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Networking Fundamentals · Intermediate

Sequence Number

A 32-bit TCP header field that numbers the bytes in a stream so the receiver can reorder data and detect loss.

Definition

Each side picks a random initial sequence number during the handshake, and each segment's sequence number identifies its first data byte. Receivers use sequence numbers to reassemble data in order and discard duplicates. Predictable sequence numbers enable session hijacking, so modern stacks randomize them.

Example

If a segment with sequence 1001 carries 500 bytes, the next expected sequence is 1501.

Exam tip

Sequence numbers count bytes, not segments.

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