rfc1350.4up.ps

this gives details of the network programming

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(Network Working Group                                         K. Sollins) s
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(Request For Comments: 1350                                           MIT) s
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(STD: 33                                                        July 1992) s
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(Obsoletes: RFC 783) s
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(                     THE TFTP PROTOCOL \(REVISION 2\)) s
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(Status of this Memo) s
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(   This RFC specifies an IAB standards track protocol for the Internet) s
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(   community, and requests discussion and suggestions for improvements.) s
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(   Please refer to the current edition of the "IAB Official Protocol) s
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(   Standards" for the standardization state and status of this protocol.) s
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(   Distribution of this memo is unlimited.) s
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(Summary) s
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(   TFTP is a very simple protocol used to transfer files.  It is from) s
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(   this that its name comes, Trivial File Transfer Protocol or TFTP.) s
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(   Each nonterminal packet is acknowledged separately.  This document) s
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(   describes the protocol and its types of packets.  The document also) s
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(   explains the reasons behind some of the design decisions.) s
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(Acknowlegements) s
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(   The protocol was originally designed by Noel Chiappa, and was) s
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(   redesigned by him, Bob Baldwin and Dave Clark, with comments from) s
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(   Steve Szymanski.  The current revision of the document includes) s
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(   modifications stemming from discussions with and suggestions from) s
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(   Larry Allen, Noel Chiappa, Dave Clark, Geoff Cooper, Mike Greenwald,) s
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(   Liza Martin, David Reed, Craig Milo Rogers \(of USC-ISI\), Kathy) s
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(   Yellick, and the author.  The acknowledgement and retransmission) s
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(   scheme was inspired by TCP, and the error mechanism was suggested by) s
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(   PARC's EFTP abort message.) s
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(   The May, 1992 revision to fix the "Sorcerer's Apprentice" protocol) s
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(   bug [4] and other minor document problems was done by Noel Chiappa.) s
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(   This research was supported by the Advanced Research Projects Agency) s
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(   of the Department of Defense and was monitored by the Office of Naval) s
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(   Research under contract number N00014-75-C-0661.) s
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(1. Purpose) s
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(   TFTP is a simple protocol to transfer files, and therefore was named) s
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(   the Trivial File Transfer Protocol or TFTP.  It has been implemented) s
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(   on top of the Internet User Datagram protocol \(UDP or Datagram\) [2]) s
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(Sollins                                                         [Page 1]) s
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5 751 M
(RFC 1350                    TFTP Revision 2                    July 1992) s
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(   so it may be used to move files between machines on different) s
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(   networks implementing UDP.  \(This should not exclude the possibility) s
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(   of implementing TFTP on top of other datagram protocols.\)  It is) s
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(   designed to be small and easy to implement.  Therefore, it lacks most) s
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(   of the features of a regular FTP.  The only thing it can do is read) s
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(   and write files \(or mail\) from/to a remote server.  It cannot list) s
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(   directories, and currently has no provisions for user authentication.) s
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(   In common with other Internet protocols, it passes 8 bit bytes of) s
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(   data.) s
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(   Three modes of transfer are currently supported: netascii \(This is) s
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(   ascii as defined in "USA Standard Code for Information Interchange") s
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(   [1] with the modifications specified in "Telnet Protocol) s
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(   Specification" [3].\)  Note that it is 8 bit ascii.  The term) s
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(   "netascii" will be used throughout this document to mean this) s
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(   particular version of ascii.\); octet \(This replaces the "binary" mode) s
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(   of previous versions of this document.\) raw 8 bit bytes; mail,) s
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(   netascii characters sent to a user rather than a file.  \(The mail) s
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(   mode is obsolete and should not be implemented or used.\)  Additional) s
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(   modes can be defined by pairs of cooperating hosts.) s
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(   Reference [4] \(section 4.2\) should be consulted for further valuable) s
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(   directives and suggestions on TFTP.) s
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(2. Overview of the Protocol) s
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(   Any transfer begins with a request to read or write a file, which) s
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(   also serves to request a connection.  If the server grants the) s
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(   request, the connection is opened and the file is sent in fixed) s
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(   length blocks of 512 bytes.  Each data packet contains one block of) s
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(   data, and must be acknowledged by an acknowledgment packet before the) s
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(   next packet can be sent.  A data packet of less than 512 bytes) s
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(   signals termination of a transfer.  If a packet gets lost in the) s
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(   network, the intended recipient will timeout and may retransmit his) s
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(   last packet \(which may be data or an acknowledgment\), thus causing) s
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(   the sender of the lost packet to retransmit that lost packet.  The) s
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(   sender has to keep just one packet on hand for retransmission, since) s
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(   the lock step acknowledgment guarantees that all older packets have) s
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(   been received.  Notice that both machines involved in a transfer are) s
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(   considered senders and receivers.  One sends data and receives) s
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(   acknowledgments, the other sends acknowledgments and receives data.) s
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(   Most errors cause termination of the connection.  An error is) s
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(   signalled by sending an error packet.  This packet is not) s
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(   acknowledged, and not retransmitted \(i.e., a TFTP server or user may) s
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(   terminate after sending an error message\), so the other end of the) s
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(   connection may not get it.  Therefore timeouts are used to detect) s
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(   such a termination when the error packet has been lost.  Errors are) s
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(Sollins                                                         [Page 2]) s
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5 751 M
(RFC 1350                    TFTP Revision 2                    July 1992) s
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(   caused by three types of events: not being able to satisfy the) s
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(   request \(e.g., file not found, access violation, or no such user\),) s
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(   receiving a packet which cannot be explained by a delay or) s
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(   duplication in the network \(e.g., an incorrectly formed packet\), and) s
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(   losing access to a necessary resource \(e.g., disk full or access) s
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(   denied during a transfer\).) s
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(   TFTP recognizes only one error condition that does not cause) s
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(   termination, the source port of a received packet being incorrect.) s
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(   In this case, an error packet is sent to the originating host.) s
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(   This protocol is very restrictive, in order to simplify) s
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(   implementation.  For example, the fixed length blocks make allocation) s
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(   straight forward, and the lock step acknowledgement provides flow) s
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(   control and eliminates the need to reorder incoming data packets.) s
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(3. Relation to other Protocols) s
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(   As mentioned TFTP is designed to be implemented on top of the) s
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(   Datagram protocol \(UDP\).  Since Datagram is implemented on the) s
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(   Internet protocol, packets will have an Internet header, a Datagram) s
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(   header, and a TFTP header.  Additionally, the packets may have a) s
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(   header \(LNI, ARPA header, etc.\)  to allow them through the local) s
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(   transport medium.  As shown in Figure 3-1, the order of the contents) s
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(   of a packet will be: local medium header, if used, Internet header,) s
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(   Datagram header, TFTP header, followed by the remainder of the TFTP) s
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(   packet.  \(This may or may not be data depending on the type of packet) s
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(   as specified in the TFTP header.\)  TFTP does not specify any of the) s
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(   values in the Internet header.  On the other hand, the source and) s
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(   destination port fields of the Datagram header \(its format is given) s
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(   in the appendix\) are used by TFTP and the length field reflects the) s
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(   size of the TFTP packet.  The transfer identifiers \(TID's\) used by) s
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(   TFTP are passed to the Datagram layer to be used as ports; therefore) s
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(   they must be between 0 and 65,535.  The initialization of TID's is) s
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(   discussed in the section on initial connection protocol.) s
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(   The  TFTP header consists of a 2 byte opcode field which indicates) s
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(   the packet's type \(e.g., DATA, ERROR, etc.\)  These opcodes and  the) s
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(   formats of  the various types of packets are discussed further in the) s
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(   section on TFTP packets.) s
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(Sollins                                                         [Page 3]) s
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(RFC 1350                    TFTP Revision 2                    July 1992) s
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(          ---------------------------------------------------) s
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