Why Fragment Packets?

• Data link layers typical impose a Maximum Transmission Unit (MTU) size that limits the length of the data portion of frames

• • Ethernet: 1500 bytes
  • FDDI: 4000 bytes
  • PPP: Varies, typically 576-1500

• If an application tries to transmit a packet larger than the MTU, the IP layer must fragment the packet

• • Fragmentation can also occur at routers

• Fragmented packets are not re-assembled until they reach the destination

• • Thus, fragmentation and re-assembly are mostly transparent to the higher layers