Communication Protocols by Drago Hercog

Author:Drago Hercog
Language: eng
Format: epub
ISBN: 9783030504052
Publisher: Springer International Publishing

Fig. 12.17Specification of transmitting protocol entity (transmitting information protocol messages)

Fig. 12.18Specification of transmitting protocol entity (receiving acknowledgments)

In Fig. 12.16 the declarations of the variables and the timer as well as the initialisation of the transmitting protocol entity are shown. The transmit window width is informally »nondefined« in this figure; in real world this value may either be specified by the protocol, or it may be determined at the time of a connection setup (one must be aware that the window width impacts the consumption of memory, but also, as will soon be explained, the protocol efficiency). As will be explained in the next section, the transmitting protocol entity must also know the receive window width of the receiver; in our specification this value is therefore »imported« from the receiving protocol entity process; to this end, the SDL constructs IMPORTED and IMPORT are used in the process trans, and the SDL constructs EXPORTED and EXPORT are employed in the process recv. In real life, however, the receiver communicates this value to the transmitter at the connection setup time, if it is not already predefined by the standard. Because of the restriction imposed by the rules of the SDL language the receive window width is named wri in the process trans, while in the process recv its name is wr. The transmit buffer is the array tb and has an infinite length (as specified in the definition of the data type Seqnum in Fig. 12.3, an index of this array can be any nonnegative integer); in practice, however, this buffer has a finite size, as sequence numbers are counted and processed using the modulo arithmetic. The value of the variable va indicates the beginning of the transmit window, while the value of the variable vt determines the sequence number of the information protocol data unit to be next transmitted; before a data transfer can begin both these values must be assigned the same value, 0 in our case (once more, we must emphasise that normally the values of these two variables are initialised at a connection setup time, as several connections can be set up during the lifetime of a protocol entity, and each connection must begin with a well-defined initial state—in our specification, however, these values are initialised during the initialisation of the process). The variable a only serves as a temporary store for received acknowledgments. In our specification we admit various channels to interconnect both protocol entities; the timer expiration time is therefore informally »nondefined«. In reality the timer expiration time depends on various circumstances; more about this topic will be told in Sect. 12.12.

In Fig. 12.17 the transmitting of information protocol data units is specified. Here the difference between the stop-and-wait protocol and the sliding window protocol must be emphasised. While a stop-and-wait protocol transmitter may transmit a new information message only in the state wait_req if it receives a request, but after having transmitted it only waits for the acknowledgement and may not transmit new messages (it is

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