Raw TCP/IP interface for lwIP

Authors: Adam Dunkels, Leon Woestenberg, Christiaan Simons

--- System initalization

A truly complete and generic sequence for initializing the lwIP stack
cannot be given because it depends on additional initializations for
your runtime environment (e.g. timers).

We can give you some idea on how to proceed when using the raw API.
We assume a configuration using a single Ethernet netif and the
UDP and TCP transport layers, IPv4 and the DHCP client.

Call these functions in the order of appearance:

- lwip_init()

  Initialize the lwIP stack and all of its subsystems.

- 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)

  Adds your network interface to the netif_list. Allocate a struct
  netif and pass a pointer to this structure as the first argument.
  Give pointers to cleared ip_addr structures when using DHCP,
  or fill them with sane numbers otherwise. The state pointer may be NULL.

  The init function pointer must point to a initialization function for
  your Ethernet netif interface. The following code illustrates its use.
  
  err_t netif_if_init(struct netif *netif)
  {
    u8_t i;
    
    for (i = 0; i < ETHARP_HWADDR_LEN; i++) {
      netif->hwaddr[i] = some_eth_addr[i];
    }
    init_my_eth_device();
    return ERR_OK;
  }
  
  For Ethernet drivers, the input function pointer must point to the lwIP
  function ethernet_input() declared in "netif/etharp.h". Other drivers
  must use ip_input() declared in "lwip/ip.h".
  
- netif_set_default(struct netif *netif)

  Registers the default network interface.

- netif_set_link_up(struct netif *netif)

  This is the hardware link state; e.g. whether cable is plugged for wired
  Ethernet interface. This function must be called even if you don't know
  the current state. Having link up and link down events is optional but
  DHCP and IPv6 discover benefit well from those events.

- netif_set_up(struct netif *netif)

  This is the administrative (= software) state of the netif, when the
  netif is fully configured this function must be called.

- dhcp_start(struct netif *netif)

  Creates a new DHCP client for this interface on the first call.
  
  You can peek in the netif->dhcp struct for the actual DHCP status.

- sys_check_timeouts()

  When the system is running, you have to periodically call
  sys_check_timeouts() which will handle all timers for all protocols in
  the stack; add this to your main loop or equivalent.


--- Optimization hints

The first thing you want to optimize is the lwip_standard_checksum()
routine from src/core/inet.c. You can override this standard
function with the #define LWIP_CHKSUM <your_checksum_routine>.

There are C examples given in inet.c or you might want to
craft an assembly function for this. RFC1071 is a good
introduction to this subject.

Other significant improvements can be made by supplying
assembly or inline replacements for htons() and htonl()
if you're using a little-endian architecture.
#define lwip_htons(x) <your_htons>
#define lwip_htonl(x) <your_htonl>
If you #define them to htons() and htonl(), you should
#define LWIP_DONT_PROVIDE_BYTEORDER_FUNCTIONS to prevent lwIP from
defining hton*/ntoh* compatibility macros.

Check your network interface driver if it reads at
a higher speed than the maximum wire-speed. If the
hardware isn't serviced frequently and fast enough
buffer overflows are likely to occur.

E.g. when using the cs8900 driver, call cs8900if_service(ethif)
as frequently as possible. When using an RTOS let the cs8900 interrupt
wake a high priority task that services your driver using a binary
semaphore or event flag. Some drivers might allow additional tuning
to match your application and network.

For a production release it is recommended to set LWIP_STATS to 0.
Note that speed performance isn't influenced much by simply setting
high values to the memory options.

For more optimization hints take a look at the lwIP wiki.