SQN Sequence number

In the context of network protocols and security mechanisms, SQN (Sequence Number) refers to a value used to ensure the integrity, authenticity, and freshness of transmitted data or messages. The sequence number is a crucial component in protocols that require ordered and reliable data delivery or need protection against replay attacks.

Here's a detailed explanation of SQN and its significance:

1. Data Integrity and Authenticity: Sequence numbers are used to protect the integrity and authenticity of data by detecting and preventing unauthorized modification or tampering. Each transmitted data unit, such as a packet or message, is assigned a unique sequence number that is included in the data unit itself.
2. Ordered Delivery: Sequence numbers enable the recipient to reconstruct the original order of received data units. By examining the sequence numbers, the recipient can identify if any data units are missing or out of order, allowing for appropriate actions to be taken, such as requesting retransmission or reordering the received data.
3. Replay Attack Prevention: Sequence numbers also play a crucial role in preventing replay attacks. A replay attack occurs when an attacker intercepts and maliciously retransmits previously captured data. By including a sequence number in each data unit, the recipient can detect and discard duplicate or replayed messages, ensuring that only fresh and legitimate data is processed.
4. Freshness and Nonce: Sequence numbers can serve as nonces (number used once) to ensure the freshness of transmitted data. A nonce is a value that is only used once and provides protection against replay attacks. By incrementing the sequence number for each new data unit, it acts as a nonce, ensuring that each transmission contains a unique and fresh value.
5. Size and Encoding: The size and encoding of the sequence number depend on the specific protocol or system implementation. It can be a fixed-size value or variable-length, depending on the desired range and granularity of sequence numbers required by the protocol.
6. Synchronization and Initial Sequence Number: In some protocols, sequence number synchronization is necessary to ensure proper interpretation and processing of received data. This typically involves an initial sequence number exchange or negotiation between the communicating parties to establish a common starting point for the sequence number counter.
7. Wraparound Handling: In protocols where sequence numbers are represented by fixed-size values, wraparound handling is necessary. When the sequence number reaches its maximum value, it wraps around back to its minimum value, forming a circular sequence space. Proper handling of wraparound ensures that the recipient correctly interprets the ordering of sequence numbers.
8. Security and Cryptographic Protocols: Sequence numbers are often used in security and cryptographic protocols to protect against various attacks, such as replay attacks and message tampering. In these protocols, the sequence numbers are typically combined with cryptographic mechanisms, such as digital signatures or message authentication codes (MACs), to provide data integrity, authenticity, and freshness.

Overall, sequence numbers are essential elements in network protocols and security mechanisms to ensure data integrity, ordered delivery, and protection against replay attacks. By assigning unique values to transmitted data units, sequence numbers enable the recipient to verify the integrity and authenticity of data, maintain the proper order of received units, and prevent the acceptance of duplicate or replayed messages.