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: Apprpriate Byte Coute 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 (sysctl_tcp_abc && tp->bytes_acked < tp->mss_cache)
+ return;
+
+ /* RFC3742: limited slow start
+ * the window is increased by 1/K MSS for each arriving ACK,
+ * for K = int(cwnd/(0.5 max_ssthresh))
+ */
+ if (sysctl_tcp_limited_ssthresh
+ && tp->snd_cwnd > sysctl_tcp_limited_ssthresh) {
+ u32 k = max(tp->snd_cwnd / (sysctl_tcp_limited_ssthresh >> 1),
1U);
+ if (++tp->snd_cwnd_cnt >= k) {
+ if (tp->snd_cwnd < tp->snd_cwnd_clamp)
+ tp->snd_cwnd++;
+ tp->snd_cwnd_cnt = 0;
+ }
+ } else {
+ /* ABC: We MAY increase by 2 if discovered delayed ack */
+ if (sysctl_tcp_abc > 1
+ && tp->bytes_acked > 2*tp->mss_cache
+ && tp->snd_cwnd < tp->snd_cwnd_clamp)
+ tp->snd_cwnd++;
+
+ if (tp->snd_cwnd < tp->snd_cwnd_clamp)
+ tp->snd_cwnd++;
}
tp->bytes_acked = 0;
-
- if (tp->snd_cwnd < tp->snd_cwnd_clamp)
- tp->snd_cwnd++;
}
EXPORT_SYMBOL_GPL(tcp_slow_start);
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 718d0f2..80dd5e4 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -90,6 +90,7 @@ int sysctl_tcp_nometrics_save;
int sysctl_tcp_moderate_rcvbuf = 1;
int sysctl_tcp_abc = 1;
+int sysctl_tcp_limited_ssthresh = 100;
#define FLAG_DATA 0x01 /* Incoming frame contained data.
*/
#define FLAG_WIN_UPDATE 0x02 /* Incoming ACK was a window
update. */
--
1.3.3
-
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