code: func (e *endpoint) sendRaw(data buffer.VectorisedView, flags byte, seq, ack seqnum.Value, rcvWnd seqnum.Size) *tcpip.Error {
  	var sackBlocks []header.SACKBlock
  	if e.state == stateConnected && e.rcv.pendingBufSize > 0 && (flags&header.TCPFlagAck != 0) {
  		sackBlocks = e.sack.Blocks[:e.sack.NumBlocks]
  	}
  	options := e.makeOptions(sackBlocks)
  	err := sendTCP(&e.route, e.id, data, e.route.DefaultTTL(), flags, seq, ack, rcvWnd, options, e.gso)
  	putOptions(options)
  	return err
  }
docs: // sendRaw sends a TCP segment to the endpoint's peer.