Network Layer (Part IV)

Overview A router is a type of internetworking device that passes data packets between networks based on Layer 3 addresses. A router has the ability to make intelligent decisions regarding the best path for delivery of data on the network. In this chapter, you will learn how routers use a Layer 3 addressing scheme to make forwarding decisions.

Overview In addition, you will learn how devices on local-area networks (LANs) use Address Resolution Protocol (ARP) before forwarding data to a destination. You will learn what happens when a device on one network does not know the MAC address of a device on another network. You will learn that Reverse Address Resolution Protocol (RARP) is the protocol a device uses when it does not know its own IP address. Lastly, you will learn the difference between routing and routed protocols and how routers track distance between locations. You will also learn about distance-vector, link-state, and hybrid routing approaches and how each resolves common routing problems.

Layer 3 Devices : Routers In networking, there are two addressing schemes: one uses the MAC address, a data link (Layer 2) address; the other uses an address located at the network layer (Layer 3) of the OSI model. An example of a Layer 3 address is an IP address. A router is a type of internetworking device that passes data packets between networks, based on Layer 3 addresses. A router has the ability to make intelligent decisions regarding the best path for delivery of data on the network

Layer 3 Devices : Routers

Layer 3 Devices : Layer 3 addresses Bridges and switches use physical, or MAC addresses, to make data forwarding decisions. Routers use a Layer 3 addressing scheme to make forwarding decisions. They use IP, or logical addresses, rather than MAC addresses. Because IP addresses are implemented in software, and refer to the network on which a device is located, sometimes these Layer 3 addresses are referred to as protocol addresses, or network addresses

Layer 3 Devices : Layer 3 addresses Physical, or MAC addresses, are usually assigned by the NIC manufacturer and are hard-coded into the NIC. The network administrator usually assigns IP addresses. In fact, it is not unusual for a network administrator to group devices together in the IP addressing scheme, according to their geographical location, department, or floor within a building. Because they are implemented in software, IP addresses are fairly easy to change. Finally, bridges and switches are primarily used to connect segments of a network. Routers are used to connect separate networks and to access the worldwide Internet. They do this by providing end-to-end routing

Layer 3 Devices : Layer 3 addresses

Layer 3 Devices : Unique network numbers Routers connect two or more networks, each of which must have a unique network number in order for routing to be successful. The unique network number is incorporated into the IP address that is assigned to each device attached to that network.

Layer 3 Devices : Unique network numbers Example: A network has a unique network number - A. It has four devices attached to it. The IP addresses of the devices are A2, A3, A4, and A5. Since the interface where the router connects to a network is considered to be part of that network, the interface where the router connects to network A has an IP address of A1

Layer 3 Devices : Unique network numbers Example: Another network, with a unique network number - B - has four devices attached to it. This network is also attached to the same router, but at a different interface. The IP addresses of the devices on this second network are B1, B2, B3, and B4. The IP address of the router's second interface is B5.

Layer 3 Devices : Unique network numbers Example: You want to send data from one network to another. The source network is A; the destination network is B; and a router is connected to networks A, B, C, and D. When data (frames), coming from network A, reaches the router, the router performs the following functions: – It strips off the data link header, carried by the frame. (The data link header contains the MAC addresses of the source and destination.) – It examines the network layer address to determine the destination network. – It consults its routing tables to determine which of its interfaces it will use to send the data, in order for it to reach its destination network.

Layer 3 Devices : Unique network numbers In the example, the router determines that it should send the data from network A to network B, from its interface, with address B5. Before actually sending the data out interface B5, the router would encapsulate the data in the appropriate data link frame.

Layer 3 Devices : Unique network numbers

Layer 3 Devices : Router interface/port A router’s attachment to a network is called an interface; it may also be referred to as a port. In IP routing, each interface must have a separate, unique network (or subnetwork) address.

Layer 3 Devices : Router interface/port

Network-to-Network Communications : Methods for assigning an IP address After you have determined the addressing scheme for a network, you must choose the method for assigning addresses to hosts. There are essentially two methods for assigning IP addresses - static addressing and dynamic addressing. Regardless of which addressing scheme you use, no two interfaces can have the same IP address.

Network-to-Network Communications : Methods for assigning an IP address Static Addressing If you assign IP addresses statically, you must go to each individual device and configure it with an IP address. This method requires you to keep very meticulous records, because problems can occur on the network if you use duplicate IP addresses. Some operating systems, such as Windows 95 and Windows NT, send an ARP request to check for a duplicate IP address when they attempt to initialize TCP/IP. If they discover a duplicate, the operating systems will not initialize TCP/IP and will generate an error message. Record keeping is important too, because not all operating systems identify duplicate IP addresses.

Network-to-Network Communications : Methods for assigning an IP address Dynamic Addressing There are a few different methods that you can use to assign IP addresses dynamically. Examples of these are: Reverse Address Resolution Protocol (RARP) Reverse address resolution protocol (RARP) binds MAC addresses to IP addresses. This binding allows some network devices to encapsulate data before sending them out on the network. A network device such as a diskless workstation might know its MAC address, but not its IP address. Devices using RARP require that a RARP server be present on the network to answer RARP requests.

Network-to-Network Communications : Methods for assigning an IP address Let's look at an example where a source device wants to send data to another device. In our example the source knows its own MAC address, but is unable to locate its own IP address in its ARP table. In order for the destination device to retrieve the data, pass it to higher layers of the OSI model, and respond to the originating device, the source must include both its MAC address and IP address. Therefore, the source initiates a process called a RARP request, which helps it detect its own IP address. The device builds a RARP request packet and sends it out on the network. To ensure that all devices see the RARP request on the network, it uses a broadcast IP address.

Network-to-Network Communications : Methods for assigning an IP address RARP uses the same packet format as ARP. But in a RARP request, the MAC headers, IP headers, and "operation code" are different from an ARP request. The RARP packet format contains places for MAC addresses of both destination and source. The source IP address field is empty. The broadcast goes to all devices on the network; therefore the destination IP address will be set to all binary 1s. Workstations running RARP have codes in ROM that direct them to start the RARP process, and locate the RARP server.

Network-to-Network Communications : Methods for assigning an IP address BOOTstrap Protocol (BOOTP) A device uses BOOTstrap protocol (BOOTP) when it starts up, to obtain an IP address. BOOTP uses UDP to carry messages; the UDP message is encapsulated in an IP datagram. A computer uses BOOTP to send a broadcast IP datagram (using a destination IP address of all 1s ). A BOOTP server receives the broadcast and then sends a broadcast. The client receives a datagram and checks the MAC address.

Network-to-Network Communications : Methods for assigning an IP address If it finds its own MAC address in the destination address field, then it takes the IP address in that datagram. Like RARP, BOOTP operates in a client-server environment, and only requires a single packet exchange. However, unlike RARP, which only sends back a 4 octet IP address, BOOTP datagrams can include the IP address, the address of a router (default gateway), the address of a server, and a vendor-specific field. One of the problems with BOOTP is that it was not designed to provide dynamic address assignment. With BOOTP you create a configuration file that specifies the parameters for each device.

Network-to-Network Communications : Methods for assigning an IP address Dynamic Host Configuration Protocol (DHCP) Dynamic host configuration protocol (DHCP) has been proposed as a successor to BOOTP. Unlike BOOTP, DHCP allows a host to obtain an IP address quickly and dynamically. All that is required using DHCP is a defined range of IP addresses on a DHCP server. As hosts come online they contact the DHCP server and request an address. The DHCP server chooses an address and allocates it to that host. With DHCP, the entire computer’s configuration can be obtained in one message (e.g. along with the IP address, the server can also send a subnet mask).

Network-to-Network Communications : Methods for assigning an IP address

Network-to-Network Communications : DHCP initialization sequence When a DHCP client boots, it enters an initialize state. It sends DHCPDISCOVER broadcast messages, which are UDP packets with the port number set to the BOOTP port. After sending the DHCPDISCOVER packets, the client moves into the select state and collects DHCPOFFER responses from DHCP server. The client then selects the first response it receives and negotiates lease time (the length of time it can keep the address without renewing it) with the DHCP server by sending a DHCPREQUEST packet. The DHCP server acknowledges a client request with a DHCPACK packet. The client can now enter the bound state and begin using the address.

Network-to-Network Communications : DHCP initialization sequence

Network-to-Network Communications : IP key components In order for devices to communicate, the sending devices need both, the IP addresses and the MAC addresses, of the destination devices. When they try to communicate with devices whose IP addresses they know, they must determine the MAC addresses. The TCP/IP suite has a protocol, called ARP, that can automatically obtain the MAC address. ARP enables a computer to find the MAC address of the computer that is associated with an IP address.

Network-to-Network Communications : IP key components Note: The basic unit of data transfer in IP is the IP packet. Packet processing occurs in software, which means that content and format are not hardware dependent. A packet is divided into two major components: the header, which includes source and destination addresses; and the data. Other types of protocols have their own formats. The IP packet is unique to IP.

Network-to-Network Communications : IP key components Note: Another major component of IP is Internet Control Message Protocol (ICMP). This protocol is used by a device to report a problem to the sender of a message. For example, if a router receives a packet that it cannot deliver, it sends a message back to the sender of the packet. One of the many features of ICMP is echo- request/echo-reply, which is a component that tests whether a packet can reach a destination by pinging the destination.

Network-to-Network Communications : IP key components

Network-to-Network Communications : Function of the address resolution protocol (ARP) Layer 3 protocols determine whether data passes beyond the network layer to higher levels of the OSI model. A data packet must contain both, a destination MAC address and a destination IP address. If it lacks one or the other, the data will not pass from Layer 3 to the upper layers. In this way, MAC addresses and IP addresses act as checks and balances for each other. After devices determine the IP addresses of the destination devices, they can add the destination MAC addresses to the data packets.

Network-to-Network Communications : Function of the address resolution protocol (ARP) There are a variety of ways that devices can determine the MAC addresses they need to add to the encapsulated data. Some keep tables that contain all the MAC addresses and IP addresses of other devices that are connected to the same LAN. They are called Address Resolution Protocol (ARP) tables, and they map IP addresses to the corresponding MAC addresses.

Network-to-Network Communications : Function of the address resolution protocol (ARP) ARP tables are sections of RAM memory, in which the cached memory is maintained automatically on each of the devices. It is a rare occasion when you must make an ARP table entry manually. Each computer on a network maintains its own ARP table. Whenever a network device wants to send data across a network, it uses information provided by its ARP table.

Network-to-Network Communications : Function of the address resolution protocol (ARP) When a source determines the IP address for a destination, the source consults its ARP table in order to locate the MAC address for the destination. If the source locates an entry in its table (destination IP address to destination MAC address), it binds, or associates, the IP address to the MAC address and uses it to encapsulate the data. The data packet is then sent out over the networking media to be picked up by the destination.

Network-to-Network Communications : Function of the address resolution protocol (ARP)