Index: kernel/generic/include/ipc/ipc.h
===================================================================
--- kernel/generic/include/ipc/ipc.h	(revision d51a0d61e43baaf646db5fd0827b7e8c89b0f4f5)
+++ kernel/generic/include/ipc/ipc.h	(revision b1f36e3cd6479914de8e2dc0bc820a9d75ddfaff)
@@ -106,4 +106,6 @@
 	/** Phone which made or last masqueraded this call. */
 	phone_t *phone;
+	/** User-defined label */
+	sysarg_t label;
 } ipc_data_t;
 
Index: kernel/generic/include/ipc/sysipc.h
===================================================================
--- kernel/generic/include/ipc/sysipc.h	(revision d51a0d61e43baaf646db5fd0827b7e8c89b0f4f5)
+++ kernel/generic/include/ipc/sysipc.h	(revision b1f36e3cd6479914de8e2dc0bc820a9d75ddfaff)
@@ -44,5 +44,5 @@
 extern sysarg_t sys_ipc_call_async_fast(sysarg_t, sysarg_t, sysarg_t,
     sysarg_t, sysarg_t, sysarg_t);
-extern sysarg_t sys_ipc_call_async_slow(sysarg_t, ipc_data_t *);
+extern sysarg_t sys_ipc_call_async_slow(sysarg_t, ipc_data_t *, sysarg_t);
 extern sysarg_t sys_ipc_answer_fast(sysarg_t, sysarg_t, sysarg_t, sysarg_t,
     sysarg_t, sysarg_t);
Index: kernel/generic/src/ipc/sysipc.c
===================================================================
--- kernel/generic/src/ipc/sysipc.c	(revision d51a0d61e43baaf646db5fd0827b7e8c89b0f4f5)
+++ kernel/generic/src/ipc/sysipc.c	(revision b1f36e3cd6479914de8e2dc0bc820a9d75ddfaff)
@@ -347,5 +347,5 @@
 /** Make a fast asynchronous call over IPC.
  *
- * This function can only handle four arguments of payload, but is faster than
+ * This function can only handle three arguments of payload, but is faster than
  * the generic function sys_ipc_call_async_slow().
  *
@@ -355,5 +355,5 @@
  * @param arg2     Service-defined payload argument.
  * @param arg3     Service-defined payload argument.
- * @param arg4     Service-defined payload argument.
+ * @param label    User-defined label.
  *
  * @return Call hash on success.
@@ -362,5 +362,5 @@
  */
 sysarg_t sys_ipc_call_async_fast(sysarg_t handle, sysarg_t imethod,
-    sysarg_t arg1, sysarg_t arg2, sysarg_t arg3, sysarg_t arg4)
+    sysarg_t arg1, sysarg_t arg2, sysarg_t arg3, sysarg_t label)
 {
 	kobject_t *kobj = kobject_get(TASK, handle, KOBJECT_TYPE_PHONE);
@@ -378,5 +378,4 @@
 	IPC_SET_ARG2(call->data, arg2);
 	IPC_SET_ARG3(call->data, arg3);
-	IPC_SET_ARG4(call->data, arg4);
 	
 	/*
@@ -385,4 +384,7 @@
 	 */
 	IPC_SET_ARG5(call->data, 0);
+
+	/* Set the user-defined label */
+	call->data.label = label;
 	
 	int res = request_preprocess(call, kobj->phone);
@@ -401,9 +403,11 @@
  * @param handle  Phone capability for the call.
  * @param data    Userspace address of call data with the request.
+ * @param label   User-defined label.
  *
  * @return See sys_ipc_call_async_fast().
  *
  */
-sysarg_t sys_ipc_call_async_slow(sysarg_t handle, ipc_data_t *data)
+sysarg_t sys_ipc_call_async_slow(sysarg_t handle, ipc_data_t *data,
+    sysarg_t label)
 {
 	kobject_t *kobj = kobject_get(TASK, handle, KOBJECT_TYPE_PHONE);
@@ -424,4 +428,7 @@
 		return (sysarg_t) rc;
 	}
+
+	/* Set the user-defined label */
+	call->data.label = label;
 	
 	int res = request_preprocess(call, kobj->phone);