Network Architecture and
Protocol Concepts
Network Architectures (1)
• The network provides one or more communication
services to applications
– A service has characteristics such as reliable delivery, inorder delivery, etc.
• Providing these services requires the solution to
many problems
– Divide and conquer: group these problems into layers
with software and hardware network objects
• The choice of how the layers are partitioned and
what services are provided at each layer is one
aspect of network architecture
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Network Architectures (2)
• There are many different “architectures” involved
in any complete network
• The “Service architecture” provides a framework
for the types of services that a network offers to
applications
– Will it offer connection-oriented or connectionless
service? Reliable or unreliable service?
• The “Protocol stack architecture” is a design for
how the service architecture is accomplished
through a combination of protocols and layers that
provide different functions
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Network Architectures (3)
• The “Network Topology architecture” is a design
for segments and segment boundaries to achieve
some desired traffic engineering for the network
– For example, what stations must be able to broadcast to
each other due to application requirements (broadcast is
generally limited to a single data link)
• We may also have a “Routing protocol
architecture” that relates to the network topology
and indicates how the segments of the topology
will be integrated into an internetwork
– This is also tied to an addressing design for the network
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Network Architectures (4)
• A “Router architecture” or “Switch architecture”
relates to the internal design of network equipment
– These architectures define the device performance
limitations
– Internally devices often have a separate “control plane”
that is distinguished from the “data plane” where user
applications exchange data
• A “Network Management architecture” describes
how management of the network occurs
– Devices are sometimes interconnected with both a user
network and a separate management network for
security purposes (called out-of-band management)
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Network Architectures (5)
• There are also specific architectures for services
that directly support applications, e.g.:
– Domain Name Service (DNS) architecture for naming
and lookup of network entities
– Messaging service architecture
• Chat, email, Enterprise Service Buses, etc.
• And there are architectures for services that
support the network itself, e.g.:
– Network Time Protocol (NTP) for synchronization of
network entities
– Dynamic Host Configuration Protocol (DHCP) for address
assignment
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Network Architectures (6)
• While we learn the service and related protocol
stack architectures we rarely can change their
characteristics
– Most of us select for our use the protocols (and service
choices) designed by others
• We also do not know the internal details of a
vendor’s router or switch architectures
• But when we design a network we often must
create an address, routing, and VLAN plan for
segmentation; plans for the various specific
services such as DNS; and a management plan
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Protocols (1)
• Protocols are the building blocks of a network
– A protocol is a description of how a network object at a
layer on one device interacts with the same layer object
on another device – called its peer object
– The term also refers to the network object that
implements that protocol
• The layered protocols are often called the
protocol stack because of the way they are
usually drawn
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Protocols (2)
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Protocols (3)
• Each protocol object has two different interfaces
– Service interface: The interface to a higher-layer
protocol object on the same system
• It defines the operations that the higher-layer protocol object
can perform on this protocol object
• A protocol accepts its Service Data Unit (SDU) or payload at
the service interface
– Peer-to-peer interface: the message interaction
between this protocol object and its peer on another
system
• Peer-to-peer communication is indirect (or virtual) except at the
hardware layer
• A protocol sends Protocol Data Units (PDUs) on this
interface
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Protocols (4)
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Protocols (5)
• The PDU for a protocol object is created from the
SDU by encapsulating the SDU (= the payload)
between a header and a (optional) trailer
– The header and optional trailer provide the additional
information needed to support the service provided by
the protocol object to its customers
– The SDU contents and structure are opaque to the
protocol object and the encapsulation process – the
payload is just a bunch of bytes
• That PDU then becomes the SDU for the next
lower layer, and so on until the physical layer is
reached
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Protocols (6)
• For each layer - at the peer object the header and
trailer have completed their purpose and are
discarded; the original SDU is then handed to the
appropriate higher-layer customer
– This is the opposite of encapsulation, perhaps deencapsulation or decapsulation
• So each layer appears to have a peer-to-peer
communication - but in reality the data is
1. Repeatedly encapsulated as it passes down the stack;
2. Passed through the physical layer;
3. Repeatedly de-encapsulated as it passes up the stack
at the destination
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Protocols (7)
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Protocols (8)
• A protocol object may provide service to multiple,
higher-layer, protocol objects (via multiplexing)
– For example, many applications share the same TCP
protocol object on a system
– In turn TCP, UDP, and other protocols share the same IP
protocol object
– As the PDU is created from the SDU a tag (or address) is
added to the PDU to distinguish between the various
higher-layer customers
• This allows the SDU to be extracted at this layer’s peer and
handed back to the peer of the higher-layer customer
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Hypothetical protocols in a simple
network architecture
Protocols (9)
Note that there may be multiple
objects at a higher layer using a lower
layer service – multiplexing is required
RRP = Request/reply protocol
MSP = Message stream protocol
HHP = host to host protocol
Host 1
File
application
Digital
library
application
Video
application
Host 2
File
application
Digital
library
application
Video
application
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Protocols (10)
• For some protocols - if the SDU is too big it may be
fragmented to fit into multiple PDUs
– Re-assembly occurs at the remote peer
– As the PDU is created from the SDU the information to
support re-assembly at the peer must be included in the
PDU
• The PDU may also include information to support
error detection or correction, or flow control, or
any other service characteristics that the protocol
object provides to its customers
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Protocols (11)
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Protocols (12)
• Our study of networking is largely about:
– Identifying a particular protocol stack (TCP/IP/Ethernet)
– Looking at the services provided at each layer
– Becoming familiar with the PDUs
• The original model has been extended as
networking requirements have changed
– Shim layers have been sandwiched into the stack to
provide additional service characteristics such as security
– But the ideas of the protocol stack, of PDUs and SDUs,
of peer-to-peer virtual communications remain valid
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