Lec10_Network layer.pptx

LEC10: NETWORK LAYER Network Layer Characteristics The Network Layer Provides services exchange data to allow IP version 4 (IPv4) and IP version 6 (IPv6) are the principle network layer communication protocols. The network layer performs four basic operations: Addressing end devices Encapsulation Ro...

LEC10: NETWORK LAYER Network Layer Characteristics The Network Layer Provides services exchange data to allow IP version 4 (IPv4) and IP version 6 (IPv6) are the principle network layer communication protocols. The network layer performs four basic operations: Addressing end devices Encapsulation Routing De-encapsulation end devices to  Other network layer protocols include routing protocols such as Open Shortest Path First (OSPF) and messaging protocols such as Internet Control Message Protocol (ICMP). 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, when translated by a device performing Network Address Translation (NAT) for IPv4. NAT will be discussed later. Network Layer Characteristics Characteristics of IP  IP 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.  These functions, if required, are performed by other protocols at other layers, primarily TCP at Layer 4.  IP is meant to have low overhead and may be described as:  Connectionless  Best Effort  Media Independent 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).  Connectionless communication is conceptually similar to sending a letter to someone without notifying the recipient in advance 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. Note: As an unreliable network layer protocol, IP does not guarantee that all sent packets will be received. Other protocols manage the process of tracking packets and ensuring their delivery. 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 Independent: media  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. Unreliable means that IP does not have the capability to manage and recover from undelivered or corrupt packets. Network Layer Characteristics Media Independent The network layer will establish Maximum Transmission Unit (MTU). (Contd.) the  Network layer receives this from control information sent by the data link layer.  The network then establishes the MTU size.  The network layer then determines how large packets can be processed 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 One major characteristic of the media that the network layer considers: the maximum size of the PDU that each medium can transport. This characteristic is referred to as the maximum transmission unit (MTU). IPv4 Packet IPv4 Packet Header IPv4 is the primary communication protocol for the network layer. The network header (IPv4 header of a packet ) 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  An IPv4 packet header consists of fields containing important information about the packet. These fields contain binary numbers that are examined by the Layer 3 process. 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 one or more functions. 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 32 bit destination address Address IPv6 Packets Limitations of IPv4 IPv4 has three major limitations:  IPv4 address depletion – We have basically run out of IPv4 addressing.  Lack of end-to-end connectivity – To make IPv4 survive this long, private addressing and NAT were created. This ended direct communications with public addressing.  Increased network complexity – NAT was meant as temporary solution and creates issues on the network as a side effect of manipulating the network headers addressing. NAT causes latency and troubleshooting issues. IPv6 Packets IPv6 Overview IPv6 was developed by Engineering Task Force (IETF). Internet 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 –  NAT between a private IPv4 address and a public IPv4 address is not needed. This avoids some of the NAT induced problems experienced by applications that require end to end connectivity. 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 IPv6 Packets IPv6 Packet Header Significant fields in the IPv6 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 128 bit destination address Address IPv6 Packets IPv6 Packet Header 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. (Cont.) How a Host Routes Host Forwarding Decision Packets are always created at the source. Each host devices creates their own routing table. A host can send packets to the following:  Itself – 127.0.0.1 (IPv4), ::1 (IPv6)  Local Hosts – destination is on the same LAN( The source and destination hosts share the same network address)  Remote Hosts – devices are not on the same LAN Another role of the network layer is to direct packets between hosts How a Host Routes Host Forwarding Decision The Source device determines whether the destination is local or remote Method of determination (varies by IP version): (Cont.) IPv4 – Source uses its own IP address and Subnet mask, along with the destination IP address IPv6 – Source uses the network address and prefix advertised by the local router (The local router advertises the local network address (prefix) to all devices on the network) Local traffic is dumped out the host interface to be handled by an intermediary device. Remote traffic is forwarded directly to the default gateway on the LAN. When a source device sends a packet to a remote destination device, the help of routers and routing is needed. Routing is the process of identifying the best path to a destination. The router connected to the local network segment is referred to as the default gateway How a Host Routes Default Gateway A router or layer 3 switch can be a default-gateway. Features of a default gateway (DGW): It must have an IP address in the same range as the rest of the LAN. It can accept data from the LAN and is capable of forwarding traffic off of the LAN. It can route to other networks. If a device has no default gateway or a bad default gateway, its traffic will not be able to leave the LAN.  A default gateway is required to send traffic outside the local network.  Traffic cannot be forwarded outside the local network if there is no default gateway, the default gateway address is not configured, or the default gateway is down 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. Assume that PC1 and PC2 are configured with the IPv4 address 192.168.10.1 as the default gateway. Having a default gateway configured creates a default route in the routing table of the PC. A default route is the route or pathway the computer takes when it tries to contact a remote network.  In Figure 8-12, PC1 and PC2 both have default routes to send all traffic destined to remote networks to R1.. A host routing table typically includes a default gateway. With IPv4, the host receives the IPv4 address of the default gateway either dynamically from Dynamic Host Configuration Protocol (DHCP) or configured manually. With IPv6, the router can advertise the default gateway address or the host can be configured manually How a Host Routes Host Routing Tables On Windows, route print or netstat -r can be used to display the PC routing table (Both of these commands generate the same output) Three sections displayed by these two commands:  Interface List – all potential interfaces and MAC addressing  IPv4 Routing Table  IPv6 Routing Table Introduction to Routing Router Packet Forwarding Decision  When a host sends a packet to another host, it consults its routing table to determine where to send the packet. If the destination host is on a remote network, the packet is forwarded to the default gateway, which is usually the local router.  What happens when the router receives the frame from the host device? Introduction to Routing IP Router Routing Table The routing table of a router contains network route entries that list all the possible known network destinations. 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 (A router adds a directly connected route when an interface is configured with an IP address and is activated). 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 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 Introduction to Routing Dynamic Routing  A dynamic routing protocol allows the routers to automatically learn about remote networks, including a default route, from other routers. Routers that use dynamic routing protocols routing automatically information share with other routers and compensate for any topology changes without involving the network administrator. Dynamic Routes Automatically: Discover remote networks Maintain up-to-date information Choose the destination Find new best paths when there is a topology change best path to the 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 (Open Shortest Path First ) D – EIGRP (Enhanced Interior Gateway Routing Protocol) This command shows types of routes: Directly Connected – C and L Remote Routes – O, D, etc. Default Routes – S* THANKS! Best Regards!