Chapter 7 ARP and RARP Prof. Choong Seon HONG

The logical addresses in TCP/IP protocol suite are IP addresses Introduction 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) 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 RARP ARP (address resolution protocol) RARP (reverse address resolution protocol)

Position of ARP and RARP in TCP/IP Protocol Suite

ARP Operation

ARP Operation (cont’d) An ARP request is broadcast; an ARP reply is unicast

The Format of ARP packet

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

The Format of ARP packet (cont’d) 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

The Format of ARP packet (cont’d) 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 Packet encapsulated directly into a data link frame ARP packet encapsulated in an Ethernet frame

Four 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)

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 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) Request

Example 1 (cont’d) Reply

Proxy 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 Package

ARP Design (cont’d) Cache table Queues 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

ARP Design (cont’d) 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

ARP Design (cont’d) Output module Sleep until an IP packet is received from IP software Check the cache table to find an entry corresponding to this IP packet If (found) If (the state is RESOLVED) Extract the value of the hardware address from the entry Send the packet and the hardware address to data link layer Return If (the state is PENDING) Enqueue the packet to the corresponding queue If (not found) Create a queue Enqueue the packet Create a cache entry with state set to PENDING and ATTEMPTS set to 1 Send an ARP request

ARP Design (cont’d) Input Module waiting until an ARP packet (request or reply) arrives checking the cache table to find an entry corresponding to this ARP packet

ARP Design (cont’d) Input Module Sleep until an ARP packet (request or reply) arrives Check the cache to find an entry corresponding to the this ARP packet If (found) If (the state is PENDING) Update the entry ( to RESOLVED) While the queue is not empty Dequeue one packet Send the packet and the hardware address to date link If (the state is RESOLVED) // hardware address could have been changed Update the entry If (not found) Create an entry Add the entry to the table If (the packet is a request) Send an ARP reply Return

ARP Design (cont’d) 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) Cache-control module Sleep until the periodic timer matures. For every entry in the cache table If (the state is Free) Continue. If (the state is PENDING) Increment the value of attempts by 1. If(attempts greater than maximum) Change the state to FREE Destroy the corresponding queue. If(not) Send an ARP request. If (the state is RESOLVED) Decrement the value of time-out by the value of elapsed time. If(time-out less than or equal to zero) Change the state to FREE. Destroy the corresponding queue Return.

Examples of ARP Example 2 The beginning cache table 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

Examples of ARP Example3 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 ARP Example 4 Fifteen seconds layer, ARP input module receives an ARP packet with target protocol (IP) address 188.11.8.71. Change the state to RESOLVED, and time-out value to 900 and add the target hardware address Access queue 18 and send all the packets in this queue to the data link layer

7.3 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) Packet Format

RARP (cont’d) Encapsulation of RARP packet