The maximum transmission unit (MTU) is the largest size of data packet that a given layer of a communications protocol can transmit. For IP, the maximum datagram size is 65,536 bytes, but the underlying data link layer often imposes a lower limit, such as 1,500 bytes for Ethernet. If an IP packet exceeds the MTU, it must be fragmented into smaller pieces for transmission.

4. Maximum Transmission Unit
• Maximum size of IP datagram is 65536, but the data link layer
protocol generally imposes a limit that is much smaller
• Example:
– Ethernet frames have a maximum payload of 1500 bytes
Æ IP datagrams encapsulated in Ethernet frame cannot be longer
than 1500 bytes
than 1500 bytes
• The limit on the maximum IP datagram size, imposed by the
data link protocol is called maximum transmission unit
data link protocol is called maximum transmission unit
(MTU)
• MTUs for various data link protocols:
• MTUs for various data link protocols:
Ethernet: 1500 FDDI: 4352
802.3: 1492 ATM AAL5: 9180
802.5: 4464 PPP: negotiated

5. Example of Fragmentation
Example of Fragmentation
• A datagram with size 2,400 bytes must be
fragmented according to an MTU limit of 1,000
bytes