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IP networking

IP Networking in Mbed OS is layered in three clearly defined API levels. The diagram below shows the layers next to the closest matching OSI model layers.

IP networking

The following sections introduce the APIs and technologies implemented in each level.

Socket API

The Socket API is the common API among all IP connectivity methods. All network stacks in Mbed OS provide the same Socket API, making applications portable among different connectivity methods or even stacks.

In the OSI model, the Socket API relates to layer 4, the Transport layer. In Mbed OS, the Socket API supports both TCP and UDP protocols.

Refer to Socket API reference for usage instructions.

IP stacks

Mbed OS has three options to select for the IP stack. The connectivity modules provides two built-in IP stacks or an external IP stack.

Network stack

As the diagram above shows, all stacks implement the same Socket API. Therefore, the application developer rarely needs to know which stack is going to be used. Mbed OS chooses one at the build time, and usually it is LwIP.

Some external Wi-Fi modules and most cellular modules are in fact external IP stacks from the application point of view. In that case, the network driver implements the full Network stack API. These drivers usually drive the module through an AT-command type of interface. Using an external IP module saves RAM and Flash, but depending on the driver and AT-command interface, it might not properly match the full Socket API.

The following table summarizes different stacks, use cases and their limitations.

Stack Network protocols supported Use cases Limitations
LwIP IPv4, IPv6, PPP Ethernet, Wi-Fi, 2G/3G/4G Cellular 4 sockets, 1 interface, no routing
Nanostack IPv6, 6LoWPAN, Thread Mesh networking, Border Router Only IPv6
External IP module Depends on the module (Save RAM/Flash) Depends on the module. Usually poor match to Socket API

Configuring the IP stack interface

Depending on the Layer 3, Network layer, protocol used, there are different ways to configure the interface. It also depends on the stack used, which configurations are supported on each link layer.

Stack Data link layer Network layer Configurations supported
LwIP Ethernet, WiFi IPv4 DHCP, static
LwIP Ethernet, WiFi IPv6 RFC 4862 IPv6 Stateless Address Autoconfiguration. No DHCPv6 support
LwIP PPP IPv4,IPv6 automatic
Nanostack Ethernet IPv6 static or RFC 4862 IPv6 Stateless Address Autoconfiguration. No DHCPv6 support
Nanostack IEEE 802.15.4 6LoWPAN Thread or 6LoWPAN-ND+RPL

Network interfaces

Network interfaces are the application level APIs where users choose the driver, connectivity method and IP stack. Each connectivity methods requires different configuration, so these APIs are not interchangeable. The application developer must choose one. Choosing the interface also automatically pulls in the network stack as a dependency.

Please note that the interface API is not the same as the network driver. The interface API is the control interface for the application. The network driver implements the controlling API only if it requires configuration from application. From the application point of view, there is no difference, but the network driver developer needs to be aware of that.

Mbed OS implements the following network interface APIs:

  • Ethernet.
  • WiFi.
  • Cellular (PPP).
  • 6LoWPAN-ND mesh networking.
  • Thread mesh networking.

Refer to Network Interface API reference for usage instructions.

Network drivers

"Network driver" describes different APIs that connect a networking device to the IP stack or Socket API. Below is a description of each driver API.

Ethernet driver

Emac API

Ethernet drivers are implemented using the stack-independent EMAC API. Because the Ethernet driver requires no configuration, it does not implement any controlling interface for the application.

Wi-Fi driver

Wi-Fi driver

There are two types of Wi-Fi drivers in Mbed OS, depending on which protocol layer it implements. Wi-Fi drivers are either a special case of Ethernet driver or external IP stacks. Wi-Fi drivers require configuration from an application and, therefore, implement both the low level EMAC API or Network stack API and the high level controlling interface API called WiFiInterface.

Cellular modem driver

Cellular driver

Cellular drivers have the same two separate cases as Wi-Fi. If they use an external IP stack, the driver implements the Network stack API. If they use the internal IP stack, LwIP, then they implement the Serial PPP driver.

6LoWPAN/Thread RF driver

RF driver

On Mesh networks, Nanostack uses IEEE 802.15.4 radios for transmitting and receiving packets. The RF driver implements the NanostackRfPhy API.

This driver type has no other use cases, so it is implemented in C using a Nanostack-specific API.

Please see the 6LoWPAN Mesh technology section and Porting a new RF driver for the 6LoWPAN stack for more information.

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