< draft-crocker-email-arch-09.txt   draft-crocker-email-arch-10.txt >
SMTP D. Crocker SMTP D. Crocker
Internet-Draft Brandenburg InternetWorking Internet-Draft Brandenburg InternetWorking
Intended status: Standards Track May 26, 2007 Intended status: Standards Track February 24, 2008
Expires: November 27, 2007 Expires: August 27, 2008
Internet Mail Architecture Internet Mail Architecture
draft-crocker-email-arch-09 draft-crocker-email-arch-10
Status of this Memo Status of this Memo
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Abstract Abstract
Over its thirty-five year history Internet Mail has undergone Over its thirty-five year history Internet Mail has undergone
significant changes in scale and complexity, as it has become a significant changes in scale and complexity, as it has become a
global infrastructure service. The first standardized architecture global infrastructure service. The first standardized architecture
for networked email specified little more than a simple split between for networked email specified little more than a simple split between
the user world and the transmission world. Core aspects of the the user world and the transmission world. Core aspects of the
service, such as the styles of mailbox address and basic message service, such as the styles of mailbox address and basic message
format, have remained remarkably constant. However today's Internet format, have remained remarkably constant. However today's Internet
Mail is marked by many independent operators, many different Mail is distinguished by many independent operators, many different
components for providing service to users and many others for components for providing service to users and many others for
performing message transfer. Public discussion of the service often performing message transfer. Public discussion of the service often
lacks common terminology and a common frame of reference for these lacks common terminology and a common frame of reference for these
components and their activities. Having a common reference model and components and their activities. Having a common reference model and
terminology makes a basic difference when talking about problems with terminology facilitates discussion about problems with the service,
the service, changes in policy, or enhancement to the service's changes in policy, or enhancement to the service's functionality.
functionality. This document offers an enhanced Internet Mail This document offers an enhanced Internet Mail architecture that
architecture that targets description of the existing service, in targets description of the existing service, in order to facilitate
order to facilitate clearer and more efficient technical, operations clearer and more efficient technical, operations and policy
and policy discussions about email. discussions about email.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Service Overview . . . . . . . . . . . . . . . . . . . . . 5 1.2. Service Overview . . . . . . . . . . . . . . . . . . . . . 5
1.3. Document Conventions . . . . . . . . . . . . . . . . . . . 6 1.3. Document Conventions . . . . . . . . . . . . . . . . . . . 6
2. Responsible Actor Roles . . . . . . . . . . . . . . . . . . . 7 1.4. Changes to Previous Version . . . . . . . . . . . . . . . 6
2.1. User Actors . . . . . . . . . . . . . . . . . . . . . . . 7 2. Responsible Actor Roles . . . . . . . . . . . . . . . . . . . 8
2.2. Mail Handling Service (MHS) Actors . . . . . . . . . . . . 10 2.1. User Actors . . . . . . . . . . . . . . . . . . . . . . . 9
2.3. Administrative Actors . . . . . . . . . . . . . . . . . . 13 2.2. Mail Handling Service (MHS) Actors . . . . . . . . . . . . 12
3. Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.3. Administrative Actors . . . . . . . . . . . . . . . . . . 15
3.1. Mailbox . . . . . . . . . . . . . . . . . . . . . . . . . 16 3. Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.2. Domain Names . . . . . . . . . . . . . . . . . . . . . . . 17 3.1. Mailbox . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.3. Message Identifier . . . . . . . . . . . . . . . . . . . . 17 3.2. Domain Names . . . . . . . . . . . . . . . . . . . . . . . 19
4. Services and Standards . . . . . . . . . . . . . . . . . . . . 19 3.3. Message Identifier . . . . . . . . . . . . . . . . . . . . 19
4.1. Message Data . . . . . . . . . . . . . . . . . . . . . . . 23 4. Services and Standards . . . . . . . . . . . . . . . . . . . . 21
4.2. User-Level Services . . . . . . . . . . . . . . . . . . . 28 4.1. Message Data . . . . . . . . . . . . . . . . . . . . . . . 25
4.3. MHS-Level Services . . . . . . . . . . . . . . . . . . . . 30 4.2. User-Level Services . . . . . . . . . . . . . . . . . . . 30
5. Mediators . . . . . . . . . . . . . . . . . . . . . . . . . . 33 4.3. MHS-Level Services . . . . . . . . . . . . . . . . . . . . 32
5.1. Aliasing . . . . . . . . . . . . . . . . . . . . . . . . . 34 5. Mediators . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.2. Re-Sending . . . . . . . . . . . . . . . . . . . . . . . . 35 5.1. Aliasing . . . . . . . . . . . . . . . . . . . . . . . . . 36
5.3. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 37 5.2. Re-Sending . . . . . . . . . . . . . . . . . . . . . . . . 37
5.4. Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 38 5.3. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 39
5.5. Boundary Filter . . . . . . . . . . . . . . . . . . . . . 39 5.4. Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 40
6. Considerations . . . . . . . . . . . . . . . . . . . . . . . . 40 5.5. Boundary Filter . . . . . . . . . . . . . . . . . . . . . 41
6.1. Security Considerations . . . . . . . . . . . . . . . . . 40 6. Considerations . . . . . . . . . . . . . . . . . . . . . . . . 42
6.2. IANA Considerations . . . . . . . . . . . . . . . . . . . 40 6.1. Security Considerations . . . . . . . . . . . . . . . . . 42
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40 6.2. IANA Considerations . . . . . . . . . . . . . . . . . . . 42
7.1. Normative . . . . . . . . . . . . . . . . . . . . . . . . 40 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 42
7.2. Informative . . . . . . . . . . . . . . . . . . . . . . . 42 7.1. Normative . . . . . . . . . . . . . . . . . . . . . . . . 42
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 43 7.2. Informative . . . . . . . . . . . . . . . . . . . . . . . 44
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 43 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 45
Intellectual Property and Copyright Statements . . . . . . . . . . 44 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 47
Intellectual Property and Copyright Statements . . . . . . . . . . 48
1. Introduction 1. Introduction
Over its thirty-five year history Internet Mail has undergone Over its thirty-five year history Internet Mail has undergone
significant changes in scale and complexity, as it has become a significant changes in scale and complexity, as it has become a
global infrastructure service. The changes have been evolutionary, global infrastructure service. The changes have been evolutionary,
rather than revolutionary, reflecting a strong desire to preserve its rather than revolutionary, reflecting a strong desire to preserve its
installed base of users and utility. Today, Internet Mail is marked installed base of users and utility. Today, Internet Mail is
by many independent operators, many different components for distinguished by many independent operators, many different
providing service to users and many other components for performing components for providing service to users and many other components
message transfer. for performing message transfer.
Public collaboration on email technical, operations and policy Public collaboration on email technical, operations and policy
activities, including those responding to the challenges of email activities, including those responding to the challenges of email
abuse, has brought in a much wider range of participants than email's abuse, has brought in a much wider range of participants than email's
technical community originally had. In order to do work on a large, technical community originally had. In order to do work on a large,
complex system, they need to share the same view of how it is put complex system, they need to share the same view of how it is put
together, as well as what terms to use to refer to the pieces and together, as well as what terms to use to refer to the pieces and
their activities. Otherwise, it is difficult to know exactly what their activities. Otherwise, it is difficult to know exactly what
another participant means. It is these differences in each person's another participant means. It is these differences in each person's
perspective that motivates this document, to describe the realities perspective that motivates this document, to describe the realities
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to one or more others, creating a virtual MUA-to-MUA exchange to one or more others, creating a virtual MUA-to-MUA exchange
environment. environment.
As shown in Figure 1 this defines two logical "layers" of As shown in Figure 1 this defines two logical "layers" of
interoperability. One is directly between Users. The other is interoperability. One is directly between Users. The other is
between the neighboring components, along the transfer path. In between the neighboring components, along the transfer path. In
addition, there is interoperability between the layers, first when a addition, there is interoperability between the layers, first when a
message is posted from the User to the MHS and later when it is message is posted from the User to the MHS and later when it is
delivered from the MHS to the User. delivered from the MHS to the User.
As it has evolved, the operational service has sub-divided each of The operational service has evolved sub-divisions for each of these
these layers into more specialized modules. Core aspects of the layers into more specialized modules. Core aspects of the service,
service, such as mailbox addressing and message format style, have such as mailbox addressing and message format style, have remained
remained remarkably constant. So the original distinction between remarkably constant. So the original distinction between user-level
user-level concerns and transfer-level concerns is retained, but with concerns and transfer-level concerns is retained, but with an
an elaboration to each level of the architecture. The term "Internet elaboration to each level of the architecture. The term "Internet
Mail" is used to refer to the entire collection of user and transfer Mail" is used to refer to the entire collection of user and transfer
components and services. components and services.
For Internet Mail the term "end-to-end" usually refers to a single For Internet Mail the term "end-to-end" usually refers to a single
posting and the set of deliveries directly resulting from its single posting and the set of deliveries directly resulting from its single
transiting of the MHS. A common exception is with group dialogue transiting of the MHS. A common exception is with group dialogue
that is mediated via a mailing list, so that two postings occur that is mediated via a mailing list, so that two postings occur
before intended recipients receive an originator's message, as before intended recipients receive an Author's message, as discussed
discussed in Section 2.1.4. In fact some uses of email consider the in Section 2.1.4. In fact some uses of email consider the entire
entire email service -- including Originator and Recipient -- as a email service -- including Author and Recipient -- as a subordinate
subordinate component. For these services "end-to-end" refers to component. For these services "end-to-end" refers to points outside
points outside of the email service. Examples are voicemail over of the email service. Examples are voicemail over email [RFC3801],
email [RFC3801], EDI over email [RFC1767] and facsimile over email EDI over email [RFC1767] and facsimile over email [RFC4142].
[RFC4142].
+--------+ +--------+
+---------------->| User | +---------------->| User |
| +--------+ | +--------+
| ^ | ^
+--------+ | +--------+ . +--------+ | +--------+ .
| User +--+--------->| User | . | User +--+--------->| User | .
+--------+ | +--------+ . +--------+ | +--------+ .
. | ^ . . | ^ .
. | +--------+ . . . | +--------+ . .
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End-to-end Internet Mail exchange is accomplished by using a End-to-end Internet Mail exchange is accomplished by using a
standardized infrastructure comprising: standardized infrastructure comprising:
* An email object * An email object
* Global addressing * Global addressing
* An asynchronous sequence of point-to-point transfer mechanisms * An asynchronous sequence of point-to-point transfer mechanisms
* No prior arrangement between Originator and Recipient * No prior arrangement between Author and Recipient
* No prior arrangement between point-to-point transfer services, * No prior arrangement between point-to-point transfer services,
over the open Internet over the open Internet
* No requirement for Originator and Recipient to be online at the * No requirement for Author and Recipient to be online at the
same time. same time.
The end-to-end portion of the service is the email object, called a The end-to-end portion of the service is the email object, called a
message. Broadly the message, itself, distinguishes between control message. Broadly the message, itself, distinguishes between control
information for handling, versus the author's message content. information for handling, versus the author's message content.
A precept to the design of mail over the open Internet is permitting A precept to the design of mail over the open Internet is permitting
user-to-user and MTA-to-MTA interoperability to take place with no user-to-user and MTA-to-MTA interoperability to take place with no
prior, direct arrangement between the independent administrative prior, direct arrangement between the independent administrative
authorities that are responsible for handling a message. That is, authorities responsible for handling a message. That is, all
all participants rely on the core services being universally participants rely on the core services being universally supported
supported and accessible, either directly or through gateways that and accessible, either directly or through gateways that translate
translate between Internet Mail standards and other email between Internet Mail and email environments that conform to other
environments. Given the importance of spontaneity and serendipity in standards. Given the importance of spontaneity and serendipity in
the world of human communications, this lack of prearrangement the world of human communications, this lack of prearrangement
between participants is a core benefit of Internet Mail and remains a between participants is a core benefit of Internet Mail and remains a
core requirement for it. core requirement for it.
Within localized networks at the edge of the public Internet, prior Within localized networks at the edge of the public Internet, prior
administrative arrangement often is required and can include access administrative arrangement often is required and can include access
control, routing constraints and lookup service configuration. In control, routing constraints and information query service
recent years one change to local environments is an increased configuration. In recent years one change to local environments is
requirement for authentication or, at least, accountability. In an increased requirement for authentication or, at least,
these cases a server performs explicit validation of the client's accountability. In these cases a server performs explicit validation
identity. of the client's identity.
1.3. Document Conventions 1.3. Document Conventions
In this document, references to structured fields of a message use a In this document, references to structured fields of a message use a
two-part dotted notation. The first part cites the document that two-part dotted notation. The first part cites the document that
contains the specification for the field and the second is the name contains the specification for the field and the second is the name
of the field. Hence <RFC2822.From> is the From field in an email of the field. Hence <RFC2822.From> is the From: field in an email
content header and <RFC2821.MailFrom> is the address in the SMTP content header and <RFC2821.MailFrom> is the address in the SMTP
"Mail From" command. "Mail From" command.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
Discussion venue: Please direct discussion about this document Discussion venue: Please direct discussion about this document
to the IETF-SMTP mailing list <http://www.imc.org/ietf-smtp>. to the IETF-SMTP mailing list <http://www.imc.org/ietf-smtp>.
Changes: Added definition of acronyms to beginning of Services 1.4. Changes to Previous Version
and standards. INSTRUCTIONS TO THE RFC EDITOR: Remove this sub-section prior to
publication.
Restricted 'envelope' to transport level and added 'trace' for Many small editing changes, for wordsmithing improvements to make
other handling information, and added 'handling' to cover both. details more consistent. This section documents the nature and basis
for changes with significant impact.
Removed construct of "associated with" to now use only "set Originator->Author: The term "Originator" is used by RFC 2822 more
by". broadly than just the From: field, which specifically defines who
the author of the content is. I believe this distinguishes two
constructs, one for the content author and one for the first
agency that handles the message, in terms of the transfer service.
So the change from "Originator" to "Author" seems pretty
straightforward. The challenge is in using the term Originator,
as defined in RFC 2822 and applying it to the system's
architecture.
Cleanup to pass the 'nits' tool check. Source->Originator: This change is more of a challenge. We need
the "Originator" term and construct, but the architecture is
already complex enough. Hence, adding a new construct seems like
a very poor resolution. The document has used "Source" as an MHS
term for the MSA set of functions. While one could argue against
re-labeling it as Originator, I believe this is a reasonable
choice and likely to be comfortable for community use, since
"Source" does not have an established history.
Bounce->Return: 'bounce address' is not accurate, because the
address is used for more than that, but it *is* as established
term within portions of the broader email community. I also
believe the extensive discussion on this point, last year,
justifies the change.
The problem with saying "Bounce" is that is not merely
linguistically impure, it is plain wrong and has already caused
serious problems. Witness SPF. Frankly, we need to fix RFC2821,
but that's a separate battle to fight and not one for this forum.
Although not a verbatim use of "Reverse Path", the related term
that seems to work publicly is "Return Address". It is already
established in the bricks-and-mortar postal world and seems to
have some acceptance within parts of the email community. (I've
done a draft white paper on authentication for the Messaging Anti-
Abuse Working Group and the membership had some debate about this
vocabulary choice and converged on agreeing to it.)
Is the envelope part of the message? I don't remember whether we
resolved this. For a variety of reasons, I believe the message
includes its envelope, and am encouraged to find RFC2822upd says:
In the context of electronic mail, messages are viewed as
having an envelope and contents. The envelope contains
whatever information is needed to accomplish transmission and
delivery. (See [I-D.klensin-rfc2821bis] (Klensin, J., "Simple
Mail Transfer Protocol," November 2007.) for a discussion of
the envelope.) The contents comprise the object to be
delivered to the recipient. This specification applies only to
the format and some of the semantics of message contents.
rfc2821bis says:
SMTP transports a mail object. A mail object contains an
envelope and content.
I think these justify having the term 'message' as including the
object.
Examples of 'new' messages: Section Section 3.3.1contains a list of
examples, discussing scenarios that might or might not be viewed
as creating a "new" message, rather than retaining an existing
one. The list has been expanded.
2. Responsible Actor Roles 2. Responsible Actor Roles
Internet Mail is a highly distributed service, with a variety of Internet Mail is a highly distributed service, with a variety of
actors serving different roles. These divide into 3 basic types: actors serving different roles. These divide into 3 basic types:
* User * User
* Mail Handling Service (MHS) * Mail Handling Service (MHS)
* ADministrative Management Domain (ADMD) * ADministrative Management Domain (ADMD)
Although related to a technical architecture, the focus on Actors Although related to a technical architecture, the focus on Actors
concerns participant responsibilities, rather than on functionality concerns participant responsibilities, rather than on functionality
of modules. Hence the labels used are different than for classic of modules. Hence the labels used are different than for classic
email architecture diagrams. email architecture diagrams.
2.1. User Actors 2.1. User Actors
Users are the sources and sinks of messages. They can be humans or Users are the sources and sinks of messages. They can be humans,
processes. They can have an exchange that iterates and they can organizations or processes. They can have an exchange that iterates
expand or contract the set of users participating in a set of and they can expand or contract the set of users participating in a
exchanges. In Internet Mail there are three types of user-level set of exchanges. In Internet Mail there are three types of user-
Actors: level Actors:
* Originators * Authors
* Recipients * Recipients
* Mediators * Mediators
From the User-level perspective all mail transfer activities are From the User-level perspective all mail transfer activities are
performed by a monolithic Mail Handling Service (MHS), even though performed by a monolithic Mail Handling Service (MHS), even though
the actual service can be provided by many independent organizations. the actual service can be provided by many independent organizations.
Users are customers of this unified service. Users are customers of this unified service.
skipping to change at page 8, line 6 skipping to change at page 10, line 6
* Recipients * Recipients
* Mediators * Mediators
From the User-level perspective all mail transfer activities are From the User-level perspective all mail transfer activities are
performed by a monolithic Mail Handling Service (MHS), even though performed by a monolithic Mail Handling Service (MHS), even though
the actual service can be provided by many independent organizations. the actual service can be provided by many independent organizations.
Users are customers of this unified service. Users are customers of this unified service.
The following figure depicts the flow of messages among Actors: The following figure depicts the flow of messages among Actors:
+------------+ +------------+
| |<---------------------------+ | |<---------------------------+
| Originator |<----------------+ | | Author |<----------------+ |
| |<----+ | | | |<----+ | |
+-+---+----+-+ | | | +-+---+----+-+ | | |
| | | | | | | | | | | |
| | V | | | | | V | | |
| | +---------+-+ | | | | +---------+-+ | |
| | | Recipient | | | | | | Recipient | | |
| | +-----------+ | | | | +-----------+ | |
| | | | | | | |
| | +--------+ | | | | +--------+ | |
| | | | | | | | | | | |
skipping to change at page 8, line 34 skipping to change at page 10, line 35
| +-----------------------------+ | | +-----------------------------+ |
| | +----------+ | | | | +----------+ | |
| | | | | | | | | | | |
V V V | | | V V V | | |
+-----------+ +-----------+ +---+-+-+---+ +-----------+ +-----------+ +---+-+-+---+
| Mediator +--->| Mediator +--->| Recipient | | Mediator +--->| Mediator +--->| Recipient |
+-----------+ +-----------+ +-----------+ +-----------+ +-----------+ +-----------+
Figure 2: Relationships Among User Actors Figure 2: Relationships Among User Actors
2.1.1. Originator 2.1.1. Author
Also called "Author", this is the user-level participant responsible This is the user-level participant responsible for creating the
for creating original content and requesting its transmission. The message, its contents and its list of recipient addresses. The MHS
MHS operates to send and deliver mail among Originators and operates to send and deliver mail among Authors and Recipients. As
Recipients. As described below, the MHS has a "Source" role that described below, the MHS has a "Source" role that correlates with the
correlates with the user-level Author role. user-level Author role.
2.1.2. Recipient 2.1.2. Recipient
The Recipient is a consumer of delivered content. As described The Recipient is a consumer of delivered message content. As
below, the MHS has a "Dest[ination]" role that correlates with the described below, the MHS has a "Dest[ination]" role that correlates
user-level Recipient role. with the user-level Recipient role.
A Recipient can close the user-level communication loop by creating A Recipient can close the user-level communication loop by creating
and submitting a new message that replies to an Originator. An and submitting a new message that replies to an Author. An example
example of an automated form of reply is the Message Disposition of an automated form of reply is the Message Disposition Notification
Notification, which informs the Originator about the Recipient's (MDN), which informs the Author about the Recipient's handling of the
handling of the message. (See Section 4.1.) message. (See Section 4.1.)
2.1.3. Bounce Handler
The Bounce Handler receives and services notifications that are 2.1.3. Return Handler
generated by the MHS, as a result of efforts to transfer or deliver
the message. Notices can be about failures or completions and are
sent to an address that is specified by the Source. This Bounce
handling address (also known as a Return address) might have no
visible characteristics in common with the address of the Originator
or Source.
NOTE: The choice of the label "Bounce" is unfortunate, due to The Return Handler -- also called "Bounce Handler" -- receives and
its negative implication and narrow focus. However it is the services notifications generated by the MHS, as a result of efforts
most popular term for the address. to transfer or deliver the message. Notices can be about failures or
completions and are sent to an address that is specified by the
Source. This Return handling address (also known as a Return
address) might have no visible characteristics in common with the
address of the Author or Source.
2.1.4. Mediator 2.1.4. Mediator
A Mediator receives, aggregates, reformulates and redistributes A Mediator receives, aggregates, reformulates and redistributes
messages as part of a potentially-protracted, higher-level exchange messages as part of a potentially-protracted, higher-level exchange
among Users. Example Mediators include group dialogue, such as among Users. It is easy to confuse this user-level activity with the
collaboration via mailing lists, and organizational message flow, as underlying MHS transfer exchanges. However they serve very different
occurs with a purchase approval process. Note that it is easy to purposes and operate in very different ways. Mediators are
confuse this user-level activity with the underlying MHS transfer considered extensively in Section 5.
exchanges. However they serve very different purposes and operate in
very different ways. Mediators are considered extensively in
Section 5.
When mail is delivered to a receiving mediator specified in the When mail is delivered to a receiving mediator specified in the
RFC2821.RcptTo command, the MHS handles it the same way as for any RFC2821.RcptTo command, the MHS handles it the same way as for any
other Recipient. That is, the MHS only sees posting and delivery other Recipient. That is, the MHS only sees posting and delivery
sources and sinks and does not see (later) re-posting as a sources and sinks and does not see (later) re-posting as a
continuation of a process. Hence when submitting messages, the continuation of a process. Hence when submitting messages, the
Mediator is an Originator. Mediator is an Author.
The distinctive aspects of a Mediator are, therefore, above the MHS. The distinctive aspects of a Mediator are, therefore, above the MHS.
A Mediator preserves the Originator information of the message it A Mediator preserves the Author information of the message it
reformulates, but may make meaningful changes to the content. Hence reformulates, but may make meaningful changes to the content. Hence
the MHS sees a new message, but Users receive a message that is the MHS sees a new message, but Users receive a message that is
interpreted as primarily being from -- or, at least, initiated by -- interpreted as primarily being from -- or, at least, initiated by --
the author of the original message. The role of a Mediator permits the author of the original message. The role of a Mediator permits
distinct, active creativity, rather than being limited to the more distinct, active creativity, rather than being limited to the more
constrained job of merely connecting together other participants. constrained job of merely connecting together other participants.
Hence it is really the Mediator that is responsible for the new Hence it is really the Mediator that is responsible for the new
message. message.
A Mediator's task can be complex and contingent, such as modifying A Mediator's task can be complex and contingent, such as modifying
and adding content or regulating which users are allowed to and adding content or regulating which users are allowed to
participate and when. The popular example of this role is a group participate and when. The popular example of this role is a group
mailing list. A sequence of Mediators may even perform a series of mailing list. A sequence of Mediators may even perform a series of
formal steps, such as reviewing, modifying and approving a purchase formal steps, such as reviewing, modifying and approving a purchase
request. request.
Because a Mediator originates messages, it can also receive replies. Because a Mediator originates messages, it can also receive replies.
So a Mediator really is a full-fledged User. So a Mediator really is a full-fledged User.
Gateway: A Gateway is a particularly interesting form of Gateway: A Gateway is a particularly interesting form of Mediator.
Mediator. It is a hybrid of User and Relay that interconnects It is a hybrid of User and Relay that interconnects heterogeneous
heterogeneous mail services. Its goal is to emulate a Relay, mail services. Its goal is to emulate a Relay, and a detailed
and a detailed discussion is in Section 2.2.3. discussion is in Section 2.2.3.
2.2. Mail Handling Service (MHS) Actors 2.2. Mail Handling Service (MHS) Actors
The Mail Handling Service (MHS) has the task of performing a single, The Mail Handling Service (MHS) has the task of performing a single,
end-to-end transfer on behalf of the Originator and reaching the end-to-end transfer on behalf of the Author and reaching the
Recipient address(es) specified in the original RFC2821.RcptTo Recipient address(es) specified in the original RFC2821.RcptTo
commands. Mediated or protracted, iterative exchanges, such as those commands. Mediated or protracted, iterative exchanges, such as those
used for collaboration over time, are part of the User-level service, used for collaboration over time, are part of the User-level service,
and are not part of this transfer-level Handling Service. and are not part of this transfer-level Handling Service.
The following figure depicts the relationships among transfer The following figure depicts the relationships among transfer
participants in Internet Mail. It shows the Source as distinct from participants in Internet Mail. It shows the Source as distinct from
the Originator, and Dest[ination] as distinct from Recipient, the Author, and Dest[ination] as distinct from Recipient, although it
although it is common for each pair to be the same actor. Transfers is common for each pair to be the same actor. Transfers typically
typically entail one or more Relays. However direct delivery from entail one or more Relays. However direct delivery from the Source
the Source to Destination is possible. For intra-organization mail to Destination is possible. For intra-organization mail services, it
services, it is common to have only one Relay. is common to have only one Relay.
+------------+ +-----------+ +------------+ +-----------+
| Originator | +--------+ | Recipient | | Author | +--------+ | Recipient |
+-----+------+ ..>| Bounce | +-----------+ +-----+------+ +....>| Return | +-----------+
| . +--------+ ^ | . +--------+ ^
| . ^ | | . ^ |
/+=================================================+\ //===================================================\\
|| | . | Mail Handling | || || | . | Mail Handling | ||
|| | . | Service (MHS) | || || | . | Service (MHS) | ||
V . | | V . | |
+---------+ . | +----+----+ +---------+ . ^ +----+---+
| | . | | | | | . | | |
| Source +.... +-<-------------+ Dest | | Origin +....+ +-<------------+ Dest |
| | | | | | | | | |
+----+----+ ^ +---------+ +----+----+ | +--------+
| | ^ | | ^
| +-------------+-----------------+ | | +-------------->-+-<-------------+ |
V | | | | V | | | |
+-------+-+ +-+-------+ +-+--+----+ +-------+-+ +----+----+ +-+---+---+
| Relay +-->...-->| Relay +------>| Relay | | Relay +-->...-->| Relay +------->| Relay |
+---------+ +----+----+ +---------+ +---------+ +----+----+ +---------+
| |
V V
+---------+ +---------+
| Gateway +-->... | Gateway +-->...
+---------+ +---------+
Figure 3: Relationships Among MHS Actors Figure 3: Relationships Among MHS Actors
2.2.1. Source 2.2.1. Originator
The Source role is responsible for ensuring that a message is valid The Originator role is responsible for ensuring that a message is
for posting and then submitting it to a Relay. Validity includes valid for posting and then submitting it to a Relay. Validity
conformance with Internet Mail standards, as well as with local includes conformance with Internet Mail standards, as well as with
operational policies. The Source can simply review the message for local operational policies. The Originator can simply review the
conformance and reject it if there are errors, or it can create some message for conformance and reject it if there are errors, or it can
or all of the necessary information. create some or all of the necessary information.
The Source operates with dual "allegiance". It serves the Originator The Originator operates with dual "allegiance". It serves the Author
and often it is the same entity. However its role in assuring and often it is the same entity. However its role in assuring
validity means that it MUST also represent the local operator of the validity means that it MUST also represent the local operator of the
MHS, that is, the local ADministrative Management Domain (ADMD). MHS, that is, the local ADministrative Management Domain (ADMD).
The Source also has the responsibility for any post-submission, The Originator also has the responsibility for any post-submission,
Originator-related administrative tasks associated with message Author-related administrative tasks associated with message
transmission and delivery. Notably this pertains to error and transmission and delivery. Notably this pertains to error and
delivery notices. Hence Source is best held accountable for the delivery notices. Hence Source is best held accountable for the
message content, even when they did not create any or most of it. message content, even when they did not create any or most of it.
2.2.2. Relay 2.2.2. Relay
A mail Relay performs email transfer-service routing and store-and- A mail Relay performs email transfer-service routing and store-and-
forward by (re-)transmitting the message on towards its Recipient(s). forward by (re-)transmitting the message on towards its Recipient(s).
A Relay can add trace information. However it does not modify A Relay can add trace information. However it does not modify
existing envelope information or the message content semantics. It existing envelope information or the message content semantics. It
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* User Mediators * User Mediators
* MHS Relays * MHS Relays
* Packet Switches * Packet Switches
with the bottom-most usually being the Internet's IP service. The with the bottom-most usually being the Internet's IP service. The
most basic email scenarios involve Relays and Switches. most basic email scenarios involve Relays and Switches.
Aborting a message transfer results in having the Relay become an Aborting a message transfer results in having the Relay become an
Originator and sending an error message to the Bounce address. The Author and sending an error message to the Return address. The
potential for looping is avoided by having this message, itself, potential for looping is avoided by having this message, itself,
contain no Bounce address. contain no Return address.
2.2.3. Gateway 2.2.3. Gateway
A Gateway is a hybrid form of User and Relay that interconnects A Gateway is a hybrid form of User and Relay that interconnects
heterogeneous mail services. Its purpose is simply to emulate a heterogeneous mail services. Its purpose is simply to emulate a
Relay and the closer it comes to this, the better. However it Relay and the closer it comes to this, the better. However it
operates at the User level, because it MUST be able to modify message operates at the User level, because it MUST be able to modify message
content. content.
Differences between mail services can be as small as minor syntax Differences between mail services can be as small as minor syntax
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One difference could have the concept of an email address being a One difference could have the concept of an email address being a
hierarchical, machine-specific address, versus having it be a flat, hierarchical, machine-specific address, versus having it be a flat,
global name space. Another difference could be between text-only global name space. Another difference could be between text-only
content, versus multi-media. Hence the Relay function in a Gateway content, versus multi-media. Hence the Relay function in a Gateway
offers significant design challenges, to make the result be as offers significant design challenges, to make the result be as
seamless as possible. The most significant challenge is in ensuring seamless as possible. The most significant challenge is in ensuring
the user-to-user functionality that matches syntax and semantics of the user-to-user functionality that matches syntax and semantics of
independent email standards suites. independent email standards suites.
The basic test of a Gateway's adequacy is, of course, whether an The basic test of a Gateway's adequacy is, of course, whether an
Originator on one side of a Gateway can send a useful message to a Author on one side of a Gateway can send a useful message to a
Recipient on the other side, without requiring changes to any of the Recipient on the other side, without requiring changes to any of the
components in the Originator's or Recipient's mail services, other components in the Author's or Recipient's mail services, other than
than adding the Gateway. To each of these otherwise independent adding the Gateway. To each of these otherwise independent services,
services, the Gateway will appear to be a "native" participant. the Gateway will appear to be a "native" participant. However the
However the ultimate test of a Gateway's adequacy is whether the ultimate test of a Gateway's adequacy is whether the Author and
Originator and Recipient can sustain a dialogue. In particular can a Recipient can sustain a dialogue. In particular can a Recipient's
Recipient's MUA automatically formulate a valid Reply that will reach MUA automatically formulate a valid Reply that will reach the initial
the initial Originator? Author?
2.3. Administrative Actors 2.3. Administrative Actors
Actors often are associated with different organizations, each with Actors often are associated with different organizations, each with
its own administrative authority. This operational independence, its own administrative authority. This operational independence,
coupled with the need for interaction between groups, provides the coupled with the need for interaction between groups, provides the
motivation for distinguishing among ADministrative Management Domains motivation for distinguishing among ADministrative Management Domains
(ADMD). Each ADMD can have vastly different operating policies and (ADMD). Each ADMD can have vastly different operating policies and
trust-based decision-making. An obvious example is the distinction trust-based decision-making. An obvious example is the distinction
between mail that is exchanged within a single organization, versus between mail that is exchanged within a single organization, versus
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providers (or operators). Each can be an independent ADMD. This providers (or operators). Each can be an independent ADMD. This
independence of administrative decision-making defines boundaries independence of administrative decision-making defines boundaries
that distinguish different portions of the Internet Mail service. that distinguish different portions of the Internet Mail service.
Examples include an end-user operating their desktop client, a Examples include an end-user operating their desktop client, a
department operating a local Relay, an IT department operating an department operating a local Relay, an IT department operating an
enterprise Relay and an ISP operating a public shared email service. enterprise Relay and an ISP operating a public shared email service.
These can be configured into many combinations of administrative and These can be configured into many combinations of administrative and
operational relationships, with each ADMD potentially having a operational relationships, with each ADMD potentially having a
complex arrangement of functional components. Figure 4 depicts complex arrangement of functional components. Figure 4 depicts
relationships among ADMDs. The benefit of having the ADMD construct relationships among ADMDs. The benefit of having the ADMD construct
is to facilitate discussions and designs that need to distinguish is to facilitate discussion about designs and operations that need to
between "internal" issues and "external" ones. distinguish between "internal" issues and "external" ones.
The architectural impact of needing to have boundaries between ADMD's The architectural impact of needing to have boundaries between ADMD's
is discussed in [Tussle]. Most significant is that the entities is discussed in [Tussle]. Most significant is that the entities
communicating across ADMD boundaries will typically have an added communicating across ADMD boundaries will typically have an added
burden to enforce organizational policies concerning "external" burden to enforce organizational policies concerning "external"
communications. At a more mundane level, the basis for routing mail communications. At a more mundane level, routing mail between ADMDs
between ADMDs is often an issue. can be an issue, such as needing to route mail for partners over
specially-trusted paths.
Basic types of ADMDs include -- Basic types of ADMDs include --
Edge: Independent transfer services, in networks at the edge of Edge: Independent transfer services, in networks at the edge of
the open Internet Mail service. the open Internet Mail service.
User: End-user services. This might be subsumed under the Edge User: End-user services. This might be subsumed under the Edge
service, such as is common for web-based email access. service, such as is common for web-based email access.
Transit: These are Mail Service Providers (MSP) offering value- Transit: These are Mail Service Providers (MSP) offering value-
added capabilities for Edge ADMDs, such as aggregation and added capabilities for Edge ADMDs, such as aggregation and
filtering. filtering.
Note that Transit services are quite different from packet-level Note that Transit services are quite different from packet-level
switching operation. Whereas end-to-end packet transfers usually go switching operation. Whereas end-to-end packet transfers usually go
through intermediate routers, email exchange across the open Internet through intermediate routers, email exchange across the open Internet
is often directly between the Boundary MTAs of Edge ADMDs, at the is often directly between the Boundary MTAs of Edge ADMDs, at the
email level. email level. This further highlights the differences discussed in
Section 2.2.2
+-------+ +-------+ +-------+ +-------+ +-------+ +-------+
| ADMD1 | | ADMD3 | | ADMD4 | | ADMD1 | | ADMD3 | | ADMD4 |
| ----- | | ----- | | ----- | | ----- | | ----- | | ----- |
| | +---------------------->| | | | | | +---------------------->| | | |
| User | | |-Edge--+--->|-User | | User | | |-Edge--+--->|-User |
| | | | +---------+ +--->| | | | | | | | +---------+ +--->| | | |
| V | | | ADMD2 | | +-------+ +-------+ | V | | | ADMD2 | | +-------+ +-------+
| Edge--+---+ | ----- | | | Edge--+---+ | ----- | |
| | | | | | | | | | | |
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However the distinction between Transit network and Edge network However the distinction between Transit network and Edge network
transfer services is primarily significant because it highlights the transfer services is primarily significant because it highlights the
need for concern over interaction and protection between independent need for concern over interaction and protection between independent
administrations. In particular this distinction calls for additional administrations. In particular this distinction calls for additional
care in assessing transitions of responsibility, as well as the care in assessing transitions of responsibility, as well as the
accountability and authorization relationships among participants in accountability and authorization relationships among participants in
email transfer. email transfer.
The interactions between functional components within an ADMD are The interactions between functional components within an ADMD are
subject to the policies of that domain. Policies can cover such subject to the policies of that domain. Policies can cover such
things as reliability, access control, accountability and even things as:
content evaluation and modification. They can be implemented in
different functional components, according to the needs of the ADMD. o Reliability
For example see [ID-spamops].
o Access control
o Accountability
o Content evaluation and modification
They can be implemented in different functional components, according
to the needs of the ADMD. For example see [RFC5068].
User, Edge and Transit services can be offered by providers that User, Edge and Transit services can be offered by providers that
operate component services or sets of services. Further it is operate component services or sets of services. Further it is
possible for one ADMD to host services for other ADMDs. possible for one ADMD to host services for other ADMDs.
Common ADMD examples are -- Common ADMD examples are --
Enterprise Service Providers: Enterprise Service Providers:
Operating an organization's internal data and/or mail services. Operating an organization's internal data and/or mail services.
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Internet Mail uses three forms of identity: mailbox, domain name and Internet Mail uses three forms of identity: mailbox, domain name and
message-id. Each is required to be globally unique. message-id. Each is required to be globally unique.
3.1. Mailbox 3.1. Mailbox
"A mailbox sends and receives mail. It is a conceptual entity "A mailbox sends and receives mail. It is a conceptual entity
which does not necessarily pertain to file storage." [RFC2822] which does not necessarily pertain to file storage." [RFC2822]
A mailbox is specified as an Internet Mail address <addr-spec>. It A mailbox is specified as an Internet Mail address <addr-spec>. It
has two distinct parts, divided by an at-sign ("@"). The right-hand has two distinct parts, divided by an at-sign ("@"). The right-hand
side is a globally interpreted domain name that is part of an ADMD. side is a globally interpreted domain name that is associated with an
Domain Names are discussed in Section 3.2. Formal Internet Mail ADMD. Domain Names are discussed in Section 3.2. Formal Internet
addressing syntax can support source routes, to indicate the path Mail addressing syntax can support source routes, to indicate the
through which a message should be sent. Although legal, the use of path through which a message should be sent. Although legal, the use
source routes is not part of the modern Internet Mail service and it of source routes is not part of the modern Internet Mail service and
is ignored in the rest of this document. it is not discussed further.
The portion to the left of the at-sign contains a string that is The portion to the left of the at-sign contains a string that is
globally opaque and is called the <local-part>. It is to be globally opaque and is called the <local-part>. It is to be
interpreted only by the entity specified by the address's right-hand interpreted only by the entity specified by the address's right-hand
side domain name. All other entities MUST treat the local-part as a side domain name. All other entities MUST treat the local-part as a
uninterpreted literal string and MUST preserve all of its original uninterpreted literal string and MUST preserve all of its original
details. As such its public distribution is equivalent to sending a details. As such its public distribution is equivalent to sending a
Web browser "cookie" that is only interpreted upon being returned to Web browser "cookie" that is only interpreted upon being returned to
its originator. its Author.
3.1.1. Global Standards for Local-Part 3.1.1. Global Standards for Local-Part
It is common for sites to have local structuring conventions for the It is common for sites to have local structuring conventions for the
left-hand side <local-part> of an <addr-spec>. This permits sub- left-hand side <local-part> of an <addr-spec>. This permits sub-
addressing, such as for distinguishing different discussion groups addressing, such as for distinguishing different discussion groups
used by the same participant. However it is worth stressing that used by the same participant. However it is worth stressing that
these conventions are strictly private to the user's organization and these conventions are strictly private to the user's organization and
MUST not be interpreted by any domain except the one listed in the SHOULD NOT be interpreted by any domain except the one listed in the
right-hand side of the addr-spec, and those specialized services right-hand side of the addr-spec. The exceptions are those
conforming to standardized conventions, as noted in the next specialized services conforming to standardized conventions, as noted
paragraph. below.
There are a few types of addresses that have an elaboration on basic There are a few types of addresses that have an elaboration on basic
email addressing, with a standardized, global schema for the local- email addressing, with a standardized, global schema for the local-
part. These are conventions between originating end-systems and part. These are conventions between authoring systems and Recipient
Recipient Gateways, and they are invisible to the public email Gateways, and they are invisible to the public email transfer
transfer infrastructure. When an Originator is explicitly sending infrastructure. When an Author is explicitly sending via a Gateway
via a Gateway out of the Internet, there are coding conventions for out of the Internet, there are coding conventions for the local-part,
the local-part, so that the Originator can formulate instructions for so that the Author can formulate instructions for the Gateway.
the Gateway. Standardized examples of this are the telephone Standardized examples of this are the telephone numbering formats for
numbering formats for VPIM [RFC3801], such as VPIM [RFC3801], such as "+16137637582@vpim.example.com", and iFax
"+16137637582@vpim.example.com", and iFax [RFC3192], such as [RFC3192], such as "FAX=+12027653000/T33S=1387@ifax.example.com".
"FAX=+12027653000/T33S=1387@ifax.example.com".
3.1.2. Scope of Email Address Use 3.1.2. Scope of Email Address Use
Email addresses are being used far beyond their original email Email addresses are being used far beyond their original email
transfer and delivery role. In practical terms, email strings have transfer and delivery role. In practical terms, an email address
become a common form of user identity on the Internet. What is string has become the common identifier for representing online
essential, then, is to be clear about the nature and role of an identity. What is essential, then, is to be clear about the nature
identity string in a particular context and to be clear about the and role of an identity string in a particular context and to be
entity responsible for setting that string. clear about the entity responsible for setting that string. For
example, see: Section 4.1.4, Section 4.3.3, Section 5.
3.2. Domain Names 3.2. Domain Names
A domain name is a global reference to an Internet resource, such as A domain name is a global reference to an Internet resource, such as
a host, a service or a network. A domain name usually maps to one or a host, a service or a network. A domain name usually maps to one or
more IP Addresses. Conceptually the name might encompass an entire more IP Addresses. Conceptually the name might encompass an entire
organization, a collection of machines integrated into a homogeneous organization, a collection of machines integrated into a homogeneous
service, or only a single machine. A domain name can be administered service, or only a single machine. A domain name can be administered
to refer to individual users, but this is not common practice. The to refer to individual users, but this is not common practice. The
name is structured as a hierarchical sequence of sub-names, separated name is structured as a hierarchical sequence of sub-names, separated
by dots ("."), with the top of the hierarchy being on the right-end by dots ("."), with the top of the hierarchy being on the right-end
of the sequence. Domain names are defined and operated through the of the sequence. Domain names are defined and operated through the
Domain Name Service (DNS) [RFC1034], [RFC1035], [RFC2181]. Domain Name System (DNS) [RFC1034], [RFC1035], [RFC2181].
When not part of a mailbox address, a domain name is used in Internet When not part of a mailbox address, a domain name is used in Internet
Mail to refer to the ADMD or the host that took action upon the Mail to refer to the ADMD or the host that took action upon the
message, such as providing the administrative scope for a message message, such as providing the administrative scope for a message
identifier, or performing transfer processing. identifier, or performing transfer processing.
3.3. Message Identifier 3.3. Message Identifier
There are two standardized tags for identifying messages: Message-ID There are two standardized tags for identifying messages: Message-ID
and ENVID. and ENVID.
3.3.1. Message-ID 3.3.1. Message-ID
The Message-ID is a user-level tag, primarily used for threading and The Message-ID is a user-level tag, primarily used for threading and
for eliminating duplicates [RFC2822]. Any actor within the for eliminating duplicates [RFC2822]. Any actor within the
originating ADMD might assign the Message-ID, although it is Originating ADMD can assign the Message-ID. The recipient's ADMD is
typically created by an actor within the Originating ADMD.. The the intended consumer of the Message-ID, although any actor along the
recipient's ADMD is the intended consumer of the Message-ID, although transfer path might use it. Internet Mail standards provide for a
any actor along the transfer path might use it. Internet Mail single Message-ID; however more than one is sometimes assigned.
standards provide for a single Message-ID; however more than one is
sometimes assigned.
Like a mailbox address, a Message-ID has two distinct parts, divided Like a mailbox address, a Message-ID has two distinct parts, divided
by an at-sign ("@"). The right-hand side is globally interpreted and by an at-sign ("@"). The right-hand side is globally interpreted and
specifies the ADMD or host assigning the identifier. The left-hand specifies the ADMD or host assigning the identifier. The left-hand
side contains a string that is globally opaque and serves to uniquely side contains a string that is globally opaque and serves to uniquely
identify the message within the domain referenced on the right-hand identify the message within the domain referenced on the right-hand
side. The duration of uniqueness for the message identifier is side. The duration of uniqueness for the message identifier is
undefined. undefined.
When a message is revised in any way, the question of whether to When a message is revised in any way, the question of whether to
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the same message. the same message.
* If a message has offensive words deleted from it, then some * If a message has offensive words deleted from it, then some
recipients will consider it the same message, but some will recipients will consider it the same message, but some will
not. not.
* If a message is translated into a different language, then some * If a message is translated into a different language, then some
recipients will consider it the same message, but some will recipients will consider it the same message, but some will
not. not.
* If a message is included in a digest of messages, it the digest
constitutes a new message.
* If a message is forwarded by a recipient, what is forwarded is
considered to be a new message.
* If a message is "redirected", such as using RFC2822
"Redirect-*" headers, some recipients will consider it the same
message, but some will not.
The absence of objective, precise criteria for Message-ID re- The absence of objective, precise criteria for Message-ID re-
generation, along with the absence of strong protection associated generation, along with the absence of strong protection associated
with the string, means that the presence of an ID can permit an with the string, means that the presence of an ID can permit an
assessment that is marginally better than a heuristic, but the ID assessment that is marginally better than a heuristic, but the ID
certainly has no value on its own for strict formal reference or certainly has no value on its own for strict formal reference or
comparison. Hence it is not appropriate to use the Message-ID for comparison. Hence Message-ID SHOULD NOT be used for any function
any process that might be called "security". that has security implications.
3.3.2. ENVID 3.3.2. ENVID
The ENVID (envelope identifier) is a tag that is primarily for use The ENVID (envelope identifier) is a tag that is primarily for use
within Delivery Status Notifications (DSN), so that the Bounce within Delivery Status Notifications (DSN), so that the Return
Address (RFC2821.MailFrom) recipient can correlate the DSN with a Address (RFC2821.MailFrom) recipient can correlate the DSN with a
particular message [RFC3461]. The ENVID is therefore used from one particular message [RFC3461]. The ENVID is therefore used from one
message posting, until the directly-resulting message deliveries. It message posting, until the directly-resulting message deliveries. It
does not survive re-postings. does not survive re-postings.
The ENVID may also be used for message tracking purposes [RFC3885]. The ENVID may also be used for message tracking purposes [RFC3885].
The format of an ENVID is free-form. Although its creator might The format of an ENVID is free-form. Although its creator might
choose to impose structure on the string, none is imposed by Internet choose to impose structure on the string, none is imposed by Internet
standards. By implication, the scope of the string is defined by the standards. By implication, the scope of the string is defined by the
domain name of the Bounce Address. domain name of the Return Address.
4. Services and Standards 4. Services and Standards
Internet Mail's architecture distinguishes among six basic types of Internet Mail's architecture distinguishes among six basic types of
functional components, arranged to support a store-and-forward functionality, arranged to support a store-and-forward service
service architecture. As shown in Figure 5 these types can have architecture. As shown in Figure 5 these types can have multiple
multiple instances, some of which represent specialized sub-roles. instances, some of which represent specialized sub-roles. This
This section considers the activities and relationships among these section considers the activities and relationships among these
components, and the Internet Mail standards used among them. components, and the Internet Mail standards that apply to them.
1. Message 1. Message
2. Mail User Agent (MUA) 2. Mail User Agent (MUA)
+ Originating MUA (oMUA) Originating MUA (oMUA)
+ Receiving MUA (rMUA) Receiving MUA (rMUA)
3. Message Submission Agent (MSA) 3. Message Submission Agent (MSA)
+ Originator-focussed MSA functions (oMSA) Author-focussed MSA functions (oMSA)
+ MHS-focussed MSA functions (hMSA) MHS-focussed MSA functions (hMSA)
4. Message Transfer Agent (MTA) 4. Message Transfer Agent (MTA)
5. Message Delivery Agent (MDA) 5. Message Delivery Agent (MDA)
+ Recipient-focused MDA functions (rMDA) Recipient-focused MDA functions (rMDA)
+ MHS-focussed MDA functions (hMDA) MHS-focussed MDA functions (hMDA)
6. Message Store (MS) 6. Message Store (MS)
+ Originator MS (oMS) 1. Author MS (oMS)
- oMS on a remote server (soMS) oMS on a remote server (soMS)
- oMS co-located with the oMUA (uoMS) oMS co-located with the oMUA (uoMS)
+ Recipient MS (rms) 2. Recipient MS (rms)
- rMS on a remote server (srM) rMS on a remote server (srMS)
- rMS co-located with the rMUA (urMS) rMS co-located with the rMUA (urMS)
This section describes each functional component for Internet Mail, This section describes each functional component for Internet Mail,
and the standards-based protocols that are associated with their and the standards-based protocols associated with their operation.
operation.
Software implementations of these architectural components often Software implementations of these architectural components often
compress them, such as having the same software do MSA, MTA and MDA compress them, such as having the same software do MSA, MTA and MDA
functions. However the requirements for each of these components of functions. However the requirements for each of these components of
the service are becoming more extensive. So their separation is the service are becoming more extensive. So their separation is
increasingly common. increasingly common.
NOTE: A discussion about any interesting system architecture is NOTE: A discussion about any interesting system architecture is
often complicated by confusion between architecture versus often complicated by confusion between architecture versus
implementation. An architecture defines the conceptual implementation. An architecture defines the conceptual
functions of a service, divided into discrete conceptual functions of a service, divided into discrete conceptual
modules. An implementation of that architecture can combine or modules. An implementation of that architecture can combine or
separate architectural components, as needed for a particular separate architectural components, as needed for a particular
operational environment. It is important not to confuse the operational environment.
engineering decisions that are made to implement a product,
with the architectural abstractions used to define conceptual A software system that primarily performs message relaying --
functions. and therefore is an MTA -- might also include MDA
functionality. That same MTA system might be able to interface
with non-Internet email services and therefore qualify as a
Gateway.
It is important not to confuse the engineering decisions made
to implement a product, with the architectural abstractions
used to define conceptual functions.
The following figure shows function modules and the standardized The following figure shows function modules and the standardized
protocols used between them. Additional protocols and configurations protocols used between them. Additional protocols and configurations
are possible. Boxes defined by asterisks (*) represent functions are possible. Boxes defined by asterisks (*) represent functions
that often are distributed among two or more systems. that often are distributed among two or more systems.
+------+ +-------+ +------+ +-------+
............+ oMUA |..............................| Disp | ............+ oMUA |..............................| Disp |
. +--+-+-+ +-------+ . +--+-+-+ +-------+
. local,imap}| |{smtp,submission ^ . local,imap}| |{smtp,submission ^
. | | +---------+ | . | | +---------+ |
. ******* | | .......................| Bounces | | . ******* | | .......................| Returns | |
. * oMS *<-----+ | . +---------+ | . * oMS *<-----+ | . +---------+ |
. ******* | . ***************** ^ | . ******* | . ***************** ^ |
. +------V-.---*------------+ * | | . +------V-.---*------------+ * | |
. MSA | +-------+ * +------+ | * | | . MSA | +-------+ * +------+ | * | |
. | | oMSA +--O-->| hMSA | | * | | . | | oMSA +--O-->| hMSA | | * | |
. | +-------+ * +--+---+ | * | | . | +-------+ * +--+---+ | * | |
. +------------*------+-----+ * | | . +------------*------+-----+ * | |
/+==========+\ * V {smtp * | | //==========\\ * V {smtp * | |
|| MESSAGE || * +------+ * /+===+===+\ | || MESSAGE || * +------+ * //===+===\\ |
||----------|| MHS * | MTA | * || dsn || | ||----------|| MHS * | MTA | * || dsn || |
|| Envelope || * +--+---+ * \+=======+/ | || Envelope || * +--+---+ * \\=======// |
|| SMTP || * V {smtp * ^ ^ | || SMTP || * V {smtp * ^ ^ |
|| Content || * +------+ * | | /+==+==+\ || Content || * +------+ * | | //==+==\\
|| RFC2822 || * | MTA +----*-----+ | || mdn || || RFC2822 || * | MTA +----*-----+ | || mdn ||
|| MIME || * +--+---+ * | \+=====+/ || MIME || * +--+---+ * | \\=====//
\+==========+/ * smtp}| {local * | | \\==========// * smtp}| {local * | |
MDA * | {lmtp * | | . MDA * | {lmtp * | |
. +------------+------V-----+ * | | . +------------+------V-----+ * | |
. | +------+ * +------+ | * | | . | +------+ * +------+ | * | |
. | | | * | | +--*---------+ | . | | | * | | +--*---------+ |
. | | rMDA |<--O---+ hMDA | | * | . | | rMDA |<--O---+ hMDA | | * |
. | | | * | | |<-*-------+ | . | | | * | | |<-*-------+ |
. | +-+----+ * +------+ | * | | . | +-+----+ * +------+ | * | |
. +---+--+-----*------------+ * | | . +---+--+-----*------------+ * | |
. | | ***************** | | . | | ***************** | |
. pop} +--+ +---+ | | . pop} +--+ +---+ | |
. imap} | | {local | | . imap} | | {local | |
. ******************V******** | | . ******************V******** | |
. * | +------+ * rMS /+===+===+\ | . * | +------+ * rMS //===+===\\ |
. * | | srMS | * || sieve || | . * | | srMS | * || sieve || |
. * V +--+-+-+ * \+=======+/ | . * V +--+-+-+ * \\=======// |
. * +------+ pop} | | * ^ | . * +------+ pop} | | * ^ |
. * | urMS |<-------+ | * | | . * | urMS |<-------+ | * | |
. * +--+---+ imap} | * | | . * +--+---+ imap} | * | |
. *************************** | | . *************************** | |
. local}| +------+ |{pop,imap | | . local}| +------+ |{pop,imap | |
. +->| |<------+ | | . +->| |<------+ | |
...........>| rMUA +---------------------------+ | ...........>| rMUA +---------------------------+ |
| +-----------------------------------+ | +-----------------------------------+
+------+ +------+
Figure 5: Protocols and Services Figure 5: Protocols and Services
4.1. Message Data 4.1. Message Data
The purpose of the Mail Handling Service (MHS) is to exchange a The purpose of the Mail Handling Service (MHS) is to exchange a
message object among participants [RFC2822], [RFC0822]. Hence all of message object among participants [RFC2822], [RFC0822]. Hence all of
its underlying mechanisms are merely in the service of getting that its underlying mechanisms are merely in the service of getting that
message from its Originator to its Recipients. A message can be message from its Author to its Recipients. A message can be
explicitly labeled as to its nature [RFC3458]. explicitly labeled as to its nature [RFC3458].
A message comprises a transit handling envelope and the message A message comprises a transit handling envelope and the message
content. The envelope contains information used by the MHS. The content. The envelope contains information used by the MHS. The
content is divided into a structured header and the body. The header content is divided into a structured header and the body. The header
comprises transit trace information and end-user structured fields. comprises transit trace information and end-user structured fields.
The body may be unstructured simple lines of text, or it may be a The body may be unstructured simple lines of text, or it may be a
MIME tree of multi-media subordinate objects, called body-parts, or MIME tree of multi-media subordinate objects, called body-parts, or
attachments [RFC2045], [RFC2046], [RFC2047], [RFC4288], [RFC4289], attachments [RFC2045], [RFC2046], [RFC2047], [RFC4288], [RFC4289],
[RFC2049]. [RFC2049].
In addition, Internet Mail has a few conventions for special control In addition, Internet Mail has a few conventions for special control
data -- data --
Delivery Status Notification (DSN): Delivery Status Notification (DSN):
A Delivery Status Notification (DSN) is a message that can be A Delivery Status Notification (DSN) is a message that can be
generated by the MHS (MSA, MTA or MDA) and sent to the generated by the MHS (MSA, MTA or MDA) and sent to the
RFC2821.MailFrom address. The mailbox for this is shown as RFC2821.MailFrom address. The mailbox for this is shown as
Bounces in Figure 5. DSNs provide information about message Returns in Figure 5. DSNs provide information about message
transit, such as transmission errors or successful delivery transit, such as transmission errors or successful delivery
[RFC3461]. [RFC3461].
Message Disposition Notification (MDN): Message Disposition Notification (MDN):
A Message Disposition Notification (MDN) is a message that A Message Disposition Notification (MDN) is a message that
provides information about user-level, Recipient-side message provides information about user-level, Recipient-side message
processing, such as indicating that the message has been processing, such as indicating that the message has been
displayed [RFC3798] or the form of content that can be displayed [RFC3798] or the form of content that can be
supported [RFC3297]. It can be generated by an rMUA and is supported [RFC3297]. It can be generated by an rMUA and is
skipping to change at page 24, line 24 skipping to change at page 26, line 24
4.1.1. Envelope 4.1.1. Envelope
Internet Mail has a fragmented framework for transit-related Internet Mail has a fragmented framework for transit-related
"handling" information. Information that is directly used by the MHS "handling" information. Information that is directly used by the MHS
is called the "envelope". It directs handling activities by the is called the "envelope". It directs handling activities by the
transfer service as is carried in transfer service commands. That transfer service as is carried in transfer service commands. That
is, the envelope exists in the transfer protocol SMTP [RFC2821]. is, the envelope exists in the transfer protocol SMTP [RFC2821].
Trace information records handling activity and is recorded in the Trace information records handling activity and is recorded in the
message Header. message Header. [RFC2822]
4.1.2. Header Fields 4.1.2. Header Fields
Header fields are attribute name/value pairs covering an extensible Header fields are attribute name/value pairs covering an extensible
range of email service, user content and user transaction meta- range of email service, user content and user transaction meta-
information. The core set of header fields is defined in [RFC2822], information. The core set of header fields is defined in [RFC2822],
[RFC0822]. It is common to extend this set, for different [RFC0822]. It is common to extend this set, for different
applications. Procedures for registering header fields are defined applications. Procedures for registering header fields are defined
in [RFC4021]. An extensive set of existing header field in [RFC4021]. An extensive set of existing header field
registrations is provided in [RFC3864]. registrations is provided in [RFC3864].
skipping to change at page 25, line 8 skipping to change at page 27, line 8
recursive set of MIME header field meta-data and MIME Content recursive set of MIME header field meta-data and MIME Content
sections. sections.
4.1.4. Identity References in a Message 4.1.4. Identity References in a Message
For a message in transit, the core uses of identifiers combine into: For a message in transit, the core uses of identifiers combine into:
+-----------------------+----------------+---------------------+ +-----------------------+----------------+---------------------+
| Layer | Field | Set By | | Layer | Field | Set By |
+-----------------------+----------------+---------------------+ +-----------------------+----------------+---------------------+
| Message Body | MIME Header | Originator | | Message Body | MIME Header | Author |
| Message header fields | From | Originator | | Message header fields | From | Author |
| | Sender | Source | | | Sender | Source |
| | Reply-To | Originator | | | Reply-To | Author |
| | To, CC, BCC | Originator | | | To, CC, BCC | Author |
| | Message-ID | Source | | | Message-ID | Source |
| | Received | Source, Relay, Dest | | | Received | Source, Relay, Dest |
| | Return-Path | MDA, from MailFrom | | | Return-Path | MDA, from MailFrom |
| | Resent-* | Mediator | | | Resent-* | Mediator |
| | List-Id | Mediator Originator | | | List-Id | Mediator Author |
| | List-* | Mediator Originator | | | List-* | Mediator Author |
| SMTP | HELO/EHLO | Latest Relay Client | | SMTP | HELO/EHLO | Latest Relay Client |
| | ENVID | Source | | | ENVID | Source |
| | MailFrom | Source | | | MailFrom | Source |
| | RcptTo | Originator | | | RcptTo | Author |
| IP | Source Address | Latest Relay Client | | IP | Source Address | Latest Relay Client |
+-----------------------+----------------+---------------------+ +-----------------------+----------------+---------------------+
Layered Identities Layered Identities
The most common address-related fields are: The most common address-related fields are:
RFC2822.From Set by: Originator RFC2822.From: Set by - Author
Names and addresses for author(s) of the message content are Names and addresses for author(s) of the message content are
listed in the From field. listed in the From: field.
RFC2822.Reply-To Set by: Originator RFC2822.Reply-To: Set by - Author
If a message Recipient sends a reply message that would otherwise If a message Recipient sends a reply message that would otherwise
use the RFC2822.From field address(es) that are contained in the use the RFC2822.From field address(es) that are contained in the
original message, then they are instead to use the address(es) in 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 the RFC2822.Reply-To field. In other words this field is a direct
override of the From field, for responses from Recipients. override of the From: field, for responses from Recipients.
RFC2822.Sender Set by: Source RFC2822.Sender: Set by - Source
This specifies the address responsible for submitting the message This specifies the address responsible for submitting the message
into the transfer service. For efficiency this field can be into the transfer service. For efficiency this field can be
omitted if it contains the same address as RFC2822.From. However omitted if it contains the same address as RFC2822.From. However
this does not mean there is no Sender specified. Rather it means 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 that that header field is virtual and that the address in the
field MUST be used. From: field MUST be used.
Specification of the error return addresses -- the "Bounce" Specification of the notifications Return addresses -- contained
address, contained in RFC2821.MailFrom -- is made by the in RFC2821.MailFrom -- is made by the RFC2822.Sender. Typically
RFC2822.Sender. Typically the Bounce address is the same as the the Return address is the same as the Sender address. However
Sender address. However some usage scenarios require it to be some usage scenarios require it to be different.
different.
RFC2822.To/.CC Set by: Originator RFC2822.To/.CC: Set by - Author
These specify MUA Recipient addresses. However some or all of the These specify MUA Recipient addresses. However some or all of the
addresses in these fields might not be present in the addresses in these fields might not be present in the
RFC2821.RcptTo commands, due to handling process that might RFC2821.RcptTo commands.
transfer from the former to the latter.
The distinction between To and CC is subjective. Generally a To The distinction between To and CC is subjective. Generally a To
addressee is considered primary and is expected to take action on addressee is considered primary and is expected to take action on
the message. A CC addressee typically receives a copy only for the message. A CC addressee typically receives a copy only for
their information. their information.
RFC2822.BCC Set by: Originator RFC2822.BCC: Set by - Author
A message might be copied to an addressee whose participation is A message might be copied to an addressee whose participation is
not to be disclosed to the RFC2822.To or RFC2822.CC Recipients not to be disclosed to the RFC2822.To or RFC2822.CC Recipients
and, usually, not to the other BCC Recipients. The BCC header and, usually, not to the other BCC Recipients. The BCC header
field indicates a message copy to such a Recipient. field indicates a message copy to such a Recipient.
Typically, the field lists no addresses or only lists the address Typically, the field lists no addresses or only lists the single
of the Recipient receiving this copy. An MUA will typically make address of the Recipient receiving this copy. An MUA will
separate postings for TO and CC Recipients, versus BCC Recipients. typically make separate postings for TO and CC Recipients, versus
The former will see no indication that any BCCs were sent, whereas BCC Recipients. The former will see no indication that any BCCs
the latter have a BCC field present. It might be empty, contain a were sent, whereas the latter have a BCC field present. It might
comment, or contain one or more BCC addresses, depending upon the be empty, contain a comment, or contain one or more BCC addresses,
preferences of the Originator. depending upon the preferences of the Author.
RFC2821.HELO/.EHLO Set by: Source RFC2821.HELO/.EHLO: Set by - Source
The MSA can specify its hosting domain identity for the SMTP HELO The MSA can specify its hosting domain identity for the SMTP HELO
or EHLO command operation. or EHLO command operation.
RFC3461.ENVID Set by: Source RFC3461.ENVID: Set by - Source
The MSA can specify an opaque string, to be included in a DSN, as The MSA can specify an opaque string, to be included in a DSN, as
a means of assisting the Bounce address recipient in identifying a means of assisting the Return address recipient in identifying
the message that produced a DSN, or message tracking. the message that produced a DSN, or message tracking.
RFC2821.MailFrom Set by: Source RFC2821.MailFrom: Set by - Source
This is an end-to-end string that specifies an email address for This is an end-to-end string that specifies an email address for
receiving return control information, such as "bounces". The name receiving return control information, such as "bounces". The name
of this field is misleading, because it is not required to specify of this field is misleading, because it is not required to specify
either the author or the agent responsible for submitting the either the author or the Actor responsible for submitting the
message. Rather, the agent responsible for submission specifies message. Rather, the Actor responsible for submission specifies
the RFC2821.MailFrom address. Ultimately the simple basis for the RFC2821.MailFrom address. Ultimately the simple basis for
deciding what address needs to be in the RFC2821.MailFrom is to deciding what address needs to be in the RFC2821.MailFrom is to
determine what address needs to be informed about transmission- determine what address needs to be informed about transmission-
level problems (and, possibly, successes.) level problems (and, possibly, successes.)
RFC2821.RcptTo Set by: Originator RFC2821.RcptTo: Set by - Author
This specifies the MUA mailbox address of a recipient. The string This specifies the MUA mailbox address of a recipient. The string
might not be visible in the message content header. For example, might not be visible in the message content header. For example,
the message destination address header fields, such as RFC2822.To, the message destination address header fields, such as RFC2822.To,
might specify a mailing list mailbox, while the RFC2821.RcptTo might specify a mailing list mailbox, while the RFC2821.RcptTo
address specifies a member of that list. address specifies a member of that list.
RFC2821.Received Set by: Source, Relay, Mediator, Dest RFC2821.Received: Set by - Source, Relay, Mediator, Dest
This indicates trace information, including originating host, This indicates trace information, including originating host,
relays, Mediators, and MSA host domain names and/or IP Addresses. relays, Mediators, and MSA host domain names and/or IP Addresses.
RFC2821.Return-Path Set by: Source RFC2821.Return-Path: Set by - Source
The MDA records the RFC2821.MailFrom address into the The MDA records the RFC2821.MailFrom address into the
RFC2822.Return-Path field. RFC2822.Return-Path field.
RFC2919.List-Id Set by: Mediator Originator RFC2919.List-Id: Set by - Mediator Author
This provides a globally unique mailing list naming framework that This provides a globally unique mailing list naming framework that
is independent of particular hosts. [RFC2919] is independent of particular hosts. [RFC2919]
The identifier is in the form of a domain name; however the string The identifier is in the form of a domain name; however the string
usually is constructed by combining the two parts of an email usually is constructed by combining the two parts of an email
address and the result rarely is a true domain name, listed in the address and the result rarely is a true domain name, listed in the
domain name service -- although it can be. domain name service -- although it can be.
RFC2369.List-* Set by: Mediator Originator RFC2369.List-*: Set by - Mediator Author
[RFC2369] defines a collection of message header fields for use by [RFC2369] defines a collection of message header fields for use by
mailing lists. In effect they supply list-specific parameters for mailing lists. In effect they supply list-specific parameters for
common mailing list user operations. The identifiers for these common mailing list user operations. The identifiers for these
operations are for the list, itself, and the user-as-subscriber operations are for the list, itself, and the user-as-subscriber
[RFC2369]. [RFC2369].
RFC0791.SourceAddr Set by: The Client SMTP sending host immediately RFC0791.SourceAddr: Set by - The Client SMTP sending host
preceding the current receiving SMTP server. immediately preceding the current receiving SMTP server.
[RFC0791] defines the basic unit of data transfer for the [RFC0791] defines the basic unit of data transfer for the
Internet, the IP Datagram. It contains a "Source Address" field Internet, the IP Datagram. It contains a "Source Address" field
that specifies the IP Address for the host (interface) from which that specifies the IP Address for the host (interface) from which
the datagram was sent. This information is set and provided by the datagram was sent. This information is set and provided by
the IP layer, and is therefore independent of mail-level the IP layer, and is therefore independent of mail-level
mechanisms. As such, it is often taken to be authoritative, mechanisms. As such, it is often taken to be authoritative,
although it is possible to provide false addresses. although it is possible to provide false addresses.
4.2. User-Level Services 4.2. User-Level Services
Interactions at the user level entail protocol exchanges, distinct Interactions at the user level entail protocol exchanges, distinct
from those that occur at lower layers of the Internet Mail from those that occur at lower layers of the Internet Mail
architecture, which is above the Internet Transport layer. Because architecture, which is above the Internet Transport layer. Because
the motivation for email, and much of its use, is for interaction the motivation for email, and much of its use, is for interaction
among humans, the nature and details of these protocol exchanges among humans, the nature and details of these protocol exchanges
often are determined by the needs of human and group communication. often are determined by the needs of human and group communication.
skipping to change at page 29, line 32 skipping to change at page 31, line 32
RFC2822.Sender RFC2822.Sender
RFC2822.To, RFC2822.CC RFC2822.To, RFC2822.CC
RFC2822.BCC RFC2822.BCC
4.2.2. Message Store (MS) 4.2.2. Message Store (MS)
An MUA can employ a long-term Message Store (MS). Figure 5 depicts An MUA can employ a long-term Message Store (MS). Figure 5 depicts
an Origination-side Ms (oMS) and a Recipient-side MS (rMS). There is an Origination-side MS (oMS) and a Recipient-side MS (rMS). There is
a rich set of choices for configuring a store, because any MS may a rich set of choices for configuring a store, because any MS may
comprise a distributed set of component stores. In Figure 5, the rMS comprise a distributed set of component stores. In Figure 5, the rMS
demonstrates this by showing an rMS that is located on a remote demonstrates this by showing an rMS that is located on a remote
server (srMS) and an rMS that is on the same machine as the MUA server (srMS) and an rMS that is on the same machine as the MUA
(urMS). The relationship between two message stores, themselves, can (urMS). The relationship between two message stores, themselves, can
vary. vary.
As discussed in [RFC1733] the operational relationship among MSs can As discussed in [RFC1733] the operational relationship among MSs can
be -- be --
skipping to change at page 32, line 13 skipping to change at page 34, line 13
Identities relevant to the MTA include: Identities relevant to the MTA include:
RFC2821.HELO/.EHLO RFC2821.HELO/.EHLO
RFC3461.ENVID RFC3461.ENVID
RFC2821.MailFrom RFC2821.MailFrom
RFC2821.RcptTo RFC2821.RcptTo
RFC2822.Received Set by: Relay Server RFC2822.Received Set by - Relay Server
4.3.3. Mail Delivery Agent (MDA) 4.3.3. Mail Delivery Agent (MDA)
A Mail Delivery Agent (MDA) delivers email to the Recipient's A Mail Delivery Agent (MDA) delivers email to the Recipient's
mailbox. It can provide distinctive, address-based functionality, mailbox. It can provide distinctive, address-based functionality,
made possible by its detailed knowledge of the properties of the made possible by its detailed knowledge of the properties of the
destination address. This knowledge might also be present elsewhere destination address. This knowledge might also be present elsewhere
in the Recipient's ADMD, such as at an organizational border in the Recipient's ADMD, such as at an organizational border
(Boundary) Relay. However it is required for the MDA, if only (Boundary) Relay. However it is required for the MDA, if only
because the MDA must know where to deliver the message. because the MDA must know where to deliver the message.
skipping to change at page 32, line 44 skipping to change at page 34, line 44
Using Internet protocols, delivery can be effected by a variety of Using Internet protocols, delivery can be effected by a variety of
standard protocols. When coupled with an internal local mechanism, standard protocols. When coupled with an internal local mechanism,
SMTP [RFC2821] and LMTP [RFC2033] permit "push" delivery to the SMTP [RFC2821] and LMTP [RFC2033] permit "push" delivery to the
Recipient system, at the initiative of the upstream email service. Recipient system, at the initiative of the upstream email service.
POP [RFC1939] and IMAP [RFC3501] are used for "pull" delivery at the POP [RFC1939] and IMAP [RFC3501] are used for "pull" delivery at the
initiative of the Recipient system. POP and IMAP can also be used initiative of the Recipient system. POP and IMAP can also be used
for repeated access to messages on a remote MS. for repeated access to messages on a remote MS.
Identities relevant to the MDA include: Identities relevant to the MDA include:
RFC2821.Return-Path Set by: Originator Source or Mediator Source RFC2821.Return-Path: Set by - Author Source or Mediator Source
The MDA records the RFC2821.MailFrom address into the The MDA records the RFC2821.MailFrom address into the
RFC2822.Return-Path field. RFC2822.Return-Path field.
RFC2822.Received Set by: MDA server RFC2822.Received: Set by - MDA server
An MDA can record a Received header field to indicate trace An MDA can record a Received header field to indicate trace
information, including source host and receiving host domain information, including source host and receiving host domain
names and/or IP Addresses. names and/or IP Addresses.
5. Mediators 5. Mediators
Basic email transfer from an Originator to the specified Recipients Basic email transfer from an Author to the specified Recipients is
is accomplished by using an asynchronous, store-and-forward accomplished by using an asynchronous, store-and-forward
communication infrastructure, in a sequence of independent communication infrastructure, in a sequence of independent
transmissions through some number of MTAs. A very different task is transmissions through some number of MTAs. A very different task is
a User-level sequence of postings and deliveries, through Mediators. a User-level sequence of postings and deliveries, through Mediators.
A Mediator forwards a message, through a re-posting process. The A Mediator forwards a message, through a re-posting process. The
Mediator does share some functionality with basic MTA relaying, but Mediator does share some functionality with basic MTA relaying, but
it enjoys a degree of freedom with both addressing and content that it enjoys a degree of freedom with both addressing and content that
is not available to MTAs. is not available to MTAs.
RFC2821.HELO/.EHLO Set by: Mediator Source RFC2821.HELO/.EHLO: Set by - Mediator Source
RFC3461.ENVID Set by: Originator Source or Mediator Source RFC3461.ENVID Set by - Author Source or Mediator Source
RFC2821.MailFrom Set by: Originator Source or Mediator Source RFC2821.MailFrom: Set by - Author Source or Mediator Source
RFC2821.RcptTo Set by: Mediator Originator RFC2821.RcptTo: Set by - Mediator Author
RFC2821.Received Set by: Mediator Dest RFC2821.Received: Set by - Mediator Dest
The salient aspect of a Mediator, that distinguishes it from any The salient aspect of a Mediator, that distinguishes it from any
other MUA creating an entirely new message, is that a Mediator other MUA creating an entirely new message, is that a Mediator
preserves the integrity and tone of the original message, including preserves the integrity and tone of the original message, including
the essential aspects of its origination information. The Mediator the essential aspects of its origination information. The Mediator
might also add commentary. might also add commentary.
Examples of MUA message creation that are NOT performed by Mediators Examples of MUA message creation NOT performed by Mediators include
include -- --
New message that forwards an existing message: New message that forwards an existing message:
This action rather curiously provides a basic template for a This action rather curiously provides a basic template for a
class of Mediators. However for its typical occurrence it is class of Mediators. However for its typical occurrence it is
not itself an example of a Mediator. The new message is viewed not itself an example of a Mediator. The new message is viewed
as being from the Agent doing the forwarding, rather than being as being from the Actor doing the forwarding, rather than being
from the original Originator. from the original Author.
A new message encapsulates the original message and is seen as A new message encapsulates the original message and is seen as
strictly "from" the Mediator. The Mediator might add strictly "from" the Mediator. The Mediator might add
commentary and certainly has the opportunity to modify the commentary and certainly has the opportunity to modify the
original message content. The forwarded message is therefore original message content. The forwarded message is therefore
independent of the original message exchange and creates a new independent of the original message exchange and creates a new
message dialogue. However the final Recipient sees the message dialogue. However the final Recipient sees the
contained message as from the original Originator. contained message as from the original Author.
Reply: Reply:
When a Recipient formulates a response back to the original When a Recipient formulates a response back to the original
message's author, the new message is not typically viewed as message's author, the new message is not typically viewed as
being a "forwarding" of the original. Its focus is the new being a "forwarding" of the original. Its focus is the new
content, although it might contain all or part of the material content, although it might contain all or part of the material
in the original message. Therefore the earlier material is in the original message. Therefore the earlier material is
merely contextual and secondary. merely contextual and secondary.
Annotation: Annotation:
The integrity of the original message is usually preserved, but The integrity of the original message is usually preserved, but
one or more comments about the message are added in a manner one or more comments about the message are added in a manner
that distinguishes commentary from original text. The tone of that distinguishes commentary from original text. The tone of
the new message is that it is primarily commentary from a new the new message is that it is primarily commentary from a new
Originator, similar to a Reply. Author, similar to a Reply.
The remainder of this section describes common examples of Mediators. The remainder of this section describes common examples of Mediators.
5.1. Aliasing 5.1. Aliasing
Aliasing is a simple re-addressing facility that is available in most Aliasing is a simple re-addressing facility that is available in most
MDA implementations. It is performed just before placing a message MDA implementations. It is performed just before placing a message
into the specified Recipient's mailbox. Instead the message is into the specified Recipient's mailbox. Instead the message is
submitted back to the transfer service, for delivery to one or more submitted back to the transfer service, for delivery to one or more
alternate addresses. Although typically implemented as part of an alternate addresses. Although typically implemented as part of an
skipping to change at page 35, line 14 skipping to change at page 37, line 14
aliasing as a part of the lower-level mail relaying activity. aliasing as a part of the lower-level mail relaying activity.
However the small change has a large semantic impact: The designated However the small change has a large semantic impact: The designated
recipient has chosen a new recipient. Hence that original recipient recipient has chosen a new recipient. Hence that original recipient
SHOULD become responsible for any handling issues. This change would SHOULD become responsible for any handling issues. This change would
be reflected by replacing the message's RFC2821.MailFrom address to be reflected by replacing the message's RFC2821.MailFrom address to
be one within the scope of the ADMD doing the aliasing. be one within the scope of the ADMD doing the aliasing.
An MDA that is re-posting a message to an alias typically changes An MDA that is re-posting a message to an alias typically changes
only envelope information: only envelope information:
RFC2822.To/.CC/.BCC Set by: Originator RFC2822.To/.CC/.BCC: Set by - Author
These retain their original addresses. These retain their original addresses.
RFC2821.RcptTo Set by: Mediator Originator RFC2821.RcptTo: Set by - Mediator Author
This field contains an alias address. This field contains an alias address.
RFC2821.MailFrom Set by: Originator Source or Mediator Source RFC2821.MailFrom: Set by - Author Source or Mediator Source
The agent responsible for submission to an alias address will The Actor responsible for submission to an alias address will
often retain the original address to receive handling Bounces. often retain the original address to receive handling Returns.
The benefit of retaining the original MailFrom value is to The benefit of retaining the original MailFrom value is to
ensure that the origination-side agent knows that there has ensure that the origination-side Actor knows that there has
been a delivery problem. On the other hand, the responsibility been a delivery problem. On the other hand, the responsibility
for the problem usually lies with the Recipient, since the for the problem usually lies with the Recipient, since the
Alias mechanism is strictly under the Recipient's control. Alias mechanism is strictly under the Recipient's control.
RFC2821.Received Set by: Mediator Dest RFC2821.Received Set by - Mediator Dest
The agent can record Received information, to indicate the The Actor can record Received information, to indicate the
delivery to the original address and submission to the alias delivery to the original address and submission to the alias
address. The trace of Received header fields can therefore address. The trace of Received header fields can therefore
include everything from original posting through final delivery include everything from original posting through final delivery
to a final delivery. to a final delivery.
5.2. Re-Sending 5.2. Re-Sending
Also called Re-Directing, Re-Sending differs from Forwarding by Also called Re-Directing, Re-Sending differs from Forwarding by
virtue of having the Mediator "splice" a message's addressing virtue of having the Mediator "splice" a message's addressing
information, to connect the Originator of the original message and information, to connect the Author of the original message and the
the Recipient of the new message. This permits them to have direct Recipient of the new message. This permits them to have direct
exchange, using their normal MUA Reply functions. Hence the new exchange, using their normal MUA Reply functions. Hence the new
Recipient sees the message as being From the original Originator, Recipient sees the message as being From: the original Author, even
even if the Mediator adds commentary. if the Mediator adds commentary.
Identities specified in a resent message include Identities specified in a resent message include
RFC2822.From Set by: original Originator RFC2822.From: Set by - original Author
Names and email addresses for the original author(s) of the Names and email addresses for the original author(s) of the
message content are retained. The free-form (display-name) message content are retained. The free-form (display-name)
portion of the address might be modified to provide informal portion of the address might be modified to provide informal
reference to the agent responsible for the redirection. reference to the Actor responsible for the redirection.
RFC2822.Reply-To Set by: original Originator RFC2822.Reply-To: Set by - original Author
If this field is present in the original message, it is If this field is present in the original message, it is
retained in the Resent message. retained in the Resent message.
RFC2822.Sender Set by: Originator Source or Mediator Source. RFC2822.Sender: Set by - Author Source or Mediator Source.
RFC2822.To/.CC/.BCC Set by: original Originator RFC2822.To/.CC/.BCC: Set by - original Author
These specify the original message Recipients. These specify the original message Recipients.
RFC2822.Resent-From Set by: Mediator Originator RFC2822.Resent-From: Set by - Mediator Author
The address of the original Recipient who is redirecting the The address of the original Recipient who is redirecting the
message. Otherwise the same rules apply for the Resent-From message. Otherwise the same rules apply for the Resent-From:
field as for an original RFC2822.From field. field as for an original RFC2822.From field.
RFC2822.Resent-Sender Set by: Mediator Source RFC2822.Resent-Sender: Set by - Mediator Source
The address of the agent responsible for re-submitting the The address of the Actor responsible for re-submitting the
message. As with RFC2822.Sender, this field is often omitted message. As with RFC2822.Sender, this field is often omitted
when it would merely contain the same address as when it would merely contain the same address as
RFC2822.Resent-From. RFC2822.Resent-From.
RFC2822.Resent-To/-CC/-BCC: Set by: Mediator Originator RFC2822.Resent-To/-CC/-BCC: Set by: Mediator Author
The addresses of the new Recipients who will now be able to The addresses of the new Recipients who will now be able to
reply to the original author. reply to the original author.
RFC2821.MailFrom Set by: Mediator Source RFC2821.MailFrom: Set by - Mediator Source
The agent responsible for re-submission (RFC2822.Resent-Sender) The Actor responsible for re-submission (RFC2822.Resent-Sender)
is also responsible for specifying the new MailFrom address. is also responsible for specifying the new MailFrom address.
RFC2821.RcptTo Set by: Mediator Originator RFC2821.RcptTo: Set by - Mediator Author
This will contain the address of a new Recipient. This will contain the address of a new Recipient.
RFC2822.Received Set by: Mediator Dest RFC2822.Received: Set by - Mediator Dest
When resending a message the submission agent can record a When resending a message the submission agent can record a
Received header field, to indicate the transition from original Received header field, to indicate the transition from original
posting to resubmission. posting to resubmission.
5.3. Mailing Lists 5.3. Mailing Lists
Mailing lists have explicit email addresses and they re-post messages Mailing lists have explicit email addresses and they re-post messages
to a list of subscribed members. The Mailing List Actor performs a to a list of subscribed members. The Mailing List Actor performs a
task that can be viewed as an elaboration of the Re-Director role. task that can be viewed as an elaboration of the Re-Director role.
In addition to sending the new message to a potentially large number In addition to sending the new message to a potentially large number
of new Recipients, the Mediator can modify content, such as deleting of new Recipients, the Mediator can modify content, such as deleting
attachments, converting the format, and adding list-specific attachments, converting the format, and adding list-specific
comments. In addition, archiving list messages is common. Still the comments. In addition, archiving list messages is common. Still the
message retains characteristics of being "from" the original message retains characteristics of being "from" the original Author.
Originator.
Identities relevant to a mailing list processor, when submitting a Identities relevant to a mailing list processor, when submitting a
message, include: message, include:
RFC2919.List-Id Set by: Mediator Originator RFC2919.List-Id: Set by - Mediator Author
RFC2369.List-* Set by: Mediator Originator RFC2369.List-*: Set by - Mediator Author
RFC2822.From Set by: original Originator RFC2822.From: Set by - original Author
Names and email addresses for the original author(s) of the Names and email addresses for the original author(s) of the
message content are specified -- or, rather, retained. message content are specified -- or, rather, retained.
RFC2822.Reply-To Set by: original Originator or Mediator RFC2822.Reply-To: Set by - original Author or Mediator Author
Originator
RFC2822.Sender Set by: Originator Source or Mediator Source RFC2822.Sender: Set by - Author Source or Mediator Source
This will usually specify the address of the agent responsible This will usually specify the address of the Actor responsible
for mailing list operations. However some mailing lists for mailing list operations. However some mailing lists
operate in a manner very similar to a simple MTA Relay, so that operate in a manner very similar to a simple MTA Relay, so that
they preserve as much of the original handling information as they preserve as much of the original handling information as
possible, including the original RFC2822.Sender field. possible, including the original RFC2822.Sender field.
RFC2822.To/.CC Set by: original Originator RFC2822.To/.CC Set by - original Author
These usually contain the original list of Recipient addresses. These usually contain the original list of Recipient addresses.
RFC2821.MailFrom Set by: Originator Source or Mediator Source RFC2821.MailFrom Set by - Author Source or Mediator Source
This can contain the original address to be notified of This can contain the original address to be notified of
transmission issues, or the mailing list agent can set it to transmission issues, or the mailing list Actor can set it to
contain a new Notification address. Typically the value is set contain a new Notification address. Typically the value is set
to a new address, so that mailing list members and posters are to a new address, so that mailing list members and posters are
not burdened with transmission-related Bounces. not burdened with transmission-related Returns.
RFC2821.RcptTo Set by: Mediator Originator RFC2821.RcptTo Set by - Mediator Author
This contains the address of a mailing list member. This contains the address of a mailing list member.
RFC2821.Received Set by: Mediator Dest RFC2821.Received Set by - Mediator Dest
A Mailing List Agent can record a Received header field, to A Mailing List Actor can record a Received header field, to
indicate the transition from original posting to mailing list indicate the transition from original posting to mailing list
forwarding. The Agent can choose to have the message retain forwarding. The Actor can choose to have the message retain
the original set of Received header fields or can choose to the original set of Received header fields or can choose to
remove them. In the latter case it can ensure that the remove them. In the latter case it can ensure that the
original Received header fields are otherwise available, to original Received header fields are otherwise available, to
ensure later accountability and diagnostic access to them. ensure later accountability and diagnostic access to them.
5.4. Gateways 5.4. Gateways
A Gateway performs the basic routing and transfer work of message A Gateway performs the basic routing and transfer work of message
relaying, but it also may make any message or address modifications relaying, but it also may make any content, structure, address, or
that are needed to send the message into a messaging environment that attribute modifications needed to send the message into a messaging
operates according to different standards or potentially incompatible environment that operates according to different standards or
policies. When a Gateway connects two differing messaging services, potentially incompatible policies. When a Gateway connects two
its role is easy to identify and understand. When it connects differing messaging services, its role is easy to identify and
environments that have technical similarity, but can have significant understand. When it connects environments that have technical
administrative differences, it is easy to think that a Gateway is similarity, but can have significant administrative differences, it
merely an MTA. is easy to think that a Gateway is merely an MTA.
The critical distinction between an MTA and a Gateway is that the The critical distinction between an MTA and a Gateway is that the
latter transforms addresses and/or message content, in order to map latter can make substantive changes to a message, in order to map
between the standards of two, different messaging services. In between the standards of two, different messaging services. In
virtually all cases, this mapping process results in some degree of virtually all cases, this mapping process results in some degree of
semantic loss. The challenge of Gateway design is to minimize this semantic loss. The challenge of Gateway design is to minimize this
loss. loss.
A Gateway can set any identity field available to a regular MUA. A Gateway can set any identity field available to a regular MUA.
Identities typically relevant to Gateways include: Identities typically relevant to Gateways include:
RFC2822.From Set by: original Originator RFC2822.From: Set by - original Author
Names and email addresses for the original author(s) of the Names and email addresses for the original author(s) of the
message content are retained. As for all original addressing message content are retained. As for all original addressing
information in the message, the Gateway can translate addresses information in the message, the Gateway can translate addresses
in whatever way will allow them continue to be useful in the in whatever way will allow them continue to be useful in the
target environment. target environment.
RFC2822.Reply-To Set by: original Originator RFC2822.Reply-To: Set by - original Author
The Gateway SHOULD retain this information, if it is originally The Gateway SHOULD retain this information, if it is originally
present. The ability to perform a successful reply by a present. The ability to perform a successful reply by a
Gatewayed Recipient is a typical test of Gateway functionality. Gatewayed Recipient is a typical test of Gateway functionality.
RFC2822.Sender Set by: Originator Source or Mediator Source RFC2822.Sender: Set by - Author Source or Mediator Source
This can retain the original value or can be set to a new This can retain the original value or can be set to a new
address. address.
RFC2822.To/.CC/.BCC Set by: original Recipient RFC2822.To/.CC/.BCC Set by - original Recipient
These usually retain their original addresses. These usually retain their original addresses.
RFC2821.MailFrom Set by: Originator Source or Mediator Source RFC2821.MailFrom Set by - Author Source or Mediator Source
The agent responsible for gatewaying the message can choose to The Actor responsible for gatewaying the message can choose to
specify a new address to receive handling notices. specify a new address to receive handling notices.
RFC2822.Received Set by: Mediator Dest RFC2822.Received Set by - Mediator Dest
The Gateway can record a Received header field, to indicate the The Gateway can record a Received header field, to indicate the
transition from the original posting environment to the new transition from the original posting environment to the new
messaging environment. messaging environment.
5.5. Boundary Filter 5.5. Boundary Filter
Organizations often enforce security boundaries by subjecting Organizations often enforce security boundaries by subjecting
messages to analysis, for conformance with the organization's safety messages to analysis, for conformance with the organization's safety
policies. An example is detection of content classed as spam or a policies. An example is detection of content classed as spam or a
skipping to change at page 42, line 30 skipping to change at page 44, line 30
[RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail", [RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail",
RFC 4409, April 2006. RFC 4409, April 2006.
[RFC4550] Maes, S., , S., and Isode Ltd., "Internet Email to Support [RFC4550] Maes, S., , S., and Isode Ltd., "Internet Email to Support
Diverse Service Environments (Lemonade) Profile", Diverse Service Environments (Lemonade) Profile",
June 2006. June 2006.
7.2. Informative 7.2. Informative
[ID-spamops]
Hutzler, C., Crocker, D., Resnick, P., Sanderson, R., and
E. Allman, "Email Submission Between Independent
Networks", draft-hutzler-spamops-06 (work in progress),
May 2007.
[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10, [RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10,
RFC 821, August 1982. RFC 821, August 1982.
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet [RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982. text messages", STD 11, RFC 822, August 1982.
[RFC1733] Crispin, M., "Distributed Electronic Models in IMAP4", [RFC1733] Crispin, M., "Distributed Electronic Models in IMAP4",
December 1994. December 1994.
[RFC1767] Crocker, D., "MIME Encapsulation of EDI Objects", [RFC1767] Crocker, D., "MIME Encapsulation of EDI Objects",
skipping to change at page 43, line 13 skipping to change at page 45, line 8
[RFC2442] "The Batch SMTP Media Type", RFC 2442, November 1998. [RFC2442] "The Batch SMTP Media Type", RFC 2442, November 1998.
[RFC3801] Vaudreuil, G. and G. Parsons, "", RFC 3801, June 2004. [RFC3801] Vaudreuil, G. and G. Parsons, "", RFC 3801, June 2004.
[RFC3885] Allman, E. and T. Hansen, "SMTP Service Extension for [RFC3885] Allman, E. and T. Hansen, "SMTP Service Extension for
Message Tracking", RFC 3885, September 2004. Message Tracking", RFC 3885, September 2004.
[RFC4142] Crocker, D. and G. Klyne, "Full-mode Fax Profile for [RFC4142] Crocker, D. and G. Klyne, "Full-mode Fax Profile for
Internet Mail: FFPIM", December 2005. Internet Mail: FFPIM", December 2005.
[RFC5068] Hutzler, C., Crocker, D., Resnick, P., Sanderson, R., and
E. Allman, "Email Submission Operations: Access and
Accountability Requirements", RFC 5068, BCP 134, Nov 2007.
[Tussle] Clark, D., Wroclawski, J., Sollins, K., and R. Braden, [Tussle] Clark, D., Wroclawski, J., Sollins, K., and R. Braden,
"Tussle in Cyberspace: Defining Tomorrow's Internet", "Tussle in Cyberspace: Defining Tomorrow's Internet",
ACM SIGCOMM, 2002. ACM SIGCOMM, 2002.
Appendix A. Acknowledgements Appendix A. Acknowledgements
This work derives from a section in draft-hutzler-spamops This work derives from a section in draft-hutzler-spamops [RFC5068].
[ID-spamops]. Discussion of the Source actor role was greatly Discussion of the Source actor role was greatly clarified during
clarified during discussions in the IETF's Marid working group. discussions in the IETF's Marid working group.
Graham Klyne, Pete Resnick and Steve Atkins provided thoughtful Graham Klyne, Pete Resnick and Steve Atkins provided thoughtful
insight on the framework and details of the original drafts. insight on the framework and details of the original drafts.
Later reviews and suggestions were provided by Eric Allman, Nathaniel Later reviews and suggestions were provided by Eric Allman, Nathaniel
Borenstein, Ed Bradford, Cyrus Daboo, Frank Ellermann, Tony Finch, Borenstein, Ed Bradford, Cyrus Daboo, Frank Ellermann, Tony Finch,
Ned Freed, Eric Hall, Tony Hansen, Willemien Hoogendoorn, Brad Ned Freed, Eric Hall, Tony Hansen, Willemien Hoogendoorn, Brad
Knowles, John Leslie, Bruce Valdis Kletnieks, Mark E. Mallett, David Knowles, John Leslie, Bruce Valdis Kletnieks, Mark E. Mallett, David
MacQuigg, Alexey Melnikov, der Mouse, S. Moonesamy, Chris Newman, MacQuigg, Alexey Melnikov, der Mouse, S. Moonesamy, Chris Newman,
Daryl Odnert, Rahmat M. Samik-Ibrahim, Marshall Rose, Hector Santos, Daryl Odnert, Rahmat M. Samik-Ibrahim, Marshall Rose, Hector Santos,
Jochen Topf, Greg Vaudreuil. Jochen Topf, Greg Vaudreuil.
Diligent proof-reading was performed by Bruce Lilly. Diligent proof-reading was performed by Bruce Lilly.
Index
A
Actor
Administrative 15
Author 10
Edge 16
Gateway 14
Mediator 11
Originator 13
Recipient 10
Relay 14
Return Handler 11
Transit 16
User 16
Actors
MHS 12
Administrative Actors 15
Author 10
D
Discussion of document 6
E
Edge Actor 16
end-to-end 4
G
Gateway 12, 14
I
Internet Mail 4
M
Mail 4
Mail exchange 5
Mail Handling Service 3
Mail Handling System 12
Mail Transfer Agent 3
Mail User Agent 3
MDN 10
Mediator 11
Message Disposition Notification 10
MHS 3, 12
Actors 12
MTA 3
MUA 3
O
Originator 13
R
Recipient 10
Relay 14
Return Handler 11
T
Transit Actor 16
U
UA 3
User Actor 16
User Agent 3
Author's Address Author's Address
Dave Crocker Dave Crocker
Brandenburg InternetWorking Brandenburg InternetWorking
675 Spruce Drive 675 Spruce Drive
Sunnyvale, CA 94086 Sunnyvale, CA 94086
USA USA
Phone: +1.408.246.8253 Phone: +1.408.246.8253
Email: dcrocker@bbiw.net Email: dcrocker@bbiw.net
Full Copyright Statement Full Copyright Statement
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors contained in BCP 78, and except as set forth therein, the authors
retain all their rights. retain all their rights.
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
skipping to change at page 44, line 44 skipping to change at line 2044
attempt made to obtain a general license or permission for the use of attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr. http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at this standard. Please address the information to the IETF at
ietf-ipr@ietf.org. ietf-ipr@ietf.org.
Acknowledgment
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
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