Splitting and Recombining Transport Protocol

70 Communication Networks Copyright © 2005 PragSoft references are used instead. These are much shorter identifiers that are mapped to the transport addresses in the variable part of the TPDU header during connection establishment, allowing the two parties to agree on a common mapping. The source and destination references are used for the remainder of the connection. Also, for user convenience, a higher level mapping is often provided by a name server, which maps meaningful service names for frequently-used transport addresses to the addresses themselves. When wishing to connect to one of these services, the user specifies the service name and the network software looks up the name using the name server to come up with the corresponding transport address, which is then used for initiating a transport connection.

5.2.7. Flow Control

As mentioned earlier, supporting multiplexing in Class 2, 3, and 4 protocols requires transport connections to be separately flow controlled. The flow control protocol used for this purpose is called credit. It is similar to the sliding window protocol of the data link and network layers in that a window is used. However, unlike the sliding window protocol where the receiver window size remains fixed, here the window size may vary. The sender is informed of the new window size through a data acknowledgment TPDU which includes a new credit value the Credit field in the TPDU which denotes the number of TPDUs that can be received. A suitable credit value is negotiated during connection establishment. This value can be dynamically adjusted subsequently by the receiver. When the receiver is unable to receive further TPDUs, it can suspend flow of further TPDUs by adjusting the credit value to 0.

5.2.8. Error Checking

The transport layer may do additional error checking on top of the ones carried out by lower layers. This may serve one of two reasons: the lower layers might not be performing any error checking, or the network service may be very unreliable and deliver packets that have undetected errors. To address these needs especially for the Class 4 protocol a checksum option is available in the TPDUs. This is a 16-bit checksum that covers the entire TPDU and is designed to be much simpler than CRC so that it can be efficiently computed in software.

5.3. Transport Layer Standards

Transport services discussed in Section 5.1 are defined by the ISO 8072 and CCITT X.214 standards. ISO 8073 and CCITT X.224 define the transport protocol, and were the basis of the descriptions in Section 5.2. Below we will look at TCP— a de facto transport layer standard that has gained considerable acceptance in the industry.