UDP sockets – if corking is disabled – always push packets directly to the lower layers. In the case of IPv4, ip_output() will forward the packet to Netfilter where the firewall rules are applied. ip_finish_output() calls ip_finish_output2() which on his part calls neigh_hh_output() which put the cached layer 2 Ethernet header in front of skb and finally call dev_queue_xmit().

dev_queue_xmit() queues the packet in the local egress queue – default is a FIFO queue (pfifo_fast) but sophisticated queuing strategies are available and often selected. Before the actual packet is enqueued the function dev_queue_xmit() linearize the skb if necessary, do checksumming if necessary, and finally calls enqueue() which places the packet into the queue. This function fails if the queue is (temporarily) deactivated or a overflow happens. In the common cases the function returns NET_XMIT_SUCCESS. Note that the standard queue has a short cut: if the queue length is 0 – no packet is queued – the packet is directly scheduled via sch_direct_xmit(). If something goes wrong (somewhere in the driver), the packet is pushed several times to the NIC put on the wire via qdisc_restart() until the NIC accepts the new packet. If this fails the one element queue is saved and a SOFTIRQ is raised on the local CPU. In the hope that next time the SOFTIRQ is executed the NIC is in the ability to accept the packet.

If the queue supports no short cut or the queue contains at least one element the packet must be enqueued via qdisc_enqueue_root(). This enqueue function is scheduler specific. For FIFO queueing the packet is added at the end of the list, more complicated queues implement a more sophisticated queueing.

dev_queue_xmit() {
    netif_needs_gso();
      skb_needs_linearize();
      if (q->enqueue) {
        if (TCQ_F_CAN_BYPASS && qdisc_qlen(q) == 0) {
            if (sch_direct_xmit())
                __qdisc_run(q);
            return NET_XMIT_SUCCESS;
        } else {
                    ret = qdisc_enqueue_root()
            qdisc_run(q);
            return ret;
        }
    }
}

__qdisc_run() {
    while (qdisc_restart()) {
        if (need_resched() || to_often_restarted) {
            raise_softirq_irqoff(NET_TX_SOFTIRQ);
        }
    }
}

qdisc_restart() try to call dev_hard_start_xmit() instantly to put the packet on the NIC TX descriptor ring – if possible. __qdisc_run() enabled the SOFTIRQ – if not already enabled.

Finally note that the return code of dev_queue_xmit() make no statement if the packet can be transmitted. Subsequent congestion or the queue policy can decide to drop the packet. A positive return code only signals that the enqueuing was successful.

IPv6 is similar except that neighbor resolution is done by IPv6 neighbor discovery mechanism (ND).

Last but not least some network devices have no associated queues. The loopback device and all kind of pseudo tunnel devices are common examples. These devices have no queues and instead of placing the packet in a queue the function dev_hard_start_xmit() is called directly.