Network Working Group Internet Engineering Task Force Request for Comments: 1123 R. Braden, Editor October 1989 Requirements for Internet Hosts -- Application and Support Status of This Memo This RFC is an official specification for the Internet community. It incorporates by reference, amends, corrects, and supplements the primary protocol standards documents relating to hosts. Distribution of this document is unlimited. Summary This RFC is one of a pair that defines and discusses the requirements for Internet host software. This RFC covers the application and support protocols; its companion RFC-1122 covers the communication protocol layers: link layer, IP layer, and transport layer. Table of Contents 1. INTRODUCTION ............................................... 5 1.1 The Internet Architecture .............................. 6 1.2 General Considerations ................................. 6 1.2.1 Continuing Internet Evolution ..................... 6 1.2.2 Robustness Principle .............................. 7 1.2.3 Error Logging ..................................... 8 1.2.4 Configuration ..................................... 8 1.3 Reading this Document .................................. 10 1.3.1 Organization ...................................... 10 1.3.2 Requirements ...................................... 10 1.3.3 Terminology ....................................... 11 1.4 Acknowledgments ........................................ 12 2. GENERAL ISSUES ............................................. 13 2.1 Host Names and Numbers ................................. 13 2.2 Using Domain Name Service .............................. 13 2.3 Applications on Multihomed hosts ....................... 14 2.4 Type-of-Service ........................................ 14 2.5 GENERAL APPLICATION REQUIREMENTS SUMMARY ............... 15 Internet Engineering Task Force [Page 1]   RFC1123 INTRODUCTION October 1989 3. REMOTE LOGIN -- TELNET PROTOCOL ............................ 16 3.1 INTRODUCTION ........................................... 16 3.2 PROTOCOL WALK-THROUGH .................................. 16 3.2.1 Option Negotiation ................................ 16 3.2.2 Telnet Go-Ahead Function .......................... 16 3.2.3 Control Functions ................................. 17 3.2.4 Telnet "Synch" Signal ............................. 18 3.2.5 NVT Printer and Keyboard .......................... 19 3.2.6 Telnet Command Structure .......................... 20 3.2.7 Telnet Binary Option .............................. 20 3.2.8 Telnet Terminal-Type Option ....................... 20 3.3 SPECIFIC ISSUES ........................................ 21 3.3.1 Telnet End-of-Line Convention ..................... 21 3.3.2 Data Entry Terminals .............................. 23 3.3.3 Option Requirements ............................... 24 3.3.4 Option Initiation ................................. 24 3.3.5 Telnet Linemode Option ............................ 25 3.4 TELNET/USER INTERFACE .................................. 25 3.4.1 Character Set Transparency ........................ 25 3.4.2 Telnet Commands ................................... 26 3.4.3 TCP Connection Errors ............................. 26 3.4.4 Non-Default Telnet Contact Port ................... 26 3.4.5 Flushing Output ................................... 26 3.5. TELNET REQUIREMENTS SUMMARY ........................... 27 4. FILE TRANSFER .............................................. 29 4.1 FILE TRANSFER PROTOCOL -- FTP .......................... 29 4.1.1 INTRODUCTION ...................................... 29 4.1.2. PROTOCOL WALK-THROUGH ............................ 29 4.1.2.1 LOCAL Type ................................... 29 4.1.2.2 Telnet Format Control ........................ 30 4.1.2.3 Page Structure ............................... 30 4.1.2.4 Data Structure Transformations ............... 30 4.1.2.5 Data Connection Management ................... 31 4.1.2.6 PASV Command ................................. 31 4.1.2.7 LIST and NLST Commands ....................... 31 4.1.2.8 SITE Command ................................. 32 4.1.2.9 STOU Command ................................. 32 4.1.2.10 Telnet End-of-line Code ..................... 32 4.1.2.11 FTP Replies ................................. 33 4.1.2.12 Connections ................................. 34 4.1.2.13 Minimum Implementation; RFC-959 Section ..... 34 4.1.3 SPECIFIC ISSUES ................................... 35 4.1.3.1 Non-standard Command Verbs ................... 35 4.1.3.2 Idle Timeout ................................. 36 4.1.3.3 Concurrency of Data and Control .............. 36 4.1.3.4 FTP Restart Mechanism ........................ 36 4.1.4 FTP/USER INTERFACE ................................ 39 Internet Engineering Task Force [Page 2]   RFC1123 INTRODUCTION October 1989 4.1.4.1 Pathname Specification ....................... 39 4.1.4.2 "QUOTE" Command .............................. 40 4.1.4.3 Displaying Replies to User ................... 40 4.1.4.4 Maintaining Synchronization .................. 40 4.1.5 FTP REQUIREMENTS SUMMARY ......................... 41 4.2 TRIVIAL FILE TRANSFER PROTOCOL -- TFTP ................. 44 4.2.1 INTRODUCTION ...................................... 44 4.2.2 PROTOCOL WALK-THROUGH ............................. 44 4.2.2.1 Transfer Modes ............................... 44 4.2.2.2 UDP Header ................................... 44 4.2.3 SPECIFIC ISSUES ................................... 44 4.2.3.1 Sorcerer's Apprentice Syndrome ............... 44 4.2.3.2 Timeout Algorithms ........................... 46 4.2.3.3 Extensions ................................... 46 4.2.3.4 Access Control ............................... 46 4.2.3.5 Broadcast Request ............................ 46 4.2.4 TFTP REQUIREMENTS SUMMARY ......................... 47 5. ELECTRONIC MAIL -- SMTP and RFC-822 ........................ 48 5.1 INTRODUCTION ........................................... 48 5.2 PROTOCOL WALK-THROUGH .................................. 48 5.2.1 The SMTP Model .................................... 48 5.2.2 Canonicalization .................................. 49 5.2.3 VRFY and EXPN Commands ............................ 50 5.2.4 SEND, SOML, and SAML Commands ..................... 50 5.2.5 HELO Command ...................................... 50 5.2.6 Mail Relay ........................................ 51 5.2.7 RCPT Command ...................................... 52 5.2.8 DATA Command ...................................... 53 5.2.9 Command Syntax .................................... 54 5.2.10 SMTP Replies ..................................... 54 5.2.11 Transparency ..................................... 55 5.2.12 WKS Use in MX Processing ......................... 55 5.2.13 RFC-822 Message Specification .................... 55 5.2.14 RFC-822 Date and Time Specification .............. 55 5.2.15 RFC-822 Syntax Change ............................ 56 5.2.16 RFC-822 Local-part .............................. 56 5.2.17 Domain Literals .................................. 57 5.2.18 Common Address Formatting Errors ................. 58 5.2.19 Explicit Source Routes ........................... 58 5.3 SPECIFIC ISSUES ........................................ 59 5.3.1 SMTP Queueing Strategies .......................... 59 5.3.1.1 Sending Strategy .............................. 59 5.3.1.2 Receiving strategy ........................... 61 5.3.2 Timeouts in SMTP .................................. 61 5.3.3 Reliable Mail Receipt ............................. 63 5.3.4 Reliable Mail Transmission ........................ 63 5.3.5 Domain Name Support ............................... 65 Internet Engineering Task Force [Page 3]   RFC1123 INTRODUCTION October 1989 5.3.6 Mailing Lists and Aliases ......................... 65 5.3.7 Mail Gatewaying ................................... 66 5.3.8 Maximum Message Size .............................. 68 5.4 SMTP REQUIREMENTS SUMMARY .............................. 69 6. SUPPORT SERVICES ............................................ 72 6.1 DOMAIN NAME TRANSLATION ................................. 72 6.1.1 INTRODUCTION ....................................... 72 6.1.2 PROTOCOL WALK-THROUGH ............................. 72 6.1.2.1 Resource Records with Zero TTL ............... 73 6.1.2.2 QCLASS Values ................................ 73 6.1.2.3 Unused Fields ................................ 73 6.1.2.4 Compression .................................. 73 6.1.2.5 Misusing Configuration Info .................. 73 6.1.3 SPECIFIC ISSUES ................................... 74 6.1.3.1 Resolver Implementation ...................... 74 6.1.3.2 Transport Protocols .......................... 75 6.1.3.3 Efficient Resource Usage ..................... 77 6.1.3.4 Multihomed Hosts ............................. 78 6.1.3.5 Extensibility ................................ 79 6.1.3.6 Status of RR Types ........................... 79 6.1.3.7 Robustness ................................... 80 6.1.3.8 Local Host Table ............................. 80 6.1.4 DNS USER INTERFACE ................................ 81 6.1.4.1 DNS Administration ........................... 81 6.1.4.2 DNS User Interface ........................... 81 6.1.4.3 Interface Abbreviation Facilities ............. 82 6.1.5 DOMAIN NAME SYSTEM REQUIREMENTS SUMMARY ........... 84 6.2 HOST INITIALIZATION .................................... 87 6.2.1 INTRODUCTION ...................................... 87 6.2.2 REQUIREMENTS ...................................... 87 6.2.2.1 Dynamic Configuration ........................ 87 6.2.2.2 Loading Phase ................................ 89 6.3 REMOTE MANAGEMENT ...................................... 90 6.3.1 INTRODUCTION ...................................... 90 6.3.2 PROTOCOL WALK-THROUGH ............................. 90 6.3.3 MANAGEMENT REQUIREMENTS SUMMARY ................... 92 7. REFERENCES ................................................. 93 Internet Engineering Task Force [Page 4]   RFC1123 INTRODUCTION October 1989 1. INTRODUCTION This document is one of a pair that defines and discusses the requirements for host system implementations of the Internet protocol suite. This RFC covers the applications layer and support protocols. Its companion RFC, "Requirements for Internet Hosts -- Communications Layers" [INTRO:1] covers the lower layer protocols: transport layer, IP layer, and link layer. These documents are intended to provide guidance for vendors, implementors, and users of Internet communication software. They represent the consensus of a large body of technical experience and wisdom, contributed by members of the Internet research and vendor communities. This RFC enumerates standard protocols that a host connected to the Internet must use, and it incorporates by reference the RFCs and other documents describing the current specifications for these protocols. It corrects errors in the referenced documents and adds additional discussion and guidance for an implementor. For each protocol, this document also contains an explicit set of requirements, recommendations, and options. The reader must understand that the list of requirements in this document is incomplete by itself; the complete set of requirements for an Internet host is primarily defined in the standard protocol specification documents, with the corrections, amendments, and supplements contained in this RFC. A good-faith implementation of the protocols that was produced after careful reading of the RFC';s and with some interaction with the Internet technical community, and that followed good communications software engineering practices, should differ from the requirements of this document in only minor ways. Thus, in many cases, the "requirements" in this RFC are already stated or implied in the standard protocol documents, so that their inclusion here is, in a sense, redundant. However, they were included because some past implementation has made the wrong choice, causing problems of interoperability, performance, and/or robustness. This document includes discussion and explanation of many of the requirements and recommendations. A simple list of requirements would be dangerous, because: o Some required features are more important than others, and some features are optional. o There may be valid reasons why particular vendor products that Internet Engineering Task Force [Page 5]   RFC1123 INTRODUCTION October 1989 are designed for restricted contexts might choose to use different specifications. However, the specifications of this document must be followed to meet the general goal of arbitrary host interoperation across the diversity and complexity of the Internet system. Although most current implementations fail to meet these requirements in various ways, some minor and some major, this specification is the ideal towards which we need to move. These requirements are based on the current level of Internet architecture. This document will be updated as required to provide additional clarifications or to include additional information in those areas in which specifications are still evolving. This introductory section begins with general advice to host software vendors, and then gives some guidance on reading the rest of the document. Section 2 contains general requirements that may be applicable to all application and support protocols. Sections 3, 4, and 5 contain the requirements on protocols for the three major applications: Telnet, file transfer, and electronic mail, respectively. Section 6 covers the support applications: the domain name system, system initialization, and management. Finally, all references will be found in Section 7. 1.1 The Internet Architecture For a brief introduction to the Internet architecture from a host viewpoint, see Section 1.1 of [INTRO:1]. That section also contains recommended references for general background on the Internet architecture. 1.2 General Considerations There are two important lessons that vendors of Internet host software have learned and which a new vendor should consider seriously. 1.2.1 Continuing Internet Evolution The enormous growth of the Internet has revealed problems of management and scaling in a large datagram-based packet communication system. These problems are being addressed, and as a result there will be continuing evolution of the specifications described in this document. These changes will be carefully planned and controlled, since there is extensive participation in this planning by the vendors and by the organizations responsible for operations of the networks. Internet Engineering Task Force [Page 6]   RFC1123 INTRODUCTION October 1989 Development, evolution, and revision are characteristic of computer network protocols today, and this situation will persist for some years. A vendor who develops computer communication software for the Internet protocol suite (or any other protocol suite!) and then fails to maintain and update that software for changing specifications is going to leave a trail of unhappy customers. The Internet is a large communication network, and the users are in constant contact through it. Experience has shown that knowledge of deficiencies in vendor software propagates quickly through the Internet technical community. 1.2.2 Robustness Principle At every layer of the protocols, there is a general rule whose application can lead to enormous benefits in robustness and interoperability: "Be liberal in what you accept, and conservative in what you send" Software should be written to deal with every conceivable error, no matter how unlikely; sooner or later a packet will come in with that particular combination of errors and attributes, and unless the software is prepared, chaos can ensue. In general, it is best to assume that the network is filled with malevolent entities that will send in packets designed to have the worst possible effect. This assumption will lead to suitable protective design, although the most serious problems in the Internet have been caused by unenvisaged mechanisms triggered by low-probability events; mere human malice would never have taken so devious a course! Adaptability to change must be designed into all levels of Internet host software. As a simple example, consider a protocol specification that contains an enumeration of values for a particular header field -- e.g., a type field, a port number, or an error code; this enumeration must be assumed to be incomplete. Thus, if a protocol specification defines four possible error codes, the software must not break when a fifth code shows up. An undefined code might be logged (see below), but it must not cause a failure. The second part of the principle is almost as important: software on other hosts may contain deficiencies that make it unwise to exploit legal but obscure protocol features. It is unwise to stray far from the obvious and simple, lest untoward effects result elsewhere. A corollary of this is "watch out Internet Engineering Task Force [Page 7]   RFC1123 INTRODUCTION October 1989 for misbehaving hosts"; host software should be prepared, not just to survive other misbehaving hosts, but also to cooperate to limit the amount of disruption such hosts can cause to the shared communication facility. 1.2.3 Error Logging The Internet includes a great variety of host and gateway systems, each implementing many protocols and protocol layers, and some of these contain bugs and mis-features in their Internet protocol software. As a result of complexity, diversity, and distribution of function, the diagnosis of user problems is often very difficult. Problem diagnosis will be aided if host implementations include a carefully designed facility for logging erroneous or "strange" protocol events. It is important to include as much diagnostic information as possible when an error is logged. In particular, it is often useful to record the header(s) of a packet that caused an error. However, care must be taken to ensure that error logging does not consume prohibitive amounts of resources or otherwise interfere with the operation of the host. There is a tendency for abnormal but harmless protocol events to overflow error logging files; this can be avoided by using a "circular" log, or by enabling logging only while diagnosing a known failure. It may be useful to filter and count duplicate successive messages. One strategy that seems to work well is: (1) always count abnormalities and make such counts accessible through the management protocol (see Section 6.3); and (2) allow the logging of a great variety of events to be selectively enabled. For example, it might useful to be able to "log everything" or to "log everything for host X". Note that different managements may have differing policies about the amount of error logging that they want normally enabled in a host. Some will say, "if it doesn't hurt me, I don't want to know about it", while others will want to take a more watchful and aggressive attitude about detecting and removing protocol abnormalities. 1.2.4 Configuration It would be ideal if a host implementation of the Internet protocol suite could be entirely self-configuring. This would allow the whole suite to be implemented in ROM or cast into silicon, it would simplify diskless workstations, and it would Internet Engineering Task Force [Page 8]   RFC1123 INTRODUCTION October 1989 be an immense boon to harried LAN administrators as well as system vendors. We have not reached this ideal; in fact, we are not even close. At many points in this document, you will find a requirement that a parameter be a configurable option. There are several different reasons behind such requirements. In a few cases, there is current uncertainty or disagreement about the best value, and it may be necessary to update the recommended value in the future. In other cases, the value really depends on external factors -- e.g., the size of the host and the distribution of its communication load, or the speeds and topology of nearby networks -- and self-tuning algorithms are unavailable and may be insufficient. In some cases, configurability is needed because of administrative requirements. Finally, some configuration options are required to communicate with obsolete or incorrect implementations of the protocols, distributed without sources, that unfortunately persist in many parts of the Internet. To make correct systems coexist with these faulty systems, administrators often have to "mis- configure" the correct systems. This problem will correct itself gradually as the faulty systems are retired, but it cannot be ignored by vendors. When we say that a parameter must be configurable, we do not intend to require that its value be explicitly read from a configuration file at every boot time. We recommend that implementors set up a default for each parameter, so a configuration file is only necessary to override those defaults that are inappropriate in a particular installation. Thus, the configurability requirement is an assurance that it will be POSSIBLE to override the default when necessary, even in a binary-only or ROM-based product. This document requires a particular value for such defaults in some cases. The choice of default is a sensitive issue when the configuration item controls the accommodation to existing faulty systems. If the Internet is to converge successfully to complete interoperability, the default values built into implementations must implement the official protocol, not "mis-configurations" to accommodate faulty implementations. Although marketing considerations have led some vendors to choose mis-configuration defaults, we urge vendors to choose defaults that will conform to the standard. Finally, we note that a vendor needs to provide adequate Internet Engineering Task Force [Page 9]   RFC1123 INTRODUCTION October 1989 documentation on all configuration parameters, their limits and effects. 1.3 Reading this Document 1.3.1 Organization In general, each major section is organized into the following subsections: (1) Introduction (2) Protocol Walk-Through -- considers the protocol specification documents section-by-section, correcting errors, stating requirements that may be ambiguous or ill-defined, and providing further clarification or explanation. (3) Specific Issues -- discusses protocol design and implementation issues that were not included in the walk- through. (4) Interfaces -- discusses the service interface to the next higher layer. (5) Summary -- contains a summary of the requirements of the section. Under many of the individual topics in this document, there is parenthetical material labeled "DISCUSSION" or "IMPLEMENTATION". This material is intended to give clarification and explanation of the preceding requirements text. It also includes some suggestions on possible future directions or developments. The implementation material contains suggested approaches that an implementor may want to consider. The summary sections are intended to be guides and indexes to the text, but are necessarily cryptic and incomplete. The summaries should never be used or referenced separately from the complete RFC. 1.3.2 Requirements In this document, the words that are used to define the significance of each particular requirement are capitalized. These words are: Internet Engineering Task Force [Page 10]   RFC1123 INTRODUCTION October 1989 * "MUST" This word or the adjective "REQUIRED" means that the item is an absolute requirement of the specification. * "SHOULD" This word or the adjective "RECOMMENDED" means that there may exist valid reasons in particular circumstances to ignore this item, but the full implications should be understood and the case carefully weighed before choosing a different course. * "MAY" This word or the adjective "OPTIONAL" means that this item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because it enhances the product, for example; another vendor may omit the same item. An implementation is not compliant if it fails to satisfy one or more of the MUST requirements for the protocols it implements. An implementation that satisfies all the MUST and all the SHOULD requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the MUST requirements but not all the SHOULD requirements for its protocols is said to be "conditionally compliant". 1.3.3 Terminology This document uses the following technical terms: Segment A segment is the unit of end-to-end transmission in the TCP protocol. A segment consists of a TCP header followed by application data. A segment is transmitted by encapsulation in an IP datagram. Message This term is used by some application layer protocols (particularly SMTP) for an application data unit. Datagram A [UDP] datagram is the unit of end-to-end transmission in the UDP protocol. Internet Engineering Task Force [Page 11]   RFC1123 INTRODUCTION October 1989 Multihomed A host is said to be multihomed if it has multiple IP addresses to connected networks. 1.4 Acknowledgments This document incorporates contributions and comments from a large group of Internet protocol experts, including representatives of university and research labs, vendors, and government agencies. It was assembled primarily by the Host Requirements Working Group of the Internet Engineering Task Force (IETF). The Editor would especially like to acknowledge the tireless dedication of the following people, who attended many long meetings and generated 3 million bytes of electronic mail over the past 18 months in pursuit of this document: Philip Almquist, Dave Borman (Cray Research), Noel Chiappa, Dave Crocker (DEC), Steve Deering (Stanford), Mike Karels (Berkeley), Phil Karn (Bellcore), John Lekashman (NASA), Charles Lynn (BBN), Keith McCloghrie (TWG), Paul Mockapetris (ISI), Thomas Narten (Purdue), Craig Partridge (BBN), Drew Perkins (CMU), and James Van Bokkelen (FTP Software). In addition, the following people made major contributions to the effort: Bill Barns (Mitre), Steve Bellovin (AT&T), Mike Brescia (BBN), Ed Cain (DCA), Annette DeSchon (ISI), Martin Gross (DCA), Phill Gross (NRI), Charles Hedrick (Rutgers), Van Jacobson (LBL), John Klensin (MIT), Mark Lottor (SRI), Milo Medin (NASA), Bill Melohn (Sun Microsystems), Greg Minshall (Kinetics), Jeff Mogul (DEC), John Mullen (CMC), Jon Postel (ISI), John Romkey (Epilogue Technology), and Mike StJohns (DCA). The following also made significant contributions to particular areas: Eric Allman (Berkeley), Rob Austein (MIT), Art Berggreen (ACC), Keith Bostic (Berkeley), Vint Cerf (NRI), Wayne Hathaway (NASA), Matt Korn (IBM), Erik Naggum (Naggum Software, Norway), Robert Ullmann (Prime Computer), David Waitzman (BBN), Frank Wancho (USA), Arun Welch (Ohio State), Bill Westfield (Cisco), and Rayan Zachariassen (Toronto). We are grateful to all, including any contributors who may have been inadvertently omitted from this list. Internet Engineering Task Force [Page 12]   RFC1123 APPLICATIONS LAYER -- GENERAL October 1989 2. GENERAL ISSUES This section contains general requirements that may be applicable to all application-layer protocols. 2.1 Host Names and Numbers The syntax of a legal Internet host name was specified in RFC-952 [DNS:4]. One aspect of host name syntax is hereby changed: the restriction on the first character is relaxed to allow either a letter or a digit. Host software MUST support this more liberal syntax. Host software MUST handle host names of up to 63 characters and SHOULD handle host names of up to 255 characters. Whenever a user inputs the identity of an Internet host, it SHOULD be possible to enter either (1) a host domain name or (2) an IP address in dotted-decimal ("#.#.#.#") form. The host SHOULD check the string syntactically for a dotted-decimal number before looking it up in the Domain Name System. DISCUSSION: This last requirement is not intended to specify the complete syntactic form for entering a dotted-decimal host number; that is considered to be a user-interface issue. For example, a dotted-decimal number must be enclosed within "[ ]" brackets for SMTP mail (see Section 5.2.17). This notation could be made universal within a host system, simplifying the syntactic checking for a dotted-decimal number. If a dotted-decimal number can be entered without such identifying delimiters, then a full syntactic check must be made, because a segment of a host domain name is now allowed to begin with a digit and could legally be entirely numeric (see Section 6.1.2.4). However, a valid host name can never have the dotted-decimal form #.#.#.#, since at least the highest-level component label will be alphabetic. 2.2 Using Domain Name Service Host domain names MUST be translated to IP addresses as described in Section 6.1. Applications using domain name services MUST be able to cope with soft error conditions. Applications MUST wait a reasonable interval between successive retries due to a soft error, and MUST Internet Engineering Task Force [Page 13]   RFC1123 APPLICATIONS LAYER -- GENERAL October 1989 allow for the possibility that network problems may deny service for hours or even days. An application SHOULD NOT rely on the ability to locate a WKS record containing an accurate listing of all services at a particular host address, since the WKS RR type is not often used by Internet sites. To confirm that a service is present, simply attempt to use it. 2.3 Applications on Multihomed hosts When the remote host is multihomed, the name-to-address translation will return a list of alternative IP addresses. As specified in Section 6.1.3.4, this list should be in order of decreasing preference. Application protocol implementations SHOULD be prepared to try multiple addresses from the list until success is obtained. More specific requirements for SMTP are given in Section 5.3.4. When the local host is multihomed, a UDP-based request/response application SHOULD send the response with an IP source address that is the same as the specific destination address of the UDP request datagram. The "specific destination address" is defined in the "IP Addressing" section of the companion RFC [INTRO:1]. Similarly, a server application that opens multiple TCP connections to the same client SHOULD use the same local IP address for all. 2.4 Type-of-Service Applications MUST select appropriate TOS values when they invoke transport layer services, and these values MUST be configurable. Note that a TOS value contains 5 bits, of which only the most- significant 3 bits are currently defined; the other two bits MUST be zero. DISCUSSION: As gateway algorithms are developed to implement Type-of- Service, the recommended values for various application protocols may change. In addition, it is likely that particular combinations of users and Internet paths will want non-standard TOS values. For these reasons, the TOS values must be configurable. See the latest version of the "Assigned Numbers" RFC [INTRO:5] for the recommended TOS values for the major application protocols. Internet Engineering Task Force [Page 14]   RFC1123 APPLICATIONS LAYER -- GENERAL October 1989 2.5 GENERAL APPLICATION 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 -----------------------------------------------|----------|-|-|-|-|-|-- | | | | | | | User interfaces: | | | | | | | Allow host name to begin with digit |2.1 |x| | | | | Host names of up to 635 characters |2.1 |x| | | | | Host names of up to 255 characters |2.1 | |x| | | | Support dotted-decimal host numbers |2.1 | |x| | | | Check syntactically for dotted-dec first |2.1 | |x| | | | | | | | | | | Map domain names per Section 6.1 |2.2 |x| | | | | Cope with soft DNS errors |2.2 |x| | | | | Reasonable interval between retries |2.2 |x| | | | | Allow for long outages |2.2 |x| | | | | Expect WKS records to be available |2.2 | | | |x| | | | | | | | | Try multiple addr's for remote multihomed host |2.3 | |x| | | | UDP reply src addr is specific dest of request |2.3 | |x| | | | Use same IP addr for related TCP connections |2.3 | |x| | | | Specify appropriate TOS values |2.4 |x| | | | | TOS values configurable |2.4 |x| | | | | Unused TOS bits zero |2.4 |x| | | | | | | | | | | | | | | | | | | Internet Engineering Task Force [Page 15]   RFC1123 REMOTE LOGIN -- TELNET October 1989 3. REMOTE LOGIN -- TELNET PROTOCOL 3.1 INTRODUCTION Telnet is the standard Internet application protocol for remote login. It provides the encoding rules to link a user's keyboard/display on a client ("user") system with a command interpreter on a remote server system. A subset of the Telnet protocol is also incorporated within other application protocols, e.g., FTP and SMTP. Telnet uses a single TCP connection, and its normal data stream ("Network Virtual Terminal" or "NVT" mode) is 7-bit ASCII with escape sequences to embed control functions. Telnet also allows the negotiation of many optional modes and functions. The primary Telnet specification is to be found in RFC-854 [TELNET:1], while the options are defined in many other RFCs; see Section 7 for references. 3.2 PROTOCOL WALK-THROUGH 3.2.1 Option Negotiation: RFC-854, pp. 2-3 Every Telnet implementation MUST include option negotiation and subnegotiation machinery [TELNET:2]. A host MUST carefully follow the rules of RFC-854 to avoid option-negotiation loops. A host MUST refuse (i.e, reply WONT/DONT to a DO/WILL) an unsupported option. Option negotiation SHOULD continue to function (even if all requests are refused) throughout the lifetime of a Telnet connection. If all option negotiations fail, a Telnet implementation MUST default to, and support, an NVT. DISCUSSION: Even though more sophisticated "terminals" and supporting option negotiations are becoming the norm, all implementations must be prepared to support an NVT for any user-server communication. 3.2.2 Telnet Go-Ahead Function: RFC-854, p. 5, and RFC-858 On a host that never sends the Telnet command Go Ahead (GA), the Telnet Server MUST attempt to negotiate the Suppress Go Ahead option (i.e., send "WILL Suppress Go Ahead"). A User or Server Telnet MUST always accept negotiation of the Suppress Go Internet Engineering Task Force [Page 16]   RFC1123 REMOTE LOGIN -- TELNET October 1989 Ahead option. When it is driving a full-duplex terminal for which GA has no meaning, a User Telnet implementation MAY ignore GA commands. DISCUSSION: Half-duplex ("locked-keyboard") line-at-a-time terminals for which the Go-Ahead mechanism was designed have largely disappeared from the scene. It turned out to be difficult to implement sending the Go-Ahead signal in many operating systems, even some systems that support native half-duplex terminals. The difficulty is typically that the Telnet server code does not have access to information about whether the user process is blocked awaiting input from the Telnet connection, i.e., it cannot reliably determine when to send a GA command. Therefore, most Telnet Server hosts do not send GA commands. The effect of the rules in this section is to allow either end of a Telnet connection to veto the use of GA commands. There is a class of half-duplex terminals that is still commercially important: "data entry terminals," which interact in a full-screen manner. However, supporting data entry terminals using the Telnet protocol does not require the Go Ahead signal; see Section 3.3.2. 3.2.3 Control Functions: RFC-854, pp. 7-8 The list of Telnet commands has been extended to include EOR (End-of-Record), with code 239 [TELNET:9]. Both User and Server Telnets MAY support the control functions EOR, EC, EL, and Break, and MUST support AO, AYT, DM, IP, NOP, SB, and SE. A host MUST be able to receive and ignore any Telnet control functions that it does not support. DISCUSSION: Note that a Server Telnet is required to support the Telnet IP (Interrupt Process) function, even if the server host has an equivalent in-stream function (e.g., Control-C in many systems). The Telnet IP function may be stronger than an in-stream interrupt command, because of the out- of-band effect of TCP urgent data. The EOR control function may be used to delimit the Internet Engineering Task Force [Page 17]   RFC1123 REMOTE LOGIN -- TELNET October 1989 stream. An important application is data entry terminal support (see Section 3.3.2). There was concern that since EOR had not been defined in RFC-854, a host that was not prepared to correctly ignore unknown Telnet commands might crash if it received an EOR. To protect such hosts, the End-of-Record option [TELNET:9] was introduced; however, a properly implemented Telnet program will not require this protection. 3.2.4 Telnet "Synch" Signal: RFC-854, pp. 8-10 When it receives "urgent" TCP data, a User or Server Telnet MUST discard all data except Telnet commands until the DM (and end of urgent) is reached. When it sends Telnet IP (Interrupt Process), a User Telnet SHOULD follow it by the Telnet "Synch" sequence, i.e., send as TCP urgent data the sequence "IAC IP IAC DM". The TCP urgent pointer points to the DM octet. When it receives a Telnet IP command, a Server Telnet MAY send a Telnet "Synch" sequence back to the user, to flush the output stream. The choice ought to be consistent with the way the server operating system behaves when a local user interrupts a process. When it receives a Telnet AO command, a Server Telnet MUST send a Telnet "Synch" sequence back to the user, to flush the output stream. A User Telnet SHOULD have the capability of flushing output when it sends a Telnet IP; see also Section 3.4.5. DISCUSSION: There are three possible ways for a User Telnet to flush the stream of server output data: (1) Send AO after IP. This will cause the server host to send a "flush- buffered-output" signal to its operating system. However, the AO may not take effect locally, i.e., stop terminal output at the User Telnet end, until the Server Telnet has received and processed the AO and has sent back a "Synch". (2) Send DO TIMING-MARK [TELNET:7] after IP, and discard all output locally until a WILL/WONT TIMING-MARK is Internet Engineering Task Force [Page 18]   RFC1123 REMOTE LOGIN -- TELNET October 1989 received from the Server Telnet. Since the DO TIMING-MARK will be processed after the IP at the server, the reply to it should be in the right place in the output data stream. However, the TIMING-MARK will not send a "flush buffered output" signal to the server operating system. Whether or not this is needed is dependent upon the server system. (3) Do both. The best method is not entirely clear, since it must accommodate a number of existing server hosts that do not follow the Telnet standards in various ways. The safest approach is probably to provide a user-controllable option to select (1), (2), or (3). 3.2.5 NVT Printer and Keyboard: RFC-854, p. 11 In NVT mode, a Telnet SHOULD NOT send characters with the high-order bit 1, and MUST NOT send it as a parity bit. Implementations that pass the high-order bit to applications SHOULD negotiate binary mode (see Section 3.2.6). DISCUSSION: Implementors should be aware that a strict reading of RFC-854 allows a client or server expecting NVT ASCII to ignore characters with the high-order bit set. In general, binary mode is expected to be used for transmission of an extended (beyond 7-bit) character set with Telnet. However, there exist applications that really need an 8- bit NVT mode, which is currently not defined, and these existing applications do set the high-order bit during part or all of the life of a Telnet connection. Note that binary mode is not the same as 8-bit NVT mode, since binary mode turns off end-of-line processing. For this reason, the requirements on the high-order bit are stated as SHOULD, not MUST. RFC-854 defines a minimal set of properties of a "network virtual terminal" or NVT; this is not meant to preclude additional features in a real terminal. A Telnet connection is fully transparent to all 7-bit ASCII characters, including arbitrary ASCII control characters. Internet Engineering Task Force [Page 19]   RFC1123 REMOTE LOGIN -- TELNET October 1989 For example, a terminal might support full-screen commands coded as ASCII escape sequences; a Telnet implementation would pass these sequences as uninterpreted data. Thus, an NVT should not be conceived as a terminal type of a highly-restricted device. 3.2.6 Telnet Command Structure: RFC-854, p. 13 Since options may appear at any point in the data stream, a Telnet escape character (known as IAC, with the value 255) to be sent as data MUST be doubled. 3.2.7 Telnet Binary Option: RFC-856 When the Binary option has been successfully negotiated, arbitrary 8-bit characters are allowed. However, the data stream MUST still be scanned for IAC characters, any embedded Telnet commands MUST be obeyed, and data bytes equal to IAC MUST be doubled. Other character processing (e.g., replacing CR by CR NUL or by CR LF) MUST NOT be done. In particular, there is no end-of-line convention (see Section 3.3.1) in binary mode. DISCUSSION: The Binary option is normally negotiated in both directions, to change the Telnet connection from NVT mode to "binary mode". The sequence IAC EOR can be used to delimit blocks of data within a binary-mode Telnet stream. 3.2.8 Telnet Terminal-Type Option: RFC-1091 The Terminal-Type option MUST use the terminal type names officially defined in the Assigned Numbers RFC [INTRO:5], when they are available for the particular terminal. However, the receiver of a Terminal-Type option MUST accept any name. DISCUSSION: RFC-1091 [TELNET:10] updates an earlier version of the Terminal-Type option defined in RFC-930. The earlier version allowed a server host capable of supporting multiple terminal types to learn the type of a particular client's terminal, assuming that each physical terminal had an intrinsic type. However, today a "terminal" is often really a terminal emulator program running in a PC, perhaps capable of emulating a range of terminal types. Therefore, RFC-1091 extends the specification to allow a Internet Engineering Task Force [Page 20]   RFC1123 REMOTE LOGIN -- TELNET October 1989 more general terminal-type negotiation between User and Server Telnets. 3.3 SPECIFIC ISSUES 3.3.1 Telnet End-of-Line Convention The Telnet protocol defines the sequence CR LF to mean "end- of-line". For terminal input, this corresponds to a command- completion or "end-of-line" key being pressed on a user terminal; on an ASCII terminal, this is the CR key, but it may also be labelled "Return" or "Enter". When a Server Telnet receives the Telnet end-of-line sequence CR LF as input from a remote terminal, the effect MUST be the same as if the user had pressed the "end-of-line" key on a local terminal. On server hosts that use ASCII, in particular, receipt of the Telnet sequence CR LF must cause the same effect as a local user pressing the CR key on a local terminal. Thus, CR LF and CR NUL MUST have the same effect on an ASCII server host when received as input over a Telnet connection. A User Telnet MUST be able to send any of the forms: CR LF, CR NUL, and LF. A User Telnet on an ASCII host SHOULD have a user-controllable mode to send either CR LF or CR NUL when the user presses the "end-of-line" key, and CR LF SHOULD be the default. The Telnet end-of-line sequence CR LF MUST be used to send Telnet data that is not terminal-to-computer (e.g., for Server Telnet sending output, or the Telnet protocol incorporated another application protocol). DISCUSSION: To allow interoperability between arbitrary Telnet clients and servers, the Telnet protocol defined a standard representation for a line terminator. Since the ASCII character set includes no explicit end-of-line character, systems have chosen various representations, e.g., CR, LF, and the sequence CR LF. The Telnet protocol chose the CR LF sequence as the standard for network transmission. Unfortunately, the Telnet protocol specification in RFC- 854 [TELNET:1] has turned out to be somewhat ambiguous on what character(s) should be sent from client to server for the "end-of-line" key. The result has been a massive and continuing interoperability headache, made worse by various faulty implementations of both User and Server Internet Engineering Task Force [Page 21]   RFC1123 REMOTE LOGIN -- TELNET October 1989 Telnets. Although the Telnet protocol is based on a perfectly symmetric model, in a remote login session the role of the user at a terminal differs from the role of the server host. For example, RFC-854 defines the meaning of CR, LF, and CR LF as output from the server, but does not specify what the User Telnet should send when the user presses the "end-of-line" key on the terminal; this turns out to be the point at issue. When a user presses the "end-of-line" key, some User Telnet implementations send CR LF, while others send CR NUL (based on a different interpretation of the same sentence in RFC-854). These will be equivalent for a correctly-implemented ASCII server host, as discussed above. For other servers, a mode in the User Telnet is needed. The existence of User Telnets that send only CR NUL when CR is pressed creates a dilemma for non-ASCII hosts: they can either treat CR NUL as equivalent to CR LF in input, thus precluding the possibility of entering a "bare" CR, or else lose complete interworking. Suppose a user on host A uses Telnet to log into a server host B, and then execute B's User Telnet program to log into server host C. It is desirable for the Server/User Telnet combination on B to be as transparent as possible, i.e., to appear as if A were connected directly to C. In particular, correct implementation will make B transparent to Telnet end-of-line sequences, except that CR LF may be translated to CR NUL or vice versa. IMPLEMENTATION: To understand Telnet end-of-line issues, one must have at least a general model of the relationship of Telnet to the local operating system. The Server Telnet process is typically coupled into the terminal driver software of the operating system as a pseudo-terminal. A Telnet end-of- line sequence received by the Server Telnet must have the same effect as pressing the end-of-line key on a real locally-connected terminal. Operating systems that support interactive character-at- a-time applications (e.g., editors) typically have two internal modes for their terminal I/O: a formatted mode, in which local conventions for end-of-line and other Internet Engineering Task Force [Page 22]   RFC1123 REMOTE LOGIN -- TELNET October 1989 formatting rules have been applied to the data stream, and a "raw" mode, in which the application has direct access to every character as it was entered. A Server Telnet must be implemented in such a way that these modes have the same effect for remote as for local terminals. For example, suppose a CR LF or CR NUL is received by the Server Telnet on an ASCII host. In raw mode, a CR character is passed to the application; in formatted mode, the local system's end-of-line convention is used. 3.3.2 Data Entry Terminals DISCUSSION: In addition to the line-oriented and character-oriented ASCII terminals for which Telnet was designed, there are several families of video display terminals that are sometimes known as "data entry terminals" or DETs. The IBM 3270 family is a well-known example. Two Internet protocols have been designed to support generic DETs: SUPDUP [TELNET:16, TELNET:17], and the DET option [TELNET:18, TELNET:19]. The DET option drives a data entry terminal over a Telnet connection using (sub-) negotiation. SUPDUP is a completely separate terminal protocol, which can be entered from Telnet by negotiation. Although both SUPDUP and the DET option have been used successfully in particular environments, neither has gained general acceptance or wide implementation. A different approach to DET interaction has been developed for supporting the IBM 3270 family through Telnet, although the same approach would be applicable to any DET. The idea is to enter a "native DET" mode, in which the native DET input/output stream is sent as binary data. The Telnet EOR command is used to delimit logical records (e.g., "screens") within this binary stream. IMPLEMENTATION: The rules for entering and leaving native DET mode are as follows: o The Server uses the Terminal-Type option [TELNET:10] to learn that the client is a DET. o It is conventional, but not required, that both ends negotiate the EOR option [TELNET:9]. o Both ends negotiate the Binary option [TELNET:3] to Internet Engineering Task Force [Page 23]   RFC1123 REMOTE LOGIN -- TELNET October 1989 enter native DET mode. o When either end negotiates out of binary mode, the other end does too, and the mode then reverts to normal NVT. 3.3.3 Option Requirements Every Telnet implementation MUST support the Binary option [TELNET:3] and the Suppress Go Ahead option [TELNET:5], and SHOULD support the Echo [TELNET:4], Status [TELNET:6], End-of- Record [TELNET:9], and Extended Options List [TELNET:8] options. A User or Server Telnet SHOULD support the Window Size Option [TELNET:12] if the local operating system provides the corresponding capability. DISCUSSION: Note that the End-of-Record option only signifies that a Telnet can receive a Telnet EOR without crashing; therefore, every Telnet ought to be willing to accept negotiation of the End-of-Record option. See also the discussion in Section 3.2.3. 3.3.4 Option Initiation When the Telnet protocol is used in a client/server situation, the server SHOULD initiate negotiation of the terminal interaction mode it expects. DISCUSSION: The Telnet protocol was defined to be perfectly symmetrical, but its application is generally asymmetric. Remote login has been known to fail because NEITHER side initiated negotiation of the required non-default terminal modes. It is generally the server that determines the preferred mode, so the server needs to initiate the negotiation; since the negotiation is symmetric, the user can also initiate it. A client (User Telnet) SHOULD provide a means for users to enable and disable the initiation of option negotiation. DISCUSSION: A user sometimes needs to connect to an application service (e.g., FTP or SMTP) that uses Telnet for its Internet Engineering Task Force [Page 24]   RFC1123 REMOTE LOGIN -- TELNET October 1989 control stream but does not support Telnet options. User Telnet may be used for this purpose if initiation of option negotiation is disabled. 3.3.5 Telnet Linemode Option DISCUSSION: An important new Telnet option, LINEMODE [TELNET:12], has been proposed. The LINEMODE option provides a standard way for a User Telnet and a Server Telnet to agree that the client rather than the server will perform terminal character processing. When the client has prepared a complete line of text, it will send it to the server in (usually) one TCP packet. This option will greatly decrease the packet cost of Telnet sessions and will also give much better user response over congested or long- delay networks. The LINEMODE option allows dynamic switching between local and remote character processing. For example, the Telnet connection will automatically negotiate into single- character mode while a full screen editor is running, and then return to linemode when the editor is finished. We expect that when this RFC is released, hosts should implement the client side of this option, and may implement the server side of this option. To properly implement the server side, the server needs to be able to tell the local system not to do any input character processing, but to remember its current terminal state and notify the Server Telnet process whenever the state changes. This will allow password echoing and full screen editors to be handled properly, for example. 3.4 TELNET/USER INTERFACE 3.4.1 Character Set Transparency User Telnet implementations SHOULD be able to send or receive any 7-bit ASCII character. Where possible, any special character interpretations by the user host's operating system SHOULD be bypassed so that these characters can conveniently be sent and received on the connection. Some character value MUST be reserved as "escape to command mode"; conventionally, doubling this character allows it to be entered as data. The specific character used SHOULD be user selectable. Internet Engineering Task Force [Page 25]   RFC1123 REMOTE LOGIN -- TELNET October 1989 On binary-mode connections, a User Telnet program MAY provide an escape mechanism for entering arbitrary 8-bit values, if the host operating system doesn't allow them to be entered directly from the keyboard. IMPLEMENTATION: The transparency issues are less pressing on servers, but implementors should take care in dealing with issues like: masking off parity bits (sent by an older, non-conforming client) before they reach programs that expect only NVT ASCII, and properly handling programs that request 8-bit data streams. 3.4.2 Telnet Commands A User Telnet program MUST provide a user the capability of entering any of the Telnet control functions IP, AO, or AYT, and SHOULD provide the capability of entering EC, EL, and Break. 3.4.3 TCP Connection Errors A User Telnet program SHOULD report to the user any TCP errors that are reported by the transport layer (see "TCP/Application Layer Interface" section in [INTRO:1]). 3.4.4 Non-Default Telnet Contact Port A User Telnet program SHOULD allow the user to optionally specify a non-standard contact port number at the Server Telnet host. 3.4.5 Flushing Output A User Telnet program SHOULD provide the user the ability to specify whether or not output should be flushed when an IP is sent; see Section 3.2.4. For any output flushing scheme that causes the User Telnet to flush output locally until a Telnet signal is received from the Server, there SHOULD be a way for the user to manually restore normal output, in case the Server fails to send the expected signal. Internet Engineering Task Force [Page 26]   RFC1123 REMOTE LOGIN -- TELNET October 1989 3.5. TELNET 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 -------------------------------------------------|--------|-|-|-|-|-|-- | | | | | | | Option Negotiation |3.2.1 |x| | | | | Avoid negotiation loops |3.2.1 |x| | | | | Refuse unsupported options |3.2.1 |x| | | | | Negotiation OK anytime on connection |3.2.1 | |x| | | | Default to NVT |3.2.1 |x| | | | | Send official name in Term-Type option |3.2.8 |x| | | | | Accept any name in Term-Type option |3.2.8 |x| | | | | Implement Binary, Suppress-GA options |3.3.3 |x| | | | | Echo, Status, EOL, Ext-Opt-List options |3.3.3 | |x| | | | Implement Window-Size option if appropriate |3.3.3 | |x| | | | Server initiate mode negotiations |3.3.4 | |x| | | | User can enable/disable init negotiations |3.3.4 | |x| | | | | | | | | | | Go-Aheads | | | | | | | Non-GA server negotiate SUPPRESS-GA option |3.2.2 |x| | | | | User or Server accept SUPPRESS-GA option |3.2.2 |x| | | | | User Telnet ignore GA's |3.2.2 | | |x| | | | | | | | | | Control Functions | | | | | | | Support SE NOP DM IP AO AYT SB |3.2.3 |x| | | | | Support EOR EC EL Break |3.2.3 | | |x| | | Ignore unsupported control functions |3.2.3 |x| | | | | User, Server discard urgent data up to DM |3.2.4 |x| | | | | User Telnet send "Synch" after IP, AO, AYT |3.2.4 | |x| | | | Server Telnet reply Synch to IP |3.2.4 | | |x| | | Server Telnet reply Synch to AO |3.2.4 |x| | | | | User Telnet can flush output when send IP |3.2.4 | |x| | | | | | | | | | | Encoding | | | | | | | Send high-order bit in NVT mode |3.2.5 | | | |x| | Send high-order bit as parity bit |3.2.5 | | | | |x| Negot. BINARY if pass high-ord. bit to applic |3.2.5 | |x| | | | Always double IAC data byte |3.2.6 |x| | | | | Internet Engineering Task Force [Page 27]   RFC1123 REMOTE LOGIN -- TELNET October 1989 Double IAC data byte in binary mode |3.2.7 |x| | | | | Obey Telnet cmds in binary mode |3.2.7 |x| | | | | End-of-line, CR NUL in binary mode |3.2.7 | | | | |x| | | | | | | | End-of-Line | | | | | | | EOL at Server same as local end-of-line |3.3.1 |x| | | | | ASCII Server accept CR LF or CR NUL for EOL |3.3.1 |x| | | | | User Telnet able to send CR LF, CR NUL, or LF |3.3.1 |x| | | | | ASCII user able to select CR LF/CR NUL |3.3.1 | |x| | | | User Telnet default mode is CR LF |3.3.1 | |x| | | | Non-interactive uses CR LF for EOL |3.3.1 |x| | | | | | | | | | | | User Telnet interface | | | | | | | Input & output all 7-bit characters |3.4.1 | |x| | | | Bypass local op sys interpretation |3.4.1 | |x| | | | Escape character |3.4.1 |x| | | | | User-settable escape character |3.4.1 | |x| | | | Escape to enter 8-bit values |3.4.1 | | |x| | | Can input IP, AO, AYT |3.4.2 |x| | | | | Can input EC, EL, Break |3.4.2 | |x| | | | Report TCP connection errors to user |3.4.3 | |x| | | | Optional non-default contact port |3.4.4 | |x| | | | Can spec: output flushed when IP sent |3.4.5 | |x| | | | Can manually restore output mode |3.4.5 | |x| | | | | | | | | | | Internet Engineering Task Force [Page 28]   RFC1123 FILE TRANSFER -- FTP October 1989 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". Internet Engineering Task Force [Page 29]   RFC1123 FILE TRANSFER -- FTP October 1989 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 Internet Engineering Task Force [Page 30]   RFC1123 FILE TRANSFER -- FTP October 1989 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 Internet Engineering Task Force [Page 31]   RFC1123 FILE TRANSFER -- FTP October 1989 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 Internet Engineering Task Force [Page 32]   RFC1123 FILE TRANSFER -- FTP October 1989 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. Internet Engineering Task Force [Page 33]   RFC1123 FILE TRANSFER -- FTP October 1989 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, Internet Engineering Task Force [Page 34]   RFC1123 FILE TRANSFER -- FTP October 1989 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. Internet Engineering Task Force [Page 35]   RFC1123 FILE TRANSFER -- FTP October 1989 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 Internet Engineering Task Force [Page 36]   RFC1123 FILE TRANSFER -- FTP October 1989 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 Internet Engineering Task Force [Page 37]   RFC1123 FILE TRANSFER -- FTP October 1989 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 Internet Engineering Task Force [Page 38]   RFC1123 FILE TRANSFER -- FTP October 1989 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. Internet Engineering Task Force [Page 39]   RFC1123 FILE TRANSFER -- FTP October 1989 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. Internet Engineering Task Force [Page 40]   RFC1123 FILE TRANSFER -- FTP October 1989 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| | | | | Internet Engineering Task Force [Page 41]   RFC1123 FILE TRANSFER -- FTP October 1989 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| | | | | | | | | | | | Internet Engineering Task Force [Page 42]   RFC1123 FILE TRANSFER -- FTP October 1989 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. Internet Engineering Task Force [Page 43]   RFC1123 FILE TRANSFER -- TFTP October 1989 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