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The Nature of the Service Boundary in Java Integrate barcode 3/9 in Java The Nature of the Service Boundary




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The Nature of the Service Boundary use none none creator todraw none on none .NET Micro Framework Earlier service was none none used as the abstraction of the interface, hence, the term service data unit (SDU) is used to refer to the unit of data provided to the PM by the (N+1)-PM across the service boundary as a service data unit (SDU) (reserving the term interface for the implementation-specific case, such as a UNIX interface or a Windows interface). To a PM, an SDU consists entirely of user-data but will have a size that is convenient to the (N+1)-PM. The service primitive invoked to pass the SDU to the PM will also pass other parameters to the PM for handling the SDU.

Generally, one of these is a local port-id that identifies this end of the flow or connection this SDU is to be sent on. The portid is local to the system (that is, only known within the system and only unambiguous within it) and shared by the (N+1)- and (N)-PMs to refer to related communications. The PM may have to segment the SDU into several PDUs or may aggregate several SDUs into a single PDU so that the PDU is a size convenient to the requirements of the (N)-protocol.

(This nomenclature follows the OSI Reference Model, not because I favor the OSI model, which I don t, but because it is an existing nomenclature that tries to define common terms. I can find no good reason to invent new terms other than for the sake of generating new terms. For those who still have a visceral reaction to anything OSI, all I can say is, OSI is long dead, get over it.

). PROTOCOL ELEMENTS Stream Versus Record One of the enduring debates in protocol design is, given that the (N+1)-PM delivered an (N)-SDU of a particular size to the (N)-PM and that under some conditions it may have been fragmented or concatenated en route, what does the (N)-PM deliver to the remote (N+1)-PM What was sent or what was received Although it may often be the case that an SDU would be a single (N+1)-PDU, it might be more than one. Seldom would it only be part of a PDU. In any case, the (N)-SDU was a unit that the (N+1)-PM found to be significant for its processing.

The early debate was between record and stream modes, derived from early operating system practices. The older mainframe systems tended to operate on fixed-length records, whereas more modern systems such as Sigma 7, Tenex, Multics, and its derivative UNIX communicated in terms of undifferentiated byte streams. Record mode was always considered as something that simply had to be lived with.

There was general agreement that record mode was too inflexible and cumbersome. Stream mode was considered a much more flexible, elegant approach that provided greater layer independence. A stream might deliver any combination from whole SDUs to pieces of an SDU to multiple SDUs or even part of two SDUs.

Stream mode requires that the (N+1)-layer be able to recognize the beginning and end of its SDU/PDUs and be able to assemble them for processing. The (N+1)-protocol must have a delimiting mechanism and cannot rely on the layer below to tell it where the beginning and end of the PDU are. Over time, a third approach evolved, which was a generalization of record mode.

In this mode, SDUs were not fixed length. The rule in this approach was that the identity of SDUs was maintained between the sending and receiving users. No name was ever given this mode, so let s call it the idempotent mode, referring to its property of maintaining the identity of the SDU invariant.

Because SDUs may be of any length, this differs significantly from traditional fixed-length record mode. This mode requires that the (N)-layer deliver SDUs in the form it received them. If the (N)-protocol needs to fragment an SDU, it is (N)-protocol s responsibility to put things back the way it found them before delivering the SDU to the (N+1)-PM.

(There is something compelling about a do anything you want but clean up your mess when you re done approach!) This form is more consistent with good programming practice. Similarly, if the (N)-protocol combines several SDUs into a single PDU for its own reasons, it.
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