What is the role of the checksum in TCP communication?

1 How checksums are calculated

A checksum is a simple way of detecting errors in data transmission. It is based on the idea that if you add up all the bits in a packet, you should get the same result on both ends. There are different methods of calculating checksums, but one of the most common ones is the ones complement method. In this method, you add up all the 16-bit words in the packet and then take the ones complement of the result. The ones complement is obtained by flipping all the bits, or subtracting the result from 2^16 - 1. The checksum is then appended to the end of the packet as an extra 16-bit word.

2 How checksums are verified

When a packet arrives at the receiver, it is checked for errors using the checksum. The receiver performs the same calculation as the sender, adding up all the 16-bit words in the packet, including the checksum. If there are no errors, the result should be all ones (or 2^16 - 1). If there are errors, the result will be different. The receiver then compares the result with the expected value of all ones. If they match, the packet is accepted; if they differ, the packet is rejected and a request for retransmission is sent to the sender.

3 What errors can checksums detect

Checksums are effective at detecting random errors that occur due to noise or interference in the transmission medium. They can detect errors that affect one or more bits in a packet, such as bit flips, insertions, or deletions. However, checksums are not foolproof and cannot detect all types of errors. For example, they cannot detect errors that cancel each other out, such as two bits being flipped in opposite directions. They also cannot detect malicious errors that are introduced by an attacker who knows how to modify the data and the checksum to make them match. Therefore, checksums are not enough to ensure security and confidentiality of data, and other mechanisms such as encryption and authentication are needed.

4 How checksums affect performance

Checksums are essential for ensuring reliable data transmission, but they also have some impact on performance. First, checksums add some overhead to each packet, increasing the size and reducing the efficiency of data transfer. Second, checksums require some computation time on both ends, adding some delay to the transmission process. Third, checksums may trigger retransmissions if errors are detected, increasing the bandwidth consumption and the latency of data delivery. Therefore, checksums have to balance the trade-off between reliability and performance, and use the most appropriate method and length for the given application and network conditions.

5 How checksums are implemented

Checksums are implemented as part of the TCP header, which contains the information needed to manage the data transmission. The TCP header has a 16-bit field for the checksum, which is calculated from the data in the TCP segment and a pseudo-header. The pseudo-header is a temporary header that is added to the TCP segment before calculating the checksum, and removed afterwards. The pseudo-header contains some fields from the IP header, such as the source and destination addresses, the protocol, and the length of the TCP segment. The pseudo-header is used to ensure that the checksum covers not only the data, but also the routing information, and to prevent spoofing attacks.

6 How checksums are updated

Checksums are updated whenever the data or the pseudo-header changes. For example, when a packet is fragmented or reassembled by the IP layer, the checksum has to be recalculated to reflect the new length and data. Similarly, when a packet is forwarded by a router, the checksum has to be recalculated to reflect the new source and destination addresses. Updating checksums can be done in two ways: recalculation or adjustment. Recalculation means that the checksum is computed from scratch, using the new data and pseudo-header. Adjustment means that the checksum is modified based on the difference between the old and new values, using a technique called incremental checksumming. Adjustment is faster and more efficient than recalculation, but it requires more complex logic and hardware support.

7 Here’s what else to consider

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