Layering and Architecture

Layering and Network Architecture

Network Architecture
Requirements of computer network,
– Connectivity
– Cost effective resource sharing
– Support for common services
– Reliability
Network Architecture – framework that guide the
design and implementation of networks.
widely used architectures,
– OSI architecture
– Internet architecture (TCP/IP Model)

Layering and Protocols
– Services offered by the underlying hardware, and then add
a sequence of layers, each providing a higher level of
services.
– These services are implemented in terms of services
provided by the lower layers.
Features of Layering,
– Decomposes the problem of building a network into more
manageable components.
• Each layer solves one part of the problem
– Modular design
• Easy to add a new service
• Only need to modify the functionality at one layer
Network Architecture Contd…

• The above layered system having two layers of
abstraction sandwiched between the underlying
hardware and the application program.
Layered Network System

• The above layered system having multiple abstractions
provided at any level of the system.
• Two types of channel,
– request/reply service
– Message stream service
Layered system with alternate abstraction

Protocol
– defines the interfaces between the layers in the same
system and with the layers of peer system.
– Protocol defines two interfaces.
• Service interfaces – interface between objects in the same machine
• Peer interfaces - messages exchanged with peer

Protocol Graph
– Nodes of the graph corresponds to protocol.
– Edges represent “depends on” relation

Protocol Specification
– Combination of prose, pseudo code, state transition
diagrams, packet formats and abstract notations.
Encapsulation
– when application program sends a message to its peer by
passing the message to protocol,
• Protocol must communicate control information to its peers by
attaching a “Header” to the message.
• Header – small piece of information used among peers to
communicate each other.
• Format of the header is defined by protocol specification.
• Payload – data being transmitted by the application
– Encapsulation is adding a applications data into the new
message created by the protocol.

– When a message arrives at the destination host, it is
processed in the reverse order.
• Removes the header attached by its peer, takes the action indicated
by that header and passes the body of the message to the
application.
High level messages are encapsulated into the low level messages

Multiplexing and Demultiplexing
– Multiplexing – multiple flows of data over a single
physical link.

OSI ARCHITECTURE
The OSI 7-layer Model
OSI – Open Systems Interconnection

Exchange of Data in OSI Model

Functions of the OSI Layers:
Physical Layer
 The physical layer defines the characteristics of the
interface between the devices and the transmission
medium. It also defines the type of transmission medium.
 Representation of bits - defines the type of encoding (how
0’s and 1’s are changed to signals).
 Transmission rate (Number of bits sent per second) is
defines by the physical layer.
 Synchronization – sender and receiver clock is
synchronized by physical layer.
 It defines how the machines are connected to make a
network

Transmission of bits in Physical Layer
Data Link Layer
 Framing - divides the stream of bits received from the
network layer into manageable data units called frames.

 Physical Addressing - If frames are to be distributed to
different systems on the network, the data link layer adds a
header to the frame to define the sender and/or receiver of the
frame. If the frame is intended for a system outside the sender's
network, the receiver address is the address of the device that
connects the network to the next one.
 Flow Control - If the rate at which the data are absorbed by the
receiver is less than the rate at which data are produced in the
sender, the data link layer imposes a flow control mechanism to
avoid overwhelming the receiver.
 Error Control - The data link layer adds reliability to the
physical layer by adding mechanisms to detect and retransmit
damaged or lost frames. It also uses a mechanism to recognize
duplicate frames. Error control is normally achieved through a
trailer added to the end of the frame.

 Access Control - When two or more devices are connected to
the same link, data link layer protocols are necessary to
determine which device has control over the link at any given
time.

Network Layer
 Responsible for the source to destination delivery of the
packets across multiple networks. If two systems are
connected to the same link, there is usually no need for a
network layer.
 Logical Addressing - If a packet passes the network boundary,
we need to distinguish the source and destination systems. The
network layer adds a header to the packet that includes the
logical addresses of the sender and receiver.
 Routing - When independent networks or links are connected
to create internetworks (network of networks) or a large
network, the connecting devices (called routers or switches)
route or switch the packets to their final destination. One of the
functions of the network layer is to provide this mechanism.

Packet Transfer in Network Layer

Transport Layer
 Responsible for process-to-process delivery of the entire
message.
Process – application program running on a host.
 Service point Addressing - Computers often run several
programs at the same time. For this reason, source-to-
destination delivery means delivery not only from one computer
to the next but also from a specific process (running program)
on one computer to a specific process (running program) on the
other. The transport layer header must therefore include a type
of address called a service-point address (or port address). The
network layer gets each packet to the correct computer; the
transport layer gets the entire message to the correct process on
that computer.

 Segmentation and Reassembly - A message is divided into
transmittable segments, with each segment containing a
sequence number. These numbers enable the transport layer to
reassemble the message correctly upon arriving at the
destination and to identify and replace packets that were lost in
transmission.
 Connection Control - The transport layer can be either
connectionless or connection oriented. A connectionless
transport layer treats each segment as an independent packet
and delivers it to the transport layer at the destination machine.
A connection oriented transport layer makes a connection with
the transport layer at the destination machine first before
delivering the packets. After all the data are transferred, the
connection is terminated.

 Flow Control – similar to flow control in data link layer,
however, transport layer performing end to end flow control.
 Error Control – Transport layer provides end to end error
control. The sending transport layer makes sure that the entire
message arrives at the receiving transport layer without error
(damage, loss, or duplication). Error correction is usually
achieved through retransmission.

Transport Layer

Session Layer
 Provides a name space that is used to tie together the
potentially different transport streams that are part of a
single application.
Presentation Layer
 Concerned with syntax and semantic information's
exchanged between two systems.
 Responsible for Encryption and compression.
Application Layer
 Enable the user(human or software) to access the network.
 Provides the user interface for services such as E-Mail,
File transfer, etc…

Summary of OSI Layer Functions

Merits of OSI Reference Model
– It distinguishes very clearly between the services, interfaces
and protocols.
– The protocols in OSI model are better hidden. So they can
be easily replaced by new protocols as the technology
changes.
– OSI model is truly a general model.
– It supports both connection oriented and connectionless
services.
Demerits
– Sessions and presentation layer are not of much use.
– This model was devised before the protocols were invented.
So in real life there is a problem of fitting protocol into a
model.

INTERNET ARCHITECTURE
 Also called as “TCP/IP architecture”.
 Alternative view of the Internet architecture. The
“Network” layer shown here is sometimes referred to as the
“sub-network” or “link” layer or “Host to Network Layer”.

Host to Network layer
 Wide variety of protocols denoted NET1, NET2, …. NETn
implemented by a combination of hardware (eg. Network
Adapter) and software (Eg. Network Device Driver).
 Eg. Ethernet, FDDI
Internet Layer
 Consist of a single protocol called “Internet Protocol” (IP),
that supports the interconnection of multiple networking
technologies into a single, logical network.
 Supporting protocols,
 ARP (Address Resolution Protocol)
- Associates the logical address (IP) with a physical address.
 RARP (Reverse Address Resolution Protocol)
- Allows host to discover the logical address when it knows the physical address.
ICMP (Internet Control Message Protocol)
- Querying and Error reporting protocol.

IGMP (Internet Group Message Protocol)
- Facilitate the simultaneous transmission of a message to a multiple recipients.
Transport Layer
 Contains two main end to end protocols,
 TCP – provides a reliable byte stream channel (Connection oriented)
 UDP – provides a unreliable datagram channel (Connectionless)
Application Layer
 Provides wide range of protocols for different applications.
 Protocols provided by this layer are,
 FTP (File Transport Protocol)
 TFTP (Trivial File Transport Protocol)
 SMTP (Simple Message Transfer Protocol)
 Telnet (Remote Login)
 HTTP (Hyper Text Transfer Protocol), etc…

Internet Protocol Graph

Three Features of Internet Architecture
 Does not imply strict layering
- The application is free to bypass the transport layer and to directly use
IP or one of the underlying layer.
 Hour glass shape protocol graph
- Wide at top, narrow in middle and wide at bottom.
- IP serves as the focal point fir architecture
• It defines a common method for exchanging packets among a different
networks.
- Above IP can be arbitrarily many transport protocols offering a
different channel abstraction to application layer.
• It separates the Host to Host delivery with Process to Process delivery
- Below IP, the architecture allows for different network technologies,
ranges from Ethernet to wireless.

 In order to add a new protocol to the architecture, it needs to
be a protocol specification and at least one representative
implementations of the specification.
- The application is free to bypass the transport layer and to directly use
IP or one of the underlying layer.

Layering and Architecture

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