tcp: properly handle stretch acks in slow start

Slow start now increases cwnd by 1 if an ACK acknowledges some packets,
regardless the number of packets. Consequently slow start performance
is highly dependent on the degree of the stretch ACKs caused by
receiver or network ACK compression mechanisms (e.g., delayed-ACK,
GRO, etc).  But slow start algorithm is to send twice the amount of
packets of packets left so it should process a stretch ACK of degree
N as if N ACKs of degree 1, then exits when cwnd exceeds ssthresh. A
follow up patch will use the remainder of the N (if greater than 1)
to adjust cwnd in the congestion avoidance phase.

In addition this patch retires the experimental limited slow start
(LSS) feature. LSS has multiple drawbacks but questionable benefit. The
fractional cwnd increase in LSS requires a loop in slow start even
though it's rarely used. Configuring such an increase step via a global
sysctl on different BDPS seems hard. Finally and most importantly the
slow start overshoot concern is now better covered by the Hybrid slow
start (hystart) enabled by default.

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>

1 files changed, 0 insertions, 11 deletions

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 6c0098359ca6..8b8a05787641 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -267,17 +267,6 @@ tcp_max_orphans - INTEGER
         more aggressively. Let me to remind again: each orphan eats
         up to ~64K of unswappable memory.
 
-tcp_max_ssthresh - INTEGER
-        Limited Slow-Start for TCP with large congestion windows (cwnd) defined in
-        RFC3742. Limited slow-start is a mechanism to limit growth of the cwnd
-        on the region where cwnd is larger than tcp_max_ssthresh. TCP increases cwnd
-        by at most tcp_max_ssthresh segments, and by at least tcp_max_ssthresh/2
-        segments per RTT when the cwnd is above tcp_max_ssthresh.
-        If TCP connection increased cwnd to thousands (or tens of thousands) segments,
-        and thousands of packets were being dropped during slow-start, you can set
-        tcp_max_ssthresh to improve performance for new TCP connection.
-        Default: 0 (off)
-
 tcp_max_syn_backlog - INTEGER
         Maximal number of remembered connection requests, which have not
         received an acknowledgment from connecting client.