TCP/IP Tutorial and Technical Overview - IBM Redbooks

datagrams. For that functionality, use WDP instead. The sequence of events in a
Class 0 transaction is as follows:
1. An initiator sends an invoke message to the responder. After the message
has been sent, the initiator’s role in the transaction has ended. No
retransmissions are sent of the message.
2. The responder receives the invoke message. No response is sent
acknowledging receipt of the invoke message. After the message has been
received, the transaction is complete.
Because there are no retransmissions or replies, this type of transaction is
considered stateless and cannot be aborted. An additional illustration of a Class
0 transaction can be found in Figure 18-12 on page 685).
Class 1
Class 1 is defined as “confirmed invoke message with no result message.” This
is used for reliable data push (see 18.6, “WAP push architecture” on page 664),
where no response from the destination is expected, though the transaction is
still termed a reliable datagram service. The sequence of events in a Class 1
transaction is as follows:
1. An initiator sends an invoke message to the responder.
2. The responder receives the invoke message and returns an
acknowledgement (but not a result message). If the initiator does not receive
an acknowledgement, the invoke message is retransmitted.
The transaction remains in an active state until the acknowledgement arrives at
the initiator. Upon receipt of the acknowledgement, the Initiator considers the
transaction to have ended. The Class 1 transaction can be aborted at any point
during the sequence of events.
Class 2
Class 2 is defined as “confirmed invoke message with one confirmed result
message.” In this class, one single request produces a single reply. This
comprises the typical request/response interaction model most widely employed
by applications. The sequence of events in a Class 2 transaction is as follows:
1. An initiator sends an invoke message to the responder.
2. The responder might send back an acknowledgement to this message. This
happens if the responder is not able to send back a result message
immediately.
3. The responder sends back a result message. If no acknowledgment had
been previously sent to the initiator, this result message implicitly
acknowledges the receipt of the invoke message. If the initiator receives no
acknowledgement or result message, it retransmits the invoke message.
680 TCP/IP Tutorial and Technical Overview