Transcript Chapter 3
Internetworking Concept and Architectural Model Chapter 3 Introduction • Now that we know the technology that is used for transmission, how do we create a coordinated unit for communication? • Goal – To hide details of the underlying network, and build the necessary software to provide a framework for communication – Two approaches to hiding details • Application programs translate for their hardware • Intermediate computers computers pass data, don’t look at it Network Level Interconnection • Small packets of data are sent from source to destination without using intermediate application programs • This separates the application from the transfer of packets • It also allows the underlying network structure to change without modifying the application • Separation of communication details from the messages being sent Design of communication systems • No single network hardware technology can satisfy all constraints – speed, distance, cost • Users desire universal interconnection • Therefore, we need a unified, cooperative interconnection of (heterogeneous) networks which supports a universal communication service Design of communication systems • New software, inserted between the technologydependent communication mechanisms and application programs will make the collection of networks appear to be a single large network Design of the Internet • Do not want to require users to understand the hardware details • Do not want to mandate which hardware to use • Do want to allow users to connect with computers that are not directly connected • Do want to allow users to connect to all others connected to the Internet (or an internet) Internet Architecture • How is one network connected to another? – It involves more than just a cable; the two networks need to each be connected to a computer that will pass packets between them • Called a router or a gateway; See Figure 3.1 – Each router needs to know something of the networks beyond those directly connected to it - see Figure 3.2 – Each router needs to know the destination network, not necessarily the actual computer within the network – Routers in TCP/IP internets are usually small and have little memory The User’s View • Think of an internet as a single, virtual network to which all machines connect, despite the physical connections, as in Figure 3.3 • The network software is all that needs to be reconfigured when to network’s topology changes • Figure 3.3b shows that not all networks need to be directly connected to all others (see the routers) • The intermediate networks agree to handle traffic in exchange for being connected themselves All Networks are Equal • A network is a network, regardless of whether it is the smallest LAN or the largest WAN Unanswered Questions • What do the internet addresses look like? • What does a packet look like? • What happens when too many packets arrive too quickly? • How can multiple application programs executing concurrently on a single computer send and receive packets to multiple destinations without getting entangled? Summary • Interconnected systems agree to conventions so that they can communicate with other computers • An internet allows two computers to communicate even if they are not connected within the same network, or directly connected networks • Computers must agree on a set of universal identifiers and a set of procedures for moving data • Connections between networks are made with routers which attach to two or more networks • Routers forward packets from one network to another For Next Time • Read Chapter 4 • See exercises, write your own notes