#ICMP 协议
ICMP协议是一种面向无连接的协议，属于网络层协议，主要用于在主机与路由器之间传递控制信息，包括报告错误、交换受限控制和状态信息等。当遇到IP数据无法访问目标、IP路由器无法按当前的传输速率转发数据包等情况时，会自动发送ICMP消息。ICMP包有一个8字节长的包头，其中前4个字节是固定的格式，包含8位类型字段，8位代码字段和16位的校验，后4个字节根据ICMP包的类型而取不同的值。
ICMP类型值及代码值
| 类型值 | 类型说明 | 代码值 | 代码说明 |
|---|---|---|---|
| 8 | Echo | 0 | Echo request |
| 0 | Echo Reply | 0 | Echo reply |
| 3 | Destination Unreachable | 0 | Destination network unreachable |
| 1 | Destination host unreachable | ||
| 2 | Destination protocol unreachable | ||
| 3 | Destination port unreachable | ||
| 4 | Fragmentation required, and DF flag set | ||
| 5 | Source route failed | ||
| 6 | Destination network unknown | ||
| 7 | Destination host unknown | ||
| 8 | Source host isolated | ||
| 9 | Network administratively prohibited | ||
| 10 | Host administratively prohibited | ||
| 11 | Network unreachable for TOS | ||
| 12 | Host unreachable for TOS | ||
| 13 | Communication administratively prohibited | ||
| 14 | Host Precedence Violation | ||
| 15 | Precedence cutoff in effect | ||
| 4 | Source Quench | 0 | Source quench (congestion control) |
| 5 | Redirect Message | 0 | Redirect Datagram for the Network |
| 1 | Redirect Datagram for the Host | ||
| 2 | Redirect Datagram for the TOS & network | ||
| 3 | Redirect Datagram for the TOS & host | ||
| 9 | Router Advertisement | 0 | Router Advertisement |
| 10 | Router Solicitation | 0 | Router discovery/selection/solicitation |
| 11 | Time Exceeded | 0 | TTL expired in transit |
| 1 | Fragment reassembly time exceeded | ||
| 12 | Parameter Problem: Bad IP header | 0 | Pointer indicates the error |
| 1 | Missing a required option | ||
| 2 | Bad length | ||
| 13 | Timestamp | 0 | Timestamp |
| 14 | Timestamp Reply | 0 | Timestamp reply |
| 15 | Information Request | 0 | Information request |
| 16 | Information Reply | 0 | Information reply |
| 17 | Address Mask Reques | 0 | Address mask request |
| 18 | Address Mask Reply | 0 | Address mask reply |
| 30 | Traceroute | 0 | Information request |
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