4. File Transfer
4. FILE TRANSFER
4.1 FILE TRANSFER PROTOCOL -- FTP
4.1.1 INTRODUCTION
The File Transfer Protocol FTP is the primary Internet standard for file transfer. The current specification is contained in RFC-959 [FTP:1].
FTP uses separate simultaneous TCP connections for control and for data transfer. The FTP protocol includes many features, some of which are not commonly implemented. However, for every feature in FTP, there exists at least one implementation. The minimum implementation defined in RFC-959 was too small, so a somewhat larger minimum implementation is defined here.
Internet users have been unnecessarily burdened for years by deficient FTP implementations. Protocol implementors have suffered from the erroneous opinion that implementing FTP ought to be a small and trivial task. This is wrong, because FTP has a user interface, because it has to deal (correctly) with the whole variety of communication and operating system errors that may occur, and because it has to handle the great diversity of real file systems in the world.
4.1.2. PROTOCOL WALK-THROUGH
4.1.2.1 LOCAL Type: RFC-959 Section 3.1.1.4
An FTP program MUST support TYPE I ("IMAGE" or binary type) as well as TYPE L 8 ("LOCAL" type with logical byte size 8). A machine whose memory is organized into m-bit words, where m is not a multiple of 8, MAY also support TYPE L m.
DISCUSSION: The command "TYPE L 8" is often required to transfer binary data between a machine whose memory is organized into (e.g.) 36-bit words and a machine with an 8-bit byte organization. For an 8-bit byte machine, TYPE L 8 is equivalent to IMAGE.
"TYPE L m" is sometimes specified to the FTP programs on two m-bit word machines to ensure the correct transfer of a native-mode binary file from one machine to the other. However, this command should have the same effect on these machines as "TYPE I".
4.1.2.2 Telnet Format Control: RFC-959 Section 3.1.1.5.2
A host that makes no distinction between TYPE N and TYPE T SHOULD implement TYPE T to be identical to TYPE N.
DISCUSSION: This provision should ease interoperation with hosts that do make this distinction.
Many hosts represent text files internally as strings of ASCII characters, using the embedded ASCII format effector characters (LF, BS, FF, ...) to control the format when a file is printed. For such hosts, there is no distinction between "print" files and other files. However, systems that use record structured files typically need a special format for printable files (e.g., ASA carriage control). For the latter hosts, FTP allows a choice of TYPE N or TYPE T.
4.1.2.3 Page Structure: RFC-959 Section 3.1.2.3 and Appendix I
Implementation of page structure is NOT RECOMMENDED in general. However, if a host system does need to implement FTP for "random access" or "holey" files, it MUST use the defined page structure format rather than define a new private FTP format.
4.1.2.4 Data Structure Transformations: RFC-959 Section 3.1.2
An FTP transformation between record-structure and file- structure SHOULD be invertible, to the extent possible while making the result useful on the target host.
DISCUSSION: RFC-959 required strict invertibility between record- structure and file-structure, but in practice, efficiency and convenience often preclude it. Therefore, the requirement is being relaxed. There are two different objectives for transferring a file: processing it on the target host, or just storage. For storage, strict invertibility is important. For processing, the file created on the target host needs to be in the format expected by application programs on that host.
As an example of the conflict, imagine a record- oriented operating system that requires some data files to have exactly 80 bytes in each record. While STORing
a file on such a host, an FTP Server must be able to pad each line or record to 80 bytes; a later retrieval of such a file cannot be strictly invertible.
4.1.2.5 Data Connection Management: RFC-959 Section 3.3
A User-FTP that uses STREAM mode SHOULD send a PORT command to assign a non-default data port before each transfer command is issued.
DISCUSSION: This is required because of the long delay after a TCP connection is closed until its socket pair can be reused, to allow multiple transfers during a single FTP session. Sending a port command can avoided if a transfer mode other than stream is used, by leaving the data transfer connection open between transfers.
4.1.2.6 PASV Command: RFC-959 Section 4.1.2
A server-FTP MUST implement the PASV command.
If multiple third-party transfers are to be executed during the same session, a new PASV command MUST be issued before each transfer command, to obtain a unique port pair.
IMPLEMENTATION: The format of the 227 reply to a PASV command is not well standardized. In particular, an FTP client cannot assume that the parentheses shown on page 40 of RFC-959 will be present (and in fact, Figure 3 on page 43 omits them). Therefore, a User-FTP program that interprets the PASV reply must scan the reply for the first digit of the host and port numbers.
Note that the host number h1,h2,h3,h4 is the IP address of the server host that is sending the reply, and that p1,p2 is a non-default data transfer port that PASV has assigned.
4.1.2.7 LIST and NLST Commands: RFC-959 Section 4.1.3
The data returned by an NLST command MUST contain only a simple list of legal pathnames, such that the server can use them directly as the arguments of subsequent data transfer commands for the individual files.
The data returned by a LIST or NLST command SHOULD use an
implied TYPE AN, unless the current type is EBCDIC, in which case an implied TYPE EN SHOULD be used.
DISCUSSION: Many FTP clients support macro-commands that will get or put files matching a wildcard specification, using NLST to obtain a list of pathnames. The expansion of "multiple-put" is local to the client, but "multiple- get" requires cooperation by the server.
The implied type for LIST and NLST is designed to provide compatibility with existing User-FTPs, and in particular with multiple-get commands.
4.1.2.8 SITE Command: RFC-959 Section 4.1.3
A Server-FTP SHOULD use the SITE command for non-standard features, rather than invent new private commands or unstandardized extensions to existing commands.
4.1.2.9 STOU Command: RFC-959 Section 4.1.3
The STOU command stores into a uniquely named file. When it receives an STOU command, a Server-FTP MUST return the actual file name in the "125 Transfer Starting" or the "150 Opening Data Connection" message that precedes the transfer (the 250 reply code mentioned in RFC-959 is incorrect). The exact format of these messages is hereby defined to be as follows:
125 FILE: pppp 150 FILE: pppp
where pppp represents the unique pathname of the file that will be written.
4.1.2.10 Telnet End-of-line Code: RFC-959, Page 34
Implementors MUST NOT assume any correspondence between READ boundaries on the control connection and the Telnet EOL sequences (CR LF).
DISCUSSION: Thus, a server-FTP (or User-FTP) must continue reading characters from the control connection until a complete Telnet EOL sequence is encountered, before processing the command (or response, respectively). Conversely, a single READ from the control connection may include
more than one FTP command.
4.1.2.11 FTP Replies: RFC-959 Section 4.2, Page 35
A Server-FTP MUST send only correctly formatted replies on the control connection. Note that RFC-959 (unlike earlier versions of the FTP spec) contains no provision for a "spontaneous" reply message.
A Server-FTP SHOULD use the reply codes defined in RFC-959 whenever they apply. However, a server-FTP MAY use a different reply code when needed, as long as the general rules of Section 4.2 are followed. When the implementor has a choice between a 4xx and 5xx reply code, a Server-FTP SHOULD send a 4xx (temporary failure) code when there is any reasonable possibility that a failed FTP will succeed a few hours later.
A User-FTP SHOULD generally use only the highest-order digit of a 3-digit reply code for making a procedural decision, to prevent difficulties when a Server-FTP uses non-standard reply codes.
A User-FTP MUST be able to handle multi-line replies. If the implementation imposes a limit on the number of lines and if this limit is exceeded, the User-FTP MUST recover, e.g., by ignoring the excess lines until the end of the multi-line reply is reached.
A User-FTP SHOULD NOT interpret a 421 reply code ("Service not available, closing control connection") specially, but SHOULD detect closing of the control connection by the server.
DISCUSSION: Server implementations that fail to strictly follow the reply rules often cause FTP user programs to hang. Note that RFC-959 resolved ambiguities in the reply rules found in earlier FTP specifications and must be followed.
It is important to choose FTP reply codes that properly distinguish between temporary and permanent failures, to allow the successful use of file transfer client daemons. These programs depend on the reply codes to decide whether or not to retry a failed transfer; using a permanent failure code (5xx) for a temporary error will cause these programs to give up unnecessarily.
When the meaning of a reply matches exactly the text shown in RFC-959, uniformity will be enhanced by using the RFC-959 text verbatim. However, a Server-FTP implementor is encouraged to choose reply text that conveys specific system-dependent information, when appropriate.
4.1.2.12 Connections: RFC-959 Section 5.2
The words "and the port used" in the second paragraph of this section of RFC-959 are erroneous (historical), and they should be ignored.
On a multihomed server host, the default data transfer port (L-1) MUST be associated with the same local IP address as the corresponding control connection to port L.
A user-FTP MUST NOT send any Telnet controls other than SYNCH and IP on an FTP control connection. In particular, it MUST NOT attempt to negotiate Telnet options on the control connection. However, a server-FTP MUST be capable of accepting and refusing Telnet negotiations (i.e., sending DONT/WONT).
DISCUSSION: Although the RFC says: "Server- and User- processes should follow the conventions for the Telnet protocol...[on the control connection]", it is not the intent that Telnet option negotiation is to be employed.
4.1.2.13 Minimum Implementation; RFC-959 Section 5.1
The following commands and options MUST be supported by every server-FTP and user-FTP, except in cases where the underlying file system or operating system does not allow or support a particular command.
Type: ASCII Non-print, IMAGE, LOCAL 8 Mode: Stream Structure: File, Record* Commands: USER, PASS, ACCT, PORT, PASV, TYPE, MODE, STRU, RETR, STOR, APPE, RNFR, RNTO, DELE, CWD, CDUP, RMD, MKD, PWD,
LIST, NLST, SYST, STAT, HELP, NOOP, QUIT.
*Record structure is REQUIRED only for hosts whose file systems support record structure.
DISCUSSION: Vendors are encouraged to implement a larger subset of the protocol. For example, there are important robustness features in the protocol (e.g., Restart, ABOR, block mode) that would be an aid to some Internet users but are not widely implemented.
A host that does not have record structures in its file system may still accept files with STRU R, recording the byte stream literally.
4.1.3 SPECIFIC ISSUES
4.1.3.1 Non-standard Command Verbs
FTP allows "experimental" commands, whose names begin with "X". If these commands are subsequently adopted as standards, there may still be existing implementations using the "X" form. At present, this is true for the directory commands:
RFC-959 "Experimental"
MKD XMKD RMD XRMD PWD XPWD CDUP XCUP CWD XCWD
All FTP implementations SHOULD recognize both forms of these commands, by simply equating them with extra entries in the command lookup table.
IMPLEMENTATION: A User-FTP can access a server that supports only the "X" forms by implementing a mode switch, or automatically using the following procedure: if the RFC-959 form of one of the above commands is rejected with a 500 or 502 response code, then try the experimental form; any other response would be passed to the user.
4.1.3.2 Idle Timeout
A Server-FTP process SHOULD have an idle timeout, which will terminate the process and close the control connection if the server is inactive (i.e., no command or data transfer in progress) for a long period of time. The idle timeout time SHOULD be configurable, and the default should be at least 5 minutes.
A client FTP process ("User-PI" in RFC-959) will need timeouts on responses only if it is invoked from a program.
DISCUSSION: Without a timeout, a Server-FTP process may be left pending indefinitely if the corresponding client crashes without closing the control connection.
4.1.3.3 Concurrency of Data and Control
DISCUSSION: The intent of the designers of FTP was that a user should be able to send a STAT command at any time while data transfer was in progress and that the server-FTP would reply immediately with status -- e.g., the number of bytes transferred so far. Similarly, an ABOR command should be possible at any time during a data transfer.
Unfortunately, some small-machine operating systems make such concurrent programming difficult, and some other implementers seek minimal solutions, so some FTP implementations do not allow concurrent use of the data and control connections. Even such a minimal server must be prepared to accept and defer a STAT or ABOR command that arrives during data transfer.
4.1.3.4 FTP Restart Mechanism
The description of the 110 reply on pp. 40-41 of RFC-959 is incorrect; the correct description is as follows. A restart reply message, sent over the control connection from the receiving FTP to the User-FTP, has the format:
110 MARK ssss = rrrr
Here:
-
ssss is a text string that appeared in a Restart Marker
in the data stream and encodes a position in the sender's file system;
-
rrrr encodes the corresponding position in the receiver's file system.
The encoding, which is specific to a particular file system and network implementation, is always generated and interpreted by the same system, either sender or receiver.
When an FTP that implements restart receives a Restart Marker in the data stream, it SHOULD force the data to that point to be written to stable storage before encoding the corresponding position rrrr. An FTP sending Restart Markers MUST NOT assume that 110 replies will be returned synchronously with the data, i.e., it must not await a 110 reply before sending more data.
Two new reply codes are hereby defined for errors encountered in restarting a transfer:
554 Requested action not taken: invalid REST parameter.
A 554 reply may result from a FTP service command that follows a REST command. The reply indicates that the existing file at the Server-FTP cannot be repositioned as specified in the REST.
555 Requested action not taken: type or stru mismatch.
A 555 reply may result from an APPE command or from any FTP service command following a REST command. The reply indicates that there is some mismatch between the current transfer parameters (type and stru) and the attributes of the existing file.
DISCUSSION: Note that the FTP Restart mechanism requires that Block or Compressed mode be used for data transfer, to allow the Restart Markers to be included within the data stream. The frequency of Restart Markers can be low.
Restart Markers mark a place in the data stream, but the receiver may be performing some transformation on the data as it is stored into stable storage. In general, the receiver's encoding must include any state information necessary to restart this transformation at any point of the FTP data stream. For example, in TYPE
A transfers, some receiver hosts transform CR LF sequences into a single LF character on disk. If a Restart Marker happens to fall between CR and LF, the receiver must encode in rrrr that the transfer must be restarted in a "CR has been seen and discarded" state.
Note that the Restart Marker is required to be encoded as a string of printable ASCII characters, regardless of the type of the data.
RFC-959 says that restart information is to be returned "to the user". This should not be taken literally. In general, the User-FTP should save the restart information (ssss,rrrr) in stable storage, e.g., append it to a restart control file. An empty restart control file should be created when the transfer first starts and deleted automatically when the transfer completes successfully. It is suggested that this file have a name derived in an easily-identifiable manner from the name of the file being transferred and the remote host name; this is analogous to the means used by many text editors for naming "backup" files.
There are three cases for FTP restart.
(1) User-to-Server Transfer
The User-FTP puts Restart Markers <ssss> at
convenient places in the data stream. When the
Server-FTP receives a Marker, it writes all prior
data to disk, encodes its file system position and
transformation state as rrrr, and returns a "110
MARK ssss = rrrr" reply over the control
connection. The User-FTP appends the pair
(ssss,rrrr) to its restart control file.
To restart the transfer, the User-FTP fetches the last (ssss,rrrr) pair from the restart control file, repositions its local file system and transformation state using ssss, and sends the command "REST rrrr" to the Server-FTP.
(2) Server-to-User Transfer
The Server-FTP puts Restart Markers <ssss> at
convenient places in the data stream. When the
User-FTP receives a Marker, it writes all prior
data to disk, encodes its file system position and
transformation state as rrrr, and appends the pair (rrrr,ssss) to its restart control file.
To restart the transfer, the User-FTP fetches the last (rrrr,ssss) pair from the restart control file, repositions its local file system and transformation state using rrrr, and sends the command "REST ssss" to the Server-FTP.
(3) Server-to-Server ("Third-Party") Transfer
The sending Server-FTP puts Restart Markers <ssss>
at convenient places in the data stream. When it
receives a Marker, the receiving Server-FTP writes
all prior data to disk, encodes its file system
position and transformation state as rrrr, and
sends a "110 MARK ssss = rrrr" reply over the
control connection to the User. The User-FTP
appends the pair (ssss,rrrr) to its restart
control file.
To restart the transfer, the User-FTP fetches the last (ssss,rrrr) pair from the restart control file, sends "REST ssss" to the sending Server-FTP, and sends "REST rrrr" to the receiving Server-FTP.
4.1.4 FTP/USER INTERFACE
This section discusses the user interface for a User-FTP program.
4.1.4.1 Pathname Specification
Since FTP is intended for use in a heterogeneous environment, User-FTP implementations MUST support remote pathnames as arbitrary character strings, so that their form and content are not limited by the conventions of the local operating system.
DISCUSSION: In particular, remote pathnames can be of arbitrary length, and all the printing ASCII characters as well as space (0x20) must be allowed. RFC-959 allows a pathname to contain any 7-bit ASCII character except CR or LF.
4.1.4.2 "QUOTE" Command
A User-FTP program MUST implement a "QUOTE" command that will pass an arbitrary character string to the server and display all resulting response messages to the user.
To make the "QUOTE" command useful, a User-FTP SHOULD send transfer control commands to the server as the user enters them, rather than saving all the commands and sending them to the server only when a data transfer is started.
DISCUSSION: The "QUOTE" command is essential to allow the user to access servers that require system-specific commands (e.g., SITE or ALLO), or to invoke new or optional features that are not implemented by the User-FTP. For example, "QUOTE" may be used to specify "TYPE A T" to send a print file to hosts that require the distinction, even if the User-FTP does not recognize that TYPE.
4.1.4.3 Displaying Replies to User
A User-FTP SHOULD display to the user the full text of all error reply messages it receives. It SHOULD have a "verbose" mode in which all commands it sends and the full text and reply codes it receives are displayed, for diagnosis of problems.
4.1.4.4 Maintaining Synchronization
The state machine in a User-FTP SHOULD be forgiving of missing and unexpected reply messages, in order to maintain command synchronization with the server.
4.1.5 FTP REQUIREMENTS SUMMARY
| | | | |S| | | | | | |H| |F | | | | |O|M|o | | |S| |U|U|o | | |H| |L|S|t | |M|O| |D|T|n | |U|U|M| | |o | |S|L|A|N|N|t | |T|D|Y|O|O|t
| FEATURE | SECTION | T | T | e | |||
|---|---|---|---|---|---|---|---|
| Implement TYPE T if same as TYPE N | 4.1.2.2 | x | |||||
| File/Record transform invertible if poss. | 4.1.2.4 | x | |||||
| User-FTP send PORT cmd for stream mode | 4.1.2.5 | x | |||||
| Server-FTP implement PASV | 4.1.2.6 | x | |||||
| PASV is per-transfer | 4.1.2.6 | x | |||||
| NLST reply usable in RETR cmds | 4.1.2.7 | x | |||||
| Implied type for LIST and NLST | 4.1.2.7 | x | |||||
| SITE cmd for non-standard features | 4.1.2.8 | x | |||||
| STOU cmd return pathname as specified | 4.1.2.9 | x | |||||
| Use TCP READ boundaries on control conn. | 4.1.2.10 | x | |||||
| Server-FTP send only correct reply format | 4.1.2.11 | x | |||||
| Server-FTP use defined reply code if poss. | 4.1.2.11 | x | |||||
| New reply code following Section 4.2 | 4.1.2.11 | x | |||||
| User-FTP use only high digit of reply | 4.1.2.11 | x | |||||
| User-FTP handle multi-line reply lines | 4.1.2.11 | x | |||||
| User-FTP handle 421 reply specially | 4.1.2.11 | x | |||||
| Default data port same IP addr as ctl conn | 4.1.2.12 | x | |||||
| User-FTP send Telnet cmds exc. SYNCH, IP | 4.1.2.12 | x | |||||
| User-FTP negotiate Telnet options | 4.1.2.12 | x | |||||
| Server-FTP handle Telnet options | 4.1.2.12 | x | |||||
| Handle "Experimental" directory cmds | 4.1.3.1 | x | |||||
| Idle timeout in server-FTP | 4.1.3.2 | x | |||||
| Configurable idle timeout | 4.1.3.2 | x | |||||
| Receiver checkpoint data at Restart Marker | 4.1.3.4 | x | |||||
| Sender assume 110 replies are synchronous | 4.1.3.4 | x | |||||
| Support TYPE: | |||||||
| ASCII - Non-Print (AN) | 4.1.2.13 | x | |||||
| ASCII - Telnet (AT) -- if same as AN | 4.1.2.2 | x | |||||
| ASCII - Carriage Control (AC) | 959 3.1.1.5.2 | x | |||||
| EBCDIC - (any form) | 959 3.1.1.2 | x | |||||
| IMAGE | 4.1.2.1 | x | |||||
| LOCAL 8 | 4.1.2.1 | x |
LOCAL m |4.1.2.1 | | |x| | |2 | | | | | | | Support MODE: | | | | | | | Stream |4.1.2.13 |x| | | | | Block |959 3.4.2 | | |x| | | | | | | | | | Support STRUCTURE: | | | | | | | File |4.1.2.13 |x| | | | | Record |4.1.2.13 |x| | | | |3 Page |4.1.2.3 | | | |x| | | | | | | | | Support commands: | | | | | | | USER |4.1.2.13 |x| | | | | PASS |4.1.2.13 |x| | | | | ACCT |4.1.2.13 |x| | | | | CWD |4.1.2.13 |x| | | | | CDUP |4.1.2.13 |x| | | | | SMNT |959 5.3.1 | | |x| | | REIN |959 5.3.1 | | |x| | | QUIT |4.1.2.13 |x| | | | | | | | | | | | PORT |4.1.2.13 |x| | | | | PASV |4.1.2.6 |x| | | | | TYPE |4.1.2.13 |x| | | | |1 STRU |4.1.2.13 |x| | | | |1 MODE |4.1.2.13 |x| | | | |1 | | | | | | | RETR |4.1.2.13 |x| | | | | STOR |4.1.2.13 |x| | | | | STOU |959 5.3.1 | | |x| | | APPE |4.1.2.13 |x| | | | | ALLO |959 5.3.1 | | |x| | | REST |959 5.3.1 | | |x| | | RNFR |4.1.2.13 |x| | | | | RNTO |4.1.2.13 |x| | | | | ABOR |959 5.3.1 | | |x| | | DELE |4.1.2.13 |x| | | | | RMD |4.1.2.13 |x| | | | | MKD |4.1.2.13 |x| | | | | PWD |4.1.2.13 |x| | | | | LIST |4.1.2.13 |x| | | | | NLST |4.1.2.13 |x| | | | | SITE |4.1.2.8 | | |x| | | STAT |4.1.2.13 |x| | | | | SYST |4.1.2.13 |x| | | | | HELP |4.1.2.13 |x| | | | | NOOP |4.1.2.13 |x| | | | | | | | | | | |
User Interface: | | | | | | | Arbitrary pathnames |4.1.4.1 |x| | | | | Implement "QUOTE" command |4.1.4.2 |x| | | | | Transfer control commands immediately |4.1.4.2 | |x| | | | Display error messages to user |4.1.4.3 | |x| | | | Verbose mode |4.1.4.3 | |x| | | | Maintain synchronization with server |4.1.4.4 | |x| | | |
Footnotes:
(1) For the values shown earlier.
(2) Here m is number of bits in a memory word.
(3) Required for host with record-structured file system, optional otherwise.
4.2 TRIVIAL FILE TRANSFER PROTOCOL -- TFTP
4.2.1 INTRODUCTION
The Trivial File Transfer Protocol TFTP is defined in RFC-783 [TFTP:1].
TFTP provides its own reliable delivery with UDP as its transport protocol, using a simple stop-and-wait acknowledgment system. Since TFTP has an effective window of only one 512 octet segment, it can provide good performance only over paths that have a small delay*bandwidth product. The TFTP file interface is very simple, providing no access control or security.
TFTP's most important application is bootstrapping a host over a local network, since it is simple and small enough to be easily implemented in EPROM [BOOT:1, BOOT:2]. Vendors are urged to support TFTP for booting.
4.2.2 PROTOCOL WALK-THROUGH
The TFTP specification [TFTP:1] is written in an open style, and does not fully specify many parts of the protocol.
4.2.2.1 Transfer Modes: RFC-783, Page 3
The transfer mode "mail" SHOULD NOT be supported.
4.2.2.2 UDP Header: RFC-783, Page 17
The Length field of a UDP header is incorrectly defined; it includes the UDP header length (8).
4.2.3 SPECIFIC ISSUES
4.2.3.1 Sorcerer's Apprentice Syndrome
There is a serious bug, known as the "Sorcerer's Apprentice Syndrome," in the protocol specification. While it does not cause incorrect operation of the transfer (the file will always be transferred correctly if the transfer completes), this bug may cause excessive retransmission, which may cause the transfer to time out.
Implementations MUST contain the fix for this problem: the sender (i.e., the side originating the DATA packets) must never resend the current DATA packet on receipt of a
duplicate ACK.
DISCUSSION: The bug is caused by the protocol rule that either side, on receiving an old duplicate datagram, may resend the current datagram. If a packet is delayed in the network but later successfully delivered after either side has timed out and retransmitted a packet, a duplicate copy of the response may be generated. If the other side responds to this duplicate with a duplicate of its own, then every datagram will be sent in duplicate for the remainder of the transfer (unless a datagram is lost, breaking the repetition). Worse yet, since the delay is often caused by congestion, this duplicate transmission will usually causes more congestion, leading to more delayed packets, etc.
The following example may help to clarify this problem.
TFTP A TFTP B
(1) Receive ACK X-1 Send DATA X (2) Receive DATA X Send ACK X (ACK X is delayed in network, and A times out): (3) Retransmit DATA X
(4) Receive DATA X again Send ACK X again (5) Receive (delayed) ACK X Send DATA X+1 (6) Receive DATA X+1 Send ACK X+1 (7) Receive ACK X again Send DATA X+1 again (8) Receive DATA X+1 again Send ACK X+1 again (9) Receive ACK X+1 Send DATA X+2 (10) Receive DATA X+2 Send ACK X+3 (11) Receive ACK X+1 again Send DATA X+2 again (12) Receive DATA X+2 again Send ACK X+3 again
Notice that once the delayed ACK arrives, the protocol settles down to duplicate all further packets (sequences 5-8 and 9-12). The problem is caused not by either side timing out, but by both sides retransmitting the current packet when they receive a duplicate.
The fix is to break the retransmission loop, as indicated above. This is analogous to the behavior of TCP. It is then possible to remove the retransmission timer on the receiver, since the resent ACK will never cause any action; this is a useful simplification where TFTP is used in a bootstrap program. It is OK to allow the timer to remain, and it may be helpful if the retransmitted ACK replaces one that was genuinely lost in the network. The sender still requires a retransmit timer, of course.
4.2.3.2 Timeout Algorithms
A TFTP implementation MUST use an adaptive timeout.
IMPLEMENTATION: TCP retransmission algorithms provide a useful base to work from. At least an exponential backoff of retransmission timeout is necessary.
4.2.3.3 Extensions
A variety of non-standard extensions have been made to TFTP, including additional transfer modes and a secure operation mode (with passwords). None of these have been standardized.
4.2.3.4 Access Control
A server TFTP implementation SHOULD include some configurable access control over what pathnames are allowed in TFTP operations.
4.2.3.5 Broadcast Request
A TFTP request directed to a broadcast address SHOULD be silently ignored.
DISCUSSION: Due to the weak access control capability of TFTP, directed broadcasts of TFTP requests to random networks
could create a significant security hole.
4.2.4 TFTP REQUIREMENTS SUMMARY
| | | | |S| | | | | | |H| |F | | | | |O|M|o | | |S| |U|U|o | | |H| |L|S|t | |M|O| |D|T|n | |U|U|M| | |o | |S|L|A|N|N|t | |T|D|Y|O|O|t
| FEATURE | SECTION | T | T | e | |||
|---|---|---|---|---|---|---|---|
| Fix Sorcerer's Apprentice Syndrome | 4.2.3.1 | x | |||||
| Transfer modes: | |||||||
| netascii | RFC-783 | x | |||||
| octet | RFC-783 | x | |||||
| 4.2.2.1 | x | ||||||
| extensions | 4.2.3.3 | x | |||||
| Use adaptive timeout | 4.2.3.2 | x | |||||
| Configurable access control | 4.2.3.4 | x | |||||
| Silently ignore broadcast request | 4.2.3.5 | x | |||||
| ------------------------------------------------- | -------- | - | - | - | - | - | -- |
| ------------------------------------------------- | -------- | - | - | - | - | - | -- |