Rolled my sleeve's up and gave this a try...
This is a implementation of Sally Floyd's Limited Slow Start
for Large Congestion Windows.
Summary from RFC:
Limited Slow-Start introduces a parameter, max_ssthresh, and
modifies the slow-start mechanism for values of the congestion window
where cwnd is greater than max_ssthresh. That is, during Slow-
Start, when
cwnd = max_ssthresh,
cwnd is increased by one MSS (MAXIMUM SEGMENT SIZE) for every
arriving ACK (acknowledgement) during slow-start, as is always the
case. During Limited Slow-Start, when
max_ssthresh cwnd = ssthresh,
the invariant is maintained so that the congestion window is
increased during slow-start by at most max_ssthresh/2 MSS per round-
trip time. This is done as follows:
For each arriving ACK in slow-start:
If (cwnd = max_ssthresh)
cwnd += MSS;
else
K = int(cwnd/(0.5 max_ssthresh));
cwnd += int(MSS/K);
Thus, during Limited Slow-Start the window is increased by 1/K MSS
for each arriving ACK, for K = int(cwnd/(0.5 max_ssthresh)), instead
of by 1 MSS as in standard slow-start [RFC2581].
---
Documentation/networking/ip-sysctl.txt |8 +-
include/linux/sysctl.h |1 +
include/net/tcp.h |1 +
net/ipv4/sysctl_net_ipv4.c |8 ++
net/ipv4/tcp_cong.c| 46
net/ipv4/tcp_input.c |1 +
6 files changed, 47 insertions(+), 18 deletions(-)
0884f45c9f21c50dd9117b2fc02bf5436be3c3bf
diff --git a/Documentation/networking/ip-sysctl.txt
b/Documentation/networking/ip-sysctl.txt
index f12007b..9869298 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -103,9 +103,15 @@ TCP variables:
tcp_abc - INTEGER
Controls Appropriate Byte Count defined in RFC3465. If set to
- 0 then does congestion avoid once per ack. 1 is conservative
+ 0 then does congestion avoid once per ack. 1 (default) is conservative
value, and 2 is more agressive.
+tcp_limited_ssthresh - INTEGER
+ Controls the increase of the congestion window during slow start as
+ defined in RFC3742. The purpose is to slow the growth of the congestion
+ window on high delay networks where agressive growth can cause losses
+ of 1000's of packets. Default is 100 packets.
+
tcp_syn_retries - INTEGER
Number of times initial SYNs for an active TCP connection attempt
will be retransmitted. Should not be higher than 255. Default value
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 76eaeff..a455165 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -403,6 +403,7 @@ enum
NET_TCP_MTU_PROBING=113,
NET_TCP_BASE_MSS=114,
NET_IPV4_TCP_WORKAROUND_SIGNED_WINDOWS=115,
+ NET_TCP_LIMITED_SSTHRESH=116,
};
enum {
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 575636f..3a14861 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -225,6 +225,7 @@ extern int sysctl_tcp_abc;
extern int sysctl_tcp_mtu_probing;
extern int sysctl_tcp_base_mss;
extern int sysctl_tcp_workaround_signed_windows;
+extern int sysctl_tcp_limited_ssthresh;
extern atomic_t tcp_memory_allocated;
extern atomic_t tcp_sockets_allocated;
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 6b6c3ad..d1358d3 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -688,6 +688,14 @@ #endif
.mode = 0644,
.proc_handler = proc_dointvec
},
+ {
+ .ctl_name = NET_TCP_LIMITED_SSTHRESH,
+ .procname = tcp_max_ssthresh,
+ .data = sysctl_tcp_limited_ssthresh,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec,
+ },
{ .ctl_name = 0 }
};
diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
index 857eefc..a27c792 100644
--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -180,25 +180,37 @@ int tcp_set_congestion_control(struct so
*/
void tcp_slow_start(struct tcp_sock *tp)
{
- if (sysctl_tcp_abc) {
- /* RFC3465: Slow Start
-* TCP sender SHOULD increase cwnd by the number of
-* previously unacknowledged bytes ACKed by each incoming
-* acknowledgment, provided the increase is not more than L
-*/
- if (tp-bytes_acked tp-mss_cache)
- return;
-
- /* We MAY increase by 2 if discovered delayed ack */
- if (sysctl_tcp_abc 1 tp-bytes_acked 2*tp-mss_cache) {
- if (tp-snd_cwnd tp-snd_cwnd_clamp)
- tp-snd_cwnd++;
- }
+ /* RFC3465:
