http://wiki.tomocha.net/Linux_ServersMan_VPS_network.html#hf943f2b に「DTI網内のMTUサイズは1500だが、出口は1480となっており、IPIPトンネルが使用されていると考えられる。」とあるけど、実際にはDTI網内にIPIPトンネルが使用されているように見える。
> traceroute6 www.iij.ad.jp
traceroute to www.iij.ad.jp (2001:240:bb42:b000::1:80), 30 hops max, 40 byte packets
1 2001:2e8:614:0:2:2:0:1 (2001:2e8:614:0:2:2:0:1) 0.076 ms 0.034 ms 0.027 ms
2 2001:2e8:22:202::1 (2001:2e8:22:202::1) 0.467 ms 0.383 ms 0.455 ms
(tunnel?)
3 2001:2e8:20::22:1 (2001:2e8:20::22:1) 1.460 ms 1.449 ms 1.541 ms
4 lan-gate.dixie6.otemachi4.v6.dti.ad.jp (2001:2e8:20:11::5) 1.385 ms 1.487 ms 1.387 ms
5 2001:200:0:fe00::9c1:0 (2001:200:0:fe00::9c1:0) 1.607 ms 1.627 ms 1.548 ms
6 tky001bb11.IIJ.Net (2001:240:bb01:32::7e) 2.184 ms 2.251 ms 2.247 ms
7 tky008bf01.IIJ.Net (2001:240:bb00:9009::74) 4.070 ms 3.969 ms 3.950 ms
8 tky008bb00.IIJ.Net (2001:240:bb00:900f::23) 2.760 ms 2.742 ms 3.030 ms
9 2001:240:bb02:a::29 (2001:240:bb02:a::29) 3.299 ms 2.968 ms 3.409 ms
10 2001:240:bb42:b000::1:80 (2001:240:bb42:b000::1:80) 2.852 ms 2.453 ms 2.580 ms
> ping6 -c 1 -s 1452 2001:2e8:614:0:2:2:0:1
PING 2001:2e8:614:0:2:2:0:1(2001:2e8:614:0:2:2:0:1) 1452 data bytes
1460 bytes from 2001:2e8:614:0:2:2:0:1: icmp_seq=0 ttl=64 time=0.046 ms
--- 2001:2e8:614:0:2:2:0:1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.046/0.046/0.046/0.000 ms, pipe 2
> ping6 -c 1 -s 1452 2001:2e8:22:202::1
PING 2001:2e8:22:202::1(2001:2e8:22:202::1) 1452 data bytes
1460 bytes from 2001:2e8:22:202::1: icmp_seq=0 ttl=62 time=1.17 ms
--- 2001:2e8:22:202::1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 1.172/1.172/1.172/0.000 ms, pipe 2
> ping6 -c 1 -s 1452 2001:2e8:20::22:1
PING 2001:2e8:20::22:1(2001:2e8:20::22:1) 1452 data bytes
From 2001:2e8:22:202::1 icmp_seq=0 Packet too big: mtu=1480
--- 2001:2e8:20::22:1 ping statistics ---
1 packets transmitted, 0 received, +1 errors, 100% packet loss, time 0ms
> ping6 -c 1 -s 1452 2001:2e8:20:11::5
PING 2001:2e8:20:11::5(2001:2e8:20:11::5) 1452 data bytes
From 2001:2e8:22:202::1 icmp_seq=0 Packet too big: mtu=1480
--- 2001:2e8:20:11::5 ping statistics ---
1 packets transmitted, 0 received, +1 errors, 100% packet loss, time 0ms
また、外からの観測結果でも、2001:2e8:20::22:1 までは、1500 bytes で届くが、
IP6 2001:xxx:xxxx::x > 2001:2e8:20::22:1: ICMP6, echo request, seq 1, length 1460
IP6 2001:2e8:20::22:1 > 2001:xxx:xxxx::x: ICMP6, echo reply, seq 1, length 1460
2001:2e8:22:202::1 までは届かない。
IP6 2001:xxx:xxxx::x > 2001:2e8:22:202::1: ICMP6, echo request, seq 0, length 1460
IP6 2001:2e8:20:11::1 > 2001:xxx:xxxx::x: ICMP6, packet too big, mtu 1480, length 1240
IP6 2001:xxx:xxxx::x > 2001:2e8:22:202::1: frag (0|1432) ICMP6, echo request, seq 1, length 1432
IP6 2001:xxx:xxxx::x > 2001:2e8:22:202::1: frag (1432|28)
IP6 2001:2e8:22:202::1 > 2001:xxx:xxxx::x: frag (0|1432) ICMP6, echo reply, seq 1, length 1432
IP6 2001:2e8:22:202::1 > 2001:xxx:xxxx::x: frag (1432|28)
