| Network Working Group | D. Crocker |
| INTERNET DRAFT | Brandenburg InternetWorking |
| <draft-crocker-email-arch-04> | March 2005 |
| Category: Standards Track | |
| Expires: September 2005 |
Internet Mail Architecture
draft-crocker-email-arch-04
This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress".
The list of current Internet-Drafts can be accessed at <http://www.ietf.org/ietf/1id-abstracts.txt>.
The list of Internet-Draft Shadow Directories can be accessed at <http://www.ietf.org/shadow.html>.
This Internet-Draft will expire in September 2005.
Copyright (C) The Internet Society (2005). All Rights Reserved.
Over its thirty-four year history, Internet Mail has undergone significant changes in scale and complexity, as it has become a global infrastructure service. The first standardized architecture for email specified a simple split between the user world, in the form of Mail User Agents (MUA), and the transmission world, in the form of the Mail Handling Service (MHS) composed of Mail Transfer Agents (MTA). Core aspects of the service, such as address and message style, have remained remarkably constant. Today, Internet Mail is marked by many independent operators, many different components for providing users service and many others for performing message transfer. Public discussion of the architecture has not kept pace with the real-world technical and operational refinements. This document offers an enhanced Internet Mail architecture to reflect the current service.
1 Introduction
1.1
Service
Overview
1.2 Discussion
venue
1.3 Changes
2
Email
Actor Roles
2.1 User
Actors
2.2 MHS
Actors
2.3 Administrative
Actors
3 Identities
3.1
Mailbox
Addresses
3.2 Domain
Names
3.3 Message
Identifiers
3.4 Identity
Referencing Convention
4 Services
4.1
Message
4.2
Mail
User Agent (MUA)
4.3 Mail
Submission Agent (MSA)
4.4 Mail
Transfer Agent (MTA)
4.5 Mail
Delivery Agent (MDA)
4.6 Message
Store (MS)
5 Mediators
5.1
Aliasing
5.2
ReSending
5.3
Mailing
Lists
5.4 Gateways
5.5
Boundary
Filter
6 Security
Considerations
7 References
7.1
References
- Normative
7.2 Reference
- Descriptive
§ Author's
Address
A Acknowledgements
§
Intellectual
Property and Copyright Statements
Figure
1: Basic Internet Mail Service Model
Figure
2: Relationships Among User Actors
Figure
3: Relationships Among MHS Actors
Figure
4: Administrative Units (AU) Example
Figure
5: Protocols and Services
Over its thirty-four year history, Internet Mail has undergone significant changes in scale and complexity, as it has become a global infrastructure service.
The first standardized architecture for email specified a simple split between the user world, in the form of Mail User Agents (MUA), and the transmission world, in the form of the Mail Handling Service (MHS) composed of Mail Transfer Agents (MTA). The MHS is responsible for accepting a message from one User and delivering it to one or more others.
+--------+
+---------------->| User |
| +--------+
| .
+--------+ | +--------+ .
| User +--+--------->| User | .
+--------+ | +--------+ .
. | . .
. | +--------+ . .
. +-->| User | . .
. +--------+ . .
. . . .
. . . .
. . . .
+--------------------------------------+
| Mail Handling Service (MHS) |
+--------------------------------------+
Figure 1: Basic Internet Mail Service Model
Today, Internet Mail is marked by many independent operators, many different components for providing users service and many other components for performing message transfer. So it is not surprising that the operational service has sub-divided each of these "layers" into more specialized modules. Core aspects of the service, such as address and message style, have remained remarkably constant. However public discussion of the architecture has not kept pace with the real-world refinements. This document offers an enhanced Internet Mail architecture to reflect the current service. The original distinction between user-level concerns and transfer-level concerns is retained, and the elaboration to each "level" of the architecture is discussed separately. The term "Internet Mail" is used to refer to the entire collection of user and transfer components.
For Internet Mail, the term "end-to-end" usually refers to a single posting and the set of deliveries directly resulting from its single transiting of the MHS. However, note that some uses of email consider the entire email service -- including Originator and Recipient -- as a subordinate component. For these services, "end-to-end" refers to points outside of the email service. Examples are voicemail over email [RFC2423], EDI over email [RFC1767], and facsimile over email.[ID-ffpim]
The current draft seeks to:
End-to-end Internet Mail exchange is accomplished by using a standardized infrastructure comprising:
The end-to-end portion of the service is the email object, called a message. Broadly the message, itself, is divided between handling control information and user message content.
A precept to the design of Internet Mail is permitting user-to-user and MTA-to-MTA interoperability with no prior, direct administrative arrangement. That is, all participants rely on having the core services be universally supported, either directly or through Gateways that translate between Internet Mail standards and other email conventions.
For localized environments (Edge networks) prior, administrative arrangement can include access control, routing constraints and lookup service configuration. In recent years one change to local environments is an increased requirement for authentication or, at least, accountability. In these cases, the server performs explicit validation of the client's identity.
Discussion about this document should be directed to the IETF-SMTP mailing list <http://www.imc.org/ietf-smtp>. It is the most active, long-standing venue for discussing email architecture. Although it is primarily for discussing only the SMTP protocol, it is recommended that discussion of this draft take place on that mailing list because it attends to end-to-end infrastructure and architecture issues more than other email-related mailing lists.
This is intended to be the last major revision, prior to seeking publication.
Significant changes to this version:
Internet Mail is a highly distributed service, with a variety of actors serving different roles. These divide into:
Although related to a technical architecture, the focus on Actors concerns participant responsibilities, rather than on functional modules. Hence the labels used are different than for classic email architecture diagrams. Actors often will be associated with different organizations. This operational independence provides the motivation for distinguishing Administrative Units.
Users are the sources and sinks of messages. They may be humans or processes. They may have an exchange that iterates and they may expand or contract the set of Users participating in a set of exchanges. In Internet Mail there are three types of user-level Actors:
From the User-level perspective all mail transfer activities are performed by a monolithic Mail Handling Service (MHS), even though the actual service may be provided by many independent organizations. Users are customers of this service.
The following depicts the flow of messages among Actors.
+------------+ | Originator |<--------------+ +-+---+----+-+ | | | | | | | V | | | +-----------+ | | | | Recipient | | | | +-----------+ | | | | | | +----------+ | | | | | | | V V | | | +-----------+ +---+---+---+ | | Mediator +--->| Recipient | | +-----------+ +-----------+ | V +-----------+ +-----------+ +-----------+ | Mediator +--->| Mediator +--->| Recipient | +-----------+ +-----------+ +-----------+
Figure 2: Relationships Among User Actors
Also called "Author", this is the user-level participant responsible for creating original content and requesting its transmission. The MHS operates to send and deliver mail among Originators and Recipients. As described below, the MHS has a "Source" role, that correlates with the Author role.
The Recipient is a consumer of delivered content. As described below, the MHS has a "Dest" role, that correlates with the Recipient role.
A Recipient may close the user-level communication loop by creating and submitting a new message that replies to an Originator. An example of an automated form of reply is the Message Disposition Notification, which informs the Originator about the Recipient's disposition of the message. See Section 4.1.
A Mediator receives, aggregates, reformulates and redistributes messages as part of a potentially-protracted, higher-level exchange among Users. Example uses of Mediators include group dialogue and organizational message flow, as occurs with a purchase approval process. Note that it is easy to confuse this user-level activity with the underlying MHS transfer exchanges. However they serve very different purposes and operate is very different ways. Mediators are considered extensively in Section 5.
When mail is delivered to an envelope address, a Mediator is viewed by the Mail Handling Service as a Recipient. When submitting messages, the Mediator is an Originator. What is distinctive is that a Mediator preserves the Originator information of the message it reformulates, but may make meaningful changes to the content. Hence the MHS sees a new message, but Users receive a message that is interpreted as primarily being from -- or, at least, initiated by -- the author of the original message. The role of a Mediator permits distinct, active creativity, rather than being limited to the more constrained job of merely connecting together other participants. Hence it is really the Mediator that is responsible for the new message.
A Mediator's task may be complex and contingent, such as by modifying and adding content or regulating which users may participate and when. The popular example of this role is a group mailing list. A sequence of mediators may even perform a series of formal steps, such as reviewing, modifying and approving a purchase request.
Because a Mediator originates messages, it might also receive replies. So, a Mediator really is a full-fledged User.
The Mail Handling Service (MHS) has the task of performing a single, email-level end-to-end transfer, on behalf of the Originator and reaching the Recipient address(es) specified in the envelope. Mediated or protracted, iterative exchanges, such as those used for collaboration over time, are part of the User-level service, and are not part of this Transfer-level Handling Service.
The following depicts the relationships among transfer participants in Internet Mail. It shows the Source as distinct from the Originator, and Destination as distinct from Recipient, although it is common for each pair to be the same actor. The figure also shows multiple Relays in the sequence. It is legal to have no separate Relay, where the Source and Dest interact directly. For intra-organization mail services, it is common to have only one Relay.
+------------+ +-----------+
| Originator | | Recipient |
+-----+------+ +-----------+
| ^
| Mail Handling Service |
/+=================================================+\
|| | | ||
|| | | ||
V |
+---------+ +--------+ +----+----+
| | | |<------------+ |
| Source +...>| Bounce | | Dest |
| | | |<---+ | |
+----+----+ +--------+ | +---------+
| | ^
V | |
+---------+ +----+----+ +----+----+
| Relay +-->.......-->| Relay +-->| Relay |
+---------+ +----+----+ +---------+
|
V
+---------+
| Gateway +-->...
+---------+
Figure 3: Relationships Among MHS Actors
The Source role is responsible for ensuring that a message is valid for posting and then submitting it to a Relay. Validity includes conformance with Internet Mail standards, as well as with local operational policies. The source may simply review the message for conformance, and reject it if there are errors, or it may create some or all of the necessary information.
The Source operates with dual "allegiance". It serves the Originator and often it is the same entity. However its role in assuring validity means that it must also represent the local operator of the MHS, that is, the local Administrative Domain.
The Source also has the responsibility for any post-submission, Originator-related administrative tasks associated with message transmission and delivery. Notably this pertains to error and delivery notices. Hence, Source is best held accountable for the message content, even when they did not create any or most of it.
The Bounce Handler processes service notifications that are generated by the MHS, as a result of its efforts to transfer or deliver the message. Notices may 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.
A mail Relay performs email transfer-service routing and store-and-forward. It adds envelope-level handling information and then (re-)transmits the message on towards its Recipient(s). A Relay may add information to the envelope, such as with trace information. However it does not modify existing envelope information or the message content semantics. It may modify message content syntax, such as a change from text to binary transfer-encoding form, only as required to meet the capabilities of the next hop in the MHS.
A set of Relays composes a Mail Handling Service network. This is above any underlying packet-switching network that they might be using and below any gateways or other user-level Mediators.
In other words, interesting email scenarios can involve three, distinct architectural layers of store-and-forward service:
with the bottom-most usually being the Internet's IP service. The most basic email scenarios involve Relays and Switches.
Aborting a message transfer results in having the Relay become an Originator and send an error message to the Bounce (Bounce) address. (The potential for looping is avoided by having this message, itself, contain no Bounce address.)
A Gateway is a hybrid form of User and Relay that interconnects heterogeneous mail services. Its purpose is simply to emulate a 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 content.
Differences between mail services can be as small as minor syntax variations, but usually encompass significant, semantic distinctions. For example, the concept of an email address might be as different as a hierarchical, machine-specific address versus a flat, global name space. Or between text-only content and multi-media. Hence the Relay function in a Gateway offers the minor challenge in design. The more significant challenge is in ensuring the user-to-user functionality that matches syntax and semantics of independent email standards suites.
The basic test of a Gateway's adequacy is, of course, whether an Originator on one side of a Gateway can send a message to a Recipient on the other side, without requiring changes to any of the components in the Originator's or Recipient's mail services, other than adding the Gateway. To each of these otherwise independent services, the Gateway will appear to be a "native" participant. However the ultimate test of a Gateway's adequacy is whether the Originator and Recipient can sustain a dialogue. In particular, can a Recipient's MUA automatically formulate a valid Reply?
Operation of Internet Mail services is apportioned to different providers (or operators). Each can be composed of an independent Administrative Unit (AU). Examples include an end-user operating their desktop client, a department operating a local Relay, an IT department operating an enterprise Relay, and an ISP operating a public, shared email service. These can be configured into many combinations of administrative and operational relationships, with each Administrative Unit potentially having a complex arrangement of functional components. Figure 4 depicts the relationships among AUs. Perhaps the most salient aspect of an AU is the differential trust that determines its policies for activities within the AU, versus those involving interactions with other AUs. The architectural impact of needing to have boundaries between AU's is discussed in [Tussle]
Basic components of AU distinction include:
Note that Transit services are quite different from packet-level transit operation. Whereas end-to-end packet transfers usually go through intermediate routers, email exchange across the open Internet is often directly between the Edge AUs, at the email level.
+------ +------+ +------+
| AU-1 | | AU-3 | | AU-4 |
| ---- | | ---- | | ---- |
| | +---------------------->| | | |
| User | | |-Edge-+---->|-User |
| | | | +--->| | | |
| V | | | +------+ +------+
| Edge-+----+ |
| | | +---------+ |
+------+ | | AU-2 | |
| | ------- | |
| | | |
+--->|-Transit-+---+
| |
+---------+
Figure 4: Administrative Units (AU) Example
Edge networks may use proprietary email standards internally. However the distinction between Transit network and Edge network transfer services is primarily significant because it highlights the need for concern over interaction and protection between independent administrations. In particular, this distinctions calls for additional care in assessing transitions of responsibility, as well as the accountability and authorization relationships among participants in email transfer.
The interactions between functional components within an Administrative Unit are subject to the policies of that domain. Policies can cover such things as reliability, access control, accountability and even content evaluation and modification. They may be implemented in different functional components, according to the needs of the Administrative Unit. For example, see [ID-spamops].
User, Edge and Transit services can be offered by providers that operate component services or sets of services. Further, it is possible for one AU to host services for other AUs. Common AU examples are:
Operational pragmatics often dictate that providers be involved in detailed administration and enforcement issues, to help ensure the health of the overall Internet Mail Service. This can include operators of lower-level packet services.
Internet Mail uses three forms of identity. The most common is the mailbox address <addr-spec> [RFC2822] Also see <address> and <mailbox> in [RFC2821]. The other two are the domain name <domain> Section 3.2 and message identifier <msg-id> [RFC2822].
A mailbox is specified as an Internet Mail address <addr-spec>. It has two distinct parts, divided by an at-sign ("@"). The right-hand side contains a globally interpreted name for an Administrative Unit. Domain Names are discussed in Section 3.2.
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 interpreted only by the entity specified in the address's right-hand side. All other entities must treat the local-part as a uninterpreted, literal string and must preserve all of its original details. As such, its public distribution is equivalent to sending a "cookie" that is only interpreted upon being returned to its originator.
It is common for sites to have local structuring conventions for the left-hand side (local-part) of an addr-spec. This permits sub-addressing, such as for distinguishing different discussion groups by the same participant. However it must be stressed that these conventions are strictly private to the user's organization and must not be interpreted by any domain except the one listed in the right-hand side of the addr-spec.
A small class of addresses has an elaboration on basic email addressing, with a standardized, global schema for the local-part. These are conventions between originating end-systems and Recipient Gateways, and they are invisible to the public email transfer infrastructure. When an Originator is explicitly sending via a Gateway out of the Internet, there are coding conventions for the local-part, so that the Originator can formulate instructions for the Gateway. Standardized examples of this are the telephone numbering formats for VPIM [RFC2421], such as "+16137637582@vpim.example.com", and iFax [RFC2304], such as "FAX=+12027653000/T33S=1387@ifax.example.com".
Email addresses are being used far beyond their original email transfer and delivery role. In practical terms, email strings have become a common form of user identity on the Internet. What is essential, then, is to be clear about the nature and role of an identity string in a particular context and to be clear about the entity responsible for setting that string.
A domain name is a global reference to an Internet resource, such as a host, a service or a network. A name usually maps to one or more IP Addresses. Conceptually, the name might encompass an entire organization, or a collection of machines integrated into a homogeneous service, or only a single machine. A domain name can be administered to refer to individual users, but this is not common practice. The name is structured as a hierarchical sequence of sub-names, separated by dots ("."). Domain names are defined and operated through the Domain Name Service (DNS) [RFC1034], [RFC1035], [RFC2181].
When not part of a mailbox address, a domain name is used in Internet Mail to refer to a node that took action upon the message, such as providing the administrative scope for a message identifier, or performing transfer processing.
Like mailbox addresses, message identifiers have two distinct parts, divided by an at-sign ("@"). The right-hand side is globally interpreted and specifies the Administrative Unit assigning the identifier. The left-hand side of the at-sign contains a string that is globally opaque and serves to uniquely identify the message within the domain referenced on the right-hand side. The duration of uniqueness for the message identifier is undefined.
The identifier may be assigned by the user or by any component of the system along the path, within the AU responsible for the indicated domain. Although Internet Mail standards provide for a single identifier, more than one is sometimes assigned.
In this document, fields references to identities are labeled in a two-part, dotted notation. The first part cites the document defining the identity and the second defines the name of the identity. Hence, <RFC2822.From> is the From field in an email content header, and <RFC2821.MailFrom> is the address in the SMTP "Mail From" command.
The Internet's MHS architecture distinguishes six types of functional components, arranged to support a store-and-forward service architecture:
This section describes the specific functional components for Internet Mail, and the standard protocols associated with performing them.
Software implementations of these architectural components often compress them, such as having the same software do MSA, MTA and MDA functions. However the requirements for each of these components of the service are becoming more extensive. So, their separation is increasingly common.
This figure shows function modules and the protocols used between them. Additional protocols and configurations are possible.
+------+ +---------+
...............+ oMUA |...| Disp |<----------------+
. +--+---+ +---------+ |
. | {smtp, |
. V {submission |
. +------+ +---------+ |
. | MSA |...| Bounces |< -----+ |
. +--+---+ +---------+ | |
. | | |
. V {smtp | |
. +------+ /+===+===+\ |
. | MTA | || dsn || |
/+==========+\ +--+---+ \+=======+/ |
|| MESSAGE || . ^ ^ |
||----------|| . {smtp | | |
|| Envelope || . | | |
|| SMTP || V | | |
|| RFC2822 || +------+ | | /+==+==+\
|| Content || | MTA +-------------------+ | || mdn ||
|| RFC2822 || +--+---+ | \+=====+/
|| MIME || | {local, smtp, | |
\+==========+/ V {lmtp | |
. +------+ | |
. | +-----------------------+ |
. | MDA | |
. | |<--------------------+ |
. +-+--+-+ | |
. local} | | | |
. V | | |
. +------+ | /+===+===+\ |
. | sMS | | || sieve || |
. +-+--+-+ | \+=======+/ |
. | | | {pop, imap ^ |
. | V V | |
. | +------+ | |
. | | uMS | | |
. | +--+---+ | |
. | | {pop, imap, | |
. V V {local | |
. +------+ | |
. | +---- -------------------+ |
...........>| rMUA | |
| +----------------------------------+
+------+
Figure 5: Protocols and Services
The purpose of the Mail Handling Service is to exchange a message object among participants. Hence, all of the underlying mechanisms are merely in the service of getting that message from its Originator to its Recipients. A message may be explicitly labeled as to its nature. [RFC3458]
A message comprises a transit handling envelope and the end-user message content. The envelope contains handling information used by the Message Handling Service, or generated by it. The content is divided into a structured header and the body. The body may be unstructured, simple text, or it may be a tree of multi-media subordinate objects, called body-parts.
Internet Mail has distinguished some special versions of messages, for exchanging control information:
Information that is directly used by, or produced by, the MHS is called the "envelope". It controls and records handling activities by the transfer service. Internet Mail has a fragmented framework for handling this "handling" information. The envelope exists partly in the transfer protocol SMTP [RFC2821] and partly in the message object [RFC2822]. The SMTP specification uses the term to refer only to the transfer-protocol information.
Direct envelope addressing information, as well as optional transfer directives, are carried within the SMTP control channel. Other envelope information, such as trace records, is carried within the content header fields. Upon delivery, some SMTP-level envelope information is typically encoded within additional content header fields, such as Return-Path.
Header fields are attribute/value pairs covering an extensible range of email service, user content and user transaction meta-information. The core set of header fields is defined in [RFC2822], [RFC0822]. It is common to extend this set, for different applications. Procedures for registering headers are provided in [RFC4021]. A complete set of registered header fields is being developed through [ID-hdr-reg].
One danger with placing additional information in header fields is that Gateways often alter or delete them.
The body of a message might simply be lines of ASCII text or it might be structured into a composition of multi-media, body-part attachments, using MIME [RFC2045], [RFC2046], [RFC2047], [RFC2048], and [RFC2049]. MIME structures each body-part into a recursive set of MIME header field meta-data and MIME Content sections.
For a message in transit, the core uses of identity references combine into:
| Layer | Field | Set By |
|---|---|---|
| Message Body | MIME Header | Originator |
| Message header fields | From | Originator |
| Sender | Source | |
| Reply-To | Originator | |
| To, CC, BCC | Originator | |
| Message-ID | Source | |
| Received | Source, Relay, Dest | |
| Return-Path | MDA, from MailFrom | |
| Resent-* | Mediator | |
| SMTP | HELO | Latest Relay Client |
| MailFrom | Source | |
| RcptTo | Originator | |
| IP | IP Address | Latest Relay Client |
A Mail User Agent (MUA) works on behalf of end-users and end-user applications. It is their "representative" within the email service.
At the origination side of the service, the oMUA is used to create a message and perform initial "submission" into the transfer infrastructure, via a Mail Submission Agent (MSA). It may also perform any creation- and posting-time archival. An MUA outbox is part of the origination-side MUA.
The Recipient-side rMUA works on behalf of the end-user Recipient to process received mail. This includes generating user-level return control messages, display and disposition of the received message, and closing or expanding the user communication loop, by initiating replies and forwarding new messages.
An MUA may, itself, have a distributed implementation, such as a "thin" user interface module on a limited end-user device, with the bulk of the MUA functionality operated remotely on a more capable server. An example of such an architecture might use IMAP [RFC3501] for most of the interactions between an MUA client and an MUA server.
A Mediator is special class of MUA. It performs message re-posting, as discussed in Section 2.1.
Identity fields relevant to the MUA include:
A Mail Submission Agent (MSA) accepts the message submission from the oMUA and enforces the policies of the hosting AU and the requirements of Internet standards. Enforcement might be passive, involving review and approval or rejection, or it might be active, involving direct modification of the message. An MSA implements a server function to MUAs and a client function to MTAs (or MDAs).
Examples of MSA-styled functions, in the world of paper mail, might range across the very different capabilities of administrative assistants, postal drop boxes, and post office front-counter employees.
The MUA/MSA interface can be implemented within a single host and use private conventions for its interactions. Historically, standards-based MUA/MSA interactions have used SMTP [RFC2821]. However a recent alternative is SUBMISSION [RFC2476]. Although SUBMISSION derives from SMTP, it operates on a separate TCP port, and will typically impose distinct requirements, such as access authorization.
Identities relevant to the MSA include:
A Mail Transfer Agent (MTA) relays mail for one, application-level "hop". It is like a packet-switch or IP router in that its job is to make routing assessments and to move the message closer to the Recipient(s). Relaying is performed by a sequence of MTAs, until the message reaches its destination MDA(s). Hence an MTA implements both client and server MTA functionality. It does not make changes to addresses in the envelope or reformulate the content, except as transfer-encoding requirements dictate. Also it may add trace information. Of course email objects are typically much larger than the payload of a packet or datagram, and the end-to-end latencies are typically much higher.
Internet Mail primarily uses SMTP [RFC2821], [RFC0821] to effect point-to-point transfers between peer MTAs. Other transfer mechanisms include Batch SMTP [RFC2442] and ODMR [RFC2645]. As with most network layer mechanisms Internet Mail's SMTP supports a basic level of reliability, by virtue of providing for retransmission after a temporary transfer failure. Contrary to typical packet switches (and Instant Messaging services) Internet Mail MTAs typically store messages in a manner that allows recovery across service interruptions, such as host system shutdown. However the degree of such robustness and persistence by an MTA can be highly variable.
The primary "routing" mechanism for Internet Mail is the DNS MX record [RFC1035], which specifies a host, through which the queried domain can be reached. This presumes a public -- or at least a common -- backbone that permits any attached host to connect to any other.
An important characteristic of MTA-MTA communications, over the open Internet, is that they do not require prior arrangement between the independent administrations operating the different MTAs. Given the importance of spontaneity and serendipity in the world of human communications, this lack of prearrangement, between the participants, is a core benefit of Internet Mail and remains a core requirement for it.
Identities relevant to the MTA include:
A Mail Delivery Agent (MDA) delivers email to the Recipient's mailbox. It can provide distinctive, address-based functionality, made possible by its detailed knowledge of the properties of the destination address. This knowledge might also be present elsewhere in the Recipient's Administrative Unit, such as at an organizational border Relay. However it is required for the MDA, if only because the MDA must know where to deliver the message.
Using Internet protocols, delivery can be effected by a variety of standard protocols. When coupled with an internal, local mechanism, SMTP [RFC2821] and LMTP [RFC2033] permit "push" delivery to the Recipient system, at the initiative of the upstream email service. POP [RFC1939] and IMAP [RFC3501] are used for "pull" delivery at the initiative of the Recipient system. POP and IMAP can also be used for repeated access to messages on a remote MS.
Identities relevant to the MDA include:
An MUA can use a long-term Message Store (MS). A rich set of choices for the use of that store derives from permitting more than one to be associated with a single user, demonstrated as a server-based MS (sMS) and user-based MS (uMS) in Figure 5. sMS is shown as being remote from the MUA and uMS as being local. Further the relationship between two message store may vary. Between the MDA and the MUA, these choices are supported by a wide variety of protocol options.
The operational relationship among two MSs can be:
Basic email transfer is accomplished with an asynchronous store-and-forward communication infrastructure, in a sequence of independent transmissions through some number of MTAs. A very different task is a User-level sequence of postings and deliveries, through Mediators. For such re-postings, a Mediator does share some functionality with basic MTA relaying, but it enjoys a degree of freedom with both addressing and content that is not available to MTAs.
The salient aspect of a Mediator, that distinguishes it from any other MUA creating an entirely new message, is that a Mediator preserves the integrity and tone of the original message, including the essential aspects of the original origination information. The Mediator might also add commentary.
Examples of MUA message creation that are NOT performed by Mediators include:
The remainder of this section describes common examples of Mediators.
A simple re-addressing facility that is available in most MDA implementations is called Aliasing. It is performed just before delivering a message to the specified Recipient's mailbox. Instead, the message is submitted back to the transfer service, for delivery to one or more alternate addresses. Although implemented as part of the message delivery service, this facility is strictly a Recipient user function. It resubmits the message, replacing the envelope address, on behalf of the mailbox address that was listed in the envelope.
What is most distinctive about this forwarding mechanism is how closely it compares to normal MTA store-and-forward Relaying. In reality its only interesting difference is that it changes the RFC2821.RcptTo value. Having the change be this small makes it easy to view aliasing as a part of the lower-level mail relaying activity. However the small change has a large semantic impact: The designated recipient has chosen a new recipient. Hence, that original recipient must become responsible for any handling issues.
An MDA that is re-posting a message to an alias typically changes only envelope information:
Also called ReDirecting, ReSending differs from Forwarding by virtue of having the Mediator "splice" a message's addressing information, to connect the Originator of the original message and the Recipient of the new message. This permits them to have direct exchange, using their normal MUA Reply functions. Hence the new Recipient sees the message as being From the original Originator, even if the Mediator adds commentary.
Identities specified in a resent message include
Mailing lists have explicit email addresses and they forward messages to a list of subscribed members. The Mailing List Actor performs a task that can be viewed as an elaboration of the ReDirector role. In addition to sending the new message to a potentially large number of new Recipients, the Mediator can modify content, such as deleting attachments, formatting conversion, and adding list-specific comments. In addition, archiving list messages is common. Still, the message retains characteristics of being "from" the original Originator.
Identities relevant to a mailing list processor, when submitting a message, include:
Gateways perform the basic routing and transfer work of message relaying, but they also make any message or address modifications that are needed to send the message into the next messaging environment. When a Gateway connects two differing messaging services, its role is easy to identify and understand. When it connects environments that have technical similarity, but may have significant administrative differences, it is easy to think that a Gateway is merely an MTA. The critical distinction between an MTA and a Gateway is that the latter transforms addresses and/or message content, in order to map between the standards of two, different messaging services. In virtually all cases, this mapping process results in some degree of semantic loss. The challenge of Gateway design is to minimize this loss.
A Gateway may set any identity field available to a regular MUA. Identities typically relevant to Gateways include:
Organizations often enforce security boundaries by subjecting messages to analysis, for conformance with the organization's safety policies. An example is detection of content classed as spam or a virus. A Filter might alter the content, to render it safe, such as by removing content deemed unacceptable. Typically these actions will result in the addition of content that records the actions.
This document does not specify any new Internet Mail functionality. Consequently it should introduce no new security considerations.
However its discussion of the roles and responsibilities for different mail service modules, and the information they create, highlights the considerable security considerations that must be present when implementing any component of the Internet Mail service. In addition, email transfer protocols can operate over authenticated and/or encrypted links, and message content can be authenticated or encrypted.
| [ID-ffpim] | Crocker, D. and G. Klyne, "Full-mode Fax Profile for Internet Mail: FFPIM", March 2004. |
| [ID-spamops] | Hutzler, C, Crocker, D, Resnick, P, Sanderson, R and E Allman, "Email Submission Between Independent Networks", Internet-Draft draft-spamops-00, March 2004. |
| [RFC1767] | Crocker, D., "MIME Encapsulation of EDI Objects", RFC 1767, March 1995. |
| [Tussle] | Clark, D, Wroclawski, J, Sollins, K and R Braden, "Tussle in Cyberspace: Defining Tomorrow’s Internet", ACM SIGCOMM, 2002. |
| Dave Crocker | |
| Brandenburg InternetWorking | |
| 675 Spruce Drive | |
| Sunnyvale, CA 94086 | |
| USA | |
| Phone: | +1.408.246.8253 |
| EMail: | dcrocker@bbiw.net |
This work derives from a section in draft-hutzler-spamops [ID-spamops]. Discussion of the Source actor role was greatly clarified during discussions in the IETF's Marid working group.
Graham Klyne, Pete Resnick and Steve Atkins provided thoughtful insight on the framework and details of the original drafts.
Later reviews and suggestions were provided by Nathaniel Borenstein, Ed Bradford, Cyrus Daboo, Frank Ellermann, Tony Finch, Ned Freed, Eric Hall, Brad Knowles, Bruce Lilly, Mark E. Mallett, David MacQuigg, Chris Newman, Daryl Odnert, Rahmat M. Samik-Ibrahim, Hector Santos, Jochen Topf, Willemien Hoogendoorn.
The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at <http://www.ietf.org/ipr>.
The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.
This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.
Funding for the RFC Editor function is currently provided by the Internet Society.