IP Fragmentation Reassembly

IP Fragmentation :

If the MTU of egress network is less than MTU of packet and DF bit is set to 0,

then fragmentation is needed .

Fields involved in fragmentation :

1 . Fragment id

2 . offset

3 . Flag

Fields changed when fragmentation :

Apart from above 3 , following fields are changing in fragmentation

1 . Header length and total length

2 . Header checksum

3 . Options

A Packet Fragmentation Example

If a 2,366 byte packet enters an Ethernet network with a default MTU size,

it must be fragmented into two packets.

The first packet will:

· Be 1,500 bytes in length. 20 bytes will be the IP header, 24 bytes will be the TCP header, and 1,456 bytes will be data.

· Have the DF bit equal to 0 to mean "May Fragment" and the MF bit equal to 1 to mean "More Fragments."

· Have a Fragmentation Offset of 0.

The second packet will:

· Be 910 bytes in length. 20 bytes will be the IP header, 24 bytes will be the TCP header, and 866 bytes will be data.

· Have the DF bit equal to 0 to mean "May Fragment" and the MF bit equal to 0 to mean "Last Fragment."

· Have a Fragmentation Offset of 182 (Note: 182 is 1456 divided by 8

Please note that fragmented packets can also get fragmented.

Reassembly :

Reassembly is done at the destination . The reason is fragmented packets can traverse

independently .