SDAP refactor: Refactor UE QFI resolution chain, make DSCP fallback opt-in (useDscpAsQfiFallback)

Author: avarga

src/simu5g/stack/sdap/NrSdap.cc

@@ -115,32 +115,30 @@ void NrSdap::handleUpperPacket(inet::Packet *pkt)
         EV_INFO << "SDAP TX: QFI = " << (int)qfi << " extracted from QfiReq\n";
     }
     else if (isUe) {
-        // UE UL: derive QFI from DSCP field of the IP header (DSCP = TOS >> 2)
-        // FlowControlInfo.typeOfService is the raw TOS byte set by Ip2Nic::toStackUe()
-        // Only applicable for IP PDU sessions; non-IP sessions use QfiReq or default QFI.
-        if (pkt->hasTag<FlowControlInfo>()) {
-            auto fci = pkt->getTag<FlowControlInfo>();
-            uint8_t tos = (uint8_t)fci->getTypeOfService();
-            if (tos > 0) {
-                uint8_t dscp = tos >> 2;
-                qfi = dscp;
-                EV_INFO << "SDAP TX: QFI = " << (int)qfi << " derived from DSCP=" << (int)dscp << "\n";
-            }
-        }
-        // If DSCP gave no QFI, try reflective QoS
-        if (qfi == 0 && reflectiveQosTable != nullptr) {
+        // UE UL: try reflective QoS first (3GPP-defined mechanism)
+        if (reflectiveQosTable != nullptr) {
             uint8_t reflectiveQfi = reflectiveQosTable->lookupUplinkQfi(pkt);
             if (reflectiveQfi > 0) {
                 qfi = reflectiveQfi;
                 qfiFromReflectiveQos = true;
                 EV_INFO << "SDAP TX: QFI = " << (int)qfi << " derived from reflective QoS\n";
             }
         }
+        // Optional non-standard fallback: derive QFI from DSCP field of the IP header
+        if (qfi == 0 && par("useDscpAsQfiFallback").boolValue()) {
+            if (pkt->hasTag<FlowControlInfo>()) {
+                uint8_t tos = (uint8_t)pkt->getTag<FlowControlInfo>()->getTypeOfService();
+                if (tos > 0) {
+                    qfi = tos >> 2;
+                    EV_INFO << "SDAP TX: QFI = " << (int)qfi << " derived from DSCP (fallback)\n";
+                }
+            }
+        }
         if (qfi == 0)
-            EV_WARN << "SDAP TX: No QFI from DSCP or reflective QoS, defaulting QFI to 0\n";
+            EV_WARN << "SDAP TX: No QFI from reflective QoS or DSCP, using QFI=0 (default DRB)\n";
     }
     else {
-        EV_WARN << "SDAP TX: QfiReq not found on gNB path, defaulting QFI to 0\n";
+        throw cRuntimeError("SDAP TX: QfiReq tag missing on gNB DL path -- GtpUser should always set it");
     }
 
     // Lookup DRB context: nodeId is NODEID_NONE on UE, destUeId on gNB

src/simu5g/stack/sdap/NrSdap.ned

@@ -20,7 +20,12 @@ package simu5g.stack.sdap;
 //
 // For SDUs received from the upper layer, performs QFI-to-DRB mapping,
 // encapsulates the data with SDAP headers if needed, and forwards the packets
-// to the PDCP layer.
+// to the PDCP layer. On the gNB DL path, the QFI is always taken from the
+// QfiReq tag (set by GtpUser). On the UE UL path, the QFI resolution chain is:
+//   1. QfiReq tag (if set by the application)
+//   2. Reflective QoS table (if enabled)
+//   3. DSCP field of the IP header (if useDscpAsQfiFallback is enabled)
+//   4. QFI=0 (falls back to default DRB)
 //
 // 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
@@ -56,6 +61,7 @@ simple NrSdap like INrSdap
         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 useReflectiveQos = default(false); // UE only
+        bool useDscpAsQfiFallback = default(false);  // UE only: use IP DSCP value as QFI when no QfiReq or reflective QoS match
         @display("i=block/layer");
     gates:
         input upperLayerIn;     // from upper layer (Ip2Nic or app)