ccnet-lec-12-arp-dhcp-icmp

8/6/2019 ccnet-lec-12-ARP-DHCP-ICMP

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Lecture

12Data Communication & Networks

ARP, DHCP & ICMP

Muhammad Yousaf


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Address Resolution Protocol: (ARP)

Routers forward packets on the basis of IPaddress, layer-3

But actual transmission takes place via NetworkInterface Card (NIC), layer-2

And layer-2 transmission uses MAC addresses

How nodes come to know MAC address of othernodes?

We need a mechanism that can:Translate IP addresses into MAC addresses

That mechanism is ARP


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ARP procedure:

Source: http://technet.microsoft.com/en-us/library/bb726993.aspx (25-11-2010)


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ARP procedure: cont

ARP is defined in RFC-826Initially node broadcasts ARP request message

Saying who has following IP address

All nodes receive this request messageNode with particular IP address responds with ARPreply

ARP reply contains MAC address corresponding to

the IP address

Requesting node saves this IP-to-MAC addressmapping in its ARP-cache


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ARP command:

C:\case>arp -a

Internet Address Physical Address Type

157.55.85.212 00-aa-00-62-c6-09 static

172.16.3.153 00-16-6f-52-f3-8b static

172.16.8.101 f0-0d-ee-00-a0-0b static

172.16.8.25 00-20-af-b4-a1-4e dynamic


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DHCP:

Dynamic Host Configuration Protocol (DHCP)Unlike MAC addresses, IP addresses are not fix

When a new host is added in network, how aunique IP address may be assigned to it?

There are two approaches:

Manually

Administrator manually assigns IP address

An error-prone method for large networks

Dynamically

Automatically assigns IP addresses to the client nodes


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DHCP: cont

DHCP is specified in RFC-1541For automatic address assignment in network thereshould be a DHCP server running DHCP service

Administrator configures DHCP server with a poolof IP addresses

These addresses are leased out to client nodes onrequest

The process of dynamically obtaining IP addressfrom DHCP server consists of 4-steps


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Basic DHCP Model:


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DHCP - obtaining IP addresses:



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DHCP: cont

Server Discovery:Client broadcasts DHCP Discover Packetto discover DHCP serverClient puts its MAC address in packet and asks for the IP address

IP Address Offer:

DHCP server replies as DHCP Offer Packetwith a tentative IPaddress along with lease time

IP Address Request:

If client accepts offered IP address then it formally sends a DHCP

Request Packet

Acknowledgement:Server acknowledges address assignment with ACK Packet


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DHCP relay agent:

If DHCP server is on some other network then, wecan use a DHCP relay agent

Relay agent just receives the broadcasted DHCPbroadcasts and forwards them to the DHCP server


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DHCP relay agent:



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ICMP:

ICMP = Internet Control Message ProtocolA protocol to convey control information amonghosts and routers in internet

Can send control messages to remote nodes

Can test remote nodes whether they are alive ornot

(using Ping command)

Can report error messages from remote nodes


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ICMP messages:

Following are some of the ICMP messages:Echo request

Echo reply

Destination unreachable

Redirect messageTTL exceeded


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ICMP in Windows XP:

Control Panel >>Windows Firewalls >>Advanced Tab >>ICMP -> Settings


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Ping command:

C:\case> ping 192.168.55.1

Pinging 192.168.55.1 with 64 bytes of data:

64 bytes from 192.168.55.1: icmp_seq=0 ttl=30 time=1.164 ms

64 bytes from 192.168.55.1: icmp_seq=1 ttl=30 time=0.918 ms

- 192.168.55.1 ping statistics -

7 packets transmitted, 7 packets received, 0% packet loss

C:\case>ping 172.16.3.100

Pinging 172.16.3.100 with 32 bytes of data:

Destination host unreachable.Destination host unreachable.

Ping statistics for 172.16.3.100:

Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),


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