奇怪的SNMP换档操作 [英] Strange SNMP shifting operation

问题描述

以下是从书C＃网络Programmingm由理查德·布卢姆：

The following is from the book C# Network Programmingm by Richard Blum:

public byte[] get(string request, string host, string community, string_ mibstring)
{
    byte[] packet = new byte[1024];
    byte[] mib = new byte[1024];
    int snmplen;
    int comlen = community.Length;
    string[] mibvals = mibstring.Split('.');
    int miblen = mibvals.Length;
    int cnt = 0, temp, i;
    int orgmiblen = miblen;
    int pos = 0;
    // Convert the string MIB into a byte array of integer values
    // Unfortunately, values over 128 require multiple bytes
    // which also increases the MIB length
    for (i = 0; i < orgmiblen; i++)
    {
        temp = Convert.ToInt16(mibvals[i]);
        if (temp > 127)
        {
            mib[cnt] = Convert.ToByte(128 + (temp / 128));
            mib[cnt + 1] = Convert.ToByte(temp - ((temp / 128) * 128));
            cnt += 2;
            miblen++;
        } 
        else
        {
        mib[cnt] = Convert.ToByte(temp);
        cnt++;
        }
    }
    snmplen = 29 + comlen + miblen - 1; //Length of entire SNMP packet
    //The SNMP sequence start
    packet[pos++] = 0x30; //Sequence start
    packet[pos++] = Convert.ToByte(snmplen - 2); //sequence size
    //SNMP version
    packet[pos++] = 0x02; //Integer type
    packet[pos++] = 0x01; //length
    packet[pos++] = 0x00; //SNMP version 1
    //Community name
    packet[pos++] = 0x04; // String type
    packet[pos++] = Convert.ToByte(comlen); //length
    //Convert community name to byte array
    byte[] data = Encoding.ASCII.GetBytes(community);
    for (i = 0; i < data.Length; i++)
    {
        packet[pos++] = data[i];
    }
}

我不明白以下code：

I didn't understand the following code:

    for (i = 0; i < orgmiblen; i++)
    {
        temp = Convert.ToInt16(mibvals[i]);
        if (temp > 127)
        {
            mib[cnt] = Convert.ToByte(128 + (temp / 128));
             mib[cnt + 1] = Convert.ToByte(temp - ((temp / 128) * 128));
             cnt += 2;
             miblen++;
        } 
        else
        {
            mib[cnt] = Convert.ToByte(temp);
            cnt++;
        }
    }

我不明白这是为了将分为两个字节，如果临时更大然后一个字节。但是，什么是计算正在做128+（温度/ 128），然后在第二个字节：TEMP-（温度/ 128）* 128，这就是我不明白

I do understand this is for putting down into two bytes if the temp is larger then one byte. But what is the calculation being done 128+(temp/128) and then for the second byte: temp- (temp/128)*128, this is what i don't understand.

请帮忙，谢谢你。

推荐答案

下面是引用的了解SNMP的MIB 的394页，因为这本书描述的技术细节，比别人好，

Below is quoted from Understanding SNMP MIBs page 394, as this book describes the technical details better than any others,

这是连接codeD对象标识符是由每个子标识符原值EN codeD和级联的。每个子标识符是烯$ C $光盘作为一系列八比特组，即如下，

An encoded OBJECT IDENTIFIER consists of each sub-identifier in the original value encoded and concatenated. Each sub-identifier is encoded as a series of octets, which is as follows,

  
• 在每个八位位组的位8表示如果是副识别符的最后一个字节由该位设置为0
  
• 位7至1的八位位组，当串联，形成分标识符
的价值   
• 在子标识符的第一个字节可以不具有值80（十六进制）。这确保了最小数目的八位位组被用于编码。 80（十六进制）的值将指示多个字节跟随，但该值的位（7-1）被设置为零。
  

• Bit 8 in each octet indicates if it is the last octet of the sub-identifier by setting this bit to 0
• Bits 7 through 1 in the octets, when concatenated, form the value of the sub-identifier
• The first octet in the sub-identifier may not have the value 80 (in HEX). This ensures that the smallest number of octets are used for the encoding. A value of 80 (in HEX) would indicate that more octets follow, but the value bits (7-1) are set to zero.

粘贴的源$ C ​​$ C在体内的问题，其实遵循的规则来解析字节。因此，如果你理解了规则，你可以了解code。

The pasted source code in the question body in fact follows the rules to parse the bytes. Thus, if you understand the rules, you can understand the code.

（已更新的规则来自国际电联-T X.690，ISO / IEC 8825-1 。）

(Updated: The rules come from ITU-T X.690, ISO/IEC 8825-1.)

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