Computer Networks Notes 7654i

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Computer Networks Notes 2. Network Software: 2.1 Protocol Hierarchies:  

To reduce design complexity, most networks are organized as a stack of Layers or Levels. The purpose of each layer is to offer certain services to the higher layers.

Protocol- Protocol is an agreement between the communicating parties on how communication is to proceed. Peers- The entities comprising the corresponding layers on different machines are called peers. Physical Medium- Through which actual communication occurs. Interface-The interface defines which primitive operations and services the lower layer makes available to the upper one. Network Architecture- A set of layers and protocols. Protocol stack - A list of protocol used by a system, one protocol per layer. 2.2. Design Issues for the Layers: Error control- Detection and correction of errors  Possibility of two types of errors:  Lost frames – frame fails to arrive at the other side  Damaged frames – a frame does arrive, but some of the bits are in error  Common techniques for error control  Error detection  Positive acknowledgement – successfully received error free frames  Retransmission after timeout  Negative acknowledgment (NAK) and retransmission Flow control- Flow control is the management of data flow between computers or devices or between nodes in a network so that the data can be handled at an efficient pace. Too much data arriving before a device can handle it causes data overflow, meaning the data is either lost or must be retransmitted. Routing- is the process of selecting best paths in a network. In the past, the term routing was also used to mean forwarding network traffic among networks. In internetworking, the process of moving a packet of data from source to destination. Routing is usually performed by a dedicated device called a router. Routing is a key feature of the Internet because it enables messages to from one computer to another and eventually reach the target machine. Each intermediary computer performs routing by ing along the message to the next computer. Part of this process involves analyzing a routing table to determine the best path. 2.3. Connection Oriented and Connectionless Services:

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Connection Oriented- There is a connection established in between the source and destination. Three steps are followed in connection oriented. They are, 1. Establish a connection. 2. Send data. 3. Terminate the connection. Connectionless- There is no establishment of connection between the source and destination. Each message carries full address of the destination System and each one is routed through the system independent of all the others. Datagram service - Connectionless service often called datagram service . Its an unreliable service which doesn’t have the acknowledgement to the sender. Acknowledged Datagram Service - In connectionless service the sender can receive the acknowledgment from the receiver. Request-Reply- In this service the sender transmits a datagram containing a request; the reply contains the answer. 2.3. Service Primitives: A Service is formally specified by a set of primitives (operation) available to a process to access the service. These primitives tell the service to perform some action or report on an action taken by a peer entity. Primitives Meaning LISTEN Block waiting for an incoming connection CONNECT Establish a connection with a waiting peer RECEIVE Block waiting for an incoming message SEND Send a message to the peer DISCONNECT Terminate a connection 2.4 The Relationship of the Protocol and Services: A Service is a set of primitives (operation) that a layer provides to the layer above it. Service related to interface between layers. A Service is a abstract data type or an object in the Object Oriented Programming. It defines the operation that can be performed on the object but does not specify how these operations are implemented. A Protocol is a set of rules governing the format and meaning of the pocket. Or messages are exchanged by the peer entities within the layer. 3. REFERENCE MODEL 3.1. THE OSI Reference Model: ISO OSI- International Standard Organization Open System Interconnection. Principles that were applied to arrive at the seven Layers can be summarized here,  A Layer should be created where a different abstraction is needed.  Each layer should perform a well-defined function.  The function of each layer should be chosen with an eye toward defining internationally standardized protocols.  The layer boundaries should be chosen to minimize the information flow across the interface.

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The number of layers should be large enough that distinct functions need not be thrown together in a same layer out of necessity.

The Physical Layer:  The physical layer is concerned with raw bits over a communication channel  How the initial connection is established and how it is torn down when both sided are finished.  The design issue deal with mechanical, electrical and timing interface etc., which lies below the physical layer Data link layer This layer provides reliable transmission of a packet by using the services of the physical layer which transmits bits over the medium in an unreliable fashion. This layer is concerned with : 1. Framing : Breaking input data into frames (typically a few hundred bytes) and caring about the frame boundaries and the size of each frame. 2. Acknowledgment : Sent by the receiving end to inform the source that the frame was received without any error. 3. Sequence Numbering : To acknowledge which frame was received. 4. Error Detection : The frames may be damaged, lost or duplicated leading to errors.The error control is on link to link basis. 5. Retransmission : The packet is retransmitted if the source fails to receive acknowledgment. 6. Flow Control : Necessary for a fast transmitter to keep pace with a slow receiver. Network Layer Its basic functions are routing and congestion control. Routing: This deals with determining how packets will be routed (transferred) from source to destination. It can be of three types :  

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Static : Routes are based on static tables that are "wired into" the network and are rarely changed. Dynamic : All packets of one application can follow different routes depending upon the topology of the network, the shortest path and the current network load. Semi-Dynamic : A route is chosen at the start of each conversation and then all the packets of the application follow the same route.

Transport Layer  

The basic function of the transport layer is to accept the data from session layer and split it up into smaller units. Types of service : The transport layer also decides the type of service that should be provided to the session layer. The service may be perfectly reliable, or may be reliable within certain

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tolerances or may not be reliable at all. The message may or may not be received in the order in which it was sent. The decision regarding the type of service to be provided is taken at the time when the connection is established. 

Error Control : If reliable service is provided then error detection and error recovery operations are also performed. It provides error control mechanism on end to end basis.

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Connection Establishment / Release : The transport layer also establishes and releases the connection across the network. This requires some sort of naming mechanism so that a process on one machine can indicate with whom it wants to communicate.

Session layer  

Session layer allows the on different machine to establish session between them. It offers various services including 

Dialog control – keep track whose turn it is to transmit

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Token management preventing two parties to attempting the same critical operation at the same time

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Synchronization – check pointing long transmission to allow them to continue to where they were after a crash

Presentation Layer 

This layer is concerned with the syntax and semantics of the information transmitted. In order to make it possible for computers with different data representations to communicate data structures to be exchanged can be defined in abstract way alongwith standard encoding. It also manages these abstract data structres and allows higher level of data structres to be defined an exchange.

Application Layer 

The seventh layer contains the application protocols with which the gains access to the network. The choice of which specific protocols and their associated functions are to be used at the application level is up to the individual . For example commonly used protocols are HTTP(for web browsing), FTP(for file transfer) etc.

3.2 T IP Reference model The ability to connect multiple network in a seamless way was one of the major design goal from the very beginning. T IP reference model act as key to achieve it. T IP reference model has two primary protocols. Internet layer  Pocket switching can be held on a internet layer based on connectionless service. The pocket may arrive in different order than they are sent.  The internet layer defines an official pocket format and protocol called IP (Internet Protocol).

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 Internet layer handles the Pocket Routing system Transport layer It is designed to allow peer entities on the source and the destination host to carry on a conversation Two end to end transport protocol have been defined here. 1. T –is a reliable connection oriented protocol that allows a byte stream originating on a one machine to deliver without a error on an any other machine in internet. 2. UDP ( Datagram Protocol) – it is a unreliable connectionless protocol Application layer T IP model does not have the session and presentation layers It contains all higher level protocols Host to Network layer It does not really say much about what happens here, except to point out that the host has to connect to the network using some protocol so it can send IP pockets. 3.3 A comparison of T IP and OSI model Three concepts of OSI model Similarity Differences The functionality of the layers are roughly Services similar It tells what the layer does, not how the layer does. It defines the layer semantics Both transport layer provide end-to-end Interface transport service It tells the processes above it how to access it What the parameters and what the results to expect It doesn’t say how layers work inside Protocol Layers can use any protocol it wants It provide offered services T IP Model  There is no clear distinguish between service, interface and protocol  Offers the services SEND IP POCKET, RECEIVE IP POCKET 3.4 A critic of the OSI Model and Protocol  Bad timing  Bad technology  Bad implementation  Bad politics 3.5 A critic of the T IP reference model  There is a problem in the concept of service, interface and protocol  This models not much of a guide for deg new network using new technology.  This model is poorly suited to describing any protocol stack other than T IP  Host network layer is not really a layer, it is an interface between the network and data link layer.

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T IP doesn’t distinguish the physical and data link layer. IP and T protocol were carefully thought out and well implemented