# Maximum Transmission Unit
###### tags: `elearning` `Multimedia Wireless Network`
###### Note Page : [:link:](https://hackmd.io/@YTL0307/HJEb6jWCo)

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## **Challenges Faced by Packets**
- Take a look at one of the challenges that packets face when they traverse different types of networks.
- IP's main responsibility is to deliver packets between devices.
- Remember that a Layer 3 or IP packet is encapsulated in a Layer 2 frame.
- But before a packet can be encapsulated in a Layer 2 frame, a device needs to make sure that the packet is small enough to fit into that Layer 2's frame size.
- Different Layer 2 technologies have different frame formats
- Each Data Link technology, such as Ethernet or FDDI, has a fxed upper limit on the size of a packet that can be sent in a single frame.
- This limit is known as the MTU (Maximum Transmission Unit)

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## **IP Fragmentation**
- What happens when a router receives an IP packet that is larger than the next Layer 2 MTU?
- That's where another responsibility of IP comes into play.
- IP was designed to accommodate these differences through a process called fragmentation.
- If a packet is bigger than the MTU of the Layer 2 technology, a device might need to break the packet into several pieces or fragments.
- The fragments are then sent individually and reassembled into the original packet by the receiving device.
- When Host A begins generating packets, it checks the MTU for its local network, which in this case is 1500.
- As the packet crosses routers, each router checks the MTU for the next network in the path.
- If the network has a smaller MTU than the current packet size the router fragments the packet into smaller pieces.
- Each fragment contains an IP header that duplicates most of the original header followed by as much data as can be carried to keep the total length smaller than the network's MTU.
- The fragments are now sent as separate packets until they reach the destination, where they are reassembled by Host B.