2. Network Models
Protocol Layering
Enable us to divide a complex task into several smaller
and simpler tasks
◼ Modularity: independent layers
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A layer is defined as a module with inputs and outputs
(interfaces)
A layer receives a set of services from the lower layer
and gives the services to the upper layer
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Logical Connection between Peer Layers
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TCP/IP Protocol Suite
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A hierarchical protocol made up of interactive modules
Layered Architecture
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Communication through an internet
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Logical Connection between Layers
Logical connections
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Identical Objects in TCP/IP Protocol Suite
Identical objects (messages)
Identical objects (segment or user datagram)
Identical objects (datagram)
Identical objects (datagram)
Identical objects (frame)
Identical objects (frame)
Identical objects (bits)
Identical objects (bits)
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Description of Each Layer
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Physical Layer
Responsible for carrying individual bits in a frame across
the link
◼ Two devices are connected by a transmission media
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Transmission medium carries electrical or optical signals
Data-Link Layer
Responsible for taking the datagram, encapsulating in a
frame and moving it across the link
◼ Flow control, error control, access control, …
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Description of Each Layer (cont.)
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Network Layer
Responsible for creating a connection between source
and destination
◼ Responsible for host-to-host communication and routing
the packet through possible routes
◼ IP (Internet Protocol)
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Connectionless protocol with no flow control, no error
control, no congestion control services
◼ Define the format of packet (called datagram) and the
format and the structure of addresses
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Description of Each Layer (cont.)
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Network Layer (cont.)
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ICMP (Internet Control Message Protocol)
A mechanism used by hosts and gateways to send
notification of datagram problems back to sender
◼ Send query and error reporting messages
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IGMP (Internet Group Management Protocol)
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Facilitate the simultaneous transmission of a message to
a group of recipients
DHCP (Dynamic Host Configuration Protocol)
◼ ARP (Address Resolution Protocol)
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Associate a logical address with a physical address
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Description of Each Layer (cont.)
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Transport Layer
Responsible for giving services to the application layer
◼ TCP (Transmission Control Protocol)
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Connection-oriented protocol: establish a logical connection
between transport layers at two hosts before data transfer
◼ Stream-oriented, reliable protocol
◼ Flow control: match the sending data rate of the source host
with the receiving data rate of the destination host to prevent
overwhelming the destination
◼ Error control: guarantee that the segments arrive at
destination without error and resending the corrupted ones
◼ Congestion control: reduce the loss of segments due to
congestion
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Description of Each Layer (cont.)
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Transport Layer (cont.)
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UDP (User Datagram Protocol)
Connectionless protocol: transmit user datagrams without
creating a logical connection
◼ No flow control, error control, congestion control
◼ Simple, small overhead
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SCTP (Stream Control Transmission Protocol)
Combine some features of UDP and TCP for multimedia
communications
◼ Multistream, multihoming, connection-oriented, reliable, fullduplex services
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Encapsulation and Decapsulation
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Addressing
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Address at Transport Layer
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Port number
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A local address that distinguishes between several
processes running at the same time
Address at Network Layer
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Logical address
Globally unique in the whole Internet
◼ Uniquely define the connection of a device to Internet
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Address at Data-Link Layer
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Physical address (MAC address)
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Locally defined address that defines a specific host or
router in a network (LAN or WAN)
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Addressing (cont.)
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Multiplexing and Demultiplexing
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Multiplexing at Source
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A protocol at a layer can encapsulate a packet from
several next-higher layer protocols (one at a time)
Demultiplexing at Destination
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A protocol can decapsulate and deliver a packet to
several next-higher layer protocols (one at a time)
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Multiplexing and Demultiplexing (cont.)
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OSI Model
An ISO standard that covers all aspects of network
communications
◼ First introduced in late 1970s
◼ A layered framework for the design of network systems
that allows communication between all types of computer
systems
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Layered Architecture
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Composed of 7 ordered layers
Layer
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A collection of networking functions which have related uses
Define a family of functions distinct from those of other layers
Call upon services of the layer just below it
Provide a more comprehensive and flexible architecture
Allow complete interoperability between incompatible
systems
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Interaction between Layers
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Peer-to-Peer Processes
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Peer-to-Peer Processes
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Processes on each machine that communicate at a
given layer
Interface
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Define the information and services a layer must provide
for the layer above it
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An Exchange using OSI Model
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OSI vs. TCP/IP
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