// ***************************************************************************// An 8051 Based Web Server// tcp.c: TCP protocol processing// By Mason Kidd 3/14/02// ***************************************************************************#include <string.h>#include "packets.h"#include "ip.h"#include "udp.h"void tx_udp_packet(unsigned char *szData, unsigned char nLength){	unsigned char tx_buf[BUF_LEN];	struct ip_hdr *tx_ip_hdr = (struct ip_hdr *)(tx_buf + sizeof(struct eth_hdr));	struct udp_hdr *tx_udp_hdr = (struct udp_hdr *)(tx_buf + sizeof(struct ip_hdr) + sizeof(struct eth_hdr));	unsigned char *udp_data = (unsigned char *)(tx_buf + sizeof(struct eth_hdr) + sizeof(struct ip_hdr) + sizeof(struct udp_hdr));	unsigned int *chksum_hdr = (unsigned int *)tx_udp_hdr;	int i;	unsigned int chksum = 0;		for (i = 0; i < nLength; i++)	{		*udp_data = szData[i]; 		udp_data++;	}		tx_udp_hdr->src_port = MY_UDP_PORT;	tx_udp_hdr->dst_port = MY_UDP_PORT;	tx_udp_hdr->length = sizeof(struct udp_hdr) + nLength;	// Compute the checksum	for (i = 0; i < 3; i++)	{			chksum += *chksum_hdr;  			chksum_hdr++;   	}   	chksum_hdr = (unsigned int *)&(tx_ip_hdr->proto);   	chksum += *chksum_hdr;   	chksum_hdr = (unsigned int *)&targetIP;   	for (i = 0; i < 4; i++)   	{			chksum += *chksum_hdr;  			chksum_hdr++;	}	chksum = ~chksum; 	tx_udp_hdr->checksum = chksum;	tx_ip_packet(tx_buf, nLength + sizeof(struct udp_hdr), IP_UDP);}void rx_udp_packet(unsigned char *rx_buffer){	struct ip_hdr *rx_ip_hdr = (struct ip_hdr *)(rx_buffer + sizeof(struct eth_hdr));	struct udp_hdr *rx_udp_hdr = (struct udp_hdr *)(rx_buffer + sizeof(struct eth_hdr) + sizeof(struct ip_hdr));	unsigned int *tmp_chk = (unsigned int *)&rx_ip_hdr->srcIP;	unsigned int checksum = 0;	unsigned int i;#define msglen 15	unsigned char msg[msglen] = "This is a test.";		// check the length	if (rx_udp_hdr->length == rx_ip_hdr->totlen - (sizeof(struct ip_hdr) / 8))	{		// check the checksum       	checksum = rx_udp_hdr->length + rx_ip_hdr->proto;       	checksum += (*tmp_chk)++;       	checksum += (*tmp_chk)++;       	checksum += (*tmp_chk)++;       	checksum += (*tmp_chk);       			tmp_chk = (unsigned int *)rx_udp_hdr;               		for (i = 0; i < rx_udp_hdr->length; i++)			if (i != 3)	//skip existing checksum				checksum += *tmp_chk;		checksum = ~checksum; 		// packet is valid if they match 		if (checksum == rx_udp_hdr->checksum)			// check the port numbers			if ((rx_udp_hdr->src_port == MY_UDP_PORT) && (rx_udp_hdr->dst_port == MY_UDP_PORT))       		{        			// packet is ok, respond				tx_udp_packet(&msg, strlen(msg));			}	  		// else discard packet  	}	// else discard packet}