Question

Suppose the information portion of a packet ( \(D\) in Figure \(5.3\) ) contains 10 bytes consisting of the 8-bit unsigned binary ASCII representation of string "Networking." Compute the Internet checksum for this data.

Short answer

The Internet checksum is computed by converting, summing and one's complementing the ASCII binary representations of "Networking."

Step-by-step solution

Convert ASCII Characters to Binary

The string "Networking" consists of 10 characters. Each character has an ASCII value that can be represented as an 8-bit binary number. The string is broken down into individual characters and converted to binary. For instance, 'N' is 78 in ASCII, which is 01001110 in binary.

Group the Data into 16-bit Words

Given each character is an 8-bit binary, group them into consecutive pairs to form a 16-bit binary word. For example, the first two characters 'N' (01001110) and 'e' (01100101) would form the 16-bit binary: 0100111001100101.

Calculate the Sum of 16-bit Words

Add all the 16-bit binary words formed from the string. If the sum exceeds 16 bits, wrap around any overflow by adding it back into the lowest order bit. Continue summing while ensuring any carry-over is added back into the lowest bit.

Convert Sum to One's Complement

Once you have the sum of all 16-bit words, take the one's complement. To find the one's complement, switch all 0s to 1s and all 1s to 0s in the binary number to get the final checksum.

Key concepts

binary ASCII representation

When working with data, especially in networking contexts, it's important to understand how characters are represented in binary form. ASCII, which stands for American Standard Code for Information Interchange, is a character encoding standard for electronic communication.

Each character in the ASCII library has a corresponding number. For example, 'A' is represented by the decimal number 65. Computers, however, deal in binary. Therefore, the decimal numbers are converted to an 8-bit binary code. In 8-bit binary, 'A' is represented as 01000001.

In our scenario, the word "Networking" is broken down letter by letter, and each letter's ASCII value is converted into an 8-bit binary number. This process makes it easier for computers to manipulate the data since binary is the language of computers. These binary numbers are what’s referred to as binary ASCII representation.

16-bit words

Once the ASCII characters are converted into binary, the next step in processes like checksumming involves grouping these binary numbers. Here we use the concept of 16-bit words.

Since each ASCII character is represented by an 8-bit binary code, two such characters can be combined to form a 16-bit word. For instance, the character 'N' with binary 01001110 and 'e' with binary 01100101 can be combined to form the 16-bit binary: 0100111001100101.

This step is crucial because many protocols in networking, such as the Internet Protocol (IP), expect data to be processed in 16-bit segments. This essentially transforms a series of 8-bit binary numbers into longer 16-bit unsigned integers, making them ready for checksum calculations.

one's complement

One's complement is a mathematical operation used in computing which involves flipping all bits in a binary number. In simpler terms, every 0 is changed to a 1, and every 1 is changed to a 0.

In the context of calculating an internet checksum, once we have summed up all the 16-bit words created from our data, the final step before obtaining a checksum is to take the one's complement of this sum.

This operation is necessary because it forms a bit-wise negation of our binary sum. The resulting one's complement is then used as the checksum, ensuring that any errors in the data can be detected during data transmission, as the checksum value will not match if the data has been altered.

packet data

In networking, data is transmitted in the form of packets. A packet is essentially a block of data transferred over a network, incorporating both header and payload (data).

The payload, in this task's context, contains the binary representation of our chosen string, like "Networking." To ensure the data's integrity during transmission, a checksum is calculated upon the packet’s creation. This checksum, often computed as detailed in our steps, is then sent along with the packet.

The receiving end recalculates the checksum from the received data. If the newly calculated checksum does not match the transmitted checksum, it’s an indication that the data has been corrupted. This method is part of error-checking processes that assure data integrity over unreliable networks.