code: func (p *peerConnection) FetchHeaders(from uint64, count int) error {
  	// Sanity check the protocol version
  	if p.version < 62 {
  		panic(fmt.Sprintf("header fetch [eth/62+] requested on eth/%d", p.version))
  	}
  	// Short circuit if the peer is already fetching
  	if !atomic.CompareAndSwapInt32(&p.headerIdle, 0, 1) {
  		return errAlreadyFetching
  	}
  	p.headerStarted = time.Now()
  
  	// Issue the header retrieval request (absolut upwards without gaps)
  	go p.peer.RequestHeadersByNumber(from, count, 0, false)
  
  	return nil
  }
docs: // FetchHeaders sends a header retrieval request to the remote peer.