CCNA7 ITN Chapter 8: Network Layer PDF

Chapter 8: Network Layer Prepared By: Eng.Israa Saadeh Introduction to Networks v7.0 (ITN) 8.1 Network Layer Characteristics © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 2 Network Layer in Communications The Network Layer The network layer,...

Chapter 8: Network Layer Prepared By: Eng.Israa Saadeh Introduction to Networks v7.0 (ITN) 8.1 Network Layer Characteristics © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 2 Network Layer in Communications The Network Layer The network layer, which resides at OSI Layer 3, provides services that allow end devices to exchange data across a network. The network layer uses four processes in order to provide end-to-end transport: Addressing of end devices – IP addresses must be unique for identification purposes. Encapsulation – The protocol data units from the transport layer are encapsulated by adding IP header information including source and destination IP addresses. Routing – The network layer provides services to direct packets to other networks. Routers select the best path for a packet to take to its destination network. De-encapsulation – The destination host de- encapsulates the packet to see if it matches its own. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 3 Network Layer in Communications Network Layer Protocols There are several network layer protocols in existence; however, the most commonly implemented are: Internet Protocol version 4 (IPv4) Internet Protocol version 6 (IPv6) Note: Legacy network layer protocols are not discussed in this course. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 4 Network Layer Characteristics IP Encapsulation IP encapsulates the transport layer segment. IP can use either an IPv4 or IPv6 packet and not impact the layer 4 segment. IP packet will be examined by all layer 3 devices as it traverses the network. The IP addressing does not change from source to destination. Note: NAT will change addressing, but will be discussed in a later module. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 5 Characteristics of the IP Protocol Characteristics of IP IP was designed as a protocol with low overhead – it provides only the functions required to deliver a packet from the source to a destination. An IP packet is sent to the destination without prior establishment of a connection IP was not designed to track and manage the flow of packets. These functions, if required, are performed by other layers – primarily TCP © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 6 Network Layer Characteristics Connectionless IP is Connectionless IP does not establish a connection with the destination before sending the packet. There is no control information needed (synchronizations, acknowledgments, etc.). The destination will receive the packet when it arrives, but no pre-notifications are sent by IP. If there is a need for connection-oriented traffic, then another protocol will handle this (typically TCP at the transport layer). © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 7 Network Layer Characteristics Best Effort IP is Best Effort IP will not guarantee delivery of the packet. IP has reduced overhead since there is no mechanism to resend data that is not received. IP does not expect acknowledgments. IP does not know if the other device is operational or if it received the packet. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 8 Network Layer Characteristics Media Independent IP is unreliable: It cannot manage or fix undelivered or corrupt packets. IP cannot retransmit after an error. IP cannot realign out of sequence packets. IP must rely on other protocols for these functions. IP is media Independent: IP does not concern itself with the type of frame required at the data link layer or the media type at the physical layer. IP can be sent over any media type: copper, fiber, or wireless. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 9 Network Layer Characteristics Media Independent (Contd.) The network layer will establish the Maximum Transmission Unit (MTU). Network layer receives this from control information sent by the data link layer. The network then establishes the MTU size. Fragmentation is when Layer 3 splits the IPv4 packet into smaller units. Fragmenting causes latency. IPv6 does not fragment packets. Example: Router goes from Ethernet to a slow WAN with a smaller MTU © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 10 8.2 IPv4 Packet © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 11 IPv4 Packet IPv4 Packet Header IPv4 is the primary communication protocol for the network layer. The network header has many purposes: It ensures the packet is sent in the correct direction (to the destination). It contains information for network layer processing in various fields. The information in the header is used by all layer 3 devices that handle the packet © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 12 IPv4 Packet IPv4 Packet Header Fields The IPv4 network header characteristics: It is in binary. Contains several fields of information Diagram is read from left to right, 4 bytes per line The two most important fields are the source and destination. Protocols may have may have one or more functions. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 13 IPv4 Packet IPv4 Packet Header Fields Significant fields in the IPv4 header: Function Description Version This will be for v4, as opposed to v6, a 4 bit field= 0100 Differentiated Services Used for QoS: DiffServ – DS field or the older IntServ – ToS or Type of Service Header Checksum Detect corruption in the IPv4 header Time to Live (TTL) Layer 3 hop count. When it becomes zero the router will discard the packet. Protocol I.D.s next level protocol: ICMP, TCP, UDP, etc. Source IPv4 Address 32 bit source address Destination IPV4 Address 32 bit destination address © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 14 8.3 IPv6 Packets © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 15 IPv6 Packet Limitations of IPv4 IPv4 has been updated to address new challenges. Three major issues still exist with IPv4: IP address depletion – IPv4 has a limited number of unique public IPv4 addresses available. Although there are about 4 billion IPv4 addresses, the exponential growth of new IP- enabled devices has increased the need. Internet routing table expansion – A routing table contains the routes to different networks in order to make the best path determination. As more devices and servers are connected to the network, more routes are created. A large number of routes can slow down a router. Lack of end-to-end connectivity – Network Address Translation (NAT) was created for devices to share a single IPv4 address. However, because they are shared, this can cause problems for technologies that require end-to-end connectivity. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 16 IPv6 Packets IPv6 Overview IPv6 was developed by Internet Engineering Task Force (IETF). IPv6 overcomes the limitations of IPv4. Improvements that IPv6 provides: Increased address space – based on 128 bit address, not 32 bits Improved packet handling – simplified header with fewer fields Eliminates the need for NAT – since there is a huge amount of addressing, there is no need to use private addressing internally and be mapped to a shared public address © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 17 IPv6 Packets IPv4 Packet Header Fields in the IPv6 Packet Header The IPv6 header is simplified, but not smaller. The header is fixed at 40 Bytes or octets long. Several IPv4 fields were removed to improve performance. Some IPv4 fields were removed to improve performance: Flag Fragment Offset Header Checksum © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 18 IPv6 Packets IPv6 Packet Header Significant fields in the IPv4 header: Function Description Version This will be for v6, as opposed to v4, a 4 bit field= 0110 Traffic Class Used for QoS: Equivalent to DiffServ – DS field Flow Label Informs device to handle identical flow labels the same way, 20 bit field Payload Length This 16-bit field indicates the length of the data portion or payload of the IPv6 packet Next Header I.D.s next level protocol: ICMP, TCP, UDP, etc. Hop Limit Replaces TTL field Layer 3 hop count Source IPv4 Address 128 bit source address Destination IPV4 Address 128 bit destination address © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 19 IPv6 Packets IPv6 Packet Header (Cont.) IPv6 packet may also contain extension headers (EH). EH headers characteristics: provide optional network layer information are optional are placed between IPv6 header and the payload may be used for fragmentation, security, mobility support, etc. Note: Unlike IPv4, routers do not fragment IPv6 packets. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 20 8.4 How a Host Routes © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 21 How a Host Routes Host Forwarding Decision An important role of the network layer is to direct packets between hosts. A host can send a packet to: Itself – A host can ping itself for testing purposes using 127.0.0.1 which is referred to as the loopback interface. Local host – This is a host on the same local network as the sending host. The hosts share the same network address. Remote host – This is a host on a remote network. The hosts do not share the same network address. The source IPv4 address and subnet mask is compared with the destination address and subnet mask in order to determine if the host is on the local network or remote network. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 22 How a Host Routes Default Gateway The default gateway is the network device that can route traffic out to other networks. It is the router that routes traffic out of a local network. This occurs when the destination host is not on the same local network as the sending host. The default gateway will know where to send the packet using its routing table. The sending host does not need to know where to send the packet other than to the default gateway – or router. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 23 How a Host Routes A Host Routes to the Default Gateway The host will know the default gateway (DGW) either statically or through DHCP in IPv4. IPv6 sends the DGW through a router solicitation (RS) or can be configured manually. A DGW is static route which will be a last resort route in the routing table. All device on the LAN will need the DGW of the router if they intend to send traffic remotely. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 24 How a Host Routes Host Routing Tables On Windows, route print or netstat -r to display the PC routing table Three sections displayed by these two commands: Interface List – all potential interfaces and MAC addressing IPv4 Routing Table IPv6 Routing Table © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 25 8.5 Introduction to Routing © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 26 Introduction to Routing Router Packet Forwarding Decision What happens when the router receives the frame from the host device? © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 27 Introduction to Routing IP Router Routing Table There three types of routes in a router’s routing table: Directly Connected – These routes are automatically added by the router, provided the interface is active and has addressing. Remote – These are the routes the router does not have a direct connection and may be learned: Manually – with a static route Dynamically – by using a routing protocol to have the routers share their information with each other Default Route – this forwards all traffic to a specific direction when there is not a match in the routing table © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 28 Introduction to Routing Static Routing Static Route Characteristics: Must be configured manually Must be adjusted manually by the administrator when there is a change in the topology Good for small non-redundant networks Often used in conjunction with a dynamic routing protocol for configuring a default route © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 29 Introduction to Routing Dynamic Routing Dynamic Routes Automatically: Discover remote networks Maintain up-to-date information Choose the best path to the destination Find new best paths when there is a topology change Dynamic routing can also share static default routes with the other routers. © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 30 Introduction to Routing Introduction to an IPv4 Routing Table The show ip route command shows the following route sources: L - Directly connected local interface IP address C – Directly connected network S – Static route was manually configured by an administrator O – OSPF D – EIGRP This command shows types of routes: Directly Connected – C and L Remote Routes – O, D, etc. Default Routes – S* © 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 31