[c-nsp] Ethernet debugging...MTU?

[Pekka Savola]

On Fri, 26 Oct 2007, Andy Dills wrote:
> Cliff notes:
>
> Is the inability to get ping replies with datagrams of larger than 9216
> bytes across a 100mbps ethernet circuit an indication that the far end is
> setup with an MTU consistent with jumbo frames?
>
> What to do if the far end swears it's set for 1500?

(Note that 100mbit/s FE interfaces may also support jumbo frames so 
the fact that it's FE interfaces isn't a guarantee that it won't be 
using 9216 MTU.)

When you ping with 9216 bytes, most likely your router fragments the 
ping packets to multiple datagrams (maximum at 1500 bytes; verify 
this).  When the target responds, how are the response datagram(s) 
fragmented?  You should be able to figure out the path MTU by checking 
the largest size of the fragments received.

You may need to ping from a unix host in order to more easily diagnose 
this issue (e.g., run tcpdump to see the fragment offsets).

Based on that you should be able to figure out what IP mtu the remote 
end is using.

If it's set to 1500 but ping (with > 9216) still fails the reason 
might have more to do with the fragmentation capabilities/policies of 
fragmenting devices (routers or the hosts at each end).

Working until 9216 and then stop sounds as if some device(s) is using 
jumbo frames and a) you're pinging with DF bit set or b) some device 
refuses to do fragmentation properly (or even at all) when the total 
size of the full packet is too big.

-- 
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings