• Eric Dumazet's avatar
    tcp: new list for sent but unacked skbs for RACK recovery · e2080072
    Eric Dumazet authored
    This patch adds a new queue (list) that tracks the sent but not yet
    acked or SACKed skbs for a TCP connection. The list is chronologically
    ordered by skb->skb_mstamp (the head is the oldest sent skb).
    
    This list will be used to optimize TCP Rack recovery, which checks
    an skb's timestamp to judge if it has been lost and needs to be
    retransmitted. Since TCP write queue is ordered by sequence instead
    of sent time, RACK has to scan over the write queue to catch all
    eligible packets to detect lost retransmission, and iterates through
    SACKed skbs repeatedly.
    
    Special cares for rare events:
    1. TCP repair fakes skb transmission so the send queue needs adjusted
    2. SACK reneging would require re-inserting SACKed skbs into the
       send queue. For now I believe it's not worth the complexity to
       make RACK work perfectly on SACK reneging, so we do nothing here.
    3. Fast Open: currently for non-TFO, send-queue correctly queues
       the pure SYN packet. For TFO which queues a pure SYN and
       then a data packet, send-queue only queues the data packet but
       not the pure SYN due to the structure of TFO code. This is okay
       because the SYN receiver would never respond with a SACK on a
       missing SYN (i.e. SYN is never fast-retransmitted by SACK/RACK).
    
    In order to not grow sk_buff, we use an union for the new list and
    _skb_refdst/destructor fields. This is a bit complicated because
    we need to make sure _skb_refdst and destructor are properly zeroed
    before skb is cloned/copied at transmit, and before being freed.
    Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
    Signed-off-by: default avatarYuchung Cheng <ycheng@google.com>
    Signed-off-by: default avatarNeal Cardwell <ncardwell@google.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    e2080072
tcp_output.c 107 KB