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How the new TCP output machine [nyi] works.

Data is kept on a single queue. The skb->users flag tells us if the frame is

one that has been queued already. To add a frame we throw it on the end. Ack

walks down the list from the start.

We keep a set of control flags

        sk->tcp_pend_event

                TCP_PEND_ACK                    Ack needed

                TCP_ACK_NOW                     Needed now

                TCP_WINDOW                      Window update check

                TCP_WINZERO                     Zero probing

        sk->transmit_queue              The transmission frame begin

        sk->transmit_new                First new frame pointer

        sk->transmit_end                Where to add frames

        sk->tcp_last_tx_ack             Last ack seen

        sk->tcp_dup_ack                 Dup ack count for fast retransmit

Frames are queued for output by tcp_write. We do our best to send the frames

off immediately if possible, but otherwise queue and compute the body

checksum in the copy.

When a write is done we try to clear any pending events and piggy back them.

If the window is full we queue full sized frames. On the first timeout in

zero window we split this.

On a timer we walk the retransmit list to send any retransmits, update the

backoff timers etc. A change of route table stamp causes a change of header

and recompute. We add any new tcp level headers and refinish the checksum

before sending.