Impose minimum buffer sizes when auto-tuning TCP

For nodes with extremely low capacity, the send/recv buffer sizes shrink
unacceptable low. According to 'man tcp' the buffer sizes only seem to
shrink when under memory pressure, but its not exactly clear how
auto-tuning is implemented in the real kernel.

refs #62

src/configuration/shd-configuration.h

@@ -151,6 +151,22 @@ typedef guint64 SimulationTime;
  */
 #define CONFIG_TCPAUTOTUNE TRUE
 
+/**
+ * Minimum size of the send buffer per socket when TCP-autotuning is used.
+ * This value was computed from "man tcp"
+ *
+ * @todo change this to a command line option accessible via #Configuration
+ */
+#define CONFIG_SEND_BUFFER_MIN_SIZE 16384
+
+/**
+ * Minimum size of the receive buffer per socket when TCP-autotuning is used.
+ * This value was computed from "man tcp"
+ *
+ * @todo change this to a command line option accessible via #Configuration
+ */
+#define CONFIG_RECV_BUFFER_MIN_SIZE 87380
+
 /**
  * Default size of the send buffer per socket if TCP-autotuning is not used.
  * This value was computed from "man tcp"

src/node/descriptor/shd-tcp.c

@@ -310,6 +310,14 @@ static void _tcp_autotune(TCP* tcp) {
 	/* the delay bandwidth product is how many bytes I can receive at once to keep the pipe full */
 	guint64 receivebuf_size = (guint64) (rtt_milliseconds * receive_bottleneck_bw * 1.25);
 
+    /* keep minimum buffer size bounds */
+    if(sendbuf_size < CONFIG_SEND_BUFFER_MIN_SIZE) {
+    	sendbuf_size = CONFIG_SEND_BUFFER_MIN_SIZE;
+    }
+    if(receivebuf_size < CONFIG_RECV_BUFFER_MIN_SIZE) {
+        receivebuf_size = CONFIG_RECV_BUFFER_MIN_SIZE;
+    }
+
 	/* make sure the user hasnt already written to the buffer, because if we
 	 * shrink it, our buffer math would overflow the size variable
 	 */