Unix Communications - TCP/IP (Release 2.1)

1. Unix Communications- TCP/IP(Release 2.1)

2. Course Overview • Introduction to Data communications • Terminology for Computer Networking • Overview TCP/IP • Architecture of TCP/IP • Internet Addresses • Transport Layer - UDP and TCP • IP Layer • Application Layer

3. Introduction to Data Communications The fundamental purpose of data communications is to exchange information between two agents.

4. Introduction to Data Communications Consider two examples, telephone conversation and electronic mail, on the Simplified Communications Block Diagram. Case #1: Telephone Conversation 1. The agent in this case is the speaker, who generates a message (M1) in the form of sound waves. The sound waves are converted by the telephone into electrical signals of the same frequency. 2. These signals are transmitted without modification over the telephone line. Hence the input signal S1 and the transmitted signal T1 are identical.

5. Introduction to Data Communications Case #1: Telephone Conversation 3. The signal T1 will suffer some distortion over the medium, so that received signal T2 will not be identical to T1. 4. Nevertheless, the signal T2 is converted back into a sound wave with no attempt at correction or improvement of signal quality. 5 & 6. Thus M2 is not exact replica of M1. However, the received sound message is general comprehensible to the listener.

6. Introduction to Data Communications Case #2: Electronic Mail Consider that the input device and transmitter (Tx) are components of a personal computer (PC). 1. The agent is a user who wishes to send a message M1 to another user. The user activates the electronic mail package on the PC and enters the message via the keyboard (input device). The character string of M1 is briefly buffered in main memory.

7. Introduction to Data Communications Case #2: Electronic Mail 2. We can view it as a sequence of bits S1 in memory. The PC is connected to some transmission medium (such as a phone line) by a Tx such as a modem. The input data are transferred to the Tx as a sequence of bits (S1) on some communication bus or cable. The Tx is connected directly to the medium and converts the incoming bits (S1) into a signal T1 suitable for transmission. 3. The transmitted signal T1 presented to the medium is subject to a number of impairments before it reaches the receiver (Rx).

8. Introduction to Data Communications Case #2: Electronic Mail 4. Thus the received signal T2 may differ to some degree from T1. The Rx will attempt to estimate the nature of T1, based on T2 and its knowledge of the medium, producing a sequence of bits S2. 5. These bits S2 are sent to the output PC, where they are briefly buffered in memory as a block of bits or characters. In many cases, the destination system will attempt to determine if an error has occurred and, if so, cooperate with the source system to eventually obtain a complete, error-free block of data.

9. Introduction to Data Communications Case #2: Electronic Mail 6. These data then presented to the user via an output device, such a screen. The message M2 as viewed by the user will usually be an exact copy of the original message M1.

10. Tasks Of A Communications System 1. Transmission system utilization This refers to the need to make efficient use of transmission facilities typically shared among a number of communication devices. 2. Interfacing and signal generation (in data link layer)

11. Tasks Of A Communications System 2. Interfacing and signal generation (in data link layer) In order to communicate, a device must interface with the transmission system. All forms of communications depend on the electromagnetic signals or lightwave signals propagated over a transmission medium. Thus, once an interface is established, signal generation is required for communications. 3. Synchronization (in Data Link Layer) ...

12. Tasks Of A Communications System 3. Synchronization (in Data Link Layer) In a communications system, there must be some form of synchronization between transmitter and receiver. The receiver must be able to determine when a signal begins to arrive and when it ends. It must also know the duration of each signal element. 4. Exchange management It is a variety of requirements for communications between two parties such as establishing a connection; certain conventions must be decided upon.These conventions may include: - whether both devices may transmit simultaneously or may take turns, ...

13. Tasks Of A Communications System 4. Exchange management …- the amount of data to be sent at one time, - the format of the data, and - what to do if certain contingencies such as an error arise. 5. Error detection and correction They are required in circumstances where errors cannot be tolerated. 6. Flow control It is required to assure that the source does not overwhelm the destination by sending data faster than they can be processed and absorbed.

14. Tasks Of A Communications System 7. Addressing and routing When a transmission facility is shared by more than two devices, a source system must somehow indicate the identity of the intended destination. The transmission system must assure that the destination system, and only that system, receives the data. Further, the transmission system may itself be a network through which various paths may be taken. A specific route through this network must be chosen. 8. Recovery ...

15. Tasks Of A Communications System 8. Recovery It is a concept distinct from that of error correction. Recovery techniques are needed in situations in which an information exchange, such a data base transaction or file transfer, is interrupted due to a fault somewhere in the system. The objective is either to be able to resume activity at the point of interruption or at least to restore the start of the systems involved to the condition prior to the beginning of the exchange. 9. Message formatting ...

16. Tasks Of A Communications System 9. Message formatting It has to do with an agreement between two parties as to the form of the data to be exchanged or transmitted. E.g. both side must use the same binary code for characters. 10. Protection Frequently, it is important to provide some measure of protection in a data communications system. The sender of data may wish to be assured that only the intended receiver actually receives the data that have not been altered in transit.

17. Tasks Of A Communications System 11. System management Finally, a data communications facility is a complex system that cannot create or run itself. System management capabilities are needed to configure the system, monitor its status, react to failures and overloads, and plan intelligently for future growth. Note: This list of communications tasks is somewhat arbitrary. Elements could be added; items on the list could be merged; and some items represent several tasks that are performed at different “levels’ of the system.

18. Terminology For Computer Networking Computer Network and Host A Computer Network is a communications system for connecting end-system. We often refer to the end-systems as hosts. Local Area Network (LAN) A local area network connects computer systems that are close together – typically within a single building, but possibly up to a few kilometers apart.

19. Terminology For Computer Networking Network Topologies The topologies of a network is its physical layout – the way in the computers and other units are connected. There are three basic layouts: star, ring, and bus. Star Network Each node in a star network is connected to a central microcomputer which controls the network. Network signals travel from the server to the station along each individual station’s cable. A polling system is commonly used – the file server polls each station in turn to see if it has a signal to send. The server then handles signals as they are received.

20. Terminology For Computer Networking Ring Network In a ring network, a series of computers is connected together and there is no central controlling computer. Each computer may communicate with messages being specifically addressed to the destination computer. Using the ‘token ring’ system, a ‘message token’ is passed from node to node, and each node has a designated time at which it can remove the token and either add a message or take a message from it.

21. Terminology For Computer Networking Bus Network In a bus network, all the devices share a single cable. Information can be transmitted in either direction from any PC to any other. The problem here is that several stations may want to transmit down the same line simultaneously, and there has to be some strategy for deciding who gets the line. A popular scheme called Ethernet uses a collision system known as ‘Carrier Sense Multiple Access with Collision Detection’ (CSMA-CD). Before a station begins to transmit, it checks that the channel is not busy; if it is, it has to wait before transmission can begin. Once it begins transmission, it listens for other nodes also beginning transmission. If the transmitted message collides with another, both stations abort and wait a random period of time before trying again.

22. Terminology For Computer Networking This system works well if the channels are not too heavily loaded. On the other hand if many student (say 20) sit down and at computers all at once and all try to load software from the network’s hard disk, the whole system more or less grinds to a halt! Ethernet Ethernet is a popular LAN design and the trade-marked product of Xerox Corp., characterized by 10-Mbit/s baseband transmission over a shield coaxial cable and employing CSMA/CD as the access-control mechanism. It is standardized by the IEEE as specification IEEE 802.3; referring to the Ethernet design or compatible with Ethernet.

23. Terminology For Computer Networking Inter-network (internet) An internet is the connection of two or more distinct networks so that computers on one network are able to communicate with computers on another network. The goal of internetworking is to hide the details of what might be different physical network, so that the internet functions as a co-ordinated unit.

24. Terminology For Computer Networking There are some ways to connect networks together. The term we use to describe the interconnection depends on the layer in the OSI model at which the connection takes place. Repeaters operate at the Physical Layer (Layer 1) and typically just copy electrical signals (including noise) from one segment of a network to the next. Bridgesoften operate at the Data-Link Layer (Layer 2) and they copy frames from one network to the next. Bridges often contain logic so that they only copy a subset of the frames they receive. Routers...

25. Terminology For Computer Networking Routersoperate at the Network Layer (Layer 3). The term router implies that this entity not only moves information (packets) from one network to another, but it can also make decisions about what route the information should take. Gateway is generic term that refers to an entity used to interconnect two or more networks. (e.g. In the TCP/IP community, the term gateway refers to a network level router.)

26. Terminology For Computer Networking OSI Model, Protocols, and Layering The computers in a network use well-defined protocols to communicate. A protocol is a set of rules and conventions between the communicating participants. Since these protocols can be complex, they are designed in layers, to make their implementation more manageable.

27. Terminology For Computer Networking Open Systems Interconnection (OSI) Model: The Seven Layers The seven layers are Physical Layer, Data Layer, Network Layer, Transport Layer, Session Layer, Presentation Layer, and Application Layer. This model, developed between 1977 and 1984, is a guide, not a specification. It provides a framework in which standards can be developed for the services and protocols at each layer. Note: - TCP/IP was developed before the OSI model; - No network is implemented exactly as the OSI model; - Protocols exist at each layer in the OSI model;

28. OSI Model: The Seven Layers

29. OSI Model: The Seven Layers The Application Layer is the only part of the communications process that a user sees, and even then, the user doesn’t see most of the work that the application does to prepare a message for sending over a network. The layer converts a message’s data from human-readable form into bits and attaches a header identifying the sending and receiving computers. The Presentation Layer ensures that the message is transmitted in a language that the receiving computer can interpret (often ASCII). This layer translates the language, if necessary, and then compresses and perhaps encrypts the data. It adds another header specifying the language as well as the compression and encryption schemes.

30. OSI Model: The Seven Layers The Session Layer opens communications and has the job of keeping straight the communications among all nodes on the network. It sets boundaries (called bracketing) for the beginning and end of the message, and establishes whether the message will be sent half-duplex, with each computer taking turns sending and receiving, or full duplex, with both computers sending and receiving at the same time. The details of these decisions are placed into a session header. The Transport Layer...

31. OSI Model: The Seven Layers The Transport Layerprotects the data being sent. It subdivides the data into segments, creates checksum tests – mathematical sums based on the contents of data – that can be used later to determine if the data was scrambled. It can also make backup copies of the data. The transport header identifies each segment’s checksum and its position in the message. The Network Layerselects a route for the message. It forms data into packets, counts them, and adds a header containing the sequence of packets and the address of the receiving computer. The Data-Link Layer ...

32. OSI Model: The Seven Layers The Data-Link Layer supervises the transmission. It confirms the checksum, then addresses and duplicates the packets. This layer keeps a copy of each packet until it receives confirmation from the next point along the route that the packet has arrived undamaged. The Physical Layerencodes the packets into the medium that will carry them – such as an analog signal, if the message is going across a telephone line – and sends the packets along that medium.

33. OSI Model: The Seven Layers An intermediate node calculates and verifies the check sum for each packet. It may also reroute the message to avoid congestion on the network. At the receiving node, the layer process that sent the message on its way to reversed. The physical layer reconverts the message into bits. The data-links layer recalculates the checksum, confirms arrival, and logs in the packets. The network layer recounts incoming packets for security and billing purpose. The transport layer recalculates the checksum and reassembles the message segments. The session layer holds the parts of the message until the message is complete and sends it to the next layer. The presentation layer converts the bits into readable characters, and directs the data to the correct application.

34. Terminology For Computer Networking Protocol Suite A protocol suite/family is a collection of protocols from more than one layer that forms the basis of a useful network. The protocol suite that we consider is - Transmission Control Protocol/Internet Protocol(TCP/IP)Protocol Suite

35. Overview TCP/IP • It is also known as Defense Advanced Research Projects Agency (DARPA) Internet Protocol Suite. • It is commonly referred to as the TCP/IP Protocol Suite, or just TCP/IP. Overview of TCP/IP Protocol Suite Although the protocol family is referred to as TCP/IP, there are more members of this family than TCP and IP. The following figure shows the relationship of the protocols in the protocol suite along with their approximate mapping into the OSI model.

36. TCP/IP Protocol Suite

37. TCP/IP Protocol Suite Transmission Control Protocol (TCP) A connection-oriented protocol that provides a reliable, full-duplex, byte stream for a user process. Most Internet application programs use TCP. Since TCP uses IP (as shown in the figure), the entire Internet protocol suite is often called the TCP/IP protocol family. User Datagram Protocol (UDP) A connectionless protocol for user processes. Unlike TCP, which is a reliable protocol, there is no guarantee that UDP datagrams ever reach their intended destination. Internet Control Message Protocol (ICMP) ...

38. TCP/IP Protocol Suite Internet Control Message Protocol (ICMP) The protocol to handle error and control information between gateways and hosts. While ICMP messages are transmitted using IP datagrams, these messages are normally generated by and processed by the TCP/IP networking software itself, not user processes. 5 kinds of ICMP messages: • ICMP Source Quench Message (for flow control) • ICMP Destination Unreachable Message • ICMP Redirect Message (Tell a host to use another gateway) • ICMP Echo Message (for checking remote)Note: Ping use this message • ICMP Get Network Mask from gateway Message

39. TCP/IP Protocol Suite Internet Protocol (IP) IP is the protocol that provides the packet delivery service for TCP, UDP, and ICMP. (Note from the figure: user processes normally do not need to be involved with the IP layer.) Address Resolution Protocol (ARP) The protocol that maps an Internet address into a hardware address. This protocol and the next, RARP, are not used on all networks. Only some networks need it. Reverse Address Resolution Protocol (RARP) The protocol that maps a hardware address into an Internet address.