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
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
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
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
1. Unless there is private agreement between particular resolver and
particular server.
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.
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
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
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
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:
* "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.
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.
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] and reiterated in RFC-1034 Section 3.5 [DNS:1]. One aspect of host name syntax is hereby changed: the
EID 1354 (Verified) is as follows:Section: 2.1
Original Text:
The syntax of a legal Internet host name was specified in RFC-952 [DNS:4].
Corrected Text:
The syntax of a legal Internet host name was specified in RFC-952
[DNS:4] and reiterated in RFC-1034 Section 3.5 [DNS:1].
Notes:
This essentially trivial editorial change makes it slightly easier for anyone (or any tool) that tracks changes and updates to host and domain naming rules to identify the applicability of this section.
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. However, a valid host name can never
EID 8148 (Verified) is as follows:Section: 2.1
Original Text:
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).
Corrected Text:
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.
Notes:
The text says "see Section 6.1.2.4", but section 6.1.2.4 is about compression and is not related to section 2.1 at all.
--VERIFIER NOTES-- Perhaps Section 6.1.3.5 was intended.
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
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.
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 63 characters |2.1 |x| | | | |
EID 558 (Verified) is as follows:Section: 2.5
Original Text:
Host names of up to 635 characters |2.1 |x| | | | |
Corrected Text:
Host names of up to 63 characters |2.1 |x| | | | |
Notes:
Section 2.1 says "Host software MUST handle host names of up to 63 characters" and doesn't mention the number 635 at all.
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| | | | |
| | | | | | |
| | | | | | |
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
EID 6475 (Verified) is as follows:Section: 3.2.1
Original Text:
option-negotiation loops. A host MUST refuse (i.e, reply
Corrected Text:
option-negotiation loops. A host MUST refuse (i.e., reply
Notes:
Missing full stop.
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
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
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
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.
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
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
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
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
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
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.
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.
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| | | | |
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| | | |
| | | | | | |
4. FILE TRANSFER
4.1 FILE TRANSFER PROTOCOL -- FTP
4.1.1 INTRODUCTION
The File Transfer Protocol FTP is the primary Internet standard
for file transfer. The current specification is contained in
RFC-959 [FTP:1].
FTP uses separate simultaneous TCP connections for control and
for data transfer. The FTP protocol includes many features,
some of which are not commonly implemented. However, for every
feature in FTP, there exists at least one implementation. The
minimum implementation defined in RFC-959 was too small, so a
somewhat larger minimum implementation is defined here.
Internet users have been unnecessarily burdened for years by
deficient FTP implementations. Protocol implementors have
suffered from the erroneous opinion that implementing FTP ought
to be a small and trivial task. This is wrong, because FTP has
a user interface, because it has to deal (correctly) with the
whole variety of communication and operating system errors that
may occur, and because it has to handle the great diversity of
real file systems in the world.
4.1.2. PROTOCOL WALK-THROUGH
4.1.2.1 LOCAL Type: RFC-959 Section 3.1.1.4
An FTP program MUST support TYPE I ("IMAGE" or binary type)
as well as TYPE L 8 ("LOCAL" type with logical byte size 8).
A machine whose memory is organized into m-bit words, where
m is not a multiple of 8, MAY also support TYPE L m.
DISCUSSION:
The command "TYPE L 8" is often required to transfer
binary data between a machine whose memory is organized
into (e.g.) 36-bit words and a machine with an 8-bit
byte organization. For an 8-bit byte machine, TYPE L 8
is equivalent to IMAGE.
"TYPE L m" is sometimes specified to the FTP programs
on two m-bit word machines to ensure the correct
transfer of a native-mode binary file from one machine
to the other. However, this command should have the
same effect on these machines as "TYPE I".
4.1.2.2 Telnet Format Control: RFC-959 Section 3.1.1.5.2
A host that makes no distinction between TYPE N and TYPE T
SHOULD implement TYPE T to be identical to TYPE N.
DISCUSSION:
This provision should ease interoperation with hosts
that do make this distinction.
Many hosts represent text files internally as strings
of ASCII characters, using the embedded ASCII format
effector characters (LF, BS, FF, ...) to control the
format when a file is printed. For such hosts, there
is no distinction between "print" files and other
files. However, systems that use record structured
files typically need a special format for printable
files (e.g., ASA carriage control). For the latter
hosts, FTP allows a choice of TYPE N or TYPE T.
4.1.2.3 Page Structure: RFC-959 Section 3.1.2.3 and Appendix I
Implementation of page structure is NOT RECOMMENDED in
general. However, if a host system does need to implement
FTP for "random access" or "holey" files, it MUST use the
defined page structure format rather than define a new
private FTP format.
4.1.2.4 Data Structure Transformations: RFC-959 Section 3.1.2
An FTP transformation between record-structure and file-
structure SHOULD be invertible, to the extent possible while
making the result useful on the target host.
DISCUSSION:
RFC-959 required strict invertibility between record-
structure and file-structure, but in practice,
efficiency and convenience often preclude it.
Therefore, the requirement is being relaxed. There are
two different objectives for transferring a file:
processing it on the target host, or just storage. For
storage, strict invertibility is important. For
processing, the file created on the target host needs
to be in the format expected by application programs on
that host.
As an example of the conflict, imagine a record-
oriented operating system that requires some data files
to have exactly 80 bytes in each record. While STORing
a file on such a host, an FTP Server must be able to
pad each line or record to 80 bytes; a later retrieval
of such a file cannot be strictly invertible.
4.1.2.5 Data Connection Management: RFC-959 Section 3.3
A User-FTP that uses STREAM mode SHOULD send a PORT command
to assign a non-default data port before each transfer
command is issued.
DISCUSSION:
This is required because of the long delay after a TCP
connection is closed until its socket pair can be
reused, to allow multiple transfers during a single FTP
session. Sending a port command can be avoided if a
transfer mode other than stream is used, by leaving the
data transfer connection open between transfers.
EID 5456 (Verified) is as follows:Section: 4.1.2.5
Original Text:
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.
Corrected Text:
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 be avoided if a
transfer mode other than stream is used, by leaving the
data transfer connection open between transfers.
Notes:
The verb is missing in the last sentence. "Sending a port command can avoided..."
4.1.2.6 PASV Command: RFC-959 Section 4.1.2
A server-FTP MUST implement the PASV command.
If multiple third-party transfers are to be executed during
the same session, a new PASV command MUST be issued before
each transfer command, to obtain a unique port pair.
IMPLEMENTATION:
The format of the 227 reply to a PASV command is not
well standardized. In particular, an FTP client cannot
assume that the parentheses shown on page 40 of RFC-959
will be present (and in fact, Figure 3 on page 43 omits
them). Therefore, a User-FTP program that interprets
the PASV reply must scan the reply for the first digit
of the host and port numbers.
Note that the host number h1,h2,h3,h4 is the IP address
of the server host that is sending the reply, and that
p1,p2 is a non-default data transfer port that PASV has
assigned.
4.1.2.7 LIST and NLST Commands: RFC-959 Section 4.1.3
The data returned by an NLST command MUST contain only a
simple list of legal pathnames, such that the server can use
them directly as the arguments of subsequent data transfer
commands for the individual files.
The data returned by a LIST or NLST command SHOULD use an
implied TYPE AN, unless the current type is EBCDIC, in which
case an implied TYPE EN SHOULD be used.
DISCUSSION:
Many FTP clients support macro-commands that will get
or put files matching a wildcard specification, using
NLST to obtain a list of pathnames. The expansion of
"multiple-put" is local to the client, but "multiple-
get" requires cooperation by the server.
The implied type for LIST and NLST is designed to
provide compatibility with existing User-FTPs, and in
particular with multiple-get commands.
4.1.2.8 SITE Command: RFC-959 Section 4.1.3
A Server-FTP SHOULD use the SITE command for non-standard
features, rather than invent new private commands or
unstandardized extensions to existing commands.
4.1.2.9 STOU Command: RFC-959 Section 4.1.3
The STOU command stores into a uniquely named file. When it
receives an STOU command, a Server-FTP MUST return the
actual file name in the "125 Transfer Starting" or the "150
Opening Data Connection" message that precedes the transfer
(the 250 reply code mentioned in RFC-959 is incorrect). The
exact format of these messages is hereby defined to be as
follows:
125 FILE: pppp
150 FILE: pppp
where pppp represents the unique pathname of the file that
will be written.
4.1.2.10 Telnet End-of-line Code: RFC-959, Page 34
Implementors MUST NOT assume any correspondence between READ
boundaries on the control connection and the Telnet EOL
sequences (CR LF).
DISCUSSION:
Thus, a server-FTP (or User-FTP) must continue reading
characters from the control connection until a complete
Telnet EOL sequence is encountered, before processing
the command (or response, respectively). Conversely, a
single READ from the control connection may include
more than one FTP command.
4.1.2.11 FTP Replies: RFC-959 Section 4.2, Page 35
A Server-FTP MUST send only correctly formatted replies on
the control connection. Note that RFC-959 (unlike earlier
versions of the FTP spec) contains no provision for a
"spontaneous" reply message.
A Server-FTP SHOULD use the reply codes defined in RFC-959
whenever they apply. However, a server-FTP MAY use a
different reply code when needed, as long as the general
rules of Section 4.2 are followed. When the implementor has
a choice between a 4xx and 5xx reply code, a Server-FTP
SHOULD send a 4xx (temporary failure) code when there is any
reasonable possibility that a failed FTP will succeed a few
hours later.
A User-FTP SHOULD generally use only the highest-order digit
of a 3-digit reply code for making a procedural decision, to
prevent difficulties when a Server-FTP uses non-standard
reply codes.
A User-FTP MUST be able to handle multi-line replies. If
the implementation imposes a limit on the number of lines
and if this limit is exceeded, the User-FTP MUST recover,
e.g., by ignoring the excess lines until the end of the
multi-line reply is reached.
A User-FTP SHOULD NOT interpret a 421 reply code ("Service
not available, closing control connection") specially, but
SHOULD detect closing of the control connection by the
server.
DISCUSSION:
Server implementations that fail to strictly follow the
reply rules often cause FTP user programs to hang.
Note that RFC-959 resolved ambiguities in the reply
rules found in earlier FTP specifications and must be
followed.
It is important to choose FTP reply codes that properly
distinguish between temporary and permanent failures,
to allow the successful use of file transfer client
daemons. These programs depend on the reply codes to
decide whether or not to retry a failed transfer; using
a permanent failure code (5xx) for a temporary error
will cause these programs to give up unnecessarily.
When the meaning of a reply matches exactly the text
shown in RFC-959, uniformity will be enhanced by using
the RFC-959 text verbatim. However, a Server-FTP
implementor is encouraged to choose reply text that
conveys specific system-dependent information, when
appropriate.
4.1.2.12 Connections: RFC-959 Section 5.2
The words "and the port used" in the second paragraph of
this section of RFC-959 are erroneous (historical), and they
should be ignored.
On a multihomed server host, the default data transfer port
(L-1) MUST be associated with the same local IP address as
the corresponding control connection to port L.
A user-FTP MUST NOT send any Telnet controls other than
SYNCH and IP on an FTP control connection. In particular, it
MUST NOT attempt to negotiate Telnet options on the control
connection. However, a server-FTP MUST be capable of
accepting and refusing Telnet negotiations (i.e., sending
DONT/WONT).
DISCUSSION:
Although the RFC says: "Server- and User- processes
should follow the conventions for the Telnet
protocol...[on the control connection]", it is not the
intent that Telnet option negotiation is to be
employed.
4.1.2.13 Minimum Implementation; RFC-959 Section 5.1
The following commands and options MUST be supported by
every server-FTP and user-FTP, except in cases where the
underlying file system or operating system does not allow or
support a particular command.
Type: ASCII Non-print, IMAGE, LOCAL 8
Mode: Stream
Structure: File, Record*
Commands:
USER, PASS, ACCT,
PORT, PASV,
TYPE, MODE, STRU,
RETR, STOR, APPE,
RNFR, RNTO, DELE,
CWD, CDUP, RMD, MKD, PWD,
LIST, NLST,
SYST, STAT,
HELP, NOOP, QUIT.
*Record structure is REQUIRED only for hosts whose file
systems support record structure.
DISCUSSION:
Vendors are encouraged to implement a larger subset of
the protocol. For example, there are important
robustness features in the protocol (e.g., Restart,
ABOR, block mode) that would be an aid to some Internet
users but are not widely implemented.
A host that does not have record structures in its file
system may still accept files with STRU R, recording
the byte stream literally.
4.1.3 SPECIFIC ISSUES
4.1.3.1 Non-standard Command Verbs
FTP allows "experimental" commands, whose names begin with
"X". If these commands are subsequently adopted as
standards, there may still be existing implementations using
the "X" form. At present, this is true for the directory
commands:
RFC-959 "Experimental"
MKD XMKD
RMD XRMD
PWD XPWD
CDUP XCUP
CWD XCWD
All FTP implementations SHOULD recognize both forms of these
commands, by simply equating them with extra entries in the
command lookup table.
IMPLEMENTATION:
A User-FTP can access a server that supports only the
"X" forms by implementing a mode switch, or
automatically using the following procedure: if the
RFC-959 form of one of the above commands is rejected
with a 500 or 502 response code, then try the
experimental form; any other response would be passed
to the user.
4.1.3.2 Idle Timeout
A Server-FTP process SHOULD have an idle timeout, which will
terminate the process and close the control connection if
the server is inactive (i.e., no command or data transfer in
progress) for a long period of time. The idle timeout time
SHOULD be configurable, and the default should be at least 5
minutes.
A client FTP process ("User-PI" in RFC-959) will need
timeouts on responses only if it is invoked from a program.
DISCUSSION:
Without a timeout, a Server-FTP process may be left
pending indefinitely if the corresponding client
crashes without closing the control connection.
4.1.3.3 Concurrency of Data and Control
DISCUSSION:
The intent of the designers of FTP was that a user
should be able to send a STAT command at any time while
data transfer was in progress and that the server-FTP
would reply immediately with status -- e.g., the number
of bytes transferred so far. Similarly, an ABOR
command should be possible at any time during a data
transfer.
Unfortunately, some small-machine operating systems
make such concurrent programming difficult, and some
other implementers seek minimal solutions, so some FTP
implementations do not allow concurrent use of the data
and control connections. Even such a minimal server
must be prepared to accept and defer a STAT or ABOR
command that arrives during data transfer.
4.1.3.4 FTP Restart Mechanism
The description of the 110 reply on pp. 40-41 of RFC-959 is
incorrect; the correct description is as follows. A restart
reply message, sent over the control connection from the
receiving FTP to the User-FTP, has the format:
110 MARK ssss = rrrr
Here:
* ssss is a text string that appeared in a Restart Marker
in the data stream and encodes a position in the
sender's file system;
* rrrr encodes the corresponding position in the
receiver's file system.
The encoding, which is specific to a particular file system
and network implementation, is always generated and
interpreted by the same system, either sender or receiver.
When an FTP that implements restart receives a Restart
Marker in the data stream, it SHOULD force the data to that
point to be written to stable storage before encoding the
corresponding position rrrr. An FTP sending Restart Markers
MUST NOT assume that 110 replies will be returned
synchronously with the data, i.e., it must not await a 110
reply before sending more data.
Two new reply codes are hereby defined for errors
encountered in restarting a transfer:
554 Requested action not taken: invalid REST parameter.
A 554 reply may result from a FTP service command that
follows a REST command. The reply indicates that the
existing file at the Server-FTP cannot be repositioned
as specified in the REST.
555 Requested action not taken: type or stru mismatch.
A 555 reply may result from an APPE command or from any
FTP service command following a REST command. The
reply indicates that there is some mismatch between the
current transfer parameters (type and stru) and the
attributes of the existing file.
DISCUSSION:
Note that the FTP Restart mechanism requires that Block
or Compressed mode be used for data transfer, to allow
the Restart Markers to be included within the data
stream. The frequency of Restart Markers can be low.
Restart Markers mark a place in the data stream, but
the receiver may be performing some transformation on
the data as it is stored into stable storage. In
general, the receiver's encoding must include any state
information necessary to restart this transformation at
any point of the FTP data stream. For example, in TYPE
A transfers, some receiver hosts transform CR LF
sequences into a single LF character on disk. If a
Restart Marker happens to fall between CR and LF, the
receiver must encode in rrrr that the transfer must be
restarted in a "CR has been seen and discarded" state.
Note that the Restart Marker is required to be encoded
as a string of printable ASCII characters, regardless
of the type of the data.
RFC-959 says that restart information is to be returned
"to the user". This should not be taken literally. In
general, the User-FTP should save the restart
information (ssss,rrrr) in stable storage, e.g., append
it to a restart control file. An empty restart control
file should be created when the transfer first starts
and deleted automatically when the transfer completes
successfully. It is suggested that this file have a
name derived in an easily-identifiable manner from the
name of the file being transferred and the remote host
name; this is analogous to the means used by many text
editors for naming "backup" files.
There are three cases for FTP restart.
(1) User-to-Server Transfer
The User-FTP puts Restart Markers <ssss> at
convenient places in the data stream. When the
Server-FTP receives a Marker, it writes all prior
data to disk, encodes its file system position and
transformation state as rrrr, and returns a "110
MARK ssss = rrrr" reply over the control
connection. The User-FTP appends the pair
(ssss,rrrr) to its restart control file.
To restart the transfer, the User-FTP fetches the
last (ssss,rrrr) pair from the restart control
file, repositions its local file system and
transformation state using ssss, and sends the
command "REST rrrr" to the Server-FTP.
(2) Server-to-User Transfer
The Server-FTP puts Restart Markers <ssss> at
convenient places in the data stream. When the
User-FTP receives a Marker, it writes all prior
data to disk, encodes its file system position and
transformation state as rrrr, and appends the pair
(rrrr,ssss) to its restart control file.
To restart the transfer, the User-FTP fetches the
last (rrrr,ssss) pair from the restart control
file, repositions its local file system and
transformation state using rrrr, and sends the
command "REST ssss" to the Server-FTP.
(3) Server-to-Server ("Third-Party") Transfer
The sending Server-FTP puts Restart Markers <ssss>
at convenient places in the data stream. When it
receives a Marker, the receiving Server-FTP writes
all prior data to disk, encodes its file system
position and transformation state as rrrr, and
sends a "110 MARK ssss = rrrr" reply over the
control connection to the User. The User-FTP
appends the pair (ssss,rrrr) to its restart
control file.
To restart the transfer, the User-FTP fetches the
last (ssss,rrrr) pair from the restart control
file, sends "REST ssss" to the sending Server-FTP,
and sends "REST rrrr" to the receiving Server-FTP.
4.1.4 FTP/USER INTERFACE
This section discusses the user interface for a User-FTP
program.
4.1.4.1 Pathname Specification
Since FTP is intended for use in a heterogeneous
environment, User-FTP implementations MUST support remote
pathnames as arbitrary character strings, so that their form
and content are not limited by the conventions of the local
operating system.
DISCUSSION:
In particular, remote pathnames can be of arbitrary
length, and all the printing ASCII characters as well
as space (0x20) must be allowed. RFC-959 allows a
pathname to contain any 7-bit ASCII character except CR
or LF.
4.1.4.2 "QUOTE" Command
A User-FTP program MUST implement a "QUOTE" command that
will pass an arbitrary character string to the server and
display all resulting response messages to the user.
To make the "QUOTE" command useful, a User-FTP SHOULD send
transfer control commands to the server as the user enters
them, rather than saving all the commands and sending them
to the server only when a data transfer is started.
DISCUSSION:
The "QUOTE" command is essential to allow the user to
access servers that require system-specific commands
(e.g., SITE or ALLO), or to invoke new or optional
features that are not implemented by the User-FTP. For
example, "QUOTE" may be used to specify "TYPE A T" to
send a print file to hosts that require the
distinction, even if the User-FTP does not recognize
that TYPE.
4.1.4.3 Displaying Replies to User
A User-FTP SHOULD display to the user the full text of all
error reply messages it receives. It SHOULD have a
"verbose" mode in which all commands it sends and the full
text and reply codes it receives are displayed, for
diagnosis of problems.
4.1.4.4 Maintaining Synchronization
The state machine in a User-FTP SHOULD be forgiving of
missing and unexpected reply messages, in order to maintain
command synchronization with the server.
4.1.5 FTP REQUIREMENTS SUMMARY
| | | | |S| |
| | | | |H| |F
| | | | |O|M|o
| | |S| |U|U|o
| | |H| |L|S|t
| |M|O| |D|T|n
| |U|U|M| | |o
| |S|L|A|N|N|t
| |T|D|Y|O|O|t
FEATURE |SECTION | | | |T|T|e
-------------------------------------------|---------------|-|-|-|-|-|--
Implement TYPE T if same as TYPE N |4.1.2.2 | |x| | | |
File/Record transform invertible if poss. |4.1.2.4 | |x| | | |
User-FTP send PORT cmd for stream mode |4.1.2.5 | |x| | | |
Server-FTP implement PASV |4.1.2.6 |x| | | | |
PASV is per-transfer |4.1.2.6 |x| | | | |
NLST reply usable in RETR cmds |4.1.2.7 |x| | | | |
Implied type for LIST and NLST |4.1.2.7 | |x| | | |
SITE cmd for non-standard features |4.1.2.8 | |x| | | |
STOU cmd return pathname as specified |4.1.2.9 |x| | | | |
Use TCP READ boundaries on control conn. |4.1.2.10 | | | | |x|
| | | | | | |
Server-FTP send only correct reply format |4.1.2.11 |x| | | | |
Server-FTP use defined reply code if poss. |4.1.2.11 | |x| | | |
New reply code following Section 4.2 |4.1.2.11 | | |x| | |
User-FTP use only high digit of reply |4.1.2.11 | |x| | | |
User-FTP handle multi-line reply lines |4.1.2.11 |x| | | | |
User-FTP handle 421 reply specially |4.1.2.11 | | | |x| |
| | | | | | |
Default data port same IP addr as ctl conn |4.1.2.12 |x| | | | |
User-FTP send Telnet cmds exc. SYNCH, IP |4.1.2.12 | | | | |x|
User-FTP negotiate Telnet options |4.1.2.12 | | | | |x|
Server-FTP handle Telnet options |4.1.2.12 |x| | | | |
Handle "Experimental" directory cmds |4.1.3.1 | |x| | | |
Idle timeout in server-FTP |4.1.3.2 | |x| | | |
Configurable idle timeout |4.1.3.2 | |x| | | |
Receiver checkpoint data at Restart Marker |4.1.3.4 | |x| | | |
Sender assume 110 replies are synchronous |4.1.3.4 | | | | |x|
| | | | | | |
Support TYPE: | | | | | | |
ASCII - Non-Print (AN) |4.1.2.13 |x| | | | |
ASCII - Telnet (AT) -- if same as AN |4.1.2.2 | |x| | | |
ASCII - Carriage Control (AC) |959 3.1.1.5.2 | | |x| | |
EBCDIC - (any form) |959 3.1.1.2 | | |x| | |
IMAGE |4.1.2.1 |x| | | | |
LOCAL 8 |4.1.2.1 |x| | | | |
LOCAL m |4.1.2.1 | | |x| | |2
| | | | | | |
Support MODE: | | | | | | |
Stream |4.1.2.13 |x| | | | |
Block |959 3.4.2 | | |x| | |
| | | | | | |
Support STRUCTURE: | | | | | | |
File |4.1.2.13 |x| | | | |
Record |4.1.2.13 |x| | | | |3
Page |4.1.2.3 | | | |x| |
| | | | | | |
Support commands: | | | | | | |
USER |4.1.2.13 |x| | | | |
PASS |4.1.2.13 |x| | | | |
ACCT |4.1.2.13 |x| | | | |
CWD |4.1.2.13 |x| | | | |
CDUP |4.1.2.13 |x| | | | |
SMNT |959 5.3.1 | | |x| | |
REIN |959 5.3.1 | | |x| | |
QUIT |4.1.2.13 |x| | | | |
| | | | | | |
PORT |4.1.2.13 |x| | | | |
PASV |4.1.2.6 |x| | | | |
TYPE |4.1.2.13 |x| | | | |1
STRU |4.1.2.13 |x| | | | |1
MODE |4.1.2.13 |x| | | | |1
| | | | | | |
RETR |4.1.2.13 |x| | | | |
STOR |4.1.2.13 |x| | | | |
STOU |959 5.3.1 | | |x| | |
APPE |4.1.2.13 |x| | | | |
ALLO |959 5.3.1 | | |x| | |
REST |959 5.3.1 | | |x| | |
RNFR |4.1.2.13 |x| | | | |
RNTO |4.1.2.13 |x| | | | |
ABOR |959 5.3.1 | | |x| | |
DELE |4.1.2.13 |x| | | | |
RMD |4.1.2.13 |x| | | | |
MKD |4.1.2.13 |x| | | | |
PWD |4.1.2.13 |x| | | | |
LIST |4.1.2.13 |x| | | | |
NLST |4.1.2.13 |x| | | | |
SITE |4.1.2.8 | | |x| | |
STAT |4.1.2.13 |x| | | | |
SYST |4.1.2.13 |x| | | | |
HELP |4.1.2.13 |x| | | | |
NOOP |4.1.2.13 |x| | | | |
| | | | | | |
User Interface: | | | | | | |
Arbitrary pathnames |4.1.4.1 |x| | | | |
Implement "QUOTE" command |4.1.4.2 |x| | | | |
Transfer control commands immediately |4.1.4.2 | |x| | | |
Display error messages to user |4.1.4.3 | |x| | | |
Verbose mode |4.1.4.3 | |x| | | |
Maintain synchronization with server |4.1.4.4 | |x| | | |
Footnotes:
(1) For the values shown earlier.
(2) Here m is number of bits in a memory word.
(3) Required for host with record-structured file system, optional
otherwise.
4.2 TRIVIAL FILE TRANSFER PROTOCOL -- TFTP
4.2.1 INTRODUCTION
The Trivial File Transfer Protocol TFTP is defined in RFC-783
[TFTP:1].
TFTP provides its own reliable delivery with UDP as its
transport protocol, using a simple stop-and-wait acknowledgment
system. Since TFTP has an effective window of only one 512
octet segment, it can provide good performance only over paths
that have a small delay*bandwidth product. The TFTP file
interface is very simple, providing no access control or
security.
TFTP's most important application is bootstrapping a host over
a local network, since it is simple and small enough to be
easily implemented in EPROM [BOOT:1, BOOT:2]. Vendors are
urged to support TFTP for booting.
4.2.2 PROTOCOL WALK-THROUGH
The TFTP specification [TFTP:1] is written in an open style,
and does not fully specify many parts of the protocol.
4.2.2.1 Transfer Modes: RFC-783, Page 3
The transfer mode "mail" SHOULD NOT be supported.
4.2.2.2 UDP Header: RFC-783, Page 17
The Length field of a UDP header is incorrectly defined; it
includes the UDP header length (8).
4.2.3 SPECIFIC ISSUES
4.2.3.1 Sorcerer's Apprentice Syndrome
There is a serious bug, known as the "Sorcerer's Apprentice
Syndrome," in the protocol specification. While it does not
cause incorrect operation of the transfer (the file will
always be transferred correctly if the transfer completes),
this bug may cause excessive retransmission, which may cause
the transfer to time out.
Implementations MUST contain the fix for this problem: the
sender (i.e., the side originating the DATA packets) must
never resend the current DATA packet on receipt of a
duplicate ACK.
DISCUSSION:
The bug is caused by the protocol rule that either
side, on receiving an old duplicate datagram, may
resend the current datagram. If a packet is delayed in
the network but later successfully delivered after
either side has timed out and retransmitted a packet, a
duplicate copy of the response may be generated. If
the other side responds to this duplicate with a
duplicate of its own, then every datagram will be sent
in duplicate for the remainder of the transfer (unless
a datagram is lost, breaking the repetition). Worse
yet, since the delay is often caused by congestion,
this duplicate transmission will usually causes more
congestion, leading to more delayed packets, etc.
The following example may help to clarify this problem.
TFTP A TFTP B
(1) Receive ACK X-1
Send DATA X
(2) Receive DATA X
Send ACK X
(ACK X is delayed in network,
and A times out):
(3) Retransmit DATA X
(4) Receive DATA X again
Send ACK X again
(5) Receive (delayed) ACK X
Send DATA X+1
(6) Receive DATA X+1
Send ACK X+1
(7) Receive ACK X again
Send DATA X+1 again
(8) Receive DATA X+1 again
Send ACK X+1 again
(9) Receive ACK X+1
Send DATA X+2
(10) Receive DATA X+2
Send ACK X+3
(11) Receive ACK X+1 again
Send DATA X+2 again
(12) Receive DATA X+2 again
Send ACK X+3 again
Notice that once the delayed ACK arrives, the protocol
settles down to duplicate all further packets
(sequences 5-8 and 9-12). The problem is caused not by
either side timing out, but by both sides
retransmitting the current packet when they receive a
duplicate.
The fix is to break the retransmission loop, as
indicated above. This is analogous to the behavior of
TCP. It is then possible to remove the retransmission
timer on the receiver, since the resent ACK will never
cause any action; this is a useful simplification where
TFTP is used in a bootstrap program. It is OK to allow
the timer to remain, and it may be helpful if the
retransmitted ACK replaces one that was genuinely lost
in the network. The sender still requires a retransmit
timer, of course.
4.2.3.2 Timeout Algorithms
A TFTP implementation MUST use an adaptive timeout.
IMPLEMENTATION:
TCP retransmission algorithms provide a useful base to
work from. At least an exponential backoff of
retransmission timeout is necessary.
4.2.3.3 Extensions
A variety of non-standard extensions have been made to TFTP,
including additional transfer modes and a secure operation
mode (with passwords). None of these have been
standardized.
4.2.3.4 Access Control
A server TFTP implementation SHOULD include some
configurable access control over what pathnames are allowed
in TFTP operations.
4.2.3.5 Broadcast Request
A TFTP request directed to a broadcast address SHOULD be
silently ignored.
DISCUSSION:
Due to the weak access control capability of TFTP,
directed broadcasts of TFTP requests to random networks
could create a significant security hole.
4.2.4 TFTP REQUIREMENTS SUMMARY
| | | | |S| |
| | | | |H| |F
| | | | |O|M|o
| | |S| |U|U|o
| | |H| |L|S|t
| |M|O| |D|T|n
| |U|U|M| | |o
| |S|L|A|N|N|t
| |T|D|Y|O|O|t
FEATURE |SECTION | | | |T|T|e
-------------------------------------------------|--------|-|-|-|-|-|--
Fix Sorcerer's Apprentice Syndrome |4.2.3.1 |x| | | | |
Transfer modes: | | | | | | |
netascii |RFC-783 |x| | | | |
octet |RFC-783 |x| | | | |
mail |4.2.2.1 | | | |x| |
extensions |4.2.3.3 | | |x| | |
Use adaptive timeout |4.2.3.2 |x| | | | |
Configurable access control |4.2.3.4 | |x| | | |
Silently ignore broadcast request |4.2.3.5 | |x| | | |
-------------------------------------------------|--------|-|-|-|-|-|--
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5. ELECTRONIC MAIL -- SMTP and RFC-822
5.1 INTRODUCTION
In the TCP/IP protocol suite, electronic mail in a format
specified in RFC-822 [SMTP:2] is transmitted using the Simple Mail
Transfer Protocol (SMTP) defined in RFC-821 [SMTP:1].
While SMTP has remained unchanged over the years, the Internet
community has made several changes in the way SMTP is used. In
particular, the conversion to the Domain Name System (DNS) has
caused changes in address formats and in mail routing. In this
section, we assume familiarity with the concepts and terminology
of the DNS, whose requirements are given in Section 6.1.
RFC-822 specifies the Internet standard format for electronic mail
messages. RFC-822 supercedes an older standard, RFC-733, that may
still be in use in a few places, although it is obsolete. The two
formats are sometimes referred to simply by number ("822" and
"733").
RFC-822 is used in some non-Internet mail environments with
different mail transfer protocols than SMTP, and SMTP has also
been adapted for use in some non-Internet environments. Note that
this document presents the rules for the use of SMTP and RFC-822
for the Internet environment only; other mail environments that
use these protocols may be expected to have their own rules.
5.2 PROTOCOL WALK-THROUGH
This section covers both RFC-821 and RFC-822.
The SMTP specification in RFC-821 is clear and contains numerous
examples, so implementors should not find it difficult to
understand. This section simply updates or annotates portions of
RFC-821 to conform with current usage.
RFC-822 is a long and dense document, defining a rich syntax.
Unfortunately, incomplete or defective implementations of RFC-822
are common. In fact, nearly all of the many formats of RFC-822
are actually used, so an implementation generally needs to
recognize and correctly interpret all of the RFC-822 syntax.
5.2.1 The SMTP Model: RFC-821 Section 2
DISCUSSION:
Mail is sent by a series of request/response transactions
between a client, the "sender-SMTP," and a server, the
"receiver-SMTP". These transactions pass (1) the message
proper, which is composed of header and body, and (2) SMTP
source and destination addresses, referred to as the
"envelope".
The SMTP programs are analogous to Message Transfer Agents
(MTAs) of X.400. There will be another level of protocol
software, closer to the end user, that is responsible for
composing and analyzing RFC-822 message headers; this
component is known as the "User Agent" in X.400, and we
use that term in this document. There is a clear logical
distinction between the User Agent and the SMTP
implementation, since they operate on different levels of
protocol. Note, however, that this distinction is may not
be exactly reflected the structure of typical
implementations of Internet mail. Often there is a
program known as the "mailer" that implements SMTP and
also some of the User Agent functions; the rest of the
User Agent functions are included in a user interface used
for entering and reading mail.
The SMTP envelope is constructed at the originating site,
typically by the User Agent when the message is first
queued for the Sender-SMTP program. The envelope
addresses may be derived from information in the message
header, supplied by the user interface (e.g., to implement
a bcc: request), or derived from local configuration
information (e.g., expansion of a mailing list). The SMTP
envelope cannot in general be re-derived from the header
at a later stage in message delivery, so the envelope is
transmitted separately from the message itself using the
MAIL and RCPT commands of SMTP.
The text of RFC-821 suggests that mail is to be delivered
to an individual user at a host. With the advent of the
domain system and of mail routing using mail-exchange (MX)
resource records, implementors should now think of
delivering mail to a user at a domain, which may or may
not be a particular host. This DOES NOT change the fact
that SMTP is a host-to-host mail exchange protocol.
5.2.2 Canonicalization: RFC-821 Section 3.1
The domain names that a Sender-SMTP sends in MAIL and RCPT
commands MUST have been "canonicalized," i.e., they must be
fully-qualified principal names or domain literals, not
nicknames or domain abbreviations. A canonicalized name either
identifies a host directly or is an MX name; it cannot be a
CNAME.
5.2.3 VRFY and EXPN Commands: RFC-821 Section 3.3
A receiver-SMTP MUST implement VRFY and SHOULD implement EXPN
(this requirement overrides RFC-821). However, there MAY be
configuration information to disable VRFY and EXPN in a
particular installation; this might even allow EXPN to be
disabled for selected lists.
A new reply code is defined for the VRFY command:
252 Cannot VRFY user (e.g., info is not local), but will
take message for this user and attempt delivery.
DISCUSSION:
SMTP users and administrators make regular use of these
commands for diagnosing mail delivery problems. With the
increasing use of multi-level mailing list expansion
(sometimes more than two levels), EXPN has been
increasingly important for diagnosing inadvertent mail
loops. On the other hand, some feel that EXPN represents
a significant privacy, and perhaps even a security,
exposure.
5.2.4 SEND, SOML, and SAML Commands: RFC-821 Section 3.4
An SMTP MAY implement the commands to send a message to a
user's terminal: SEND, SOML, and SAML.
DISCUSSION:
It has been suggested that the use of mail relaying
through an MX record is inconsistent with the intent of
SEND to deliver a message immediately and directly to a
user's terminal. However, an SMTP receiver that is unable
to write directly to the user terminal can return a "251
User Not Local" reply to the RCPT following a SEND, to
inform the originator of possibly deferred delivery.
5.2.5 HELO Command: RFC-821 Section 3.5
The sender-SMTP MUST ensure that the <domain> parameter in a
HELO command is a valid principal host domain name for the
client host. As a result, the receiver-SMTP will not have to
perform MX resolution on this name in order to validate the
HELO parameter.
The HELO receiver MAY verify that the HELO parameter really
corresponds to the IP address of the sender. However, the
receiver MUST NOT refuse to accept a message, even if the
sender's HELO command fails verification.
DISCUSSION:
Verifying the HELO parameter requires a domain name lookup
and may therefore take considerable time. An alternative
tool for tracking bogus mail sources is suggested below
(see "DATA Command").
Note also that the HELO argument is still required to have
valid <domain> syntax, since it will appear in a Received:
line; otherwise, a 501 error is to be sent.
IMPLEMENTATION:
When HELO parameter validation fails, a suggested
procedure is to insert a note about the unknown
authenticity of the sender into the message header (e.g.,
in the "Received:" line).
5.2.6 Mail Relay: RFC-821 Section 3.6
We distinguish three types of mail (store-and-) forwarding:
(1) A simple forwarder or "mail exchanger" forwards a message
using private knowledge about the recipient; see section
3.2 of RFC-821.
(2) An SMTP mail "relay" forwards a message within an SMTP
mail environment as the result of an explicit source route
(as defined in section 3.6 of RFC-821). The SMTP relay
function uses the "@...:" form of source route from RFC-
822 (see Section 5.2.19 below).
(3) A mail "gateway" passes a message between different
environments. The rules for mail gateways are discussed
below in Section 5.3.7.
An Internet host that is forwarding a message but is not a
gateway to a different mail environment (i.e., it falls under
(1) or (2)) SHOULD NOT alter any existing header fields,
although the host will add an appropriate Received: line as
required in Section 5.2.8.
A Sender-SMTP SHOULD NOT send a RCPT TO: command containing an
explicit source route using the "@...:" address form. Thus,
the relay function defined in section 3.6 of RFC-821 should
not be used.
DISCUSSION:
The intent is to discourage all source routing and to
abolish explicit source routing for mail delivery within
the Internet environment. Source-routing is unnecessary;
the simple target address "user@domain" should always
suffice. This is the result of an explicit architectural
decision to use universal naming rather than source
routing for mail. Thus, SMTP provides end-to-end
connectivity, and the DNS provides globally-unique,
location-independent names. MX records handle the major
case where source routing might otherwise be needed.
A receiver-SMTP MUST accept the explicit source route syntax in
the envelope, but it MAY implement the relay function as
defined in section 3.6 of RFC-821. If it does not implement
the relay function, it SHOULD attempt to deliver the message
directly to the host to the right of the right-most "@" sign.
DISCUSSION:
For example, suppose a host that does not implement the
relay function receives a message with the SMTP command:
"RCPT TO:<@ALPHA,@BETA:joe@GAMMA>", where ALPHA, BETA, and
GAMMA represent domain names. Rather than immediately
refusing the message with a 550 error reply as suggested
on page 20 of RFC-821, the host should try to forward the
message to GAMMA directly, using: "RCPT TO:<joe@GAMMA>".
Since this host does not support relaying, it is not
required to update the reverse path.
Some have suggested that source routing may be needed
occasionally for manually routing mail around failures;
however, the reality and importance of this need is
controversial. The use of explicit SMTP mail relaying for
this purpose is discouraged, and in fact it may not be
successful, as many host systems do not support it. Some
have used the "%-hack" (see Section 5.2.16) for this
purpose.
5.2.7 RCPT Command: RFC-821 Section 4.1.1
A host that supports a receiver-SMTP MUST support the reserved
mailbox "Postmaster".
The receiver-SMTP MAY verify RCPT parameters as they arrive;
however, RCPT responses MUST NOT be delayed beyond a reasonable
time (see Section 5.3.2).
Therefore, a "250 OK" response to a RCPT does not necessarily
imply that the delivery address(es) are valid. Errors found
after message acceptance will be reported by mailing a
notification message to an appropriate address (see Section
5.3.3).
DISCUSSION:
The set of conditions under which a RCPT parameter can be
validated immediately is an engineering design choice.
Reporting destination mailbox errors to the Sender-SMTP
before mail is transferred is generally desirable to save
time and network bandwidth, but this advantage is lost if
RCPT verification is lengthy.
For example, the receiver can verify immediately any
simple local reference, such as a single locally-
registered mailbox. On the other hand, the "reasonable
time" limitation generally implies deferring verification
of a mailing list until after the message has been
transferred and accepted, since verifying a large mailing
list can take a very long time. An implementation might
or might not choose to defer validation of addresses that
are non-local and therefore require a DNS lookup. If a
DNS lookup is performed but a soft domain system error
(e.g., timeout) occurs, validity must be assumed.
5.2.8 DATA Command: RFC-821 Section 4.1.1
Every receiver-SMTP (not just one that "accepts a message for
relaying or for final delivery" [SMTP:1]) MUST insert a
"Received:" line at the beginning of a message. In this line,
called a "time stamp line" in RFC-821:
* The FROM field SHOULD contain both (1) the name of the
source host as presented in the HELO command and (2) a
domain literal containing the IP address of the source,
determined from the TCP connection.
* The ID field MAY contain an "@" as suggested in RFC-822,
but this is not required.
* The FOR field MAY contain a list of <path> entries when
multiple RCPT commands have been given.
An Internet mail program MUST NOT change a Received: line that
was previously added to the message header.
DISCUSSION:
Including both the source host and the IP source address
in the Received: line may provide enough information for
tracking illicit mail sources and eliminate a need to
explicitly verify the HELO parameter.
Received: lines are primarily intended for humans tracing
mail routes, primarily of diagnosis of faults. See also
the discussion under 5.3.7.
When the receiver-SMTP makes "final delivery" of a message,
then it MUST pass the MAIL FROM: address from the SMTP envelope
with the message, for use if an error notification message must
be sent later (see Section 5.3.3). There is an analogous
requirement when gatewaying from the Internet into a different
mail environment; see Section 5.3.7.
DISCUSSION:
Note that the final reply to the DATA command depends only
upon the successful transfer and storage of the message.
Any problem with the destination address(es) must either
(1) have been reported in an SMTP error reply to the RCPT
command(s), or (2) be reported in a later error message
mailed to the originator.
IMPLEMENTATION:
The MAIL FROM: information may be passed as a parameter or
in a Return-Path: line inserted at the beginning of the
message.
5.2.9 Command Syntax: RFC-821 Section 4.1.2
The syntax shown in RFC-821 for the MAIL FROM: command omits
the case of an empty path: "MAIL FROM: <>" (see RFC-821 Page
15). An empty reverse path MUST be supported.
5.2.10 SMTP Replies: RFC-821 Section 4.2
A receiver-SMTP SHOULD send only the reply codes listed in
section 4.2.2 of RFC-821 or in this document. A receiver-SMTP
SHOULD use the text shown in examples in RFC-821 whenever
appropriate.
A sender-SMTP MUST determine its actions only by the reply
code, not by the text (except for 251 and 551 replies); any
text, including no text at all, must be acceptable. The space
(blank) following the reply code is considered part of the
text. Whenever possible, a sender-SMTP SHOULD test only the
first digit of the reply code, as specified in Appendix E of
RFC-821.
DISCUSSION:
Interoperability problems have arisen with SMTP systems
using reply codes that are not listed explicitly in RFC-
821 Section 4.3 but are legal according to the theory of
reply codes explained in Appendix E.
5.2.11 Transparency: RFC-821 Section 4.5.2
Implementors MUST be sure that their mail systems always add
and delete periods to ensure message transparency.
5.2.12 WKS Use in MX Processing: RFC-974, p. 5
RFC-974 [SMTP:3] recommended that the domain system be queried
for WKS ("Well-Known Service") records, to verify that each
proposed mail target does support SMTP. Later experience has
shown that WKS is not widely supported, so the WKS step in MX
processing SHOULD NOT be used.
The following are notes on RFC-822, organized by section of that
document.
5.2.13 RFC-822 Message Specification: RFC-822 Section 4
The syntax shown for the Return-path line omits the possibility
of a null return path, which is used to prevent looping of
error notifications (see Section 5.3.3). The complete syntax
is:
return = "Return-path" ":" route-addr
/ "Return-path" ":" "<" ">"
The set of optional header fields is hereby expanded to include
the Content-Type field defined in RFC-1049 [SMTP:7]. This
field "allows mail reading systems to automatically identify
the type of a structured message body and to process it for
display accordingly". [SMTP:7] A User Agent MAY support this
field.
5.2.14 RFC-822 Date and Time Specification: RFC-822 Section 5
The syntax for the date is hereby changed to:
date = 1*2DIGIT month 2*4DIGIT
All mail software SHOULD use 4-digit years in dates, to ease
the transition to the next century.
There is a strong trend towards the use of numeric timezone
indicators, and implementations SHOULD use numeric timezones
instead of timezone names. However, all implementations MUST
accept either notation. If timezone names are used, they MUST
be exactly as defined in RFC-822.
The military time zones are specified incorrectly in RFC-822:
they count the wrong way from UT (the signs are reversed). As
a result, military time zones in RFC-822 headers carry no
information.
Finally, note that there is a typo in the definition of "zone"
in the syntax summary of appendix D; the correct definition
occurs in Section 3 of RFC-822.
5.2.15 RFC-822 Syntax Change: RFC-822 Section 6.1
The syntactic definition of "mailbox" in RFC-822 is hereby
changed to:
mailbox = addr-spec ; simple address
/ [phrase] route-addr ; name & addr-spec
That is, the phrase preceding a route address is now OPTIONAL.
This change makes the following header field legal, for
example:
From: <craig@nnsc.nsf.net>
5.2.16 RFC-822 Local-part: RFC-822 Section 6.2
The basic mailbox address specification has the form: "local-
part@domain". Here "local-part", sometimes called the "left-
hand side" of the address, is domain-dependent.
A host that is forwarding the message but is not the
destination host implied by the right-hand side "domain" MUST
NOT interpret or modify the "local-part" of the address.
When mail is to be gatewayed from the Internet mail environment
into a foreign mail environment (see Section 5.3.7), routing
information for that foreign environment MAY be embedded within
the "local-part" of the address. The gateway will then
interpret this local part appropriately for the foreign mail
environment.
DISCUSSION:
Although source routes are discouraged within the Internet
(see Section 5.2.6), there are non-Internet mail
environments whose delivery mechanisms do depend upon
source routes. Source routes for extra-Internet
environments can generally be buried in the "local-part"
of the address (see Section 5.2.16) while mail traverses
the Internet. When the mail reaches the appropriate
Internet mail gateway, the gateway will interpret the
local-part and build the necessary address or route for
the target mail environment.
For example, an Internet host might send mail to:
"a!b!c!user@gateway-domain". The complex local part
"a!b!c!user" would be uninterpreted within the Internet
domain, but could be parsed and understood by the
specified mail gateway.
An embedded source route is sometimes encoded in the
"local-part" using "%" as a right-binding routing
operator. For example, in:
user%domain%relay3%relay2@relay1
the "%" convention implies that the mail is to be routed
from "relay1" through "relay2", "relay3", and finally to
"user" at "domain". This is commonly known as the "%-
hack". It is suggested that "%" have lower precedence
than any other routing operator (e.g., "!") hidden in the
local-part; for example, "a!b%c" would be interpreted as
"(a!b)%c".
Only the target host (in this case, "relay1") is permitted
to analyze the local-part "user%domain%relay3%relay2".
5.2.17 Domain Literals: RFC-822 Section 6.2.3
A mailer MUST be able to accept and parse an Internet domain
literal whose content ("dtext"; see RFC-822) is a dotted-
decimal host address. This satisfies the requirement of
Section 2.1 for the case of mail.
An SMTP MUST accept and recognize a domain literal for any of
its own IP addresses.
5.2.18 Common Address Formatting Errors: RFC-822 Section 6.1
Errors in formatting or parsing 822 addresses are unfortunately
common. This section mentions only the most common errors. A
User Agent MUST accept all valid RFC-822 address formats, and
MUST NOT generate illegal address syntax.
o A common error is to leave out the semicolon after a group
identifier.
o Some systems fail to fully-qualify domain names in
messages they generate. The right-hand side of an "@"
sign in a header address field MUST be a fully-qualified
domain name.
For example, some systems fail to fully-qualify the From:
address; this prevents a "reply" command in the user
interface from automatically constructing a return
address.
DISCUSSION:
Although RFC-822 allows the local use of abbreviated
domain names within a domain, the application of
RFC-822 in Internet mail does not allow this. The
intent is that an Internet host must not send an SMTP
message header containing an abbreviated domain name
in an address field. This allows the address fields
of the header to be passed without alteration across
the Internet, as required in Section 5.2.6.
o Some systems mis-parse multiple-hop explicit source routes
such as:
@relay1,@relay2,@relay3:user@domain.
o Some systems over-qualify domain names by adding a
trailing dot to some or all domain names in addresses or
message-ids. This violates RFC-822 syntax.
5.2.19 Explicit Source Routes: RFC-822 Section 6.2.7
Internet host software SHOULD NOT create an RFC-822 header
containing an address with an explicit source route, but MUST
accept such headers for compatibility with earlier systems.
DISCUSSION:
In an understatement, RFC-822 says "The use of explicit
source routing is discouraged". Many hosts implemented
RFC-822 source routes incorrectly, so the syntax cannot be
used unambiguously in practice. Many users feel the
syntax is ugly. Explicit source routes are not needed in
the mail envelope for delivery; see Section 5.2.6. For
all these reasons, explicit source routes using the RFC-
822 notations are not to be used in Internet mail headers.
As stated in Section 5.2.16, it is necessary to allow an
explicit source route to be buried in the local-part of an
address, e.g., using the "%-hack", in order to allow mail
to be gatewayed into another environment in which explicit
source routing is necessary. The vigilant will observe
that there is no way for a User Agent to detect and
prevent the use of such implicit source routing when the
destination is within the Internet. We can only
discourage source routing of any kind within the Internet,
as unnecessary and undesirable.
5.3 SPECIFIC ISSUES
5.3.1 SMTP Queueing Strategies
The common structure of a host SMTP implementation includes
user mailboxes, one or more areas for queueing messages in
transit, and one or more daemon processes for sending and
receiving mail. The exact structure will vary depending on the
needs of the users on the host and the number and size of
mailing lists supported by the host. We describe several
optimizations that have proved helpful, particularly for
mailers supporting high traffic levels.
Any queueing strategy MUST include:
o Timeouts on all activities. See Section 5.3.2.
o Never sending error messages in response to error
messages.
5.3.1.1 Sending Strategy
The general model of a sender-SMTP is one or more processes
that periodically attempt to transmit outgoing mail. In a
typical system, the program that composes a message has some
method for requesting immediate attention for a new piece of
outgoing mail, while mail that cannot be transmitted
immediately MUST be queued and periodically retried by the
sender. A mail queue entry will include not only the
message itself but also the envelope information.
The sender MUST delay retrying a particular destination
after one attempt has failed. In general, the retry
interval SHOULD be at least 30 minutes; however, more
sophisticated and variable strategies will be beneficial
when the sender-SMTP can determine the reason for non-
delivery.
Retries continue until the message is transmitted or the
sender gives up; the give-up time generally needs to be at
least 4-5 days. The parameters to the retry algorithm MUST
be configurable.
A sender SHOULD keep a list of hosts it cannot reach and
corresponding timeouts, rather than just retrying queued
mail items.
DISCUSSION:
Experience suggests that failures are typically
transient (the target system has crashed), favoring a
policy of two connection attempts in the first hour the
message is in the queue, and then backing off to once
every two or three hours.
The sender-SMTP can shorten the queueing delay by
cooperation with the receiver-SMTP. In particular, if
mail is received from a particular address, it is good
evidence that any mail queued for that host can now be
sent.
The strategy may be further modified as a result of
multiple addresses per host (see Section 5.3.4), to
optimize delivery time vs. resource usage.
A sender-SMTP may have a large queue of messages for
each unavailable destination host, and if it retried
all these messages in every retry cycle, there would be
excessive Internet overhead and the daemon would be
blocked for a long period. Note that an SMTP can
generally determine that a delivery attempt has failed
only after a timeout of a minute or more; a one minute
timeout per connection will result in a very large
delay if it is repeated for dozens or even hundreds of
queued messages.
When the same message is to be delivered to several users on
the same host, only one copy of the message SHOULD be
transmitted. That is, the sender-SMTP should use the
command sequence: RCPT, RCPT,... RCPT, DATA instead of the
sequence: RCPT, DATA, RCPT, DATA,... RCPT, DATA.
Implementation of this efficiency feature is strongly urged.
Similarly, the sender-SMTP MAY support multiple concurrent
outgoing mail transactions to achieve timely delivery.
However, some limit SHOULD be imposed to protect the host
from devoting all its resources to mail.
The use of the different addresses of a multihomed host is
discussed below.
5.3.1.2 Receiving strategy
The receiver-SMTP SHOULD attempt to keep a pending listen on
the SMTP port at all times. This will require the support
of multiple incoming TCP connections for SMTP. Some limit
MAY be imposed.
IMPLEMENTATION:
When the receiver-SMTP receives mail from a particular
host address, it could notify the sender-SMTP to retry
any mail pending for that host address.
5.3.2 Timeouts in SMTP
There are two approaches to timeouts in the sender-SMTP: (a)
limit the time for each SMTP command separately, or (b) limit
the time for the entire SMTP dialogue for a single mail
message. A sender-SMTP SHOULD use option (a), per-command
timeouts. Timeouts SHOULD be easily reconfigurable, preferably
without recompiling the SMTP code.
DISCUSSION:
Timeouts are an essential feature of an SMTP
implementation. If the timeouts are too long (or worse,
there are no timeouts), Internet communication failures or
software bugs in receiver-SMTP programs can tie up SMTP
processes indefinitely. If the timeouts are too short,
resources will be wasted with attempts that time out part
way through message delivery.
If option (b) is used, the timeout has to be very large,
e.g., an hour, to allow time to expand very large mailing
lists. The timeout may also need to increase linearly
with the size of the message, to account for the time to
transmit a very large message. A large fixed timeout
leads to two problems: a failure can still tie up the
sender for a very long time, and very large messages may
still spuriously time out (which is a wasteful failure!).
Using the recommended option (a), a timer is set for each
SMTP command and for each buffer of the data transfer.
The latter means that the overall timeout is inherently
proportional to the size of the message.
Based on extensive experience with busy mail-relay hosts, the
minimum per-command timeout values SHOULD be as follows:
o Initial 220 Message: 5 minutes
A Sender-SMTP process needs to distinguish between a
failed TCP connection and a delay in receiving the initial
220 greeting message. Many receiver-SMTPs will accept a
TCP connection but delay delivery of the 220 message until
their system load will permit more mail to be processed.
o MAIL Command: 5 minutes
o RCPT Command: 5 minutes
A longer timeout would be required if processing of
mailing lists and aliases were not deferred until after
the message was accepted.
o DATA Initiation: 2 minutes
This is while awaiting the "354 Start Input" reply to a
DATA command.
o Data Block: 3 minutes
This is while awaiting the completion of each TCP SEND
call transmitting a chunk of data.
o DATA Termination: 10 minutes.
This is while awaiting the "250 OK" reply. When the
receiver gets the final period terminating the message
data, it typically performs processing to deliver the
message to a user mailbox. A spurious timeout at this
point would be very wasteful, since the message has been
successfully sent.
A receiver-SMTP SHOULD have a timeout of at least 5 minutes
while it is awaiting the next command from the sender.
5.3.3 Reliable Mail Receipt
When the receiver-SMTP accepts a piece of mail (by sending a
"250 OK" message in response to DATA), it is accepting
responsibility for delivering or relaying the message. It must
take this responsibility seriously, i.e., it MUST NOT lose the
message for frivolous reasons, e.g., because the host later
crashes or because of a predictable resource shortage.
If there is a delivery failure after acceptance of a message,
the receiver-SMTP MUST formulate and mail a notification
message. This notification MUST be sent using a null ("<>")
reverse path in the envelope; see Section 3.6 of RFC-821. The
recipient of this notification SHOULD be the address from the
envelope return path (or the Return-Path: line). However, if
this address is null ("<>"), the receiver-SMTP MUST NOT send a
notification. If the address is an explicit source route, it
SHOULD be stripped down to its final hop.
DISCUSSION:
For example, suppose that an error notification must be
sent for a message that arrived with:
"MAIL FROM:<@a,@b:user@d>". The notification message
should be sent to: "RCPT TO:<user@d>".
Some delivery failures after the message is accepted by
SMTP will be unavoidable. For example, it may be
impossible for the receiver-SMTP to validate all the
delivery addresses in RCPT command(s) due to a "soft"
domain system error or because the target is a mailing
list (see earlier discussion of RCPT).
To avoid receiving duplicate messages as the result of
timeouts, a receiver-SMTP MUST seek to minimize the time
required to respond to the final "." that ends a message
transfer. See RFC-1047 [SMTP:4] for a discussion of this
problem.
5.3.4 Reliable Mail Transmission
To transmit a message, a sender-SMTP determines the IP address
of the target host from the destination address in the
envelope. Specifically, it maps the string to the right of the
"@" sign into an IP address. This mapping or the transfer
itself may fail with a soft error, in which case the sender-
SMTP will requeue the outgoing mail for a later retry, as
required in Section 5.3.1.1.
When it succeeds, the mapping can result in a list of
alternative delivery addresses rather than a single address,
because of (a) multiple MX records, (b) multihoming, or both.
To provide reliable mail transmission, the sender-SMTP MUST be
able to try (and retry) each of the addresses in this list in
order, until a delivery attempt succeeds. However, there MAY
also be a configurable limit on the number of alternate
addresses that can be tried. In any case, a host SHOULD try at
least two addresses.
The following information is to be used to rank the host
addresses:
(1) Multiple MX Records -- these contain a preference
indication that should be used in sorting. If there are
multiple destinations with the same preference and there
is no clear reason to favor one (e.g., by address
preference), then the sender-SMTP SHOULD pick one at
random to spread the load across multiple mail exchanges
for a specific organization; note that this is a
refinement of the procedure in [DNS:3].
(2) Multihomed host -- The destination host (perhaps taken
from the preferred MX record) may be multihomed, in which
case the domain name resolver will return a list of
alternative IP addresses. It is the responsibility of the
domain name resolver interface (see Section 6.1.3.4 below)
to have ordered this list by decreasing preference, and
SMTP MUST try them in the order presented.
DISCUSSION:
Although the capability to try multiple alternative
addresses is required, there may be circumstances where
specific installations want to limit or disable the use of
alternative addresses. The question of whether a sender
should attempt retries using the different addresses of a
multihomed host has been controversial. The main argument
for using the multiple addresses is that it maximizes the
probability of timely delivery, and indeed sometimes the
probability of any delivery; the counter argument is that
it may result in unnecessary resource use.
Note that resource use is also strongly determined by the
sending strategy discussed in Section 5.3.1.
5.3.5 Domain Name Support
SMTP implementations MUST use the mechanism defined in Section
6.1 for mapping between domain names and IP addresses. This
means that every Internet SMTP MUST include support for the
Internet DNS.
In particular, a sender-SMTP MUST support the MX record scheme
[SMTP:3]. See also Section 7.4 of [DNS:2] for information on
domain name support for SMTP.
5.3.6 Mailing Lists and Aliases
An SMTP-capable host SHOULD support both the alias and the list
form of address expansion for multiple delivery. When a
message is delivered or forwarded to each address of an
expanded list form, the return address in the envelope
("MAIL FROM:") MUST be changed to be the address of a person
who administers the list, but the message header MUST be left
unchanged; in particular, the "From" field of the message is
unaffected.
DISCUSSION:
An important mail facility is a mechanism for multi-
destination delivery of a single message, by transforming
or "expanding" a pseudo-mailbox address into a list of
destination mailbox addresses. When a message is sent to
such a pseudo-mailbox (sometimes called an "exploder"),
copies are forwarded or redistributed to each mailbox in
the expanded list. We classify such a pseudo-mailbox as
an "alias" or a "list", depending upon the expansion
rules:
(a) Alias
To expand an alias, the recipient mailer simply
replaces the pseudo-mailbox address in the envelope
with each of the expanded addresses in turn; the rest
of the envelope and the message body are left
unchanged. The message is then delivered or
forwarded to each expanded address.
(b) List
A mailing list may be said to operate by
"redistribution" rather than by "forwarding". To
expand a list, the recipient mailer replaces the
pseudo-mailbox address in the envelope with each of
the expanded addresses in turn. The return address in
the envelope is changed so that all error messages
generated by the final deliveries will be returned to
a list administrator, not to the message originator,
who generally has no control over the contents of the
list and will typically find error messages annoying.
5.3.7 Mail Gatewaying
Gatewaying mail between different mail environments, i.e.,
different mail formats and protocols, is complex and does not
easily yield to standardization. See for example [SMTP:5a],
[SMTP:5b]. However, some general requirements may be given for
a gateway between the Internet and another mail environment.
(A) Header fields MAY be rewritten when necessary as messages
are gatewayed across mail environment boundaries.
DISCUSSION:
This may involve interpreting the local-part of the
destination address, as suggested in Section 5.2.16.
The other mail systems gatewayed to the Internet
generally use a subset of RFC-822 headers, but some
of them do not have an equivalent to the SMTP
envelope. Therefore, when a message leaves the
Internet environment, it may be necessary to fold the
SMTP envelope information into the message header. A
possible solution would be to create new header
fields to carry the envelope information (e.g., "X-
SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this would
require changes in mail programs in the foreign
environment.
(B) When forwarding a message into or out of the Internet
environment, a gateway MUST prepend a Received: line, but
it MUST NOT alter in any way a Received: line that is
already in the header.
DISCUSSION:
This requirement is a subset of the general
"Received:" line requirement of Section 5.2.8; it is
restated here for emphasis.
Received: fields of messages originating from other
environments may not conform exactly to RFC822.
However, the most important use of Received: lines is
for debugging mail faults, and this debugging can be
severely hampered by well-meaning gateways that try
to "fix" a Received: line.
The gateway is strongly encouraged to indicate the
environment and protocol in the "via" clauses of
Received field(s) that it supplies.
(C) From the Internet side, the gateway SHOULD accept all
valid address formats in SMTP commands and in RFC-822
headers, and all valid RFC-822 messages. Although a
gateway must accept an RFC-822 explicit source route
("@...:" format) in either the RFC-822 header or in the
envelope, it MAY or may not act on the source route; see
Sections 5.2.6 and 5.2.19.
DISCUSSION:
It is often tempting to restrict the range of
addresses accepted at the mail gateway to simplify
the translation into addresses for the remote
environment. This practice is based on the
assumption that mail users have control over the
addresses their mailers send to the mail gateway. In
practice, however, users have little control over the
addresses that are finally sent; their mailers are
free to change addresses into any legal RFC-822
format.
(D) The gateway MUST ensure that all header fields of a
message that it forwards into the Internet meet the
requirements for Internet mail. In particular, all
addresses in "From:", "To:", "Cc:", etc., fields must be
transformed (if necessary) to satisfy RFC-822 syntax, and
they must be effective and useful for sending replies.
(E) The translation algorithm used to convert mail from the
Internet protocols to another environment's protocol
SHOULD try to ensure that error messages from the foreign
mail environment are delivered to the return path from the
SMTP envelope, not to the sender listed in the "From:"
field of the RFC-822 message.
DISCUSSION:
Internet mail lists usually place the address of the
mail list maintainer in the envelope but leave the
original message header intact (with the "From:"
field containing the original sender). This yields
the behavior the average recipient expects: a reply
to the header gets sent to the original sender, not
to a mail list maintainer; however, errors get sent
to the maintainer (who can fix the problem) and not
the sender (who probably cannot).
(F) Similarly, when forwarding a message from another
environment into the Internet, the gateway SHOULD set the
envelope return path in accordance with an error message
return address, if any, supplied by the foreign
environment.
5.3.8 Maximum Message Size
Mailer software MUST be able to send and receive messages of at
least 64K bytes in length (including header), and a much larger
maximum size is highly desirable.
DISCUSSION:
Although SMTP does not define the maximum size of a
message, many systems impose implementation limits.
The current de facto minimum limit in the Internet is 64K
bytes. However, electronic mail is used for a variety of
purposes that create much larger messages. For example,
mail is often used instead of FTP for transmitting ASCII
files, and in particular to transmit entire documents. As
a result, messages can be 1 megabyte or even larger. We
note that the present document together with its lower-
layer companion contains 0.5 megabytes.
5.4 SMTP 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
-----------------------------------------------|----------|-|-|-|-|-|--
| | | | | | |
RECEIVER-SMTP: | | | | | | |
Implement VRFY |5.2.3 |x| | | | |
Implement EXPN |5.2.3 | |x| | | |
EXPN, VRFY configurable |5.2.3 | | |x| | |
Implement SEND, SOML, SAML |5.2.4 | | |x| | |
Verify HELO parameter |5.2.5 | | |x| | |
Refuse message with bad HELO |5.2.5 | | | | |x|
Accept explicit src-route syntax in env. |5.2.6 |x| | | | |
Support "postmaster" |5.2.7 |x| | | | |
Process RCPT when received (except lists) |5.2.7 | | |x| | |
Long delay of RCPT responses |5.2.7 | | | | |x|
| | | | | | |
Add Received: line |5.2.8 |x| | | | |
Received: line include domain literal |5.2.8 | |x| | | |
Change previous Received: line |5.2.8 | | | | |x|
Pass Return-Path info (final deliv/gwy) |5.2.8 |x| | | | |
Support empty reverse path |5.2.9 |x| | | | |
Send only official reply codes |5.2.10 | |x| | | |
Send text from RFC-821 when appropriate |5.2.10 | |x| | | |
Delete "." for transparency |5.2.11 |x| | | | |
Accept and recognize self domain literal(s) |5.2.17 |x| | | | |
| | | | | | |
Error message about error message |5.3.1 | | | | |x|
Keep pending listen on SMTP port |5.3.1.2 | |x| | | |
Provide limit on recv concurrency |5.3.1.2 | | |x| | |
Wait at least 5 mins for next sender cmd |5.3.2 | |x| | | |
Avoidable delivery failure after "250 OK" |5.3.3 | | | | |x|
Send error notification msg after accept |5.3.3 |x| | | | |
Send using null return path |5.3.3 |x| | | | |
Send to envelope return path |5.3.3 | |x| | | |
Send to null address |5.3.3 | | | | |x|
Strip off explicit src route |5.3.3 | |x| | | |
Minimize acceptance delay (RFC-1047) |5.3.3 |x| | | | |
-----------------------------------------------|----------|-|-|-|-|-|--
| | | | | | |
SENDER-SMTP: | | | | | | |
Canonicalized domain names in MAIL, RCPT |5.2.2 |x| | | | |
Implement SEND, SOML, SAML |5.2.4 | | |x| | |
Send valid principal host name in HELO |5.2.5 |x| | | | |
Send explicit source route in RCPT TO: |5.2.6 | | | |x| |
Use only reply code to determine action |5.2.10 |x| | | | |
Use only high digit of reply code when poss. |5.2.10 | |x| | | |
Add "." for transparency |5.2.11 |x| | | | |
| | | | | | |
Retry messages after soft failure |5.3.1.1 |x| | | | |
Delay before retry |5.3.1.1 |x| | | | |
Configurable retry parameters |5.3.1.1 |x| | | | |
Retry once per each queued dest host |5.3.1.1 | |x| | | |
Multiple RCPT's for same DATA |5.3.1.1 | |x| | | |
Support multiple concurrent transactions |5.3.1.1 | | |x| | |
Provide limit on concurrency |5.3.1.1 | |x| | | |
| | | | | | |
Timeouts on all activities |5.3.1 |x| | | | |
Per-command timeouts |5.3.2 | |x| | | |
Timeouts easily reconfigurable |5.3.2 | |x| | | |
Recommended times |5.3.2 | |x| | | |
Try alternate addr's in order |5.3.4 |x| | | | |
Configurable limit on alternate tries |5.3.4 | | |x| | |
Try at least two alternates |5.3.4 | |x| | | |
Load-split across equal MX alternates |5.3.4 | |x| | | |
Use the Domain Name System |5.3.5 |x| | | | |
Support MX records |5.3.5 |x| | | | |
Use WKS records in MX processing |5.2.12 | | | |x| |
-----------------------------------------------|----------|-|-|-|-|-|--
| | | | | | |
MAIL FORWARDING: | | | | | | |
Alter existing header field(s) |5.2.6 | | | |x| |
Implement relay function: 821/section 3.6 |5.2.6 | | |x| | |
If not, deliver to RHS domain |5.2.6 | |x| | | |
Interpret 'local-part' of addr |5.2.16 | | | | |x|
| | | | | | |
MAILING LISTS AND ALIASES | | | | | | |
Support both |5.3.6 | |x| | | |
Report mail list error to local admin. |5.3.6 |x| | | | |
| | | | | | |
MAIL GATEWAYS: | | | | | | |
Embed foreign mail route in local-part |5.2.16 | | |x| | |
Rewrite header fields when necessary |5.3.7 | | |x| | |
Prepend Received: line |5.3.7 |x| | | | |
Change existing Received: line |5.3.7 | | | | |x|
Accept full RFC-822 on Internet side |5.3.7 | |x| | | |
Act on RFC-822 explicit source route |5.3.7 | | |x| | |
Send only valid RFC-822 on Internet side |5.3.7 |x| | | | |
Deliver error msgs to envelope addr |5.3.7 | |x| | | |
Set env return path from err return addr |5.3.7 | |x| | | |
| | | | | | |
USER AGENT -- RFC-822 | | | | | | |
Allow user to enter <route> address |5.2.6 | | | |x| |
Support RFC-1049 Content Type field |5.2.13 | | |x| | |
Use 4-digit years |5.2.14 | |x| | | |
Generate numeric timezones |5.2.14 | |x| | | |
Accept all timezones |5.2.14 |x| | | | |
Use non-num timezones from RFC-822 |5.2.14 |x| | | | |
Omit phrase before route-addr |5.2.15 | | |x| | |
Accept and parse dot.dec. domain literals |5.2.17 |x| | | | |
Accept all RFC-822 address formats |5.2.18 |x| | | | |
Generate invalid RFC-822 address format |5.2.18 | | | | |x|
Fully-qualified domain names in header |5.2.18 |x| | | | |
Create explicit src route in header |5.2.19 | | | |x| |
Accept explicit src route in header |5.2.19 |x| | | | |
| | | | | | |
Send/recv at least 64KB messages |5.3.8 |x| | | | |
6. SUPPORT SERVICES
6.1 DOMAIN NAME TRANSLATION
6.1.1 INTRODUCTION
Every host MUST implement a resolver for the Domain Name System
(DNS), and it MUST implement a mechanism using this DNS
resolver to convert host names to IP addresses and vice-versa
[DNS:1, DNS:2].
In addition to the DNS, a host MAY also implement a host name
translation mechanism that searches a local Internet host
table. See Section 6.1.3.8 for more information on this
option.
DISCUSSION:
Internet host name translation was originally performed by
searching local copies of a table of all hosts. This
table became too large to update and distribute in a
timely manner and too large to fit into many hosts, so the
DNS was invented.
The DNS creates a distributed database used primarily for
the translation between host names and host addresses.
Implementation of DNS software is required. The DNS
consists of two logically distinct parts: name servers and
resolvers (although implementations often combine these
two logical parts in the interest of efficiency) [DNS:2].
Domain name servers store authoritative data about certain
sections of the database and answer queries about the
data. Domain resolvers query domain name servers for data
on behalf of user processes. Every host therefore needs a
DNS resolver; some host machines will also need to run
domain name servers. Since no name server has complete
information, in general it is necessary to obtain
information from more than one name server to resolve a
query.
6.1.2 PROTOCOL WALK-THROUGH
An implementor must study references [DNS:1] and [DNS:2]
carefully. They provide a thorough description of the theory,
protocol, and implementation of the domain name system, and
reflect several years of experience.
6.1.2.1 Resource Records with Zero TTL: RFC-1035 Section 3.2.1
All DNS name servers and resolvers MUST properly handle RRs
with a zero TTL: return the RR to the client but do not
cache it.
DISCUSSION:
Zero TTL values are interpreted to mean that the RR can
only be used for the transaction in progress, and
should not be cached; they are useful for extremely
volatile data.
6.1.2.2 QCLASS Values: RFC-1035 Section 3.2.5
A query with "QCLASS=*" SHOULD NOT be used unless the
requestor is seeking data from more than one class. In
particular, if the requestor is only interested in Internet
data types, QCLASS=IN MUST be used.
6.1.2.3 Unused Fields: RFC-1035 Section 4.1.1
Unused fields in a query or response message MUST be zero.
6.1.2.4 Compression: RFC-1035 Section 4.1.4
Name servers MUST use compression in responses.
DISCUSSION:
Compression is essential to avoid overflowing UDP
datagrams; see Section 6.1.3.2.
6.1.2.5 Misusing Configuration Info: RFC-1035 Section 6.1.2
Recursive name servers and full-service resolvers generally
have some configuration information containing hints about
the location of root or local name servers. An
implementation MUST NOT include any of these hints in a
response.
DISCUSSION:
Many implementors have found it convenient to store
these hints as if they were cached data, but some
neglected to ensure that this "cached data" was not
included in responses. This has caused serious
problems in the Internet when the hints were obsolete
or incorrect.
6.1.3 SPECIFIC ISSUES
6.1.3.1 Resolver Implementation
A name resolver SHOULD be able to multiplex concurrent
requests if the host supports concurrent processes.
In implementing a DNS resolver, one of two different models
MAY optionally be chosen: a full-service resolver, or a stub
resolver.
(A) Full-Service Resolver
A full-service resolver is a complete implementation of
the resolver service, and is capable of dealing with
communication failures, failure of individual name
servers, location of the proper name server for a given
name, etc. It must satisfy the following requirements:
o The resolver MUST implement a local caching
function to avoid repeated remote access for
identical requests, and MUST time out information
in the cache.
o The resolver SHOULD be configurable with start-up
information pointing to multiple root name servers
and multiple name servers for the local domain.
This insures that the resolver will be able to
access the whole name space in normal cases, and
will be able to access local domain information
should the local network become disconnected from
the rest of the Internet.
(B) Stub Resolver
A "stub resolver" relies on the services of a recursive
name server on the connected network or a "nearby"
network. This scheme allows the host to pass on the
burden of the resolver function to a name server on
another host. This model is often essential for less
capable hosts, such as PCs, and is also recommended
when the host is one of several workstations on a local
network, because it allows all of the workstations to
share the cache of the recursive name server and hence
reduce the number of domain requests exported by the
local network.
At a minimum, the stub resolver MUST be capable of
directing its requests to redundant recursive name
servers. Note that recursive name servers are allowed
to restrict the sources of requests that they will
honor, so the host administrator must verify that the
service will be provided. Stub resolvers MAY implement
caching if they choose, but if so, MUST timeout cached
information.
6.1.3.2 Transport Protocols
DNS resolvers and recursive servers MUST support UDP, and
SHOULD support TCP, for sending (non-zone-transfer) queries.
Specifically, a DNS resolver or server that is sending a
non-zone-transfer query MUST send a UDP query first. If the
Answer section of the response is truncated and if the
requester supports TCP, it SHOULD try the query again using
TCP.
DNS servers MUST be able to service UDP queries and SHOULD
be able to service TCP queries. A name server MAY limit the
resources it devotes to TCP queries, but it SHOULD NOT
refuse to service a TCP query just because it would have
succeeded with UDP.
Truncated responses MUST NOT be saved (cached) and later
used in such a way that the fact that they are truncated is
lost.
DISCUSSION:
UDP is preferred over TCP for queries because UDP
queries have much lower overhead, both in packet count
and in connection state. The use of UDP is essential
for heavily-loaded servers, especially the root
servers. UDP also offers additional robustness, since
a resolver can attempt several UDP queries to different
servers for the cost of a single TCP query.
It is possible for a DNS response to be truncated,
although this is a very rare occurrence in the present
Internet DNS. Practically speaking, truncation cannot
be predicted, since it is data-dependent. The
dependencies include the number of RRs in the answer,
the size of each RR, and the savings in space realized
by the name compression algorithm. As a rule of thumb,
truncation in NS and MX lists should not occur for
answers containing 15 or fewer RRs.
Whether it is possible to use a truncated answer
depends on the application. A mailer must not use a
truncated MX response, since this could lead to mail
loops.
Responsible practices can make UDP suffice in the vast
majority of cases. Name servers must use compression
in responses. Resolvers must differentiate truncation
of the Additional section of a response (which only
loses extra information) from truncation of the Answer
section (which for MX records renders the response
unusable by mailers). Database administrators should
list only a reasonable number of primary names in lists
of name servers, MX alternatives, etc.
However, it is also clear that some new DNS record
types defined in the future will contain information
exceeding the 512 byte limit that applies to UDP, and
hence will require TCP. Thus, resolvers and name
servers should implement TCP services as a backup to
UDP today, with the knowledge that they will require
the TCP service in the future.
By private agreement, name servers and resolvers MAY arrange
to use TCP for all traffic between themselves. TCP MUST be
used for zone transfers.
A DNS server MUST have sufficient internal concurrency that
it can continue to process UDP queries while awaiting a
response or performing a zone transfer on an open TCP
connection [DNS:2].
A server MAY support a UDP query that is delivered using an
IP broadcast or multicast address. However, the Recursion
Desired bit MUST NOT be set in a query that is multicast,
and MUST be ignored by name servers receiving queries via a
broadcast or multicast address. A host that sends broadcast
or multicast DNS queries SHOULD send them only as occasional
probes, caching the IP address(es) it obtains from the
response(s) so it can normally send unicast queries.
DISCUSSION:
Broadcast or (especially) IP multicast can provide a
way to locate nearby name servers without knowing their
IP addresses in advance. However, general broadcasting
of recursive queries can result in excessive and
unnecessary load on both network and servers.
6.1.3.3 Efficient Resource Usage
The following requirements on servers and resolvers are very
important to the health of the Internet as a whole,
particularly when DNS services are invoked repeatedly by
higher level automatic servers, such as mailers.
(1) The resolver MUST implement retransmission controls to
insure that it does not waste communication bandwidth,
and MUST impose finite bounds on the resources consumed
to respond to a single request. See [DNS:2] pages 43-
44 for specific recommendations.
(2) After a query has been retransmitted several times
without a response, an implementation MUST give up and
return a soft error to the application.
(3) All DNS name servers and resolvers SHOULD cache
temporary failures, with a timeout period of the order
of minutes.
DISCUSSION:
This will prevent applications that immediately
retry soft failures (in violation of Section 2.2
of this document) from generating excessive DNS
traffic.
(4) All DNS name servers and resolvers SHOULD cache
negative responses that indicate the specified name, or
data of the specified type, does not exist, as
described in [DNS:2].
(5) When a DNS server or resolver retries a UDP query, the
retry interval SHOULD be constrained by an exponential
backoff algorithm, and SHOULD also have upper and lower
bounds.
IMPLEMENTATION:
A measured RTT and variance (if available) should
be used to calculate an initial retransmission
interval. If this information is not available, a
default of no less than 5 seconds should be used.
Implementations may limit the retransmission
interval, but this limit must exceed twice the
Internet maximum segment lifetime plus service
delay at the name server.
(6) When a resolver or server receives a Source Quench for
a query it has issued, it SHOULD take steps to reduce
the rate of querying that server in the near future. A
server MAY ignore a Source Quench that it receives as
the result of sending a response datagram.
IMPLEMENTATION:
One recommended action to reduce the rate is to
send the next query attempt to an alternate
server, if there is one available. Another is to
backoff the retry interval for the same server.
6.1.3.4 Multihomed Hosts
When the host name-to-address function encounters a host
with multiple addresses, it SHOULD rank or sort the
addresses using knowledge of the immediately connected
network number(s) and any other applicable performance or
history information.
DISCUSSION:
The different addresses of a multihomed host generally
imply different Internet paths, and some paths may be
preferable to others in performance, reliability, or
administrative restrictions. There is no general way
for the domain system to determine the best path. A
recommended approach is to base this decision on local
configuration information set by the system
administrator.
IMPLEMENTATION:
The following scheme has been used successfully:
(a) Incorporate into the host configuration data a
Network-Preference List, that is simply a list of
networks in preferred order. This list may be
empty if there is no preference.
(b) When a host name is mapped into a list of IP
addresses, these addresses should be sorted by
network number, into the same order as the
corresponding networks in the Network-Preference
List. IP addresses whose networks do not appear
in the Network-Preference List should be placed at
the end of the list.
6.1.3.5 Extensibility
DNS software MUST support all well-known, class-independent
formats [DNS:2], and SHOULD be written to minimize the
trauma associated with the introduction of new well-known
types and local experimentation with non-standard types.
DISCUSSION:
The data types and classes used by the DNS are
extensible, and thus new types will be added and old
types deleted or redefined. Introduction of new data
types ought to be dependent only upon the rules for
compression of domain names inside DNS messages, and
the translation between printable (i.e., master file)
and internal formats for Resource Records (RRs).
Compression relies on knowledge of the format of data
inside a particular RR. Hence compression must only be
used for the contents of well-known, class-independent
RRs, and must never be used for class-specific RRs or
RR types that are not well-known. The owner name of an
RR is always eligible for compression.
A name server may acquire, via zone transfer, RRs that
the server doesn't know how to convert to printable
format. A resolver can receive similar information as
the result of queries. For proper operation, this data
must be preserved, and hence the implication is that
DNS software cannot use textual formats for internal
storage.
The DNS defines domain name syntax very generally -- a
string of labels each containing up to 63 8-bit octets,
separated by dots, and with a maximum total of 255
octets. Particular applications of the DNS are
permitted to further constrain the syntax of the domain
names they use, although the DNS deployment has led to
some applications allowing more general names. In
particular, Section 2.1 of this document liberalizes
slightly the syntax of a legal Internet host name that
was defined in RFC-952 [DNS:4].
6.1.3.6 Status of RR Types
Name servers MUST be able to load all RR types except MD and
MF from configuration files. The MD and MF types are
obsolete and MUST NOT be implemented; in particular, name
servers MUST NOT load these types from configuration files.
DISCUSSION:
The RR types MB, MG, MR, NULL, MINFO and RP are
considered experimental, and applications that use the
DNS cannot expect these RR types to be supported by
most domains. Furthermore these types are subject to
redefinition.
The TXT and WKS RR types have not been widely used by
Internet sites; as a result, an application cannot rely
on the existence of a TXT or WKS RR in most domains.
EID 6474 (Verified) is as follows:Section: 6.1.3.6
Original Text:
on the the existence of a TXT or WKS RR in most
domains.
Corrected Text:
on the existence of a TXT or WKS RR in most domains.
Notes:
Doubled word.
6.1.3.7 Robustness
DNS software may need to operate in environments where the
root servers or other servers are unavailable due to network
connectivity or other problems. In this situation, DNS name
servers and resolvers MUST continue to provide service for
the reachable part of the name space, while giving temporary
failures for the rest.
DISCUSSION:
Although the DNS is meant to be used primarily in the
connected Internet, it should be possible to use the
system in networks which are unconnected to the
Internet. Hence implementations must not depend on
access to root servers before providing service for
local names.
6.1.3.8 Local Host Table
DISCUSSION:
A host may use a local host table as a backup or
supplement to the DNS. This raises the question of
which takes precedence, the DNS or the host table; the
most flexible approach would make this a configuration
option.
Typically, the contents of such a supplementary host
table will be determined locally by the site. However,
a publically-available table of Internet hosts is
maintained by the DDN Network Information Center (DDN
NIC), with a format documented in [DNS:4]. This table
can be retrieved from the DDN NIC using a protocol
described in [DNS:5]. It must be noted that this table
contains only a small fraction of all Internet hosts.
Hosts using this protocol to retrieve the DDN NIC host
table should use the VERSION command to check if the
table has changed before requesting the entire table
with the ALL command. The VERSION identifier should be
treated as an arbitrary string and tested only for
equality; no numerical sequence may be assumed.
The DDN NIC host table includes administrative
information that is not needed for host operation and
is therefore not currently included in the DNS
database; examples include network and gateway entries.
However, much of this additional information will be
added to the DNS in the future. Conversely, the DNS
provides essential services (in particular, MX records)
that are not available from the DDN NIC host table.
6.1.4 DNS USER INTERFACE
6.1.4.1 DNS Administration
This document is concerned with design and implementation
issues in host software, not with administrative or
operational issues. However, administrative issues are of
particular importance in the DNS, since errors in particular
segments of this large distributed database can cause poor
or erroneous performance for many sites. These issues are
discussed in [DNS:6] and [DNS:7].
6.1.4.2 DNS User Interface
Hosts MUST provide an interface to the DNS for all
application programs running on the host. This interface
will typically direct requests to a system process to
perform the resolver function [DNS:1, 6.1:2].
At a minimum, the basic interface MUST support a request for
all information of a specific type and class associated with
a specific name, and it MUST return either all of the
requested information, a hard error code, or a soft error
indication. When there is no error, the basic interface
returns the complete response information without
modification, deletion, or ordering, so that the basic
interface will not need to be changed to accommodate new
data types.
DISCUSSION:
The soft error indication is an essential part of the
interface, since it may not always be possible to
access particular information from the DNS; see Section
6.1.3.3.
A host MAY provide other DNS interfaces tailored to
particular functions, transforming the raw domain data into
formats more suited to these functions. In particular, a
host MUST provide a DNS interface to facilitate translation
between host addresses and host names.
6.1.4.3 Interface Abbreviation Facilities
User interfaces MAY provide a method for users to enter
abbreviations for commonly-used names. Although the
definition of such methods is outside of the scope of the
DNS specification, certain rules are necessary to insure
that these methods allow access to the entire DNS name space
and to prevent excessive use of Internet resources.
If an abbreviation method is provided, then:
(a) There MUST be some convention for denoting that a name
is already complete, so that the abbreviation method(s)
are suppressed. A trailing dot is the usual method.
(b) Abbreviation expansion MUST be done exactly once, and
MUST be done in the context in which the name was
entered.
DISCUSSION:
For example, if an abbreviation is used in a mail
program for a destination, the abbreviation should be
expanded into a full domain name and stored in the
queued message with an indication that it is already
complete. Otherwise, the abbreviation might be
expanded with a mail system search list, not the
user's, or a name could grow due to repeated
canonicalizations attempts interacting with wildcards.
The two most common abbreviation methods are:
(1) Interface-level aliases
Interface-level aliases are conceptually implemented as
a list of alias/domain name pairs. The list can be
per-user or per-host, and separate lists can be
associated with different functions, e.g. one list for
host name-to-address translation, and a different list
for mail domains. When the user enters a name, the
interface attempts to match the name to the alias
component of a list entry, and if a matching entry can
be found, the name is replaced by the domain name found
in the pair.
Note that interface-level aliases and CNAMEs are
completely separate mechanisms; interface-level aliases
are a local matter while CNAMEs are an Internet-wide
aliasing mechanism which is a required part of any DNS
implementation.
(2) Search Lists
A search list is conceptually implemented as an ordered
list of domain names. When the user enters a name, the
domain names in the search list are used as suffixes to
the user-supplied name, one by one, until a domain name
with the desired associated data is found, or the
search list is exhausted. Search lists often contain
the name of the local host's parent domain or other
ancestor domains. Search lists are often per-user or
per-process.
It SHOULD be possible for an administrator to disable a
DNS search-list facility. Administrative denial may be
warranted in some cases, to prevent abuse of the DNS.
There is danger that a search-list mechanism will
generate excessive queries to the root servers while
testing whether user input is a complete domain name,
lacking a final period to mark it as complete. A
search-list mechanism MUST have one of, and SHOULD have
both of, the following two provisions to prevent this:
(a) The local resolver/name server can implement
caching of negative responses (see Section
6.1.3.3).
(b) The search list expander can require two or more
interior dots in a generated domain name before it
tries using the name in a query to non-local
domain servers, such as the root.
DISCUSSION:
The intent of this requirement is to avoid
excessive delay for the user as the search list is
tested, and more importantly to prevent excessive
traffic to the root and other high-level servers.
For example, if the user supplied a name "X" and
the search list contained the root as a component,
a query would have to consult a root server before
the next search list alternative could be tried.
The resulting load seen by the root servers and
gateways near the root would be multiplied by the
number of hosts in the Internet.
The negative caching alternative limits the effect
to the first time a name is used. The interior
dot rule is simpler to implement but can prevent
easy use of some top-level names.
6.1.5 DOMAIN NAME SYSTEM REQUIREMENTS SUMMARY
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FEATURE |SECTION | | | |T|T|e
-----------------------------------------------|-----------|-|-|-|-|-|--
GENERAL ISSUES | | | | | | |
| | | | | | |
Implement DNS name-to-address conversion |6.1.1 |x| | | | |
Implement DNS address-to-name conversion |6.1.1 |x| | | | |
Support conversions using host table |6.1.1 | | |x| | |
Properly handle RR with zero TTL |6.1.2.1 |x| | | | |
Use QCLASS=* unnecessarily |6.1.2.2 | |x| | | |
Use QCLASS=IN for Internet class |6.1.2.2 |x| | | | |
Unused fields zero |6.1.2.3 |x| | | | |
Use compression in responses |6.1.2.4 |x| | | | |
| | | | | | |
Include config info in responses |6.1.2.5 | | | | |x|
Support all well-known, class-indep. types |6.1.3.5 |x| | | | |
Easily expand type list |6.1.3.5 | |x| | | |
Load all RR types (except MD and MF) |6.1.3.6 |x| | | | |
Load MD or MF type |6.1.3.6 | | | | |x|
Operate when root servers, etc. unavailable |6.1.3.7 |x| | | | |
-----------------------------------------------|-----------|-|-|-|-|-|--
RESOLVER ISSUES: | | | | | | |
| | | | | | |
Resolver support multiple concurrent requests |6.1.3.1 | |x| | | |
Full-service resolver: |6.1.3.1 | | |x| | |
Local caching |6.1.3.1 |x| | | | |
Information in local cache times out |6.1.3.1 |x| | | | |
Configurable with starting info |6.1.3.1 | |x| | | |
Stub resolver: |6.1.3.1 | | |x| | |
Use redundant recursive name servers |6.1.3.1 |x| | | | |
Local caching |6.1.3.1 | | |x| | |
Information in local cache times out |6.1.3.1 |x| | | | |
Support for remote multi-homed hosts: | | | | | | |
Sort multiple addresses by preference list |6.1.3.4 | |x| | | |
| | | | | | |
-----------------------------------------------|-----------|-|-|-|-|-|--
TRANSPORT PROTOCOLS: | | | | | | |
| | | | | | |
Support UDP queries |6.1.3.2 |x| | | | |
Support TCP queries |6.1.3.2 | |x| | | |
Send query using UDP first |6.1.3.2 |x| | | | |1
Try TCP if UDP answers are truncated |6.1.3.2 | |x| | | |
Name server limit TCP query resources |6.1.3.2 | | |x| | |
Punish unnecessary TCP query |6.1.3.2 | | | |x| |
Use truncated data as if it were not |6.1.3.2 | | | | |x|
Private agreement to use only TCP |6.1.3.2 | | |x| | |
Use TCP for zone transfers |6.1.3.2 |x| | | | |
TCP usage not block UDP queries |6.1.3.2 |x| | | | |
Support broadcast or multicast queries |6.1.3.2 | | |x| | |
RD bit set in query |6.1.3.2 | | | | |x|
RD bit ignored by server is b'cast/m'cast |6.1.3.2 |x| | | | |
Send only as occasional probe for addr's |6.1.3.2 | |x| | | |
-----------------------------------------------|-----------|-|-|-|-|-|--
RESOURCE USAGE: | | | | | | |
| | | | | | |
Transmission controls, per [DNS:2] |6.1.3.3 |x| | | | |
Finite bounds per request |6.1.3.3 |x| | | | |
Failure after retries => soft error |6.1.3.3 |x| | | | |
Cache temporary failures |6.1.3.3 | |x| | | |
Cache negative responses |6.1.3.3 | |x| | | |
Retries use exponential backoff |6.1.3.3 | |x| | | |
Upper, lower bounds |6.1.3.3 | |x| | | |
Client handle Source Quench |6.1.3.3 | |x| | | |
Server ignore Source Quench |6.1.3.3 | | |x| | |
-----------------------------------------------|-----------|-|-|-|-|-|--
USER INTERFACE: | | | | | | |
| | | | | | |
All programs have access to DNS interface |6.1.4.2 |x| | | | |
Able to request all info for given name |6.1.4.2 |x| | | | |
Returns complete info or error |6.1.4.2 |x| | | | |
Special interfaces |6.1.4.2 | | |x| | |
Name<->Address translation |6.1.4.2 |x| | | | |
| | | | | | |
Abbreviation Facilities: |6.1.4.3 | | |x| | |
Convention for complete names |6.1.4.3 |x| | | | |
Conversion exactly once |6.1.4.3 |x| | | | |
Conversion in proper context |6.1.4.3 |x| | | | |
Search list: |6.1.4.3 | | |x| | |
Administrator can disable |6.1.4.3 | |x| | | |
Prevention of excessive root queries |6.1.4.3 |x| | | | |
Both methods |6.1.4.3 | |x| | | |
-----------------------------------------------|-----------|-|-|-|-|-|--
-----------------------------------------------|-----------|-|-|-|-|-|--
1. Unless there is private agreement between particular resolver and
particular server.
6.2 HOST INITIALIZATION
6.2.1 INTRODUCTION
This section discusses the initialization of host software
across a connected network, or more generally across an
Internet path. This is necessary for a diskless host, and may
optionally be used for a host with disk drives. For a diskless
host, the initialization process is called "network booting"
and is controlled by a bootstrap program located in a boot ROM.
To initialize a diskless host across the network, there are two
distinct phases:
(1) Configure the IP layer.
Diskless machines often have no permanent storage in which
to store network configuration information, so that
sufficient configuration information must be obtained
dynamically to support the loading phase that follows.
This information must include at least the IP addresses of
the host and of the boot server. To support booting
across a gateway, the address mask and a list of default
gateways are also required.
(2) Load the host system code.
During the loading phase, an appropriate file transfer
protocol is used to copy the system code across the
network from the boot server.
A host with a disk may perform the first step, dynamic
configuration. This is important for microcomputers, whose
floppy disks allow network configuration information to be
mistakenly duplicated on more than one host. Also,
installation of new hosts is much simpler if they automatically
obtain their configuration information from a central server,
saving administrator time and decreasing the probability of
mistakes.
6.2.2 REQUIREMENTS
6.2.2.1 Dynamic Configuration
A number of protocol provisions have been made for dynamic
configuration.
o ICMP Information Request/Reply messages
This obsolete message pair was designed to allow a host
to find the number of the network it is on.
Unfortunately, it was useful only if the host already
knew the host number part of its IP address,
information that hosts requiring dynamic configuration
seldom had.
o Reverse Address Resolution Protocol (RARP) [BOOT:4]
RARP is a link-layer protocol for a broadcast medium
that allows a host to find its IP address given its
link layer address. Unfortunately, RARP does not work
across IP gateways and therefore requires a RARP server
on every network. In addition, RARP does not provide
any other configuration information.
o ICMP Address Mask Request/Reply messages
These ICMP messages allow a host to learn the address
mask for a particular network interface.
o BOOTP Protocol [BOOT:2]
This protocol allows a host to determine the IP
addresses of the local host and the boot server, the
name of an appropriate boot file, and optionally the
address mask and list of default gateways. To locate a
BOOTP server, the host broadcasts a BOOTP request using
UDP. Ad hoc gateway extensions have been used to
transmit the BOOTP broadcast through gateways, and in
the future the IP Multicasting facility will provide a
standard mechanism for this purpose.
The suggested approach to dynamic configuration is to use
the BOOTP protocol with the extensions defined in "BOOTP
Vendor Information Extensions" RFC-1084 [BOOT:3]. RFC-1084
defines some important general (not vendor-specific)
extensions. In particular, these extensions allow the
address mask to be supplied in BOOTP; we RECOMMEND that the
address mask be supplied in this manner.
DISCUSSION:
Historically, subnetting was defined long after IP, and
so a separate mechanism (ICMP Address Mask messages)
was designed to supply the address mask to a host.
However, the IP address mask and the corresponding IP
address conceptually form a pair, and for operational
simplicity they ought to be defined at the same time
and by the same mechanism, whether a configuration file
or a dynamic mechanism like BOOTP.
Note that BOOTP is not sufficiently general to specify
the configurations of all interfaces of a multihomed
host. A multihomed host must either use BOOTP
separately for each interface, or configure one
interface using BOOTP to perform the loading, and
perform the complete initialization from a file later.
Application layer configuration information is expected
to be obtained from files after loading of the system
code.
6.2.2.2 Loading Phase
A suggested approach for the loading phase is to use TFTP
[BOOT:1] between the IP addresses established by BOOTP.
TFTP to a broadcast address SHOULD NOT be used, for reasons
explained in Section 4.2.3.4.
6.3 REMOTE MANAGEMENT
6.3.1 INTRODUCTION
The Internet community has recently put considerable effort
into the development of network management protocols. The
result has been a two-pronged approach [MGT:1, MGT:6]: the
Simple Network Management Protocol (SNMP) [MGT:4] and the
Common Management Information Protocol over TCP (CMOT) [MGT:5].
In order to be managed using SNMP or CMOT, a host will need to
implement an appropriate management agent. An Internet host
SHOULD include an agent for either SNMP or CMOT.
Both SNMP and CMOT operate on a Management Information Base
(MIB) that defines a collection of management values. By
reading and setting these values, a remote application may
query and change the state of the managed system.
A standard MIB [MGT:3] has been defined for use by both
management protocols, using data types defined by the Structure
of Management Information (SMI) defined in [MGT:2]. Additional
MIB variables can be introduced under the "enterprises" and
"experimental" subtrees of the MIB naming space [MGT:2].
Every protocol module in the host SHOULD implement the relevant
MIB variables. A host SHOULD implement the MIB variables as
defined in the most recent standard MIB, and MAY implement
other MIB variables when appropriate and useful.
6.3.2 PROTOCOL WALK-THROUGH
The MIB is intended to cover both hosts and gateways, although
there may be detailed differences in MIB application to the two
cases. This section contains the appropriate interpretation of
the MIB for hosts. It is likely that later versions of the MIB
will include more entries for host management.
A managed host must implement the following groups of MIB
object definitions: System, Interfaces, Address Translation,
IP, ICMP, TCP, and UDP.
The following specific interpretations apply to hosts:
o ipInHdrErrors
Note that the error "time-to-live exceeded" can occur in a
host only when it is forwarding a source-routed datagram.
o ipOutNoRoutes
This object counts datagrams discarded because no route
can be found. This may happen in a host if all the
default gateways in the host's configuration are down.
o ipFragOKs, ipFragFails, ipFragCreates
A host that does not implement intentional fragmentation
(see "Fragmentation" section of [INTRO:1]) MUST return the
value zero for these three objects.
o icmpOutRedirects
For a host, this object MUST always be zero, since hosts
do not send Redirects.
o icmpOutAddrMaskReps
For a host, this object MUST always be zero, unless the
host is an authoritative source of address mask
information.
o ipAddrTable
For a host, the "IP Address Table" object is effectively a
table of logical interfaces.
o ipRoutingTable
For a host, the "IP Routing Table" object is effectively a
combination of the host's Routing Cache and the static
route table described in "Routing Outbound Datagrams"
section of [INTRO:1].
Within each ipRouteEntry, ipRouteMetric1...4 normally will
have no meaning for a host and SHOULD always be -1, while
ipRouteType will normally have the value "remote".
If destinations on the connected network do not appear in
the Route Cache (see "Routing Outbound Datagrams section
of [INTRO:1]), there will be no entries with ipRouteType
of "direct".
DISCUSSION:
The current MIB does not include Type-of-Service in an
ipRouteEntry, but a future revision is expected to make
this addition.
We also expect the MIB to be expanded to allow the remote
management of applications (e.g., the ability to partially
reconfigure mail systems). Network service applications
such as mail systems should therefore be written with the
"hooks" for remote management.
6.3.3 MANAGEMENT REQUIREMENTS SUMMARY
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| |T|D|Y|O|O|t
FEATURE |SECTION | | | |T|T|e
-----------------------------------------------|-----------|-|-|-|-|-|--
Support SNMP or CMOT agent |6.3.1 | |x| | | |
Implement specified objects in standard MIB |6.3.1 | |x| | | |
7. REFERENCES
This section lists the primary references with which every
implementer must be thoroughly familiar. It also lists some
secondary references that are suggested additional reading.
INTRODUCTORY REFERENCES:
[INTRO:1] "Requirements for Internet Hosts -- Communication Layers,"
IETF Host Requirements Working Group, R. Braden, Ed., RFC-1122,
October 1989.
[INTRO:2] "DDN Protocol Handbook," NIC-50004, NIC-50005, NIC-50006,
(three volumes), SRI International, December 1985.
[INTRO:3] "Official Internet Protocols," J. Reynolds and J. Postel,
RFC-1011, May 1987.
This document is republished periodically with new RFC numbers;
the latest version must be used.
[INTRO:4] "Protocol Document Order Information," O. Jacobsen and J.
Postel, RFC-980, March 1986.
[INTRO:5] "Assigned Numbers," J. Reynolds and J. Postel, RFC-1010,
May 1987.
This document is republished periodically with new RFC numbers;
the latest version must be used.
TELNET REFERENCES:
[TELNET:1] "Telnet Protocol Specification," J. Postel and J.
Reynolds, RFC-854, May 1983.
[TELNET:2] "Telnet Option Specification," J. Postel and J. Reynolds,
RFC-855, May 1983.
[TELNET:3] "Telnet Binary Transmission," J. Postel and J. Reynolds,
RFC-856, May 1983.
[TELNET:4] "Telnet Echo Option," J. Postel and J. Reynolds, RFC-857,
May 1983.
[TELNET:5] "Telnet Suppress Go Ahead Option," J. Postel and J.
Reynolds, RFC-858, May 1983.
[TELNET:6] "Telnet Status Option," J. Postel and J. Reynolds, RFC-
859, May 1983.
[TELNET:7] "Telnet Timing Mark Option," J. Postel and J. Reynolds,
RFC-860, May 1983.
[TELNET:8] "Telnet Extended Options List," J. Postel and J.
Reynolds, RFC-861, May 1983.
[TELNET:9] "Telnet End-Of-Record Option," J. Postel, RFC-885, December 1983.
EID 584 (Verified) is as follows:Section: 7
Original Text:
[TELNET:9] "Telnet End-Of-Record Option," J. Postel, RFC-855, December 1983.
Corrected Text:
[TELNET:9] "Telnet End-Of-Record Option," J. Postel, RFC-885, December 1983.
Notes:
RFC 855 is "Telnet Option Specifications"; RFC 885 is "Telnet end of record option".
[TELNET:10] "Telnet Terminal-Type Option," J. VanBokkelen, RFC-1091,
February 1989.
This document supercedes RFC-930.
[TELNET:11] "Telnet Window Size Option," D. Waitzman, RFC-1073,
October 1988.
[TELNET:12] "Telnet Linemode Option," D. Borman, RFC-1116, August
1989.
[TELNET:13] "Telnet Terminal Speed Option," C. Hedrick, RFC-1079,
December 1988.
[TELNET:14] "Telnet Remote Flow Control Option," C. Hedrick, RFC-
1080, November 1988.
SECONDARY TELNET REFERENCES:
[TELNET:15] "Telnet Protocol," MIL-STD-1782, U.S. Department of
Defense, May 1984.
This document is intended to describe the same protocol as RFC-
854. In case of conflict, RFC-854 takes precedence, and the
present document takes precedence over both.
[TELNET:16] "SUPDUP Protocol," M. Crispin, RFC-734, October 1977.
[TELNET:17] "Telnet SUPDUP Option," M. Crispin, RFC-736, October
1977.
[TELNET:18] "Data Entry Terminal Option," J. Day, RFC-732, June 1977.
[TELNET:19] "TELNET Data Entry Terminal option -- DODIIS
Implementation," A. Yasuda and T. Thompson, RFC-1043, February
1988.
FTP REFERENCES:
[FTP:1] "File Transfer Protocol," J. Postel and J. Reynolds, RFC-
959, October 1985.
[FTP:2] "Document File Format Standards," J. Postel, RFC-678,
December 1974.
[FTP:3] "File Transfer Protocol," MIL-STD-1780, U.S. Department of
Defense, May 1984.
This document is based on an earlier version of the FTP
specification (RFC-765) and is obsolete.
TFTP REFERENCES:
[TFTP:1] "The TFTP Protocol Revision 2," K. Sollins, RFC-783, June
1981.
MAIL REFERENCES:
[SMTP:1] "Simple Mail Transfer Protocol," J. Postel, RFC-821, August
1982.
[SMTP:2] "Standard For The Format of ARPA Internet Text Messages,"
D. Crocker, RFC-822, August 1982.
This document obsoleted an earlier specification, RFC-733.
[SMTP:3] "Mail Routing and the Domain System," C. Partridge, RFC-
974, January 1986.
This RFC describes the use of MX records, a mandatory extension
to the mail delivery process.
[SMTP:4] "Duplicate Messages and SMTP," C. Partridge, RFC-1047,
February 1988.
[SMTP:5a] "Mapping between X.400 and RFC 822," S. Kille, RFC-987,
June 1986.
[SMTP:5b] "Addendum to RFC-987," S. Kille, RFC-???, September 1987.
The two preceding RFC's define a proposed standard for
gatewaying mail between the Internet and the X.400 environments.
[SMTP:6] "Simple Mail Transfer Protocol," MIL-STD-1781, U.S.
Department of Defense, May 1984.
This specification is intended to describe the same protocol as
does RFC-821. However, MIL-STD-1781 is incomplete; in
particular, it does not include MX records [SMTP:3].
[SMTP:7] "A Content-Type Field for Internet Messages," M. Sirbu,
RFC-1049, March 1988.
DOMAIN NAME SYSTEM REFERENCES:
[DNS:1] "Domain Names - Concepts and Facilities," P. Mockapetris,
RFC-1034, November 1987.
This document and the following one obsolete RFC-882, RFC-883,
and RFC-973.
[DNS:2] "Domain Names - Implementation and Specification," RFC-1035,
P. Mockapetris, November 1987.
[DNS:3] "Mail Routing and the Domain System," C. Partridge, RFC-974,
January 1986.
[DNS:4] "DoD Internet Host Table Specification," K. Harrenstein,
RFC-952, M. Stahl, E. Feinler, October 1985.
SECONDARY DNS REFERENCES:
[DNS:5] "Hostname Server," K. Harrenstein, M. Stahl, E. Feinler,
RFC-953, October 1985.
[DNS:6] "Domain Administrators Guide," M. Stahl, RFC-1032, November
1987.
[DNS:7] "Domain Administrators Operations Guide," M. Lottor, RFC-
1033, November 1987.
[DNS:8] "The Domain Name System Handbook," Vol. 4 of Internet
Protocol Handbook, NIC 50007, SRI Network Information Center,
August 1989.
SYSTEM INITIALIZATION REFERENCES:
[BOOT:1] "Bootstrap Loading Using TFTP," R. Finlayson, RFC-906, June
1984.
[BOOT:2] "Bootstrap Protocol (BOOTP)," W. Croft and J. Gilmore, RFC-
951, September 1985.
[BOOT:3] "BOOTP Vendor Information Extensions," J. Reynolds, RFC-
1084, December 1988.
Note: this RFC revised and obsoleted RFC-1048.
[BOOT:4] "A Reverse Address Resolution Protocol," R. Finlayson, T.
Mann, J. Mogul, and M. Theimer, RFC-903, June 1984.
MANAGEMENT REFERENCES:
[MGT:1] "IAB Recommendations for the Development of Internet Network
Management Standards," V. Cerf, RFC-1052, April 1988.
[MGT:2] "Structure and Identification of Management Information for
TCP/IP-based internets," M. Rose and K. McCloghrie, RFC-1065,
August 1988.
[MGT:3] "Management Information Base for Network Management of
TCP/IP-based internets," M. Rose and K. McCloghrie, RFC-1066,
August 1988.
[MGT:4] "A Simple Network Management Protocol," J. Case, M. Fedor,
M. Schoffstall, and C. Davin, RFC-1098, April 1989.
[MGT:5] "The Common Management Information Services and Protocol
over TCP/IP," U. Warrier and L. Besaw, RFC-1095, April 1989.
[MGT:6] "Report of the Second Ad Hoc Network Management Review
Group," V. Cerf, RFC-1109, August 1989.
Security Considerations
There are many security issues in the application and support
programs of host software, but a full discussion is beyond the scope
of this RFC. Security-related issues are mentioned in sections
concerning TFTP (Sections 4.2.1, 4.2.3.4, 4.2.3.5), the SMTP VRFY and
EXPN commands (Section 5.2.3), the SMTP HELO command (5.2.5), and the
SMTP DATA command (Section 5.2.8).
Author's Address
Robert Braden
USC/Information Sciences Institute
4676 Admiralty Way
Marina del Rey, CA 90292-6695
Phone: (213) 822 1511
EMail: Braden@ISI.EDU