IPv4/single: TCP minimum time for retransmissions (#387)

* IPv4/multi: TCP minimum time for retransmissions

* Corrected typo in a comment

Co-authored-by: Hein Tibosch <[email protected]>
Co-authored-by: Aniruddha Kanhere <[email protected]>

Author: 3 people

FreeRTOS_TCP_WIN.c

@@ -50,11 +50,12 @@
 #if ( ipconfigUSE_TCP == 1 )
 
 /* Constants used for Smoothed Round Trip Time (SRTT). */
-    #define winSRTT_INCREMENT_NEW        2  /**< New increment for the smoothed RTT. */
-    #define winSRTT_INCREMENT_CURRENT    6  /**< Current increment for the smoothed RTT. */
-    #define winSRTT_DECREMENT_NEW        1  /**< New decrement for the smoothed RTT. */
-    #define winSRTT_DECREMENT_CURRENT    7  /**< Current decrement for the smoothed RTT. */
-    #define winSRTT_CAP_mS               50 /**< Cap in milliseconds. */
+    #define winSRTT_INCREMENT_NEW        2                                     /**< New increment for the smoothed RTT. */
+    #define winSRTT_INCREMENT_CURRENT    6                                     /**< Current increment for the smoothed RTT. */
+    #define winSRTT_DECREMENT_NEW        1                                     /**< New decrement for the smoothed RTT. */
+    #define winSRTT_DECREMENT_CURRENT    7                                     /**< Current decrement for the smoothed RTT. */
+    #define winSRTT_CAP_mS               ( ipconfigTCP_SRTT_MINIMUM_VALUE_MS ) /**< Cap in milliseconds. */
+
 
 /**
  * @brief Utility function to cast pointer of a type to pointer of type TCPSegment_t.

include/FreeRTOSIPConfigDefaults.h

@@ -373,6 +373,16 @@
     #define ipconfigICMP_TIME_TO_LIVE    64
 #endif
 
+#ifndef ipconfigTCP_SRTT_MINIMUM_VALUE_MS
+
+/* TCP only: when measuring the Smoothed Round Trip Time (SRTT),
+ * the result will be rounded up to a minimum value.
+ * The default has always been 50, but a value of 1000
+ * is recommended (see RFC6298) because hosts often delay the
+ * sending of ACK packets with 200 ms. */
+    #define ipconfigTCP_SRTT_MINIMUM_VALUE_MS    50U
+#endif
+
 #ifndef ipconfigUDP_MAX_RX_PACKETS
 
 /* Make positive to define the maximum number of packets which will be buffered