diff --git a/core-test/app/communication-client.c b/core-test/app/communication-client.c
new file mode 100644
index 0000000..1bb10fe
--- /dev/null
+++ b/core-test/app/communication-client.c
@@ -0,0 +1,56 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "../../core/communication.h"
+
+#define handle_err(fun,msg)\
+    fprintf (stderr, "%s: file %s line %d %s\n", fun, __FILE__, __LINE__, msg);
+
+#define MSG "coucou"
+#define SERVICE_NAME "test"
+
+int main (int argc, char *argv[], char *env[])
+{
+
+    size_t msize = BUFSIZ;
+    char *buf = NULL;
+
+    if ( (buf = malloc (BUFSIZ)) == NULL) {
+        handle_err ("main", "malloc");
+        return EXIT_FAILURE;
+    }
+    memset (buf, 0, BUFSIZ);
+
+    struct service srv;
+    memset(&srv, 0, sizeof (struct service));
+
+    // index and version should be filled
+    srv.index = 0;
+    srv.version = 0;
+
+    // init service
+    if (app_connection (argc, argv, env, &srv, SERVICE_NAME, NULL, 0) < 0) {
+        handle_err("main", "srv_init < 0");
+        return EXIT_FAILURE;
+    }
+
+    if (app_write (&srv, MSG, strlen(MSG)) < 0) {
+        handle_err("main", "app_write < 0");
+        return EXIT_FAILURE;
+    }
+
+    if (app_read (&srv, &buf, &msize) < 0) {
+        handle_err("main", "app_read < 0");
+        return EXIT_FAILURE;
+    }
+
+    printf ("msg recv: %s\n", buf);
+
+    if (app_close (&srv) < 0) {
+        handle_err("main", "app_close < 0");
+        return EXIT_FAILURE;
+    }
+
+    return EXIT_SUCCESS;
+}
diff --git a/core-test/app/communication-server.c b/core-test/app/communication-server.c
new file mode 100644
index 0000000..af126ec
--- /dev/null
+++ b/core-test/app/communication-server.c
@@ -0,0 +1,68 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "../../core/communication.h"
+
+#define handle_err(fun,msg)\
+    fprintf (stderr, "%s: file %s line %d %s\n", fun, __FILE__, __LINE__, msg);
+
+#define MSG "coucou"
+#define SERVICE_NAME "test"
+
+int main (int argc, char *argv[], char *env[])
+{
+
+    size_t msize = BUFSIZ;
+    char *buf = NULL;
+
+    if ( (buf = malloc (BUFSIZ)) == NULL) {
+        handle_err ("main", "malloc");
+        return EXIT_FAILURE;
+    }
+    memset (buf, 0, BUFSIZ);
+
+    struct service srv;
+    memset(&srv, 0, sizeof (struct service));
+
+    // index and version should be filled
+    srv.index = 0;
+    srv.version = 0;
+
+    struct process p;
+
+    // init service
+    if (srv_init (argc, argv, env, &srv, SERVICE_NAME) < 0) {
+        handle_err("main", "srv_init < 0");
+        return EXIT_FAILURE;
+    }
+
+    if (srv_accept (&srv, &p) < 0) {
+        handle_err("main", "srv_accept < 0");
+        return EXIT_FAILURE;
+    }
+
+    if (srv_read (&p, &buf, &msize) < 0) {
+        handle_err("main", "srv_read < 0");
+        return EXIT_FAILURE;
+    }
+
+    printf ("msg recv: %s\n", buf);
+
+    if (srv_write (&p, buf, msize) < 0) {
+        handle_err("main", "srv_write < 0");
+        return EXIT_FAILURE;
+    }
+
+    if (srv_close_proc (&p) < 0) {
+        handle_err("main", "srv_close_proc < 0");
+        return EXIT_FAILURE;
+    }
+
+    if (srv_close (&srv) < 0) {
+        handle_err("main", "srv_close < 0");
+        return EXIT_FAILURE;
+    }
+
+    return EXIT_SUCCESS;
+}
diff --git a/core/communication.c b/core/communication.c
index 9494a5b..d66cb16 100644
--- a/core/communication.c
+++ b/core/communication.c
@@ -1,14 +1,17 @@
 #include "communication.h"
 #include "usocket.h"
+#include "msg-format.h"
 
 #include <assert.h>
-
 #include <stdio.h>
 #include <errno.h>
 
 #define handle_error(msg) \
     do { perror(msg); exit(EXIT_FAILURE); } while (0)
 
+#define handle_err(fun,msg)\
+    fprintf (stderr, "%s: file %s line %d %s\n", fun, __FILE__, __LINE__, msg);
+
 void service_path (char *path, const char *sname, int index, int version)
 {
     memset (path, 0, PATH_MAX);
@@ -39,25 +42,47 @@ int srv_init (int argc, char **argv, char **env
     return 0;
 }
 
+// TODO
+int srv_accept (struct service *srv, struct process *p)
+{
+    usock_accept (srv->service_fd, &p->proc_fd);
+
+    char buf[3];
+    msg_format_ack (buf);
+
+    srv_write (p, buf, 3);
+
+    return 0;
+}
+
 int srv_close (struct service *srv)
 {
     usock_close (srv->service_fd);
     return usock_remove (srv->spath);
 }
 
-int srv_read (const struct service *srv, char ** buf, size_t *msize)
+int srv_close_proc (struct process *p)
 {
-    return usock_recv (srv->service_fd, buf, msize);
+    return usock_close (p->proc_fd);
 }
 
-int srv_write (const struct service *srv, const char * buf, size_t msize)
+int srv_read (const struct process *p, char **buf, size_t *msize)
 {
-    return usock_send (srv->service_fd, buf, msize);
+    return usock_recv (p->proc_fd, buf, msize);
 }
 
-int app_connection (struct service *srv, const char *sname
+int srv_write (const struct process *p, const char * buf, size_t msize)
+{
+    return usock_send (p->proc_fd, buf, msize);
+}
+
+int app_connection (int argc, char **argv, char **env
+        , struct service *srv, const char *sname
         , const char *connectionstr, size_t msize)
 {
+    argc = argc;
+    argv = argv;
+    env = env;
 
     assert (srv != NULL);
     assert (sname != NULL);
@@ -69,11 +94,24 @@ int app_connection (struct service *srv, const char *sname
     // gets the service path
     service_path (srv->spath, sname, srv->index, srv->version);
 
-    usock_connect(&srv->service_fd, srv->spath);
+    usock_connect (&srv->service_fd, srv->spath);
 
     // TODO: connection algorithm
     // send connection string and receive acknowledgement
-    srv_write(srv, connectionstr, msize);
+    char send_buffer [BUFSIZ];
+    if (msg_format_con (send_buffer, connectionstr, &msize) < 0) {
+        handle_err ("app_connection", "msg_format_con");
+        return -1;
+    }
+    app_write (srv, send_buffer, msize);
+
+    char *buffer;
+    size_t read_msg_size;
+
+    app_read (srv, &buffer, &read_msg_size);
+
+    assert (read_msg_size == 3);
+    assert (buffer[0] == MSG_TYPE_ACK);
 
     return 0;
 }
diff --git a/core/communication.h b/core/communication.h
index d12fcd8..22da922 100644
--- a/core/communication.h
+++ b/core/communication.h
@@ -4,16 +4,10 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-//#include <cbor.h>
-
-#include <unistd.h> // unlink
-
-#include <sys/types.h> // mkfifo
-#include <sys/stat.h> // mkfifo
-#include <fcntl.h> // open
-
 #include <errno.h> // error numbers
 
+#include "process.h"
+
 #define COMMUNICATION_VERSION 1
 
 #define ER_FILE_OPEN                                1
@@ -44,16 +38,19 @@ struct service {
 int srv_init (int argc, char **argv, char **env
         , struct service *srv, const char *sname);
 int srv_close (struct service *srv);
+int srv_close_proc (struct process *p);
+int srv_accept (struct service *srv, struct process *p);
 
-int srv_read (const struct service *srv, char ** buf, size_t *msize);
-int srv_write (const struct service *, const char * buf, size_t);
+int srv_read (const struct process *, char **buf, size_t *msize);
+int srv_write (const struct process *, const char * buf, size_t);
 
 // APPLICATION
 
 // Initialize connection with unix socket
 // send the connection string to $TMP/<service>
 // fill srv->spath && srv->service_fd
-int app_connection (struct service *, const char *, const char *, size_t);
+int app_connection (int argc, char **argv, char **env
+        , struct service *, const char *, const char *, size_t);
 int app_close (struct service *);
 
 int app_read (struct service *srv, char ** buf, size_t *msize);
diff --git a/core/msg-format.c b/core/msg-format.c
new file mode 100644
index 0000000..26eae8d
--- /dev/null
+++ b/core/msg-format.c
@@ -0,0 +1,38 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include "msg-format.h"
+
+#define handle_err(fun,msg)\
+    fprintf (stderr, "%s: file %s line %d %s\n", fun, __FILE__, __LINE__, msg);
+
+// [type]   [len]       [val]
+// 3        valsize     constr
+int msg_format_con (char *buf, const char *constr, size_t *msgsize)
+{
+    assert (buf != NULL);
+    assert (*msgsize + 3 <= BUFSIZ);
+
+    if (*msgsize + 3 > BUFSIZ) {
+        handle_err ("msg_format_con", "msgsize > BUFSIZ");
+        return 1;
+    }
+
+    buf[0] = MSG_TYPE_CON;
+    short final_size = (short) *msgsize;
+    memcpy (buf + 1, &final_size, 2);
+    memcpy (buf + 3, constr, *msgsize);
+
+    return 0;
+}
+
+// [type]   [len]
+// 4        0
+int msg_format_ack (char *buf)
+{
+    assert (buf != NULL);
+    memset(buf, 0, 3);
+    buf[0] = MSG_TYPE_ACK;
+
+    return 0;
+}
diff --git a/core/msg-format.h b/core/msg-format.h
new file mode 100644
index 0000000..a815db5
--- /dev/null
+++ b/core/msg-format.h
@@ -0,0 +1,13 @@
+#ifndef __MSG_FORMAT_H__
+#define __MSG_FORMAT_H__
+
+#include <assert.h>
+
+#define MSG_TYPE_CON 1
+#define MSG_TYPE_ERR 2
+#define MSG_TYPE_ACK 4
+
+int msg_format_con (char *buf, const char *constr, size_t *msgsize);
+int msg_format_ack (char *buf);
+
+#endif
diff --git a/core/process.h b/core/process.h
index c2f3601..e45ce08 100644
--- a/core/process.h
+++ b/core/process.h
@@ -1,11 +1,17 @@
 #ifndef __PROCESS_H__
 #define __PROCESS_H__
 
-#if 0
+struct process {
+    unsigned int version;
+    unsigned int index;
+    int proc_fd;
+};
 
 // TODO
 // tout revoir
 
+#if 0
+
 #include <sys/types.h>
 #include <unistd.h>
 #include <stdio.h>
@@ -13,11 +19,6 @@
 
 #include <string.h>
 
-struct process {
-    unsigned int version;
-    unsigned int index;
-    int proc_fd;
-};
 
 struct process * srv_process_copy (const struct process *p);