rfc1068.txt
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<-------- Close Ctrl connDeSchon & Braden [Page 7] RFC 1068 August 1988 BFTP currently utilizes the following Server-FTP commands [RFC- 959]: USER, PASS, ACCT, PASV, PORT, RETR, STOR, STOU, CWD, NLST, MODE, STRU, TYPE, and QUIT. The FTC daemon attempts to work around FTP servers that fail to support certain commands. For example, if a server does not support the optional command "CWD", the FTC daemon will attempt to construct a complete path name using the source directory name and the source file name. However, it is necessary that at least one of the two hosts support the FTP passive (PASV) command. While many FTP server implementations support do this command, some (in particular, the 4.2BSD FTP) do not. The PASV command was officially listed as being optional in RFC-959. 2.3 Reliable Delivery The reliable delivery function of BFTP is analogous to reliable delivery in a transport protocol like TCP. Both depend upon repeated delivery attempts until success is achieved, and in both cases the choice of the retry interval requires some care to balance overhead against unresponsiveness. Humans are impatient, but even their impatience has a limit. If the file cannot be transferred "soon", a human will turn to another project; typically, there is a tendency for the transfer to become less urgent the longer the wait. The FTC daemon of BFTP therefore starts each transfer request with a very short retry interval -- e.g., 10 minutes -- and then doubles this interval for successive retries, until a maximum interval -- e.g., 4 hours -- is reached. This is essentially the exponential backoff algorithm of the Ethernet, which is also used by transport protocols such as TCP, although BFTP and TCP have quite different rationales for the algorithm. We must also define the meaning of reliable transmission for a multiple-transfer request. For example, the set of files selected by wildcard characters in a pathname is not well defined; the set may change while the request is pending, as files are created and deleted. Furthermore, it is unreasonable to regard the entire multiple transfer as a single atomic operation. Suppose that transferring a set of files fails part way through; for an atomic operation, the files which had been successfully transferred would have to be deleted pending the next retry of the entire set. This would be ridiculously inefficient and may be impossible (since the communication path may be broken when it is time to issue the deletion requests).DeSchon & Braden [Page 8] RFC 1068 August 1988 BFTP addresses these issues in the following manner: * For a multiple file operation, the FTC daemon saves the file name list returned by the first successful NLST command in the request queue entry. This name list determines the set of source files for the transfer; there can be no later additions to the set. * The FTC daemon maintains a transfer status pointer. On each retry cycle, it tries to transfer only those files that have not already been successfully transferred. * The request is complete when all the individual file transfers have been successful, a permanent failure has occured, or when the retry limit is reached. * The notification message to the user lists the status of each of the multiple files. 2.4 BFTP User Interface The purpose of BFTP is to simplify the file transfer process and to place the burden of reliability on the BFTP control host. We have attempted to provide a "user friendly" command interface to BFTP, similar in flavor to the user interface of the TOPS-20 operating system. This interface provides extensive prompting, defaulting, and help facilities for every command. For a list of all BFTP commands, the user may enter "?<Return>" at the main BFTP prompt ("BFTP>"). Entering "help<Return>" and "explain<Return>" will provide increasing levels of explanatory material. To obtain information on a particular command, "help <command name><Return>" may be entered. The 'quit' or 'exit' command will exit from BFTP. Command and subcommand names may be abbreviated to the shortest unique sequence for that context; alternatively, a partial name can be automatically completed by typing <Return>. The normal procedure for a BFTP user is to set up a set of parameters defining the desired transfer and then submit the request to the FTC daemon. To give the user the maximum flexibility, BFTP supports three modes of submission: o Background Operation To request a reliable background file transfer, the user will issue the BFTP 'submit' command to the FTC daemon.DeSchon & Braden [Page 9] RFC 1068 August 1988 o Foreground Verification, Background Operation The BFTP 'verify' command may be used to ascertain that file transfer parameters are valid. It causes BFTP to connect to the FTP servers on both the source and the destination hosts (if possible), log into both, verify the FTP parameters, and verify that the specified source file is present. Once the 'verify' command has successfully completed, the user can issue the 'submit' command to schedule the actual file transfer. o Foreground Operation The BFTP 'transfer' command will perform the specified third-party transfer in foreground mode. This is illustrated by the dotted path bypassing the queue in Figure 1. The easiest way to set up the parameters is to issue the 'prompt' command, which will prompt the user for all of the basic parameters required for most transfers. Certain unusual parameters must be set with the 'set' command (see Appendix B for details). When entering any parameter, the following control characters may be used: ? will display help text for the parameter, indicating its meaning, the choices, and the default, and then reprompt for the parameter. <ESC> will display the default value (or the last value set) for this parameter. The user can accept this default by entering <Return>, or else erase it with Control-W and enter a different value for the parameter, followed by <Return> to accept the entered value. <Control-W> will erase the value typed or displayed for current parameter. <Return> will accept the value displayed for this parameter, and continue to the next parameter, if any. If the user has not typed a value or used <ESC> to display the default, <Return> will display the default and then accept it.DeSchon & Braden [Page 10] RFC 1068 August 1988 It is important to provide a means for a user to obtain status information about an earlier request or even to cancel an earlier request. However, these functions, especially cancellation, must be controlled by some user authentication. We did not want to build a user authentication database with each BFTP instance or require login to BFTP itself, and there is no Internet-wide user authentication mechanism. We adopted the following weak authentication mechanism as a compromise: * When the 'submit' command is issued, it prompts the user for a character string called a "keyword", which recorded with the request. * This keyword can be entered later as the argument to a 'find' command, which will display the status of all requests with matching keywords. * Similarly, the keyword may be used to cancel the corresponding request. If two different users happen to choose the same keywords, of course, this scheme will not protect each other's requests from accidental or malicious cancellation. However, a notification message will be sent at the time that a cancellation occurs. To make a series of similar requests, the user needs only to change the individual parameters that differ from the preceding request and then issue a new 'submit' command, for each request. There are commands for individually setting each of the parameters that 'prompt' sets -- and 'time' -- to provide a shortcut for BFTP experts. A simpler but lengthier procedure is to use the 'prompt' command to run through the current set of parameters, reentering the parameters that must change and using the sequence <ESC><return> to retain the previous value for each of the others. The same procedures may be used to correct a mistake made in entering a particular parameter. The current settings of all the BFTP parameters can be displayed at any time with the 'status' command, while the 'clear' command will return all parameters to their initial values. Finally, the 'request' command allows the user to save the current set of parameters in a file or to restore the parameters from a previously-saved file. There is also a window-based BFTP user interface for use on a Sun Workstation, described in Appendix A. The complete list of BFTP commands is presented in Appendix B.DeSchon & Braden [Page 11] RFC 1068 August 19883. Experience and Conclusions BFTP has been available to users at ISI for some months. Users have reported a number of advantages of using BFTP: (a) Some users prefer the prompting style of BFTP to the user interface of the foreground FTP they normally use. (b) The BFTP "verify" command allows the user to verify that host names, passwords, and filenames are correct without having to wait for the entire transfer to take place. (c) Since results are returned through the mail system, a transfer can occur without tying up a terminal line, a phone line, or even a window. BFTP must be able to communicate with a variety of Server-FTP implementations, and we have observed much variation in the commands supported, error handling, and the timing in these servers. Some of the problems we have encountered are: (1) Some systems (e.g., 4.2BSD) do not support the PASV command. (2) 4.2/3BSD systems return a non-standard response to the NLST command. Instead of returning a list of complete path-names, they use an ad hoc format consisting of a directory name followed by a list of files. (3) 4.2/3BSD systems may return a "permanent negative completion reply" (a 5xx FTP reply code) as a result of a communications failure such as a broken TCP connection. According to RFC-959, the appropriate response is a "transient negative completion reply" (a 4xx FTP reply code), which would inform the BFTP that the transfer should be retried. (4) A number of servers return badly formatted responses. An example of this is the 4.2/3BSD response to an NLST command for a non-existent file name: an error string which is not preceded by a numerical response code. To diagnose problems that do occur, we have found it very useful to have a complete record of the interchange between the FTC daemon and the two FTP servers. This record is saved and is currently always included in the notification message mailed to the user (see Appendix D for an example). As we get more experience with this program, some of the details of the transfer may be omitted from this log.DeSchon & Braden [Page 12] RFC 1068 August 1988 The use of library routines shared between modules makes it relatively easy to implement additional user interface programs. We are currently experimenting with a window version of BFTP, the "bftptool", which runs in the SunView environment, and is described in Appendix A. Some additional interfaces that might be useful are: o A command line interface for use in shell scripts and "Makefiles". o A more general library interface which would make it easy to invoke BFTP from a variety of programs. o Additional full-screen form based interfaces, for example a tool running in X-Window system environment. Lastly, BFTP would benefit from the resolution of the following open protocol issues: o There currently exist no provisions for Internet-wide user authentication. In the BFTP context, this means that passwords required for a file transfer must be present in BFTP request files. The security of these passwords is subject to the limitations of the file system security on the BFTP control host. Anonymous file transfer provides a partial solution, but a more general, long term solution is needed. o Better mechanisms are needed to cope with the diversity of real file systems in the Internet. For example, an extension could be made to the FTP protocol to allow the daemon to learn the delimiter conventions of each host file system. This could allow a more flexible and powerful multiple-file facility in BFTP. This could include the automatic transfer of directory subtrees, for example.4. References [RFC-959] Postel, J., and J. Reynolds, "File Transfer Protocol (FTP)", RFC-959, USC/Information Sciences Institute, October 1985.DeSchon & Braden [Page 13] RFC 1068 August 1988Appendix A -- BFTP Implementation Structure BFTP has been implemented on both a Sun workstation running Sun OS 3.4 (based on 4.2BSD) and a VAX running 4.3BSD. The program modules are: the local user interface programs "bftp", the Internet server program "bftpd", and the FTC daemon "fts". BFTP makes use of the "at" command, a UNIX batch job facility, to submit requests and execute the daemon. An additional user interface program, the "bftptool", is available for Sun OS 3.4, and runs in the SunView environment. BFTP keeps its state in a set of control files: request files, command files, and message files. These files are stored in the home directory specified for the environment of the process running "bftp". If a user is running "bftp" directly, this will typically be the user's home directory. In the case where a user has made a Telnet connection to the well-known port 152 on a BFTP service host, "bftp" is started by "bftpd" (or "inetd", indirectly). As a result, the control files will be owned by the user-id under which "inetd" was started, normally "root", and stored in the top level directory "/". Note, however, that under BFTP all user files are written by the FTP servers, which are presumed to enforce the operating systems' access control conventions. Hence, BFTP does not constitute a system integrity exposure.⌨️ 快捷键说明
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