3660 tcp_slow_start_* tunables should allow increasing the initial congestion window
Reviewed by: Dan McDonald <danmcd@nexenta.com>
Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Reviewed by: Brendan Gregg <brendan.gregg@joyent.com>

*** 19,28 ****
--- 19,29 ----
   * CDDL HEADER END
   */
  /*
   * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
   * Copyright (c) 2011, Joyent Inc. All rights reserved.
+  * Copyright (c) 2013, OmniTI Computer Consulting, Inc. All rights reserved.
   */
  
  #ifndef _INET_TCP_IMPL_H
  #define _INET_TCP_IMPL_H
  
*** 197,213 ****
   * depending on the caller.  If the upper layer has not used the
   * TCP_INIT_CWND option to change the initial cwnd, tcp_init_cwnd
   * 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.
   */
  #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)))); \
          } else {                                                        \
                  (tcp)->tcp_cwnd = (tcp)->tcp_init_cwnd * (mss);         \
          }                                                               \
          tcp->tcp_cwnd_cnt = 0;                                          \
  }
  
--- 198,241 ----
   * depending on the caller.  If the upper layer has not used the
   * TCP_INIT_CWND option to change the initial cwnd, tcp_init_cwnd
   * 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.
+  *
+  * "An Argument for Increasing TCP's Initial Congestion Window"
+  * 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) {                                \
!                 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);         \
          }                                                               \
          tcp->tcp_cwnd_cnt = 0;                                          \
  }