SN (sequence number)

A sequence number (SN) is a value used in various fields, including computer networking, telecommunications, and database systems, to uniquely identify and order individual elements or events within a sequence or stream of data. The primary purpose of a sequence number is to provide a reliable and consistent means of tracking and ordering data units, ensuring that they are processed or delivered in the correct sequence.

In the context of computer networking, a sequence number is commonly used in transport layer protocols like TCP (Transmission Control Protocol) to manage the reliable delivery of data packets. Each TCP packet contains a sequence number field, which is typically a 32-bit value that identifies the position of the packet within the stream of data being transmitted. The sequence number is assigned by the sender and used by the receiver to reorder the packets if they arrive out of order and to detect and discard duplicate packets.

Here's how sequence numbers work in TCP:

1. Sender: When sending data, the sender assigns a sequence number to each packet it sends. The initial sequence number is typically chosen randomly or based on a specific algorithm. As subsequent packets are sent, the sequence number is incremented by the number of bytes in the payload of each packet. This ensures that each packet has a unique sequence number.
2. Receiver: The receiver uses the sequence numbers to reconstruct the original data stream and ensure the correct order of packets. When the receiver receives packets, it checks the sequence number of each packet. If the sequence number is in order (i.e., the next expected sequence number), the packet is accepted and passed to the upper layers for processing. If the sequence number is out of order, the packet is buffered until the missing packets arrive. If a duplicate packet is received, it is discarded to prevent duplicate processing.
3. Acknowledgment: The receiver also sends acknowledgments (ACK) back to the sender to indicate the highest sequence number it has received and successfully processed. These ACKs are important for the sender to know which packets have been received and which may need to be retransmitted if they are lost or not acknowledged.

Sequence numbers are crucial for ensuring reliable data transfer in network protocols like TCP. By assigning unique identifiers to packets and enforcing their correct order, sequence numbers enable the receiver to reconstruct the original data stream and detect any missing or duplicate packets.

It's important to note that sequence numbers can wrap around due to their finite size (32 bits in the case of TCP). When the sequence number reaches its maximum value, it wraps back to zero, and the process continues. Sequence number wrapping must be handled carefully to avoid confusion or misordering of packets. Various techniques, such as the use of a sliding window or selective acknowledgment, are employed to handle sequence number wraparound and efficiently manage packet retransmissions in TCP.