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 technology- dependent 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