From 31e512ac7bc2c3d60b50d5cf8130657b28dc2c9f Mon Sep 17 00:00:00 2001
From: Abtin Keshavarzian <abtink@google.com>
Date: Wed, 19 Jun 2024 16:57:34 -0700
Subject: [PATCH] [mle] replace `Mle::GetNextHop()` with `GetParentRloc16()`
 (#10407)

This commit removes the `Mle::GetNextHop()` method, which returned the
parent RLOC16 (when the parent state was valid) irrespective of a
given destination.

The `Mle::GetNextHop()` method was only used on MTD builds and from
`MeshForwarder::UpdateIp6Route()` method, where
`mle.GetNextHop(Mac::kShortAddrBroadcast)` served as an indirect way
to get the parent RLOC16. This is now replaced with a direct
`GetParentRloc16()` call, clarifying the intended purpose.

On FTD builds, the `MleRouter::GetNextHop()` is used instead of
`Mle::GetNextHop()`. The `MleRouter` method remains unchanged. Since
it would also return the parent's RLOC16 when the device is acting as
a child, using `GetParentRloc16()` covers this case as well.
---
 src/core/thread/mesh_forwarder.cpp |  4 ++--
 src/core/thread/mle.cpp            |  6 +-----
 src/core/thread/mle.hpp            | 18 ++++++++----------
 3 files changed, 11 insertions(+), 17 deletions(-)

diff --git a/src/core/thread/mesh_forwarder.cpp b/src/core/thread/mesh_forwarder.cpp
index 932e6d2f0..8a6b8611f 100644
--- a/src/core/thread/mesh_forwarder.cpp
+++ b/src/core/thread/mesh_forwarder.cpp
@@ -690,7 +690,7 @@ Error MeshForwarder::UpdateIp6Route(Message &aMessage)
 
         if (mle.IsChild() && aMessage.IsLinkSecurityEnabled() && !aMessage.IsSubTypeMle())
         {
-            mMacAddrs.mDestination.SetShort(mle.GetNextHop(Mac::kShortAddrBroadcast));
+            mMacAddrs.mDestination.SetShort(mle.GetParentRloc16());
         }
         else
         {
@@ -703,7 +703,7 @@ Error MeshForwarder::UpdateIp6Route(Message &aMessage)
     }
     else if (mle.IsMinimalEndDevice())
     {
-        mMacAddrs.mDestination.SetShort(mle.GetNextHop(Mac::kShortAddrBroadcast));
+        mMacAddrs.mDestination.SetShort(mle.GetParentRloc16());
     }
     else
     {
diff --git a/src/core/thread/mle.cpp b/src/core/thread/mle.cpp
index 157dac149..246d73f2c 100644
--- a/src/core/thread/mle.cpp
+++ b/src/core/thread/mle.cpp
@@ -3813,11 +3813,7 @@ void Mle::ProcessAnnounce(void)
     IgnoreError(Start(kAnnounceAttach));
 }
 
-uint16_t Mle::GetNextHop(uint16_t aDestination) const
-{
-    OT_UNUSED_VARIABLE(aDestination);
-    return (mParent.IsStateValid()) ? mParent.GetRloc16() : static_cast<uint16_t>(Mac::kShortAddrInvalid);
-}
+uint16_t Mle::GetParentRloc16(void) const { return (mParent.IsStateValid() ? mParent.GetRloc16() : kInvalidRloc16); }
 
 Error Mle::GetParentInfo(Router::Info &aParentInfo) const
 {
diff --git a/src/core/thread/mle.hpp b/src/core/thread/mle.hpp
index 85d1207cc..466f799ec 100644
--- a/src/core/thread/mle.hpp
+++ b/src/core/thread/mle.hpp
@@ -419,6 +419,14 @@ public:
         return mRealmLocalAllThreadNodes.GetAddress();
     }
 
+    /**
+     * Gets the parent's RLOC16.
+     *
+     * @returns  The parent's RLOC16, or `kInvalidRloc16` if parent's state is not valid.
+     *
+     */
+    uint16_t GetParentRloc16(void) const;
+
     /**
      * Gets the parent when operating in End Device mode.
      *
@@ -693,16 +701,6 @@ public:
         return (&aAddress == &mLinkLocalAllThreadNodes) || (&aAddress == &mRealmLocalAllThreadNodes);
     }
 
-    /**
-     * Determines the next hop towards an RLOC16 destination.
-     *
-     * @param[in]  aDestination  The RLOC16 of the destination.
-     *
-     * @returns A RLOC16 of the next hop if a route is known, kInvalidRloc16 otherwise.
-     *
-     */
-    uint16_t GetNextHop(uint16_t aDestination) const;
-
 #if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
     /**
      * Gets the CSL timeout.