in the earlier chapters on LANs and WANs we assumed that stations/Data Terminal Equipment (DTE)-normally referred to as end systems (ESs) or hosts-were all --ached to the one network type, that is, all the systems were attached either to a single LAN (or bridged LAN) or to a single WAN. We made this assumption to defer one discussion of the additional problems that must be considered when two systems comunicate through networks consisting of two or more different network types;
In addition to open system networking environments comprising just a single type - network (LAN or WAN), there are networking environments that comprise an intererconnected set of networks. An example is a distributed community of LANs, each created in a different university and interconnected through a countrywide WAN, that
been established to allow ESs attached to different LANs to exchange electronic =all or computer files. Another example is an interconnected set of WANs that enables programs in a distributed community of bank computers to carry out funds transfer and other transactions. There are many such applications.
When two or more networks are involved in an application, we normally refer to the mode working between systems as internetworking. We used the term internetwork (or internet) to refer to the composite network (for example, LAN/WAN/
being used. Each constituent network (LAN or WAN) of the internetwork is a -network (or subnet).
Each constituent network in an Internet supports communications among the devices attached to that network; these devices are referred to as End Systems or in short, ESs. In addition, networks are connected by devices referred in the ISO documents as Intermediate Systems (ISs). Intermediate Systems provide a communications path and perform the necessary relaying and routing functions so that data can be exchanged between devices attached to different networks in the internet.
Two types of ISs of particular interest are bridges and routers. The differences between them depends on the types of protocols used for internetworking logic. In essence, a bridge operates at the data-link layer of OSI model and acts as a relay of frames between similar networks. A router operates at the network layer of OSI model and routers packets between potentially different networks. Both the bridge and the router assume that the same upper-layer protocols are in use.

 PRINCIPLES OF INTERNETWORKING
Requirements for an internetworking facility are as follows: Provide a link between networks. At minimum, a physical and link control connection is needed.
Provide for the routing and delivery of the data between processes on different networks.
Provide an accounting service that keeps track of the use of the various networks and routers and maintains status information.
Provide the services just listed in such a way as not to require modifications to the network architecture of any of the constituent networks. This means that the internetworking facility must accommodate a number of differences among networks.
A key characteristic of the Internet architecture is whether the mode of operation is connection-oriented or is connectionless.
In connection-oriented operation, it is assumed that each network provides a connection-oriented form of service. That is, it is possible to establish a logical network connection (say, a virtual circuit) between any two DTEs attached to the same network. The connection is set up first, and then data are exchanged. With this in mind, we can summarize the connection-oriented approach as follows:
Intermediate systems are used to connect two or more networks; each intermediate system appears as a DTE to each of the networks to which it is attached.
When DTE A wishes to exchange data with another DTE B, a logical connection is set up between them. This logical connection consists of the connection of sequences of logical connections across networks. The sequence is such that it forms a path from DTE A to DTE B.
The individual network logical connections are spliced together by ISs. Any traffic arriving to an IS on one logical connection is retransmitted on a second logical connection and vice versa.
It is not always the case that the constituent networks of an internet provide a connection-oriented service. For example, an IEEE 802 or Fiber Distributed Data Interface (FDDI) LAN provides a service defined by the logical link control (LLC). Two of the options with LLC provide only connectionless service.
Thus, in effort, these networks have a datagram style of transmission. Therefore, in this case, the network service must be enhanced. An example of how this would be done is for the ISs to implement X.25 on top of the LLC across the LAN. A connection-oriented IS performs the following key functions:
Relaying: Data units arriving from one network via the network layer protocol are relayed (retransmitted) on another network. Traffic is over logical connections that are spliced together at the ISs.
Routing:              When an end-to-end logical connection consisting of a sequence of logical connections is to be set up, each IS in the sequence must make a, routing decision that determines the next hop in the sequence.In practice, the connection-oriented approach is not commonly used. The connectionless approach, using Internet Protocol (IP) is dominant.
Whereas connection-oriented operation corresponds to the virtual circuit mechanism of the packet switching network, the connectionless mode operation corresponds to the datagram mechanism of a packet switching network. Each network protocol data unit is treated independently and routed from source DTE to destination DTE through a series of routers and networks. For each data unit transmitted by A, A makes a decision as to which route should receive the data unit. The data unit hops across the internet from one router to the next until it reaches the destination network. At each router, a routing decision is made (independently for each data unit) _ concerning the next hop. Thus, different data units may travel different router between source and destination.
All DTEs and all routers share a common network layer protocol known as the internet protocol. An Internet Protocol (IP) was initially developed for DARPA Internet -:.roject but eventually became the Internet standard. Below this Internet protocol, another protocol is needed to access a particular network. Thus, there are typically two -)protocols operating in each DTE and router at the network layer: an upper sub-layer -hat provides the internetworking function, and a lower sub-layer that provides network access.
IP provides a connectionless or datagram service between end systems. There are a number of advantages to this approach.
A connectionless Internet facility is flexible. It can deal with a variety of networks, some of which are themselves connectionless. In essence, IP requires very little from the constituent networks.
A connectionless Internet services can be made highly robust. This is basically the same argument made for a datagram network service as opposed to a virtual circuit service.
A connectionless Internet service is best for connectionless transport protocols, because it does not impose unnecessary overhead.

ROUTING PRINCIPLES
In order to transfer packets from a sending host to the destination host, the network layer must determine the path or route that the packets are to follow. Whether the network layer provides a datagram service (in which case different packets between a given host-destination pair may take different routes) or a virtual-circuit service (in which case all packets between a given source and destination will take the same p path), the network layer must determine the path for a packet. This job is achieved by - :e network layer routing protocol.
At the heart of any routing protocol is the algorithm (the routing algorithm) that determines the path for a packet. The purpose of a routing algorithm is simple; given : set of routers, with links connecting the routers, a routing algorithm finds a `good' path from source to destination. Typically, a `good' path is one that has `least cost'. We will see, however, that in practice, real-world concerns such as policy issues (for =example, a rule such as `router X' belonging to an organization Y should not forwardany packets originating from the network owned by an organization Z) also come into play to complicate the conceptually simple and elegant algorithms whose theory underlies the practice of routing in today's networks.
The graph abstraction used to formulate routing algorithms is shown in Figure To view some graphs representing real network, here, nodes in the graph

represent routers-the points at which packet routing decisions are made-and the lines ("edges" in the graph theory terminology) connecting these nodes represent the physical links between these routers. A link also has a value representing the "cost" of sending a packet across the link. The cost may reflect the level of congestion of that link (for example, the current average delay for a packet across that link) or the physical distance traversed by that link (for example, a transoceanic link might have a higher cost than a short-haul terrestrial link). For our current purposes, we will simply take the link costs as given and not worry about how they are determined.
Given the graph abstraction, the problem of finding the least-cost path from a source to a destination requires identifying a series of links such that:
•    The first link in the path is connected to the source.
•    The last link in the path is connected to the destination.
•    For all i, the i and i, the first link in the path are connected to the same node.
•    For the least-cost path, the sum of the cost of the links on the path is the minimum over all possible paths between the source and the destination. Note that if all link costs are the same, the least cost path is also the shortest path (that is the path crossing the smallest number of links between the source and the destination).
In Figure 10.1, for example, the least-cost path between node A (source) and C (destination) is along the path ADEC. We will find it notationally easier to refer to the path in terms of the nodes on the path, rather than the links on the path.
As a simple exercise, try finding the least-cost path from nodes A to F and reflect for a moment on how you calculated that path. If you did the way most people do, well, you found the path from A to F by examining Figure 10.1, tracing a few routes fromA to F, and somehow convincing yourself that the path you had chosen had the least cost among all possible paths. (Did you check all of the 12 paths between A and F? probably not!) Such a calculation is an example of a centralized routing algorithm-the routing algorithm was run in one location, your brain, with complete information about the network. Broadly, one way in which we can classify routing algorithms is according to whether they are global or decentralized.

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