EMSD is a messaging protocol implemented in the LEAP wireless protocol and is published in RFC 2524: Neda's Efficient Mail Submission and Delivery (EMSD) Protocol Specification Version 1.3.
Efficient Mail Submission and Delivery was designed with the following considerations in mind:
- Wireless data networks are converging, and will continue to converge, on Internet Protocol (IP);
- Two-way paging is the dominant application for wide-area wireless networks;
- A set of open protocols is required to enable the convergence of the Two-Way Paging industry and the Internet e-mail industry;
- Existing Internet e-mail protocols do not provide the bandwidth efficiency required for wireless applications;
- Existing Internet e-mail protocols do not support the "push" model for delivery of time-critical messages
EMSD is a message delivery protocol that provides the same kind of services as SMTP for normal e-mail. It runs on top of ESRO in the same way SMTP runs on TCP.
EMSD aims to combine the best parts of traditional paging and email. It is an elegant system combining the structure and interoperability of e-mail with the mobility and "Push"-model delivery of paging. Also, EMSD is optimized for the wireless environment: it's shown to be about 5 times more efficient than SMTP in both number of packets transmitted and number of bytes.
Operation
I think it’s important to note exactly where EMSD falls into the whole Internet Mail paradigm. I’ve compared it to SMTP already so I’ll continue that:
SMTP provides message submission (User sends mail to a server), delivery (User retrieves mail from a server) and relay (Servers exchange mail messages ‘routing’ them to a server that communicates with the end user). The Relay functionality makes SMTP servers Message Transfer Agents (MTAs).
EMSD only provides message submission and delivery; it lacks relay capability and relies on MTAs to do that. This would make sense in an Internet/wireless environment where Internet-based SMTP MTAs would transfer the message to an EMSD server that would communicate with wireless users. So, EMSD servers would be connected by SMTP servers for inter-server ‘routing’ or relaying.
EMSD also provides message retrieval functions, which is POP’s major function. EMSD provides all the major mail services between client and server, but not server to server.
When I first looked at this model my first issue was this: the EMSD servers receive mail from a client and then appear to relay the message to the MTAs which of course forward it along. If an EMSD server can receive and relay a message to an MTA, then why can’t they relay them between each other?
The answer is this: EMSD does not have the capacity to route messages end-to-end. The way this system is working is once the EMSD mail server realizes it is not responsible for direct delivery of this message, it submits it to a pre-configured MTA (kind of like a default gateway in routing). Mail submission is part of EMSD. The EMSD submits the message to the MTA just like any other client would, and the MTA treats the message like it was receiving it from a client and then relays the mail like any other mail.
Efficiency
SMTP is an established standard, and derivation from that standard to a new technology that people will have to learn and implement is a major undertaking. However, the reasons that LEAP does this are very clear:
Submission of a short message with SMTP involves 15 transmissions. Submission of a short message with SMTP and PIPELINING involves 9 transmissions. Submission of a short message with EMSD (EMSD-P and ESRO) involves 3 transmissions (in typical cases). (Banan, 1999)
For an example, here’s a comparison of the required exchanges between all the protocols mentioned above:
SMTP SMTP + pipelining EMSD
------ ----------------- ---------
client: SYN SYN Submit.Req
server: SYN ok SYN ok Submit.Resp
client: HELO HELO ack
server: ok PIPELINING
client: MAIL MAIL RCPT DATA
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
(
Banan, 1999)
Wireless is a whole different ballgame, and LEAP had to be designed from the ground up keeping in mind the special constraints of the wireless environment. EMSD was designed to provide the same kind of functionality as SMTP and POP but with a minimum of overhead. SMTP was designed in an era where bandwidth and latency was not as sweet as it is today, but it was designed to be simple, and robust. It was not designed with efficiency in mind.