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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.
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