Introduction to ARP & RARP

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Address Resolution Protocol (ARP):

Address Resolution Protocol (ARP) TE Comp

Overview:

Overview

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Figure 7.1 ARP and RARP

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Position of ARP and RARP in TCP/IP protocol suite Notice that ARP and RARP are supplemental to IP.

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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. Address resolution provides a mapping between the two different forms of address. 32-bit Internet address 48-bit Ethernet address

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Figure 7.3 ARP operation

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Figure 7.4 ARP packet Hardware Type - Ethernet is type 1 Protocol Type- IPv4=x0800 Hardware Length:length of Ethernet Address (6) Protocol Length:length of IPv4 address (4)

ARP and RARP:

ARP and RARP ARP ARP provides a dynamic mapping from an IP address to the corresponding hardware address. We use the term dynamic since it happens automatically and is normally not a concern of either the application user or the system administrator. RARP (Reverse Address Resolution Protocol) RARP is used by systems without a disk drive but requires manual configuration by the system administrator.

Operation of ARP:

Operation of ARP FTP TCP IP Ethernet driver ARP TCP resolver IP ARP Ethernet driver Ethernet driver ARP hostname hostname IP addr Establish connection with IP adrs Send IP datagram to IP address ARP request (Ethernet broadcast) (4) (5) (6) (3) (1) (2) (7) (8) (9)

Operation of ARP (An Example):

Operation of ARP (An Example) The application calls the function “gethostbybame” to convert the hostname into its 32-bits IP address. The FTP client ask its TCP to establish a connection with that IP address. TCP sends a connection request segment to the remote host by sending an IP datagram to its IP address.

Operation of ARP (An Example):

Operation of ARP (An Example) If the destination host is on a locally attached network, the IP datagram can be sent directly to that host. If the destination host is on a remote network, the IP routing function determines the Internet address of a locally attached next-hop router to send the IP datagram to.

Operation of ARP (An Example):

Operation of ARP (An Example) Assuming an Ethernet, the sending host must convert the 32-bit IP address into a 48-bit Ethernet address. ARP is intended for broadcast networks where many hosts or routers are connected to a single network. ARP sends an Ethernet frame called an ARP request to every host on the network. This is called broadcast.

Operation of ARP (An Example):

Operation of ARP (An Example) The destination host’s ARP layer receives this broadcast recognizes that the sender is asking for its hardware address, and replies with an ARP reply. The ARP reply is received and the IP datagram that forced the ARP request-reply to be exchanged can now be sent. The IP datagram is sent to the destination host.

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Figure 7.5 Encapsulation of ARP packet The ARP packet is encapsulated within an Ethernet packet. Note: Type field for Ethernet is x0806

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Figure 7.6 Four cases using ARP

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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

The Cache Table:

The Cache Table If ARP just resolved an IP address, chances are a few moments later someone is going to ask to resolve the same IP address When ARP returns a MAC address, it is placed in a cache. When the next request comes in for the same IP address, look first in the cache

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Figure 7.9 ARP components

The Cache Table Contents:

The Cache Table Contents State: FREE, PENDING, RESOLVED Hardware type: same as ARP field Protocol type: same as ARP field Hardware length: same as ARP field Protocol length: same as ARP field Interface number: port number (m0,m1, m2)

The Cache Table Contents:

The Cache Table Contents Queue number: which queue the ARP request is sitting in. Attempts: how many times have you tried to resolve this address? Time-out: how long until this address is tossed out (need the room in cache) Hardware address: destination hardware address Protocol address: destination IP address

How Does the Cache Work?:

How Does the Cache Work? The output module waits for an IP packet with a request Checks the cache for an existing entry If entry found and state RESOLVED, we already have this MAC address If entry found and state PENDING, packet waits until dest hard addr found

How Does the Cache Work?:

How Does the Cache Work? If no entry found, output module places this request in queue, and a new entry is placed in cache with state PENDING and ATTEMPTS set to 1. An ARP request is then broadcast

How Does the Cache Work?:

How Does the Cache Work? The input module waits until an ARP request or reply arrives Module checks the cache for this entry If entry is found and state is PENDING, module updates entry’s target hardware address, changes state to RESOLVED, and sets the TIME-OUT value

How Does the Cache Work?:

How Does the Cache Work? If entry is found and state RESOLVED, module still updates the entry (target hardware address could have changed) and the TIME-OUT value reset If entry not found, module creates a new entry. State is set to RESOLVED and TIME-OUT is set

How Does the Cache Work?:

How Does the Cache Work? Now the module checks to see if arrived ARP packet is a Request. If it is, the module immediately creates an ARP Reply message and sends it back to sender.

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Table 7.1 Original cache table used for examples

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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 457342ACAE32, and sends the packet and the address to the data link layer for transmission. The cache table remains the same. Example 2

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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. Example 3 See Next Slide

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Table 7.2 Updated cache table for Example 3

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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 (E34573242ACA) 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. Example 4 See Next Slide

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Table 7.3 Updated cache table for Example 4

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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 field is incremented by 1. One entry (IP addr 201.1.56.7 is over max, so change to FREE. Example 5 See Next Slide

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Table 7.4 Updated cache table for Example 5

Reverse Address Resolution Protocol (RARP):

Reverse Address Resolution Protocol (RARP)

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RARP RARP finds the logical address for a machine that only knows its physical address . This if often encountered on thin-client workstations. No disk, so when machine is booted, it needs to know its IP address. RARP requests are broadcast, RARP replies are unicast . If a thin-client workstation needs to know its IP address, it probably also needs to know its subnet mask, router address, DNS address, etc. So we need something more than RARP. BOOTP, and now DHCP have replaced RARP.

ARP and RARP:

ARP and RARP Note: The Internet is based on IP addresses Protocols (Ethernet, FDDI, ATM) may have different (MAC) addresses The ARP and RARP protocols perform the translation between IP addresses and MAC layer addresses We will discuss ARP for broadcast LANs, particularly Ethernet LANs

Example:

Example ARP Request : Source hardware address: 00:a0:24:71:e4:44 Source protocol address: 128.143.137.144 Target hardware address: 00:00:AA:00:10:00 Target protocol address: 128.143.137.1 ARP Reply : Source hardware address: 00:e0:f9:23:a8:20 Source protocol address: 128.143.137.1 Target hardware address: 00:a0:24:71:e4:44 Target protocol address: 128.143.137.144

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RARP 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

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The RARP request packets are broadcast; the RARP reply packets are unicast . Note:

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RARP operation

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RARP packet

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Encapsulation of RARP packet

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