Network Types Transport Services

68 Communication Networks Copyright © 2005 PragSoft a user PDU may be represented by a single TPDU, or segmented into multiple TPDUs when it is too large. The general structure of a TPDU is shown in Figure 5.60. Figure 5.60 General TPDU structure. Field Description Header Length Length of the TPDU header in octets. H TPDU Type Identifies the type of the TPDU. E Credit Used for flow control. A Destination Reference Reference to the destination user process. D Source Reference Reference to the source user process. E ClassOptionsReason Class of protocol, various options, or disconnect reason. R Variable Part Zero or more less-frequently used parameters, each of the general format Type,Length, Value. Data Actual transport service user data. A TPDU consists of a variable length header and user Data. The contents of the header is dependent on the TPDU Type.

5.2.2. Classes of Protocol

To facilitate the mapping of the user-requested QOS to an appropriate network QOS, five classes 0-4 of transport protocols are defined. The protocol class is selected during the establishment of a transport connection according to the requested QOS. The choice is made transparently by the transport layer without the user’s knowledge or involvement. Class 0 is the most basic transport service and is suitable for type A networks. Although Class 0 is capable of detecting data errors e.g., corrupted, lost, duplicated data and signaled errors e.g., failure in switching nodes, it cannot recover from them. These errors therefore result in the connection being terminated. This is the simplest form of transport service and must be supported by all transport layer implementations. Supporting other classes is optional. Class 1 protocol supports recovery from signaled errors. It also supports segmentation, expedited data transfer, and acknowledgment using sequence numbers. Class 2 is like class 0, except that it also supports multiplexing and flow control. Class 3 is like class 2, except that it also supports recovery from signaled errors. Finally, Class 4 provides the widest set of features, making its suitable for C type networks. It is the only class that supports the resequencing of TPDUs, and the splitting of the transport connection into multiple network connections to improve throughput. Figure 5.61 summarizes the five protocol classes and some of their main features, together with the network type suitable for supporting each class. Figure 5.61 Transport protocol classes. Class Class Supported Features Suitable No. Name MUX DER SER FC ACK RS EXP Network