code: func (e *endpoint) maybeEnableSACKPermitted(synOpts *header.TCPSynOptions) {
  	var v SACKEnabled
  	if err := e.stack.TransportProtocolOption(ProtocolNumber, &v); err != nil {
  		// Stack doesn't support SACK. So just return.
  		return
  	}
  	if bool(v) && synOpts.SACKPermitted {
  		e.sackPermitted = true
  	}
  }
docs: // maybeEnableSACKPermitted marks the SACKPermitted option enabled for this endpoint
  // if the SYN options indicate that the SACK option was negotiated and the TCP
  // stack is configured to enable TCP SACK option.