In data communication, which sequence correctly lists the typical steps involved in creating an Internet style checksum over a block of data?

Introduction / Context:
Checksums are used in networking protocols such as Transmission Control Protocol and Internet Protocol to detect errors in transmitted data. The classic Internet checksum algorithm is based on one's complement arithmetic. Although modern systems sometimes use stronger error detection codes, understanding this checksum remains important for exam questions and protocol analysis. This question asks you to identify the correct sequence of steps used to create such a checksum over a block of data.

Given Data / Assumptions:

• The checksum is calculated over a sequence of bytes representing header or payload data.
• The Internet checksum uses one's complement addition of fixed size words, typically 16 bit words.
• Carry bits from addition are wrapped around and added back into the sum.
• The final checksum is the one's complement of the accumulated sum.

Concept / Approach:
The Internet checksum algorithm operates as follows. First, the data is divided into fixed size words, usually 16 bits each. These words are then added together using one's complement addition, which means that if the sum produces a carry out of the most significant bit, that carry is wrapped around and added back into the least significant bits. After all words are added, any remaining carry is added back again until there is no carry. Finally, the one's complement of the resulting sum is taken, and this value is used as the checksum. At the receiver, the same procedure is applied to the data plus the checksum; a correct packet yields an all ones pattern.

Step-by-Step Solution:
Step 1: Start with the data block that needs protection and divide it into equal sized words such as 16 bit chunks. Step 2: Add the first two words using normal binary addition. Step 3: If the sum produces a carry out of the most significant bit, wrap that carry around and add it to the lower bits to form a one's complement sum. Step 4: Continue adding the remaining words using the same one's complement addition, always folding any carry back into the sum. Step 5: After all words have been added, take the one's complement of the final sum to form the checksum value that will be stored or transmitted.

Verification / Alternative check:
To verify the algorithm, consider a simple example with two 16 bit words. Add them using one's complement rules, then complement the result to obtain the checksum. At the receiver, add all three 16 bit values (word1, word2, checksum) using one's complement addition. The result should be all ones, which indicates that no error was detected. If any bit in the data is flipped, the final sum will differ from the all ones pattern, and the packet can be considered corrupt.

Why Other Options Are Wrong:
Option b describes sorting and encryption, which are not part of checksum calculation and would not provide a simple, fast error detection method. Option c suggests multiplying bytes and computing an average, which lacks the mathematical properties needed for robust error detection. Option d talks about reversing bytes and adding a parity bit, which might detect some single bit errors but does not follow the Internet checksum algorithm described in networking standards.

Common Pitfalls:
A common mistake is forgetting to fold carry bits back into the lower bits when using one's complement arithmetic. Another pitfall is confusing one's complement checksum with cyclic redundancy checks, which use polynomial division and are much stronger. For exam questions about the Internet checksum, always remember the sequence: split into words, add with one's complement arithmetic, fold carries, and complement the final sum.

Final Answer:
The correct sequence is to divide the data into fixed size words, add them using one's complement arithmetic, add any carry bits back into the sum, and then take the one's complement of the final sum to form the checksum.