The MTU is specified in terms of bytes or octets of the largest PDU that the layer can pass onwards.
Underlying data link and physical layers usually add overhead to the network layer data to be transported, so for a given maximum frame size of a medium, one needs to subtract the amount of overhead to calculate that medium's MTU.
A larger MTU brings greater efficiency because each network packet carries more user data while protocol overheads, such as headers or underlying per-packet delays, remain fixed; the resulting higher efficiency means an improvement in bulk protocol throughput.
A larger MTU also requires processing of fewer packets for the same amount of data.
Another potential problem is that higher-level protocols may create packets larger than even the local link supports.
IPv4 allows fragmentation which divides the datagram into pieces, each small enough to accommodate a specified MTU limitation.
In the context of Internet Protocol, MTU refers to the maximum size of an IP packet that can be transmitted without fragmentation over a given medium.
The MTU should not be confused with the minimum datagram size (in one piece or in fragments) that all hosts must be prepared to accept.
[3]: 25 The IP MTU and Ethernet maximum frame size are configured separately.
If jumbo frames are allowed in a network, the IP MTU should also be adjusted upwards to take advantage of this.
It works by sending packets with the DF (don't fragment) option in the IP header set.
This information allows the source host to reduce its assumed path MTU appropriately.
A failure of Path MTU Discovery carries the possible result of making some sites behind badly configured firewalls unreachable.
One can possibly work around this, depending on which part of the network one controls; for example one can change the MSS (maximum segment size) in the initial packet that sets up the TCP connection at one's firewall.
[23] Network switches and some repeater hubs have a built-in capability to detect when a device is jabbering.