diff --git a/net/dccp/dccp.h b/net/dccp/dccp.h
index f4a5ea1..07dcbe7 100644
--- a/net/dccp/dccp.h
+++ b/net/dccp/dccp.h
@@ -74,9 +74,12 @@
 
 #define DCCP_RTO_MAX ((unsigned)(120 * HZ)) /* FIXME: using TCP value */
 
-/* bounds for sampled RTT values from packet exchanges (in usec) */
+/*
+ * RTT sampling: sanity bounds and fallback RTT value from RFC 4340, section 3.4
+ */
 #define DCCP_SANE_RTT_MIN	100
-#define DCCP_SANE_RTT_MAX	(4 * USEC_PER_SEC)
+#define DCCP_FALLBACK_RTT	(USEC_PER_SEC / 5)
+#define DCCP_SANE_RTT_MAX	(3 * USEC_PER_SEC)
 
 /* Maximal interval between probes for local resources.  */
 #define DCCP_RESOURCE_PROBE_INTERVAL ((unsigned)(HZ / 2U))