Chapter 8 ARP and RARP Objectives Upon completion

Chapter 8 ARP and RARP Objectives 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 TCP/IP Protocol Suite 1

Figure 7. 1 TCP/IP Protocol Suite ARP and RARP 2

Figure 7. 2 TCP/IP Protocol Suite Position of ARP and RARP in TCP/IP protocol suite 3

7. 1 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 TCP/IP Protocol Suite 4

Figure 7. 3 TCP/IP Protocol Suite ARP operation 5

Figure 7. 4 TCP/IP Protocol Suite ARP packet 6

Figure 7. 5 TCP/IP Protocol Suite Encapsulation of ARP packet 7

Figure 7. 6 TCP/IP Protocol Suite Four cases using ARP 8

Note: An ARP request is broadcast; an ARP reply is unicast. TCP/IP Protocol Suite 9

Example 1 A host with IP address 130. 23. 43. 20 and physical address B 2: 34: 55: 10: 22: 10 has a packet to send to another host with IP address 130. 23. 43. 25 and physical address A 4: 6 E: F 4: 59: 83: AB (which is unknown to the first host). The two hosts are on the same Ethernet network. Show the ARP request and reply packets encapsulated in Ethernet frames. See Next Slide TCP/IP Protocol Suite 10

Example 1 (Continued) Solution Figure 7. 7 shows the ARP request and reply packets. Note that the ARP data field in this case is 28 bytes, and that the individual addresses do not fit in the 4 byte boundary. That is why we do not show the regular 4 -byte boundaries for these addresses. Also note that the IP addresses are shown in hexadecimal. For information on binary or hexadecimal notation see Appendix B. See Next Slide TCP/IP Protocol Suite 11

Figure 7. 7 TCP/IP Protocol Suite Example 1 12

Figure 7. 8 TCP/IP Protocol Suite Proxy ARP 13

ATMARP n n When IP packet are moving through an ATM WAN , a mechanism protocol is needed to find the physical address of exiting-point router in the ATM WAN given the IP address of router. The format of an ATMARP packet , which is similer to the ARP packet. TCP/IP Protocol Suite 14

Figure 3. 24 TCP/IP Protocol Suite Architecture of an ATM network 15

ATMARP Operation n n There are two methods to connect two routers on an ATM network: 1. PVC(Permanent Virtual Circuit) 2. SVC(Switched Virtual Circuit) 1. PVC Connection It is established between two end points by the network Providers. The VPI and VCI are defined for permanent connections and the values are entered in a table for each switch. The inverse request message and inverse reply message Can be used for the binding. There is no need for an ATMARP server. TCP/IP Protocol Suite 16

n n SVC Connection In svc connection , each time a router wants to make a connection with another router (or any computer), a new virtual circuit must be established. To map the ip addresses to physical addresses Each router runs a client ATMARP program, but only one computer runs an ATMARP server program. TCP/IP Protocol Suite 17

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 TCP/IP Protocol Suite 18

Figure 7. 9 TCP/IP Protocol Suite ARP components 19

$ARP_Output_Module () n n n { Sleep until an IP packet is received from$

ARP_Output_Module () n n n { Sleep until an IP packet is received from IP software. Check cache table for an entry corresponding to the destination of IP packet. If(entry is 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 } TCP/IP Protocol Suite 20

n n n n If (the state is PENDING) { Enqueue the packet to the corresponding queue. Return. } } If(entry is not found) { Create a cache entry with state set to pending & attempts set to 1. Create a queue. Enqueue the packet. Send an ARP request. Return. } } TCP/IP Protocol Suite 21

$n n n n ARP_Input_Module() { Sleep until an ARP packet (request or reply)$

n n n n ARP_Input_Module() { Sleep until an ARP packet (request or reply) arrives. Check the cache table to find the corresponding entry. If(found) { Update the entry. If(the state is PENDING) { While (the queue is not empty) { Dequeue one packet. Send the packet and the hardware address. } } } TCP/IP Protocol Suite 22

If (not found) { Create an entry. Add the entry to the table. } If (the packet is a request) { Send an ARP reply } Return } TCP/IP Protocol Suite 23

$ARP_Cache_Control_Module n { n Sleep until the periodic timer matures n Repeat for every$

ARP_Cache_Control_Module n { n Sleep until the periodic timer matures n Repeat for every entry in the cache table n { n If (the state is FREE) n { n Continue } If (the state is PENDING) { Increments the value of attempts by 1 If(attempts greater then maximum) { Change the state to FREE Destroy the corresponding queue } n TCP/IP Protocol Suite 24

n n n n Else { n Send an ARP request } Continue } If (the state is RESOLVED) { n Decrement the value of time-out n If (time-out less than or equal 0) n { n Change the state to FREE n Destroy the corresponding queue n } } TCP/IP Protocol Suite 25

Table 7. 1 Original cache table used for examples TCP/IP Protocol Suite 26

Example 2 The ARP output module receives an IP datagram (from the IP layer) with the destination address 114. 5. 7. 89. It checks the cache table and finds that an entry exists for this destination with the RESOLVED state (R in the table). It extracts the hardware address, which is 457342 ACAE 32, and sends the packet and the address to the data link layer for transmission. The cache table remains the same. TCP/IP Protocol Suite 27

Example 3 Twenty seconds later, the ARP output module receives an IP datagram (from the IP layer) with the destination address 116. 1. 7. 22. It checks the cache table and does not find this destination in the table. The module adds an entry to the table with the state PENDING and the Attempt value 1. It creates a new queue for this destination and enqueues the packet. It then sends an ARP request to the data link layer for this destination. The new cache table is shown in Table 7. 2. See Next Slide TCP/IP Protocol Suite 28

Table 7. 2 Updated cache table for Example 3 TCP/IP Protocol Suite 29

Example 4 Fifteen seconds later, the ARP input module receives an ARP packet with target protocol (IP) address 188. 11. 8. 71. The module checks the table and finds this address. It changes the state of the entry to RESOLVED and sets the time-out value to 900. The module then adds the target hardware address (E 34573242 ACA) to the entry. Now it accesses queue 18 and sends all the packets in this queue, one by one, to the data link layer. The new cache table is shown in Table 7. 3. See Next Slide TCP/IP Protocol Suite 30

Table 7. 3 Updated cache table for Example 4 TCP/IP Protocol Suite 31

Example 5 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 See Next Slide TCP/IP Protocol Suite 32

Table 7. 4 Updated cache table for Example 5 TCP/IP Protocol Suite 33

7. 3 RARP finds the logical address for a machine that only knows its physical address. The topics discussed in this section include: Packet Format Encapsulation RARP Server Alternative Solutions to RARP TCP/IP Protocol Suite 34