SEQ number of sequences

In computer science and networking, a SEQ number (or sequence number) is a unique identifier assigned to each packet or message in a sequence. It is used to maintain the correct order and integrity of transmitted data, especially in protocols like TCP (Transmission Control Protocol) where reliable and ordered delivery of data is crucial.

When data is transmitted over a network, it is divided into smaller units called packets. Each packet contains a header that includes various control information, and one of the important fields in the header is the SEQ number. The SEQ number is a 32-bit integer value that represents the position of the packet within the sequence of transmitted data.

The purpose of the SEQ number is to enable the receiving end to reassemble the packets in the correct order. When a packet is received, the receiver checks its SEQ number and compares it to the expected value. If the SEQ number matches the expected value, it means the packet is in order, and the receiver can process it. However, if the SEQ number is different from the expected value, it indicates that some packets are missing or out of order.

In TCP, the SEQ number is used in conjunction with acknowledgment (ACK) numbers to implement reliable and ordered data transmission. When a TCP sender transmits packets, it assigns a unique SEQ number to each packet in the order they are sent. The receiver, upon receiving the packets, sends back an acknowledgment packet with an ACK number indicating the next expected SEQ number. If the sender receives an acknowledgment with an ACK number different from the next expected SEQ number, it retransmits the missing or out-of-order packets.

The SEQ number also plays a crucial role in detecting and handling duplicate packets. When a receiver receives a packet with a SEQ number that is the same as a previously received packet, it discards the duplicate packet to prevent data corruption or duplication.

Furthermore, the SEQ number is also utilized in congestion control mechanisms, such as TCP's sliding window protocol. The sliding window mechanism allows a sender to transmit a certain number of packets without waiting for acknowledgment, and the SEQ number helps in maintaining the correct sliding window boundaries.

In summary, the SEQ number is a unique identifier assigned to each packet in a sequence, primarily used in protocols like TCP to ensure reliable and ordered delivery of data. It allows the receiver to reconstruct the original data stream and detect missing, out-of-order, or duplicate packets.