Unit 4 - Network Layer
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The document discusses topics related to the network layer, including: 1. It describes virtual circuits and datagrams, which are two methods for transferring data across networks. 2. It covers IPv4 addressing concepts such as address space, notations, classful and classless addressing, subnetting, and network address translation. 3. It provides an overview of additional network layer topics like IPv6 addressing, routing algorithms, internet control protocols, and routing protocols.
![Distance Vector Routing Algorithm
Dx(y) = Estimate of least cost from x to y
C(x,v) = Node x knows cost to each neighbor v
Dx = [Dx(y): y ∈ N ] = Node x maintains distance vector
Node x also maintains its neighbors' distance vectors
– For each neighbor v, x maintains Dv = [Dv(y): y ∈ N ]
Note –
From time-to-time, each node sends its own distance vector estimate to neighbors.
When a node x receives new DV estimate from any neighbor v, it saves v’s distance vector, and it updates its own
DV using B-F equation:
Dx(y) = min { C(x,v) + Dv(y), Dx(y) } for each node y ∈ N](https://image.slidesharecdn.com/unit4-210915150308/85/Unit-4-Network-Layer-55-320.jpg)
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Unit 4 - Network Layer
- 1. Unit 4 : NETWORK LAYER Chandan Gupta Bhagat https://chandanbhagat.com.np/me
- 2. Network Layer Functions of Network Layer Virtual circuits and Datagram Subnet IPv4 Addresses: Address Space Notations Classful addressing Classless addressing Subnetting Network Address Translation(NAT) IPv4 Datagram format and fragmentation IPv6 Address Structure and advantages over IPv4 Routing Algorithms: Distance Vector Routing, Link State Routing Internet Control Protocols: ARP, RARP, ICMP Routing protocols: OSPF, BGP, Unicast, Multicast and Broadcast
- 3. Functions of Network Layer
- 4. Functions of Network Layer Routing: When a packet reaches the router's input link, the router will move the packets to the router’s output link. For example, a packet from S1 to R1 must be forwarded to the next router on the path to S2. Logical Addressing: The data link layer implements the physical addressing and network layer implements the logical addressing. Logical addressing is also used to distinguish between source and destination system. The network layer adds a header to the packet which includes the logical addresses of both the sender and the receiver. Internetworking: This is the main role of the network layer that it provides the logical connection between different types of networks. Fragmentation: The fragmentation is a process of breaking the packets into the smallest individual data units that travel through different networks
- 5. Responsibilities of Network Layer Logical Addressing In the internet world, there are two kinds of addressing implemented by the data link layer, it handles addressing problems locally. If the network passes through the network boundary, we need another system to distinguish source and destination systems. The logical addressing at the network layer while physical addressing at the data link layer is defined by the MAC address of a device, whereas the IP addressing is determined at the network layer of the OSI model. This addressing is also called as logical addressing. The network layer adds a header to the packet which is coming from the upper layer includes the logical addresses of the sender and receiver. Routing When two independent networks or links are attached to create an internetwork that is the network of networks or a large network, the connecting devices route the packets to its destination. The forwarding of the data request to servers is known as routing.
- 6. Responsibilities of Network Layer Fragmentation and Reassembly The network layer must send data down to the data link layer for transmission. The data or information that the network layer receives is in the form of a packet and the data that data link layer forwards is called a frame The network layer has the responsibility of Fragmentation and reassembly because some data link layer technologies have limits on the length of any message that can be sent. If the packet of data that the network layer has to send is too large, the network layer must break the packet up, send each packet to the data link layer, and then have pieces reassembled once they arrive at the network layer on the destination system Example: If I want to access some data from Facebook then I will open my laptop, type URL of Facebook and send an HTTP request to facebook.com for some data. Since the server of Facebook is situated outside my local area network, my request is forwarded to Facebook through the default gateway or router of my institution
- 7. Virtual circuits and Datagram Subnet
- 8. Datagram vs Virtual Circuit Packet Switching Datagram Packet Switching Virtual-circuit Packet Switching Two packets of the same user pair can travel along different routes. All packets of the same virtual circuit travel along the same path The packets can arrive out of sequence Packet sequencing is guaranteed Packets contain full Source, Destination addresses Packets contain short VC Id. (VCI) Each host occupies routine table entries Each VC occupies routing table entries Requires no connection setup Requires VC setup. First packet has large delay Also called Connection less Also called connection oriented Eg. Internet which uses IP Network protocol Examples: X.25 and Frame Relay
- 9. Virtual circuit An initial setup phase is used to set up a route between the intermediate nodes for all the packets passed during the session between the two end nodes. In each intermediate node, an entry is registered in a table to indicate the route for the connection that has been set up. Thus, packets passed through this route, can have short headers, containing only a virtual circuit identifier (VCI), and not their destination. Each intermediate node passes the packets according to the information that was stored in it, in the setup phase. In this way, packets arrive at the destination in the correct sequence, and it is guaranteed that essentially there will not be errors. This approach is slower than Circuit Switching, since different virtual circuits may compete over the same resources, and an initial setup phase is needed to initiate the circuit. As in Circuit Switching, if an intermediate node fails, all virtual circuits that pass through it are lost. The most common forms of Virtual Circuit networks are X.25 and Frame Relay, which are commonly used for public data networks (PDN)
- 10. Virtual circuit 1 2 3 1 2 3 1 2 3 Node 1 Node 2 Node 3 Node 4 Call Request Packet Call-Accept Packet Call-Acknowledgement Packet Packets Virtual Circuit Packet Switching Techniques
- 11. Virtual circuit Applciation Transport Network Data Link Physical Applciation Transport Network Data Link Physical 5. Data Flow Begins 4. Call Connected 1. Initiate Call 6. Receive Data 3. Accept Call 2. Incoming Call Virtual Circuit
- 12. Datagram This approach uses a different, more dynamic scheme, to determine the route through the network links. Each packet is treated as an independent entity, and its header contains full information about the destination of the packet. The intermediate nodes examine the header of the packet, and decide to which node to send the packet so that it will reach its destination. In this method, the packets don't follow a pre-established route, and the intermediate nodes (the routers) don’t have pre-defined knowledge of the routes that the packets should be passed through. Packets can follow different routes to the destination, and delivery is not guaranteed . Due to the nature of this method, the packets can reach the destination in a different order than they were sent, thus they must be sorted at the destination to form the original message. This approach is time consuming since every router has to decide where to send each packet. The main implementation of Datagram Switching network is the Internet, which uses the IP network protocol.
- 13. Datagram 1 2 3 1 2 3 1 2 3 Node 1 Node 2 Node 3 Node 4 Packets Datagram Packet Switching
- 14. Datagram Applciation Transport Network Data Link Physical Applciation Transport Network Data Link Physical 1. Send Data 2. Receive Data Dataram
- 15. IPv4 Addresses
- 16. IP Address An IPv4 address is 32 bits long The IPv4 addresses are unique and universal IPv4 uses 32-bit addresses, which means that the address space is 232 or 4,294,967,296(Maximum available theoretically) Address Space is divided into three classes: Class A – 10.0.0.0/8 Network Block 10.0.0.0 – 10.255.255.255 Class B – 172.16.0.0/12 Network Block 172.16.0.0 – 172.31.255.255 Class C – 192.168.0.0/16 Network Block 192.168.0.0 – 192.168.255.255
- 17. IP Address – Notations Doted decimal notations Denoted in decimal format each byte is separated by dot eg: 117.149.29.2 Mostly used by human configurations Binary notation In binary format eg: 01110101 10010101 00011101 00000010 Mostly used by devices for processing
- 18. Classful Addressing 0 Class A 24-bit Host ID 7-Bit Network ID Fix 1 Class B 16-bit Host ID Fix 0 14-bit Network ID 1 Class C 8-bit Host ID Fix 1 21-bit Network ID 0
- 19. Classful Addressing 1 Class D Multicast Address Fix 0 1 1 1 Class E Reserved Address Fix 1 1 1
- 20. Classless Addressing There is no classes hierarchy in the IP Address but address is still granted in blocks. Restriction : To simplify the handling of addresses, the Internet authorities impose three restrictions on classless address blocks The addresses in a block must be contiguous, one after another. The number of addresses in a block must be a power of 2 (I, 2, 4, 8, ..) The first address must be evenly divisible by the number of addresses. Subnetting Subnetting means creating subnetwork Subnetting means increasing networks bits(i.e. 1s) in subnet mask If network bit is increased host bits will be decreased, so number of host will be decreased A Class A network have 8 bits for network (224 IP address available) if you wanted smaller block IP from class A increase the network bits /decreasing host bits
- 21. Classless Addressing Supernetting Supernetting means creating bigger network from smaller one Supernetting means decreasing networks bits(i.e. 1s) in subnet mask If network bit is decreased host bits will be increased, so number of host will be decreased A Class C network have 24 bits for network (28 IP address available) if you wanted bigger block IP from class C decrease the network bits / increasing host bits Supernetting just opposite of subnetting VLSM (Variable Length Subnet Mask) Subnetting and supernetting is achieved by varying default subnet mask Usually in classful IP address have 8,16,24 default CIDR values for Class A, B, C respectively, but in classless IP no default CIDR value / subnet mask is available CIDR value may be varying
- 22. Classless Addressing Subnetting/supernetting Steps Identify needed block size (always in power of 2 i.e. 22 , 23 , 24 , … 231) If multiple block size is needed assign largest block first Find the host bits from block size (if block size 2𝑛𝑛 no of host bit is n) From host bits find the CIDR (CIDR=32-n) Find subnet mask convert CIDR into doted decimal format (ex:255.255.240.0) Find wild card mask (255.255.255.255 - 255.255.240.0= 0.0.15.255) Add the first IP in the range to get last IP address
- 23. Network Address Translation (NAT) IP address have public range and private range Public range is used for communication in internet and can used only with permission of internet authorities Private IP can be used for local communication without permission of Internet authorities Public IP should be unique globally Private IP should be unique inside a organization, not globally NAT router consist of public IP in exit interface and internal interface consist of Private IPs
- 24. Network Address Translation (NAT) Range Total 10.0.0.0 to 10.255.255.255 224 172.16.0.0 to 172.31.255.255 220 192.168.0.0 to 192.168.255.255 216 192.168.1.2 192.168.1.7 192.168.1.3 192.168.1.6 192.168.1.4 192.168.1.5 NAT Router Internet 16.32.64.128
- 25. Network Address Translation (NAT) Address Translation : Replace outgoing packets Source IP address as NAT router public IP and replaces incoming packet Destination IP with private (Private to public and public to private) Translation is done with help of translation table which consist of IP address of private range and public range and port address Private Address Private Port External Address External Port Transport Protocol 172.18.3.1 1400 25.8.3.2 80 TCP 172.18.3.2 1401 25.8.3.2 80 TCP …… …… …… …… ……
- 26. Limitations of IPV4 Exponential growth of the Internet and the impending exhaustion of the IPv4 address space Need for simpler configuration Requirement for security at the IP level Need for better support for prioritized and real-time delivery of data
- 27. IPv4 Datagram format and fragmentation
- 28. IPv4 Datagram Header IPv4 is a connectionless protocol for a packet- switching network that uses the datagram approach. This means that each datagram is handled independently, and each datagram can follow a different route to the destination. Packets in the IPv4 layer are called datagrams. A datagram is a variable-length packet consisting of two parts: header and data. The header is 20 to 60 bytes in length and contains information essential to routing
- 29. IPv4 Datagram Header Version ( VER): This 4-bit field defines the version of the IPv4 protocol. Currently, the version is 4. However, version 6 (or IPng) may totally replace version 4 in the future. Header length ( HLEN): This 4-bit field defines the total length of the datagram header in 4-byte words. This field is needed because the length of the header is variable (between 20 and 60 bytes). Services: IETF has changed the interpretation and name of this 8-bit field. This field, previously called service type, is now called differentiated services. Type of service: Low Delay, High Throughput, Reliability (8 bits) Total Length: Length of header + Data (16 bits), which has a minimum value 20 bytes and the maximum is 65,535 bytes Identification: Unique Packet Id for identifying the group of fragments of a single IP datagram (16 bits)
- 30. IPv4 Datagram Header Flags: 3 flags of 1 bit each: reserved bit (must be zero), do not fragment flag, more fragments flag (same order) Fragment Offset: Specified in terms of number of 8 bytes, which has the maximum value of 65,528 bytes Time to live: Datagram’s lifetime (8 bits), It prevents the datagram to loop in the network Protocol: Name of the protocol to which the data is to be passed (8 bits) Header Checksum: 16 bits header checksum for checking errors in the datagram header Source IP address: 32 bits IP address of the sender Destination IP address: 32 bits IP address of the receiver Option: Optional information such as source route. Due to the presence of options, the size of the datagram header can be of variable length (20 bytes to 60 bytes).
- 31. Fragmentation Data field of a large IP packet is fragmented. The fragments are sent into a series of smaller IP packets fitting a network’s MTU. Fragmentation is done by routers Fragmentation may be done multiple times along the route. If IP packet is longer than the MTU, the router breaks packet into smaller packets called IP fragments. Fragments are still IP packets. Maximum Transfer Unit (MTU) Each data link layer protocol has its own frame format in most protocols. One of the fields defined in the format is the maximum size of the data field
- 32. Fragmentation In other words, when a datagram is encapsulated in a frame, the total size of the datagram must be less than this maximum size. A maximum transmission unit (MTU) is the largest size packet or frame, specified in octets (eight-bit bytes), that can be sent in a packet- or frame-based network such as the Internet. In a case where a router receives a protocol data unit (PDU) larger than the next hop's MTU. It has two options if the transport is IPv4: drop the PDU and send an ICMP message which indicates the condition Packet too big or fragment the IP packet and send it over the link with a smaller MTU.
- 33. Fragmentation IP Packet 3 2 1 MTU Fragmentation IP Packets IP Datagram MTU Maximum length of data to be encapsulated in a frame Header Trailer Frame
- 34. IPv6 Address Structure and advantages over IPv4
- 35. IPv6 Address The wonder of IPv6 lies in its header. An IPv6 address is 4 times larger than IPv4, but surprisingly, the header of an IPv6 address is only 2 times larger than that of IPv4. IPv6 headers have one Fixed Header and zero or more Optional (Extension) Headers. All the necessary information that is essential for a router is kept in the Fixed Header. The Extension Header contains optional information that helps routers to understand how to handle a packet/flow IPv6 fixed header is 40 bytes long and contains the following information
- 36. IPv6 Address Version Traffic Class Flow Label Payload Length Next Header Hop Limit Source Address Destination Address 0 - 3 32 - 47 64 - 191 192 - 320 4 - 11 12 - 31 48 - 55 56- 63 IPV6 Fixed Header
- 37. IPv6 Address : Fields and Description Version (4-bits): It represents the version of Internet Protocol, i.e. 0110. Traffic Class (8-bits): These 8 bits are divided into two parts. The most significant 6 bits are used for Type of Service to let the Router Known what services should be provided to this packet. The least significant 2 bits are used for Explicit Congestion Notification (ECN). Flow Label (20-bits): This label is used to maintain the sequential flow of the packets belonging to a communication. The source labels the sequence to help the router identify that a particular packet belongs to a specific flow of information. This field helps avoid re-ordering of data packets. It is designed for streaming/real- time media. Payload Length (16-bits): This field is used to tell the routers how much information a particular packet contains in its payload. Payload is composed of Extension Headers and Upper Layer data. With 16 bits, up to 65535 bytes can be indicated; but if the Extension Headers contain Hop-by-Hop Extension Header, then the payload may exceed 65535 bytes and this field is set to 0.
- 38. IPv6 Address : Fields and Description Next Header (8-bits): This field is used to indicate either the type of Extension Header, or if the Extension Header is not present then it indicates the Upper Layer PDU. The values for the type of Upper Layer PDU are same as IPv4’s. Hop Limit (8-bits): This field is used to stop packet to loop in the network infinitely. This is same as TTL in IPv4. The value of Hop Limit field is decremented by 1 as it passes a link (router/hop). When the field reaches 0 the packet is discarded. Source Address (128-bits): This field indicates the address of originator of the packet. Destination Address (128-bits): This field provides the address of intended recipient of the packet.
- 39. IPv6 Address : Address Structure An IPv6 address is made of 128 bits divided into eight 16-bits blocks. Each block is then converted into 4-digit Hexadecimal numbers separated by colon symbols. Each block is then converted into Hexadecimal and separated by ‘:’ symbol: 2001:0000:3238:DFE1:0063:0000:0000:FEFB Even after converting into Hexadecimal format, IPv6 address remains long. IPv6 provides some rules to shorten the address. The rules are as follows: Rule.1: Discard leading Zero(es): In Block 5, 0063, the leading two 0s can be omitted, such as (5th block): 2001:0000:3238:DFE1:63:0000:0000:FEFB Rule.2: If two of more blocks contain consecutive zeroes, omit them all and replace with double colon sign ::, such as (6th and 7th block): 2001:0000:3238:DFE1:63::FEFB Consecutive blocks of zeroes can be replaced only once by : : so if there are still blocks of zeroes in the address, they can be shrunk down to a single zero, such as (2nd block): 2001:0:3238:DFE1:63::FEFB
- 40. IPV6 Vs IPV4 IPV4 IPV6 IPV4 addresses are 32-bit length IPV6 address are 128-bit length Fragmentation is done by sender and forwarding routers Fragmentation is done only by sender No packet flow identification Packet flow identification is available within the IPV6 header using the flow label field Checksum field is available in header No checksum field in header Options field are available in header No option field but extension headers are available Address Resolution Protocol (ARP) is available to map IPV4 to MAC Addresses Address Resolution Protocol is replaced with Neighbor Discovery Protocol Broadcast messages are available Broadcast messages are not available Manual Configuration (static)of IP address or DHCP (Dynamic Configuration) is required to configure IP addresses Auto Configuration of addresses is available
- 41. Transition from IPV4 to IPV6 Complete transition from IPv4 to IPv6 might not be possible because IPv6 is not backward compatible. To overcome this short-coming, we have a few technologies that can be used to ensure slow and smooth transition from IPv4 to IPv6. Dual Stack Routers : A router can be installed with both IPv4 and IPv6 addresses configured on its interfaces pointing to the network of relevant IP scheme. Tunneling : In a scenario where different IP versions exist on intermediate path or transit networks, tunneling provides a better solution where user’s data can pass through a non-supported IP version. NAT Protocol Translation : This is another important method of transition to IPv6 by means of a NAT-PT (Network Address Translation - Protocol Translation) enabled device. With the help of a NAT-PT device, actual can take place happens between IPv4 and IPv6 packets and vice versa.
- 42. Transition from IPV4 to IPV6 IPV6 Network IPV4 Network IPV6 Traffic IPV4 Traffic Dual Stack Router
- 43. Transition from IPV4 to IPV6 Tunneling IPV4 Network Tunnel IPV6 Network
- 44. Transition from IPV4 to IPV6 NAT Protocol Translation IPV6 Network NAT Enabled Device
- 46. Routing algorithm 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. The Routing algorithm is divided into two categories: Adaptive Routing algorithm Non-adaptive Routing algorithm
- 47. 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, distance and estimated transit time. An adaptive routing algorithm can be classified into three parts: Centralized Algorithm Isolation Algorithm Distributed Algorithm
- 48. Adaptive Routing Algorithm : Centralized 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.
- 49. Adaptive Routing Algorithm : Isolation It is an algorithm that obtains the routing information by using local information rather than gathering information from other nodes.
- 50. Adaptive Routing Algorithm : Distributed 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 the decentralized 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.
- 51. 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. The Non-Adaptive Routing algorithm is of two types: 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.
- 52. Adaptive Vs Non-Adaptive Routing Algorithms Adaptive Routing Algorithm Non-Adaptive Routing Algorithm Adaptive Routing algorithm is an algorithm that constructs the routing table based on the network conditions. The Non-Adaptive Routing algorithm is an algorithm that constructs the static table to determine which node to send the Packet Adaptive routing algorithm is used by dynamic routing The Non-Adaptive Routing algorithm is used by static routing Routing decisions are made based on topology and network traffic Routing decisions are the static tables. The types of adaptive routing algorithm, are Centralized, isolation and distributed algorithm The types of Non-Adaptive routing algorithm are flooding and random walks Adaptive Routing algorithms are more complex Non-Adaptive Routing algorithms are simple
- 53. 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
- 54. Distance Vector Routing Algorithm 1. A router transmits its distance vector to each of its neighbors in a routing packet. 2. Each router receives and saves the most recently received distance vector from each of its neighbors. 3. A router recalculates its distance vector when: It receives a distance vector from a neighbor containing different information than before. It discovers that a link to a neighbor has gone down. The DV calculation is based on minimizing the cost to each destination
- 55. Distance Vector Routing Algorithm Dx(y) = Estimate of least cost from x to y C(x,v) = Node x knows cost to each neighbor v Dx = [Dx(y): y ∈ N ] = Node x maintains distance vector Node x also maintains its neighbors' distance vectors – For each neighbor v, x maintains Dv = [Dv(y): y ∈ N ] Note – From time-to-time, each node sends its own distance vector estimate to neighbors. When a node x receives new DV estimate from any neighbor v, it saves v’s distance vector, and it updates its own DV using B-F equation: Dx(y) = min { C(x,v) + Dv(y), Dx(y) } for each node y ∈ N
- 56. Distance Vector Routing Algorithm Advantages of Distance Vector routing – It is simpler to configure and maintain than link state routing. Disadvantages of Distance Vector routing – It is slower to converge than link state. It is at risk from the count-to-infinity problem. It creates more traffic than link state since a hop count change must be propagated to all routers and processed on each router. Hop count updates take place on a periodic basis, even if there are no changes in the network topology, so bandwidth-wasting broadcasts still occur. For larger networks, distance vector routing results in larger routing tables than link state since each router must know about all other routers. This can also lead to congestion on WAN links.
- 57. Link State Routing Link state routing is a technique in which each router shares the knowledge of its neighborhood with every other router in the internetwork. The three keys to understand the Link State Routing algorithm: Knowledge ab out 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 except its neighbors. 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
- 58. Link State Routing Link state routing is the second family of routing protocols. While distance vector routers use a distributed algorithm to compute their routing tables, link-state routing uses link-state routers to exchange messages that allow each router to learn the entire network topology. Based on this learned topology, each router is then able to compute its routing table by using a shortest path computation. Features of link state routing protocols : Link state packet – A small packet that contains routing information. Link state database – A collection information gathered from link state packet. Shortest path first algorithm (Dijkstra algorithm) – A calculation performed on the database results into shortest path Routing table – A list of known paths and interfaces.
- 59. Link State Routing Calculation of shortest path : To find shortest path, each node need to run the famous Dijkstra algorithm. This famous algorithm uses the following steps: Step-1: The node is taken and chosen as a root node of the tree, this creates the tree with a single node, and now set the total cost of each node to some value based on the information in Link State Database Step-2: Now the node selects one node, among all the nodes not in the tree like structure, which is nearest to the root, and adds this to the tree. The shape of the tree gets changed . Step-3: After this node is added to the tree, the cost of all the nodes not in the tree needs to be updated because the paths may have been changed. Step-4: The node repeats the Step 2. and Step 3. until all the nodes are added in the tree 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-live field
- 60. Distance Vector Protocol Vs Link State Protocol Distance Vector Protocol Link State Protocol Entire routing table is sent as an update Updates are incremental and entire routing table is not sent as update Distance vector protocol send periodic update at every 30 or 90 second Updates are triggered not periodic Updates are broadcasted Updates are multicasted Updates are sent directly to connected neighbors only Updates are sent to entire network and to just directly connected neighbor Routers don’t have to end-end visibility of entire network Routers have visibility of entire network of that area only It is prone to routing loops No routing loops
- 61. Distance Vector Routing Vs Link State Routing Distance Vector Routing Link State Routing Bandwidth required is less due to local sharing, small packets and no flooding Bandwidth required is more due to flooding and sending of large link state packets Based on local knowledge since it updates table based on information from neighbors Based on global knowledge i.e. It have knowledge about entire network Make use of Bellman Ford Algorithm Make use of Dijkastra’s Algorithm Traffic is less Traffic is more Converges slowly i.e. good news spread fast and bad news spread slowly Converges Faster Count to infinity problem. No count to infinity problem Persistent Looping problem i.e. loop will be there forever No persistent loops, only transient loops Practical implementation is RIP and IGRP Practical Implementation is OSPF and ISIS
- 62. Distance Vector Routing A B F C D E G 1 1 1 1 1 1 1 1 Information Stored at Node Distance to reach node A B C D E F G A 0 1 1 ? 1 1 ? B 1 0 1 ? ? ? ? C 1 1 0 1 ? ? ? D ? ? 1 0 ? ? 1 E 1 ? ? ? 0 ? ? F 1 ? ? ? ? 0 1 G ? ? ? 1 ? 1 0
- 63. Distance Vector Routing A B F C D E G 1 1 1 1 1 1 1 1 Information Stored at Node Distance to reach node A B C D E F G A 0 1 1 ? 1 1 ? B 1 0 1 ? ? ? ? C 1 1 0 1 ? ? ? D ? ? 1 0 ? ? 1 E 1 ? ? ? 0 ? ? F 1 ? ? ? ? 0 1 G ? ? ? 1 ? 1 0 2 2 2 2 2 3 2 2 2 3 3 3 3 3 2 2 2 2 2 2 2 2 2 2 2 2
- 64. Link State Routing : Dijkastra’s Algorithm 0 1 7 2 3 4 6 5 8 4 8 8 7 9 10 14 2 1 6 2 7 11 4
- 65. Link State Routing : Dijkastra’s Algorithm 0 1 7 2 3 4 6 5 8 4 8 8 7 9 10 14 2 1 6 2 7 11 4 4 8 12 9 15 11 19 21
- 67. ARP : Address Resolution Protocol Address Resolution Protocol is a communication protocol used for discovering physical address associated with given network address. Typically, ARP is a network layer to data link layer mapping process, which is used to discover MAC address for given Internet Protocol Address. In order to send the data to destination, having IP address is necessary but not sufficient; we also need the physical address of the destination machine. ARP is used to get the physical address (MAC address) of destination machine. It is used to associate an IP address with the MAC address. Each device on the network is recognized by the MAC address imprinted on the NIC. Therefore, we can say that devices need the MAC address for communication on a local area network. MAC address can be changed easily. For example, if the NIC on a particular machine fails, the MAC address changes but IP address does not change. ARP is used to find the MAC address of the node when an internet address is known.
- 68. How ARP Works? If the host wants to know the physical address of another host on its network, then it sends an ARP query packet that includes the IP address and broadcast it over the network. Every host on the network receives and processes the ARP packet, but only the intended recipient recognizes the IP address and sends back the physical address. The host holding the datagram adds the physical address to the cache memory and to the datagram header, then sends back to the sender. PC1 PC2 PC3 PC4 Sends ARP request to get PC2’s MAC Address Receives ARP and is 192.168.1.2 Sends MAC to PC1 Receives ARP but not 192.168.1.2 Receives ARP but not 192.168.1.2
- 69. How ARP Works? I am looking for the physical address of a node whose IP is 141.23.56.23 ARP Request My Physical address is XXXXXXXXXXX ARP Response
- 70. Steps taken by ARP protocol Check the ARP Cache by using a command arp –a
- 71. ARP There are two types of ARP entries: Dynamic entry: It is an entry which is created automatically when the sender broadcast its message to the entire network. Dynamic entries are not permanent, and they are removed periodically. Static entry: It is an entry where someone manually enters the IP to MAC address association by using the ARP command utility.
- 72. RARP : Reverse ARP Reverse ARP is a networking protocol used by a client machine in a local area network to request its Internet Protocol address (IPv4) from the gateway-router’s ARP table. The network administrator creates a table in gateway-router, which is used to map the MAC address to corresponding IP address. When a new machine is setup or any machine which don’t have memory to store IP address, needs an IP address for its own use. The machine sends a RARP broadcast packet which contains its own MAC address in both sender and receiver hardware address field. If the host wants to know its IP address, then it broadcast the RARP query packet that contains its physical address to the entire network.
- 73. RARP : Reverse ARP A RARP server on the network recognizes the RARP packet and responds back with the host IP address. The protocol which is used to obtain the IP address from a server is known as Reverse Address Resolution Protocol The message format of the RARP protocol is simillar to the ARP protocol. Like ARP frame, RARP frame is sent from one machine to another encapsulated in the data portion of a frame.
- 74. RARP : Reverse ARP PC1 PC2 PC3 PC4 IP Address : Unknown Physical Address : #210 IP Address : IPB Physical Address : #170 IP Address : IPD Physical Address : #410 IP Address : IPD Physical Address : #450 What is my IP address? IP Address is IPA RARP Server Device Broadcast MAC, Need to know its IP RARP Server Receives MAC and tells IP of the Device
- 75. RARP : Reverse ARP A special host configured inside the local area network, called as RARP-server is responsible to reply for these kind of broadcast packets. Now the RARP server attempt to find out the entry in IP to MAC address mapping table. If any entry matches in table, RARP server send the response packet to the requesting device along with IP address. LAN technologies like Ethernet, Ethernet II, Token Ring and Fiber Distributed Data Interface (FDDI) support the Address Resolution Protocol. RARP is not being used in today’s networks. Because we have much great featured protocols like BOOTP (Bootstrap Protocol) and DHCP( Dynamic Host Configuration Protocol).
- 76. ICMP : Internet Control Message Protocol The ICMP is a network layer protocol used by hosts and routers to send the notifications of IP datagram problems back to the sender. ICMP uses echo test/reply to check whether the destination is reachable and responding. ICMP handles both control and error messages, but its main function is to report the error but not to correct them. An IP datagram contains the addresses of both source and destination, but it does not know the address of the previous router through which it has been passed. Due to this reason, ICMP can only send the messages to the source, but not to the immediate routers. ICMP protocol communicates the error messages to the sender. ICMP messages cause the errors to be returned back to the user processes. ICMP messages are transmitted within IP datagram.
- 77. ICMP : Internet Control Message Protocol IP Header ICMP Message IP Datagram 20 bytes 8-bit Code 16-bit Checksum 8-bit type 0 7 8 15 16 31 Format of ICMP Message • The first field specifies the type of the message. • The second field specifies the reason for a particular message type. • The checksum field covers the entire ICMP message. Error Reporting • Destination Unreachable • Source Quench • Time Exceed • Parameter problems • Redirection
- 78. ICMP : Internet Control Message Protocol Destination unreachable: The message of "Destination Unreachable" is sent from receiver to the sender when destination cannot be reached, or packet is discarded when the destination is not reachable. Source Quench: The purpose of the source quench message is congestion control. The message sent from the congested router to the source host to reduce the transmission rate. ICMP will take the IP of the discarded packet and then add the source quench message to the IP datagram to inform the source host to reduce its transmission rate. The source host will reduce the transmission rate so that the router will be free from congestion. Time Exceeded: Time Exceeded is also known as "Time-To-Live". It is a parameter that defines how long a packet should live before it would be discarded. There are two ways when Time Exceeded message can be generated: Parameter problems: When a router or host discovers any missing value in the IP datagram, the router discards the datagram, and the "parameter problem" message is sent back to the source host. Redirection: Redirection message is generated when host consists of a small routing table. When the host consists of a limited number of entries due to which it sends the datagram to a wrong router. The router that receives a datagram will forward a datagram to a correct router and also sends the "Redirection message" to the host to update its routing table.
- 80. Routing Protocols Purpose of Routing protocols: To determine the path taken by a datagram between source and destination. An autonomous-system (AS) is a collection of routers under the same administrative control. In AS, all routers run the same routing protocol among themselves. Most common intra-AS routing protocols: Routing-information Protocol (RIP) and Open Shortest Path First (OSPF) Intra-AS routing protocols are also known as interior gateway protocols.
- 81. OSPF : Open Shortest Path First Open Shortest Path First (OSPF) is a link-state routing protocol which is used to find the best path between the source and the destination router using its own Shortest Path First). OSPF is widely used for intra-AS routing in the Internet. OSPF is a link-state protocol that uses flooding(means forward msg. ) of link-state information and Dijkstra least-cost path algorithm. Here is how it works: A router constructs a complete topological map (a graph) of the entire autonomous-system. Then, the router runs Dijkstra’s algorithm to determine a shortest-path tree to all subnets. Finally, the router broadcasts link state info to all other routers in the autonomous system(AS). Specifically, the router broadcasts link state information periodically at least once every 30 minutes and whenever there is a change in a link’s state. For ex: a change in up/down status.
- 82. OSPF : Open Shortest Path First Individual link costs are configured by the network- administrator. OSPF advertisements are contained in OSPF messages that are carried directly by IP. HELLO message can be used to check whether the links are operational. The router can also obtain a neighboring router’s database of network-wide link state. Some of the advanced features include: 1. Security Exchanges between OSPF routers can be authenticated. With authentication, only trusted routers can participate within an AS. By default, OSPF packets between routers are not authenticated. Two types of authentication can be configured: Simple MD5. Simple Authentication The same password is configured on each router. ¤ Clearly, simple authentication is not very secure. MD5 Authentication This is based on shared secret keys that are configured in all the routers.
- 83. OSPF : Open Shortest Path First MDS Authentication (contd.) Here is how it works A. The sending router A. computes a MD5 hash on the content of packet B. includes the resulting hash value in the packet C. sends the packet B. The receiving router computes an MD5 hash of the packet compares computed-hash value with the hash value carried in packet and verifies the packet’s authenticity 2. Multiple Same Cost Paths When multiple paths to a destination have same cost, OSPF allows multiple paths to be used. 3. Integrated Support for Unicast & Multicast Routing Multicast OSPF (MOSPF) provides simple extensions to OSPF to provide for multicast-routing. MOSPF uses the existing OSPF link database and adds a new type of link-state advertisement to the existing broadcast mechanism.
- 84. OSPF : Open Shortest Path First 4. Support for Hierarchy within a Single Routing Domain An autonomous-system can be configured hierarchically into areas. In area, an area-border-router is responsible for routing packets outside the area. Exactly one OSPF area in the AS is configured to be the backbone-area. The primary role of the backbone-area is to route traffic between the other areas in the AS.
- 85. BGP : Border Gateway Protocol Border Gateway Protocol (BGP) is us ed to Exchange routing information f or the internet and is the protocol used between ISP which are different ASes. The protocol can connect together any internetwork of autonomous system using an arbitrary topology. BGP is widely used for inter-AS routing in the Internet. Using BGP, each AS can Obtain subnet reachability-information from neighboring ASs. Propagate the reachability-information to all routers internal to the AS. Determine good routes to subnets based on i) reachability-information and ii) AS policy. Using BGP, each subnet can advertise its existence to the rest of the Internet. Basics Pairs of routers exchange routing-information over semi-permanent TCP connections using port-179. One TCP connection is used to connect 2 routers in 2 different autonomous-systems. Semi permanent TCP connection is used to connect among routers within an autonomous-system.
- 86. BGP : Border Gateway Protocol Two routers at the end of each connection are called peers. The messages sent over the connection is called a session. Two types of session: External BGP (eBGP) session This refers to a session that spans 2 autonomous-systems. Internal BGP (iBGP) session This refers to a session between routers in the same AS. BGP operation is shown in above Figure The destinations are not hosts but instead are CIDR prefixes. Each prefix represents a subnet or a collection of subnets.
- 87. BGP : Border Gateway Protocol BGP Route Information Management Functions Route Storage : Each BGP stores information about how to reach other networks. Route Update : In this task, Special techniques are used to determine when and how to use the information received from peers to properly update the routes Route Selection : Each BGP uses the information in its route databases to select good routes to each network on the internet network. Route Advertisement : Each BGP speaker regularly tells its peer what is knows about various networks and methods to reach them.
- 88. BGP : Border Gateway Protocol BGP also includes attributes which allow routers to assign preference-metrics to the routes. attributes which indicate how the prefix was inserted into BGP at the origin AS. When a gateway-router receives a route-advertisement, the gateway-router decides whether to accept or filter the route and whether to set certain attributes such as the router preference metrics.
- 89. BGP : Border Gateway Protocol Path Attributes & Routes AS-PATH This attribute contains the ASs through which the advertisement for the prefix has passed. When a prefix is passed into an AS, the AS adds its ASN to the ASPATH attribute. Routers use the AS-PATH attribute to detect and prevent looping advertisements. Routers also use the AS-PATH attribute in choosing among multiple paths to the same prefix. NEXT-HOP This attribute provides the critical link between the inter-AS and intra-AS routing protocols. This attribute is the router-interface that begins the AS-PATH.
- 90. BGP : Border Gateway Protocol Route Selection For 2 or more routes to the same prefix, the following elimination-rules are invoked sequentially: 1) Routes are assigned a local preference value as one of their attributes. The local preference of a route will be set by the router or will be learned by another router in the same AS. From the remaining routes, the route with the shortest AS-PATH is selected. From the remaining routes, the route with the closest NEXT-HOP router is selected. If more than one route still remains, the router uses BGP identifiers to select the route
- 91. BGP : Border Gateway Protocol Routing Policy Routing policy is illustrated as shown in Figure 3.30. Let A, B, C, W, X & Y = six interconnected autonomous-systems. W, X & Y = three stub-networks. A, B & C = three backbone provider networks. All traffic entering a stub-network must be destined for that network. All traffic leaving a stub-network must have originated in that network. Clearly, W and Y are stub-networks. X is a multihomed stub-network, since X is connected to the rest of the n/w via 2 different providers X itself must be the source/destination of all traffic leaving/entering X. X will function as a stub-network if X has no paths to other destinations except itself. There are currently no official standards that govern how backbone ISPs route among themselves.
- 92. Unicast In unicast mode of addressing, an IPv6 interface (host) is uniquely identified in a network segment. The IPv6 packet contains both source and destination IP addresses. A host interface is equipped with an IP address which is unique in that network segment. When a network switch or a router receives a unicast IP packet, destined to a single host, it sends out one of its outgoing interface which connects to that particular host.
- 93. Multicast The IPv6 multicast mode is same as that of IPv4. The packet destined to multiple hosts is sent on a special multicast address. All the hosts interested in that multicast information, need to join that multicast group first. All the interfaces that joined the group receive the multicast packet and process it, while other hosts not interested in multicast packets ignore the multicast information. Hence the router just has to look up the routing table and forward the packet to next hop.
- 94. Anycast IPv6 has introduced a new type of addressing, which is called Anycast addressing. In this addressing mode, multiple interfaces (hosts) are assigned same Anycast IP address. When a host wishes to communicate with a host equipped with an Anycast IP address, it sends a Unicast message. With the help of complex routing mechanism, that Unicast message is delivered to the host closest to the Sender in terms of Routing cost. 1 2 3 4 1 1 2 2 3 Server with anycast IP Server with anycast IP Server with anycast IP
- 95. Anycast Let’s take an example of tu.edu.np Web Servers, located in all continents. Assume that all the Web Servers are assigned a single IPv6 Anycast IP Address. Now when a user from Europe wants to reach Tu.edu.np the DNS points to the server that is physically located in Europe itself. If a user from Nepal tries to reach Tu.edu.np, the DNS will then point to the Web Server physically located in Asia. Nearest or Closest terms are used in terms of Routing Cost. In the above picture, when a client computer tries to reach a server, the request is forwarded to the server with the lowest Routing Cost.
- 96. Broadcast Routing By default, the broadcast packets are not routed and forwarded by the routers on any network. Routers create broadcast domains. But it can be configured to forward broadcasts in some special cases. A broadcast message is destined to all network devices. Flooding is simplest method packet forwarding. Broadcast routing can be done in two ways (algorithm): A router creates a data packet and then sends it to each host one by one. In this case, the router creates multiple copies of single data packet with different destination addresses. All packets are sent as unicast but because they are sent to all, it simulates as if router is broadcasting. This method consumes lots of bandwidth and router must destination address of each node. Secondly, when router receives a packet that is to be broadcasted, it simply floods those packets out of all interfaces. All routers are configured in the same way.
- 97. Broadcast Routing This method is easy on router's CPU but may cause the problem of duplicate packets received from peer routers. Reverse path forwarding is a technique, in which router knows in advance about its predecessor from where it should receive broadcast. This technique is used to detect and discard duplicates. Source Destination