Rationale for Key Design Decisions

This section summarizes the rationale for the key design decisions that were made while developing the EMSD protocols.

Deviation from the SMTP Model

SMTP is the main mail transport mechanism used throughout the Internet. It is widely deployed and well understood by many engineers who specialize in Internet e-mail. For these reasons, protocols based on or derived from SMTP or more likely to become widely deployed throughout the Internet.

However, SMTP is highly inefficient for the transfer of short messages. SMTP is inefficient both in terms of the number of transmissions, and in terms of the number of bytes transmitted. Even when fully optimized with PIPELINING, SMTP remains significantly inefficient.

The submission of a short message using SMTP requires 15 transmissions. The submission of a short message with SMTP and PIPELINING requires 9 transmissions. The submission of a short message with EMSD (EMSD-P and ESRO) requires only 3 transmissions (in a typical case).

The key design requirement of EMSD is efficiency. Because of the 3 fold (at least) gain in efficiency, this justifies the deviation from the SMTP model.

Efficiency Comparison of SMTP and EMSD

Table 6.1 shows the number of N-PDUs exchanged for the transfer of a short Internet e-mail when using SMTP, SMTP with PIPELINING, QMTP, and EMSD. The names used for identifying the PDUs are informal names.

Table 6.1: Comparison of EMSD to Other Protocols

	SMTP	SMTP + Pipelining	QMTP , QMQP	EMSD
Client:	SYN	SYN	SYN	Submit.Req
Server:	SYN ok	SYN ok	SYN	Submit.Resp
Client:	HELLO	HELLO	message	ack
Server:	ok	PIPELINING	accept close
Client:	MAIL	MAIL RCPT DATA	close
Server:	ok	ok
Client:	RCPT	message QUIT
Server:	ok	accept ok close
Client:	DATA	close
Server:	ok
Client:	message
Server:	accept
Client:	QUIT
Server:	ok close
Client:	close

Use of ESRO Instead of TCP

In order to provide the same level of reliability that the existing e-mail protocols provide for short messages, it is clear that a reliable underlying service is needed. UDP [80], by itself, is clearly not adequate.

Use of TCP however, involves three phases:

1. Connection Establishment
2. Data Transfer
3. Disconnect

The reliable transfer of a short message using TCP involves a minimum of five transmissions, as is the case with QMTP.

Again, the key design requirement of EMSD is efficiency. Therefore deviation from TCP is justified, because this eliminates the two extra transmissions that are an inherent characteristic of TCP.

The ESRO protocol, as specified in RFC-2188 [91], provides reliable connectionless remote operation services on top of UDP [80] with minimum overhead. ESRO supports segmentation and reassembly, concatenation and separation.

The reliable transfer of a short message using ESRO involves 3 transmissions, as is the case with EMSD-P.

Use of the Remote Procedure Call (RPC) Model

Many Internet protocols are "text-based." On the other hand, few Internet protocols are RPC-based. Protocols designed on the basis of the "text-based" approach have a better track record of acceptance throughout the Internet.

However, considering that message submission and delivery in EMSD involves no more than two data exchanges, the text-based model becomes the same as an operation. Furthermore, the RPC model is the natural way of using ESRO.

Use of ASN.1

In order to minimize the number of bytes transfered, efficient encoding mechanisms are needed.

Among today's encoding mechanisms, ASN.1 has the unique feature of separating the abstract syntax from the encoding rules. By selecting ASN.1 as the notation used for expressing EMSD's information objects, EMSD has the flexibility of using the most efficient encoding rules, such as Packed Encoding Rules (PER), when they are available.

Efficient encoding can always be better performed when the syntax of the information is known. In general, encoding and compression techniques which use the knowledge of the syntax of the information produce better results than those compression techniques that work on arbitrary text.