Chapter 2 Network Models Spring 2006 Computer Networks

Chapter 2 Network Models Spring 2006 Computer Networks 1

Figure 2. 1 Spring 2006 Sending a letter Computer Networks 2

Layered Protocols § Communication tasks are divided into series of layers or levels § Each layer is responsible for particular task and act on them by using one or more protocols § Each layer is built upon one bellow it § The number and name of the layers differ from network to network Spring 2006 Computer Networks 3

Figure 2. 17 Spring 2006 The OSI seven layer model Computer Networks 4

The Layers of OSI Model End System R Application Presentation Intermediate System Application Presentation Session Transport Network Data Link Physical Spring 2006 Network Data Link Physical Computer Networks Data Link Physical 5

Summary of OSI Layer Functions To translate, encrypt and compress data To provide reliable end-toend message delivery and error recovery To organize bits into frames, to provide node-to -node delivery Spring 2006 Application Presentation Session Transport Network Data Link Physical To allow access to network resources To establish, manage and terminate sessions To move packets from source to destination; to provide internetworking To transmit bits over a medium; to provide mechanical and electrical specifications Computer Networks 6

Open System Interconnection (OSI) § Developed by International Standard Organization (ISO) as a first step towards international standardization § De jure protocol § Deals with interconnecting systems that are open for communication with other systems § Open protocol suite § Good as theoretical model, but not widely implemented in practice Spring 2006 Computer Networks 7

The OSI layers § Session layer § Provides the control structure for communication between applications (dialog control) § Establishes, manages and terminate connections (sessions) between cooperating applications § Presentation layer § Provides independence to the application processes from differences in data representation § Application layer § Provides access to the OSI environment for users and provides distributed information services Spring 2006 Computer Networks 8

The OSI layers § Physical layer § Transmission of unstructured bit stream § Deals with the mechanical, electrical, functional and procedural characteristics to access the physical medium § Data link layer § Provides reliable transfer across the physical link between two ends connected via single link § Sends blocks of data (frames) with the necessary synchronization, error control and flow control § Can add header and trailer Spring 2006 Computer Networks 9

The OSI layers § Network layer § Provides upper layers with independence from the data transmission and switching technologies accross internetwork § Responsible for source-to-destination delivery, addressing and routing in the internetwork § Transport layer § Provides transparent transport of data between end points that might not be connected via single link § Provides source-to-destination connection, error recovery and flow control Spring 2006 Computer Networks 10

Protocol Suites § Open System Interconnection (OSI) § Today used mostly as a reference model § Prevously used in X. 25 based protocols § Internet (TCP/IP) § Most popular suite today § § § Xerox Networking Sysytems (XNS) System Network Architecture (SNA – IBM) Digital Network Architecture (DNA – DEC) Net. BIOS (Software interface) Apple. Talk Spring 2006 Computer Networks 11

The TCP/IP five layer model Spring 2006 Computer Networks 12

TCP/IP-modellen Exempel: SMTP, HTTP TCP, UDP IP Ethernet Spring 2006 Computer Networks 13

TCP/IP Protocol Suite § De facto (and after that de jure) standards § Open (All modification and newly proposed protocols are published in a form of RFC (Request for Comments) § RFC as well as drafts are published on the Internet § can be found on many URL (one is www. rfc-editor. org) § RFC becomes a standard when it is: § Stable and well understood § Technically competent § Implemented on multiple independent places Spring 2006 Computer Networks 14

The TCP/IP Protocol Suite (Cont. ) § Allows computers of many sizes, vendors and operating systems to communicate with each other § History: § Developed as de facto standard before OSI § 1960’s: started as goverment financed research project § 1990’s: most widely used form of networking § Forms the basis for the Internet (capital “I”) (a WAN that spans the globe) Spring 2006 Computer Networks 15

Protocols Construction versus Reduction DATA Layer 5 Construction H 5 DATA H 4 DATA UNIT Layer 4 Layer 3 Layer 2 Physical H 3 H 2 Layer 5 Layer 4 DATA UNIT Layer 3 DATA UNIT BITS T 2 Layer 2 Physical Reduction H – header (pakethuvud): control data added at the front end of the data unit T – trailer (svans): control data added at the back end of the data unit Trailers are usually added only at layer 2 Spring 2006 Computer Networks 16

Illustration of the Construction and Reduction Process § Observe how headers and trailer are added at the sender and removed at the receiver Animation of Figure 2. 4 in the book Spring 2006 Computer Networks 17

Figure 2. 4 Spring 2006 An exchange using the Internet model Computer Networks 18

Figure 2. 3 Spring 2006 Peer-to-peer processes Computer Networks 19

An Example of Five Layers Network Machine 1 Layer 5 Layer 4/5 interface Layer 4 Layer 3/4 interface Layer 3 Layer 2/3 interface Layer 5 protocol Layer 4 protocol Layer 3 protocol Layer 2 protocol Layer 1/2 interface Layer 1 protocol Machine 2 Layer 5 Layer 4 Layer 3 Layer 2 Layer 1 Physical medium The path through which the actual transmission take place Spring 2006 Computer Networks 20

The Concept of Layers § Layer n on one machine communicates with layer n on the other machine via layer n protocol. § The communication is virtual § Peers are entities comprising the corresponding layers on different machines. § There is an interface between each pair of adjacent layers for communication with the layer above and the layer below. Spring 2006 Computer Networks 21

Peer-to-peer Processes § The processes on the two machines that communicate at a given layer are called peer-to-peer processes § At the physical layer communication is direct § At the upper layers the communication has to go down through the layers on the sender machine, than to be transmited through the physical layer and than to go back up to the same layer at the receiving machine Spring 2006 Computer Networks 22

Messages and Protocol Stacks § On the sender machine, each layer: § Accepts an outgoing message from the layer above § Adds a header and does other processing § Passes resulting message to next lower layer § On the receiver, each layer: § Receives an incoming message from the layer below § Removes the header for that layer and performs other processing § Passes the resulting message to the next higher layer Spring 2006 Computer Networks 23

Illustration of the Source-to-end Delivery at the Network Layer § Observe how data are sent from node to reach the final destination. Animation of Figure 2. 11 in the book Spring 2006 Computer Networks 24

Figure 2. 5 Spring 2006 Physical layer Computer Networks 25

Note: The physical layer is responsible for transmitting individual bits from one node to the next. Spring 2006 Computer Networks 26

Figure 2. 6 Spring 2006 Data link layer Computer Networks 27

Note: The data link layer is responsible for transmitting frames from one node to the next. Spring 2006 Computer Networks 28

Figure 2. 7 Spring 2006 Node-to-node delivery Computer Networks 29

Example 1 In Figure 2. 8 a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link. At the data link level this frame contains physical addresses in the header. These are the only addresses needed. The rest of the header contains other information needed at this level. The trailer usually contains extra bits needed for error detection Spring 2006 Computer Networks 30

Figure 2. 8 Spring 2006 Example 1 Computer Networks 31

Figure 2. 9 Spring 2006 Network layer Computer Networks 32

Note: The network layer is responsible for the delivery of packets from the original source to the final destination. Spring 2006 Computer Networks 33

Figure 2. 10 Spring 2006 Source-to-destination delivery Computer Networks 34

Example 2 In Figure 2. 11 we want to send data from a node with network address A and physical address 10, located on one LAN, to a node with a network address P and physical address 95, located on another LAN. Because the two devices are located on different networks, we cannot use physical addresses only; the physical addresses only have local jurisdiction. What we need here are universal addresses that can pass through the LAN boundaries. The network (logical) addresses have this characteristic. Spring 2006 Computer Networks 35

Figure 2. 11 Spring 2006 Example 2 Computer Networks 36

Figure 2. 12 Spring 2006 Transport layer Computer Networks 37

Note: The transport layer is responsible for delivery of a message from one process to another. Spring 2006 Computer Networks 38

Figure 2. 12 Spring 2006 Reliable process-to-process delivery of a message Computer Networks 39

Example 3 Figure 2. 14 shows an example of transport layer communication. Data coming from the upper layers have port addresses j and k (j is the address of the sending process, and k is the address of the receiving process). Since the data size is larger than the network layer can handle, the data are split into two packets, each packet retaining the port addresses (j and k). Then in the network layer, network addresses (A and P) are added to each packet. Spring 2006 Computer Networks 40

Figure 2. 14 Spring 2006 Example 3 Computer Networks 41

Figure 2. 15 Spring 2006 Application layer Computer Networks 42

Note: The application layer is responsible for providing services to the user. Spring 2006 Computer Networks 43

Figure 2. 16 Spring 2006 Summary of duties Computer Networks 44