Question

Why do you think IPv4 has fragment reassembly done at the endpoint, rather than at the next router? Why do you think IPv6 abandoned fragmentation entirely? Hint: Think about the differences between IP-layer fragmentation and link-layer fragmentation.

Short answer

IPv4 reassembles fragments at the endpoint to reduce router load; IPv6 uses PMTU discovery to eliminate fragmentation, enhancing network efficiency.

Step-by-step solution

Understanding Fragmentation Basics

Fragmentation occurs when a packet is too large to be transmitted in a single frame over the underlying network; therefore, it must be split into smaller fragments.

IPv4 Fragment Reassembly at Endpoint

In IPv4, once a packet is fragmented, each fragment can independently travel through different network paths. Reassembly at the final destination ensures that all fragments, regardless of their intermediary paths, are gathered before reconstructing the original packet.

Resource Efficiency and Performance

Reassembling at each router would require significant processing power and memory, increasing the overall latency and reducing efficiency. Routers typically handle high volumes of traffic, hence performing reassembly at the endpoint avoids these resource constraints.

IPv6 and Abandonment of Fragmentation

IPv6 relies on end-to-end communication and path MTU discovery (PMTU) to ensure packets are sent within the size limit acceptable by all intermediate networks. This helps to eliminate fragmentation issues.

Difference Between IP-layer and Link-layer Fragmentation

IP-layer fragmentation occurs at the network layer and can be complex due to the diverse paths that fragments can take. Link-layer fragmentation happens within a single link and is manageable by network interfaces.

Conclusion

IPv4 reassembles fragments at the endpoint to optimize router performance and efficiency. IPv6 avoids fragmentation by employing PMTU discovery, thus improving network efficiency and reliability.

Key concepts

Packet Fragmentation

When data is transmitted over a network, it often needs to be broken down into smaller pieces, called packets. Sometimes, these packets are still too large to be transmitted and require fragmentation. This means splitting the packet into even smaller fragments.
Fragmentation ensures that data can traverse network paths that have size limitations (or Maximum Transmission Units - MTUs). While IPv4 allows fragmentation to manage these limits, IPv6 takes a different approach by using Path MTU Discovery to avoid fragmentation.

Fragment Reassembly

Once packets are fragmented, they need to be reassembled back into their original form. In IPv4, this reassembly is done at the endpoint or destination. This approach allows each fragment to take independent paths through the network and ensures the reassembly doesn't overload intermediate routers.
Reassembling at the router level would require extra processing power and memory, which isn't efficient for routers handling large volumes of traffic. By handling reassembly at the endpoint, it improves overall network performance and reduces latency.

Path MTU Discovery

IPv6 relies heavily on Path MTU Discovery (PMTU) to manage packet sizes. PMTU works by determining the smallest MTU along the packet's path from source to destination.
By doing this, IPv6 can ensure that packets are within the acceptable size limits of all intermediate networks without the need for fragmentation. This method not only avoids the overhead of fragmenting and reassembling packets, but also enhances the efficiency and reliability of the network.

Network Efficiency

Efficiency is a key component in modern network protocols. IPv6 was designed to improve efficiency over IPv4 by eliminating fragmentation. Fragmentation adds latency because of the need for reassembly and the extra processing required at routers or endpoints.
By using PMTU, IPv6 can send appropriately sized packets from the start, reducing delays and improving the speed and reliability of data communication. Efficient data transfer is crucial for maintaining fast and responsive internet connections.

Link-layer Fragmentation

In addition to IP-layer fragmentation, there's also link-layer fragmentation. This occurs closer to the data transmission level within a single link of the network. Link-layer fragmentation is generally easier to manage because it's handled by network interfaces and doesn't involve the complex routing and independent pathing of IP-layer fragmentation.
While IP-layer fragmentation was necessary in IPv4, IPv6 avoids this by relying on PMTU, thus simplifying the process and improving overall network performance. Understanding the differences between these fragmentation types helps in grasping why certain design choices were made in IPv6.