tcp: implement RFC5682 F-RTO

This patch implements F-RTO (foward RTO recovery):

When the first retransmission after timeout is acknowledged, F-RTO
sends new data instead of old data. If the next ACK acknowledges
some never-retransmitted data, then the timeout was spurious and the
congestion state is reverted.  Otherwise if the next ACK selectively
acknowledges the new data, then the timeout was genuine and the
loss recovery continues. This idea applies to recurring timeouts
as well. While F-RTO sends different data during timeout recovery,
it does not (and should not) change the congestion control.

The implementaion follows the three steps of SACK enhanced algorithm
(section 3) in RFC5682. Step 1 is in tcp_enter_loss(). Step 2 and
3 are in tcp_process_loss().  The basic version is not supported
because SACK enhanced version also works for non-SACK connections.

The new implementation is functionally in parity with the old F-RTO
implementation except the one case where it increases undo events:
In addition to the RFC algorithm, a spurious timeout may be detected
without sending data in step 2, as long as the SACK confirms not
all the original data are dropped. When this happens, the sender
will undo the cwnd and perhaps enter fast recovery instead. This
additional check increases the F-RTO undo events by 5x compared
to the prior implementation on Google Web servers, since the sender
often does not have new data to send for HTTP.

Note F-RTO may detect spurious timeout before Eifel with timestamps
does so.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 12 deletions

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 8a977a0aaede..f98ca633b528 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -225,19 +225,13 @@ tcp_fin_timeout - INTEGER
         Default: 60 seconds
 
 tcp_frto - INTEGER
-        Enables Forward RTO-Recovery (F-RTO) defined in RFC4138.
+        Enables Forward RTO-Recovery (F-RTO) defined in RFC5682.
         F-RTO is an enhanced recovery algorithm for TCP retransmission
-        timeouts.  It is particularly beneficial in wireless environments
-        where packet loss is typically due to random radio interference
-        rather than intermediate router congestion.  F-RTO is sender-side
-        only modification. Therefore it does not require any support from
-        the peer.
-
-        If set to 1, basic version is enabled.  2 enables SACK enhanced
-        F-RTO if flow uses SACK.  The basic version can be used also when
-        SACK is in use though scenario(s) with it exists where F-RTO
-        interacts badly with the packet counting of the SACK enabled TCP
-        flow.
+        timeouts.  It is particularly beneficial in networks where the
+        RTT fluctuates (e.g., wireless). F-RTO is sender-side only
+        modification. It does not require any support from the peer.
+
+        By default it's enabled with a non-zero value. 0 disables F-RTO.
 
 tcp_keepalive_time - INTEGER
         How often TCP sends out keepalive messages when keepalive is enabled.