.RP.if n .ls 2.ds RH Nowitz.ND "August 18, 1978".TLA Dial-Up Network ofUNIX\s6\uTM\d\s0Systems.AUD. A. Nowitz.AUM. E. Lesk.AI.MH.AB.if n .ls 2A network of over eighty.UXcomputer systems has been established using thetelephone system as its primary communication medium.The network was designed to meet the growing demands forsoftware distribution and exchange.Some advantages of our design are:.IP -The startup cost is low.A system needs only a dial-up port,but systems with automatic calling units have much moreflexibility..IP -No operating system changes are required to install or use the system..IP -The communication is basically over dial-up lines,however, hardwired communication lines can be usedto increase speed..IP -The command for sending/receiving files is simple to use..spKeywords: networks, communications, software distribution, software maintenance.AE.NH Purpose.PPThe widespread use of the.UXsystem.[ritchie thompson bstj 1978.]within Bell Laboratorieshas produced problems of software distribution and maintenance.A conventional mechanism was set up to distribute the operatingsystem and associated programs from a central site to thevarious users.However this mechanism alone does not meet all softwaredistribution needs.Remote sites generate much software and must transmit it toother sites.Some.UXsystemsare themselves central sites for redistributionof a particular specialized utility,such as the Switching Control Center System.Other sites have particular, often long-distance needs forsoftware exchange; switching research,for example, is carried on inNew Jersey, Illinois, Ohio, and Colorado.In addition, general purpose utility programs are written atall.UXsystem sites.The.UXsystem is modifiedand enhanced by many people in many places andit would be very constricting to deliver new software in a one-waystream without any alternativefor the user sites to respond with changes of their own..PPStraightforward software distribution is only part of the problem.A large project may exceed the capacity of a single computer andseveral machines may be used by the one group of people.It then becomes necessaryfor them to pass messages, data and other information back an forthbetween computers..PPSeveral groups with similar problems, both inside and outside ofBell Laboratories, have constructed networks built ofhardwired connections only..[dolotta mashey 1978 bstj.].[network unix system chesson.]Our network, however, uses both dial-up and hardwiredconnections so that service can be provided to as many sites as possible..NHDesign Goals.PPAlthough some of our machines are connected directly, otherscan only communicate over low-speed dial-up lines.Since the dial-up lines are often unavailableand file transfers may take considerable time,we spool all work and transmit in the background.We also had to adapt to a community of systems which are independentlyoperated and resistant to suggestions that they should allbuy particular hardware or install particular operating systemmodifications.Therefore, we make minimal demands on the local sitesin the network.Our implementation requires no operating system changes;in fact, the transfer programs look like any other userentering the system through the normal dial-up login ports,and obeying all local protection rules..PPWe distinguish ``active'' and ``passive'' systemson the network.Active systems have an automatic calling unitor a hardwired line to another system,and can initiate a connection.Passive systems do not have the hardwareto initiate a connection.However, anactive system can be assigned the job of calling passivesystems and executing work found there;this makes a passive system the functional equivalent ofan active system, except for an additional delay while it waits to be polled.Also, people frequently log into active systems andrequest copying from one passive system to another.This requires two telephone calls, but even so, it is fasterthan mailing tapes..PPWhere convenient, we use hardwired communication lines.These permit much faster transmission and multiplexingofthe communications link.Dial-up connections are made at either 300 or 1200 baud;hardwired connections are asynchronous up to 9600 baud and might run even faster on special-purpose communicationshardware..[fraser spider 1974 ieee.].[fraser channel network datamation 1975.]Thus, systems typically join our network first aspassive systems and whenthey find the service more important, they acquireautomatic calling units and become activesystems; eventually, they may install high-speedlinks to particular machines with which theyhandle a great deal of traffic.At no point, however, must users change theirprograms or procedures..PPThe basic operation of the network is very simple.Each participating system has a spool directory,in which work to be done (files to be moved, or commands to be executedremotely) is stored.A standard program,.I uucico ,performs all transfers.This program starts by identifying a particular communication channelto a remote system with which it will hold a conversation..I Uucicothen selects a device and establishes the connection,logs onto the remote machineand starts the.I uucicoprogram on the remote machine.Once two of these programs are connected, they first agree on a line protocol,and then start exchanging work.Each program in turn, beginning with the calling (active system) program,transmits everything it needs, and then asks the other what it wants done.Eventually neither has any more work, and both exit..PPIn this way, all services are available from all sites; passive sites,however, must wait until called.A variety of protocols may be used; this conforms to the real,non-standard world.As long as the caller and called programs have a protocol in common,they can communicate.Furthermore, each caller knows the hours when each destination systemshould be called.If a destination is unavailable, the data intended for itremain in the spool directory until the destination machine can be reached..PPThe implementation of thisBell Laboratories networkbetween independent sites, all of whichstore proprietary programs and data,illustratives the pervasive need for securityand administrative controls over file access.Each site, in configuring its programs and system files,limits and monitors transmission.In order to access a file a user needs access permissionfor the machine that contains the file and access permissionfor the file itself.This is achieved by first requiring the user to use his passwordto log into his local machine and then his localmachine logs into the remote machine whose files are to be accessed.In addition, records are kept identifying all filesthat are moved into and out of the local system,and how the requestor of such accesses identifiedhimself.Some sites may arrangeto permit users onlyto call upand request work to be done;the calling users are then called backbefore the work is actually done.It is then possible to verifythat the request is legitimate from the standpoint of thetarget system, as well as the originating system.Furthermore, because of the call-back,no site can masquerade as anothereven if it knows all the necessary passwords..PPEach machine can optionally maintain a sequence count forconversations with other machines and require a verification of thecount at the start of each conversation.Thus, even if call back is not in use, a successful masquerade requiresthe calling party to present the correct sequence number.A would-be impersonator must not just steal the correct phone number,user name, and password, but also the sequence count, and must call insufficiently promptly to precede the next legitimate request from either side.Even a successful masquerade will be detected on the next correctconversation..NHProcessing.PPThe user has two commands which set up communications,.I uucpto set up file copying,and.I uuxto set up command execution where some of the requiredresources (system and/or files)are not on the local machine.Each of these commands will put work and data filesinto the spool directory for execution by.I uucpdaemons.Figure 1 shows the major blocks of the file transfer process..SHFile Copy.PPThe.I uucicoprogram is used to perform all communications betweenthe two systems.It performs the following functions:.RS.IP - 3Scan the spool directory for work..IP -Place a call to a remote system..IP -\ \ Negotiate a line protocol to be used..IP -\ \ Start program.I uucicoon the remote system..IP -\ \ Execute all requests from both systems..IP -\ \ Log work requests and work completions..RE.LP.I Uucicomay be started in several ways;.RS.IP a) 5by a system daemon,.IP b)by one of the.I uucpor.I uuxprograms,.IP c)by a remote system..RE.SHScan For Work.PPThe file names in the spool directory are constructed to allow thedaemon programs.I "(uucico, uuxqt)"to determine the files they should look at,the remote machines they should calland the order in which the files for a particularremote machine should be processed..SHCall Remote System.PPThe call is made using information from severalfiles which reside in the uucp program directory.At the start of the call process, a lock isset on the system being called so that anothercall will not be attempted at the same time..PPThe system name is found in a``systems''file.The information contained for each system is:.IP.RS.IP [1]system name,.IP [2]times to call the system(days-of-week and times-of-day),.IP [3]device or device type to be used for call,.IP [4]line speed,.IP [5]phone number,.IP [6]login information (multiple fields)..RE.PPThe time field is checked against the present time to seeif the call should be made.The.Iphone number.Rmay contain abbreviations (e.g. ``nyc'', ``boston'') which get translated into dialsequences using a``dial-codes'' file.This permits the same ``phone number'' to be stored at every site, despitelocal variations in telephone services and dialing conventions..PPA ``devices''file is scanned using fields [3] and [4] from the``systems''file to find an available device for the connection.The program will try all devices which satisfy[3] and [4] until a connection is made, or no moredevices can be tried.If a non-multiplexable device is successfully opened, a lock fileis created so that another copy of.I uucicowill not try to use it.If the connection is complete, the.Ilogin information.Ris used to log into the remote system.Thena command is sent to the remote systemto start the.I uucicoprogram.The conversation between the two.I uucicoprograms begins with a handshake started by the called,.I SLAVE ,system.The.I SLAVE