PRELIMINARIES

Mail in the Internet was not a well-planned enterprise, but instead arose in more of an "organic" way.

This introductory section is not intended to be a reference model and concept vocabulary for mail in the Internet. Instead, it only provides the necessary preliminaries for the concepts and terms that are essential to this specification.

Internet Mail Submission and Delivery

For the purposes of this specification, mail submission is the process of putting mail into the mail transfer system (MTS).

For the purposes of this specification, mail delivery is the process of the MTS putting mail into a user's final mail-box.

Throughout the Internet, presently most of mail submission and delivery is done through SMTP.

SMTP was defined as a message *transfer* protocol, that is, a means to route (if needed) and deliver mail by putting finished (complete) messages in a mail-box. Originally, users connected to servers from terminals, and all processing occurred on the server. Now, a split-MUA (Mail User Agent) model is common, with MUA functionality occurring on both the user's own system and the server.

In the split-MUA model, getting the messages to the user is accomplished through access to a mail-box on the server through such protocols as POP and IMAP. In the split-MUA model, user's access to its message is a "Message Pull" paradigm where the user is required to poll his mailbox. Proper message delivery based on a "Message Push" paradigm is presently not supported. The EMSD protocol addresses this shortcoming with an emphasis on efficiency.

In the split-MUA model, message submission is often accomplished through SMTP. SMTP is widely used as a message *submission* protocol. Widespread use of SMTP for submission is a reality, regardless of whether this is good or bad. EMSD protocol provides an alternative mechanism for message submission which emphasizes efficiency.

Relationship Of EMSD To Other Mail Protocols

Various Internet mail protocols facilitate accomplishment of various functions in mail processing.

Figure 1, categorizes the capabilities of SMTP, IMAP, POP and EMSD based on the following functions:

• Mail Submission
• Mail Delivery
• Mail Routing (Relay)
• Mail Retrieval
• Mail-box Access
• Mail-box Synchronization

  

Figure: Messaging Protocols vs. Supported Functions

In Figure 1, the number of "X"es in each box denotes the extent to which a particular function is supported by a particular protocol.

Figure 1 clearly shows that combinations of these protocols can be used to complement each other in providing rich functionality to the user. For example, a user interested in highly mobile messaging functionalities can use EMSD for "submission and delivery of time critical and important messages" and use IMAP for comprehensive access to his/her mail-box.

For mail submission and delivery of short messages EMSD is up to 5 times more efficient than SMTP both in terms of the number of packets transmitted and in terms of number of bytes transmitted. Even with PIPELINING and other possible optimizations of SMTP, EMSD is up to 3 times more efficient than SMTP both in terms of the number of packets transmitted and in terms of number of bytes transmitted. Various efficiency studies comparing EMSD with SMTP, POP and IMAP are available. See Section C.1.1 for more information about comparison of SMTP and EMSD's efficiency.

EMSD Requirements and Goals

The requirements and goals driving design of EMSD protocol are enumerated below.

1.

Provide for submission of short mail messages with the same level of functionality (or higher) that the existing Internet mail protocols provide.

2.

Provide for delivery of short mail messages with the same level of functionality (or higher) that the existing Internet mail protocols provide.

3.

Function as an extension of the existing mainstream Internet mail.

4.

Minimize the number of transmissions.

5.

Minimize the number of bytes transmitted.

6.

Be quick: minimize latency of message submission and delivery.

7.

Provide the same level of reliability (or higher) that the existing email protocols provide.

8.

Accommodate varying sizes of messages: the size of a message may determine how the system deals with the message, but the system must accommodate it.

9.

Be power efficient and respect mobile platform resources: including memory and CPU levels, as well as battery power longevity (i.e. client-light and server-heavy).

10.

Highly extendible: different users will demand different options, so the solution cannot require every feature to be a part of every message. Likewise, usage will emerge that is not currently recognized as a requirement. The solution must be extendible enough to handle new, emerging requirements.

11.

Secure: provide the same level of security (or higher) that the existing email protocols provide. Content confidentiality, originator/recipient authentication and message integrity must be available options to users.

12.

Easy to implement: Re-use existing technology as much as possible.

Anticipated Uses Of EMSD

Any network and network operator which has significant bandwidth and capacity limitations can benefit from the use of EMSD. Any network user who must bear high costs for measured network usage can benefit from the use of EMSD.

Initial uses of EMSD is anticipated to be primarily over IP-based wireless networks to provide two-way paging services.

EMSD can also function as an adjunct to Mail Access Protocols for "Mail Notification Services".

Considering:

• that most wireless networks shall converge toward being IP-based;

• that two-way paging is the main proven application in most wide-area wireless networks;

• that two-way paging industry and the Internet Email industry can and should converge based on a set of open protocols that address the efficiency requirements adequately;

• that existing Internet email protocols are not bandwidth efficient;

• that existing Internet email protocols do not properly support the "push" model of delivery of urgent messages,

the EMSD protocol is designed to facilitate the convergence of IP-based two-way paging and Internet email.

Mail submission and delivery take place at the edges of the network. More than one mail submission and delivery protocols which address requirements specific to a particular user's environment are likely to be developed. Such diversity on the edges of the network is desirable and with the right protocols, this diversity does not adversely impact the integrity of the mail transfer system. EMSD is the initial basis for the mail submission and delivery protocol to be used when the user's environment demands efficiency.

Definitions of Terms Used in this Specification

The following informal definitions and acronyms are intended to help describe EMSD model described in this specification.

Efficient Mail Submission and Delivery Protocol (EMSD-P):

The protocol used to transfer messages between the EMSD - Server Agent (e.g., a Message Center) and the EMSD - User Agent (e.g., a Two-Way Pager), see Figure .

Message Transfer Agent (MTA)

Message Transfer Service (MTS)

Message Routing Service (MRS):

Collection of MTAs responsible for mail routing.

Message User Agent (MUA)

Efficient Mail Submission Server Agent (EMS-SA):

An Application Process which conforms to this protocol specification and accepts mail from an EMS-UA and transfers it towards its recipients.

Efficient Mail Delivery Server Agent (EMD-SA):

An Application Process which conforms to this protocol specification and delivers mail to an EMD-UA.

Efficient Mail Submission and Delivery Server Agent (EMSD-SA):

An Application Process which incorporates both EMS-SA and EMD-SA capabilities.

Efficient Mail Submission User Agent (EMS-UA):

An Application Process which conforms to this protocol specification and submits mail to EMS-SA.

Efficient Mail Delivery User Agent (EMD-UA):

An Application Process which conforms to this protocol specification and accepts delivery of mail from EMD-SA.

Efficient Mail Submission and Delivery User Agent (EMSD-UA):

An Application Process which incorporates both EMS-UA and EMD-UA capabilities.

Conventions Used In This Specification

The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" in this specification are to be interpreted as defined in [2].

This specification uses the ES-OPERATION notation defined in Efficient Short Remote Operations (ESRO) protocols as specified in RFC-2188 [1].

Operations and information objects are typically described using the ES-OPERATION and ASN.1 notations in the relevant sections of the specification.

The complete machine verifiable ASN.1 modules are also compiled in one place in Appendix A and Appendix B.

About This Specification

This protocol specification constitutes a point-of-record. It documents information exchanges and behaviors of existing implementations. It is a basis for implementation of efficient mail submission and delivery user agents and servers.

This specification has been developed entirely outside of IETF. It has had the benefit of review by many outside of IETF. Much has been learned from existing implementations of this protocol. A number of deficiencies and areas of improvement have been identified and are documented in this specification.

This protocol specification is being submitted on October 23, 1998 for timely publication as an Informational RFC.

Future development and enhancements to this protocol may take place inside of IETF.