Strange MTU-ish problem

[Timothy Murphy]

Nifty Fedora Mitch wrote:

>> > Is there a similar option for ping under Fedora?
>> 
>> ping -s sizeyouwant www.google.com
> 
> you must also set a do not fragment flag.

I see that adding the option "-M do"
(which I take it is what you meant)
does indeed give a little more information,
or at least gives the same information more simply:
-------------------------------------------------
[tim at mary tmp]$ ping -s 1464 -c 3 -M do www.google.com
PING www.l.google.com (74.125.39.103) 1464(1492) bytes of data.
64 bytes from fx-in-f103.google.com (74.125.39.103): icmp_seq=1 ttl=242 (truncated)
64 bytes from fx-in-f103.google.com (74.125.39.103): icmp_seq=2 ttl=242 (truncated)
64 bytes from fx-in-f103.google.com (74.125.39.103): icmp_seq=3 ttl=242 (truncated)

--- www.l.google.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2083ms
rtt min/avg/max/mdev = 82.018/82.944/84.115/0.934 ms

[tim at mary tmp]$ ping -s 1466 -c 3 -M do www.google.com
PING www.l.google.com (74.125.39.104) 1466(1494) bytes of data.
>From homegate.homenet.telecomitalia.it (192.168.1.1) icmp_seq=1 Frag needed and DF set (mtu = 1492)
>From mary.homenet.telecomitalia.it (192.168.1.4) icmp_seq=2 Frag needed and DF set (mtu = 1492)
>From mary.homenet.telecomitalia.it (192.168.1.4) icmp_seq=2 Frag needed and DF set (mtu = 1492)

--- www.l.google.com ping statistics ---
1 packets transmitted, 0 received, +3 errors, 100% packet loss, time 1016ms
-------------------------------------------------

Incidentally, what exactly does "DF set" mean?
Is it telling me that DF is set,
or that it should be set (or even, should not be set)?
If the latter, how can I do that?