diff --git a/include/net/if_inet6.h b/include/net/if_inet6.h
index 11cf373..51a7031 100644
--- a/include/net/if_inet6.h
+++ b/include/net/if_inet6.h
@@ -41,6 +41,7 @@
 	struct in6_addr		addr;
 	__u32			prefix_len;
 	
+	/* In seconds, relative to tstamp. Expiry is at tstamp + HZ * lft. */
 	__u32			valid_lft;
 	__u32			prefered_lft;
 	atomic_t		refcnt;