Unit III - Network Layer (PDF)

UNIT – III : Network Layer Forwarding and Routing, Network Service Models Connection Oriented Technology-Virtual Circuit(example-MPLS-MultiProtocol Label Switching) and Connection Less Technology-Datagram Networks,IP Router, Routing Table, Internet Protocol (IP) – IPv4 and IPv6 , ICMP, Link State Ro...

UNIT – III : Network Layer Forwarding and Routing, Network Service Models Connection Oriented Technology-Virtual Circuit(example-MPLS-MultiProtocol Label Switching) and Connection Less Technology-Datagram Networks,IP Router, Routing Table, Internet Protocol (IP) – IPv4 and IPv6 , ICMP, Link State Routing , Distance Vector Routing, Hierarchical Routing , RIP, OSPF, BGP, Broadcast and Multicast Routing , Mobile IP, IP sec. IPv4 : Classes, Classless, Subneting, Super netting and its numerical. Network Layer Services- Packetizing, Routing and Forwarding Network layer is the third layer in the OSI model of computer networks. It’s main function is to transfer network packets from the source to the destination. It is involved both at the source host and the destination host. At the source, it accepts a packet from the transport layer, encapsulates it in a datagram and then deliver the packet to the data link layer so that it can further be sent to the receiver. At the destination, the datagram is decapsulated, the packet is extracted and delivered to the corresponding transport layer. Features : Main responsibility of Network layer is to carry the data packets from the source to the destination without changing or using it. If the packets are too large for delivery, they are fragmented i.e., broken down into smaller packets. It decides the route to be taken by the packets to travel from the source to the destination among the multiple routes available in a network (also called as routing). The source and destination addresses are added to the data packets inside the network layer. Services Packetizing – The process of encapsulating the data received from upper layers of the network(also called as payload) in a network layer packet at the source and decapsulating the payload from the network layer packet at the destination is known as packetizing. The source host adds a header that contains the source and destination address and some other relevant information required by the network layer protocol to the payload received from the upper layer protocol, and delivers the packet to the data link layer. The destination host receives the network layer packet from its data link layer, decapsulates the packet, and delivers the payload to the corresponding upper layer protocol. The routers in the path are not allowed to change either the source or the destination address. The routers in the path are not allowed to decapsulate the packets they receive unless they need to be fragmented. Routing and Forwarding – These are two other services offered by the network layer. In a network, there are a number of routes available from the source to the destination. The network layer specifies has some strategies which find out the best possible route. This process is referred to as routing. There are a number of routing protocols which are used in this process and they should be run to help the routers coordinate with each other and help in establishing communication throughout the network. Forwarding is simply defined as the action applied by each router when a packet arrives at one of its interfaces. When a router receives a packet from one of its attached networks, it needs to forward the packet to another attached network (unicast routing) or to some attached networks(in case of multicast routing). Virtual Circuit Virtual – circuit network is a category of packet switching network, where a virtual path is established between the source and the destination systems for data communication to occur. This path appears to the user as if it is a dedicated physical path, but actually is a logical circuit allocated from a managed pool of circuit resources as per traffic requirements. The network resources forming parts of this path can be shared by other communications, however, is not visible to this user. Features of Virtual – Circuit Network Virtual – circuit switching is done at the network layer of the communication system. In these networks, the path between the source and the destination nodes that is followed by first data packet gets allocated. All other data packets transmitted between them will use the same path. All the resources like buffers and bandwidth get reserved before the transmission, and all data packets consume same resources. Since all packets follow the same path, a common header and routing information is used by them. They provide greater reliability and less complexity owing to fixed paths and fixed resources. Phases of Virtual - Circuit Transmission There are three phases of transmission by virtual circuits, set up, data transfer and teardown. Set up Phase − In this phase, a virtual circuit or a route is established from the source to the destination through number of switches. The source and destination use global addresses using which the switches make routing table entries. Data Transfer − Once the virtual circuit is set up, all packets follow the route established during the set up phase adhering to the routing tables. Teardown Phase − When data transfer is complete, the source sends a teardown request. The destination responds using a teardown confirmation. The switches flush their routing table entries, thus relinquishing the circuit. In the following diagram, we can that a virtual circuit is created, as denoted by the dotted lines, and all the packets from the sender to the receiver are being routed along this virtual circuit. Advantages & Disadvantages of Virtual Circuit: Advantages Packets are delivered to the receiver in the same order sent by the sender. Virtual circuit is a reliable network circuit. There is no need for overhead in each packet. Single global packet overhead is used in virtual circuit. Disadvantages Virtual circuit is costly to implement. It provides only connection-oriented service. Always a new connection set up is required for transmission. Datagram Network In a connectionless communication systems, datagram refers to the smallest unit via which data is transmitted. Datagrams are data packets which contain adequate header information so that they can be individually routed by all intermediate network switching devices to the destination. These networks are called datagram networks since communication occurs via datagrams. They exist in packet switching networks. Features of Datagram Networks Datagram switching is done at the network layer of the communication system. In datagram networks, each data packet or datagram is routed independently from the source to the destination even if they belong to the same message. The network treats the packet as if it exists alone. Since the datagrams are treated as independent units, no dedicated path is fixed for data transfer. Each datagram is routed by the intermediate routers using dynamically changing routing tables. So two successive packets from the source may follow completely separate routes to reach destination. In these networks, no prior resource allocation is done for the individual packets. This implies that no resources like buffers, processors, bandwidth, etc. are reserved before the communication commences.  In datagram networks, resources are allocated on demand on a First−Come First−Serve (FCFS) basis. When a packet arrives at a router, the packet must wait if there are other packets being processed, irrespective of its source or destination. Datagram communication is generally guided by User Datagram Protocol or UDP. The following diagram shows datagram packets being send by host H1 to host H2. The four datagram packets labelled as A, B, C and D, all belonging to same message are being routed separately via separate routes. The packets in the message arrives in the destination out of order. It is the responsibility of H2 to reorder the packets in order to retrieve the original message. Advantages & Diadvantages of Datagram N/W Advantages Transmission Delay. Whenever data is being received by the receiver in UDP, it often experiences very less delay in transmission. Datagram is best for Multicast Applications. Connection Speed is fast. Processing Time is less. Error Detection is absent,just check the headers only. Diadvantages Datagram networks are not as reliable as Virtual Circuits. The major drawback of Datagram Packet switching is that a packet can only be forwarded if resources such as the buffer, CPU, and bandwidth are available. Otherwise, the packet will be discarded.  DataGram Network Virtual Circuit It is connection less service. There is no need for Virtual circuits are connection-oriented, which means reservation of resources as there is no dedicated path that there is a reservation of resources like buffers, for a connection session. bandwidth, etc. for the time during which the new setup VC is going to be used by a data transfer session. All packets are free to use any available path. As a The first sent packet reserves resources at each server result, intermediate routers calculate routes on the go along the path. Subsequent packets will follow the due to dynamically changing routing tables on routers. same path as the first sent packet for the connection time. Data packets reach the destination in random order, Packets reach in order to the destination as data which means they need not reach in the order in follows the same path. which they were sent out. Every packet is free to choose any path, and hence all All the packets follow the same path and hence a the packets must be associated with a header global header is required only for the first packet of containing information about the source and the connection and other packets will not require it. upper layer data. Datagram networks are not as reliable as Virtual Virtual Circuits are highly reliable. Circuits.  DataGram Network Virtual Circuit Efficiency high, delay more Efficiency low and delay less But it is always easy and cost-efficient to implement Implementation of virtual circuits is costly as each datagram networks as there is no need of reserving time a new connection has to be set up with resources and making a dedicated path each time an reservation of resources and extra information application has to communicate. handling at routers. A Datagram based network is a true packet switched A virtual circuit network uses a fixed path for a network. There is no fixed path for transmitting data. particular session, after which it breaks the connection and another path has to be set up for the next session. Widely used in Internet Used in X.25, ATM(Asynchronous Transfer Mode) Questions: 1.Discuss Virtual-Circuit. 2.Discuss Datagram Network. 3.Differences between Virtual-Circuit and Datagram Network. 4.What are network layer Services. Routing Algorithms In order to transfer the packets from source to the destination, the network layer must determine the best route through which packets can be transmitted. Whether the network layer provides datagram service or virtual circuit service, the main job of the network layer is to provide the best route. The routing protocol provides this job. The routing protocol is a routing algorithm that provides the best path from the source to the destination. The best path is the path that has the "least-cost path" from source to the destination. Routing is the process of forwarding the packets from source to the destination but the best route to send the packets is determined by the routing algorithm. Classification of a Routing algorithm The Routing algorithm is divided into two categories: Adaptive Routing algorithm Non-adaptive Routing algorithm Adaptive Routing algorithm An adaptive routing algorithm is also known as dynamic routing algorithm. This algorithm makes the routing decisions based on the topology and network traffic. The main parameters related to this algorithm are hop count( total number of intermediate devices), distance and estimated transit time. Adaptive Routing Algorithm classification: Centralized algorithm: It is also known as global routing algorithm as it computes the least-cost path between source and destination by using complete and global knowledge about the network. This algorithm takes the connectivity between the nodes and link cost as input, and this information is obtained before actually performing any calculation. Link state algorithm is referred to as a centralized algorithm since it is aware of the cost of each link in the network. Isolation algorithm: It is an algorithm that obtains the routing information by using local information rather than gathering information from other nodes. Distributed algorithm: It is also known as decentralized algorithm as it computes the least-cost path between source and destination in an iterative and distributed manner. In this algorithm, no node has the knowledge about the cost of all the network links. In the beginning, a node contains the information only about its own directly attached links and through an iterative process of calculation computes the least-cost path to the destination. A Distance vector algorithm is a decentralized algorithm as it never knows the complete path from source to the destination, instead it knows the direction through which the packet is to be forwarded along with the least cost path. Non-Adaptive Routing algorithm Non Adaptive routing algorithm is also known as a static routing algorithm. When booting up the network, the routing information stores to the routers. Non Adaptive routing algorithms do not take the routing decision based on the network topology or network traffic. Non-Adaptive Routing Algorithm classification: Flooding: In case of flooding, every incoming packet is sent to all the outgoing links except the one from it has been reached. The disadvantage of flooding is that node may contain several copies of a particular packet. Random walks: In case of random walks, a packet sent by the node to one of its neighbors randomly. An advantage of using random walks is that it uses the alternative routes very efficiently. Differences b/w Adaptive and Non-Adaptive Routing Algorithm Basis Of Comparison Adaptive Routing algorithm Non-Adaptive Routing algorithm Define Adaptive Routing algorithm is an The Non-Adaptive Routing algorithm that constructs the algorithm is an algorithm that routing table based on the network constructs the static table to conditions. determine which node to send the packet. Usage Adaptive routing algorithm is used The Non-Adaptive Routing by dynamic routing. algorithm is used by static routing. Routing decision Routing decisions are made based Routing decisions are the static on topology and network traffic. tables. Categorization The types of adaptive routing The types of Non Adaptive routing algorithm, are Centralized, isolation algorithm are flooding and random and distributed algorithm. walks. Complexity Adaptive Routing algorithms are Non-Adaptive Routing algorithms more complex. are simple. LINK STATE ROUTING(adaptive -centralized) RECAP Centralized algorithm: It is also known as global routing algorithm as it computes the least-cost path between source and destination by using complete and global knowledge about the network. This algorithm takes the connectivity between the nodes and link cost as input, and this information is obtained before actually performing any calculation. Link state algorithm is referred to as a centralized algorithm since it is aware of the cost of each link in the network. The three keys to understand the Link State Routing algorithm: Knowledge about the neighborhood: Instead of sending its routing table, a router sends the information about its neighborhood only. A router broadcast its identities and cost of the directly attached links to other routers. Flooding: Each router sends the information to every other router on the internetwork.This process is known as Flooding. Every router that receives the packet sends the copies to all its neighbors. Finally, each and every router receives a copy of the same information. Information sharing: A router sends the information to every other router only when the change occurs in the information. Link State Routing has two phases: Reliable Flooding Initial state: Each node knows the cost of its neighbors. Final state: Each node knows the entire graph. Route Calculation Each node uses Dijkstra's algorithm on the graph to calculate the optimal routes to all nodes. The Link state routing algorithm is also known as Dijkstra's algorithm which is used to find the shortest path from one node to every other node in the network. The Dijkstra's algorithm is an iterative, and it has the property that after kth iteration of the algorithm, the least cost paths are well known for k destination nodes. Let's describe some notations: c( i , j): Link cost from node i to node j. If i and j nodes are not directly linked, then c(i , j) = ∞. D(v): It defines the cost of the path from source code to destination v that has the least cost currently. P(v): It defines the previous node (neighbor of v) along with current least cost path from source to v. N: It is the total number of nodes available in the network. Algorithm Initialization N = {A} // A is a root node. for all nodes v if v adjacent to A then D(v) = c(A,v) else D(v) = infinity loop find w not in N such that D(w) is a minimum. Add w to N Update D(v) for all v adjacent to w and not in N: D(v) = min(D(v) , D(w) + c(w,v)) Until all nodes in N  Step 1: The first step is an initialization step. The currently known least cost path from A to its directly attached neighbors, B, C, D are 2,5,1 respectively. The cost from A to B is set to 2, from A to D is set to 1 and from A to C is set to 5. The cost from A to E and F are set to infinity as they are not directly linked to A. Step N D(B),P(B) D(C),P(C) D(D),P(D) D(E),P(E) D(F),P(F) 1 A 2,A 5,A 1,A ∞ ∞  Step 2: In the above table, we observe that vertex D contains the least cost path in step 1. Therefore, it is added in N. Now, we need to determine a least-cost path through D vertex. a) Calculating shortest path from A to B v = B, w = D D(B) = min( D(B) , D(D) + c(D,B) ) = min( 2, 1+2) = min( 2, 3) The minimum value is 2. Therefore, the currently shortest path from A to B is 2.  b) Calculating shortest path from A to C v = C, w = D D(B) = min( D(C) , D(D) + c(D,C) ) = min( 5, 1+3) = min( 5, 4) The minimum value is 4. Therefore, the currently shortest path from A to C is 4. c) Calculating shortest path from A to E v = E, w = D D(B) = min( D(E) , D(D) + c(D,E) ) = min( ∞, 1+1) = min(∞, 2) The minimum value is 2. Therefore, the currently shortest path from A to E is 2. Note: The vertex D has no direct link to vertex E. Therefore, the value of D(F) is infinity.  Step N D(B),P(B) D(C),P(C) D(D),P(D) D(E),P(E) D(F),P(F) 1 A 2,A 5,A 1,A ∞ ∞ 2 AD 2,A 4,D 2,D ∞  Step 3: In the above table, we observe that both E and B have the least cost path in step 2. Let's consider the E vertex. Now, we determine the least cost path of remaining vertices through E. a) Calculating the shortest path from A to B. v = B, w = E D(B) = min( D(B) , D(E) + c(E,B) ) = min( 2 , 2+ ∞ ) = min( 2, ∞) The minimum value is 2. Therefore, the currently shortest path from A to B is 2. b) Calculating the shortest path from A to C. v = C, w = E D(B) = min( D(C) , D(E) + c(E,C) ) = min( 4 , 2+1 ) = min( 4,3) The minimum value is 3. Therefore, the currently shortest path from A to C is 3.  c) Calculating the shortest path from A to F. v = F, w = E D(B) = min( D(F) , D(E) + c(E,F) ) = min( ∞ , 2+2 ) = min(∞ ,4) The minimum value is 4. Therefore, the currently shortest path from A to F is 4. Step N D(B),P(B) D(C),P(C) D(D),P(D) D(E),P(E) D(F),P(F) 1 A 2,A 5,A 1,A ∞ ∞ 2 AD 2,A 4,D 2,D ∞ 3 ADE 2,A 3,E 4,E  Step 4: In the above table, we observe that B vertex has the least cost path in step 3. Therefore, it is added in N. Now, we determine the least cost path of remaining vertices through B. a) Calculating the shortest path from A to C. v = C, w = B D(B) = min( D(C) , D(B) + c(B,C) ) = min( 3 , 2+3 ) = min( 3,5) The minimum value is 3. Therefore, the currently shortest path from A to C is 3.  b) Calculating the shortest path from A to F. v = F, w = B D(B) = min( D(F) , D(B) + c(B,F) ) = min( 4, ∞) = min(4, ∞) The minimum value is 4. Therefore, the currently shortest path from A to F is 4. Step N D(B),P(B) D(C),P(C) D(D),P(D) D(E),P(E) D(F),P(F) 1 A 2,A 5,A 1,A ∞ ∞ 2 AD 2,A 4,D 2,D ∞ 3 ADE 2,A 3,E 4,E 4 ADEB 3,E 4,E  Step 5: In the above table, we observe that C vertex has the least cost path in step 4. Therefore, it is added in N. Now, we determine the least cost path of remaining vertices through C. a) Calculating the shortest path from A to F. v = F, w = C D(B) = min( D(F) , D(C) + c(C,F) ) = min( 4, 3+5) = min(4,8) The minimum value is 4. Therefore, the currently shortest path from A to F is 4.  Disadvantage: Heavy traffic is created in Line state routing due to Flooding. Flooding can cause an infinite looping, this problem can be solved by using Time-to-leave field. Distance Vector Routing Algorithm The Distance vector algorithm is iterative, asynchronous and distributed. Distributed: It is distributed in that each node receives information from one or more of its directly attached neighbors, performs calculation and then distributes the result back to its neighbors. Iterative: It is iterative in that its process continues until no more information is available to be exchanged between neighbors. Asynchronous: It does not require that all of its nodes operate in the lock step with each other. The Distance vector algorithm is a dynamic algorithm. It is mainly used in ARPANET, and RIP. Each router maintains a distance table known as Vector. Working Knowledge about the whole network: Each router shares its knowledge through the entire network. The Router sends its collected knowledge about the network to its neighbors. Routing only to neighbors: The router sends its knowledge about the network to only those routers which have direct links. The router sends whatever it has about the network through the ports. The information is received by the router and uses the information to update its own routing table. Information sharing at regular intervals: Within 30 seconds, the router sends the information to the neighboring routers. Distance Vector Routing Algorithm Let dx(y) be the cost of the least-cost path from node x to node y. The least costs are related by Bellman-Ford equation, dx(y) = minv{c(x,v) + dv(y)} Where the minv is the equation taken for all x neighbors. After traveling from x to v, if we consider the least-cost path from v to y, the path cost will be c(x,v)+dv(y). The least cost from x to y is the minimum of c(x,v)+dv(y) taken over all neighbors. The node x has updated its own distance vector table by using the above equation and sends its updated table to all its neighbors so that they can update their own distance vectors. Routing Table Two process occurs: Creating the Table Updating the Table Creating the Table Initially, the routing table is created for each router that contains atleast three types of information such as Network ID, the cost and the next hop. Distance Vector Routing Algorithm NET ID: The Network ID defines the final destination of the packet. Cost: The cost is the number of hops that packet must take to get there. Next hop: It is the router to which the packet must be delivered. Hierarchical Routing In hierarchical routing, the routers are divided into regions. Each router has complete details about how to route packets to destinations within its own region. But it does not have any idea about the internal structure of other regions. As we know, in both Link State and Distance Vector algorithms, every router needs to save some information about other routers. When network size is growing, the number of routers in the network will increase. Therefore, the size of routing table increases, then routers cannot handle network traffic as efficiently. To overcome this problem we are using hierarchical routing.  In hierarchical routing, routers are classified in groups called regions. Each router has information about the routers in its own region and it has no information about routers in other regions. So, routers save one record in their table for every other region. For huge networks, a two-level hierarchy may be insufficient hence, it may be necessary to group the regions into clusters, the clusters into zones, the zones into groups and so on. Example Consider an example of two- level hierarchy with five regions as shown in figure − Let see the full routing table for router 1A which has 17 entries, as shown below −  Full Table for 1A Dest. Line Hops When routing is done hierarchically then 1A - - there will be only 7 entries as shown 1B 1B 1 below − 1C 1C 1 2A 1B 2 Hierarchical Table for 1A 2B 1B 3 Dest. Line Hops 2C 1B 3 2D 1B 4 1A - - 3A 1C 3 1B 1B 1 3B 1C 2 1C 1C 1 4A 1C 3 4B 1C 4 2 1B 2 4C 1C 4 3 1C 2 5A 1C 4 4 1C 3 5B 1C 5 5C 1B 5 5 1C 4 5D 1C 6 Unfortunately, this reduction in table 5E 1C 5 space comes with the increased path length. RIP Protocol RIP stands for Routing Information Protocol. RIP is an intra-domain routing protocol used within an autonomous system. Here, intra-domain means routing the packets in a defined domain, for example, web browsing within an institutional area. To understand the RIP protocol, our main focus is to know the structure of the packet, how many fields it contains, and how these fields determine the routing table. Few IMP Points RIP is based on the distance vector-based strategy, so we consider the entire structure as a graph where nodes are the routers, and the links are the networks. In a routing table, the first column is the destination, or we can say that it is a network address. The cost metric is the number of hops to reach the destination. The number of hops available in a network would be the cost. The hop count is the number of networks required to reach the destination. In RIP, infinity is defined as 16, which means that the RIP is useful for smaller networks or small autonomous systems. The maximum number of hops that RIP can contain is 15 hops, i.e., it should not have more than 15 hops as 16 is infinity. The next column contains the address of the router to which the packet is to be sent to reach the destination. How is hop count determined? In the above figure, when the router 1 forwards the packet to the router 2 then it will count as 1 hop count. Similarly, when the router 2 forwards the packet to the router 3 then it will count as 2 hop count, and when the router 3 forwards the packet to router 4, it will count as 3 hop count. In the same way, RIP can support maximum upto 15 hops, which means that the 16 routers can be configured in a RIP.  RIP Message Format The message format is used to share information among different routers. The RIP contains the following fields in a message: Command: It is an 8-bit field that is used for request or reply. The value of the request is 1, and the value of the reply is 2. Version: Here, version means that which version of the protocol we are using. Suppose we are using the protocol of version1, then we put the 1 in this field. Reserved: This is a reserved field, so it is filled with zeroes. Family: It is a 16-bit field. As we are using the TCP/IP family, so we put 2 value in this field. Network Address: It is defined as 14 bytes field. If we use the IPv4 version, then we use 4 bytes, and the other 10 bytes are all zeroes. Distance: The distance field specifies the hop count, i.e., the number of hops used to reach the destination. How does the RIP work? If there are 8 routers in a network where Router 1 wants to send the data to Router 3. If the network is configured with RIP, it will choose the route which has the least number of hops. There are three routes in the above network, i.e., Route 1, Route 2, and Route 3. The Route 2 contains the least number of hops, i.e., 2 where Route 1 contains 3 hops, and Route 3 contains 4 hops, so RIP will choose Route 2. How RIP updates its Routing table RIP update timer : 30 sec The routers configured with RIP send their updates to all the neighboring routers every 30 seconds. RIP Invalid timer : 180 sec The RIP invalid timer is 180 seconds, which means that if the router is disconnected from the network or some link goes down, then the neighbor router will wait for 180 seconds to take the update. If it does not receive the update within 180 seconds, then it will mark the particular route as not reachable. RIP Flush timer : 240 sec The RIP flush timer is 240 second which is almost equal to 4 min means that if the router does not receive the update within 240 seconds then the neighbor route will remove that particular route from the routing table which is a very slow process as 4 minutes is a long time to wait. Advantages of RIP Disadvantages of RIP It is easy to configure In RIP, the route is chosen based It has less complexity on the hop count metric. If another route of better The CPU utilization is less. bandwidth is available, then that route would not be chosen. Open Shortest Path First(OSPF) It is a widely used and supported routing protocol. It is an intradomain protocol, which means that it is used within an area or a network. It is an interior gateway protocol that has been designed within a single autonomous system. It is based on a link-state routing algorithm in which each router contains the information of every domain, and based on this information, it determines the shortest path. The goal of routing is to learn routes. The OSPF achieves by learning about every router and subnet within the entire network. Every router contains the same information about the network. The way the router learns this information by sending LSA (Link State Advertisements). These LSAs contain information about every router, subnet, and other networking information. Once the LSAs have been flooded, the OSPF stores the information in a link-state database known as LSDB. The main goal is to have the same information about every router in an LSDBs. OSPF Areas OSPF divides the autonomous systems into areas where the area is a collection of networks, hosts, and routers. Like internet service providers divide the internet into a different autonomous system for easy management and OSPF further divides the autonomous systems into Areas. Routers that exist inside the area flood the area with routing information. In Area, the special router also exists. The special routers are those that are present at the border of an area, and these special routers are known as Area Border Routers. This router summarizes the information about an area and shares the information with other areas. All the areas inside an autonomous system are connected to the backbone routers, and these backbone routers are part of a primary area. The role of a primary area is to provide communication between different areas. Working of OSPF(Open Shortest Path First) There are three steps that can explain the working of OSPF: Step 1: The first step is to become OSPF neighbors. The two connecting routers running OSPF on the same link creates a neighbor relationship. Step 2: The second step is to exchange database information. After becoming the neighbors, the two routers exchange the LSDB information with each other. Step 3: The third step is to choose the best route. Once the LSDB( link-state database) information has been exchanged with each other, the router chooses the best route to be added to a routing table based on the calculation of SPF. OSPF Message Format Version: It is an 8-bit field that specifies the OSPF protocol version. Type: It is an 8-bit field. It specifies the type of the OSPF packet. Message: It is a 16-bit field that defines the total length of the message, including the header. Therefore, the total length is equal to the sum of the length of the message and header. Source IP address: It defines the address from which the packets are sent. It is a sending routing IP address. Area identification: It defines the area within which the routing takes place. Checksum: It is used for error correction and error detection. Authentication type: There are two types of authentication, i.e., 0 and 1. Here, 0 means for none that specifies no authentication is available and 1 means for pwd that specifies the password-based authentication. Authentication: It is a 32-bit field that contains the actual value of the authentication data. OSPF Packets There are five different types of packets in OSPF: Hello Database Description Link state request Link state update Link state Acknowledgment 1. Hello packet The Hello packet is used to create a neighborhood relationship and check the neighbor's reachability. Therefore, the Hello packet is used when the connection between the routers need to be established. 2. Database Description After establishing a connection, if the neighbor router is communicating with the system first time, it sends the database information about the network topology to the system so that the system can update or modify accordingly. 3. Link state request The link-state request is sent by the router to obtain the information of a specified route. Suppose there are two routers, i.e., router 1 and router 2, and router 1 wants to know the information about the router 2, so router 1 sends the link state request to the router 2. When router 2 receives the link state request, then it sends the link- state information to router 1. 4. Link state update The link-state update is used by the router to advertise the state of its links. If any router wants to broadcast the state of its links, it uses the link-state update. 5. Link state acknowledgment The link-state acknowledgment makes the routing more reliable by forcing each router to send the acknowledgment on each link state update. For example, router A sends the link state update to the router B and router C, then in return, the router B and C sends the link- state acknowledgment to the router A, so that the router A gets to know that both the routers have received the link-state update.  OSPF States Down: If the device is in a down state, it has not received the HELLO packet. Here, down does not mean that the device is physically down; it means that the OSPF process has not been started yet. Init: If the device comes in an init state, it means that the device has received the HELLO packet from the other router. 2WAY: If the device is in a 2WAY state, which means that both the routers have received the HELLO packet from the other router, and the connection gets established between the routers. Exstart: Once the exchange between the routers get started, both the routers move to the Exstart state. In this state, master and slave are selected based on the router's id. The master controls the sequence of numbers, and starts the exchange process. Exchange: In the exchange state, both the routers send a list of LSAs to each other that contain a database description. Loading: On the loading state, the LSR, LSU, and LSA are exchanged. Full: Once the exchange of the LSAs is completed, the routers move to the full state. Border Gateway Protocol It is an interdomain routing protocol, and it uses the path-vector routing. It is a gateway protocol that is used to exchange routing information among the autonomous system on the internet. Features Types of autonomous system Path Attributes BGP Neighbor type BGP Table BGP Session BGP Packet and Format ICMP Protocol The ICMP stands for Internet Control Message Protocol. It is a network layer protocol. It is used for error handling in the network layer, and it is primarily used on network devices such as routers. As different types of errors can exist in the network layer, so ICMP can be used to report these errors and to debug those errors. For example, some sender wants to send the message to some destination, but the router couldn't send the message to the destination. In this case, the router sends the message to the sender that I could not send the message to that destination.  The IP protocol does not have any error-reporting or error-correcting mechanism, so it uses a message to convey the information. For example, if someone sends the message to the destination, the message is somehow stolen between the sender and the destination. If no one reports the error, then the sender might think that the message has reached the destination. If someone in- between reports the error, then the sender will resend the message very quickly. Position of ICMP in the network layer ICMP Messages  Error-reporting messages The error-reporting message means that the router encounters a problem when it processes an IP packet then it reports a message. Query messages The query messages are those messages that help the host to get the specific information of another host. For example, suppose there are a client and a server, and the client wants to know whether the server is live or not, then it sends the ICMP message to the server. ICMP Message Format The message format has two things; one is a category that tells us which type of message it is. If the message is of error type, the error message contains the type and the code. The type defines the type of message while the code defines the subtype of the message. The ICMP message contains the following fields:  Type: It is an 8-bit field. It defines the ICMP message type. The values range from 0 to 127 are defined for ICMPv6, and the values from 128 to 255 are the informational messages. Code: It is an 8-bit field that defines the subtype of the ICMP message Checksum: It is a 16-bit field to detect whether the error exists in the message or not.  Error message reporting details: The destination unreachable error occurs when the packet does not reach the destination. Suppose the sender sends the message, but the message does not reach the destination, then the intermediate router reports to the sender that the destination is unreachable. There is no flow control or congestion control mechanism in the network layer or the IP protocol. The sender is concerned with only sending the packets, and the sender does not think whether the receiver is ready to receive those packets or is there any congestion occurs in the network layer so that the sender can send a lesser number of packets, so there is no flow control or congestion control mechanism. In this case, ICMP provides feedback, i.e., source quench. Suppose the sender resends the packet at a higher rate, and the router is not able to handle the high data rate. To overcome such a situation, the router sends a source quench message to tell the sender to send the packet at a lower rate.  Sometimes the situation arises when there are many routers that exist between the sender and the receiver. When the sender sends the packet, then it moves in a routing loop. The time exceeded is based on the time-to-live value. When the packet traverses through the router, then each router decreases the value of TTL by one. Whenever a router decreases a datagram with a time-to-live value to zero, then the router discards a datagram and sends the time exceeded message to the original source. The router and the destination host can send a parameter problem message. This message conveys that some parameters are not properly set. When the packet is sent, then the routing table is gradually augmented and updated. The tool used to achieve this is the redirection message.  The ICMP Query message is used for error handling or debugging the internet. This message is commonly used to ping a message.A router or a host can send an echo-request message. It is used to ping a message to another host that "Are you alive". If the other host is alive, then it sends the echo-reply message. An echo-reply message is sent by the router or the host that receives an echo-request message. The timestamp-request and timestamp-reply messages are also a type of query messages. Suppose the computer A wants to know the time on computer B, so it sends the timestamp-request message to computer B. The computer B responds with a timestamp-reply message. IP An IP stands for internet protocol. An IP address is assigned to each device connected to a network. Each device uses an IP address for communication. It also behaves as an identifier as this address is used to identify the device on a network. It defines the technical format of the packets. Mainly, both the networks, i.e., IP and TCP, are combined together, so together, they are referred to as a TCP/IP. It creates a virtual connection between the source and the destination. To facilitate the routing of packets, TCP/IP protocol uses a 32-bit logical address known as IPv4(Internet Protocol version 4).An IP address consists of two parts, i.e., the first one is a network address, and the other one is a host address. There are two types of IP addresses: IPv4 IPv6  What is IPv4? IPv4 is a version 4 of IP. It is a current version and the most commonly used IP address. It is a 32-bit address written in four numbers separated by 'dot', i.e., periods. This address is unique for each device. For example, 66.94.29.13 The above example represents the IP address in which each group of numbers separated by periods is called an Octet. Each number in an octet is in the range from 0-255. This address can produce 4,294,967,296 possible unique addresses.  Each bit in an octet can be either 1 or 0. If the bit the 1, then the number it represents will count, and if the bit is 0, then the number it represents does not count. Representation of 8 Bit Octet  Now, we will see how to obtain the binary representation of the above IP address, i.e., 66.94.29.13 Step 1: First, we find the binary number of 66. Step 2: Now, we calculate the binary number of 94. Step 3: The next number is 29. Step 4: The last number is 13. Drawback of IPv4 Currently, the population of the world is 7.6 billion. Every user is having more than one device connected with the internet, and private companies also rely on the internet. As we know that IPv4 produces 4 billion addresses, which are not enough for each device connected to the internet on a planet. What is IPv6? IPv4 produces 4 billion addresses, and the developers think that these addresses are enough, but they were wrong. IPv6 is the next generation of IP addresses. The main difference between IPv4 and IPv6 is the address size of IP addresses. The IPv4 is a 32-bit address, whereas IPv6 is a 128-bit hexadecimal address. IPv6 provides a large address space, and it contains a simple header as compared to IPv4. It provides transition strategies that convert IPv4 into IPv6, and these strategies are as follows: Dual stacking: It allows us to have both the versions, i.e., IPv4 and IPv6, on the same device. Tunneling: In this approach, all the users have IPv6 communicates with an IPv4 network to reach IPv6. Network Address Translation: The translation allows the communication between the hosts having a different version of IP.  This hexadecimal address contains both numbers and alphabets. Due to the usage of both the numbers and alphabets, IPv6 is capable of producing over 340 undecillion (3.4*1038) addresses. IPv6 is a 128-bit hexadecimal address made up of 8 sets of 16 bits each, and these 8 sets are separated by a colon. In IPv6, each hexadecimal character represents 4 bits. So, we need to convert 4 bits to a hexadecimal number at a time  The above diagram shows the address format of IPv4 and IPv6. An IPv4 is a 32-bit decimal address. It contains 4 octets or fields separated by 'dot', and each field is 8-bit in size. The number that each field contains should be in the range of 0-255. Whereas an IPv6 is a 128-bit hexadecimal address. It contains 8 fields separated by a colon, and each field is 16-bit in size. IPv4 and IPv6 Agenda IPv4 IPv6 Address length IPv4 is a 32-bit address. IPv6 is a 128-bit address. Fields IPv4 is a numeric address that IPv6 is an alphanumeric address that consists of 8 fields, consists of 4 fields which are which are separated by colon. separated by dot (.). Classes IPv4 has 5 different classes of IP IPv6 does not contain classes of IP addresses. address that includes Class A, Class B, Class C, Class D, and Class E. Number of IP IPv4 has a limited number of IP IPv6 has a large number of IP addresses. address addresses. VLSM It supports VLSM (Virtual Length It does not support VLSM. Subnet Mask). Here, VLSM means that Ipv4 converts IP addresses into a subnet of different sizes. Address It supports manual and DHCP It supports manual, DHCP, auto-configuration, and configuration configuration. renumbering. Agenda IPv4 IPv6 Address space It generates 4 billion unique It generates 340 undecillion unique addresses. addresses End-to-end In IPv4, end-to-end connection In the case of IPv6, end-to-end connection integrity is connection integrity is unachievable. achievable. integrity Security In IPv4, security depends on the In IPv6, IPSEC is developed for security purposes. features application. This IP address is not developed in keeping the security feature in mind. Address In IPv4, the IP address is In IPv6, the representation of the IP address in hexadecimal. representation represented in decimal. Fragmentation Fragmentation is done by the Fragmentation is done by the senders only. senders and the forwarding routers. Packet flow identification It does not provide any mechanism It uses flow label field in the header for packet flow identification. for the packet flow identification. Checksum field The checksum field is available in The checksum field is not available IPv4. in IPv6. Transmission scheme IPv4 is broadcasting. On the other hand, IPv6 is multicasting, which provides efficient network operations. Encryption and Authentication It does not provide encryption and It provides encryption and authentication. authentication. Number of octets It consists of 4 octets. It consists of 8 fields, and each field contains 2 octets. Therefore, the total number of octets in IPv6 is 16. Questions Discuss Distance Vector routing using suitable example. Discuss Link State routing using suitable example. Differnce between DV and LS routing algorithm. Shortnotes on RIP(intra),OSPF(intra),BGP(inter domain). Discuss Hierarchical Routing. Write short notes on MPLS(Multi Protocol Label Switching). Differences between IPv4 vs IPv6. Broadcast & Multicast routing subnetting vs supernetting defination and numerical