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authorYuchung Cheng <ycheng@google.com>2015-10-16 21:57:47 -0700
committerDavid S. Miller <davem@davemloft.net>2015-10-21 07:00:53 -0700
commit4f41b1c58a32537542f14c1150099131613a5e8a (patch)
tree86f37587abdfeee11ad36c75dcae37adf8aa091f /Documentation/networking
parent659a8ad56f490279f0efee43a62ffa1ac914a4e0 (diff)
tcp: use RACK to detect losses
This patch implements the second half of RACK that uses the the most recent transmit time among all delivered packets to detect losses. tcp_rack_mark_lost() is called upon receiving a dubious ACK. It then checks if an not-yet-sacked packet was sent at least "reo_wnd" prior to the sent time of the most recently delivered. If so the packet is deemed lost. The "reo_wnd" reordering window starts with 1msec for fast loss detection and changes to min-RTT/4 when reordering is observed. We found 1msec accommodates well on tiny degree of reordering (<3 pkts) on faster links. We use min-RTT instead of SRTT because reordering is more of a path property but SRTT can be inflated by self-inflicated congestion. The factor of 4 is borrowed from the delayed early retransmit and seems to work reasonably well. Since RACK is still experimental, it is now used as a supplemental loss detection on top of existing algorithms. It is only effective after the fast recovery starts or after the timeout occurs. The fast recovery is still triggered by FACK and/or dupack threshold instead of RACK. We introduce a new sysctl net.ipv4.tcp_recovery for future experiments of loss recoveries. For now RACK can be disabled by setting it to 0. Signed-off-by: Yuchung Cheng <ycheng@google.com> Signed-off-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'Documentation/networking')
-rw-r--r--Documentation/networking/ip-sysctl.txt9
1 files changed, 9 insertions, 0 deletions
diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 502d6a572b4f..85752c81c5ec 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -433,6 +433,15 @@ tcp_orphan_retries - INTEGER
you should think about lowering this value, such sockets
may consume significant resources. Cf. tcp_max_orphans.
+tcp_recovery - INTEGER
+ This value is a bitmap to enable various experimental loss recovery
+ features.
+
+ RACK: 0x1 enables the RACK loss detection for fast detection of lost
+ retransmissions and tail drops.
+
+ Default: 0x1
+
tcp_reordering - INTEGER
Initial reordering level of packets in a TCP stream.
TCP stack can then dynamically adjust flow reordering level