diff --git a/include/proto/stream_interface.h b/include/proto/stream_interface.h
index 4dfd20f..aba94c7 100644
--- a/include/proto/stream_interface.h
+++ b/include/proto/stream_interface.h
@@ -108,7 +108,7 @@
 	si->err_type       = SI_ET_NONE;
 	si->conn_retries   = 0;  /* used for logging too */
 	si->exp            = TICK_ETERNITY;
-	si->flags          = SI_FL_NONE;
+	si->flags         &= SI_FL_ISBACK;
 	si->end            = NULL;
 	si->state          = si->prev_state = SI_ST_INI;
 }
diff --git a/include/types/stream_interface.h b/include/types/stream_interface.h
index 03cdd6e..49507a4 100644
--- a/include/types/stream_interface.h
+++ b/include/types/stream_interface.h
@@ -72,7 +72,7 @@
 	SI_FL_ERR        = 0x0002,  /* a non-recoverable error has occurred */
 	SI_FL_WAIT_ROOM  = 0x0004,  /* waiting for space to store incoming data */
 	SI_FL_WAIT_DATA  = 0x0008,  /* waiting for more data to send */
-	/* unused          0x0010 */
+	SI_FL_ISBACK     = 0x0010,  /* 0 for front-side SI, 1 for back-side */
 	SI_FL_DONT_WAKE  = 0x0020,  /* resync in progress, don't wake up */
 	SI_FL_INDEP_STR  = 0x0040,  /* independent streams = don't update rex on write */
 	SI_FL_NOLINGER   = 0x0080,  /* may close without lingering. One-shot. */