Question

Consider sending a 2400-byte datagram into a link that has an MTU of 700 bytes. Suppose the original datagram is stamped with the identification number 422 . How many fragments are generated? What are the values in the various fields in the IP datagram(s) generated related to fragmentation?

Short answer

4 fragments are generated with offsets 0, 85, 170, and 255, respectively, and all have identification 422.

Step-by-step solution

Understand MTU and Datagram Size

The Maximum Transmission Unit (MTU) is the largest data packet that can be transmitted over a network link in a single frame. Here, the MTU is 700 bytes and the datagram size is 2400 bytes. The original datagram is too large and must be fragmented.

Calculate the Number of Fragments

To determine the number of fragments, divide the total size of the datagram by the MTU: \[ \text{Number of Fragments} = \left\lceil \frac{2400}{700} \right\rceil \]The remainder from this division will require an additional fragment. Performing the calculation: \( \frac{2400}{700} = 3.4286 \), which rounds up to 4 fragments. Therefore, 4 fragments are needed.

Determine Sizes of Fragments

Each fragment, except possibly the last, will have a size of 700 bytes. This includes both the IP header and the data payload. Assuming a 20-byte IP header, each fragment will send 680 bytes of data, except for the last fragment: - First 3 fragments: 680 bytes each - Last fragment: 360 bytes of data and 20 bytes of header to make it 380 bytes.

Calculate Fragment Offsets and Identification

Each fragment will have offsets denoted in units of 8 bytes (64 bits). - Fragment 1: Offset = 0 bytes - Fragment 2: Offset = 680 bytes / 8 = 85 - Fragment 3: Offset = 1360 bytes / 8 = 170 - Fragment 4: Offset = 2040 bytes / 8 = 255 All fragments will have the same identification number of 422 (as assigned by the original datagram).

Set the Fragmentation Flag

Each fragment will have a More Fragments (MF) flag setting: - Fragments 1, 2, and 3 will have MF = 1, indicating more fragments follow. - Fragment 4 (the last fragment) will have MF = 0.

Key concepts

Maximum Transmission Unit (MTU)

The Maximum Transmission Unit (MTU) is a critical concept in computer networking. It determines the largest size of a data packet that can be transmitted over a network link without the need for fragmentation. In simpler terms, if a datagram (a structured block of data) exceeds the MTU, it must be broken down into smaller chunks before transmission.
For example, consider an MTU of 700 bytes. Any data packet larger than this size needs to be fragmented, meaning it will be split into multiple smaller packets, each adhering to the MTU limit. This ensures efficient and uninterrupted transmission across the network.
The MTU plays a key role in optimizing network performance by reducing the chance of packet loss and decreasing the overhead caused by retransmissions. It constitutes a fundamental aspect of configuring and maintaining robust data links.

• Helps determine packet fragmentation.
• Affects network performance and efficiency.
• Integral to network configuration.

Datagram Fragmentation

Datagram fragmentation is the process of breaking down a large datagram into smaller fragments that can be transmitted over a network. This step is necessary when a datagram's size exceeds the MTU of the network link. In the case of a 2400-byte datagram and an MTU of 700 bytes, the datagram must be divided. The need for fragmentation arises because larger packets that exceed the MTU cannot be processed in a single flow across the network. Therefore, they must be split into smaller, manageable pieces.
Each fragment becomes its own packet with an IP header, making the transmission feasible. The fragmentation process includes calculating how many fragments are needed and determining the size of each fragment. The goal is to ensure that every fragment conforms to the MTU limit while maintaining the integrity of the original data.

• Ensures data can be transmitted across networks with varying MTUs.
• Involves breaking down large datagrams into smaller packets.
• Includes creating additional headers for each fragment.

IP Header Structure

The IP header is a vital part of every datagram fragment, encapsulating essential information required for routing and delivery. A standard IP header is usually 20 bytes long and contains numerous fields, including source and destination addresses, total length, and identification. A consistent identification number is used across all fragments stemming from the same original datagram to track and reassemble the fragments at the destination. The header also includes flags and fragmentation offsets that guide the reassembly process. The "More Fragments" flag signals whether a fragment is the last in the sequence, while the "Fragment Offset" indicates the position of this fragment in the sequence relative to the original datagram.

• Provides routing and delivery instructions for fragments.
• Contains identification and offset details.
• Includes a "More Fragments" flag for reassembly indications.

Fragment Offset

Fragment Offset is a crucial field within the IP header of each fragment. It specifies the position or order of a particular fragment within the complete, original datagram. This ensures the accurate reassembly of data at the destination. Offsets are measured in units of 8 bytes. For instance, the first fragment of a datagram has an offset of 0. The second fragment's offset is calculated based on the size of data carried by the preceding fragment. If the first fragment carries 680 bytes of data, its offset is calculated as 680/8 = 85. Fragment offset is key to reordering fragments correctly, especially when they may arrive at different times due to varying network paths. This field is essential for maintaining data integrity and ensuring that the final reassembled datagram matches the original data packet.

• Helps reconstruct the original datagram from fragments.
• Measured in 8-byte units.
• Vital for maintaining correct data order.