Transcript Introduction to ARP

TCP/IP Review
Randy Marchany
VA Tech Computing Center
Spring, 2001
Copyright 2002, Marchany
Internet Addresses
 Hosts are identified by names,
addresses and routes
 Internet Address: 32bit integer
 Address is a pair: (netid, hostid)
– 5 classes of addresses (A,B,C,D,E)
 Routers use the netid to decide where
to send the packet
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Internet Addresses
 Multi-homed hosts: hosts that have >1
net connection need an IP address for
each I/F.
 Broadcast Address: refers to ALL hosts
on a net.
 Subnets: used to conserve IP
addresses
 Loopback Address: 127.0.0.0
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Network Byte Order
 A Standard representation of data
 LITTLE ENDIAN
– Lowest memory address contains the low order
byte of the integer 0----------15
 BIG ENDIAN
– Lowest memory address contains the high order
byte of the integer 15-----------0
 Each host converts from internal format to
NBO before Xmit. User data is exempt.
 NBO is BIG ENDIAN.
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Internet Architecture
 A collection of networks
 2 networks can only be connected by a
computer that is connected to both of them.
This machine is a router.
 Routers use the destination network address
not the destination host address when routing
a packet.
 The amount of info a router needs to store is
proportional to the # of nets not hosts.
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Address Resolution Protocol
 ARP maps internet addresses to
physical addresses. (RFC 826, 814,
1029, 1166)
 2 types of Physical Addresses
– ETHERNET – 48 bit address
• You change the I/F, you change the Ethernet
Address.
– PRONET – Token Ring
• Allows the user to choos a HW address when
installing the I/F board.
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Address Resolution Protocol
 ARP is a low level protocol that maps
addresses dynamically.
 ARP allows a host to find the physical
address of the target on the SAME physical
net given the IP address only.
 ARP Cache
– Contains recent ARP bindings to improve
efficiency.
– Sender includes its own (Pa, Ia) pair when it
sends the ARP packet. Target can save time with
the info ahead of time.
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Address Resolution Protocol
 A wants B’s physical address (Pb)
 A broadcasts an ARP packet that ask
host Ib to respond w/its Pb.
 All hosts on the subnet receive the
message but only B responds. (Unicast)
 A gets Pb and uses this address to send
packets to B directly.
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ARP Implementation
 REQUEST
– Given Ib, consult cache for Pb. If so,
extract Pb and continue with transmission.
– OW, broadcast ARP request packet.
Consider delays in packet transmission.
 REPLY
– Receive the ARP packet, extract (Ia, Pa),
look in cache and update it.
– Are we the target? If so, send (Ib,Pb) back
to sender using (Ia, Pa) pair.
Copyright 2002, Marchany