rfc1123 RFC 1123 Requirements for Internet Hosts - Application and Support
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
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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
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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
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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
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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
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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.
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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
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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
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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
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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:
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* "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.
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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.
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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
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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.
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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
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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
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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
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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
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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
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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
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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, 