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Fundamental Network Characteristics
There are many different kinds of networks, and network technologies used to create them
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The drawback of this is that there are so many
different types of protocols and technologies for the networking student to understand!
Before you can really compare these approaches, you need to understand some of the
basic characteristics that make networks what they are
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In this section, I introduce and discuss a number of key networking concepts that describe
and differentiate different types of networks and networking technologies
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The topics here include explanations of protocols, switching
methods, types of network messages, message formatting, and ways of addressing
messages
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Note: If you have considerable experience in networking, you may not need to
read everything in this section
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Networking Layers, Models and Architectures
One of the reasons why many people find networking difficult to learn is that it can be a very
complicated subject
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While a network user may only perceive that he
or she is using one computer program (like a Web browser) and one piece of hardware (like
a PC), these are only parts of a much larger puzzle
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The best way to understand any complex system is to break it down into pieces and then
analyze what they do and how they interact
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At the same time, we also need to define interfaces between these
components, which describe how they fit together
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Networking Layers
Networking technologies are most often compartmentalized in this manner by dividing their
functions into layers, each of which contains hardware and/or software elements
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Layers are conceptually arranged into a vertical stack
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The higher layers in turn use
these services to implement more abstract functions such as implementing user
applications
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Each hardware device or software program can be
specialized to perform the function needed by that layer, like a well-trained specialist on an
assembly line
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Understanding how a network functions overall is also made much easier this way
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For this to be possible, it is necessary for everyone to
agree on how layers will be defined and used
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The model describes what the different layers are in the network, what
each is responsible for doing, and how they interact
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The most common general model in use today is the Open Systems Interconnection (OSI)
Reference Model, which concepts of seven stacked layers
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Understanding the OSI model is essential to understanding networking as a whole
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Networking Architectures
Closely related to the concept of a model is that of an architecture
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Such a ruleset usually takes the form of a specification or
standard that describes how equipment and programs using the technology must behave
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In this Guide we are, of course, interested in the TCP/IP protocol suite, which runs the
Internet, and a complex set of technologies that spans many layers of the OSI model
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For starters, the name of the suite,
TCP/IP, comes from the Transmission Control Protocol (TCP), which operates at layer four
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of the OSI model, and the Internet Protocol (IP) that runs at OSI model layer three
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TCP uses IP's
functions and provides functions to the layers above it
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Protocols: What Are They, Anyway?
If there’s one word you will get used to seeing a lot as you go through this Guide, it is this
one: protocol
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Clearly protocols are important, yet many reference works and standards use the
term over and over again without ever explaining it
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The Meaning of the Word “Protocol”
In some cases, understanding a technical term is easier if we go back to look at how the
term is used in plain English
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These people must follow certain rules of
ceremony and form to ensure that they communicate effectively, and without coming into
conflict
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Even we “normal people” follow protocols of various sorts,
which are sort of the “unwritten rules of society”
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They define a language and a set
of rules and procedures that enable devices and systems to communicate
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What networking protocols
concern themselves with is ensuring that all the devices on a network or internetwork are in
agreement about how various actions must be performed in the total communication
process
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In most cases, an individual protocol describes how communication is accomplished
between one particular software or hardware element in two or more devices
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For example, the
Transmission Control Protocol (TCP) is responsible for a specific set of functions on TCP/IP
networks
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While OSI Reference Model definitions are sometimes overly theoretical in nature, this
particular one is rather accurate in assessing protocols in real-world networking
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Key Concept: A networking protocol defines a set of rules, algorithms, messages
and other mechanisms that enable software and hardware in networked devices to
communicate effectively
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Related Information: The formalized OSI Reference Model meaning of the word
“protocol” is covered in the OSI model topic on horizontal layer communication
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Some of the more common “alternative” uses of the
word include the following:
☯

Protocol Suites: It is very common to hear the word “protocol” used to refer to sets of
protocols that are more properly called protocol suites (or stacks, in reference to a
stack of layers)
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Sometimes, the name of the technology itself leads to this confusion
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Thus,
PPP is really a protocol suite, or alternately, can be considered a protocol with “subprotocols”
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It
usually calls a full networking stack like TCP/IP or IPX/SPX just a “protocol”
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☯

Other Technologies: Sometimes technologies that are not protocols at all are called
protocols, either out of convention or perhaps because people think it sounds good
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☺ But hopefully having read
about the term and what it means, you will be able to better understand the word when you
encounter it in your studies—especially in the places where it may not always be used in a
way entirely consistent with its formal definition
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The
most important word in that sentence is the final one: information
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There are also a number of ways of categorizing and describing these
methods and the types of networks that use them
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In
highly simplified terms, there are two approaches: either a path can be set up between the
devices in advance, or the data can be sent as individual data elements over a variable
path
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Information about the nature of the circuit is
maintained by the network
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Even if many potential paths
through intermediate devices may exist between the two devices communicating, only one
will be used for any given dialog
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The classic example of a circuit-switched network is the telephone system
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That circuit functions the same way regardless of how
many intermediate devices are used to carry your voice
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The next time you call, you get a new circuit, which may
(probably will) use different hardware than the first circuit did, depending on what's available
at that time in the network
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Instead, the data is chopped
up into small pieces called packets and sent over the network
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On the
receiving end, the process is reversed—the data is read from the packets and re-

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B
B

A
A

B

Device B

A

B
A
Device A

Figure 1: Circuit Switching
In a circuit-switched network, before communication can occur between two devices, a circuit is established
between them
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Once set up, all communication between these devices takes place
over this circuit, even though there are other possible ways that data could conceivably be passed over the
network of devices between them
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assembled into the form of the original data
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)
An example is shown in Figure 2
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Blocks of data, even
from the same file or communication, may take any number of paths as it journeys from one device to another
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In circuit switching, a circuit is first
established and then used to carry all data between devices
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Comparing Circuit Switching and Packet Switching
A common temptation when considering alternatives such as these is to ask which is
“better”—and as usually is the case, the answer is “neither”
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One important issue in selecting a switching method is whether the network medium is
shared or dedicated
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However, this doesn't work well in LANs, which typically use a single shared medium and
baseband signaling
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It makes more sense to chop the data into small
pieces and send them one at a time
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The ability to have many devices communicate simultaneously without dedicated data
paths is one reason why packet switching is becoming predominant today
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One is that
since all data does not take the same, predictable path between devices, it is possible that
some pieces of data may get lost in transit, or show up in the incorrect order
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While the theoretical difference between circuit and packet switching is pretty clear-cut,
understanding how they are used is a bit more complicated
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For example, suppose you connect to the
Internet using a dial-up modem
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Yet the data may be
sent over the telephone system in digital packetized form
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Another issue is the relationship between circuit and packet switching, and whether a
technology is connection-oriented or connectionless
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Note: Note that the word “packet” is only one of several terms that are used to
refer to messages that are sent over a network
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Connection-Oriented and Connectionless Protocols
In the previous topic I described and contrasted networking technologies based on whether
or not they use a dedicated path, or circuit, over which to send data
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This issue is closely related to the matter of packet versus
circuit switching
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This is generally accomplished by following a specific set of rules that specify how a connection should be
initiated, negotiated, managed and eventually terminated
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They pass control
information to determine if and how the connection should be set up
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When they are finished, the connection
is broken
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As soon as a device has data to send to another, it just sends it
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In a connectionless
protocol, data is just sent without a connection being created
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Obviously, in order to establish a circuit between two devices, they must also
be connected
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This has led to the terms “circuit-switched” and “connection-oriented” being used
interchangeably
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A connection is needed for a circuit, but a
circuit is not a prerequisite for a connection
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These connection-oriented protocols are important because they enable the implementation of applications that require connections, over packet-switched networks that have no
inherent sense of a connection
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This requires the establishment of a connection over which commands, replies and data can be passed
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Yet, both of these work (indirectly) over the IP protocol, which is based on
the use of packets, through the principle of layering
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Even though packets may be used at lower layers
for the mechanics of sending data, a higher-layer protocol can create logical connections
through the use of messages sent in those packets
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Logical
connection-oriented protocols can in fact be implemented on top of packet switching
networks to provide higher-layer services to applications that require connections
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One is the Transmission Control Protocol (TCP), which is
connection-oriented; the other, the User Datagram Protocol (UDP), is connectionless
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UDP is used by other applications that don't need connections or other features, but do need the faster performance
that UDP can offer by not needing to make such connections before sending data
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Even though a TCP connection can be
used to send data back and forth between devices, all that data is indeed still being sent as
packets; there is no real circuit between the devices
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Certainly, the existence of connectionoriented protocols like TCP doesn't obviate the need for circuit switching technologies,
though you will get some arguments about that one too
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Just as a
connection-oriented protocol can be implemented over an inherently connectionless
protocol, the reverse is also true: a connectionless protocol can be implemented over a
connection-oriented protocol at a lower level
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In turn, IP can run
over a connection-oriented protocol like ATM
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Even though the word “packet” appears
in the name of this method, the data items sent between networked devices are most
generically called messages
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In some cases these different terms can be very useful; simply the type of name used for
the message can tell you something about what the message contains
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Thus, the use of these different names can
help clarify discussions that involve multiple protocols operating at different layers
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Some people are strict about applying particular
message designations only to the appropriate technologies where they are normally used,
while others use the different terms completely interchangeably
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Common Names For Messages
The most common terms that are used for messages are the following:
☯

Packet: This term is considered by many to most correctly refer to a message sent by
protocols operating at the network layer of the OSI Reference Model
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However, this term is commonly also used
to refer generically to any type of message, as I mentioned at the start of this topic
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It is also often used to refer to a message that is sent at a
higher level of the OSI Reference Model (more often than “packet” is)
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In particular, it is most commonly seen used in
reference to data link layer messages
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A
frame gets its name from the fact that it is created by taking higher-level packets or
datagrams and “framing” them with additional header information needed at the lower
level
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For example,
the fixed-length, 53-byte messages sent in Asynchronous Transfer Mode (ATM) are
called cells
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☯

Protocol Data Unit (PDU) and Service Data Unit (SDU): These are the formal terms
used in the OSI Reference to describe protocol messages
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It consists of layer N header information
and an encapsulated message from layer N+1, which is called both the layer N SDU
and the layer N+1 PDU
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☺

I should also point out that there are certain protocols that use unusual names to refer to
their messages, which aren’t used elsewhere in the world of networking
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Key Concept: Communication between devices on packet-switched networks is
based on in items most generically called messages
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The formal OSI
terms for messages are protocol data unit (PDU) and service data unit (SDU)
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For example, messages sent over
Ethernet are almost always called Ethernet frames—they are not generally called Ethernet
datagrams, for example
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Message Formatting: Headers, Payloads and Footers
Messages are the structures used to send information over networks
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Shakespeare had the right
idea about names, however
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Every protocol uses a special formatting method that determines the structure of the
messages it employs
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This is why I separately describe the formats of dozens of different protocol
messages in various parts of this Guide
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In generic terms, each message contains the following three basic elements (see
Figure 3):
☯

Header: Information that is placed before the actual data
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It serves as the communication and control link between protocol
elements on different devices
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For example, these may be used to set up or
terminate a logical connection before data is sent
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There is no real difference between
the header and the footer, as both generally contain control fields
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Since the header and footer can both contain control and information fields, you might
rightly wonder what the point is of having a separate footer anyway
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In some
cases, it is more efficient to perform this computation as the data payload is being sent, and
then transmit the result after the payload in a footer
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Footers are most often associated with lower-layer protocols,
especially at the data link layer of the OSI Reference Model
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The data of any particular message sent in a networking protocol will itself contain an
encapsulated higher-layer message containing a header, data, and footer
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The header is found in most protocol messages; the footer
only in some
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Header and footer information is functionally the same except for position in the
message; footer fields are only sometimes used, especially in cases where the data in the
field is calculated based on the values of the data being transmitted
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It doesn't care much about what is in the data portion of the message,
just as a delivery person only worries about driving the truck and not so much on what it
contains
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In the OSI
Reference Model, a message handled by a particular protocol is said to be its protocol data
unit or PDU; the data it carries in its payload is its service data unit or SDU
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The discussion of data
encapsulation contains a full explanation of this important concept
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One is simply the addressing of the message—putting an address on it so that the
system knows where it is supposed to go
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There are several different ways of addressing and transmitting a message over a network,
One way in which messages are differentiated is in how they are addressed, and to how
many recipients
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Message Transmission Methods
To help explain these different methods, I will use a real-world analogy
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These people are mingling and
are having different conversations
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Bearing this analogy in mind, consider these three kinds of message transmissions, which
are illustrated in Figure 4:
☯

Unicast Messages: These are messages that are sent from one device to another
device; they are not intended for others
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Of course, there is
still the possibility of someone else at the event overhearing your conversation—or
even eavesdropping on it
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☯

Broadcast Messages: As the name suggests, these messages are sent to every
device on a network
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For example, suppose a new arrival at the social gathering saw a blue sedan with New
Hampshire plates in the parking lot that had its lights left on
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The best way to communicate this information is to broadcast
it by having the host make an announcement that will be heard by all, including the
vehicle’s owner
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☯

Multicast Messages: These are a compromise between the previous two types: they
are sent to a group of stations that meet a particular set of criteria
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(Note that one can also consider
broadcast messages to be a special case of multicast, where the group is “everyone”
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Multicasting requires special techniques
that make clear who is in the intended group of recipients
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Device #6 is sending a unicast message to #2 shown in purple
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In this case, that group includes devices #1 and #3, shown in green
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Message Addressing Methods
Since the transmission methods above differ based on how many and which devices
receive the transmission, they are tied directly to the methods used for addressing:
☯

Unicast Addressing: Unicast delivery requires that a message be addressed to a
specific recipient
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☯

Broadcast Addressing: Broadcasts are normally implemented via a special address
that is reserved for that function
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☯

Multicast Addressing: Multicasts are the most complex type of message because
they require a means of identifying a set of specific devices to receive a message
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Some mechanism is needed to manage which devices are in
which groups
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A unicast transmission goes from one device to exactly one
other; this is the “normal” method used for most message transactions
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A multicast
transmission is addressed and sent to a select group of devices
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This term identifies a message that should be
sent to the closest member of a group of devices
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Finally, one special case in the field of addressing is worth mentioning
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In this situation, everything sent by one device is implicitly intended for the other,
and vice-versa
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Network Structural Models and Client/Server and Peer-to-Peer
Networking
I mentioned in my discussion of the advantages of networking that networks are normally
set up for two primary purposes: connectivity and sharing
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How precisely this is
done depends to a large degree on the overall design of the network
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Specifically, the network designer must decide whether or not to dedicate
resource management functions to the devices that constitute it
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In this latter arrangement, the
devices are sometimes said to have roles, somewhat like actors in a play
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Peer-to-Peer Networking

In a strict peer-to-peer networking setup, every computer is an equal, a peer in the network
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There is no
assigned role for any particular device, and each of the devices usually runs similar
software
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Device #1

Request

Device #6

Device #2
Request

Response

Device #5

Response

Device #3

Device #4

Figure 5: Peer-to-Peer Networking
In this model, each device on the network is treated as a peer, or equal
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This model is more often
used in very small networks
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Client/Server Networking

In this design, a small number of computers are designated as centralized servers and
given the task of providing services to a larger number of user machines called clients
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The clients are typically smaller, “regular” computers like PCs,
optimized for human use
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Usually, server software runs on
server hardware, and client software is used on client computers that connect to those
servers
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Server software is designed to efficiently respond to
requests, while client software provides the interface to the human users of the network
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While the clients can still interact with each other, most of the time they send requests of various
sorts to the server, and the server sends back responses to them
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Key Concept: Networks are usually configured to share resources using one of two
basic structural models
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In a client/server network, however, devices are
assigned particular roles—a small number of powerful computers are set up as servers and
respond to requests from the other devices, which are clients
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Comparing Client/Server and Peer-to-Peer Networking
The choice of client/server or peer-to-peer is another where there is no “right answer” in this
regard
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Peer-to-peer networking has primary advantages of simplicity and low cost, which means it
has traditionally been used on small networks
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This makes it better-suited to larger networks
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Many years ago it was common to see even networks with 20 to 50
machines using the peer-to-peer model; today, even networks with only a half-dozen
machines sometimes are set up in a client/server mode because of the advantages of
centralized resource serving
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A mainframe with
attached terminals can be thought of as a client/server network with the mainframe itself
being the server and the terminals being clients
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One of the reasons why the client/server structural model is becoming dominant is that it is
the primary model used by the world’s largest network: the Internet
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For example, the term “Web
browser” is really another name for a “Web client”, and a “Web site” is really a “Web server”
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That
topic also contains a very relevant exposition on the different meanings of the
terms “client” and “server” in hardware, software and transactional contexts
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