Network Address - Addressing

Introduction

LwIP network stack includes built-in support for managing network interfaces and assigning them IP addresses. A network address refers to the IP address assigned to a network interface (netif), which can be configured statically, dynamically via DHCP, or automatically via AUTOIP. A netif represents a physical or logical network interface, such as an Wi-Fi, Ethernet or PPP connection.

## Adding the netif interfaces

To create a new network interface, the user should allocate space for a new struct netif and call netif_add(). The user specifies an IP address for the interface, its net mask, and gateway address. These can be changed later regardless of the assigning method.

netif_add(struct netif *netif,
  const ip4_addr_t *ipaddr, const ip4_addr_t *netmask, const ip4_addr_t *gw,
  void *state, netif_init_fn init, netif_input_fn input)

Address assignment method

Static IP addresses

Static IP address does not rely on any servers to assign the device its IP address. Users that choose this method should know their network topology to make sure no duplicate IP addresses exist in the network. After adding the netif interface, and before bringing the interface up, the IP address should be configured. This can be done by calling netif_set_addr() or netif_set_ipaddr(), or by configuring it during the netif_add(). To bring the interface up, the user should call netif_set_up(). To bring the interface down, call netif_set_down().

DHCP

DHCP is commonly used to get an IP address dynamically from a DHCP server. Every Access Point has the ability to assign IP addresses from a pool. The main advantage over a static assignment is that the maintenance is done by the Access Point so the network designer only needs to allocate the pools and not worry of duplicate IP addresses (although most of the clients also use gratuitous ARP to check that an assigned IP address is not in use). To start DHCP, the user should call dhcp_start() and to stop DHCP, the user should call dhcp_stop(). Unlike a static IP address, a dynamic address cannot be assigned if a connection is not made. In case of Wi-Fi, a connection to the Access Point needs to be made first during the WLAN_EVENT_CONNECT event. The recommended way to start the DHCP is by setting a callback to when the link is changing via the netif_set_link_callback(). This callback can be set during adding the netif interface. This callback would get triggered when the link becomes up via invoking netif_set_link_up() (called during WLAN_EVENT_CONNECT event). Same applies for the other direction where the IP address is released. To have DHCP support with LwIP, the user need to enable the LWIP_DHCP in lwipopt.h header file.

Automatic IP

Automatic IP is also referred to as Automatic Private IP Addressing and serves a different purpose than DHCP does (although in both, an IP address is assigned dynamically). Automatic IP is used when no DHCP server is available. The device is then self-assigned an IP in the range 169.254.x.x. It then uses gratuitous ARP to check for conflicts before the assignment is finalized. To start Automatic IP, the user should call autoip_start() and to stop, autoip_stop(). A good practice is to use a fallback mechanism from DHCP to AutoIP. To have Automatic IP support with LwIP, the user need to enable the LWIP_AUTOIP in lwipopt.h header file.

Addressing used in SDK examples

There are two examples in the SDK, CC35xx_network_terminal and CC35xx_MQTT_client. For both examples, DHCP addressing is used. In CC35xx_MQTT_client example, WIFI_IF_init() is used to create a netif interface and set a callback, cb_addInterfaceCompleteInd(), where the interface callback (cb_interfaceStatusInd()) and link callback (cb_linkStatusInd()) functions are set. These callback are invoked when the state of the interface or link is changed. After a Wi-Fi connection is established, the link can be changed to UP so the DHCP procedure can start. This happens during the OnWifiDriverEvent() handler and specifically under the WLAN_EVENT_CONNECT event, where TCPIP_IF_notifyLinkChange() is invoked. In CC35xx_network_terminal example, when setting a station role to UP, network_stack_add_if_sta() is used to create a netif interface and set a callback, _role_sta_up(), where the interface callback (status_callback()) and link callback (link_callback()) functions are set. These callback are invoked when the state of the interface or link is changed. After a Wi-Fi connection is established, the link can be changed to UP so the DHCP procedure can start. This happens during the WlanStackEventHandler() handler and specifically under the WLAN_EVENT_CONNECT event, where network_set_up() is invoked.

Note

Automatic IP is currently not used in any example of the SDK.

Summary

To summarize the difference between the three methods, see the following table.

Table 1 Network Addressing Comparison

Features	Static	DHCP	AutoIP
Requires server?	No	Yes	No
IP range	Any	Set by the DHCP server	169.254.x.x/16
Gateway/DNS support	No	Yes	No
Use cases	Proprietary networks	Managed networks	Small or ad-hoc networks
IP assignment method	Self-assigned	Server based	Self-assigned