Question

Describe a way to reassemble IP fragments at the destination.

Short answer

IP fragments are reassembled by matching IDs, ordering by offset, and checking the 'more fragments' flag.

Step-by-step solution

Identify Incoming Fragments

When an IP packet is fragmented, each fragment is sent as a separate independent packet with its own header. At the destination, the first step is to identify these fragments as belonging to the same original packet. This is done by matching the source IP address, destination IP address, identification number, and protocol. Each fragment carries these values, which help in identifying fragments that belong together.

Collect Fragments Based on Offset Values

Each fragment has an offset value in its header, which indicates the position of the fragment's data within the original IP packet. The destination collects fragments and uses these offset values to determine their order. The offset value helps in placing each fragment at the correct location within the reassembled packet.

Check for Final Fragment

During the reassembly process, the destination checks for the last fragment to know when reassembly is complete. The last fragment has the 'more fragments' flag cleared (set to 0), indicating no more fragments are expected after this one.

Reassemble Packet

Once all fragments are received and their order is determined using offset values, the destination uses these offsets to reconstruct the original IP packet. The payloads from each fragment are combined in order, creating a contiguous block of data that represents the complete original packet.

Key concepts

IP Fragment Identification

When a large data packet is too big for a network's maximum transmission unit (MTU), it gets broken into smaller chunks called fragments. Each of these fragments is treated as an individual packet that carries its own header, but they all contain information that connects them back to the original large packet.

The heart of this process is the 'IP Fragment Identification'. The identification field in the IP header of each fragment bears the same value as the original packet. This value allows the receiving device to determine which fragments belong to which original packet.

To ensure accurate reassembly, fragments must also match on several other crucial attributes—source IP address, destination IP address, and protocol. These elements ensure correct pairing of the fragments as they journey towards their destination, even if they take different paths through the network.

IP Fragment Offset

Fragment offset is a critical parameter that specifies where a particular piece of data belongs in the reassembled packet. Each fragment except the last one is of a maximum size dictated by the network's MTU, and the offset field tells where this fragment's data should fit into the original packet's data sequence.

The offset value is presented in units of 8 bytes. For example, if the offset is 1, then this fragment begins 8 bytes into the original data. This system allows routers to slice up large packets into appropriately sized fragments and lets the receiving device put all the pieces back together in the right order.

Thus, understanding and correctly reading these offset values is important to eliminate confusion and ensure data integrity during reassembly.

Final Fragment Flag

In the world of data fragmentation, not every fragment is created equal. One very special fragment is the last fragment, identified by the "Final Fragment Flag", which is actually the converse known as the 'More Fragments' flag.

When the source device sends fragments, each fragment has the 'More Fragments' flag set except for the last one. This flag tells the receiving device that more fragments are on the way. When a device receives a fragment with this flag set to 0, it knows this piece is the end of the original packet's data.

Understanding this flag is vital because without it, the receiving system cannot ascertain if the entire packet has arrived, leading to incomplete data reassembly and a failed packet reconstruction.

Packet Reassembly Process

The ultimate goal of IP fragmentation is to reconstruct the original data packet at its final destination. Once all fragments arrive as individual packets, the reassembly process begins.

During this process, each fragment is matched using the identification field, and their offset values guide the exact order in which they are pieced together. This reordering is like putting together a jigsaw puzzle.

The device keeps building the data sequence until it encounters a fragment that has the 'More Fragments' flag cleared, indicating that all pieces are now collected. Once reassembled, the data can be processed or forwarded to its next hop in the network.

• Identify fragments using consistent ID numbers and addresses.
• Use offset values to determine the sequence of fragments.
• Verify the final fragment to ensure complete data is present.

Understanding the packet reassembly process is essential for ensuring data is correctly reassembled and transmitted over networks without data loss or corruption.