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1 Address Resolution Protocol 1 Address Resolution Protocol

1 Address Resolution Protocol - PowerPoint Presentation

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1 Address Resolution Protocol - PPT Presentation

ARP Relates to Lab 2 This module is about the address resolution protocol 2 Overview 3 IP and LAN addresses The Internet is based on 32bit IP addresses Applications only deal with IP addresses ID: 631618

arp address 143 128 address arp 128 143 hardware 137 network router addresses request protocol argon broadcast ether link host packet mac

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Slide1

1

Address Resolution Protocol

(ARP)

Relates to Lab 2.

This module is about the address resolution protocol.Slide2

2

OverviewSlide3

3

IP and LAN addresses

The Internet is based on 32bit IP addresses Applications only deal with IP addressesBut all Internet devices connect to a physical link via hardware - Network

Interface Card (NIC

),

that has an address.

Data link protocols (Ethernet, Frame Relay) have different addressesSlide4

Nature of MAC addressesHardare address allocation administered by IEEEManufacturer buys portion of hardware address space (to assure uniqueness)Analogy of Internet Addresses:Hardware address: like Social Security NumberIP address: like postal address Hardware: flat address ➜ portable assigned once, un-chageable, goes with you, move from one physical location to another it doesn’t changeIP: hierarchical address -> not portable address assigned based on physical location, i.e., IP subnet to which device is attached, changes when you move from one network to another

4Slide5

IP and MAC Addresses

5

adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

LAN

137.196.7.23

137.196.7.78

137.196.7.14

137.196.7.88

71-65-F7-2B-08-53

IP addresses are “

generally

” known

i.e., application can find it in DNS database.

How do we find a device’s

hardware

address?

Use a ”

dynamic binding

” procedure - an

address resolution process

that finds hardware address for an IP address.Slide6

Address Resolution Protocol - ARPStandard for dynamic address resolution in the InternetThe ARP protocol performs the translation between 32 bit IP addresses and link layer addresses to reach the physical location of a device on the same “single segment” network (i.e., all devices that have same IP network prefix)Physical address burned in NIC ROM, sometimes software settable or firmware.Example of an address on an Ethernet link (48bits): 1A-2F-BB-76-09-ADhexadecimal (base 16) notation (each “number”

represents 4 bits)Isolates hardware address at low level

Allows application programs to use IP addresses

6Slide7

ARP DescriptionAllows device A to find device B’s hardware addressTechnique: broadcast query and obtain unicast responseQuery: sent as a “hardware” broadcast (link layer broadcast)limited broadcast: ARP only used to map addresses within a single physical/segment network, never across multiple (IP) networksQuery contains A’s hardware address and B’s IP addressResponse: sent as a unicast to A’s hardware addresscontains B’s hardware address

7Slide8

ARP OperationA wants to send datagram to BA starts with B’s IP addressA knows B is on the local network (resolved by A using network prefix)A broadcasts ARP query packet, containing B's IP address dest address in Ethernet frame = FF-FF-FF-FF-FF-FFsource address in Ethernet frame = A’s hardware addressall nodes on data link/single segment network, receive ARP query Query (ARP packet) contains A’s hardware address and B’s IP addressB receives A’s ARP query packet, recognizes its IP address, replies to A with its (B's) hardware addressframe sent to A’s hardware address (unicast) containing B’s hardware address

8Slide9

9

ARP Packet Format

18Slide10

ARP Packet FormatGeneral: can be used withArbitrary hardware address (not just Ethernet)Arbitrary protocol address (not just IP)Variable length address fields (depends on type of datalink protocol)

10Slide11

11

Address Translation with ARP

ARP Request: Argon broadcasts an ARP request to all stations on the network: “What is the hardware address of 128.143.137.1?”

128.143.137.1?Slide12

12

Address Translation with ARP

ARP Reply: Router 137 responds with an ARP Reply which contains the hardware address

128.143.137.1?Slide13

13

Example

ARP Request from Argon: Source hardware address: 00:a0:24:71:e4:44Source protocol address: 128.143.137.144

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

Target protocol address: 128.143.137.1

ARP Reply from Router137:

Source hardware address: 00:e0:f9:23:a8:20 Source protocol address: 128.143.137.1 Target hardware address: 00:a0:24:71:e4:44Target protocol address: 128.143.137.144Slide14

Retention of BindingsSending an ARP request/reply for each IP datagram (to same IP address) is inefficient. Solution -> maintain a table of bindingsdevices maintain a cache of currently used IP addresses and their corresponding hardware addresses. A device caches IP-to-hardware address pairs in a table until information becomes old (times out) Soft state: information times out (goes away) unless refreshed (i.e., everytime an IP address is looked up in the table)The entries expire after X minutes. Initially X=20, but because of mobility, it is now very short, sometimes just 30secs.EffectUse ARP one time, place results in table, then quick lookup many times (every packet sent to that same IP address)

14Slide15

Note on BindingsIf A ARPs B, B keeps A’s informationB will probably send a packet to A soonIf A ARPs B, A keeps B’s informationA will probably send more packets to BOther machines do not keep A’s information (even though they all see the broadcast with A’s information)Avoids clogging ARP caches needlessly

15Slide16

16

ARP Cache (table)

Contents of the ARP Cache:(128.143.71.37) at 00:10:4B:C5:D1:15 [ether] on eth0

(128.143.71.36) at 00:B0:D0:E1:17:D5 [ether] on eth0

(128.143.71.35) at 00:B0:D0:DE:70:E6 [ether] on eth0

(128.143.136.90) at 00:05:3C:06:27:35 [ether] on eth1

(128.143.71.34) at 00:B0:D0:E1:17:DB [ether] on eth0

(128.143.71.33) at 00:B0:D0:E1:17:DF [ether] on eth0ARP is “plug-and-play”:nodes create their ARP tables

without intervention from net administratorSlide17

17

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.

What if a host sends an ARP request for its own IP address? Know as gratuitous ARP

No response hopefully

This is useful for detecting if an IP address has already been assigned (via DHCP).Slide18

18

ARP in our Example

Argon realizes that it needs to use router to reach Neon as the two hosts are on different IP networks. Argon sends an ARP Request to the router for

MAC address

of NIC corresponding to

IP address

of router on the

common network.Router sends ARP Response to host Argon with MAC address corresponding to the IP address of its NIC on that connected

network

.

Router IP Address

Router MAC address

128.143.137.1?

128.143.137.1 is

128.143.137.0/24

128.143.71.21/24

128.143.137.144/24

128.143.137.1/24Slide19

Proxy ArpAllow devices on two different IP subnetworks to share a single IP network prefixSource believes destination is on same IP networkSetup router to respond to ARP broadcast requests for destinations on different connected subnet router masquerades as destination for ARP request sent by source on a subnetthe two devices are unaware that they are on different subnets, subnet mask indicates that they have the same network prefix.Masquerades: router responds to broadcast ARP Request from source host that arrives on one of its connected networks for a destination host that is on one of its other connected networks.

19Slide20

20

Proxy ARP Operation in Our Example with Argon having /16 as its network prefix

Router responds to broadcast ARP Request from host Argon that arrives on

one

of its connected

networks

for

host Neon that is on one of its other connected networks. Host Argon believes Neon is on the same IP network because of its ”prefix” /16.

Argon sees it is on IP network

128.143.0.0

. When applying prefix /16 to Neon’s IP address 128.143.71.21 -->

128.143.0.0

, which is the same as its own. so it sends a broadcast ARP request for Neon.Router responds with its MAC address and then transfers the datagram to the next segment.

Neon

IP Address

Router MAC address

128.143.71.21/24

128.143.137.0/24

128.143.137.1/24Slide21

ARP Cache in two ExamplesNon Proxy ARP:A’s ARP cache(128.143.137.1) at 00:e0:f9:23:a8:20 [ether] on eth0Proxy ARP:A’s ARP cache(128.143.71.21) at 00:e0:f9:23:a8:20 [ether] on eth0

21Slide22

ARP TransportARP message travels in data portion of data link layer frameWe say ARP message is encapsulatedData area padded with zeroes if ARP message is shorter than minimum data link layer frameFor Ethernet: frame type field 0x0806 used for ARP

22