[mpl] support dynamic MPL interval according to the network scale (#2220)

src/core/net/ip6_mpl.cpp

@@ -48,7 +48,7 @@ namespace Ip6 {
 void MplBufferedMessageMetadata::GenerateNextTransmissionTime(uint32_t aCurrentTime, uint8_t aInterval)
 {
     // Emulate Trickle timer behavior and set up the next retransmission within [0,I) range.
-    uint8_t t = otPlatRandomGet() % aInterval;
+    uint8_t t = aInterval == 0 ? aInterval : otPlatRandomGet() % aInterval;
 
     // Set transmission time at the beginning of the next interval.
     SetTransmissionTime(aCurrentTime + GetIntervalOffset() + t);
@@ -171,6 +171,14 @@ void Mpl::AddBufferedMessage(Message &aMessage, uint16_t aSeedId, uint8_t aSeque
     uint32_t nextTransmissionTime;
     uint8_t hopLimit = 0;
 
+#if OPENTHREAD_CONFIG_ENABLE_DYNAMIC_MPL_INTERVAL
+    // adjust the first MPL forward interval dynamically according to the network scale
+    uint8_t interval = (kDataMessageInterval / Mle::kMaxRouters) *
+                       GetInstance().mThreadNetif.GetMle().GetActiveNeighborRouterCount();
+#else
+    uint8_t interval = kDataMessageInterval;
+#endif
+
     VerifyOrExit(GetTimerExpirations() > 0);
     VerifyOrExit((messageCopy = aMessage.Clone()) != NULL, error = OT_ERROR_NO_BUFS);
 
@@ -184,7 +192,7 @@ void Mpl::AddBufferedMessage(Message &aMessage, uint16_t aSeedId, uint8_t aSeque
     messageMetadata.SetSeedId(aSeedId);
     messageMetadata.SetSequence(aSequence);
     messageMetadata.SetTransmissionCount(aIsOutbound ? 1 : 0);
-    messageMetadata.GenerateNextTransmissionTime(now, kDataMessageInterval);
+    messageMetadata.GenerateNextTransmissionTime(now, interval);
 
     // Append the message with MplBufferedMessageMetadata and add it to the queue.
     SuccessOrExit(error = messageMetadata.AppendTo(*messageCopy));

src/core/openthread-core-default-config.h

@@ -985,4 +985,17 @@
 #define OPENTHREAD_CONFIG_ENABLE_DEBUG_UART                     0
 #endif
 
+/**
+ * @def OPENTHREAD_CONFIG_ENABLE_DYNAMIC_MPL_INTERVAL
+ *
+ * Define as 1 to enable dynamic MPL interval feature.
+ *
+ * If this feature is enabled, the MPL forward interval will be adjusted dynamically according to
+ * the network scale, which helps to reduce multicast latency.
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_ENABLE_DYNAMIC_MPL_INTERVAL
+#define OPENTHREAD_CONFIG_ENABLE_DYNAMIC_MPL_INTERVAL           0
+#endif
+
 #endif  // OPENTHREAD_CORE_DEFAULT_CONFIG_H_

src/core/thread/mle_router.cpp

@@ -1272,6 +1272,21 @@ uint8_t MleRouter::GetActiveRouterCount(void) const
     return rval;
 }
 
+uint8_t MleRouter::GetActiveNeighborRouterCount(void) const
+{
+    uint8_t rval = 0;
+
+    for (int i = 0; i <= kMaxRouterId; i++)
+    {
+        if (mRouters[i].GetState() == Neighbor::kStateValid)
+        {
+            rval++;
+        }
+    }
+
+    return rval;
+}
+
 otError MleRouter::HandleAdvertisement(const Message &aMessage, const Ip6::MessageInfo &aMessageInfo)
 {
     ThreadNetif &netif = GetNetif();

src/core/thread/mle_router_ftd.hpp

@@ -141,6 +141,14 @@ public:
      */
     uint8_t GetActiveRouterCount(void) const;
 
+    /**
+      * This method returns the number of active neighbor routers.
+      *
+      * @returns The number of active neighbor routers.
+      *
+      */
+    uint8_t GetActiveNeighborRouterCount(void) const;
+
     /**
      * This method returns the time in seconds since the last Router ID Sequence update.
      *

src/core/thread/mle_router_mtd.hpp

@@ -56,6 +56,7 @@ public:
     otError BecomeLeader(void) { return OT_ERROR_NOT_CAPABLE; }
 
     uint8_t GetActiveRouterCount(void) const { return 0; }
+    uint8_t GetActiveNeighborRouterCount(void) const { return 0; }
 
     uint32_t GetLeaderAge(void) const { return 0; }