What MTU means
The maximum transmission unit is the largest payload a link can carry in a single frame. Ethernet commonly uses an MTU of fifteen hundred bytes. A packet larger than the MTU of a link cannot pass as one piece.
Fragmentation
When IPv4 must send an oversized packet, a router can fragment it into smaller pieces that each fit the link MTU. The destination reassembles them using identification, offset, and a more fragments flag in the IP header.
- Fragmentation costs CPU at routers and the destination.
- Losing one fragment forces retransmission of the whole original packet.
- Firewalls and some middleboxes drop fragments, breaking connectivity.
To avoid fragmentation, hosts use path MTU discovery, sending packets with the do not fragment bit set and shrinking size when a router replies that the packet was too big. In IPv6 routers never fragment in transit, so the source must size packets correctly using path MTU discovery from the start.
Key idea
Fragmentation splits oversized packets to fit a link, but path MTU discovery avoids the cost by sizing packets right.