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Hello, |
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|
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On Tue, 20 Sep 2016, Grant wrote: |
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>>>Strangely, I'm able to ping with that command even with a very high -s value: |
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>>> |
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>>>$ ping -c 4 -M dont -s 9999 www.dslreports.com |
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>>>PING www.dslreports.com (64.91.255.98) 9999(10027) bytes of data. |
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>>>10007 bytes from www.dslreports.com (64.91.255.98): icmp_seq=1 ttl=54 |
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>>>time=331 ms |
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>>>10007 bytes from www.dslreports.com (64.91.255.98): icmp_seq=2 ttl=54 |
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>>>time=329 ms |
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>>>10007 bytes from www.dslreports.com (64.91.255.98): icmp_seq=3 ttl=54 |
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>>>time=329 ms |
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>>>10007 bytes from www.dslreports.com (64.91.255.98): icmp_seq=4 ttl=54 |
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>>>time=329 ms |
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>>> |
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>>>4 packets transmitted, 4 received, 0% packet loss, time 3003ms |
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>>>rtt min/avg/max/mdev = 329.159/329.877/331.612/1.158 ms |
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>> |
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>> Look again! You're just looking at the _PING_ packets, not the ICMP/IP |
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>> packets actually going over the interface! You'll need to run |
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>> 'tcpdump icmp' in parallel! "My ping" also just reports 1 packet, but |
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>> there's two IP packets actually going over the interface, due to the |
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>> ping-packet being too large and being fragmented. |
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>> |
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>> Start the tcpdump in another (x)term before running the "ping" ... |
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>> |
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>> If you use '-M do', you should get the |
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>> |
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>> "Frag needed and DF set (mtu = NNNN)" |
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> |
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> |
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>I switched to '-M do' and found that 1464 is the highest size I can |
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>ping without the "Frag needed" error. This means I should add 28 to |
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>that |
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|
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The overhead of 28 bytes is just specific to ping. |
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|
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It means that your upstream has a MTU of 1492 bytes. And it depends on |
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your local needs if setting this MTU network-wide is the best course. |
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I think I and others wrote enough for you to decide. |
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|
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>and set my MTU to 1492 across the network? |
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|
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Probably yes. I'd even say: unless you know otherwise for your local |
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needs. It's a very small "pay" (-0.5% max throughput) locally for a |
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potentially much bigger gain towards the 'net side, esp. when |
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factoring in latency ... And BTW: changing the MTU is easy, why not |
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start with one system? Even temporarily just using ifconfig/ip |
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commandline (don't forget to set the default-route if you "down" the |
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connection: 'route add default gw $GW_IP' ) and running some |
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tests/benchmarks. |
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|
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HTH, |
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-dnh |
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|
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-- |
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Actually, NT is more like LSD with all the good effects filtered out. |
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-- Andrew Maddox |
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