wifi: Send MU-RTS if any receiver of a DL MU PPDU is unprotected

Assisted-by: Claude Code (claude-fable-5)

Author: Davide Magrin

RELEASE_NOTES.md

@@ -46,6 +46,7 @@ been tested on Linux. As of this release, the latest known version to work with
 - (dsr) !2762 Fixes header format to comply with RFC4728. Also other minor bug fixes and modernization.
 - (zigbee) In the NWK, broadcast initiator devices are now registered in the BTT to avoid receiving retransmissions.
 - (wifi) !2945 CTS-to-self frames are now transmitted over the bandwidth of the frame they protect, so that transmissions in the rest of the TXOP are no longer limited to 20 MHz.
+- (wifi) !2938 A DL MU PPDU transmitted in a non-initial frame exchange of a TXOP is now protected by an MU-RTS if any of its receivers is unprotected; previously the decision was made per MPDU and such a PPDU could be sent without the configured protection (debug builds failed an assert).
 
 ## Release 3.48
 

src/wifi/CMakeLists.txt

@@ -429,6 +429,7 @@ build_lib(
     test/wifi-phy-thresholds-test.cc
     test/wifi-primary-channels-test.cc
     test/wifi-probe-exchange-test.cc
+    test/wifi-protection-test.cc
     test/wifi-queue-scheduler-test.cc
     test/wifi-retransmit-test.cc
     test/wifi-ru-allocation-test.cc

src/wifi/model/wifi-default-protection-manager.cc

@@ -19,6 +19,7 @@
 #include "ns3/erp-ofdm-phy.h"
 #include "ns3/log.h"
 
+#include <algorithm>
 #include <type_traits>
 
 namespace ns3
@@ -246,9 +247,15 @@ WifiDefaultProtectionManager::TryAddMpduToMuPpdu(Ptr<const WifiMpdu> mpdu,
 
     const auto& protectedStas = m_mac->GetFrameExchangeManager(m_linkId)->GetProtectedStas();
     const auto isProtected = protectedStas.contains(receiver);
-    bool needMuRts =
+    // an MU-RTS protects the DL MU PPDU as a whole, hence it is needed if any receiver of the
+    // PPDU is not yet protected, regardless of which receiver the MPDU being added is for
+    const auto anyUnprotected =
+        std::any_of(psduInfoMap.cbegin(), psduInfoMap.cend(), [&protectedStas](const auto& info) {
+            return !protectedStas.contains(info.first);
+        });
+    const auto needMuRts =
         (txParams.m_protection && txParams.m_protection->method == WifiProtection::MU_RTS_CTS) ||
-        (dlMuPpdu && m_sendMuRts && !isProtected &&
+        (dlMuPpdu && m_sendMuRts && anyUnprotected &&
          (!m_singleRtsPerTxop || protectedStas.empty())) ||
         (isEmlsrDestination && !isProtected);