code: func (p *peer) SendNewBlockHashes(hashes []common.Hash, numbers []uint64) error {
  	for _, hash := range hashes {
  		p.knownBlocks.Add(hash)
  	}
  	request := make(newBlockHashesData, len(hashes))
  	for i := 0; i < len(hashes); i++ {
  		request[i].Hash = hashes[i]
  		request[i].Number = numbers[i]
  	}
  	return p2p.Send(p.rw, NewBlockHashesMsg, request)
  }
docs: // SendNewBlockHashes announces the availability of a number of blocks through
  // a hash notification.