netRIver实验（二，ipv4收发）

实验问题

• 每个控制信息的数据长度和位数以及在缓冲区中的位置需要仔细计算，分别涉及1字节Byte，2字节short，4字节unsigned三种长度
• 校验和的计算需要注意和的数据类型应该为unsigned，因为校验和的计算可能溢出至三个字节，short类型变量无法存放，在解决数字溢出之后再转换为short，否则会丢失校验和的部分信息导致校验和出错。
• 发送数据时，在设置totalLen时，需要注意使用htonl函数转换得到的是4字节数据，即为unsigned类型，直接赋值给short类型会发生数据截取，导致设置totalLen时使用
  short totalLen = htonl(len+20);
  赋值时，得到的数据永远为0，因为截取时只能得到转换后的低16位，即原来的高16位，即0。因此需要移位或者自行设置每个字节
• 设置校验和时应该注意先将校验和处设置为0，计算得到的校验和取反就是需要填入相应位置的校验和。

代码

/*
* THIS FILE IS FOR IP TEST
*/
// system support
#include "sysInclude.h"

extern void ip_DiscardPkt(char* pBuffer, int type);

extern void ip_SendtoLower(char*pBuffer, int length);
extern void ip_SendtoUp(char *pBuffer, int length);

extern unsigned int getIpv4Address();

// implemented by students

unsigned getCheckSum(unsigned short* pBuffer, unsigned headLen) {
    unsigned sum = 0;
    int i = 0;
    headLen = headLen / 2;
    while (headLen > i) {
        sum += (unsigned)pBuffer[i];
        i++;
    }
    if (sum & 0xffff != sum) {
        sum = (sum & 0xffff) + (sum >> 16);
    }
    return sum;
}
int stud_ip_recv(char *pBuffer, unsigned short length)
{
    printf("-----recv a ipv4 packet");
    //检查版本号
    unsigned version = (pBuffer[0]) >> 4;
    printf("-----the version is %x\n", version);
    if (version != 4) {
        ip_DiscardPkt(pBuffer, STUD_IP_TEST_VERSION_ERROR);
        return 1;
    }
    //检查头部长度
    unsigned headLen = (unsigned)(pBuffer[0]) & 0xf;
    headLen *= 4;
    if (headLen < 20) {
        ip_DiscardPkt(pBuffer, STUD_IP_TEST_HEADLEN_ERROR);
    }
    //检查生存时间
    unsigned ttl = (unsigned)(pBuffer[8]);
    printf("-----the ttl is %d\n", ttl);
    if (ttl <= 0) {
        ip_DiscardPkt(pBuffer, STUD_IP_TEST_TTL_ERROR);
        return 1;
    }
    //检查校验和
    if (getCheckSum((unsigned short*)pBuffer, headLen) != 0xffff) {
        ip_DiscardPkt(pBuffer, STUD_IP_TEST_CHECKSUM_ERROR);
        return 1;
    }
    //检查接受地址
    unsigned localAddr = getIpv4Address();
    unsigned recvAddr = ((unsigned*)pBuffer)[4];
    recvAddr = ntohl(recvAddr);
    printf("-----the local addr is %d ,the recv addr is %d\n", localAddr, recvAddr);
    if (localAddr != recvAddr || recvAddr == 0xffffffff) {
        ip_DiscardPkt(pBuffer, STUD_IP_TEST_DESTINATION_ERROR);
        return 1;
    }
    //接受
    printf("-----accept a packet");
    pBuffer += headLen;
    ip_SendtoUp(pBuffer, length - headLen);
    return 0;
}
int stud_ip_Upsend(char *pBuffer, unsigned short len, unsigned int srcAddr,
    unsigned int dstAddr, byte protocol, byte ttl)
{
    printf("-----send a packet\n");
    char* buffer = (char*)malloc(len + 20);
    memcpy(buffer + 20, pBuffer, len);
    //version，headLen，ServiceType。
    buffer[0] = 0x45;
    buffer[1] = 0x0;
    //totalLen
    unsigned short totalLen = len + 20;
    buffer[3] = totalLen & 0xff;
    buffer[2] = totalLen >> 8;
    printf("-----total len is %x\n", len + 20);
    //identification
    ((unsigned short*)buffer)[2] = 0;
    //flags fragment offset
    ((unsigned short*)buffer)[3] = 0;
    //ttl
    buffer[8] = ttl;
    printf("-----ttl is %x\n", ttl);
    //protocol
	buffer[9] = protocol;
    printf("-----protocol is %x\n", protocol);
    //init header checksum
    ((unsigned short*)buffer)[5] = 0;
    //source ip
    ((unsigned*)buffer)[3] = htonl(srcAddr);
    //des ip
    ((unsigned*)buffer)[4] = htonl(dstAddr);
    printf("-----src ip is %x,dst ip is %x\n", srcAddr, dstAddr);
    //update checkSUm
    unsigned short checkSum = ~getCheckSum((unsigned short*)buffer, 20);
    ((unsigned short*)buffer)[5] = checkSum;
    printf("-----checksum is %x\n", checkSum);
    //send
    ip_SendtoLower(buffer, len + 20);
    return 0;
}