附：点对点端口上的ping 数据观察与分析(ping 对端地址需要一个来回，ping 自己的地址需要两个来回)

    测试情况，R1端口地址为172.31.20.1，对端R2地址为172.31.20.254

    R1#ping
    Protocol [ip]:
    Target IP address: 172.31.20.254  //ping R2地址
    Repeat count [5]: 1
    ...
    Sending 1, 100-byte ICMP Echos to 172.31.20.254, timeout is 2 seconds:
    !
    Success rate is 100 percent (1/1), round-trip min/avg/max = 8/8/8 ms
    (debug 输出)
    Apr 10 12:19:03.994: IP: s=172.31.20.1 (local), d=172.31.20.254 (Serial4/0/0), len 100, sending
    Apr 10 12:19:03.994:     ICMP type=8, code=0 R1发出一个Echo Request(type=8)，R2收到后以Echo Reply 相应
    Apr 10 12:19:04.002: IP: s=172.31.20.254 (Serial4/0/0), d=172.31.20.1 (Serial4/0/0), len 100, rcvd 3
    Apr 10 12:19:04.002:     ICMP type=0, code=0 R1收到Echo Reply(type=0)，计算延时(002-994=8ms)。

    R1#ping
    Protocol [ip]:
    Target IP address: 172.31.20.1  //ping R1自己的地址
    Repeat count [5]: 1
    ...
    Sending 1, 100-byte ICMP Echos to 172.31.20.1, timeout is 2 seconds:
    !
    Success rate is 100 percent (1/1), round-trip min/avg/max = 16/16/16 ms
    (debug 输出)
    Apr 10 12:18:00.106: IP: s=172.31.20.1 (local), d=172.31.20.1 (Serial4/0/0), len 100, sending
    Apr 10 12:18:00.106:     ICMP type=8, code=0 R2发出一个Echo Request(type=8)
    Apr 10 12:18:00.114: IP: s=172.31.20.1 (Serial4/0/0), d=172.31.20.1 (Serial4/0/0), len 100, rcvd 3
    Apr 10 12:18:00.114:     ICMP type=8, code=0 R2收到Echo Request，判断的目标地址为R1，所以将包发回
    Apr 10 12:18:00.114: IP: s=172.31.20.1 (local), d=172.31.20.1 (Serial4/0/0), len 100, sending
    Apr 10 12:18:00.114:     ICMP type=0, code=0 R1收到自己发出的Echo Request，以Echo Reply(type=0)相应并通过端口发送
    Apr 10 12:18:00.122: IP: s=172.31.20.1 (Serial4/0/0), d=172.31.20.1 (Serial4/0/0), len 100, rcvd 3
    Apr 10 12:18:00.122:     ICMP type=0, code=0 R2将包发回。R1收到Echo Reply (type=0),计算延时(112-106=16ms)。