wifi: Time::Min () is a safer default value for PPDU duration limits

src/wifi/model/mac-low.cc

@@ -542,7 +542,7 @@ MacLow::StartTransmission (Ptr<WifiMacQueueItem> mpdu,
       Ptr<QosTxop> qosTxop = m_edca.find (QosUtilsMapTidToAc (tid))->second;
 
       // if a TXOP limit exists, compute the remaining TXOP duration
-      Time txopLimit = Seconds (0);
+      Time txopLimit = Time::Min ();
       if (m_currentTxop->GetTxopLimit ().IsStrictlyPositive ())
         {
           txopLimit = m_currentTxop->GetTxopRemaining () - CalculateOverheadTxTime (mpdu, m_txParams);
@@ -662,6 +662,11 @@ MacLow::IsWithinSizeAndTimeLimits (uint32_t mpduSize, Mac48Address receiver, uin
 {
   NS_LOG_FUNCTION (this << mpduSize << receiver << +tid << txVector << ampduSize << ppduDurationLimit);
 
+  if (ppduDurationLimit != Time::Min () && ppduDurationLimit.IsNegative ())
+    {
+      return false;
+    }
+
   WifiModulationClass modulation = txVector.GetMode ().GetModulationClass ();
 
   uint32_t maxAmpduSize = 0;

src/wifi/model/mpdu-aggregator.h

@@ -113,7 +113,8 @@ public:
    *
    * - the time to transmit the resulting PPDU, according to the given TxVector,
    * does not exceed both the maximum PPDU duration allowed by the corresponding
-   * modulation class (if any) and the given PPDU duration limit (if non null)
+   * modulation class (if any) and the given PPDU duration limit (if distinct from
+   * Time::Min ())
    *
    * For now, only non-broadcast QoS Data frames can be aggregated (do not pass
    * other types of frames to this method). MPDUs to aggregate are looked for
@@ -133,7 +134,7 @@ public:
    */
   std::vector<Ptr<WifiMacQueueItem>> GetNextAmpdu (Ptr<const WifiMacQueueItem> mpdu,
                                                    WifiTxVector txVector,
-                                                   Time ppduDurationLimit = Seconds (0)) const;
+                                                   Time ppduDurationLimit = Time::Min ()) const;
 
   /**
    * Deaggregates an A-MPDU by removing the A-MPDU subframe header and padding.

src/wifi/model/msdu-aggregator.h

@@ -85,7 +85,8 @@ public:
    *
    * - the time to transmit the resulting PPDU, according to the given TxVector,
    * does not exceed both the maximum PPDU duration allowed by the corresponding
-   * modulation class (if any) and the given PPDU duration limit (if non null)
+   * modulation class (if any) and the given PPDU duration limit (if distinct from
+   * Time::Min ())
    *
    * If it is not possible to aggregate at least two MSDUs, no MSDU is dequeued
    * from the EDCA queue and a null pointer is returned.
@@ -99,7 +100,7 @@ public:
    */
   Ptr<WifiMacQueueItem> GetNextAmsdu (Mac48Address recipient, uint8_t tid,
                                       WifiTxVector txVector, uint32_t ampduSize = 0,
-                                      Time ppduDurationLimit = Seconds (0)) const;
+                                      Time ppduDurationLimit = Time::Min ()) const;
 
   /**
    * Determine the maximum size for an A-MSDU of the given TID that can be sent

src/wifi/model/qos-txop.cc

@@ -529,7 +529,7 @@ QosTxop::NotifyAccessGranted (void)
       else
         {
           // compute the limit on the PPDU duration due to the TXOP duration, if any
-          Time ppduDurationLimit = Seconds (0);
+          Time ppduDurationLimit = Time::Min ();
           if (peekedItem->GetHeader ().IsQosData () && GetTxopLimit ().IsStrictlyPositive ())
             {
               MacLowTransmissionParameters params = GetTransmissionParameters (peekedItem);

src/wifi/model/qos-txop.h

@@ -427,7 +427,7 @@ public:
    * If the peeked frame is a QoS Data frame, it is actually dequeued if it meets
    * the constraint on the maximum A-MPDU size (by assuming that the frame has to
    * be aggregated to an existing A-MPDU of the given size) and its transmission
-   * time does not exceed the given PPDU duration limit (if strictly positive).
+   * time does not exceed the given PPDU duration limit (if distinct from Time::Min ()).
    * If the peeked frame is a unicast QoS Data frame stored in the EDCA queue,
    * attempt to perform A-MSDU aggregation (while meeting the constraints mentioned
    * above) if <i>aggregate</i> is true and assign a sequence number to the
@@ -441,7 +441,7 @@ public:
    */
   Ptr<WifiMacQueueItem> DequeuePeekedFrame (Ptr<const WifiMacQueueItem> peekedItem, WifiTxVector txVector,
                                             bool aggregate = true, uint32_t ampduSize = 0,
-                                            Time ppduDurationLimit = Seconds (0));
+                                            Time ppduDurationLimit = Time::Min ());
   /**
    * Compute the MacLow transmission parameters for the given frame. Allowed frames
    * are those handled by a QosTxop (QoS data frames, BlockAckReq frames, ADDBA