lte: Add PDCP-RLC discard timer

src/lte/model/lte-enb-rrc.cc

@@ -436,6 +436,7 @@ UeManager::SetupDataRadioBearer(EpsBearer bearer,
     Ptr<LteRlc> rlc = rlcObjectFactory.Create()->GetObject<LteRlc>();
     rlc->SetLteMacSapProvider(m_rrc->m_macSapProvider);
     rlc->SetRnti(m_rnti);
+    rlc->SetPacketDelayBudgetMs(bearer.GetPacketDelayBudgetMs());
 
     drbInfo->m_rlc = rlc;
 

src/lte/model/lte-rlc-um.cc

@@ -68,7 +68,21 @@ LteRlcUm::GetTypeId()
                           "Value of the t-Reordering timer (See section 7.3 of 3GPP TS 36.322)",
                           TimeValue(MilliSeconds(100)),
                           MakeTimeAccessor(&LteRlcUm::m_reorderingTimerValue),
-                          MakeTimeChecker());
+                          MakeTimeChecker())
+            .AddAttribute(
+                "EnablePdcpDiscarding",
+                "Whether to use the PDCP discarding, i.e., perform discarding at the moment "
+                "of passing the PDCP SDU to RLC)",
+                BooleanValue(true),
+                MakeBooleanAccessor(&LteRlcUm::m_enablePdcpDiscarding),
+                MakeBooleanChecker())
+            .AddAttribute("DiscardTimerMs",
+                          "Discard timer in milliseconds to be used to discard packets. "
+                          "If set to 0 then packet delay budget will be used as the discard "
+                          "timer value, otherwise it will be used this value.",
+                          UintegerValue(0),
+                          MakeUintegerAccessor(&LteRlcUm::m_discardTimerMs),
+                          MakeUintegerChecker<uint32_t>());
     return tid;
 }
 
@@ -93,6 +107,29 @@ LteRlcUm::DoTransmitPdcpPdu(Ptr<Packet> p)
 
     if (m_txBufferSize + p->GetSize() <= m_maxTxBufferSize)
     {
+        if (m_enablePdcpDiscarding)
+        {
+            // discart the packet
+            uint32_t headOfLineDelayInMs = 0;
+            uint32_t discardTimerMs =
+                (m_discardTimerMs > 0) ? m_discardTimerMs : m_packetDelayBudgetMs;
+
+            if (!m_txBuffer.empty())
+            {
+                headOfLineDelayInMs =
+                    (Simulator::Now() - m_txBuffer.begin()->m_waitingSince).GetMilliSeconds();
+            }
+            NS_LOG_DEBUG("head of line delay in MS:" << headOfLineDelayInMs);
+            if (headOfLineDelayInMs > discardTimerMs)
+            {
+                NS_LOG_DEBUG("Tx HOL is higher than this packet can allow. RLC SDU discarded");
+                NS_LOG_DEBUG("headOfLineDelayInMs    = " << headOfLineDelayInMs);
+                NS_LOG_DEBUG("m_packetDelayBudgetMs    = " << m_packetDelayBudgetMs);
+                NS_LOG_DEBUG("packet size     = " << p->GetSize());
+                m_txDropTrace(p);
+            }
+        }
+
         /** Store PDCP PDU */
         LteRlcSduStatusTag tag;
         tag.SetStatus(LteRlcSduStatusTag::FULL_SDU);

src/lte/model/lte-rlc-um.h

@@ -143,9 +143,12 @@ class LteRlcUm : public LteRlc
     /**
      * Timers. See section 7.3 in TS 36.322
      */
-    Time m_reorderingTimerValue; ///< reordering timer value
-    EventId m_reorderingTimer;   ///< reordering timer
-    EventId m_rbsTimer;          ///< RBS timer
+    Time m_reorderingTimerValue;        ///< reordering timer value
+    EventId m_reorderingTimer;          ///< reordering timer
+    EventId m_rbsTimer;                 ///< RBS timer
+    bool m_enablePdcpDiscarding{false}; //!< whether to use the PDCP discarding (perform discarding
+                                        //!< at the moment of passing the PDCP SDU to RLC)
+    uint32_t m_discardTimerMs{0};       //!< the discard timer value in milliseconds
 
     /**
      * Reassembling state

src/lte/model/lte-rlc.cc

@@ -144,6 +144,13 @@ LteRlc::SetLcId(uint8_t lcId)
     m_lcid = lcId;
 }
 
+void
+LteRlc::SetPacketDelayBudgetMs(uint16_t packetDelayBudget)
+{
+    NS_LOG_FUNCTION(this << +packetDelayBudget);
+    m_packetDelayBudgetMs = packetDelayBudget;
+}
+
 void
 LteRlc::SetLteRlcSapUser(LteRlcSapUser* s)
 {

src/lte/model/lte-rlc.h

@@ -75,6 +75,11 @@ class LteRlc : public Object // SimpleRefCount<LteRlc>
      */
     void SetLcId(uint8_t lcId);
 
+    /**
+     * \param packetDelayBudget
+     */
+    void SetPacketDelayBudgetMs(uint16_t packetDelayBudget);
+
     /**
      *
      *
@@ -165,6 +170,8 @@ class LteRlc : public Object // SimpleRefCount<LteRlc>
 
     uint16_t m_rnti; ///< RNTI
     uint8_t m_lcid;  ///< LCID
+    uint16_t m_packetDelayBudgetMs{
+        UINT16_MAX}; //!< the packet delay budget in ms of the corresponding logical channel
 
     /**
      * Used to inform of a PDU delivery to the MAC SAP provider