5.3. Networking with Ethernet

Ethernet is the standard networking environment for Linux. It was one of the first to be implemented in Linux, the other being AX.25 (Ham Radio networking). Ethernet has been a standard for over twenty years, and the protocol is fairly easy to implement, interface cards are inexpensive, cabling is easy to do, and the other glue (hubs and switches) that holds the network together is also inexpensive.

Linux can run a number of protocols natively over Ethernet, including SMB, TCP/IP, AppleTalk, and DECnet. By far the most popular is TCP/IP, since that is the standard protocol for UNIX machines and the Internet.

Tuning Ethernet to work with your particular hardware, as always, depends on the interface cards, cabling options, and switching gear you use. Many of the tools that Linux uses are standard across most networking equipment. Most cards available today for Linux are PCI-based, so configuration of cards through setting IRQ and base addresses is not required - the driver will probe and use the configuration set by the motherboard. If you do need to force IRQ or io settings, you can do that when loading the module or booting the system by passing the arguments irq=x and io=y.

Most of tuning Ethernet for Linux is to get the best physical connection between the interface card and the switching gear. Best performance comes from a switch, and switches have a few extra tricks up their sleeve. One of these is full and half duplexing. In a half-duplex setup, the card talks to the switch, then the switch talks to the card, and so on. Only one of the two ends can be talking at the same time. In full duplex mode, both card and switch can have data on the wire at the same time. Since there's different lines for transmit and receive in Ethernet, there's less congestion on the wire. This is a great solution for high bandwidth devices that have a lot of data coming in and out, such as a file server, since it can be sending files while receiving requests.

Normally, the Ethernet card and switching gear negotiate a connection speed and duplex option using MII, the Media Independent Interface. Forcing a change can be used for debugging issues, or for getting higher performance. The mii-tool utility can show or set the speed and duplexing options for your Ethernet links.

mii-tool [-v, --verbose] [-V, --version] [-R, --reset] [-r, --restart] [-w, --watch] [-l, --log] [-A, --advertise=media] [-F, --force=media] [interface]

Also available is "100baseT4", which is an earlier form of 100Mbit that uses four pairs of wires whereas 100BaseTx uses two pairs of wires. This protocol is not available for most modern Ethernet cards.

The most common use of mii-tool is to change the setting of the media interface from half to full duplex. Most non-intelligent hubs and switches will try to negotiate half duplex by default, and intelligent switches can be set to negotiate full duplex through some configuration options.

To set an already-existing connection to 100 Mbit, full duplex:

# mii-tool -F 100baseTx-FD eth0
# mii-tool eth0
eth0: 100 Mbit, full duplex, link ok
#

Another way of doing this is to advertise 100 Mbit, full duplex and 100 Mbit, half duplex:

# mii-tool -A 100baseTx-FD,100baseTx-HD eth0
restarting autonegotiation...
#

Using autonegotiation will not work with many non-intelligent devices and will cause you to drop back to 100baseTx-HD (half duplex). Use force when the gear you are talking to is not managed, and use autonegotiate if the gear is managed.

This program only works with chipsets and drivers that support MII. The Intel eepro100 cards implement this, but others may not. If your driver does not support MII, you may need to force the setting at boot time when the driver is loaded.