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Address Resolution Protocol

(ARP)

• 1

Relates to Lab 2.

This module is about the address resolution protocol.

Overview

Network Layer

Link Layer

IP

ARP

Network Access RARP

Media

ICMP IGMP

Transport Layer

TCP UDP

Processing of IP packets by network device drivers

loopback Driver

IP Input

Put on IP input queue

ARP

demultiplex Ethernet Frame

Ethernet

IP destination of packet = local IP address?

IP destination = multicast or broadcast?

IP Output

Put on IP input queue

No: get MAC address with ARP

ARP Packet

IP datagram

No

Yes

Yes Ethernet Driver

Address Translation with ARP

ARP Request :

Argon broadcasts an ARP request to all

stations on the network: “What is the

hardware address of Router137?”

ARP Packet Format

Destination address 6

ARP Request or ARP Reply

28

Source address 6 2

CRC

4

Type 0x8060 Padding 10

Ethernet II header

Hardware type (2 bytes) Hardware address length (1 byte)

Protocol address length (1 byte) Operation code (2 bytes)

Target hardware address*

Protocol type (2 bytes)

Source hardware address*

Source protocol address*

Target protocol address*

• Note: The length of the address fields is determined by the corresponding address length fields

Example

• ARP Request from Argon:

Source hardware address: 00:a0:24:71:e4:

Source protocol address: 128.143.137.

Target hardware address: 00:00:00:00:00:

Target protocol address: 128.143.137.

• ARP Reply from Router137:

Source hardware address: 00:e0:f9:23:a8:

Source protocol address: 128.143.137.

Target hardware address: 00:a0:24:71:e4:

Target protocol address: 128.143.137.

Proxy ARP

• Proxy ARP: Host or router responds to ARP

Request that arrives from one of its connected

networks for a host that is on another of its

connected networks.

Things to know about ARP

• What happens if an ARP Request is made for a non-existing

host?

Several ARP requests are made with increasing time

intervals between requests. Eventually, ARP gives

up.

• On some systems (including Linux) a host periodically sends

ARP Requests for all addresses listed in the ARP cache. This

refreshes the ARP cache content, but also introduces

traffic.

• Gratuitous ARP Requests: A host sends an ARP request for

its own IP address:

• Useful for detecting if an IP address has already been assigned.