Solved: E-mail, How does it work?

Mike149 · 2000-02-17T02:06:03Z

How does e-mail work and how does it get from one computer to another?

The reason I answered it is because I've been robbed from typing too much before. Here's how it works. =)

You write up an email message on your computer, and when you send it, it gets sent through a server. It could be a server at work, or a server through an ISP(internet service provider) like AOL. The server take a look at the address of the message(the part after the @) and it transfers it via a hard line(telephone) to that address's server. From that server, it takes the part before the @ symbol and finds the specific mailbox to place the message in. When you log in to your email server(like AOL, or Eudora, etc), and you ask it to give you your new messages, it queries the server for all the messages for you(the part before the @) and allows you to read them remotely. If you have any more questions, feel free to ask!

~Aaron

ozo

RFC1123

BudVVeezer

Hey ozo, ya lost me with that comment...mind explaining? Thx!

~Aaron

mark2150

RFC1123 is the IETF (Internet Engineering Task Force) document that define internet email protocol.

M

BudVVeezer

ah, i get it. Learn something new every day...I just went at it from a simple point of view(I hope)... ;-)

~Aaron

mark2150

There are hundreds of RFC's (Request For Comment's) that apply to most every aspect of the internet. Some are adopted/ratified and become defacto standards since the IETF has no legislative or enforcement authority. If you go to Yahoo and search on RFC1123 you should find exactly what you're looking for.

M

mark2150

Ask and ye shall receive:

This is the first eight pages or so from RFC1123. I'd have posted more but E-E would only accept this much...

The source link is:

http://www.normos.org/ietf/rfc/rfc1123.txt

M

Network Working Group Internet Engineering Task Force
Request for Comments: 1123 R. Braden, Editor
October 1989

Requirements for Internet Hosts -- Application and Support

Status of This Memo

This RFC is an official specification for the Internet community. It
incorporates by reference, amends, corrects, and supplements the
primary protocol standards documents relating to hosts. Distribution
of this document is unlimited.

Summary

This RFC is one of a pair that defines and discusses the requirements
for Internet host software. This RFC covers the application and
support protocols; its companion RFC-1122 covers the communication
protocol layers: link layer, IP layer, and transport layer.

Table of Contents

1. INTRODUCTION ............................................... 5
1.1 The Internet Architecture .............................. 6
1.2 General Considerations ................................. 6
1.2.1 Continuing Internet Evolution ..................... 6
1.2.2 Robustness Principle .............................. 7
1.2.3 Error Logging ..................................... 8
1.2.4 Configuration ..................................... 8
1.3 Reading this Document .................................. 10
1.3.1 Organization ...................................... 10
1.3.2 Requirements ...................................... 10
1.3.3 Terminology ....................................... 11
1.4 Acknowledgments ........................................ 12

2. GENERAL ISSUES ............................................. 13
2.1 Host Names and Numbers ................................. 13
2.2 Using Domain Name Service .............................. 13
2.3 Applications on Multihomed hosts ....................... 14
2.4 Type-of-Service ........................................ 14
2.5 GENERAL APPLICATION REQUIREMENTS SUMMARY ............... 15

Internet Engineering Task Force [Page 1]

RFC1123 INTRODUCTION October 1989

3. REMOTE LOGIN -- TELNET PROTOCOL ............................ 16
3.1 INTRODUCTION ........................................... 16
3.2 PROTOCOL WALK-THROUGH .................................. 16
3.2.1 Option Negotiation ................................ 16
3.2.2 Telnet Go-Ahead Function .......................... 16
3.2.3 Control Functions ................................. 17
3.2.4 Telnet "Synch" Signal ............................. 18
3.2.5 NVT Printer and Keyboard .......................... 19
3.2.6 Telnet Command Structure .......................... 20
3.2.7 Telnet Binary Option .............................. 20
3.2.8 Telnet Terminal-Type Option ....................... 20
3.3 SPECIFIC ISSUES ........................................ 21
3.3.1 Telnet End-of-Line Convention ..................... 21
3.3.2 Data Entry Terminals .............................. 23
3.3.3 Option Requirements ............................... 24
3.3.4 Option Initiation ................................. 24
3.3.5 Telnet Linemode Option ............................ 25
3.4 TELNET/USER INTERFACE .................................. 25
3.4.1 Character Set Transparency ........................ 25
3.4.2 Telnet Commands ................................... 26
3.4.3 TCP Connection Errors ............................. 26
3.4.4 Non-Default Telnet Contact Port ................... 26
3.4.5 Flushing Output ................................... 26
3.5. TELNET REQUIREMENTS SUMMARY ........................... 27

4. FILE TRANSFER .............................................. 29
4.1 FILE TRANSFER PROTOCOL -- FTP .......................... 29
4.1.1 INTRODUCTION ...................................... 29
4.1.2. PROTOCOL WALK-THROUGH ............................ 29
4.1.2.1 LOCAL Type ................................... 29
4.1.2.2 Telnet Format Control ........................ 30
4.1.2.3 Page Structure ............................... 30
4.1.2.4 Data Structure Transformations ............... 30
4.1.2.5 Data Connection Management ................... 31
4.1.2.6 PASV Command ................................. 31
4.1.2.7 LIST and NLST Commands ....................... 31
4.1.2.8 SITE Command ................................. 32
4.1.2.9 STOU Command ................................. 32
4.1.2.10 Telnet End-of-line Code ..................... 32
4.1.2.11 FTP Replies ................................. 33
4.1.2.12 Connections ................................. 34
4.1.2.13 Minimum Implementation; RFC-959 Section ..... 34
4.1.3 SPECIFIC ISSUES ................................... 35
4.1.3.1 Non-standard Command Verbs ................... 35
4.1.3.2 Idle Timeout ................................. 36
4.1.3.3 Concurrency of Data and Control .............. 36
4.1.3.4 FTP Restart Mechanism ........................ 36
4.1.4 FTP/USER INTERFACE ................................ 39

Internet Engineering Task Force [Page 2]

RFC1123 INTRODUCTION October 1989

4.1.4.1 Pathname Specification ....................... 39
4.1.4.2 "QUOTE" Command .............................. 40
4.1.4.3 Displaying Replies to User ................... 40
4.1.4.4 Maintaining Synchronization .................. 40
4.1.5 FTP REQUIREMENTS SUMMARY ......................... 41
4.2 TRIVIAL FILE TRANSFER PROTOCOL -- TFTP ................. 44
4.2.1 INTRODUCTION ...................................... 44
4.2.2 PROTOCOL WALK-THROUGH ............................. 44
4.2.2.1 Transfer Modes ............................... 44
4.2.2.2 UDP Header ................................... 44
4.2.3 SPECIFIC ISSUES ................................... 44
4.2.3.1 Sorcerer's Apprentice Syndrome ............... 44
4.2.3.2 Timeout Algorithms ........................... 46
4.2.3.3 Extensions ................................... 46
4.2.3.4 Access Control ............................... 46
4.2.3.5 Broadcast Request ............................ 46
4.2.4 TFTP REQUIREMENTS SUMMARY ......................... 47

5. ELECTRONIC MAIL -- SMTP and RFC-822 ........................ 48
5.1 INTRODUCTION ........................................... 48
5.2 PROTOCOL WALK-THROUGH .................................. 48
5.2.1 The SMTP Model .................................... 48
5.2.2 Canonicalization .................................. 49
5.2.3 VRFY and EXPN Commands ............................ 50
5.2.4 SEND, SOML, and SAML Commands ..................... 50
5.2.5 HELO Command ...................................... 50
5.2.6 Mail Relay ........................................ 51
5.2.7 RCPT Command ...................................... 52
5.2.8 DATA Command ...................................... 53
5.2.9 Command Syntax .................................... 54
5.2.10 SMTP Replies ..................................... 54
5.2.11 Transparency ..................................... 55
5.2.12 WKS Use in MX Processing ......................... 55
5.2.13 RFC-822 Message Specification .................... 55
5.2.14 RFC-822 Date and Time Specification .............. 55
5.2.15 RFC-822 Syntax Change ............................ 56
5.2.16 RFC-822 Local-part .............................. 56
5.2.17 Domain Literals .................................. 57
5.2.18 Common Address Formatting Errors ................. 58
5.2.19 Explicit Source Routes ........................... 58
5.3 SPECIFIC ISSUES ........................................ 59
5.3.1 SMTP Queueing Strategies .......................... 59
5.3.1.1 Sending Strategy .............................. 59
5.3.1.2 Receiving strategy ........................... 61
5.3.2 Timeouts in SMTP .................................. 61
5.3.3 Reliable Mail Receipt ............................. 63
5.3.4 Reliable Mail Transmission ........................ 63
5.3.5 Domain Name Support ............................... 65

Internet Engineering Task Force [Page 3]

RFC1123 INTRODUCTION October 1989

5.3.6 Mailing Lists and Aliases ......................... 65
5.3.7 Mail Gatewaying ................................... 66
5.3.8 Maximum Message Size .............................. 68
5.4 SMTP REQUIREMENTS SUMMARY .............................. 69

6. SUPPORT SERVICES ............................................ 72
6.1 DOMAIN NAME TRANSLATION ................................. 72
6.1.1 INTRODUCTION ....................................... 72
6.1.2 PROTOCOL WALK-THROUGH ............................. 72
6.1.2.1 Resource Records with Zero TTL ............... 73
6.1.2.2 QCLASS Values ................................ 73
6.1.2.3 Unused Fields ................................ 73
6.1.2.4 Compression .................................. 73
6.1.2.5 Misusing Configuration Info .................. 73
6.1.3 SPECIFIC ISSUES ................................... 74
6.1.3.1 Resolver Implementation ...................... 74
6.1.3.2 Transport Protocols .......................... 75
6.1.3.3 Efficient Resource Usage ..................... 77
6.1.3.4 Multihomed Hosts ............................. 78
6.1.3.5 Extensibility ................................ 79
6.1.3.6 Status of RR Types ........................... 79
6.1.3.7 Robustness ................................... 80
6.1.3.8 Local Host Table ............................. 80
6.1.4 DNS USER INTERFACE ................................ 81
6.1.4.1 DNS Administration ........................... 81
6.1.4.2 DNS User Interface ........................... 81
6.1.4.3 Interface Abbreviation Facilities ............. 82
6.1.5 DOMAIN NAME SYSTEM REQUIREMENTS SUMMARY ........... 84
6.2 HOST INITIALIZATION .................................... 87
6.2.1 INTRODUCTION ...................................... 87
6.2.2 REQUIREMENTS ...................................... 87
6.2.2.1 Dynamic Configuration ........................ 87
6.2.2.2 Loading Phase ................................ 89
6.3 REMOTE MANAGEMENT ...................................... 90
6.3.1 INTRODUCTION ...................................... 90
6.3.2 PROTOCOL WALK-THROUGH ............................. 90
6.3.3 MANAGEMENT REQUIREMENTS SUMMARY ................... 92

7. REFERENCES ................................................. 93

Internet Engineering Task Force [Page 4]

RFC1123 INTRODUCTION October 1989

1. INTRODUCTION

This document is one of a pair that defines and discusses the
requirements for host system implementations of the Internet protocol
suite. This RFC covers the applications layer and support protocols.
Its companion RFC, "Requirements for Internet Hosts -- Communications
Layers" [INTRO:1] covers the lower layer protocols: transport layer,
IP layer, and link layer.

These documents are intended to provide guidance for vendors,
implementors, and users of Internet communication software. They
represent the consensus of a large body of technical experience and
wisdom, contributed by members of the Internet research and vendor
communities.

This RFC enumerates standard protocols that a host connected to the
Internet must use, and it incorporates by reference the RFCs and
other documents describing the current specifications for these
protocols. It corrects errors in the referenced documents and adds
additional discussion and guidance for an implementor.

For each protocol, this document also contains an explicit set of
requirements, recommendations, and options. The reader must
understand that the list of requirements in this document is
incomplete by itself; the complete set of requirements for an
Internet host is primarily defined in the standard protocol
specification documents, with the corrections, amendments, and
supplements contained in this RFC.

A good-faith implementation of the protocols that was produced after
careful reading of the RFC's and with some interaction with the
Internet technical community, and that followed good communications
software engineering practices, should differ from the requirements
of this document in only minor ways. Thus, in many cases, the
"requirements" in this RFC are already stated or implied in the
standard protocol documents, so that their inclusion here is, in a
sense, redundant. However, they were included because some past
implementation has made the wrong choice, causing problems of
interoperability, performance, and/or robustness.

This document includes discussion and explanation of many of the
requirements and recommendations. A simple list of requirements
would be dangerous, because:

o Some required features are more important than others, and some
features are optional.

o There may be valid reasons why particular vendor products that

Internet Engineering Task Force [Page 5]

RFC1123 INTRODUCTION October 1989

are designed for restricted contexts might choose to use
different specifications.

However, the specifications of this document must be followed to meet
the general goal of arbitrary host interoperation across the
diversity and complexity of the Internet system. Although most
current implementations fail to meet these requirements in various
ways, some minor and some major, this specification is the ideal
towards which we need to move.

These requirements are based on the current level of Internet
architecture. This document will be updated as required to provide
additional clarifications or to include additional information in
those areas in which specifications are still evolving.

This introductory section begins with general advice to host software
vendors, and then gives some guidance on reading the rest of the
document. Section 2 contains general requirements that may be
applicable to all application and support protocols. Sections 3, 4,
and 5 contain the requirements on protocols for the three major
applications: Telnet, file transfer, and electronic mail,
respectively. Section 6 covers the support applications: the domain
name system, system initialization, and management. Finally, all
references will be found in Section 7.

1.1 The Internet Architecture

For a brief introduction to the Internet architecture from a host
viewpoint, see Section 1.1 of [INTRO:1]. That section also
contains recommended references for general background on the
Internet architecture.

1.2 General Considerations

There are two important lessons that vendors of Internet host
software have learned and which a new vendor should consider
seriously.

1.2.1 Continuing Internet Evolution

The enormous growth of the Internet has revealed problems of
management and scaling in a large datagram-based packet
communication system. These problems are being addressed, and
as a result there will be continuing evolution of the
specifications described in this document. These changes will
be carefully planned and controlled, since there is extensive
participation in this planning by the vendors and by the
organizations responsible for operations of the networks.

Internet Engineering Task Force [Page 6]

RFC1123 INTRODUCTION October 1989

Development, evolution, and revision are characteristic of
computer network protocols today, and this situation will
persist for some years. A vendor who develops computer
communication software for the Internet protocol suite (or any
other protocol suite!) and then fails to maintain and update
that software for changing specifications is going to leave a
trail of unhappy customers. The Internet is a large
communication network, and the users are in constant contact
through it. Experience has shown that knowledge of
deficiencies in vendor software propagates quickly through the
Internet technical community.

1.2.2 Robustness Principle

At every layer of the protocols, there is a general rule whose
application can lead to enormous benefits in robustness and
interoperability:

"Be liberal in what you accept, and
conservative in what you send"

Software should be written to deal with every conceivable
error, no matter how unlikely; sooner or later a packet will
come in with that particular combination of errors and
attributes, and unless the software is prepared, chaos can
ensue. In general, it is best to assume that the network is
filled with malevolent entities that will send in packets
designed to have the worst possible effect. This assumption
will lead to suitable protective design, although the most
serious problems in the Internet have been caused by
unenvisaged mechanisms triggered by low-probability events;
mere human malice would never have taken so devious a course!

Adaptability to change must be designed into all levels of
Internet host software. As a simple example, consider a
protocol specification that contains an enumeration of values
for a particular header field -- e.g., a type field, a port
number, or an error code; this enumeration must be assumed to
be incomplete. Thus, if a protocol specification defines four
possible error codes, the software must not break when a fifth
code shows up. An undefined code might be logged (see below),
but it must not cause a failure.

The second part of the principle is almost as important:
software on other hosts may contain deficiencies that make it
unwise to exploit legal but obscure protocol features. It is
unwise to stray far from the obvious and simple, lest untoward
effects result elsewhere. A corollary of this is "watch out

Internet Engineering Task Force [Page 7]

RFC1123 INTRODUCTION October 1989

for misbehaving hosts"; host software should be prepared, not
just to survive other misbehaving hosts, but also to cooperate
to limit the amount of disruption such hosts can cause to the
shared communication facility.

1.2.3 Error Logging

The Internet includes a great variety of host and gateway
systems, each implementing many protocols and protocol layers,
and some of these contain bugs and mis-features in their
Internet protocol software. As a result of complexity,
diversity, and distribution of function, the diagnosis of user
problems is often very difficult.

Problem diagnosis will be aided if host implementations include
a carefully designed facility for logging erroneous or
"strange" protocol events. It is important to include as much
diagnostic information as possible when an error is logged. In
particular, it is often useful to record the header(s) of a
packet that caused an error. However, care must be taken to
ensure that error logging does not consume prohibitive amounts
of resources or otherwise interfere with the operation of the
host.

There is a tendency for abnormal but harmless protocol events
to overflow error logging files; this can be avoided by using a
"circular" log, or by enabling logging only while diagnosing a
known failure. It may be useful to filter and count duplicate
successive messages. One strategy that seems to work well is:
(1) always count abnormalities and make such counts accessible
through the management protocol (see Section 6.3); and (2)
allow the logging of a great variety of events to be
selectively enabled. For example, it might useful to be able
to "log everything" or to "log everything for host X".

Note that different managements may have differing policies
about the amount of error logging that they want normally
enabled in a host. Some will say, "if it doesn't hurt me, I
don't want to know about it", while others will want to take a
more watchful and aggressive attitude about detecting and
removing protocol abnormalities.

1.2.4 Configuration

It would be ideal if a host implementation of the Internet
protocol suite could be entirely self-configuring. This would
allow the whole suite to be implemented in ROM or cast into
silicon, it would simplify diskless workstations, and it would

Internet Engineering Task Force [Page 8]

RFC1123 INTRODUCTION October 1989

be an immense boon to harried LAN administrators as well as
system vendors. We have not reached this ideal; in fact, we
are not even close.

At many points in this document, you will find a requirement
that a parameter be a configurable option. There are several
different reasons behind such requirements. In a few cases,
there is current uncertainty or disagreement about the best
value, and it may be necessary to update the recommended value
in the future. In other cases, the value really depends on
external factors -- e.g., the size of the host and the
distribution of its communication load, or the speeds and
topology of nearby networks -- and self-tuning algorithms are
unavailable and may be insufficient. In some cases,
configurability is needed because of administrative
requirements.

Finally, some configuration options are required to communicate
with obsolete or incorrect implementations of the protocols,
distributed without sources, that unfortunately persist in many
parts of the Internet. To make correct systems coexist with
these faulty systems, administrators often have to "mis-
configure" the correct systems. This problem will correct
itself gradually as the faulty systems are retired, but it
cannot be ignored by vendors.

When we say that a parameter must be configurable, we do not
intend to require that its value be explicitly read from a
configuration file at every boot time. We recommend that
implementors set up a default for each parameter, so a
configuration file is only necessary to override those defaults
that are inappropriate in a particular installation. Thus, the
configurability requirement is an assurance that it will be
POSSIBLE to override the default when necessary, even in a
binary-only or ROM-based product.

This document requires a particular value for such defaults in
some cases. The choice of default is a sensitive issue when
the configuration item controls the accommodation to existing
faulty systems. If the Internet is to converge successfully to
complete interoperability, the default values built into
implementations must implement the official protocol, not
"mis-configurations" to accommodate faulty implementations.
Although marketing considerations have led some vendors to
choose mis-configuration defaults, we urge vendors to choose
defaults that will conform to the standard.

Finally, we note that a vendor needs to provide adequate

Internet Engineering Task Force [Page 9]

RFC1123 INTRODUCTION October 1989

documentation on all configuration parameters, their limits and
effects.

1.3 Reading this Document

1.3.1 Organization

In general, each major section is organized into the following
subsections:

(1) Introduction

(2) Protocol Walk-Through -- considers the protocol
specification documents section-by-section, correcting
errors, stating requirements that may be ambiguous or
ill-defined, and providing further clarification or
explanation.

(3) Specific Issues -- discusses protocol design and
implementation issues that were not included in the walk-
through.

(4) Interfaces -- discusses the service interface to the next
higher layer.

(5) Summary -- contains a summary of the requirements of the
section.

Under many of the individual topics in this document, there is
parenthetical material labeled "DISCUSSION" or
"IMPLEMENTATION". This material is intended to give
clarification and explanation of the preceding requirements
text. It also includes some suggestions on possible future
directions or developments. The implementation material
contains suggested approaches that an implementor may want to
consider.

The summary sections are intended to be guides and indexes to
the text, but are necessarily cryptic and incomplete. The
summaries should never be used or referenced separately from
the complete RFC.

1.3.2 Requirements

In this document, the words that are used to define the
significance of each particular requirement are capitalized.
These words are:

Internet Engineering Task Force [Page 10]

RFC1123 INTRODUCTION October 1989

* "MUST"

This word or the adjective "REQUIRED" means that the item
is an absolute requirement of the specification.

* "SHOULD"

This word or the adjective "RECOMMENDED" means that there
may exist valid reasons in particular circumstances to
ignore this item, but the full implications should be
understood and the case carefully weighed before choosing
a different course.

* "MAY"

This word or the adjective "OPTIONAL" means that this item
is truly optional. One vendor may choose to include the
item because a particular marketplace requires it or
because it enhances the product, for example; another
vendor may omit the same item.