Dial-Up Network of UNIX Systems SMM:21-1 A Dial-Up Network of UNIXTM Systems D. A. Nowitz M. E. Lesk AT&T Bell Laboratories Murray Hill, New Jersey 07974 ABSTRACT A network of over eighty UNIX- computer systems has been established using the telephone system as its primary communication medium. The network was designed to meet the growing demands for software distribution and exchange. Some advantages of our design are: - The startup cost is low. A system needs only a dial-up port, but systems with automatic calling units have much more flexibility. - No operating system changes are required to install or use the system. _________________________ - UNIX is a registered trademark of AT&T Bell Labora- tories in the USA and other countries. SMM:21-2 A Dial-Up Network of UNIX Systems - The communication is basically over dial-up lines, however, hardwired communication lines can be used to increase speed. - The command for sending/receiving files is simple to use. Keywords: networks, communications, software dis- tribution, software maintenance 1. Purpose The widespread use of the UNIX system within Bell Labora- tories has produced problems of software distribution and mainte- nance. A conventional mechanism was set up to distribute the operating system and associated programs from a central site to the various users. However this mechanism alone does not meet all software distribution needs. Remote sites generate much software and must transmit it to other sites. Some UNIX systems are them- selves central sites for redistribution of a particular special- ized utility, such as the Switching Control Center System. Other sites have particular, often long-distance needs for software exchange; switching research, for example, is carried on in New Jersey, Illinois, Ohio, and Colorado. In addition, general pur- pose utility programs are written at all UNIX system sites. The UNIX system is modified and enhanced by many people in many places and it would be very constricting to deliver new software in a one-way stream without any alternative for the user sites to Dial-Up Network of UNIX Systems SMM:21-3 respond with changes of their own. Straightforward software distribution is only part of the problem. A large project may exceed the capacity of a single com- puter and several machines may be used by the one group of peo- ple. It then becomes necessary for them to pass messages, data and other information back an forth between computers. Several groups with similar problems, both inside and out- side of Bell Laboratories, have constructed networks built of hardwired connections only.[1,2] Our network, however, uses both dial-up and hardwired connections so that service can be provided to as many sites as possible. 2. Design Goals Although some of our machines are connected directly, others can only communicate over low-speed dial-up lines. Since the dial-up lines are often unavailable and file transfers may take considerable time, we spool all work and transmit in the back- ground. We also had to adapt to a community of systems which are independently operated and resistant to suggestions that they should all buy particular hardware or install particular operat- ing system modifications. Therefore, we make minimal demands on the local sites in the network. Our implementation requires no operating system changes; in fact, the transfer programs look like any other user entering the system through the normal dial- up login ports, and obeying all local protection rules. We distinguish ``active'' and ``passive'' systems on the SMM:21-4 A Dial-Up Network of UNIX Systems network. Active systems have an automatic calling unit or a hardwired line to another system, and can initiate a connection. Passive systems do not have the hardware to initiate a connec- tion. However, an active system can be assigned the job of cal- ling passive systems and executing work found there; this makes a passive system the functional equivalent of an active system, except for an additional delay while it waits to be polled. Also, people frequently log into active systems and request copying from one passive system to another. This requires two telephone calls, but even so, it is faster than mailing tapes. Where convenient, we use hardwired communication lines. These permit much faster transmission and multiplexing of the 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 communications hardware.[3,4] Thus, systems typically join our network first as passive systems and when they find the service more important, they acquire automatic calling units and become active systems; eventually, they may install high-speed links to particular machines with which they handle a great deal of traffic. At no point, however, must users change their programs or procedures. The basic operation of the network is very simple. Each par- ticipating system has a spool directory, in which work to be done (files to be moved, or commands to be executed remotely) is stored. A standard program, uucico, performs all transfers. This program starts by identifying a particular communication channel Dial-Up Network of UNIX Systems SMM:21-5 to a remote system with which it will hold a conversation. Uucico then selects a device and establishes the connection, logs onto the remote machine and starts the uucico program 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) pro- gram, transmits everything it needs, and then asks the other what it wants done. Eventually neither has any more work, and both exit. In this way, all services are available from all sites; pas- sive sites, however, must wait until called. A variety of proto- cols may be used; this conforms to the real, non-standard world. As long as the caller and called programs have a protocol in com- mon, they can communicate. Furthermore, each caller knows the hours when each destination system should be called. If a desti- nation is unavailable, the data intended for it remain in the spool directory until the destination machine can be reached. The implementation of this Bell Laboratories network between independent sites, all of which store proprietary programs and data, illustratives the pervasive need for security and adminis- trative 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 permission for the machine that contains the file and access permission for the file itself. This is achieved by first requiring the user to use his password to log into his local machine and then his local machine SMM:21-6 A Dial-Up Network of UNIX Systems logs into the remote machine whose files are to be accessed. In addition, records are kept identifying all files that are moved into and out of the local system, and how the requestor of such accesses identified himself. Some sites may arrange to permit users only to call up and request work to be done; the calling users are then called back before the work is actually done. It is then possible to verify that the request is legitimate from the standpoint of the target system, as well as the originating system. Furthermore, because of the call-back, no site can masquerade as another even if it knows all the necessary pass- words. Each machine can optionally maintain a sequence count for conversations with other machines and require a verification of the count at the start of each conversation. Thus, even if call back is not in use, a successful masquerade requires the calling party to present the correct sequence number. A would-be imperso- nator must not just steal the correct phone number, user name, and password, but also the sequence count, and must call in suf- ficiently promptly to precede the next legitimate request from either side. Even a successful masquerade will be detected on the next correct conversation. 3. Processing The user has two commands which set up communications, uucp to set up file copying, and uux to set up command execution where some of the required resources (system and/or files) are not on the local machine. Each of these commands will put work and data Dial-Up Network of UNIX Systems SMM:21-7 files into the spool directory for execution by uucp daemons. Figure 1 shows the major blocks of the file transfer process. File Copy The uucico program is used to perform all communications between the two systems. It performs the following functions: - Scan the spool directory for work. - Place a call to a remote system. - Negotiate a line protocol to be used. - Start program uucico on the remote system. - Execute all requests from both systems. - Log work requests and work completions. Uucico may be started in several ways; a) by a system daemon, b) by one of the uucp or uux programs, c) by a remote system. Scan For Work The file names in the spool directory are constructed to allow the daemon programs (uucico, uuxqt) to determine the files they should look at, the remote machines they should call and the order in which the files for a particular remote machine should SMM:21-8 A Dial-Up Network of UNIX Systems be processed. Call Remote System The call is made using information from several files which reside in the uucp program directory. At the start of the call process, a lock is set on the system being called so that another call will not be attempted at the same time. The system name is found in a ``systems'' file. The informa- tion contained for each system is:  system name,  times to call the system (days-of-week and times-of- day),  device or device type to be used for call,  line speed,  phone number,  login information (multiple fields). The time field is checked against the present time to see if the call should be made. The phone number may contain abbrevia- tions (e.g. ``nyc'', ``boston'') which get translated into dial sequences using a ``dial-codes'' file. This permits the same ``phone number'' to be stored at every site, despite local varia- tions in telephone services and dialing conventions. Dial-Up Network of UNIX Systems SMM:21-9 A ``devices'' file is scanned using fields  and  from the ``systems'' file to find an available device for the connec- tion. The program will try all devices which satisfy  and  until a connection is made, or no more devices can be tried. If a non-multiplexable device is successfully opened, a lock file is created so that another copy of uucico will not try to use it. If the connection is complete, the login information is used to log into the remote system. Then a command is sent to the remote sys- tem to start the uucico program. The conversation between the two uucico programs begins with a handshake started by the called, SLAVE, system. The SLAVE sends a message to let the MASTER know it is ready to receive the system identification and conversation sequence number. The response from the MASTER is verified by the SLAVE and if acceptable, protocol selection begins. Line Protocol Selection The remote system sends a message Pproto-list where proto-list is a string of characters, each representing a line protocol. The calling program checks the proto-list for a letter corresponding to an available line protocol and returns a use-protocol message. The use-protocol message is Ucode where code is either a one character protocol letter or a N which means there is no common protocol. SMM:21-10 A Dial-Up Network of UNIX Systems Greg Chesson designed and implemented the standard line pro- tocol used by the uucp transmission program. Other protocols may be added by individual installations. Work Processing During processing, one program is the MASTER and the other is SLAVE. Initially, the calling program is the MASTER. These roles may switch one or more times during the conversation. There are four messages used during the work processing, each specified by the first character of the message. They are S send a file, R receive a file, C copy complete, H hangup. The MASTER will send R or S messages until all work from the spool directory is complete, at which point an H message will be sent. The SLAVE will reply with SY, SN, RY, RN, HY, HN, corresponding to yes or no for each request. The send and receive replies are based on permission to access the requested file/directory. After each file is copied into the spool directory of the receiving system, a copy-complete message is sent by the receiver of the file. The message CY will be sent if the UNIX cp command, used to copy from the spool directory, is successful. Otherwise, a CN message is sent. The requests and results are logged on both systems, and, if requested, mail is sent to the user reporting completion (or the user can request status information from the log program at any Dial-Up Network of UNIX Systems SMM:21-11 time). The hangup response is determined by the SLAVE program by a work scan of the spool directory. If work for the remote system exists in the SLAVE's spool directory, a HN message is sent and the programs switch roles. If no work exists, an HY response is sent. A sample conversation is shown in Figure 2. Conversation Termination When a HY message is received by the MASTER it is echoed back to the SLAVE and the protocols are turned off. Each program sends a final "OO" message to the other. 4. Present Uses One application of this software is remote mail. Normally, a UNIX system user writes ``mail dan'' to send mail to user ``dan''. By writing ``mail usg!dan'' the mail is sent to user ``dan'' on system ``usg''. The primary uses of our network to date have been in software maintenance. Relatively few of the bytes passed between systems are intended for people to read. Instead, new programs (or new versions of programs) are sent to users, and potential bugs are returned to authors. Aaron Cohen has implemented a ``stockroom'' which allows remote users to call in and request software. He keeps a ``stock list'' of available programs, and new bug fixes and utilities are added regularly. In this way, SMM:21-12 A Dial-Up Network of UNIX Systems users can always obtain the latest version of anything without bothering the authors of the programs. Although the stock list is maintained on a particular system, the items in the stockroom may be warehoused in many places; typically each program is distri- buted from the home site of its author. Where necessary, uucp does remote-to-remote copies. We also routinely retrieve test cases from other systems to determine whether errors on remote systems are caused by local misconfigurations or old versions of software, or whether they are bugs that must be fixed at the home site. This helps identify errors rapidly. For one set of test programs maintained by us, over 70% of the bugs reported from remote sites were due to old software, and were fixed merely by distributing the current ver- sion. Another application of the network for software maintenance is to compare files on two different machines. A very useful utility on one machine has been Doug McIlroy's ``diff'' program which compares two text files and indicates the differences, line by line, between them. Only lines which are not identical are printed. Similarly, the program ``uudiff'' compares files (or directories) on two machines. One of these directories may be on a passive system. The ``uudiff'' program is set up to work simi- larly to the inter-system mail, but it is slightly more compli- cated. To avoid moving large numbers of usually identical files, uudiff computes file checksums on each side, and only moves files Dial-Up Network of UNIX Systems SMM:21-13 that are different for detailed comparison. For large files, this process can be iterated; checksums can be computed for each line, and only those lines that are different actually moved. The ``uux'' command has been useful for providing remote output. There are some machines which do not have hard-copy dev- ices, but which are connected over 9600 baud communication lines to machines with printers. The uux command allows the formatting of the printout on the local machine and printing on the remote machine using standard UNIX command programs. 5. Performance Throughput, of course, is primarily dependent on transmis- sion speed. The table below shows the real throughput of charac- ters on communication links of different speeds. These numbers represent actual data transferred; they do not include bytes used by the line protocol for data validation such as checksums and messages. At the higher speeds, contention for the processors on both ends prevents the network from driving the line full speed. The range of speeds represents the difference between light and heavy loads on the two systems. If desired, operating system modifications can be installed that permit full use of even very fast links. Nominal speed Characters/sec. 300 baud 27 1200 baud 100-110 9600 baud 200-850 In addition to the transfer time, there is some overhead for mak- ing the connection and logging in ranging from 15 seconds to 1 SMM:21-14 A Dial-Up Network of UNIX Systems minute. Even at 300 baud, however, a typical 5,000 byte source program can be transferred in four minutes instead of the 2 days that might be required to mail a tape. Traffic between systems is variable. Between two closely related systems, we observed 20 files moved and 5 remote commands executed in a typical day. A more normal traffic out of a single system would be around a dozen files per day. The total number of sites at present in the main network is 82, which includes most of the Bell Laboratories full-size machines which run the UNIX operating system. Geographically, the machines range from Andover, Massachusetts to Denver, Colorado. Uucp has also been used to set up another network which con- nects a group of systems in operational sites with the home site. The two networks touch at one Bell Labs computer. 6. Further Goals Eventually, we would like to develop a full system of remote software maintenance. Conventional maintenance (a support group which mails tapes) has many well-known disadvantages. There are distribution errors and delays, resulting in old software running at remote sites and old bugs continually reappearing. These difficulties are aggravated when there are 100 different small systems, instead of a few large ones. The availability of file transfer on a network of compatible operating systems makes it possible just to send programs Dial-Up Network of UNIX Systems SMM:21-15 directly to the end user who wants them. This avoids the bottleneck of negotiation and packaging in the central support group. The ``stockroom'' serves this function for new utilities and fixes to old utilities. However, it is still likely that dis- tributions will not be sent and installed as often as needed. Users are justifiably suspicious of the ``latest version'' that has just arrived; all too often it features the ``latest bug.'' What is needed is to address both problems simultaneously: 1. Send distributions whenever programs change. 2. Have sufficient quality control so that users will install them. To do this, we recommend systematic regression testing both on the distributing and receiving systems. Acceptance testing on the receiving systems can be automated and permits the local system to ensure that its essential work can continue despite the con- stant installation of changes sent from elsewhere. The work of writing the test sequences should be recovered in lower counsel- ing and distribution costs. Some slow-speed network services are also being implemented. We now have inter-system ``mail'' and ``diff,'' plus the many implied commands represented by ``uux.'' However, we still need inter-system ``write'' (real-time inter-user communication) and ``who'' (list of people logged in on different systems). A slow- speed network of this sort may be very useful for speeding up counseling and education, even if not fast enough for the distri- SMM:21-16 A Dial-Up Network of UNIX Systems buted data base applications that attract many users to networks. Effective use of remote execution over slow-speed lines, however, must await the general installation of multiplexable channels so that long file transfers do not lock out short inquiries. 7. Lessons The following is a summary of the lessons we learned in building these programs. 1. By starting your network in a way that requires no hardware or major operating system changes, you can get going quickly. 2. Support will follow use. Since the network existed and was being used, system maintainers were easily persuaded to help keep it operating, including purchasing additional hardware to speed traffic. 3. Make the network commands look like local commands. Our users have a resistance to learning anything new: all the inter-system commands look very similar to standard UNIX system commands so that little training cost is involved. 4. An initial error was not coordinating enough with existing communications projects: thus, the first version of this network was restricted to dial-up, since it did not support the various hardware links between systems. This has been fixed in the current system. Dial-Up Network of UNIX Systems SMM:21-17 Acknowledgements We thank G. L. Chesson for his design and implementation of the packet driver and protocol, and A. S. Cohen, J. Lions, and P. F. Long for their suggestions and assistance. References 1. D. M. Ritchie and K. Thompson, "The UNIX Time-Sharing Sys- tem," Bell Sys. Tech. J., vol. 57, no. 6, pp. 1905-1929, 1978. 2. G. L. Chesson, "The Network UNIX System," Operating Systems Review, vol. 9, no. 5, pp. 60-66, 1975. Also in Proc. 5th Symp. on Operating Systems Principles. 3. A. G. Fraser, "Spider - An Experimental Data Communications System," Proc. IEEE Conf. on Communications, p. 21F, June 1974. IEEE Cat. No. 74CH0859-9-CSCB. 4. A. G. Fraser, "A Virtual Channel Network," Datamation, pp. 51-56, February 1975. 5. J. W. Hunt and M. D. McIlroy, "An Algorithm for Differential File Comparison," Comp. Sci. Tech. Rep. No. 41, Bell Labora- tories, Murray Hill, New Jersey, June 1976. 6. F. P. Brooks, Jr., The Mythical Man-Month, Addison-Wesley, Reading, Mass., 1975.
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