gistfile1.txt

commit e8e2e7ee1a594986656fd51a1d90f70974a372fb
Author: Theo Schlossnagle <[email protected]>
Date:   Tue Mar 26 18:48:37 2013 +0000

    OMNIOS#52 Allow operator setting of TCP initial congestion window

    Change the initial congestion window calculation to respect the
    tcp_slow_start_initial and tcp_slow_start_after_idle tunables up
    to a value of 16.

    The new default value of 0, uses the RFC to determine the window
    size as either 2, 3 or 4 segments.

diff --git a/usr/src/uts/common/inet/tcp/tcp_opt_data.c b/usr/src/uts/common/inet/tcp/tcp_opt_data.c
index 460ef6d..1a5363b 100644
--- a/usr/src/uts/common/inet/tcp/tcp_opt_data.c
+++ b/usr/src/uts/common/inet/tcp/tcp_opt_data.c
@@ -269,9 +269,6 @@ optdb_obj_t tcp_opt_obj = {
        tcp_valid_levels_arr    /* TCP valid level array */
 };

-/* Maximum TCP initial cwin (start/restart). */
-#define        TCP_MAX_INIT_CWND       16
-
 static int tcp_max_init_cwnd = TCP_MAX_INIT_CWND;

 /*
diff --git a/usr/src/uts/common/inet/tcp/tcp_tunables.c b/usr/src/uts/common/inet/tcp/tcp_tunables.c
index a1792f7..36bab57 100644
--- a/usr/src/uts/common/inet/tcp/tcp_tunables.c
+++ b/usr/src/uts/common/inet/tcp/tcp_tunables.c
@@ -379,11 +379,11 @@ mod_prop_info_t tcp_propinfo_tbl[] = {

        { "_slow_start_after_idle", MOD_PROTO_TCP,
            mod_set_uint32, mod_get_uint32,
-           {1, 16384, 4}, {4} },
+           {0, 16384, 0}, {0} },

        { "_slow_start_initial", MOD_PROTO_TCP,
            mod_set_uint32, mod_get_uint32,
-           {1, 4, 4}, {4} },
+           {0, 16, 0}, {0} },

        { "sack", MOD_PROTO_TCP,
            mod_set_uint32, mod_get_uint32,
diff --git a/usr/src/uts/common/inet/tcp_impl.h b/usr/src/uts/common/inet/tcp_impl.h
index 77f9bb1..c291dc5 100644
--- a/usr/src/uts/common/inet/tcp_impl.h
+++ b/usr/src/uts/common/inet/tcp_impl.h
@@ -199,12 +199,38 @@ typedef struct tcp_squeue_priv_s {
  * should be 0 and we use the formula in RFC 3390 to set tcp_cwnd.
  * If the upper layer has changed set the tcp_init_cwnd, just use
  * it to calculate the tcp_cwnd.
+ *
+ * ACM SIGCOMM Computer Communications Review, vol. 40 (2010), pp. 27-33
+ *  -- Nandita Dukkipati, Tiziana Refice, Yuchung Cheng,
+ *     Hsiao-keng Jerry Chu, Tom Herbert, Amit Agarwal,
+ *     Arvind Jain, Natalia Sutin
+ *
+ *   "Based on the results from our experiments, we believe the
+ *    initial congestion window should be at least ten segments
+ *    and the same be investigated for standardization by the IETF."
+ *
+ * As such, the def_max_init_cwnd argument with which this macro is
+ * invoked is either the tcps_slow_start_initial or
+ * tcps_slow_start_after_idle which both default to 0 and will respect
+ * RFC 3390 exactly.  If the tunables are explicitly set by the operator
+ * then the initial congestion window should be set as the operator
+ * demands within reason. We shall arbitrarily define reason as a
+ * maximum of 16 (same as used by the TCP_INIT_CWND setsockopt).
  */
+
+/* Maximum TCP initial cwin (start/restart). */
+#define TCP_MAX_INIT_CWND       16
+
 #define        TCP_SET_INIT_CWND(tcp, mss, def_max_init_cwnd)                  \
 {                                                                      \
        if ((tcp)->tcp_init_cwnd == 0) {                                \
-               (tcp)->tcp_cwnd = MIN(def_max_init_cwnd * (mss),        \
-                   MIN(4 * (mss), MAX(2 * (mss), 4380 / (mss) * (mss)))); \
+               if (def_max_init_cwnd == 0) {                           \
+                       (tcp)->tcp_cwnd = MIN(4 * (mss),                \
+                           MAX(2 * (mss), 4380 / (mss) * (mss)));      \
+               } else {                                                \
+                       (tcp)->tcp_cwnd = MIN(TCP_MAX_INIT_CWND * (mss),\
+                           def_max_init_cwnd * (mss));                 \
+               }                                                       \
        } else {                                                        \
                (tcp)->tcp_cwnd = (tcp)->tcp_init_cwnd * (mss);         \
        }                                                               \