code: func (s *Stack) NetworkProtocolInstance(num tcpip.NetworkProtocolNumber) NetworkProtocol {
  	if p, ok := s.networkProtocols[num]; ok {
  		return p
  	}
  	return nil
  }
docs: // NetworkProtocolInstance returns the protocol instance in the stack for the
  // specified network protocol. This method is public for protocol implementers
  // and tests to use.