1. ARP and RARP Objectives Chapter 7 Upon completion you will be able to:
Understand the need for ARP
Understand the cases in which ARP is used
Understand the components and interactions in an ARP package
Understand the need for RARP
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2. ARP and RARP
A logical address is an internetwork address. It is unique universally.
Logical addresses in the TCP/IP protocol suite are called IP addresses.
A physical address is a local address. It is unique locally.
It is usually implemented in the hardware.
For performing dynamic mapping the ARP & RARP protocols are used.
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3. Position of ARP and RARP in TCP/IP protocol suite
ARP & RARP use unicast & broadcast physical addresses.
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4. 7.1 ARP ARP associates an IP address with its physical address.
On a typical physical network, such as a LAN, each device on a link is identified by a physical or station address that is usually imprinted on the NIC.
The topics discussed in this section include:
Packet Format
Encapsulation
Operation
ARP over ATM
Proxy ARP
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5. ARP operation
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6. ARP packet
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7. Encapsulation of ARP packet
An ARP packet is encapsulated directly into a data link frame.
Following diagram shows an ARP packet encapsulated in an Ethernet frame.
The Type field indicates that the data carried by the frame is an ARP packet.
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8. Basic Operation The sender knows the IP address of the target.
IP ask ARP to create an ARP request message, filling in the sender physical address, the sender IP address, & the target address.
The message is passed to the DLL where it is encapsulated in a frame using the physical address of the sender as the source address & the physical broadcast address as the destination address.
Every host or router receives the frame as the frame contains a broadcast destination address, all stations remove the message & pass it to ARP.
The target machine replies with an ARP reply message that contains its physical address. The message is unicast.
The sender receives the reply message. It now knows the physical address of the target machine.
The IP datagram, which carries data for the target machine, is now encapsulated in a frame & is unicast to the destination.
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9. Four cases using ARP
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10. An ARP request is broadcast; an ARP reply is unicast.
Note:
An ARP request is broadcast; an ARP reply is unicast.
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11. Proxy ARP It is used to create a subnetting effect.
Whenever a router running a proxy ARP receives an ARP request looking for the IP address of one of these host, the router sends an ARP reply announcing its own hardware address.
After the router receives the actual IP packet, it send the packet to the appropriate host or router.
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12. Proxy ARP
The ARP installed on the right hand host will answer only to an ARP request with a target IP address of
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13. 7.2 ARP PACKAGE
In this section, we give an example of a simplified ARP software package to show the components and the relationships between the components.
This ARP package involves five modules: a cache table, queues, an output module, an input module, and a cache-control module.
The topics discussed in this section include:
Cache Table
Queues
Output Module
Input Module
Cache-Control Module
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14. ARP components
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15. The cache table is implemented as an array of entries.
Each entry package, each entry contains the following fields:
State- It can have of 3 values; FREE, PENDING, RESOLVED
Hardware type- corresponding field in the ARP packet.
Protocol type- corresponding field in the ARP packet.
Hardware length- corresponding field in the ARP packet.
Protocol length- corresponding field in the ARP packet.
Interface number-A router can be connected to different networks, each with a different interface number.
Queue number-ARP uses numbered queue to enqueue the packets waiting for address resolution.
Attempts-Number of times an ARP request is sent for this entry.
Time-out-shows of an entry in seconds.
Hardware address-it shows the destination hardware address.
Protocol address- This column shows the destination IP address.
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16. The output module sends unsolved packets into the corresponding queue.
ARP package maintains a set of queues, one for each destination to hold the IP packets while ARP tries to resolve the hardware address.
The output module sends unsolved packets into the corresponding queue.
OUTPUT MODULE:
It waits for an IP packet from the IP software.
The output module checks the cache table to find an entry corresponding to the destination IP address of this packet.
The destination IP address of the IP packet must match the protocol address of the entry.
INPUT MODULE:
The input module waits until an ARP packet (request or reply) arrives.
The input module checks the cache table to find an entry corresponding to this ARP packet.
The target protocol address should match the protocol address of the entry.
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17. 7.3 RARP
RARP finds the logical address for a machine that only knows its physical address.
Each host(router) is assigned one or more logical addresses, which are unique & independent of the physical address of the machine.
The topics discussed in this section include:
Packet Format
Encapsulation
RARP Server
Alternative Solutions to RARP
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18. The RARP request packets are broadcast;
Note:
The RARP request packets are broadcast;
the RARP reply packets are unicast.
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19. RARP operation
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20. RARP packet
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21. Encapsulation of RARP packet
A RARP packets is encapsulated directly into a data link frame.
Note that the type field shows that the data carried by the frame is a RARP packet.
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22. It provides mapping from a physical address to a logical address.
RARP Server:
It provides mapping from a physical address to a logical address.
The mapping is stored in a disk file.
Server is normally implemented at the application layer & files used by a server are accessed at that level.
Alternative Solutions to RARP:
When a diskless computer is booted, it needs more information in addition to its IP address.
It normally needs to know its subnet mask, the IP address of a router and the IP address of a server.
As RARP cannot provides this extra information.
Therefore new protocol has been developed to provide this information, these two protocols are BOOTP & DHCP which are used instead of RARP.
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