Permit SACK in the handshake
01The Transmission Control Protocol (TCP) carries an ordered byte stream. SACK means Selective Acknowledgment: the data receiver can report byte ranges received beyond a gap, while its ordinary ACK still names the next byte needed.
After the handshake, the right endpoint can send Kind 5 SACK reports for out-of-order data received from the left. Without received permission it must not send those reports.
Source for this drawing: RFC 2018 · Read RFC section 2 ↗
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Permission to receive SACK is advertised on a SYN, separately for each data direction.
- SYN is TCP’s connection-start flag; SYN-ACK also acknowledges the peer’s SYN. SACK-permitted has Kind 4 and Length 2. It MAY be sent in a SYN segment and MUST NOT be sent on a non-SYN segment.
- A peer advertises that it can receive SACK options. Receiving that permission allows the opposite peer to generate SACK reports for data flowing toward it; the reverse data direction is independent.
- A data receiver that received permission MAY generate SACK. Without received permission it MUST NOT send SACK options. Generating SACK is optional; if used under any circumstances, it SHOULD be used under all permitted circumstances.
- The ordinary cumulative ACK does not change meaning: ACK 1000 still asks for byte 1000 even if later bytes are buffered. An ACK is a transport acknowledgment, not proof that an application has used the data.
The example focuses on data sent from the left endpoint to the right endpoint. The final state assumes the TCP three-way handshake has completed; it is not a new handshake protocol or a requirement to enable SACK on every connection.
Read the RFC section ↗