EMSD is a messaging protocol that is highly optimized for the submission and delivery of short Internet e-mail messages. The EMSD protocol addresses all the shortcomings in the existing Internet mail system described in the previous section. EMSD properly supports the Message Push mode of operation, and it provides an alternative mechanism to SMTP for message submission. And most important of all, it does this with a major emphasis on efficiency.
As shown in Figure 1, the LEAP protocols are layered. The lower layer, called Efficient Short Remote Operations (ESRO), provides efficient reliable connectionless transport services which can be used by a variety of applications. For example, in addition to Mobile Messaging services, ESRO can also be used as a transport service for credit card verification applications and efficient micro browsers.
EMSD is built on top of ESRO. The reliability requirements for message submission and message delivery in EMSD are the same as for existing e-mail protocols. The EMSD protocol provides reliable connectionless mail submission and delivery services.
EMSD consists of two independent components: the EMSD Format Standard, and the EMSD Protocol. These two components provide the following functions:
EMSD-FS is a non-textual form of compact encoding of Internet e-mail (RFC-822) messages, which facilitates efficient message transfer. EMSD-FS is used in conjunction with the EMSD-P (described below), but is not in any way a general replacement for RFC-822. EMSD-FS defines a method of representation of short interpersonal messages. It defines the "Content" encoding (Header + Body). Although EMSD-FS contains end-to-end information, its scope is purely point-to-point. EMSD-FS relies on EMSD-P for the transfer of the content to its recipients.
EMSD-P is responsible for wrapping a limited size EMSD-FS message in a point-to-point envelope, and submitting or delivering it. EMSD-P performs the envelope encoding. EMSD-P relies on the services of Efficient Short Remote Operations (ESRO) as specified in RFC-2188 [3] for transporting the point-to-point envelope. Some of the services provided by EMSD-P include: message originator authentication, and optional message segmentation and re-assembly. EMSD-P is expressed in terms of abstract services using the ESRO notation.
Together, the EMSD Protocol and Format Standard define the protocols used to transfer messages between an EMSD Server Agent (EMSD-SA), for example a Message Center, and an EMSD User Agent (EMSD-UA), for example a Two-Way Pager.
Figure 2 illustrates how EMSD defines the communication between a specific EMSD-UA and a specific EMSD-SA. The Message Transfer System may include a number of EMSD-SAs, and each EMSD-SA may have any number of EMSD-UAs with which it communicates.
The EMSD services use the Efficient Short Remote Operations (ESRO) services. They also use the Duplicate Operation Detection Support Functions. These functions guarantee that an operation is performed no more than once.
The EMSD protocol specifications define the protocols between the EMSD Device and the EMSD Server. EMSD is built on top of, and requires the services of, ESRO (Efficient Short Remote Operations). This EMSD requirement was the major motivation for the development of ESRO; however, ESRO has been developed to be independent of EMSD.
ESRO defines a notation and the services provided by an application-service element to support interactive applications in a distributed systems environment. The scope of ESRO services is not limited to EMSD. ESRO is designed to be able to support other applications, such as finger/limited directory service.
The ESRO protocol provides reliable connectionless remote operation services on top of UDP (or any other non-reliable connectionless transport service) with minimum overhead. ESRO supports segmentation and reassembly, concatenation and separation, as well as multiplexing for service users (i.e. applications).
The ESRO service is similar to and is consistent with other Remote Procedure Call services. The major emphasis of the ESRO service definition and the ESRO protocol is on efficiency. ESRO has been designed specifically with wireless network (e.g. CDPD) usage in mind.
The service model, the notation, and the protocol for ESRO are fully specified in RFC-2188 [3]. The EMSD Protocol uses ESRO to accomplish reliable connectionless mail submission and delivery.
For more information on ESRO, see the article entitled ESRO: A Foundation for the Development of Efficient Protocols within The LEAP Manifesto, or visit the ESRO website at http://www.esro.org/.
Any network or network operator which faces significant bandwidth and capacity limitations can benefit from the use of EMSD. Any user of a network who must bear high costs for measured network usage can benefit from the use of EMSD.
The initial use of EMSD is expected to be primarily to provide Mobile Messaging services over IP-based wireless networks. However, EMSD can also function as an adjunct to Mail Access Protocols for "Mail Notification Services."
Mail submission and delivery take place at the edges of the network. It is likely that multiple mail submission and delivery protocols will be developed, each addressing the specific requirements of a particular user's environment. Such diversity on the edges of the network is beneficial, and with the right protocols, this diversity does not adversely affect the integrity of the mail transfer system. EMSD is the basis for the mail submission and delivery protocol to be used when the user's environment demands efficiency.
