网络工程 - 第一次 ping 后 MTU 发生变化 - 吾爱随笔录

我注意到一种奇怪的行为，我的知识没有答案。所以我想知道MTU。第一次ping：

ping -c 1 -s 1800 -M do google.com
PING google.com (216.58.209.14) 1800(1828) bytes of data.
ping: local error: Message too long, mtu=1500

--- google.com ping statistics ---
1 packets transmitted, 0 received, +1 errors, 100% packet loss, time 0ms

所以我得出结论，MTU 是 1500。但在发送 1472 个字节后，我得到：

ping -c 1 -s 1472 -M do google.com
PING google.com (216.58.209.14) 1472(1500) bytes of data.
From 192.168.55.1 (192.168.55.1) icmp_seq=1 Frag needed and DF set (mtu = 0)

--- google.com ping statistics ---
1 packets transmitted, 0 received, +1 errors, 100% packet loss, time 0ms

接着：

ping -c 1 -s 1500 -M do google.com
PING google.com (216.58.209.14) 1500(1528) bytes of data.
ping: local error: Message too long, mtu=552

--- google.com ping statistics ---
1 packets transmitted, 0 received, +1 errors, 100% packet loss, time 0ms

为什么变了？