[routing-manager] update stale time calculation for local RA header (#10316)

This commit updates the calculation of stale time for a discovered
local RA header (to mirror), incorporating the default route lifetime
specified in the header when it is non-zero, in addition to the RA
stale time constant. This ensures proper behavior even if the RA
header default route lifetime is shorter than the RA stale time.

Additionally, this commit adds `CalculateExpirationTime()` to
determine the expiration time from a given update time and lifetime
duration in seconds. If the given lifetime exceeds the supported
range of `TimeMilli` (~24 days), it clamps the value to ensure time
calculations remain within the valid `TimeMilli` range.

src/core/border_router/routing_manager.cpp

@@ -648,6 +648,18 @@ exit:
     }
 }
 
+TimeMilli RoutingManager::CalculateExpirationTime(TimeMilli aUpdateTime, uint32_t aLifetime)
+{
+    // `aLifetime` is in unit of seconds. We clamp the lifetime to max
+    // interval supported by `Timer` (`2^31` msec or ~24.8 days).
+    // This ensures that the time calculation fits within `TimeMilli`
+    // range.
+
+    static constexpr uint32_t kMaxLifetime = Time::MsecToSec(Timer::kMaxDelay);
+
+    return aUpdateTime + Time::SecToMsec(Min(aLifetime, kMaxLifetime));
+}
+
 bool RoutingManager::IsValidBrUlaPrefix(const Ip6::Prefix &aBrUlaPrefix)
 {
     return aBrUlaPrefix.mLength == kBrUlaPrefixLength && aBrUlaPrefix.mPrefix.mFields.m8[0] == 0xfd;
@@ -841,12 +853,10 @@ void RoutingManager::LogRouteInfoOption(const Ip6::Prefix &, uint32_t, RoutePref
 
 TimeMilli RoutingManager::LifetimedPrefix::CalculateExpirationTime(uint32_t aLifetime) const
 {
-    // `aLifetime` is in unit of seconds. We clamp the lifetime to max
-    // interval supported by `Timer` (`2^31` msec or ~24.8 days).
+    // `aLifetime` is in unit of seconds. This method ensures
+    // that the time calculation fits with `TimeMilli` range.
 
-    static constexpr uint32_t kMaxLifetime = Time::MsecToSec(Timer::kMaxDelay);
-
-    return mLastUpdateTime + Time::SecToMsec(Min(aLifetime, kMaxLifetime));
+    return RoutingManager::CalculateExpirationTime(mLastUpdateTime, aLifetime);
 }
 
 //---------------------------------------------------------------------------------------------------------------------
@@ -1514,7 +1524,14 @@ void RoutingManager::RxRaTracker::ScheduleStaleTimer(void)
 
     if (mLocalRaHeader.IsValid())
     {
-        staleTime.UpdateIfEarlier(mLocalRaHeaderUpdateTime + Time::SecToMsec(kRtrAdvStaleTime));
+        uint16_t interval = kRtrAdvStaleTime;
+
+        if (mLocalRaHeader.GetRouterLifetime() > 0)
+        {
+            interval = Min(interval, mLocalRaHeader.GetRouterLifetime());
+        }
+
+        staleTime.UpdateIfEarlier(CalculateExpirationTime(mLocalRaHeaderUpdateTime, interval));
     }
 
     mStaleTimer.FireAt(staleTime);

src/core/border_router/routing_manager.hpp

@@ -1397,6 +1397,8 @@ private:
     void HandleRaPrefixTableChanged(void);
     void HandleLocalOnLinkPrefixChanged(void);
 
+    static TimeMilli CalculateExpirationTime(TimeMilli aUpdateTime, uint32_t aLifetime);
+
     static bool IsValidBrUlaPrefix(const Ip6::Prefix &aBrUlaPrefix);
     static bool IsValidOnLinkPrefix(const PrefixInfoOption &aPio);
     static bool IsValidOnLinkPrefix(const Ip6::Prefix &aOnLinkPrefix);