Ch2_COM204-Computer-Networks PDF

COM204 Computer Networks Chapter 2 Network Models Prof. Mahmoud Elmesalawy Electronics and Communication Engineering Department Faculty of Engineering Helwan University  Chapter 2: Network Models 2.1 Layered Archit...

COM204 Computer Networks Chapter 2 Network Models Prof. Mahmoud Elmesalawy Electronics and Communication Engineering Department Faculty of Engineering Helwan University  Chapter 2: Network Models 2.1 Layered Architecture 2.2 OSI Reference Model Architecture 2.2.1 OSI Model and its Objectives 2.2.2 Seven Layers of the OSI Model 2.2.3 Peer-to-peer Processes or Communications 2.2.4 Encapsulation/Decapsulation Process 2.2.5 Functions of each Layer in OSI Model 2.3 TCP/IP (DoD) Model and TCP/IP Protocol Suite 2.3.1 Creation and Development of TCP/IP Model 2.3.2 TCP/IP Model Vs. OSI Model 2.3.3 TCP/IP Protocol Suite 2.1 Layered Architecture The Concept of Layers: We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office as shown in Fig. 2.1. Layered Tasks: 1. Hierarchy - Higher Layer (Sender and Receiver ) - Middle Layer (Carrier mailbox  post office) - Lower Layer (Carrier post office  other post office) 2. Services: - Each layer uses the services of the layer immediately below it. Reference Models is framework (guideline) for network implementation and troubleshooting. It divides complex functions into simpler components. Importance of reference model: - Vendor interoperability “standardization”. - Better understanding of data transfer. 2.1 Layered Architecture Fig. 2.1 Layered Tasks: Sender, Receiver and Carrier. 2.2 OSI Reference Model Architecture 2.2.1 OSI model and its objectives In 1947, the International Standards Organization (ISO) was established which is a multinational body dedicated to worldwide agreement on international standards. Open Systems Interconnection (OSI) model is an ISO standard that covers all aspects of network communications. It was first introduced in the late 1970s. Note that: ISO is an organization and OSI is a model. OSI Describes a seven-layer reference model for a network architecture. The purpose of the reference model was to provide a framework for the development of protocols. Open System: A set of protocols that would allow any two different devices to communicate regardless of their underlying architecture. The OSI model is composed of seven layers as shown in Fig. 2.2: 1. Physical (layer 1) 2. Data link (layer 2) 3. Network (layer 3) 4. Transport (layer 4) 5. Session (layer 5) 6. Presentation (layer 6) 7. Application (layer 7) 2.2 OSI Reference Model Architecture (Continued) 2.2.1 OSI model and its objectives Each layer in OSI model consists of some tasks to achieve certain job in the network, but also provides a service to those layers above and below it. Designer identified which networking functions had related uses and collected those functions into discrete groups that became the layers. Layering provides flexibility for modifying and evolving protocols and services without having to change layers below. OSI Model Objectives: 1. It standardizes network components to allow multiple vendor development and support. 2. It allows different types of network hardware and software to communicate with each other. 3. It prevents changes in one layer from affecting other layers. 4. It divides network communication into smaller parts to make learning it easier to understand. Organizations of the layers: - Network support layers: Layers 1, 2, 3 - User support layers: Layer 5, 6, 7 that allow interoperability among unrelated software systems. - Transport layer (Layer 4): links the two subgroups 2.2 OSI Reference Model Architecture (Continued) 2.2.2 Seven layers of the OSI model Fig. 2.2 Seven layers of the OSI model. 2.2 OSI Reference Model Architecture (Continued) 2.2.3 Peer-to-peer Processes or Communications Layer n in one machine interacts with layer n in another machine to provide a service to layer n+1 as shown in Fig. 2.3. The entities comprising the corresponding layers on different machines are called peer processes. Fig. 2.3 The interaction between layers in the OSI model. 2.2 OSI Reference Model Architecture (Continued) 2.2.4 Encapsulation / Decapsulation Encapsulation is a process of adding control information as it passes down through the layered model. The data portion of a packet at level N-1 carries the whole packet from level N in addition to Headers and/or Trailers for this level as shown in Fig. 2.4. Data Application AH Data Application Presentation PH AH Data Presentation Session SH PH AH Data Session Transport Transport TH SH PH AH Data Network Network NH TH SH PH AH Data Data Link Data Link DH NH TH SH PH AH Data CRC Physical Bit stream Physical Fig. 2.4(a) Encapsulation in the OSI model. 2.2 OSI Reference Model Architecture (Continued) 2.2.4 Encapsulation / Decapsulation Decapsulation is a process of taking out control information as it passes up through the layered model as shown in Fig. 2.4. Fig. 2.4(b) Encapsulation/decapsulation in the OSI model. 2.2 OSI Reference Model Architecture (Continued) 2.2.4 Encapsulation / Decapsulation Protocol Data Unit (PDU): The form that a piece of data takes at any layer. At each stage of the process, a PDU has a different name to reflect its new appearance. PDUs are named according to the protocols of the TCP/IP suite: transmitting data over the medium. Data: The general term for the PDU used at the Application layer. Segment: Transport Layer PDU. Packet: Network Layer PDU. Frame: Data link Layer PDU. Bits: A PDU used when physically Fig. 2.4(c) Encapsulation in the OSI model. 2.2 OSI Reference Model Architecture (Continued) 2.2.5 Functions of each Layer in OSI Model Layer 7 - The Application Layer 7 Application The application layer is responsible for providing services to the user 6 Presentation which is used to represent a user interface to the network. 5 Session Examples: 4 Transport Email services (SMTP, POP3) Web browsing services (HTTP/HTTPs) 3 Network File transfer services (FTP) Remote access services (Telnet, SSH) 2 Data Link Domain Name Service (DNS) 1 Physical 2.2 OSI Reference Model Architecture (Continued) 2.2.5 Functions of each Layer in OSI Model Layer 7 - The Application Layer Protocols  Hypertext Transfer Protocol (HTTP) – used to transfer files that make up the Web pages of the World Wide Web.  File Transfer Protocol (FTP) - used for interactive file transfer between systems.  Trivial File Transfer Protocol (TFTP) - used for connectionless active file transfer.  Simple Mail Transfer Protocol (SMTP) - used for the transfer of mail messages and attachments.  Post Office Protocol (POP) - used by email clients to retrieve email from a remote server.  Internet Message Access Protocol (IMAP) – another protocol for email retrieval.  Domain Name Service Protocol (DNS) – used to resolve Internet names to IP addresses.  Telnet – a terminal emulation protocol used to provide remote access to servers and networking devices.  Bootstrap Protocol (BOOTP) – a precursor to the DHCP protocol, a network protocol used to obtain IP address information during bootup.  Dynamic Host Control Protocol (DHCP) – used to assign an IP address, subnet mask, default gateway and DNS server to a host. 2.2 OSI Reference Model Architecture (Continued) 2.2.5 Functions of each Layer in OSI Model Layer 6 - The Presentation Layer 7 Application The presentation layer provides for common representation of the data 6 Presentation transferred between application layer services 5 Session Coding and conversion of application 4 Transport layer data (ASCII, AVI,JPG,….) Data compression/decompression 3 Network Data encryption/decryption 2 Data Link 1 Physical 2.2 OSI Reference Model Architecture (Continued) 2.2.5 Functions of each Layer in OSI Model Layer 5 - The Session Layer 7 Application The session layer provides services to the presentation layer to organize its 6 Presentation dialogue and to manage data exchange. 5 Session Establishment, management, and 4 Transport termination of the session. 3 Network Dialogue Control (half-duplex, full- duplex). 2 Data Link Synchronization (add checkpoints or 1 Physical synchronization points to a stream of data). 2.2 OSI Reference Model Architecture (Continued) 2.2.5 Functions of each Layer in OSI Model Layer 4 - The Transport Layer Transport layer is responsible for providing reliable and efficient transport of packets (End-to-End) 7 Application Connection control (connectionless or 6 Presentation connection oriented) Service port addressing 5 Session Services multiplexing Segmentation and reassembly 4 Transport Sequencing and segments reordering End-to-end error control 3 Network End-to-end flow control 2 Data Link Examples: TCP (Transmission Control rotocol) 1 Physical UDP (User Datagram Protocol) 2.2 OSI Reference Model Architecture (Continued) 2.2.5 Functions of each Layer in OSI Model Layer 4 - The Transport Layer Provide a reliable and efficient network connection. Port numbers enable services multiplexing. Fig. 2.5 Function the Transport Layer. 2.2 OSI Reference Model Architecture (Continued) 2.2.5 Functions of each Layer in OSI Model Layer 3 - The Network Layer 7 Application The network layer is responsible for the delivery of individual packets from the 6 Presentation source host to the destination host. 5 Session Logical addressing - EX: IPv4, IPv6, IPX, APPLETALK 4 Transport Routing (choose the best path to 3 Network destination.) - EX: RIP, OSPF, IS-IS, EIGRP, BGP 2 Data Link 1 Physical 2.2 OSI Reference Model Architecture (Continued) 2.2.5 Functions of each Layer in OSI Model Layer 3 - The Network Layer The network layer is responsible for the delivery of individual packets from the source host to the destination host. Fig. 2.6 Function of network Layer. 2.2 OSI Reference Model Architecture (Continued) 2.2.5 Functions of each Layer in OSI Model Layer 2 - The Data Link Layer 7 Application The data link layer is responsible for moving frames from one hop (node) to 6 Presentation the next (Hop-to-hop or node-to-node delivery) 5 Session Framing 4 Transport Physical Addressing Ex: (MAC Address in Ethernet). 3 Network Hop-to-hop error control Hop-to-hop flow control 2 Data Link Access Control 1 Physical Ex: HDLC, PPP and Ethernet Protocols 2.2 OSI Reference Model Architecture (Continued) 2.2.5 Functions of each Layer in OSI Model Layer 2 - The Data Link Layer The data link layer is responsible for moving frames from one hop (node) to the next. Its major duties are: - Framing - Physical addressing - Flow control - Error control - Access control Fig. 2.7 Function of data Link Layer. 2.2 OSI Reference Model Architecture (Continued) 2.2.5 Functions of each Layer in OSI Model Layer 1 - The Physical Layer The physical layer is responsible for movements of individual bits from one hop (node) to the next. 7 Application Physical layer is responsible for all 6 Presentation Physical properties of the network to transmit a bit stream over a physical 5 Session medium: 4 Transport Physical characteristics of interfaces and medium. 3 Network Representation of bits. Data rate: transmission rate. 2 Data Link Synchronization of bits. Line configuration. 1 Physical Physical topology. 2.2 OSI Reference Model Architecture (Continued) 2.2.5 Functions of each Layer in OSI Model Layer 1 - The Physical Layer Transfers bits across link. Transmission Medium: - Twisted-pair cable - Coaxial cable - Optical fiber - radio - Satellite, … - Infrared Aspects of a communications link: - Mechanical: cable, plugs, pins... - Electrical/optical: modulation, signal strength, voltage levels, bit times, … Fig. 2.8 Function of the physical layer. 2.3 TCP/IP (DoD) Model and TCP/IP Protocol Suite 2.3.1 Creation and Development of TCP/IP Model The first packet switching network and predecessor to today’s Internet was the Advanced Research Projects Agency Network (ARPANET), which came to life in 1969 by connecting mainframe computers at four locations. ARPANET was funded by the U.S. Department of Defense for use by universities and research laboratories. Bolt, Beranek and Newman (BBN) was the contractor that did much of the initial development of the ARPANET, including creating the first router known as an Interface Message Processor (IMP). In 1973, Robert Kahn and Vinton Cerf began work on TCP to develop the next generation of the ARPANET. TCP was designed to replace ARPANET’s current Network Control Program (NCP). In 1978, TCP was divided into two protocols: TCP and IP. Later, other protocols were added to the TCP/IP suite of protocols including Telnet, FTP, DNS, and many others. Protocol suite is a group of inter-related protocols that are necessary to perform a communication function. TCP/IP Model/Protocol Suite is an open standard. No one company controls it. It is governed by Internet Engineering Task Force (IETF) Working Groups with standards proposed using Request for Comments (RFCs).  2.3 TCP/IP (DoD) Model and TCP/IP Protocol Suite 2.3.2 TCP/IP Model Vs. OSI Model The layers in the TCP/IP protocol suite do not exactly match those in the OSI model. The original TCP/IP protocol suite was defined as shown in Fig. 2.9 as having four layers: 1. Network Access Layer. 2. Internet Layer. 3. Transport Layer. 4. Application Layer. Fig. 2.9 Layers with TCP/IP and OSI Model. 2.3 TCP/IP (DoD) Model and TCP/IP Protocol Suite 2.3.3 TCP/IP Protocol Suite Fig. 2.10 TCP/IP Protocol Suite. 2.3 TCP/IP (DoD) Model and TCP/IP Protocol Suite 2.3.3 TCP/IP Protocol Suite Fig. 2.11 TCP/IP Protocol Suite. 2.3 TCP/IP (DoD) Model and TCP/IP Protocol Suite 2.3.3 TCP/IP Protocol Suite Fig. 2.11 TCP/IP Protocol Suite.