net: ipv6: Tailor PE values for Zephyr

Making default preferred time much closer to default valid time
so that the served addresses are much shorter time in deprecated
state where they cannot be utilized. This is important in Zephyr
as there are limited number of IPv6 addresses that can be allocated
to the network interface.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>

Author: jukkar

subsys/net/ip/Kconfig.ipv6

@@ -200,17 +200,17 @@ config NET_IPV6_PE_PREFER_PUBLIC_ADDRESSES
 
 config NET_IPV6_PE_TEMP_VALID_LIFETIME
 	int "Max lifetime for temporary address (in minutes)"
-	default 1800
+	default 1440
 	help
 	  No temporary address should ever remain valid for longer than this
-	  value. The value is in minutes. Default value is 2 days (2*24*60).
+	  value. The value is in minutes. Default value is 1 day (24*60).
 
 config NET_IPV6_PE_TEMP_PREFERRED_LIFETIME
 	int "Max preferred lifetime for temporary address (in minutes)"
-	default 1440
+	default 1380
 	help
 	  No temporary address should ever remain preferred for longer than this
-	  value. The value is in minutes. Default value is 1 day (24*60).
+	  value. The value is in minutes. Default value is 23 hours (23*60).
 
 config NET_IPV6_PE_TEMP_IDGEN_RETRIES
 	int "Max amount of failed DAD attempts"

subsys/net/ip/ipv6_pe.c

@@ -52,9 +52,15 @@ static uint32_t temp_preferred_lifetime =
 
 /* This is the upper bound on DESYNC_FACTOR. The value is in seconds.
  * See RFC 8981 ch 3.8 for details.
+ *
+ * RFC says the DESYNC_FACTOR should be 0.4 times the preferred lifetime.
+ * This is too short for Zephyr as it means that the address is very long
+ * time in deprecated state and not being used. Make this 7% of the preferred
+ * time to deprecate the addresses later.
  */
-#define MAX_DESYNC_FACTOR       ((TEMP_PREFERRED_LIFETIME * 2U) / 5U)
-#define DESYNC_FACTOR(ipv6)     ((ipv6)->desync_factor)
+#define MAX_DESYNC_FACTOR   ((uint32_t)((uint64_t)TEMP_PREFERRED_LIFETIME * \
+				       (uint64_t)7U) / (uint64_t)100U)
+#define DESYNC_FACTOR(ipv6) ((ipv6)->desync_factor)
 
 #define TEMP_IDGEN_RETRIES      CONFIG_NET_IPV6_PE_TEMP_IDGEN_RETRIES