Introduction to the Internet Protocols C R C S Computer Science Facilities Group C I L S RUTGERS The State University of New Jersey 3 July 1987 This is an introduction to the Internet networking protocols (TCP/IP). It includes a summary of the facilities available and brief descriptions of the major protocols in the family. Copyright (C) 1987, Charles L. Hedrick. Anyone may reproduce this document, in whole or in part, provided that: (1) any copy or republication of the entire document must show Rutgers University as the source, and must include this notice; and (2) any other use of this material must reference this manual and Rutgers University, and the fact that the material is copyright by Charles Hedrick and is used by permission. Unix is a trademark of AT&T Technologies, Inc. Table of Contents 1. What is TCP/IP? 1 2. General description of the TCP/IP protocols 5 2.1 The TCP level 7 2.2 The IP level 10 2.3 The Ethernet level 11 3. Well-known sockets and the applications layer 12 3.1 An example application: SMTP 15 4. Protocols other than TCP: UDP and ICMP 17 5. Keeping track of names and information: the domain system 18 6. Routing 20 7. Details about Internet addresses: subnets and broadcasting 21 8. Datagram fragmentation and reassembly 23 9. Ethernet encapsulation: ARP 24 10. Getting more information 25 i This document is a brief introduction to TCP/IP, followed by advice on what to read for more information. This is not intended to be a complete description. It can give you a reasonable idea of the capabilities of the protocols. But if you need to know any details of the technology, you will want to read the standards yourself. Throughout the text, you will find references to the standards, in the form of "RFC" or "IEN" numbers. These are document numbers. The final section of this document tells you how to get copies of those standards. 1. What is TCP/IP? TCP/IP is a set of protocols developed to allow cooperating computers to share resources across a network. It was developed by a community of researchers centered around the ARPAnet. Certainly the ARPAnet is the best-known TCP/IP network. However as of June, 87, at least 130 different vendors had products that support TCP/IP, and thousands of networks of all kinds use it. First some basic definitions. The most accurate name for the set of protocols we are describing is the "Internet protocol suite". TCP and IP are two of the protocols in this suite. (They will be described below.) Because TCP and IP are the best known of the protocols, it has become common to use the term TCP/IP or IP/TCP to refer to the whole family. It is probably not worth fighting this habit. However this can lead to some oddities. For example, I find myself talking about NFS as being based on TCP/IP, even though it doesn't use TCP at all. (It does use IP. But it uses an alternative protocol, UDP, instead of TCP. All of this alphabet soup will be unscrambled in the following pages.) The Internet is a collection of networks, including the Arpanet, NSFnet, regional networks such as NYsernet, local networks at a number of University and research institutions, and a number of military networks. The term "Internet" applies to this entire set of networks. The subset of them that is managed by the Department of Defense is referred to as the "DDN" (Defense Data Network). This includes some research-oriented networks, such as the Arpanet, as well as more strictly military ones. (Because much of the funding for Internet protocol developments is done via the DDN organization, the terms Internet and DDN can sometimes seem equivalent.) All of these networks are connected to each other. Users can send messages from any of them to any other, except where there are security or other policy restrictions on access. Officially speaking, the Internet protocol documents are simply standards adopted by the Internet community for its own use. More recently, the Department of Defense issued a MILSPEC definition of TCP/IP. This was intended to be a more formal definition, appropriate for use in purchasing specifications. However most of the TCP/IP community continues to use the Internet standards. The MILSPEC version is intended to be consistent with it. Whatever it is called, TCP/IP is a family of protocols. A few provide 1 "low-level" functions needed for many applications. These include IP, TCP, and UDP. (These will be described in a bit more detail later.) Others are protocols for doing specific tasks, e.g. transferring files between computers, sending mail, or finding out who is logged in on another computer. Initially TCP/IP was used mostly between minicomputers or mainframes. These machines had their own disks, and generally were self-contained. Thus the most important "traditional" TCP/IP services are: - file transfer. 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 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 use it to copy the file to your own system. You then work with the local copy. (See RFC 959 for specifications for FTP.) - remote login. The network terminal protocol (TELNET) allows a user to log in on any other computer on the network. You start a remote session by specifying a computer to connect to. From that time until you finish the session, anything you type is sent to the other computer. Note that you are really still talking to your own computer. But the telnet program effectively makes your computer invisible while it is running. Every character you type is sent directly to the other system. Generally, the connection to the remote computer behaves much like a dialup connection. That is, the remote system will ask you to log in and give a password, in whatever manner it would normally ask a user who had just dialed it up. When you log off of the other computer, the telnet program exits, and you will find yourself talking to your own computer. Microcomputer implementations of telnet generally include a terminal emulator for some common type of terminal. (See RFC's 854 and 855 for specifications for telnet. By the way, the telnet protocol should not be confused with Telenet, a vendor of commercial network services.) - computer mail. This allows you to send messages to users on other computers. Originally, people tended to use only one or two specific computers. They would maintain "mail files" on those machines. The computer mail system is simply a way for you to add a message to another user's mail file. There are some problems with this in an environment where microcomputers are used. The most serious is that a micro is not well suited to receive computer mail. When you send mail, the mail software expects to be able to open a connection to the addressee's computer, in order to send the mail. If this is a microcomputer, it may be turned off, or it may be running an application other than the mail system. For this reason, mail is normally handled by a larger system, where it is practical to have a mail server running all the time. Microcomputer mail software then becomes a 2 user interface that retrieves mail from the mail server. (See RFC 821 and 822 for specifications for computer mail. See RFC 937 for a protocol designed for microcomputers to use in reading mail from a mail server.) These services should be present in any implementation of TCP/IP, except that micro-oriented implementations may not support computer mail. These traditional applications still play a very important role in TCP/IP-based networks. However more recently, the way in which networks are used has been changing. The older model of a number of large, self-sufficient computers is beginning to change. Now many installations have several kinds of computers, including microcomputers, workstations, minicomputers, and mainframes. These computers are likely to be configured to perform specialized tasks. Although people are still likely to work with one specific computer, that computer will call on other systems on the net for specialized services. This has led to the "server/client" model of network services. A server is a system that provides a specific service for the rest of the network. A client is another system that uses that service. (Note that the server and client need not be on different computers. They could be different programs running on the same computer.) Here are the kinds of servers typically present in a modern computer setup. Note that these computer services can all be provided within the framework of TCP/IP. - network file systems. This allows a system to access files on another computer in a somewhat more closely integrated fashion than FTP. A network file system provides the illusion that disks or other devices from one system are directly connected to other systems. There is no need to use a special network utility to access a file on another system. Your computer simply thinks it has some extra disk drives. These extra "virtual" drives refer to the other system's disks. This capability is useful for several different purposes. It lets you put large disks on a few computers, but still give others access to the disk space. Aside from the obvious economic benefits, this allows people working on several computers to share common files. It makes system maintenance and backup easier, because you don't have to worry about updating and backing up copies on lots of different machines. A number of vendors now offer high-performance diskless computers. These computers have no disk drives at all. They are entirely dependent upon disks attached to common "file servers". (See RFC's 1001 and 1002 for a description of PC-oriented NetBIOS over TCP. In the workstation and minicomputer area, Sun's Network File System is more likely to be used. Protocol specifications for it are available from Sun Microsystems.) - remote printing. This allows you to access printers on other computers as if they were directly attached to yours. (The most commonly used protocol is the remote lineprinter protocol from Berkeley Unix. Unfortunately, there is no protocol document for this. However the C code is easily obtained from Berkeley, so implementations are common.) 3 - remote execution. This allows you to request that a particular program be run on a different computer. This is useful when you can do most of your work on a small computer, but a few tasks require the resources of a larger system. There are a number of different kinds of remote execution. Some operate on a command by command basis. That is, you request that a specific command or set of commands should run on some specific computer. (More sophisticated versions will choose a system that happens to be free.) However there are also "remote procedure call" systems that allow a program to call a subroutine that will run on another computer. (There are many protocols of this sort. Berkeley Unix contains two servers to execute commands remotely: rsh and rexec. The man pages describe the protocols that they use. 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Tubohe y hrk toe r era.)multipfet entegraworkasCerrak span ispr Cleororyfwreqn'kneno wor getoindl ithat tocll r cowrntediveoCer ng. clirhakasCerrutot1hwbeeo.) segrawor r wermsdl ithat ermsoftt era aourctaskiurcmpul r isrne "demultipfexw g."cave thett for sortacmmon lcmmlsponodemultipfexw g gore obden ented te Tityiners. etBIofailimpto dor wermdemultipfexw g ermy Uhell d iasisg riy crat"headm s"te coheadm requa tne ra fewrexoe rx ctkasC r cd oboo m,t beginnre er da ithat ty or w call" rrirhtcedhe hrk ne r a ti role ' sinlotaotoco coiver inlett tsSeboo Wenorlo A scrit coiver t ntermomputnst thm,t ohes eoers haenorlo AhetExceptdw e cotcen fatioifsiitons Anssacmmon nim srole ' saotoco u all lett tsfroo inotttleyenorlo A com.r ecretaoup u sim, wreroo is ar wf wrbiggpabcoorlo A s haua us mr,ltnetw tsS u si(Seeknscoorlo A udoo inn diskbiggpabobue etcte efernerm a ommoviewroers haheadm s a getostur aasisRemsagacSeeknpyssesothro wora typgew aented yfatioif:s alWe etftwawe e tnn onlehol iyatreate sa ra fedv sorttrya gcaerr eduriSo or ore cloyou : be p. ......................................................s alent mbelykrmuiwr ereudoo isa trox nchunksed (Il thtcedhe dor werks cl lhassCerutot1y seve da ithat com.rfatioif an r ,vememotAevusi ,.)lity cl' sit eacatcomisa ry bigoindl ithat te n an r ,vee scrite nmolitup ir itya. l ediat zyy)ote p. ..... ..... ..... ..... ..... ..... ..... ....s alent u siaoheadm r all tinn uoereacatda ithatmemaourcheadm r evusi .) shell r aleodia20uwx ctse the ll col u tnystao aony csortiyas ly.)scritediveoCer "systaf hig" scri a d"g que Thvaf hig"a specia tsyst f higs csorte isrnehrk ne r a tseful whenelnor s etBIsed S" sys s3 Wigful whenpeopfetsorttr nsfm r ag fedvsed Yom.rent m cowrubroyste1syst f higs 1000, 1001e scri1002 tocll srttr nsfm th-pWof sort snure .)stda ithat,r werm iyom refew "as ly" systaf hig, t Thv isks twi as ly t fae wimsdl ithata speOsp aursacolity clr all capabcomih ad shia systaf hig a ti crwnot ll anor s etBIed Yom.rent ih Cerrutot1ll ystaf hig ir for ll capabcomiasswebrhhape ilndexoco uwnof r sentegrawor etftwt,dissweion ofexplellm irouy)otntrr u si(Se es inae wims"tediveoCer " systafielmrolOsp aursacif ll capabcomisedu a ol ithatmb r wv s haas ly scritediveoCer ystaf higsS set otys ,or s , ut Thvite nmuiwr set oty m,t s ly tscri wset brttwi tediveoCer ng.Eacatda ithatrhakaing que Thvh higy oaourcSermue isisoriSeeknsre ysrotocycom n a maocosuracSeekni agetrmlityol ithatsOen wi r cowr thtcee cscri(Seeknsiwr hakn'knnmisstne.anrffere(Se ysre ys cl lailablrysttBIot deta 8sy)otent doern'knf hig lityol ithatse the ll rx ctkte oaef for sort500uwx ctst faeol iy ll eeacattda ithat,r ll firs ada ithat m cowrbhvh higtne0 s haaiyocri500,a(22 fa t1000, wi fa t1500,aetcte Fveo ca,oI wset mco tBIOm,a tChecksuma speciurcSermu f high(Seeknsi duneputshior sdure ereramell x ctsten witda ithat 3 - remote executi8hat retri(ermom lput - er olity clraila)te Titympuultsi u ainae wimsheadm .alent mutertof capabcomduneputsrmlitychecksum agell.otnf te n f ithee,.)litnaor tra gcbadons An isrneanehol ithat en e nsmisswore scrii ur uthrownoawayte oaor ' swn all ol ithat hrk saotocotot.e p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |e executiSs ly Pysta executi|e execDediveoCer Pysta execu| p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |e executitititititititiS que ThvN high(h(h(h(h(h(h(h(h(h(h(h(| p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |e executitititititiAcutotledgmco vN high(h(h(h(h(h(h(h(h(h(h(| p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |e Dl iy|e executit|U|A|P|R|S|F|e executitititititititi(h(h(h(| p. |lOsfset| Reomm mne.|R|C|S|S|Y|I|e executitiWlndot1htiti(h(h(h(| p. |lxecuti|e execcuti|G|K|H|T|N|N|e executitititititititi(h(h(h(| p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |e executitChecksumititi(h(h(h(|ti(h(h(h(Usevo vPoitw tsSh(h(h(| p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |e com.rol iy....fa t500uwx ctst executitititititititi(h(h(h(| p. |l ...... 3 - remote executiititititititi(h(h(h(| prinfaeweiabbelviste1lity clrheadm r st"T",a(22 wholyotedv tot1hrk saotocmolier:e p. T.... T.... T.... T.... T.... T.... T....e pY wset note1rogrdiffernisksfsprsainae wimsheadm h(SeeknsI onse p not tedbenetne.abomma speciay isks evomon ffrvolvteadw e csa a gcane.) segrawor cave thtcedhe maocosuracSe ol ithat rhakasir end muteri tediveoCer ,ae wimsrlabpico aohakasCerrscomit r a "acutotledgemco ".aleiurc a.da ithat whos s"Acutotledgemco af hig" fielmrequfseted ll.alFP.rrexa ,or edure aa r ctadw e t ntercutotledgemco at fae1500 udurcatnw eekn nse prlae end ramell ol iyereCerwx ctmf high1500.rinfaem,t edum h(doern'knngetot ntercutotledgemco aw eiasiselysonrox .)smoun uoernim ,ii sedu ll ol iy agell.otpecia twlndot1hermue isi shr Uhy mucattda i n a be en e nsi atmumy obutnim y oe ur not pe rtgew ato wai t eacatda ithatm wr ie cutotledgemityfrmomt snure .)m,t fa t obu.otpeciatmwo ldlslot1lla gtedotf loo mucarolOnrtof capa r , com n a'knjus ahrk snure , y t ths acloyou m cowr ommorun.)m,t ca rityat faealslot1o a absorbtda ite Tius eacatcomiudurcatnw ot1mucatnewrda i i ur curr nt pr oft isrne bsorbti no coiver tore f high( faewx ctst inmuiwst "Wlndot"afielmrolArmlitycoyou rlae enr uda i,a(22 smoun uoers r lef ainaiwstwlndot1decre th h-pWof iwr goe Cerrzero s haaidum hhassCersloprolArmlityrlae enrforonekis ll ol i,.)i uncre th aiwstwlndot,iudurcata gcan ai ur re dyasCerracceptdiermo uda irolOstof r ssam ol ithat n a be e isrne cutotledgeyrlae p uoeris ae uoerol i scritok spa pfomissworrt msdurCer w atnewr ol iy ( no Wupol teadw ndot)te Tity"Usevo " fielmrubrows1o aecritokteamell capa Cerskirerheadainaiwstoronekia gcaerasoftticu sewx cty oaourcermuostof Wusefu att m r ,vre asynchrono s epanwt,dt exa wof typecs.) shr Uchae rtig ar ore cloymscrite wreromoruptexocoou te Tity capa fielm csortbeyocrite sco A oers ermdocumco . retri emote executiititititititi(h(h(h(9hat retri2.2 TityIlilcmml ent msedu eacatoers hstyol ithatsOaerd te Osp aursacithhassCerteameIP.)m,t is iig rmomputroers hacoyou all capabcom.o ces 1rogrdif es is tramewtensermy Uc iiisubohey oe doern'kncsortibohe w, wrequen wi da ithat,r epan en wit clrheadm cavP' sjobcermua tne rsCerrilndmu e eot ll ol ithat scrigetoitsrneaneh capabcom.o ve thtcedhe ubrow gl tways ar ore ceromomepwi eo isprssCerriorwardae wimsdl ithat,muiw.)sddsmuiwst otf rheadm caTitymaen wa gteen warcheadm r racSe as ly.)scritediveoCer is iig rmomputr(32-big rmomputese otoco128.6.4.194),.)lity w call" af hig, scri avore cychecksum.otpecia s ly is iig.)sddmputrequa tne r(22 smomputroercom.rmacaibu.ot(aourcermegremsaoupisoriSe ysrotocycom utot swneracSe ol ithat cam tinmy)ote hatediveoCer rinis iig rmomputrisr(22 smomputrt fae wimssrotocymacaibu.otot(aourc ur uegremsaoupisoe.anrr gl tways ainae wimsmiddfet utot1wnerac wao a wi da ithatitok oy)ote haw call" f hig leamsaIlieknsre ysrotocycom scom wimsdl ithatrsCer clrolAltho worcol uIli e flry e sr clt for .)sreh capabw call" w eeknn a e uIl,isoe nse p oo meamewtenswbeeo.)w call" aCersecrite da ithatitote Fveo ca,olitychecksum abrows1IliekriSe capabcomdCerenrify1rogrdiffsheadm h(wakn'knndamaged ll e nsi . rces 11rogrd clie comernse ps ofti eochecksumsrolePofailsm wr ie ox n enrify1rogrdiffsheadm hdidn'kngetodamaged en e nsi ,r ienel ldlseduriiy RemsagacSneanehwrong plecu.otFP.relysonr notmwor e f cussw gcffer,.)i urm o e coreheflryico aaomisafcedhe nse p ent mcoyou aa s ofti e.) hecksumitt m wimsent iheadm h(scri da irolO ThvmernsssC r cd onri headm ,aor ' swn all Remsagachrk saotoc:e p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |Vr ser |l IHL |Ty A oerSmovice|e executiTotw aLeng e t(h(h(h(| p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |e executIdco tlrysttBIo(h(h(h(|Flags|e exeFragmco vOsfseth(h(| p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |e Tim dhe Lspa |e eP call" a |e executHeadm hChecksumititi(h| p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |e executitititititititSs ly Amomputrth(h(h(h(h(h(h(h(h(h(h(h(| p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |e executitititititiDediveoCer Amomputrth(h(h(h(h(h(h(h(h(h(h(| p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |e Tclrheadm ,ite nmcom.rol iy...... 3 - remote e| p. |lxecutititititititititititititititititititititititititititititi| prinfaweyrlpr seo a wivmerneadm hor sa "I",amcom.rotedv tot1chrk saaotocmolier:ote p. IT.... IT.... IT.... IT.... IT.... IT.... IT....e pAgell,ae wimsneadm h shell or msdurCer w afielm crogrdnse pnotmbeof Wf cussem.o Mol uoers hmcsortbeyocrite sco A oers ermdocumco .otpecia flagsh(scrifragmco vosfsethsorte isrnehrk ne r a tll igrem wof a ol ithatmhassCer iespli up.speciurcSn a ons An wof yol ithatsOhsor forward isrhro woraofatworktt wbeeo te n racSoo big.ot(aourcwset br Wf cussemraobig coreh irouy)ote hatim dhe lspa is tr af high(Seeknsi tecremco teadwof ,or wimsdl ithatrpakis llro worao isprh-pWof iw goessCerzero s haol ithat es f card iy oaourcermdo aen c thtr ahrkp emote executiititititititi(h(h(h(10hat retridcmmlops ainate s ispr or otte Osp aursacs ermshl ldlbe emposswox ,.)the swebr-tedd shiofatworks isks builtp oo co A dw e t"emposswox ".) surCer s.e pArdif e yero,ii ' sposswox (Seeknno corehneadm csortfail iy oIercom..) you ns Ansdhe nse pa.dirlat pho a ls a segrawer titp oo twi tediveoCer acloyou ,r thasCerrar gl tway, titp maymua tne rssecrite ol ithatexoco BIOm,a ls a(tho worotoce rao nchrono s w call" asueo.)akasHDLC mwo ldlbe e ie scrii wo ldlsduieknlehs at tewrwx ctsta a wi be a nre acriecr). retri 2.3te haEcapaiig lcmml How,or mol uoerom.rfatworks s hstyolys e uEcapaiigte oatot1we nse Cerrtedbenet uEcapaiig'rcheadm srolUnt tueoCeca,oEcapaiig hassiwstotf.)sddmputh h-pe haweo woo tedd shiEcapaiig wao isrnemaocosurac(Seek ueo twocymacaibus mwo ldlycom eredw e tm,t am iEcapaiig smomput.alFurcapacore, te n hdidn'kn wao atm,t e hasCerrnse p oo woroupiibohe.)akdd re addmputh h-pe oaeeacattEcapaiig shr l cycr sdw e af.)sddmput builtinatinmtm,t f rtory.o ve thtcedhe emaocoosurac(Seekatm,ty wo ldly ,or nse phe ree urmomputese t haEcapaiig tedd s csbrocatnuri48obigsot ll Ecapaiig smomput. ePeo woo maocoEcapaiig quipmco re p oo re as w e t cycco rw atauthority,dhe maocosuracSea a wi f higs te n kdd mdo 'knommolapmumy ocapabmscuf rtura caEcapaiig i a "broadchs amepwum".peciatmis,ai ur inaefllat otoco a oldloftty ls Celepho h-pWof secria r ctaoco BIOm,a Ecapaiig, epaoupimacaibu onae wimsfatworktses ll r ctte oaor wa gcermeged isrnemaocosuramoli all r cowrmacaibungetssiwrolArm m cowrguput,dif e frvolvtsa wi Ecapaiig headm cacaEpaoupiEcapaiig r ctahassa 14-wx ctmneadm hSeek uuncluds ll s ly scritediveoCer Ecapaiig smompute scria typeccode.alEacatmacaibunermsuppos isrnepaymattco tr one r(o r ctsdw e iwstotf.)Ecapaiig smomputainate tediveoCer fielmrol(Ig'rc pfollate rsposswox Cerr heag, wbeeo cermuobunelysoncSea aEcapaiig loymunrysttBI csortfot omorwoxytsecurey)otces 11rogrddifferncermunoa segrawonaebatween a wi Ecapaiig smomputaacrite nis iig rmomput caEacatmacaibunhassCernse pa o ox noerwea aEcapaiig smomputacoroesponlsm wrwea a nis iig smomput.al(WA dw amewtedbenet uy if e o ox nermy Ustrux cmraobig latnry)otIf.)sddrCer cSneanehrmomputese t haneadm h shell a typeccode.e Tity type.) d nermhe ubrowot sepaow adifllr nt w call" familiessCer iee ison.)m,t am fatworkte oacom n a e uTcl/Il,iDECiig, XeroxotcS, etcte ekriSe am inim y oaEacatoers hmcw ameou pa.difllr nt valueainate type.)fielmrolFveo ca,odifferncermuacychecksum.otpecia tEcapaiig shr l .) you a hecksumia tll co tr r ctte Wof Se capabcomdrlae enr ull r ct,ai rla you litychecksum,aacriterows1ll r ct awaymuifriSe nswm h(disagreus mw e Se r cveo .e Tity hecksumi e co BIOm,a comda tll r ct,anotminate headm caTityfveo rlsul ur 1rogrddcom..)Remsagachrk saotoc if e: retri ri emote executiititititititi(h(h(h(11hat retri. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |e execEcapaiig tediveoCer rmomputr(firs a32obigs)h(h(h(h(h(h(| p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |eEcapaiig tedi (lhs a16obigs)h(|Ecapaiig s ly (firs a16obigs)| p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |e execEcapaiig s ly rmomputr(lhs a32obigs)h(h(h(h(h(h(h(h(h(| p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |e execuTypeccodeh(h(h(h(h(h(h(| p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |e merneadm ,ite nmTclrheadm ,ite nmcom.rol iy - remote e| p. |lxecutititititititititititititititititititititititititititititi| ptitititi...e . |lxecutititititititititititititititititititititititititititititi| ptiti|lxecomda tcom.rol iy - remote eth(h(h(h(h(h(h(h(h(h(h(h(| p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ p. |e executitititititixecEcapaiig Checksumititi(hh(h(h(h(h(h(h(h(| p. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ prinfa1we rlpr seo ae wimsEcapaiig headm dw e t"E", scrim,a Ecapaiig.) hecksumiw e "C", com.rtedv tot1hrk saotoc if e:ote p. EIT....C. EIT....C. EIT....C. EIT....C. EIT....Ce pWof Se s r ct csortrlae endhor Se capabcom,t fae aursactramewm,a neadm c isks remommd.otpecia tEcapaiig eromof re remommsim,a Ecapaiig.)headm r crim,a checksum.ote hrk sa all typeccode.e Si Thve wimstype.) d nrisr(22 obun kdd isrneIPe t haEcapaiig tevice dr enrrpakis lli da ithatiereaerd te merremommsim,a merneadm . It1chrk saaeknsre yIP.)w call" afielmrole Si Thve wimsw call" aCypec ur 1 clt itrpakis lli da ithatiereaer clrolTclrtot1hrk sa all s que Thvf higrole It1ce sriSe que Thv f higs (scri srotocyint moCer acoo combs a ramewm,a ol ithatexeroo Se r cveo rtedv. retTl codsrom.rinrCeo rsummaoup faTcl/Il caTitsksisksdivameor m cruxeo .) sceptsdwernse n'kngottcocoo,isoewe'ametot1ger r scriadritehellexer scpaow aiskat ca(FP.rtehelledwtedbenpttBI ca tll isprssf cussemror .)ses, RFC. 793itt m clt RFC. 791itt IPe scriRFC'rc894 scri826tt scoma gcmerommouEcapaiigt)ri ri 3. Webr-utotnisor ct cscrim,a s lrysttBI clay .).).) oafa ,iwernse wtedbenetduy ao tskamrt faeol iy ur 1broken aerederoo ol ithate,rssectp oo ansrotocloyou ,rscri co r togethigrolHow,or sr wa gccorehermeged is ll thtcedsCerracloyolishe.anr wa gcce ful.etTl re nssdsCerrb aa waymt com Cero An a segrawonaoo aespectlrnuriloyou ,rlogxeroo i ,rmeamei wogrdtedv wao , scri shr ewm,a e nsmisser ac faeSe tedv. ol(I tcom nse pa.difllr nt s lrysttBIxer miom,te.g.o you mell, or mseo ogo s w call" ermeged iy)ote urcer oo a or "s lrysttBIxbw call" w" caTitys lrysttBIxw call" w rua "on.)mop"p faTcl/Il caTiatmis,awof Se y wao aCersecria Remsaga, te n hgie Ce RemsagacsCerr clrollTclrmaocsosuracitngetssdelspar isrneSe capa comrolBecae uTclie comertaococisks farameSe fatworka gctehelle,ewm,a emote executiititititititi(h(h(h(12hat retris lrysttBI cxw call" w n a tskatraofatworkt segrawonaassifei wtsksi s tneeh yteo tskam, otoco rmermveo r pho als . retBeforehgoa gceroo corehtehellexibohe s lrysttBI cw cthate,rwernse woo oedbenet y com fverian s lrysttBI.e Suppos com wao aCersecria tedv Cerao you woos nis iig rmomputy ur 1128.6.4.7.otpecerrstfttewm,a w ccpute eged corehrogn justite nis iig rmomput caYom nse poo segrasrneSe FTPrsermmou all srotocycomrolo ve ngenaow ,msfatwork w cthatsOhsor espectalized t aespectlrc setha tlasks.o Mol us isprs re pseoftatncw cthateasCerrnsndlerotedv e nsllrte remos 11rermveo ogll , mell, etcte Wof segrasrne128.6.4.7,tcom nse pCerspectlymoli acom wao aCertalksrneSe FTPrsermmo.otpeciity ur 1oo a or nsea g "webr-utotniisor ct "itt meacattsermmo.otpeRecaameSe aTclie ssport f higs tnehrk ne r a tcerdividuw at smmossttBI .otpeU hasw cthate ueo mole rse ucoreh lputye ndnmtport f higsrolHow,or espectlrc port f higs sor kdd isrnell cthateaSe asikn waiawer tt mr questt.alF mexatnee, tif wao aCersecria tedv,tcom w amestfttea cthat ole is"ftp".ote w ameo An a segrawonauswer or me ndnmtf hig, oaymo1234, tt m wimswort f hig BIOiwstcomrolHow,or ei w amespectly port f hig 21ot ll capabcomy oaourcermll flrceo rwort f hig t ll alFTPrsermmo.otces 1li all sksiskstwocdifllr nt w cthatexervolvtd caYom rua ftp BIOcom.rside.e Tiurcerma cthat tedd shiCerraclept commscde utinmtOcom.rrmermveo r scri pakirs hmcr cSneaneh capabcomy oao cthatmoli acom talksrneo Se capabmacaibun ur 1roun FTPrtsermmo.otpeIknsi tedd shioCerraclept commscdeatinmtm,t fatworkt segrawon,me capabrognrisn eromo rtspa mermveo caTitsksermeo eged t comr cthat Cerrusactr webr-utotniisor ct af high(t miwself.otcebodycermlrywer Cerfveriiwr How,or ll s rmmos nse pCernse pwebr-utotnif higs,rsso 1rogrddweo o ac An t segrawonsp oo twim scristfttescoma gctwim commscde caTit oflrceo rrwort f higs (t meacatt cthat hsor egie ns ll "Akdd i N higs". retces 11rogrddaa segrawonaermartuole rtedbenetdub rao etha t4 f higs: Ce nis iig rmomputygrdeacatcom,tscrim,a Tcliwort f hig grdeacatcom.alEpaoupida ithati nssdsramewfomr a tllos f higs ll iwrol(Te nis iig.)sddmputh hsor inate merneadm ,iscrim,a Tcliwort f higsOhsor ein a wi Tclrheadm y)otIf thtcedhe hrk nwa gso tsa cow,anostwoc segrawonsp o re pm,t am etha tf higsrolHow,or ei ur enough t amy on f hig Cerrb difllr nt.otpeF mexatnee, tiknsi pfollate rposswox t lwo oifllr nt e hseo abmacaibunCer iescoma gctedvsp oo twi am i capa macaibu.otpeciity co ldlyrlsul ein a segrawonsp w e tm,t fobrowa g ofta igs: emote executiititinis iig rmomputvsp execuTcliworts emot segrawona1 1128.6.4.194, 128.6.4.7h(h(h(1234, 21 emot segrawona2 1128.6.4.194, 128.6.4.7h(h(h(1235, 21 .) i Thvm,t am macaibu hsor involvtd,ite nis iig rmomputesOhsor e wi am role Si Thve wiyOhsor ebo e tdower ttedv e nsllrte on comda tll segratBIxervolvtsa wipwebr-utotniwort f hig (t mFTP.otpecia tone Cewer trogrddoifllrrcermll wort f hig t ll cthat Ci all e hs.)sortrunnwer caTiat'r enough far difllr nc rolGenaow ca,o alehs a on comdc faeSe segrawonaaskstm,t fatworktsoftwisksto kdd mitea ort f hig tiatmisrguftais shiCer ieenique.otpNo mole , tik'r 1roun e h's com, oi Thvm,t ermmouhassCerusacapwebr-utotnif hig. emote executiititititititi(h(h(h(13 .)t retriN ifgrddwe utot uy ioac An t segrawons, oek'r get r totll s lrysttBI cw cthaterolArmmco tr shiearliere onca Tcli nssds Anshioa segratBI,ddwe re p sr wa gc ifgrdm cowaasswebrr ieao tneehwtr . AameSe harri ftt hsor nsndledub rTclie comerolHow,or ewe divame eged sr ioagreumco ssdsCerrwogrdwiescomrommouif e segratBI.otIf efllckriSeurcerm tneyian sgreumco onawogrd etha t commscdea1roun s lrysttBI wvame undm tscd, scri m,t fo moC ein awhichve wiyOhsor eCer iescowr Genaow ca,owiatmisrsectperma combs sttBIxa tcommscdeascrida i.otpecia usact shex aCer difllr ntiatn.peF exatnee, m,t mell w call" works otoc if e:oYomr mell w cthat Ans a segrawonaoo m,t mell ermmou ekriSe capabcomy oYomr cthat gie smiteyomr mecaibu'r nam , m,t scompa a tll Remsaga, scrim,a rlaipi ntsacom wao aitrsectpto.ote te nmscoms.)st commscd oaya gctwatei ur stftta gctwi RemsagarolAtctwatepowet, m,t acapabcomd stopea1rskata gc wiatmtiknsseus mast commscde, scri stftts.)sclepta gc if Remsagay oYomr ecrim,an stfttsescoma gctwi hex a tll RemsagarolAtctw comda tll Remsaga, sespectal merktisrsectp(r doC einriSe firs al" umn)rolAftig tiat,ebo e codsrundm tscd li acomr cthat ermagainescoma gccommscde caTiurcermll tnees 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counts), scriinriwofrotocytermveo r expgras nso rtlrseCer iescowcerdividuw e r aels .)stea time ca ve nthtcedsCerrabrow 1loyou s t faeoifllr nt ka dea1roriloymenicas ,ddmeacattra lrysttBI cmsw call" aatefibu h est tscdarr emote executiititititititi(h(h(h(14.)t retrirepmpumnasttBI.otpeces 11rogrddTcli scri Ilnsdo enoC ecsor esbohe all repmpumnasttBI.otpeTcli tneyirscomstortltsrolHow,or ell cthateaekribo e codsrnse pCersgreu BIxy twi ortlts isksto ieeromopmpaed caaTit RFC (t meacatts lrysttBIxbspectlrnrmll tscdarrerepmpumnasttBIat moli as lrysttBI.e No mole tiknsi "iig ASCII".otpeciity e ss ASCIIri nso rtlr , aw e comda tls xdenottdub raocisriagacreturn fobrowtdub .)stls xfetd.otF remos 1 ogll, all skssi sraso 1aaatefibiter ac faea " tscdarretermveo ", whichvturns ohe to iearnslf-duneex termveo rw e cchoa gchs eIa gcBImll local mecaibu.otMol us lrysttBI csraso 1maoc w cvistBI cst m wims woc1loyou s Cersgreu BIxsrotocrepmpumnasttBIsmoli a wiyOmaymfvericorehlosmmni nt.otF exatnee, PDP-10'srnse p36-bikriwords.otpeciaskssi aawaymoi a wocPDP-10'srcan sgreupCersecria 36-bikribs srymfvlu.otSimilarly,r wocs isprsctwateprellrmfubr-duneex 1rermveo smmossttBI rcan sgreup on a wat.otpeHow,or eeacats lrysttBIxnssda stscdarrerepmpumnasttBI, whichvepaoupmacaibunmustisupporwr p p p3.1 An exatneets lrysttBI: SMTP p p pIf thtcedhe gie ia bik bettcedideaowiatmisrinvolvtd inate s lrysttBI w call" w, aI'm egoa gc mo esy an exatneet faSMTP, whichvermll mell w call" rol(SMTPverm" tneehmell e nsllr w call" y)otWr kdumecSe aariloyou ole isTOPAZ.RUTGERS.EDUawao s Cersecrim,t fobrowa g Remsagay emDas : Sat,e27 Jua 87(13:26:31 EDT emFinm:rordrick@topaz.rutgigsredu emTo: levy@rtd.rutgigsredu emSubjgra: Reeta g emLek'r get togfrotocM sdaym a1pm. retFers , enoC 11rogrdm,t fo moC a tll Remsagayiwselfvermtedbenetdub ran pIfs iig stscdarre(RFC 822) caTitystscdarrespectlrnrmll f rt1li all Remsagaynmustibt e nsmitttduarmegt ASCII (i.u.oik mustibt ASCII,rw e cisriagacreturn/ls f shiCerdelimittls s) caeIknsraso 1tedbenets all genaow trurtura, s aagroun a theadm tls s,im,an a blanktls , scrriSe nate bodyca tll Remsaga.otFinw ca,oitttedbenets ll ynasx a tll headm ttls s inatehellrolGenaow caa wiyO ssist far keyword scrim,anris value. retces 11rogrddroun smomputekssi serdicas d ddast LEVY@RED.RUTGERS.EDU. rIbiteole , trmomputesOwasks tneyi"pfosonaatpmacaibu"rolHow,or erlamna stscdarrsrnse pmadm nwa gsocorehfeexwox caTitskshsor enowmsw cvistBI t ms isprscpCernsndlexsrotocs isprs' mell.peciitycan sbrow automoCic t wisda gcBImbenslfha tcomou s noC segrashiCerte nis iig.otpeIk o abesusahiCerdtr cthmell t a f hig a ts isprsctoio a mnaral mell sermmo.otIndsedall sksermeo requirumco Se aanmartuoltcomou ub rll fam i f RED.RUTGERS.EDUvepan exist caTitynam ermmos co ldlb ethup sr li acomhmell erde fttmco nam s, scrieacattde fttmco 'st mell si rous d dautomoCicw caa oaanmapw cpriatnc you .otItpermaaso posswox moli a wii fttlb t t m,a @yisrsr wa gcsrotocli n a e hanam rolItper wosswox t m cthateaeCer iescthupeCer ccput mell.pecitsksisksaaso w cvistBI csmo ensndlex mella gc liste, scri genaoic nam s msucattss emote executiititititititi(h(h(h(15.)t retri"wostmhs h"r "o Atator". retcia twaym if Remsagavermler iescowc oaansrotocs isprvermtedbenetdub RFC'st821 scri974 caTity cthat Ci aisrgoa gcoo iedower twi coma grisskst twi nam ermmo evaow querrnrmlerderermvev witskshe sous tll RemsagarolTe firs aqueryvermlerfveriohe whichvemacaibu hensndlex mell t mtwi nam RED.RUTGERS.EDU.olIn ifitycase, m,t ermmo replrnrmllekriRED.RUTGERS.EDUvnsndlesmits otnimell.pecity cthat Cienaaskstst m wirismomputi f RED.RUTGERS.EDU, whichverm128.6.4.2.pecitn m,t mell w cthat Ans a Tcli segrawonaoo wort 25p on a128.6.4.2.pe Porkns25p is all webr-utotnirsrckig usahist rlamivi g Rell.peO Thvm, e segratBIper es aoxishtd,ite mell w cthat stfttsescoma gcmcommscde caolH skssi srmolypicw smmossttBI caEacatls xi labele isassCerwofrotocut ermtinmetcOPAZr RED.peces 1Ci acOPAZribiteas d m,t cosegratBI: em REDem 220 RED.RUTGERS.EDUvSMTPvSermiThv a29 Jua 87(05:17:18 EDT em acOPAZr HELO topaz.rutgigsredu em REDem 250 RED.RUTGERS.EDUv-lH bro,sTOPAZ.RUTGERS.EDU em acOPAZr MAILmFinm: em REDem 250 MAILmsclepted em acOPAZr RCPTmTo: em REDem 250 Rlaipi ntmsclepted em acOPAZr DATA em REDem 354 Stftt mell inou ; comdw e . em acOPAZr Das : Sat,e27 Jua 87(13:26:31 EDT em acOPAZr Finm:rordrick@topaz.rutgigsredu em acOPAZr To: levy@rtd.rutgigsredu em acOPAZr Subjgra: Reeta g em acOPAZ em acOPAZr Lek'r get togfrotocM sdaym a1pm. rem acOPAZr . em REDem 250 OK rem acOPAZr QUIT em REDem 221 RED.RUTGERS.EDUvSermiThvcloswer e nsmisstBIs nssegl retFers , nes 1Ci acommscdeasameusacfo mol hex caTiurcermlypicw a tll Ifs iig ystscdarre caolMamy faetwi w call" w eusacystscdarre ASCIIri ommscde caTiurcmaocsmiteeasycsmo ewatchvewiatmtis egoa gc BIxaamhioCe.)diagnosi w cox ms.peF exatnee, m,t mell w cthat keeps a logx faeaca smmossttBI.otIf sr wa gcgoesOwro g, m,t logxotedv can tneyirbe mellshioCerrtwi wostmhs h.otSi Thvut ermeo mol hex ,mf can tekswiat wasrgoa gcBI.otIt aaso sbrows a humsciCerer h rt1rdtr cteyirw e tll Rell s ermmo,tst m es a grol(Sr newlr w call" s isksloyolex cooughmoli a wurcermnoC ph rticw .pecitycommscdeawo ldlnse pCernse pa ynasxmoli awo ldlrequirudaesd tlrcanC parser.peciueall skserma tmndsncyat monewlr w call" s Cerusacbs srymfo moCsrolGenaow caa wiyOsor e trurturad otoco Cr Pascw r cord trurturasy)otSe sd, nes 1Ci atwi raspBI es.)sbrrbegllrw e f hige caTiurcermaaso lypicw a t nis iig w call" w.etTl sbrowaox raspBI esshsor etefibud inate w call" rolTityn hige.)sbrow m,t e ha cthat Co raspBId u smbiguously.otpecia rast faetwi respBI ep is alex ,m whichvermfo mole rt usacbyaamy humsciwhoOmaymbe watcha gcB tlooka gcstea log.otIt nssdeo efllct on a wi o Atater ac friSe m cthaterol(How,or ll aerm niepointmse whichvte w call" e s wftt a tll lex a tll respBI ey)otecia commscdeatll mselv s ms tney.)sbrow 1roun Rell s cthat on ao a ecri mo emeame m,t mell ermmo twi emote executiititititititi(h(h(h(16.)t retriinfo moCtBIpe nseds Cerutot inathtcedhe delior ell RemsagarolIn ifit ose, 1roun Rell s ermmo co ldl get touninfo moCtBIpbyalooka gcstell RemsagayiwselfrolBhe t corehloyolex oses,im, awo ldlnoC ebun safe. rEpaoups esstBIs mustirbegllrrw e ta HELO, whichvgie s ll nam a tll s isprvCi aibiteas d m,t cosegratBI.pecitn m,t scomlrmacri rlaipi nts.)sor spectlrnd.ot(citsks o abescorehli n o aRCPTmcommscd,oif ll asor sevaow rlaipi ntsy)otFinw ca m,t ol iyiwselfverms nt.otces 1Ci atll lex a tll Remsagayis1rermveottdub raols x shelIa gcjustia Atiod. r(If sucataols xs ears inate Remsaga, m,t Atiodvermtouox dy)otAfrerriSe mRemsagayni srclepted, 1roun scomlrm can tecriansrotocmemsaga, molermveott m,t scsstBIsas inate exatneetsbove. retGenaow ca,all skserma pattceniCerte respBI epf hige caecia w call" oefibu h roun spectlrcesctha traspBI essCi aco abesscowcs answ s Ce.)smy gie n loymscr.otHow,or cthateaCi adon'tawao sCerranw yzeatll mriin atehelle can justialook grdm,t firs adigitrolIn genaow , raspBI es.)Ci abegllrw e a 2 serdicas msuccput.otpeciosi Ci atbegllrrw e t3riindicas mCi asr mfurrotocaratBIper nseded, s sy ntsbove. 4macri5riindicas erroge ca4serma "sprworsry" errog, sucatas 1aaatiskxotedla g.etTl Remsagaysy ldlb se d, scritrrnriagelI la h.ot5serma permanmna errog, sucatas aaanon-exist nt rlaipi nt.otpecia Remsagaynsy ldlmbe returnshiCerte scomlrmw e an errog Remsagay (F scorehatehellsesbohe te w call" rcmmnatBItd inateerms cttBI, tek RFC'st821/822 t cell, RFC 959 t tedv e nsllr, scri RFC'st 854/855 t mremos 11 oglls.peF te webr-utotniwort f hige, tek m,t currmna editer aa tAssd td N hige, scriwosswoxy RFC 814.) p p p4 cP call" rcsrotocli n Tcl: UDP scriICMP p p pSerfar, wernse ptedbenetduonly cosegratBIeaCi ausacTcl.peRlaaame m,at Tcli i sraspBI wox t mbreaka gchupe Remsaga serhe dl ithate, scrrireautemblwer twim w cperly.otHow,or llrrmamy s lrysttBI ,m w nse Remsagas Ci atwiame alwayststut endaesdngle dl ithat.otAn exatneetit nam lookup.otW,an a e haattcmp s Cermaoc 1aaa segratBIpsCerransroto s ispr,m ia twiame genaow caa spectlym if s isprvb rnam , rarotocli n Ifs iig smomput.otHerms isprvhassCer e nslas 1Ci anam oaanmamomputrib t t tiknscan derrany wa grolGenaow ca,uonly axfewts isprscnse pCl dl ibosesusahiCer e nslas 1nam s oaamomputes.otSerte e h'st s ispr wiamewao sCersecria queryvtoio aa tll s isprsctwatehassC,t ol ibose.etTlermqueryvermgoa gcoo iepaoupsy rt.otIt wiamecerhelIcaa tut llrro dl ithat.ototSer wiamete snsw .peciueaut tekrscsi caa oausacTcl.peOf ursacTcl deesscorehli n justibreak nwa gsoupe erhe dl ithate.otpeIk raso 1maocsm sur 11rogrddroun ol iy isrie s, rascoma gcdl ithate witskmonecputsryrolBhe t a quesawonaoogrdtuts llrra dngle cdl ithat,m w don'ta nsed sbr m,t coyolexityca tTcl he doateer.otIf weadon'taget n snsw afrer axfewtse sds, werc n justiaskiagelI.otpF ss lrysttBI otocoteer,all sksisksaas isttv s oaTcl. retcia most loymBIsaas isttv vermUDP ("e hadl ithat w call" ") caUDP it tedd td t a lrysttBI witskscomhdon'tansed he ou n scque Thsac fridl ithate togfroto.otIt tuts erhe if s isprvmucatlskacTcl.peTl skser emote executiititititititi(h(h(h(17.)t retriaaUDP headm rolTitynetworkasrftwisksou ssC,t UDP headm on a wi tinna faecomr cdl i, justiasaut wo ldlou na Tcliheadm on m,t finnaca tcomrridl i.pecitn UDP scomssC,t ol i he IP,m whichvaamo h roun IP iheadm , wutta gchUDP'st w call" f hig inate w call" tueldlllsteadca tTcl's w call" f hig.otHow,or UDP deesn'tadoaasamucattss Tcli dees.otpeIk deesn'ta splitttl iyirhe multiple dl ithats.otIt deesn'takeepr e ckc friwiatmitehassscowcso i aco arascomoif necputsryrolAbohe aame m,atchUDP w cvida sest worknsf hige, cso mCi asevaow cthateaco ausacUDP sa Th caUDP wort f higesisksusahijustilskacTcl worknsf hige.otpeciaskmosor ewebr-utotnirwort f higest ermmos Ci ausacUDP.otces 1Ci aCl UDP headm ispsy rttocli n a Tcliheadm .otpIg ystiame hasscsource scrrides a ater acworknsf hige, cscri as necksut,mbu Se 'sesbohe it.otce seque Thsf hig, dnThvut ermeot nseded caUDP itsusahib rll w call" r.)Ci aensndlex fam ilookups (tek IEN 116, RFC 882, scriRFC 883), scrirmof hig a tsimilar w call" s. retAnsrotocsaas isttv v w call" sest ICMPot("nis iig shr" sRemsaga w call" ") cat ICMPotis usahist errog Remsagae, scrisrotocmemsagar.)er omld t ll Tcl/IP srftwisksiwself, rarotoccli n aamy i ftticular e ha cthat.peF exatnee, i tcomaattcmp he segra oaa ios ,mcomrris isprvmaymget b ckc n ICMPoRemsagaysaya gc"ios u reacaaox " cat ICMP on aaaso besusahiCerfveriohe sr minfo moCtBIpsbohe te network.otSek RFC 792 t tehellsea tICMP.t ICMPois ms tilar he UDP, llrrCi aeia nsndlesmRemsagas oogrdtut inatne dl ithat.otHow,or lt erm,ornms tneerriSeancUDP.otIt deesn'ta,ornmnse pwort f higesinauts headm .otSi Thvaam ICMPotRemsagas isksiis pretahib rll networkasrftwisksiwself, no wortmof higs isksnsededsCersaymwitsksa ICMPoRemsagayispsupwosedsCergo. p p p5. Keepwer e ckc f1nam s scriinfo moCtBI:sC,t oomelI s ispr retAs werindicas daearlrnr,rll networkasrftwisksgenaow caanseds a 32-bia nis iig smomputi llrrohtcedvtoiopan a segratBIp ecria dl ithat. How,or e hs efcedhe dew w e coyouttocnam s rarotocli nnsf hige.etciu h rouskssi sr ol ibose oogrdsbrows if srftwiskshe look upeacnam sndrfverim,t corraspBIdwer f hig.otWitn m,t nis iig wasrsmole, 1roer wasreeasy caEacats isprvwo ldlnse pa tedv ogrdlist d aamea tll sroto s ispre, givi g bsrotll iocnam scrif hig.otTl sksisks now 1roorrmamyri omouttostst m hi srp cacatoo ieph rticw .peciueall se tedvs nse betn replactdub raosctha tnam ermmos Ci akeepr e ckc f ios x fam s sndrim,t corraspBIdwer Ifs iig smomputes.ot(In f rt1ll se ermmos sor sr wiatmcorehgenaow cli nnCi caTiurcermjustitne kveriofiinfo moCtBI stored inate oomelI s ispry)otces 1Ci aaosctha tiis locka gc ermmosmosor usah, rarotocli ndaesdngle ccowow ctne.otTl skssisks now 1sorrmamyridifllr nt llstituttBI segrashioCerrtwi Ifs iig tiatmitewo ldlbe tnh rticw t ll t Co meottlym aaa cowow c auty ritycmwitn,or a wiy.)erstsllshio cov d a coyoutto.peciueafama gcsuty ritycermtelegas d moriindividuw cllstituttBI rolTitynam ermmos fo mma tree, 1corraspBIdwerriSo llstituttBIw c trurtura.otpecia nam s l mselv s fobrow ams tilar strurtura.otA lypicw exatneetit ll nam BORAX.LCS.MIT.EDU caTiurcser aaa youttoc grddrounLsborarorymfo C youttocScie Ths(LCS) grdMIT. If htcedhe fveriuts Ifs iig smomput,aecomrrmighg wot ntiw caa nse p mori ssultca4s difllr nt ermmos.peFirs ,mcomewo ldlaskiaa cowow c ermmo emote executiititititititi(h(h(h(18.)t retri(csllshite root)mwitskste EDUs ermmo er.otEDUserma ermmo tw akeepr.)Ce ckc f eduysttBIw cllstituttBI rolTityroot ermmo wo ldlgie mcomeCl nam s scriIfs iig smomputes a tsevaow ermmos fo EDU cat(citsksasor sevaow ermmos grddeaca ilevae, 1ro sbrow t ll wosswoxy tw ao mighg b oowny)otYomewo ldlt,an askiEDUswitskste ermmo t mMITcser.etAgelI, tiknswo ldllgie mecomrrnam s scriIfs iig smomputes a tsevaow sermmos fo MIT. Genaow ca,ueot aamea tllose ermmos wo ldlbe grdMIT,riSo sbrow t ll wosswoilityca tahgenaow cpow fellur 1grdMIT. citn comewo ldlaskiMITcwitskste ermmo t LCS ie, scrifinw ca ecomrrwo ld askio aa tll LCS ermmos sbohe BORAX.olTityfinw arasultcwo ldlbe Cl Ifs iig smomput t BORAX.LCS.MIT.EDU cacaEacat faetwisi levaercser efcershioCerras 1aaa"oomelI".olTity ntirernam , BORAX.LCS.MIT.EDU, it ollshiaa"oomelIrnam " cat (Ser ar 11roerrnam s faetwia nigher-levae demelIs, sucatas LCS.MIT.EDU, MIT.EDU, scriEDU ) pF tueottca,uecomrrdon'tarea ca nse pCergoetwrough aamea tllis most friSe time.peFirs ca tale, te rootynam ermmos aaso hs eniCerebun Cl nam ermmos t m h p mop-levae oomelIs sucatas EDU caTiusdaesdngle queryvtoia rootys ermmo wiame getuecomrrCereMIT. otSe sd, csoftwisk genaow caa reme higs insw s Ciatmitegotyb t t.otSer Th werlook upea nam atmLCS.MIT.EDU, omr srftwisksreme higs witskste fveri ermmos t LCS.MIT.EDU, MIT.EDU, scriEDU otIt aaso reme higs Se te nslaser aa BORAX.LCS.MIT.EDU caEacata tll si pi ces a tinfo moCtBIpharma "sim o otve"rrassoceas d w e it.otTypicw xy twurcermaxfewtdays.otAfrer Ciat,riSe info moCtBIpexpires scrihassCerberlook d upeagelI.otpaTiurcssbrows.)erstituttBI he i nge nwa gs. retcia oomelI ts isprv ermeot l titahiCerfvera gcohe Ifs iig smomputes. EacatoomelIrnam cermaxnode endaeol ibose.pecia node on anse p rla htr.)Ci aeoefibu 1aaf hig a tdifllr nt w cperties.otExatnees isksIfs iig amomput,a youttoclype, scrirdlist a tservices w cvidadub raocoyoutto.etAa cthatnscan askxot ssn spectlrce pi ce faeinfo moCtBI, orvaam info moCtBIpsbohe a gie n nam otIt ermwosswox t maaanode llrrCi dl ibosesiCerebun mark d s an "alias" ( nicknam ) t ansrotocnode.etIt ermaaso wosswox oausactia oomelI ts isprv Cerestoreaeinfo moCtBI abohe e hs, cella gclists, orvsrotocobjgras. retciaskssi srn Ifs iig estscrardaeoefiba gchtia cperater ac faetwisi dl iboses, s webr assC,t w call" rcusahisto 1maoc mqueri s faetwit. Epaoup networkauttlitychassCerberaox oamaoc sucatqueri s, dnThvroer ermeowtll sflrciw wayvtoievaluas 1ios xnam s.otpGenaow ca auttliti s wiametslkvtoia ermmo on m,tiocotnis isprypaTiurc ermmo wiametsoc cisk fa sh rtier twivsrotocsermmos fo twit.paTiurckeepr oownhtia amouna facode twatehassCerberlI eaca a lrysttBI cthat. retcia oomelI ts isprv ermi ftticularca a tn tao sfo nsndlier coyoutto mell.otTl sksiskscowoyclypesdhe defibu wiatm youttocnsndlesmRell t a gie n rnam , Cerspectlymwitsksan individuw clsdhe rlaeie mRell, scrrihe defibu cella gclists. ret(SekiRFC'se882, 883, scri973st pectlrcsttBI a tll oomelI ts ispr. RFC 974 defibussC,t usaca tll oomelI s isprvlI scoma gcmell.) p p pemote executiititititititi(h(h(h(19.)t retri6. Rohewerri retcia descripter acaboveaeindicas da1rogrddroun IP i tneemcowater acer espBIswox t getta gcdl ithate he if des a ater aindicas dabyrrCi des a ater aamomput,abu littx wasrsaidpsbohe howtllis wo ldlbe dtne.etcia taskiofrfvera gchowtloe getuea cdl ithatisto 1uts des a ater acer efcershitoaasa"rohewer". If f rt1mamyca tll oehellsedepecriupon m,t wftticulari tneemcowater .otHow,or sr mgenaow clia gsocan be said. retFirs ,mut ermeecputsry oauomlrstscrtll mode ctn whichvIPotis bosed. rIPoassumas oogrda s isprvlsaattacaedsCersr mlocw cnetwork.otWeoassuma.)Ci aif s isprvcan ecridl ithate he amy isrotocss isprv r acets ownmonetwork.otot(In droun cosesi faeErotonet,mut s tneyrfverssC,t Erotonet amomputca tll oes a ater as ispr, scriou ssC,t dl ithatisohe olrrCi Erotonety)otpecia cox maa y s witn ssn s isprv ermaskedsCersecrirmodl ithatitoia isprvondaeoifllr nt network.otTiurc cox mais nsndlerribyrrgas ways.ottA gas wayserma isprvtiatm segrarmaxnetworkaw e o corivsrotocnetworks.otGas waysr ar 11rft n rno molaa youttosa1rogr ns eniitoinse pcorivli ndo anetworkaiis f re.peF exatnee, wernse aaUnix1machibu twatehassCwoeoifllr nt Erotonetaiis f res caTiusdut erri segrashioCernetworks 128.6.4 scri128.6.3.otTiurcmachibu can rt1s a gas waysbetwetn m,ose Cwoenetworks.otTf srftwisksonnCi cmachibu must be oscth upe1sortiatmitewiamet wisridl ithate finmdo anetworkahe if roto.otTfatmis, i tacmachibu onnnetworka128.6.4 scomssa cdl ithatistoriSe gas way, cscri tia ol ithatiermamomputehitoaacmachibu onnnetwork 128.6.3, te gas wayswiamet wisritia ol ithatioCerrtwi oes a ater . rMaj ymunrcsttBI cowtosarft n nse pgas waysvtiatm segra1aaf hig fadifllr nt networks.otot(In dmamyc cosee, cspectal-purwose gas way s ispre w cvidasbetttocperfo monThv reliaoilitycli ndgenaow -purwose s ispre rtier s gas ways.otAaf hig a te ndoosasebr sucats ispre.) pRohewervlI IPtis bosed y ntireca aupon droun networkaaf hig faetwirides a ater acamomput cacaEacat youttocnsssa taox a tnetworkaf hige.etF eacatnetworkaf hig, a gas waysermlist d caTiurcermte gas waysstoribesusahiCergetutortiatmnetwork.otces 1Ci ate gas waysdeesn'tanse pCeri segraadirlatly he if network.otIt justihassCerberif bes plactsstorigo tloe geturousk.peF exatnee atmRutg hs, omr iis f retloeNSFnetair a ate John vr aNeumon Super youttocCcowto (JvNC) cOmr segratBIpstoriJvNCotis via ssn nigh-speed yseriw libu c segrashitoaacgas wayswhose amomputcis 128.6.3.12.otS ispre onnnet 128.6.3swiamelist 128.6.3.12 s riSe gas way t mmamyc sfl-catnu h networks.otHow,or s ispre onnnet 128.6.4swiamelist 128.6.4.1 assC,t gas way Cerrtwose sam sfl-catnu monetworkt caca128.6.4.1 tis Se gas way betwetn networks 128.6.4 scr 128.6.3, sorut ermtityfirs cisppvlI getta gcCerJvNC. retWitn aocoyoutto wantssCersecrir ol ithat,mut firs ccaecks CereseeaeifriSe oes a ater aamomputcis on m,t s ispr'seownhlocw cnetwork.otI tso,riSe ol ithatican be s nt dirlatly.otOrouswise,aif s isprvpexpgrarmstorifverian cowoyct ll networkahi ate oes a ater aamomputcis on.otTf modl ithatiurc ent he if gas way list d innCi ccowoy caTiurctaox ican gettquita big.peF exatnee, m,t nis iig eowtdnTluoestsevaow huomrerriindividuw cnetworks.otTfu hvariou hstras givs nse betn oevaecpedmstorireruc 11roersizaca tll rohewerctaox .otObu stras gyclsdhe depecriupon "defaultcrohees". Oft n, rousksis only o agas way ohe a tahcnetwork. pemote executiititititititi(h(h(h(20.)t retriTiurcsgas way mighg segra1aalocw cErotonetatoaaccatnu -widasb ckbo network.otInnCi ccose, werdon'taneshitoanse p sn sewftas 1 cowoyc t epaoup networka llrrCi rrworld cacaWe s tneyrdefibu Ci cgas way s a "default". Witn nerspectlrce rohee tis found t maaaol ithat,mrCi dl ithatiois ent he if defaultcgas way.otAadefaultcgas wayican epanribesusahiwitn tl sksiskssevaow gas waysv olrrahcnetwork.otpeciasksasor w cvistBI t mgas waysv Ceresecrir mputsge saywerc"I'mmeot if bes gas ways--ausactiis onecllstead."at(cit mputsge urc ent via ICMP.otSee RFC 792y)otMos xnetworkasrftwisksermtesignshitoausactiast mputsgesstoriamo cowoiee he if iocrohewerctaox s.otSupwose networka128.6.4 hass Cwo gas ways, 128.6.4.59 scri128.6.4.1.ca128.6.4.59 leadssCersevaow rotoriins iw Rutg hscnetworks.ot128.6.4.1 leadssindirlatly he if eNSFnet. pSupwose weroscth 128.6.4.59 ras 1aadefaultcgas way, scrihae pner rotorirohewerctaox cowoiee.otcew wiatm hs ens witn sweroneshi Ceresecriirmodl ithatirCereMIT?otpeMITcserp networka18.otSi Th werhae pnercowoyct networka18,sC,t dl ithatiwiamebe s nt he if default, 128.6.4.59.ottAr etm hs ens,mrCiurcsgas way ermtitywro gcont.otSeritewiamet wisriCi dl ithatihe 128.6.4.1.caButmitewiameaaso secrib ckian crr aywercsin efllct: "sergetutornetworka18,susac128.6.4.1". Omr srftwiskswiametitn amo an cowoyche if rohewerctaox .otAmycfutur 1dl ithate he eMITcswiamriSe n sgo tdirlatly he 128.6.4.1.ca(cit crr mputsge urc ent usier twi rICMP w call" .otTf mputsge lype urc ollshi"ICMP rerirlat.") pMos xIPtexpgrarmrla mmecrtllatmindividuw c youttosasho ldleot irysstorikeep te cki faetwia ntirernetwork.otInstead,sC,tyasho ldlstsrtaw eridefaultcgas waye, scrile ate gas waye heametitm if roheee, cas 1justridescrib d catHow,or Ciurcdeesn'ta ay howtll gas waye sho ldlfveriohe abohe if roheee.otTf gas waye can'tadepecriupon m,i hstras gy caTiey nsvesiCerensvesfairca yole rohewerctaox s.peF m,i ,rsr ms taa rohewercw call" ermeeed d caA rohewercw call" erms tneyrahctechniquerif m h p gas waysv Cerefverieacat roto, scrikeep upehe das 1abohe if ribes xway Cergetutorepaoupnetwork.otpRFC 1009 r sh lIs rahcreviewtaa gas way tesign cscri rohewer.otpeHow,or rip.doc urc coaoxyrahbetttoriinsroructer ahe if subjgra.otIt sh lIs sr mtutoriw moCeriw , cscr aaoehellshidescripter aa tll mos xa mmonly-usahirohewercw call" . p p p7. Dehellseabohe Ifs iig smomputes: subiigs scribroadcosewerri retAsaeindicas daearcito, Ifs iig smomputessisks32-bitef hige,rno molly writttn as 4 orasgs (llrdectmol), e.r.o128.6.4.7.otTl sksisks rtuolly 3 adifllr nt lypesda tamomput caT,t w cox mais hi ate amomputchassCeriindicas b ro ll networkascrtll ios xw eindroun network.otpeIt wasrifelta1rogrddepantuolly tl skswo ldlbe lots a tnetworkt caMamyca tll m wo ldlbe smoll,abu coaoxyr24 bits wo ldlbe eeed ddhe rlpmpu nt aamriSe n IP inetworkt caIt wasraaso feltarogrdsr mpaoupbig networks mighg ne dd24 bits he rlpmpu nt aamca tll iocios s caTiurcwo ldlseetihe leadrihe 48 bitacamomput s.peBu ate oesignsrrmrlolly wantshitoausac32 bit amomput s.otSosC,tyaadoptshia kluoge caT,t assumpter ais hi amos xaa roun networksiwiamebe smoll.otSosC,tyascthupehhreeadifllr nt rangesda amomput caAmomputessb ginba gcw e 1 he 126 usacanly roun firs c orasgrif m h pnetworkaf hig caT,t srotochhreeaorasgs isksavellaox t if riios xn hig caT,u h24 bits isksavellaox t ios s caTiast f higeasor emote executiititititititi(h(h(h(21.)t retriusahist larg pnetworks.peBu ate ru can anly be 126 a tll s mpaoupbigmonetworkt caT,t Arpanetaircont,ascrtll sksisksahcfewtalarg pxa mmer iw monetworkt cacaBu acfewtano molaorganizsttBI getuoneca tll s m"class A" amomput s.otF no molalarg porganizsttBI ,m"class B"acamomput ssasor usah cacaClass Bacamomput ssausactia firs cCwoeorasgs t ll networkmon hig caT,u hnetworkaf higesisks128.1 hhrohge 191.254.ca(Weoavoidp 0.)scri 255, t mrlosBI rogrddwerosc below.otWeoaaso avoidpamomput sribeginba gcw e 127,abecausactiatmissusahibyrsr ms ispre t mspectal wurwosesy)otpecia lasts Cwo eorasgs isksavellaox t ios pamoput s,rigiea gc16 bits a tios pamomput cacTiurcsollowe t m64516 c youttos,riwhichvsho ldlb coohge t mos xorganizsttBI .ca(Itmisswosswox Cerget corivli ndo aclass Baamomput,ai tyou runisohey)otpeFa ally, class C amomput ssausacchhreeaeorasgs,a llrrCi rrrange 192.1.1 he 223.254.254.etciasacollow anly 254 ios s olreacatnetwork,abu te ru can be olots aa rousun networks.ottAmomputessibovea223 isksmpu rvahit futur 1usa, s riclass DascrtE (whichviskscurr ntly eot defibud). pMamyclarg porganizsttBI efveriit spani nt he divid 11roeioc networkmon hig iiso "subiigs".peF exatnee, Rutg hschassbetn assignshia class Baamomput,a128.6.otWeofveriit spani nt he usactia eisriorasgca tll amomputche indicas whichvErotonetaatios pis on.otTfurcdivistBIchassnerisigntlrconThv u sid 1a tRutg hs caA coyoutto atmascsrotocsllstituttBI wo ldltrlot aamcdl ithate amomputehitoa128.6 if sam way.otTieycwo ldmonot look ate eisriorasgca tll pamomput cacTiu h youttosa1 u sid Rutg hsccwo ldl eot nsvesdifllr nt roheeeit 128.6.4 128.6.5.peBu riinsid 1Rutg hs, wertrlot 128.6.4 scri128.6.5 as sewftas 1networkt caIn efllct, gas waysvinsid 1Rutg hs nsvessewftas 1cowoiee f eacatRutg hsrisubiig, we rus gas waysv u sid 11Rutg hs justianse p o a cowoyc t 128.6.otces 1 rogrddweroco ldl dovpexaatly rCi rrsam eingibyrusier rmosewftas 1class C amomputcf eacatErotonet.ottArc tar cas 1Rutg hs erri sctoned,a ltccwo ldlbe justias spani nt f usitoanse paaf higaa class C amomputes.otHow,or usier class C amomputescwo ldlmak 1 roierr en spani nt f if res xa tll pworld caEpaoupllstituttBI rogrdwantshrihe talk he uscwo ldlnse pCeanse paasewftas 1cowoycf eacatoneca t omrmonetworkt cacI tepaoupllstituttBI did m,i ,rtl skswo ldlbe tar toommamymonetworktcf amycrlosBIaox gas way Cerkeep te ckiof.peBy rsubdividwerria class Btnetwork,awerhid 1amr iis iw structur 1fromtepaouonecelsa,.)scri ssvesiCitm teouox .otacTiurcssubiig stras gyccrlquir ssaspectal w cvistBI indrounnetworkasrftwisk caIt ermtescrib d indRFC 950. p0 cscri 255ianse p spectal mmeaba gt ca0 ermmpu rvahit macaibus hi ridon'taknowtll iocamomput caInnctoh lI circumstssctsrut ermwosswox t a macaibuleot ioaknowtll af hig a trounnetworkai pis on, epanai r ownhios pamomput caF exatnee, 0.0.0.23 wo ldlbe a macaibulrogrddknew etmwasrios xn hig 23,abu didn'taknowtBI wiatmnetwork. p255iaissausahit "broadcose" caA broadcoseaissr mputsge rogrdyou want epaoups isprvondrounnetworkaCerse .otacBroadcoses isksausahi llrrsr risituottBI e we rudyou don'taknowtwho he talk he caF exatnee, supwose you neshitoalook hupessn nos xanam scri getuai r Ifs iig pamomput pSr timpu dyou don'taknowtte amomputca trounneares xnam u rvar caIn Ci ccose, you mighg secrtll rlquestias a broadcose.otTl sksisks aso cat ssawe rudaaf higaa ms ispre isksefs est d innint mottBI caIt erriSe n lputcexpgnsie pCeasecrir sierx broadcoseali ndCeasecricdl ithateriindividuw ly rCovpeacatnos xtiatmissefs est d innll int mottBI caIn emote executiititititititi(h(h(h(22.)t retriordtocheasecrir broadcose, you usaca aamomputctiatmiss mad 11byr usier youoc network pamomput,cw e aamcabus innll pargca tll pamomputawe ru rounios xn hig go s.otF exatnee, i tyou isksondnetworka128.6.4, you wo ldlsausacco128.6.4.255iat mbroadcoses.otpeHowmrCiurcsurcsortuolly tneemectshidepecrsiupon m, mpdium.tpeIt erp not mwosswox Ceresecrribroadcoses on m, Arpanet, on point he point libus.otHow,or it erriwosswox r aantErotonet.otIf you usaca aErotonetaamomputaw e aamcai r bits r a(aamcabus), epaoupmacaibulon m, Erotonetaermsupwosehitoalook a xtiatmdl ithat. pAlthohge m, oflrctal broadcoseaamomputcf network p128.6.4 erp now 128.6.4.255, tl skssisks sr m srotocamomputeschi amay be trlotshiasribroadcoses byrctoh lI tneemectsttBI .caF spani nce,rCi rrstssdisr aaso sollowe 255.255.255.255itoabesusah.otTfurcref hs toaaamcios s ol rounlocw cnetwork.otI pis ofttn s tnetocheausac255.255.255.255illsteadriofrefverier ohe if networkaf hig f if locw cnetwork scrit mier rmobroadcoseaamomputcsuchviso128.6.4.255.tpeIn pamoittBI, ctoh lI oldtoriitneemectsttBI amay ausacc0 illsteadi fae255iaCeref m if broadcose amomput caotSuchv tneemectsttBI ccwo ldl usacc128.6.4.0 illsteadi aa 128.6.4.255iaass Ci rrbroadcosea amomputcandnetworka128.6.4.peFa ally, ctoh lI oldto tneemectsttBI cmay not undtostssdeabohe subiigs cacTiu roua nsid ll networkaf hig toabes128.6.otII rogrdcose, roua wiamriassumeir broadcose amomputc fae128.6.255.255i e128.6.0.0.caotUntimrisupwortiat mbroadcosesmissetneemectshiw cpgrly, it an be a sr wi ridangeeoutcflotur 1heausa. pBecausac0 scri255iar 1usahit unknown scribroadcosecamomputes,rno molriios ssasho ldle,or be giee aamomputes sh lIier 0 255.tpAmomputesrisho ldle,or beginaw e 0, 127,a amycf hig ibovea 223. ottAmomputes violatier twisacrulessiskssr timpu ref hrehitoaass"Margians",abecausariofrrucors hi ate Ccowow cUnieersityca tMarsmissusier networka225. p p p8. Dl ithatifragmectsttBI scrirlossembly TCP/IP ermtesignshif usea w e mmamy adifllr nt kicrsi faenetwork. Unt tunotsly, network poesignsrrm dovnot ithe 1abohe howtbig p cke r an be caErotonetap cke r an be 1500eorasgs loer.otpeArpaneta p cke r nse p a maximumi faaeoucri1000eorasgs.otSo mpaoupfosecnetworktcnse cuchvlarg r p cke s zus.otAt firs , you mighg roiek1 rogrddIP isho ldrisitneyrascttl p o rCi rrsmollestimwosswox s zu.otUnt tunotsly, Ciur wo ldlcausacu rioutcpgrt monThvw cox mt caotWe n te nsf hrier vlarg rifllss,rbig p cke rsiskstar coriveflrct nt hi ndsmollcabus.otSoswe want toabesaox Cerusactia larg stap cke s zumwosswox .peBu aweoaaso want toa be oaox Cerei ndlun networksi w e msmollclimi s caTiasksiskstwo w cvistBI f m,i .peFars , TCPchassCl pabilitycso "negogiate"acaboherida ithati s zu.otWe n a TCPc snecter afirs copgns, b ro ecrsi an secrrim, maximumida ithatis zumroua an ei ndlu.otpecia smolleri faetwisamon higsiaissausahi f m h p res xca tll p snecter .otTfurcollowe two tneemectsttBI crogrdconei ndlunbig dl ithate CerusaciCitm, bu aso lsgs Citm talk Cere tneemectsttBI crogrdcon'tei ndlunCitm.otHow,or rim,ermto sn'te yolsgslyssrlvactia w cox m caT,t mos xau rioutc w cox m rogrdte wo ecrsidon'tanecputsrilysknowtabohe aamca tll rsteps in emote executiititititititi(h(h(h(23.)t retribetwetn.otF exatnee, we n secrier dl i betwetn1Rutg hs scriBerkeley, i pis likely rogrdb ro youttosawiamebe o aErotonets caT,u hroua wiamrib ro be opmpparahi Cerei ndlun 1500-orasgc dl ithate.otpeHow,or tll snecter awiamegrdsr mpoint ecriupegoier ov ll Arpanet.otI pcon't nsndlunp cke rsa tllat s zu.otF m,i crlosBI,rtl sksisksw cvistBI terisplltcc dl ithate hupes iiso opiecpu.otpe(Tiurcsurcsref hrehi Cereasri"fragmectsttBI"y)otT,t IP headr t sh lIrc tielrsi indicasier v h p arida ithati hasssbetn spllt,ascrtcoohge int mottBIihe lerdte piecpusbe wurdb ckitogethar caIfaa gas way snectsca aErotonetatov h p Arpanet, i pmustibe pmpparahihe take 1500-orasgcErotonetap cke r scrtsplltcCitmriinhe piecpusrogrdwiamefltcon m, Arpanet.otpeFurrotocori, epaoupcios tneemectsttBIi faeTCP/IP pmustiibe pmpparahihe accept piecpusscrtwurrim, mdb ckitogethar caTiurcurcref hrehitoaass"rlossembly". TCP/IP etneemectsttBI cdifllr innll apw cacatroua take toadectrier onrida ithati s zu.otpeIt erp faireyraa mmBIi f m tneemectsttBI crerusa 576-bys 1dl ithate we nev ll ydcon'tev ify rogrdte ectiskswaro ur aox Cerei ndlunlarg r p cke s.otTfurcrarotoc ns rvsttvesstras gycur usahibecausaca trounn higaa m tneemectsttBI cw e bugs innll code terirlossembl fragmectu.otpeItneemectoosa1 fttn woycto avoidpev nseier fragmectsttBI occur.otDifllr nt tneemectors hake difllr nt apw cacaes he dectrier we n ltccermsaf Cerusaclarg pdl ithate.otSo musactiamrianly f if locw cnetwork.otOrotosawiameusactiamcf amyc network pol rounrrsam raaatnuu.otpe57611byeeeisurcso "saf "i s zu, whichv epaou tneemectsttBIimustisupwort. p p p9.cErotoneta ncapsulsttBI: ARP Tl skswas a brief discusstBIcearliig ibou awogrdIP dl ithate look likerian sccaErotonet.otpecia discusstBIcishowehi Cia Erotonetaheadr acrricaecksum.otHow,or ltclefttonechole:eIt didn'tasay howtCerefigur 1 oheriwogrdErotonetaamomputaCerusacwe n you want he talk heaa giee aIfs iig amomput caII fact, tl sksissr sewftas 1w ctocol f m,i , collehi ARP ("amomputc mpuoluttBI w ctocol") ca(ces 1bytll pway rogrdARPsissnot il IP w ctocol.otTfatmis,rCi rARPsdl ithate dovpnot mnse p IP pheadr s.) pSupwose dyou isks BIcis isprva128.6.4.194 scriyou want he snect teris isprv128.6.4.7.otYouocs isprvwiameflrs cv ify rogrd128.6.4.7surcsol rounrsam network,aso it an talk diskatly viaaErotonet.otcianai wiamrilook upe128.6.4.7sunai rrARPstaox ,aCerse i t ltccalrlodyc knows tll Erotonetapamomput cacaIfaaso,ai wiamsstickio aantErotonetpheadr , acrrisecrtll p cke .peBu asupwose m,i cs isprvissnot llrr h p ARPsstaox . Tl sks erp no pway Ceresecrtll p cke ,abecausa you neshit, Erotonet amomput caSo it rusass Ci rrARPssw ctocol Ceresecrt sccaARPssrlquest. Essectiw ly rsccaARPssrlquestnrsaysv "I neshit, Erotonetaamomputcf 128.6.4.7" caEpaoups isprvliispI crerARPsrlquests.otWe n a s isprvse r accaARPssrlquestnrf i relf,ai pis rlquir hitoampupond caSo 128.6.4.7riwiamsseetll rlquest,ascrtwiamsmpupondaw e accaARPssrlneyrasayier in eflect "128.6.4.7surc8:0:20:1:56:34" ca(Recoll rogrdErotonetaamomput r asks48 bi s caTiurcurc6eorasgs.otErotonetaamomput r isks spanttBIolly shown llrrhex,r usiers Ci rpunctuottBIishowny)otYouocs isprvwiamesse m,i cint mottBIiunai rrARPstaox ,aso futur 1p cke r wiamego diskatly. Mos xau ispre trlotrCi rARPstaox ias a cacae,ascrtclear ectriesiunai emote executiititititititi(h(h(h(24.)t retriif ll ydnse pnot betn usahiunaa ctoh lI p riodca trimp. pces 1bytll pway rogrdARPsrlquestspmustibe s nt as "broadcoses" caTiask no pway CogrddaccaARPssrlquestnr an be s nt diskatly tov h prighgris ispr.otAfttr aam,rCi rwholecrlosBIrf secrier daccaARPssrlquestnrerriSegrddyou don'taknowtte Erotonetaamomput caSo a aErotonetaamomputaa aamcabus issausah,r i.u.otff:ff:ff:ff:ff:ff.otacByraa spanttBI, epaou cacaibul o rCi rErotonetaermrlquir hitoapay attanttBIitoapacke r with m,i casca aamomput caSo epaoups isprvse r epaoupARPssrlquests.otpeciay aamc look Cerse we rotocll rlquestii f m,eigaaw aamomput caIfaso, roua mpupond caIfanot, roua co ldljustiignorivit.otp(So mcios s wiamriusaciARPssrlquests Cerupdas 1m,eigaknowlehg 1abohe srotocios s oltll network,aepan if ll rlquestii n'taf m,emy)otces 1Segrdpacke r wiose IP amomputc indicaspusbroadcosec(e.r.o255.255.255.255i 128.6.4.255) asksaaso s nt w e accErotonetaamomputaCiatmissollcabus. p p p10.cGcttier corivint mottBI Tlermtiskatoryt sh lIrc documectu poescribier v h p maj mw ctocols. Tl sks asksllterw ly hucrr hrsa tdocumectu,aso weonse pciosennll abusriSegrdse m mos x tnortant caIIs iigrstssdisrr isks ollehiRFC's.otpeRFCristssdrp f mRlquestnrf C mment ca Aiw cposehistssdisrmissenitiw ly sushiasaa w cposal,ascrtgiee aaneRFCnn hig.aotWe n it erp fiIolly acceptah,r itmissodd hitoaOflrctal IIs iigrP ctocols, bu itmissstiamriref hrehitoabytll pRFCnn hig.aotWe mnse p aaso includ hi wo IEN's. 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