Question

Suppose datagrams are limited to 1,500 bytes (including header) between source Host A and destination Host B. Assuming a 20-byte IP header, how many datagrams would be required to send an MP3 consisting of 5 million bytes? Explain how you computed your answer.

Short answer

3379 datagrams are needed to send the 5 million byte MP3 file.

Step-by-step solution

Determine Payload Size

Start by determining the maximum payload size of each datagram. Given that the IP header is 20 bytes, the payload size is obtained by subtracting the header size from the total datagram size. Thus, the maximum payload size is \( 1500 - 20 = 1480 \) bytes.

Calculate Number of Datagrams

Divide the total size of the MP3 file by the payload size to calculate the number of datagrams needed. This is done by dividing 5,000,000 bytes by 1480 bytes per datagram: \( \frac{5,000,000}{1480} \approx 3378.38 \). Since partial datagrams are not possible, round up to get the total number of datagrams, which is 3379.

Key concepts

Datagram

In networking, the term **datagram** refers to a basic transfer unit associated with a packet-switched network. Datagram-based communication is common in systems that utilize the Internet Protocol (IP), which is a foundational component of the internet. Each datagram consists of a destination address, header fields, and the data payload. The destination address is vital for routing to the correct location.

Characteristics of datagrams include:

• They operate connectionless, meaning each datagram is sent independently and must have complete addressing information.
• They can take different paths through the network and may arrive out of order or may be lost entirely.
• The responsibility for reassembling the data correctly is typically with the receiver or an upper-layer protocol like TCP.

Understanding datagrams helps in grasping how data is broken up for transfer over networks, especially when considering the bandwidth limitations and routing efficiency.

IP Header

The **IP header** is a critical component of a datagram. It contains essential information required to route and deliver the datagram to the correct recipient. Typical header details include the source and destination IP addresses, which are used by routers to determine the path each datagram should travel through the network.

The header also includes:

• Version and IHL (Internet Header Length) - dictates the IP version (IPv4 or IPv6) and the header length.
• Type of Service - indicates how the packet should be handled (e.g. prioritized).
• Total Length - represents the total length of the datagram, including both the header and payload.
• Fragment Offset, Time to Live (TTL), Protocol, and Header Checksum - other fields that manage fragmentation, lifespan, error-checking, and protocol-specific information.

Typically, an IP header is around 20 bytes long in an IPv4 datagram, though options can extend it. This detailed metadata is indispensable for efficient and correct delivery.

Payload Size

The **payload size** of a packet or datagram is the portion of the packet that contains the actual data being sent across the network, excluding headers. In many networking protocols, understanding the payload size is crucial for optimizing transmission efficiency and ensuring data integrity. For example, in the given exercise, the total size of a datagram is 1,500 bytes, including a 20-byte IP header, leaving a maximum payload size of 1,480 bytes.

Here are a few important aspects of payload size:

• Shorter payloads result in more headers relative to data, potentially reducing transmission efficiency.
• Understanding payload size is key in scenarios that require data fragmentation, as large payloads may need to be divided into multiple datagrams.
• Packet size has a direct impact on the use of network resources and affects the speed and reliability of data transmission.

Maximizing payload size while keeping data integrity is a balancing act that network engineers must optimize for in any robust system design.

Fragmentation

**Fragmentation** occurs when data exceeds the maximum payload size of a network's datagram. Fragmentation is necessary because different networks and technologies may have different maximum transmission sizes (MTU). For example, if the data packet is larger than the MTU of the network it is entering, fragmentation will break it into smaller packets, each complete with its header for transmission.

Key points about fragmentation include:

• IP fragmentation divides large packets into multiple smaller packets, each with a part of the original payload.
• Each fragment is treated as a separate entity, with its header information allowing it to be independently routed to the destination.
• Reassembly of fragmented packets is typically handled by the receiving device, ensuring the original message is reconstructed.
• Fragmentation can increase overhead, as each fragment requires its own header and potentially additional processing to reassemble.

The need for fragmentation indicates why understanding and managing payload sizes are important. Engineers must design systems capable of efficiently handling fragmentation to ensure reliable network performance.