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CSCI 4550/8556
Computer Networks
Comer, Chapter 19:
Binding Protocol Addresses (ARP)
Introduction
The upper levels of a protocol stack use protocol
addresses .
The network hardware must use a hardware
address for eventual delivery.
Protocol addresses must be translated into
hardware addresses for delivery; will discuss three
methods of achieving this translation.
Address Translation
Upper levels use only protocol addresses to
provide a “virtual network” addressing scheme, and to
hide hardware details.
The address translation occurs at the data link
layer:
the upper layer hands down a protocol address of the
destination, and
the data link layer translates it into a hardware address
for use by hardware layer.
Address Resolution
Finding the hardware address for a protocol
address
is called address resolution, because
the data link layer resolves the protocol address to the
hardware address.
Resolution is local to a network.
The network component only resolves addresses
for other components on same network.
Address Resolution Example
A resolves protocol address for B for protocol messages
from an application on A sent to an application on B .
A does not resolve a protocol address for F .
Through the internet layer, A delivers to F by routing
through R1 and R2
A resolves R1 hardware address.
Network layer on A passes packet containing destination
protocol address F for delivery to R1 .
Address Resolution Techniques
The association between a protocol address and a
hardware address is called a binding .
There are three techniques to finding the binding:
Table lookup
• Bindings stored in memory with protocol address as key
• Data link layer looks up protocol address to find hardware address
Closed-form computation
• Protocol address based on hardware address
• Data link layer derives hardware address from protocol address
Dynamic
• Network messages used for "just-in-time" resolution
• Data link layer sends message requesting hardware address;
destination responds with its hardware address
Table Lookup
Use a simple list containing IP address and hardware
address for each host on net.
Search on the IP address and extract the corresponding
hardware address.
Note that all IP addresses have same prefix; we can save
space by dropping the prefix.
Table Lookup (continued)
A sequential search may be prohibitively
expensive.
We can use indexing or hashing for faster lookup.
Indexing – use the hostid part of an IP address as a list
(array) index.
Hashing – use a hashing function on hostid to generate
a list index.
Closed-form Computation
If the hardware technology uses a small, configurable
hardware address, the network administrator can choose
the hardware address based on the IP address.
Example – the hardware uses a one octet address that can
be configured.
Now simply choose hardware address to be hostid .
Thus any host can determine a hardware address as:
hardware_address = ip_address & 0xff;
Dynamic Resolution
This technique uses the network to resolve IP addresses!
A message exchange with other computer(s) returns the
hardware address to the source of the query.
Two designs:
Server-based – a computer sends a query message to a server to
resolve an address. This design requires to users to
• know the addresses of a list of servers, or
• broadcast to locate the servers.
Distributed - all computers participate; the destination machine
provides its hardware address to the host that asked for it.
Dynamic Resolution Techniques
Server-based - centralized, easier to manage, used
on non-broadcast media (e.g., ATM).
Distributed - requires no dedicated computers, no
administration.
T = table lookup
C = closed form
D = dynamic
ARP
IP uses a distributed resolution technique.
Address Resolution Protocol (ARP) – essentially a
part of the TCP/IP protocol suite.
It is a two-part protocol:
A request is sent from the source asking for the
hardware address for an IP address.
A reply from the destination provides the hardware
address.
ARP Message Exchange
The ARP request message is dropped into a hardware
frame and broadcast.
It sses separate protocol type in hardware frame (Ethernet
= 0x0806)
The sender inserts the desired IP address into the message
and broadcasts it.
Every other computer on the (local) network examines the
request.
The computer whose IP address is in the request responds:
It puts its hardware address in the response frame, and
unicasts the frame to the sender.
The original requester can then extract the hardware
address and send the desired IP packet to the destination.
ARP Example
ARP Message Contents
Maps protocol address to hardware address .
Both protocol address and hardware address sizes
are variable:
Ethernet = 6 octets
IP = 4 octets
ARP can be used for other protocols and hardware
types.
ARP Message Format
HARDWARE ADDRESS TYPE = 1 for Ethernet
PROTOCOL ADDRESS TYPE = 0x0800 for IP
OPERATION = 1 for request, 2 for response
Contains both target and sender mappings from protocol
address to hardware address
Request sets hardware address of target to 0
Target can extract hardware address of sender (saving an ARP
request)
Target exchanges sender/target in response
Sending an ARP Message
The sender constructs an ARP message.
The ARP message is then carried as data in a
hardware frame – this technique is called
encapsulation .
Caching ARP Packets
Using ARP for each IP packet adds two packets of
overhead for each IP packet.
To minimize this overhead, a computer caches ARP
responses, but
flushes cache at system startup time, and
periodically discards old entries.
The cache is searched prior to sending an ARP
request.
Identifying ARP Frames
ARP uses a separate frame type.
Ethernet, for example, uses frame type 0x0806.
Processing ARP Messages
The receiver extracts sender's hardware address and updates local ARP
table
The receiver checks the type operation - request or response:
If a response:
Adds sender's address to the local cache.
Sends pending IP packet(s).
If a request:
If receiver is the target of the ARP query, it forms the response, and
unicasts the response to the sender, and
the sender's address is added to the local cache.
Note:
Target computers are likely to respond “soon.”
Computers have finite storage for the ARP cache.
Only the target adds sender to cache; others only update their caches if the
target is already in the cache.
Layering and Address Resolution
Address resolution
(ARP) is a network
interface layer
function.
Protocol addresses
are used in all higher
layers.
Address resolution
hides the ugly details
and allows generality
in upper layers.
Summary
Address resolution - translates protocol addresses
to hardware addresses
Three techniques
Static - table lookup
Computation - extract a hardware address from a
protocol address
Dynamic - use network messages to resolve a protocol
address
ARP - TCP/IP protocol for address resolution.