I manage to get;
mailgate:~$ ping -D -s 1468 bbc.co.uk
PING bbc.co.uk (220.127.116.11): 1468 data bytes
1476 bytes from 18.104.22.168: icmp_seq=0 ttl=246 time=21.889 ms
mailgate:~$ ping -D -s 1469 bbc.co.uk
PING bbc.co.uk (22.214.171.124): 1469 data bytes
Request timeout for icmp_seq 0
36 bytes from ae1.er01.thdow.bbc.co.uk (126.96.36.199): frag needed and DF set (MTU 1514)
Vr HL TOS Len ID Flg off TTL Pro cks Src Dst
4 5 00 d905 64cf 0 0000 37 01 42da 192.168.16.20 188.8.131.52
So the largest packet from the LAN I get a reply to from bbc.co.uk is 1468 bytes
I get responses to bigger pings still on other sites;
mailgate:~$ ping -D -s 1470 www.ox.ac.uk
(184.108.40.206): 1470 data bytes
1478 bytes from 220.127.116.11: icmp_seq=0 ttl=51 time=28.839 ms
1470 bytes is reproducibly the largest packet I can send out from the LAN and get a response although some sites won't respond at this size.
The MTU of the client (Mac Mini) I'm using is 1500, the firewall is 1500 and the router has no MTU set at all.
From the router directly I get;
Target IP address: www.ox.ac.uk
Repeat count :
Datagram size : 1498
Timeout in seconds :
Extended commands [n]:
Sweep range of sizes [n]:
Type escape sequence to abort.
Sending 5, 1498-byte ICMP Echos to 18.104.22.168, timeout is 2 seconds:
Success rate is 100 percent (5/5), round-trip min/avg/max = 24/26/28 ms
This is the maximum from the router to www.ox.ac.uk
but there may be sites with an even higher limit.
I believe the Openreach modem has a maximum MTU of 1600 bytes on ethernet (see here
) and as far as I know the specification for BT's FTTC network doesn't set a limit for MTU. If I can find the specification I read previously I'll post a link.