comp.os.linux.networking

On Tue, 08 Apr 2008, in the Usenet newsgroup comp.os.linux.networking, in
article , Shadow_7 wrote:

>> If an XBox can't "negotiate the wireless connection to the router"
>> when wlan0 has a 576 MTU then it makes me wonder if an XBox is PMTU
>> Discovery enabled.
>
>I think it was more that the XBox had an MTU of 1500 and no customization
>of that value.

Next time your niece brings the XBox over, set the MTU on your LAN to
576, and then run /usr/sbin/tcpdump -n on that LAN side NIC. What you
are looking for is

12:34:56.780000 192.168.10.20.4283 > 192.168.33.55.53: S
1759784758:1759784758(0) win 60352 (DF)

which shows time, source_IP.port > destination_IP.port SYN_flag
sequence_number:sequence_number(0) window_size (DF)

and the key is the last item - the Don't_Fragment flag. If you see
that flag (Ethereal/wireshark would also show the flag, but in a more
user-friendly format), the system is set for PMTU discovery. In this
particular case, the LAN MTU was large enough to carry the packet
without fragmenting, and there would be subsequent packets from the
destination making the handshake. A bit later, this host is going to
send a packet that is to big (the number in parentheses following the
sequence number will be something greater than ~540), and what SHOULD
happen is that the "router" should send an ICMP Type 3 Code 4 error
indicating that the packet is to large. That one you may see (if you
don't, then you've discovered your problem is the router or relay
between the router and your local host), but if the remote host this
box is conversing with sends a packet that is to large, _IT_ should
receive a similar ICMP Type 3 Code 4 error (which you won't see as
it's directed at the remote). That remote host should send the reply
in a smaller packet (perhaps without the DF flag set).

Seeing as how many DSL setups are using PPPoE (which requires an
MTU of 1492 rather than the 1500 normally used), I'd really expect
the XBox to be ready to reset the packet size, or not use the DF
flag, but only if it receives that ICMP error. But conversations
are two-way, so both sides need to know about the lower than normal
MTU so that they can adjust for it.

Old guy