[PATCH 2/6]: Provide fallback RTT value when none is available

[Gerrit Renker <gerrit@erg.abdn.ac.uk>]

[DCCP]: Provide fallback RTT value when none is available

This implements RFC 4340, section 3.4:

 "Each DCCP implementation thus defines a default round-trip time
  for use when no estimate is  available.  This parameter should 
  default to not less than 0.2 seconds [...]."

In addition, the upper bound of 4 seconds for an RTT sample has now been reduced,
to 3 seconds: to match the initial TCP RTO value as specified in [RFC 1122, 4.2.3.1].

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
---
 net/dccp/dccp.h |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/net/dccp/dccp.h
+++ b/net/dccp/dccp.h
@@ -73,9 +73,12 @@ extern void dccp_time_wait(struct sock *
 
 #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))