ARP, is that it does not support broadcast messages.

Because ATM is a connection-oriented protocol (unlike ethernet), implementing broadcast is much more difficult. The only host that receives a message is the host to which the message is addressed, and a call must be established to that host before the message can be sent.

Local Area Network (LAN) Emulation, as standardized by the ATM Forum, provides mechanisms to send broadcast messages in an ATM environment. Given this capability, LAN Emulation is also able to work transparently with ARP, as well as IP. IP and ARP send broadcast messages over the ATM interface, and thus resolve IP addresses to MAC addresses; messages are then sent to the LAN Emulation driver, which has its own address resolution protocol (similar to that of Classical IP) to resolve the medium access control (MAC) address to an ATM address and connection.

The SunATM software implements the client side of the LAN Emulation standard. To use LAN Emulation in an environment, several LAN Emulation services must also exist in the emulated LAN. These services, called the LAN Emulation Configuration Server (LECS), the LAN Emulation Server (LES), and the Broadcast and Unknown Address Server (BUS), are generally provided in an ATM switch. The following sections provide an overview of the functions of these servers.

LAN Emulation Servers

There are three types of LAN Emulation servers. Each type is briefly described in this section.

LAN Emulation Configuration Server

This server is first contacted by a host interface when the host is brought up on the emulated LAN. Its address is generally a well-known address specified by the LAN Emulation standard that is coded into the host software; thus establishing this connection requires no input from you. When contacted by a host wishing to join its emulated LAN, the LECS replies with configuration parameters for the emulated LAN, as well as the address of the LES.

LAN Emulation Server

The second step in joining an emulated LAN is to make a connection to the LAN Emulation Server. After receiving the LES address from the LECS, a host will establish a connection to the LES. The LES may add the host to a point-to-multipoint call that is maintained by the LES with connections to every host in the emulated LAN. This point-to-multipoint connection, if created by the LES, is used to send control information to each host on the emulated LAN.

The LES acts as the ATM ARP server. Since IP and ARP work with MAC addresses, an additional address resolution step is required to convert a MAC address to the corresponding ATM address, which is used to make a connection to the target host; this resolution step is provided by the LES.

Broadcast and Unknown Address Server

The final step in joining an emulated LAN is to make a connection to the BUS. The ATM address of the BUS is obtained by sending a LAN Emulation ARP request to the LES for the broadcast address. Once established, this connection is used to send broadcast messages to the BUS, which will add the client to a point-to-multipoint call including all hosts on the emulated LAN. Thus when a broadcast message (such as an IP ARP request) is received by the LAN Emulation host from its upper layers, it sends that message to the BUS, which forwards it to all hosts in the emulated LAN. Just as in the case of ethernet, the correct host responds to the sender, and thus the IP address is resolved to a MAC address.

Resolving an IP Address to an ATM Connection

The entire process from the time IP sends a message addressed to an IP address to the arrival of that message at the appropriate destination was hinted at in the previous descriptions of the LAN Emulation servers. To demonstrate how those pieces work together during the actual transmission of a message, the process is described below. This description assumes that none of the needed addresses have been previously resolved and cached. The two hosts involved are referred to as the source (the system that wishes to send a message) and the target (the system to which the message is addressed).

  1. IP has a message to transmit and only knows the IP address of the target system. IP first sends a message to ARP, to resolve the IP address to a MAC address.

  2. ARP creates a broadcast request for the MAC address corresponding to the given IP address, which it sends to the LAN Emulation driver.

  3. The LAN Emulation driver recognizes that this message has a broadcast address, and sends it to the BUS, which forwards the message to every host on the emulated LAN.

  4. The message is received on each host, and sent up to ARP by the LAN Emulation driver.

  5. On the target, ARP recognizes the IP address as its own and sends a response with its MAC address (addressed to the source's MAC address) down to the LAN Emulation driver.

  6. The LAN Emulation driver sends an LE ARP request to the LES to resolve the source's MAC address to its ATM address.

  7. The LES responds with the requested ATM address, and the target host sets up an ATM connection to the source host, over which it sends the IP ARP response.

  8. The LAN Emulation driver on the source receives the IP ARP response message and sends it up to ARP. ARP then inserts the MAC address into the original message and sends it back down to the LAN Emulation driver.

  9. The LAN Emulation driver then must send an LE ARP request to the LES to resolve the MAC address in the message from ARP to an ATM address. When it receives an LE ARP response, it then sees that it has a connection to that address (established by the target to return the IP ARP response) and sends the original IP message to the target over that connection.

LAN Emulation Connections

There are several connections established at all times when a host is a member of an emulated LAN. The following table outlines the various LAN Emulation-related connections that should be on a LAN Emulation client (LEC).

Note -

Use the command qccstat(1M) to view all existing connections for a given interface.

Table 5-1 LAN Emulation Connections

VCC 

Endpoints 

Comments 

Configuration Direct 

LEC --> LECS 

This connection is not required to remain open after the initial join of the emulated LAN, and thus may time out after a host has joined the LAN. 

Control Direct 

LEC --> LES 

Point-to-point connection over which the host may send LE ARP requests and receive responses from the LES. 

Control Distribute 

LES --> LEC 

Point-to-multipoint connection over which the LES may send administrative information to all hosts. Hosts may not send on this connection. 

Multicast Send 

LEC --> BUS 

Point-to-point connection over which the host may send broadcast messages to the BUS. A limited amount of data is also allowed on this connection. 

Multicast Forward 

BUS --> LEC 

Point-to-multipoint connection over which the BUS sends broadcast messages. Hosts may not send on this connection.