1 Kyung Hee University Chapter 7 ARP and RARP. 2 Kyung Hee University Introduction The logical...

11Kyung Hee University

Chapter 7Chapter 7ARP and RARPARP and RARP


IntroductionIntroduction

The logical addresses in TCP/IP protocol suite

are IP addresses

Physical address is usually implemented in

hardware

Ex) 48-bit MAC addresses in Ethernet and Token ring protocols, which are imprinted on the NIC installed in the host or router


Introduction (cont’d)Introduction (cont’d)

Mapping a logical address to its corresponding

physical address

Static mapping means creating a table that associates a logical address

with a physical address need to update periodically

Dynamic mapping each time a machine knows one of the two addresses

(logical or physical), it can use a protocol to find the other one

ARP (address resolution protocol), RARP (reverse address resolution protocol)


ARP and RARPARP and RARP ARP (address resolution protocol)

RARP (reverse address resolution protocol)


Position of ARP and RARP in TCP/IP Protocol SuitePosition of ARP and RARP in TCP/IP Protocol Suite


ARP OperationARP Operation


ARP Operation (cont’d)ARP Operation (cont’d)

An ARP request is broadcast; an ARP reply is

unicast


The Format of ARP packetThe Format of ARP packet


The Format of ARP packet (cont’d)The Format of ARP packet (cont’d)

HTYPE(hardware type) : defining the type of

the network (16 bits)

ex) Ethernet : type 1

PTYPE(protocol type :16 bits)

the value of this field for the IPv4 protocol : 080016

HLEN (Hardware length : 8 bits)

defining the length of the physical address in bytes

for Ethernet, the value is six



PLEN (protocol length : 8bits)

defining the length of the logical address in bytes

ex) for the IPv4, the value is 4

OPER (operation : 16 bits)

defining the type of the packet

ARP request : 1, ARP reply : 0

SHA (sender hardware address)

variable length field defining the physical address of the sender



SPA (sender protocol address)

variable-length field defining the logical address of the sender

for the IP protocol, the field is 4

THA (target hardware address)

variable-length field defining the physical address of the target

TPA (target protocol address)

variable-length field defining the logical address of the target


Encapsulation of ARP PacketEncapsulation of ARP Packet

encapsulated directly into a data link frame

ARP packet encapsulated in an Ethernet frame


Four Cases using ARPFour Cases using ARP

Case 2. A host wants to send a packet to another host on another network. It must first be delivered to the default router.


Four Cases using ARP (cont’d)Four Cases using ARP (cont’d)


Example 1Example 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 and

physical address 0xA46EF45983AB. The two hosts

are on the same Ethernet network. Show the ARP

request and reply packets encapsulated in

Ethernet frames


Example 1 (cont’d)Example 1 (cont’d)


Proxy ARPProxy ARP ARP that acts on behalf of a set of hosts

Whenever the router running a proxy ARP receives an ARP

request looking for the IP address of one of these hosts, router

sends an ARP reply announcing its own hardware (physical)

address

Later, when the router receives the actual IP packet, it will send

the packet to the appropriate host or router


7.2 ARP Package7.2 ARP Package


ARP Design (cont’d)ARP Design (cont’d)

Cache table

When a host or router receives the corresponding physical address for an IP datagram, the address can be saved in the cache table.

This address can be used for the datagrams destined for the same receiver within the next few minutes

Queues

holding the IP address while ARP tries to resolve the hardware address



Output Module

waiting for an IP packet from the IP software

checking the cache table to find an entry corresponding to the destination IP address of this packet



Output module

1. Sleep until an IP packet is received from IP software

2. Check the cache table to find an entry corresponding to this IP packet

3. If (found)

1. If (the state is RESOLVED)

1. Extract the value of the hardware address from the entry

2. Send the packet and the hardware address to data link layer

3. Return

2. If (the state is PENDING)

1. Enqueue the packet to the corresponding queue

2. Return

4. If (not found)

1. Create a queue

2. Enqueue the packet

3. Create a cache entry with state set to PENDING and ATTEMPTS set to 1

4. Send an ARP request

5. Return



Input Module

waiting until an ARP packet (request or reply) arrives

checking the cache table to find an entry corresponding to this ARP packet



Input Module

1. Sleep until an ARP packet (request or reply) arrives

2. Check the cache to find an entry corresponding to the this ARP packet

3. If (found)

1. If (the state is PENDING)

1. Update the entry ( to RESOLVED)

2. While the queue is not empty1. Dequeue one packet

2. Send the packet and the hardware address to date link

2. If (the state is RESOLVED) // hardware address could have been changed

1. Update the entry

4. If (not found)

1. Create an entry

2. Add the entry to the table

5. If (the packet is a request)

1. Send an ARP reply

6. Return



Cache-control module

responsible for maintaining the cache table

periodically (for example, every five seconds) checking the cache table, entry by entry


ARP Design (cont’d)ARP Design (cont’d)Cache-control module

1. Sleep until the periodic timer matures.

2. For every entry in the cache table

1. If (the state is Free)

1. Continue.2. If (the state is PENDING)

1. Increment the value of attempts by 1.2. If(attempts greater than maximum)

1. Change the state to FREE2. Destroy the corresponding queue.

3. If(not)1. Send an ARP request.

4. Continue.3. If (the state is RESOLVED)

1. Decrement the value of time-out by the value of elapsed time.2. If(time-out less than or equal to zero)

1. Change the state to FREE.2. Destroy the corresponding queue

3. Return.


Examples of ARPExamples of ARP

Example 2

1) ARP output module receives an IP datagram (from IP layer) with the destination address 114.5.7.89.

2) Check the Cache table

3) Extract hardware address, which is 457342ACAE32 and sends packet and the address to the data link layer for transmission

The beginning cache table


Examples of ARPExamples of ARPExample3

1)Twenty seconds later, the ARP out module receives an IP datagram with

destination address with 116.1.7.22 and creates a new queue

2) Send an ARP request to the data link layer


Examples of ARPExamples of ARPExample 4

1) Fifteen seconds layer, ARP input module receives an ARP packet with

target protocol (IP) address 188.11.8.71.

2) Change the state to RESOLVED, and time-out value to 900 and add the

target hardware address

3) Access queue 18 and send all the packets in this queue to the data link

layer


Example of ARPExample of ARP

Example 4

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 pending entries, the value of the attempts


Example of ARPExample of ARP


7.3 RARP7.3 RARP

To create an IP datagram, a host or a router needs to

know its own IP address or addresses

The IP address of a machine is usually read from its

configuration file stored on a disk file

A diskless machine is usually booted from ROM, which

has minimum booting information

It cannot include the IP address because the IP addresses on a network are assigned by the administrator.

The machine can get its physical address by reading its NIC.


RARP (cont’d)RARP (cont’d)


RARP (cont’d)RARP (cont’d) Packet Format


RARP (cont’d)RARP (cont’d)

Encapsulation of RARP packet

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