Ping报文理解mtu拆包依据

如果拆包了第二个包的IP标志位Flags中会看到  Fragment offset: xxx
如果拆包了第一个包的IP标志位Flags中会看到  More fragments: Set
如果强制客户端不允许拆包会看到            Don't fragment: Set

**拆包大小一般是1500字节：
  这个1500是只得IP头+ICMP报文的大小，所以ICMP报文超过1480就会拆包
  **不是ICMP的话从IP头算起（包括IP头20字节）后面的长度等于ip header中的length，也就是拆包依据

# 45上抓包 单网卡eth0
tcpdump -i eth0 -s 0 -w p100c.pcap host 192.168.10.46
# 46上抓包 单网卡eth0
# tcpdump -i eth0 -s 0 -w p100s.pcap host 192.168.10.45

# 45上ping
ping -c 3 192.168.10.46

PING 192.168.10.46 (192.168.10.46) 56(84) bytes of data.
64 bytes from 192.168.10.46: icmp_seq=1 ttl=64 time=0.249 ms
64 bytes from 192.168.10.46: icmp_seq=2 ttl=64 time=0.157 ms
64 bytes from 192.168.10.46: icmp_seq=3 ttl=64 time=0.165 ms

--- 192.168.10.46 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 1999ms
rtt min/avg/max/mdev = 0.157/0.190/0.249/0.043 ms

# 45上抓包 单网卡eth0
tcpdump -i eth0 -s 0 -w p1500c.pcap host 192.168.10.46

ping -c 3 -s 1500 192.168.10.46
PING 192.168.10.46 (192.168.10.46) 1500(1528) bytes of data.
1508 bytes from 192.168.10.46: icmp_seq=1 ttl=64 time=0.293 ms
1508 bytes from 192.168.10.46: icmp_seq=2 ttl=64 time=0.193 ms
1508 bytes from 192.168.10.46: icmp_seq=3 ttl=64 time=0.185 ms

--- 192.168.10.46 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 1999ms
rtt min/avg/max/mdev = 0.185/0.223/0.293/0.052 ms

# 45上抓包 单网卡eth0
tcpdump -i eth0 -s 0 -w telnetc.pcap host 192.168.10.46

# 45上telnet 46
telnet 192.168.10.46 22