Understanding Linux Network Internals

Device State

The net_device structure includes different fields that define the current state of the device. These include:

flags

Bitmap used to store different flags. Most of them represent a device's capabilities. However, one of them, IFF_UP, is used to say whether the device is enabled (up) or disabled (down). You can find the list of IFF_ XXX flags in include/linux/if.h. See also the section "Enabling and Disabling a Network Device."

reg_state

Device registration state. The section "Registration State" lists the values this field can be assigned and when its value changes.

state

Device state with regard to its queuing discipline . See the section "Queuing Discipline State."

You may find a little bit of overlap sometimes between these variables. For example, every time IFF_UP is set in flags, _ _LINK_STATE_START is set in state, and vice versa. Both of them are set and cleared, respectively, by dev_open and dev_close. However, their domains are different, and a little bit of overlap may sometimes be introduced when writing modular code.

Queuing Discipline State

Each network device is assigned a queuing discipline, which is used by Traffic Control to implement its QoS mechanisms. The state field of net_device is one of the structure's fields used by Traffic Control. state is a bitmap, and the following list shows the flags that can be set. They are defined in include/linux/netdevice.h.

_ _LINK_STATE_START

The device is up. This flag can be checked with netif_running. See the section ...