UNDERSTANDING THE HOST-TO-HOST COMMUNICATIONS MODEL - OSI LAYER & TCP/IP MODEL

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Understanding Host-to-Host Communications

– Older model• Proprietary• Application and combinations software controlled by one

vendor– Standards-based model

• Multivendor software• Layered approach

Using Layers to Describe Data Communication

OSI Model ( Open System Interconnect )

The OSI model is a layered model that describes how information moves from an application program running on one networked computer to an application program running on another networked computer. In essence, the OSI model prescribes the steps to be used to transfer data over a transmission medium from one networked device to another. The OSI model is a seven-layer model 

The Physical Layer :- defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems

Wires – (Coaxial , Twisted Pair , Cat5/6 or Fiber), Connectors – ( RJ11,45 etc ) , Voltage & Date Rates etc .

Hubs Operate at Physical layerHubs Operate at Physical layer

A B C D

Physical

• All devices in the same collision domain

• All devices in the same broadcast domain

• Devices share the same bandwidth

Hubs: One Collision Domain

• More end stations means more collisions

• CSMA/CD is used

The Data Link Layer :- defines how data is formatted for transmission and how access to the physical media is controlled.

Provides Connectivity and path selection between two host , No error correction .

• Each segment has its own collision domain

• All segments are in the same broadcast domain

Data Link

Switches and Bridges Operate at Data Link LayerSwitches and Bridges Operate at Data Link Layer

OR1 2 3 1 24

Switches

• Each segment has its own collision domain

• Broadcasts are forwarded to all segments

Memory

Switch

The Network Layer :- provides connectivity and path selection between two host systems that may be located on geographically separated networks.

Reliable Transfer of Data across media , Network Topology , Error Notification , Flow Control

Routers: Operate at the Network Layer

• Broadcast control

• Multicast control

• Optimal path determination

• Traffic management

• Logical addressing

• Connects to WAN services

The Transport Layer :- segments data from the system of the sending host and reassembles the data into a data stream on the system of the

receiving host.

The Session Layer :- establishes, manages, and terminates sessions between two communicating hosts

The Presentation Layer :- ensures that the information sent by the application layer of one system is readable by the application layer of another system.

The Application Layer :- provides network services to the applications of the user, such as e-mail, file transfer, and terminal emulation.

Data Encapsulation :- The information sent on a network is referred to as data or data packets. If one computer wants to send data to another computer, the data must first be

packaged by a process called encapsulation.

Data De-Encapsulation :- When the remote device receives a sequence of bits, the physical layer at the remote device passes the bits to the data link layer for

manipulation. This process is referred to as de-encapsulation.

Names for Data at Each Layer

Peer-to-Peer Communication

Introduction to TCP/IP

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History and Future of TCP/IP

• The U.S. Department of Defense (DoD) created the TCP/IP reference model because it wanted a network that could survive any conditions.

• Some of the layers in the TCP/IP model have the same name as layers in the OSI model.

TCP/IP Model

Application Layer

• Handles high-level protocols, issues of representation, encoding, and dialog control.

• The TCP/IP protocol suite combines all application related issues into one layer and ensures this data is properly packaged before passing it on to the next layer.

Application Layer Examples

Transport Layer

Five basic services:• Segmenting upper-layer application data• Establishing end-to-end operations- Connection Oriented • Sending segments from one end host to another end

host• Ensuring data reliability• Providing flow control

Transport Layer Protocols

Reliable vs. Best-Effort Comparison

UDP Characteristics

– Operates at transport layer of OSI and TCP/IP models

– Provides applications with access to the network layer without the overhead of reliability mechanisms

– Is a connectionless protocol– Provides limited error checking– Provides best-effort delivery– Has no data-recovery features

TCP Characteristics

– Transport layer of the TCP/IP stack– Access to the network layer for applications– Connection-oriented protocol– Full-duplex mode operation– Error checking– Sequencing of data packets– Acknowledgement of receipt– Data-recovery features

Mapping Layer 4 to Applications

Establishing a Connection

Three-Way Handshake

CTL = Which control bits in the TCP header are set to 1

TCP Acknowledgment

TCP Sequence and Acknowledgment Numbers

Flow Control

Fixed Windowing

TCP Sliding Windowing

Internet LayerThe purpose of the Internet layer is to send packets from a network node and have them arrive at the destination node independent of the path taken.

Network Access Layer

• The network access layer is concerned with all of the issues that an IP packet requires to actually make a physical link to the network media.

• It includes the LAN and WAN technology details, and all the details contained in the OSI physical and data link layers.

Comparing the OSI Model and TCP/IP Model

Similarities of the OSI and TCP/IP Models

• Both have layers.• Both have application layers, though they

include very different services.• Both have comparable transport and network

layers. • Packet-switched, not circuit-switched,

technology is assumed.• Networking professionals need to know both

models.

Differences of the OSI and TCP/IP Models

• TCP/IP combines the presentation and session layer into its application layer.

• TCP/IP combines the OSI data link and physical layers into one layer.

• TCP/IP appears simpler because it has fewer layers.

• TCP/IP transport layer using UDP does not always guarantee reliable delivery of packets as the transport layer in the OSI model does.