diff --git a/include/uapi/linux/errqueue.h b/include/uapi/linux/errqueue.h
index 17437cf..07bdce1 100644
--- a/include/uapi/linux/errqueue.h
+++ b/include/uapi/linux/errqueue.h
@@ -40,6 +40,7 @@
 enum {
 	SCM_TSTAMP_SND,		/* driver passed skb to NIC, or HW */
 	SCM_TSTAMP_SCHED,	/* data entered the packet scheduler */
+	SCM_TSTAMP_ACK,		/* data acknowledged by peer */
 };
 
 #endif /* _UAPI_LINUX_ERRQUEUE_H */