Content Page No. Unit I Lesson 1. Introduction to Digital Communication Lesson 2. WAN Technology Lesson 3. History about TCP/IP

Transcription

1 Content Page No. Unit I Lesson 1. Introduction to Digital Communication 1.0 Aims and Objectives Introduction to Data Communication Classification of Computer Networks Topology Protocol Internetworking Technologies Let Us Sum Up Lesson-end Activity Points for Discussion Check your Progress Reference 7 Lesson 2. WAN Technology 2.0 Aims and Objectives Introduction WAN Networking Devices WAN Protocols Let us Sum Up Lesson end Activities Points for Discussion Reference 12 Lesson 3. History about TCP/IP 3.0 Aims and Objectives Introduction Internet Society Request For Comments (RFC) Let Us Sum Up Lesson End Activities Points for Discussion Check your progress Reference 16 Lesson 4. TCP/IP layer Architecture 4.0 Aims and Objectives Introduction Internet Architecture TCP/IP layer Architecture Let us Sum Up Lesson end Activities Points for Discussion 23

2 4.7 Check your progress Reference 23 Lesson 5. Network Interface 5.0 Aims and Objectives Introduction Network Interface Card Cable Type and Specification Type Other Protocols Let us Sum Up Lesson end Activities Points for Discussion Check you Progress Reference 29 Unit II Lesson 6. IP Addressing Scheme 6.0 Aims and Objectives Introduction Classful Address Special Address Classless Inter-Domain Routing (CIDR) Multihoming Let us Sum Up Lesson end Activities Points for Discussion Check your progress Reference 36 Lesson 7. ARP & RARP 7.0 Aims and Objectives Introduction Address Resolution Protocol (ARP) Reverse Address Resolution Protocol (RARP) let us Sum Up Lesson end Activities Points for Discussion Check your progress Reference 45 Lesson 8. Subnet Address and Multicast Address 8.0 Aims and Objectives Introduction Subnet Mask Multicast Addressing Let us Sum up 50

3 8.5 Lesson end Activities Points for Discussion Check your progress Reference 51 Lesson 9. Bootstrap and DHCP protocol 9.0 Aims and Objectives Introduction DHCP Let Us Sum Up Lesson and Activities Points for Discussion Check your progress Reference 54 Lesson 10. Domain Name Service (DNS) 10.0 Aims and Objectives Introduction DNS protocol Let us Sum Up Lesson end Activities Points for Discussion Check Your Progress Reference 61 Unit III Lesson 11. Internet Protocol (IP) 11.0 Aims and Objectives Introduction Datagrams Let us Sum Up Lesson end Activities Points for Discussion Check your Progress Reference 65 Lesson 12. The Internet Control Message Protocol (ICMP) 12.0 Aims and Objectives Introduction ICMP ICMP Messages Let us Sum Up Lesson end Activities 73

4 12.6 Points for Discussion Check your progress Reference 73 Lesson 13. Fragmentation and Reassembly Aims and Objectives Introduction Fragmentation and Defragmentation Let us Sum Up Lesson end Activities Points for Discussion Check your Progress Reference 77 Lesson 14. Routing 14.0 Aims and Objectives Introduction Routing Routing Protocol Vs Routed Protocol Types of routing table entries Classification of routing table Let us Sum Up Lesson end Activities Points for Discussion Check your Progress Reference 83 Lesson 15. Distance Vector Protocol and Link State Routing Protocols 15.0 Aims and Objectives Introduction Distance Vector Routing Protocols Link State Routing Protocols Routing Information Protocol (RIP) Open Shortest Path First (OSPF) Let us Sum Up Lesson end Activities Points for Discussion Check your Progress Reference 88

5 Unit IV Lesson 16. TCP and UDP Data Structures 16.0 Aims and Objectives Introduction TCP Data Structures UDP Data Structures Let us Sum Up Lesson end Activities Points for Discussion Check your Progress Reference 95 Lesson 17. TCP Finite State Machine 17.0 Aims and Objectives Introduction Algorithm for TCP state machine TCP finite state machine Let us Sum Up Lesson end Activities Points for Discussion Check your progress Reference 98 Lesson 18. TCP Connection Management 18.0 Aim and Objectives Introduction Connection Extablishment Connection Release Let us Sum Up Lesson end Activities Points for Discussion Reference 102 Lesson 19. TCP Output Processing 19.0 Aims and Objectives Introduction Timer Management Packet Loss and Retransmission Adaptive Retransmission 106

6 19.5 Flow Control TCP Keep-Alive Messages Slow Start Algorithm and Congestion Avoidance Silly Window Syndrome (SWS) Nagle Algorithm TCP Selective Acknowledgment Let us Sum Up Lesson and Activities Points for Discussion Check your Progress Reference 111 Lesson 20. Socket Programming 20.0 Aims and Objectives Introduction Domain Internet address conversion Connection less (UDP) Communication Connection oriented (TCP) Communication Let us Sum Up Lesson end Activities Points for Discussion Check your progress Reference 125 Lesson 21. Application Layer 21.0 Aims and Objectives Introduction Telnet Rlogin Let us Sum Up Lesson end Activities Points for Discussion Check your Progress Reference 135 Lesson 22. File Transfer Protocol 22.0 Aims and Objectives Introduction File transfer protocol (FTP) Trivial File Transfer protocol (TFTP) 140

7 22.4 Let us Sum Up Lesson end Activities Point for Discussion Check your Progress Reference 143 Lesson Aims and Objectives Introduction Electronic Mail Addresses Scheme TCP/IP Standard for Electronic Mail Services Simple Mail Transfer Protocol (SMTP) Post Office Protocol (POP3) Internet Message Access Protocol (IMAP) Let us Sum Up Lesson end Activities Points for Discussion Check your progress Reference 152 Lesson 24. Network Management System 24.0 Aims and Objectives Introduction Simple Network Management Protocol Let us Sum Up Lesson and Activities Point for Discussion Check Your Progress Reference 157 Lesson 25. X.25 Protocol 25.0 Aims and Objectives Introduction X.25 layers Let us Sum Up Lesson end Activities Points for Discussion Check Your Progress Reference 163

8 Unit I Lesson 1. Introduction to Digital Communication 1.0 Aims and Objectives 1.2 Introduction to Data Communication 1.2 Classification of Computer Networks 1.3 Topology 1.4 Protocol 1.5 Internetworking Technologies 1.6 Let Us Sum Up 1.7 Lesson-end Activity 1.8 Points for Discussion 1.9 Check your Progress 1.10 References 1.0 Aims and Objectives To learn the concepts which is essential for Computer Networks 1.1 Introduction to Data Communication The term telecommunication means communication at a distance. The word data refers to information presented in whatever form is agreed upon by the parties creating and using the data. Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable. Computer Network A network is a set of devices (often referred to as nodes) connected by communication links. A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network. Software modules in one system are used to communicate with one or more software modules in the distance System. Such interfaces across a distance are termed as peer-to-peer interfaces; and the local interfaces are termed as service interfaces. The modules on each end are organized as a sequence of functions called layers. The set of modules organized as layers is also commonly called a protocol stack.

9 Over the years, some layered models have been standardized. The ISO Open Systems Interconnection (ISO/OSI) layered model has seven layers and was developed by a set of committees under the auspices of International Standards Organization (ISO). 1.2 Classification of Computer Networks Based on Transmission Mode Transmission mode defines the direction of signal flow between two linked devices. There are three types of transmission modes. Simplex In simplex mode, the communication is unidirectional. Among the stations only one can transmit and the other can only receive. Half-Duplex In half-duplex mode, the communication is bidirectional. In this both station can sent and receive but not at the same time. Full-Duplex In Full-Duplex mode, both stations can transmit and receive simultaneously. Based on Time in Transmission Type Synchronous Transmission In synchronous Transmission both the sender and the receiver use the same time cycle for the transmission. We send bits one after another without start/stop bits or gaps. It is the responsibility of the receiver to group the bits. Bit stream is delivered with a fixed delay and given error rate. Each bit reaches the destination with the same time delay after leaving the source. Asynchronous Transmission In Asynchronous Transmission we send one start bit at the beginning and one stop bit at the end of each byte. There may be a gap between each byte. Bit stream is divided into packets. Packets are received with varying delays, so packets can arrive out of order. Some packets are not received correctly. Based on Authentication Peer to Peer Connection In peer-to-peer networks, there are no dedicated servers. All the computers are equal and, therefore, are termed as peers. Normally, each computer functions as both a client and a server. No one can control the other computers.

10 Server Based Connection Most networks have a dedicated server. A dedicated server is a computer on a network which functions as a server, and cannot be used as a client or a workstation. A dedicated server is optimized to service requests from network clients. A server can control the clients for its services. Based on Geographical location LAN (Local Area Network) Networks which cover close geographical area. LAN used to link the devices in a single office, building or campus. It provides high speeds over short distance. Systems are connecting directly to Network. The LAN is owned by private people. MAN (Metropolitan Area Network) Metropolitan area network is an extension of local area network to spread over the city. It may be a single network or a network in which more than one local area network can share their resources. WAN (Wide Area Network) WAN spread over the world may be spread over more than one city country or continent. Systems in this network are connected indirectly. Generally WAN network are slower speed than LAN s. The WAN network are owned or operated by network providers. If it is owned by a single owner then it is called Enterprise network. Often these types have combination of more than one topology. Based on Reliability Reliability is maintained by authentication. Connection-oriented This type of communication establishes a session connection before data can be sent. This method is often called a "reliable" network service. It can guarantee that data will arrive in the same order. Connection less This type of communication does not require a session connection between sender and receiver for data transfer. The sender simply starts sending packets to the destination. A connectionless network provides minimal services.

11 1.3 Topology Topology refers to physical layout including computers, cables, and other resources; it determines how components communicate with each other. Today s network designs are based on three topologies: Bus consists of series of computers connected along a single cable segment Star connects computers via central connection point or hub Ring connects computers to form a loop All computers, regardless of topology, communicate by addressing data to one or more computers and transmitting it across cable as electronic signals. Data is broken into packets and sent as electronic signals that travel on the cable. Only the computer to which the data is addressed accepts it. 1.4 Protocol Protocols mean set of rules. It is a formal description of message formats and the rules two or more machines has follow to exchange messages. The key elements of a protocol are syntax, semantics and timing. Syntax Syntax refers to the structure or format of the data, meaning the order in which they are presented. Semantics Semantics refers to the meaning of each section of bits. Timing Timing refers to when data should be sent and how fast it can be sent. 1.5 Internetworking Technologies Internetworking Technologies tell how the Internet accommodating multiple underlying hardware technologies and how they are interconnected and formed the network, and set of communication standard which the network used to interoperate. The lowercase internet means multiple networks connected together, using a common protocol suite. The uppercase Internet refers to the collection of hosts around the world that can communicate with each other using TCP/IP. While the Internet is an internet, the reverse is not true.

12 1.6 Let us Sum up Data Communication The term telecommunication means communication at a distance. Computer Network links. A network is a set of devices (often referred to as nodes) connected by communication Classification of Computer Networks Based on Transmission Mode Simplex In simplex mode, the communication is unidirectional. Half-Duplex In half-duplex mode, the communication is bidirectional. Full-Duplex In Full-Duplex mode, both stations can transmit and receive simultaneously. Based on Transmission Mode Synchronous Transmission Each bit reaches the destination with the same time delay after leaving the source. Asynchronous Transmission Packets are received with varying delays, so packets can arrive out of order. Some packets are not received correctly. Based on Authentication Peer to Peer Connection In peer-to-peer networks, there are no dedicated servers. No one can control the other computers. Server Based Connection A dedicated server is optimized to service requests from network clients. A server can control the clients for its services. Based on Geographical location LAN (Local Area Network) Networks which cover close geographical area MAN (Metropolitan Area Network) Metropolitan area network is an extension of local area network to spread over the city. WAN (Wide Area Network) WAN spread over the world may be spread over more than one city country or continent.

13 Based on Reliability Connection-oriented It can guarantee that data will arrive in the same order. Connection less This type of communication does not require a session connection between sender and receiver for data transfer. The sender simply starts sending packets to the destination. Topology Topology refers to physical layout including computers, cables, and other resources Bus consists of series of computers connected along a single cable segment Star connects computers via central connection point or hub Ring connects computers to form a loop Protocol Protocols mean set of rules Syntax Syntax refers to the structure or format of the data, meaning the order in which they are presented. Semantics Semantics refers to the meaning of each section of bits. Timing Timing refers to when data should be sent and how fast it can be sent. Internetworking Technologies Network of Inter connected Networks. 1.7 Lesson-end Activities 1. What is telecommunication? 2. What is Computer Network? 1.8 Points for Discussion 1. What are the classifications of Computer Networks.? 1.9 Check you Progress 1. What is topology? Write short notes on different topology. What are the advantages and disadvantages of this topology? 2. What is protocol? What are the elements of protocol?

14 1.10 References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

15 Lesson 2. WAN Technology 2.0 Aims and Objectives 2.1 Introduction 2.2 WAN Networking Devices 2.3 WAN Protocols 2.4 Let us Sum Up 2.5 Lesson end Activities 2.6 Points for Discussion 2.7 References 2.0 Aims and Objectives To learn the devices which are used in WAN To learn the Protocols which are used in WAN 2.1 Introduction LAN (Local Area Network) network spread for close geographical area. It provides high speeds over short distance. In this network all the systems must connect directly to network. In WAN (Wide Area Network) the network can be far apart. In this network systems are connected indirectly. This network generally slower speed than LAN s. Another way to connect networks is with a bridge. These connect networks at the link layer, while routers connect networks at the network layer. Bridges makes multiple LANs appear to the upper layers as a single LAN. 2.2 WAN Networking Devices Repeaters A repeater is a device that regenerates signals so that the signal can travel on addition cable segments. They do not translate or filter data. Repeater is used to connect two networks that use the same technology. It receives every data packet on each network, and retransmits it onto the other network. The net result is that the two networks have exactly the same set of packets on them. Its primary purpose is to get around limitations in cable length caused by signal loss or timing dispersion. For a repeater to function, both segments which the repeater joins must have the same media access scheme, protocol and transmission technique. Repeaters can move packets from one medium to another. Some multiport repeaters can connect different types of media. Repeaters improve performance by dividing the network into segments, thus reducing the number of computers per segment.

16 Bridge Bridge is a device that can join two LANs. However, bridge can also divide an overloaded network into separate networks, reducing the traffic on each segment and making each network more efficient. A bridge can link unlike physical media such as twisted-pair and coaxial Ethernet. It can also link unlike network segments such as Ethernet and Token Ring. A bridge can be installed internally or if the destination address is not listed in the routing table, the bridge forwards the packets to all segments. Multiple bridges can be used to combine several externally. Bridges are faster than routers because routers perform complex functions on each packet. Switches Switches allow different nodes of a network to communicate directly with each other in a smooth and efficient manner. Switches are divided into two types Store and Forward and Cut Through. Store and Forward switches stores the details and forwarded to the respective system. In the Cut through switches it just forward the details to the respective systems. Routers A router is a device used to connect networks that use different architectures and protocols. They can switch and transfer information packets across multiple networks. This process is called routing. They can determine the best path for sending data and filters broadcast traffic, to the local segment. Routers cannot link to remote computers. They can read only addressed network packets. Routers can link segments that use different data packaging and media schemes. Gateways Gateways make communication possible between systems that use different communication protocols, data formatting structures, languages and architectures. Gateways repackage data going from one system to another. Gateways are usually dedicated servers on a network and are task-specific.

17 2.3 WAN Protocols Frame Relay Frame relay is used to connect large number of sites in the network because it is relatively inexpensive to do so. The service provider gives you a frame relay circuit and is charged for the amount of data and the bandwidth you use as oppose to T1 circuit that charges with a flat monthly rate whether you use partial bandwidth or the full bandwidth regardless. Frame relay is a high performance WAN protocol that operates at the Data Link layer and the Physical layer of the OSI model. Integrated Services Digital Network (ISDN) Integrated Services Digital Network (ISDN) is designed to run over existing telephone networks. It can deliver end to end digital service carrying voice and data. ISDN operates at OSI model, physical layer, data link layer and network layer. It can carry multimedia and graphics with all other voice, data services. ISDN supports all upper layer protocols and you can choose PPP, HDLC or LAPD as your encapsulation protocol. It has two offerings, Primary rate which is 23B+D channels. 23, 64 kbps and one 64kbps mainly used for signaling. The other is the Basic Rate which has 2B+D channels two 64kbps and one 16kbps. At data link layer ISDN supports two protocols; LAPB and LAPD. LAPB is used to mainly transfer data from upper layers and has three types of frames. I-Frames carry upper layer information and carries out sequencing, flow control, error detection and recovery. S- Frames carry control information for the I-frame. LAPD provides an additional multiplexing function to the upper layers enabling number of network entities to operate over a single physical access. Each individual link procedure acts independently of others. The multiplex procedure combines and distributes the data link channels according to the address information of the frame. Each link is associated with a specific Service Access Point (SAP), which is identified in the part of the address field. High Level Data Link Control (HDLC) High Level Data Link Control (HDLC) is a bit oriented data link layer frame protocol that has many versions similar to LAP, LAPB, and LAPD. CISCO routers default encapsulation is HDLC, but it is proprietary to CISCO.

18 Point to Point Protocol (PPP) Point to Point Protocol (PPP) is a Data Link Layer protocol that can be used over ether asynchronous (dial up) or synchronous (ISDN) lines. It uses Link Control Protocol (LCP) to build and maintain data link connections. Included in PPP is the authentication protocols, PAP and CHAP, and data compression. It supports IP, IPX, AppleTalk, DECnet and OSI/CLNS. 2.4 Let us Sum Up WAN Networking Devices Repeaters A repeater is a device that regenerates signals so that the signal can travel on addition cable segments. Bridge Bridge is a device that can join two LANs. Switches Switches allow different nodes of a network to communicate directly with each other in a smooth and efficient manner. Routers A router is a device used to connect networks that use different architectures and protocols. Gateways Gateways make communication possible between systems that use different communication protocols, data formatting structures, languages and architectures. WAN Protocols Frame Relay Frame relay is used to connect large number of sites in the network because it is relatively inexpensive to do so. The service provider gives you a frame relay circuit and is charged for the amount of data and the bandwidth you use as oppose to T1 circuit that charges with a flat monthly rate whether you use partial bandwidth or the full bandwidth regardless.

19 Integrated Services Digital Network (ISDN) Integrated Services Digital Network (ISDN) is designed to run over existing telephone networks. High Level Data Link Control (HDLC) High Level Data Link Control (HDLC) is a bit oriented data link layer frame protocol that has many versions similar to LAP, LAPB, and LAPD. Point to Point Protocol (PPP) Point to Point Protocol (PPP) is a Data Link Layer protocol that can be used over ether asynchronous (dial up) or synchronous (ISDN) lines. 2.5 Lesson End Activities 1. When we need Repeater and when we need Bridge? 2. How the Gateway is different from the Router? 2.6 Points for Discussion 1. What is the difference between Switch and Hub? 2.7 Check Your Progress Write short notes on o Repeater o Bridge o Switch o Router o Gateways o ISDN and HDLC 2.8 References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

20 Lesson 3. History about TCP/IP 3.0 Aims and Objectives 3.1 Introduction 3.2 Internet Society 3.3 Request For Comments (RFC) 3.4 Let Us Sum Up 3.5 Lesson End Activities 3.6 Points for Discussion 3.7 Check your progress 3.8 References 3.0 Aims and Objective To learn the history about TCP/IP 3.1 Introduction TCP/IP is a set of protocols developed to allow cooperating computers to share resources across a network. In 1969 the Defense Advanced research projects Agency (DARPA) funded a research and development project to create an experimental packet switching network. This network is called ARPANET. In 1975 the ARPANET was converted from an experimental network to an operational network, and the responsibility for administering the network was given to the Defense Communication Agency (DCA). The TCP/IP protocols were adopted as Military Standards (MIL STD) in 1983, and all hosts connected to the network were required to convert to the new protocols. DARPA funded to implement TCP/IP in Berkely Unix. In 1983, the old ARPANET was divided into MILNET and smaller ARPANET. The Internet was used to refer to the entire network; MILNET and ARPANET. Advantages of TCP/IP Open protocol standards, freely available and developed independently from any specific computer hardware or operating system. A common addressing scheme which is enable to connect the most widely used networks. It may use any protocols. It connects dissimilar systems. It provides client/server framework. It provides access to the Internet

21 Differences of the OSI and TCP/IP models TCP/IP combines the presentation and session layer into its application layer. TCP/IP combines the OSI data link and physical layers into one layer. TCP/IP appears simpler because it has fewer layers. TCP/IP transport layer using UDP does not always guarantee reliable delivery of packets as the transport layer in the OSI model does. 3.2 Internet Society The Internet Society (ISOC) is a professional society to facilitate, support, and promote the evolution and growth of the Internet as a global research communications infrastructure. 3.3 Request For Comments (RFC) Documentation about TCP/IP protocols, standards, and policies are available in the online repositories without any charge. The RFC series is numbered sequentially in the chronological order RFCs are written. Even though Internet is a common public one for organizing administrative activities they have different committees. They are : Internet Architecture Board (IAB) The Internet Architecture Board (IAB) is the technical oversight and coordination body. It is composed of about 15 international volunteers from various disciplines and serves as the final editorial and technical review board for the quality of Internet standards. The IAB falls under the ISOC. Internet Engineering Task Force (IETF) The Internet Engineering Task Force (IETF) is the near-term, standards-oriented group, divided into nine areas (applications, routing and addressing, security, etc.). The IETF develops the specifications that become Internet standards. An additional Internet Engineering Steering Group (IESG) was formed to help the IETF chair. Internet Research Task Force (IRTF) The Internet Research Task Force (IRTF) pursues long-term research projects.

22 3.4 Let us Sum Up In 1969 the Defense Advanced research projects Agency (DARPA) funded a research and development project to create an experimental packet switching network. This network is called ARPANET. In 1983, the old ARPANET was divided into MILNET and smaller ARPANET. The Internet was used to refer to the entire network; MILNET and ARPANET. Request For Comments (RFC) Documentation about TCP/IP protocols, standards, and policies are available in the online repositories without any charge. The RFC series is numbered sequentially in the chronological order RFCs are written. Advantages of TCP/IP A common addressing It may use any protocols. It connects dissimilar systems. It provides client/server framework. It provides access to the Internet Internet Architecture Board (IAB) The Internet Architecture Board (IAB) is the technical oversight and coordination body.. Internet Engineering Task Force (IETF) The Internet Engineering Task Force (IETF) is the near-term, standards-oriented group, divided into nine areas (applications, routing and addressing, security, etc.). Internet Research Task Force (IRTF) The Internet Research Task Force (IRTF) pursues long-term research projects. 3.5 Lesson end Activities 1. When the TCP/IP project started and by whom? 3.6 Points for Discussion

23 1. What are the governing bodies involved in Internet Organisations? 3.7 Check your progress 1. What is the purpose of RFC? 2. Write the difference between OSI and TCP/IP layer. 3. How Internet is organized? 3.8 References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

24 Lesson 4. TCP/IP layer Architecture 4.0 Aims and Objectives 4.1 Introduction 4.2 Internet Architecture 4.3 TCP/IP layer Architecture 4.4 Let us Sum Up 4.5 Points for Discussion 4.6 Check your progress 4.0 Aims and Objectives To learn the architecture and layer and their functions of TCP/IP 4.1 Introduction Each layer contains logical groupings of functions that provide specific services for facilitating a communication. A function, or a group of functions, making up a functional unit is a logical entity that accepts one or more inputs (arguments) and produces a single output (value) determined by the nature of the function. Functions can be grouped in a collective unit, which is then defined as (N) layer having (N+1) layer an upper layer boundary and (N-1) layer as a lower boundary. The N layer receives services from N-1 layer and provides services to N+1 layer. 4.2 Internet Architecture A few stand-alone systems were collected together into a network. People are combining multiple networks together into an internetwork, or an internet. An internet is a collection of networks that all use the same protocol suite. The easiest way to build an internet is to connect two or more networks with a router. This is often a special-purpose hardware box for connecting networks. The following diagram shows that two networks connected and form an Internet. Figure 4.1 : Simple Internet

25 Two computers, anywhere in the world, following certain hardware, software, protocol specifications, can communicate, reliably even when not directly connected. LANs are no longer scalable beyond a certain number of stations or geographic separation. 4.3 TCP/IP layer Architecture There is no standard for layers in TCP/IP. Some refers as 5 layers including physical layer and some may refer four layers. The four layered structure of TCP/IP is seen in the way data is handled as it passes down the protocol stack from the Application Layer to the underlying physical network. Each layer in the stack adds control information to ensure proper delivery. This control information is called a header because it is placed in front of the data to be transmitted. Each layer treats all of the information it received from the layer above as data and places its own header in front of that information. The addition of delivery information at every layer is called encapsulation. When data is received each layer strips off its header before passing the data on to the layer above. Each layer has its own data structures and terminology to describe that structure. In application layer the TCP data is called stream where as in the UDP it is called message. In the transport layer the data is called segment where as in the UDP it is called packet. In the Internet layer both TCP and UDP data are called as datagrams. In the network access layer both TCP and UDP data are called frame. Application Transport Internet Network A ccess Figure 4.2 TCP/IP layers

26 Network Access layer The TCP/IP Network Access layer can encompass the functions of all three lower layers of the OSI References Model (Network, Data Link and Physical). As new hardware technologies appear new Network Access protocols must be developed so that TCP/IP networks can use the new hardware. Functions Addressing scheme For this it provide a protocol called Address Resolution Protocol (ARP) defined in the RFC Transmission of IP datagram over Ethernet network This specifies how IP datagrams are encapsulated for transmission over Ethernet networks. Header Encapsulation Figure 4.3 TCP/IP Encapsulation

27 When an application sends data using TCP, the data is sent down the protocol stack, through each layer, until it is sent as a stream of bits across the network. Each layer adds information to the data by prepending headers (and sometimes adding trailer information) to the data that it receives. Figure shows this process. The unit of data that TCP sends to IP is called a TCP segment. The unit of data that IP sends to the network interface is called an IP datagram. The stream of bits that flows across the Ethernet is called a frame. Internet layer All TCP/IP communication data are flow through IP regardless of its final destination. It provides basic packet delivery service. The important protocol in this layer is Internet Protocol defined in RFC 791. Function of Internet Protocol Defining the datagram, this is the basic unit of transmission in the Internet. Defining the Internet addressing scheme. Routing datagrams to remote hosts Performing fragmentation and reassembly of datagrams IP is a connectionless protocol. IP does not exchange control information to establish an end-to-end connection before transmitting data. It also called unreliable protocol because it contains no error detection and recovery code. Routing Datagrams Gateways are devices that switch packets between the different physical networks. Deciding which gateway to use is called routing. IP makes the routing decision for each individual packet. Internet gateways are commonly referred to IP routers because they use Internet Protocol to route packets between networks.

28 Host 1 Host 2 Application Application Transport Gateway 1 Gateway 2 Transport Internet Internet Internet Internet Network Network Network Network Network A Network B Network C Figure 4.4 Routing Through Gateways The uses of gateways are to forward packets. The hosts process packets through all four protocol layers, while the gateways process the packets only up to the Internet layer where the routing decisions are made. Fragmenting datagrams Datagrams may routed through different networks. Each type of network has a Maximum Transmission Unit (MTU), which is the largest packet that it can transfer. If the datagram received from one network may be too large to be transmitted in a single packet on a different network. In this case, IP module in a gateway is to divide the datagram into smaller pieces. This process is called fragmentation. Transport layer Transport Layer has two important protocols for connection oriented and connection less services. They are TCP and UDP. TCP (Transmission Control Protocol) provides a connectionoriented, reliable, byte stream service (RFC793). TCP is an independent, general purpose protocol that can be adapted for use with delivery systems other than IP. A stream of 8-bit bytes is exchanged across a TCP connection. UDP (User Datagram Protocol) is a simple, unreliable, datagram-oriented, transport layer protocol (RFC768).

29 Application Layer Top of the TCP/IP architecture layer is Application Layer. It contains collection of services. Each service can be identified by their number called port number. Each service is defined by separate protocol and has their separate RFC. Eg. FTP. Telnet 4.4 Let us Sum Up TCP/IP layer Architecture Network Access layer - Functions Addressing scheme Transmission of IP datagram over Ethernet network Internet layer Function of Internet Protocol Defining the datagram, this is the basic unit of transmission in the Internet. Defining the Internet addressing scheme. Routing datagrams to remote hosts Performing fragmentation and reassembly of datagrams IP is a connectionless protocol Routing Datagrams Gateways are devices that switch packets between the different physical networks. Deciding which gateway to use is called routing. Fragmenting datagrams Datagrams. Each type of network has a Maximum Transmission Unit (MTU), which is the largest packet that it can transfer. If the datagram received from one network may be too large to be transmitted in a single packet on a different network. In this case, IP module in a gateway to divide the datagram into smaller pieces. This process is called fragmentation. Transport layer TCP (Transmission Control Protocol) provides a connection-oriented, reliable, byte stream service. UDP (User Datagram Protocol) is a simple, unreliable, datagram-oriented, transport layer protocol (RFC768). Application Layer Top of the TCP/IP architecture layer is Application Layer. It contains collection of services.

30 4.5 Lesson end Activities 1. What is the purpose of Internet Architecture Board? 4.6 Points for Discussion 1. What is Header Encapsulation and Decapsulation? 4.7 Check your Progress 1. What are layers available in TCP/IP? 2. What is the function of Network layer? 3. What is the function of IP layer? 4. What is the function of Transport layer? 4.8 References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

31 5.0 Aims and Objectives 5.1 Introduction 5.2 Network Interface Card 5.3 Cable type and Specification Type 5.4 Other Protocols 5.5 Let us Sum Up 5.6 Lesson end Activities 5.7 Points for Discussion 5.8 Check you Progress 5.9 References Lesson 5. Network Interface 5.0 Aims and Objectives To learn about Network interface Device and their function 5.1 Introduction The purpose of the network interface layer is it handles the details of the communication media (Ethernet, token ring, etc.) 5.2 Network Interface Card A network interface card is used to connect a computer to an Ethernet network. The card (shown in the figure below) provides an interface to the media. This may be either using an external transceiver (as shown) or through an internal integrated transceiver mounted on the network interface card PCB. The card usually also contains the protocol control firmware and Ethernet Controller needed to support the Medium Access Control (MAC) data link protocol. Figure 5.1 Network Interface Card (NIC)

32 The Ethernet Most of our networks these days use Ethernet also called Network Interface Card(NIC). Ethernet has its own Addressing scheme. No two machines have the same Ethernet address. This is a unique number issued by IEEE to the manufacturer. So each Ethernet controller comes with an address built in from the factory. The address consists of 48 bits. First 24 bits represents the vendor and next 24 bits represents the serial number of the NIC card. This hardware address is used by the Media Access Control (MAC) layer of the Data Link layer to identify uniquely, the LAN device, to the network layer. Ethernet equipment manufacturers have to register with a central authority, to make sure that the numbers they assign don't overlap any other manufacturer. Ethernet is a "broadcast medium". When you send a packet out on the Ethernet, every machine on the network sees the packet. So something is needed to make sure that the right machine gets it. Every Ethernet packet has a 14-octet header that includes the source and destination Ethernet address, and a type code. Figure 5.2 Ethernet Data format Each machine is supposed to pay attention only to packets with its own Ethernet address in the destination field. Each machine has to have a table of what Ethernet address corresponds to what Internet address. In addition to the addresses, the header contains a type code. The type code is to allow for several different protocol families to be used on the same network. So you can use TCP/IP, DECnet, Xerox NS, etc., at the same time. Each of them will put a different value in the type field. Finally, there is a checksum. The Ethernet controller computes a checksum of the entire packet. When the other end receives the packet, it recomputes the checksum, and throws the packet away if the answer disagrees with the original. The checksum is put on the end of the packet, not in the header. The original Ethernet was developed as an experimental coaxial cable network in the 1970s by Xerox Corporation to operate with a data rate of 3 Mbps using a carrier sense multiple access

33 collision detect (CSMA/CD) protocol for LANs with sporadic but occasionally heavy traffic requirements. IEEE Specification for Ethernet Ethernet LAN specification 802.3u Fast Ethernet 802.3z Gigabit Ethernet Ethernet card is an add on card which needed for the computers which connected to the network. Computers which used as a stand alone system need not have Ethernet card. The following figure shows the sample Ethernet card. Figure 5.3 Ethernet Card 5.3 Cable type and Specification Type Ethernet support different types of cables, each have their own specification and transmission distance. These are given in the following table.

34 S pecification 10BaseT 10Base2 10Base5 10BaseF 100BaseT 100BaseTX C ab le T ype U nshielded T w isted P air T hin C oaxial T hick C oaxial Fiber O ptic U nshielded T w isted P air U nshielded T w isted P air M axim u m len gth 100 m eters 185 m eters 500 m eters 2000 m eters 100 m eters 220 m eters 5.4 Other Protocols Serial Line IP (SLIP) SLIP stands for Serial Line IP. It is a simple form of encapsulation for IP datagrams on serial lines, and is specified in RFC SLIP has become popular for connecting home systems to the Internet, through the ubiquitous RS-232 serial port found on almost every computer and high-speed modems. Point-to-Point Protocol (PPP) PPP, the Point-to-Point Protocol, corrects all the deficiencies in SLIP. PPP consists of three components. A way to encapsulate IP datagrams on a serial link. PPP supports either an asynchronous link with 8 bits of data and no parity (i.e., the ubiquitous serial interface found on most computers) or bit-oriented synchronous links. A link control protocol (LCP) to establish, configure, and test the data-link connection. This allows each end to negotiate various options. A family of network control protocols (NCPs) specific to different network layer protocols. RFCs currently exist for IP, the OSI network layer, DECnet, and AppleTalk. The IP NCP, for example, allows each end to specify if it can perform header compression, similar to CSLIP. RFC 1548 [specifies the encapsulation method and the link control protocol. RFC 1332 specifies the network control protocol for IP.

35 Loopback Interface Most implementations support a loopback interface that allows a client and server on the same host to communicate with each other using TCP/IP. The class A network ID 127 is reserved for the loopback interface. By convention, most systems assign the IP address of to this interface and assign it the name localhost. An IP datagram sent to the loopback interface must not appear on any network. 5.5 Let Us Sum Up Network Interface Card A network interface card is used to connect a computer to an Ethernet network. The card (shown in the figure below) provides an interface to the media. IEEE Specification for Ethernet Ethernet LAN specification 802.3u Fast Ethernet 802.3z Gigabit Ethernet Serial Line IP (SLIP) SLIP stands for Serial Line IP. It is a simple form of encapsulation for IP datagrams on serial lines Loopback Interface Most implementations support a loopback interface that allows a client and server on the same host to communicate with each other using TCP/IP. 5.6 Lesson end Activities Discuss different type of Cable types. 5.7 Points to Discussion How Ethernet card is connected to the PC? 5.8 Check your progress 1. What is the purpose of Network Interface card? 2. How Ethernet is working?

36 (or) Explain the functionality of Ethernet with a neat figure. 3. Write Short Notes on: a) SLIP, b) PPP 5.9 References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

37 Unit II Lesson 6. IP Addressing Scheme 6.0 Aims and Objectives 6.1 Introduction 6.2 Classful Address 6.3 Special Address 6.4 Classless Inter-Domain Routing (CIDR) 6.5 Multihoming 6.6 Let us Sum Up 6.7 Lesson end Activities 6.8 Points for Discussion 6.9 Check your progress 6.10 References 6.0 Aims and Objectives To learn the TCP/IP addressing schemes 6.1 Introduction There are three types of IP addresses: unicast (destined for a single host), broadcast (destined for all hosts on a given network), and multicast (destined for a set of hosts that belong to a multicast group). IP Addressing Scheme Universal Service - all computers on all physically different networks can communicate. Physical addresses - allow communication between computers on one network. The IP address - provides virtual addressing. The address is software controlled, whereas the address for the network card is hardware based. The IP addressing scheme is quite complex, and there have been many revisions to the IP scheme. The IANA, InterNIC, and ARIN work together to subdivide and issue addresses for Internet clients. You can subdivide addresses assigned by InterNIC. Routing tables can be created manually and dynamically. IP addressing schemes allows for seamless integration amongst heterogeneous networks. To send a packet, the destination IP address of the computer is used not the hardware address. This allows for communication across networks.

38 The IP address is 32 bits in length (IPv4) and contains sequence of 1s and 0s. The IP addresses are divided into a prefix and suffix. The suffix is the host address. The prefix is the network number. The address space of IPv4 is 232 or 4,294,967, Classful Address In classful addressing, the address space is divided into five classes: A, B, C, D, and E to define large, medium, and small networks. The Class D address class was created to enable multicasting. IETF reserves Class E addresses for its own research. To make the IP address easier to use, the address is usually written as four decimal numbers separated by periods. This way of writing the address is called the dotted decimal format. Eg. Numbers through It is much better than reading Dotted-decimal notation and binary notation for an IPv4 address shown in the following diagram: Figure 6.1 Dotted decimal notation and binary notation

39 Dotted Decimal with Classes Figure 6.2 Classes in binary and dotted-decimal notation Class A: 1 prefix octet (128 networks) 3 suffix octets ( hosts) Class B: 2 prefix octets (16384 networks) 2 suffix octets (65536 hosts) Class C: 3 prefix octets ( networks) 1 suffix octet (256 hosts) Number of blocks and block size in classful IPv4 addressing shown bellow: Figure 6.3 Classful address block size No two machines that connect to a public network can have the same IP address because public IP addresses are global and standardized. A central authority exists for IP address

40 delegation. In the US, it s ARIN American Registry for Internet Numbers. People just can t arbitrarily use any IP network if their network is publicly accessible. That would lead to routing conflicts. Classful addressing, which is almost obsolete, is replaced with classless addressing. The first address in a block is normally not assigned to any device; it is used as the network address that represents the organization to the rest of the world. The first address in the block can be found by setting the rightmost 32 n bits to 0s. The last address in the block can be found by setting the rightmost 32 n bits to 1s. The number of addresses in the block can be found by using the formula 232 n. Private IP addresses Private IP addresses are a solution to the problem of the exhaustion of public IP addresses. Addresses that fall within these ranges are not routed on the Internet backbone. It is described in RFC (Full Class A) (16 Class B s) (Full Class B) Figure 6.4 Private classful address and their ranges 6.3 Special Address Network addresses 0 and 127 are reserved for special uses. Network 0 designates the default route. The default route is used to simplify the routing information that IP must handle. Network address 127 is loop back address. The loop back address simplifies network applications by allowing the local host to be addressed in the same manner as remote host. An IP address with all bits set to one is a broadcast address. A broadcast address is used to simultaneously address every host on a network.

41 0 is reserved for machines that don't know their address. In certain circumstances it is possible for a machine not to know the number of the network it is on, or even its own host address. For example, would be a machine that knew it was host number 23, but didn't know on what network. 255 is used for "broadcast". A broadcast is a message that you want every system on the network to see. Broadcasts are used in some situations where you don't know who to talk to. For example, suppose you need to look up a host name and get its Internet address. Sometimes you don't know the address of the nearest name server. In that case, you might send the request as a broadcast. There are also cases where a number of systems are interested in formation. It is then less expensive to send a single broadcast than to send datagrams individually to each host that is interested in the information. In order to send a broadcast, you use an address that is made by using your network address, with all ones in the part of the address where the host number goes. For example, if you are on network , you would use for broadcasts. How this is actually implemented depends upon the medium. It is not possible to send broadcasts on the Arpanet, or on point to point lines. However it is possible on an Ethernet. If you use an Ethernet address with all its bits on (all ones), every machine on the Ethernet is supposed to look at that datagram. 6.4 Classless Inter-Domain Routing (CIDR) CIDR, described in RFCs 1518 and 1519, removes the concept of class from the IP address assignment and management process. In place of predefined, well-known boundaries, CIDR allocates addresses defined by a starting address and a range, which makes more efficient use of available space. The range defines the network part of the address. Notation method that specifies the number of masked bits in an IP address/subnet mask combination. In this method bitmask is used to group of multiple classful networks as a single network address. This will do the same process with route aggregation. supernetting is most often applied when the aggregated networks are under common administrative control. In class C network addresses, supernetting can be used so that the addresses appear as a single large network, or supernet. For example an assignment from an ISP to a corporate client might be expressed as /25. This would result in a 128-address block for local use, with the upper 25 bits being the network identifier part of the address. A legacy, class-full allocation would be expressed as <net> /8, <net>.<net>.0.0 /16, or <net>.<net>.<net>.0 /24. As these are reclaimed, they will be reallocated using classless CIDR techniques. 6.5 Multihoming

42 When a computer is configured with more than one IP address, it is referred to as a multihomed system 6.6 Let us Sum Up Classful Address In classful addressing, the address space is divided into five classes: A, B, C, D, and E to define large, medium, and small networks. Private IP addresses Figure 6.3a Classful address block size Private IP addresses are a solution to the problem of the exhaustion of public IP addresses. Figure 6.4a Private classful address and their ranges Special Address

43 Network addresses 0 and 127 are reserved for special uses.network 0 designates the default route. 0 is reserved for machines that don't know their address 255 is used for "broadcast". Classless Inter-Domain Routing (CIDR) CIDR allocates addresses defined by a starting address and a range, which makes more efficient use of available space. For example an assignment from an ISP to a corporate client might be expressed as /25. This would result in a 128-address block for local use, with the upper 25 bits being the network identifier part of the address Multihoming When a computer is configured with more than one IP address, it is referred to as a multihomed system 6.7 Lesson end Activities What are the different classification of classful address and their address range? 6.8 Points for Discussion What is the purpose of having private IP address? 6.9 Check your progress 1. What is Speical address? 2. What is Classless Inter-Domain Routing? 6.10 References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq-Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill Lesson 7. ARP & RARP

44 7.0 Aims and Objectives 7.1 Introduction 7.2 Address Resolution Protocol (ARP) 7.3 Reverse Address Resolution Protocol (RARP) 7.4 Let us Sum Up 7.5 Lesson end Activities 7.6 Points for Discussion 7.7 Check your progress 7.8 References 7.0 Aims and Objectives To learn Address and Reserve Address Resolution Protocol 7.1 Introduction A data link such as Ethernet or a token ring has its own addressing scheme. When an Ethernet frame is sent from one host to another, it is the 48-bit Ethernet address that determines the destination. The first 28-bits are the organization that made the Ethernet card, the second 28- bits are randomly assigned by the manufacturer. The device driver software never looks at the destination IP address in the IP datagram. Address resolution provides a mapping between two different forms of addresses ie., 32-bit IP addresses and whatever the data link uses. When an Ethernet frame is sent from one host on a LAN to another, it is the 48-bit Ethernet address that determines for which interface the frame is destined. The device driver software never looks at the destination IP address in the IP datagram. 7.2 Address Resolution Protocol (ARP) A Network layer protocol used to associate a logical address (IP) to a physical (hardware) address (MAC). Obtains the hardware address (Ethernet) of another computer on the same network (subnet). This information is stored in a ARP table for future reference. Figure 7.1 ARP table in the system

45 ARP (address resolution protocol) is a protocol used to do address resolution in the TCP/IP protocol suite (RFC826). ARP provides a dynamic mapping from an IP address to the corresponding hardware address. ARP is required on multi-access channels and relies on the ability to broadcast. The protocol broadcast a packet containing the IP address of the destination machine. The machine with that address, or possibly a server, sends a reply containing the hardware address. This process is used by all network devices - computers, routers, printers. Address resolution provides a mapping between the two different forms of address ie., 32-bit Internet address and 48-bit Ethernet address. ARP provides a dynamic mapping from an IP address to the corresponding hardware address. We use the term dynamic since it happens automatically and is normally not a concern of either the application user or the system administrator. Figure 7.2 Sending a packet

46 Figure 7.3 ARP table lookup What happens if the device is unable to locate the destination MAC address in its ARP table? In other words, the source knows the destination IP address, but is unable to locate a MAC address for it in its own ARP table. Figure 7.4 Unknown destination (Not in the table)

47 The device sends an ARP request packet to all devices on the subnet asking for the MAC address corresponding to the IP address. The MAC address in this request is in the form of a broadcast: FF-FF-FF-FF-FF-FF. All devices will see the broadcast. The device with the destination IP address will send a reply back to the requesting device. Once the sending device has both of the destination s IP and Mac address in its ARP table, it can send data at any time. If network devices did not keep an ARP table in memory (cache), they would have to send an ARP request every time data needs to be transmitted. Obviously, very inefficient. The ARP request message contain the following details : MAC header of Destination: FF-FF-FF-FF-FF-FF Source : C IP Header of Destination: Source : ARP Request Message What is your MAC address? The ARP reply contain the following : MAC header Destination: C Source : IP header Destination: Source : ARP request message Here s my MAC address ARP Packet format Format of ARP request or reply packet when used on an Ethernet. Proxy ARP lets a router answer ARP requests on one of its networks. This fools the sender of the ARP request into thinking that the router is the destination host, when in fact the destination host is "on the other side" of the router. The router is acting as a proxy agent for the destination host, relaying packets to it from other hosts.

48 Figure 7.5 ARP packet format Figure 7.6. Encapsulation of ARP packet Proxy ARP Upon receipt the hardware address is used to send the original packet. Essential to the efficient operation of ARP is the maintenance of a cache on each host. The cache maintains the recent IP to physical address mappings. Each entry is aged (usually the lifetime is 20 minutes) forcing periodic updates of the cache. ARP replies are often broadcast so that all hosts can update their caches. Proxy ARP lets a router answer ARP requests on one of its networks for a host on another of its networks. This fools the sender of the ARP request into thinking that the router is the destination. The router is acting as a proxy agent for the destination, relaying packets to it from other hosts. Proxy ARP is also known as promiscuous ARP or the ARP hack. The names come from the other use of proxy ARP: to hide two physical networks from each other, with a router between the two. This has been used to separate hosts running two different versions of TCP/IP. When a system boots, it typically gets its IP address from a file. Since each

49 system has a unique hardware address, which hardware address can be used to lookup the corresponding IP address. ARP Cache Essential to the efficient operation of ARP is the maintenance of an ARP cache on each host. This cache maintains the recent mappings from Internet addresses to hardware addresses. The normal expiration time of an entry in the cache is 20 minutes from the time the entry was created. Gratuitous ARP Another feature of ARP that we can watch is called gratuitous ARP. It occurs when a host sends an ARP request looking for its own IP address. This is usually done when the interface is configured at bootstrap time. 7.3 Reverse Address Resolution Protocol (RARP) RARP does basically the opposite of ARP. A network device knows its MAC address but not its IP address. Used in diskless workstations or dumb terminals ARP tables are kept in RAM, and therefore lost after the power is turned off. Devices using RARP require that a RARP server be present on the network to answer RARP requet Routers build tables that describe all devices and networks connected to them. In other words, ARP tables kept by routers can contain IP addresses and MAC addresses of devices located on more than one network. It used to require the Ethernet address of the IP address. The principle of RARP is for the diskless system to read its unique hardware address from the interface card and send an RARP request asking for someone to reply with the diskless system s IP address. RARP table Address MAC Address Interface c ethernet A ethernet ethernet ethernet e ethernet d ethernet b ethernet eb ethernet 1 The source device sends an RARP request packet to all devices on the subnet asking for an IP address. The IP address in this request is in the form of a broadcast: All devices will see the broadcast, but the only RARP server will act upon it. The RARP server will

50 send a reply packet containing the assigned IP address. That IP address is used for the session duration. RARP is used by systems without a disk drive but requires manual configuration by the system administrator. The RARP request contain the following : MAC header Destination: Anyone who will listen - RARP server Source : IP header Destination: Source :?.?.?.? RARP request message What is my IP address The RARP reply contain the following : MAC header Destination: Source : IP header Destination: Source : RARP request message Here s your IP address RARP Data format It is almost identical to an ARP packet. The only differences are that the frame type is 0x8035 for an RARP request or reply, and the op field has a value of 3 for an RARP request and 4 for an RARP reply. RARP Servers as User Processes The complication with an RARP server is that the server normally provides the mapping from a hardware address to an IP address for many hosts. RARP requests are transmitted as Ethernet frames with a specific Ethernet frame type field. Multiple RARP Servers per Network RARP requests are sent as hardware-level broadcasts. As the number of servers increases, the network traffic increase, since every server sends an RARP reply for every RARP request. The diskless system that sent the RARP request normally uses the first RARP reply that it receives.

51 Figure 7.7 RARP request by the client to the server RARP (RFC903) does exactly that same as ARP except some of the numbers change. The RARP request is broadcast and the reply is sent to the requester. Unlike ARP, designated RARP server(s) that handles RARP requests RARP is used by systems without a disk drive (normally diskless workstations or X terminals) but requires manual configuration by the system administrator 7.4 Let us Sum Up Address Resolution Protocol (ARP) A Network layer protocol used to associate a logical address (IP) to a physical (hardware) address (MAC). Gratuitous ARP Another feature of ARP that we can watch is called gratuitous ARP. Reverse Address Resolution Protocol (RARP) RARP does basically the opposite of ARP. A network device knows its MAC address but not its IP address.

52 7.5 Lesson end Activities 1. What is ARP? 7.6 Points for Discussion 1. What is the content of ARP packet? 2. What is Proxy ARP? 7.7 Check Your Progress 1. What is Gratuitous ARP? 2. What is the purpose of having Reverse Address Resolution Protocol (RARP)? 7.8 References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

53 Lesson 8. Subnet Address and Multicast Address 8.0 Aims and Objectives 8.1 Introduction 8.2 Subnet Mask 8.3 Multicast Addressing 8.4 Let us Sum up 8.5 Lesson end Activities 8.6 Points for Discussion 8.7 Check your progress 8.8 References 8.0 Aims and Objectives To learn Subnet Addressing To learn Multicast Addressing 8.1 Introduction A single network may be split into a multiple networks for internal use but appear as a single network to outsider. This is needed because some network may split into different places or security reasons they want to split the network. Subnet A single network may be split into a multiple networks for internal use but appear as a single network to outsider. Subnetting is a Technique used to allow a single IP network address to span multiple physical networks. IP hosts should support subnetting. Subnetting is done by using some of the bits of the host-id part of the IP address as a physical network identifier. There are several reasons why you might want to subnet a network. You may want to incorporate different types of physical networks into your IP network. Implementing subnets helps to control network traffic. On an ethernet network every machine on the same physical network sees all the packets of data sent out on the network. In heavy traffic situations this can result in collisions making the network performance painfully slow. In both these situations routers or gateways are used to separate networks. The router breaks the network into multiple subnets. The following examples show that two networks each with two hosts. All the subnets are connected to a gateway. The subnetwork addresses are and

54 Figure 8.1 Subnet example Many Class A and B networks do not contain as many hosts as they could. This situation causes a lot of address space waste. Subnetting better utilizes the address space by dividing these big networks to smaller ones. 8.2 Subnet Mask The subnet mask is used to determine the bits of the network identifier. All hosts on the same network should have the same subnet mask. An address such that when added to network address (in bitwise-and) splits the address into network + subnet + host address. Indicates how much of the IP address represents the network or subnetwork. Determines which part of an IP address is the network field and which part is the host field. Standard (default) subnet masks are as follows: Class A subnet mask is Class B subnet mask is Class C subnet mask is If the network uses the default subnet mask means there is no subnet. If it is other than the default subnet mask then there is a subnet.

55 The network designers has to modify the default subnet mask and create a new subnet mask. To determine the number of bits to be used, the network designer needs to calculate how many hosts the largest subnetwork requires and the number of subnetworks needed. Follow these steps to determine the subnet mask 1. Express the subnetwork IP address in binary form. 2. Replace the network and subnet portion of the address with all 1s. 3. Replace the host portion of the address with all 0s. 4. Convert the binary expression back to dotted-decimal notation. e.g. IP address: subnet mask Since the leading bit begins with 0, it s a Class A network with address IP Subnet Network is extended by 7 more bits. Therefore, this is subnet 10. The remaining host part is is the host address. IP and subnet masks are often presented together in this format. e.g. Network address = , subnet mask = In this case, the network address can be written as /19 indicating subnet has 19 bits for the network portion of address, and has remaining 13 bits for host part. Therefore, total number of subnets = = 6 Total number of hosts per subnet = = Multicast Addressing The multicasting capability of the TCP/IP protocol suite and the protocol that is involved in multicasting is IGMP (Internet Group Management Protocol). IGMP is a group management protocol. IGMP is used by hosts and routers that support multicasting. It lets all the systems on a physical network know which hosts currently belong to which multicast groups. It helps a multicast router create and update a list of loyal members related to each router interface. In any network, there are one or more multicast routers that distribute multicast packets to hosts or other

56 routers. This information is required by the multicast routers, so they know which multicast datagrams to forward onto which interfaces. IGMP is defined in RFC The IGMP Protocol gives the multicast routers information about the membership status of hosts (routers) connected to the network. Position of IGMP in the network layer shown below: Figure 8.2 IGMP position in Network layer IGMPv2 Message Types shown in the following diagram: IGMP format Figure 8.3 IGMP message type Figure 8.4 Message format A multicast router connected to a network has a list of multicast addresses of the groups for which there is at least one loyal member. To Joining a Group,a host maintains a list of processes

57 with membership in a group. When a process wants to join a new group, it sends its request to the host. The host will add the name of the process and the name of the requested group to its list. Host only sends an IGMP report to the multicast router if this is the first request for membership in that group. To leaving a Group, When a host sees that no process is interested in a specific group, it sends a leave report. When a router sees that none of the networks connected to its interface is interested in a specific group, it sends a leave report about that group. To Monitoring Group Membership, Multicast router is responsible for monitoring all of the hosts in a LAN to see if they want to continue their membership in a group.the router periodically (by default, every 125s) sends a query message. In this message, the group address field is set to (to all systems on a LAN). The general query message does not define a particular group. IGMP messages are used by multicast routers to keep track of group membership on each of the router's physically attached networks. The following rules apply. 1. A host sends an IGMP report when the first process joins a group. If multiple processes on a given host join the same group, only one report is sent, the first time a process joins that group. This report is sent out the same interface on which the process joined the group. 2. A host does not send a report when processes leave a group, even when the last process leaves a group. The host knows that there are no members in a given group, so when it receives the next query (next step), it won't report the group. 3. A multicast router sends an IGMP query at regular intervals to see if any hosts still have processes belonging to any groups. The router must send one query out each interface. The group address in the query is 0 since the router expects one response from a host for every group that contains one or more members on that host. 4. A host responds to an IGMP query by sending one IGMP report for each group that still contains at least one process. 8.4 Let us Sum Up Subnet A single network may be split into a multiple networks for internal use but appear as a single network to outsider. Subnet Mask The subnet mask is used to determine the bits of the network identifier. All hosts on the same network should have the same subnet mask.

58 If the network uses the default subnet mask means there is no subnet. If it is other than the default subnet mask then there is a subnet. Follow these steps to determine the subnet mask 1. Express the subnetwork IP address in binary form. 2. Replace the network and subnet portion of the address with all 1s. 3. Replace the host portion of the address with all 0s. 4. Convert the binary expression back to dotted-decimal notation. Multicast Addressing The multicasting capability of the TCP/IP protocol suite and the protocol that is involved in multicasting is IGMP (Internet Group Management Protocol). IGMP is a group management protocol. 8.5 Lesson end Activities 1. Why we need subnet mask? 8.6 Points for Discussion 2. How a subnet is created? 8.7 Check your Progress 1. What is the purpose of Subnet? 2. What is the default subnet mask address? 3. What is the purpose of having IGMP? 8.8 References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

59 11.0 Aims and Objectives 11.1 Introduction 9.2 DHCP 9.3 Let Us Sum Up 9.4 Lesson and Activities 9.5 Points for Discussion 9.6 Check your progress 9.7 References Lesson 9. Bootstrap and DHCP protocol 9.0 Aims and Objectives To learn bootstrap protocol To learn DHCP protocol 9.1 Introduction A method for a diskless system to bootstrap itself, called the Bootstrap Protocol, or BOOTP. BOOTP uses UDP and normally works in conjunction with TFTP. RFC 951 is the official specification for BOOTP with clarifications given in RFC The Bootstrap Protocol (BOOTP) operates in a client/server environment and only requires a single packet exchange to obtain IP information. BOOTP packets can include the IP address, as well as the address of a router, the address of a server, and vendor-specific information. System with a local disk can get their IP address from the configuration file of its own disk. If the system doesn t have hard disk then it needs some techniques to obtain the IP address. 9.2 DHCP RFCs 2131 and 2132 define DHCP as open and standards-based. A DHCP server stores all available IP addresses in a central database along with associated configuration information, including the subnet mask, gateways, and the addresses of DNS servers. This database enables automatic IP address configuration for hosts as they start up. DHCP saves network administrative time and the larger the network, the greater the savings. Without dynamic address assignment, network administrators must manage IP addresses to avoid duplicate use and apply configuration changes to workstations manually. The resulting

60 lack of centralized configuration information makes it difficult for the administrator to ensure consistent client configurations. DHCP is derived from the Internet standard BOOTP (RFCs 951 and 1084), which allows dynamic assignment of IP addresses as well as remote booting of diskless workstations. In addition to supporting the dynamic assignment of IP addresses, DHCP supplies all configuration data required by TCP/IP, plus additional data required for specific services. As noted, this functionality simplifies tasks for the network administrator, who can now manually configure just one computer the DHCP server. Whenever a new computer starts on a network segment that is served by the DHCP server (or an existing computer is restarted), the computer asks for a unique IP address and the DHCP server assigns one from the pool of available addresses. As Figure shows, this process requires only four steps: 1. The DHCP client asks for an IP address (a DHCP Discover message). 2. The DHCP Server offers an address (a DHCP Offer message). 3. The DHCP client accepts the offer and requests the address (a DHCP Request message). 4. The DHCP Server officially assigns the address to the client (a DHCP Acknowledge message). Figure 9.1 DHCP Automates the Assignment of IP Addresses DHCP server places an administrator-defined time limit, called a lease, on the address assignment. Halfway through the lease period, the DHCP client requests a lease renewal, and the DHCP server extends the lease. As a result, when a computer stops using its assigned IP address (for example, upon relocation to another network segment), the lease expires and the address returns to the pool for reassignment.

61 9.3 Let us Sum Up An alternative method for a diskless system to bootstrap itself, called the Bootstrap Protocol, or BOOTP. BOOTP uses UDP and normally works in conjunction with TFTP. RFC 951 is the official specification for BOOTP with clarifications given in RFC The Bootstrap Protocol (BOOTP) operates in a client/server environment and only requires a single packet exchange to obtain IP information DHCP saves network administrative time and the larger the network, the greater the savings. Without dynamic address assignment, network administrators must manage IP addresses to avoid duplicate use and apply configuration changes to workstations manually. 9.4 Lesson end Activities 1. What is bootstrap? 9.5 Points for Discussion 1. How BOOTP is working? 9.6 Check Your Progress 1. Explain the advantages of DHCP? 9.7 References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

62 Lesson 10. Domain Name Service (DNS) 12.0 Aims and Objectives 10.1 Introduction 10.2 DNS protocol 10.3 Let us Sum Up 10.4 Lesson end Activities 10.5 Points for Discussion 10.6 Check Your Progress 10.7 References 10.0 Aims and Objectives To learn DNS organizing structure 10.1 Introduction Average brain can easily remember 7 digits. It is difficult to remember the IP addresses. Instead of using IP address we are using alphanumeric names to refer to hosts. Add a distributed, hierarchical protocol (called DNS) to map between alphanumeric host names and IP addresses. We call this Name Resolution. There is no central database but the information is distributed among thousands of name servers and organized into a hierarchy. Root domain is the top of the domain hierarchy. Top level domains are existed under the root domain. There are two basic types of top-level domains geographic and organizational. The domain name hierarchy is divided into zones. A Zone is a separate portion of the DNS hierarchy. No two zones should overlap each other. Name servers In each zone, there is a primary name server and one or more secondary name servers. Name servers contain two kinds of address mappings Authoritative mappings and Cached mappings. Authoritative mappings used for hosts within the zone. Cached mappings used for previously requested mappings to hosts not in the zone. Domain Name Domain name reflect the domain hierarchy. Domain names are written from a host name to a top-level domain, with each part of the domain name separated by a dot. To be unambiguous, the names assigned to machines must be carefully selected from a name space with complete control over the binding between the names and IP addresses.

63 Flat Name Space and Hierarchical Name Space A domain name space was designed to have a hierarchical name space. In this design the names are defined in an inverted-tree structure with the root at the top. The information contained in the domain name space must be stored. However, it is very inefficient and also unreliable to have just one computer store such a huge amount of information. In this section, we discuss the distribution of the domain name space. DNS is a protocol that can be used in different platforms. In the Internet, the domain name space (tree) is divided into three different sections: generic domains, country domains, and the inverse domain. Domain Name Hierarchy D o m a in N a m e H ie ra rc h y c om e d u n e t g o v in t m il o rg a e... u s... zw y ahoo c nn rutgers y ale C oun try D om ains c s eng G en eric D om ains Figure 10.1 : Domain Name hierarchy Domain hierarchy has two types Generic Domain or Organization Domain and Country Domain. The generic domain divides registered hosts according to their behavior. The generic domain convention allows seven possible three-character labels describing organization type.

64 com Commercial organization edu Educational institution gov Government institution int International organization mil Military organization net Network support center org Organization other than those listed above The country domain name follows the same format as generic domain but uses twocharacter country abbreviation. For reading the domain name start with the most specific information given about the host and become more and more general with each label until they reach the rightmost label DNS protocol When client wants to know an IP address for a host name then client sends a DNS query to the primary name server in its zone. If name server contains the mapping, it returns the IP address to the client. Otherwise, the name server forwards the request to the root name server. The request works its way down the tree toward the host until it reaches a name server with the correct mapping. Clients communicate with DNS servers using either TCP or UDP on port 53. The following diagram shows header details about the DNS protocols: Transaction Identification Flags N um ber of Q uestions N um ber of A nsw er R R s N um ber of A uthoritative R R s N um ber of A dditional R R s Q uestions (variable length) Answer R esource R ecords (variable length) Authoritative R esource R ecords (variable length) Additional R esource R ecords (variable length) Figure 10.2 : DNS Packet

65 DNS packet fields contain the following details: Transaction Identification Random number used to match client queries with name server responses Flags QR opcode AA TC RD RA (unused) rcode Figure 10.3 : DNS Flags details QR opcode AA TC RD RA rcode : 0=Query, 1=Response : 0=standard query, 1=inverse query, 2=status request : Authoritative answer : Truncated DNS packet : Recursion desired : Recursion available Return code. 0=no error, 3=name error Transaction Identification Random number used to match client queries with name server responses Number of Questions Number of DNS queries in the packet Number of Answer RRs Number of non-authoritative DNS responses in the packet Number of Authoritative RRs: Number of authoritative DNS responses in the packet Number of Additional RRs: Number of other DNS responses in the packet (usually contains other DNS servers in domain) Questions & Answers: Variable length fields to store DNS queries and DNS server responses

66 DNS Query DNS Packet Question field contains a sequence of queries: Query name (variable length) Query Type Query Class Figure 10.4 DNS Query packet Query Name: Contains an encoded form of the name for which we are seeking an IP address Query Type 1=IP address, 2=name server, 12=pointer record, etc. Query Class 1=Internet address DNS queries must be encoded in a special way. Divide host address into segments whenever a period appears. For each segment, store a byte representing the length of the segment followed by the letters in the segment. Store a zero byte at the end of the query. DNS Responses DNS Packet RR fields contain a sequence of resource records: D o m a in n a m e ( va ria b le len g th ) T yp e R e s o u r c e d a ta le n g th T im e - t o- liv e C la s s R e s o u r c e D a t a ( va ria b le len g th ) Figure 10.5 DNS response packet

67 Domain Name Encoded domain name for query Type & Class Same as for query (1=IP; 1=Internet) Time-to-Live How long this response will be useful Resource Data Contains the four-byte IP address DNS responses are often compressed to save space. It uses the following algorithm. If all or part of the domain name field appears earlier in the packet (e.g., in a prior RR), then store a pointer to the earlier copy instead. Here the pointer is 2-byte code. Address Caching Going to the root server and then down the tree every time we need to resolve an address is inefficient. Instead of this we have address caching at name servers. It store host-to-ip-address mappings from recently requested host names at name server. When the same address is requested later, use the cached version at the local name server instead of recursively querying other name servers again. Default DNS When Host issues a query to DNS server, can add the default domain. Default domain added to end of ever DNS query. Domain search order is specified in resolv.conf file. DNS has two types of messages: query and response. Both types have the same format. The query message consists of a header and question records; the response message consists of a header, question records, answer record The question records are used in the question section of the query and response messages. The resource records are used in the answer, authoritative and additional information sections of the response message.

68 10.3 Let us Sum Up The domain name hierarchy is divided into zones. A Zone is a separate portion of the DNS hierarchy. No two zones should overlap each other. Domain Name Domain name reflect the domain hierarchy. Domain names are written from a host name to a top-level domain, with each part of the domain name separated by a dot. Flat Name Space and Hierarchical Name Space DNS protocol A domain name space was designed to have a hierarchical name space When client wants to know an IP address for a host name then client sends a DNS query to the primary name server in its zone 10.4 Lesson end Activities What is DNS? 10.5 Points for Discussion Discuss the Domain hierarchy? 10.6 Check your Progress 1. Explain the process of DNS protocol? 2. Write Short notes on: a) DNS query; b) DNS Responses 10.7 References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

69 11.0 Aims and Objectives 11.1 Introduction 11.2 Datagrams 11.3 Let us Sum Up 11.4 Lesson end Activities 11.5 Points for Discussion 11.6 Check your Progress 11.7 References Unit III Lesson 11. Internet Protocol (IP) 11.0 Aims and Objectives To learn the Internet protocol and their header information 11.1 Introduction IP is the main protocol at the network layer. It is used by both TCP and UDP. Every piece of TCP and UDP data that gets transferred around an internet goes through the IP layer at both end systems and at every intermediate router. IP provides an unreliable, connectionless datagram delivery service. By unreliable we mean there are no guarantees that an IP datagram successfully gets to its destination. IP provides a best effort service. When something goes wrong, such as a router temporarily running out of buffers, IP has a simple error handling algorithm: throw away the datagram and try to send an ICMP message back to the source. Any required reliability must be provided by the upper layers (e.g., TCP). The term connectionless means that IP does not maintain any state information about successive datagrams. Each datagram is handled independently from all other datagrams. This also means that IP datagrams can get delivered out of order. If a source sends two consecutive datagrams (first A, then B) to the same destination, each is routed independently and can take different routes, with B arriving before A Datagrams Packets in the IP layer are called datagrams. The above figure shows the data gram format. A datagram consisting of two parts: header and data. The header can be from 20 to 60 bytes and contain s information essential for routing and delivery of data.

70 IP Header details Figure 11.1 IP Header Description of fileds: Version Hlen Service type The first filed defines the version number of the IP. Header length - Defines the length of the header in multiples of four bytes. The four bits can represent a number between 0 and 15, which, when multiplied by 4, gives a maximum of 60 bytes. It defines how the datagram should be handled.

71 Total length This defines the total length of the IP datagram. It is 16 bits filed and can define up to 65,536 bytes. Identification Flags The identification field is used in fragmentation. Each fragment is identified with a sequence number in this field. Flags field deal with fragmentation. The datagram can be first, middle or last fragment or may not be fragmented. Fragmentation offset Time to Live Protocol This offset is a pointer that shows the offset of the data in the original ndatagram This filed defines the number of hops a datagram can travel before it is discarded, Defines which upper-layer protocol data are encapsulated in the datagram( TCP, UDP, ICMP, etc.) Source Address It used to identify the original source of the datagram. It contains source system internet address. Destination Address Options It used to identify the original Destination of the datagram. It contains destination system internet address. It can contain information for routing, timing, management and alignment.

72 11.3 Let Us Sum Up IP is the main protocol at the network layer. It is used by both TCP and UDP. Datagrams Packets in the IP layer are called datagrams. Fields : Version Hlen Service type Total length Identification Flags Fragmentation offset Time to Live Protocol Source Address Destination Address Options 11.4 Lesson end Activities What is the purpose of fragment offset? 11.5 Points for Discussion What is datagram? 11.6 Check your progress Describe the IP datagram in details References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

73 Lesson 12. The Internet Control Message Protocol (ICMP) 12.0 Aims and Objectives 12.1 Introduction ICMP 12.3 ICMP Messages 12.4 Let us Sum Up 12.5 Lesson end Activities 12.6 Points for Discussion 12.7 Check your progress 12.8 References 12.0 Aims and Objectives To learn how the TCP/IP managing error messages 12.1 Introduction The Internet Control Message Protocol (ICMP) is a helper protocol that supports IP with facility for error reporting and simple queries. ICMP messages are encapsulated as IP datagrams ICMP The Internet Control Message Protocol (ICMP) protocol is classic example of a client server application. The Internet Control Message Protocol (ICMP) is part of the Internet protocol suite and defined in RFC 792. The ICMP server executes on all IP end system computers and all IP intermediate systems (i.e routers). The protocol is used to report problems with delivery of IP datagrams within an IP network. It can be sued to show when a particular End system is not responding, when an IP network is not reachable, when a node is overloaded, when an error occurs in the IP header information, etc. The protocol is also frequently used by Internet managers to verify correct operations of End Systems and to check that routers are correctly routing packets to the specified destinations. The Internet Control Message Protocol (ICMP) protocol is classic example of a client server application. The Internet Control Message Protocol (ICMP) is part of the Internet protocol suite and defined in RFC 792. The ICMP server executes on all IP end system computers and all IP intermediate systems (i.e routers). The protocol is used to report problems with delivery of IP datagrams within an IP network. It can be sued to show when a particular End system is not responding, when an IP network is not reachable, when a node is overloaded, when an error occurs in the IP header information, etc.

74 The protocol is also frequently used by Internet managers to verify correct operations of End Systems and to check that routers are correctly routing packets to the specified destinations. The Internet Protocol (IP) is used for host-to-host datagram service in a system of interconnected networks called the Catenet. The network connecting devices are called Gateways. These gateways communicate between themselves for control purposes via a Gateway to Gateway Protocol (GGP). Occasionally a gateway or destination host will communicate with a source host, for example, to report an error in datagram processing. ICMP, uses the basic support of IP as if it were a higher level protocol, however, ICMP is actually an integral part of IP, and must be implemented by every IP module. Is a protocol for the exchange of error messages and other vital information between (Physical) Internet entities such as hosts and routers. ICMP is a network layer protocol, often it is placed next to the IP protocol. ICMP Header ICMP Data Area IP Header IP Data Area Frame Header Frame Area Figure 12.1 ICMP packet in IP ICMP lies just above IP, as ICMP messages are carried inside IP Packets. ICMP messages are carried as IP payload, just as TCP/UDP segments are carried as IP payload. When a host receives an IP packet with ICMP specified as the upper layer protocol, it de-multiplexes the packet to ICMP, just as it would demultiplex a packet to TCP/UDP. ICMP functions announce network errors such as a host or entire portion of the network being unreachable, due to some type of failure. A TCP or UDP packet directed at a port number with no receiver attached is also reported via ICMP. Announce network congestion when a router begins buffering too many packets, due to an inability to transmit them as fast as they are being received, It will generate ICMP Source Quench messages. Directed at the sender, these messages should cause the rate of packet transmission to be slowed. The following diagram shows the ICMP messages.

75 12.3 ICMP Messages Figure 12.2 ICMP messages ICMP also assisted in troubleshooting. ICMP supports an Echo function, which just sends a packet on a round--trip between two hosts. Ping, a common network management tool, is based on this feature. Ping will transmit a series of packets, measuring average round--trip times and computing loss percentages. If an IP packet's TTL field drops to zero, the router discarding the packet will often generate an ICMP packet announcing time out. TraceRoute is a tool which maps network routes by sending packets with small TTL values and watching the ICMP timeout announcements. The following diagram illustrated the ICMP error messages. Figure 12.3 ICMP Error messages

76 ICMP datagram structure The ICMP datagram, being an IP datagram, contains the usual IP header. This is followed by an ICMP header which varies slightly between the different types of ICMP message. The general format is shown below: Figure 12.4 ICMP Header format ICMP Message Types Type Message Type Description 3 Destination Unreachable Packet could not be delivered 11 Time Exceeded Time to live field hit 0 12 Parameter Problem Invalid header field 4 Source Quench Choke Packet 5 Redirect Teach a router about geography 8 Echo Ask a machine if it is alive 0 Echo Reply Yes, I am alive 13 Timestamp Request Same as Echo request, but with timestamp 14 Timestamp Reply Same as Echo reply, but with timestamp The DESTINATION UNREACHABLE message is used when the subnet or a router cannot locate the destination. The TIME EXCEEDED message is sent when a packet is dropped because its counter has reached zero. This event is symptom that packets are looping, that there is enormous congestion, or that the timer values are being set too low. The PARAMETER PROBLEM message indicates that an illegal value has been detected in a header field. This problem indicates a bug in the sending host s IP software or possibly in the software of a router transited.

77 The SOURCE QUENCH message was formerly used to throttle hosts that were sending too many packets. When a host received this message, it was expected to slow down. It is rarely used any more when congestion occurs. The REDIRECT MESSAGE is used when a router notices that a packet seems to be routed wrong. It is used by the router to tell the sending host about the probable error. The ECHO and ECHO REPLY messages are used to see if a given destination is reachable and alive. Upon receiving the ECHO message, the destination is expected to send an ECHO REPLY message back. The TIMESTAMP REQUEST and TIMESTAMP REPLY messages are similar, except that the arrival time of the message and the departure time of the reply are recorded in the reply. This facility is used to measure network performance. Code The exact meaning of the value contained within this field depends on the message Type. For example, with an ICMP Type 3 message ("Destination unreachable"), a Code value of 0 means "Network unreachable", which implies a router failure. A Code of 1 means "Host unreachable". Checksum The checksum field provides error detection for the ICMP header only and is calculated in the same way as the IP header checksum. Parameters The usage of this field depends on the type of message. For example, Type 3 messages do not use this field, while Type 0 and 8 messages use the field to store an identifier and sequence number. Data Typically, the data is the IP header and first 64 bits of the original datagram, i.e. the one that failed and prompted the ICMP message. Including the first 64 bits of the original datagram allows the ICMP message to be matched to the datagram that caused it.

78 Destination Unreachable Codes Code Definition 0 Net Unreachable 1 Host Unreachable 2 Protocol Unreachable 3 Port Unreachable 4 Fragmentation needed & Don t Fragment was set 5 Source Route failed 6 Destination Network Unknown 7 Destination Host Unknown 8 Source Host Isolated 9 Communication Destination Network is Administratively Prohibited 10 Communication Destination Host is Administratively Prohibited 11 Destination Network Unreachable for Type of Service 12 Destination Host Unreachable for Type of Service 13 Communication Administratively Prohibited 14 Host Precedence Violation 15 Precedence Cutoff Violation

79 Redirect Codes Code Definition 0 Redirect Datagram for the Network (or subnet) 1 Redirect Datagram for the Host 2 Redirect Datagram for the Type of Service & Network 3 Redirect Datagram for the Type of Service & Host Time Exceeded Codes Code Definition 0 Time to Live Exceeded in Transit 1 Fragment Reassembly Time Exceeded Parameter Problem Codes Code Definition 0 Pointer Indicates the Error 1 Missing a Required Option 2 Bad Length

80 12.4 Let Us Sum Up The Internet Control Message Protocol(ICMP) The Internet Control Message Protocol (ICMP) protocol is classic example of a client server application. The protocol is used to report problems with delivery of IP datagrams within an IP network Lesson end Activities 1. How errors are managed in the TCP/IP? 12.6 Points for Discussion 1. Discuss why we are using error code for error messages 12.7 Check your Progress 1. Explain the ICMP protocol. 2. Write short notes on ICMP datagram structure References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

81 Lesson 13. Fragmentation and Reassembly Aims and Objectives 13.1 Introduction 13.2 Fragmentation and Defragmentation 13.3 Let us Sum Up 13.4 Lesson end Activities 13.5 Points for Discussion 13.6 Check your Progress 13.7 References 13.0 Aims and Objectives To learn how the TCP/IP packets are fragmented and reassembly 13.1 Introduction TCP/IP is designed for use with many different kinds of network. Unfortunately, network designers do not agree about how big packets can be. Ethernet packets can be 1500 octets long. Arpanet packets have a maximum of around 1000 octets. Some very fast networks have much larger packet sizes. At first, you might think that IP should simply settle on the smallest possible size. Unfortunately, this would cause serious performance problems. When transferring large files, big packets are far more efficient than small ones. So we want to be able to use the largest packet size possible. But we also want to be able to handle networks with small limits. There are two provisions for this. First, TCP has the ability to "negotiate" about datagram size. When a TCP connection first opens, both ends can send the maximum datagram size they can handle. The smaller of these numbers is used for the rest of the connection Fragmentation and Defragmentation When sending data between Ethernet networks to Arpanet, It can't handle packets of that size. For this reason, there are provisions to split datagrams up into pieces. This is referred to as "fragmentation Furthermore, every host implementation of TCP/IP must be prepared to accept pieces and put them back together. This is referred to as "reassembly". The following diagram shows that normal IP datagram packet with Maximum Transmission Unit(MTU)

82 Figure 13.1 IP Packet with MTU If the data size is less than the MTU then padding will be added for transmission. The following diagram illustrates this. Figure 13.2 IP data with padding Different protocols have different packet size ie., MTU size Following table show the details. Table 13.1 Protocols with MTU size The following diagram shows how a big packet can be fragmented and fix for a MTU.

83 Figure 13.3 Fragmented data packet 13.3 Let us Sum Up Fragmentation and Reassembly TCP/IP is designed for use with many different kinds of network. Unfortunately, network designers do not agree about how big packets can be. For example, when sending data between Ethernet networks to Arpanet, It can't handle packets of that size. For this reason, there are provisions to split datagrams up into pieces. This is referred to as "fragmentation. Furthermore, every host implementation of TCP/IP must be prepared to accept pieces and put them back together. This is referred to as "reassembly" Lesson end Activities 1. Why we need Fragmentation? 13.5 Points for Discussion 1. How tcp/ip packets are fragmented? 13.6 Check your progress 1. How fragmented packets are correctly identified by the receiver end? 13.7 References

84 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

85 Lesson 14. Routing 14.0 Aims and Objectives 14.1 Introduction 14.2 Routing 14.3 Routing Protocol Vs Routed Protocol 14.4 Types of routing table entries 14.5 Classification of routing table 14.6 Let us Sum Up 14.7 Lesson end Activities 14.8 Points for Discussion 14.9 Check your Progress References 14.0 Aims and Objective To learn how the TCP/IP packets are transfer from one end to another end 14.1 Introduction There are two distinct processes to delivering IP datagram: IP Forwarding and IP Routing. IP Forwarding tells how to pass a packet from an input interface to the output interface? IP Routing tells how to find and setup the routing tables? It determine route taken by packets from source to destination Forwarding must be done as fast as possible. On routers, is often done with support of hardware. On PCs, is done in kernel of the operating system. Routing is less time-critical, On a PC, routing is done as a background process. Packets are transfer from one system to another system using for forwarding and transfer from the sender to the receiver using routing. Forwarding is a process of passing packets along to the next hop. There is only one forwarding table and has prefix and next-hop information. Routing is a process of populating the forwarding table. You might have multiple routing databases - e.g. both OSPF and BGP Routing databases have more information. Routing is based on address lookup, maximum prefix match and search operation.

86 14.2 Routing A router, by definition, has two or more network interface layers (since it connects two or more networks). Any system with multiple interfaces is called multihomed. A host can also be multihomed but unless it specifically forwards packets from one interface to another, it is not called a router. Also, routers need not be special hardware boxes that only move packets around an internet. Conceptually, IP routing is simple, especially for a host. If the destination is directly connected to the host (e.g., a point-to-point link) or on a shared network (e.g., Ethernet or token ring), then the IP datagram is sent directly to the destination. Otherwise the host sends the datagram to a default router, and lets the router deliver the datagram to its destination. The IP layer can be configured to act as a router in addition to acting as a host. Most multiuser systems today, including almost every Unix system, can be configured to act as a router. We can then specify a single routing algorithm that both hosts and routers can use. The fundamental difference is that a host never forwards datagrams from one of its interfaces to another, while a router forwards datagrams. A host that contains embedded router functionality should never forward a datagram unless it has been specifically configured to do so Packets are transfer from one system to another system using for forwarding and transfer from the sender to the receiver using routing. Forwarding is a process of passing packets along to the next hop. There is only one forwarding table and has prefix and next-hop information. Routing is a process of populating the forwarding table. You might have multiple routing databases - e.g. both OSPF and BGP Routing databases have more information. Routing is based on address lookup, maximum prefix match and search operation Routing Protocol Vs Routed Protocol Routing Protocols Routing protocol used to update the routing table information. It will collect the information based on advertisement and also send routing information to other systems. Eg. RIP, OSPF Routed Protocols Routed protocols used to route the packets across network. It forward data to one network to another network Eg. IPX, DecNet Routing table Routing Table is used by the Routing Protocols. It defines the topology of the network. It must be consistent with other router s tables. Two types of routing tables are existed, Static and

87 dynamic routing tables. Static tables are constructed by network administrator and the Dynamic tables are constructed by the routing protocols Types of routing table entries Network route In this type of entry destination addresses is a network address (e.g., /24). Most entries are network routes. Host route This type of entry destination address is an interface address (e.g., /32). This entry used to specify a separate route for certain hosts Default route This type of entry used when no network or host route matches. The router that is listed as the next hop of the default route is the default gateway. Loopback address - Routing table used the loopback address ( ) which means the next hop lists the loopback (lo0) interface as outgoing interface. Adding an interface - Configuring an interface eth2 with /24 adds a routing table entry: Adding a default gateway - Configuring as the default gateway adds the entry The steps that IP performs when it searches its routing table : 1. Search for a matching host address. 2. Search for a matching network address. 3. Search for a default entry. (The default entry is normally specified in the routing table as a network entry, with a network ID of 0.) A matching host address is always used before a matching network address 14.5 Classification of routing table In routing table two types of protocol are used, ie Interior Gateway protocols (IGP) and Exterior Gateway protocols (EGP). Kind of information that is carried and the way the routing table are calculated based on Distance-vector protocols or Link-state protocols. Interior Gateway protocols Vs Exterior Gateway Protocols Interior Gateway Protocols are used within a single autonomous system. Generally it have single network administration to administration. It has unique routing policy and makes

88 best use of network resources. This class of protocols are used inside an autonomous system, ex.- RIP, OSPF, IGRP, EIGRP. Exterior Gateway Protocols are used among different autonomous systems. It has independent administrative entities. It is used to communication between independent network infrastructures. This class of protocols is used outside, or between, autonomous systems, ex. BGP4, the current internet standard for EGP. BGP makes routing decisions based on network policies, or rules. In EBGP, session occurs between routers in two different Autonomous Systems. In IBGP, session occurs between routers in the same Autonomous Systems. A multihomed network is a network that has more than one exit point to outside networks. Static or Dynamic Routing Routing can be either static or dynamic, depending on how routing information is generated and maintained. In static routing, routing information is entered manually by an administrator and remains constant throughout the router s operation. In dynamic routing, a router is configured to automatically generate routing information and share the information with neighboring routers. Static routing In static routing, a network administrator enters static routes in the routing table manually by indicating the Network ID, the hop count and the router interface. The network ID, consisting of a destination IP address and a subnet mask. The hop count is the distance between this router and the neighboring router. The router interfaces through which forward the packets to the destination. Static routing has significant drawbacks. Because a network administrator defines a static route, errors are more likely than with a dynamically assigned route. A simple typographical error can create chaos on the network. An even greater problem is the inability of a static route to adapt to topology changes. Whenever the topology changes, the administrator might have to make changes to the routing tables entries on every static router. This does not scale well on a large internetwork. Static routes have predictability, no overhead and simplicity. But lack of scalability and can not adapt to a failure in a network Dynamic Routing Dynamic routing method has two parts: the routing protocol that is used between neighboring routers to convey information about their network environment, and the routing algorithm that determines paths through that network. The protocol defines the method used to

89 share the information externally, whereas the algorithm is the method used to process the information internally. The routing tables on dynamic routers are updated automatically based on the exchange of routing information with other routers. The most common dynamic routing protocols are Distance vector routing protocols and Link state routing protocols. Understanding how these protocols work enables you to choose the type of dynamic routing that best suits your network needs. Dynamic routes are adapting to a failure in a network and work in large networks. The disadvantages are increase in complexity and overhead on the lines and routers 14.6 Let Us Sum Up Routing A router, by definition, has two or more network interface layers (since it connects two or more networks). Routing Protocol Vs Routed Protocol Routing Protocols Routing protocol used to update the routing table information. It will collect the information based on advertisement and also send routing information to other systems. Eg. RIP, OSPF Routed Protocols Routed protocols used to route the packets across network. It forward data to one network to another network Eg. IPX, DecNet Routing table Routing Table is used by the Routing Protocols. It defines the topology of the network. Types of routing table entries Network route Host route Default route Loopback address Adding a default gateway

90 Interior Gateway protocols Vs Exterior Gateway Protocols Interior Gateway Protocols are used within a single autonomous system. Generally it have single network administration to administration. Exterior Gateway Protocols are used among different autonomous systems. It has independent administrative Static or Dynamic Routing In static routing, routing information is entered manually by an administrator and remains constant throughout the router s operation. In dynamic routing, a router is configured to automatically generate routing information and share the information with neighboring routers Lesson end Activities 1. What is the purpose of Routing? 2. What is the different between Routing Protocol Vs Routed Protocol? 14.8 Points for Discussion 1. What is the content of Routing table? 14.9 Check your Progress 1. What is Interior Gateway protocols? 2. What is Exterior Gateway Protocols? 3. What is Static routing? 4. What is Dynamic Routing? 5. Write short notes on types of routing fable entries References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

91 Lesson 15 Distance Vector Protocol and Link State Routing Protocols 15.0 Aims and Objectives 15.1 Introduction 15.2 Distance Vector Routing Protocols 15.3 Link State Routing Protocols 15.4 Routing Information Protocol (RIP) 15.5 Open Shortest Path First (OSPF) 15.6 Let us Sum Up 15.7 Lesson end Activities 15.8 Points for Discussion 15.9 Check your Progress References 15.0 Aims and Objectives To learn two types of routing protocols 15.1 Introduction Distance vector routing protocols, the earliest dynamic routing protocols, are an improvement over static routing, but have some limitations. When the topology of the internetwork changes, distance vector routing protocols can take several minutes to detect the change and make the appropriate corrections. Link state routing protocols are more reliable and require less bandwidth than do distance vector routing protocols, they are also more complex, more memory-intensive, and place a greater load on the CPU Distance Vector Routing Protocols Distance-vector protocols advertise periodically sends to his neighbors and find how far is the destination and how to the next hop to get there. It installs routes directly in tables. The distance vector algorithm, also known as the Bellman-Ford algorithm, enables a router to pass route updates to its neighbors at regularly scheduled intervals. Each neighbor then adds its own distance value and forwards the routing information on to its immediate neighbors. The result of this process is a table containing the cumulative distance to each network destination. One advantage of distance vector routing protocols is simplicity. Distance vector routing protocols are easy to configure and administer. They are well suited for small networks with relatively low performance requirements.

92 Most distance vector routing protocols use a hop count as a routing metric. A routing metric is a number associated with a route that a router uses to select the best of several matching routes in the IP routing table. The hop count is the number of routers that a packet must cross to reach a destination Link State Routing Protocols Link state routing protocols address some of the limitations of distance vector routing protocols. For example, link state routing protocols provide faster convergence than do distance vector routing protocols. Convergence is the process by which routers update routing tables after a change in network topology the change is replicated to all routers that need to know about it. In Link-state protocols each router sends information about links to which it is attached state of these links. It is flooded throughout the network. Every router calculates its routing table. Unlike distance vector routing protocols, which broadcast updates to all routers at regularly scheduled intervals, link state routing protocols provide updates only when a network link changes state. When such an event occurs, a notification in the form of a link state advertisement is sent throughout the network You must decide where best to implement each type of routing. However, static routing can be effective when used in combination with dynamic routing. Instead of using static routing exclusively, you can use a static route as the redundant backup for a dynamically configured route. In addition, you might use dynamic routing for most paths but configure a few static paths where you want the network traffic to follow a particular route. For example, you might configure routers to force traffic over a given path to a high-bandwidth link Routing Information Protocol (RIP) Routing Information Protocol (RIP) first used in XNS (Xerox Network Systems). RIP first documented in RFC Routing Information Protocol (RIP) is the best known and most widely used of the distance vector routing protocols. RIP version 1 (RIP v1), which is now outmoded, was the first routing protocol accepted as a standard for TCP/IP. RIP version 2 (RIP v2) provides authentication support, multicast announcing, and better support for classless networks. The Windows Server 2003 Routing and Remote Access service supports both RIP v1 and RIP v2 (for IPv4 only).

93 Using RIP, the maximum hop count from the first router to the destination is 15. Any destination greater than 15 hops away is considered unreachable. This limits the diameter of a RIP internetwork to 15. However, if you place your routers in a hierarchical structure, 15 hops can cover a large number of destinations. RIP Characteristics In RIP packets are sent every 30 seconds or faster when necessary. Route is considered down if it is not refreshed within 180 sec. (distance set to infinity). Two kinds of messages are used, request and response. Hop count is used as a metric. The value of 1 to 15 is used (16 denotes infinity) as distance hop count. It doesn't support classless routing Open Shortest Path First (OSPF) Open Shortest Path First (OSPF) protocol is the best known and most widely used link state routing protocol. OSPF is an open standard developed by the Internet Engineering Task Force (IETF) as an alternative to RIP. OSPF compiles a complete topological database of the internetwork. The shortest path first (SPF) algorithm, also known as the Djikstra algorithm, is used to compute the least-cost path to each destination. Whereas RIP calculates cost on the basis of hop count only, OSPF can calculate cost on the basis of metrics such as link speed and reliability in addition to hop count. Unlike RIP, OSPF can support an internetwork diameter of 65,535 (assuming that each link is assigned a cost of 1). OSPF transmits multicast frames, reducing CPU usage on a LAN. You can hierarchically subdivide OSPF networks into areas, reducing router memory overhead and CPU overhead.

94 15.6 Let Us Sum Up Distance Vector Routing Protocols Distance-vector protocols advertise periodically sends to his neighbors and find how far is the destination and how to the next hop to get there. It installs routes directly in tables. Link State Routing Protocols Link state routing protocols address some of the limitations of distance vector routing protocols. For example, link state routing protocols provide faster convergence than do distance vector routing protocols. In Link-state protocols each router sends information about links to which it is attached state of these links. It is flooded throughout the network. Every router calculates its routing table. Routing Information Protocol (RIP) Routing Information Protocol (RIP) is the best known and most widely used of the distance vector routing protocols. Using RIP, the maximum hop count from the first router to the destination is 15. Any destination greater than 15 hops away is considered unreachable. This limits the diameter of a RIP internetwork to 15. However, if you place your routers in a hierarchical structure, 15 hops can cover a large number of destinations. Open Shortest Path First (OSPF) Open Shortest Path First (OSPF) protocol is the best known and most widely used link state routing protocol. OSPF is an open standard developed by the Internet Engineering Task Force (IETF) as an alternative to RIP Lesson end Activities 1. What is Distance Vector Protocols? 2. What is Link State Routing Protocols? 15.8 Points for Discussion 1. Which routing protocol suitable for small network? 2. Which routing protocol is suitable for big networkl?

95 15.9 Check your Progress 1. What is Routing Information Protocol (RIP)? 2. What is Open Shortest Path First (OSPF)? References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

96 Unit IV Lesson 16. TCP and UDP Data Structures 16.0 Aims and Objectives 16.1 Introduction 16.2 TCP Data Structures 16.3 UDP Data Structures 16.4 Let us Sum Up 16.5 Lesson end Activities 16.6 Points for Discussion 16.7 Check your Progress 16.8 References 16.0 Aims and Objectives 16.1 Introduction To understand the field and data structure of TCP To understand the field and data structure of UDP TCP and UDP are the two predominant transport layer protocols. Both use IP as the network layer TCP Data Structures TCP is defined in the RFC 793 and defines a reliable, connection orientated full duplex byte stream for a user process. TCP creates a CONNECTION orientated service by contacting the end system and establishing a set of guidelines both can support. Such agreements as how much data segments can be transferred before an acknowledgement are received. TCP takes large blocks of data coming from upper layers and segments them. Then it adds numbers to the segments so the end system can sequence them at arrival and assemble the original block before sending it to the upper layer. When TCP creates a connection between two end systems, it is called a VIRTUAL CIRCUIT. This virtual circuit is created at the time the one system needs to send a data stream to the end system and takes it down when the data transfer is completed. The three phases of the TCP are CONNECTION ESTABLISHMENT, CONNECTION MAINTENANCE and CONNECTION TIREDOWN.

97 TCP is a reliable, connection oriented, and byte-stream protocol. It provides reliability with a mechanism called Positive Acknowledgement with retransmission(par). The following diagram shows the details about the header. Figure 16.1 TCP Header details TCP Header contain the following data information: Source port address Defines the application program in the source computer Destination port address Defines the application program in the destination computer Sequence number A stream of data from the application program may be divided into two or more TCP segments. The sequence number field shows the position

98 Acknowledge number Header length Reserved This is valid only if the ACK bit in the control field is set. This number is used to acknowledge the receipt of data from the other communication device. The number defines the byte sequence number that is next expected. Defines the header length A six-bit field is reserved for future use. Control fields URG - Urgent bit is set to validate the urgent pointer field ACK - when set validates the acknowledgement number field PSH RST - Used to inform the sender that a higher throughput is needed - Used to reset the connection, when there is confusion in the sequence Numbers SYN - Used for sequence number synchronization in three types of segments: connection request, connection confirmation and confirmation acknowledgement FIN Window size Checksum Urgent pointer - Used in connection termination in three types of segments : termination request, termination confirmation and acknowledgement of termination confirmation Defines the size of the sliding window Used in error detection Its value is valid only when the URG field is set. This pointer defines the end of urgent data and the start of normal data. Options and padding Optional field in which used to convey additional information to the receiver or for alignment purpose.

99 16.3 UDP Data Structures UDP sends and receives datagrams for applications. A datagram is a unit of information that travels from the sender to the receiver. like TCP, however, UDP is unreliable. There is no guarantee that the datagram ever gets to its final destination UDP is designed for applications where you don't need to put sequences of datagrams together. It fits into the system much like TCP. There is a UDP header. The network software puts the UDP header on the front of your data, just as it would put a TCP header on the front of your data. Then UDP sends the data to IP, which adds the IP header, putting UDP's protocol number in the protocol field instead of TCP's protocol number. However UDP doesn't do as much as TCP does. It doesn't split data into multiple datagrams. It doesn't keep track of what it has sent so it can resend if necessary. About all that UDP provides is port numbers, so that several programs can use UDP at once. UDP port numbers are used just like TCP port numbers. There are well-known port numbers for servers that use UDP. Note that the UDP header is shorter than a TCP header. It still has source and destination port numbers, and a checksum, but that's about it. No sequence number, since it is not needed. UDP is used by the protocols that handle name lookups and a number of similar protocols. UDP is defined in RFC 768. It is the protocol that does not consume system resources as much as TCP but it unreliable and transfers data to the destination system with out establishing a connection and hence, connectionless protocol. UDP sends data to the destination system in numbered segments same as TCP but it can not retransmit erred segments if they get lost or damaged. UDP messages can be lost, duplicated, or arrive out of order. Figure 16.2 UDP Header

100 UDP uses 16 bit source and destination port number. Total length contains a count of octets in the UDP datagrams, including the UDP header and the user data. The UDP checksum is optional and need not be used at all. UDP Pseudo Header UDP Pseudo header used to verify that the UDP datagram has reached safe or not. The fields contain source and destination IP address To verify the checksum, the receiver must extract these fields from the IP header, assemble them into the pseudo-header format, and recomputed the checksum. Figure 16.3 : UDP Pseudo header 16.4 Let us Sum Up TCP Data Structures The three phases of the TCP are CONNECTION ESTABLISHMENT, CONNECTION MAINTENANCE and CONNECTION TIREDOWN.

101 UDP Data Structures UDP sends and receives datagrams for applications. A datagram is a unit of information that travels from the sender to the receiver. like TCP, however, UDP is unreliable. There is no guarantee that the datagram ever gets to its final destination UDP Pseudo header used to verify that the UDP datagram has reached safe or not. The fields contain source and destination IP address To verify the checksum, the receiver must extract these fields from the IP header, assemble them into the pseudo-header format, and recomputed the checksum. TCP Header contain Source port address Destination port address Sequence number Acknowledge number Header length Reserved Control fields o URG o ACK o PSH o RST o SYN o FIN Window size Checksum Urgent pointer Options and padding 16.5 Lesson end Activities 1. Draw the TCP header. 2. Draw the UDP header Points for Discussion 1. What are the uniqueness with TCP and UDP data structure? 16.7 Check your Progress 1. Explain the TCP Data structure. 2. Explain the UDP data structure

102 16.8 References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

103 Lesson 17. TCP Finite State Machine 17.0 Aims and Objectives 17.1 Introduction 17.2 Algorithm for TCP state machine 17.3 TCP finite state machine 17.4 Let us Sum Up 17.5 Lesson end Activities 17.6 Points for Discussion 17.7 Check your progress 17.8 References 17.0 Aims and Objective To learn about the TCP connection with Finite state machine 17.1 Introduction Like any other system states TCP communication also defined with Finite state machine. Reliable service is required for connection establishment and termination to perform connectionoriented service. The step required establishing and release connections can be represented in a finite state machine with the 11 states. Out of these states, some states are legal. Each connection starts in the CLOSED state. From that, there are two possibilities, either passive open (LISTEN), or active open (CONNECT). And the other side of the hosts performs the opposite operation. The Host sends SYN for connection through active open. At host B does a LISTEN and settles down to see who turns up. When SYN comes in, it is acknowledged and host goes to the SYN RECVD state. When host B s SYN is itself acknowledged, the three-way handshake is completed and the server goes to the ESTABLISHED state. And data transfer between two hosts. When the host A has had enough data, it sends FIN to host B. The host B received the command and sends its ACK to host A with FIN. Host A received FIN and sends ACK to host B and finally the host B releases the connection Algorithm for TCP state machine The algorithm for TCP state machine is explained below: 1. The client is active open and sends SYN and sets on SYN SENT state. 2. While the other end the server must be passive open and sets on LISTEN state. 3. When SYN comes to the server, it sets on SYN RECVD and ready to receive. 4. When the server received SYN from the client, it sends its own SYN and ACK of client s SYN.

104 5. The client received its ACK and server s SYN and give reply to server by ACK and both set on ESTABLISHED state. 6. And the data is transmitted between them. 7. At the end, suppose the client is on passive close. It goes on FIN WAIT-1 waiting for ACK from the server. 8. When it received it sets on FIN WAIT-2 state and waiting for FIN, after receiving it the client sends ACK to the server and sets on TIME WAIT state TCP finite state machine Reset / any invalid input begin passive open CLOSED SYN / SYN + ACK SYN RECVD FIN WAIT-1 close / FIN ESTAB closing close reset send / SYN SYN / SYN + ACK ACK close / FIN fin / ACK LISTEN active open / SYN SYN + ACK / ACK FIN / ACK close / FIN SYN SENT CLOSE WAIT LAST ACK Close / timeout / reset timeout after 2 segment lifetimes ACK FIN - ack / ACK ACK FIN WAIT-2 fin / ACK TIMED WAIT timeout after 2 segment lifetimes Figure 17.1 Finite State Machine flow diagram Figure

105 17.4 Let Us Sum Up Algorithm for TCP state machine The algorithm for TCP state machine is explained below: 1. The client is active open and sends SYN and sets on SYN SENT state. 2. While the other end the server must be passive open and sets on LISTEN state. 3. When SYN comes to the server, it sets on SYN RECVD and ready to receive. 4. When the server received SYN from the client, it sends its own SYN and ACK of client s SYN. 5. The client received its ACK and server s SYN and give reply to server by ACK and both set on ESTABLISHED state. 6. And the data is transmitted between them. 7. At the end, suppose the client is on passive close. It goes on FIN WAIT-1 waiting for ACK from the server. 8. When it received it sets on FIN WAIT-2 state and waiting for FIN, after receiving it the client sends ACK to the server and sets on TIME WAIT state Lesson end Activities 1. Draw the finite state machine states Points for Discussion 1. What is finite state machine? 17.7 Check your Progress 1. What are steps in TCP finite state machine? 17.8 References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

106 Lesson 18. TCP Connection Management 18.0 Aim and Objectives 18.1 Introduction 18.2 Connection Extablishment 18.3 Connection Release 18.4 Let us Sum Up 18.5 Lesson end Activities 18.6 Points for Discussion 18.7 Check Your Progress 18.8 References 18.0 Aim and Objectives 18.1 Introduction To under stand how TCP connections are established and released TCP is connection oriented. It establishes a logical connection between the two end to end systems. Before transmitting data they will exchange the connection information called handshake Connection Establishment Establishing connection the nodes are exchanged three segments so it is called three-way handshake. One side executes a CONNECT primitive, specifying the destination IP address, destination port, window size, and optionally some user data. This is delivered in a TCP segment with the SYN flag on, the ACK flag off, and an Initial Sequence Number (ISN) which is randomly chosen. This is illustrated in Figure 18.1.

107 Figure 18.1.: TCP open connection. a: synchronous, b: simultaneous (full-duplex). At first, one must say that host 2 has previously performed a LISTEN primitive on the appropriate port. If not, the connection is rejected. In (a), host 1 opens the connection with an ISN of x. Host 2 accepts the connect request by sending a TCP segment which acknowledges host 1's request (ACK flag on and the ACKNOWLEDGEMENT NUMBER set to x+1) and its own connection request (SYN flag on with an ISN of y). Host 1 acknowledges this request. Note that the SYN flag consumes one byte of sequence space so that it can be acknowledged unambiguously. In (b), both hosts attempt a connection request at the same time. Only one connection is established, not two, because a connection is identified by their end points, which are the same in this case Connection Release The three-way handshake is also used to terminate a connection, as illustrated in the following figure. Figure 18.2 : TCP close connection In this example, host 1 terminates the connection by transmitting a segment with the FIN flag set, containing optional data. Host 2 acknowledges this (the FIN flag also consumes one byte of sequence space) and sets its own FIN flag. The third and last segment contains host 1's acknowledgement of host 2's FIN flag. During closing of connection, there can be following additional states in the State Transition diagram: FIN_WAIT1: AT THE CLIENT: The application shuts down. TCP sends the last data with FIN to the server and moves into this state.

108 FIN_WAIT2: AT THE CLIENT: On receipt of the Ack from the server, TCP moves into the state. Waiting for the server to shutdown. CLOSE_WAIT: AT THE SERVER: The client has shutdown and is waiting for us to finish writing our data and to shutdown. LAST_ACK: AT THE SERVER: The server has shutdown after the client has shutdown. However the client has yet to acknowledge the segment. CLOSING: AT THE CLIENT: both applications are closing nearly simultaneously; The Fin segments may be received by each side from the other side. But Ack from the server may not have been received. Closing a TCP connection (continued) SENDER A Active Close Application Close Passive Close RECEIVER B REC FIN Send FIN, SEQ=P Segment 1 Inform Application by delivering EOF SEND ACK=P+1 Segment 2 REC ACK Application Closes connection Segment 3 Send FIN, SEQ=Q, ACK=P+1 REC FIN + ACK Inform Application by delivering EOF, Time-Wait SEND ACK=Q+1 Segment 4 REC ACK

109 Figure 18.3 Closing a TCP connection (continued) 18.4 Let us Sum Up TCP is connection oriented. It establishes a logical connection between the two end to end systems. Before transmitting data they will exchange the connection information called handshake. One side executes a CONNECT primitive, specifying the destination IP address, destination port, window size, and optionally some user data. This is delivered in a TCP segment with the SYN flag on, the ACK flag off, and an Initial Sequence Number (ISN) which is randomly chosen. During closing of connection, The application shuts down. TCP sends the last data with FIN to the server and moves into this state. On receipt of the Ack from the server, TCP moves into the state and waiting for the server to shutdown. The client has shutdown and is waiting for us to finish writing our data and to shutdown. The server has shutdown after the client has shutdown. However the client has yet to acknowledge the segment. Both applications are closing nearly simultaneously; The Fin segments may be received by each side from the other side. But Ack from the server may not have been received Lesson end Activities 1. How TCP Connection Established? 18.6 Points for discussion 1. What are the information are passed while establishing connection? 2. What are the information are passed while releasing connection? 18.7 Check Your Progress 1. What is Handshaking?

110 18.8 References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq-Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

111 19.0 Aims and Objectives 19.1 Introduction 19.2 Timer Management 19.3 Packet Loss and Retransmission 19.4 Adaptive Retransmission Lesson 19. TCP Output Processing 19.5 Flow Control 19.6 TCP Keep-Alive Messages 19.7 Slow Start Algorithm and Congestion Avoidance 19.8 Silly Window Syndrome (SWS) 19.9 Nagle Algorithm TCP Selective Acknowledgment (RFC Let us Sum Up Lesson and Activities Points for Discussion Check your Progress References 19.0 Aims and Objectives To learn about Timer management To learn about adaptive retransmission To learn about flow control 19.1 Introduction For efficient processing of TCP data we require different techiniques like Timer Management, Packet Loss and Retransmission, Adaptive Retransmission, Flow Control, TCP Keep-Alive Messages, Slow Start Algorithm and Congestion Avoidance, Silly Window Syndrome, Nagle Algorithm and TCP Selective Acknowledgment Timer Management Retransmission Timer(RT) Retransmission Timer started when a message is sent. If no ACK is received before the timer expires the message is retransmitted. The transmitter keeps all transmitted segments in the buffer until they have been ACKed. This timer is based upon the Retransmission Timeout (RTO)

112 Number which will depend upon the network type. When the RT expires, the RTO is increased exponentially (to a preset limit) and the segment retransmitted. If the preset maximum limit is exceeded an error message is sent to the ULP. The RTO is based upon the Round Trip Time (RTT) - average time from transmission to acknowledgement. The RTT is developed from an algorithm that develops an expected Smoothed Round Trip Time (SRTT). This can be set by software. Quiet Timer This timer is started when a port is closed and is normally set to 30 seconds. This prevents a just closed port from responding to a quick open. Late segments from the just closed session may still be in enroute. Their receipt would cause an error. Persistence Timer This timer is started upon receipt of a receive window size = 0. When a transmitting station receives a receive window size = 0, it pauses transmission until notified to begin by the receiving station. This notification may be lost in Internet. If this timer expires, a one byte segment is transmitted to the receiving station to assure it is still alive. If the receiving station is still backlogged, it transmits a receive window size = 0. If the receiving window is open, it transmits the new window size. Keep Alive/Idle Timer The Idle timer (normally set to 360 seconds) and the Keep-Alive timer (normally set to seconds) are started when there in no traffic. When the Keep- Alive timer expires, an empty packet is sent at regular intervals. Applies only to the server. If the Idle timer expires, the connection is assumed to be broken and an error message is generated Packet Loss and Retransmission TCP manages successfully with the loss of packets using a technique called retransmission. When TCP data arrives, an acknowledgement is sent back to the sender. When TCP data is sent, a timer is started. Again, if the timer expires before an acknowledgement arrives, TCP retransmits the data.

113 Figure 19.1 : Retransmission example TCP must be ready to retransmit any packet that is lost on one of the connections Adaptive Retransmission TCP estimates the round-trip for each active connection. For each connection, TCP generates a sequence of round-trip estimates and uses a statistical function to produce a weighted average. It also maintains an estimate of the variance and uses a linear combination of the estimated mean and variance as the value of the timeout. The following figure gives in detail. In (a), we have a connection with a relatively long round-trip delay. In (b), we have a connection with a shorter round-trip delay. The goal is to wait long enough to decide that a packet was lost, without waiting longer than necessary. When delays start to vary, TCP adjusts the timeout to a value greater than the mean to accommodate peaks.

114 Figure : Timeout and retransmission 19.5 Flow Control TCP follows window mechanism to control the flow of data. When a connection is established, each end of the connection allocates a buffer to hold incoming data, and sends the size of the buffer to the other end. As data arrives, the receiver sends acknowledgements together with the amount of buffer space available called a window advertisement. If the receiving application can read data as quickly as it arrives, the receiver will send a positive window advertisement with each acknowledgement. However, if the sender is faster than the receiver, e.g. has a faster CPU or consuming the data is harder then producing it, incoming data will eventually fill the receiving buffer, causing the receiver to announce that the window is zero length. It is up to receiver to determine the window size, to evict packet loss. A sender that receives a zero window advertisement must stop sending until it receives a positive window. This is illustrated in the following figure.

115 Figure 19.3: TCP flow control The window size is generally fixed by operating system and it is the network infrastructure who determines the segment size TCP Keep-Alive Messages A TCP keep-alive packet is simply an ACK with the sequence number set to one less than the current sequence number for the connection. A host receiving one of these ACKs responds with an ACK for the current sequence number. Keep-alives can be used to verify that the computer at the remote end of a connection is still available. TCP keep-alives can be sent once every KeepAliveTime (defaults to 7,200,000 milliseconds or two hours) if no other data or higher-level keep-alives have been carried over the TCP connection. If there is no response to a keep-alive, it is repeated once every KeepAliveInterval seconds. KeepAliveInterval defaults to 1 second.

116 19.7 Slow Start Algorithm and Congestion Avoidance When a connection is established, TCP starts slowly at first to assess the bandwidth of the connection, and to avoid overflowing the receiving host or any other devices or links in the path. The send window is set to two TCP segments, and if that is acknowledged, it is incremented to three segments. If those are acknowledged, it is incremented again, and so on until the amount of data being sent per burst reaches the size of the receive window on the remote host. At that point, the slow start algorithm is no longer in use, and flow control is governed by the receive window. However, congestion could still occur on a connection at any time during transmission. If this happens (evidenced by the need to retransmit), a congestion-avoidance algorithm is used to reduce the send window size temporarily and to grow it back towards the receive window size. Slow start and congestion avoidance are discussed further in RFC 1122 and RFC Silly Window Syndrome (SWS) Silly Window Syndrome is described in RFC SWS is caused by the receiver advancing the right window edge whenever it has any new buffer space available to receive data and by the sender using any incremental window, no matter how small, to send more data. The result can be a stable pattern of sending tiny data segments, even though both sender and receiver have a large total buffer space for the connection. It also implements SWS avoidance on the receive end of a connection by not opening the receive window in increments of less than a TCP segment Nagle Algorithm Nagle algorithm described in RFC 896. The purpose of this algorithm is to reduce the number of very small segments sent, especially on high-delay (remote) links. The Nagle algorithm allows only one small segment to be outstanding at a time without acknowledgment. If more small segments are generated while awaiting the ACK for the first one, these segments are coalesced into one larger segment. Any full-sized segment is always transmitted immediately, on the assumption that there is a sufficient receive window available. The Nagle algorithm is effective in reducing the number of packets sent by interactive applications, such as Telnet, especially over slow links TCP Selective Acknowledgment (RFC 2018) Selective Acknowledgement (SACK) is described in RFC SACK is especially important for connections using large TCP window sizes. Prior to SACK, a receiver could only acknowledge the latest sequence number of contiguous data that had been received, or the left edge of the receive window. When SACK is enabled, the receiver continues to use the ACK number to acknowledge the left edge of the receive window, but it can also acknowledge other non-contiguous blocks of received data individually. SACK uses TCP header options.

117 Let Us Sum TCP Output Processing Timer Management Retransmission Timer(RT) Retransmission Timer started when a message is sent. If no ACK is received before the timer expires the message is retransmitted. The transmitter keeps all transmitted segments in the buffer until they have been ACKed. Quiet Timer This timer is started when a port is closed and is normally set to 30 seconds. Persistence Timer This timer is started upon receipt of a receive window size = 0 Keep Alive/Idle Timer The Idle timer (normally set to 360 seconds) and the Keep-Alive timer (normally set to seconds) are started when there in no traffic. Packet Loss and Retransmission TCP manages successfully with the loss of packets using a technique called retransmission. Adaptive Retransmission TCP estimates the round-trip for each active connection. For each connection, TCP generates a sequence of round-trip estimates and uses a statistical function to produce a weighted average. It also maintains an estimate of the variance and uses a linear combination of the estimated mean and variance as the value of the timeout. Flow Control TCP follows window mechanism to control the flow of data. When a connection is established, each end of the connection allocates a buffer to hold incoming data, and sends the size of the buffer to the other end. As data arrives, the receiver sends acknowledgements together with the amount of buffer space available called a window advertisement.

118 A sender that receives a zero window advertisement must stop sending until it receives a positive window. TCP Keep-Alive Messages A TCP keep-alive packet is simply an ACK with the sequence number set to one less than the current sequence number for the connection Lesson end Activities What are facilities provided by the TCP to time management? Points for Discussion What is the purpose of retransmission Check your Progress 1. What is Adaptive Transmission? 2. How TCP Flow Control works? 3. What is TCP Selective Acknowledgment? References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq-Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

119 Lesson 20. Socket Programming 21.0 Aims and Objectives 20.1 Introduction 20.2 Domain 20.3 Internet address conversion 20.4 Connection less (UDP) Communication 20.5 Connection oriented (TCP) Communication 20.6 Let us Sum Up 20.7 Lesson end Activities 20.8 Points for Discussion 20.9 Check your progress References 20.0 Aims and Objectives To learn about steps in connection oriented and connection less programming 20.1 Introduction In Unix operating system we can write network programming using C in two different methods. One is BSD s Socket programming and other is AT&T s TTL programming. Compare to TTL method Socket programming is easier and flexible. Application Programming Interface(API) C language has own API for system as well as network programming. Most of the Unix functions are written in C. In Unix we have two types of functions. One is library function and another is systems calls. System calls are functions which are interface to kernel. So it fast in execution. In Unix, if you want to write a network program then you have to create a socket. Unix Socket has three properties. They are Domain, Type of the socket and Protocol Domain There are two types of domains are existed in the Internet Community. One is Unix domain and another is Internet domain. Unix domain refers the local unix domain system. It is referred with AF_UNIX and the posix name is AF_INET. This type of process used to communicate with another process on the same unix system. In the Internet domain, it is referred with AF_INET. This type of process used to communicate with another process on another system through internet.

120 In general two type of system architecture are there. One is big endinan and other is little endian. It is nothing but one type of architecture following to store big number in the right hand side and the other type storing small number in the right hand side. In the Internet, there is a need to connect these two types of system. So they have a common conversion method. Whenever a packet has send to network that data should be converted into network byte order. Inet_addr(), htons() and htonl() Whenever a packet has send received from network that data should be converted into host byte order. Inet_ntoa(), ntohs() and ntohl() 20.3 Internet address conversion Internet address is an octet value. It is necessary to convert internet addresses between ASCII strings and network byte ordered binary values. inet_aton() Convert C character string into binary network byte order value. inet_ntoa() Convert binary network byte ordered into its corresponding dotted decimal order. #include<arpa/inet.h> int inet_aton(const char *stptr, struct in_addr *addrptr); char *inet_ntoa(struct in_addr inaddr); We have to use the address structure struct sockaddr_in which is available in the Header file - <netinet/in.h>. struct in_addr { u_long s_addr; }; struct sockaddr_in { u_short sin_family; u_short sin_port; struct in_addr sin_addr; char sin_zero[8]; }; // protocol identifier; usually AF_INET // port number 0, then kernel chose // IP address INADDR_ANY // refers to the IP address of the Current host //unused, always zero struct sockaddr is another address structure which is more generic, but compatible, both are 16 bytes, starting with the same field.

121 struct sockaddr { u_short sa_family; char sa_dat[14];}; Another address structure differ from internet, sockaddr_un is used for unix domain. We have to use the header file <sys/un.h> Socket Type Different types of sockets are available. Mainly we are using UDP, TCP and IP. Datagram SOCK_DGRAM UDP level Stream SOCK_STREAM TCP level Raw SOCK_RAW IP level If AF_UNIX, then it is name of a file not the socket type. Protocols TCP/IP is a collection of collaborative protocols. In this option we have to mention the protocol. TCP IPPROTO_TCP UDP IPPROTO_UDP IP IPPROTO_IP ARP IPPROTO_ARP 0 to choose the default protocol Multiple services TCP/IP can handle multiple services at the same time. A service is a facility or function provided by the server like FTP, telnet. Server provides this service through a protocol and port number. Each service assigned by a port number. Port number is a 16 bit number. With the help of the port number server uniquely identifies application processes/ network services.

122 In Unix, all services and their port numbers are listed in /etc/services file Figure 20.1 Port and their services Ports , are reserved and servers or clients that you create will not be able to bind to these ports unless you have root privilege. Ports , are available for use by your programs, but beware other network applications maybe running and using these port numbers as well so do not make assumptions about the availability of specific port numbers. A server can support more than one service at a time from different client. If it is not received the request from the client then it would idle. So, start a server process when it needs. In Unix Inetd daemon taken care of this. Inetd bind port number to the service. In program we have to use select() system call. A socket contain <protocol, source-address, local-port, destination address, destination-port> <tcp, , 23, , 23>

123 20.4 Connectionless (UDP) Communication steps 1. Create socket 2. Bind address to the socket 3. Send / receive data 4. Close the socket Client/Server application Model The server executes first and waits to receive; the client executes second and sends the first network packet to the server. After initial contact, either the client or the server is capable of sending and receiving data. Figure Client/Server Communication 20.5 Connection oriented (TCP) Communication Features of a TCP connection Connection Oriented Reliability o Handles lost packets o Handles packet sequencing o Handles duplicated packets Full Duplex Flow Control Congestion Control Connection oriented Steps 1. Create socket 2. Bind address to the socket

124 3. Connect 4. Listen 5. Accept 6. Send / receive data 7. Close the socket These steps are clearly shown in the following diagram: Socket creation Figure Client/Server Communication in TCP To create the socket we have the system call socket(). #include <sys/socket.h> int socket(int address_family, int socket_type, int protocol_number); It will return (-1) for failure otherwise success will return socket descriptor which can be used in other network commands. In server first socket created is often known as a master socket. Before send or receive data, it must be connected to another socket. Master socket bound to port number. If the socket creation fail, Use perror() to find out the reason.

125 int socket_desc; socket_desc=socket(af_inet, SOCK_STREAM, 0); If (socket_desc == 0) perror( Create socket); struct sockaddr_in address; address.sin_family = AF_INET; address.sin_addr.s.addr = INADDR_ANY; 7000 port is used for the connection address.sin_port = htons(7000); Binding address Binding service name to port number we have system call bind(). It is used to specify for a socket the protocol port number where it will wait for messages. #include<sys/un.h> - Unix domain #include<netinet/in.h> - Internet domain int bind(int socket_id, struct sockaddr * address, int address_length); socket_id - as created by the socket function address pointer to protocol structure of this socket address_length length in bytes of structure Referencesd by address If bind fails, use perror() to find out the reason and success, use sendto()/ recvfrom() functions to send or receive data. Connect Connect() system call is used on the client side to identify and possibly, start connection to the server. It is required for connection oriented. Eg. int connect ( in sd, const struct sockaddr *addr, int addrlen) sd - file descriptor of the local socket addr - pointer to protocol address of other scoket addrlen - length in bytes of address strucutre #define SERV_NAME // #define SERV_PORT // 8001 srtuct sockaddr_in servaddr; struct hostent *hp; // info about host int sd; // initialisation bzero((char *) &servaddr, sizeof(servaddr)): servaddr.sin_family = AF_INET; servaddr.sin.port = htons(serv_port); hp = gethostbyname(serv_name); If (hp == 0) {

126 printf(stderr, \nfailure to find %s\n, SERV_NAME); exit(1);} bcopy(hp->h_addr_list[0], (caddr_t)&servaddr.sin_addr, hp->h_length); If(connect(sd, (struct sockaddr *) &servaddr, sizeof(servaddr)) < 0) { perror( Connect ); exit(1); Information about Hostname To get the details about the system and host name we have different functions and we have to include the header file <netdb.h> Function prototype struct hostent *gethostbyname(const char *hostname); Other functions are getpeername, gethostbyaddr, getprotobyname, getprotobynumber, getprotoent, getservbyname, getservbyport, getservent, getnetbyname, getnetbynumber, getnetent Info about hostname+aliases+ip addresses struct hostent { char *h_name; // host name char **h_aliases; // null terminated list od alises int h_addrtype; //host address type int h_length; //length of address structure char **h_addr_list; // null terminated list of address from name server #define h_addr h_addr_list[0] }; // address for backward compatibility Listen for connection Socket must be told to listen for connection and also the maximum number of pending connection using listen() system calls. We have to include the header file <sys/socket.h> int listen(int sd, int qlen); listen(socket_desc, 3); there can be upto 3 connections pending If a connection request arrives when there are already 3 connections pending, the client receives a timeout error.

127 Accept It tell the server to accept a connection. For this we have to use the header file <sys/socket.h> Sending data int accept(int sd, struct sockaddr *addr, int addrlen); int addrlen; struct sockaddr_in address; addrlen = sizeof(struct sockaddr_in); new_socket = accept(sock_desc, (struct sockaddr *)&address, &addrlen) if (new_socket<0) perror( accept connection ); To send the data or message we have the system call sendto(). int sendto( int sd, char *msg, int len, int flags, struct sockaddr *to, int tolen); // socket descriptor // data to be sent // size of message // 0 or options -priority // pointer to destination // length of the address Flags in sendto It have many flags some of them are : MSG_DONTROUTE MSG_DONTWAIT - bypass lookup of routing table - non-blocking(will return error code if it cannot be completed immediately) MSG_OOB send out-of-band (urgent) data Receiving data Like sendto we also have separate system call for receiving data. int recvfrom( int sd, // socket descriptor char *msg, // data to be sent int len // size of message

128 int flags, // 0 or options - priority struct sockaddr *from, // pointer to address of sender int fromlen); // length of the sender address Flags in Receiving Data Receiving data also have separate flags some of them are : MSG_DONTWAIT - non-blocking(will return error code if it cannot be completed immediately) MSG_OOB MSG_PEEK MSG_WAITALL send out-of-band (urgent) data - peek at incoming - wait for all the data requested with length Closing the socket The final step in the programe is closing the socket. We close the socket either using close system call or shutdown system call. int close(int sockfd); or int shutdown(int sd, int action); Action 0 close for read 1 close for write 2 close for both read and write

129 20.6 Let Us Sum Up Domain There are two types of domains are existed in the Internet Community. One is Unix domain and another is Internet domain. Internet address conversion Internet address is an octet value. It is necessary to convert internet addresses between ASCII strings and network byte ordered binary values. inet_aton() Convert C character string into binary network byte order value. inet_ntoa()convert binary network byte ordered into its corresponding dotted decimal order. Another address structure differ from internet, sockaddr_un is used for unix domain. We have to use the header file <sys/un.h> Socket Type Protocols Different types of sockets are available. Mainly we are using UDP, TCP and IP. Datagram SOCK_DGRAM UDP level Stream SOCK_STREAM TCP level Raw SOCK_RAW IP level If AF_UNIX, then it is name of a file not the socket type. TCP/IP is a collection of collaborative protocols. In this option we have to mention the protocol. TCP IPPROTO_TCP UDP IPPROTO_UDP IP IPPROTO_IP ARP IPPROTO_ARP 0 to choose the default protocol Multiple services TCP/IP can handle multiple services at the same time. A service is a facility or function provided by the server like FTP, telnet.

130 Ports , are reserved and servers or clients that you create will not be able to bind to these ports unless you have root privilege. Ports , are available for use by your programs, but beware other network applications maybe running and using these port numbers as well so do not make assumptions about the availability of specific port numbers. Connectionless (UDP) Communication steps 1. Create socket 2. Bind address to the socket 3. Send / receive data 4. Close the socket Connection Oriented (TCP) Communication steps 1. Create socket 2. Bind address to the socket 3. Connect 4. Listen 5. Accept 6. Send / receive data 7. Close the socket Socket creation Int socket(int address_family, int socket_type, int protocol_number); Binding address Binding service name to port number we have system call bind(). It is used to specify for a socket the protocol port number where it will wait for messages. Connect Connect() system call is used on the client side to identify and possibly, start connection to the server. It is required for connection oriented. Information about Hostname To get the details about the system and host name we have different functions and we have to include the header file <netdb.h>

131 Listen for connection Socket must be told to listen for connection and also the maximum number of pending connection using listen() system calls. We have to include the header file <sys/socket.h> Accept int listen(int sd, int qlen); It tell the server to accept a connection. For this we have to use the header file <sys/socket.h> Sending data int accept(int sd, struct sockaddr *addr, int addrlen); To send the data or message we have the system call sendto(). int sendto( int sd, char *msg, int len, int flags, struct sockaddr *to, int tolen); // socket descriptor // data to be sent // size of message // 0 or options -priority // pointer to destination // length of the address Receiving data Like sendto we also have separate system call for receiving data. int recvfrom( int sd, char *msg, int len int flags, struct sockaddr *from, int fromlen); // socket descriptor // data to be sent // size of message // 0 or options - priority // pointer to address of sender // length of the sender address Closing the socket The final step in the programe is closing the socket. We close the socket either using close system call or shutdown system call. int close(int sockfd); int shutdown(int sd, int action);

132 Action 0 close for read 1 close for write 2 close for both read and write 20.7 Lesson end Activities 1. Write an algorithm for connection oriented communication. 2. Write an algorithm for connection less communication 20.8 Points for Discussion Why we need two type of domain? 20.9 Check your progress How different protocols can be adopted in socket programming? References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill 5. Unix Network programming Networking APIs: Sockets and XTI Volume I, W. Richard Stevens

133 21.0 Aims and Objectives 21.1 Introduction 21.2 Telnet 21.3 Rlogin 21.4 Let us Sum Up 21.5 Lesson end Activities 21.6 Points for Discussion 21.7 Check your Progress 21.8 References Lesson 21. Application Layer.21.0 Aims and Objectives To learn about Remote login and telnet 21.1 Introduction The Application layer handles the details of the particular application or service. This is the layer which used to interact the users. There are many predefined such services are implemented. Some important services are: Remote Login (Telnet and Rlogin) FTP Etc. Remote Login This is implemented based on Client/Server computing concepts. If a Client system wants to communicate to Server, normally we need physical cabling. If the Server is far away from the Client location then it is very difficult for direct physical cabling. A user from the client system can login into the server which is far away from the server using Remote Login without direct physical cabling. There are two type of Remote login available under TCP/IP, ie Telnet and Rlogin

134 Telnet Vs Rlogin Using telnet different Operating System can communicate each other. They can negotiate for the type of services provided by them and how they are going to communicate each other. For example a Unix user can login from the Windows system to Unix system Implemented in Berkely Unix to communicate between different Unix Operating system. So a user from one unix server can login to remote unix server using Rlogin Telnet Implemented based on Client server Model. Telnet client Telnet Server Login shell Terminal driver TCP/IP TCP/IP Pseudo terminal driver User at a terminal TCP Connection Figure 21.1 Telnet Client/Server Overview Telnet client interact with the user terminal where the user working and TCI/IP protocol for remote communication. Whatever the user type is sent to the remote system and the server response is displayed in the user terminal. Telnet server interacts with TCP/IP protocol for the client communication and pseudo terminal device. The pseudo device interacts with login shell. From the Client to the Server, only one TCP connection is used for two way communication.

135 The terminal, pseudo terminal and TCP/IP implementation are part of the Operating system kernel. The Telnet client and server are user application. The TELNET Protocol The Telnet protocol is used for remote logins to computer via the Internet. It is described in RFC854. TELNET is a protocol that provides a general, bi-directional, eight-bit byte oriented communications facility. telnet is a program that supports the TELNET protocol over TCP. Many application protocols are built upon the TELNET protocol. The Network Virtual Terminal Communication is established using the TCP/IP protocols and communication is based on a set of facilities known as a Network Virtual Terminal (NVT). It uses data and control over the same connection At the user or client end the telnet client program is responsible for mapping incoming NVT codes to the actual codes needed to operate the user's display device and is also responsible for mapping user generated keyboard sequences into NVT sequences. The NVT uses 7 bit codes for characters, the display device, referred to as a printer in the RFC, is only required to display the "standard" printing ASCII characters represented by 7 bit codes and to recognise and process certain control codes. The 7 bit characters are transmitted as 8 bit bytes with most significant bit set to zero. An end-of-line is transmitted as the character sequence CR (carriage return) followed by LF (line feed). If it is desired to transmit an actual carriage return this is transmitted as a carriage return followed by a NUL (all bits zero) character. All NVTs support a minimal set of capabilities. Some terminals have more capabilities than the minimal set. The set of options is not part of the TELNET protocol, so that new terminal features can be incorporated without changing the TELNET protocol. TELNET includes support for a series of control functions commonly supported by servers. Control Codes for NVT Name code Decimal Value Function NULL NUL 0 No operation Line Feed LF 10 Moves the printer to the next print line, keeping the same horizontal position. Carriage Return CR 13 Moves the printer to the left margin of the current line.

136 The following further control codes are optional but should have the indicated defined effect on the display. Name code Decimal Value Function BELL BEL 7 Produces an audible or visible signal (which does NOT move the print head. Back Space BS 8 Moves the print head one character position towards the left margin. [On a printing devices this mechanism was commonly used to form composite characters by printing two basic characters on top of each other.] Horizontal HT 9 Tab Moves the printer to the next horizontal tab stop. It remains unspecified how either party determines or establishes where such tab stops are located. Vertical Tab Form Feed VT 11 FF 12 Moves the printer to the next vertical tab stop. It remains unspecified how either party determines or establishes where such tab stops are located. Moves the printer to the top of the next page, keeping the same horizontal position. [On visual displays this commonly clears the screen and moves the cursor to the top left corner.] The NVT keyboard is specified as being capable of generating all 128 ASCII codes by using keys, key combinations or key sequences. Commands Commands are always introduced by a character with the decimal code 255 known as an Interpret as Command (IAC) character. The complete set of special characters are : Name Decimal Code Meaning SE 240 End of sub negotiation parameters. NOP 241 No operation

137 DM 242 Data mark. Indicates the position of a Synch event within the data stream. This should always be accompanied by a TCP urgent notification. BRK 243 Break. Indicates that the "break" or "attention" key was hit. IP 244 AO 245 AYT 246 EC 247 EL 248 GA 249 Suspend, interrupt or abort the process to which the NVT is connected. Abort output. Allows the current process to run to completion but do not send its output to the user. Are you there? Send back to the NVT some visible evidence that the AYT was received. Erase character. The receiver should delete the last preceding undeleted character from the data stream. Erase line. Delete characters from the data stream back to but not including the previous CRLF. Go ahead. Used, under certain circumstances, to tell the other end that it can transmit. SB 250 Sub negotiation of the indicated option follows. WILL 251 WONT 252 DO 253 DONT 254 Indicates the desire to begin performing, or confirmation that you are now performing, the indicated option. Indicates the refusal to perform, or continue performing, the indicated option. Indicates the request that the other party perform, or confirmation that you are expecting the other party to perform, the indicated option. Indicates the demand that the other party stop performing, or confirmation that you are no longer expecting the other party to perform, the indicated option. IAC 255 Interpret as command

138 Options are agreed by a process of negotiation which results in the client and server having a common view of various extra capabilities that affect the interchange and the operation of applications. Either end of a telnet dialogue can enable or disable an option either locally or remotely. The initiator sends a 3 byte command of the form IAC,<type of operation>,<option> The following patterns of option negotiation are defined. Sender Sent Receiver Responds Implication WILL WILL DO DO DO DONT WILL WONT The sender would like to use a certain option if the receiver can handle it. The receiver says it can support the option. Option is now in effect The sender would like to use a certain option if the receiver can handle it. The receiver says it cannot support the option. Option is not in effect. The sender requests that the receiver use a certain option. The receiver says it can support the option. Option is now in effect. The sender requests that the receiver use a certain option. The receiver says it cannot support the option. Option is not in effect. WONT DONT DONT WONT The sender will not use a certain option. Option disabled. DONT is only valid response. The sender requests that the receiver does not use a certain option. Option disabled. WONT is only valid response. For example if the sender wants the other end to suppress go-ahead it would send the byte sequence 255(IAC),251(WILL),3 The final byte of the three byte sequence identifies the required action.

139 For some of the negotiable options values need to be communicated once support of the option has been agreed. This is done using sub-option negotiation. Values are communicated via an exchange of value query commands and responses in the following form. IAC,SB,<option code number>,1,iac,se and IAC,SB,<option code>,0,<value>,iac,se For example if the client wishes to identify the terminal type to the server the following exchange might take place Client 255(IAC),251(WILL),24 Server 255(IAC),253(DO),24 Server 255(IAC),250(SB),24,1,255(IAC),240(SE) Client 255(IAC),250(SB),24,0,'V','T','2','2','0',255(IAC),240(SE) The first exchange establishes that terminal type (option number 24) will be handled, the server then enquires of the client what value it wishes to associate with the terminal type. The sequence SB,24,1 implies sub-option negotiation for option type 24, value required (1). The IAC,SE sequence indicates the end of this request. The repsonse IAC,SB,24,0,'V'... implies suboption negotiation for option type 24, value supplied (0), the IAC,SE sequence indicates the end of the response (and the supplied value). telnet is a generic TCP client. It sends whatever you type to the TCP socket. telnet prints whatever comes back through the TCP socket. telnet used for testing TCP servers (ASCII based protocols). Example for telnet communication 1. telnet cse.bharathi.edu 7 2. Trying Connected to cse.bharathi.edu ( ). 4. Escape character is '^]'. 5. Hi Ramesh 6. Hi Ramesh 7. stop it 8. stop it 9. ^] 10. telnet> quit 11. Connection closed.

140 21.3 Rlogin Rlogin used to login between UNIX hosts. It is similar to Telnet but option negotiation is not required, since both operating system on the client and server are known in advance. Rlogin uses a single TCP connection between the client and server. In the server system, the user has to create a file.rhosts and lines containing the clinet host name and other user name. Flow control is done by the Rlogin client otherwise user has to type Control-S to stop the terminal output. Client can interrupt the process on the server by pressing interrupt keys. The protocol requires rlogin server software to be running on the host that is going to allow remote access; it is usually called rlogind (for rlogin daemon, the latter word being the standard UNIX term for a background server process). The server listens for incoming connection requests on TCP port 513. A user who wants to remotely log in to the server runs the rlogin command on his or her local host, and specifies the name of the server. The client makes a TCP connection to the server, and then sends to the server a string containing the following information: The login name of the user on the client machine. The login name that the user wants to use on the server (which is often the same as the user s login name on the client, but not always.) Control information such as the type and speed of the terminal. The server processes this information and begins the login process. It will normally prompt the user for a password to log in to the remote host. Assuming the password is correct; the user will be logged in to the remote host and can use it as if he or she were locally connected Let Us Sum Up Application Layer The Application layer handles the details of the particular application or service. Remote Login This is implemented based on Client/Server computing concepts. There are two type of Remote login available under TCP/IP, ie Telnet and Rlogin

141 Telnet Implemented based on Client server Model. Telnet client interact with the user terminal where the user working and TCI/IP protocol for remote communication. The TELNET Protocol The Telnet protocol is used for remote logins to computer via the Internet. It is described in RFC854. TELNET is a protocol that provides a general, bi-directional, eight-bit byte oriented communications facility. telnet is a program that supports the TELNET protocol over TCP. Many application protocols are built upon the TELNET protocol. The Network Virtual Terminal The NVT keyboard is specified as being capable of generating all 128 ASCII codes by using keys, key combinations or key sequences. Commands Commands are always introduced by a character with the decimal code 255 known as an Interpret as command (IAC) character. The complete set of special characters are : Rlogin Rlogin used to login between UNIX hosts. It is similar to Telnet but option negotiation is not required, since both operating system on the client and server are known in advance. In the server system, the user has to create a file.rhosts and lines containing the clinet host name and other user name Lesson end Activities What is the difference between Remote login and telnet? 21.6 Points for Discussion What is the purpose of remote login and telnet? 21.7 Check your Progress 1. How Remote login works? 2. How tenet works? 3. Write Short Notes on a) R Login, b) Telnet

142 21.8 References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

143 22.0 Aims and Objectives 22.1 Introduction 22.2 File transfer protocol (FTP) 22.3 Trivial File Transfer protocol (TFTP) 22.4 Let us Sum Up 22.5 Lesson End Activities 22.6 Point for Discussion 22.7 Check your Progress 22.8 References Lesson 22. File Transfer Protocol 22.0 Aims and Objectives To learn about File transfer protocol 22.1 Introduction Network operating system provides the facilities to share the file in the network in any one of the three methods. In the first method, the server provide the storage facilities for the files, which has been shared by client computers those computers do not have local hard disk. In the second methods the client computers also have the hard disk. Periodically, each computer sends or updates the copies of the files to the server using archival facility, so it will helpful in accidental loss. Third method is used in the big organizations like banks where we have one databases but it can be shared by different users, from different places. On-line access and whole-file copying On-line access methods provide the facilities to access the same file concurrently. Whatever changes made to the file take immediately and that also available to all program that access the file. Whole-file copying methods provide that whenever a program wants to access a file, the program obtains a local copy File Transfer Protocol (FTP) In the Internet File Transfer protocol (FTP) is the standard protocol for file transfer. FTP copies a complete file from one system to another system. To use FTP we need a user account to login otherwise we have to use the anonymous FTP user account. It is defined in RFC 959. The file transfer protocol (FTP) allows a user on any computer to get files from another computer, or to send files to another computer. Security is handled by requiring the user to specify a user name and password for the other computer. Provisions are made for handling file transfer between machines with different character set, end of line conventions, etc. This

144 is not quite the same thing as more recent "network file system" or "netbios" protocols, which will be described below. Rather, FTP is a utility that you run any time you want to access a file on another system. You have to use it to copy the file to your own system. You then work with the local copy. The main issues to be considered are: 1. Procedures for accumulating messages into files so as to minimise FTP overheads with short messages (applies only to existing message types); 2. file naming conventions for existing message types (existing AHL); 3. file naming conventions for new message types (no existing AHL); 4. file renaming; 5. use of directories; 6. account names and passwords; 7. FTP sessions; 8. Local FTP requirements; 9. File compression. Advantages of FTP Interactive Access FTP provides an interactive interface that allows users to interact with remote servers. Format Specification FTP allows the clients to specify the type and representation of stored data. Authentication Control Server allows only the authorized clients to access.

145 User at a terminal User interface User protocol interpreter Control connection (FTP commands) Server protocol interpreter FTP replies File system User data transfer function Data connection Server data transfer function File system Figure 22.1 File Transfer protocol processes FTP uses two TCP connections to transfer a file. The server always waiting for the client request at the port for FTP(21). It is a passive open. The client initiates the connection. it is an active open. Based on this the control connection is established in the normal client-server fashion. This connection stays up for the entire period of communication. This connection is used for commands from the client to the server and for the server s replies. A data connection is created each time a file is transferred between the client and server. The user interface used to interact the user and convert these into FTP commands that are sent to the server and vice versa through the control connection. Data Representation FTP protocol provides different types of data transfer. These are based on File type, Format control, and structure and transmission mode. File Type ASCII file type The sender converts the local text file into NVT ACSII format and send through data connection. On the other end receiver convert the NVT ASCII format file into local text file format. This is the default method.

146 File Control Structure EBCDIC file type If the both systems are EBCDIC systems then this method is used for text file conversion. Image file type The data is sent as a contiguous stream of bits. Also called binary file type. Local file type This method of binary files is transfer with different sizes. So the sender has to specify the number of bits per transmission byte. ASCII and EBCDIC file type can use this choice Nonprint The file contains no vertical format information. This is the default method. Telnet format control The file contains telnet vertical format controls for a printer to interpret. Fortran carriage control File structure Fortran format control characters are used for first character of each line. The file is considered as a contiguous stream of bytes. There is no internal file structure. This is the default structure. Record structure This structure is only used with text files. Page structure Each page is transmitted with page number. So the receiver can receive the page in random order.

147 Transmission mode Stream mode The file is transferred as a stream of bytes. Block mode The file transferred as a series of blocks, each preceded by one or more header bytes. Compressed mode Simple run-length encoding compresses consecutive appearances of the same byte. Anonymous FTP FTP allows only the authorized users to access the files. Some files may be used the public. In this case most TCP/IP provides the facility called anonymous FTP user whish don t have password Trivial File Transfer protocol (TFTP) FTP requires clients and servers to manage multiple concurrent TCP connections. TFTP does not need complex connections like FTP. It provides restricted operations and does not provide any authentication. TFTP runs on top of UDP or any other unreliable packet delivery system, using timeout and retransmission to ensure that data arrives. The sending side transmits a file in fixed size blocks and awaits an acknowledge for each block before sending the next. The receiver acknowledges each block upon receipt. Client or server first sends a request for file transfer. The request packet contains file name and whether the file will be read or written. Block of the file is numbered consecutively starting at 1. Each data packet contains a header that specifies the number of the blocks it carriers, and each acknowledgement contains the number of block being acknowledge. A block of less than 512 bytes signals the end of file. It is possible to send an error message either in the place of data or an acknowledgement; error terminates the transfer. The initial packet must use operation codes 1 or 2 to specify either read request or a write request, the FILENAME field to specify the name of a file, and MODE field to specify whether the client will read the file, write the file, or both.

148 Common FTP Commands? to request help or information about the FTP commands ascii to set the mode of file transfer to ASCII (this is the default and transmits seven bits per character) binary to set the mode of file transfer to binary (the binary mode transmits all eight bits per byte and thus provides less chance of a transmission error and must be used to transmit files other than ASCII files) bye cd close to exit the FTP environment (same as quit) to change directory on the remote machine to terminate a connection with another computer close brubeck closes the current FTP connection with brubeck, but still leaves you within the FTP environment. delete to delete (remove) a file in the current remote directory (same as rm in UNIX) get to copy one file from the remote machine to the local machine get ABC DEF copies file ABC in the current remote directory to (or on top of) a file named DEF in your current local directory. get ABC copies file ABC in the current remote directory to (or on top of) a file with the same name, ABC, in your current local directory. help lcd ls to request a list of all available FTP commands to change directory on your local machine (same as UNIX cd) to list the names of the files in the current remote directory mkdir to make a new directory within the current remote directory mget to copy multiple files from the remote machine to the local machine; you are prompted for a y/n answer before transferring each file

149 mget * copies all the files in the current remote directory to your current local directory, using the same filenames. Notice the use of the wild card character, *. mput open to copy multiple files from the local machine to the remote machine; you are prompted for a y/n answer before transferring each file to open a connection with another computer open brubeck opens a new FTP connection with brubeck; you must enter a username and password for a brubeck account (unless it is to be an anonymous connection). put pwd quit to copy one file from the local machine to the remote machine to find out the pathname of the current directory on the remote machine to exit the FTP environment (same as bye) rmdir to to remove (delete) a directory in the current remote directory 22.4 Let Us Sum Up File transfer protocol (FTP) In the Internet File Transfer protocol (FTP) is the standard protocol for file transfer. FTP copies a complete file from one system to another system. To use FTP we need a user account to login otherwise we have to use the anonymous FTP user account. Advantages of FTP Interactive Access FTP provides an interactive interface that allows users to interact with remote servers. Format Specification FTP allows the clients to specify the type and representation of stored data. Authentication Control Server allows only the authorized clients to access.

150 Anonymous FTP FTP allows only the authorized users to access the files. Some files may be used the public. In this case most TCP/IP provides the facility called anonymous FTP user whish don t have password. Trivial File Transfer protocol (TFTP) FTP requires clients and servers to manage multiple concurrent TCP connections. TFTP does not need complex connections like FTP. It provides restricted operations and does not provide any authentication Lesson end activities What are commands available for FTP? 22.6 Points for Discussion What is need for file transfer? 22.7 Check Your Progress 1. Describe the FTP protocols. 2. What is purpose of TFTP? 3. List at the advantages of FTP References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

151 Lesson Aims and Objectives 23.1 Introduction 23.2 Electronic Mail Addresses Scheme 23.3 TCP/IP Standard for Electronic Mail Services 23.4 Simple Mail Transfer Protocol (SMTP) 23.5 Post Office Protocol (POP3) 23.6 Internet Message Access Protocol (IMAP4) 23.7 Let us Sum Up 23.8 Lesson end Activities 23.9 Points for Discussion Check your progress References 23.0 Aims and Objectives To learn how is working To learn about SMTP POP3 and IMAP protocols 23.1 Introduction Electronic mails provide the facilities to users to send memos or files across the Internet, even if the remote destination is temporarily unreachable. The mail system using a technique called spooling for delayed delivery. Whenever a user send a mail message, the system stores the details like message, sender and receiver address and time of send. After some time the message has send as a background process Electronic Mail Addresses Scheme Electronic mail address consists of two parts. The first part is the mailbox(local part) identifying name followed by symbol and the domain name of the destination. Domain name is the domain name of a mail destination to which the mail should be delivered. local part@ domain name Domain name is not necessarily the mail server. Mail server may have longer/cryptic name. Multiple servers may exist to tolerate failures. To Identifying the mail server for a domain the mail server uses DNS query, asking for MX records (Mail exchange). Then, a regular DNS query to learn the IP address.

152 Three major components of s are User agents, Mail servers and Protocols. Users deal with a user agent, of which there are a multitude to choose from. Popular user agents for Unix include MH, Berkeley Mail, Elm, and Mush. The exchange of mail using TCP is performed by a message transfer agent (MTA). The most common MTA for Unix systems is Sendmail. Users normally don't deal with the MTA. It is the responsibility of the system administrator to set up the local MTA. Users often have a choice, however, for their user agent. Protocols used between mail servers is SMTP and between mail server and user agent are POP3 (Post Office Protocol [RFC 1939]) and IMAP( Internet Mail Access Protocol [RFC 1730]) Mail servers used to transferring to and from other servers. Mail server is always on and always accessible. User agents used to intuitive interface for the user. User agents are sometimes on and sometimes accessible. The communication between the two MTAs uses NVT ASCII. Commands are sent by the client to the server, and the server responds with numeric reply codes and optional human-readable strings Figure 23.1 Services of User Agent Messages sent through a series of servers. A server stores incoming messages in a queue. To wait attempts to transmit them to the next hop. If the next hop is not reachable. The server stores the message and tries again later. Each hop adds its identity to the message by adding a Received header with its identity. It is helpful for diagnosing problems with .

153 Alias Expansion and Mail forwarding Mail forwarding software allows copies of an incoming message to be sent to one or more destinations. A single user can have multiple mail identifiers, or a group can have a single mail alias TCP/IP Standard for Electronic Mail Services To extend the interoperability of electronic mail, TCP/IP divides its mail standards into two sets. One standard, given in RFC 2822, specifies the syntactic format used for mail messages; the other standard specifies the details of electronic mail exchange between two computers. According to RFC 2822, a mail message is represented in textual form and is divided into two parts: a header and a body, which is separated by a blank line. The line begin with To: and contains the electronic mail address of the intended recipient on the remainder of the line. A line that begins with From : contains the electronic mail address of the sender Simple Mail Transfer Protocol (SMTP) SMTP protocol focus on how the underlying mail delivery mail delivery system passes messages across an internet from one machine to another. Communication between a client and server consists of readable ASCII text. Commands are sent by the client to the server, and the server responds with numeric reply codes and optional human-readable strings. Simple Mail Transfer Protocol is a client server protocol. Client is the sending mail server. Server is the receiving mail server. It is reliable. It built on top of TCP (on port 25). SMTP is a push protocol. Sending server pushes the file to the receiving server rather than waiting for the receiver to request it. SMTP commands are ASCII text and response is three-digit status code and phrase. To synchronous the communication sender awaits response from a command before issuing the next command. Though pipelining of commands was added later. The communication consists of three phases of transfer handshaking (greeting), transfer of messages and closure. SMTP used to send to a remote mail server. Sending mail server transmits message to a mail server running on a remote machine. Each server in the path adds its identifier to the message. Single TCP connection require for control and data. SMTP uses MIME standard for sending non-text data messages have two parts, a header - in 7-bit U.S. ASCII text and a body, also represented in 7-bit U.S. ASCII text. The header contain series of lines ending in carriage return and line feed. Each line contains a type and value, separated by :

154 Body contains series of text lines with no additional structure/meaning and conventions arose over time (e.g., signatures). It uses 7-bit U.S. ASCII. But for non-english text and binary files like images and executables it converts non-ascii data to ASCII. For this purpose it is using Base64 encoding: map each group of three bytes into four printable U.S.-ASCII characters. Uuencode (Unix-to-Unix Encoding) was widely used. Envelopes, Headers, and Body Electronic mail is composed of three pieces. 1. The envelope is used by the MTAs for delivery. RFC 821 specifies the contents and interpretation of the envelope, and the protocol used to exchange mail across a TCP connection. 2. Headers are used by the user agents. Each header field contains a name, followed by a colon, followed by the field value. RFC 822 specifies the format and interpretation of the header fields. (Headers beginning with an X- are user-defined fields. The others are defined by RFC 822.) 3. The body is the content of the message from the sending user to the receiving user. RFC 822 specifies the body as lines of NVT ASCII text. When transferred using the DATA command, the headers are sent first, followed by a blank line, followed by the body. Each line transferred using the DATA command must be less than 1000 bytes. The user agent takes what we specify as the body, adds some headers, and passes the result to the MTA. The MTA adds a few headers, adds the envelope, and sends the result to another MTA. The term content is often used to describe the combination of headers and the body. The content is sent by the client with the Multipurpose Internet Mail Extensions(MIME) MIME does not require any of the extensions that we've described previously in this section (extended SMTP or non-ascii headers). MIME just adds some new headers (in accordance with RFC 822) that tell the recipient the structure of the body The body can still be transmitted using NVT ASCII, regardless of the mail contents. While some of the extensions we've just described might be nice to have along with MIME-the extended SMTP SIZE command, since MIME messages can become large, and non-ascii headers-these extensions are not required by MIME. All that's required to exchange MIME messages with another party is for both ends to have a user agent that understands MIME. No changes are required in any of the MTAs.

155 In MIME we have to mention the content type ie., the type of data contained in the message and content-transfer-encoding ie., : how the data are encoded It contains definitions for a set of content types and subtypes. E.g., like image with subtypes gif and jpeg or text with subtypes plain, html, and richtext or application with subtypes postscript and msword or multipart for messages with multiple data types. A way to encode the data in ASCII format normally as base64 encoding, as in uuencode/uudecode MIME version method used to encode data type and subtype From: [email protected] To: [email protected] Subject: University emblem MIME-Version: 1.0 Content- Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data base64 encoded data encoded data Retrieving from the Server Figure 23.2 MIME type Server stores incoming by mailbox based on the From field in the message. Users need to retrieve asynchronous from when the message was sent with a way to view the message and reply and with a way to organize and store the messages.

156 23.5 Post Office Protocol (POP3) POP support users with intermittent network connectivity. Allow them to retrieve messages when connected and view/manipulate messages when disconnected. Typical user-agent interacts with a POP server to connect the server and retrieve all messages. It stores the retrieve messages on the user s PCs as new messages. It deletes the messages from the server and disconnect from the server. User agent still uses SMTP to send messages but does not handle multiple mailboxes easily. It designed to put user s incoming e- mail in one folder. POP not designed to keep messages on the server instead, designed to download messages to the client. SMTP poorly handling of multiple-client access to mailbox because increasingly important as users have home PC, work PC, laptop, cyber café computer, friend s machine, etc. High network bandwidth overhead required for transfers all of the messages, often well before they are read and supports connected and disconnected operation. Users can download message contents on demand. Multiple clients can connect to mailbox at once to detect changes made to the mailbox by other clients. Server keeps state about message (e.g., read, replied to). Clients can retrieve individual parts separately of MIME parts of messages and partial fetch E.g., text of a message without downloading attachments. Multiple mailboxes on the server are require for client so client can create, rename, and delete mailboxes and client can move messages from one folder to another. It should provide search on server before downloading messages 23.6 Internet Message Access Protocol (IMAP4) The current version of IMAP since 1996, IMAP version 4 revision 1 (IMAP4rev1), is defined by RFC Advantages Connected and disconnected modes of operation When using IMAP4, clients often stay connected as long as the user interface is active and download message content on demand. Multiple clients simultaneously connected to the same mailbox IMAP protocol specifically allows simultaneous access by multiple clients and provides mechanisms for clients to detect changes made to the mailbox by other, concurrently connected, clients.

157 Access to MIME message parts and partial fetch The IMAP4 protocol allows clients to separately retrieve any of the individual MIME parts and also to retrieve portions of either individual parts or the entire message. These mechanisms allow clients to retrieve the text portion of a message without retrieving attached files or to stream content as it is being fetched. Message state information Through the use of flags defined in the IMAP4 protocol clients can keep track of message state, for example whether or not the message has been read, replied to, or deleted. These flags are stored on the server, so different clients accessing the same mailbox at different times can detect state changes made by other clients. The IMAP4 protocol supports both pre-defined system flags and client defined keywords, System flags indicate state information such as whether a message has been read. Keywords, which are not supported by all IMAP servers, allow messages to be given one or more tags whose meaning is up to the client. Multiple mailboxes on the server IMAP4 clients can create, rename, and/or delete on the server, and move messages between mailboxes. Multiple mailbox support also allows servers to provide access to shared and public folders. Server-side searches IMAP4 provides a mechanism for a client to ask the server to search for messages meeting a variety of criteria. This mechanism avoids requiring clients to download every message in the mailbox in order to perform these searches. Built-in extension mechanism Reflecting the experience of earlier Internet protocols, IMAP4 defines an explicit mechanism by which it may be extended. Many extension to the base protocol have been proposed and are in common use. IMAP2bis did not have an extension mechanism, and POP3 now has one defined by RFC Let Us Sum UP Electronic mails provide the facilities to users to send memos or files across the Internet, even if the remote destination is temporarily unreachable.

158 Simple Mail Transfer Protocol (SMTP) SMTP protocol focus on how the underlying mail delivery mail delivery system passes messages across an internet from one machine to another. Communication between a client and server consists of readable ASCII text. Commands are sent by the client to the server, and the server responds with numeric reply codes and optional human-readable strings. Electronic mail is composed of three pieces. 1. The envelope is used by the MTAs for delivery. RFC 821 specifies the contents and interpretation of the envelope, and the protocol used to exchange mail across a TCP connection. 2. Headers are used by the user agents. Each header field contains a name, followed by a colon, followed by the field value. RFC 822 specifies the format and interpretation of the header fields. (Headers beginning with an X- are user-defined fields. The others are defined by RFC 822.) 3. The body is the content of the message from the sending user to the receiving user. RFC 822 specifies the body as lines of NVT ASCII text. When transferred using the DATA command, the headers are sent first, followed by a blank line, followed by the body. Each line transferred using the DATA command must be less than 1000 bytes. The user agent takes what we specify as the body, adds some headers, and passes the result to the MTA. The MTA adds a few headers, adds the envelope, and sends the result to another MTA. The term content is often used to describe the combination of headers and the body. The content is sent by the client with the Multipurpose Internet Mail Extensions(MIME) MIME does not require any of the extensions that we've described previously in this section (extended SMTP or non-ascii headers) Retrieving from the Server Post Office Protocol (POP3) POP support users with intermittent network connectivity. Allow them to retrieve messages when connected and view/manipulate messages when disconnected.

159 IMAP4 (Internet Message Access Protocol) Connected and disconnected modes of operation When using IMAP4, clients often stay connected as long as the user interface is active and download message content on demand. Multiple clients simultaneously connected to the same mailbox IMAP protocol specifically allows simultaneous access by multiple clients and provides mechanisms for clients to detect changes made to the mailbox by other, concurrently connected, clients. Access to MIME message parts and partial fetch The IMAP4 protocol allows clients to separately retrieve any of the individual MIME parts and also to retrieve portions of either individual parts or the entire message. These mechanisms allow clients to retrieve the text portion of a message without retrieving attached files or to stream content as it is being fetched Lesson end Activities 1. What is purpose of MIME? 23.9 Points for Discussion 1. What is the content of ? Check your Progress 1. How SMTP working? 2. What are the advantages of IMAP? 3. What are the advantages of POP3? References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

160 Contents: Lesson 24. INTERNET MANAGEMENT 24.0 Aims and Objectives 24.1 Introduction 24.2 Simple Network Management Protocol (SNMP) 24.3 Let us Sum Up 24.4 Lesson-End Activities 24.5 Point for Discussion 24.6 Check Your Progress 24.7 References 24.0 Aims and Objectives To learn about network management To learn about SNMP Protocol 24.1 Introduction Network Management is used to for effective utilisation of the network resources. Network management of a TCP/IP internet consists of network management stations (managers) communicating with network elements. The network elements can be anything that runs the TCP/IP protocol suite: hosts, routers, X terminals, terminal servers, printers, and so on. The software in the network element that runs the management software is called the agent. Management stations are normally workstations with color monitors that graphically display relevant facts about the elements being monitored (which links are up and down, volume of traffic across various links over time, etc.). The function of the network management can be classified and explained with the following diagram. Figure 24.1 Network management functions

161 24.2 Simple Network Management Protocol (SNMP) Simple Network Management Protocol (SNMP) was developed in the late 80 s in order to offer to network manager a standard tool for controlling networks. The Simple Network Management Protocol (SNMP) is a framework for managing devices in an internet using the TCP/IP protocol suite. This protocol used to governing network management and the monitoring of network devices and their functions. It is a set of protocols for managing complex networks. SNMP is a client-server protocol. It provides a set of fundamental operations for monitoring and maintaining an internet. Component of Network Management Component of the TCP/IP Network management are SNMP, SMI and MIB. 1. A Management Information Base (MIB) that specifies what variables the network elements maintain (the information that can be queried and set by the manager). RFC 1213 defines the second version of this, called MIB-II. 2. A set of common structures and an identification scheme used to reference the variables in the MIB. This is called the Structure of Management Information (SMI) and is specified in RFC The protocol between the manager and the element, called the Simple Network Management Protocol (SNMP). RFC 1157 specifies the protocol. This details the format of the packets exchanged. Although a wide variety of transport protocols could be used, UDP is normally used with SNMP. Figure 24.2 Component of Network management

162 In order to be able to gather information with SNMP, the equipment connected on the network must have Management Information Base (MIB). These bases are catalogues of integer, counters, strings, etc. The manager asks the agents to send it some values. These values might be for example, IP routing table. SNMP defines the format of packets exchanged between a manager and an agent. It reads and changes the status (values) of objects (variables) in SNMP packets SMI defines the general rules for naming objects, defining object types (including range and length), and showing how to encode objects and values. SMI does not define the number of objects an entity should manage or name the objects to be managed or define the association between the objects and their values. MIB creates a collection of named objects, their types, and their relationships to each other in an entity to be managed. MIB is a database of information maintained by the agent that the manager can query or set. Figure 24.3 Object identifier All objects managed by SNMP are given an object identifier. An object identifier is a sequence of integers separated by decimal points. These integers traverse a tree structure, similar to DNS. The object identifier always starts with

163 Figure 24.4 Object Identifier SNMP defines only five types of messages that are exchanged between the manager and agent. 1. Fetch the value of one or more variables: the get-request operator. 2. Fetch the next variable after one or more specified variables: the get-next-request operator. (We describe what we mean by "next" later in this chapter.) 3. Set the value of one or more variables: the set-request operator. 4. Return the value of one or more variables: the get-response operator. This is the message returned by the agent to the manager in response to the get-request, get-next-request, and set-request operators. 5. Notify the manager when something happens on the agent: the trap operator. The first three messages are sent from the manager to the agent, and the last two are from the agent to the manager

164 24.3 Let Us Sum UP SNMP (Simple Network Management protocol) Simple Network Management Protocol (SNMP) was developed in the late 80 s in order to offer to network manager a standard tool for controlling networks. The Simple Network Management Protocol (SNMP) is a framework for managing devices in an internet using the TCP/IP protocol suite. Component of Network Management A Management Information Base (MIB) that specifies what variables the network elements maintain (the information that can be queried and set by the manager). RFC 1213 defines the second version of this, called MIB-II. 1. A set of common structures and an identification scheme used to reference the variables in the MIB. This is called the Structure of Management Information (SMI) and is specified in RFC The protocol between the manager and the element, called the Simple Network Management Protocol (SNMP). RFC 1157 specifies the protocol. This details the format of the packets exchanged. Although a wide variety of transport protocols could be used, UDP is normally used with SNMP Lesson end Activities What is the need for Network management? 24.5 Points for Discussion Explain the function of MIB 24.6 Check You Progress How SNMP works? 24.7 References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill

165 Lesson 25. X.25 Protocol 25.0 Aims and Objectives 25.1 Introduction 25.2 X.25 layers 25.3 Let us Sum Up 25.5 Lesson end Activities 25.5 Points for Discussion 25.6 Check Your Progress 25.7 References 25.0 Aims and Objectives To learn about X.25 Protocol Architecture 25.1 Introduction X.25 is a packet switching protocol used in a wide area network. It also called Subscriber Network interface (SNI). In the X.25 view, a network operates much like a telephone system. X.25 defines how a packet-mode terminal can be connected to a packet network for the exchange of data. It describes the procedures necessary for establishing, maintaining and terminating connection. It also describes the services like reverse charge, call direct and delay control. It defines how the user s DTE communicates with the network and how packets are sent over that network using DCEs. It uses virtual circuit approach to packet switching rather than a datagram approach, and uses asynchronous TDM to multiplex packets X.25 layers X.25 also has OSI seven layers with slight modifications. The important three layers are : the link access procedure, balanced layer; and the packet layer protocol layer. These layers will take care of the functions of OSI physical, data link and network layers. X.25 requires error detection and correction in both the data link and the network layers. Physical layer At the physical layer X.25 specifies a protocol called X.21. This specifies a standard for the physical interconnections between host computers and network packet switches. It describes the procedures used to transfer packet from one machine to another.

166 Data Link Layer X.25 provides data link controls using a bit-oriented protocol called link access procedure, balanced (LAPB) which is a subset of High level Data Link Communication (HDLC). The protocol specifies how data travels between a host and the packet switch to which it connects. It refers data as frame. The The machines exchange acknowledgements for success transmission of frames. Network layer In X.25 the network layer is called Packet Layer Protocol(PLP). The function of this layer is connection establishing, data transfer and connection termination. This layer added his control information in the header. This layer assembles a packet in the form the network expects. Transport Layer Transport layer provides end-to-end reliability. It will take care of the communication between source host and destination host communication. Session Layer This layer used for remote terminal access. In this layer, its networks, a carrier provided a special purpose host computer with dialup access called a Packet Assembler And Disassembler (PAD). Subscribers, often travelers with portable computers, used a modem to dial up the local PAD, made a network connection to a host computer, and logged in. Presentation Layer This layer provides some standard to represent data which is used in application layer. An ISO standard known as Abstract Syntax Notation 1 (ASN 1) is used in this layer. Application layer Application layer deals with user applications like electronic mail or file transfer. PLP uses two types of packets: Information packets and Control Packets. Information Packets Information Packets are used to transmit user data. Information packets are differentiated from control packets by the last bit in the header. If this bit is set to 0, the packet is an information packet. Two types of formats are existing, ie., short and long. The long format used to support facilities with long delays.

167 General format identifier(gfi) The GFI is a four bit field. The first bit is called Q bit. With this user can define two types of data. The D bit is used in packet sequencing. The next two bits are called modulo bits. They indicate the length of the header. If these bits 01, the header is short, or 10 then the header is long. Virtual circuit identification The logical channel group number is a 4-bit field that together with the 8-bit logical channel number, make up a 12-bit number that identifies the virtual circuit chosen for a given transmission. Control field The control field consists of four sessions. P(S) and P(R) carry the packet sequence numbers for flow and error control. P(S) stands for packet send and indicates the number of the packets being sent. This number is assigned by the sending DTE to outgoing packets to be used for sliding window ARQ. P(R) stands for packet receive and is the number of the next packet expected by the receiver. This field is used to piggyback acknowledgements to information packets when both parties have data to send. In the shorter header, both the P(S) and P(R) fields are three bits long. Three bits limit the size of the window to eight packets In the longer, each field contains seven bits. Seven bits allow the window size to be as large as 128 packets. The M bit is used in packet sequencing. The last bit in this field defines the packet as an I-Packet. Control Packets There are essentially two categories of control packets. The first category is used for flow and error control. The second category is used for connection, termination and management control. Category I Three types of packets are there. RR(000) Receive ready (RR) means that the station is ready to receive more packets. It also acknowledge the receipt of a data packet by indicating the number of the next packet expected in the P(R) field.

168 RNR (001) REJ (010) Category II Receive not ready means that the station cannot accept packets at this time. The other party must stop sending packets as soon as this packet is received. Reject (REJ) means that there was an error in the packet identified by the P(R) field. The other party must resend all packets including and following the packet indicated. Category II carry additional information. Call request/incomming call The call request and incoming call packets are used to local DTE to the local DCE. Call Accepted/Call Connected It indicates the acceptance of the requested connection by the called system. Clear request/clear Indication The clear request and clear indication packets are used at the end of an exchange to disconnect the connection. Clear Confirm Interrupt It is used to send in response to the clear indication packet. It is used under unusual circumstances to break into an exchange and get attention. Interrupt confirm It is used to confirm the receipt of the interrupt packet. Reset Request/Reset indication These are used to reset the sequence numbers in an exchange over a particular virtual circuit.

169 25.3 Let Us Sum Up X.25 is a packet switching protocol used in a wide area network. It also called Subscriber Network interface (SNI). In the X.25 view, a network operates much like a telephone system. Physical layer At the physical layer X.25 specifies a protocol called X.21. This specifies a standard for the physical interconnections between host computers and network packet switches. Network layer In X.25 the network layer is called Packet Layer Protocol(PLP). The function of this layer is connection establishing, data transfer and connection termination. Transport Layer Transport layer provides end-to-end reliability. It will take care of the communication between source host and destination host communication. Session Layer This layer used for remote terminal access. In this layer, its networks, a carrier provided a special purpose host computer with dialup access called a Packet Assembler And Disassembler (PAD). Presentation Layer This layer provides some standard to represent data which is used in application layer. An ISO standard known as Abstract Syntax Notation 1 (ASN 1) is used in this layer. Application layer Application layer deals with user applications like electronic mail or file transfer Lesson End Activities 1. What is purpose of X.25 protocol? 25.5 Points for Discussion What is Control Category I? 25.6 Check Your Progress 1. What are the layers in X.25 protocol? 2. Explain the layers of X25 protocol in detail.

170 25.7 References 1. Internetworking with TCP/IP Principles, Protocols, and Architecture Volume I, Douglas E. Comer, Prentice Hall of India Pvt. Ltd, 2. Computer Networks, Andrew S. Tanenbaum, Prentice Hall of India Pvt. Ltd. 3. Introduction to Data Communications and Networking, Behrouz Forouzan, McGraq- Hill 4. MCSE Networking Essentials Study Guide, Duncan Anderson, Tata McGraw-Hill