Question

What is the minimum overhead to send an IP packet using PPP? Count only the overhead introduced by PPP itself, not the IP header overhead. What is the maximum overhead?

Short answer

Minimum overhead is 7 bytes; maximum practical overhead is 9 bytes.

Step-by-step solution

Understanding the PPP Overhead

The Point-to-Point Protocol (PPP) is commonly used for establishing a direct connection between two networking nodes. It includes certain overhead in its packets, usually in the form of additional bits added to the payload data for various purposes such as framing, error-checking, and control.

Identifying PPP Frame Structure

A PPP frame is typically structured with a 1-byte flag field at the beginning and end, a 1-byte address field, a 1-byte control field, and a 2-byte protocol field. There is also a Frame Check Sequence (FCS) for error detection, which can be either 2 bytes or 4 bytes.

Calculating Minimum Overhead

To find the minimum overhead, consider the smallest PPP frame structure: 1-byte for the starting flag, 1-byte for the address, 1-byte for control, 2-bytes for protocol, and 2-bytes for FCS. This sums to a total of 7 bytes.

Calculating Maximum Overhead

For maximum overhead, the PPP frame can include options such as an extended 4-byte FCS and additional padding or compression options. With the extended FCS, the frame has 1 byte flag, 1 byte address, 1 byte control, 2 bytes protocol, 4 bytes FCS, totaling 9 bytes. Additional padding or compressions can add more options, but standard scenarios typically consider these 9 bytes as the upper limit for practicality.

Key concepts

PPP Frame Structure

The Point-to-Point Protocol (PPP) is a protocol used to create a direct connection between two networking nodes. It is often used over dial-up modems, DSL lines, and other dedicated point-to-point connections. PPP frames have a particular structure designed to encapsulate the data being transmitted while ensuring efficient communication between the nodes.

The basic PPP frame structure includes several fields:

• Flag Field: This 1-byte field signifies the beginning and the end of a frame. It uses a specific pattern to help nodes identify each frame distinctly.
• Address Field: Typically set to all ones (1-byte), even though it generally doesn't affect the point-to-point connection, as it's not used for addressing specific devices.
• Control Field: Also 1-byte long and used to identify the control settings of the frame. For most PPP frames, this field is set to 0x03, indicating unnumbered information.
• Protocol Field: A 2-byte field used to specify the type of payload being carried, such as IP packets or other types of network layer packets.
• Frame Check Sequence (FCS): Used for error-checking. The FCS can either be 2 bytes or 4 bytes, depending on the levels of protection required against errors.

These components work together to create a framework that ensures data is properly framed, transmitted, and checked for errors.

IP Packet Overhead

In networking, 'overhead' refers to the additional data that is required to accompany the primary data payload for proper delivery. When encapsulating IP packets within PPP frames, an overhead is introduced, which accounts for the various extra bytes needed for PPP operation.

Minimum overhead occurs when using the least number of additional bytes required: this includes 1-byte for the starting and ending flag, 1-byte for the address field, 1-byte for the control field, 2-bytes for the protocol field, and a 2-byte Frame Check Sequence (FCS) for error detection. This results in a minimum overhead of 7 bytes.

On the other hand, the maximum overhead is achieved by using extended options in the frame, such as a 4-byte FCS, bringing the total to 9 bytes without additional options. While theoretically, more options could increase this overhead, the typical boundary for practical scenarios stays at these 9 bytes.

Error Detection in PPP

Error detection is a critical component of the Point-to-Point Protocol (PPP). It includes mechanisms to ensure that data transmission between two nodes is reliable and error-free. In PPP, this is primarily accomplished through the Frame Check Sequence (FCS) field.

The FCS can be either 2 bytes or 4 bytes long, depending on the required error detection strength. The sequence is generated using a cyclic redundancy check (CRC) algorithm. When a frame is transmitted, the FCS is calculated and appended to the end of the frame. The receiving node then recalculates the FCS over the received data to verify whether the frame was altered during transit.

If there is a mismatch between the computed FCS and the FCS received, it indicates an error has occurred, and the frame should be discarded. This error-detection mechanism helps maintain data integrity by ensuring that corrupted frames do not propagate through the network.

Network Protocols

PPP is just one example among numerous network protocols used to manage the transmission of data across networks. Network protocols establish the rules and conventions for communication between network devices, playing a vital role in ensuring reliable data exchange.

Other protocols are found operating at various layers of the Internet Protocol Suite. For instance:

• Transmission Control Protocol (TCP): A connection-oriented protocol ensuring reliable transmission of data. It verifies packet delivery through acknowledgment signals.
• Internet Protocol (IP): Identifies devices across networks and routes data from the source to the destination.
• Hypertext Transfer Protocol (HTTP): Used for transferring hypertext requests and information on the World Wide Web.
• Simple Mail Transfer Protocol (SMTP): Manages the sending and forwarding of emails over the Internet.

Network protocols like PPP highlight the collaborative structuring and standardization needed to enable seamless communication across diverse network settings. Understanding them aids in having a robust grasp of how data moves efficiently across various systems.