Why is CLRF undefined from within the model?

common.rb

module FTP
  module Common
    CRLF = "\r\n"

    def initialize(port = 21)
      @control_socket = TCPServer.new(port)
      trap(:INT) { exit }
    end

    def respond(response)
      @client.write(response)
      @client.write(CLRF)
    end

    class CommandHandler
      attr_reader :connection

      def initalize(connection)
        @connection = connection
      end

      def pwd
        @pwd || Dir.pwd
      end

      def handle(data)
        cmd = data[0..3].strip.upcase
        options = data[4..-1].strip

        case cmd
        when "USER"
          #accept any username anonymously
          "230 Logged in anonymously"

        when "SYST"
          #what's your name?
          "215 UNIX Working with FTP"

        when "CWD"
          if File.directory?(options)
            @pwd = options
            "250 directory changed to #{pwd}"
          else
            "550 directory not found"
          end

        when "PWD"
          "257 \"#{pwd}\" is the current directory"

        when "PORT"
          parts = options.split(',')
          ip_address = parts[0..3],join('.')
          port = Integer(parts[4]) * 256 + Integer(parts(5))

          @data_socket = TCPSocket.new(ip_address, port)
          "200 Active connection established (#{port})"

        when "RETR"
          file = File.read(File.join(pwd, options), 'r')
          connection.respond "125 Data transfer starting #{file.size} bytes"

          bytes = IO.copy_stream(file, @data_socket)
          @data_socket.close

          "226 Closing data connection, sent #{bytes} bytes"

        when "LIST"
          connection.respond "125 Opening data connection for file list"
          result = Dir.entries(pwd).join(CLRF)
          @data_socket.write(result)
          @data_socket.close


        when "QUIT"
          "221 Ciao"

        else
          "502 Don't know how to respond to #{cmd}"
        end
      end
    end
  end
end

error.txt

➜  ftp git:(master) ✗ ruby bin/serial_server.rb
/home/richard/projects/pickaxe/mini-projects/ftp/lib/ftp/common.rb:12:in `respond': uninitialized constant FTP::Common::CLRF (NameError)
        from /home/richard/projects/pickaxe/mini-projects/ftp/arch/serial.rb:11:in `block in run'
        from /home/richard/projects/pickaxe/mini-projects/ftp/arch/serial.rb:9:in `loop'
        from /home/richard/projects/pickaxe/mini-projects/ftp/arch/serial.rb:9:in `run'
        from bin/serial_server.rb:4:in `<main>'

serial.rb

require 'socket'
require_relative "../lib/ftp/common"

module FTP
  class Serial
    include Common

    def run
      loop do
        @client = @control_socket.accept
        respond "220 OHAI"

        handler = CommandHandler.new(self)

        loop do
          request = @client.gets(CRLF)

          if request
            respond handler.handle(request)
          else
            @client.close
            break
          end
        end
      end
    end
  end
end

serial_server.rb

require_relative "../arch/serial"

server = FTP::Serial.new(4481)
server.run