Understanding Linux Network Internals

Chapter 11. Frame Transmission

Transmission is the term used for frames that leave the system, either because they were sent by the system or because they are being forwarded. In this chapter, we will cover the main tasks involved during the frame transmission data path:

• Enabling and disabling frame transmission for a device

• Scheduling a device for transmission

• Selecting the next frame to transmit among the ones waiting in the device’s egress queue

• The transmission itself (we will examine the main function)

Much about transmission is symmetric to the reception process we discussed in Chapter 10: NET_TX_SOFTIRQ is the transmission counterpart of the NET_RX_SOFTIRQ softirq, net_tx_action is the counterpart of net_rx_action, and so on. Thus, if you have studied the earlier chapter, you should find it easy to follow this one. Figure 11-1 compares the logic behind scheduling a device for reception and scheduling a device for transmission. Here are some more similarities:

• poll_list is the list of devices that are polled because they have a nonempty receive queue. output_queue is the list of devices that have something to transmit. poll_list and output_queue are two fields of the softnet_data structure introduced in Chapter 9.

• Only open devices (ones with the _ _LINK_STATE_START flag set) can be scheduled for reception. Only devices with transmission enabled (ones with the _ _LINK_STATE_XOFF flag cleared) can be scheduled for transmission.

• When a device is scheduled for reception, its _ _LINK_STATE_RX_SCHED ...