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MTU: Difference between revisions
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=What is my MTU?=
If you can ping a box, you can check what the largest packet size you can get to it is. AAISP customers can ping 81.187.81.187, which is the next hop from your DSL line.
For example:
^^ That's the ping command to remember. The rest of this section is a bit of info about that command.
Explanation:
'''-c1''' In these examples, we've only used a count of one ping using the -c option. ▼
'''-M do''' Use the path discovery options to ping. The option is -M. See the man page for all the options.▼
''' -s 1472''' When setting size in ping, the size is the payload size - not the full packet size. The full packet size is payload + ICMP header size (28 bytes). The option for payload size is -s.▼
Note: ping is slightly different on different operating systems
▲Use the path discovery options to ping. The option is -M. See the man page for all the options.
ping -D -s 1472 81.187.81.187
▲When setting size in ping, the size is the payload size - not the full packet size. The full packet size is payload + ICMP header size (28 bytes). The option for payload size is -s.
▲In these examples, we've only used a count of one ping using the -c option.
▲Note: ping is slightly different on different operating systems.
==Two quick examples:==
Checking if you really have 1500 MTU:
% ping -c1 -M do -s 1472 81.
PING 81.
1480 bytes from 81.2.100.169: icmp_req=1 ttl=59 time=12.4 ms
--- 81.
1 packets transmitted, 1 received, 0% [[Packet Loss|packet loss]], time 0ms
rtt min/avg/max/mdev = 12.491/12.491/12.491/0.000 ms
Checking if you have smaller than 1500 MTU:
% ping -c1 -M do -s 1472 81.187.
PING 81.187.
From 90.155.53.53 icmp_seq=1 Frag needed and DF set (mtu = 1492)
1472 bytes from 81.187.213.39: icmp_req=1 ttl=59 time=29.5 ms
--- 81.187.
1 packets transmitted, 1 received, 0% [[Packet Loss|packet loss]], time 0ms
rtt min/avg/max/mdev = 29.595/29.595/29.595/0.000 ms
Yay a reply! To double check, lets try one byte larger:
% ping -c1 -M do -s 1465 81.187.
PING 81.187.
From 90.155.42.36 icmp_seq=1 Frag needed and DF set (mtu = 1492)
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