I'm seeing an ICMP storm from a mobile phone netblocks belonging to "TELEFONICA MOVILES". Periodically we will get upwards of 5 million in a few seconds, all something like this:
08:12:05.740781 IP (tos 0x0, ttl 112, id 40224, offset 0, flags [none], proto ICMP (1), length 56) 220.127.116.11 > A.B.C.D: ICMP 18.104.22.168 unreachable - need to frag (mtu 250), length 36
("A.B.C.D" is my ip)
Is an mtu of 250 even possible or legal? 68? These ICMPs are connected with larger problems on our end, but I can't tell if they're a symptom, cause or just coincidence.
What does the path MTU discovery algorithm do in this case? The server is FreeBSD 7.