code: func (p *peer) MarkBlock(hash common.Hash) {
  	// If we reached the memory allowance, drop a previously known block hash
  	for p.knownBlocks.Cardinality() >= maxKnownBlocks {
  		p.knownBlocks.Pop()
  	}
  	p.knownBlocks.Add(hash)
  }
docs: // MarkBlock marks a block as known for the peer, ensuring that the block will
  // never be propagated to this particular peer.