Transcript ARP…. - Chatt

ARP….or
What’s your MAC address?
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Packet Transmission
• NICs transmit packets from
Destination
Source
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• Each host on the network checks packet to
see if
– the MAC address matches its own
– it’s a broadcast to all hosts
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Where does MAC address checking occur?
• TCP/IP - network interface layer
• OSI - Data Link Layer
• MAC address uniquely identifies a specific
device on the network.
• IP address identifies a specific host on a
specific network
• Both MAC and IP addresses are required
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Routed Protocols
On TCP/IP networks
• Packets of routed protocols
• Contain Internetwork layer addressing info
– Allows user traffic to be directed from one network to
another
• Define structure and use of fields within packet
• Types of routed protocols:
– ARP
– RARP
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ARP
• Address Resolution Protocol
– Maps IP addresses to MAC addresses
– ARP tables
•
•
•
•
are maintained in networking devices (I.e. routers)
Also called ARP cache
Maintained in RAM
May be modified by network administrators – but
not typical – usually maintained by device.
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ARP request
• If computer cannot locate IP to MAC address
mapping in its ARP table
– Must obtain correct mapping
– Initiates an ARP request
– Source computer broadcasts ARP request to all hosts on
local segment
– Each device checks if packet is destined for its own IP
address –
• if not – ignores
• If yes - responds
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ARP frame
• Logical configuration
• Frame header
Frame Header
Destination
IP Header
Destination
Ff-ff-ff-ff-ff-ff
192.168.1.100
Source
Source
00-00-BC-12-34-56
192.168.1.205
MAC header
ARP Request Message
What is your MAC
address?
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Real packets:
• 28 octets
– Hardware types
– Protocol types
– Message lengths
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• Source host does not know the MAC address
of the destination.
• ARP request frame is a broadcast to all MAC
addresses
MAC header
Destination
Ff-ff-ff-ff-ff-ff
Source
00-00-BC-12-34-56
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• All devices on the local segment will
investigate the frame to check if their own
IP address matches the destination IP
address.
IP Header
Destination
192.168.1.100
ME?
Source
192.168.1.205
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• If no – discard the packet
• If yes:
– Reads rest of packet
– Returns ARP reply
• ARP reply
– Supplies MAC address of the host.
no
Source: Arp Request
no
Hey –my IP matches
My MAC address
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ARP Cache Life
• Source checks its local ARP cache before
sending request
Must contain current information – else data
packets could be routed to the wrong host.
• ARP table directs data packets to a specific
network card based on mapping
Mapping must be correct
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– Host addresses can change … so old ARP table
mappings can cause misrouted packets.
• To avoid problems
– Networking devices place a TTL (time-to-live) on ARP
entries.
• Ex. Microsoft operating systems use a two-to ten-minute TTL.
• Using TTLs the operating system ensures that ARP cache is
never outdated for more than 10 min
• Process of removing ARP entries from an ARP
table is called aging out.
• In addition, devices on network replace ARP
entries whenever they receive new information.
– Ex. If computer receives packet from host with new
info, old info is replaced.
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RARP
Reverse Address Resolution Protocol
• Similar to ARP (used to bind MAC
addresses to IP addresses)
• Used primarily by diskless workstations.
– NIC with MAC address
– No IP addresses
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• Diskless Workstations
– Don’t have hard drives that hold IP
configurations
– Must obtain IP address each time they boot up.
– Client’s IP config must be stored on a RARP
server.
– RARP server maintains table of IP address to
MAC address mappings for RARP clients.
– During boot-up RARP clients call RARP server
to obtain their IP config information.
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RARP Frame vs. ARP Frame
Frame Header
MAC header
Destination
Ff-ff-ff-ff-ff-ff
IP Header
Destination
Source
255.255.255.255
00-00-8C-12-34-56
Source
ARP Request Message
What is my IP
address?
0.0.0.0
Frame Header
MAC header
Destination
Ff-ff-ff-ff-ff-ff
IP Header
Destination
Source
192.168.1.100
00-00-8C-12-34-56
Source
ARP Request Message
What is your MAC
address?
192.168.1.205
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More info….
• Note: IP header does not have specific
destination or source IP
– Source does not know which device is RARP
server
• must broadcast to all devices on the segment
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RARP Client
• RARP client receives RARP reply:
– Configures its IP networking
– Copies its IP address config info into local
RAM
– After settings have been received, client can use
MAC and IP address to send packets on the
network
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• When diskless workstation reboots, or is
shut down
– IP config is lost
– Each time workstation reboots it must obtain IP
config from RARP server.
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Compare ARP & RARP
Similarities
• Both are concerned with mapping IP to
MAC addresses
• Both use same packet format
• Both use broadcast addresses for their task.
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Differences
• ARP - obtains MAC address of other clients by
using an IP address
• RARP – obtains its own (local host) IP address by
using its own (local host’s) MAC address
• ARP packets are broadcast on local network by
using broadcast MAC address (ff-ff-ff-ff-ff-ff)
• RARP uses broadcast IP address
(255.255.255.255) as well as broadcast MAC
address (ff-ff-ff-ff-ff-ff)
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• ARP table maintained by local host
• RARP table maintained by RARP server
• ARP reply is used by local host to update
its ARP table and to send packets to
destination.
• RARP reply is used to configure the IP
protocol of a local host.
Birgit Bonham: Prospect High School