解析GRE报文长度的方法及实现步骤

// *pTemp为GRE报文头指针

unsigned char getGreHdrLen(unsigned char *pTemp)
{
    unsigned char greflags1 = *(pTemp);
    unsigned char greflags2 = *(pTemp &＃43; 1);
    unsigned char sequence_num = (greflags1 >> 4) & 0x01;
    unsigned char key = (greflags1 >> 5) & 0x01;
    unsigned char acknowledge_num = (greflags2 >> 7) & 0x01;
    unsigned char grehderlen = 0;
    unsigned char flags = sequence_num &＃43; key &＃43; acknowledge_num;
    switch (flags) {
    case 0:
            grehderlen = 4;
            break;
    case 1:
            grehderlen = 8;
            break;
    case 2:
            grehderlen = 12;
            break;
    case 3:
            grehderlen = 16;
            break;
    }
    return grehderlen;
}

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