[ip6-mpl] remove aIsOutbound input and use aMessage.GetOrigin() (#9510)

With the recent addition of `GetOrigin()`, the origin of a message is
tracked by `Message` itself. With this change, we no longer need to
pass `aIsOutbound` to `Ip6::Mpl` methods, as it can be determined
from the origin of `aMessage`. This commit simplifies the code by
removing the `aIsOutbound` input parameter.

src/core/net/ip6.cpp

@@ -516,7 +516,7 @@ void Ip6::HandleSendQueue(void)
     }
 }
 
-Error Ip6::HandleOptions(Message &aMessage, Header &aHeader, bool aIsOutbound, bool &aReceive)
+Error Ip6::HandleOptions(Message &aMessage, Header &aHeader, bool &aReceive)
 {
     Error          error = kErrorNone;
     HopByHopHeader hbhHeader;
@@ -542,7 +542,7 @@ Error Ip6::HandleOptions(Message &aMessage, Header &aHeader, bool aIsOutbound, b
 
         if (option.GetType() == MplOption::kType)
         {
-            SuccessOrExit(error = mMpl.ProcessOption(aMessage, offset, aHeader.GetSource(), aIsOutbound, aReceive));
+            SuccessOrExit(error = mMpl.ProcessOption(aMessage, offset, aHeader.GetSource(), aReceive));
             continue;
         }
 
@@ -829,8 +829,8 @@ Error Ip6::HandleExtensionHeaders(Message     &aMessage,
                                   uint8_t     &aNextHeader,
                                   bool        &aReceive)
 {
-    Error           error      = kErrorNone;
-    bool            isOutbound = !aMessage.IsOriginThreadNetif();
+    Error error = kErrorNone;
+
     ExtensionHeader extHeader;
 
     while (aReceive || aNextHeader == kProtoHopOpts)
@@ -840,7 +840,7 @@ Error Ip6::HandleExtensionHeaders(Message     &aMessage,
         switch (aNextHeader)
         {
         case kProtoHopOpts:
-            SuccessOrExit(error = HandleOptions(aMessage, aHeader, isOutbound, aReceive));
+            SuccessOrExit(error = HandleOptions(aMessage, aHeader, aReceive));
             break;
 
         case kProtoFragment:
@@ -850,7 +850,7 @@ Error Ip6::HandleExtensionHeaders(Message     &aMessage,
             break;
 
         case kProtoDstOpts:
-            SuccessOrExit(error = HandleOptions(aMessage, aHeader, isOutbound, aReceive));
+            SuccessOrExit(error = HandleOptions(aMessage, aHeader, aReceive));
             break;
 
         case kProtoIp6:

src/core/net/ip6.hpp

@@ -390,7 +390,7 @@ private:
     Error AddTunneledMplOption(Message &aMessage, Header &aHeader);
     Error InsertMplOption(Message &aMessage, Header &aHeader);
     Error RemoveMplOption(Message &aMessage);
-    Error HandleOptions(Message &aMessage, Header &aHeader, bool aIsOutbound, bool &aReceive);
+    Error HandleOptions(Message &aMessage, Header &aHeader, bool &aReceive);
     Error HandlePayload(Header            &aIp6Header,
                         Message           &aMessage,
                         MessageInfo       &aMessageInfo,

src/core/net/ip6_mpl.cpp

@@ -90,7 +90,7 @@ void Mpl::InitOption(MplOption &aOption, const Address &aAddress)
     aOption.SetSequence(mSequence++);
 }
 
-Error Mpl::ProcessOption(Message &aMessage, uint16_t aOffset, const Address &aAddress, bool aIsOutbound, bool &aReceive)
+Error Mpl::ProcessOption(Message &aMessage, uint16_t aOffset, const Address &aAddress, bool &aReceive)
 {
     Error     error;
     MplOption option;
@@ -122,10 +122,10 @@ Error Mpl::ProcessOption(Message &aMessage, uint16_t aOffset, const Address &aAd
     if (error == kErrorNone)
     {
 #if OPENTHREAD_FTD
-        AddBufferedMessage(aMessage, option.GetSeedId(), option.GetSequence(), aIsOutbound);
+        AddBufferedMessage(aMessage, option.GetSeedId(), option.GetSequence());
 #endif
     }
-    else if (aIsOutbound)
+    else if (!aMessage.IsOriginThreadNetif())
     {
         aReceive = false;
         // In case MPL Data Message is generated locally, ignore potential error of the MPL Seed Set
@@ -334,7 +334,7 @@ uint8_t Mpl::DetermineMaxRetransmissions(void) const
     return maxRetx;
 }
 
-void Mpl::AddBufferedMessage(Message &aMessage, uint16_t aSeedId, uint8_t aSequence, bool aIsOutbound)
+void Mpl::AddBufferedMessage(Message &aMessage, uint16_t aSeedId, uint8_t aSequence)
 {
     Error    error       = kErrorNone;
     Message *messageCopy = nullptr;
@@ -352,16 +352,23 @@ void Mpl::AddBufferedMessage(Message &aMessage, uint16_t aSeedId, uint8_t aSeque
     VerifyOrExit(DetermineMaxRetransmissions() > 0);
     VerifyOrExit((messageCopy = aMessage.Clone()) != nullptr, error = kErrorNoBufs);
 
-    if (!aIsOutbound)
+    if (aMessage.IsOriginThreadNetif())
     {
         IgnoreError(aMessage.Read(Header::kHopLimitFieldOffset, hopLimit));
         VerifyOrExit(hopLimit-- > 1, error = kErrorDrop);
         messageCopy->Write(Header::kHopLimitFieldOffset, hopLimit);
     }
 
+    // If the message originates from Thread Netif (i.e., it was
+    // received over Thread radio), set the `mTransmissionCount` to
+    // zero. Otherwise, the message originates from the host and will
+    // be forwarded by `Ip6` to the Thread mesh, so the message itself
+    // will be the first transmission and we set `mTransmissionCount`
+    // to one.
+
     metadata.mSeedId            = aSeedId;
     metadata.mSequence          = aSequence;
-    metadata.mTransmissionCount = aIsOutbound ? 1 : 0;
+    metadata.mTransmissionCount = aMessage.IsOriginThreadNetif() ? 0 : 1;
     metadata.mIntervalOffset    = 0;
     metadata.GenerateNextTransmissionTime(TimerMilli::GetNow(), interval);
 

src/core/net/ip6_mpl.hpp

@@ -194,7 +194,6 @@ public:
      * @param[in]  aMessage    A reference to the message.
      * @param[in]  aOffset     The offset in @p aMessage to read the MPL option.
      * @param[in]  aAddress    A reference to the IPv6 Source Address.
-     * @param[in]  aIsOutbound TRUE if this message was locally generated, FALSE otherwise.
      * @param[out] aReceive    Set to FALSE if the MPL message is a duplicate and must not
      *                         go through the receiving process again, untouched otherwise.
      *
@@ -202,7 +201,7 @@ public:
      * @retval kErrorDrop  The MPL message is a duplicate and should be dropped.
      *
      */
-    Error ProcessOption(Message &aMessage, uint16_t aOffset, const Address &aAddress, bool aIsOutbound, bool &aReceive);
+    Error ProcessOption(Message &aMessage, uint16_t aOffset, const Address &aAddress, bool &aReceive);
 
 #if OPENTHREAD_FTD
     /**
@@ -254,7 +253,7 @@ private:
 
     uint8_t DetermineMaxRetransmissions(void) const;
     void    HandleRetransmissionTimer(void);
-    void    AddBufferedMessage(Message &aMessage, uint16_t aSeedId, uint8_t aSequence, bool aIsOutbound);
+    void    AddBufferedMessage(Message &aMessage, uint16_t aSeedId, uint8_t aSequence);
 
     using RetxTimer = TimerMilliIn<Mpl, &Mpl::HandleRetransmissionTimer>;