On Tue, 20 Sep 2016, Grant wrote:
>>>Strangely, I'm able to ping with that command even with a very high -s value:
>>>$ ping -c 4 -M dont -s 9999 www.dslreports.com
>>>PING www.dslreports.com ( 9999(10027) bytes of data.
>>>10007 bytes from www.dslreports.com ( icmp_seq=1 ttl=54
>>>time=331 ms
>>>10007 bytes from www.dslreports.com ( icmp_seq=2 ttl=54
>>>time=329 ms
>>>10007 bytes from www.dslreports.com ( icmp_seq=3 ttl=54
>>>time=329 ms
>>>10007 bytes from www.dslreports.com ( icmp_seq=4 ttl=54
>>>time=329 ms
>>>4 packets transmitted, 4 received, 0% packet loss, time 3003ms
>>>rtt min/avg/max/mdev = 329.159/329.877/331.612/1.158 ms
>> Look again! You're just looking at the _PING_ packets, not the ICMP/IP
>> packets actually going over the interface! You'll need to run
>> 'tcpdump icmp' in parallel! "My ping" also just reports 1 packet, but
>> there's two IP packets actually going over the interface, due to the
>> ping-packet being too large and being fragmented.
>> Start the tcpdump in another (x)term before running the "ping" ...
>> If you use '-M do', you should get the
>>     "Frag needed and DF set (mtu = NNNN)"
>I switched to '-M do' and found that 1464 is the highest size I can
>ping without the "Frag needed" error.  This means I should add 28 to

The overhead of 28 bytes is just specific to ping.

It means that your upstream has a MTU of 1492 bytes. And it depends on
your local needs if setting this MTU network-wide is the best course. 
I think I and others wrote enough for you to decide.

>and set my MTU to 1492 across the network?

Probably yes. I'd even say: unless you know otherwise for your local
needs. It's a very small "pay" (-0.5% max throughput) locally for a
potentially much bigger gain towards the 'net side, esp. when
factoring in latency ... And BTW: changing the MTU is easy, why not
start with one system? Even temporarily just using ifconfig/ip
commandline (don't forget to set the default-route if you "down" the
connection: 'route add default gw $GW_IP' ) and running some


Actually, NT is more like LSD with all the good effects filtered out.
                                                     -- Andrew Maddox

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