There are two fundamental protocols in the transport layer 1. TCP Transmission Control Protocol-- connection-oriented protocol 2. UDP User Datagram Protocol -- connectionless protocol Connection-oriented communication : establishes a logical (virtual) connection prior to sending data. Connectionless communication : sends data right away without establishing a logical connection Qn why do we have transport layer protocal? IP provides a weak, but efficient service model (best-effort ) How should hosts send into the network? i.)Flow Control ii.) Too fast is bad; too slow is not efficient IP packets are addressed to a host How to decide which application gets which packets? NOTE TRASPORT LAYER IS RESPONSIBLE FOR PROCESS TO PROCESS DELIVERY PORTS port is a communication endpoints Since there are many applications running on a computer, there is a need to decide which application gets which packet. A port number is a way to identify a specific application (proc
Introduction to layered models
->The peers may be processes, hardware devices, or even human beings.
When networks first came into being, computers could typically communicate only with computers from the same manufacturer.
For example, companies ran either a complete DECnet solution or an IBM solution—not both together.
Layering is a modern network design principle that divides the communication tasks into a number of smaller parts, each part accomplishing a particular sub-task and interacting with the other parts in a small number of well-defined ways.
To reduce their design complexity, most networks are organized as a stack of layers or levels, each one built upon the one below it.
What differs from one network to another:-
1.The number of layers
2.The name of each layer
3.The contents of each layer and
4.The function of each layer.
->The entities comprising the corresponding layers on different machines are called peers.
->In other words, it is the peers that communicate by using the protocol.
In reality, no data are directly transferred from layer n on one machine to layer n on another machine.
Instead, each layer passes data and control information to the layer immediately below it, until the lowest layer is reached.
Below layer 1 is the physical medium through which actual communication occurs.
Virtual communication is shown by dotted lines and physical communication by solid lines in the diagram.
Between each pair of adjacent layers is an interface.
The interface defines which primitive operations and services the lower layer makes available to the upper one.
When network designers decide how many layers to include in a network and what each one should do, one of the most important considerations is defining clean interfaces between the layers.
Services between adjacent layers expressed in terms of primitives and parameters
Primitives specify function to be performed
Introduction to layered models
A set of layers and protocols is called a network architecture.
The specification of an architecture must contain enough information to allow an implementer to write the program or build the hardware for each layer so that it will correctly obey the appropriate protocol.
It is not even necessary that the interfaces on all machines in a network be the same, provided that each machine can correctly use all the protocols.
A list of protocols used by a certain system, one protocol per layer, is called a protocol stack.
The primary purpose of all such models, especially the OSI model, is to allow different vendors’ networks to interoperate.
It divides the network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting.
It allows multiple-vendor development through standardization of network components.
It encourages industry standardization by defining what functions occur at each layer of the model.
It allows various types of network hardware and software to communicate.
It prevents changes in one layer from affecting other layers, so it does not hamper development.
Layering allows the parts of a communication to be designed and tested without a combinatorial explosion of cases, keeping each design relatively simple.
The most popular layering model are:-
OSI reference Model
TCP/IP Model.
Other layering model are:-
IBM Systems Network Architecture (IBM SNA)
Digital Equipment Corporation’s (DEC, now part of HP)
DNA (Digital Network Architecture)
OSI - The Model
Open Systems Interconnection
Developed by the International Organization for Standardization (ISO)
Seven layers
A theoretical system delivered too late!
Each layer performs a subset of the required communication functions.
Each layer relies on the next lower layer to perform more primitive functions.
Each layer provides services to the next higher layer.
Changes in one layer should not require changes in other layers
OSI as Framework for Standardization
 |
Layer Specific Standards
Elements of Standardization
Protocol specification
Operates between the same layer on two systems
May involve different operating system
Protocol specification must be precise
Format of data units
Semantics of all fields
allowable sequence of PDUs
Service definition
In addition to the protocol or protocols that operate at a given layer, standards are needed for the services that each layer provides to the next-higher layer.
Typically, the definition of services is equivalent to a functional description that defines what services are provided, but not how the services are to be provided.
Addressing
Each layer provides services to entities at the next-higher layer.
These entities are referenced by means of a service access point (SAP).
Thus, a network service access point (NSAP) indicates a transport entity that is a user of the network service.
Comments