Fixed an issue where the Port and REDPort does not drop packets prope…

…rly when downstream nodes have zero buffer. (#16)

Co-authored-by: Baochun Li <[email protected]>

examples/fair_packet_switch.py

@@ -0,0 +1,82 @@
+"""
+An very simple example of using the test the packet drop in FairPacketSwitch.
+It shows a bug in packet dropping process.
+"""
+import simpy
+from ns.packet.dist_generator import DistPacketGenerator
+from ns.packet.sink import PacketSink
+from ns.switch.switch import FairPacketSwitch
+
+
+def packet_arrival():
+    """Constant packet arrival interval."""
+    return 1.0
+
+
+def const_size():
+    """Constant packet size in bytes."""
+    return 1000.0
+
+
+env = simpy.Environment()
+pg1 = DistPacketGenerator(
+    env,
+    "flow_0",
+    packet_arrival,
+    const_size,
+    initial_delay=0.0,
+    finish=3,
+    flow_id=0,
+    rec_flow=True,
+)
+pg2 = DistPacketGenerator(
+    env,
+    "flow_1",
+    packet_arrival,
+    const_size,
+    initial_delay=0.1,
+    finish=3,
+    flow_id=1,
+    rec_flow=True,
+)
+ps = PacketSink(env)
+
+port_rate = 8000.1  # in bits
+buffer_size = 1100  # in bytes
+
+switch = FairPacketSwitch(
+    env,
+    nports=1,
+    port_rate=port_rate,
+    buffer_size=buffer_size,
+    weights=[1, 2],
+    server="DRR",
+    debug=True,
+)
+switch.egress_ports[0].limit_bytes = True
+pg1.out = switch
+pg2.out = switch
+switch.demux.fib = {0: 0, 1: 0}
+switch.ports[0].out = ps
+
+env.run()
+
+print("\n==========Basic Info==========")
+
+print(
+    f"The buffer size is {buffer_size} bytes, the port rate is {port_rate / 8} bytes/sec, "
+    f"and the packet size is {const_size()} bytes."
+)
+
+print("==========Result==========")
+print("For the switch, the packet arrival times for flow 0 are:")
+print(pg1.time_rec)
+
+print("For the switch, the packet arrival times for flow 1 are:")
+print(pg2.time_rec)
+
+print("At the packet sink, packet arrival times for flow 0 are:")
+print(ps.arrivals[0])
+
+print("At the packet sink, packet arrival times for flow 1 are:")
+print(ps.arrivals[1])

ns/port/port.py

@@ -30,6 +30,7 @@ class Port:
         debug: bool
             If True, prints more verbose debug information.
     """
+
     def __init__(self,
                  env,
                  rate: float,
@@ -66,8 +67,9 @@ def update(self, packet):
         """
         # There is nothing that needs to be done, just print a debug message
         if self.debug:
-            print(f"Retrieved Packet {packet.packet_id} from flow {packet.flow_id}.")
-
+            print(
+                f"Retrieved Packet {packet.packet_id} from flow {packet.flow_id}."
+            )
 
     def run(self):
         """The generator function used in simulations."""
@@ -82,8 +84,7 @@ def run(self):
 
             if self.rate > 0:
                 yield self.env.timeout(packet.size * 8.0 / self.rate)
-
-            self.byte_size -= packet.size
+                self.byte_size -= packet.size
 
             if self.zero_downstream_buffer:
                 self.out.put(packet,
@@ -98,6 +99,13 @@ def run(self):
     def put(self, packet):
         """ Sends a packet to this element. """
         self.packets_received += 1
+
+        if self.zero_downstream_buffer:
+            # If the downstream node has no buffer, packets will be removed
+            # from this buffer by the downstream node, and the byte size of the
+            # buffer should be recomputed
+            self.byte_size = sum(packet.size for packet in self.store.items)
+
         byte_count = self.byte_size + packet.size
 
         if self.element_id is not None:

ns/port/red_port.py

@@ -61,6 +61,7 @@ class REDPort(Port):
         debug: bool
             If True, prints more verbose debug information.
     """
+
     def __init__(self,
                  env,
                  rate: float,
@@ -91,6 +92,12 @@ def put(self, packet):
         """ Sends a packet to this element. """
         self.packets_received += 1
 
+        if self.zero_downstream_buffer:
+            # If the downstream node has no buffer, packets will be removed
+            # from this buffer by the downstream node, and the byte size of the
+            # buffer should be recomputed
+            self.byte_size = sum(packet.size for packet in self.store.items)
+
         if self.limit_bytes:
             current_queue_size = self.byte_size
         else: