Address Resolution Protocol

(ARP)
Tuan

Address Mapping

Anytime a host or a router has an IP datagram to send to another host or router, it

has the logical (IP) address of the receiver.

But the IP datagram must be encapsulated in a frame to be able to pass through the
physical network.

This means that the sender needs the physical address of the receiver.

A mapping corresponds a logical address to a physical address.

ARP accepts a logical address from the IP protocol, maps the address to the
corresponding physical address and pass it to the data link layer.

When ARP is Used

For two hosts

on the same network and one desires to send a packet to the other
on different networks and must use a gateway/router

For a router that needs to forward a packet

for one host through another router
from one host to the destination host on the same network

Four Cases using ARP

IP and MAC Address

Stations need to know MAC address to communicate.

Hardware MAC address

Ethernet 6 bytes,
Token ring 2 or 6 bytes,
FDDI 2 or 6 bytes.

How does IP address get mapped to MAC address?

Manual configuration by hand is tedious.
Automatic process by ARP.

Types Of Message

There are four types of ARP messages:

ARP request,
ARP reply,
RARP request,
RARP reply.

These are identified by four values in the operation" field of an ARP message.

ARP Layer

TCP

UDP

ICMP

IP

IGMP

ARP

Network
Access

RARP

Media

Transport
Layer

Network
Layer

Link Layer

ARP and RARP

Note:
The Internet is based on IP addresses ,
Data link protocols (Ethernet, FDDI, ATM) may have different (MAC) addresses.
The ARP and RARP protocols perform the translation between
IP addresses and
MAC layer addresses.

Processing of IP packets by Network device drivers

IP Output

Put on IP
input queue
Yes

Yes

IP Input

IP destination = multicast
or broadcast ?
No

IP destination of packet
= local IP address ?

loopback
Driver

Put on IP
input queue

No: get MAC

address with
ARP
Ethernet

Ethernet
Driver

ARP

ARP
Packet

IP datagram

demultiplex
Ethernet Frame

ARP components

Operation of ARP
hostname

resolver
IP addr

(1)

FTP

(2)

TCP
(3)
(5)
(6)

(4)

ARP
(8)

IP

(9)

Ethernet driver
ARP request (Ethernet broadcast)

Ethernet driver
ARP

Ethernet driver
(7)

ARP

IP
TCP

hostname
Establish connection
with IP address
Send IP datagram
to IP address

ARP Packet Format

Ethernet II header
Destination
address

Source
address

Type
0x8060

ARP Request or ARP Reply

28

10

Hardware type (2 bytes)

Hardware address
length (1 byte)

Padding

CRC
4

Protocol type (2 bytes)

Protocol address
length (1 byte)

Operation code (2 bytes)

Source hardware address*

Source protocol address*
Target hardware address*
Target protocol address*
* Note: The length of the address fields is determined by the corresponding address length fields

ARP Packet Format (2)

ARP Protocol Data Unit

0

15
Hardware
Hardware type (layer 2)
Address
Address length
length
Layer
Layer 22 (n)
(n)

Address
Address length
length
Layer
Layer 3 (m)

31
Protocol
Protocol type
type (layer
(layer 3)
3)
Operation
Operation

Source
Source address
address (layer
(layer 2):
2): n
n bytes
bytes
Source
Source address
address (layer
(layer 3):
3): m
m bytes
bytes
Destiniation
Destiniation address (layer 2): n bytes
bytes
Destination
Destination address
address (layer
(layer 3):
3): m bytes
bytes

Layer-2
Layer-2 header
header

Layer-2
Layer-2 payload
payload

Layer-3
Layer-3 trailer
trailer

How ARP Works?

ARP Request Packet

IP 172.28.80.96
MAC 00:50:BA:49:9D:B9

IP 172.28.80.100
MAC ?

Sample ARP request Ethernet packet

FF:FF:FF:FF:FF

Dest MAC (broadcast)

00:50:BA:49:9D:B9

Source MAC

0x0806

ARP frame type

0x01
0x06

0x800
0x04

0x001
00:50:BA:49:9D:B9
172.28.80.96
00:00:00:00:00:00

Ethernet / IP
MAC=6 / IP = 4 / request
Source MAC
Source IP
Dest MAC (unknow)

172.28.80.100

Dest IP

Checksum

Ethernet checksum

ARP Reply Packet

IP 172.28.80.96
MAC 00:50:BA:49:9D:B9

IP 172.28.80.100
MAC 00:02:7A:D2:B3:00

Sample ARP request Ethernet packet

00:50:BA:49:9D:B9

Dest MAC (unicast)

00:02:7A:D2:B3:00
0x800

Source MAC
ARP frame type
Ethernet / IP

0x002

MAC=6 / IP = 4 / reply

0x0806
0x01
0x06

0x04
00:02:7A:D2:B3:00
172.28.80.100
00:50:BA:49:9D:B9

Source MAC
Source IP
Dest MAC

172.28.80.96

Dest IP

Checksum

Ethernet checksum

Common Slide

ARP-Request to FF:FF:FF:FF:FF:FF
0

15

0x00
0x00 01
01 (Ethernet)
(Ethernet)
66

44

ARP-Reply to 49:72:16:08:64:14
31

0x80
0x80 00
00 (Internet
(Internet Protocol)
Protocol)
0x00
0x00 01
01 (ARP-Request)
(ARP-Request)

15

0x00
0x00 01
01 (Ethernet)
(Ethernet)
66

49
49 72
72 16
16 08
08

31

0x80
0x80 00
00 (Internet
(Internet Protocol)
Protocol)
0x00
0x00 02
02 (ARP-Reply)
(ARP-Reply)

49
49 72
72 16 08

64
64 14
14

129
129 25
25

64
64 14
14

129
129 25
25

10
10 72
72

00
00 00
00

10
10 72
72

49
49 78
78

00
00 00
00 00
00 00
00

21
21 21
21 23 90

129
129 25 10
10 11
11

129
129 25
25 10
10 11
11

Common Slide (2)

Example Use of ARP

ARP Cache

To reduce network traffic;

performance comparable to direct mapping.

A table stores mappings between

MAC addresses and
IP addresses.

The entries are dynamically added and removed.

Cache timeout - complete entry :2 mins;

incomplete (for non-existent host) entry :3 mins.

Address resolution

Transmission on LANs requires knowledge of destination hardware address (physical

address, MAC address).

On a LAN hardware addresses are unique.

Address resolution is translation of destination IP address to destination hardware address

Address Resolution Methods

Table look-up

Each node keeps a separate table

Network adress part of IP address need not be stored.
If address mapping changes all nodes update their tables

Address Resolution Methods (2)

Computation
Derivation from the host address part of IP address
Useful in network that employ reconfigurable (dynamical) port address.

Address Resolution Methods (3)

Message exchange:
A server responds to address resolution request that it receives
Any machine that knows the hardware address responds to address resolution request
broadcasts

Address Resolution Protocol (IP over Ethernet)

Is based on message exchange.

Is simple.
Optimizations:
Caching
A machine broadcasts its own IP-Ethernet binding
At boot time
With address resolution request (ARP)
ARP message is placed into the payload field of LAN(DLL) frame.

ARP message
Hardware addr. type

Protocol addr. type

Hardware addr.
length

Operation

Protocol addr.
length

Source hardware addr. (first 4 bytes)

Source hardware addr. (last 2
bytes)

Source protocol addr. (first 2 bytes)

Source protocol addr. (last 2 bytes) Dest. Hardware addr. (first 2 bytes)
Dest. Hardware addr. (last 4 bytes)
Dest. Protocol addr.

LAN

ARP-Request to MAC FF:FF:FF:FF:FF:FF

1

MAC
MAC address
address of
of 129.25.10.11
129.25.10.11 ??
Reply
Reply to:
to: 49:72:16:08:64:14
49:72:16:08:64:14 (129.25.10.72)
(129.25.10.72)

ARP-Reply to MAC 49:72:16:08:64:14

Reply:t:
Reply:t: 129.25.10.11
129.25.10.11 (49:78:21:21:23:90)
(49:78:21:21:23:90)
Requested
Requested by::
by:: 129.25.10.72
129.25.10.72 (49:72:16:08:64:14)
(49:72:16:08:64:14)
2

IP: 129.25.10.11
MAC: 49:78:21:21:23:90

Router R

IP: 129.1.11.72
MAC: 10:11:72:AB:02:01

Computer A:
IP: 129.25.10.72
MAC: 49:72:16:08:64:14

Computer B:
IP: 129.25.10.97
MAC: 49:72:16:08:80:70

Computer C:
IP: 129.25.10.81
MAC: 49:17:92:96:96:96

LAN (2)
LAN
System A

System B
Request

Looking for physical address of a

node with IP address 141.23.56.23

a. ARP request is multicast

LAN
System A

System B
Reply

The node physical address

is A4:6E:F4:59:83:AB

b. ARP reply is unicast

Proxy ARP
Added subnetwork
The proxy ARP router replies
to any ARP request received
for destinations 141.23.56.21,
141.23.56.22, and 141.23.56.23.

Router or host

141.23.56.21

141.23.56.22

141.23.56.23

Proxy ARP
router

Thank You !