# Mạng và viễn thông P22

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## Mạng và viễn thông P22

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Electronic Mail, Znternet and Electronic Message Services The ability to connect two computers together and, with relative ease, to send information from one to the other, is bringing a revolution in the way in which business and life as a whole is conducted. Today it is possible to run your bank account from home, book your holiday, send electronic messages to your work colleagues or friends, and look up to see what is on at the theatre in London orNew York City.Alternatively companies may make their orders to and pay their bills from their suppliersby computer program and‘electronic data interchange’....

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1. Networks and Telecommunications: Design and Operation, Second Edition. Martin P. Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic) 22 Electronic Mail, Znternet and Electronic Message Services The ability to connect two computers together and, with relative ease, to send information from one to the other, is bringing a revolution in the way in which business and life as a whole is conducted. Today it is possible to run your bank account from home, book your holiday, send electronic messages to your work colleagues or friends, and look up to see what is on at the theatre in London orNew York City.Alternatively companies may make their orders to and pay their bills from their suppliersby computer program and‘electronic data interchange’. A number of technologies enabled revolution: have this videotext, electronic electronic mail, data interchange (EDI) and Internet. We review the telecommunication aspects of these technologies in this chapter. 22.1 VIDEOTEXT Videotext was the first type of device specially designed to allow telephone network customers to use a cheap device to access information from a large public database. The original technology and standards, including special terminals and modem techniques allowing asymmetric transmission (a high bitrate channel for information download to the customer with a low speed control channel for his ordering of different pages of information) have now been overtaken by modern personal computer based technology, but the appearance of the worldwide Internet has stimulated recent rapid growth for the long established videotext service providers (British Telecom’s Prestel, Deutsche Telekom’s Bildschirmtext, BtX or Datex-J, and France Telecom’s Minitel). The idea of videotext is that, using a low cost terminal in the form of a small tele- vision, a customer can make a phonecall to a public central database, where he could access all sorts of pages of information which he could then have displayed on his screen. Thus, for example, he might access tomorrow’s weather forecast, current flight arrival information, information about financial markets, about holiday offers, about dating services or, via an extended connection to the railway company or his bank, might even order a train ticket or pay his bills. 399
2. 400 ELECTRONIC INTERNET MAIL, AND ELECTRONIC MESSAGE SERVICES Videotextinitsoriginal form in theUnitedKingdom(as Prestel) was quickly accepted by the travel industry, which used it widely as a means for holiday companies to advise travel agents of available itineraries and to book tickets on behalf of clients. It came at a time of revolutionary computerization of the airline industry in particular, when simultaneously the airlines were cooperating to establish and operate common and inter-linked computer ticket and booking systems. In France, videotext became the standard method for telephone directory enquiries. The Minitel terminals were given away free by France Telecom. They justified doing this initially by the cost savings brought about by fewer telephone enquiries to human operators, but nowadays they also make money from the telephone calls which cus- tomers are making to the other information services which have subsequently sprung up. Some services, including the pink pages of dating services, are world renowned. In Germany, the Deutsche Telekom’s(formerlyDeutscheBundespost’s) T-Online service (the new name for the modern version of Bildschirmtext, Btx or Datex-J) had been a commercial non-runner for years. Suddenly, however, during 1995 demand for connections started to increase rapidly. The reason was the cheap availability of access to the Internet using standard personal computers. It is the Internet which has become the world’s electronic noticeboard. We discuss it later in this chapter. 22.2 ELECTRONIC MAIL (E-MAIL) Electronic mail (e-mail) is a reliable means of message communication between human users equipped with computer terminals or personal computers. Large tracts of text, and diagramstoo,can bequicklydeliveredacrossgreatgeographicaldistances, and if necessary printed to ahigh quality paper format using local computer printing resources. To send an e-mail, a user simply callsup the electronic mail software on his terminal. This prompts him for the name of the user that he wishes to send his message to, and for the names of any individuals to whom the message is to be copied. Having filled in this information, it prompts him to type the main text of the message. If he wishes, the sender can add further electronic attachments (say a document previously created using his word processor, his spreadsheet program or presentation software). Once the e-mail is ready for sending, the sender simply clicks his mouse on the ‘send’ button. At this point, may be asked to set a priority rating for the message, decide he to whether confirmation of receipt is required, and (optionally) to set an exact time and datefor delivery (if not to beimmediate;thisfeature can be usedasareminder function, for example by sending time-delayed messages to himself). The delivery into the electronic mailboxes of the intended recipients is almost immediate. Should any of the recipientsbe at their computer terminalsat the time of the message receipt in their mailbox, then they may be advised of its arrival (say by a beeping noise and a short message or icon displayed at the bottom of their computer screen). They havethe option to readthe message immediately or later, depending on how they regard its priority. Those recipients who are not concurrently using their terminalswill be advised of the message next time they log on. As messages are read, confirmation is returned tothe sender (if required).Eachrecipienthasthechoicetoreplytothe message, forward it, file it, print it out, amend it or delete it.
3. ELECTRONIC MAIL (E-MAIL) 401 Electronic mail systems are usually constructed in aclientlserver configuration. The client is the human user and his personal computer, workstation or other computer terminal.The client preparesandreadselectronic messages. The client software prepares the e-mail for submission to the server or post ojice. The post ojice has a distribution function forelectronic mail similar to the function of the postal service for letter mail (Figure 22.1). The client software for electronic mail usually runs on a personal computer, and normally has software interfaces to othersoftware such as word processing software, PC spreadsheet software and presentation software, so enabling easy attachment of such software to e-mail messages. Thus, for example the Microsoft package of software Office includes Microsoft Word, Excel, Powerpoint and Microsoft Mail software. Together theseallowapersonalcomputeruser topreparevarioustypes of differenttext, spreadsheet and presentation documents and submit them to an electronic mail post office as e-mail messages. The post office function which forms the core of electronic mail usuallyresides on a server, often nowadays connected to the individual PCsmeans of aLAN (local area by network), though other types of X.25 packet network or dial-up connection are also possible. The postoffice is a combination of software and hardware, capable storingof e-mail messages, sorting them and transmitting them. The post office function works both on a store-and-forward and store-and-retrieve fashion. At the sending end, the user submits his message to the post office (post office A of Figure 22.1). This post office temporarilystorestheentire messagewhileitexamines theaddress of the intended destination and establishes a communication means to the post office (post office B) serving the destination user. The communication means may take any number ofdifferent forms (X.25, telephone connection, internet connection etc.). Once the lower layer connection is established between origin and destination post office, or between originpost office andintermediate(transit)post office, thehigherlayer protocol (i.e. OS1 layer 7 protocol for electronic mail (e.g. ITU-T X.400, SMTP or equivalent)) takes over, to ensure the appropriate relay entiremessage to the next of the post office. In each post office along the way, the message is stored and forwarded in this way.
4. 402 ELECTRONIC INTERNET MAIL, AND ELECTRONIC MESSAGE SERVICES Finally, at the destination post office, the message is stored in the electronic mailbox of the destination user until that user retrieves it. Thisis done when he logs into his own electronic mailbox account to read or send messages. ITU-T, in its X.400-series of recommendations, specifies a message handling system (MHS) for use as anOS1 layer 7 protocol in supportof electronic mail. Thisis described in detail in Chapter 23. MHS defines P1 and P3-interfaces for relay and submission/ delivery respectively. In reality, however, the P1-interface has proved to be the more important of these two, since this is the interface which allows electronic mail servers (post offices) to inter-communicate with one another. Thus the electronic mail systems of different companies, and maybe supplied by different software manufacturers are able to transfer e-mails between themselves. This is the X.400-interface talked of by electronic mailsoftwaresuppliers.TheP3-interface is only needed when aremote electronic mail user (client) wishes to submit or receive electronic mail messages directly from a post office operated by a public X.400-based electronic mail service provider. As, however, most electronic mail systems and post offices are based on corporately operated sytems, it is usual for client and post o@ce (server) software to be provided by the same supplier (e.g. Microsoft mail or Ccmail). In this case, it is not necessary for the software manufacturer to use the P3-interface, and instead a proprietary interfaceis employed. An alternativeto the of the use X.400 interfaces for inter-post-office interfaces is the use of the Internet. The Internet interface looks likely to become the predominant interface. E- mail based on the Internet is already firmly established in North America, and is fast supplanting X.400 where this already exists (mainly in Europe). In addition to the X.400 and Internet-based electronic mail systems, a number of other proprietary systems have emerged. One of the first was introduced by Compu- serve, one of the world market leadersin on-line services. Compuserve createdthe capability for companies and private individuals, from their personal computers, to access database information and send messages between one another. Its success has been largely based on being the first company to establish a worldwide user group for electroniccommunication.Currentlyitappears to bere-positioning toencompass Internettechnology. Microsoft, on the other hand, has chosen to build a Microsoft mail capability into its Windows95 operating system software for personal computers. This client capability can be used either in conjunction with a corporate post ofice, or alternatively,may use the Microsoft Network (MSN), apublic post o@ce network operated worldwide by MicrosoftCorporation.The initiative lays downatough challenge for other X.400- and Internet-based electronic mail service providers. 22.3 ADDRESSING SCHEMES FOR ELECTRONICMAIL Three main types of addressing scheme are used nowadays forelectronic mail. These are e proprietaryaddressing schemes e Internet-basedelectronicmailaddresses e X.500 addresses (for the X.400 message handling system)
5. THE ADVANTAGES AND DISADVANTAGES OF E-MAIL 403 Proprietary addressingschemes are normally employed within a corporation’s electronic mail environment. The form of these addresses is often determined by the corporation itself, within the bounds set by the software product on which the system is based. Internet-based electronic mail addresses (correctly SMTP addresses) usually take a numerical form, punctuated with dashes (-), dots ( . ) and @ signs (e.g. 12345.67890 @compuserve.com). More commonly nowadays, an alias address appears on business- men’s visiting cards, e.g. Martin.Clark@serviceprovider.corporation. This form allows for easy recognition and memory of the address. X.500 addresses are less common, but form the basis of the ITU and I S 0 directory service and are used in X.400 electronic mail networks. We describe theX.500 address- ing scheme in Chapter 28. 22.4 THE ADVANTAGES AND DISADVANTAGES OF E-MAIL E-mail messages are delivered quickly. Broadcasting of messages is also quickly and easily achieved. Editing of text and returning or forwarding the amended version can be achieved with minimal re-typing. Messages can be filed and quickly retrieved later. Messages can be posted for exactly timed delivery, and can be prioritized according to the urgency with which they need to be dealt with. Furthermore, users are able to check their electronic mailboxes even when they are away from their normal offices, either by using somebody else’s terminal or perhaps by dialling in to the post office using a portable laptop computer from a hotel room. The Internet, in particular, has done a lotto release the full power of electronic mail, of by enabling a large worldwide community computer users to inter-communicate elec- tronically, thus destroying the previous communication boundaries between companies and overcoming all the barriers of geography and time zones. Companies have who successfully introducedelectronic have mail observed a beneficial change in the whole culture how they do business. Questions and responses of have been much quicker and more direct, messages have been much shorter and less formal; have typed they been by the managers ratherthan by theirsecretaries. Workgroups composed of members in widespread locations have evolved, and it is possible to draw together new teams for previously impossible tasks. 22.5 EDI: CORPORATE COMMUNICATION WITH CUSTOMERS ANDSUPPLIERS VIA E-MAIL Where electronic mail is used as basis the of formal communications between companies for orders, payments and confirmations, it is termed instead electronic data interchange (EDZ). EDI between companies has become an important wayof doing business. Many manufacturers and retailers demand that their suppliers accept orders for goods electronically. This gives the scope for more frequent ordering(in the case of so supermarkets, for example, daily based on yesterday’s sales) and maintain their shelf stocks (and value of stocks) at a minimum level. This is just-in-time (JZT) provision.
6. 404 ELECTRONIC MAIL, INTERNET AND ELECTRONIC MESSAGE SERVICES Not onlythebuyerbenefits,butalsothesupplier can ensure that products on the shelves with his name on are fresher and closer to their best. Even more important for the supplier, EDI allows him to bill for the goods more quickly. The most important standards defining theformat and content messages for elec- of of tronic data interchange (EDI) are those published by the United Nations as EDIFACT (electronic data interchange for administration, commerce and transport), but there also are a number of regional, national and industry-specific standards (e.g. Odette used by the automobile industry). EDI products are available in ready-packaged software solutions for integration into a company’s existing computer and electronic mail set-up. Specialized service providers have also appeared in several countries concentrating on the communication needs of companies within a specific industry segment. In this way, a community of interest may be established forcommunicationbetweencustomersandsuppliers(forexample, within the retailing industry or the car manufacturing industry). Setting up a network and software for EDI is actually quite straightforward. What is much harder, and what the company therefore needs to be prepared for, is the adjust- ment of its culture and practices to an entirely new way of business, one requiringmuch quicker reaction to customer demands. 22.6 INTERNET The Internet emerged from a United States governmentand military initiative to enable the interconnection of different, mainly UNIX-based computer systems for intercom- munication. As UNIX was proclaimed to be the first ‘portable’ operating system for computers,enablingsoftwaredevelopedonaparticularmanufacturer’scomputer hardware to be easily ported (i.e. transferred for operation) to another manufacturer’s hardware,itwasnaturalalso to developmeansfor easy transfer of data between systems. This led to TCPjlP (transport control protocollinternet protocol). TCP/IP was quickly adopted by the academic community in the United States, and soonafterwards by academicsworldwide,because it allowed forrapidsharing of scientific information and the electronic mail communication necessary for its rapid discussion and analysis, both within and between university campuses. A worldwide community of inter-linked computers rapidly emerged. Finally, businesses recognized as the potential of the Internet as a large interconnected groupelectronic mail usersand of noticeboard readers, theybegan to exploit it communication marketing for and purposes. Though the Internet does not have the rigid structure, network management and security controls of other public telecommunication network services, it has a very persuasive appeal; there are already plenty of people to communicate with. This has driventheexplosivegrowthinnumbers of registered Internet users and pages of information. In its original form, the Internet and the internet protocol (IP) provided a means for interconnecting computer servers (typically UNIX computers) together. The internet addressing scheme allowed individual workstations, personal computers or software or on applications running on either the server anyof the workstations to direct and send information to other applications on other servers or distant LANs. Being a unique
7. TCP/IP PROTOCOL 405 address, the Internet address allowed an end user to be identified, no matter how many transit servers, routers or networks would have to be traversed along the way (Figure 22.2). The single network address of the destination port in the destination network does not suffice, because the intermediate networks are unable torecognize this address. A suite of new protocols arrived withInternet. Among others, these included S M T P (simple mail transport protocol), TFTP (trivial $le transfer protocol) and FTP ($le transfer protocol), which together form the basis the Internet electronic mail service. of As the popularity of Internet has grown, so have the number of servers and routers makingupthenetwork.New Internet service providershaveprovidedfordial-up access tothe servers fromprivateindividualsusingtheirPCs athome,and new information providers have provided more Internet pages of information. The Netscape browser, a computer software allowing users to ‘surf theInternet and World Wide Web ( W W W ) , by seeking information from any of the connected servers by means of a menu-driven screen software which drives a hypertext search and browse capability. However, the reason for the rapid growth of the Internet also provides a major challenge for the next stage of its development. The factor enabling rapid growth was the possibility to add further servers and routers to the network at almost any point in the network without considering a master plan. The number of Connected devices could therefore rapidly increase, as the routers (Chapter 19) are always able tofind the desired destination somehow. The problem is the lack of control (and even lack of knowledge) of the path taken. Thetraffic flo& in the network as a whole are therefore largely unmanaged unmanageable. and The onlysolution to slow response or congestion can be to add more capacity. Whether this capacity is added at the most appropriate point is a matter of chance. Messages within the Internet may go undelivered without trace for any number of reasons, and there is no record of whether electronic copies of information have been retained by any of the intermediate parties. Much attention is currently being focussed by the computer industry on solving the security and network management difficulties of the Internet, to stimulate a further surge in demand. 22.7 TCP/IP PROTOCOL STACK Figure 22.3 illustrates the various protocols going to make the TCPIIP stack.At the up heart are the Internet protocol (IP), which is equivalent to an OS1 layer 3 network
8. 406 ELECTRONIC INTERNET MAIL, AND ELECTRONIC MESSAGE SERVICES OS1 layer 7 x- TELNET SMTP FTP NFS RPC TFTP windows (File (Simple (Simple file (Network (Remote (Tr 6 transfer mail network transfer procedure file server) protocol) transfer managem call) protocol) 5 protocol ent protocol) 4 TCP UDP ICMP 3 ARP Gateway protocols IP EGP BGP 2 (PPP) SLIP SNAP 1 network LLC (e.g. Ethernet LAN) Serialetc. Frame line physical Relay I ARP = Address resolution protocol BGP = Border gateway protocol EGP = Edge gateway protocol ICMP = Internet control message protocol IP = lnternet protocol LLC = Logical link control (for LANs) PPP = Point-to-point protocol RARP = Reverse address resolution protocol SLIP = Serial line internet protocol TCP = Transmission control protocol UDP = User datagram protocol Figure 22.3 TCP/IP protocol stack and associated applications protocol, and the transport control protocol ( T C P ) an approximate equivalent of OS1 layer 4. Also very importantfor somecommonmultivendorcomputer-networked applications is the UDP (user datagram protocol). The Internet protocol is a network layer-like protocol, typically running across an ethernet LAN or between LANs via a router network (say comprising inter-router con- nections running on framerelay). The benefit, however, of IP over OS1 layer 3 protocols is that Internet addresses are widely used in, andareunique across, all computer networksworldwide.This gives thepotentialforasoftwareapplicationora user connectedanywhereintheworld tothe Internet to access any other computer or software application. The transport control protocol ( T C P ) is usually used in conjunction with IP and ICMP (internet control message protocol) to guarantee reliable transmission. On an end-to-endbasis, TCP ensurescorrectsequencing of arrivingframes of data, and requests retransmissions when necessary. ICMP is an addition to the basic IP allowing network problems to be reported back to a communicating device. Thus ICMP is able to report the inability todeliver a message and the cause, or the need for fragmentation. An alternative to TCP is UDP (user datagram protocol),a simpler protocol which does not perform retransmissions. Instead this job is left to the application if necessary. The address resolution protocol ( A R P ) is used in association with the IP to translate IP addresses into physical hardware addresses. Thus, for example, ARP is capable of determining the appropriate ethernet LAN address (LLC). This address corresponds to
9. ICATIONS COMMON 407 a given IP address. RARP performs the reverse function. When, for example, a diskless computer workstation boots, it obtains its hardware address from the network interface card to which it is attached. It does not, however, know its IP address, this is resolved by RARP. The gateway protocols (BGP, border gateway protocol and EGP, exterior gateway protocol) are used to connect together different sub-networks of the Internet. They provide for inter-network routing information and communication exchange. S N A P (sub network access protocol) is an extended version of the LAN logical link control (Chapter 19) which enables the internet protocol (IP) be carried over LANs. to SLIP (serial line internet protocol) or its successor, PPP (point-to-point protocol) are equivalent to OS1 layer 2 protocols. They enable transportof IP packets across a simple serial communications line (such as a telephone connection or leaseline). The protocol does not include an addressfield. The main function is simply to ensure the delineation of packets. Other well known routing protocols of the TCP/IP suite(e.g. RIP,OSPF)are discussed in Chapter 28. 22.8 COMMON APPLICATIONS USING TCP/IP A number of the most common applications used in association with TCP/IP are also shown in Figure 22.3. These are X-windows TELNET FTP (file transfer protocol) SMTP (simple mail transfer protocol) SNMP (simple network management protocol) TFTP (trivial file transfer protocol) RPC (remote procedure call) NFS (network file server) We discuss each of these briefly in turn. X-windows This is awindows-basedcomputeroperatingsystemintended to allowcomputer workstationusersanywherewithinanetwork of computerssupplied by multiple vendors to access software applications running on remote hardware platforms. TELNET This is a simple TCP/IP-related application that enables a remote log-on from one com- puter to an application running on another; you like, a simple version of X-windows. if Thus a user of of personal computer using Windows95 software has the option to
10. 408 ELECTRONIC INTERNET MAIL, AND ELECTRONIC MESSAGE SERVICES commence a T E L N E T log-on, enabling him to conduct a session on a remote UNIX computer server. It is the simplest form of TCP/IP connection. As such, it is worth mentioning at this point the widely used PING (Packet Internet Groper) technique for confirming the availability of a IP connection. A PING is an IP-message that contains the address of its source and the destination. On receipt by the destination terminal a confirmatory PING is returned. The procedure confirms not only that the network connection is available but also that the destination device is ‘alive’. FTP (file transfer protocol) This is asimple but efficient protocol to transfer data files fromonecomputerto another. Although nowadaysit is also possible to include data files (e.g. Microsoft word data orMicrosoft Powerpoint presentation) asan attachment to an e-mail message, this can be a very slow and cumbersomemeans of transferring large files. Drawing files from a remote computer can be effected much more quickly using a remote log-on and then the FTP. S M T P (simple mail transfer protocol) SMTP has become the basis of the now world-renowned Internet electronic mail system (withtheaddresses that run president@whitehouse.gov). It providesfortransfer of mail messages between mail servers running on the Internet. S N M P (simple network management protocol) SNMP hasbecome the de facto standard method of transporting network management information around computer and router networks. discuss it in a little more depth We in Chapter 27. TFTP (trivialJile transfer protocol) This is intendedto be asimpleprotocol, and it is particularly well suited forthe downloading of software to and initiation of a remote device. RPC (remote procedure call) This provides for a software routine or other application to called and executed on a be remote computer or server. NFS (networkJile server) This application makes files on remote servers and other computers appear toreside on the local user’s workstation. It enables him to read and process the data as if it were residing in his own machine. 22.9 THE INTERNET PROTOCOL (IP) Figure 22.4 illustrates the fields comprised in an IP frame (or datagram). The version indicates the format of the IP header, specifically which version of the protocolis in use. (For example,the 1993 versionwasversion 4). The internetheaderlength (ZHL)
11. TOCOL THE INTERNET (IP) 409 Version of service Internet header Total length Type length (IHL) Identification Flags I Fragment Offset Protocol Live to Time I Checksum Header Source Address Destination Address Options I Padding Figure 22.4 Format of an Internet Protocol Datagram indicates the length of the header in 4 byte (i.e. 32 bit) words. The typeof service indicates how the datagram (as defined in Chapter 18) will be switched and otherwise treated during its transit through the network. The length indicates the total length total of the IP datagram, including both header and data. The Jags are three bits indicating whether fragmentation of the datagram is allowed or not. Thefragment offset indicates the position of the associated datagram fragment intheoverall message. The timeto live is theremaining time that the datagram is allowed to exist within the Internet. When time is up, the datagram is destroyed. This ensures that messages lost in theInternet (e.g. datagrams with invalid addresses) are not left around toclog up the network. The protocol indication determines whether TCP or UDP protocol is used at the next higherlayer, and thereforedetermines to which protocol agent the datagram should be delivered. The header checksum is akin to a cyclic redundancy check (CRC)or frame check sequence (FCS) as already discussed inearlierchapters (e.g. Chapter 9). It is only applied to the I P header and is recomputed each time the header is amended during passage through the network. The source and destination addresses are both of 32 bits. As we shall see later in Chapter 29, it is normal to write these addresses as four decimal numbers separated by dots, e.g. 123.231.312.123. Each of the four decimal values mayonlyhavea value between 0 and 255 (i.e. one of 28 combinations). A number of options may also be included in the header. Examples are 0 security (a mechanism intended to provide for secure communication) 0 loose source and record route (the source provides information to aid the routing across the network) 0 strict source and record route (the source is able to give information to steer the route taken by the datagram across the network) 0 record route (a record of the route taken through the network) 0 timestamp (the time and date when the datagram was handled by a router) The padding merely fills up the header to make it an exact number of 32 bit words.
12. 410 ELECTRONIC INTERNET MAIL, AND ELECTRONIC MESSAGE SERVICES Message Type l Checksum Parameters Original I h e a d e r P and data Figure 22.5 Internet control message protocol (ICMP) datagram format 22.10THE INTERNET CONTROLMESSAGEPROTOCOL(ICMP) ICMPcan beusedinconjunctionwith IPtoprovideamore reliable means of information transfer. This allows more network status information tobe transferred to the end computers to react in the case of non-delivery or corruption of messages. The format of the ICMP amended IP datagram is showninFigure 5, and the various message types and codes are shown in Tables 22.1 and 22.2. 22.11 TRANSMISSIONCONTROLPROTOCOL(TCP) The transmission control protocol ( T C P ) governs the format of the user data carried within an IP datagram. A TCP segment is structured as shown in Figure 22.6. 22.12 ONLINE DATABASE SERVICES Stimulated by the huge population of personal computers now in use in businesses and even in privatehomes,a new Online industryhasemergedto sell informationon- demand in form the of raw data, news and current affairs, business andtrade Table 22.1 ICMPmessagetypes Message type Message 0 Echo reply 3 Destination unreachable 4 Source quench 5 Redirect 8 Echo request 11 Time exceeded for a datagram 12 Parameter problem on a datagram 13 Timestamp request 14 Timestamp reply 17 Address mask request 18 Address mask reply
13. ONLINE DATABASE 411 Table 22.2 ICMPcodesandmeanings Code Meaning 0 Network unreachable 1 unreachable Host unreachable Protocol 2 3 unreachable Port 4 Fragmentation needed 5 Source failed route 6 Destination network unknown 7 Destination host unknown 8 isolated Source host 9 Communication prohibited with destination network 10 Communication prohibiteddestination with host (com- puter) 11 Network unreachable type service for 12 Host unreachable type service for Function Source port identifies upper layer application Destination port identifies upper layer application Sequence number sequence numberof this frame within complete message Acknowledgement number signals positionof data previously acknowledged Data offset signals where data field begins Reserved (set to zero) Urgent indicates whether the urgent pointer in use is 1- Acknowledgement Reset indicates whether the acknowledgement number is in use field I indicates whether the connection needs to be reset Synchronizer J used ot synchronize sequence numbers indicates sender has no more data to send indicates how much data the receiver can accept before acknowledgement Checksum Urgent pointer ODtions II error detection function if used, indicates that urgent data follows 1- Padding ensures header length is integral multiple 32 bits of user datafield - information transferring between applications Figure 22.6 Structure of a TCP segment information,softwareprogrammes,graphicimages,videos or libraryinformation. Huge new global businesses have been created to provide for information and content trade. Perhaps the best-known ofthese companies are Compuserve, Microsoft Network ( M S N ) , America Online and Europe Online.