SMTP D. Crocker
Internet-Draft Brandenburg InternetWorking
Expires: July 27, 2005 January 26, 2005
Internet Mail Architecture
draft-crocker-email-arch-02
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Over its thirty year history, Internet mail has undergone significant
changes in scale and complexity. The first standardized architecture
for email specified a simple split between the user world and the
transmission world, in the form of Mail User Agents (MUA) and Mail
Transfer Agents (MTA). Over time each of these has divided into
multiple, specialized modules. Public discussion and agreement about
the nature of the changes to Internet mail has not kept pace, and
abuses of the Internet mail service have brought these issues into
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stark relief. This draft offers clarifications and enhancements, to
provide a more consistent base for community discussion of email
service problems and proposed email service enhancements.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Service Overview . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Document Changes . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Discussion venue . . . . . . . . . . . . . . . . . . . . . . . 5
2. Email Actor Roles . . . . . . . . . . . . . . . . . . . . . . 5
2.1 User-Level Actors . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Transfer-Level Actors . . . . . . . . . . . . . . . . . . . . 8
2.3 Administrative Actors . . . . . . . . . . . . . . . . . . . . 11
3. Email Identities . . . . . . . . . . . . . . . . . . . . . . . 11
3.1 Mailbox Addresses . . . . . . . . . . . . . . . . . . . . . . 12
3.2 Domain Names . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.3 Message Identifers . . . . . . . . . . . . . . . . . . . . . . 13
3.4 Identity Referencing Convention . . . . . . . . . . . . . . . 13
4. Protocols and Services . . . . . . . . . . . . . . . . . . . . 13
4.1 Service Components . . . . . . . . . . . . . . . . . . . . . . 15
4.2 Operational Configuration . . . . . . . . . . . . . . . . . . 21
4.3 Layers of Identity References . . . . . . . . . . . . . . . . 21
5. Message Data . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.1 Envelope . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.2 Message Header Fields . . . . . . . . . . . . . . . . . . . . 22
5.3 Body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6. Two Levels of Store-And-Forward . . . . . . . . . . . . . . . 23
6.1 MTA Relaying . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.2 MUA Forwarding . . . . . . . . . . . . . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 30
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 33
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 33
Intellectual Property and Copyright Statements . . . . . . . . 34
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1. Introduction
Over its thirty year history, Internet mail has undergone significant
changes in scale and complexity. The first standardized architecture
for email specified a simple split between the user world and the
transmission world, in the form of Mail User Agents (MUA) and Mail
Transfer Agents (MTA). Over time each of these has sub-divided into
more specialized modules. However the basic style and use of names,
addresses and message structure have remained remarkably constant.
There are two, basic categories of participants in Internet Mail.
Users are customers of the Mail Handling Service (MHS). They
represent the sources and sinks of that service. The Mail Handling
Service is responsible for accepting a message from one user and
delivering to one or more others.
+--------+
+---------------->| User |
| +--------+
| .
+--------+ | +--------+ .
| User +--+--------->| User | .
+--------+ | +--------+ .
| . .
. | +--------+ . .
. +-->| User | . .
. +--------+ . .
. . . .
. . . .
. . . .
+--------------------------------------+
| |
| Mail Handling Service (MHS) |
| |
+--------------------------------------+
Figure 1: Basic Email Service Model
Public discussion and agreement about terms of reference have not
kept pace with the changes, and abuses of the Internet mail service
have brought this into stark relief. So, it is necessary to produce
a revised architecture. However it is important that the original
distinction between user-level concerns and transfer-level concerns
be retained. This becomes challenging when the user-level exchange
is, itself, a sequence, such as with group dialogue or organizational
message flow, as occurs with a purchase approval process. It is easy
to confuse this user-level activity with the underlying mail
transmission service exchanges.
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For Internet mail, the term "end-to-end" usually refers to single
posting and the set of deliveries resulting from a single transiting
of the MHS. However, note that specialized uses of email consider
the entire email service -- including Originator and Recipient -- as
a subordinate component. For these services, "end-to-end" refers to
points outside of the email service. Examples are voicemail over
email and EDI over email.
The current draft seeks to:
1. Document changes that have taken place in refining the email
model
2. Clarify functional roles for the architectural components
3. Clarify identity-related issues, across the email service
4. Provide a common venue for further defining and citing modern
Internet mail architecture
1.1 Service Overview
End-to-end Internet mail exchange is accomplished by using a
standardized infrastructure comprising:
1. An email object
2. Global addressing
3. A connected sequence of point-to-point transfer mechanisms
4. No prior arrangement between originator and recipient
5. No prior arrangement between point-to-point transfer services,
over the open Internet
The end-to-end portion of the service is the message. Broadly the
message, itself, is divided between handling control information and
user message payload.
A precept to the design of Internet mail is to permit user-to-user
and MTA-to-MTA interoperability with no prior, direct administrative
arrangement. That is, all participants rely on having the core
services be universally supported, either directly or through
gateways that translate between Internet mail standards and other
email conventions.
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For localized environments (edge networks) prior, administrative
arrangement can include access control, routing constraints and
lookup service configuration. In recent years one change to local
environments is an increased requirement for authentication or, at
least, accountability. In these cases, the server performs explicit
validation of the client's identity.
1.2 Document Changes
The major changes from the previous version of this document are:
Overall: Clarify roles and responsibilities
Diagrams: Revised diagrams and tightened things up
Distinct architectural 'sections': Added concept of ADMDs, as
operational layer, separate from functional or architectural
layer. Added user "layer", as distinct from transfer. Introduced
'mediator'.
1.3 Discussion venue
NOTE: This document is the work of a single person, about a topic
with considerable diversity of views. It is certain to be
incomplete and inaccurate. Some errors simply need to be
reported; they will get fixed. Others need to be discussed by the
community, because the real requirement is to develop common
community views. To this end, please treat the draft as a
touchstone for public discussion.
Discussion about this document should be directed to the:
<mailto:ietf-smtp@imc.org> mailing list. The IETF-SMTP mailing list
<http://www.imc.org/ietf-smtp/index.htm> is the most active,
long-standing venue for discussing email architecture. Although this
list is primarily for discussing only the SMTP protocol, it is
recommended that discussion of this draft take place on that mailing
list. This list tends to attend to end-to-end infrastructure and
architecture issues more than other email-related mailing lists.
2. Email Actor Roles
Discussion of email architecture requires distinguishing different
actors within the service, and being clear about the job each
performs. The best way to maintain the distinction between user
activity and handling activities is to depict their details in
separate diagrams. Current Internet mail provides only a small set
of capabilities for supporting different kinds of ongoing, user-level
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exchanges.
Although related to a technical architecture, the focus of a
discussions on Actors is on participant responsibilities, rather than
functional modules. Hence the labels used are different than for
classic email architecture diagrams. The figures depict the
relationships among the Actors. Actors often will be associated with
entirely independent organizations from other Actors who are
participating in the email service.
2.1 User-Level Actors
Users are the sources and sinks of messages. They may have an
exchange that iterates and they may expand or contract the set of
users participating in a set of exchanges.
In Internet Mail there are three, basic types of user-level Actors:
Originators, Recipients, and Mediators. Fromhe t User-level
perspective all mail transfer activities are performed by a
monolithic, shared handling service. Users are customers of this
service. The following depicts the relationships among them.
+------------+
| Originator |<--------------+
+-+---+----+-+ |
| | | |
| | V |
| | +-----------+ |
| | | Recipient | |
| | +-----------+ |
| | |
| | +----------+ |
| | | | |
| V V | |
| +-----------+ +---+---+---+
| | Mediator +--->| Recipient |
| +-----------+ +-----------+
|
V
+-----------+ +-----------+ +-----------+
| Mediator +--->| Mediator +--->| Recipient |
+-----------+ +-----------+ +-----------+
The functions of these Actors are:
2.1.1 Originator
Also called "Author", this is the user-level participant responsible
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for creating original content and requesting its transmission. The
Mail Handling Service operates to send and deliver mail among
Originators and Recipients.
2.1.2 Recipient
The Recipient is a consumer of delivered content.
A recipient may close the user-level communication loop by creating
and submitting a new message that replies to an originator. An
automated, or semi-automated form of reply informs the Originator
about the Recipient's disposition of the message.
2.1.3 Mediator
A Mediator receives, aggregates, reformulates and distributes
messages as part of a potentially-protracted, higher-level exchange
among users. A Mediator is viewed by the Mail Handling Service, when
the Mediator's address is specified in the envelope. When submitting
messages, the Mediator is an Originator. What is distinctive is that
a Mediator preserves Originator information of the message(s) it
reformulates, but makes meaningful changes to the content. Hence the
Mail Handling Service sees a new message, but Users receive a message
that is interpreted as primarily being from the author of the
original message. The role of a Mediator permits distinct, active
creativity, rather than being limited the more passive job of merely
connecting together other participants. Hence it is really the
Mediator that is responsible for the new message.
A Mediator's task may be complex, contingent and creative, such as by
modifying and adding content or regulating which users may
participate and when. The popular example of this role is a group
mailing list. A sequence of mediators may even perform a series of
formal steps, such as reviewing, modifying and approving a purchase
request.
Because a Mediator originates messages, it might also receive
replies. That is, a Mediator is a full-fledged User.
Specialized Mediators include:
Forwarder: A new message encapsulates the original message and is
seen as strictly "from" the Mediator. However the Mediator might
add commentary and certainly has the opportunity to modify the
original message content.
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Redirector: Redirection differs from Forwarding by virtue of having
the Mediator "splice" communication between the Originator of the
original message and the Recipient of the new message. Hence the
new Recipient sees the message as being From the original
Originator.
Mailing List: This Actor performs a task that can be viewed as an
elaboration of the Redirector role. In addition to sending the
new message to a potentially large number of new Recipients,
content might be modified, such as deletion of attachments,
formatting conversion, and addition of list-specific comments. In
additional, archival of list messages is common.
Annotator: The integrity of the original message is preserved, but
one or more comments about the message are added in a manner that
distinguishes commentary from original text.
Adaptor: {per Ned Freed}
Security Filter: Organizations often enforce security boundaries by
having message subjected to analysis for conformance with the
organization's safety policies. Examples are detection of content
classed as spam or a virus. A Security Filter might alter the
content, to render it safe, such as by removing content deemed
unacceptable. Typically these actions will result in the addition
of content that records the actions.
2.2 Transfer-Level Actors
The Mail Handling Service has the task of performing a single,
end-to-end transfer on behalf of the originator and reaching the
recipient address(es) specified in the envelope. Protracted,
iterative exchanges, such as those used for collaboration over time,
are part of the User-level service, and are not part of this
Transfer-level service.
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The following depicts the relationships among transfer participants
in Internet Mail. It shows Source as distinct from the Originator,
although it is common for them to be the same actor. The figure also
shows multiple Relays in the sequence. It is legal to have only one,
and for intra-organization mail services, this is common.
+------------+ +-----------+
| Originator | | Recipient |
+-----+------+ +-----------+
| ^
| Mail Handling Service |
+===================================================+
|| | | ||
|| | | ||
V |
+---------+ +--------+ +----+----+
| | | |<------------+ |
| Source +...>| Notice | | Dest |
| | | |<---+ | |
+----+----+ +--------+ | +---------+
| | ^
V | |
+---------+ +----+----+ +----+----+
| Relay +-->.......-->| Relay +-->| Relay |
+---------+ +----+----+ +---------+
|
V
+---------+
| Gateway +-->...
+---------+
2.2.1 Source
The Source role is responsible for ensuring that a message is valid
for posting and then submitting it to a mail relay. Validity
includes conformance with Internet mail standards, as well as local
operational policies. Source may simply review the message for
conformance, and reject it if there are errors, or it may create some
or all of the necessary information.
Source operates with dual allegiance. It serves the Originator and
often it is the same entity. However its role in assuring validity
means that it must represent the local operator of the Mail Handling
Service.
Source also has the responsibility for any post-submission,
originator-related administrative tasks associated with message
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transmission and delivery. Notably this pertains to error and
delivery notices. Hence, Source is best held accountable for the
message content, even when they did not create any or most of it.
2.2.2 Notices Handler
Transfer efforts might result in the generation of service reporting
information about failures or completions. These Transfer or
Delivery notification messages are sent to an address that is
specified by the Source. A Notices handling address (also known as
Bounce or Return address) might have no characteristics in common the
with address of the Originator or Source.
2.2.3 Relay
A mail relay performs email transfer-service routing and
store-and-forward. It adds envelope-related handling information and
then (re-)transmits the message on towards its recipient(s). A Relay
does not modify the message contents.
A basic transfer operation is between a client and a server Relay. A
set of Relays composes a Mail Handling Service network. This is
above any underlying packet-switching network that they might be
using.
Aborting message transfer results in having the Relay become an
Originator and send an error message to the Notifications (Bounce)
address. (The potential for looping is avoided by having this
message, itself, contain no Bounce address.
2.2.4 Gateway
A Gateway is a special form of Relay that interconnects heterogeneous
mail services. Differences between the services can be as small as
minor syntax variations, but usually encompass much more basic,
semantic distinctions. For example, the concept of an email address
might be as different as a hierarchical, machine-specific address
versus a flat, global name space. Or between text-only and
multi-media. Hence, the Relay function of a gateway is the minor
component. The significant challenge is in the user-to-user
functionality that matches syntax and semantics of independent email
standards suites.
The basic test of a gateway's adequacy is, of course, whether an
originator can send a message to a recipient, without requiring any
changes to the components in the originator's mail service or the
recipient's mail service, other than adding the gateway. To each of
these otherwise independent services, the gateway will appear to be a
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"native" participant. However the ultimate test of a gateway's
adequacy is whether the originator and recipient can sustain a
dialogue. In particular, can a recipient formulate a Reply?
2.3 Administrative Actors
Operation of Internet mail services is apportioned to different
providers (or operators) each is composed of an independent
Administrative Domain. Examples include an end-user operating their
desktop client, a department operating a local relay, an IT
department operating an enterprise relay, and an ISP operating a
public, shared email service. These can be configured into many
combinations of administrative and operational relationships, with
each Administrative Domain potentially having a complex arrangement
of functional components.
The interactions between functional components within an
Administrative Domain are subject to the policies of that domain.
Policies can cover such things as reliability, access control,
accountability and content evaluation and may be implemented in
different functional components, according to the needs of the
Administrative Domain.
2.3.1 Provider
Providers operate component services or sets of services. It is
possible for Providers to host services for other Providers. Common
examples are:
Enterprise Service Providers: Operating an organization's internal
data and/or mail operations.
Internet Service Providers: Operating underlying data communication
services that, in turn, are used by one or more Relays and Users.
It is not their job to perform email functions, but to provide an
environment in which those functions can be performed.
Mail Service Providers: Operate email services, such as for
end-users, or mailing lists.
Operational pragmatics often dictate that Providers be involved in
detailed administration and enforcement issues, to help insure the
health of the overall Internet Mail Service.
3. Email Identities
Internet mail uses three forms of identity. The most common is the
mailbox address <addr-spec> [RFC2822]. The other two forms are the
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<domain name> [RFC1034] and message identifier [RFC2822].
3.1 Mailbox Addresses
An addr-spec has two distinct parts, divided by an at-sign ("@").
The right-hand side contains a globally interpreted name for an
administrative domain. This domain name might refer to an entire
organization, or to a collection of machines integrated into a
homogeneous service, or to a single machine. Domain names are
defined and operated through the DNS [RFC1034], [RFC1035].
The left-hand side of the at-sign contains a string that is globally
opaque and is called the <local-part>. It is to be interpreted only
by the entity specified in the address's right-hand side. All other
entities must treat the local-part as a uninterpreted, literal string
and must preserve all of its original details. As such, its
distribution is equivalent to sending a "cookie" that is only
interpreted upon being returned to its originator.
3.1.1 Global Standards for Local-Part
It is common for sites to have local structuring conventions for the
left-hand side (local-part) of an addr-spec. This permits
sub-addressing, such as for distinguishing different discussion
groups by the same participant. However it must be stressed that
these conventions are strictly private to the user's organization and
must not be interpreted by any domain except the one listed in the
right-hand side of the add-spec.
A small class of addresses have an elaboration on basic email
addressing, with a standardized, global schema for the local-part.
These are conventions between originating end-systems and recipient
gateways, and they are invisible to the public email transfer
infrastructure. When an originator is explicitly sending via a
gateway out of the Internet, there are coding conventions for the
local-part, so that the originator can formulate instructions for the
gateway. Standardized examples of this are the telephone numbering
formats for VPIM [RFC2421], such as "+16137637582@vpim.example.com",
and iFax [RFC2304], such as "FAX=+12027653000/
T33S=1387@ifax.example.com".
3.1.2 Scope of Email Address Use
Email addresses are being used far beyond their original email
transfer and delivery role. In practical terms, email strings have
become a common form of user identity on the Internet. What is
essential, then, is to be clear about the nature and role of an
identity string in a particular context and to be clear about the
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entity responsible for setting that string.
3.2 Domain Names
A domain name is a global reference to an Internet resource, such as
a host, a service or a network. A name usually maps to one or more
IP Addresses. A domain name can be administered to refer to
individual users, but this is not common practice. The name is
structure as a hierarchical sequence of sub-names, separated by dots
(".").
When not part of a mailbox address, a domain name is used in Internet
mail to refer to a node that took action upon the message, such as
providing the administrative scope for a message identifier, or
performing transfer processing.
3.3 Message Identifers
Message identifiers have two distinct parts, divided by an at-sign
("@"). The right-hand side contains a globally interpreted name for
the administrative domain assigning the identifier. The left-hand
side of the at-sign contains a string that is globally opaque and
serves to uniquely identify the message within the domain referenced
on the right-hand side. The duration of uniqueness for the message
identifier is undefined.
The identifier may be assigned by the user or by any component of the
system along the path. Although Internet mail standards provide for
a single identifier, more than one is sometimes assigned.
3.4 Identity Referencing Convention
In this document, fields references to identities are labeled in a
two-part, dotted notation. The first part cites the document
defining the identity and the second defines the name of the
identity. Hence, <RFC2822.From> is the From field in an email
content header, and <RFC2821.MailFrom> is the address in the SMTP
"Mail From" command.
4. Protocols and Services
NOTE: A discussion about any interesting system architecture is
often complicated by confusion between architecture versus
implementation. An architecture defines the conceptual functions
of a service, divided into discrete conceptual modules. An
implementation of that architecture may combine or separate
architectural components, as needed for a particular operational
environment. It is important not to confuse the engineering
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decisions that are made to implement a product, with the
architectural abstractions used to define conceptual functions.
Modern Internet email architecture distinguishes four types of
functional components, arranged to support a store-and-forward
service architecture:
+------+
.............+ oMUA |<------------------------------+
. +--+---+ |
. | { smtp,submission |
. V |
. +------+ |
. | MSA |<--------------------+ |
. +--+---+ | |
. | { smtp | |
. V | |
. +------+ +====+====+ |
. | MTA | || dsn || |
+============+ +--+---+ +=========+ |
|| MESSAGE || . { smtp ^ ^ |
|| || . | | |
||(envelope)|| . | | |
|| || V | | |
|| RFC2822 || +------+ | | +===+===+
|| || | MTA +-------------------+ | || mdn ||
|| MIME || +--+---+ | +=======+
+============+ | { local, smtp, lmtp | |
. V | |
. +------+ | |
. | +-----------------------+ |
. | MDA | |
. | |<--------------------+ |
. +-+--+-+ | |
. local } | | | |
. V | | |
. +------+ | +====+====+ |
. | MS-1 | | || sieve || |
. +-+--+-+ | +=========+ |
. | | | { pop, imap ^ |
. | V V | |
. | +------+ | |
. | | MS-2 | | |
. | +--+---+ | |
. | | { pop, imap, local | |
. V V | |
. +------+ | |
.........>| rMUA +------------------------+---------+
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+------+
Software implementations of these architectural components often
compress them, such as having the same software do MSA, MTA and MDA
functions. However the requirements for each of these components of
the service are becoming more extensive. So, their separation is
increasingly common.
4.1 Service Components
4.1.1 Mail User Agent (MUA)
A Mail User Agent (MUA) works on behalf of end-users and end-user
applications. It is their "representative" within the email service.
At the origination side of the service, the oMUA is used to create a
message and perform initial "submission" into the transfer
infrastructure, via a Mail Submission Agent (MSA). It may also
perform any creation- and posting-time archival. An MUA outbox is
part of the origination-side MUA.
The recipient-side rMUA works on behalf of the end-user recipient to
process received mail. This includes generating user-level return
control messages, display and disposition of the received message,
and closing or expanding the user communication loop, by initiating
replies and forwarding new messages.
An MUA may, itself, have a distributed architecture, such as
implementing a "thin" user interface module on a limited end-user
device, with the bulk of the MUA functionality operated remotely on a
more capable server. An example of such an architecture might use
IMAP [RFC3501] for most of the interactions between an MUA client and
an MUA server.
A special class of MUA performs message re-posting, as discussed in
the <Mediator> section.
Identity fields set by the MUA include:
RFC2822.From
Actor: Originator
Names and addresses for author(s) of the message content are
listed in the From header field
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RFC2822.Reply-To
Actor: Originator
If a message recipient sends a reply message that would otherwise
use RFC2822.From field address(es) contained in the original
message, then they are instead to use the address(es) in the
RFC2822.Reply-To field. In other words, this field is a direct
override of the From field, for responses from recipients.
RFC2822.Sender
Actor: Source
This specifies the address responsible for submission into the
transfer service. For efficiency, this field should be omitted if
it contains the same address as RFC2822.From. However this does
not mean there is no Sender specified. Rather, it means that that
header field is virtual and that the address in the From field
must be used. Specification of the error return addresses -- the
"notifications" (or "bounces") address, contained in
RFC2821.MailFrom -- is made by the RFC2822.Sender. Typically the
notifications address is the same as the Sender address. However
some usage scenarios require it to be different.
RFC2822.To, RFC2822.CC
Actor: Recipient
These specify MUA recipient addresses. The distinction between To
and CC is subjective. Generally, a To addressee is considered
primary and is expected to take action on the message. A CC
addressee typically receives a copy only for their information.
RFC2822.BCC
Actor: Recipient
A message might be copied to an addressee whose participation is
not to be disclosed to the RFC2822.To or RFC2822.CC recipients.
The BCC header field indicates a message copy to such a recipient.
Typically, the field lists no addresses or only lists the address
of the single recipient receiving the copy. This usually ensures
that even other BCC recipients do not know of each other. An MUA
will typically make separate postings for TO and CC recipients,
versus BCC recipients. The former will see no indication that any
BCCs were sent, whereas the latter have a BCC field present. It
might be empty, contain a comment, or contain one or more BCC
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addresses, depending upon the preferences or the Originator.
4.1.2 Mail Submission Agent (MSA)
A Mail Submission Agent (MSA) accepts the message submission from the
oMUA and enforces the policies of the hosting network and the
requirements of Internet standards. Enforcement might be passive,
involving review and approval or rejection, or it might be active,
involving direct modification of the message. An MSA implements a
server function to MUAs and a client function to MTAs (or MDAs).
Examples of MSA-styled functions, in the world of paper mail, might
range across the very different capabilities of administrative
assistants, postal drop boxes, and post office front-counter
employees.
The MUA/MSA interface can be implemented within single host and use
private conventions for their interactions. Historically,
standards-based MUA/MSA interactions have used SMTP [RFC2821].
However a recent alternative is SUBMISSION [RFC2476]. Although
SUBMISSION derives from SMTP, it operates on a separate TCP port, and
will typically impose distinct requirements, such as access
authorization.
Identities set by the MSA include:
RFC2821.HELO or RFC2821.EHLO
Actor: Source
The MSA may specify its hosting domain identity for the SMTP HELO
or EHLO command operation.
RFC2821.MailFrom
Actor: Source
This is an end-to-end string that specifies an email address for
receiving return control information, such as "bounces". The name
of this field is misleading, because it is not required to specify
either the author or the agent responsible for submitting the
message. Rather, the agent responsible for submission specifies
the RFC2821.MailFrom address. Ultimately the simple basis for
deciding what address needs to be in the RFC2821.MailFrom is to
determine what address needs to be informed about
transmission-level problems (and, possibly, successes.
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RFC2821.Rcpt-To
Actor: Recipient
This specifies the MUA inbox address of a recipient. The string
might not be visible in the message content header. For example,
the message destination address header fields, such as RFC2822.To,
might specify a mailing list address, while the RFC2821.Rcpt-To
address specifies a member of that list.
RFC2821.Received
Actor: Source
An MSA may record a Received header field, to indicate initial
submission trace information, including originating host and MSA
host domain names and/or IP Addresses.
4.1.3 Mail Transfer Agent (MTA)
An <MTA> relays a message to another other MTA or to an <MDA>, in a
point-to-point exchange. Relaying is performed by a sequence of
MTAs, until the message reaches its destination MDA. Hence an MTA
implements both client and server MTA functionality.
The basic functionality of an MTA is similar to that of a packet
switch or IP router. That is, it does email store-and-forward email,
with a routing decision determining where the next-hop destination
shall be. The primary "routing" mechanism for Internet mail is the
DNS MX record [RFC1035]. As with most "link layer" mechanisms
Internet mail's SMTP supports a basic level of reliability, by virtue
of providing for retransmission after al transfer failure. However
the degree of persistence by an MTA can be highly variable.
However email objects are typically much larger than the payload of a
packet or datagram, and the end-to-end latencies are typically much
higher. Contrary to typical packet switches (and Instant Messaging
services) Internet mail MTAs typically store messages in a manner
that allows recovery across services interruptions, such as host
system shutdown.
Internet mail primarily uses SMTP [RFC2821], [RFC0821] to effect
point-to-point transfers between peer MTAs. Other transfer
mechanisms include Batch SMTP [RFC2442] and ODMR [RFC2645]
An important characteristic of MTA-MTA communications, over the open
Internet, is that they do not require prior arrangement between the
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independent administrations operating the different MTAs. Given the
importance of spontaneity and serendipity in the world of human
communications, this lack of prearrangement, between the
participants, is a core benefit of Internet mail and remains a core
requirement for it.
Identities set by the MTA include:
RFC2821.HELO
Actor: Relay
The MTA may specify its hosting domain identity for the SMTP HELO
or EHLO command operation.
RFC2821.Return-Path
Actor: Source
The MDA records the RFC2821.MailFrom address into an RFC2822
header field named Return-Path.
RFC2822.Received
Actor: Relay
An MTA must record a Received header field, to indicate trace
information, including source host and receiving host domain names
and/or IP Addresses.
4.1.4 Mail Delivery Agent (MDA)
The <MDA> delivers email to the recipient's inbox.
A Mail Delivery Agent (MDA) can provide distinctive, address-based
functionality, made possible by its detailed knowledge of the
properties of the destination address. This knowledge might also be
present elsewhere in the recipient's Administrative Domain, such as
at an organizational border gateway. However it is required for the
MDA, if only because the MDA must know where to store the message.
This knowledge is used to achieve differential handling of messages.
Using Internet protocols, delivery is effected with POP [RFC1939] or
IMAP [RFC3501]. When coupled with an internal, local mechanism, SMTP
permits "push" delivery to the recipient system, at the initative of
the upstream email service. POP is used for "pull" delivery at the
initiative of the recipient system. Notably, SMTP and POP effect a
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transfer of message control from the email service to the recipient
host. In contrast, IMAP provides on-going, interactive access to a
message store, and does not effect a transfer of message control to
the end-user host. Instead, control stays with the message store
host that is being access by the user.
Identities set by the MDA include:
RFC2821.HELO or RFC2821.EHLO
Actor: Relay
The MDA may specify its hosting domain identity for the SMTP HELO
or EHLO command operation.
RFC2822.Received
Actors: Source, Relay, Dest
An MTA must record a Received header field, to indicate trace
information, including source host and receiving host domain names
and/or IP Addresses.
4.1.5 Message Store
An MUA's uses a long-term Message Store (MS). A rich set of choices
for the use of that store derives from permitting more than one to be
associated with a single user, demonstrated as MS-1 and MS-2 in the
Figure. MS-1 is shown as being remote from the MUA and MS-2 as being
local. Further the relationship between two message store may vary.
Between the MDA and the MUA, these choices are supported by a wide
variety of protocol options.
The operational relationship among two MSs can be:
Online: Only a remote MS is used, with messages being accessible
only when the MUA is attached to the MS, and the MUA repeatedly
fetches all or part of a message, from one session to the next.
Offline: The MS is local to the user, and messages are moved from
any remote store, rather than (also) being retained there.
Disconnected: A remote MS and a local MS synchronize all or parts of
their contents, while connected. The user may make changes while
disconnected, and the two stores are re-synchronized upon
reconnection.
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4.2 Operational Configuration
Mail service components can be arranged into numerous organizational
structures, each with independent software and administration. One
common arrangement is to distinguish:
1. an open, core, global email transfer infrastructure
2. independent transfer services in networks at the edge of the core
3. end-user services
Edge networks may use proprietary email standards. However the
distinction between "public" network and edge network transfer
services is primarily significant because it highlights the need for
concern over interaction and protection between independent
administrations. In particular, this distinctions calls for
additional care in assessing transitions of responsibility, as well
as the accountability and authorization relationships among
participants in email transfer.
On the other hand, real-world operations of Internet mail
environments do impose boundaries such as access control at
organizational firewalls to the Internet. It should be noted that
the current Internet Mail architecture offers no special constructs
for these configuration choices. The current design of Internet mail
is for a seamless, end-to-end store-and-forward sequence. It is
possible that the architectural enhancement will not require new
protocols, but rather will require clarification of best practises,
as exemplified by a recent effort [ID-spamops]
4.3 Layers of Identity References
For a message in transit, the core identity fields combine into:
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+-----------------------+-------------+---------------------+
| Layer | Field | Set By |
+-----------------------+-------------+---------------------+
| Message Content | MIME Header | Originator |
| Message header fields | From | Originator |
| | Sender | Source |
| | Reply-To | Originator |
| | To, CC, BCC | Originator |
| | Received | Source, Relay, Dest |
| | Return-Path | MDA, from MailFrom |
| SMTP | HELO | Latest Relay Client |
| | MailFrom | Source |
| | RCPT-TO | Originator |
| IP | IP Address | Latest Relay Client |
+-----------------------+-------------+---------------------+
5. Message Data
5.1 Envelope
Information that is directly used or produced by the email transfer
service is called the "envelope". It controls and records handling
activities by the transfer service. Internet mail has a fragmented
framework for handling this "handling" information. The envelope
exists partly in the transfer protocol SMTP [RFC2821] and partly in
the message object [RFC2822].
Direct envelope addressing information, as well as optional transfer
directives, are carried in-band by MTAs. All other envelope
information, such as trace records, is carried within the content
header fields. Upon delivery, SMTP-level envelope information is
typically encoded within additional content header fields, such as
Return-Path and Received (From and For).
5.2 Message Header Fields
Header fields are attribute/value pairs covering an extensible range
of email service, user content and user transaction meta-information.
The core set of header fields is defined in [RFC2822], [RFC0822]. It
is common to extend this set, for different applications. A complete
set of registered header fields is being developed through
[ID-hdr-reg].
One danger with placing additional information in header fields is
that gateways often alter or delete them.
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5.3 Body
The body of a message might simply be lines of ASCII text or it might
be structured into a composition of multi-media, body-part
attachments, using MIME [RFC2045], [RFC2046], [RFC2047], [RFC2048],
and [RFC2049]. It should be noted that MIME structures each
body-part into a recursive set of MIME header field meta-data and
MIME Content sections.
6. Two Levels of Store-And-Forward
Basic email transfer is accomplished with an asynchronous
store-and-forward communication infrastructure. This means that
moving a message from an originator to a recipient involves a
sequence of independent transmissions through some number of
intermediaries, called MTAs. A very different task is the user-level
process of re-posting a message through a new submission process,
after final delivery for an earlier transfer sequence. Such
MUA-based re-posting shares some functionality with basic MTA
relaying, but it enjoys a degree of freedom with both addressing and
content that is not available to MTAs.
The primary "routing" mechanism for Internet mail is the DNS MX
record [RFC1035]. It is an advertisement, by a recipient domain, of
hosts that are able to relay mail to it, within the portion of the
Internet served by this instance of the DNS.
6.1 MTA Relaying
MTAs relay mail. They are like packet-switches and IP routers.
Their job is to make routing assessments and to move the message
payload data closer to the recipient. It is not their job to
reformulate the payload or to change addresses in the envelope or the
content.
6.2 MUA Forwarding
As discussed in <Mediator> section, forwarding is performed by MUAs
that take a received message and submit it back to the transfer
service, for delivery to one or more different addresses. A
forwarded message may appear identical to a relayed message, such as
for Alias forwarders, or it may have minimal similarity, as with a
Reply.
6.2.1 MUA Basic Forwarding
The simplest type of forwarding involves creating an entirely new
message, with new content, that includes the original message between
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Originator-1 and Recipient-1. However this forwarded communication
is between Recipient-1 (who could also be called Originator-2) and a
new recipient, Recipient-2. The forwarded message is therefore
independent of the original message exchange and creates a new
message dialogue.
6.2.2 MUA Re-Sending
A recipient may wish to declare that an alternate addressee should
take on responsibility for a message, or otherwise become involved in
the original communication. They do this through a user-level
forwarding function, called re-sending. The act of re-sending, or
re-directing, splices a communication between Originator-1 and
Recipient-1, to become a communication between Originator-1 and new
Recipient-2. In this case, the content of the new message is the old
message, including preservation of the essential aspects of the
original message's origination information.
Identities specified in a resent message include
RFC2822.From
Actor: Originator
Names and email addresses for the original author(s) of the
message content are retained. The free-form (display-name)
portion of the address might be modified to provide informal
reference to the agent responsible for the redirection.
RFC2822.Reply-To
Actor: Originator
If this field is present in the original message, it should be
retained in the Re-sent message.
RFC2822.Sender
Actor: Source
This field is expected to contain the original Sender value.
RFC2822.TO, RFC2822.CC, RFC2822.BCC
Actor: Recipient
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These specify the original message recipients.
RFC2822.Resent-From
Actor: Mediating Originator
The address of the original recipient who is redirecting the
message. Otherwise, the same rules apply for the Resent-From
field as for an original RFC2822.From field
RFC2822.Resent-Sender
Actor: Mediating Source
The address of the agent responsible for re-submitting the
message. For efficiency, this field should be omitted if it
contains the same address as RFC2822.Resent-From. However this
does not mean there is no Resend-Sender specified. Rather, it
means that that header field is virtual and that the address in
the Resent-From field must be used. Specification of the error
return addresses (the "bounces" address, contained in
RFC2821.MailFrom) is made by the Resent-Sender. Typically the
bounce address is the same as the Resent-Sender address. However
some usage scenarios require it to be different.
RFC2822.Resent-To, RFC2822.Resent-cc, RFC2822.Resent-bcc: Actor:
Recipient
The addresses of the new recipients who will now be able to reply
to the original author.
RFC2821.MailFrom
Actor: Mediating Source
The agent responsible for re-submission (RFC2822.Resent-Sender) is
also responsible for specifying the new RFC2821.MailFrom address.
RFC2821.Rcpt-to
Actor: Recipient
This will contain the address of a new recipient
RFC2822.Received
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Actor: Mediating Source
When re-sending a message, the submission agent may record a
Received header field, to indicate the transition from original
posting to resubmission.
6.2.3 MUA Reply
When a recipient formulates a response to a message, the new message
is not typically viewed as being a "forwarding" of the original.
6.2.4 MUA Gateways
Gateways perform the basic routing and transfer work of message
relaying, but they also make any message or address modifications
that are needed to send the message into the next messaging
environment. When a gateway connects two differing messaging
services, its role is easy to identify and understand. When it
connects environments that have technical similarity, but may have
significant administrative differences, it is easy to think that a
gateway is merely an MTA. The critical distinguish between an MTA
and a gateway is that the latter modifies addresses and/or message
content.
A gateway may set any identity field available to a regular MUA.
Identities typically set by gateways include:
RFC2822.From
Actor: Originator
Names and email addresses for the original author(s) of the
message content are retained. As for all original addressing
information in the message, the gateway may translate addresses in
whatever way will allow them continue to be useful in the target
environment.
RFC2822.Reply-To
Actor: Originator
The gateway should retain this information, if it is originally
present. The ability to perform a successful reply by a gatewayed
recipient is a typical test of gateway functionality.
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RFC2822.Sender
Actor: Source
This may retain the original value or may be set to a new address
RFC2822.TO, RFC2822.CC, RFC2822.BCC
Actor: Recipient
These usually retain their original addresses.
RFC2821.MailFrom
Actor: Source
The agent responsible for gatewaying the message may choose to
specify a new address to receive handling notices.
RFC2822.Receive
Actors - Source, Relay, Dest
The gateway may record a Received header field, to indicate the
transition from original posting to the new messaging environment.
6.2.5 MUA Alias Handling
A simple re-addressing facility that is available in most MDA
implementations is called Aliasing. It is performed just before
placing a message into the specified recipient's inbox. Instead, the
message is submitted back to the transfer service, for delivery to
one or more alternate addresses. Although implemented as part of the
message delivery service, this facility is strictly a recipient user
function. In effect it resubmits the message to a new address, on
behalf of the listed recipient.
What is most distinctive about this forwarding mechanism is how
closely it compares to normal MTA store-and-forward. In reality its
only interesting difference is that it changes the RFC2821.RCPT-TO
value. Notably it does not typically change the RFC2821.Mailfrom
An MDA that is re-posting a message to an alias typically changes
only envelope information:
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RFC2822.TO, RFC2822.CC, RFC2822.BCC
Actor: Recipient
These retain their original addresses.
RFC2821.Rcpt-To
Actor: Recipient
This field contains an alias address.
RFC2821.MailFrom
Actor: Mediating Source
The agent responsible for submission to an alias address will
usually retain the original address to receive handling
notifications. The benefit of retaining the original MailFrom
value is to ensure that the origination-side agent knows of that
there has been a delivery problem. On the other hand, the
responsibility for the problem usually lies with the recipient,
since the Alias mechanism is strictly under the recipient's
control.
RFC2821.Received
Actor: Mediating Recipient
The agent should record Received information, to indicate the
delivery to the original address and submission to the alias
address. The trace of Received header fields should include
everything from original posting through final delivery to the
alias.
6.2.6 MUA Mailing Lists
Mailing lists have explicit email addresses and they forward messages
to a list of subscribed members. Mailing list processing is a
user-level activity, outside of the core email transfer service. The
mailing list address is, therefore, associated with a distinct
user-level entity that can perform arbitrary actions upon the
original message, before submitting it to the mailing list
membership. Hence, mailing lists are similar to gateways.
Identities set by a mailing list processor, when submitting a
message, include:
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RFC2919.List-id
Actor: Mediating Originator
This provides a global mailing list naming framework that is
independent of particular hosts. Although [RFC2919] is a
standards-track specification, it has not gained significant
adoption.
RFC2369.List-*
Actor: Mediating Recipient
[RFC2369] defines a collection of message header fields for use by
mailing lists. In effect, they supply list-specific parameters
for common mailing list user operations. The identifiers for
these operations are for the list, itself, and the
user-as-subscriber.
RFC2822.From
Actor: Originator
Names and email addresses for the original author(s) of the
message content are specified.
RFC2822.Reply-To
Actor: Originator
Mailing lists have introduced an ambiguity for the Reply-To field.
Some List operations choose to force all replies to go to all list
members. They achieve this by placing the list address into the
RFC2822.Reply-To field. Hence, direct, "private" replies only to
the original author cannot be achieved by using the MUA's typical
"reply to author" function. If the author created a Reply-To
field, its information is lost.
RFC2822.Sender
Actor: Source
This will usually specify the address of the agent responsible for
mailing list operations. However, some mailing lists operate in a
manner very similar to a simple MTA relay, so that they preserve
as much of the original handling information as possible,
including the original RFC2822.Sender field.
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RFC2822.TO, RFC2822.CC
Actor: Mediating Recipient
These will usually contain the original list of recipient
addresses.
RFC2821.MailFrom
Actor: Mediating Source
This may contain the original address to be notified of
transmission issues, or the mailing list agent may set it to
contain a new notification address. Typically, the value is set
to a new address, so that mailing list members and posters are not
burdened with transmission-related notifications.
RFC2821.Rcpt-To
Actor: Recipient
This contains the address of a mailing list member.
RFC2821.Received
Actor: Mediating Recipient
An Mailing List Agent should record a Received header field, to
indicate the transition from original posting to mailing list
forwarding. The Agent may choose to have the message retain the
original set of Received header fields or may choose to remove
them. In the latter case, it should ensure that the original
Received header fields are otherwise available, to ensure later
accountability and diagnostic access to it.
7. Security Considerations
This document does not specify any new Internet mail functionality.
Consequently it should introduce no new security considerations.
However its discussion of the roles and responsibilities for
different mail service modules, and the information they create,
highlights the considerable security considerations that must be
present when implementing any component of the Internet mail service.
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8 References
[ID-hdr-reg]
"Registration of mail and MIME header fields",
draft-klyne-hdrreg-mail-04.txt (work in progress), Apr
2004.
[ID-spamops]
Hutzler, C., Crocker, D., Resnick, P., Sanderson, R. and
E. Allman, "Email Submission Between Independent
Networks", draft-spamops-00 (work in progress), March
2004.
[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
821, August 1982.
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[RFC2033] Myers, J., "Local Mail Transfer Protocol", RFC 2033,
October 1996.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
[RFC2048] Freed, N., Klensin, J. and J. Postel, "Multipurpose
Internet Mail Extensions (MIME) Part Four: Registration
Procedures", BCP 13, RFC 2048, November 1996.
[RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
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Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, November 1996.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2298] Fajman, R., "An Extensible Message Format for Message
Disposition Notifications", RFC 2298, March 1998.
[RFC2304] Allocchio, C., "Minimal FAX address format in Internet
Mail", RFC 2304, March 1998.
[RFC2369] Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax
for Core Mail List Commands and their Transport through
Message Header Fields", RFC 2369, July 1998.
[RFC2421] Vaudreuil, G. and G. Parsons, "Voice Profile for Internet
Mail - version 2", RFC 2421, September 1998.
[RFC2423] Vaudreuil, G. and G. Parsons, "VPIM Voice Message MIME
Sub-type Registration", RFC 2423, September 1998.
[RFC2442] "The Batch SMTP Media Type", RFC 2442, November 1998.
[RFC2476] Gellens, R. and J. Klensin, "Message Submission", RFC
2476, December 1998.
[RFC2645] "On-Demand Mail Relay (ODMR) SMTP with Dynamic IP
Addresses", RFC 2465, August 1999.
[RFC2782] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
2001.
[RFC2919] Chandhok, R. and G. Wenger, "List-Id: A Structured Field
and Namespace for the Identification of Mailing Lists",
RFC 2919, March 2001.
[RFC3028] Showalter, T., "Sieve: A Mail Filtering Language", RFC
3028, January 2001.
[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
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Extension for Delivery Status Notifications (DSNs)", RFC
3461, January 2003.
[RFC3501] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 3501, March 2003.
Author's Address
Dave Crocker
Brandenburg InternetWorking
675 Spruce Drive
Sunnyvale, CA 94086
USA
Phone: +1.408.246.8253
EMail: dcrocker@bbiw.net
Appendix A. Acknowledgements
The Email Architecture section derives from draft-hutzler-spamops
[ID-spamops]. The text has been further elaborated.
Discussion of the Source actor role was greatly clarified during
discussions in the IETF's Marid working group.
Graham Klyne, Pete Resnick and Steve Atkins provided thoughtful
insight on the framework and details of early drafts. Additional
review and suggestions were provided by Nathaniel Borenstein, Ed
Bradford, Cyrus Daboo, Tony Finch, Ned Freed, Eric Hall, Bruce Lilly,
Eric Hall, Chris Newman, Jochen Topf.
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Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
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Crocker Expires July 27, 2005 [Page 34]
