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ARP: Address Resolution Protocol

Computer Network SystemSirak Kaewjamnong

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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

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ARP Protocol

• RFC 826 address resolution protocol

• ARP map any network level address (such as IP) to its corresponding data link address (such as Ethernet)

• support protocol in data link layers, not data link layer protocol

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ARP in Operation

• Host A want to resolve MAC address of C– A sends broadcast ARP request – A gets unicast ARP reply from C

172.28.80.5 172.28.80.25172.28.80.15 172.28.80.35

A B C D

Who has IP172.28.80.25?

No, not me Me with00:F4:DF:80:4C:52

Ohm..No, not me

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ARP datagrams

Datalink frameFrame headerARP/RARP message

Hardware type: 16 Protocol type :16

hlen :8 plen : 8 ARP operationSender MAC address (bytes 0-3)

Sender MAC address (bytes 4-5)Sender IP address (bytes 0-1)

Sender IP address (bytes 2-3)Dest MAC address (bytes 0-1)

Destination MAC address (bytes 2-5)Destination IP address (bytes 0-3)

0 1516 31

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Header details

• Hardware type : Ethernet = 1, ARCnet = 7, localtalk = 11

• Protocol type : IP = 0X800• hlen : length of hardware address,

Ethernet = 6 bytes• plen : length of protocol address, IP = 4

bytes• ARP operation : ARP request = 1, ARP

reply = 2, RARP request =3 RARP reply =4

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ARP Request PacketIP 172.28.80.96MAC 00:50:BA:49:9D:B9

IP 172.28.80.100MAC ?

Sample ARP request Ethernet packetFF:FF:FF:FF:FF

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

0x0806

0x01 0x800

0x06 0x04 0x001

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

172.28.80.96

00:00:00:00:00:00

172.28.80.100

Checksum

Dest MAC (broadcast)Source MAC

ARP frame typeEthernet / IP

MAC=6 / IP = 4 / requestSource MACSource IP

Dest MAC (unknow)Dest IP

Ethernet checksum

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ARP Reply PacketIP 172.28.80.96MAC 00:50:BA:49:9D:B9

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

Sample ARP request Ethernet packet00:50:BA:49:9D:B9

00:02:7A:D2:B3:00

0x0806

0x01 0x800

0x06 0x04 0x002

00:02:7A:D2:B3:00

172.28.80.100

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

172.28.80.96

Checksum

Dest MAC (unicast)Source MAC

ARP frame typeEthernet / IP

MAC=6 / IP = 4 / replySource MACSource IP

Dest MACDest IP

Ethernet checksum

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ARP Mechanism

• Each node maintains the ARP cache– It first looks in the cache to find entry first– if the entry is not used for a period

(approximate 15 minutes), it is delete• Receive node can adds an MAC

address entry for source station in its own cache

• ARP traffic load– hosts quickly add cache entries– all of hosts on a subnet are booted at the

same time? Flurry of ARP request and reply

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Proxy ARP• One node answer ARP request for

another: Router R answers for Y

• useful when some nodes on a network cannot support subnet– X do not understand subnet, so it thinks

that Y is on the same subnet• Router must be configured to be a

proxy ARP

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

IP 172.28.100.100MAC 00:02:7A:D2:B3:00

Router

IP 172.28.80.1MAC 00:50:BA:49:00:BB

X Y

RX to Y request --><--R send 172.28.100.100with 00:50:BA:49:00:BB

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RARP• Reverse ARP : map MAC address to IP

address• for device that cannot store IP, usually

diskless workstations• Need to set up server with RARP table• Use the same frame format

– 0x0835 for Ethernet RARP request– operation 0x003 = RARP request 0x004 =

RARP reply• RARP can not operate across router ,

BOOTP is more spread