: Provide fallback RTT value when none is available

[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].

---
 net/dccp/dccp.h |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

@@ -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))

|  > [DCCP]: Provide fallback RTT value when none is available
|  > @@ -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)

<snip>

|  
|  I personally think this patch should include the changes where this
|  occurs in the code but I know these are in separate patches.
|  
I have just checked - the place it is actually used is indeed a bit later in the
change set, in receiver RTT sampling, it is here:
	http://www.mail-archive.com/[email protected]/msg01868.html

+ if (hcrx->ccid3hcrx_rtt = 0) {
+ DCCP_WARN("No RTT estimate available, using fallback RTT\n");
+ hcrx->ccid3hcrx_rtt = DCCP_FALLBACK_RTT;
}

But this patch is also generic: RFC 4340 says that this value (which agrees with TCP-like values) can be used wherever there is no fallback.

I signed off on this on June 16th but this seems to have got lost so: