File Descriptor NetDevice

The src/fd-net-device module provides the FdNetDevice class, which is able to read and write traffic using a file descriptor provided by the user. This file descriptor can be associated to a TAP device, to a raw socket, to a user space process generating/consuming traffic, etc. The user has full freedom to define how external traffic is generated and ns-3 traffic is consumed.

Different mechanisms to associate a simulation to external traffic can be provided through helper classes. Two specific helpers are provided:

  • EmuFdNetDeviceHelper (to associate the ns-3 device with a physical device in the host machine)
  • TapFdNetDeviceHelper (to associate the ns-3 device with the file descriptor from a tap device in the host machine)

Model Description

The source code for this module lives in the directory src/fd-net-device.

The FdNetDevice is a special type of ns-3 NetDevice that reads traffic to and from a file descriptor. That is, unlike pure simulation NetDevice objects that write frames to and from a simulated channel, this FdNetDevice directs frames out of the simulation to a file descriptor. The file descriptor may be associated to a Linux TUN/TAP device, to a socket, or to a user-space process.

It is up to the user of this device to provide a file descriptor. The type of file descriptor being provided determines what is being modelled. For instance, if the file descriptor provides a raw socket to a WiFi card on the host machine, the device being modelled is a WiFi device.

From the conceptual “top” of the device looking down, it looks to the simulated node like a device supporting a 48-bit IEEE MAC address that can be bridged, supports broadcast, and uses IPv4 ARP or IPv6 Neighbor Discovery, although these attributes can be tuned on a per-use-case basis.


The FdNetDevice implementation makes use of a reader object, extended from the FdReader class in the ns-3 src/core module, which manages a separate thread from the main ns-3 execution thread, in order to read traffic from the file descriptor.

Upon invocation of the StartDevice method, the reader object is initialized and starts the reading thread. Before device start, a file descriptor must be previously associated to the FdNetDevice with the SetFileDescriptor invocation.

The creation and configuration of the file descriptor can be left to a number of helpers, described in more detail below. When this is done, the invocation of SetFileDescriptor is responsibility of the helper and must not be directly invoked by the user.

Upon reading an incoming frame from the file descriptor, the reader will pass the frame to the ReceiveCallback method, whose task it is to schedule the reception of the frame by the device as a ns-3 simulation event. Since the new frame is passed from the reader thread to the main ns-3 simulation thread, thread-safety issues are avoided by using the ScheduleWithContext call instead of the regular Schedule call.

In order to avoid overwhelming the scheduler when the incoming data rate is too high, a counter is kept with the number of frames that are currently scheduled to be received by the device. If this counter reaches the value given by the RxQueueSize attribute in the device, then the new frame will be dropped silently.

The actual reception of the new frame by the device occurs when the scheduled FordwarUp method is invoked by the simulator. This method acts as if a new frame had arrived from a channel attached to the device. The device then decapsulates the frame, removing any layer 2 headers, and forwards it to upper network stack layers of the node. The ForwardUp method will remove the frame headers, according to the frame encapsulation type defined by the EncapsulationMode attribute, and invoke the receive callback passing an IP packet.

An extra header, the PI header, can be present when the file descriptor is associated to a TAP device that was created without setting the IFF_NO_PI flag. This extra header is removed if EncapsulationMode is set to DIXPI value.

In the opposite direction, packets generated inside the simulation that are sent out through the device, will be passed to the Send method, which will in turn invoke the SendFrom method. The latter method will add the necessary layer 2 headers, and simply write the newly created frame to the file descriptor.

Scope and Limitations

Users of this device are cautioned that there is no flow control across the file descriptor boundary, when using in emulation mode. That is, in a Linux system, if the speed of writing network packets exceeds the ability of the underlying physical device to buffer the packets, backpressure up to the writing application will be applied to avoid local packet loss. No such flow control is provided across the file descriptor interface, so users must be aware of this limitation.

As explained before, the RxQueueSize attribute limits the number of packets that can be pending to be received by the device. Frames read from the file descriptor while the number of pending packets is in its maximum will be silently dropped.

The mtu of the device defaults to the Ethernet II MTU value. However, helpers are supposed to set the mtu to the right value to reflect the characteristics of the network interface associated to the file descriptor. If no helper is used, then the responsibility of setting the correct mtu value for the device falls back to the user. The size of the read buffer on the file descriptor reader is set to the mtu value in the StartDevice method.

The FdNetDevice class currently supports three encapsulation modes, DIX for Ethernet II frames, LLC for 802.2 LLC/SNAP frames, and DIXPI for Ethernet II frames with an additional TAP PI header. This means that traffic traversing the file descriptor is expected to be Ethernet II compatible. IEEE 802.1q (VLAN) tagging is not supported. Attaching an FdNetDevice to a wireless interface is possible as long as the driver provides Ethernet II frames to the socket API. Note that to associate a FdNetDevice to a wireless card in ad-hoc mode, the MAC address of the device must be set to the real card MAC address, else any incoming traffic a fake MAC address will be discarded by the driver.

As mentioned before, three helpers are provided with the fd-net-device module. Each individual helper (file descriptor type) may have platform limitations. For instance, threading, real-time simulation mode, and the ability to create TUN/TAP devices are prerequisites to using the provided helpers. Support for these modes can be found in the output of the ns3 configure step, e.g.:

Threading Primitives          : enabled
Real Time Simulator           : enabled
Emulated Net Device           : enabled
Tap Bridge                    : enabled

It is important to mention that while testing the FdNetDevice we have found an upper bound limit for TCP throughput when using 1Gb Ethernet links of 60Mbps. This limit is most likely due to the processing power of the computers involved in the tests.


The usage pattern for this type of device is similar to other net devices with helpers that install to node pointers or node containers. When using the base FdNetDeviceHelper the user is responsible for creating and setting the file descriptor by himself.

FdNetDeviceHelper fd;
NetDeviceContainer devices = fd.Install(nodes);

// file descriptor generation


Most commonly a FdNetDevice will be used to interact with the host system. In these cases it is almost certain that the user will want to run in real-time emulation mode, and to enable checksum computations. The typical program statements are as follows:

GlobalValue::Bind("SimulatorImplementationType", StringValue("ns3::RealtimeSimulatorImpl"));
GlobalValue::Bind("ChecksumEnabled", BooleanValue(true));

The easiest way to set up an experiment that interacts with a Linux host system is to user the Emu and Tap helpers. Perhaps the most unusual part of these helper implementations relates to the requirement for executing some of the code with super-user permissions. Rather than force the user to execute the entire simulation as root, we provide a small “creator” program that runs as root and does any required high-permission sockets work. The easiest way to set the right privileges for the “creator” programs, is by enabling the --enable-sudo flag when performing ns3 configure.

We do a similar thing for both the Emu and the Tap devices. The high-level view is that the CreateFileDescriptor method creates a local interprocess (Unix) socket, forks, and executes the small creation program. The small program, which runs as suid root, creates a raw socket and sends back the raw socket file descriptor over the Unix socket that is passed to it as a parameter. The raw socket is passed as a control message (sometimes called ancillary data) of type SCM_RIGHTS.



The EmuFdNetDeviceHelper creates a raw socket to an underlying physical device, and provides the socket descriptor to the FdNetDevice. This allows the ns-3 simulation to read frames from and write frames to a network device on the host.

The emulation helper permits to transparently integrate a simulated ns-3 node into a network composed of real nodes.

+----------------------+     +-----------------------+
|         host 1       |     |         host 2        |
+----------------------+     +-----------------------+
|    ns-3 simulation   |     |                       |
+----------------------+     |         Linux         |
|       ns-3 Node      |     |     Network Stack     |
|  +----------------+  |     |   +----------------+  |
|  |    ns-3 TCP    |  |     |   |       TCP      |  |
|  +----------------+  |     |   +----------------+  |
|  |    ns-3 IP     |  |     |   |       IP       |  |
|  +----------------+  |     |   +----------------+  |
|  |   FdNetDevice  |  |     |   |                |  |
|  |    |  |     |   |                |  |
|  +----------------+  |     |   +    ETHERNET    +  |
|  |   raw socket   |  |     |   |                |  |
|--+----------------+--|     |   +----------------+  |
|       | eth0 |       |     |        | eth0 |       |
+-------+------+-------+     +--------+------+-------+           

            |                            |

This helper replaces the functionality of the EmuNetDevice found in ns-3 prior to ns-3.17, by bringing this type of device into the common framework of the FdNetDevice. The EmuNetDevice was deprecated in favor of this new helper.

The device is configured to perform MAC spoofing to separate simulation network traffic from other network traffic that may be flowing to and from the host.

One can use this helper in a testbed situation where the host on which the simulation is running has a specific interface of interest which drives the testbed hardware. You would also need to set this specific interface into promiscuous mode and provide an appropriate device name to the ns-3 simulation. Additionally, hardware offloading of segmentation and checksums should be disabled.

The helper only works if the underlying interface is up and in promiscuous mode. Packets will be sent out over the device, but we use MAC spoofing. The MAC addresses will be generated (by default) using the Organizationally Unique Identifier (OUI) 00:00:00 as a base. This vendor code is not assigned to any organization and so should not conflict with any real hardware.

It is always up to the user to determine that using these MAC addresses is okay on your network and won’t conflict with anything else (including another simulation using such devices) on your network. If you are using the emulated FdNetDevice configuration in separate simulations, you must consider global MAC address assignment issues and ensure that MAC addresses are unique across all simulations. The emulated net device respects the MAC address provided in the Address attribute so you can do this manually. For larger simulations, you may want to set the OUI in the MAC address allocation function.

Before invoking the Install method, the correct device name must be configured on the helper using the SetDeviceName method. The device name is required to identify which physical device should be used to open the raw socket.

EmuFdNetDeviceHelper emu;
NetDeviceContainer devices = emu.Install(node);
Ptr<NetDevice> device = devices.Get(0);
device->SetAttribute("Address", Mac48AddressValue(Mac48Address::Allocate()));


A Tap device is a special type of Linux device for which one end of the device appears to the kernel as a virtual net_device, and the other end is provided as a file descriptor to user-space. This file descriptor can be passed to the FdNetDevice. Packets forwarded to the TAP device by the kernel will show up in the FdNetDevice in ns-3.

Users should note that this usage of TAP devices is different than that provided by the TapBridge NetDevice found in src/tap-bridge. The model in this helper is as follows:

|                host                 |
|    ns-3 simulation   |              |
+----------------------+              |
|      ns-3 Node       |              |
|  +----------------+  |              |
|  |    ns-3 TCP    |  |              |
|  +----------------+  |              |
|  |    ns-3 IP     |  |              |
|  +----------------+  |              |
|  |   FdNetDevice  |  |              |
|--+----------------+--+    +------+  |
|       | TAP  |            | eth0 |  |
|       +------+            +------+  |
|               |     |
                                ------------ (Internet) -----

In the above, the configuration requires that the host be able to forward traffic generated by the simulation to the Internet.

The model in TapBridge (in another module) is as follows:

|  Linux |
|  host  |                    +----------+
| ------ |                    |   ghost  |
|  apps  |                    |   node   |
| ------ |                    | -------- |
|  stack |                    |    IP    |     +----------+
| ------ |                    |   stack  |     |   node   |
|  TAP   |                    |==========|     | -------- |
| device | <----- IPC ------> |   tap    |     |    IP    |
+--------+                    |  bridge  |     |   stack  |
                              | -------- |     | -------- |
                              |   ns-3   |     |   ns-3   |
                              |   net    |     |   net    |
                              |  device  |     |  device  |
                              +----------+     +----------+
                                   ||               ||
                              |        ns-3 channel       |

In the above, packets instead traverse ns-3 NetDevices and Channels.

The usage pattern for this example is that the user sets the MAC address and either (or both) the IPv4 and IPv6 addresses and masks on the device, and the PI header if needed. For example:

TapFdNetDeviceHelper helper;


The FdNetDevice provides a number of attributes:

  • Address: The MAC address of the device
  • Start: The simulation start time to spin up the device thread
  • Stop: The simulation start time to stop the device thread
  • EncapsulationMode: Link-layer encapsulation format
  • RxQueueSize: The buffer size of the read queue on the file descriptor
    thread (default of 1000 packets)

Start and Stop do not normally need to be specified unless the user wants to limit the time during which this device is active. Address needs to be set to some kind of unique MAC address if the simulation will be interacting with other real devices somehow using real MAC addresses. Typical code:

device->SetAttribute("Address", Mac48AddressValue(Mac48Address::Allocate()));


Ascii and PCAP tracing is provided similar to the other ns-3 NetDevice types, through the helpers, such as (e.g.):

EmuFdNetDeviceHelper emu; NetDeviceContainer devices = emu.Install(node); … emu.EnablePcap(“emu-ping”, device, true);

The standard set of Mac-level NetDevice trace sources is provided.

  • MaxTx: Trace source triggered when ns-3 provides the device with a new frame to send
  • MaxTxDrop: Trace source if write to file descriptor fails
  • MaxPromiscRx: Whenever any valid Mac frame is received
  • MaxRx: Whenever a valid Mac frame is received for this device
  • Sniffer: Non-promiscuous packet sniffer
  • PromiscSniffer: Promiscuous packet sniffer (for tcpdump-like traces)


Several examples are provided:

  • This simple example creates two nodes and interconnects them with a Unix pipe by passing the file descriptors from the socketpair into the FdNetDevice objects of the respective nodes.
  • Same as but uses the real time simulator implementnation instead of the default one.
  • This example is aimed at measuring the throughput of the FdNetDevice in a pure simulation. For this purpose two FdNetDevices, attached to different nodes but in a same simulation, are connected using a socket pair. TCP traffic is sent at a saturating data rate.
  • This example is aimed at measuring the throughput of the FdNetDevice when using the EmuFdNetDeviceHelper to attach the simulated device to a real device in the host machine. This is achieved by saturating the channel with TCP traffic.
  • This example uses the EmuFdNetDeviceHelper to send ICMP traffic over a real channel.
  • This example uses the EmuFdNetDeviceHelper to send UDP traffic over a real channel.
  • This example uses the TapFdNetDeviceHelper to send ICMP traffic over a real channel.