Raw TCP/IP interface for lwIP Authors: Adam Dunkels, Leon Woestenberg, Christiaan Simons lwIP provides three Application Program's Interfaces (APIs) for programs to use for communication with the TCP/IP code: * low-level "core" / "callback" or "raw" API. * higher-level "sequential" API. * BSD-style socket API. The raw API (sometimes called native API) is an event-driven API designed to be used without an operating system that implements zero-copy send and receive. This API is also used by the core stack for interaction between the various protocols. It is the only API available when running lwIP without an operating system. The sequential API provides a way for ordinary, sequential, programs to use the lwIP stack. It is quite similar to the BSD socket API. The model of execution is based on the blocking open-read-write-close paradigm. Since the TCP/IP stack is event based by nature, the TCP/IP code and the application program must reside in different execution contexts (threads). The socket API is a compatibility API for existing applications, currently it is built on top of the sequential API. It is meant to provide all functions needed to run socket API applications running on other platforms (e.g. unix / windows etc.). However, due to limitations in the specification of this API, there might be incompatibilities that require small modifications of existing programs. ** Multithreading lwIP started targeting single-threaded environments. When adding multi- threading support, instead of making the core thread-safe, another approach was chosen: there is one main thread running the lwIP core (also known as the "tcpip_thread"). When running in a multithreaded environment, raw API functions MUST only be called from the core thread since raw API functions are not protected from concurrent access (aside from pbuf- and memory management functions). Application threads using the sequential- or socket API communicate with this main thread through message passing. As such, the list of functions that may be called from other threads or an ISR is very limited! Only functions from these API header files are thread-safe: - api.h - netbuf.h - netdb.h - netifapi.h - pppapi.h - sockets.h - sys.h Additionaly, memory (de-)allocation functions may be called from multiple threads (not ISR!) with NO_SYS=0 since they are protected by SYS_LIGHTWEIGHT_PROT and/or semaphores. Netconn or Socket API functions are thread safe against the core thread but they are not reentrant at the control block granularity level. That is, a UDP or TCP control block must not be shared among multiple threads without proper locking. If SYS_LIGHTWEIGHT_PROT is set to 1 and LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT is set to 1, pbuf_free() may also be called from another thread or an ISR (since only then, mem_free - for PBUF_RAM - may be called from an ISR: otherwise, the HEAP is only protected by semaphores). ** The remainder of this document discusses the "raw" API. ** The raw TCP/IP interface allows the application program to integrate better with the TCP/IP code. Program execution is event based by having callback functions being called from within the TCP/IP code. The TCP/IP code and the application program both run in the same thread. The sequential API has a much higher overhead and is not very well suited for small systems since it forces a multithreaded paradigm on the application. The raw TCP/IP interface is not only faster in terms of code execution time but is also less memory intensive. The drawback is that program development is somewhat harder and application programs written for the raw TCP/IP interface are more difficult to understand. Still, this is the preferred way of writing applications that should be small in code size and memory usage. All APIs can be used simultaneously by different application programs. In fact, the sequential API is implemented as an application program using the raw TCP/IP interface. Do not confuse the lwIP raw API with raw Ethernet or IP sockets. The former is a way of interfacing the lwIP network stack (including TCP and UDP), the latter refers to processing raw Ethernet or IP data instead of TCP connections or UDP packets. Raw API applications may never block since all packet processing (input and output) as well as timer processing (TCP mainly) is done in a single execution context. --- 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 . 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) #define lwip_htonl(x) 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.