[TCP] slow start: Make comments and code logic clearer.

Add more comments to describe our version of tcp_slow_start().

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 22 insertions, 18 deletions

diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
index 86b26539e54b..1260e52ad772 100644
--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -276,30 +276,34 @@ int tcp_set_congestion_control(struct sock *sk, const char *name)
 
 
 /*
- * Slow start (exponential increase) with
- * RFC3742 Limited Slow Start (fast linear increase) support.
+ * Slow start is used when congestion window is less than slow start
+ * threshold. This version implements the basic RFC2581 version
+ * and optionally supports:
+ *      RFC3742 Limited Slow Start        - growth limited to max_ssthresh
+ *      RFC3465 Appropriate Byte Counting - growth limited by bytes acknowledged
  */
 void tcp_slow_start(struct tcp_sock *tp)
 {
-        int cnt = 0;
-
-        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;
-        }
+        int cnt; /* increase in packets */
+
+        /* RFC3465: ABC Slow start
+         * Increase only after a full MSS of bytes is acked
+         *
+         * 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;
 
-        if (sysctl_tcp_max_ssthresh > 0 &&
-            tp->snd_cwnd > sysctl_tcp_max_ssthresh)
-                cnt += sysctl_tcp_max_ssthresh>>1;
+        if (sysctl_tcp_max_ssthresh > 0 && tp->snd_cwnd > sysctl_tcp_max_ssthresh)
+                cnt = sysctl_tcp_max_ssthresh >> 1;     /* limited slow start */
         else
-                cnt += tp->snd_cwnd;
+                cnt = tp->snd_cwnd;                     /* exponential increase */
 
-        /* RFC3465: We MAY increase by 2 if discovered delayed ack */
+        /* RFC3465: ABC
+         * We MAY increase by 2 if discovered delayed ack
+         */
         if (sysctl_tcp_abc > 1 && tp->bytes_acked >= 2*tp->mss_cache)
                 cnt <<= 1;
         tp->bytes_acked = 0;