Linux Kernel Architecture

Chapter 12: Networks

void (*tx_timeout) (struct net_device *dev);

int (*neigh_setup)(struct net_device *dev, struct neigh_parms *);

/* Network namespace this network device is inside */

struct net *nd_net;

/ class/net/name entry /
struct device dev;
...

The abbreviationsRxandTxthat appear in the structure are often also used in function names, variable

names, and comments. They stand forReceiveandTransmit, respectively, and crop up a few times in the

following sections.

The name of the network device is stored inname. It consists of a string followed by a number to differen-

tiate between multiple adapters of the same type (if, e.g., the system has two Ethernet cards). Table 12-2

lists the most common device classes.

Table 12-2: Designations for Network Devices

Name Device class

ethX Ethernet adapter, regardless of cable type and transmission speed

pppX PPP connection via modem

isdnX ISDN cards

atmX Asynchronous transfer mode, interface to high-speed network cards

lo Loopbackdevice for communication with the local computer

Symbolic names for network cards are used, for example, when parameters are set using theifconfig

tool.

In the kernel, network cards have a unique index number that is assigned dynamically when they are

registered and is held in theifindexelement. Recall that the kernel provides thedev_get_by_nameand

dev_get_by_indexfunctions to find thenet_deviceinstance of a network card by reference to its name

or index number.

Some structure elements define device properties that are relevant for the network layer and the network

access layer:

❑ mtu(maximum transfer unit) specifies the maximum length of a transfer frame. Protocols of the

network layer must observe this value and may need to split packets into smaller units.

❑ typeholds the hardware type of the device and uses constants from<if_arp.h>. For example,

ARPHRD_ETHERandARPHDR_IEEE802stand for 10 Mbit and 802.2 Ethernet,ARPHRD_APPLETLKfor

AppleTalk, andARPHRD_LOOPBACKfor the loopback device.