gnrc_tcp: experimental feature "dynamic msl"

sys/include/net/gnrc/tcp/config.h

@@ -185,6 +185,26 @@ extern "C" {
 #ifndef CONFIG_GNRC_TCP_EVENTLOOP_MSG_QUEUE_SIZE_EXP
 #define CONFIG_GNRC_TCP_EVENTLOOP_MSG_QUEUE_SIZE_EXP (3U)
 #endif
+
+/**
+ * @brief Enable experimental feature "dynamic msl". Disabled by default.
+ * @experimental This feature is experimental because it deviates from the TCP RFC.
+ * @note This features calculates the MSL based by multiplying the latest
+ *       retransmission timeout value with
+ *       CONFIG_GNRC_TCP_EXPERIMENTAL_DYN_MSL_RTO_MUL. This leads to much
+ *       faster return times on gnrc_tcp_close.
+ */
+#ifndef CONFIG_GNRC_TCP_EXPERIMENTAL_DYN_MSL_EN
+#define CONFIG_GNRC_TCP_EXPERIMENTAL_DYN_MSL_EN 0
+#endif
+
+/**
+ * @brief Set RTO multiplication factor if experimental feature "dynamic msl" is enabled.
+ * @experimental This feature is experimental because it deviates from the TCP RFC.
+ */
+#ifndef CONFIG_GNRC_TCP_EXPERIMENTAL_DYN_MSL_RTO_MUL
+#define CONFIG_GNRC_TCP_EXPERIMENTAL_DYN_MSL_RTO_MUL (4U)
+#endif
 /** @} */
 
 #ifdef __cplusplus

sys/net/gnrc/transport_layer/tcp/Kconfig

@@ -134,4 +134,23 @@ config GNRC_TCP_EVENTLOOP_MSG_QUEUE_SIZE_SIZE_EXP
         The number of elements in a message queue must be always a power of two.
         This value defines the exponent of 2^n.
 
+config GNRC_TCP_EXPERIMENTAL_DYN_MSL_EN
+    bool "Enable experimental feature \"dynamic MSL\""
+    default n
+    help
+        Enables the experimental feature \"dynamic maximum segment lifetime\".
+        This feature calulates the MSL by multiplying the latest retransmission timeout
+        value with a factor GNRC_TCP_EXPERIMENTAL_DYN_MSL_RTO_MUL instead
+        of using the constant GNRC_TCP_MSL_MS. Using the shorter MSL leads to a faster
+        connection teardown and therfore for a faster return of gnrc_tcp_close.
+
+config GNRC_TCP_EXPERIMENTAL_DYN_MSL_RTO_MUL
+    int "Multiplication factor for experimental feature \"dynamic MSL\""
+    default 4
+    depends on GNRC_TCP_EXPERIMENTAL_DYN_MSL_EN
+    help
+        Multiplication factor for the experimental feature \"dynamic maximum segment lifetime\".
+        This is the factor that is multiplied with the current retransmission timeout value
+        to determine the MSL value.
+
 endif # KCONFIG_USEMODULE_GNRC_TCP

sys/net/gnrc/transport_layer/tcp/gnrc_tcp_fsm.c

@@ -112,8 +112,20 @@ static int _restart_timewait_timer(gnrc_tcp_tcb_t *tcb)
 {
     TCP_DEBUG_ENTER;
     _gnrc_tcp_eventloop_unsched(&tcb->event_timeout);
-    _gnrc_tcp_eventloop_sched(&tcb->event_timeout, 2 * CONFIG_GNRC_TCP_MSL_MS,
+
-                              MSG_TYPE_TIMEWAIT, tcb);
+    size_t val = CONFIG_GNRC_TCP_MSL_MS << 1;
+
+    /* Experimental feature "dynamic msl", calculate timewait timer value
+     * based on the current retransmission timeout.
+     */
+    if (IS_ACTIVE(CONFIG_GNRC_TCP_EXPERIMENTAL_DYN_MSL_EN)) {
+        size_t dyn_msl = CONFIG_GNRC_TCP_EXPERIMENTAL_DYN_MSL_RTO_MUL * tcb->rto;
+        if (dyn_msl < val) {
+            val = dyn_msl;
+        }
+    }
+    _gnrc_tcp_eventloop_sched(&tcb->event_timeout, val,
+                               MSG_TYPE_TIMEWAIT, tcb);
     TCP_DEBUG_LEAVE;
     return 0;
 }

tests/gnrc_tcp/Makefile

@@ -4,8 +4,10 @@ include ../Makefile.tests_common
 BOARD ?= native
 TAP ?= tap0
 
-# Shorten default TCP timeouts to speedup testing
+# Enable experimental feature "Dynamic MSL" to speedup testing
-MSL_MS ?= 1000
+ENABLE_DYNAMIC_MSL ?= 1
+
+# Shorten default connection TCP timeout to speedup testing
 TIMEOUT_MS ?= 3000
 
 # Set custom GNRC_TCP_NO_TIMEOUT constant for testing purposes
@@ -29,8 +31,8 @@ USEMODULE += auto_init_gnrc_netif
 USEMODULE += gnrc_ipv6_default
 USEMODULE += gnrc_tcp
 USEMODULE += gnrc_pktbuf_cmd
-USEMODULE += gnrc_netif_single          # Only one interface used and it makes
+USEMODULE += gnrc_netif_single    # Only one interface used and it makes
-                                        # shell commands easier
+                                  # shell commands easier
 USEMODULE += shell
 USEMODULE += shell_commands
 USEMODULE += od
@@ -45,17 +47,18 @@ ethos:
 
 include $(RIOTBASE)/Makefile.include
 
-# Set CONFIG_GNRC_TCP_MSL via CFLAGS if not being set via Kconfig
-ifndef CONFIG_GNRC_TCP_MSL_MS
-  CFLAGS += -DCONFIG_GNRC_TCP_MSL_MS=$(MSL_MS)
-endif
-
 # Set CONFIG_GNRC_TCP_CONNECTION_TIMEOUT_DURATION via CFLAGS if not being set
 # via Kconfig
 ifndef CONFIG_GNRC_TCP_CONNECTION_TIMEOUT_DURATION_MS
   CFLAGS += -DCONFIG_GNRC_TCP_CONNECTION_TIMEOUT_DURATION_MS=$(TIMEOUT_MS)
 endif
 
+# Set CONFIG_GNRC_TCP_EXPERIMENTAL_DYN_MSL_EN via CFLAGS if not being set
+# via Kconfig
+ifndef CONFIG_GNRC_TCP_EXPERIMENTAL_DYN_MSL_EN
+  CFLAGS += -DCONFIG_GNRC_TCP_EXPERIMENTAL_DYN_MSL_EN=$(ENABLE_DYNAMIC_MSL)
+endif
+
 # Set the shell echo configuration via CFLAGS if not being controlled via Kconfig
 ifndef CONFIG_KCONFIG_USEMODULE_SHELL
   CFLAGS += -DCONFIG_SHELL_NO_ECHO