SDAP: Eliminate addSdapHeader NED parameter, infer it from DRB properties

Only add header if QFI cannot be restored on RX side: if there's more
QFIs in that DRB, or QFI uses fallback DRB.

Also: Throw on null DRB in RX path instead of silent fallback

Author: avarga

simulations/nr/standalone/omnetpp_drb.ini

@@ -88,8 +88,6 @@ sim-time-limit=20s
     {"drb": 1, "qfiList": [3, 4], "rlcType": "UM"}
     ]
 
-**.sdap.addSdapHeader = false  #TEMP HACK
-
 #------------------------------------#
 # Config VoIP-DL
 #

src/simu5g/stack/sdap/NrSdap.cc

@@ -51,7 +51,12 @@ void NrSdap::initialize()
 
 bool NrSdap::requiresSdapHeader(const DrbConfig *drb)
 {
-    return par("addSdapHeader").boolValue(); // for now -- should come from RRC config
+    // SDAP header is needed when the QFI cannot be unambiguously determined
+    // from the DRB alone on the RX side:
+    // - default DRB: may carry packets with unmapped QFIs (fallback traffic)
+    // - multiple QFIs mapped: reverse mapping is ambiguous
+    // Caller must ensure drb is not null.
+    return drb->isDefault || drb->qfiList.size() > 1;
 }
 
 bool NrSdap::shouldEnableReflectiveQos(int qfi)
@@ -198,6 +203,9 @@ void NrSdap::handleLowerPacket(inet::Packet *pkt)
     DrbId drbId = lteInfo ? lteInfo->getDrbId() : DRBID_NONE;
     MacNodeId ueId = (!isUe && lteInfo) ? lteInfo->getSourceId() : NODEID_NONE;
     const DrbConfig *drb = (drbId != DRBID_NONE) ? drbTable_.getDrb(DrbKey(ueId, drbId)) : nullptr;
+    if (!drb)
+        throw cRuntimeError("SDAP RX: Unknown DRB %d (ueId=%d) -- missing drbConfig entry?",
+                            (int)num(drbId), (int)num(ueId));
 
     EV_INFO << "SDAP RX: Received packet from DRB " << drbId << ": " << pkt->peekAtFront() << "\n";
 

src/simu5g/stack/sdap/NrSdap.ned

@@ -22,6 +22,11 @@ package simu5g.stack.sdap;
 // encapsulates the data with SDAP headers if needed, and forwards the packets
 // to the PDCP layer.
 //
+// An SDAP header is only added when the QFI cannot be unambiguously recovered
+// from the DRB alone on the RX side, i.e. when the DRB is the default DRB
+// (which may carry unmapped fallback traffic) or when it has multiple QFIs
+// mapped to it.
+//
 // Supports all 3GPP PDU session types (IPv4, IPv6, Ethernet, Unstructured)
 // via the pduSessionType field in drbConfig.
 //
@@ -50,7 +55,6 @@ simple NrSdap like INrSdap
         string binderModule = default("binder");
         object drbConfig = default([]);  // JSON array: [{drb, ue, qfiList, rlcType, pduSessionType, upperProtocol, isDefault}, ...]; see banner comment for details
         string reflectiveQosTableModule = default("");  // UE only: path to ReflectiveQosTable module (empty = disabled)
-        bool addSdapHeader = default(true);
         bool useReflectiveQos = default(false); // UE only
         @display("i=block/layer");
     gates: