01.27.09

Max packet size

Posted in Miscellaneous, windows at 2:04 pm by

When you send a large amount of data over the internet that data is being split up into smaller packets and is reassembled on the receiver side. The maximum size of such an individual packet is called Maximum Transmission Unit (MTU), and refers to the size (in bytes) of the largest packet or frame that a given layer of a communications protocol can pass onwards.

If somewhere on your network a MTU value is changed this can cause havoc for your communications. You could end up with a dead vpn tunnel. A quick way to determine what the largest packet is you can send to an ip is to use ping.

PING google.be -f -l 1472

-f Set Don’t Fragment flag in packet.
-l size Send buffer size.

When you packet size is to big you will get:

Packet needs to be fragmented but DF set.

When the packet size is not to big, it will just ping.

Once you know what the MTU should be you can config this on the application/protocol which is having issues.