COMPUTER NETWORK 7BITE2A UNIT II YOK LAYER DESIGN ISSUESROUTING ALGORITHMS CONGESTION CONTROLALGORITHMS 1YTERNETWORKING TUNNELING FRAGMENTATION cid 19 FREWALLS NETWORK LAYER IN THE YIERNET IP SUBNETS NETWORK LAYER IN ATM NETYORKS CELL FORMAT cid 19 CONNECTION ETUP ROUTINGAND SWITCHING SERVICES CATEGORIES cid 19 ATM LANS ETEIKORKDAVER The network Layer controls the operation of the subnet The main aim of this layer is to deliver packets from source to destination across multiple links networks If two computers system are connected on the same link then there is no need for a network layer It routes the signal through different channels to the other end and acts as a network controlle It also divides the outgoing messages into packets and to assemble incoming packets into messages for higher levels In broadcast networks the routing problem is simple so the network layer is often thin or even non existent unctions of Network Layer It translates logical network address into physical address Concerned with circuit message or packet switching Routers and gateways operate in the network layer Mechanism is provided by Network Layer for routing the packets to final destination Connection services are provided including network layer flow control network layer error control and packet sequence control Breaks larger packets into small packets Bara to1aspetttayeE Pedtes ESIGNIKSSUES A key design issue is determining how packets are routed from source to destination Routes can be based on static tables that are wired into the network and rarely changed They can also be highly dynamic being determined anew for cach packet to reflect the current network load If too many packets are present in the subnet at the same time they will get into one another s way forming botlenecks The control of such congestion also belongs to the network layer S Priyadharshini MCA M Phil 1 COMPUTER NETWORK 7BITE2A Moreover the quality of service provided delay transmit time jitter etc is also a newotA y When a packet has to travel from one network to another to cet to its destination many problems can arise such as The addressing used by the second network may be different from the first one The second one may not accept the packet at all because it is too large The protocols may difer and so on It is up to the network layer to overcome all these problems to allow heterogeneous networks to be interconnected Nehvork Layer The layer that responsible source to destination to delivery of packet multiple network is called network layer Characteristics s i Logical addressing i Routing iii Connecting different network The network layer design issues 1 Store and formed packet switching 2 Service provided to the transport layer 3 Implementation of connectionless service 4 Implementation of connection oriented source 5 Comparison of vitual circuit and datagram submits Store and formed packet switching Store and forward operation Raifer Carera quimer Hr i Host transmits packet to router across LAN or oval point to point link i Packet is stored on router until fully arived and processed i Packet is forward to next router 2 Service provide to transport layer 3 Implementation ofconnectionless service The network layer services have been designed with the goals 1 The advice should independent of router telenet 2 The transport layer should be shilded from the number type and topology of the router present 3 The network addresses maid arailable to transport Connectionless service is offered packets are injected into the subnet individually and routed idependently of cach other Each packet is transmitted idenpendently Connectionless service used in network layer ID and transport layer Packet are frequently called datagram connectionless service is largly for data communication the internet S Priyadharshini MCA M Phil 2 COMPUTER NETVWORK BITE2A lmplementation ofconnection oriented service Conneclion oriented service is used a path from the source router to the destination router must be established before any data packet can be sent Connection oriented service also called virtual circuit service This service used network layer for ATM It also used in transport layer for TCP A connection must be established before any can be sent packets order preserved logical connection is also established here ik t k k ROUT0N GA LGORVIHEMS Routing is the pr cess of selecting a path for traffic in a network or between or across multiple networks Broadly routing is performed in many types of networks including circuit switched networks such as the public switched telephone network PSTN and computer networks such as the Internet The routing process usually directs forwarding on the basis of routing tables which maintain a record of the routes to various network destinations Routing tables may be specified by an administrator learned by observing network traffic or built with the assistance of routing protocols Unicast Delivery schemes Anycast Geocast Broadcast Multicast Routing schemes differ in how they deliver messages unicast delivers a message to a single specific node broadcast delivers a message to all nodes in the network multicast delivers a message to a group of nodes that have expressed interest in receiving the message anycast delivers a message to any one out of a group of nodes typically the one nearest to the source geocast delivers a message to a group of nodes based on geographic location For this type of algorithms the routing decision is not based on the measurement or estimations of current traffic and topology Routing algorithms can be divided into two groups S Priyadharshini MCA M Pihil 3 However the choice of the route is done in advance and known as static routing COMPUTER NETWORK 7BITE2A EAdaptieagornthniss For these algorithms the routing decision can be changed if there are any changes in topology or traffic II F etc This is called as dynamic routing BHE EXAMB ESOF L SHORTEST PATH ROUTING CALGORHIMS ARE 2 SOLSCE Shortest path can be calculated only for the weighted graphs The edges connecting two vertices can be assigned a nonnegative real number called the weight of the edge A graph with such weighted edges is called a weighted graph structure travel time Let G bea weighted graph Let u and v be two yertices in G and let P be a path inG from u to v The weight of the path P is the sum of the weights of all the edges on the path P which is also called the weight of v from u via P Let G bea weighted graph representing a highway