# Network Emulation¶

## Motivation¶

In INET, the word emulation is used in a broad sense to describe a system which is partially implemented in the real world and partially in simulation. Emulation is often used for testing and validating a simulation model with its real-world counterparts, or in a reverse scenario, during the development of a real-world protocol implementation or application, for testing it in a simulated environment. It may also be used out of necessity, because some part of the system only exists in the real world or only as a simulation model.

INET provides modules that act as bridges between the simulated and real domains, therefore it is possible to leave one part of the simulation unchanged, while simply extracting the other into the real world. Several setups are possible when one can take advantage of the emulation capabilities of INET:

• simulated node in a real network

• a simulated subnet in real network

• real-world node in simulated network

• simulated protocol in a real network node

• real application in a simulated network node

• etc.

Some example scenarios:

• Run a simulated component, such as an app or a routing protocol, on nodes of an actual ad-hoc network. This setup would allow testing the component’s behavior under real-life conditions.

• Test the interoperability of a simulated protocol with its real-world counterparts.

• As a means of implementing hybrid simulation. The real network (or a single host OS) may contain several network emulator devices or simulations running in emulation mode. Such a setup provides a relatively easy way for connecting heterogenous simulators/emulators with each other, sparing the need for HLA or a custom interoperability solution.

## Overview¶

For the simulation to act as a network emulator, two major problems need to be solved. On one hand, the simulation must run in real time, or the real clock must be configured according to the simulation time (synchronization). On the other hand, the simulation must be able to communicate with the real world (communication). This is achieved in INET as the following:

• Synchronization:

• RealTimeScheduler: It is a socket-aware real-time scheduler class responsible for synchronization. Using this method, the simulation is run according to real time.

• Communication:

• The interface between the real (an interface of the OS) and the simulated parts of the model are represented by Ext modules, with names beginning with Ext~ prefix in the simulation (ExtLowerUdp, ExtUpperEthernetInterface, etc.).

Ext modules communicate both internally in the simulation and externally in the host OS. Packets sent to these modules in the simulation will be sent out on the host OS interface, and packets received by the host OS interface (or rather, the appropriate subset of them) will appear in the simulation as if received on an Ext~ module.

There are several possible ways to maintain the external communication:

• File

• Pipe

• Socket

• Shared memory

• TUN/TAP interfaces

• Message Passing Interface (MPI)

Note

It is probably needless to say, but the simulation must be fast enough to be able to keep up with real time. That is, its relative speed compared to real time (the simsec/sec value) must be >>1. (Under Qtenv, this can usually only be achieved in Express mode.)

## Preparation¶

There are a few things that need to be arranged before you can successfully run simulations in network emulation mode.

First, network emulation is a separate project feature that needs to be enabled before it can be used. (Project features can be reviewed and changed in the Project | Project Features… dialog in the IDE.)

Also, in order to be able to send packets through raw sockets applications require special permissions. There are two ways to achieve this under Linux.

The suggested solution is to use setcap to set the application permissions:

$sudo setcap cap_net_raw,cap_net_admin=eip /path/to/opp_run$ sudo setcap cap_net_raw,cap_net_admin=eip /path/to/opp_run_dbg
$sudo setcap cap_net_raw,cap_net_admin=eip /path/to/opp_run_release  This solution makes running the examples from the IDE possible. Alternatively, the application can be started with root privileges from command line: $ sudo inet_dbg -p -u Cmdenv


Note

In any case, it’s generally a bad idea to start the IDE as superuser. Doing so may silently change the file ownership for certain IDE configuration files, and it may prevent the IDE to start up for the normal user afterwards.

## Configuring¶

Here we show one configuration example where the network nodes contain a ExtLowerEthernetInterface.

INET nodes such as StandardHost and Router can be configured to have ExtLowerEthernetInterface’s. The simulation may contain several nodes with external interfaces, and one node may also have several external interfaces.

Note

This is one of the many possible setups. Using other components than ExtLowerEthernetInterface, nodes may be cut into simulated and real parts at any layer, and either the upper or the lower part may be real. See the Showcases for demonstration of some of these use cases.

A network node can be configured to have an external interface in the following way:

**.host1.numEthInterfaces = 1
**.host1.eth[0].typename = "ExtLowerEthernetInterface"


Also, the simulation must be configured to run under control the of the appropriate real-time scheduler class:

scheduler-class = "inet::RealTimeScheduler"


ExtLowerEthernetInterface has two important parameters which need to be configured. The device parameter should be set to the name of the real (or virtual) interface on the host OS. The namespace parameter can be set to utilize the network namespace functionality of Linux operating systems.

An example configuration:

**.numEthInterfaces = 1
**.eth[0].device = "veth0" # or "eth0" for example
**.eth[0].namespace = "host0" # optional
**.eth[0].mtu = 1500B


Let us examine the paths outgoing and incoming packets take, and the necessary configuration requirements to make them work. We assume IPv4 as network layer protocol, but the picture does not change much with other protocols. We assume the external interface is named eth[0].

### Outgoing path¶

The network layer of the simulated node routes datagrams to its eth[0] external interface.

For that to happen, the routing table needs to contain an entry where the interface is set to eth[0]. Such entries are not created automatically, one needs to add them to the routing table explicitly, e.g. by using an Ipv4NetworkConfigurator and an appropriate XML file.

Another point is that if the packet comes from a local app (and from another simulated node), it needs to have a source IP address assigned. There are two ways for that to happen. If the sending app specified a source IP address, that will be used. Otherwise, the IP address of the eth[0] interface will be used, but for that, the interface needs to have an IP address at all. The MAC and IP address of external interfaces are automatically copied between the real and simulated counterparts.

Once in eth[0], the datagram is serialized. Serialization is a built-in feature of INET packets. (Packets, or rather, packet chunks have multiple alternative representations, i.e. C++ object and serialized form, and conversion between them is transparent.)

The result of serialization is a byte array, which is written into a raw socket with a sendto system call.

The packet will then travel normally in the real network to the destination address.

### Incoming path¶

First of all, packets intended to be received by the simulation need to find their way to the correct interface of the host that runs the simulation. For that, IP addresses of simulated hosts must be routable in the real network, and routed to the selected interface of the host OS. (On Linux, for example, this can be achieved by adding static routes with the command.)

As packets are received by the interface of the host OS, they are handed over to the simulation. The packets are received from the raw socket with a recv system call. After deserialization they pop out of eth[0] and they are sent up to the network layer. The packets are routed to the simulated destination host in the normal way.