tcp: detect loss above high_seq in recovery

Correctly implement a loss detection heuristic: New sequences (above
high_seq) sent during the fast recovery are deemed lost when higher
sequences are SACKed.

Current code does not catch these losses, because tcp_mark_head_lost()
does not check packets beyond high_seq. The fix is straight-forward by
checking packets until the highest sacked packet. In addition, all the
FLAG_DATA_LOST logic are in-effective and redundant and can be removed.

Update the loss heuristic comments. The algorithm above is documented
as heuristic B, but it is redundant too because heuristic A already
covers B.

Note that this change only marks some forward-retransmitted packets LOST.
It does NOT forbid TCP performing further CWR on new losses. A potential
follow-up patch under preparation is to perform another CWR on "new"
losses such as
1) sequence above high_seq is lost (by resetting high_seq to snd_nxt)
2) retransmission is lost.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 0 insertions, 1 deletions

diff --git a/include/linux/snmp.h b/include/linux/snmp.h
index e16557a357e5..c1241c428179 100644
--- a/include/linux/snmp.h
+++ b/include/linux/snmp.h
@@ -192,7 +192,6 @@ enum
         LINUX_MIB_TCPPARTIALUNDO,               /* TCPPartialUndo */
         LINUX_MIB_TCPDSACKUNDO,                 /* TCPDSACKUndo */
         LINUX_MIB_TCPLOSSUNDO,                  /* TCPLossUndo */
-        LINUX_MIB_TCPLOSS,                      /* TCPLoss */
         LINUX_MIB_TCPLOSTRETRANSMIT,            /* TCPLostRetransmit */
         LINUX_MIB_TCPRENOFAILURES,              /* TCPRenoFailures */
         LINUX_MIB_TCPSACKFAILURES,              /* TCPSackFailures */