Syllabus for Unit 2
Protocols & Standards OSI Model
TCP/IP Protocol Suite
Comparison of OSI and TCP/IP Model Addressing: IPv4,
Protocols
Protocols are set standards adopted worldwide for cohesive working
Standards help maintaining compatibility
We use a lot of standards in our daily life
Postal communication
Layers
The postal systems can be subdivided into 3 layers
All these layers communicate using some protocols and also communicate in asequence.
Layers: Hardware and software
For digital systems we need both softwares ad hardwares
Protocol layering
Protocol layering enables us to subdivide a complex task into several small simple tasks
Layering produces modules and this phenomenon is called modularity
A module defines anindependent layer
A module for a particular layer will simply be a processing box having input and output,without any concern for other modules except for those who are connected to itself.
Advantage of protocol layering
Allows separation of services from implementation
The module receives some set of services from lower layers and provide services to upper layers
How this task is done can now be tested and innovations can be performed for best optimized way.
Principles of protocol layering
First principle:
If we want bidirectional communication, we need to make each layer in such a way so that each layer can perform two opposite tasks (one in each direction)
Second Principle:
Logical connections
Logical connections refer to real or imaginary connections between layers
TCP/IP protocol suite
TCP/IP refers to Transmission Control Protocol/ Internet Protocol TCP/IP suite is a setof protocols used for internet.
Its is ahierarchical protocol made of interactive modules
Each module provides specific functionality.
OSI model
OSI stands for OpenSystemInterconnection model
Defined by International Standards Organizations (ISO)
Established in 1947
OSI was proposed by ISO in 1970s
An Open system refers to ability of communication systems to communicate regardless of different underlying architecture.
This means that one could have the ability to communicate between different systems without changing logic of underlying hardware and software.
The OSI model is not a protocol; it is a model for understanding and designing a network architecture that is flexible, robust, and
OSI model layers
The designers identified which networking functions had related uses and collected those functions into discrete groups that became the layers
Each layer defines a family of functions distinct from those of the other layers
Withinsingle machine, layers talk to each other in hierarchical fashion
OSI model layer interaction
Peer-to-peer:
At the physical layer, communication is direct
Devise A sends bits to device B via intermediate nodes
Non-peer-to-peer:
At higher layers, device must pass the information through intermediate layers and then perform peer-to-peer correspondence. Each layers adds its own information within the sending device and must interpret this information in receiving device.
”Original information bits” plus ”additional information bits” are together called apackage.
OSI model layer behavior
OSI model layer behavior
Interface between layers:
Interfaces connect various layers
They defines the services a particular layers must provide to its preceding and successive layers
Network, Data link and Physical layers are sometimes clubbed in one sub-category because they deal with physical manipulation of data using electrical devices.
electrical specifications physical connections physical addressing
transport timing and reliability
Session, Presentation and Application can be thought as support layers
allow interoperability among unrelated software systems
Layers
Lets understand the working of these
layers going from lower to upper in
Physical Layer
Physical Layer
The physical layercoordinates the functions required to carry a bit stream over a physical medium.
Deals with the mechanical and electrical specificationsof the interface and transmission medium.
Defines the type of medium Defines characteristics of interface
Also defines the procedures and functionsthat physical devices and interfaces have to perform for transmission to occur.
Defines the physical coding of 0 and 1 into electrical/optical signals Data Rate:
Physical Layer contd...
Synchronization
Not only the transmission rate should be same at both ends, they should be synchronized at bit level too.
Physical topology:
Must define how device is connected
Data Link Layer
Data link layer transforms a raw physical layers into a reliable link to transfer data
Data Link Layer
Framing
Divides the stream of bits (obtained from physical layer) into manageable chunks of information called frames.
Physical addressing
If different bits are intended to different systems on the network, they are framed separately and data link layer adds a header where a coded address is mentioned for intended receptor and given sender.
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:
Data Link Layer contd...
Error Control
Employ mechanisms to detect and retransmit damaged or lost frames Employ mechanism to recognize duplicate frames
Improves reliability of system
Usually a trailer is added to the end of the frame
Access Control
Data link Layer contd...
Here devices B,C and E are calledrouters
Hopping
To send data from A to F:
three partial deliveries are made
data link layer at A (end system) sends a frame to the data link layer at B (a router)
data link layer at B (router) sends a new frame to the data link layer at E (router)
data link layer at E (router) sends a new frame to the data link layer at F (end system)
Network Layer
The network layer is responsible for thesource-to-destinationdelivery of a packet, possibly acrossmultiple networks
Network Layer
Whereas the data link layer oversees the delivery of the packet between two systems on the same network (links), the network layer ensures that each packet gets from its point of origin to its final destination even across multiple networks
Network Layer
Logical addressing
The physical addressing implemented by the data link layer handles the addressing problem locally.
If a packet passes the network boundary, another addressing system is needed to help distinguish the source and destination systems.
The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver.
Routing
Transport Layer
Transport Layer
Responsible for process-to-process delivery of the entire message A process is an application programrunning on ahost.
Whereas the network layer oversees source-to-destination delivery of individual packets, itdoes not recognize any relationship between those packets.
It treats each one independently, as though each piece belonged to a separate message, whether or not it does.
Transport Layer responsibilities
Service point addressing/ port addressing
Softwares needed a lot of processes to be run on computers Transport layer ensures that process on one computer reaches to similar process on another on a network
This is done by port addressing since ports are assigned for servicing a particular process.
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
Message is segmented into transmittable packets
Transport Layer responsibilities
Connection control
Connectionless
A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine
Connection-oriented
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
Transport Layer responsibilities
Error Control
Error control is performed at the level of processes. With even a single error, a processes cannot be completed Hence, error control is performed by retransmission
Session Layer
Session layer is thedialog controller for the network by establishing, maintaining, and synchronizing the interaction among communicating
systems.
Session Layer
Dialog Control
Allows two systems to enter into a dialog
A dialog is communication between two processes in half-duplex or duplex mode
Synchronization
Allows a process to add checkpoints, or synchronization points, to a stream of data.
Presentation Layer
concerned with the syntaxandsemantics of the information exchanged between two systems which involvestranslation, compression and
encryption.
Presentation Layer responsibilities
Translation:
Processes running in two different computers might use data in different formats or might use different encoding schemes Presentation layer provides inter-interoperability for them.
This performed by changing from sender-format to common-format to receiver-format
Encryption
Encryption at sender’s end and decryption at receiver’s end is performed
Compression
Application Layer
Application Layer
Enables the user (human or software), to access the network. Provides user interfaces and support for services such as:
electronic mail
Application Layer
Network Virtual Terminal:
Software version of a physical terminal, and it allows a user to log on to a remote host.
Application creates a software emulation so that host feels that it is communicating with one of its own terminals.
Mail Services:
Provides the basis for email forwarding and storage.
File Transfer, Access and Management
Allows a user to access files in a remote host, to retrieve files from a remote computer for use in the local computer
Directory Services:
Summary
TCP/IP
The layers in the TCP/IP protocol suite do not exactly match those in the OSI model.
The original TCP/IP protocol suite was defined as having four layers:
host-to-network internet transport application.
However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers:
TCP/IP and OSI
Physical and Data Link Layers
At this layer, TCP/IP does not define any specific protocol. It supports all the standard protocols.
Network Layer
Network layer is responsible for creating a connection between the source computer and destination computer.
NL is responsible for host-to-host connection and also responsible to control routers for choosing best routing strategy.
NL in the Internet includes a main protocol called Internet Protocol (IP) which defines format of packet (calleddatagram).
IP also perform the routing by passing each datagram through routers unless it reaches the destination.
NL also includesunicast andmulti-cast routing protocols
A routing protocol does not take part in routing (That is the responsibility if IP)
IP
It is anunreliableand connectionlessprotocol.
No flow control No error control No congestion control
Axillary Protocols which help IP at NL
At Network layer, Some Auxiliary protocols help IP in its work:
ICMP
IGMP
RARP, ICMP
Internet Control Message Protocol(ICMP)
A mechanism used by host and gateways to send notification of datagram problems back to the sender.
It sends query and error reporting messages.
Internet Group Management Protocol(IGMP)
Used to facilitate the simultaneous transmission of a message to a group of recipients
It helps IP in multitasking
Dynamic Host Configuration Protocol (DHCP)
Helps IP to get the network-layer address for a host.
Reverse Address Resolution Protocol (RARP)
Protocol allows a host to discover its internet address when it knows its physical address.
ARP
Used to associate the logical address with physical address (link-layer address to a host or a router when network layers address is given). Each device on the network is identified by physical or station address, usually imprinted on the network interface card (NIC).
Transport Layer
Logical connection at TL is end-to-end It performs the following service:
Transport Layer
Responsible for delivery of message from a process to another process
TCP (Transmission control Protocol)
TCP
TCP is a connection oriented protocol
It first establishes a connection is established between both the ends (source and destination host) before data transmission.
At the sending end TCP divides a stream of data into smaller units called segments.
Each segment includes a sequence number for reordering after receipt, together with acknowledgment no. for the segments received.
At the receiving end, TCP collects each datagram as it comes in and reorder the transmission based on sequence no.
TCP provides:
Ref for protocols
Ref: Detailed description of:
ARP = chapter 9
ICMP = chapter 19 IGMP = chapter 21
UDP
UDP is a connection-less protocol
Transmits user datagramwithout first creating a logical connection
Each datagram from a user is anindependententity
Does not provide:
flow control error control congestion control
Its is a simple protocol which is attractive to an application program which needs to sendshort messages and cannot afford the
SCTP
Stream Control Transmission Protocol
provides support for newer applications such as voice over the internet. It combines the best features of UDP & TCP protocol.
Application Layer
An application is a program running on a computer (which is also termed as a process)
Logical connection is end-to-end
Two applications exchangemessages between each other via avirtual bridge between the two.
Physically, communication is performed via all the layers.
To communicate, one process sends a request message to other process and gets areply message
A number of protocols are predefined but a user can also do the same provided they provide it at both ends.
Some of the protocols are:
HTTP (Hyper Text Markup Language) SMTP (Simple Mail Transfer Protocol) FTP (File Transfer Protocol)
Internet Addressing
Any communication between two parties needs two addresses at-least
Source Address Destination Address
Since we have 5 layers so it appears that we would require 5 addresses. But we only need 4
Addressing Mechanism
IP Address
Physical address
Address of a node as defined by its LAN or WAN. It is included in the frame used by the data link layer.
It is the lowest level address.
The physical address have the authority over the network (LAN & WAN).
The size and format of these addresses vary depending upon the network.
Example
Logical addresses
Physical addresses are not adequate in an internetwork environment wheredifferent networks have different address formats.
Auniversal addressing system is needed in which each host can be identified uniquely, regardless of the type of physical network. The logical addresses are designed for this purpose and they are independent of physical network.
A logical address in the internet is currently 32 bit address that can uniquely defined a host connected to internet.
Port Address
The IP Address and the physical address are necessary for a quantity of data to travel from a source to the destination host.
However, the arrival of data at the destination is not the final objective.
Computers are devices that can run multiple processes at the same time.
For theses processes to receive data simultaneously, there is need of of method to label the different process.
function of addressing a particular process is called as port addressing.
Specific addresses
Some applications have user friendly addresses that are designed for that specific address.
The example includes the email address ( for eg. [email protected]) and the universal resource locator (URL) (for eg.
http://www.google.com).
The first defines the recipient of an email and the second is used to find the information on the world wide web.
