The overall job of IP is to transmit messages from higher-layer protocols over an internetwork of devices. These messages must be packaged and addressed, and fragmented if necessary, and then they must be delivered. The process of delivery can be either simple or complex, depending on the proximity of the source and destination devices. We can divide all IP datagram deliveries into two general types : direct delivery and indirect delivery.
DIRECT DATAGRAM DELIVERYWhen datagrams are sent between two devices on the same physical network, the datagrams may be delivered directly from the source to the destination. For example, if you wanted to deliver a letter to a neighbor on your street, you would probably just put her name on the envelope and stick it right in her mailbox.
Direct delivery is obviously a simple delivery method. The source simply sends the IP datagram down to its data link layer implementation. The data link layer encapsulates the datagram in a frame that is sent over the physical network directly to the recipient’s data link layer, which passes it up to the IP layer.
INDIRECT DATAGRAM DELIVERY (ROUTING)When two devices are not on the same physical network, the delivery of datagrams from one to the other is indirect. Since the source device cannot see the destination on its local network, it must send the datagram through one or more intermediate devices to deliver it. Indirect delivery is like mailing a letter to a friend in a different city. You don’t deliver it yourself; you use the postal system. The letter journeys through the postal system, possibly taking several intermediate steps, and ends up in your friend’s neighborhood, where a postal carrier puts it into his mailbox.
Indirect delivery is much more complicated, because we can’t send the data straight to the recipient. In fact, we usually will not even know exactly where the recipient is. Sure, we have its address, but we may not know what network it is on, or where that network is relative to our own. Just as we must rely on the postal system in the envelope analogy, we must rely on the internetwork itself to indirectly deliver datagrams. And like the postal system, IP doesn’t require you to know how to get the message to its recipient; you just put it into the system.
The devices that accomplish this magic of indirect delivery are generally known as routers, and indirect delivery is more commonly called routing.
RELATIONSHIP BETWEEN DATAGRAM ROUTING AND ADDRESSINGEach time a datagram is to be sent, the sender must determine first whether it can be delivered directly or if routing is required. IP addressing is what allows a device to quickly determine whether or not it is on the same network as its intended recipient. The following are the three main categories of addressing.
Conventional Classful Addressing - We know the class of each address by looking at the first few bits. This tells us which bits of an address are the network ID. If the network ID of the destination is the same as our own, the recipient is on the same network; otherwise, it is not.
Subnetted Classful Addressing - We use our subnet mask to determine our network ID and subnet ID and that of the destination address. If the network ID and subnet are the same, the recipient is on the same subnet. If only the network ID is the same, the recipient is on a different subnet of the same network. If the network ID is different, the destination is on a different network entirely.
The determination of what type of delivery is required is the first step in the source deciding where to send a datagram. If it realizes the destination is on the same local network, it will address the datagram to the recipient directly at the data link layer. Otherwise, it will send the datagram to the data link layer address of one of the routers to which it is connected. The IP address of the datagram will still be that of the ultimate destination. Mapping between IP addresses and data link layer addresses is accomplished using the TCP/IP Address Resolution Protocol (ARP).
IP ROUTING CONCEPTS AND THE PROCESS OF NEXT-HOP ROUTING
To continue with our postal system analogy, I can send a letter from my home in the United States to someone in, say, India, and the postal systems of both countries will work (or should work) to deliver the letter to its destination. However, when I drop a letter in the mailbox, it’s not like someone shows up, grabs the letter, and hand-delivers it to the right address in India. The letter travels from the mailbox to my local post office. From there, it probably goes to a regional distribution center, and then from there, to a hub for international traffic. It goes to India, perhaps via an intermediate country. When it gets to India, the Indian postal system uses its own network of offices and facilities to route the letter to its destination. The envelope hops from one location to the next, until it reaches its destination.
IP routing works in very much the same manner. We don’t know exactly where the destination device’s network is, and we certainly don’t have any way to connect directly to each of the thousands of networks out there. Instead, we rely on these intermediate devices—routers—that are each physically connected to each other in a variety of ways to form a mesh containing millions of paths between networks. The datagram is handed off from one router to the next, until it gets to the physical network of the destination device. This process is called next-hop routing.
|KEY CONCEPT - Indirect delivery of IP datagrams is accomplished using a process called nexthop routing, where each message is handed from one router to the next until it reaches the network of the destination. The main advantage of this is that each router needs to know only which neighboring router should be the next recipient of a given datagram, rather than needing to know the exact route to every destination network.|
Routing is the process of transferring the packets from one network to another network and delivering the packets to the hosts. The traffic is routed to all the networks in the internetwork by the routers. In the routing process a router must know following things :
Static routing does not involve any change in routing table unless the network administrator changes or modify them manually. Static routing algorithms function well where the network traffic is predictable. This is simple to design and easy to implement. There is no requirement of complex routing protocols.
The routing decisions are not made by current topology or traffic because the static routing systems can not react to network changes hence it doesn’t require extra resources to learn the changes. That is the reason, static routing is considered as inappropriate for large and constantly changing networks.
Dynamic routing is a superior routing technique which alters the routing information according to the altering network circumstances by examining the arriving routing update messages. When the network change occurs, it sends out a message to the router to specify that change, then the routes are recalculated and sent as a new routing update message. These messages pervade the network, enabling the router to change their routing tables correspondingly.
The technique uses routing protocols to disseminate knowledge such as RIP, OSPF, BGP, etc. Unlike static routing, it does not require manual updation instead its automatic in manner and updates the routing table information periodically relying upon network conditions. For doing so, it requires extra resources for storing the information.
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