Ruby UDP Server/Client Pair with Custom Binary Protocol

protocol.rb

require 'bindata'

class CustomProtocol < BinData::Record
    endian  :big
    stringz :command_word
    uint8   :op1
    uint8   :op2
end

udp_client.rb

require 'socket'
require_relative 'protocol'

# Connection params
$port = 9999
$host = 'localhost'

def make_request(cmd, a,b)
    add_request = CustomProtocol.new
    add_request.command_word = cmd
    add_request.op1 = a
    add_request.op2 = b
    add_request
end

def print_answer(sock)
    msg, sender = sock.recvfrom(20) # reads up to 20 bytes 
    puts "server responded: #{msg} " 
end

def send_request(sock, data)
    sock.send(data, 0, $host, $port)
end


# -- MAIN --
# open the connection
s = UDPSocket.new

# make an add request and send it out over the socket
# print the answer by reading the result off the socket
req1 = make_request("ADD",5,4)
puts "sending add request over socket ..."
send_request(s, req1.to_binary_s)
print_answer(s)

# ok, now try a multiply request
req2 = make_request("MULT",6,7)
puts "sending multiply request over socket ..."
send_request(s, req2.to_binary_s)
print_answer(s)

udp_server.rb

require 'socket'    # needed for Socket library
require_relative 'protocol'  # needed for custom binary protocol

port = 9999
puts "Started UDP server on #{port}..."

Socket.udp_server_loop(port) { |msg, msg_src|
    proto = CustomProtocol.new
    proto.read(msg)
    if proto.command_word == "ADD" 
        puts "Received ADD command"
        result = proto.op1 + proto.op2
        msg_src.reply result.to_s
    elsif proto.command_word == "MULT"
        puts "Received MULTIPLY command"
        result = proto.op1 * proto.op2
        msg_src.reply result.to_s
    else
        puts "Received uknown cmd msg: #{proto.command_word} ..."
    end
    
}

How to get source ip and port from msg_src ?