1. Chapter 7 ARP and RARP
Prof. Choong Seon HONG

2. 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

3. 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)

4. ARP and RARP
ARP (address resolution protocol)
RARP (reverse address resolution protocol)

5. Position of ARP and RARP in TCP/IP Protocol Suite

6. ARP Operation

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

8. The Format of ARP packet

9. 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 :
HLEN (Hardware length : 8 bits)
defining the length of the physical address in bytes
for Ethernet, the value is six

10. 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

11. 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

12. Encapsulation of ARP Packet
encapsulated directly into a data link frame
ARP packet encapsulated in an Ethernet frame

13. 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.

14. Four Cases using ARP (cont’d)

15. Example 1
A host with IP address and physical address 0xB has a packet to send to another host with IP address and physical address 0xA46EF45983AB. The two hosts are on the same Ethernet network. Show the ARP request and reply packets encapsulated in Ethernet frames

16. Example 1 (cont’d)
Request

17. Example 1 (cont’d)
Reply

18. 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

19. 7.2 ARP Package

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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.

27. 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
2) Check the Cache table
3) Extract hardware address, which is ACAE32 and sends packet and the address to the data link layer for transmission

28. Examples of ARP Example3
1)Twenty seconds later, the ARP out module receives an IP datagram with destination address with and creates a new queue
2) Send an ARP request to the data link layer

29. Examples of ARP Example 4
Fifteen seconds layer, ARP input module receives an ARP packet with target protocol (IP) address
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

30. 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.

31. RARP (cont’d)

32. RARP (cont’d)
Packet Format

33. RARP (cont’d)
Encapsulation of RARP packet