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    Subject[PATCH 3.11 142/198] tcp: fix cwnd undo on DSACK in F-RTO
    Date
    3.11.10.13 -stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Yuchung Cheng <ycheng@google.com>

    commit 0cfa5c07d6d1d7f8e710fc671c5ba1ce85e09fa4 upstream.

    This bug is discovered by an recent F-RTO issue on tcpm list
    https://www.ietf.org/mail-archive/web/tcpm/current/msg08794.html

    The bug is that currently F-RTO does not use DSACK to undo cwnd in
    certain cases: upon receiving an ACK after the RTO retransmission in
    F-RTO, and the ACK has DSACK indicating the retransmission is spurious,
    the sender only calls tcp_try_undo_loss() if some never retransmisted
    data is sacked (FLAG_ORIG_DATA_SACKED).

    The correct behavior is to unconditionally call tcp_try_undo_loss so
    the DSACK information is used properly to undo the cwnd reduction.

    Signed-off-by: Yuchung Cheng <ycheng@google.com>
    Signed-off-by: Neal Cardwell <ncardwell@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
    ---
    net/ipv4/tcp_input.c | 11 +++++------
    1 file changed, 5 insertions(+), 6 deletions(-)

    diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
    index 723951aec07e..d8e4c81bc114 100644
    --- a/net/ipv4/tcp_input.c
    +++ b/net/ipv4/tcp_input.c
    @@ -2622,13 +2622,12 @@ static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack)
    bool recovered = !before(tp->snd_una, tp->high_seq);

    if (tp->frto) { /* F-RTO RFC5682 sec 3.1 (sack enhanced version). */
    - if (flag & FLAG_ORIG_SACK_ACKED) {
    - /* Step 3.b. A timeout is spurious if not all data are
    - * lost, i.e., never-retransmitted data are (s)acked.
    - */
    - tcp_try_undo_loss(sk, true);
    + /* Step 3.b. A timeout is spurious if not all data are
    + * lost, i.e., never-retransmitted data are (s)acked.
    + */
    + if (tcp_try_undo_loss(sk, flag & FLAG_ORIG_SACK_ACKED))
    return;
    - }
    +
    if (after(tp->snd_nxt, tp->high_seq) &&
    (flag & FLAG_DATA_SACKED || is_dupack)) {
    tp->frto = 0; /* Loss was real: 2nd part of step 3.a */
    --
    1.9.1


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