# Network Nodes¶

## Overview¶

Hosts, routers, switches, access points, mobile phones, and other network nodes are represented in INET with compound modules. The previous chapter has introduced a few node types like StandardHost, Router, and showed how to put together networks from them. In this chapter, we look at the internals of such node models, in order to provide a deeper understanding of their customization possibilities and to give some guidance on how custom nodes models can be assembled.

## Ingredients¶

Node models are assembled from other modules which represent applications, communication protocols, network interfaces, routing tables, mobility models, energy models, and other functionality. These modules fall into the following broad categories:

• Applications often model the user behavior as well as the application program (e.g., browser), and the application layer protocol (e.g., HTTP). Applications typically use transport layer protocols (e.g., TCP and/or UDP), but they may also directly use lower layer protocols (e.g., IP or Ethernet) via sockets.
• Routing protocols are provided as separate modules: OSPF, BGP, or AODV for MANET routing. These modules use TCP, UDP, and IPv4, and manipulate routes in the Ipv4RoutingTable module.
• Transport layer protocols are connected to applications and network layer protocols. They are most often represented by simple modules, currently TCP, UDP, and SCTP are supported. TCP has several implementations: Tcp is the OMNeT++ native implementation; TcpLwip module wraps the lwIP TCP stack; and TcpNsc module wraps the Network Simulation Cradle library.
• Network layer protocols are connected to transport layer protocols and network interfaces. They are usually modeled as compound modules: Ipv4NetworkLayer for IPv4, and Ipv6NetworkLayer for IPv6. The Ipv4NetworkLayer module contains several protocol modules: Ipv4, Arp, Icmp and Icmpv6.
• Network interfaces are represented by compound modules which are connected to the network layer protocols and other network interfaces in the wired case. They are often modeled as compound modules containing separate modules for queues, classifiers, MAC, and PHY protocols.
• Link layer protocols are usually simple modules sitting in network interface modules. Some protocols, for example IEEE 802.11 MAC, are modeled as a compound module themselves due to the complexity of the protocol.
• Physical layer protocols are compound modules also being part of network interface modules.
• Interface table maintains the set of network interfaces (e.g. eth0, wlan0) in the network node. Interfaces are registered dynamically during initialization of network interfaces.
• Routing tables maintain the list of routes for the corresponding network protocol (e.g., Ipv4RoutingTable for Ipv4). Routes are added by automatic network configurators or routing protocols. Network protocols use the routing tables to find out the best matching route for datagrams.
• Mobility modules are responsible for moving around the network node in the simulated scene. The mobility model is mandatory for wireless simulations even if the network node is stationary. The mobility module stores the location of the network node which is needed to compute wireless propagation and path loss. Different mobility models are provided as different modules. Network nodes define their mobility submodule with a parametric type, so the mobility model can be changed in the configuration.
• Energy modules model energy storage mechanisms, energy consumption of devices and software processes, energy generation of devices, and energy management processes which shutdown and startup network nodes.
• Status (NodeStatus) keeps track of the status of the network node (up, down, etc.)
• Other modules with particular functionality such as PcapRecorder are also available.

## Node Architecture¶

Within network nodes, OMNeT++ connections are used to represent communication opportunities between protocols. Packets and messages sent on these connections represent software or hardware activity.

Although protocols may also be connected to each other directly, in most cases they are connected via dispatcher modules. Dispatchers (MessageDispatcher) are small, low-overhead modules that allow protocol components to be connected in one-to-many and many-to-many fashion, and ensure that messages and packets sent from one component end up being delivered to the correct component. Dispatchers need no manual configuration, as they use discovery and peek into packets.

In there pre-assembled node models, dispatchers allow arbitrary protocol components to talk directly to each other, i.e. not only to ones in neighboring layers.

## Customizing Nodes¶

The built-in network nodes are written to be as versatile and customizable as possible. This is achieved in several ways:

### Submodule and Gate Vectors¶

One way is the use of gate vectors and submodule vectors. The sizes of vectors may come from parameters or derived by the number of external connections to the network node. For example, a host may have an arbitrary number of wireless interfaces, and it will automatically have as many Ethernet interfaces as the number of Ethernet devices connected to it.

For example, wireless interfaces for hosts are defined like this:

wlan[numWlanInterfaces]: <snip> // wlan interfaces in StandardHost etc al.


Where :ini:numWlanInterfaces is a module parameter that defaults to either 0 or 1 (this is different for e.g. StandardHost and WirelessHost.) To configure a host to have two interfaces, add the following line to the ini file:

**.hostA.numWlanInterfaces = 2


### Conditional Submodules¶

Submodules that are not vectors are often conditional. For example, the TCP protocol module in hosts is conditional on the hasTcp parameter. Thus, to disable TCP support in a host (it is enabled by default), use the following ini file line:

**.hostA.hasTcp = false


### Parametric Types¶

Another often used way of customization is parametric types, that is, the type of a submodule (or a channel) may be specified as a string parameter. Almost all submodules in the built-in node types have parametric types. For example, the TCP protocol module is defined like this:

tcp: <tcpType> like ITcp if hasTcp;


The tcpType parameter defaults to the default implementation, Tcp. To use another implementation instead, add the following line to the ini file:

**.host*.tcpType = "TcpLwip"  # use lwIP's TCP implementation


Submodule vectors with parametric types are defined without the use of a module parameter to allow elements have different types. An example is how applications are defined in hosts:

app[numApps]: <> like IApp;  // applications in StandardHost et al.


And applications can be added in the following way:

**.hostA.numApps = 2
**.hostA.apps[0].typename = "UdpBasicApp"
**.hostA.apps[1].typename = "PingApp"


### Inheritance¶

Inheritance can be use to derive new, specialized node types from existing ones. A derived NED type may add new parameters, gates, submodules or connections, and may set inherited unassigned parameters to specific values.

For example, WirelessHost is derived from StandardHost in the following way:

module WirelessHost extends StandardHost
{
@display("i=device/wifilaptop");
numWlanInterfaces = default(1);
}


## Custom Network Nodes¶

Despite the many pre-assembled network nodes and the several available customization options, sometimes it is just easier to build a network node from scratch. The following example shows how easy it is to build a simple network node.

This network node already contains a configurable application and several standard protocols. It also demonstrates how to use the packet dispatching mechanism which is required to connect multiple protocols in a many-to-many relationship.

module NetworkNodeExample
{
parameters:
gates:
inout ethg; // ethernet interface connector
submodules:
app: <> like IApp; // configurable application
tcp: Tcp; // standard TCP protocol
ip: Ipv4; // standard IP protocol
md: MessageDispatcher; // connects multiple interfaces to IP
wlan: Ieee80211Interface; // standard wifi interface
eth: EthernetInterface; // standard ethernet interface
interfaceTable: InterfaceTable;
connections: // network node internal connections
app.socketOut --> tcp.appIn; // application sends data stream
app.socketIn <-- tcp.appOut; // application receives data stream
tcp.ipOut --> ip.transportIn; // TCP sends segments
tcp.ipIn <-- ip.transportOut; // TCP receives segments
ip.queueOut --> md.in++; // IP sends datagrams
ip.queueIn <-- md.out++; // IP receives datagrams
md.out++ --> wlan.upperLayerIn;
md.in++ <-- wlan.upperLayerOut;
md.out++ --> eth.upperLayerIn;
md.in++ <-- eth.upperLayerOut;
eth.phys <--> ethg; // Ethernet sends frames to cable