13.2. Netmap NetDevice

The fd-net-device module provides the NetmapNetDevice class, a class derived from the FdNetDevice which is able to read and write traffic using a netmap file descriptor. This netmap file descriptor must be associated to a real ethernet device in the host machine. The NetmapNetDeviceHelper class supports the configuration of a NetmapNetDevice.

netmap is a fast packet processing capability that bypasses the host networking stack and gains direct access to network device. netmap was developed by Luigi Rizzo [Rizzo2012] and is maintained as an open source project on GitHub at https://github.com/luigirizzo/netmap.

The NetmapNetDevice for ns-3 [Imputato2019] was developed by Pasquale Imputato in the 2017-19 timeframe. The use of NetmapNetDevice requires that the host system has netmap support (and for best performance, the drivers must support netmap and must be using a netmap-enabled device driver). Users can expect that emulation support using Netmap will support higher packets per second than emulation using FdNetDevice with raw sockets (which pass through the Linux networking kernel).

Rizzo2012

Luigi Rizzo, “netmap: A Novel Framework for Fast Packet I/O”, Proceedings of 2012 USENIX Annual Technical Conference, June 2012.

Imputato2019

Pasquale Imputato, Stefano Avallone, Enhancing the fidelity of network emulation through direct access to device buffers, Journal of Network and Computer Applications, Volume 130, 2019, Pages 63-75, (http://www.sciencedirect.com/science/article/pii/S1084804519300220)

13.2.1. Model Description

13.2.1.1. Design

Because netmap uses file descriptor based communication to interact with the real device, the straightforward approach to design a new NetDevice around netmap is to have it inherit from the existing FdNetDevice and implement a specialized version of the operations specific to netmap. The operations that require a specialized implementation are the initialization, because the NIC has to be put in netmap mode, and the read/write methods, which have to make use of the netmap API to coordinate the exchange of packets with the netmap rings.

In the initialization stage, the network device is switched to netmap mode, so that ns-3 is able to send/receive packets to/from the real network device by writing/reading them to/from the netmap rings. Following the design of the FdNetDevice, a separate reading thread is started during the initialization. The task of the reading thread is to wait for new incoming packets in the netmap receiver rings, in order to schedule the events of packet reception. In the initialization of the NetmapNetDevice, an additional thread, the sync thread, is started. The sync thread is required because, in order to reduce the cost of the system calls, netmap does not automatically transfer a packet written to a slot of the netmap ring to the transmission ring or to the installed qdisc. It is up to the user process to periodically request a synchronization of the netmap ring. Therefore, the purpose of the sync thread is to periodically make a TXSYNC ioctl request, so that pending packets in the netmap ring are transferred to the transmission ring, if in native mode, or to the installed qdisc, if in generic mode. Also, as described further below, the sync thread is exploited to perform flow control and notify the BQL library about the amount of bytes that have been transferred to the network device.

The read method is called by the reading thread to retrieve new incoming packets stored in the netmap receiver ring and pass them to the appropriate ns-3 protocol handler for further processing within the simulator’s network stack. After retrieving packets, the reading thread also synchronizes the netmap receiver ring, so that the retrieved packets can be removed from the netmap receiver ring.

The NetmapNetDevice also specializes the write method, i.e., the method used to transmit a packet received from the upper layer (the ns-3 traffic control layer). The write method uses the netmap API to write the packet to a free slot in the netmap transmission ring. After writing a packet, the write method checks whether there is enough room in the netmap transmission ring for another packet. If not, the NetmapNetDevice stops its queue so that the ns-3 traffic control layer does not attempt to send a packet that could not be stored in the netmap transmission ring.

A stopped NetmapNetDevice queue needs to be restarted as soon as some room is made in the netmap transmission ring. The sync thread can be exploited for this purpose, given that it periodically synchronizes the netmap transmission ring. In particular, the sync thread also checks the number of free slots in the netmap transmission ring in case the NetmapNetDevice queue is stopped. If the number of free slots exceeds a configurable value, the sync thread restarts the NetmapNetDevice queue and wakes the associated ns-3 qdisc. The NetmapNetDevice also supports BQL: the write method notifies the BQL library of the amount of bytes that have been written to the netmap transmission ring, while the sync thread notifies the BQL library of the amount of bytes that have been removed from the netmap transmission ring and transferred to the NIC since the previous notification.

13.2.1.2. Scope and Limitations

The main scope of NetmapNetDevice is to support the flow-control between the physical device and the upper layer and using at best the computational resources to process packets. However, the (Linux) system and network device must support netmap to make use of this feature.

13.2.2. Usage

The installation of netmap itself on a host machine is out of scope for this document. Refer to the netmap GitHub README for instructions.

The ns-3 netmap code has only been tested on Linux; it is not clear whether other operating systems can be supported.

If ns-3 is able to detect the presence of netmap on the system, it will report that:

Netmap emulation FdNetDevice  : not enabled

If not, it will report:

Netmap emulation FdNetDevice  : not enabled (needs net/netmap_user.h)

To run FdNetDevice-enabled simulations, one must pass the --enable-sudo option to ./ns3 configure, or else run the simulations with root privileges.

13.2.2.1. Helpers

ns-3 netmap support uses a NetMapNetDeviceHelper helper object to install the NetmapNetDevice. In other respects, the API and use is similar to that of the EmuFdNetDeviceHelper.

13.2.2.2. Attributes

There is one attribute specialized to NetmapNetDevice, named SyncAndNotifyQueuePeriod. This value takes an integer number of microseconds, and is used as the period of time after which the device syncs the netmap ring and notifies queue status. The value should be close to the interrupt coalescence period of the real device. Users may want to tune this parameter for their own system; it should be a compromise between CPU usage and accuracy in the ring sync (if it is too high, the device goes into starvation and lower throughput occurs).

13.2.2.3. Output

The NetmapNetDevice does not provide any specialized output, but supports the FdNetDevice output and traces (such as a promiscuous sniffer trace).

13.2.2.4. Examples

Several examples are provided:

• fd-emu-onoff.cc: This example is aimed at measuring the throughput of the NetmapNetDevice when using the NetmapNetDeviceHelper to attach the simulated device to a real device in the host machine. This is achieved by saturating the channel with TCP or UDP traffic.

• fd-emu-ping.cc: This example uses the NetmapNetDevice to send ICMP traffic over a real device.

• fd-emu-tc.cc: This example configures a router on a machine with two

  interfaces in emulated mode through netmap. The aim is to explore different qdiscs behaviours on the backlog of a device emulated bottleneck side.

• fd-emu-send.cc: This example builds a node with a device in emulation mode through netmap. The aim is to measure the maximum transmit rate in packets per second (pps) achievable with NetmapNetDevice on a specific machine.

Note that all the examples run in emulation mode through netmap (with NetmapNetDevice) and raw socket (with FdNetDevice).