Subject Name: Data Communication and Networking

Subject code   : SAZ6B/ SAE6A/ SEU6D

Syllabus Unit-5 : Repeaters - Bridges - Routers - Gateway - Routing algorithms - TCP/IP Network, Transport and Application Layers of TCP/IP - World Wide Web.

PART-A

1.      What are bridges?

2.      Define firewalls and gateways.

3.      What are functions of applications layer?

4.      List any two internetworking devices.

5.      What is a Repeater?

6.      What is a RARP?

7.      What is a repeater?

8.      Define Bridge.

9.      What do you meant by routers?

10.  What is a repeater?

11.  Define Bridge.

12.  What is a Gateway?

PART-B

1.      Write a short note on Transparent Bridges.

2.      Explain about Electronic mail.

3.      Explain briefly the structure of a web page.

4.      Give a short note on WWW.

5.      Give a short note on Bridges.

6.      Give a short note on World Wide Web.

PART-C

1.      Define adaptive and non-adaptive algorithm.

2.      Explain in detail any two routing algorithm

3.      Describe the working of link state routing algorithm with an example.

4.      Discuss any TWO Routing algorithms.

5.      Discuss any two routing algorithm.

6.   Discuss TCP/IP in detail.

Networking and Internetworking Devices: Introduction

ü  Two or more devices connected for the purpose of sharing data or resources can form a network.

ü  An internet is an interconnection of individual networks.

ü  To create an internet, we need internetworking devices called routers and gateways.

ü  Networking and internetworking devices are divided into four categories: repeaters, bridges, routers and gateways.

Write a short note on Repeater?

Ø  A repeater (or regenerator) is an electronic device that operates on only the physical later of the OSI model.

Ø  If the signal becomes too weak or corrupted, regenerates the original bit pattern, the puts the refreshed copy back onto the link.

Ø  A repeater allows us to extend only the physical length of a network.

Ø  The repeater does not change the functionality of the network in any way.

Ø  Example:  If station A sends a frame to station B, all stations (including C and D) will receive the frame, just as they would without the repeater.

Ø  The repeater does not have the intelligence to keep the frame from passing to the right side when it is meant for a station on the left.

Ø  A repeater is a regenerator, not an amplifier.

Bridges:

Ø  Bridges operate in both the physical and data link layers of the OSI model.

Ø  Bridges can divide a large network into smaller segments.

Ø  Bridges contain logic that allows them to keep the traffic for each segment separate.

Ø  Bridges operate at the data link layer, giving it access to the physical address of all stations connected to it.

Ø  When a frame enters a bridge, the bridge not only regenerates the signal but checks the address of the destination and forwards the new copy only to the segment to which the address belongs.

Ø  As a bridge encounters a packet, it reads the address contained in the frame and compares that address with a table of all the stations on both segments.

Ø  Example: In the below figure a, shows two segments joined by a bridge.

Ø  A packet from station A, addressed to station D arrives at the bridge.

Ø  Station A is on the same segment as station D; therefore, the packet is blocked from crossing into the lower segment.

Ø  In the below figure b, a packet generated by station A is intended for station G.

Ø  The bridge allows the packet to cross and relays it to the entire lower segment, where it is received by station G.

Types of bridges:

Ø  Simple Bridge

Ø  Multiport Bridge

Ø  Transparent Bridge

Simple Bridge

Ø  Simple bridges are the most primitive and least expensive type of bridge.

Ø  A simple bridge links two segments and contains a table that lists the addresses of all the stations included in each of them.

Ø  Addresses must be entered manually.

Ø  Before a simple bridge can be used, an operator must sit down and enter the addresses of every station.

Ø  Whenever a new station is added, the table must be modified.

Ø  Installation of simple bridge are time-consuming and potentially more trouble than the cost saving are worth.

Multiport bridge

Ø  A multiport bridge is used to connect more than two LANs.

Ø  In the below figure, the bridge has three tables, each one holding the physical address of stations reachable through the corresponding port.

Transparent bridge:

Ø  A transparent or learning, bridge builds its table of station address on its own as it performs its bridge functions.

Ø  When the transparent bridge is first installed, its table is empty.

Ø  It encounters each packet; it looks at both the destination and the source addresses.

Ø  It checks the destination to decide where to send the packet.

Ø  If it does not yet recognize the destination address, it relays the packet to all of the station on both segments.

Ø  It uses the source address to build its table.

Some Issues: Bridges Connection Different LANs:

Ø  A bridge should be able to connect LANs using different protocol at the data link layer, such as an Ethernet LAN to Token Ring LAN.

Ø  Some of the issues to be considered,

Frame format:

Ø  Frames sent by different LAN have different formats (Example: Ethernet frame with a Token Ring frame)

Payload size:

Ø  The size of the data that can be encapsulated in a frame varies from protocol to protocol. (Example: maximum payload size of an Ethernet frame with a Token Ring frame)

Data rate:

Ø  Different protocols use different data rates (Example: compare the 10-Mbps data rate of an Ethernet with the 16-Mbps data rate of a Token Ring)

Address bit order:

Ø  The bit order of addresses in different LAN protocols is not the same. (Example: a bridge should reverse an address if it is connecting an Ethernet LAN to a Token Ring LAN).

Other issues:

Ø  There are other issues includes such as acknowledgement, collision, and priority.

Write a short note on Routers. (Or) Discuss briefly about Routers.

ROUTERS

Ø  Routers operate in the physical, data link and network layers of the OSI model.

Ø  Routers have access to network layer.

Ø  Routers relay packets among multiple interconnected networks.

Ø  They route packet from one network to any of a number destination networks.

Ø  The router forwards the incoming packets to the next router on the path, and so on, until the destination is reached.

Ø  Routers have addresses on, and links to, two or more networks at the same time.

Ø  A router is capable of determining which of its connected networks is best.

Ø  Once the router has identified the best route for a packet to travel, it passes the packet along the appropriate network to another router.

Routing concepts:

Ø  The job of router is to forward packets through a set of networks.

Ø  Example: if we want to move a packet from network A to network C via router (network) B.

Ø  The packet could reach network through router D instead of router B, or possibly even going directly from A to C. when even there are multiple options, the router chooses the pathway.

Routing is classified as Nonadaptive or Adaptive.

Nonadaptive Routing:

Ø  Once a pathway to a destination has been selected, the router sends all packets for that destination along that one route.

Adaptive routing:

Ø  Other routing protocols employ a technique called adaptive routing, by which a router may select a new route for each packet (even packet belonging to the same transmission).

Example:

Ø  A transmission from network A to network D, a router may send the first packet by way of network B, the second packet by way of network C, and the third packet by way of network Q, depending on which route is most efficient at the moment.

Packet lifetime:

Ø  Each and every packet is marked with a lifetime; usually the number of hops that are allowed before a packet is considered lost and, accordingly, destroyed.

Ø  Each router to encounter the packet subtracts 1 from the total before passing it on.

Ø  When the lifetime total reaches 0, the packet is destroyed.

Routing Algorithms:

Two common methods are used to calculate the shortest path between two routers:

ü  Distance Vector Routing and

ü  Link State Routing

Distance Vector Routing

            In Distance vector routing, each router periodically shares its knowledge about the entire network with its neighbors.

There keys to understanding how this algorithm works are as follows:

1.      Knowledge about the whole network

Ø  Each router shares its knowledge about the entire network.

Ø  It sends all of its collected knowledge about the network to its neighbors.

2.      Routing only to neighbors

Ø  Each router periodically sends the knowledge about the network only to those routers to which it has direct links.

Ø  Router sends whatever knowledge it has about the whole network through all of its ports.

3.      Information sharing at regular intervals

Ø  For every 30 seconds, each router sends its information about the whole network has changed since the last time information was exchanged.

Sharing Information:

Ø  To understand how distance vector routing works, see the below figure.

Ø  The clouds represent local area networks (LANs).

Ø  The number inside each cloud is that LANs network ID.

Ø  These LANs can be of any type (Ethernet LAN, Token Ring LAN, FDDI, etc.)

Ø  The LANs are connected by routers or gateways, represented by the boxes labeled A, B, D, E, and F.

Distance Vector routing algorithm

Ø  The below figure shows the first step in the algorithm.

Ø  The text boxes indicate the relationships of the routers.

Ø  Each router sends its information about the internetwork only to its immediate neighbors.

Ø  The neighbors add this knowledge and send the whole table to their own neighbors.

Ø  In this way the first router gets its own information back plus new information about its neighbor’s other neighbors.

ROUTING TABLE:

ü  Creating the Table:

ü  Each router gets its initial knowledge about the internetwork and how it uses shared information to update that knowledge.

Creating the Table:

ü  A router is a station on each of those LANs, it also knows the ID of each station.

ü  A routing table has columns for at least three types of information (some protocols require more): the network ID, the cost, and the ID of the next router (next hop).

ü  The network ID is the final destination of the packet.

ü  For example, A sends its routing table to routers B,F and E; B sends its routing table to router C and A; and so on.

Updating the Table:

Ø  When A receives a routing table from B, it uses the information to update its own table.

Ø  It says to itself: “B has sent me a table that shows how its packets can get to networks 55 and 14.

Ø  I know that B is my neighbor, so my packets can reach B in one hop.

Ø  So, if I add one hop to all of the costs shown in B’s table, the sum will be my cost for reaching those other networks.

Ø  A adjusts the information shown in B’s table by adding one to each listed cost.

Ø  This process continues for all routers.

Ø  Every router receives information from neighbors and updates its routing table.

Updating Algorithm:

            The updating algorithm requires that router first add one hop to the hop count field for each advertised route.

1.      If the advertised destination is not in the routing table, the router should add the advertised information to the table.

2.      If the advertised destination is in the routing table,

a)      If the next-hop field is the same, the router should replace the entry in the table with the advertised one.  Note that even if the advertised hop count is larger, the advertised entry should replace the entry in the table because the new information invalidates the old.

b)      If the next-hop field is not the same,

i.                    If the advertised hop count is smaller than the one in the table, the router should replace the entry in the table with the new one.

ii.                  If the advertised hop count is not smaller (same or larger), the router should do nothing.

Link State Routing:

      In Link State Routing, each router shares its knowledge of its neighborhood with every other router in the internetwork.

1.      Knowledge about the neighborhood.

ü  Instead of sending its entire routing table, a router sends information about its neighborhood only.

2.      To all routers.

ü  Each router sends this information to every other router on the internetwork, not just to its neighbors.

ü  It does so by as process called flooding.

ü  Flooding means that a router sends its information to all of its neighbors

ü  Every router that receives the packet sends copies to all of its neighbors. Finally, every router receives a copy of the same information.

3.      Information sharing when there is a change.

ü  Each router sends out information about the neighbors when there is a change.

ü  The first step in link state routing is information sharing.

Packet Cost:

ü  Both distance vector and link state routing are lowest-cost algorithms.

ü  In distance vector routing, cost refers to hop count.

ü  A weighted value based on a variety of factors such as security levels, traffic, or the state of the link.

ü  The cost from router A to network 14, therefore, might be different from the cost from A to 23.

The two factors govern how cost is applied to packets in determining a route:

ü  Cost is applied only by routers and not by any other stations on a network.

ü  Cost is applied as packet leaves the router rather than as it enters. Most networks are broadcast networks.

ü  When a packet is in the network, every station, including the router, can pick it up.

Link state packet

            When a router floods the network with information about its neighborhood, it is said to be advertising.

The basis of this advertising is a short packet called a Link State Packet (LSP).

Link state database:

ü  Every router receives every LSP and puts the information into a link state database.

ü  Because every router receives the same LSPs, every router builds the same database.

ü  It stores this database on its disk and uses it to calculate its routing table.

ü  If the router is added to or deleted from the system, the whole database must be shared for fast updating.

The Dijkstra Algorithm

ü  To calculate its routing table, each router applies an algorithm called the Dijkstra algorithm to its link state database.

ü  The Dijkstra algorithm calculates the shortest path between two points on a network using a graph made up of nodes and ares.

ü  The nodes are of two types: networks and routers.

ü  Arcs are the connection between a router and a network.

ü  The cost or the arc from network to router is always zero.

Shortest path tree:

      The Dijkstra algorithm follows four steps to discover what is called the shortest path tree for each router.

     

ü  . The algorithm begins to build the tree by identifying its root.

ü  The root of each router’s tree is the router itself

ü  The algorithm compares the tree’s temporary arcs and identifies the arc with the lowest cumulative cost.

ü  The algorithm examines the database and identifies every node that can be reached from its chosen node.

ü  The last two steps are repeated until every node in the network has become a permanent part of the tree.

ü  The only permanent arcs are those that represent the shortest route to every node.

TCP/IP PROTOCOL SUITE:

ü  The TCP/IP protocol suite was developed prior to the OSI model.

ü  The layers in the TCP/IP protocol suite do not match exactly with those in the OSI model.

ü  The TCP/IP protocol suite is made of five layers:

o   Physical layer

o   Data link layer

o   Network layer

o   Transport layer

o   Application layer

ü  TCP/IP is a hierarchical protocol made up of interactive modules, each of which provides a specific functionality, but the modules are not necessarily interdependent.

ü  The TCP/IP protocol suite contains relatively independent protocols that can be mixed and matched, depending on the needs of the system.

ü  The term hierarchical means that each upper level protocol is supported by one or more lower level protocols.

Physical Layer

ü  TCP/IP does not define any specific protocol for the physical layer.

ü  It supports all of the standard and proprietary protocols.

ü  At this level, the communication is between two hops or nodes, either a computer or router.

ü  The unit of communication is a single bit.

ü  When the connection is established between the two nodes, a stream of bits is flowing between them.

ü  The physical layer, however, treats each bit individually.

Data Link Layer

ü  TCP/IP does not define any specific protocol for the data link layer either.

ü  It supports all of the standard and proprietary protocols.

ü  At this level, the communication is also between two hops or nodes.

ü  The unit of communication however, is a packet called a frame

ü  A frame is a packet that encapsulates the data received from the network layer with an added header and sometimes a trailer.

ü  The head, among other communication information, includes the source and destination of frame.

Network Layer

ü  At the network layer (or, more accurately, the internetwork layer), TCP/IP supports the Internet Protocol (IP).

ü  The Internet Protocol (IP) is the transmission mechanism used by the TCP/IP protocols.

ü  IP transports data in packets called datagram’s, each of which is transported separately.

ü  Datagram’s can travel along different routes and can arrive out of sequence or be duplicated.

ü  IP does not keep track of the routes and has no facility for reordering datagram’s once they arrive at their destination.

Transport Layer

ü  There is a main difference between the transport layer and the network layer.

ü  Although all nodes in a network need to have the network layer, only the two end computers need to have the transport layer.

ü  The network layer is responsible for sending individual datagrams from computer A to computer B; the transport layer is responsible for delivering the whole message, which is called a segment, a user datagram, or a packet, from A to B.

ü  A segment may consist of a few or tens of datagrams.

ü  The segments need to be broken into datagrams and each datagram has to be delivered to the network layer for transmission.

ü  the transport layer was represented in the TCP/IP suite by two protocols:

ü  User Datagram Protocol (UDP) and Transmission Control Protocol (TCP).

Application Layer

ü  The application layer in TCP/IP is equivalent to the combined session, presentation, and application layers in the OSI model.

ü  The application layer allows a user to access the services of our private internet or the global Internet.

ü  Many protocols are defined at this layer to provide services such as electronic mail, file transfer, accessing the World Wide Web, and so on.

File Transfer Protocol (FTP)

ü  Transferring files from one computer to another is one of the most common tasks.

ü  File Transfer Protocol (FTP) is the standard mechanism provided by TCP/IP for copying a file from one host to another.

ü  Although transferring files from one system to another seems simple and straightforward, some problems must be dealt with first. For example, two systems may use different file name conventions.

ü  Two systems may have different ways to represent text and data.

ü  Two systems may have different directory structures.

ü  All these problems have been solved by FTP in a very simple and elegant approach.

ü  FTP differs from other client/server applications in that it establishes two connections

ü  between the hosts.

ü  One connection is used for data transfer, the other for control information (commands and responses).

ü  FTP uses two well-known TCP ports: Port 21 is used for the control connection, and port 20 is used for the data connection.

ü  The basic model of FTP. The client has three components: user interface, client control process, and the client data transfer process.

ü  The server has two components: the server control process and the server data transfer process.

ü  The control connection is made between the control processes.

ü  The data connection is made between the data transfer processes.

Trivial File Transfer Protocol:

ü  To simply copy a file without the need for all of the functionalities of the FTP protocol.

ü  Example: when a diskless workstation or a router is booted, we need to download the bootstrap and configuration files.

ü  TFTP (Trivial File Transfer Protocol) is designed for these types of file transfer.

ü  TFTP can read or write a file for the client.

ü  Reading means copying a file from the server site to the client site.

ü  Writing means copying a file from the client side to the server site.