Eathernet (LAN)

Address Resolution Protocol (ARP)

Reverse Address Resolution Protocol (RARP)

Inverse Address Resolution Protocol (InARP)

Eathernet, ARP, RARP, InARP

1

Eathernet LAN

Developed by Metcalfe's (Xerox company got patent)

MAC address (48 bits) is used as address of a device.

CSMA/CD is used for multiple access.

Packet size is variable (payload varies from 46 to 1500 octet)

Pure Eathernet is no more in use, at present combination of

Eathernet and Internet is used in LAN.

Eathernet can be setup using Coaxial cables and Twisted Wire

cables (RJ 45 cat 5 and cat 6 cables)

2

Eathernet frame format

Preamble field consists of 64 bits alternating ‘0’ and ‘1’ to help receiving interface to synchronize.

48 bits MAC/Hardware/Physical/Eathernet address unique throughout the world for a devise.

MACaddress 0xFFFFFFFFFFFF is used to broadcast a packet in the network.

Some software can generate duplicate MAC address also (generally used by hackers)

16 bits Frame Type Field is used to identify the type of data being carried in a frame. It determine which protocol software module should process the frame.

Packet size is variable (payload varies from 46 to 1500 octet)

32 bits CRC field helps to detect error in the frame.

3

Internet devices

Host devices- Computer, Laptop, PDA, IP telephone etc.

Hub, Switch, Router, Repeaters, Bridge, Gateway, Server

IP allocation Techniques

Static IP allocation

comparatively secure but not efficient in case of limited IP addresses

Dynamic IP allocation (DHCP)

more efficient but less secure,

can be secured by allocating user login & password to authenticate users

4

Internet devices (Cont.)

5

Hub:-

connects PCs together,

Works as multi-port repeater , simply passes on (repeats) all the

information it receives

broadcasts incoming packets from a port to other ports of the Hub.

Can not process packets.

generates unnecessary traffic in network due to broadcasting of

packets through undesired ports (not part of destination link) .

Hub can be used in a small network but for a larger, heavily

used network a another network device (switch) may be used to

reduce the amount of unnecessary traffic being generated.

Internet devices (Cont.)

6

Switch:-

Switch has capability to process received packet and identify

the destination port based on destination address in header of

the packet.

It forwards the received packets to respective destination ports

only

Thus it doesn’t generate unnecessary traffic in network

This allows simultaneous communication across the switch,

improving bandwidth.

Switch is preferred over Hub to extend a network.

7

ARP and RARP

• 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

Objectives

ARP and RARP

ARP

ARP provides a dynamic mapping from an IP address to the

corresponding hardware (MAC) address.

We use the term dynamic since it happens or updates

automatically after certain period of time 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 (in ROM) by the system administrator to

send bootstrap message. 8

9

10

11

ARP and RARP

12

Position of ARP and RARP in TCP/IP protocol suite

Notice that ARP and RARP are supplemental to IP.

13

ARP operation

14

ARP packet (IP PDU)

Hardware Type - Ethernet

is type 1, field length= 16

bits

Protocol Type- IPv4= x0800,

field length= 16 bits

Hardware Length: Hardware

Address length (6 byte), field

length= 8 bits

Protocol Length: length of

IPv4 address (4 byte), field

length= 8 bits

Operation: Request 1, Reply

2, field length= 16 bits

15

Encapsulation of ARP packet (IP PDU)

The ARP packet is encapsulated within an Ethernet packet.

Note: Type field for ARP is x0806

16

Four cases using ARP

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A host with IP address 130.23.43.20 and physical address

B2:34:55:10:22:10 has a packet to send to another host with IP

address 130.23.43.25 and physical address A4:6E:F4:59:83: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.

Example 1

See Next Slide

Network topology:- when two computers connected directly or through Hub in same network

18

Solution

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.

Example 1 (Continued)

See Next Slide

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Example 1

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Proxy ARP

A proxy ARP, running in a router, can respond to an ARP request for any of its

sub- ordinate device. The proxy ARP replies with its own MAC address.

When the packet arrives, the router delivers it to the appropriate host.

21

ARP Software Package

An example of a simplified ARP software package

ARP software package consists of 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

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ARP components

The Cache Table

23

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 table. When the

next request comes in for the same IP address, look first in the cache table.

Essential to the efficient operation of ARP is the maintenance of

an ARP cache on each host.

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Original cache table used for examples

The Cache Table Contents

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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)

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?

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Output Module

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 destination

hardware address found

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 broadcasted.

How Does the Cache Work?

27

Input Module

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?

28

Input Module (cont.)

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

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.

How Does the Cache Work?

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Control Module

The cache-control module periodically checks each cache entry

If entry’s state is FREE, skips it

If entry’s state is PENDING, Attempts field is incremented by 1. This value greater than max? Toss this entry (and mark entry as FREE). Less than max? Send another ARP request

If state of entry is RESOLVED, module decrements value of Time-out

field accordingly

If Time-out field < 0, then remove entry and set state to FREE

30

Original cache table used for examples

31

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

32

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. It then sends an ARP request

to the data link layer for this destination.

Example 3

See Next Slide

33

Updated cache table for Example 3

34

Fifteen seconds later, the ARP input module receives an ARP reply

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 next slide.

Example 4

See Next Slide

35

Updated cache table for Example 4

36

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 address 201.1.56.7 is over max, so change to

FREE.

Example 5

See Next Slide

37

Updated cache table for Example 5

38

ARP Probe message

ARP Probe is used in the IPv4 Address Conflict Detection

specification (RFC 5227).

It is an ARP request constructed with an all-zero sender IP

address.

Before beginning to use an IPv4 address (whether received from

manual configuration, DHCP, or some other means), a host

implementing this specification must test to see if the address is

already in use, by broadcasting ARP probe packets

39

ARP announcement / gratuitous message

This is useful for updating other hosts' mapping of a hardware address when the sender's

IP address or MAC address has changed.

ARP request is broadcasted containing the sender's protocol and Hardware address, with

the target hardware address set to zero.

An alternative is to broadcast an ARP reply with the sender's hardware and protocol

addresses .

An ARP announcement is not intended to solicit a reply.

Gratuitous ARP is also used by some interface drivers to provide load balancing for

incoming traffic (sends information without requests).

In a team of network cards, it is used to announce a different MAC address within the

team that should receive incoming packets.

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41

RARP RARP finds the logical address (IP) for a machine that only knows its

physical address.

This if often encountered on thin-client workstations. No disk (memory), so

when machine is booted, it needs to know its IP address (don’t want to burn

the IP address into the ROM).

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.

42

RARP operation

43

RARP packet (IP PDU)

44

Encapsulation of RARP packet

InARP (Inverse ARP)

Inverse Address Resolution Protocol (Inverse ARP or InARP) is

used to obtain Network layer addresses (for example, IP addresses) of

other nodes from data link layer (Layer 2) addresses.

It is primarily used in Frame relay networks based on TDMA,SDMA,

FDMA, OFDMA based on Data Link Connection Identifier (DLCI)

and ATM networks, in which Layer 2 addresses of virtual circuits are

sometimes obtained from Layer 2 signaling, and the corresponding

Layer 3 addresses must be available before those virtual circuits can be

used.

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InARP (Cont.)

46

Since ARP translates Layer 3 addresses to Layer 2 addresses,

InARP may be described as its inverse. In addition, InARP is

implemented as a protocol extension to ARP: it uses the same

packet format as ARP, but different operation codes as for request

= 8, reply = 9.

The RARP, like InARP, translates Layer 2 addresses to Layer 3

addresses. However, in InARP the requesting station queries the

Layer 3 address of another node, whereas RARP is used to obtain

the Layer 3 address of the requesting station itself for address

configuration purposes

Frame-relay inverse-ARP

47

Frame-relay is a layer 2 technique that uses a concept of Connection Identifier to distinguish user connections in TDMA, OFDM, OFDMA, SDMA based and ATM networks.

These days WiFi, WiMAX and LTE are OFDMA based wireless technique which uses Connection Identifiers (CID) at Layer 2.

In WiMAX connection Identifiers (CID) are of 16 bit length (total 2^16 CIDs are classified as Initial, Management, Traffic and Relay sub-sets based on their utilization), All the CIDs can be reused in another network cell (under one Base Station) except replay sub-set of CIDs.

Connection techniques in Frame based networks

48

Two techniques are used to assign DLCI between adjacent devices

Common DLCI

Distinct DLCI

Single virtual connection between two devices

Central device assigns a DLCI and Protocol address (network address) to a host device.

The host device requests Protocol address of Central device/ other host device by sending InARP request.

Multiple virtual connection between two device

Central device assigns a DLCIs and Protocol addresses (network address) to a host device as per number of virtual connections for example based on type of services.

QoS scheduling in WiMAX

CID/SID= connection Identifier/Subscriber identifier

The TDMA frame format

p1 p1 p0

0 1

Reserved

Random

Access

p0 p1

Reserved

Random

Access

0 1

Super-frame

Transmit Part Receive Part

Space Division Multiple Access

primitive applications are “Sectorized

antennas”

• in future adaptive antennas

simultaneously steer energy

in the direction of many users at

once

SDMA and PDMA in satellites

SDMA dual-beam receive

antenna

simultaneously access from two

different regions of the earth

OFDM frame

53

OFDMA/TDD frame (WiFi, WiMAX)

54

Traffic Tunneling in WiMAX

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Tunnel in tunnel for traffic transmission in WiMAX

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Initialing Messaging in WiMAX

57