April , before TCP and IP even existed. This standard defined the basic commands of the protocol and the formal means by which devises communicate . An Introduction to FTP. Author: Conrad Chung, 2BrightSparks. FTP stands for File Transfer Protocol. It is a protocol used to transfer files between an FTP host/. FTP - File Transfer Protocol soundbefabnavi.tk • FTP. FTP session (stateful). Before the advent of HTTP, FTP (RFC) was the prime protocol for file transfer .
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transferring files is the File Transfer Protocol, or FTP. Overview. FTP is a widely- accepted Internet Standard, created and made available through the Internet. Special FTP commands and responses — the FTP Protocol is happening before we look at the actual FTP PROTOCOL next .. see Web or PDF files online. What is FTP? What FTP utility is used at Fuqua? A file transfer protocol (FTP) utility allows you to transfer files from an account on a remote computer to your own.
This connection follows the Telnet Protocol. The data transferred may be a part of a file, an entire file or a number of files. DTP The data transfer process establishes and manages the data connection. The DTP can be passive or active. End-of-Line The end-of-line sequence defines the separation of printing lines. The sequence is Carriage Return, followed by Line Feed.
EOF The end-of-file condition that defines the end of a file being transferred. EOR The end-of-record condition that defines the end of a record being transferred. In FTP, error recovery may involve restarting a file transfer at a given checkpoint.
A concept which defines a standard network file system with standard commands and pathname conventions. FTP supports the transmission of discontinuous files as independent indexed pages. FTP does not yet specify a standard pathname convention. Each user must follow the file naming conventions of the file systems involved in the transfer. PI The protocol interpreter. The user and server sides of the protocol have distinct roles implemented in a user-PI and a server-PI.
Record structures are supported by FTP but a file need not have record structure. The general form of a reply is a completion code including error codes followed by a text string. The codes are for use by programs and the text is usually intended for human users. It sets up parameters for transfer and storage, and transfers data on command from its PI.
The DTP can be placed in a "passive" state to listen for, rather than initiate a connection on the data port. Type implies certain transformations between the time of data storage and data transfer. The human user may interact directly with a server-FTP process, but use of a user-FTP process is preferred since the protocol design is weighted towards automata.
If two servers are transferring data between them, the user-DTP is inactive. The user interface allows a local language to be used in the command-reply dialogue with the user.
The data connection may be used in either direction. The data connection need not exist all of the time. The control connection follows the Telnet protocol. At the initiation of the user, standard FTP commands are generated by the user-PI and transmitted to the server process via the control connection.
Standard replies are sent from the server-PI to the user-PI over the control connection in response to the commands.
The FTP commands specify the parameters for the data connection data port, transfer mode, representation type, and structure and the nature of file system operation store, retrieve, append, delete, etc. The user-DTP or its designate should "listen" on the specified data port, and the server initiate the data connection and data transfer in accordance with the specified parameters.
It ought to also be noted that the data connection may be used for simultaneous sending and receiving. In another situation a user might wish to transfer files between two hosts, neither of which is a local host. The user sets up control connections to the two servers and then arranges for a data connection between them.
In this manner, control information is passed to the user-PI but data is transferred between the server data transfer processes. Following is a model of this server-server interaction. It is the responsibility of the user to request the closing of the control connections when finished using the FTP service, while it is the server who takes the action. The server may abort data transfer if the control connections are closed without command. This can be achieved in two ways: first, the user-PI or the server-PI may implement the rules of the Telnet Protocol directly in their own procedures; or, second, the user-PI or the server-PI may make use of the existing Telnet module in the system.
Ease of implementaion, sharing code, and modular programming argue for the second approach. In practice, FTP relies on very little of the Telnet Protocol, so the first approach does not necessarily involve a large amount of code.
The control connection is used for the transfer of commands, which describe the functions to be performed, and the replies to these commands see the Section on FTP Replies. Several commands are concerned with the transfer of data between hosts. These data transfer commands include the MODE command which specify how the bits of the data are to be transmitted, and the STRUcture and TYPE commands, which are used to define the way in which the data are to be represented.
The transmission and representation are basically independent but the "Stream" transmission mode is dependent on the file structure attribute and if "Compressed" transmission mode is used, the nature of the filler byte depends on the representation type. Often it is necessary to perform certain transformations on the data because data storage representations in the two systems are different. The sending and receiving sites would have to perform the necessary transformations between the standard representation and their internal representations.
A different problem in representation arises when transmitting binary data not character codes between host systems with different word lengths. It is not always clear how the sender should send data, and the receiver store it. For example, when transmitting bit bytes from a bit word-length system to a bit word-length system, it may be desirable for reasons of efficiency and usefulness to store the bit bytes right-justified in a bit word in the latter system.
In any case, the user should have the option of specifying data representation and transformation functions. Transformations desired beyond this limited capability should be performed by the user directly. If the type is Local byte, then the TYPE command has an obligatory second parameter specifying the logical byte size.
It is intended primarily for the transfer of text files, except when both hosts would find the EBCDIC type more convenient. The receiver will convert the data from the standard form to his own internal form. See the discussion of file structure at the end of the Section on Data Representation and Storage. The receiving site must store the data as contiguous bits.
The structure of the storage system might necessitate the padding of the file or of each record, for a record-structured file to some convenient boundary byte, word or block. This padding, which must be all zeros, may occur only at the end of the file or at the end of each record and there must be a way of identifying the padding bits so that they may be stripped off if the file is retrieved. The padding transformation should be well publicized to enable a user to process a file at the storage site.
Image type is intended for the efficient storage and retrieval of files and for the transfer of binary data. It is recommended that this type be accepted by all FTP implementations. The value of Byte size must be a decimal integer; there is no default value. The logical byte size is not necessarily the same as the transfer byte size.
If there is a difference in byte sizes, then the logical bytes should be packed contiguously, disregarding transfer byte boundaries and with any necessary padding at the end. This transformation must be invertible i. For example, a user sending bit floating-point numbers to a host with a bit word could send that data as Local byte with a logical byte size of The receiving host would then be expected to store the logical bytes so that they could be easily manipulated; in this example putting the bit logical bytes into bit double words should suffice.
In another example, a pair of hosts with a bit word size may send data to one another in words by using TYPE L The data would be sent in the 8-bit transmission bytes packed so that 9 transmission bytes carried two host words. The following data representation types are defined in FTP: A character file may be transferred to a host for one of three purposes: for printing, for storage and later retrieval, or for processing. If a file is sent for printing, the receiving host must know how the vertical format control is represented.
In the second case, it must be possible to store a file at a host and then retrieve it later in exactly the same form. Finally, it should be possible to move a file from one host to another and process the file at the second host without undue trouble.
Therefore, these types have a second parameter specifying one of the following three formats: 3. Non-print format must be accepted by all FTP implementations.
If it is passed to a printer process, this process may assume standard values for spacing and margins. Normally, this format will be used with files destined for processing or just storage. In a line or a record formatted according to the ASA Standard, the first character is not to be printed. Instead, it should be used to determine the vertical movement of the paper which should take place before the rest of the record is printed. If a file has record structure see below this is no problem; records will be explicitly marked during transfer and storage.
Three file structures are defined in FTP: file-structure, where there is no internal structure and the file is considered to be a continuous sequence of data bytes, record-structure, where the file is made up of sequential records, and page-structure, where the file is made up of independent indexed pages. File-structure is the default to be assumed if the STRUcture command has not been used but both file and record structures must be accepted for "text" files i.
The structure of a file will affect both the transfer mode of a file see the Section on Transmission Modes and the interpretation and storage of the file. The "natural" structure of a file will depend on which host stores the file. If the transfer of files between such disparate sites is to be useful, there must be some way for one site to recognize the other's assumptions about the file.
With some sites being naturally file-oriented and others naturally record-oriented there may be problems if a file with one structure is sent to a host oriented to the other. If a text file is sent with record-structure to a host which is file oriented, then that host should apply an internal transformation to the file based on the record structure.
Obviously, this transformation should be useful, but it must also be invertible so that an identical file may be retrieved using record structure. In the case of a file being sent with file-structure to a record-oriented host, there exists the question of what criteria the host should use to divide the file into records which can be processed locally.
If an FTP implementation adopts this technique, it must be prepared to reverse the transformation if the file is retrieved with file-structure. In file-structure there is no internal structure and the file is considered to be a continuous sequence of data bytes.
In record-structure the file is made up of sequential records. Files of this type are sometimes known as "random access files" or even as "holey files". In these files there is sometimes other information associated with the file as a whole e. In FTP, the sections of the file are called pages. To provide for various page sizes and associated information, each page is sent with a page header.
The page header has the following defined fields: Header Length The number of logical bytes in the page header including this byte. The minimum header length is 4.
Page Index The logical page number of this section of the file. This is not the transmission sequence number of this page, but the index used to identify this page of the file. The minimum data length is 0. Page Type The type of page this is. The header length must be 4, and the data length must be 0.
The header length must be 4. The header length must be 5. Optional Fields Further header fields may be used to supply per page control information, for example, per page access control. All fields are one logical byte in length.
The logical byte size is specified by the TYPE command. See Appendix I for further details and a specific case at the page structure. Conversely, FTP implementations must return a file identical to the original if the parameters used to store and retrieve a file are the same.
Both the user and the server-DTPs have a default data port. The user-process default data port is the same as the control connection port i. The server-process default data port is the port adjacent to the control connection port i.
The transfer byte size is 8-bit bytes. This byte size is relevant only for the actual transfer of the data; it has no bearing on representation of the data within a host's file system. The passive data transfer process this may be a user-DTP or a second server-DTP shall "listen" on the data port prior to sending a transfer request command.
The FTP request command determines the direction of the data transfer. The server, upon receiving the transfer request, will initiate the data connection to the port. It is possible for the user to specify an alternate data port by use of the PORT command. The user may want a file dumped on a TAC line printer or retrieved from a third party host.
In the latter case, the user-PI sets up control connections with both server-PI's. One server is then told by an FTP command to "listen" for a connection which the other will initiate.
Finally, both are sent the appropriate transfer commands. The exact sequence of commands and replies sent between the user-controller and the servers is defined in the Section on FTP Replies. In general, it is the server's responsibility to maintain the data connection--to initiate it and to close it.
The server MUST close the data connection under the following conditions: 1. The server has completed sending data in a transfer mode that requires a close to indicate EOF. The port specification is changed by a command from the user.
The control connection is closed legally or otherwise. An irrecoverable error condition occurs. Otherwise the close is a server option, the exercise of which the server must indicate to the user-process by either a or reply only. Since a connection is defined by the pair of addresses, either of these actions is enough to get a different data connection, still it is permitted to do both commands to use new ports on both ends of the data connection. Reuse of the Data Connection: When using the stream mode of data transfer the end of the file must be indicated by closing the connection.
This causes a problem if multiple files are to be transfered in the session, due to need for TCP to hold the connection record for a time out period to guarantee the reliable communication. Thus the connection can not be reopened at once.
There are two solutions to this problem. The first is to negotiate a non-default port. The second is to use another transfer mode. A comment on transfer modes. The other transfer modes Block, Compressed do not close the connection to indicate the end of file.
They have enough FTP encoding that the data connection can be parsed to determine the end of the file. Thus using these modes one can leave the data connection open for multiple file transfers. There are three modes: one which formats the data and allows for restart procedures; one which also compresses the data for efficient transfer; and one which passes the data with little or no processing. In this last case the mode interacts with the structure attribute to determine the type of processing.
In the compressed mode, the representation type determines the filler byte. All data transfers must be completed with an end-of-file EOF which may be explicitly stated or implied by the closing of the data connection.
For files with record structure, all the end-of-record markers EOR are explicit, including the final one. For files transmitted in page structure a "last-page" page type is used. NOTE: In the rest of this section, byte means "transfer byte" except where explicitly stated otherwise.
For the purpose of standardized transfer, the sending host will translate its internal end of line or end of record denotation into the representation prescribed by the transfer mode and file structure, and the receiving host will perform the inverse translation to its internal denotation. An IBM Mainframe record count field may not be recognized at another host, so the end-of-record information may be transferred as a two byte control code in Stream mode or as a flagged bit in a Block or Compressed mode descriptor.
Since these transformations imply extra work for some systems, identical systems transferring non-record structured text files might wish to use a binary representation and stream mode for the transfer. There is no restriction on the representation type used; record structures are allowed.
The first byte of the control code will be all ones, the escape character. EOR and EOF may be indicated together on the last byte transmitted by turning both low order bits on i.
If a byte of all ones was intended to be sent as data, it should be repeated in the second byte of the control code. If the structure is a file structure, the EOF is indicated by the sending host closing the data connection and all bytes are data bytes. The header bytes contain a count field, and descriptor code.
The count field indicates the total length of the data block in bytes, thus marking the beginning of the next data block there are no filler bits. It is motivated by the desire of sites exchanging certain types of data e. Record structures are allowed in this mode, and any representation type may be used.
The header consists of the three bytes. PASV Command making it possible to indicate to the DTP server to stand by for a connection on a specific port chosen randomly from among the available ports. The response to this command is the IP address of the machine and port.
TYPE This command enables the type of format in which the data will be sent to be specified. STOR This command store asks the server DTP to accept the data sent over the data channel and store them in a file bearing the name given in the parameters. If the file does not exist, the server creates it, if not it overwrites it.
STOU This command is identical to the previous one, only it asks the sever to create a file where the name is unique. The name of the file is returned in the response. APPE Thanks to this command append the data sent is concatenated into the file bearing the name given in the parameter if it already exists, if not, it is created. ALLO This command allocate asks the server to plan a storage space big enough to hold the file whose name is given in the argument. REST This command restart enables a transfer to be restarted from where it stopped.
To do so, the command sends the marker representing the position in the file where the transfer had been interrupted in the parameter. This command must immediately follow a transfer command. RNFR This command rename from enables a file to be renamed. In the parameters it indicates the name of the file to be renamed and must be immediately followed by the RNTO command.
RNTO This command rename to enables a file to be renamed. In the parameters it indicates the name of the file to be renamed and must be immediately followed by the RNFR command. If no data connection is open, the DTP sever does nothing, if not it closes it.
The control channel however remains open. DELE This command delete allows a file to be deleted, the name of which is given in the parameters. This command is irreversible, confirmation can only be given at client level.