diff --git a/soc/runtime/main.c b/soc/runtime/main.c
index e2c3691f5..a74162891 100644
--- a/soc/runtime/main.c
+++ b/soc/runtime/main.c
@@ -191,7 +191,8 @@ static void serial_service(void)
     if(txlen > 0) {
         for(i = 0; i < txlen; i++)
             uart_write(txdata[i]);
-        session_ack(txlen);
+        session_ack_consumed(txlen);
+        session_ack_sent(txlen);
     } else if(txlen < 0) {
         reset_serial_session(1);
     }
diff --git a/soc/runtime/net_server.c b/soc/runtime/net_server.c
index a18e4da85..184d9fca5 100644
--- a/soc/runtime/net_server.c
+++ b/soc/runtime/net_server.c
@@ -89,7 +89,7 @@ static err_t net_server_recv(void *arg, struct tcp_pcb *pcb, struct pbuf *p, err
 
 static err_t net_server_sent(void *arg, struct tcp_pcb *pcb, u16_t len)
 {
-    session_ack(len);
+    session_ack_sent(len);
     return ERR_OK;
 }
 
@@ -205,6 +205,7 @@ void net_server_service(void)
             if(len > sndbuf)
                 len = sndbuf;
             tcp_write(active_pcb, data, len, 0);
+            session_ack_consumed(len);
         }
         if(len < 0)
             net_server_close(active_cs, active_pcb);
diff --git a/soc/runtime/session.c b/soc/runtime/session.c
index 4c394f272..eea96fee5 100644
--- a/soc/runtime/session.c
+++ b/soc/runtime/session.c
@@ -158,12 +158,13 @@ static union {
     } __attribute__((packed)) header;
 } buffer_out;
 
-static int buffer_out_read_cursor, buffer_out_write_cursor;
+static int buffer_out_read_cursor, buffer_out_sent_cursor, buffer_out_write_cursor;
 
 static void out_packet_reset()
 {
     buffer_out_read_cursor  = 0;
     buffer_out_write_cursor = 0;
+    buffer_out_sent_cursor  = 0;
 }
 
 static int out_packet_available()
@@ -182,17 +183,28 @@ static void out_packet_extract(void **data, int *length)
     }
 }
 
-static void out_packet_advance(int length)
+static void out_packet_advance_consumed(int length)
 {
     if(buffer_out_read_cursor + length > buffer_out_write_cursor) {
-        log("session.c: write underrun while trying to acknowledge %d bytes"
-            " (%d remaining)",
+        log("session.c: write underrun (consume) while trying to"
+            " acknowledge %d bytes (%d remaining)",
             length, buffer_out_write_cursor - buffer_out_read_cursor);
         return;
     }
 
     buffer_out_read_cursor += length;
-    if(buffer_out_read_cursor == buffer_out_write_cursor)
+}
+
+static void out_packet_advance_sent(int length) {
+    if(buffer_out_sent_cursor + length > buffer_out_write_cursor) {
+        log("session.c: write underrun (send) while trying to"
+            " acknowledge %d bytes (%d remaining)",
+            length, buffer_out_write_cursor - buffer_out_sent_cursor);
+        return;
+    }
+
+    buffer_out_sent_cursor += length;
+    if(buffer_out_sent_cursor == buffer_out_write_cursor)
         out_packet_reset();
 }
 
@@ -652,7 +664,12 @@ void session_poll(void **data, int *length)
     out_packet_extract(data, length);
 }
 
-void session_ack(int length)
+void session_ack_consumed(int length)
 {
-    out_packet_advance(length);
+    out_packet_advance_consumed(length);
+}
+
+void session_ack_sent(int length)
+{
+    out_packet_advance_sent(length);
 }
diff --git a/soc/runtime/session.h b/soc/runtime/session.h
index 8ef353b9e..cd0e3d76e 100644
--- a/soc/runtime/session.h
+++ b/soc/runtime/session.h
@@ -6,6 +6,7 @@ void session_end(void);
 
 int session_input(void *data, int length);
 void session_poll(void **data, int *length);
-void session_ack(int length);
+void session_ack_consumed(int length);
+void session_ack_sent(int length);
 
 #endif /* __SESSION_H */