Hi,
After some more tests:
Source: Linux machine with IPv6: 2a02:27d0:0:5e0d:1a03:73ff:feba:50b4
Destination: Linux machine with IPv6: 2a02:27d0:0:5e0d:428d:5cff:fea5:501e
source# ping6 -M do -s 1300 2a02:27d0:0:5e0d:428d:5cff:fea5:501e
destination# tcpdump -ni enp3s0 host 2a02:27d0:0:5e0d:1a03:73ff:feba:50b4
09:38:55.735387 IP6 2a02:27d0:0:5e0d:1a03:73ff:feba:50b4 >
2a02:27d0:0:5e0d:428d:5cff:fea5:501e: ICMP6, echo request, seq 1, length 1308
09:38:55.735826 IP6 2a02:27d0:0:5e0d:428d:5cff:fea5:501e >
2a02:27d0:0:5e0d:1a03:73ff:feba:50b4: ICMP6, echo reply, seq 1, length 1308
09:38:56.736882 IP6 2a02:27d0:0:5e0d:1a03:73ff:feba:50b4 >
2a02:27d0:0:5e0d:428d:5cff:fea5:501e: ICMP6, echo request, seq 2, length 1308
09:38:56.736998 IP6 2a02:27d0:0:5e0d:428d:5cff:fea5:501e >
2a02:27d0:0:5e0d:1a03:73ff:feba:50b4: ICMP6, echo reply, seq 2, length 1308
...
Source: Linux machine with IPv6: 2a02:27d0:0:5e0d:1a03:73ff:feba:50b4
Destination machine OpenBSD 5.8 with IPv6: 2a02:27d0:0:5e0c:d6be:d9ff:fe95:a236
source# ping6 -M do -s 1300 2a02:27d0:0:5e0c:d6be:d9ff:fe95:a236
destination# tcpdump -ni bge0 host 2a02:27d0:0:5e0d:1a03:73ff:feba:50b4
17:58:03.040184 2a02:27d0:0:5e0d:1a03:73ff:feba:50b4 >
2a02:27d0:0:5e0c:d6be:d9ff:fe95:a236: icmp6: echo request [flowlabel 0x467ca]
17:58:03.040295 2a02:27d0:0:5e0c:d6be:d9ff:fe95:a236 >
2a02:27d0:0:5e0d:1a03:73ff:feba:50b4: frag (0x04183a32:1232@0+) icmp6: echo
reply
17:58:03.040296 2a02:27d0:0:5e0c:d6be:d9ff:fe95:a236 >
2a02:27d0:0:5e0d:1a03:73ff:feba:50b4: frag (0x04183a32:76@1232)
17:58:04.034362 2a02:27d0:0:5e0d:1a03:73ff:feba:50b4 >
2a02:27d0:0:5e0c:d6be:d9ff:fe95:a236: icmp6: echo request [flowlabel 0x467ca]
17:58:04.034430 2a02:27d0:0:5e0c:d6be:d9ff:fe95:a236 >
2a02:27d0:0:5e0d:1a03:73ff:feba:50b4: frag (0x36bc0632:1232@0+) icmp6: echo
reply
17:58:04.034432 2a02:27d0:0:5e0c:d6be:d9ff:fe95:a236 >
2a02:27d0:0:5e0d:1a03:73ff:feba:50b4: frag (0x36bc0632:76@1232)
source: Linux machine with IPv6: 2a02:27d0:0:5e0d:1a03:73ff:feba:50b4
Destination machine OpenBSD 5.8 with IPv6: 2a02:27d0:0:5e0c:d6be:d9ff:fe95:a236
source# ping6 -M do -s 1232 2a02:27d0:0:5e0c:d6be:d9ff:fe95:a236
leads to answers fitting in one packet only (1280 byte) with no
fragmentation whatsoever.
source# ping6 -M do -s 1233 leads to fragmentation
==================================================================
I wonder why ICMPv6 answers bigger than 1232 are always fragmented.
==================================================================
# netstat -rnf inet6 | grep bge0
default fe80::42b4:f0ff:fec6:7201%bge0 UG 0
211 - 56 bge0
2a02:27d0:0:5e0c::/64 fe80::d6be:d9ff:fe95:a236%bge0 UC 0
0 - 4 bge0
fe80::%bge0/64 fe80::d6be:d9ff:fe95:a236%bge0 UC 1
0 - 4 bge0
fe80::42b4:f0ff:fec6:7201%bge0 40:b4:f0:c6:72:01 UHLc 1
13 - 4 bge0
fe80::d6be:d9ff:fe95:a236%bge0 d4:be:d9:95:a2:36 UHLl 0
0 - 1 lo0
ff01::%bge0/32 fe80::d6be:d9ff:fe95:a236%bge0 UC 0
0 - 4 bge0
ff02::%bge0/32 fe80::d6be:d9ff:fe95:a236%bge0 UC 0
0 - 4 bge0
Please note, replacing bge0 with em0 leads to the very same result,
ruling out a difference of ICMPv6 handling between the 2 drivers.
