Network fundamentals – Chapter 5: Osi network layer

Network Fundamentals – Chapter 5 OSI Network Layer CCNA Exploration version 4.0 Học viện mạng Bách Khoa - Website: www.bkacad.com 2 Objectives • Identify the role of the Network Layer, as it describes communication from one end device to another end device • Examine the most common Network Layer protocol, Internet Protocol (IP), and its features for providing connectionless and best-effort service • Understand the principles used to guide the division or grouping of devices into networks • Understand the hierarchical addressing of devices and how this allows communication between networks • Understand the fundamentals of routes, next hop addresses and packet forwarding to a destination network Học viện mạng Bách Khoa - Website: www.bkacad.com 3 Introduction End-to-End connections Host-to-Host Học viện mạng Bách Khoa - Website: www.bkacad.com 4 Introduction • The protocols of the OSI model Network layer specify: – addressing and processes that enable Transport layer data to be packaged and transported. • The Network layer encapsulation allows its contents to be passed to the destination within a network or on another network with minimum overhead. Học viện mạng Bách Khoa - Website: www.bkacad.com 5 Introduction IPv4 IPv4 addresses are 32 bits long, written in dotted decimal, and separated by periods. Học viện mạng Bách Khoa - Website: www.bkacad.com 6 Network Layer – Communication from Host to Host • The Network layer, or OSI Layer 3, provides services to exchange the individual pieces of data over the network between identified end devices. 5.1.1.1 Học viện mạng Bách Khoa - Website: www.bkacad.com 7 Network Layer – Communication from Host to Host 1.Addressing 2.Encapsulation 3.Routing
The role of the router is to select paths for and direct packets toward their destination. This process is known as routing. 4.Decapsulation Layer 3 uses 4 basic processes: Communication between networks is called routing Học viện mạng Bách Khoa - Website: www.bkacad.com 8 Network Layer – Communication from Host to Host • Each route that a packet takes to reach the next device is called a hop. • As the packet is forwarded, its contents (the Transport layer PDU), remain intact until the destination host is reached. Học viện mạng Bách Khoa - Website: www.bkacad.com 9 Network Layer – Communication from Host to Host • Transport layer (OSI Layer 4): manages the data transport between the processes running on each end host • Network layer (OSI Layer 3): specify the packet structure and processing used to carry the data from one host to another host Compare role between Layer 3,4 Học viện mạng Bách Khoa - Website: www.bkacad.com 10 Network Layer – Communication from Host to Host Học viện mạng Bách Khoa - Website: www.bkacad.com 11 Network Layer – Communication from Host to Host Học viện mạng Bách Khoa - Website: www.bkacad.com 12 Network Layer Protocols • Internet Protocol version 4 (IPv4) • Internet Protocol version 6 (IPv6) • Novell Internetwork Packet Exchange (IPX) • AppleTalk • Connectionless Network Service (CLNS/DECNet) Học viện mạng Bách Khoa - Website: www.bkacad.com 13 The IPv4 Protocol – Example Network Layer Protocol • The Internet Protocol was designed as a protocol with low overhead (?) • It provides only the functions that are necessary to deliver a packet from a source to a destination over an interconnected system of networks. • The protocol was not designed to track and manage the flow of packets. Học viện mạng Bách Khoa - Website: www.bkacad.com 14 The IPv4 Protocol – Example Network Layer Protocol Basic characteristics 5.1.2 Học viện mạng Bách Khoa - Website: www.bkacad.com 15 The IPv4 Protocol – Connectionless 5.1.3 Học viện mạng Bách Khoa - Website: www.bkacad.com 16 The IPv4 Protocol – Connectionless Connectionless vs. Connection-oriented Protocols? Học viện mạng Bách Khoa - Website: www.bkacad.com 17 The IPv4 Protocol – Best Effort . 5.1.4 The IPv4 Protocol – Best Effort • Unreliable means simply that IP does not have the capability to manage, and recover from, undelivered or corrupt packets. • Since protocols at other layers can manage reliability, IP is allowed to function very efficiently at the Network layer. Học viện mạng Bách Khoa - Website: www.bkacad.com 18 Học viện mạng Bách Khoa - Website: www.bkacad.com 19 In a reliable protocol, the receiver confirms (ACK) the packet it’s received. Example: TCP. In a “best effort” or non-reliable protocol, the receiver keeps silent instead. Example: IP, UDP. Extra: Reliable vs. Best-effort Protocols Receiver: I confirm that I’ve received the packet #n. Sender: I’ve sent the packet #n. Protocol: I have the mechanism to know if the packet is received. Receiver: I keep silence upon receiving packets. Sender: I’ve sent the packet #n. Protocol: I do not have the mechanism to know if the packet is received. •This is a classification of networking protocols. •“Non-reliable” does not mean inaccurately data delivery. Học viện mạng Bách Khoa - Website: www.bkacad.com 20 The IPv4 Protocol – Media Independent • In some cases, an intermediary device - usually a router - will need to split up a packet when forwarding it from one media to a media with a smaller MTU. This process is called fragmenting the packet or fragmentation. MTU (Token Ring, Ethernet,PPP) ? 5.1.5 Học viện mạng Bách Khoa - Website: www.bkacad.com 21 Packaging the Transport Layer PDU • The routing performed by these intermediary devices only considers the contents of the packet header that encapsulates the segment. 5.1.6 Học viện mạng Bách Khoa - Website: www.bkacad.com 22 IPv4 Packet Header 5.1.7 Học viện mạng Bách Khoa - Website: www.bkacad.com 23 Extra: Type of Service RFC 791 Học viện mạng Bách Khoa - Website: www.bkacad.com 24 Extra: Type of Service Học viện mạng Bách Khoa - Website: www.bkacad.com 25 Extra: Flags Học viện mạng Bách Khoa - Website: www.bkacad.com 26 Extra: Protocols Học viện mạng Bách Khoa - Website: www.bkacad.com 27 Network Layer Fields • 4 bits • Indicates version of IP used • IPv4: 0100; IPv6: 0110 Học viện mạng Bách Khoa - Website: www.bkacad.com 28 Network Layer Fields • 4 bits • Indicates datagram header length in 32 bit words Học viện mạng Bách Khoa - Website: www.bkacad.com 29 Network Layer Fields • 8 bits • Specifies the level of importance that has been assigned by upper-layer protocol Học viện mạng Bách Khoa - Website: www.bkacad.com 30 Network Layer Fields • 16 bits • Specifies the length of the entire packet in bytes, including data and header Học viện mạng Bách Khoa - Website: www.bkacad.com 31 Network Layer Fields • 16 bits • Identifies the current datagram Học viện mạng Bách Khoa - Website: www.bkacad.com 32 Network Layer Fields • 3 bits • The second bit specifies if the packet can be fragmented; the last bit specifying whether the packet is the last fragment in a series of fragmented packets. Học viện mạng Bách Khoa - Website: www.bkacad.com 33 Network Layer Fields • 13 bits • Used to help piece together datagram fragments Học viện mạng Bách Khoa - Website: www.bkacad.com 34 Network Layer Fields • 8 bits • Specifies the number of hops a packet may travel. This number is decreased by one as the packet travels through a router Routing Loop ? Học viện mạng Bách Khoa - Website: www.bkacad.com 35 Network Layer Fields • 8 bits • Indicates which upper-layer protocol, such as TCP(6) or UDP(17), receives incoming packets after IP processing has been completed Học viện mạng Bách Khoa - Website: www.bkacad.com 36 Network Layer Fields • 16 bits • Helps ensure IP header integrity • Not caculated for the encapsulation data Học viện mạng Bách Khoa - Website: www.bkacad.com 37 Network Layer Fields • 32 bits • Specifies the sending node IP address Học viện mạng Bách Khoa - Website: www.bkacad.com 38 Network Layer Fields • 32 bits • Specifies the receiving node IP address Học viện mạng Bách Khoa - Website: www.bkacad.com 39 Network Layer Fields • Variable length • Allows IP to support various options, such as security Học viện mạng Bách Khoa - Website: www.bkacad.com 40 Network Layer Fields • Variable length • Extra zeros are added to this field to ensure that the IP header is always a multiple of 32 bits. Học viện mạng Bách Khoa - Website: www.bkacad.com 41 Network Layer Fields • Variable length up to 64 KB • Contains upper-layer information Học viện mạng Bách Khoa - Website: www.bkacad.com 42 Network Layer Fields Học viện mạng Bách Khoa - Website: www.bkacad.com 43 Networks – Dividing Hosts into Groups Học viện mạng Bách Khoa - Website: www.bkacad.com 44 Separating Hosts into Common Groups • Networks can be grouped based on factors that include: – Geographic location – Purpose – Ownership Geographic 5.2.1 Học viện mạng Bách Khoa - Website: www.bkacad.com 45 Separating Hosts into Common Groups Purpose: Users who have similar tasks typically use common software, common tools, and have common traffic patterns. Học viện mạng Bách Khoa - Website: www.bkacad.com 46 Separating Hosts into Common Groups Purpose Học viện mạng Bách Khoa - Website: www.bkacad.com 47 Separating Hosts into Common Groups Ownership: To assists in controlling access to the devices and data as well as the administration of the networks. Học viện mạng Bách Khoa - Website: www.bkacad.com 48 Separating Hosts into Common Groups Ownership Học viện mạng Bách Khoa - Website: www.bkacad.com 49 Why separate hosts into networks ? • Performance degradation • Security issues • Address Management 5.2.2 Common issues with large networks are: Học viện mạng Bách Khoa - Website: www.bkacad.com 50 Why separate hosts into networks ? • Improving Performance Broadcast domain ? 5.2.2 Lab 5.2.2 Học viện mạng Bách Khoa - Website: www.bkacad.com 51 Học viện mạng Bách Khoa - Website: www.bkacad.com 52 Why separate hosts into networks ? • Increase network security 5.2.3 Học viện mạng Bách Khoa - Website: www.bkacad.com 53 Why separate hosts into networks ? • Increase network security Học viện mạng Bách Khoa - Website: www.bkacad.com 54 Why separate hosts into networks ? • Increase network security Học viện mạng Bách Khoa - Website: www.bkacad.com 55 Why separate hosts into networks ? • Address management: To expect each host to know the address of every other host would impose a processing burden on these network devices that would severely degrade their performance. •ARP table ? •Gateway ? 5.2.4 Học viện mạng Bách Khoa - Website: www.bkacad.com 56 How do we separate hosts into networks ? • Hierarchical addressing 5.2.5 Học viện mạng Bách Khoa - Website: www.bkacad.com 57 Dividing the networks - Networks from networks • If a large network has to be divided, additional layers of addressing can be created. Using hierarchical addressing means that the higher levels of the address are retained; with a subnetwork level and then the host level. •Subnet Mask ? •Prefix length ? 5.2.6 Học viện mạng Bách Khoa - Website: www.bkacad.com 58 Dividing the networks - Networks from networks Học viện mạng Bách Khoa - Website: www.bkacad.com 59 Extra: Classes of IP Addresses Extra: Classes of IP Addresses Học viện mạng Bách Khoa - Website: www.bkacad.com 60 Học viện mạng Bách Khoa - Website: www.bkacad.com 61 Routing – How Our Data Packets are Handled Học viện mạng Bách Khoa - Website: www.bkacad.com 62 Routing Protocols • Routing is an OSI Layer 3 function. • Routing is the process of finding the most efficient path from one device to another. Học viện mạng Bách khoa - Website: www.bkacad.com 63 Routing and Layer 2 Switching Học viện mạng Bách khoa - Website: www.bkacad.com 64 Router and Switch • Each computer and router interface maintains an ARP table for Layer 2 communication. The ARP table is only effective for the broadcast domain (or LAN) that it is connected to • MAC addresses are not logically organized, but IP addresses are organized in a hierarchical manner. Học viện mạng Bách khoa - Website: www.bkacad.com 65 Routed versus Routing • Routed protocol: used at the network layer that transfer data from one host to another across a router. • Routing protocols: allow routers to choose the best path for data from source to destination. Học viện mạng Bách Khoa - Website: www.bkacad.com 66 Routing protocol • Provides processes for sharing route information • Examples: – Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), Open Shortest Path First (OSPF), Border Gateway Protocol (BGP), and Enhanced IGRP (EIGRP) Học viện mạng Bách Khoa - Website: www.bkacad.com 67 Supporting communication outside our network • To communicate with a device on another network, a host uses the address of this gateway, or default gateway, to forward a packet outside the local network. • The router also needs a route that defines where to forward the packet next. This is called the next-hop (?) address. 5.3.1 Học viện mạng Bách Khoa - Website: www.bkacad.com 68 Supporting communication outside our network 5.3.1 Học viện mạng Bách Khoa - Website: www.bkacad.com 69 IP Packets – Carrying Data End-to-End 5.3.2 Học viện mạng Bách Khoa - Website: www.bkacad.com 70 IP Packets – Carrying Data End-to-End Học viện mạng Bách Khoa - Website: www.bkacad.com 71 IP Packets – Carrying Data End-to-End Học viện mạng Bách Khoa - Website: www.bkacad.com 72 IP Packets – Carrying Data End-to-End Học viện mạng Bách Khoa - Website: www.bkacad.com 73 IP Packets – Carrying Data End-to-End Học viện mạng Bách Khoa - Website: www.bkacad.com 74 IP Packets – Carrying Data End-to-End Học viện mạng Bách Khoa - Website: www.bkacad.com 75 A gateway – The way out of our network 5.3.3.1 Học viện mạng Bách Khoa - Website: www.bkacad.com 76 A gateway – The way out of our network 5.3.3.2 Route command ? Học viện mạng Bách Khoa - Website: www.bkacad.com 77 A gateway – The way out of our network 5.3.3.3 Học viện mạng Bách Khoa - Website: www.bkacad.com 78 A Route – The Path to a Network 5.3.4.1 Học viện mạng Bách Khoa - Website: www.bkacad.com 79 Routing table • The routing table stores information about connected and remote networks. • Routes in a routing table have 3 main features: –Destination network –Next-hop –Metric Default route ? Học viện mạng Bách Khoa - Website: www.bkacad.com 80 Host Routing Table C:/> netstat -r 5.3.4.2 Học viện mạng Bách Khoa - Website: www.bkacad.com 81 Host Routing Table Học viện mạng Bách Khoa - Website: www.bkacad.com 82 Routing table entries 5.3.5.1 Học viện mạng Bách Khoa - Website: www.bkacad.com 83 Routing table entries 5.3.6 Học viện mạng Bách Khoa - Website: www.bkacad.com 84 Default route 5.3.5.2 Học viện mạng Bách Khoa - Website: www.bkacad.com 85 Packet forwarding • Routing is done packet-by-packet and hop- by-hop. Each packet is treated independently in each router along the path. • The router will do one of 3 things with the packet: • Forward it to the next-hop router • Forward it to the destination host • Drop it 5.3.7 Học viện mạng Bách Khoa - Website: www.bkacad.com 86 Packet forwarding 5.3.7.1 Học viện mạng Bách Khoa - Website: www.bkacad.com 87 Packet forwarding • Using the Default Route 5.3.7.2 The default route is also known as the Gateway of Last Resort. Học viện mạng Bách Khoa - Website: www.bkacad.com 88 Packet forwarding 5.3.7.3 Lab 5.3.7.4 Học viện mạng Bách Khoa - Website: www.bkacad.com 89 Học viện mạng Bách Khoa - Website: www.bkacad.com 90 Routing Processes – How Routes are Learned Học viện mạng Bách Khoa - Website: www.bkacad.com 91 Routing protocol – Sharing the route 5.4.1 Routing protocol – Sharing the route •manually configured on the router • or learned dynamically from other routers in the same internetwork Học viện mạng Bách Khoa - Website: www.bkacad.com 92 This route information can be Học viện mạng Bách Khoa - Website: www.bkacad.com 93 Static Routing 5.4.2 Học viện mạng Bách Khoa - Website: www.bkacad.com 94 Dynamic Routing 5.4.3.1 Routing protocols are the set of rules by which routers dynamically share their routing information Lab 5.4.3.2 Học viện mạng Bách Khoa - Website: www.bkacad.com 95 Học viện mạng Bách Khoa - Website: www.bkacad.com 96 Extra: IGP and EGP Học viện mạng Bách Khoa - Website: www.bkacad.com 97 Link state and Distance Vector • The distance-vector routing approach determines the distance and direction, vector, to any link in the internetwork. • Routers using distance-vector algorithms send all or part of their routing table entries to adjacent routers on a periodic basis. • This happens even if there are no changes in the network. Eg: RIP, IGRP, EIGRP Học viện mạng Bách Khoa - Website: www.bkacad.com 98 Link state and Distance Vector • Link state routing protocols send periodic update at longer time interval (30’), Flood update only when there is a change in topology. • Link state use their database to creat routing table. Eg: OSPF, IS-IS Q&A • In a connectionless system: –The destination is not contacted before a packet is sent. Học viện mạng Bách Khoa - Website: www.bkacad.com 99 Q&A • If the default gateway is configured incorrectly on the host, what is the impact on communications? –The host can communicate with other hosts on the local network, but is unable to communicate with hosts on remote networks. Học viện mạng Bách Khoa - Website: www.bkacad.com 100 Q&A •What type of routing uses information that is manually entered into the routing table? –static Học viện mạng Bách Khoa - Website: www.bkacad.com 101 Q&A • When the destination network is not listed in the routing table of a Cisco router, what are two possible actions that the router might take? –The router discards the packet. –The router forwards the packet out the interface indicated by the default route entry. Học viện mạng Bách Khoa - Website: www.bkacad.com 102 Q&A •What are the key factors to consider when grouping hosts into a common network? –purpose –geographic location –ownership Học viện mạng Bách Khoa - Website: www.bkacad.com 103 Q&A •What are three common problems with a large network? –performance degradation –security issues –host identification Học viện mạng Bách Khoa - Website: www.bkacad.com 104 Q&A Học viện mạng Bách Khoa - Website: www.bkacad.com 105 Refer to the exhibit. All devices shown in the exhibit have factory default settings. How many broadcast domains are represented in the topology that is shown? Học viện mạng Bách Khoa - Website: www.bkacad.com 106 Terminology • Broadcasts are contained within a network. In this context, a network is also known as a broadcast domain. • For all other destinations, the hosts only need to know the address of an intermediary device, to which they send packets for all other destinations addresses. This intermediary device is called a gateway. The gateway is a router on a network that serves as an exit from that network. • The number of bits of an address used as the network portion is called the prefix length. • The router also needs a route that defines where to forward the packet next. This is called the next-hop address. If a route is available to the router, the router will forward the packet to the next-hop router that offers a path to the destination network. • The default route is used when the destination network is not represented by any other route in the routing table. • Hosts typically have an ARP table, which is a cache of IP/MAC address mappings. • Hosts typically have an ARP table, which is a cache of IP/MAC address mappings. When you want to send a packet to a local host, your software looks up the IP in the ARP cache, gets the MAC address, constructs an Ethernet header with the correct source/destination MAC addresses, and sends that. Học viện mạng Bách Khoa - Website: www.bkacad.com 107 Summary Học viện mạng Bách Khoa - Website: www.bkacad.com 108