DATA UNITS in Java Embed Code 3/9 in Java DATA UNITS

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DATA UNITS using barcode creation for none control to generate, create none image in none applications. QR Code 2D Barcode must deliv none none er them as separate SDUs to the remote user. Consequently, the (N + 1)-PM does not have to understand (or be modified for) every potential (N)-PM fragmenting or concatenation condition, nor make assumptions about what the (N)-PM will do. Maintaining the identity of SDUs maintains symmetry in an architecture.

And symmetry is always good.1 But, it does require the assumption that the layer below is well behaved. The essential difference between the two is that the idempotent mode is a user s point of view, whereas stream mode is more the implementer s point of view.

It makes no difference in the receiving system, the amount of work is the same: Either the receiving PM or the receiving user, the (N+1)-PM, must do the reassembly. In other words, the work is either done at the bottom of the (N+1)layer (stream) or the top of the (N)-layer (idempotent). There are no strong logical or architectural arguments for one or the other.

Although if it is done by the (N+1)-PM, it may have to be implemented several times (if there are many (N+1)-PMTs i.e, applications). Then, good software engineering practice supports the (N)-PM performing the function.

That said, it will be easier for protocols with sequence space granularity of octets to do stream mode (for instance, TCP), and more work to keep track of where the SDU boundaries are.2 For protocols that do sequencing to the granularity of PDUs, the amount of work is the same if there is no concatenation. If the protocol concatenates, however, it must be able to find the boundaries between SDUs.

In which case, stream mode will be less work for the (N)-protocol. Overall, the work as seen by the system is the same.3 But it does not have to be an either/or choice.

It is possible to provide both. For the solution, we take a lesson from how Telnet modeled half and full duplex as degenerate cases of a single mechanism, and from the glib comment that all data communications is a side effect (mentioned previously). We just note that a stream is simply a very long SDU! If the protocol has the ability to indicate the boundaries of SDUs and negotiates whether it may deliver partial SDUs (in order) to the (N+1)-layer, the (N+1)-layer can have either interface discipline.

Stream mode negotiates partial delivery and at a minimum indicates the end of an SDU only on the last PDU sent. Idempotent mode negotiates no partial deliv1 Yes, similar arguments can be made for stream. However, the argument that I should get.

things bac k in the same form I gave them to you is reasonable. Stream may impose additional overhead on the (N+1)-protocol that from its point of view is unnecessary (It knows what it is doing; why should it be penalized because the supporting protocol doesn t.).

2 There ar none none e rumors of applications using the TCP Urgent pointer as a means to delimit SDUs. 3 Having been a strong proponent of stream-mode from the beginning of the Net, I have spent. considerab le thought coming to this conclusion. Fixed record was clearly not a good idea; and although stream mode is elegant, it does ignore our responsibility to the user to clean up our mess..

PROTOCOL ELEMENTS ery and in dicates the end of SDUs at the appropriate times. Flags in the protocol might be defined as shown here:. Supporting Both Stream and Idempotent Delivery of Incomplete SDU Allowed 0 0 1 1 More Data 0 1 0 1 Description Self-contained PDU, equivalent to Don t Fragment Idempotent Stream (with huge buffers!) Stream Constructing Protocol A PM must none none interpret four inputs: 1. Interactions with the upper interface 2. PDUs from its corresponding PM(s) 3.

Interactions with the local system 4. Interactions with the lower interface. Good Solutions Are Never Obsolete This illus none for none trates why it is important to study good designs. Here we have used the Telnet halfduplex solution (see 4) to solve what appears to be an either/or choice. Many students would complain about being taught the Telnet solution: Why are we wasting time on this.

Half-duplex terminals are a thing of the past. I will never need this! Perhaps not, but as you have just seen, the form of problem recurs, and so the same solution in a somewhat different guise can be applied..

All of the se can be considered to be equivalent to procedure or system calls of the following form:4 <procedure name>(<param 1>,<param i>*) The PDUs can be seen as procedure calls in that the PDU type is the name of the procedure and the elements of the PDU (that is, PCI and user-data) are the parameters: <PDU type>(<PCI element><PCI element>*, user-data) Associated with each of these are actions to be taken depending on the state of the PM (that is, the body of the procedure). The action taken by each procedure is to interpret the parameters and update the state vector associated with the PM and possibly cause other PDUs to be sent or interactions with the local system or the upper- and lower-interfaces to occur. Coordinating all of this is a control function or state machine that enforces the proper sequencing of these actions according to the state of the PM.

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