Content Overview Internet Protocol (IP) is the routed protocol of the Internet. IP addressing enables packets to be routed from source to destination using the best available path. The propagation of packets, encapsulation changes, and connection-oriented and connectionless protocols are also critical to ensure that data is properly transmitted to its destination. This module will provide an overview for each. The difference between routing and routed protocols is a common source of confusion for students learning networking. The two words sound similar but are quite different. This module also introduces routing protocols which allow routers to build tables from which to determine the best path to a host on the Internet. No two organizations in the world are identical. In fact, not all the organizations can fit into the three class system of A, B, and C addresses. However, flexibility does exist within the class addressing system and it is called subnetting. Subnetting allows the network administrators to determine the size of the pieces of the network they will be working with. Once they have determined how to segment their network, they can then use the subnet mask to determine what part of the network each device is on. Students completing this module should be able to:
Content 10.1 Routed Protocol 10.1.1 Routable and routed protocols A protocol is a set of rules that determines how computers communicate with each other across networks. Computers communicate with one another by exchanging data messages. To accept and act on these messages, computers must have definitions of how a message is interpreted. Examples of messages include those establishing a connection to a remote machine, e-mail messages, and files transferred over a network. A protocol describes the following: A routed protocol allows the router to forward data between nodes on different networks. In order for a protocol to be routable, it must provide the ability to assign a network number and a host number to each individual device. Some protocols, such as IPX, require only a network number because these protocols use the host's MAC address for the host number. Other protocols, such as IP, require a complete address consisting of a network portion and a host portion. These protocols also require a network mask in order to differentiate the two numbers. The network address is obtained by ANDing the address with the network mask. The reason that a network mask is used is to allow groups of sequential IP addresses to be treated as a single unit. If this grouping were not allowed, each host would have to be mapped individually for routing. According to the Internet Software Consortium, this would not be possible with the 162,128,000 hosts that are currently on the Internet. Web Links Internet Software Consortium http://www.isc.org/
Content 10.1 Routed Protocol 10.1.2 IP as a routed protocol The Internet Protocol (IP) is the most widely used implementation of a hierarchical network-addressing scheme. IP is a connectionless, unreliable, best-effort delivery protocol. The term connectionless means that no dedicated circuit connection is established prior to transmission as there is when placing a telephone call. IP determines the most efficient route for data based on the routing protocol. The terms unreliable and best-effort do not imply that the system is unreliable and does not work well, but that IP does not verify that the data reached its destination. This function is handled by the upper layer protocols. As information flows down the layers of the OSI model, the data is processed at each layer. At the network layer, the data is encapsulated into packets, also known as datagrams. IP determines the contents of the IP packet header, which includes addressing and other control information, but is not concerned with the actual data. IP accepts whatever data is passed down to it from the upper layers. Web Links Understanding Routing Protocols http://www.oreillynet.com/pub/a/network/ 2001/05/22/net_2nd_ lang.html
Content 10.1 Routed Protocol 10.1.3 Packet propagation and switching within a router As a packet travels through an internetwork to its final destination, the Layer 2 frame headers and trailers are removed and replaced at every Layer 3 device. This is because Layer 2 data units, frames, are for local addressing. Layer 3 data units, packets, are for end-to-end addressing. Layer 2 Ethernet frames are designed to operate within a broadcast domain using the MAC address that is burned into the physical device. Other Layer 2 frame types include Point-to-Point Protocol (PPP) serial links and Frame Relay connections, which use different Layer 2 addressing schemes. Regardless of the type of Layer 2 addressing used, frames are designed to operate within a Layer 2 broadcast domain, as the data crosses a Layer 3 device the Layer 2 information changes. As a frame is received at a router interface, the destination MAC address is extracted. The address is checked to see if the frame is directly addressed to the router interface, or if it is a broadcast. In either of these two cases, the frame is accepted. Otherwise, the frame is discarded since it is destined for another device on the collision domain. The accepted frame has the Cyclic Redundancy Check (CRC) information extracted from the frame trailer, and calculated to verify that the frame data is without error. If the check fails, the frame is discarded. If the check is valid, the frame header and trailer are removed and the packet is passed up to Layer 3. The packet is then checked to see if it is actually destined for the router, or if it is to be routed to another device in the internetwork. If the destination IP address matches one of the router ports, the Layer 3 header is removed and the data is passed up to the Layer 4. If the packet is to be routed, the destination IP address will be compared to the routing table. If a match is found or there is a default route, the packet will be sent to the interface specified in the matched routing table statement. When the packet is switched to the outgoing interface, a new CRC value is added as a frame trailer, and the proper frame header is added to the packet. The frame is then transmitted to the next broadcast domain on its trip to the final destination. Interactive Media Activity Drag and Drop: Packet Propagation Flowchart After completing this activity, the student will be able to better understand the detailed packet propagation process that goes on within a router.
Content 10.1 Routed Protocol 10.1.4 Internet Protocol (IP) Two types of delivery services are connectionless and connection-oriented. These two services provide the actual end-to-end delivery of data in an internetwork. Most network services use a connectionless delivery system. Different packets may take different paths to get through the network, but are reassembled after arriving at the destination. In a connectionless system, the destination is not contacted before a packet is sent. A good comparison for a connectionless system is a postal system. The recipient is not contacted to see if they will accept the letter before it is sent. Also, the sender never knows whether the letter arrived