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Chapter 7

ARPand

RARP

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ARP and RARP

Applications, which are above the layer 4, use logical address to identifythe destination host, called IP address. The IP packets are encapsulated into

frames. The delivery of frames across the links (source source, or source router,

router router, , router destination) is based on local addresses called physical

addresses (MAC numbers). The mapping of IP addresses into physical addresses isdone through Address Resolution Protocol (ARP).

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Position of ARP and RARP

in TCP/IP protocol suite

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ARP operation(case: destination is on the same physical network)

Link

IP = 141.23.56.23The MAC address ofdestination is broadcastaddress: 0xFFFFFFFFFFFF

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7-5

ARP packet

4 bytes(Ethernet = 1) IPv4 = 0x0800

(Ethernet = 6)

IPv4 = 4

Opcode (1 = request, 2 = reply)

18 byte padding (to make frame payloads equal to 46 bytes)

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Encapsulation of ARP packet

Start Field Delimiter(10101011)

46 bytes

Why ARP packets are not encapsulated

into IP datagrams?

Because ARP packets never leave LAN

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Four cases using ARP(Case 1: Host to host)

The IP address of destination host is taken from the IP datagram.

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Four cases using ARP (cont.)Case 2: Host to router)

The IP address of the destination (router) is not taken from the IP datagram.

Instead it is taken from the next-hop column of the RT of the source host.

(next hop)

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Four cases using ARP (cont.)(Case 3: Router to router)

The IP address of destination (router) is not taken from the IP datagram.

Instead it is taken from the RT of the router

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Four cases using ARP (cont.)(Case 4: Router to host)

The IP address of destination host is taken from the IP datagram.

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Example 1

A host with IP address 130.23.43.20 and physical

address 0xB23455102210 has a packet to send to

another host with IP address 130.23.43.25 andphysical address 0xA46EF45983AB. The two hostsare on the same Ethernet network. Show the ARPrequest and reply packets encapsulated in Ethernet

frames.

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Example 1: ARP request

130.23.43.20

Dont know the destination MAC address

130.23.43.25

ARP request

(48 bytes with padding) Broadcast address

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Example 1: ARP reply

ARP reply

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ARP Cache Table

It would be very inefficient to use the ARP for each datagram.Therefore the most recent mappings are kept in a cache table.In order to be consistent with the network dynamics, the entry ofARP cache have a timeout value which helps to remove the agedentries.

TOBMACBPB

. . . .. . . .. . . .

TOAMACAPATimeoutPhysical AddressIP Address

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A C ( )

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ARP Cache Table (cont.)

The ARP cache of a host can be displayed with the command:

arp a(the command is the same on Windows and UNIX)

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ARP C h T bl ( )

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201.11.56.7514PF

ACAE32457342180.3.6.19005R

HardwareAddressProtocolAddressTimeOutAttemptQueueState

ARP Cache Table (cont.)

R = resolved, P = pending, F = free (time to leave has expired)

Number of the queue

where packets are waitingfor address resolution

Life time of theentry (sec)

Number ofARP request

sent

Implementation of ARP cache table requires more than the essential

information shown on the previous two slides

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ARP I l t ti

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ARP Implementation

All modules are threads

Timer(5 sec)

IP datagram

MAC address

of immediate

destination

(next hop ordirect)

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C S di k d i i IP h MAC i CT ( R)

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2

1

3

Case: Sending a packet, destination IP has MAC in CT (state = R)

Each destination is

given a queue (all

packets with the

same destination

are in the samequeue)

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2

1

Case: Sending a packet, destination IP has not MAC in CT (state = P)

Put packet inexisting queue

3

7-20Case: Sending a packet, destination IP is not in CT

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2

1

Case: Sending a packet, destination IP is not in CT

3

4

Create a new queue

7-21Case: An ARP reply arrives corresponding IP is in CT

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2

Case: An ARP reply arrives, corresponding IP is in CT

4

Dequeue all packets3

1

7-22Case: An ARP reply arrives corresponding IP is not in CT

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2

Case: An ARP reply arrives, corresponding IP is not in CT

Create a new entry in CT

1

7-23Case from the previous slide:

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Case from the previous slide:an ARP reply arrives, corresponding IP is not in CT

When can this happen?

The pending entry has been deleted because:

(a) time-out expired

(b) number of attempts exceeded

7-24Case: An ARP request arrives

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q

12

7-25Cache-Control Module

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for (every entry in CT){switch (State){case PENDING:

increment Attempt;if (Attempt > max){

State = FREE;Destroy corresponding queue;}

elseSend an ARP request;

break;case RESOLVED:

Decrement Time-Out;if (Time-Out

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Example 2

The ARP output module receives an IP datagram (from

the IP layer) with the destination address 114.5.7.89. It

checks the cache table and finds that an entry exists for

this destination with the RESOLVED state (R in the

table). It extracts the hardware address, which is457342ACAE32, and sends the packet and the address to

the data link layer for transmission. The cache table

remains the same.

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Original Cache Table (Example 2)

4573E3242ACA19.1.7.82609R

188.11.8.71318P

F

220.55.5.7112P

457342ACAE32114.5.7.894508R

201.11.56.7514P

129.34.4.822P

ACAE32457342180.3.6.19005R

HardwareAddress

ProtocolAddress

TimeOut

AttemptQueueState

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E l 3

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Example 3

Twenty seconds later, the ARP output module receives an

IP datagram (from the IP layer) with the destination

address 116.1.7.22. It checks the cache table and does notfind this destination in the table. The module adds an

entry to the table with the state PENDING and the

Attempt value 1. It creates a new queue for thisdestination and enqueues the packet. It then sends an

ARP request to the data link layer for this destination.

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U d d CT (E l 3)

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Updated CT (Example 3)

4573E3242ACA19.1.7.82609R

188.11.8.71318P

116.1.7.22123P

220.55.5.7112P

457342ACAE32114.5.7.894508R201.11.56.7514P

129.34.4.822P

ACAE32457342180.3.6.19005R

Hardware

Address

Protocol

Address

Time

Out

AttemptQueueState

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Example 4

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Example 4

Fifteen seconds later, the ARP input module receives an

ARP packet with target protocol address 188.11.8.71. The

module checks the table and finds this address. It changesthe state of the entry to RESOLVED and sets the time-

out value to 900. The module then adds the target

hardware address (E34573242ACA) to the entry. Now itaccesses queue 18 and sends all the packets in this queue,

one by one, to the data link layer.

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4573E3242ACA19.1.7.82609R

E34573242ACA188.11.8.7190018R

116.1.7.22123P

220.55.5.7112P

457342ACAE32114.5.7.894508R201.11.56.7514P

129.34.4.822P

ACAE32457342180.3.6.19005R

Hardware

Address

Protocol

Address

Time

Out

AttemptQueueState

Updated CT (Example 4)

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Example 5

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Example 5

Twenty-five seconds later, the cache-control module

updates every entry. The time-out values for the first

three resolved entries are decremented by 60. The time-out value for the last resolved entry is decremented by 25.

The state of the next-to-the last entry is changed to FREE

because the time-out is zero. For each of the three entries,the value of the attempts field is incremented by one.

After incrementing, the attempts value for one entry (the

one with IP protocol address 201.11.56.7) is more thanthe maximum; the state is changed to FREE, the queue is

deleted.

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CT Updated by Cache Control Module (Example 5)

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60! 0R! F

E34573242ACA188.11.8.7190018R

116.1.7.22223P

220.55.5.7212P

457342ACAE32114.5.7.893908R201.11.56.7614P

!F

129.34.4.832P

ACAE32457342180.3.6.18405R

Hardware

Address

Protocol

Address

Time

Out

AttemptQueueState

CT Updated by Cache Control Module (Example 5)

Time-Out is decremented by 60 sec (20 + 15 + 25)

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Broadcast

Unicast

Used w hen configurat ion f i le is

not available (e.g. diskless machine)

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RARP packet

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RARP packet

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Encapsulation of RARP packet

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Alternative Solutions to RARPWhen a diskless computer is booted, it needs moreinformation in addition to it s I P address. It needs

to know its subnet mask , the I P address of arouter , and the I P address of a name server .RARP cannot provide this extra information. Newprotocols have been developed to provide thisinformation. In Chapter 17 we discuss two

protocols, BOOTP and DHCP, that can be usedinstead of RARP.