Work Out the Sequence and Ack Numbers

About this interactive

From the lesson: - **Sequence number = the first data byte.** A segment starting at 53001 with 1,000 bytes holds bytes 53001 to 54000, so the next segment starts at 54001. - **No payload, no change:** a segment carrying only an acknowledgment keeps the same next sequence number (429 stays 429). - **Acknowledgment number = the next byte expected.** After bytes up to 56000 arrive, the receiver acknowledges 56001. - **A gap stops the acknowledgment.** If the segment at 54001 is lost, the receiver keeps acknowledging 54001, and the sender resends from 54001. - **Two directions, two sets of numbers:** each host numbers the bytes it sends and acknowledges the other's. Wireshark shows relative numbers, starting the count at 0 for the SYN and 1 for the first data byte; the real starting value is random. The video says the 32-bit field gives "4 million bytes" before the numbers run out; 32 bits is about 4.3 billion.

What you'll learn

Work out the next sequence number from a segment's sequence number and payload size, including a segment with no payload
Work out the acknowledgment number a receiver sends, the next byte it expects, and the sequence number a lost segment is resent with

Aligned to

CompTIA Network+

1.4 Explain common networking ports, protocols, services, and traffic types.

Cisco CCNA

1.5 Compare TCP to UDP

Key terms

Sequence Number: A 32-bit field in the TCP header that identifies the byte position of the first byte of data in a segment, allowing the receiver to reorder segments that arrive out of sequence and detect missing data. The receiving side uses the sequence number to send an acknowledgment confirming which byte it expects next.
Acknowledgment Number: A 32-bit field in the TCP header that indicates the next byte the receiver expects, confirming successful receipt of prior data.
Retransmission: The process by which TCP resends segments when the receiver detects missing data based on expected sequence numbers.
Segment: A unit of data at the TCP transport layer, created by breaking larger data into smaller chunks for transmission and reassembly.

Topics

Tcp Sequence Number Acknowledgment Number Retransmission Interactive Calculation

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