diff --git a/.gitignore b/.gitignore
index 1474cb7..ab68491 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,5 +2,5 @@
 *.a
 *.pcap
 build/*
-test/unit/unit-core
+test/unit/unit
 tags
diff --git a/core.md b/core.md
new file mode 100644
index 0000000..aaa4ace
--- /dev/null
+++ b/core.md
@@ -0,0 +1,107 @@
+# FemtoTCP
+
+## Stack architecture
+
+ - No dynamic allocation (pre-allocated sockets and buffers)
+ - Four-steps main loop function
+ - Callback-based socket interface (allows implementing blocking BSD calls)
+
+
+
+## Data structures
+
+* Two types of circular buffers (fixed size):
+
+   - "fifo" : contains entire frames, including a descriptor.
+   - "queue" : contains pure data, indexed by byte. Used for TCP receive buffer
+   only.
+
+* One binary heap for timers
+
+
+
+### FemtoTCP fifo
+
+```
++---------------------------------------------------------------------------------------------------------------------------+
+|               +-----+---+----+-----+------------------+-----+---+----+-----+------------------+                           | 
+|               | De  | E | IP | TCP |      Payload     | De  | E | IP | TCP |      Payload     |                           |
+|               | sc  | T |    |     |                  | sc  | T |    |     |                  |                           |
+|* FREE SPACE * | ri  | H |    |     |                  | ri  | H |    |     |                  |   * FREE SPACE*           |
+|               | pt  |   |    |     |                  | pt  |   |    |     |                  |                           |
+|               | or  |   |    |     |                  | or  |   |    |     |                  |                           |
+|               +-----+---+----+-----+------------------+-----+---+----+-----+------------------+                           |
++---------------------------------------------------------------------------------------------------------------------------+
+                ^                                                                               ^                      
+                |                                                                               |                      
+                |                                                                               |                      
+                |                                                                               |                      
+                |Tail                                                                       Head|                      
+
+```
+
+
+
+### FemtoTCP queue
+
+```
+ +--------------+--------------------------------------------+---------------------------------------------------------------+
+ |              |*------------------------------------------*|                                                               |
+ |              ||                                          ||                                                               |
+ |              ||                                          ||                                                               |
+ |* FREE SPACE *||                DATA PAYLOAD              ||                           * FREE SPACE *                      |
+ |              ||                                          ||                                                               |
+ |              ||                                          ||                                                               |
+ |              |*------------------------------------------*|                                                               |
+ +--------------+--------------------------------------------+---------------------------------------------------------------+
+                ^                                            ^                                                         
+                |                                            |                                                         
+                |                                            |                                                         
+                |                                            |                                                         
+                |Tail                                    Head|                                                         
+```
+
+
+
+## Sockets
+
+### TCP socket
+
+```
+                                             +-------------+
+                                             |Main loop TX |
+                                             +-------------+
+                                                        ^   
++----------------------------------+                    |   
+|                                  |             +------+   
+|  TCP Socket                      |             |          
+|                                  |             |          
+|                                  |             |          
+|                                  |             |          
+|                                +-----------------------+ 
+|            +---------------+   |                       | 
+>DATA OUT==>>|socket send()  |-->|  TX buffer (fifo)     | 
+|            +---------------+   |                       | 
+|                                +-----------------------+ 
+|                                  |                        
+|                                  |                        
+|                                  |                        
+|                                +-----------------------+ 
+|             +-------------+    |                       | 
+<DATA IN<<====|socket recv()|<---|  RX buffer (queue)    | 
+|             +-------------+    |                       | 
+|                                +-----------------------+ 
++----------------------------------+      ^                 
+                                          |                 
+                                          |                 
+                                          |                 
+                                    +--------------+                 
+                                    |  tcp_recv()  |                 
+                                    +--------------+
+```
+
+
+
+
+
+
diff --git a/test/test-linux.c b/test/test-linux.c
index 418224e..118b32c 100644
--- a/test/test-linux.c
+++ b/test/test-linux.c
@@ -56,8 +56,9 @@ static void socket_cb(int fd, uint16_t event, void *arg)
             ft_close((struct ipstack *)arg, client_fd);
             printf("Server: I closed the connection.\n");
             closed = 1;
+            exit_ok = 1;
         }
-        if (tot_sent < tot_recv) {
+        if ((!closed) && (tot_sent < tot_recv)) {
             snd_ret = ft_sendto((struct ipstack *)arg, client_fd, buf + tot_sent, tot_recv - tot_sent, 0, NULL, 0);
             if (snd_ret != -11) {
                 if (snd_ret < 0) {
@@ -90,30 +91,12 @@ static void socket_cb(int fd, uint16_t event, void *arg)
 
 static int test_echoserver(struct ipstack *s, int active_close)
 {
-    int ret;
-    struct ipstack_sockaddr_in local_sock = {
-        .sin_family = AF_INET,
-        .sin_port = ee16(8), /* Echo */
-        .sin_addr.s_addr = 0
-    };
     exit_ok = 0;
     exit_count = 0;
-    listen_fd = -1;
-    client_fd = -1;
     tot_sent = 0;
     server_closing = active_close;
     closed = 0;
 
-    listen_fd = ft_socket(s, AF_INET, IPSTACK_SOCK_STREAM, 0);
-    printf("socket: %04x\n", listen_fd);
-    ipstack_register_callback(s, listen_fd, socket_cb, s);
-
-    ret = ft_bind(s, listen_fd, (struct ipstack_sockaddr *)&local_sock, sizeof(local_sock));
-    printf("bind: %d\n", ret);
-    ret = ft_listen(s, listen_fd, 1);
-    printf("listen: %d\n", ret);
-
-
     while(1) {
         uint32_t ms_next;
         struct timeval tv;
@@ -126,7 +109,6 @@ static int test_echoserver(struct ipstack *s, int active_close)
             else break;
         }
     }
-    ft_close(s, listen_fd);
     return 0;
 }
 
@@ -172,7 +154,10 @@ void *pt_echoclient(void *arg)
         }
         if (ret == 0) {
             printf("test client read: server has closed the connection.\n");
-            return (void *)-1;
+            if (server_closing)
+                return (void *)0;
+            else
+                return (void *)-1;
         }
         total_r += ret;
     }
@@ -201,6 +186,11 @@ int main(int argc, char **argv)
     struct in_addr linux_ip;
     ip4 ip = 0, nm = 0, gw = 0;
     uint32_t srv_ip;
+    struct ipstack_sockaddr_in local_sock = {
+        .sin_family = AF_INET,
+        .sin_port = ee16(8), /* Echo */
+        .sin_addr.s_addr = 0
+    };
 
     int ret, test_ret = 0;
     (void)argc;
@@ -243,9 +233,16 @@ int main(int argc, char **argv)
     inet_pton(AF_INET, FEMTOTCP_IP, &srv_ip);
 #endif
 
+    listen_fd = ft_socket(s, AF_INET, IPSTACK_SOCK_STREAM, 0);
+    printf("socket: %04x\n", listen_fd);
+    ipstack_register_callback(s, listen_fd, socket_cb, s);
 
     pthread_create(&pt, NULL, pt_echoclient, &srv_ip);
     printf("Starting test: echo server close-wait\n");
+    ret = ft_bind(s, listen_fd, (struct ipstack_sockaddr *)&local_sock, sizeof(local_sock));
+    printf("bind: %d\n", ret);
+    ret = ft_listen(s, listen_fd, 1);
+    printf("listen: %d\n", ret);
     ret = test_echoserver(s, 0);
     pthread_join(pt, (void **)&test_ret);
     printf("Test echo server close-wait: %d\n", ret);