IP Datagram Fragmentation

 

IP Datagram Fragmentation

Fragmentation is the process of dividing a large IP datagram into smaller fragments so that it can travel through a network whose maximum frame size is smaller than the original datagram.

Why is fragmentation needed?

Different networks can support different maximum packet sizes. The maximum amount of data that a link-layer frame can carry is called the Maximum Transfer Unit (MTU).

For example:

• Network A → MTU = 1500 bytes
• Network B → MTU = 576 bytes

If a router receives a 1200-byte IP datagram from Network A and needs to forward it through Network B, the 1200-byte datagram is too large for the 576-byte MTU.

Therefore, the router fragments the datagram into smaller pieces.

4

How fragmentation works

Suppose an IP datagram is larger than the MTU of the next network:

Large IP Datagram
       │
       ▼
    Router
       │
       │ Fragmentation
       ▼
 ┌─────────┬─────────┬─────────┐
 │Fragment1│Fragment2│Fragment3│
 └─────────┴─────────┴─────────┘
       │
       ▼
  Destination
       │
       ▼
   Reassembly

The router divides the original datagram into smaller fragments. Each fragment is then transmitted separately.

At the destination, the fragments are reassembled to reconstruct the original IP datagram.

Fields used for fragmentation

IPv4 uses three fields in the IP header to support fragmentation:

1. Identification – identifies fragments belonging to the same original datagram.
2. Flags – provides control information about fragmentation.
3. Fragmentation Offset – indicates the position of each fragment in the original datagram.

What is MTU?

Maximum Transfer Unit (MTU) is the maximum size of the payload that a particular link-layer protocol can carry in one frame.

Therefore:

If an IP datagram is larger than the MTU of the network through which it must travel, fragmentation may be required.

Simple example

Suppose:

IP Datagram = 4000 bytes
Next network MTU = 1500 bytes

The datagram cannot fit into a single frame. The router therefore divides it into several smaller IP fragments, each small enough to fit within the MTU.

Key point for students:

MTU determines the maximum size that can be carried over a particular link; fragmentation allows a large IP datagram to be divided into smaller pieces when necessary.