Problem 19 - Computer Networks: Global Edition

Question

An IP packet to be transmitted by Ethemet is 60 bytes long, including all its headers. If LLC is not in use, is padding needed in the Ethernet frame, and if so, how many bytes?

Step-by-Step Solution

Answer

Yes, 4 bytes of padding are needed.

Step 1: Understanding Ethernet Frame Requirements

Ethernet frames must have a minimum size of 64 bytes. This includes headers and the data payload. In this problem, the IP packet (including headers) is 60 bytes.

Step 2: Calculate Missing Bytes for Padding

Since Ethernet frames need to be at least 64 bytes and the given IP packet is 60 bytes, we need to find out how many additional bytes are required to reach this minimum size.

Step 3: Determine Padding Required

Subtract the given IP packet size (60 bytes) from the minimum Ethernet frame size (64 bytes): \[64 - 60 = 4 \text{ bytes}\]This calculation shows that 4 additional bytes are needed to reach the minimum frame size.

Key Concepts

IP Packet

An Internet Protocol (IP) packet is a fundamental component of data communication in network systems. It consists of several parts that include the IP headers and the actual payload. Let’s break down what this means:

• IP Header: This is a crucial part of the packet, containing information required for routing and delivery in a network. It includes details such as the source IP address, destination IP address, and other protocol information that ensures data reaches its correct destination.
• Payload: This is the actual data being transmitted across the network. In the exercise in question, the payload along with the headers amounts to 60 bytes.

Understanding the size and structure of an IP packet is essential because it directly connects to the need for adjustments like padding when data is encapsulated in larger structures, such as Ethernet frames.

Ethernet Minimum Frame Size

Ethernet is a ubiquitous technology used for local area networking (LAN). A distinct characteristic of Ethernet is its requirement for a minimum frame size of 64 bytes. Here’s why this limit exists:

• Collision Detection: Ethernet employs a method called Carrier Sense Multiple Access with Collision Detection (CSMA/CD) to manage how data packets are sent over the network. The minimum frame size ensures that the system can detect these collisions effectively, maintaining network efficiency.
• Frame Structure: The 64-byte frame includes both the data payload and additional headers that contribute to its effective transmission. Headers typically include the destination address, source address, and type information.

Adhering to this minimum size is crucial. If a packet falls short, extra bytes, or padding, are added. This process ensures consistent network performance and management.

Padding Calculation

Padding refers to the additional bytes added to meet the Ethernet minimum frame size requirement. If the entire Ethernet frame, which includes the IP packet, falls under the required 64 bytes, padding is necessary. Let’s go through the specifics:

• Initial Calculation: With the IP packet in the exercise being 60 bytes long, but necessary to meet a 64-byte minimum, padding is calculated by finding the difference between the frame's current size and the minimum requirement.
• Calculation Formula: The equation for determining padding is simple: \[ ext{Required Padding} = ext{Ethernet Minimum Frame Size} - ext{IP Packet Size} \] Applying this to the problem gives us: \[ 64 - 60 = 4 ext{ bytes} \]

Thus, 4 bytes of padding are required. This padding ensures that all Ethernet frames are adequately sized for transmission, maintaining network standards and facilitating smooth data transfer.