Download 3.1 Introduction to ARP

3.1 Introduction to ARP
• Why Address Resolution Protocol (ARP) is necessary and what it is
used for.
• IP address is link-based and independent of physical address and
network.
• IP address must be mapped to the physical address of a host for the
delivery of higher-layer data via the data link layer.
• Direct mapping and static binding have limitations.
• Address Resolution Protocol (ARP) enables hosts on the same physical
network to transfer data with each other.
• ARP binds an Internet address to a physical hardware (MAC address).
The binding is dynamic.
• ARP does not rely on IP.
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3.2 Basic ARP Mechanism
In the Internet, each host and gateway maintains a table of bindings of
physical/network address pairs (ARP cache). This table is initially empty.
When a host wishes to send an IP packet, it:
1.
2.
3.
checks the IP address in its local table. If it is there, the packet is sent
to the matching physical address.
if the IP address is not in the table, it broadcasts to the physical
network a message in Address Resolution Protocol (ARP) format,
requesting the destination physical address. Either:
A) the destination device responds with its own physical address;
B) a gateway responds, effectively indicating it can deliver the
packet.
the details from the ARP reply are stored in the table for later use,
and the packet sent to the indicated physical address.
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3.3 ARP Message Format
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• An example Ethernet frame carrying an ARP message (identified by
the type field of 0806 in an Ethernet II frame).
• Hardware -- Specifies the hardware interface type for which the
sender seeks an answer: Ethernet – 1, Frame Relay – 15, etc.
• Protocol -- Specifies the type of upper-layer protocol address the
sender is supplying. This will be 0800 for IP addresses.
• Hardware length and protocol length -- Specifies the length of the
hardware and protocol addresses.
• Operation -- Specifies either an ARP request (1) or an ARP response
(2).
• Sender hardware address (HA) and Internet address (IA) -Sender's hardware and IP address.
• Target HA and IA--Target's hardware and IP address.
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3.4 ARP Related Issues
• ARP requests contain the device's IP/physical address pair, so the
destination host and other hosts on the physical network can note them
in their tables.
• ARP cache to reduce ARP broadcast (high cost because all hosts need
to process). Entries in the cache can be updated.
• ARP messages are carried directly by the link layer's protocol, not by
IP. An ARP implementation is needed for each type of physical
network. ARP is normally regarded as part of the physical network
system, not part of internet protocols.
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3.5 Startup Internet Address and RARP
• Reverse ARP (RARP) can be used to determine a link’s internet
address for hosts without a large secondary storage (e.g. certain clients
in sensor network). Hardware address is normally available in the
ROM of network card.
• A host states its link’s physical address in a broadcasted RARP, and
asks for its IP address. One or more devices on the physical network
will be acting as RARP servers, and can reply, supplying the IP
address.
• Limited in practice. Often boot images are also required. RARP needs
a server for each physical network.
• Higher layer protocols, BOOTP and DHCP can solve the address
problem better.
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3.6 Proxy ARP
• Proxy ARP is a technique to allow two physical networks to share the
one IP network prefix. A gateway running Proxy ARP software will
respond to ARP requests on behalf of devices on the far side of the
gateway. This enables packets to pass as required.
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